chapter 5 protection of synchronous generator...

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CHAPTER 5

PROTECTION OF SYNCHRONOUS GENERATOR

5.1 INTRODUCTION

There is a constant need for the reduction of operational and

maintenance costs of large sized synchronous generators. The most efficient

way of reducing these costs would be continuous monitoring of the condition

of these generators. This allows for early detection of the degeneration of the

generator’s health, facilitating a proactive response, minimizing downtime,

and maximizing productivity. There are many techniques and tools available,

which are used to monitor the condition of these machines and prolonging

their life span. Stator winding faults of synchronous generator are considered

serious problems because of the damage associated with high currents and

high cost of maintenance. A high speed bias differential relay is normally

used to detect three phase, phase to phase and double phase to ground faults.

Detection of single line to ground faults depends on the generator grounding

type which can be classified into low and high impedance grounding. In case

of low impedance grounding, a differential relay can detect and provide

protection of only about, 95% of the windings. However, for high impedance

grounding, ground faults are not normally detectable by the differential relay

because the fault current is usually less than the sensitivity of the relay. In

such case, an over-voltage relay connected across the grounding resistor has

been used to sense the zero sequence voltage. This relay should be set to

avoid tripping of normal unbalance which yields reduced sensitivity.

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In this work, Computational Intelligence based approaches for

identifying Stator Earth Fault and Inter-Turn fault of Synchronous Generator

are proposed. The detector uniquely responds to the winding inter-turn fault

and Stator Earth Fault with remarkable high sensitivity. Discrimination of

different percentage of winding affected by these faults is provided via ANN

or GA-BPN or ANFIS which are given with eight dimensional input vector.

This input vector is obtained from features extracted from DWT of faulty

current leaving the generator phase winding for the case of Inter-Turn fault.

For stator earth fault protection subharmonic injection technique is followed.

So the neutral current of the generator is taken for structuring input vector.

Training data for ANN, GA-BPN and ANFIS are generated via a simulation

of generator with these faults using MATLAB Simulink. The proposed

algorithm using ANFIS is giving satisfied performance than ANN and GA-

BPN with selected statistical data of decomposed levels of faulty current. The

sample power system is shown in Figure 5.1.

Figure 5.1 Sample Power System

5.2 STATOR EARTH FAULT PROTECTION

Synchronous generators are very important elements in power

systems since they are in-charge of providing an uninterrupted power supply

to the consumers. Therefore, their reliability and good functioning are crucial.

The construction as well as maintenance cost is high depending upon the

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complexity and the size of the generators. The important role of generators in

the power system and the high cost of repair in case of damage require a good

protection system against faults. It must be protected against the damage

caused by abnormal conditions in the electrical network or in the generator

itself. Generators are protected against external faults by several circuit

breakers that isolate all faults that occur in the network (i.e. transformers,

buses, lines, etc). At the same time, the generators must be protected against

faults that occur inside the machine. There are several ways to detect these

faults and avoid the damages caused by them. This work focuses on stator

winding ground faults to provide 100% protection in synchronous machines.

A stator winding ground fault is the most common type of fault to which

generators are subjected. Stator ground faults could be caused by the

insulation degradation in the windings as well as environmental influences

such as moisture or oil in combination with dirt which settles on the coil

surfaces and outside the stator slots. This often leads to electrical tracking

discharges in the end winding which eventually punctures the insulation.

A stator ground fault referred in this work is a single-phase to

ground fault. Generators must be protected against the fault for two reasons:

The first and obvious faults which occur during abnormal

situations in the functioning of the machine causing

undesirable voltages, currents, oscillations or damage.

The second reason, an undetected and uncleared ground fault

could develop into a phase-to-phase fault or into an inter-

winding fault, if another single-phase to ground fault occurs.

These faults are associated with immediate damage to the

generator since the resulting short-circuit current would be of

devastating magnitude.

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The conventional methods like Stator winding zero-sequence

neutral over voltage protection, Instantaneous ground overcurrent protection,

Ground time-overcurrent protection, Ground differential protection and

Percentage phase differential protection can detect ground faults for only

about 95% of the stator winding since there is not enough voltage to drive

current when the fault occurs near the neutral. In the remaining 5% of the

stator winding (the closest part to the neutral), the relays cannot be operated.

Therefore, additional protection methods are used to provide a 100% stator

ground fault protection. Special protection systems based on the third

harmonic analysis and on the subharmonic voltage injection can detect stator

ground faults close to the neutral. These protection methods are strongly

recommended for large generators because the entire stator winding must be

protected.

5.2.1 Subharmonic Injection Method

The subharmonic injection method used to protect the stator

winding of unit-connected generators against stator ground faults is described

as follows:

5.2.1.1 Principle of operation

A subharmonic voltage (usually one fourth of the system

frequency) is injected through an injection transformer between the grounding

element of the generator (i.e. resistor, distribution transformer or reactor) and

ground. The operation theory of this principle is based on measuring the

change of the subharmonic current resulting from the injected voltage when a

stator ground fault occurs. Since the impedance of the stator to ground varies

when the fault occurs, the subharmonic protection scheme can detect this

change of the injected current and initiate the tripping-off of the generator.

During normal operation, the resulting subharmonic current is limited by the

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grounding impedance (resistor, reactor or distribution transformer with

resistor loaded at the secondary), by the internal impedance of the injection

circuit and by the shunt leakage capacitance to ground of the stator winding,

bus, step-up transformers, etc. The inductances of the stator windings can be

neglected when compared to the impedances of the grounding element and

the shunt capacitances to ground.

When the ground fault occurs, the fault resistance appears in

parallel with the shunt capacitances to ground. Thus, the impedance limits the

subharmonic current changes and also makes the current change. The

subharmonic injection principle of operation is based on this change in the

subharmonic current. Therefore, detecting and measuring this change and

operating if it is necessary will be the function of the subharmonic injection

scheme inorder to protect the stator winding. Figure 5.2 shows the principle of

operation of this method.

Figure 5.2 Principle of operation

The subharmonic voltage could also be injected at the terminals of

the generator but as stated in Tai et al (2000) the injection voltage scheme

injected from the generator neutral is more flexible to be carried out and

presents better results than injected from the terminals.

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As discussed in Reimert (2005), the scheme uses a subharmonic

signal for two reasons. First the lower frequency increases the impedance of

the stator capacitive reactance. This improves scheme sensitivity. Secondly,

by measuring the current as an integrated value over full 12.5 Hz cycles all

other harmonics of 50 Hz are eliminated, allowing more sensitive

determination of the injected current.

The generators considered in this work are unit-connected

generators. This means that a step-up transformer will be connected in

between the terminals of the generator and the electrical network. Moreover,

circuit breakers are placed in between the generator and the bus in order to

isolate the generator from external faults. At this point, it is necessary to make

some assumptions in order to obtain a simple model which means, substitute

some elements of the scheme but taking into account their effect on the

circuit.

First of all, the measuring circuit will be taken out. The injection

transformer and all the elements of the injection circuit (signal controllers, test

circuits, etc) will be substituted for injection impedance (Zinj = Rinj+j·Xinj).

Therefore, the 12.5 Hz voltage source will have to be reflected (E'inj) to the

generator side of the transformer using the turns ratio (rt). The internal

resistance of the subharmonic source will be reflected as well to the generator

side of the transformer and will also be included in the injection impedance

(Zinj).

The generator is modelled as follows:

- One reactor per phase whose value is Xd or Xd' or Xd'' which

are the synchronous, transient and subtransient reactance

respectively.

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- The stator winding capacitance to ground will be modelled as

one capacitor per phase placed after the reactance.

- In order to model the 50 Hz power generation of the

synchronous machine, one 50 Hz AC source is placed in each

phase. Their amplitude is the nominal voltage of the generator

divided by square root of 3 (Un/√3) and their phase will be 0º,

-120º and 120º depending on the phase (A, B and C

respectively).

Finally, the capacitor between the circuit breaker and the step-up

transformer, the bus and the step-up up transformer can be modelled just

taking into account their capacitances to ground. Thus, three capacitors per

phase is placed to model these elements. Figure 5.3 shows the equivalent of

the unit-connected generator with the injection scheme.

Figure 5.3 Equivalent of the unit-connected generator with the

injection scheme

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5.2.1.2 Typical values for a unit-connected generator

Table 5.1 shows the values and characteristics of the unit-connected

generator used in the calculations and simulations of this work.

Table 5.1 Typical values for a unit-connected generator

Rated Power(SN) 850MVA

Rated Rotational Speed 3000 rpm

Rated Frequency (fo) 50 Hz

Rotor Type Round rotor

Nominal Voltage (Un) 21kV

Synchronous Reactance (Xd) 2.44 p.u.

Transient Reactance (Xd’) 0.43 p.u.

Sub-transient Reactance (Xd”) 0.25 p.u.

Zero Sequence Reactance (X0) 0.13 p.u.

Negative Sequence Reactance (X2) 0.24 p.u.

Zero Sequence Resistance (R0) 0.0025 p.u.

Positive Sequence Resistance (R1) 0.0034 p.u.

Negative Sequence Resistance (R2) 0.04 p.u.

Capacitance to ground of the stator winding (Cgnd) 0.385µF

Power Factor 0.882

Grounding Resistor (Rn) Rated 100A, 21/√3V, Rn=1212Ω

Bus Capacitance (Cbus) 0.1 µF/phase

Surge capacitor between step-up transformer and circuit breakers capacitance (Csurge)

0.25 µF/phase

Step-up transformer capacitance (Ctrafo) 0.2 µF/phase

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5.2.1.3 Typical values for a subharmonic injection scheme

According to the Figure 5.3, there are two values of the

subharmonic injection scheme that must be defined: Einj’ and Zinj. These

values will be taken from an example presented in Pope (1984). Einj’ is the

subharmonic voltage source reflected to the generator side of the injection

transformer. In Pope (1984), the injection transformer has a turns ratio of

generator _ sidet

source _ side

V 1rV 2.5

(5.1)

and the voltage source has a magnitude of 140 V (source side), which means

that the reflected voltage to the generator side is given by

gen._ side source _ side1V V . 56V

2.5 (5.2)

Therefore, Einj’= 56 V. Zinj is the equivalent impedance of the injection

circuit. It also includes the internal resistance of the subharmonic voltage

source. In Pope (1984), the leakage impedance of the injection transformer is

Zinj = 36 + j·125 in a 60 Hz system frequency as given in equation (5.3). This

means that,

R 36

Zinj 36 j.125 125L 0.331H2. .60

(5.3)

Thus, it will be assumed that Zinj (impedance of the injection circuit) will be

calculated with R=36 Ω and L=0.331 H. One must realize that depending on

the system frequency this impedance will vary since X=j2πfL.

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5.2.2 Mathematical equations

In this section, the mathematical equation for the subharmonic

current is presented. The subharmonic current in a non-fault scenario is

calculated as

inj

inj n C total

E'I

z R Z

(5.4)

As one knows the values of the injection circuit impedance, the grounding

resistor and the total capacitance to ground, it is possible to calculate the non-

fault current.

non _ fault

6

56I 136 j 0.331 2 12.5 1212 j2.035 10 2 12.5

0.0088A 78.67

The magnitude of the non-fault current is very small since the impedance of

the capacitances to ground is very large. The angle of the non-fault current is

close 90 º due to the influence of this large capacitance. When the ground

fault occurs, the impedance that limits the subharmonic current changes.

Consequently, the subharmonic current is divided in two currents: one that

flows through the capacitor and the other one through the fault resistance. The

total subharmonic current in a fault scenario is:

inj injfault C Rf

Ctotal finj n C total finj n

Ctotal f

E ' E 'I I I Z RZ R Z || R Z R

Z R

(5.5)

One can substitute the known values and find Ifault

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faultf f f

f f f

56 56I j 6257 R j 6257 R (R j 6257)36 j 26 1212 1248 j 26j 6257 R (R j 6257) (R j 6257)

2

f f2 2f f

5639150049 R 6257 R1248 j 26R 39150049 R 39150049

The fault current depends on the fault resistance. It is not easy to

imagine the curve that represents the fault current when varying the fault

resistance. Therefore, MATLAB SIMULINK software is employed to obtain

and analyze the values of the subharmonic current.

The 50 Hz power generation is included in the model of the

subharmonic injection scheme. It could be modeled by one AC source per

phase. When considering the effect of the 50 Hz generation, the measured

current will not only have the 12.5 Hz but also it will contain the 50 Hz.

Therefore, the equations for the current will not be calculated and the whole

study will be based on the data obtained in the simulation in MATLAB

Simulink.

5.2.3 The Simulation Model

In the faulted phase, instead of having just one AC 50 Hz source,

one can introduce two sources whose amplitudes are αUn/√3 and (1- α)Un/√3.

The fault resistance Rf is placed between these two sources. Moreover, the

capacitance to ground of the stator winding is divided in two capacitors: one

placed at the neutral whose value is αCstator and the other is placed with the

rest of capacitances to ground and its value is (1-α)Cstator. The aim of

splitting the stator ground capacitance and the power generation in the faulted

phase is modelling the fault location as follows:

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α=1 means that the fault is placed at the terminals of the

generator since all the capacitance of the stator winding comes

before the fault resistance.

α=0 means that the fault is placed at the neutral since all the

capacitance of the stator winding is placed after the fault

resistance. Figure 5.4 shows the subharmonic protection scheme

with the 50 Hz power generation.

Figure 5.4 Subharmonic protection scheme with the 50 Hz power

generation

There are two well-differentiated parts in the current curve shown

in Figure 5.5. In the first 0.4 seconds, the current has a small magnitude but

in the second 0.4 seconds, after the ground fault occurs, the current magnitude

increases a lot. Moreover, the frequencies of the fault and non-fault currents

are different. The non-fault part is a 12.5 Hz sinusoid while the fault part is a

50 Hz curve.

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Figure 5.5 Injected current per Rf = 1000 Ω and α =1

In Figure 5.6, one can see the 12.5 Hz sine wave. Once again, the

amplitude of the non-fault subharmonic current is 8.8 mA.

Figure 5.6 Non-fault current

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5.3 STATOR INTER-TURN FAULT PROTECTION

In this work, Inter-turn winding fault is also considered for the

protection of synchronous generator since all kind of faults develop into inter

winding fault by damaging the inter-winding insulation. So, it is necessary to

protect the synchronous generator against inter winding faults. For inter

winding protection, differential method cannot be implemented as the current

on both side of the fault will be the same. The Inter-turn fault is simulated by

connecting a resistor across the winding which will reduce the resultant value

of both resistance and reactance of the winding as shown in Figure 5.7 which

is reported by Douglas et al (2005).

The Interturn fault is simulated by connecting a resistor across the

winding which will reduce the resultant value of both resistance and reactance

of the circuit. The Figure 5.7 shows the modified circuit for fault simulation.

A variable resistor is connected across the stator winding A. The percentage

of the winding fault can be changed by varying the value of the resistor.

Figure 5.7 Generator with fault simulating resistor

The values of resistor to simulate winding Inter-turn fault have

been calculated from 0% to 100% of winding fault. The calculation is as

follows:

A

C

B

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Zph - Impedance of phase winding

Under x% of winding short circuit fault effective Impedance is:

Zph (eff) = Zph – (x/100)Zph = Zph || R

Zph (1- (x/100)) = (Zph*R)/(R+Zph)

R = Zph (1- (x/100)) /(x/100) (5.6)

Using the equation (5.6), Values of resistor to be connected across the

winding have been calculated for simulating different % of winding fault.

The values of resistor to be connected across the phase winding of synchronous generator are listed in Table 5.2.

Table 5.2 Values of Resistor for simulating various % of inter-turn fault in the winding of synchronous generator

% of Fault

Resistance ( in Ohms) % of Fault Resistance

( in Ohms) 1 5.265649 17 0.672677 2 3.047938 18 0.645939 3 2.254692 19 0.621447 4 1.833225 20 0.598896 5 1.566449 21 0.578036 6 1.379858 22 0.558657 7 1.240637 23 0.540585 8 1.131947 24 0.523673 9 1.044203 25 0.507796 10 0.97152 26 0.492846 11 0.910069 27 0.478731 12 0.857247 28 0.46537 13 0.811213 29 0.452693 14 0.770627 30 0.440638 15 0.73449 31 0.429152 16 0.702037 32 0.418186

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Table 5.2 (Continued)

% of Fault

Resistance ( in Ohms) % of Fault Resistance

( in Ohms) 33 0.407698 67 0.189578 34 0.397649 68 0.185073 35 0.388007 69 0.180607 36 0.378739 70 0.176176 37 0.369819 71 0.171778 38 0.361221 72 0.167407 39 0.352924 73 0.163059 40 0.344905 74 0.158731 41 0.337147 75 0.154418 42 0.329633 76 0.150115 43 0.322346 77 0.145817 44 0.315272 78 0.14152 45 0.308398 79 0.137219 46 0.301712 80 0.132907 47 0.295201 81 0.128578 48 0.288856 82 0.124227 49 0.282667 83 0.119845 50 0.276623 84 0.115423 51 0.270718 85 0.110954 52 0.264942 86 0.106426 53 0.259289 87 0.101825 54 0.25375 88 0.097139 55 0.24832 89 0.092349 56 0.242992 90 0.087434 57 0.23776 91 0.082366 58 0.232618 92 0.077111 59 0.227561 93 0.071624 60 0.222584 94 0.065843 61 0.217682 95 0.059679 62 0.212849 96 0.052992 63 0.208082 97 0.045553 64 0.203376 98 0.036906 65 0.198726 99 0.025877 66 0.194128 100 0.000002

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5.4 PROPOSED ALGORITHM

In this proposed scheme, with Ia phase current data measured under

winding inter-turn fault, In neutral current data under stator earth fault of

generator are recorded for 0.25 cycles from the instant of fault. For four

decomposition levels of these current, maximum and range values are taken

as features for framing input vector under faulty condition. Extracted features

may be of anything like maximum, mean, minimum, absolute mean deviation

etc. Output vector of ANN or GA-BPN or ANFIS reveals the percentage of

winding affected by fault. If the disturbance is classified as a fault on the

winding, the circuit breaker of the generator will be tripped. The flowchart of

the proposed algorithm is shown in Figure 5.8.

5.4.1 Data Encoding

For the protection of synchronous generator against stator earth

fault, four decomposition levels (level V, level VI, level VII and level VIII)

are used. First four levels are not considered since they are not giving wide

variation in their statistical data. But for the inter-turn fault protection, first

four levels are used as they are showing wide variations in their statistical

data.

For protecting the generator against stator earth fault training data

for ANN, GA-BPN and ANFIS are encoded as specified in Figure 5.9. The

last column of the data is representing the percentage of winding affected by

earth fault which is to be the output of Computational Intelligence

Techniques.

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Figure 5.8 Flowchart of proposed algorithm

Fault Current of 3phase Synchronous Generator

Sampling of Fault current

Sampling counts and Data Recorder

Check For

¼ cycles (n = 10)

Check for I, II

quadrant of faulty

waveform

Check for III, IV

quadrant of faulty

waveform

Feature Extraction (DWT), Data Encoding

ANN GABPNANFIS Trained

for I

quadrant faulty Data


for III

Quadrant faulty

Data


for II



for IV


I II III IV

Yes

No

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In

Level V

Max.

In

Level V

Range

In

Level

VI

Max.

In

Level

VI Range

In

Level VII

Max.

In

Level VII

Range

In

Level VIII

Max.

In

Level VIII

Range

Output (%

Wdg. Under Earth Fault)

Figure 5.9 Training Data Encoding for stator earth fault

For protecting the generator against inter-turn fault training data for

ANN, GA-BPN and ANFIS are encoded as specified in Figure 5.10. The last

column of the data is representing the percentage of winding affected by

Inter-turn fault which is to be the output of Intelligent Computational

Approaches.

Ia

Level I

Max.

Ia

Level I

Range

Ia

Level II

Max.

Ia

Level II

Range

Ia

Level III

Max.

Ia

Level III

Range

Ia

Level IV

Max.

Ia

Level IV

Range

Output (% Wdg inter-turn

Fault)

Figure 5.10 Training Data Encoding for winding inter-turn fault

The input vector elements for stator earth fault protection are

shown in the Figure 5.11.

In

Level V

Max.

In

Level V

Range

In

Level

VI

Max.

In

Level

VI Range

In

Level VII

Max.

In

Level VII

Range

In

Level VIII

Max.

In

Level VIII

Range

Figure 5.11 Input Vector for stator earth fault

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The input vector elements for Stator Inter-Turn fault protection are

shown in the Figure 5.12.

Ia Level

I Max.

Ia Level

I Range

Ia Level

II Max.

Ia Level

II Range

Ia Level

III Max.

Ia Level

III Range

Ia Level

IV Max.

Ia Level

IV Range

Figure 5.12 Input Vector for winding inter-turn fault

For different percentage of these winding faults including no fault

case, the output element of each data takes value shown in the Table 5.3.

Table 5.3 Output vector for the protection of synchronous generator

Output vector

% of winding affected by the fault

0 No Fault 0.01 1% 0.02 2% 0.03 3% 0.04 4% 0.05 5%

.

.

.

.

.

. 1 100%

5.4.2 Decision Making by Computational Intelligence Approaches

After encoding the data of fault current as per the formats shown

above it is given to the corresponding ANNs or GA-BPNs or ANFISs which

are trained for I, II, III, IV quadrants of faulty currents respectively.

Intelligent Computational approaches are giving decision about the

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percentage of winding affected by fault. The same flow can be adopted for

other phase windings of synchronous generator. So, the synchronous

generator can be protected completely from faulty condition by identifying

the percentage of winding affected by Inter-Turn and Earth fault on the phase

winding. No fault case is also taken into account for training the ANN, GA-

BPN and ANFIS.

5.5 GENERATION OF TRAINING DATA FOR STATOR INTER-TURN FAULT PROTECTION

A practical test system with unit connected generator of rating

(ABB Industry Oy / Machines) 21KV, 850MVA, 50Hz has been simulated

for various percentage of inter-turn faults of one phase winding of

synchronous generator using MATLAB Simulink. Simulation model of the

sample power system is shown in Figure 5.13.

Figure 5.13 Simulation Model of Sample Power System for Stator Inter-Turn Fault

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Training data for the ANN, GA-BPN and ANFIS are prepared by

simulating various percentage of winding short circuit faults on the phase

winding. With 95% Inter-Turn fault on phase ‘A’ winding of the generator,

the phase fault current recorded for quarter cycle is shown in Figure 5.14.

Figure 5.14 Fault Current from Phase A of Synchronous Generator

The phase current of the generator is passed through sampling

circuit. Then sampled signal performs as the input to the DWT based fault

diagnosis algorithm. The described DWT based algorithm is applied and

tested on the sample system. The test includes different percentage of short

circuit fault and no fault case under loading condition.

This current is then loaded to Wavelet Tool of MATLAB and

analyzed with ‘db-2’ wavelet for four-level decomposition. Outputs of DWT

tool for different levels are shown in Figure 5.15 to Figure 5.18 for the faulty

current in phase A.

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Figure 5.15 Decomposed Level-I of Fault current

Figure 5.16 Decomposed Level-II of Fault current

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Figure 5.17 Decomposed Level-III of Fault current

Figure 5.18 Decomposed Level-IV of Fault current

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Extracted features of statistical data, maximum and range for four

levels of the faulty phase current are arranged to form input vector for ANN,

GA-BPN and ANFIS. Training data have been developed for different

percentage of winding faults with the fault simulating resistor which is

connected across the phase winding. For various percentage of winding inter-

turn fault at different loading of generator (60%, 80%, 100%, 120%), 400

fault current data have been simulated using MATLAB Simulink. Training

data recorded at a loading of 100% of the system for inter-turn fault

protection are listed in Appendix 2.Among these data, 200 data are used to

train Intelligent Network, 100 data are used for testing and 100 data for

validation. The output element of each data takes the value as shown in the

Table 5.3 for different percentage of winding inter-turn fault including no

fault case.

Total simulated data are four hundred in number which are shown

in Appendix 2. But ten sample data are given in Table 5.4 (Unnormalized).

Table 5.5 shows corresponding normalized data. System is tested with many

data for different % of Inter-turn fault.

Table 5.4 Simulated Actual Training Data of Sample Power System for Stator Inter-Turn fault protection

Data Number

Level I Level II Level III Level IV Output Max. Range Max. Range Max. Range Max. Range % of fault

1 2.73 6.42 22.74 38.57 84.43 116.8 194 288.4 1 2 3.06 4.66 14.65 25.3 66.86 94.52 165.4 229.9 2 3 2.13 3.22 14.2 23.7 53.08 75.88 135.5 191.9 3 4 1.36 2.75 12.79 21.36 48.44 69.34 127.1 175.4 4 5 1.97 4.42 8.9 15.13 41.85 60.99 115.3 159 5 6 1.83 4.22 8.34 14.19 39.29 57.26 107.7 148.6 6 7 1.72 2.35 7.83 13.32 36.68 53.45 100.6 138.9 7 8 0.88 1.47 7.39 12.55 34.3 50.01 94.42 138.3 8 9 0.88 1.4 7.14 12.1 7.14 12.1 89.14 123.2 9 10 0.83 1.33 6.8 11.52 30.66 44.68 84.58 116.9 10

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Table 5.5 Simulated Actual Training Data of Sample Power System for

Stator Inter-Turn fault protection (Normalized)

Data Number

Level I Level II Level III Level IV Output

Max Range Max Range Max Range Max Range % of fault

1 0.892157 1 1 0.27854 0.635782 1 1 1 0.01 2 1 0.725857 0.644239 0.182708 0.503475 0.809247 0.852577 0.797157 0.02 3 0.696078 0.501558 0.62445 0.171154 0.399708 0.649658 0.698454 0.665395 0.03 4 0.444444 0.428349 0.562445 0.154255 0.364767 0.593664 0.655155 0.608183 0.04 5 0.643791 0.688474 0.391381 0.109264 0.315143 0.522175 0.59433 0.551318 0.05 6 0.598039 0.657321 0.366755 0.102476 0.295865 0.49024 0.555155 0.515257 0.06 7 0.562092 0.366044 0.344327 0.096193 0.276211 0.45762 0.518557 0.481623 0.07 8 0.287582 0.228972 0.324978 0.090632 0.258289 0.428168 0.486701 0.479542 0.08 9 0.287582 0.218069 0.313984 0.087382 0.053766 0.103596 0.459485 0.427184 0.09

10 0.271242 0.207165 0.299033 0.083194 0.230879 0.382534 0.435979 0.40534 0.1

5.6 SIMULATION RESULTS FOR STATOR INTER-TURN

FAULT PROTECTION

With the proposed procedure, sample system having one generator

has been tested and gives 100% accuracy in identifying the percentage of inter

turn fault with only two features of maximum and range of four

decomposition levels of phase current leaving the winding to be protected.

Therefore input vector will have eight components. Similar procedure can be

followed for other phase currents of generator. To ensure the isolation of the

faulty generator, relay is provided at the bus to which generator is connected.

So, CB will be operated according to the decision made by ANN, GA-BPN

and ANFIS.

75

The ANFIS structure used for the protection of synchronous

generator is shown in Figure 5.19. Details of ANFIS are:

Number of nodes : 1433

Number of linear parameters : 711

Number of nonlinear parameters : 1264

Total number of parameters : 1975

Number of training data pairs : 200

Number of checking data pairs : 100

Number of fuzzy rules : 79

ANFIS used for this purpose uses hybrid method as its optimization

method. The error tolerance is taken as zero.

Figure 5.19 ANFIS Structure for Stator Inter-Turn Fault Protection

Training error for percentage of winding fault diagnosis of

synchronous generator of sample network for ANFIS is shown in Figure 5.20.

76

Figure 5.20 Error Vs Epochs of Trained ANFIS

Test results for various fault simulated for different percentage of

winding inter turn fault of the sample system using the proposed algorithm are

shown in the Figure 5.21 to Figure 5.24. Figure 5.21 shows output of ANFIS

for 25% of winding inter-turn fault of the generator. Its average testing error

is only 1.9298x10-7 i.e., indicating the percentage of winding affected by fault

is 25.

Testing Data: ANFIS Output: *

Figure 5.21 ANFIS output for 25% of winding Inter-turn fault

Erro

r

Epochs

77

Figure 5.22 shows output of ANFIS for 15% of winding interturn fault of the generator. Its average testing error is only 0.0057901 i.e.,

indicating the percentage of winding affected by fault is 15.



Figure 5.23 shows output of ANFIS for 3% of winding interturn

fault of the generator. Its average testing error is only 8.1805x10-9 i.e., indicating the percentage of winding affected by fault is 3.



Out

put

78

Figure 5.24 shows output of ANFIS 60% of winding inter-turn fault

of the generator. Its average testing error is only 9.798x10-5 i.e., indicating the

percentage of winding affected by fault is 60.



ANN used for this application is feed forward propagation network.

Levenberg-Marquardt optimization is used for updating the weights and bias

values of the Neural Network. Figure 5.25 shows performance curve of ANN

after training the network.

Figure 5.25 Performance (MSE) curve after training of ANN

79

5.7 GENERATION OF TRAINING DATA FOR STATOR

EARTH FAULT PROTECTION

An unit connected generator of rating (ABB Industry Oy /

Machines) 21KV, 850MVA, 50Hz has been simulated for various percentage

of Stator Earth faults of ‘A’ phase winding of synchronous generator using

MATLAB Simulink. Simulation model of the practical system is shown in

Figure 5.26.

Figure 5.26 Simulation Model of Practical System with Subharmonic

Injection Scheme

80

Training data for the ANN, GA-BPN and ANFIS are prepared by

simulating various percentages of stator earth faults on the phase winding.

With 50% stator earth fault on phase ‘A’ winding of the generator, the neutral

subharmonic current recorded for quarter cycle is shown in Figure 5.27.

Figure 5.27 Neutral Subharmonic Current for 50% of ‘A’ winding earth

fault of Synchronous Generator

The neutral subharmonic current of the generator is passed through

sampling circuit. Then sampled signal acts as the input to the DWT based

fault diagnosis algorithm. The described DWT based algorithm is applied and

tested on the sample system. The test includes different percentage of stator

earth fault and no fault case.

This current is then loaded to Wavelet Tool of MATLAB and

analyzed with ‘db-2’ wavelet for four-level decomposition. Complete

output of this tool for eight decomposition levels is shown in Figure 5.28.

Figure 5.29 to Figure 5.21 shows four levels of decompositions for the

81

subharmonic current in neutral when the generator ‘A’ phase winding is under

50% of stator earth fault.

Figure 5.28 Decomposed Levels-I to VIII of subharmonic neutral current

82

Figure 5.29 Decomposed Level-V of subharmonic neutral current

Figure 5.30 Decomposed Level-VI of subharmonic neutral current

83

Figure 5.31 Decomposed Level-VII of subharmonic neutral current

Figure 5.32 Decomposed Level-VIII of subharmonic neutral current

84

Extracted features of statistical data, maximum and range for four

levels of the subharmonic neutral current of generator are arranged to form

input vector for ANN, GA-BPN and ANFIS. Training data have been

developed for different percentage of winding earth faults using subharmonic

injection scheme. For various percentage of winding involved in stator earth

fault with different values of fault resistances (Rf -0Ω,100 Ω ,200Ω, 300Ω

,400Ω, 500Ω, 600Ω, 700Ω, 800Ω, 900Ω, 1000Ω), totally 460 fault neutral

current data have been obtained using MATLAB Simulink. Among these

data, 230 data are used to train Intelligent Computational Techniques, 115

data are used for testing and 115 data are used for validation. For different

percentage of stator winding earth fault including no fault case, the output

element of each data takes the value as shown in Table 5.3.

Total simulated training data of 460 in number are given in

Appendix 2. But, ten sample data are given in Table 5.6 (Unnormalized).

Table 5.7 show corresponding normalized data. System is tested with many

data for different % of stator earth fault.


stator earth fault protection

Data Number

Level V Level VI Level VII Level VIII Output Rf in Ω Max Range Max Range Max Range Max Range % of fault

1 0.4461 0.8925 2.428 4.854 11.65 23.31 32.01 64.2 10 0

2 0.4272 0.8541 2.32 4.638 11.08 22.18 29.21 59.69 20 100

3 0.5104 1.02 2.696 5.484 12.54 24.58 29.01 57.71 30 200

4 0.5804 1.161 3.129 6.261 14.54 29.05 34.7 69.09 40 300

5 0.7388 1.478 3.967 7.937 18.05 36.07 39.32 78.68 50 400

6 0.7879 1.575 4.238 8.543 19.49 39 44.61 88.98 60 500

7 0.9157 1.832 4.917 9.831 22.25 44.55 47.42 96.05 70 600

8 0.9653 1.942 5.184 10.58 23.5 47.15 52.15 104.9 80 700

9 0.9977 2.1 5.289 10.77 24.69 49.36 5.35 111.8 90 800

10 1.132 2.424 5.699 11.28 25.65 51.32 60.08 120.2 100 900

85


stator earth fault protection (Normalized)

Data Number

Level V Level VI Level VII Level VIII Output Rf

in Ω Max Range Max Range Max Range Max Range % of fault

1 0.1741 0.1722 0.2027 0.1997 0.1285 0.2306 0.2679 0.2749 10 0

2 0.1667 0.1648 0.1937 0.1908 0.1222 0.2194 0.2444 0.2556 20 100

3 0.1991 0.1968 0.2250 0.2256 0.1383 0.2431 0.2428 0.2472 30 200

4 0.2265 0.2240 0.2612 0.2575 0.1604 0.2873 0.2904 0.2959 40 300

5 0.2883 0.2852 0.3311 0.3265 0.1991 0.3568 0.3290 0.3370 50 400

6 0.3074 0.3039 0.3538 0.3514 0.2150 0.3858 0.3733 0.3811 60 500

7 0.3573 0.3535 0.4104 0.4044 0.2455 0.4407 0.3968 0.4113 70 600

8 0.3766 0.3747 0.4327 0.4352 0.2592 0.4664 0.4364 0.4493 80 700

9 0.3893 0.4052 0.4415 0.4430 0.2724 0.4882 0.0448 0.4788 90 800

10 0.4417 0.4677 0.4757 0.4640 0.2830 0.5076 0.5028 0.5148 100 900

5.8 SIMULATION RESULTS FOR STATOR EARTH FAULT

PROTECTION

With the proposed procedure, unit connected generator has been

tested and gives 100% accuracy in identifying the percentage of stator earth

fault with only two features of maximum and range for four decomposition

levels of subharmonic neutral current. Therefore input vector will have eight

components. Similar procedure can be followed for other phases of generator.

To ensure the isolation of the faulty generator, relay is provided at the bus to

which generator is connected. So, CB will be operated according to the

decision made by ANN, GA-BPN and ANFIS.

86

The ANFIS structure used for the protection of synchronous

generator is shown in Figure 5.33. Details of ANFIS are:

Number of nodes : 36

Number of linear parameters : 81

Number of nonlinear parameters : 126

Total number of parameters : 105

Number of training data pairs : 230

Number of checking data pairs : 115

Number of fuzzy rules : 4

ANFIS used for this purpose uses hybrid method as its optimization

method. The error tolerance is taken as zero.

Figure 5.33 ANFIS Structure for Stator Earth Fault Protection

Training error for percentage of winding fault diagnosis of

synchronous generator of sample network for ANFIS is shown in Figure 5.34.

87

Figure 5.34 Error Vs Epochs of Trained ANFIS

Test results for various fault simulated for different percentage of

winding earth fault of the sample system using the proposed algorithm are

shown in the Figure 5.35 to Figure 5.38. Figure 5.35 shows output of ANFIS

for 90% of winding earth fault of the generator with fault resistance equal to

400Ω. Its average testing error is only 4.1836x10-7 i.e., indicating exactly the

percentage of winding affected by fault as 90.


Figure 5.35 ANFIS output for 90% of winding earth fault

88

Figure 5.36 shows output of ANFIS for 40% of winding earth fault

of the generator with fault resistance equal to 200Ω. Its average testing error

is only 0.0019157 i.e., indicating the percentage of winding affected by earth

fault is 40.



Figure 5.37 shows output of ANFIS for 60% of winding earth fault

of the generator with fault resistance equal to 0 Ω. Its average testing error is

only 0.00084479 i.e., indicating the percentage of winding affected by earth

fault is 60.



89

Figure 5.38 shows output of ANFIS 2.5% of winding earth fault of

the generator with the fault resistance equal to 600Ω. Its average testing error

is only 0.0011466 i.e., representing the percentage of winding affected by

fault is 2.511.


Figure 5.38 ANFIS output for 2.5% of winding earth fault

The ANN used for this protection scheme is also feed forward

network which follows Levenberg-Marquardt optimization for updating the

weights and bias values of the Neural Network. Figure 5.39 shows the

performance curve of ANN after training the network.

Figure 5.39 Performance (MSE) curve after training of ANN

90

5.9 CONCLUSION

A proposed fault detector for generator protection against stator

earth fault and stator inter-turn fault is introduced in this chapter. The detector

is a computational intelligence techniques based algorithm, fed with input

data obtained through DWT of phase current leaving the generator winding.

The detector is characterized with very high sensitivity in discriminating

percentage of winding under earth faults as well as inter-turn faults. Also, it

has high stability for external faults of the system. The proposed detector

schematic, training and testing procedures, and comparison with the

conventional differential algorithm are described in the above sections. A

method for generator stator earth fault simulation and generator inter-turn

fault simulation have been described, and validated to insure its compatibility

and efficacy.

The comparison of training performance of these three

computational intelligence techniques for the stator earth fault protection and

winding inter-turn fault protection of a practical system with unit connected

generator of rated (ABB Industry Oy / Machines) 21KV, 850MVA, 50Hz is

shown in Table 5.8

Table 5.8 Training performance of ANN, GA-BPN and ANFIS

Intelligent Computa-

tional Techniques

Stator Earth Fault Protection of sample system

Winding Interturn Fault Protection of sample system

MSE EPOCHS TIME (Sec.)

MSE EPOCHS TIME (Sec.)

ANN 0.01063 50 1.6 0.0101 50 1.4

GA-BPN 0.011 100 8 0.0099 100 8

ANFIS 0.00006 50 20 0.00005 50 20

91

From the above results, it is found that ANFIS is giving lesser MSE

but the time taken for training is more than the other two techniques. Table

5.9 shows the test results at different loading of sample system for Winding

Inter-turn Fault Protection. Table 5.10 shows the test results at different fault

resistance of the sample system for Stator Earth Fault Protection. From the

tables, it is understood that ANFIS is giving better performance than the other

two techniques.

All the proposed schemes are providing 100% stator earth fault

protection that is covering 0-100% of phase winding. Percentage of winding

nearer to the neutral of about 5% can effectively be protected using these

techniques than the conventional protection schemes where they are just

giving 5-100% protection.

The same schemes can be adopted for other two phase windings of

synchronous generator. So, the synchronous generator can be protected from

faulty condition by identifying the percentage of winding affected by the earth

fault and the percentage of winding by the inter-turn fault. No fault case is

also taken into account for training the ANN, GA-BPN and ANFIS.

92

Table 5.9 Comparison of testing performance of ANN, GA-BPN and

ANFIS at different loads for Winding Inter-turn Fault

Protection

Data No. Target Output

Actual Output Inference (% Wdg. Under Interturn Fault)

% Load ANN GA-BPN ANFIS

1 0 0.1044 0.1001 0.0005 0

60% 2 0.03 0.0316 0.0323 0.031 3 3 0.4 0.3510 0.3612 0.4001 40 4 0.6 0.6497 0.65 0.592 60 5 0.95 0.894 0.904 0.958 95 1 0 -0.0332 -0.0132 0.0002 0

80% 2 0.08 0.0541 0.0752 0.0802 8 3 0.25 0.213 0.2111 0.2508 25 4 0.8 0.74 0.7502 0.792 80 5 0.98 0.9698 0.9722 0.9803 98 1 0.15 0.175 0.166 0.142 15

100% 2 0.25 0.2761 0.2633 0.256 25 3 0.75 0.7391 0.7199 0.7508 75 4 0.875 0.8907 0.887 0.867 87.5 5 0.975 0.9454 0.956 0.983 97.5 1 0 0.0982 0.0632 0.006 0

120% 2 0.1 0.1507 0.199 0.099 10 3 0.275 0.2367 0.265 0.283 27.5 4 0.57 0.5946 0.5645 0.573 57 5 0.93 0.885 0.8995 0.9282 93

93

Table 5.10 Comparison of testing performance of ANN, GA-BPN and

ANFIS at different fault resistance (Rf) for Stator Earth

Fault Protection

Data No.

Target Output

Actual Output Inference (% Wdg.

Under Earth Fault)

Fault Resistance(Rf)

in Ohms ANN GA-BPN ANFIS

1 0 0.1068 0.1128 -0.008 0

0

2 0.05 0.0516 0.0523 0.042 5

3 0.3 0.2310 0.2612 0.292 30

4 0.7 0.7497 0.75 0.692 70

5 0.9 0.794 0.824 0.908 90

1 0 -0.0432 -0.0332 0.008 0

200

2 0.025 0.011 0.22 0.242 2.5

3 0.1 0.113 0.111 0.108 10

4 0.4 0.58 0.502 0.392 40

5 0.825 0.8198 0.822 0.833 82.5

1 0.15 0.175 0.166 0.142 15

400

2 0.25 0.2761 0.2633 0.256 25

3 0.6 0.6391 0.6199 0.608 60

4 0.875 0.8907 0.887 0.867 87.5

5 0.975 0.9454 0.956 0.983 97.5

1 0 0.0982 0.0632 0.006 0

600

2 0.1 0.2507 0.199 0.092 10

3 0.275 0.2497 0.265 0.283 27.5

4 0.575 0.6046 0.582 0.583 57.5

5 0.85 0.7585 0.7925 0.842 85

chapter 5 protection of synchronous generator...

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