Generators

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ELECTRICAL GENERATION

We have already stated that nearly every piece of equipment on board will utilize electrical power in some way or another. This can be in the form of power to operate the equipment or just to monitor it and give alarms if needed. It can normally be taken for granted that when the switch is turned, there will be power delivered to our equipment. Understanding how this power is generated and controlled will give you a better understanding of how the electrical distribution system operates and what factors affect it.

Generator Analysis

A generator converts mechanical energy to electrical energy by way of magnetic induction.  Recall the three requirements to induce a current via magnetic induction:

conductor magnetic field relative motion between the two

For a Linear Generator, a copper bar rests on two wires which form a loop.  What happens if we roll the bar to the left through the magnetic field?  A current is induced which flows as shown in Figure 1.

Figure 1

To determine which way the current will flow in a generator, the LEFT HAND RULE can be used: "Motion along the thumb, field (flux) along the forefinger, current along the middle finger." (Figure 2)

Figure 2

"My Fine Clothes" = Motion, Flux, Current

The current flows down the bar and therefore clockwise around the loop.  What happens when the bar reaches the end of the field?  There is no more induced current since one of the three requirements is not met.

Although this generator will work, it would be impractical to build because of size constraints.  It does, however, show the principle under which rotary generators work.  For practical purposes, the linear motion must be changed into rotary motion.

In a Single-Phase Generator, a source of rotation called the prime mover rotates an electromagnet (or rotor) past a conductor. A prime mover may be either a steam driven turbine, gas turbine, or diesel engine. The conductor is a pair of windings on the stator (or stationary part of the generator).  Brushes and slip rings are used to connect the DC excitation current to the rotor.  Note that the rotor field rotates at a constant speed.

Figure 3

At point A on the curve in Figure 3 there is no relative motion between the rotor field and the stator windings.  This makes the induced voltage zero.  As the rotor field moves down and toward the stator windings, it "cuts" more and more conductors. The induced voltage gradually increases and reaches a peak at point B.  As the rotor field continues down to the bottom, it cuts fewer and fewer conductors. The induced voltage gradually decreases and reaches zero again at point C.

As the rotor field moves up through and back into the stator windings, it cuts more and more conductors again and the induced voltage gradually increases and reaches another peak at point D (Figure 3).  The polarity (+,-) of the induced voltage reverses because the rotor field is passing back into the stator. The potential reaches a negative peak at point D.  As the rotor field continues up past the stator windings, it cuts fewer and fewer conductors. The induced voltage decreases until it returns to zero at point A and one cycle has been completed.

When the rotor field passes through one revolution on a two pole (N,S) generator, one cycle of AC voltage is produced.  This relationship is very important in the design of generators.  Output frequency, number of poles, and RPM are related by the following formula:

Ns = rotor speed (actually magnetic field speed) in RPM f = frequency, in Hz p = poles (per phase) Three-phase machines (both generators and motors) have several advantages over single-phase machines.  Three-phase machines make more efficient use of materials and therefore are not as expensive as single-phase equipment of the same rating.  

In the top drawing of Figure 4, you see that much of the space within a single phase generator is wasted.  In the case of three-phase motors, operating cost is less, they are easier to start, and they run with less vibration.  (A three-phase source provides more of a rotational effect since the each phase peaks sequentially; a single-phase source provides more of a simple on/off effect.)

Figure 4

The bottom drawing of Figure 4 shows two more phases added and equally spaced (120o) in the machine.  Note the more efficient use of

space within the casing and that the induced voltage is always greater than zero on at least one phase.

Standard Generator Specifications

450 Volts

        The level of RMS voltage present at the output of the generator.         Standard voltage on board; compatible with shore power.

60 Hz

        The rate at which the instantaneous value of voltage changes (frequency or cycles per second).         Compatible with shore power; standard U.S. frequency; most of Europe uses 50 Hz.

3 Phase

        Refers to the three separate output voltages as shown in Figure 4.         Allows for more efficient operation and use of space; compatible with shore power.

Three Phase AC Generator Components

Stator: The stationary inner circumference of the generator that houses one or more sets of armature windings.

Rotor: The rotating part of the machinery that houses the field windings. This is connected to the prime mover via a reduction gear and therefore rotates with a frequency proportional to the prime mover RPM.

Field windings: Coils of wire on the rotor. Form an electromagnet when DC current is passed through them. When rotated, a rotating magnetic field is produced.

Armature: The windings where the generator output voltage is produced. In most cases, this is located on the stator.

Poles (or pole pieces): The cores around which the armature or field windings are wound. There are a minimum of two per phase (one North and one South).

Brushes and Slip Rings: Provide DC current to the rotor windings while allowing the rotor to turn.

Brushes: Normally carbon or copper, held in place by brush rigging, brushes pass current from the DC source to the slip rings. Brushes are more easily replaced than slip rings so they're made of softer material than the material they contact.

Slip Rings: Smooth copper based rings pressed onto the rotor. Slip rings will rotate with the rotor and pick up the DC current off the brushes.

Output Breaker or Generator Breaker: Circuit breaker that connects the generator output to the ship's distribution system.

Static Exciter / Rotary amplifier: Both produce a DC field excitation current. This current flows through brushes and slip rings onto the rotor windings to produce the rotor's magnetic field.

Voltage Regulator: Controls the output voltage of the generator by comparing the generator's output voltage to a fixed reference and sending a differential feedback signal to the static exciter which will change the field strength (and therefore the generator’s output.)

Governor: Controls the speed of the prime mover as the load changes.

Generator Operations - Producing a Voltage

First, the prime mover needs to be rotating.  This rotates the generator rotor and produces the relative motion between field and armature windings.

Second, bring the generator RPM to the value corresponding to 60 Hz and place the governor in automatic control of frequency.

Third, produce the magnetic field. To do this we need to provide the excitation DC current to the field windings. The higher the current sent to the field windings, the stronger the magnetic field, and the higher the output voltage. The static exciter / rotary amplifier will provide the DC current for the field windings through the brushes and slip rings.  Since there is no current (and therefore no magnetism) present in the rotor at

startup, the current to "flash" the field is provided to the static exciter from a permanent magnet alternator (PMA).

Generator Operations - On Line

"Generator output" refers to voltage and frequency; the two parameters the operator can control and regulate.  To describe how a generator's output is regulated, a load will be placed on the generator and its response described.

As a load is applied to the ship's electrical distribution system, the current output of the generator increases. This is because the ship's loads are essentially arranged as a large parallel circuit. Any added load causes the overall system resistance to drop and the current to rise.

Example:

Initially, there is a 100 ohm equivalent load across the ship with a 450 volt source; the current is 4.5 A (Figure 5, left drawing).

Figure 5

Recall the relationship of resistors in parallel.  If another 100 ohm load is started in parallel with the first (Figure 5, right drawing), the new equivalent resistance will be 50 ohms; the new current is 9.0 A.

With an increase in current flowing through the armature (stator) windings, the magnetic field produced by the current through this coil(s)

increases in strength.  This phenomena, called ARMATURE REACTION has two effects:

The armature (stator) field distorts and weakens the rotor (main) field (Figure 6) causing a decrease in the induced voltage.

A counter torque is created on the rotor due to a reaction between the armature (stator) field and the rotor (main) field. This will tend to slow down the rotor and lower the frequency of the generator voltage.

Figure 6

In addition to armature reaction, the increase in current with added load will cause an increase in the voltage drop across the internal impedance of the generator. This is due to INTERNAL LOSSES in the generator.

Figure 7

Example:

1. The generator has a load R1 in the distribution system and is maintaining 450V line voltage.  Because of the current flow through the internal impedance RINT and XL, there is a voltage drop of 20v across this impedance.  This means that the generator's induced voltage, EIND, must be 470V to compensate for the internal impedance and still maintain 450V line voltage.

2. Now add another load, R2. The subsequent increase in current (and increase in internal resistance due to the heating of the wires) causes a larger voltage drop across the internal impedance, say 30v versus 20v. If EIND does not change, the line voltage will drop to 440V; not good for shipboard equipment designed to run on 450V.

3. The voltage regulator will sense the drop in line voltage through its voltage transformers. This will be compared to a reference voltage in the regulator circuitry and a differential feedback signal sent to the static exciter. The static exciter will increase the DC current sent to the field windings, raising the field strength and restoring the voltage to its original value.

Three-Phase Connections

The winding for each phase has two output leads (for a total of six in a three-phase system). If you examine an actual three phase machine, you will see only three leads coming out. This is because they are connected inside the generator. The two possible configurations are delta and wye.

DELTA                         WYE        

Figure 8

Note: The RMS voltage between any pair of output leads is designed to be 450 volts.

System nomenclature: PHASE VOLTAGE (Ephase) - The induced potential across any

single phase; measured inside the machine casing. PHASE CURRENT (Iphase) - The current passing through any

single phase; measured inside the machine casing. LINE VOLTAGE (Eline) - The potential across any pair of wires (A-

B,B-C,C-A); measured outside the machine casing. LINE CURRENT (Iline) - The current passing through any one of

the three output cables (A,B, or C); measured outside the machine casing.

Now we will look at the similarities and differences between Delta and Wye.  Assume that we are discussing 3-phase, 60 Hz, 450 volt generators which have enough load on them to draw 1000 amps as shown in Figure 9 below.

                 DELTA                                            WYE    

Figure 9

Delta Connection - Any pair of output cables are connected across a single-phase of the delta connected generator. This means that the phase and line voltages must be equal. The current passing through any output cable is supplied by the two phases connected to the cable. Since the current is 1000 amps, each phase must be supplying some portion of the 1000 amps. Each phase will supply 577.3 amps. Recall that the voltages produced by the phases are 120 degrees out of phase with each other, so the currents associated with them are also 120 degrees out-of-phase.

For a delta connection

Wye Connection - Any pair of output cables are connected across two phases of the wye connected generator. Since the line voltage is 450 volts, the two phase voltages must add up to 450 volts. Again recall the 120 degree difference. The phase voltages are each 259.8 volts. The line current is 1000 amps. All of the current passing through any cable is being supplied from a single-phase of the generator.

For a wye connection

Power in Three-Phase Systems.

The only difference in calculating power in three-phase systems versus single-phase systems is the square root of three factor.

Advantages and Disadvantages of Delta and Wye Connections

Delta

The internal voltage of a delta machine is greater than the internal voltage of a wye machine. Larger voltage requires larger insulation which causes a slight increase in weight and size of coils.

A delta connected machine has a smaller phase current, and therefore less I2R losses.

If one phase of a delta winding opens up, there will be no effect on the output voltage (since the other two windings are connected so that they can supply the load), but the total current through the system must be reduced to 57.7% of the rated current to prevent overheating of the remaining active phases. This allows a selective reduction in system loads.

Wye

For a given amount of line voltage and line current, the wye generator has a larger current. This means that more power is lost inside the generator in the form of heat.

The manufacturing and rewinding of a wye connected machine is easier due to the less complex connections of the windings.

If one phase of a wye connected machine opens up, two of the three output voltages will go to zero volts. This occurrence does not allow a selection of which loads to remove. Two thirds of the loads on that generator will lose power.

Sample Problem:

A 3 phase generator is supplying a distribution system. You observe the following switchboard indications:

Power Meter: 800 kw  Voltmeter: 450 v Ammeter: 1300 A.  Frequency: 60 Hz.

What is the power factor of this ship? (Assume a primarily inductive load.)

= 0.79 lagging

Transformer

A transformer is a device which transfers AC power from one circuit to another by means of electromagnetic induction.  During the transfer, voltage and current are "transformed" from one magnitude to another.   The frequency of the AC power is never changed by a transformer.  A transformer has no moving parts. There is no electrical connection between the two circuits; however, there is a mechanical connection.

Transformer Construction

There are three parts common to any transformer: The primary winding The secondary winding The core

The primary and secondary windings are inductors (coils). The core is the ferromagnetic material that the inductors are wrapped around. The core and the windings are not electrically connected in any way.

Figure 10

Ideal Transformer Operation

An ideal transformer is one that has no losses. That is, all of the energy that goes into the transformer will come out of it in a useable fashion.

A labeled schematic of a typical transformer:

Figure 11

Eg = voltage generated by the source  Ep = voltage that appears across the primary coil Rg = resistance inside the source  Es = voltage that appears across the secondary coil Ip = current passing through the primary coil  Is = current passing through the secondary coil RL = resistance of load on the transformer  Np: Ns = "turns ratio" for the transformer. The turns ratio indicates the number of primary turns of wire there are compared to the number of secondary turns. For instance, if a transformer had a turns ration of 4:3, that would mean that for every 4 turns of primary winding, there would be 3 turns of secondary winding. That might mean an actual count of 4000 to 3000 or 80 to 60.

The quantities above are related in the following equations:

                          Voltage:                                   Current:                                           Power:

Transformer Applications

Isolation Transformer (Figure 12) - A transformer with a turns ratio of 1:1 is an isolation transformer. Isolation transformers are used when it is necessary to prevent grounds or other power system problems from effecting a particular piece of gear. The main purpose of an isolation transformer is to minimize shock hazard when using portable electrical tools in the 120 VAC system.

Figure 12

Power Transmission (Figure 13) - It is more efficient to transmit power at high voltages. At high voltages, the power required to transmit a given amount of current will be low, and power losses (I2R) in the transmission lines will be low. In commercial power distribution, step-up and step-down transformers are used to reduce the current. Shipboard generation of 450 volt power requires that the power distribution system needs only step-down transformers to lower the generated 450 volts to about 110 volts. Some gear that requires high internal voltages uses step-up transformers to raise the 450 volt input to several thousand volts.

Figure 13

Multiwinding (Figure 14) - A transformer with multiple secondary windings is a multiwinding transformer. These transformers are often a component in a device such as a radio or radar that requires multiple levels of AC voltage. A multiwinding transformer's secondaries are independent of each other. A fault in one secondary winding will not affect any other winding.

Figure 14

A step-down transformer has an extremely high turns ratio (on the order of 1,000,000:1). These devices are mostly used in conjunction with power system installed meters.  The voltmeters and ammeters installed in shipboard distribution systems usually have an instrument transformer between them and the line voltages and currents to prevent high currents from damaging the meters.

Voltage Transformer - This is an instrument transformer connected in parallel with the voltage to be measured.  Care should be taken that the secondary side of the transformer is never shorted out, as this will produce excessively high currents in both windings and damage the meter.

Figure 15

Paralleling AC Sources

Paralleling is the procedure of placing two or more generators online to provide power to a common bus or power panel. This procedure is also used to transfer the electrical load from one generator to another. It is one of the most common electric plant configurations on board. If you have spent most of your time "topside", you probably never realized that any paralleling was occurring. If performed properly, paralleling allows a smooth transition of power without any interruption of power to the ship. When the procedure is not followed properly everyone on the ship is immediately aware of the problem due to the subsequent loss of power.

Figure 16

Reasons for Paralleling

To provide standby power in the event one machine fails. The machine in parallel with the failing one will pick up the load.

To increase the plant capacity beyond that of a single unit. In the event that a large load is started unexpectedly a single generator will not be able to handle the entire load.

To serve as additional reserve power for expected demands. This is a common practice before evolutions such as General Quarters when all of the combat systems loads are energized.

To permit starting a new machine and shutting down the running machine without interrupting the power supply. This is

perhaps the most common reason for paralleling. Run times can be equalized and maintenance can be performed.

Reasons for Synchronizing Generators To avoid severe damage, generators must be synchronized prior

to paralleling them together. Normally, one machine will be supplying an electrical distribution bus.

The oncoming machine will be synchronized with the running machine (actually it will be synchronized with the bus). Generators are in SYNCHRONIZATION when the following FOUR CONDITIONS are met:

1.  Equal terminal voltages. If the machines have different induced voltages, large currents would be generated due to the different potentials connected by a low resistance. Voltage is set by adjusting the generator's field strength.

2.  Equal frequency. This prevents generators from being paralleled out of phase. Actually, the frequency of the oncoming machine should be slightly higher than the frequency of the online machine. (See Figure 17)

Figure 17

3.  Proper phase relationship. The two generators must be "in phase". Both machines must have the same angular relationship.

Figure 18

a.  In the left drawings in Figure 19 when 1A generator's voltage is "on a peak" or +450v, 1B generator is "in a valley" or -450v. This is a large difference of potential and will cause a large current between the two machines and subsequent damage. In reality the unloaded machine will attempt to instantaneously "catch up" and match phases with the loaded machine. Generators have been known to lift deck supports to accomplish this. The two generators on the right are in-phase and will not generate the high currents and damage.

b.  With the oncoming generator at a slightly higher frequency than the running generator, the two will be alternately running in and out of phase. The operator's job is to shut the output breaker for the oncoming machine when the two generators are in phase. The synchroscope is used to accomplish this task.

4.  Proper phase sequence. The "A" phase of the oncoming machine must be connected to the "A" phase of the running machine. The same must also be true for the "B" and "C" phases. This is not so much of a concern when paralleling two shipboard generators since their connections are hard-wired into the switchboards.

The House Diagram

In the droop mode, when the power demand is increased:

The frequency will drop slightly. The action of the governor restores the frequency to 60 Hz.

The voltage will drop slightly. The action of the voltage regulator restores the voltage to 450 volts.

This characteristic can be graphed. Two generators to be operated in parallel can be graphed sharing the same vertical axis.  This graph is called the house diagram.

Figure 19

The intersection of the two dashed lines is the operating point. The horizontal dashed line indicates the operating frequency or

voltage. The vertical dashed line indicates the operating power in KW or

kVAR.

Balancing the Load

Suppose that the total load on the ship at time of the parallel is 2000 KW and that the system frequency is 60 Hz.

1A TG is on line supplying the load and 1B TG is the oncoming generator. Consider the distribution system in Figure 20 and the associated house diagram.

Figure 20

EIND = the potential (voltage) induced by the generator

EINT = the potential drop across the generator's internal impedance

ESUP = the potential supplied to the load (Z) by the generator

Note that the sum of the generators' loads must equal 2000 KW throughout this evolution.

Figure 21

1B generator's output breaker has just been shut. 1A generator is carrying most of the ship's load and subsequently has a large current output. These currents will cause a voltage drop across the generator's internal resistance, in this case 30 volts. To maintain 450 volts on the

distribution system, 1A generator's voltage regulator will ensure 480 volts is induced to overcome the 30 volts. Since 1B is not yet carrying any load, it will not have a large output current, little to no internal voltage loss, and thus will need only 450 volts of induced voltage to maintain 450 volts on the distribution system. Now balance the real load between the two machines.

Real Load

The transfer of real power between generators in parallel must be accomplished by adjustment of the governor controls. The machine taking the load must have more energy (steam or fuel) and the machine losing the load must have less energy admitted to its prime mover. In practice, the switchboard operator will raise up on the oncoming (1B) generator's governor control adjust rheostat while lowering down on the running (1A) generator's rheostat. The real load has been balanced.

Figure 22

Since 1B has picked up some of the load, its current output has increased to the point where there is now a 15 volt internal voltage drop. On the other hand 1A has given up some load so its current output is lower and its internal voltage loss drops to 15 volts also. Since the system voltage is 450 volts, the voltage regulators in the two machines will change very little (they both adjust their induced voltages on the basis of what the system voltage is). This means that with an induced voltage of 480 volts and an internal loss of 15 volts, 1A is supplying 465 volts. 1B supplies 435 volts due to an induced voltage of 450 volts and

an internal loss of 15 volts. Note that the average of the two supplied voltages is 450 volts, that appears on both generators voltmeters.

Reactive Load

With this large difference between the two generator's potentials, large currents called "circulating currents" are generated. Since the imbalance is caused by the difference in the strength of two induced magnetic fields, we say that circulating currents are caused by an imbalance in the reactive load.

To correct this problem, the switchboard operator will raise the oncoming (1B) generator's voltage adjust rheostat and lower the running (1A) generator's auto voltage adjust rheostat. The reactive load has been balanced and appears in Figure 23.

Figure 23

Steps to Parallel Two Generators

1.  Match voltage.

2.  Match frequency.

3.  Match phase rotation/phase sequence.

4.  Observe the synchroscope rotating slowly in the fast direction (clockwise).

5.  Turn the breaker switch at the "5 'til 12" position on the synchroscope.

6.  Increase the setting on the oncoming generator's governor to balance the real load.

7.  Adjust both governors to regain 60 Hz.

8.  Raise the voltage on the oncoming generator to balance the reactive load.

9.  Lower both voltage regulators together to regain a 450 volt bus voltage.

Load Changes

Consider an addition of 1000 KW of load on the system.

It should be noticed that with the Droop characteristic, the increase in load is shared equally (as with Isochronous) but that the new operating frequency has dropped to 59.75 Hz. The switchboard operator will be required to raise up on both machines' governor controls to restore frequency. The addition of load will also cause an addition of reactive load. This will show up on the generator's ammeters. The generators will share the reactive load equally and in Droop characteristic governors will require the switchboard operator to raise up on both voltage regulator controls.

Figure 24

It must be remembered that despite the load on each generator, the frequency of both machines must remain the same. If the frequencies were somehow different, large currents would be generated because of the phase differences. These large currents tend to motorize the slower machine forcing it to catch up to the faster machine (similar to the action of a synchro system). So admitting more steam to one generator and taking it away from the other will not change system frequency, just shift the load between generators.

Shore Power Parallels

There are two important things to remember about paralleling with shore power: 1.  Shore power is isochronous, and

2.  It is impossible for your ship's generators to power Norfolk or San Diego!

For these reasons it is important to ensure that the ship's generators are in droop mode and always maintain a load on the shore power system.  Consider the following diagram:

Figure 25

To transfer the load from the ship to shore, lower the ship's governor control while watching the KW meter. When the load on the ship is minimal, open the generator output breaker. To transfer from shore to

ship, the opposite procedure applies. Your ship's EOSS should be followed closely to prevent errors.

This page last modified: October 2, 2003