Cummins-Onan Power Topic White Papers - Alternator

8/10/2019 Cummins-Onan Power Topic White Papers - Alternator

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Power topic #5981 | Technical information from Cummins Power Generation

■ White Paper

Rich Scoggins — Applications Engineering Technical Specialist, Cummins Power Generation

David Matuseski, PE — Technical Counsel / Critical Protection, Cummins Power Generation

Alternator winding pitch andpower system design

Winding Pitch Considerations

In this paper we will state three conclusions:

■ 2/3 pitch is recommended for all 4-wire

distribution systems (i.e., distribution systemswhich include a neutral conductor) because2/3 pitch alternators suppress third harmoniccurrent which will circulate in the neutral.

■ 5/6 pitch alternators are acceptable in systemsthat do not include a neutral, including themajority of medium voltage/high voltage systems

■ Alternators of different pitches may be paralleledif measures are taken to eliminate or mitigate therisks of current circulating in the neutrals.

Winding pitch is one of many

parameters in an alternator design

that must be chosen to optimize the

product for its intended application.

As with most design decisions,

the best choice depends on the

application. This paper will examine

the key factors influencing the

selection of alternator winding pitch

and advantages and disadvantages

of the two most commonly used

winding pitches, 2/3 pitch and 5/6

pitch. We will also discuss concerns

related to paralleling alternators ofdifferent winding pitches and how

these concerns are mitigated.



0 2 | P o w e r T o p i c # 5 9 8 1

What is Winding Pitch?

The term “winding pitch” refers to the ratio of the

number of winding slots enclosed by each coil in the

alternator stator to the number of winding slots per

generator pole.

Figure 2 illustrates a full pitch winding. Here we have a

4-pole alternator with a 48 slot stator. In this alternator

one pole spans twelve slots. The winding spans all

twelve slots so this is referred to as a full pitch winding.

In Figure 3, the winding spans ten of the twelve slots. This winding has a pitch of 10/12, or 5/6 of the slots

on the pole. This is a 5/6 pitch winding.

In Figure 4, the winding spans eight of the twelve

slots, or 2/3 of the slots on the pole. This is a 2/3

pitch winding.

Winding Pitch and Harmonics

The voltage waveform shape created by an alternator

when operating unloaded or driving a linear load may

be described in terms of its fundamental frequency

and voltage magnitude, along with the magnitude

of the harmonic voltages and their frequencies. The

description is necessary because all alternators exhibit

some level of harmonic voltage distortion, and while

these distortions are very small relative to the distortion

that can be caused by non-linear loads, they may still

be significant, particularly in paralleling applications.

Figure 5 shows the relationship of first-order

(fundamental frequency waveform, shown in red) to

third, fifth and seventh-order harmonic waveforms

(shown in blue, green and tan, respectively). The

harmonic voltages are effectively added to thefundamental waveform, resulting in the pure sinusoidal

shape of the fundamental being somewhat distorted.

At any point in time the resultant voltage (shown in

black) will be the sum of the fundamental and all of

the harmonics.

Winding pitch is one of several design factors

that affect the harmonic content of the generated

waveform. A parameter known as pitch factor (K p )

defines the reduction of harmonic content due to using

a fractional pitch (i.e., less than full pitch) winding.

Figure 2. A full pitch winding

Figure 3. A 5/6 pitch winding

Figure 4. A 2/3 pitch winding



voltage” in this context refers to systems in which the

line voltage is less than 1000 volts. Because there aredifferent definitions for medium and high voltage in

different regions we will use the terms “medium or high

voltage” or simply “MV / HV” when referring to systems

in which the line voltage exceeds 1000 volts.) The

reason for this is that in 4-wire systems 3 rd harmonic

currents (in fact all triplen harmonic currents) from

all three phases add directly in the neutral. This can

result in high levels of harmonic distortion and potential

overheating in the neutral conductor. Single-phase

loads always have currents flowing in the neutral and

in particular single-phase rectified or switching loads

(such as switch mode power supplies) generate 3rd

K pis defined as K

p = cos(N*180(1-pitch)/2)

where:

N = the order of harmonic

Pitch is specified as a fraction (2/3, 5/6, etc.)

Note that for a full pitch winding (pitch = 1) the pitch

factor is 1 for all harmonics. There is no reduction in

voltage at the fundamental frequency or any of the

harmonics.

Table 1 illustrates the primary advantages and

disadvantages of 2/3 versus 5/6 pitch alternators.

Notice that the pitch factor for the fundamental

harmonic is 0.97 for the 5/6 pitch alternator and

0.87 for the 2/3 pitch alternator. This means that for a

5/6 pitch alternator the fundamental voltage is

97 percent of the fundamental voltage generated by

the same alternator if it were wound with a full winding

pitch, at the same level of excitation. For the 2/3 pitch

alternator the fundamental voltage is 87 percent of the

fundamental voltage generated by a full pitch winding.

This shows that an alternator with a 5/6 pitch coil will

be able to generate a higher fundamental voltage

than a 2/3 pitch coil at the same level of excitation. An alternator wound with a 5/6 pitch coil is capable of

greater kVA output than the same alternator would be

if it were wound with a 2/3 pitch coil. This is the main

advantage of a 5/6 pitch alternator as opposed to a

2/3 pitch. It allows for a more efficient use of copper

and steel, as a greater kVA output can be generated

using the same amount of material.

The primary advantage of a 2/3 pitch alternator is that

it generates no 3rd harmonic content, as can be seen in

Table 1. In fact 2/3 pitch alternators generate no triplen

harmonics at all. (The term triplen harmonics refers to

all odd multiples of the 3rd harmonic, so the 3rd, 9th, 15th

and 21st are all triplen harmonics.)

It is important to minimize all of the harmonic

voltages. The Total Harmonic Distortion (THD), a

summation of all of the harmonic voltages as a

percentage of the fundamental, is a commonly

specified alternator parameter. With good alternator

design similar values of THD can be achieved with

either a 2/3 or 5/6 pitch alternator.

While it is important to minimize all harmonic voltages,

the 3rd harmonic merits special consideration in low

voltage, 4-wire systems. (Note that the term “low

Winding Pitch Factor (K p )

2/3 Pitch 5/6 Pitch

Fundamental 0.87 0.97

3rd Harmonic 0.00 0.71

5th Harmonic 0.87 0.26

7th Harmonic 0.87 0.26

Table 1. Winding pitch factors for 2/3 and 5/6 pitch alternators

Figure 5. Waveform harmonics



0 4 | P o w e r T o p i c # 5 9 8 1

harmonic currents. Low voltage three phase rectified

(non-linear) loads also generate 3rd harmonic currents.

It is for these reasons that Cummins uses 2/3 pitch

alternators for all low voltage generator sets. Although

5/6 pitch windings minimize the 5th and 7th harmonic (as

can be seen from Table 1) this advantage is outweighed

by the advantage of the 2/3 pitch alternator eliminating

the 3rd harmonic in low voltage systems.

At medium or high voltage the effects of 3 rd harmonics

are less of a consideration because the neutral wire

isn’t typically used. With MV/HV generator sets a

transformer is typically used to step the voltagedown to line voltage. The secondary winding of the

transformer will be Y-connected, providing a neutral

conductor for single-phase loads. The primary winding

of the transformer will be delta connected to the

generator windings. 3rd harmonic currents will circulate

in the delta connected primary winding and remain on

the high voltage side of the transformer. While these 3 rd

harmonic currents will generate heat (like all harmonic

current), the effect is not as dramatic as the effect of

3rd harmonic current adding directly in the neutral wire.

When a neutral conductor is not used, as is the case

for most medium and high voltage systems, generated3rd harmonic voltage is less of a consideration. In these

applications, 5/6 pitch alternators are often used

successfully as long as the total harmonic distortion

is kept low and care is taken to eliminate or minimize

current circulating between the neutral and ground

connections of the paralleled generator sets.

Paralleling Alternators of

Dissimilar Pitch

When generators are paralleled, the voltages of the

two machines are forced to the exact same magnitude

at the point where they are connected to the paralleling

bus. Differences in electromotive force (emf) generated

by the alternators will result in current flow from

the machine with higher instantaneous emf to the

machine(s) with lower instantaneous emf. Figure 6.

illustrates this phenomenon.

In this figure, two voltage waveforms (the red and blue

lines) are superimposed upon each other. Note that

these voltage waveforms may be exactly the same

RMS voltage magnitude, but at different points in

time the blue voltage is higher than the red, and vice

versa. When the machines are connected together

on a common bus, the differences in voltage result

in current flow between the machines, which is

represented by the green line. Note that in this simple

example the magnitude of the current shown is

exaggerated to more clearly illustrate the phenomenon.

Note also that because the blue and red voltage lines

cross each other three times in each half cycle, the

current generated is a third-order harmonic current.

Figure 6. Current flow between paralleled alternators of dissimilar pitch

Figure 7. Neutral current in 4-wire paralleled generator sets



In this example either 2/3 pitch or 5/6 pitch medium

voltage alternators are commonly used. There is one

additional consideration when a 5/6 pitch alternator

is used. As stated before, a 5/6 pitch alternator will

produce some 3rd harmonic voltage. If there are

impedance differences between the generators,

there will be some amount of current that will circulate

between them through the resistors and the earth

ground connection. These impedance differences may

be due to slight alternator differences, different cable

lengths or different resistor values. In the majority of

cases, impedance differences are not big enough for

this circulating current flow to cause any problems. The alternator and resistor specifications should take

this current into account. Many times it is easiest and

most cost effective to choose an alternator and resistor

that are not affected by this current flow.

When a reduction in circulating current flow is

required, there are alternative grounding methods

that can be used. These methods include utilizing

reactors in place of resistors or using a neutral

switching scheme. Reactors could be used in place

of the resistors with the same configuration shown

in Figure 8. The reactors need to be tuned and sized

appropriately. Another reactor method is described

in the following section on “Strategies for Paralleling

Generators with Dissimilar Pitches”. The neutral

switching method is described below.

So, at any point in the cycle where there is a

voltage difference between the machines prior to

paralleling, current will flow between the machines.

This is referred to as circulating neutral current and

is apparent when there is a path through the neutral

of the system in which the current can flow. Figure 7

illustrates circulating current flowing through the neutral

conductors of paralleled generator sets.

The impact of incompatibility can be clearly seen with

proper measuring devices, and is often visible with

conventional AC current metering. The circulating

current will be most apparent by displaying currentflowing from each generator with no load on the

system. There are several methods for paralleling

5/6 pitch alternators or alternators of dissimilar pitch

and the most common of these methods will be

described here.

Examples and Best Practices

Medium / High Voltage Examples

The most common methods for paralle ling medium /

high voltage generators vary across the globe. Wewill present different two different methods, one used

commonly in North America and one used commonly

outside of North America.

Figure 8 shows the most common generator

paralleling configuration at medium voltage used in

North America. In this system the neutral point of each

generator is connected to earth through a neutral

grounding resistor. The generators parallel at medium

voltage and all power travels through a delta-wye

transformer before it reaches the actual load. In this

configuration, any harmonic distortion created by the

generators remains on the high side of the power

transformer and does not reach the actual load. As you

can see, harmonic distortion from the power source is

much less of a concern when generating at medium

voltage because it does not add to the harmonic

distortion that is created by the load. Plus, any 3 rd

harmonic voltage created by the generator will stay

on the medium voltage side of the transformer and

not be traveling through the neutral in the low voltage

distribution system. Figure 8. Paralleled medium voltage generators (typical arrangement

in North America)



0 6 | P o w e r T o p i c # 5 9 8 1

path on which the most disruptive current can flow.

(The harmonic currents will still cause heating in the

machines, but the disruptive effect of current flow in

the neutral is eliminated.)

Parallel dissimilar alternators in

4-wire systems.

When paralleled generator sets are required to

serve single-phase loads directly (with no delta-wye

transformer in the circuit to create a neutral for the

loads) there are three methods to reduce the risk

of circulating neutral currents: connecting neutrals

of similar pitch machines only, installing reactors

between the dissimilar pitch alternators, or derating the

alternator to accommodate the neutral current.

Figure 9 shows the most common generator paralleling

configuration for medium/high voltage systems

outside of North America. When the generators

are stopped, all the neutral grounding switches are

closed. When the generators are called to start, thefirst one to close its paralleling breaker will keep its

neutral grounding switch closed. Then, all the other

neutral grounding switches are opened. By having only

one generator connected to the earthing resistor, all

potential circulating currents between the generators

is eliminated. If the generator system is then paralleled

with the utility, the neutral grounding switch that is

closed should then be opened. As with all grounding

schemes, the neutral grounding resistor must be

properly sized. This method has the same advantage

of keeping all the harmonic current created by the

generators on the high side of the transformer and notadding to the harmonic content of the load.

Low Voltage Example

Figure 10 shows a typical low voltage paralleling

system. In North America most low voltage systems

operate at 480V line-to-line with some systems

operating at 416V or 600V line-to-line. There are two

major differences between a low voltage system and

a MV/HV system that require a more careful choice of

the alternator winding pitch. First, many low voltage

systems include a neutral conductor that is connected

between the generators and the paralleling switchgear. This neutral may also be connected to the various low

voltage loads. Second, many of the low voltage loads

are non-linear and generate harmonics on the power

system while they are operating. Some of these loads

include variable frequency drives (VFD), uninterruptible

power supplies (UPS) and switched mode power

supplies (SMPS). As stated before, choosing an

alternator that minimizes the total harmonic distortion

that it produces is important for low voltage systems.

A 2/3 pitch alternator will not add any third order

harmonics to the total system. Therefore a

2/3 pitch alternator is the best choice for low

voltage 4-wire systems.

Strategies for Paralleling

Generators with Dissimilar Pitches

Use a 3-wire distribution system when

paralleling alternators of dissimilar pitch.

By avoiding a solid neutral connection between the

genset bus and the loads, the most common cause

of harmonic problems is minimized by removing the

Figure 10. Paralleled low voltage generators

Figure 9. Paralleled MV/HV generators (typical arrangement in

Europe).



The major issue in the use of reactors is their cost,

and the custom nature of their design, making them

problematic to acquire and install quickly. Also, the

failure of the reactor may go undetected for a long

time resulting in a change in the effecting bondingarrangement of the system and potential

unexpected hazards.

Derating alternators to accommodate

circulating current.

The circulating current may or may not be damaging

to the alternators, depending on the magnitude of the

current, the ratings of the generators in the system and

the susceptibility of protective devices in the system to

neutral or harmonic currents. Because the harmonic

content of a generator waveform varies with the

load, the negative effects of operating with dissimilargenerators may be more apparent at some load levels

than at others, but typically the major concern will be

the magnitude of current flow at rated load, because

that is the point at which the internal temperature of

the alternator will typically be highest and is most

susceptible to failure.

In 4-wire generator installations that use dissimilar

pitch generators, generator neutral current should be

measured to verify that operation of the generators in

parallel will not result in system operation problems

or premature generator failure. If there are no other

related problems in the system, the designer may

allow system operation with the neutral current and

compensate by derating the alternator.

The derating factors can be calculated as follows:

Maximum allowable load on alternator

(KVA) = IR /[(I

R2 + I

N2 )1/2(KVA

gen )]

where:

IR = output current of the generator set atfull load and rated power factor

IN = neutral current of the generator set at

full balanced load, paralleled

KVA gen

= alternator rated KVA at

maximum temperature rise

Connect neutrals of like-pitch machines only.

Low voltage systems are usually required to have a

neutral-to-ground connection. In a parallel application

the ideal location for this bonding point is in the system

switchgear, so that there is only one neutral bond

for the system. Consideration must be given to the

magnitude of loads requiring the neutral connection

versus loads that can operate only on the three phases.

System loads will naturally balance out as long as there

is sufficient line-to-neutral capacity in the system.

Care must be taken in this scenario to make sure that

a generator without a neutral connection is not the first

to close to the bus or the system will have no neutral

to ground bond. If there is a scenario in which any

generator must be allowed to be the only generator

connected to the bus (in the event that all othergenerator sets have failed for example) use neutral

contactors to make sure that only like pitch machines

have their neutrals connected, similar to the system

shown in Figure 9. In this design it is particularly critical

for the failure modes of the neutral contactors to be

considered. Alarms should be raised by failure of a

neutral closure to operate correctly in either opening

or closing mode. Dual neutral contactor position

indicating contacts (one “a” and one “b” from different

switches) should be used to be more certain of the

state of the neutral contactor.

Install reactors in the neutral leg between

generators of different pitches.

Reactors can be installed in the neutral leg between

generators of different pitches, as shown in Figure 11,

to minimize circulating neutral current. Reactors can

be tuned to specific frequencies that are the biggest

problems, but typically they are designed for 150/180

Hz, as this is the most problematic harmonic.

Figure 11. Reactor installed in the neutral leg between paralleled

alternators of dissimilar pitches



power.cummins.com

©2014 Cummins Power Generation Inc. All right s reserved. Cummins Power Ge nerationand Cummins are registered trademarks ofCummins Inc. “Our energy working for you.”

About the authors

Rich Scroggins is a Technical Specialist

in the Application Engineering group at

Cummins Power Generation. Rich has

been with Cummins for 18 years in a variety

of engineering and product management

roles. Rich has led product developmentand application work with transfer switches,

switchgear controls and networking and

remote monitoring products and has

developed and conducted seminars and

sales and service training internationally

on several products. Rich received his

bachelors degree in electrical engineering

from the University of Minnesota and an

MBA from the University of St. Thomas.

Summary

2/3 pitch alternators have long been the preference

for low voltage distribution systems due to their

suppression of 3rd harmonic currents. In systems

that do not use a neutral conductor, 3rd harmonic

currents are less of a concern and alternators of

different pitches are acceptable provided that the total

harmonic distortion is low.

Paralleling alternators of dissimilar pitches creates

a possibility for circulating currents between the

alternators. However, there are techniques that can

be applied to eliminate or reduce circulating currents

mitigating the risk.

Cummins Power Generation recommends 2/3 pitch

alternators for all 4-wire distribution systems. 5/6 pitch

(or alternators of other pitches) are acceptable for

use in systems that do not use a neutral conductor.

Alternators of dissimilar pitches may be paralleled if

measures are taken to minimize circulating currents.

References

■“AC Generators with 2/3 and 5/6 Winding Pitch”,Dr. Jawad Al-Tayie, Chris Whitworth, Dr. AndreasBiebighaeuser.

■“Paralleling Dissimilar Gensets: Part 2 – Compatible

Alternators”, Gary Olson, Cummins Power Generation.

For additional information about onsite power systems

or other energy solutions, visit power.cummins.com.

David Matuseski graduated from the

University of Minnesota with a Bachelor

of Electrical Engineering in 1987. He

is a registered Professional Engineer

in the state of Minnesota. Dave has

been working in the power industry asa Cummins engineer and as a power

engineering consultant since 1996.

Within Cummins, Dave has held the

positions of Design Engineer, Project

Manager, Engineering Manager and

Chief Engineer. His current position is

Technical Counsel for the Cummins

Critical Protection group that specializes

in the data center, healthcare and water

treatment market segments.

P o w e r T o p i c # 5 9 8 1

http://power.cummins.com/

http://power.cummins.com/



Impact of leading powerfactor loads onsynchronous alternators

Power topic #6001 | Technical information from Cummins Power Generation

Many electrical loads incorporate elementsthat can impose a leading power factor on thepower source. While these loads are typicallynot a problem for utility power sources, leadingpower factor can cause generator set failures orthe failure of certain loads to operate properlyon a generator set. This paper briefly explainsthe phenomena, and what can be done toaddress problems when leading power factor

loads are present.

The problems seen when attempting to operate genera-

tor sets with leading power factor loads may seem

mysterious, but in reality, they are not too much different

from another energy absorption problem: the limited

ability of a generator set to absorb real kW power from

loads some elevator drives, and in crane applications.

A generator is physically unable to absorb more than a

very small amount of real (kW) or reactive (kVAR) power.

While the reverse kW power produced by a dropping

load in a crane application drove the engine into over-

speed conditions when it exceeded the ability of the

engine to absorb it, the reverse kVAR load presented by

leading power factor devices will drive the alternator into

over voltage conditions.

Over the past years, generator set manufacturers have

evolved their equipment designs to include use of

digital automatic voltage regulator (AVR) equipment,

separate excitation systems, and PWM-type control

architecture to enable the generator set to produce

and stable output voltage and successfully operate

non-linear loads. At the same time, manufacturers of

equipment that has non-linear load characteristics have

begun to commonly employ filters to limit harmonic

current distortion induced on the power supply.

Capacitive elements are also applied in facilities to

improve the power factor when operating on the utility

source to avoid higher energy charges. While filters

provide positive impacts on the overall power system,

they can be very disruptive to generator operation.

> White paper

By Gary Olson, Director, Power Systems Development

FIGURE 1 – In this example no load field required is 17 amps, while

full load is approximately 38 amps.

360

10 20 30 40 50 60

280

320

240

200

160

120

80

40

80

40

00

EXAMPLE

Field Current Amperes

Full Load Amperes

No LoadSaturation

1.0 P.F. 0.8 P.F.

Zero P.F.

Normal Volts

G e n e r a t o r V o l t s

G

0

250 kw – 312.5 kVa

240 Volts – 735 Amps



www.cumminspower.com

© 2009 Cummins Power Generation

Further, there is a tendency, particularly in data center

applications, to group UPS loads together on a common

bus. This concentrates the leading power factor load on

one bus, so that if a large group of UPS load is applied

to the first generator set available, it can easily be driven

into an excess reverse var condition, which will result

in overvoltage and shutdown. If multiple generator setsare on the bus and a large reverse var load is applied

to the genset bus, the var load sharing control system

can be disrupted, because not all load sharing control

systems include logic for reverse kVAR load sharing.

If reverse kVAR load sharing is not in the logic for the

control system the system will typically cause one or

more generator sets to exceed their reverse power

limits, which can cause pole slipping.

Generator sets in a paralleling system are maintained in

synchronism by their magnetic fields. When a leading

power factor load is applied, the voltage of the genset

or genset bus rises, and the voltage regulation system

of each generator set reduces exciter power, reducing

the strength of the magnetic field. If the field is switched

off in an attempt to reduce voltage to an acceptable

level, the generator set may slip a pole, which results in

potentially catastrophic alternator damage.

The reverse kVAR limit of the aggregated generators

is the sum of the reverse var limits of each generator.

However, the reverse var settings may not be able to

take advantage of all the capability of the alternators

due to limitations in the VAR load sharing system.

Solutions

What can be done about this? First, we need to

understand how much reactive power can be absorbed

by the generator without negative impact. The ability of

an alternator to absorb power is described by a reactive

capability curve. FIGURE 2 shows a typical generator

capability curve describing the capability of a machine

to produce and absorb power. In this curve the kVAR

produced or absorbed is on the X-axis (positive to the

right). The Y-axis shows kW (positive going up). kVAR

and kW are shown as per unit quantities based on the

rating of the alternator (not necessarily the generator

set, which may have a lower rating.

The normal operating range of a generator set is betweenzero and 100 percent of the kW rating of the alternator

(positive) and between 0.8 and 1.0 power factor (green

area on curve). The black lines on the curves show the

operating range of a specific alternator when operating

Power topic #6001 | Page 2

The generator set AVR monitors generator output volt-

age and controls alternator field strength to maintain

constant output voltage. Relatively low AVR output is

required to maintain generator voltage at no load. In the

figure shown, the no load exciter field current required

is less than half the full load level.

Filter equipment is often sized for operation at theexpected maximum load on the UPS or motor load.

At light loads there may be excess filter capacitance,

causing a leading power factor. Since rectifiers are

commonly designed to ramp on from zero load to

minimize load transients, leading power factor loads

may be imposed on the system until inductive loads are

added to the system or the load factor of the nonlinear

load increases.

A utility supply simply absorbs the reactive power

output because it is extremely large relative to the

filter system and it has many loads that can consume

this energy. With a generator set, however, the risingvoltage from the leading power factor causes the

voltage regulator to turn down and reduce alternator

field strength. If the AVR turns all the way off it looses

control of system voltage, which can result in sudden

large increases in system voltage. The increase in

voltage can result in damage to loads, or can cause

the loads to fail to operate on the generator set.

A UPS is designed to recognize high voltage as an

abnormal and undesirable condition, so it can imme-

diately switch off its rectifier. When it does that, the

high voltage condition is immediately relieved (because

the filter is disconnected from the generator set) andvoltage returns to normal. To the observer, the generator

will seem to be unable to pick up the system loads.

Paralleling problems

Generator sets that are using in isolated bus paralleling

systems have particular issues with leading power

factor loads.

When loads are applied to a parallel generator bus, the

total load on the system can be many times larger than

the capacity of a single generator set. The generator

sets close to the bus one at a time, so that if high

loads (either leading or lagging) are applied beforegenset capacity is available, the generator bus can fail.

With leading power factor loads, the failure mode will

be due to either an over voltage condition or reverse

kVAR shutdown. due to either an overvoltage condition

or due to reverse kVAR shutdown.





The solution to this problem on this specific machine

involves avoiding excess reverse kVAR levels through

proper system design and operation:

• Modify the sequence of operation for the facility so

that loads that require reactive power are present on

the bus when the UPS ramps on to the generator.

The reactive power produced by the filters will beconsumed by the system loads, and the loss of

voltage control is avoided. This requires a re-thinking

of operating sequences in some cases because

1) perhaps mechanical loads rather than UPS will

need to go on the generator first, or 2) loads will be

required to be broken into smaller blocks of UPS

and mechanical loads, rather than larger isolated

buses of each.

• Turn off the lters when operating on the generator

set or reduce the magnitude of filtering provided.

If the generator is provided with modern digital

excitation control, the filters won’t be needed tomaintain stable generator operation, but may still

be required to properly serve other loads.

outside of normal range. Notice that as power factor

drops, the machine must be de-rated to prevent

overheating. On the left quadrant, you can see that

near-normal output (yellow area) can be achieved with

some leading power factor load, in this case, down

to about 0.97 power factor, leading. At that point, the

ability to absorb additional kVAR quickly drops to nearzero (red area), indicating that the AVR is “turning off”

and any level of reverse kVAR greater than the level

shown will cause the machine to lose control of voltage.

In other words, if the machine is rated for operation at

1000 kVA and 0.8 power factor (600 kVAR rated), with

a reverse kVAR level of 0.2 per unit (rated), you will

exceed the machine’s capabilities. So, with more than

120 kVAR reverse reactive power and leading power

factor lower than 0.97 (for most people a surprisingly

low level) we have a problem.


The ability of a generator set to absorb reactivepower is defined by a reverse kVAR limit, not aspecific power factor.

Per Unit kVAr

P e r U n i t k W

Excitation

Engine

Rotor

Stability

Stator

0.9 0.8 0.7 0.70.6 0.60.5 0.5 1.00.90.80.4 0.40.3 0.30.2 0.1 0.0 0.1 0.2

1.0

1.1

0.9

0.8

0.7

0.6

0.0

0.1

0.2

0.3

0.4

0.5

1.0

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

ALTERNATOR CAPABILITY CURVE

0.8 0.9 0.90.950.95 1.0 0.8

Export (Lag)Import (Lead)

FIGURE 2 – Green area is normal operating range of a typical synchronous machine, yellow is abnormal but not damaging, and operating in

red regional will cause damage or misoperation.



Conclusions and recommendations

> Synchronous alternators have limited ability to

absorb kVAR from load devices, and exceeding

this limit will result in generator set shutdown.

> Paralleling operations require careful consideration

of the loading sequence to prevent reverse var

conditions that can damage the generator set.

> Consider modifying system sequence of operation

or limiting filter operation until adequate loads are

in place to prevent reverse kVAR conditions onthe genset.

> Specify the magnitude of reverse kVAR the genset

must be able to absorb, not the power factor alone.

> In single generator applications protective devices

can be set to the limits of the alternator. In paralleling

applications both alternator limits and Var load share

limits must be considered.

For additional technical support, please contact your

local Cummins Power Generation distributor. To locate

your distributor, visit www.cumminspower.com.

The actual limits of reverse kVAR can vary considerably

from machine to machine, both within a specific manu-

facturer’s product line, and between equipment from

different suppliers. A good rule of thumb for Cummins

equipment is that it can absorb about 20% of its rated

kVAR output in reverse kVAR without losing control of

voltage. However, since this characteristic is not univer-

sal, it is advisable for a system designer to specify the

reverse kVAR limit used in his design, or the magnitude

of the reverse kVAR load that is expected. Note that this

is not specified as a leading power factor limit, but rather

as a maximum magnitude of reverse kVAR.

Alternators are physically limited in their ability to both

produce and absorb power. When a leading power

factor load is applied to an alternator at a site, misoper-

ation of the generator, overvoltage, load misoperation,

and alternator damage can occur. There is very little

that the alternator supplier can do to resolve problems

at a specific site other than to help a system designer

understand the nature of the problem and the limits of

the machine as installed. Most of the solution will come

from changes in the system sequence of operation, or

hardware changes that prevent disruptive reverse var

conditions from affecting the generator set.

About the author

Gary Olson graduated from Iowa State

University with a BS in Mechanical

Engineering in 1977, and graduated

from the College of St. Thomas with an

MBA in 1982.



© 2009 Cummins Power Generation Inc. All rights reserved.

Cummins Power Generation and Cummins are registered

trademarks of Cummins Inc. “Our energy working for you.”

is a trademark of Cummins Power Generation.

PT-6001 (Rev 9/09)

He has been employed by Cummins Power Generation

for more than 30 years in various engineering and

management roles. His current responsibilities include

research relating to on-site power applications, technical

product support for on-site power system equipment,

and contributing to codes and standards groups.



Alternator protection, part 1:Understanding code requirements

Power topic #6002 Part 1 of 3 | Technical information from Cummins Power Generation

This paper identifies requirements for protectionof alternators based on North American code.The paper also provides insight on how thecodes should be interpreted based on thecharacteristics of alternators used in emergencyand legally required generator set applications.

North American code requirementsfor alternator protection

It is nearly universal practice to protect electricaldistribution systems from the effects of electricaloverloads and short circuits, since these conditionscan cause major damage, loss of revenue, and mayeven cause loss of life. The use of overcurrent and shortcircuit protection are well-accepted and practiced forgrid-powered distribution systems, but special attentionis needed for alternator protection due to the char-acteristics of the generator sets themselves, and thedifferences in the objectives of protection on emergencysystems compared to systems served by normal power.

The design of power systems requires an appreciationof the balance between protection and reliability. Ingeneral, as protection is made more certain, reliabilityof electrical service is compromised. North Americancodes and standards recognize not only the technicaldifferences in protection needs between generatorequipment and systems, but also the need to strikethe right balance between protection and reliability inspecific applications.

Defining “alternator damage”

One of the major objectives of established codes andstandards is to prevent alternator damage, so it isimportant to understand what constitutes “alternator

damage.” Alternator windings, like other electricalconductors, will operate at increasingly higher tempera-ture as load level increases. Under overload conditionstemperatures will increase to sufficiently high levelsto cause thermal stress and ultimately insulation (andalternator) failure. Even if the alternator does not fail,even a single overload may cause the effective lifeof the alternator to be dramatically reduced withoutcausing an immediate failure.

In a simplistic sense, an alternator is damaged due to

the effects of an overcurrent condition whenever analternator fails to function, or its life is unacceptablyshortened. “Fails to function” is pretty clear—the alter-nator ceases to produce usable output, and perhapscauses damage to the facility around it. “Unacceptablyshortened life” is not a precise engineering definition foralternator damage. Alternator manufacturers typicallywill provide curves that describe the magnitude of3-phase current and duration of current flow that canbe carried by a specific alternator without causing analternator failure, or shortening of insulation systemlife to the point that the effective life of the machine isshortened too much.

Since we know that high alternator temperature is onesymptom of impending damage, it may be expectedthat one method that might be used to detect damagewould be to instrument the alternator with temperaturesensing devices (usually thermocouples) throughoutthe windings, and then define the damage point asany time that the temperature at any thermocouplereaches a predefined value. However, the responsecharacteristics of imbedded temperature detectors andtheir monitoring devices are not suitable for reportingdamage before it becomes severe, and even if theycould detect damage, the machine can be damaged

> White paper

By Gary Olson, Technical Counsel





North American requirements for generator

(alternator) protection

The 2005 US National Electric Code® (NEC® ), NFPA 70®,makes the following references to generator protection:

240.21 Location in circuit: (G) Conductors from

Generator Terminals. Conductors from generator ter-minals that meet the size requirement in Section 445-13shall be permitted to be protected against overload bythe generator overload protective device(s) required bySection 445-12.

445.12 Overcurrent protection: (A) Constant Voltage

Generators. Constant voltage generators, except ACexciters, shall be protected from overloads by inherentdesign, circuit breakers, fuses, or other acceptableovercurrent protective means suitable for the conditionsof use. Exception to (A) through (E): Where deemed by the

Reprinted with permission from NFPA 70HB-2008, National Electric Code Handbook 2008, National Fire Protection Association, Quincy, MA 02169. This

reprinted material is not the complete and official position of the NFPA on the referenced subject, which is represented only by the standard in its entirety.

authority having jurisdiction, a generator is vital to the operationof an electrical system and the generator should operate tofailure to prevent a greater hazard to persons. The overload

sensing device(s) shall be permitted to be connected to an an- nunciator or alarm supervised by authorized personnel insteadof interrupting the generator circuit.

445.13 Ampacity of conductors: The ampacity ofthe conductors from the generator terminals to the firstdistribution device(s) containing overcurrent protectionshall not be less than 115 percent of the nameplatecurrent rating of the generator. It shall be permitted tosize the neutral conductors in accordance with Section220.61. Conductors that must carry ground faultcurrents shall not be smaller than required by Section250.24(C)… Exception: Where the design and operation of the

generator prevent overloading, the ampacity of the conductors shall not be less than 100 percent of the nameplate current rating.

Power topic #6002 Part 1 of 3 | Page 2

under other conditions, such as rotor (field) damagewhich can occur on unbalanced faults.

Consequently, alternator manufacturers define alterna-tor damage due to overloads based on a conservativeengineering estimate of the capability of the alternatorto resist damage on over current conditions. Theseestimates are based on testing of alternators undershort circuit conditions, measurement of current flowsand temperatures inside the machine, and an under-standing of the characteristics of the insulation systemused in a specific machine. (See FIGURE 1.) The curvesthat are drawn and provided to customers and consul-tants as a result of this testing and evaluation do notnecessarily define the exact failure point of a specificmachine. Rather, they provide an accepted guideline forwhen the life of the machine is unacceptably shortened.Use of the alternator manufacturer’s damage curve inconjunction with protective device operation curves willresult in optimum system protection while maintainingsystem reliability at acceptable levels.

The above references cite the need for overcurrentprotection both for the conductors connecting the

generator set to the first level of distribution devices(240.21) and the alternator itself (445.12). Several pointsare significant. Note that the authority having jurisdic-tion (AHJ) may allow an exception to the requirementfor protection, especially when the premature operationof the protection may cause hazards to people. Inother words, the code would allow the generator set tooperate without any protection at all, in some cases,

1000

100

10

1

0.1

0.01 1 0

2 0

5 0

1 0 0

2 0 0

5 0 0

1 0 0 0

2 0 0 0

5 0 0 0

1 0 0 0 0

2 0 0 0 0

5 0 0 0 0

1 0 0 0 0 0

ALTERNATOR DAMAGE CURVE

Current in Amp Res at 480 Volts

T i m e i n S e c o n d s

Generator

Damage

Curve

Generator

FLA, 752A

Cable

Damage

Curve

FIGURE 1 – Alternator and fully rated cable thermal damage curves.

Note that the alternator curve falls to the left of the cable curve,

so any device that fully protects the alternator will also protect thefeeder cable. Cable is 2-600 MCM/Phase Cu 75C.





The Canadian Electrical Code

28-902 Protection of constant voltage generator sets

(1) Constant voltage generators, whether direct-currentor alternating-current, shall be protected from excesscurrent by overcurrent devices, except that:

(a) When the type of apparatus used and the nature ofthe system operated make protective devices inadvis-able or unnecessary, the protective device need not beprovided…

CSA C282-00 Emergency electrical power supply

for buildings, requires:

7.7.1 The overcurrent devices in the emergency distributionsystem shall be coordinated to maximize the selectivetripping of branch circuit breakers when a short circuitoccurs. Short circuit currents of sufficient magnitudeshall be made available from the generator to satisfythis coordination ability.


if failure would cause a greater risk to people in thefacility served. This is a direct indication of the principalof balance noted earlier—protection of hardware shouldnot be done at expense of putting people at risk.

Note also that the code allows the alternator protectionto be used as the protective equipment for the feederconductors from the generator set to the first level ofdistribution as long as it is sized properly. Adding anotherprotective device for protection of a fully rated feederwould provide no better protection, increase the risk of

nuisance tripping, and make coordination more difficult.

This is a reasonable practice, as can be seen inFIGURE 1, which illustrates that the generator damagecurve for an alternator falls well to the left of thedamage curve for a fully rated feeder cable. So, anyprotective system that fully protects the alternator willalso protect the feeder cable.

Note also that if there is overload protection in thegenerator control system, feeder sizing can be limitedto 100% of the generator rating. (Section 445.13requires feeder protection to be 115% of the protectivedevice rating.)

The Canadian Electrical Code (22.1-1990) includesrequirements that are similar to the US NEC. Specificrequirements for generator protection are described inSection 28-902.

In addition to these requirements in both the US andCanadian codes, it is only reasonable that the system

be designed so that nuisance protective device opera-tions (i.e., circuit breaker tripping) are avoided. The lossof system operation is as disruptive and dangerouswhen on a generator set as the loss of normal facilitypower when generator set power is not available.

There is some specific recognition of this problem inthe NEC. In the exception at the end of section 445.12,it is noted that sometimes keeping the generator setrunning to protect human life is more important thanprotecting the generator set itself. NFPA 110-2002®,

which is particularly applicable to emergency andlegally required applications, states:

6.5.1 General. The overcurrent protective devices in theEPSS shall be coordinated to optimize selective trippingof the circuit overcurrent protective devices when ashort circuit occurs.

In summary, one can say that generator sets in criticalapplications can be protected in a less rigorous fashionthan other equipment, but effective protection isgenerally required; that effective protection depends oncoordinating the thermal damage curve of the alterna-tor with the protective device; and that electrical system

coordination (discrimination) is also required in mostcritical applications.

Protection for the generator set can come in manyforms. The NEC only requires that the generator setbe provided with appropriate protection. It specificallyallows overcurrent sensing devices, and inherentovercurrent protection.






That being said, it should be noted that the thermaldamage curve of an alternator generally follows anI2t characteristic, so circuit breakers that don’t haveoperating characteristics with that curve shape throughthe overload operating range of the alternator will bedifficult or impossible to apply in a way that adequately

protects an alternator. For example, in FIGURE 2 wesee an example situation where a 500 kW/480 VAC al-ternator is being “protected” by a typical 800A moldedcase circuit breaker with thermal magnetic trip unit.

Note that for this situation for nearly any overloadcondition the breaker operation curve lies to the rightof the thermal damage curve of the alternator, so itcan provide no protection at all. Obviously, protectionis not present even though a reasonably sized, typicalbreaker is provided.

System designers must verify that the protectionprovided is coordinated with the thermal damage

curve of the alternator, and verify that coordination withdownstream devices is provided. It cannot be assumedthat “any circuit breaker” will work—rather it is often

the case that breakers, especially thermal-magnetictrip breakers sized to the capability of the alternator willoften provide inadequate protection through all or partof the possible overload range. Better protection canbe provided by breakers with electronic trip units, ormicroprocessor-based protective relaying equipment.

Recommendations

Alternators must be protected from the effects ofovercurrent and short circuit conditions, but thisprotection must be carefully chosen with full knowledgeof the capabilities and limitations of the generator set.Protective devices that can provide proper protectioninclude:

• Circuit breakers with solid state trip units that

are fully coordinated with the alternator thermaldamage curve.

• Fuses also could be applied, but many owners

prefer not to use them because of the danger ofsingle phase operation and problems with fusereplacement in an emergency situation.

• Individual phase protective relaying properly

coordinated with the alternator thermal damagecurve.

Note that whatever protective device is chosen, itmust be specifically matched to the alternator thermaldamage curve so that potential nuisance tripping orincomplete protection is avoided. Protection should be3-phase sensing with I2t characteristic shape.

Whatever protective system is used, system designersshould review the capabilities of the protection systemand the thermal limits of the alternator to be certain thatthe machine is adequately protected. Circuit breakeroperation curves are commonly available, but alternatorthermal damage curves are generally not published andwidely distributed by alternator manufacturers. In anycase, they are needed to ascertain the viability of theprotection proposed by the supplier.

The protective system design should recognize thefact that single-phase faults are much more commonthan 3-phase faults, and provide proper protection for

the alternator under that condition. This may includeaccelerated tripping to compensate for the additionalheating effects of single-phase faults.

For additional technical support, please contact yourlocal Cummins Power Generation distributor. To locateyour distributor, visit www.cumminspower.com.

1000

100

10

1

0.1

0.01 1 0

2 0

5 0

1 0 0

2 0 0

5 0 0

1 0 0 0

2 0 0 0

5 0 0 0

1 0 0 0 0

2 0 0 0 0

5 0 0 0 0

1 0 0 0 0 0

ALTERNATOR THERMAL DAMAGE CURVE

AND MOLDED CASE BREAKER TRIP CURVE

Current in Amp Res at 480 Volts


GE 800A Molded Case

Circuit Breaker

Generator

FLA, 752A

MaximumInstantaneous

Trip Setting (8X)

Minimum

Instantaneous

Trip Setting (3X)

Generator

Damage

Curve

FIGURE 2 – Alternator thermal damage curve and molded casebreaker trip curve. At current levels less than 2000 amps the molded

case breaker cannot provide protection for the alternator.



An AmpSentry™ protection relay specification:

Controls shall be provided to monitor the output current of the gen-erator set and initiate an alarm when load current exceeds 110% of

the rated current of the machine for more than 60 seconds. The con-

trols shall shut down and lock out the generator set when the outputcurrent level approaches the thermal damage point of the alternator.

Controls shall be provided to monitor the kW load on the generator

set, and initiate an alarm condition when the total load on the genera-

tor set exceeds the generator set rating for more than 5 seconds.

Control shall include a load shed control to operate a set of dry

contacts for use in shedding designated loads when the generator

set is overloaded.

An individual phase AC over voltage monitoring system monitoringall three phases of the alternator shall be provided to initiate shut-

down of the generator set when alternator output voltage exceeds

110% of the operator set voltage level for more than 10 seconds.

Shutdown shall occur with no intentional delay if alternator outputvoltage exceeds 130%. Under voltage shutdown shall occur when

the alternator output voltage is less than 85% of the operator set volt-

age for more than 10 seconds.

References

• ANSI/ NFPA 70, National Electrical Code, 2005.

• ANSI/ NFPA 99, Health Care Facilities.

• ANSI/ NFPA 110, Generator Sets for Emergency

and Standby Power Applications.

• ANSI/IEEE Std 242-1986, “IEEE Recommended Practice for

Protection and Coordination of Industrial and Commercial Power

Systems” (Buff Book), Section 11.4.

• Baker, David S., “Generator Backup Protection”,

IEEE Transactions on Industry Applications, Vol. 1A-18, No. 6,November-December 1982.

• Canadian Standards Association, C282,

“Emergency Electrical Power Supply for Buildings” 2000.

• Earley, Mark W., Murray, Richard H., and Caloggerro, John M.,

“National Electrical Code Handbook” National Fire Protection Association, Fifth Edition.

• R-1053, PowerCommand® AmpSentry™ Time Overcurrent

Characteristic Curve, Onan Corporation, 1994.

He has been employed by Cummins Power Generationfor more than 25 years in various engineering andmanagement roles. His current responsibilities includeresearch relating to on-site power applications, technicalproduct support for on-site power system equipment,and contributing to codes and standards groups.He also manages an engineering group dedicated todevelopment of next generation power system designs.

About the author

Gary Olson graduated from Iowa StateUniversity with a Bachelor of ScienceDegree in Mechanical Engineering in

1977, and graduated from the College ofSt. Thomas with a Master of Business

Administration degree in 1982.



© 2009 Cummins Power Generation Inc. All rights reserved. Cummins Power Generation

and Cummins are registered trademarks of Cummins Inc. PowerCommand is a registered

trademark of Cummins Power Generation. AmpSentr y and “Our energy working for

you.” are trademarks of Cummins Power Generation. Other company, product, or service

names may be trademarks or service marks of others.

PT-6002; Part 1 of 3 (1/09)



Alternator protection, part 2: Alternatives


Generators sets are commonly to be protectedfrom the effects of overload conditions byequipment that is required by local codes andstandards. The standards generally do notstipulate what type of protection device thatis required, so a system designer can selectprotective devices based on the needs of theapplication and the preferences of the user.This document provides an overview of over-

load protection alternatives that are availablefor generator sets.

Power topic #6002 Part 1 identified requirements for

protection of alternators based on North American codes.

Inherent overcurrent protection

The “US National Electrical Code® Handbook” (page606) notes that generator sets can be designed so thata short term overcurrent condition causes a collapseof the voltage on the output of an alternator. This limitsthe current and output kW of the alternator during an

overload or short circuit condition so that it can consid-ered inherently self-protected.

In practice, this can be achieved with a shunt-typeexcitation system design. In this type of generatorexcitation system, both the sensing and power leads ofthe voltage regulator are connected to the output of thealternator. When an overload or short circuit occurs, thecollapsing voltage on the power supply to the voltageregulator effectively limits the power available to thealternator excitation system, thus causing alternatoroutput voltage to collapse until the overload is removedfrom the system.

The weakness of this design is that shunt-type systemstypically utilize single phase sensing voltage regulators,so a single-phase fault or overload may not cause thesystem to collapse immediately. If a fault or overloadoccurred on a phase which is not used for sensing orregulation power, the overcurrent condition would bemaintained until the fault spread far enough to affectthe power supply to the voltage regulator. By that time,alternator damage may have occurred.

Shunt-type excitation systems have other weaknesses

related to motor starting and coordination (discrimina-tion) on generator sets, so they are not often specified,particularly for large or critical systems. However, theirhistorical acceptance without other overcurrent protec-tion is a precedent for acceptance of other types ofinherent protection for alternators.

Other inherent protection systems are available whichaddress the concerns stated here for shunt-type systems.

Circuit breaker protection

Molded case breakers with

thermal-magnetic trip units

Generator sets that are rated for operation at less than1000 volts, and 1200 amps and smaller are often pro-vided with a molded case circuit breaker for alternatorprotection. The molded case breaker is usually providedwith a thermal-magnetic trip unit. The “conventionalwisdom” is that this device will protect the alternatorand the feeder connecting it to the first level of distribu-tion, and these devices are often accepted for alternatorprotection by authorities. However, it is difficult (if notimpossible) to design an effective alternator protectionsystem using this equipment.

> White paper







In FIGURE 1 you can see an illustration of a typicalgenerator set thermal damage curve that has beenoverlaid on a time overcurrent characteristic curve fora molded case circuit breaker with thermal-magnetictrip unit. A problem is apparent from the first glance atthe chart: The thermal damage curve of the alternatoroverlaps the trip curve of the circuit breaker. For properprotection the damage curve must be entirely to theright of the trip curve of the breaker, illustrating that forany current level, for any time duration, the breaker will

positively trip before alternator damage will occur.

In general, a molded case breaker that is sized to carrythe full output of a generator set on a continuous basiswill overlap the alternator thermal damage curve. Use ofa larger breaker would make the situation even worse.

If the generator set is exposed to a bolted fault closeto the terminals of the generator set, the circuit breakermay trip in it’s instantaneous region as much as severalseconds before it needs to trip to protect the alternator.Tripping in the instantaneous region of the breakercurve can also occur due to surge currents from start-ing of large motors, or due to transformer magnetizing

current. In many cases, this would be considered anuisance trip, because there is no potential damageto any part of the system, but the breaker has clearedanyway. If this happens in the “real world”, the operatorwould probably re-adjust the trip curve of the gensetbreaker to delay tripping, thus making the trip curvemove further to the right, and providing even lessprotection to the alternator.

1000

100

10

1

0.1

0.01 1 0

2 0

5 0

1 0 0

2 0 0

5 0 0

1 0 0 0

2 0 0 0

5 0 0 0

1 0 0 0 0

2 0 0 0 0

5 0 0 0 0

1 0 0 0 0 0

ALTERNATOR THERMAL DAMAGE CURVE

AND MOLDED CASE BREAKER TRIP CURVE

Current in Amperes at 480 Volts


GE 800A Molded Case

Circuit Breaker

Generator

FLA, 752A

Maximum

Instantaneous

Trip Setting (8X)

Minimum

Instantaneous

Trip Setting (3X)

Generator

Damage

Curve

FIGURE 1 – At current levels less than 2000 amps the molded case

breaker cannot provide protection for the alternator.




240.21 Location in circuit: (G) Conductors from

Generator Terminals. Conductors from generator ter-minals that meet the size requirement in Section 445-13shall be permitted to be protected against overload bythe generator overload protective device(s) required bySection 445-12.

445.12 Overcurrent protection: (A) Constant Voltage

Generators. Constant voltage generators, except ACexciters, shall be protected from overloads by inherentdesign, circuit breakers, fuses, or other acceptableovercurrent protective means suitable for the conditionsof use. Exception to (A) through (E): Where deemed by the

Reprinted with permission from NFPA 70HB-2008, National Electric Code Handbook 2008, National Fire Protection Association, Quincy, MA 02169. This

reprinted material is not the complete and official position of the NFPA on the referenced subject, which is represented only by the standard in its entirety.

authority having jurisdiction, a generator is vital to the operationof an electrical system and the generator should operate tofailure to prevent a greater hazard to persons. The overload

sensing device(s) shall be permitted to be connected to an an- nunciator or alarm supervised by authorized personnel insteadof interrupting the generator circuit.

445.13 Ampacity of conductors: The ampacity ofthe conductors from the generator terminals to the firstdistribution device(s) containing overcurrent protectionshall not be less than 115 percent of the nameplatecurrent rating of the generator. It shall be permitted tosize the neutral conductors in accordance with Section220.61. Conductors that must carry ground faultcurrents shall not be smaller than required by Section250.24(C)… Exception: Where the design and operation of the

generator prevent overloading, the ampacity of the conductors shall not be less than 100 percent of the nameplate current rating.






If a low level overcurrent condition occurs the alterna-tor will be damaged long before the breaker trips toprotect the alternator. For example (see FIGURE 1), witha 1000 amp load, the alternator will be damaged after300 seconds, but the breaker would not trip until thecondition had lasted at least 1000 seconds.

Clearly, a molded case breaker with a thermal-mag-netic trip, sized for the full output of the genset, doesnot provide effective alternator protection under allpotential failure modes and conditions. There is anotherfacet to the problem, and that is the reliability of serviceto loads when using molded case breakers.

Molded case breakers with thermal-magnetic trip unitsare generally not continuously rated devices. Unlessa breaker is specifically listed and labeled to carry a100% rating, it will carry only 80% of its nameplaterating on a continuous basis. Use of a breaker that isequal in rating to the generator output rating can limit

the ability of the generator set to operate at high currentlevels (which it is designed to do) for extended periods

of time (particularly at high ambient temperatures). Ifthe generator set is operating at a high load level, themolded case breaker can trip in the 80-100% currentrange, especially in high ambient conditions, causingan unnecessary interruption of service to critical loads.

Some designers attempt to address the derating

problem by application of oversized breakers or byuse of continuously rated breakers. Note that a largercapacity breaker would probably trip later, makingprotection less effective for the alternator. It also wouldrequire larger generator feeder conductors that addcost to the installation because the feeder conductorsize and the breaker must be coordinated to protectthe feeder. Any continuously rated breaker would stillneed to be matched to the thermal damage curve of thealternator in order to protect the alternator through alloverload conditions.

Another potential solution is to use multiple smaller

breakers to feed smaller transfer devices or systemloads. If a molded case breaker with thermal-magnetictrips is rated for approximately half the rating of thealternator, it will generally protect the machine throughall overcurrent levels and duration’s. (The designershould verify this for each application by comparison ofthe breaker trip curves and alternator thermal damagecurve.) This strategy can be used where the loads ona generator set can be split and fed to multiple circuitsdirectly from the generator set.

However, in general we can say that the most com-monly specified and applied protective system for low

voltage generator sets (molded case/thermal-magneticbreakers) will not provide necessary protection underall circumstances, and will subject the power systemto nuisance power failures due to unnecessary breakertripping.

An alternative that can be investigated is the use of“generator” type circuit breakers. These breakers areavailable from a few manufacturers, and allow bettercoordination of the genset thermal damage curve bymodifying the characteristic trip curve to be more likethe genset curve.

Molded case (and other) breakers

with solid state trip unitsBetter alternator protection can also be achieved by useof breakers with more capable trip units. In FIGURE 2an 800 amp insulated case breaker with solid state tripis overlaid on the alternator thermal damage curve. Inthis case the breaker characteristic curve shape moreclosely resembles the thermal damage curve of the

1

1000

100

10

0.1

0.01 1 0

2 0

5 0

1 0 0

2 0 0

5 0 0

1 0 0 0

2 0 0 0

5 0 0 0

1 0 0 0 0

2 0 0 0 0

5 0 0 0 0

1 0 0 0 0 0

ALTERNATOR PROTECTION

USING A MOLDED CASE BREAKER WITH SOLID STATE TRIP UNIT

Current in Amperes at 480 Volts


Potential

Problem Area

Generator

FLA, 752A

GE 800A RMS-9

Programmer

Generator

Damage

Curve

FIGURE 2 – This breaker provides better protection than one with a

thermal-magnetic trip unit.






alternator and it falls almost completely to the left of thealternator damage curve, so much better protection isprovided to the generator set. There is the potential fordamage due to a sustained low level overload conditionfor an extended period of time.

Insulated case breakers are not often applied on

generator sets since they are considerably moreexpensive than molded case breakers, and may be toolarge or fragile for direct mounting on many generatorsets. Most molded case breakers are available withsolid state trip units (usually higher ampacities). Theseallow the designer to more closely match the breakeroperation characteristic to the alternator provided.However, they do need to be adjusted for each alterna-tor, and they may not provide complete protection insome operating regions, or may be misadjusted inthe field due to what are perceived by an operator asnuisance trips.

All breakers that include instantaneous trips aresusceptible to nuisance tripping under some surge loadconditions on a generator set. (Tripping that occurs dueto peak inrush current which is less than the thermaldamage limit of the alternator, but higher than theinstantaneous pickup setting.)

A final point: The natural reaction to a breaker trip(and generator shutdown) by most operators wouldbe to reset the breaker and immediately restart themachine. If the fault were still present on the machine,the machine could be damaged on the second opera-tion. This is because the breaker would theoreticallytake the same time to clear as on initial trip, but sincethe machine is at higher internal temperatures at thestart of the fault, it would reach a higher than expectedtemperature, potentially damaging the machine. Thisproblem is present in all breakers, and most overcur-rent relays.

Overcurrent protective relaysSpecific overcurrent protective relays could be providedfor alternator protection. These are often used foralternators operating at over 1000 volts AC. Relaysare available in the marketplace that will provide goodprotection for the alternator, and may include overcur-rent, differential overcurrent, zero or negative sequencedevices. The drawback to these devices is cost, bothin terms of the design (device selection and settingsdecisions on the part of the system designer) andinstallation of the relay(s) selected. For small generatorsets the cost of a installed utility grade relay couldapproach the cost of the generator set! Installation cost

increases may be significant, since many protectivedevices are not designed for mounting or long termoperation on a generator set due to vibration or harshtemperature extremes that are common on a generatorset. Depending on the devices chosen, maintenancecosts may be increased due to the need for calibrationand other maintenance on the relays compared to otherprotective devices.

Another solution

Cummins PowerCommand® generator sets with AmpSentry™ protective relay provide the alternator

protection that is required by codes and standards andby facility owners desiring good alternator protection.Because AmpSentry protective relay is specificallydesigned for alternator protection, it can providenecessary protection without the compromises andlimitations that are necessary with other approaches.(FIGURE 3)

FIGURE 3 – PowerCommand generator sets include an overcurrentand short circuit protection algorithm that is designed specifically for

alternator protection. The I2t portion of the curve is matched to the

alternator provided on the genset.

1000

100

10

1

0.1

0.01

0

1

0.05

0

3

1

0

1 0

0

AMPSENTRY PROTECTIVE RELAY WITH

DECREMENT CURVE AND ALTERNATOR THERMAL DAMAGE CURVE

Current in Amp (Times Rated)


Molded Case Circuit Breaker

With Thermal-Magnetic Trip

Alternator Thermal

Damage Curve

Amsentry Protective Relay





AmpSentry™ protective relay function

Overload conditions

PowerCommand generator set controls continuouslymonitor the output current and voltage on each phaseof the alternator. On sensing a current level of morethan 110% of the steady state rating of the generator

set for more than 60 seconds on any phase, an alarmcondition is initiated to alert the operator that an abnor-mal condition is present on the machine. The controlalso logs the time and nature of the fault condition.

If the overload continues, the control continuouslymonitors the current level and duration and performsI2t calculations to determine when the overload hasreached a point that it may damage the machine. Priorto reaching that point, the excitation system is switchedoff, thus protecting the alternator and system loadsfrom the affects of the condition. Depending on theversion of the PowerCommand control provided, the

machine may or may not operate for a short time (withexcitation switched off) to allow the engine and alterna-tor to cooldown prior to being shut down. This reducesthe risk of damage to the engine and presents nodanger to the installation since the alternator excitationsystem is switched off.

Short circuit conditions

Under 3-phase short circuit conditions, the operationof the control is slightly different than operation underovercurrent conditions. Again, the PowerCommandcontrol continuously monitors the output current andvoltage on all phases of the machine. If the control

senses current on any phase greater than 3 timesrated, it will switch to current regulation mode. Thecontrol will adjust the excitation system output to thefield of the alternator based on sensed current levelrather than sensed voltage level. The control, using apermanent magnet excitation support system providedwith the alternator, will regulate the output current to300% of the rated output current. The 300% currentlevel is intended to be sufficient to clear downstreamovercurrent protective devices. The generator set mayalso enter this state during starting of motors that arelarge relative to the size of the alternator. When the faultclears (or the motor starts), the control returns to voltage

regulation mode, and softly ramps the system voltageback to normal levels, without a voltage overshoot.

The overcurrent protective functions of an AmpSentryprotective relay does not completely reset after thefault is cleared. The control system “remembers” theovercurrent energy expended by the machine, and thetime since clearing. It will progressively move the trip

curve left to maintain protection for the alternator if it isnot fully cooled down after a fault.

AmpSentry protective relay protections in PowerCommandcontrols effectively eliminates the need for a main genera-tor set circuit breaker. AmpSentry provides both positivealternator protection and protection for the feeder con-

necting the generator set to the next level of downstreamprotective devices, as long as the feeder is sized tooperate under the full rated output of the generator set.

Throughout this document another significant assump-tion has been made: That the engine has sufficienthorsepower available to drive the alternator at rated speedduring the overload or short circuit condition. It is possiblethat under some fault conditions the horsepower load onthe engine will be sufficiently large to cause the enginespeed to drop or even collapse. PowerCommand gensetswith AmpSentry protective relay include a kW overloadfunction and under frequency protection to respond to

these conditions.

Medium voltage applications

There are no specific special protection requirementsfor medium voltage applications versus low voltageapplications. Medium voltage alternators must meet thesame general protection requirements as low voltagemachines—they must be protected.

Several IEEE documents provide recommendations forsystem design for medium voltage alternators. Theseusually incorporate many more protective functionsthan are commonly provided low voltage machines.

In addition, the IEEE recommendations often includeneutral grounding resistors for medium voltage genera-tor sets. The grounding resistor will limit line to groundfault magnitude and limit the magnitude of over voltagethat occurs on a ground fault, so that there is more timeto clear downstream faults before alternator damageand less probability of damage to the alternator due tothe over voltage condition.

Generally, system designs utilize Overcurrent and otherprotective relays to provide alternator protection ratherthan depending on a circuit breaker trip unit.

Cummins generator sets that incorporate AmpSentry

protective relay functions provide the necessary overcur-rent functions that are coordinated with the alternatorthermal damage curve, other protective functions, andfault current regulation capability that limits ground faultcurrent without impeding coordination and also preventthe over voltage condition that will occur on a groundfault due to overexcitation in the system.




References

• ANSI/NFPA 70, National Electrical Code, 2005.

• ANSI/NFPA 99, Health Care Facilities,

• ANSI/NFPA 110, Generator Sets for Emergency and Standby

Power Applications.

• ANSI/IEEE Std 242-2001, “IEEE Recommended Practice for

Protection and Coordination of Industrial and Commercial Power

Systems” (Buff Book), Section 12.

• Baker, David S., “Generator Backup Protection”,

IEEE Transactions on Industry Applications, Vol. 1A-18, No. 6,November-December 1982.

• Canadian Standards Association, C282,

“Emergency Electrical Power Supply for Buildings” 2000.


“National Electrical Code Handbook”, National Fire Protection Association, Fifth Edition.

• R-1053, PowerCommand AmpSentry Time Overcurrent

Characteristic Curve, Cummins Power Generation, 2007.


Suggested protection specifications:

Alternator shall be protected per the requirements of NFPA 70 section445.12. The protection provided shall be coordinated with the thermal

damage curve of the alternator. Damage curve and protection curve

shall be submitted to verify performance.

The protection shall allow operation of the generator set continuously

at its rated output.

The protection equipment provided shall be 3rd party certified to

verify performance.

Generator set shall be provided with individual phase line to neutral

over voltage protection that is adjustable and set to operate at morethat 110% of nominal voltage for more than 0.5 seconds.


About the author

Gary Olson graduated from Iowa StateUniversity with a Bachelor of ScienceDegree in Mechanical Engineering in1977, and graduated from the College ofSt. Thomas with a Master of Business





and Cummins are registered trademarks of Cummins Inc. PowerCommand is a registered

trademark of Cummins Power Generation. AmpSentr y and “Our energy working for

you.” are trademarks of Cummins Power Generation. Other company, product, or service

names may be trademarks or service marks of others.

PT-6002; Part 2 of 3 (1/09)



Alternator protection, part 3:Generator set disconnectrequirements


In utility distribution systems protection equip-ment and disconnecting functions are oftenmerged together. However, because generatorsets can be prevented from operation, discon-necting means are often different. This paperprovides a discussion of disconnect require-ments in the USA.

Power topic #6002 Part 1 identified requirements for protection of alternators based on North American codes.

Power topic #6002 Part 2 provides an overview ofoverload protection alternatives that are available for generator sets.

Generator sets are required to have a disconnectingmeans so that the equipment can be quickly andeffectively shut down during an emergency situation(such as a fire in the facility) and to allow service tothe equipment or conductors. However, what the termdisconnect means in the context of the US National

Electrical Code® (NEC® ) does not necessarily mean aswitch or circuit breaker.

In most applications, a generator set is unlike a utilityservice, in that a generator set can be prevented from

energizing a circuit in a number of different ways. Theseinclude switching off the fuel supply, disconnectingthe starting batteries, operating a keyed auto/manualswitch, or engaging an emergency stop circuit. Thedisconnect means may also be a circuit breaker ordisconnect switch. While a disconnect switch or circuitbreaker is required to isolate a utility service from afacility distribution system, because you can’t switchoff the utility service, other means are allowed on agenerator set.

In Article 445.10 of the NEC it is clearly stated that agenerator set, unless it is in an installation where it can

be paralleled, can use any means that will prevent theengine of the generator set from operating. There arecompelling reasons to avoid use of a traditional dis-connect switch or circuit breaker as the “disconnectingmeans”:

• Addition of a molded case breaker with thermal-

magnetic trip unit can make the system less reliablebecause of the potential for nuisance tripping ofthe device due to surge loads from motor starting,or due to operation at more than the device’s

> White paper


An emergency stop switch fitted with lockout/tag-out provis ions.






continuous rating (usually 80% of nameplate tripsetting). A breaker would cause an extendedoutage in the system since the breaker cannot beautomatically reset. An operator must manuallyreset the breaker and re-close it to energize thesystem.

• When a breaker is opened for the purpose of

servicing a system, it may be inadvertently leftopen after service is completed. NFPA 110and NFPA 99 both require local and remote

indication that a generator set is disabled. Useof a control function integrated into the gensetcontrol system that automatically issues the “notin auto” remote indication provides an easy andeffective means to meet that requirement. A circuitbreaker or disconnect switch fitted with auxiliarycontacts that indicate the switch is open may beinterconnected to the control system to achieve therequired remote indication, but this adds cost andcomplexity to the design.

• When a breaker is used for the disconnecting

means, it may result in difficulty achieving selectivecoordination of the emergency electrical system, asis required by the NEC.

Whatever disconnecting means is used on a generatorset, it should have a lock-out/tag-out provision to allowtechnicians to prevent energization of the system untilthey have completed their work on it.

When generator sets are applied in paralleling applica-tions the paralleling breaker may be required to includea locking means to prevent energization of the feederfrom a generator set from the paralleling bus.





445.10 Disconnecting means required for generators:

Generators shall be equipped with a disconnect bymeans of which the generator and all protective devicesand control apparatus are able to be disconnectedentirely from the circuits supplied by the generatorexcept where;

(1) The driving means for the generator can be readilyshut down; and

(2) The generator is not arranged to operate in parallelwith another generator or source of voltage.

100.A Service: The conductors and equipment fordelivering electric energy from the serving utility to thewiring system of the premises served.

Reprinted with permission from NFPA 70HB-2008, National Electric Code Handbook 2008, National Fire Protection Association, Quincy, MA 02169. Thisreprinted material is not the complete and official position of the NFPA on the referenced subject, which is represented only by the standard in its entirety.



Suggested protection specifications:

The generator set control shall include a lock-out/tag-out devicewhich is integrated with the control-mounted emergency stop

switch. The lock-out/tag-out device shall cause the generator set

to be in emergency stop mode whenever a padlock is installed onthe device, preventing the generator set from running, producing

power, or cranking.

References

• ANSI/ NFPA 70, National Electrical Code, 2005.

• ANSI/ NFPA 99, Health Care Facilities.

• ANSI/ NFPA 110, Generator Sets for Emergency

and Standby Power Applications.


“National Electrical Code Handbook” National Fire Protection

Association, Fifth Edition.


About the author

Gary Olson graduated from Iowa StateUniversity with a Bachelor of ScienceDegree in Mechanical Engineering in1977, and graduated from the College ofSt. Thomas with a Master of Business





and Cummins are registered trademarks of Cummins Inc. “Our energy working for you.”

is a trademark of Cummins Power Generation. Other company, product, or service names

may be trademarks or service marks of others.

PT-6002; Part 3 of 3 (1/09)



Generator reactances are used for twodistinctly different purposes. One use is tocalculate the flow of symmetrical short circuitcurrent in coordination studies. A second usefor generator reactances are in specificationsthat limit the sub-transient reactance to 12%or less in order to limit the voltage distortioninduced by non-linear loads. For either shortcircuit or harmonic distortion analyses, the

stated reactances will need to be converted toa common base to make valid comparisons.Typically, generator reactances are publishedin per unit values on a specified base alterna-tor rating. Where the generator set ratingdiffers from the alternator base rating it will benecessary to convert the per unit values fromthe alternator base rating to the generatorset rating. For selecting circuit breakers withadequate AIC rating the maximum asymmetricalshort circuit current to flow in the first half

cycle may be approximated from the genera-tor sub-transient reactance (x"

d ) and a factor

to account for DC offset.

The flow of current in an AC circuit is controlled byimpedance. When a short circuit fault occurs in adistribution system the fault current that flows is afunction of:

1. the internal voltage of the connected machinesin the system (generators and motors),

2. the impedance of those machines,

3. the impedance to the point of the fault, mostlycable impedance,

4. and the impedance of the fault, if arcing.

The generator internal voltage and generator imped-ance determines the current that flows when theterminals of a generator are shorted. The effect ofarmature reaction on the generator air gap flux causesthe current to decay over time from an initial high value

to a steady state value dependant on the generatorreactances. Since the resistive component in genera-tors is negligible, for practical purposes it may beignored and only the reactances need be considered.Generator reactances, as determined by tests withfixed excitation, are:

> White paper

By Timothy A. Loehlein, Technical Specialist-Electrical

Calculating generator reactancesPower topic #6008 | Technical information from Cummins Power Generation

Name Symbol Range1 Effective Time

Importance

Sub-transient reactance X"d .09 – .17 0 to 6 cycles

Determines maximum instantaneous current and current at timemolded case circuit breakers usually open.

Transient reactance X'd .13 – .20 6 cycles to 5 sec.Determines current at short time delay of circuit breakers.

Synchronous reactance Xd 1.7 – 3.3 after 5 sec.

Determines steady state current without excitation support (PMG).

Zero sequence reactance Xo .06 – .09

A factor in L-N short circuit current.

Negative sequence reactance X2 .10 – .22

A factor in single-phase short circuit current.

1 Reactances shown are typical per unit values for generators ranging from 40 to 2000 kW.




© 2006 | Cummins Power Generation

GENERATOR SHORT CIRCUIT CURRENTS

Calculating per unitsub-transient reactance

Cummins Power Generation publishes generatorreactance values in Per-Unit (P.U.) to a base alternatorrating as specified on the alternator data sheet. Thegenerator sets however, have various base ratings, andso there is a need to convert from the alternator baseto the generator set base. This conversion is accom-plished using the following formula:


Sub-transient reactance

With a generator operating at full voltage, a symmetrical3-phase short circuit at its terminals will cause a largeamount of current to flow. This initial current is used todetermine the required interrupting rating of overcurrentdevices, circuit breakers and fuses, located at the

generator(s). The initial instantaneous current value(ISC

Sym ) is controlled by the sub-transient reactance

(X"d ) and is expressed by the voltage divided by the

sub-transient reactance, or:

ISCSym

= E AC

÷ X"d

In per unit, assuming a 0.10 sub-transient reactance,the initial symmetrical short circuit current expressed inmultiples of full load current:

ISCSym

= 1.0 ÷ 0.10 = 10 = 1000% of full load current

Reactances, including the sub-transient, are expressedwith a plus or minus tolerance of 10%, typically.Determination of the maximum current should use theworst case tolerance of minus 10%. The maximumsymmetrical current from the example above becomes:

ISCSym

= 1.0 ÷ (0.10 – 10%) = 11.1 = 1110% of full load current

The peak current in the first half cycle will also includea DC component, the magnitude of which depends onthe point in the cycle when the short circuit occurs.

A DC component offsets the symmetrical currentaround the zero axis resulting in asymmetry. If the shortoccurs at point where voltage is at its peak, the DCcomponent will be zero. If the short occurs at the point

where voltage is at zero, the DC component will be atits maximum, and the peak current will be almost twotimes the symmetrical current.

In practice the actual DC offset will likely range some-where between zero offset and the maximum offset.However, for the purpose of selecting the interruptingrating of circuit breakers the assumption that the DCoffset will be at maximum is a safe and conservativeassumption (ISC

Sym x 2). Generally, circuit breaker

interrupting ratings are expressed in RMS Symmetrical Amperes and are tested to achieve near maximumoffset at that symmetrical current magnitude. Select

breakers, then, based on the maximum allowablegenerator symmetrical short circuit current.

FIGURE 1 – Symmetrical current

FIGURE 2 – Asymmetrical current

P.U.Z.new = P.U.Z.given

base kV given

2

base kV new( )

base kVA new

base kVA given( )



related to the generator set sub-transient reactance.The higher the reactance is, the higher the voltagedistortion. Reducing the source impedance (reactance)reduces the voltage distortion. This calculation willneed to be used to check and compare suppliers’submittals against specifications. In the example, whilea 125°C rise alternator may have been specified, 105°Calternator would have to be supplied to meet a 0.12per-unit or less sub-transient reactance requirement.

Summary

The sub-transient reactance of a generator set is usedto calculate the maximum available short circuit currentfor selecting circuit breakers with adequate interruptingrating. Since nearly all of the generator impedance isreactance, addition of the DC component for the first fewcycles may almost double the symmetrical value of cur-rent. Specifications that limit the sub-transient reactance(x"

d ) to 12-per-unit or less limit the voltage distortion

caused by non-linear load currents. Calculation of theavailable short circuit current using per-unit values mayrequire converting from the alternator base rating to thegenerator set base rating using the formula provided.


For example, suppose you want to find the sub-transient reactance (X"

d ) in Per-Unit for the 105°C rise,

480 volt alternator on a 750 kW DFHA generator set.From the generator set specification sheet, S-1132,the alternator data sheet is ADS-311. ADS-311 showsa sub-transient reactance of 0.16 P.U. based on the125°C alternator base rating of 1300kVA. The base kVArating of the 750DFHA is 938 kVA. Using the formula,calculate X"

d on the generator set base as follows:

Limiting voltage distortion

It is becoming commonplace for consulting engineersto limit sub-transient reactance in specifications to 0.12per-unit or less to limit generator voltage distortioncaused by non-linear load induced harmonic currents.The alternator source voltage distortion induced bythe harmonic (sub-cyclic) current distortion is directly

Design Engineer, Technical Project Leader and ProjectManager. His current position is a Technical Specialist-Electrical in application engineering for CumminsEnergy Solutions Business, supporting combined heatand power (CHP) and peaking applications.

About the author

Timothy A. Loehlein is a graduate of theUniversity of Minnesota with a Bachelorof Electrical Engineering and a PE inMinnesota. Tim has been a CumminsPower Generation employee since 1976in positions as Application Engineer,



X"d(Genset) = X"d(ADS)

kV ADS

2

kV Genset( )

kVA Genset

kVA ADS( )

X"d(Genset)

= 0.160.48

2

0.48( ) 938

1300( )

X"d(Genset)

= 0.115 P.U.

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