06102549

My Experience

with an Australian Grid

BIMAL K. BOSE In my long professional career that has spanned over 40 years, I have

handled many technical consulting projects from industries. These

include settling technical disputes among corporations and complex

failure studies in plants with power electronics equipment. I would like

to share with the readers my experiences regarding one particular

project—hopefully they will find it very interesting.

While in my office one morning, I received a telephone call from

someone in Australia informing me of a fault that had occurred in the large

cycloconverter drive of a mining ore-crushing mill. Around midnight, when the

mill was running at full load, there was a roaring thud—followed by the mill

shutdown and blackout of power supply. Extensive damage occurred to the sys-

tem, and the plant shutdown led to a severe economic loss for the company.

Digital Object Identifier 10.1109/MIE.2011.943022

Date of publication: 9 December 2011

©CARTESIA

12 IEEE INDUSTRIAL ELECTRONICS MAGAZINE n DECEMBER 2011 1932-4529/11/$26.00&2011IEEE

The problem required critical investi-

gation. I was considered the world’s

expert on cycloconverters. My re-

sponsibility was to investigate: What

was the problem? Why did it occur?

Were there any flaws in the design of

the drive and protection system?

Were the plant personnel adequately

trained for the operation of the equip-

ment? Was there any gap in com-

munication among the equipment

designer, installer, and personnel of

the plant? What should be the possi-

ble solution and remedial measures

so that this does not happen again in

the future? The tasks appeared very

complex. I had the opportunity to

visit the damaged plant in Australia. I

agreed to undertake the project but

was not very confident whether I

could satisfy their expectations.

For confidentiality reasons, this

description is very general and does

not identify any of the parties in-

volved. The description indicates

the complexity of the project, requir-

ing an intimate understanding of

utility grid operation, power elec-

tronics, machines, control, faults, and

protection systems. First, we need to

understand the background systems

in detail.

Utility Grid OperationA utility grid is responsible for main-

taining a reliable quality power supply

to the customers [1]. Quality power

means that the three-phase power

should be balanced with the regula-

tion of voltage and frequency; there

should be no harmonic distortion or

transients; and of course, there should

not be any interruption of power sup-

ply. Typically, the bus voltage should

be regulated within �10% of the

declared voltage and the frequency

deviation within a small fraction of a

hertz from 50 Hz (Australian grid).

Note that the frequency is identical

throughout the grid, but the voltage

can vary at different locations. The

frequency accuracy is important for

electric clocks as well as the speed of

ac motors (induction and synchro-

nous) operating on the grid. Low

voltage causes light dimming and

deteriorates the speed and torque

capability of motors, besides affect-

ing the performance of voltage-sensi-

tive apparatus.

The customer power demand on a

grid always fluctuates, and at any

instant, the generated power should

match the demand power neglecting

losses and any storage of energy.

Frequency is the normal messenger

for real power-demand variation,

whereas voltage is the messenger for

reactive power-demand variation. The

increase of real power demand tends

to get extracted from the stored

kinetic energy pool of the rotating

machines on the grid that results in

the drop of frequency. However, as

the governors on the turbo genera-

tors sense the drop of speed, they

pump more fuel (steam in fossil and

nuclear power plants and water in

hydro power plants) to the driving

turbines to compensate for the speed

drop. Normally, one generating plant

is assigned the responsibility of

frequency regulation, whereas the

others carry assigned block loads. If

loading is large and sudden, and the

grid capacity is small (single genera-

tor in an extreme case), the fre-

quency drop will be large because of

the inertia delay of the rotating sys-

tem. Although the bus voltage is

affected by real power demand, the

effect is more dominant by reactive

power loading. Lagging reactive power

reduces the bus voltage that is

normally compensated by the gener-

ator excitation control. Static volt

ampere reactive (VAR) compensa-

tors (SVCs) or synchronous rotating

VAR generators can be installed in

strategic locations of the grid to

improve power factors and thus

reduce reactive current loading (and

loss) of equipment and excitation

control range of generators. Again,

the load is distributed on the grid so

that there is no overloading of any

line and equipment and the grid

operates at optimal efficiency. If the

total load on the grid exceeds the

generation capacity (indicated by

frequency dip), the excess load is

shed selectively. If the load exceeds

the line capacity, the line circuit

breaker is tripped to relieve the load.

The advanced smart grid of tomorrow

with state-of-the-art power electron-

ics, computers, and communication

equipment will permit optimum re-

source utilization and economic elec-

tricity to customers, maintaining

higher energy efficiency, reliability,

and security of the system.

Cycloconverter DriveOperationTraditionally, a thyristor phase-con-

trolled cycloconverter (CCV), as shown

in Figure 1, is used in large multimega-

watt gearless mill drive applications

[2], [3]. The applications include

ore-grinding mills, rolling mills, cement

mills, pumps and compressors, variable-

speed constant frequency systems,

limited-speed range slip power-recovery

Scherbius drives, ship propulsions,

and more. A CCV is essentially a direct

frequency changer that converts ac

input power at 50 Hz into variable volt-

age and variable frequency (VVVF)

N

a

c

b

+C –C

FIGURE 1 –A 36-thyristor phase-controlled cycloconverter supply with a synchronousmotor load.

DECEMBER 2011 n IEEE INDUSTRIAL ELECTRONICS MAGAZINE 13

output power to drive an induction or

synchronous motor. Figure 2 shows a

typical ore-grinding mill [4] that is

coupled to the motor. For a large

drive, wound-field synchronous motor

(WFSM) is preferred because of better

efficiency. The three-phase to three-

phase CCV consists of three dual-

bridge six-pulse 36-thyristor phase-

controlled converters, where each

phase group is supplied by a step-

down transformer with secondary

delta windings in isolation. The sta-

tor winding of the machine is wye

connected to the CCV. The thyris-

tors operate in a phase-control line-

commutation principle to fabricate

sinusoidal phase voltages, as shown

by the waveform in Figure 3. The

CCV operates in the blocking mode,

i.e., when the positive converter (þC)

of a dual bridge conducts positive

phase current, the negative converter

(�C) is blocked (to prevent short cir-

cuit) and vice versa. This is unlike cir-

culating current mode operation of

the component bridges where an

intergroup reactor is inserted between

the bridges. The advantages in block-

ing mode operation are that the con-

trol is somewhat simple and the device

current rating is low. However, the dis-

advantages are that the output voltage

and input line current waves contain

higher harmonic distortion that limits

the output frequency range typically

within 33% of line frequency. A CCV

operates in four quadrants (motoring

and generation in either direction).

Because of phase control with sinusoi-

dal firing angle modulation, the line

displacement power factor (DPF) is

low and harmonic patterns are very

complex [2]. The output phase volt-

age harmonic family is given by the

expression pnfi � nf0, where pn � n

is an odd integer, p is equal to six, m

and n are integers, fi is the line fre-

quency, and f0 is the load frequency.

The line harmonics family is given

by (np � 1)fi � mf0, where (np � 1) �m ¼ odd integer. The line side of CCV

normally needs an SVC to improve

DPF and an active harmonic filter

(AHF) to restore sinusoidal line cur-

rents. The machine inductance on

the load side gives adequate filtering

to give a near sinusoidal load current.

Figure 4 shows the fundamental

phase voltage and line current waves

in the motoring mode, where / is the

lagging DPF angle. The positive half

cycle of the current wave is carried

by the positive converter (þC),

whereas the negative half cycle is car-

ried by the negative converter (�C).

The figure indicates that the major

part of the half cycle operates in the

rectification mode, whereas a small

segment operates in the inverting

mode. With a WFSM drive, the field

current is normally controlled so that

the machine terminal DPF is near

unity. However, it is difficult to elimi-

nate the inverting mode operation

entirely at the trailing edge of each

half cycle of the current wave.

In a six-pulse CCV, the thyristors

in each phase group are fired syn-

chronously at 60� interval (i.e., 3.33 ms

for 50 Hz supply) by the gate driv-

ing circuit. The principle of thyristor

commutation (current transfer) is ex-

plained in the inverting mode by a sim-

plified half-wave positive converter [3]

in Figure 5. When an incoming thyr-

istor is fired at an advance angle

b b ¼ p� a, a ¼ firing angleð Þ, the in-

coming and outgoing devices overlap

in conduction for angle l before

commutation is completed at turn-off

angle c ¼ xtoff, where x is the line

frequency and toff is the turn-off time.

The variables are related as shown in

the following equation [3]:

cos c� cos (lþ c) ¼ 2xLcIdffiffiffiffiffiffiffiffi

6Vs

p , (1)

where Lc is the line leakage or com-

mutating inductance, Id is the CCV

FIGURE 3 – Fabrication of a phase voltage wave of a cycloconverter.FIGURE 2 –An ore-grinding mill.

vo

vo

io

io

φ

φRectification0<αP ≤π /2

Rectification0≤αN <π /2

π /2≤αP ≤π π /2≤αN ≤π

Inversion– Converter+ Converter

FIGURE 4 –Output phase voltage and current waves in the motoring mode (with perfectfiltering).

14 IEEE INDUSTRIAL ELECTRONICS MAGAZINE n DECEMBER 2011

load current at the instant of com-

mutation, and Vs is the supply phase

voltage (rms). The turn-off angle

c with the reverse voltage segment

across the outgoing thyristor is

crucial for the successful turn-off

of the outgoing thyristor. The equa-

tion indicates that the increase in

Lc and Id and decrease in Vs in-

creases the angle l and decreases

c angle b ¼ lþ cð Þ that tends to

cause commutation failure. Obviously,

thyristors are more likely to have

commutation failure in the inverting

mode, where the c angle and inverse

voltage can be too small (unlike the

rectification mode where their magni-

tudes are large). The commutation

failure or line short circuit may also

occur in the dual bridge if an incoming

bridge converter bank is enabled before

turning off the outgoing bank (defined

as cross-commutation failure).

The System UnderInvestigation

Grinding Mill Cycloconverter Drive

The CCV synchronous motor drive

system for the ore-grinding mill con-

sists of two identical drive units as

shown by the simplified diagram in

Figure 6. The 33-kV, three-phase, 50-Hz

power supply to the drive system

comes from the Australian grid

through the electromechanical cir-

cuit breaker CCB. Then, the 33 kV

supply steps down to 1.22 kV

through three identical wye/delta-

wye transformer banks (details are

not shown). All the secondary delta

windings supply to CCV-1, whereas

the wye windings are connected to

the CCV-2 unit. Each CCV takes the

1.22-kV, three-phase, 50-Hz power at

the input and converts to three-

phase VVVF (0–14 Hz) supply for

controlling the speed of each wye-

connected synchronous motor. The

CCV units are the same as shown in

Figure 1. The field excitation cur-

rent of the motors are controlled

so that DPF is near unity (0.99 lag)

at the machine terminal for opti-

mum efficiency and power rating

of the CCV-machine system. Each

vd′

vQ1

iQ5iQ1

iQ3

id

Q5 Q1 Q3

0

0

0

α

ωt

ωt

ωt

βμ

γ

γ = ωtoff

(a)

(b)

(c)

FIGURE 5 –Parts (a)–(c) show three-phase half-wave positive converter (+C) waveforms inthe inverting mode.

Transformer

1.22 kV

Cycloconverter 1(CCV-1)

Cycloconverter 2(CCV-2)

VVVF

VVVF

Variable VoltageVariable Frequency

Motor-1

Motor-2

TLC

TLC

Pinion-1

Pinion-2

Ore-Grinding Millwith Ring Gear

6 MW, Eight-Pole, 0–210 r/minSynchronous Motor

If

If

50 Hz

33 kVPower Supply

From Grid

CCB

Circuit Breaker

FIGURE 6 –Dual cycloconverter synchronous motor drives for the ore-grinding mill (a simplified diagram).


machine is rated at 6 MW, eight-pole

and 0–210 r/min speed range corre-

sponding to the frequency range of

0–14 Hz. Both the motors are con-

trolled to operate at an identical

speed and share the load torque

equally under all conditions.

Because of the phase shift of 30�between the delta and wye phase

groups, the two CCV units receive

gate-firing pulses with a phase skew

angle of 30� (1.67 ms). The advantage

of the phase shifting operation is to

reduce the harmonic orders in the

primary line current. With a six-pulse

operation of each CCV, ideally the

line current has a 12-pulse wave

shape, i.e., the harmonic orders are

11th, 13th, 23rd, 25th, and so on. The

machines do not have any damper

winding and their field excitation is

controlled by the thyristor rectifier.

The motor shafts contribute torque

to the respective pinion through a

mechanical torque-limited coupler

(TLC), and the pinions drive the ring

gear that is coupled directly to the

grinding mill drum as shown in the

figure. Also, the direction of rotation

of the pinions and ring gear in the

motoring mode is shown in the fig-

ure. The TLC limits the shaft torque

to the pinion and decouples the load

if the torque exceeds the safe limit.

The TLCs are shown by dotted lines

because these were not originally

connected.

The speed and torque of each

drive unit is controlled by vector or

field-oriented control with accurate

tracking so that the respective pinions

carry identical driving torque. A sim-

plified control block diagram with the

explanatory phasor diagram is shown

in Figure 7, where all the symbols are

standard [3]. The speed control in the

outer loop generates the torque com-

mand that then generates the torque

component of current IT� as shown in

the figure. The machine always oper-

ates in the constant torque region

with the stator flux (ws) as constant.

The orthogonal relation between ws

and IT and the corresponding flux link-

age relations shown in the phasor dia-

gram are difficult to maintain in the

transient condition because the field

current (If ) response is sluggish. With

the increase in the torque command,

the shortage in If is compensated by

temporarily injecting magnetizing cur-

rent IM from the stator side. Since

I�f ¼ I �M=cos d in the steady state, a

sudden increase in the torque angle dby an increase in the torque command

will increase I�f . But the response

delay of If will cause a finite I �M trying

to maintain constant ws. As If builds

up, IM decreases until it vanishes to

satisfy the steady-state phasor dia-

gram at unity DPF. The motor has a

position sensor, but all the other

feedback signals are estimated from

the machine’s terminal voltages and

currents [3].

Cycloconverter Commutation

Faults and Protection System

The reliable CCV operation on the

grid mainly depends on satisfactory

commutation of the thyristors, which

in turn is dictated by the supply volt-

age waves, as discussed before. Un-

fortunately, the utility power supply is

not 100% reliable. The common causes

of commutation failure are as follows:

n very low supply voltage [defined

as undervoltage (UV)]

n supply voltage interruption by

open or short circuit

n transients in supply voltage

n very high line inductance (Lc) (long

line or new transformer added)

n very high load current.

With the exception of the supply

interruption, the commutation fail-

ure normally occurs in the inverting

mode, as discussed before. Although

in the present system, the CCV is

operated near unity DPF, there will

be always a small angular interval of

the inverting mode operation (see

Figures 4 and 5). The load current

(except the fault condition) is con-

trolled by feedback loops. In the nor-

mal open- or short-circuit condition

of the supply, there is no voltage for

commutation, and therefore, commu-

tation failure occurs easily. In the sup-

ply open-circuit condition, however,

the commutation may be successful if

capacitor or counter electromotive

force (EMF)-type loads remain con-

nected on the power line.

When commutation failure occurs,

both the incoming and outgoing thyr-

istors continue to conduct. This will

short circuit the line voltage, and

therefore, fault current will build up. If

subsequent thyristors are fired by the

gate drive circuit, short circuit will

occur in all the three phases building

up large fault current. The fault will

be normally fed by the synchronous

machine EMF (the motor will act as a

generator, where the grinding mill

inertia supplies the energy) as well as

the power line. The machine gener-

ates the EMF due to its speed and field

excitation, and the line voltage will

feed the fault through the transformer

if the line circuit breaker is closed. If

the line CCB is opened under the fault

condition, the fault current fed by the

machine will attenuate due to large

transformer self-inductance (instead

of usual leakage or commutating in-

ductance). The scenario will be some-

what similar for line open-circuit

conditions. For the symmetrical short

circuit of the line, the fault will be fed

by the machine with the transformer

leakage inductance in the circuit.

Note that the instants of commuta-

tion fault of the two CCVs are different

because of a 30� phase-shift (1.67 ms)

operation. Therefore, fault develop-

ment and the resulting fault current

profiles will be somewhat unsymmet-

rical in the two CCVs. Again, at the

fault condition, the control system

operation is erratic in nature and the

performance prediction becomes ex-

tremely difficult.

The large fault current at the

commutation failure is usually alter-

nating and unbalanced in the three

phases with a decaying dc compo-

nent. Since the machines remain fully

excited, this current interacts with

the field flux to generate a large pul-

sating or oscillating torque. This tor-

que is transmitted to the grinding

mill through the respective pinions

and ring gear. Again, because of the

asymmetry of the faults between the

CCVs, the pulsating torques are out

of phase. The pulsating torques can

easily overload the pinions and ring

gear causing severe damage. The

plant mechanical system coupled to


the machine shafts is normally char-

acterized by multimodal natural fre-

quencies of torsional oscillation. The

pulsating torque frequency spectrum

can have a frequency component

near a natural frequency so that the

applied torque will be magnified by

the resonance condition, thus con-

tributing to more damage. There may

be some amount of damping in the

electrical circuit and the mechanical

elements, but the system is generally

underdamped. If the gate drive re-

mains activated with the erroneous

operation of the control system, the

fault scenario will be much more

complex. Eventually, the overcurrent

protection will trip the line breaker

and the fault current will become

zero when the machine stops.

The CCVs and machines in Figure 6

were designed to be robust enough

to withstand the commutation fault

successfully without causing any dam-

age. Thyristors have a large thermal

capacity so that large transient fault

currents can flow without exceeding

∗ Im∗

cos δIr =

ωr∗

ψe∗

ψs

ψsψr

ψe

ωr

ωr

SpeedCommand

+ +– –

P-I P-ITe∗

Te

IT∗

IM∗

Im∗

IT

IT

IrIT

IM

Im

IM

IM

+

+

+

+

–

–

–

–

P-I

P-I∗Vm

∗Vc

∗Va∗Vb

∗VT

cos α

cos δ

cos δ

δ

cos θo

sin θo

sin α

34 ac Line

VR Cylcoconverter

Ir

Ir

Ir

ia, ib, ic

va, vb, vc

×

÷ WFSM

Flux Program

+ –P-I

P.S.

qoqo ′

θe

θe

Io

vs

α = θe + δ

d s d e

d e ′π2

q s

(a)

(b)

FIGURE 7 – (a) A simplified control block diagram of a cycloconverter-synchronous motor drive with (b) a phasor diagram.


the safe junction temperature (TJ )

limit. The machine windings also have

a large thermal time constant. The

drive motor shaft is also robust enough

to absorb large pulsating torque. The

shaft was actually designed to with-

stand nine times the rated torque on

the drive end and 4.5 times on the non-

drive end.

The CCV theory is generally well

documented in books [2], [3] and

published literature. However, CCV

failure modes and the corresponding

generation of fault current, pulsating

torque, and torsional oscillation of

the mechanical system are very com-

plex, and the information is available

only on a piece-meal basis [4]–[6].

Generally, CCV drive manufacturers

make internal study based on the

particular system configuration, and

the information is available only as

internal documents. To have a pro-

per understanding of the fault per-

formance and to effectively design the

protection system to prevent any

damage in the plant equipment, it is

necessary to have systematic system

analysis, modeling, and computer sim-

ulation study of the drive system

along with the plant mechanical sys-

tem under fault conditions. Unfortu-

nately, such a study is laborious, time

consuming, and expensive and is

hardly done by the manufacturers.

The CCV drive system in Figure 6

was designed with the following

protections:

n Undervoltage Protection: If the sup-

ply voltage falls below 85% of the

rated value, the thyristor gate

pulses are blocked, and at the

same time, the line breaker CCB is

tripped. However, the CCB restarts

automatically (in case, restart is

enabled) if the voltage level ex-

ceeds 90% within 200 ms. This is

the only CCV protection for malop-

eration of the power supply.

n Overcurrent Protection: In a well-

grounded condition of the drive,

the load current is controlled to

be within the safe value by the

feedback control. In case of over-

current due to the CCV fault, the

CCB and gate pulse inhibit are

activated as mentioned previously.

n Transformer Differential Protec-

tion: This protection detects the

difference in the two CCV input

currents and trips the system, if

the magnitude difference exceeds

a certain limit.

n Earth Fault Monitoring: If a stator

phase winding is shorted to the

stator core (ground) by insulation

failure, it trips the system as afore-

mentioned. One-phase earth fault

is not a problem, but if two phases

have earth faults, the load short

circuit occurs that tends to cause

commutation failure.

Note that the utility system fre-

quency rarely falls below a small

fraction of a hertz. Besides, underfre-

quency in a grid does not affect the

commutation process as long as the

voltage waveforms are sinusoidal

with adequate magnitude. Therefore,

underfrequency protection is not used

in the grid-connected CCVs.

Grid-Connected Cycloconverter

Drive with Cogeneration from

Gas Turbine Generators

The present CCV drive system is con-

nected to the grid in parallel with

a cogeneration system as shown in

Figure 8. The radial grid line at 132 kV

is 310 mi long. It is stepped down to

33 kV by a transformer and con-

nected to the bus through a circuit

breaker CB1. Since the grid line has

limited power capability and is sub-

jected to reliability problems, the

local cogeneration by a private com-

pany is added for the reliable op-

eration of the grinding mill. The

cogeneration system uses two gas

turbine generators at 11 kV (40 MW

each) and feed the 33-kV bus

AustralianPower GridOre-Grinding

Mill CCV Drive

CCB

33 kV

CB1td

33 kV Bus33 kV Bus

310 mi 132 kV

Grid Supplyto Grinding Mill

CB249 Hz, td3

CB4CB3

CB5

47 Hz, td447 Hz, td4Future

Low-FrequencyProtection

(UnderfrequencyProtection–Added Later)

49.5 Hz, td5

UndervoltageProtection85%V, td2

11 kV

40 MW EachG3 G2 G!

Gas Turbine Generators

FIGURE 8 –Grinding mill operation on the Australian grid with local cogeneration from thegas turbine generators.

The turn-off angle is crucial for the successful

turn-off of the outgoing thyristor.


through the respective transformer

and circuit breaker. A third genera-

tor is planned for the future expan-

sion. The waste heat of the gas

turbines is used for space heating.

Suffice it to say that the protection

system of the CCV drive should be

designed such that it operates satis-

factorily for all the possible power

supply malfunctions, either in the

combined grid operation mode or

islanding mode with the cogenera-

tion system. All the protective

elements should have proper co-

ordination. The actual protections

in Figure 8 can be summarized as

follows:

n grid line breaker (CB1)—over-

current protection, time delay

td ¼ 10 s

n gate pulse inhibiting (both CCVs),

td1 ¼ 1:67 and 3.33 ms, respec-

tively (worst case)

n CCV breaker (CCB)—undervolt-

age protection, 85% voltage, td2 ¼150 ms þ low-pass filter (LPF)

n underfrequency protection, 49.5 Hz,

td5 ¼ 150 ms (added later)

n cogeneration system breakers:

1) line breaker (CB2)—low-

frequency back-up protection,

49 Hz, td3 ¼ 10 cycles þ 500

ms þ LPF

2) generator breakers (CB3 and

CB4)—low-frequency protection,

47 Hz, td4 ¼ 200 ms þ LPF

where LPF filters are the primary sig-

nal with a typical delay of 100 ms.

Problem Diagnosis

One night when the CCV drive system

of the grinding mill was operating at a

full load with the grid and cogenera-

tion system in parallel, there was sud-

denly a very loud bang/thud in the

mill, which lasted approximately 1 s. It

was then followed by the mill shut-

down and a power supply blackout.

The damage in the mechanical system

(including pinions and ring gear) was

extensive. It was found that this

scenario occurred simultaneously

with the gas supply failure of both the

turbines in the cogeneration plant.

Then, the investigation started with

all the recorded evidences to study

the problem and suggest a solution.

The study indicated that there

was, in fact, a commutation failure in

the CCVs, and the protection system

did not work. The study also indi-

cated that although the mechanical

system damage was extensive, there

was no damage at all to the CCVs and

drive motors. Apparently, the CCV

drive system was designed to be

robust for any commutation fault, as

mentioned previously. However, evi-

dently, as discussed before, the com-

mutation failure caused excessive

pulsating torque and the resulting

torsional oscillation caused the

mechanical damage. It appeared that

the manufacturers designed the

equipment and the protection sys-

tem to protect their own equipment,

and no consideration was given to

protect the user’s equipment. Also,

at this point, it appeared that the pro-

tection system was designed for the

usual utility system operation and no

importance was given for the opera-

tion with the cogeneration system.

The study indicated that when

the gas supply failed (with the loss

of power from the local generators),

the grinding mill power was auto-

matically transferred to the grid.

This, in turn, overloaded the grid

and tripped the line breaker CB1

with a time delay tdð Þ causing island-

ing mode operation of the drive with

the cogeneration system. The CCV

drive operated with a 50-Hz power

supply.

System Analysis

The scenario of the system (Figure 8)

under the fault condition can be

summarized as follows. When the

gas supply to the turbines of gen-

erators G1 and G2 failed, these

generators ceased to be normal

generators with the cutoff of prime

mover power supplies. Instead,

the machines (with full excitation)

operated as synchronous motors

that drove the turbines as pumps

in the same direction extracting

power from the stored kinetic

energy of the rotating system. The

mode of operation is similar to

generating-to-motoring mode op-

eration in a four-quadrant drive

system. At this condition, the mill

power demand was automatically

transferred to the grid with the

subsequent overload tripping of

the line breaker (CB1). After the

line breaker tripping, the equiva-

lent system topology is shown in

Figure 9. In the system, the grind-

ing mill power was extracted from

Turbine

InertiaLoad If

Gas Turbine Generators(Two in Parallel)

CB3CB4

CB2

CCB33 kV

ParallelLoad

CCV-Synchronous Motor Drives(Two in Parallel)

Grinding Mill Grinding MillLoadCCV

If

FIGURE 9 –An equivalent topology of a cycloconverter drive system in the islanding modefrom the grid with the gas turbine generators operating under the gas failure condition.

The reliable CCV operation on the grid

mainly depends on satisfactory commutation

of the thyristors.


the stored inertia energy of the two

gas turbine generators (parallel load

was neglected). This means that the

gas turbine generators (with field

excitation) were working as motors

for the gas turbines and as genera-

tors for the grinding mill. In this

mode of operation, the kinetic

energy of the generators, i.e., the

speed, fell rapidly. The speed was

proportional to frequency, which

was again proportional to the line

voltage, assuming that the generator

excitation (very sluggish) remained

constant. This fall of voltage and

frequency was affected by the paral-

lel load, if any.

An approximate analysis of the

CCV line frequency and voltage is

possible as shown in Figure 9,

assuming that the generator field

current If is maintained constant at

the rated value and three-phase out-

put voltages remained sinusoidal

and balanced. The valid assump-

tions are as follows.

At a constant excitation, the

generator EMF is proportional to the

speed, i.e.,

E ¼ KIf N ¼ K1N , (2)

where E is the EMF, If the field cur-

rent, and N the speed in r/min. The

generator frequency is proportional

to speed, i.e.,

f ¼ K2N : (3)

Combining (2) and (3)

E

f¼ K1N

K2N¼ K , (4)

or

E ¼ Kf , (5)

i.e., the generator EMF is propor-

tional to frequency. This means that

100% EMF (33 kV) corresponds to

100% frequency (50 Hz).

Assume, for simplicity, that the

CCV power demand is constant. The

stored kinetic energy W of the two

gas turbine generators is given as

W ¼ 1

2Jx2

m, (6)

where J is the total moment of inertia

and xm the angular mechanical speed

(rad/s).

Rated Frequencyand Voltage

50 Hz

100%

Frequency (f )and Voltage % 50%

0

49.5 Hz (99% Voltage)

49.5 Hz (98% Voltage) LF Protection47 Hz (98% Voltage) LF Protection

CCV Constant Power (P) Line

85% Voltage UV Protection (42.5 Hz)

Critical Voltage forCommunication Failure

Line of InertiaEnergy (W)

D1

B1E

D

CBA

td3

td4

td2

td1

700 ms

300 ms

250 ms

Gate Inhibit

3.33 msGate Inhibit

150 ms

C1

A1

CB3 and CB4Trip Command

UnderfrequencyProtection Trip

Command (AddedLater forSolution)

Under Voltage TripCommand (CCB)

Time (ms)

td5

td1

CB2 Trip Command

FIGURE 10 –A sketch of line frequency and voltage curves at the cycloconverter terminal under the turbine gas failure condition andtripping of circuit breakers (not to scale).

The plant mechanical system coupled

to the machine shafts is normally characterized

by multimodal natural frequencies

of torsional oscillation.


Since the generator speed is pro-

portional to frequency, from (6) we

obtain

W ¼ K 0f 2: (7)

With constant power loading of the

CCVs

dW

dt¼ P: (8)

Differentiating (7) and substituting

(8), we obtain

dW

dt¼ P ¼ 2K 0f

df

dt, (9)

or

df

dt¼ P

2K 0f¼ K 00

f: (10)

Equation (10) shows that the rate of

frequency variation is inversely pro-

portional to frequency. This means

that as the frequency decays with the

time, the slope of df=dt will be higher.

An approximate sketch of P, W , and

f as a function of time is given in Fig-

ure 10. Since the generator voltage is

proportional to frequency, it is not

shown separately. If parallel loading

and generator losses are included, the

fall of voltage and frequency will be

faster. On the other hand, if generator

excitation (If ) is increased (with satu-

ration) to compensate the voltage

drop, the voltage drop will improve to

some extent.

Figure 10 shows the trip command

activation of circuit breakers CCB,

CB2, CB3, and CB4 and their corre-

sponding tripping points with the

respective time delays. The CCVs have

only undervoltage protection at 85%

voltage (i.e., 42.5 Hz with the restart

deactivated), which is initiated at

point D. Point E indicates the typical

voltage for the commutation failure.

The underfrequency protection at 49.5

Hz (99% voltage, point A) was added

later as a remedial measure, which will

be discussed later. Both the undervolt-

age and underfrequency protections

inhibit the CCV gate trigger pulses and

trip the circuit breaker CCB with the

respective time delays as indicated in

Figures 8 and 10. As the frequency

decreases, CB2 is activated first (point

B) and then CB3 and CB4 (point C), as

shown in Figure 10. However, CB3 and

CB4 trip first at point C1 because of

less time delay (td4). Note that

although undervoltage trip is acti-

vated before opening CB3 and CB4,

the gate triggers inhibit and CCB trip-

ping actually occurs after opening

CB3 and CB4. This concludes that

CCVs experience an open-circuit con-

dition of power supply that causes

the commutation failure.

Suggested Solution and Other

Remedial Measures

As a suggested solution, an underfre-

quency protection at 49.5 Hz was pro-

posed. This protection, as shown in

Figure 10, offers faster tripping before

open circuiting of the power supply

occurs. In this protection, the line

frequency is measured accurately for

three cycles before activating the

protection. The underfrequency pro-

tection was found to protect the CCV

drive system successfully from the

commutation failure when the gas

supply failure occurred later in the

cogeneration plant. In a normal utility

system operation (with or without

cogeneration), the undervoltage pro-

tection prevents the commutation

fault, whereas the underfrequency

protection remains inactive because

the utility system’s frequency rarely

decreases to 49.5 Hz.

The question arises what other

remedial measures could have been

added to prevent the commutation

fault and reduce its severity? These

measures can be summarized as

follows.

n Add a TLC to each motor shaft,

as shown in Figure 6. In fact,

this was added later as back-up

protection.

n Provide motor field excitation with

a regenerative rectifier. A quick

decay of the field current (If ) re-

duces the severity of the motor

pulsating torque.

n Provide fast fuse protection to

the stator phases of the motor.

n Provide a mechanical brake to

the motor to prevent movement

with the pulsating torque.

n Use a shorter tripping time delay

for the circuit breaker CCB.

n Use a single CCV-motor drive in-

stead of the dual drive system

shown in Figure 6. The control

and protection becomes simple

and the equipment damage is

less severe.

n Instead of using thyristor CCV, use

a double-sided three-level neutral

point clamped (NPC) converter

system, as shown in Figure 11.

In fact, this is the recent trend.

The self-commutated devices do not

depend on power supply voltage

vo

FIGURE 11 –A two-sided voltage-fed three-level NPC inverter.

Underfrequency in a grid does not affect

the commutation process as long as the

voltage waveforms are sinusoidal with

adequate magnitude.


waves, thus give a higher reliability of

operation. Besides, the line and load

harmonics and DPF improve signifi-

cantly eliminating the need of an SVC

and an AHF. In addition, the spare

capacity of line-side converters can

be used for VAR compensation.

Conclusion andLessons LearnedThe CCV drives operating on the

utility systems are usually protected

against the commutation failure of

thyristors. Commutation failure nor-

mally occurs due to undervoltage

(including open- and short-circuit

conditions). For this reason, under-

voltage protection is provided that

blocks the gate trigger pulses and

trips the supply circuit breaker.

Underfrequency protection is not

needed because the frequency is

regulated precisely within a small

fraction of a hertz. In addition, as long

as the voltage waveforms remain

sinusoidal, underfrequency does not

cause any commutation failure.

The present system under consid-

eration was originally provided only

with the undervoltage protection.

The undervoltage protection did not

protect the CCVs from the commuta-

tion failure when the gas supply

failed in the cogeneration system. As

a result, large unbalanced and un-

symmetrical fault currents devel-

oped that caused a severe pulsating

torque on the machine shafts. This

torque was magnified by the tor-

sional resonance of the mechanical

system, causing severe damage to

the plant. The analysis indicated that

an underfrequency protection was

needed to protect the plant in the

present scenario. Therefore, an un-

derfrequency protection was added

with the undervoltage protection and

the system operation was verified

to be reliable against the commuta-

tion failure under all conditions of

operation.

Unfortunately, designers normally

design the equipment so that it pro-

tects only their equipment and often

ignores the protection of the user’s

equipment. The result is catastrophic

damage and prolonged power outage

in the plant, as illustrated in the

present case. The design and protec-

tion system of the equipment should

be satisfactory under all operating

conditions of the plant. The equip-

ment designer, installer, and user

should work with full cooperation to

ensure healthy operation of the plant.

The fault performance and optimum

protection system design of the equip-

ment are normally very complex and

may require systematic analysis, mod-

eling, and computer simulation stud-

ies. This aspect of the equipment

design is often ignored. The user is

normally not very familiar with the

complex equipment. The equipment

designer and installer should educate

the user thoroughly for successful

and reliable operation under all possi-

ble conditions because gaps in knowl-

edge and communication can cause

disastrous consequences.

AcknowledgmentThe author acknowledges the exten-

sive help and cooperation received

from the parties involved in carrying

out the investigations of this project.

BiographyBimal K. Bose ([email protected]) was

a faculty member at Bengal Engineer-

ing and Science University (BESU)

from 1960 to 1971. From 1971 to 1976,

he was an associate professor of

electrical engineering at the Rensse-

laer Polytechnic Institute, Troy, New

York. From 1976 to 1987, he was a

research engineer in the GE Corpo-

rate R&D Center, Schenectady, New

York. He has held the Condra Chair

of Excellence (endowed chair) in

power electronics at the University

of Tennessee, Knoxville since 1987.

Concurrently, he served as the

distinguished scientist (1989–2000)

and chief scientist (1987–1989) of

EPRI-Power Electronics Applications

Center, Knoxville, Tennessee. He is

a specialist in power electronics

and motor drives. He has authored

more than 200 papers, holds 21 U.S.

patents, and has authored/edited

seven books in power electronics.

IEEE Industrial Electronics Magazine

honored him by publishing a special

issue ‘‘Honoring Dr. Bimal Bose: Cel-

ebrating His Contributions in Power

Electronics’’ in June 2009. He is a

recipient of a number of awards,

including the IEEE Power Electron-

ics Society Newell Award (2005),

IEEE Millennium Medal (2000), IEEE

Meritorious Achievement Award in

Continuing Education (1997), IEEE

Lamme Gold Medal (1996), IEEE–IES

Eugene Mittelmann Award (for life-

time achievement in power electron-

ics and motor drives) (1994), IEEE

Region 3 Outstanding Engineer Award

(1994), IEEE Industry Applications So-

ciety Outstanding Achievement Award

(1993), IEEE Fellow (1989, Life Fellow

in 1996), Calcutta University Mouat

Gold (1970), GE Silver Patent Medal

(1986), GE Publication Award (1985),

Distinguished Alumnus Award (2006)

from Bengal Engineering and Science

University, and a number of IEEE

Prize Paper awards.

References[1] T. Wildi, Electrical Machines, Drives, and

Power Systems, 6th ed. Upper SaddleRiver, NJ: Prentice-Hall, 2006.

[2] B. R. Pelly, Thyristor Phase-Controlled Con-verters and Cycloconverters. New York:Wiley, 1971.

[3] B. K. Bose, Modern Power Electronics andAC Drives. Upper Saddle River, NJ: Prentice-Hall, 2002.

[4] J. Pontt, J. Rodriguez, E. Caceres, I. Illanes,and J. Rebolledo, ‘‘Cycloconverter behav-ior for a grinding mill drive under firingpulses fault conditions,’’ in Proc. IEEE IASConf. Rec., 2005, pp. 645–649.

[5] J. Rodriguez, J. Pontt, P. Newman, L.Moran, and G. Alzamora, ‘‘Technical evalu-ation and practical experience of highpower grinding mill drives in mining appli-

cations,’’ in Proc. IEEE IAS Conf. Rec., 2003,pp. 1629–1636.

[6] J. O. Pontt, J. P. Rodriguez, J. C. Rebolledo,K. Tischler, and N. Becker, ‘‘Operation ofhigh power cycloconverter-fed gearlessdrives under abnormal conditions,’’ IEEETrans. Ind. Applicat., vol. 43, no. 3, pp. 814–820, May/June 2007.

[7] H. Stemmler, ‘‘High power industrial

drives,’’ Proc. IEEE, vol. 82, pp. 1266–1286,Aug. 1994.

[8] R. A. Errath, ‘‘15000-HP gearless ball milldrive in cement—why not!’’ IEEE Trans.Ind. Applicat., vol. 32, pp. 663–669, May/June 1996.

[9] B. K. Bose, Power Electronics and MotorDrives—Advances and Trends. New York:Academy Press, 2006.

[10] B. K. Bose, ‘‘Power electronics and motordrives—Recent progress and perspec-tive,’’ IEEE Trans. Ind. Electron, vol. 56,pp. 581–588, Feb. 2009.


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