CHAPTER 4 MITIGATION OF VOLTAGE SAG / SWELL USING FUEL...

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CHAPTER 4

MITIGATION OF VOLTAGE SAG / SWELL USING FUEL

CELL BASED DYNAMIC VOLTAGE RESTORER

4.1 INTRODUCTION

The widely used energy storage systems are capacitors and

batteries. But capacitors have limitations of low storage and charging and

discharging speeds. Battery backup systems operate similarly to adding

capacitive energy storage, with the advantage that their energy per volume

ratio is much higher than standard capacitors. The batteries are easily

available with low cost; provide ride through for deep sags and full outages.

These have low life and require additional space and maintenance. A super

capacitor can overcome these limitations and provide an efficient working but

super capacitors do not have a conventional solid dielectric.

Fuel cell is used to charge the supercapacitor to restore the voltage

during distortions. Systems for electro chemical energy storage and

conversion include batteries, fuel cells and electro chemical capacitors.

Although the energy storage and conversion mechanisms are different, there

are electro chemical similarities of these three systems. In batteries and fuel

cell, electrical energy is generated by conversion of chemical energy via red-

ox reactions at anode and cathode. The negative electrode is denoted as anode

Alkaline Fuel C

ell(A

FC)

H2

Potassium

Hydroxide

(40-200) °C

60%

Molten C

arbonate Fuel C

ell (M

CFC

)C

H4, H

2, CO

Molten

Carbonate

(650) °C

45-50%

The following are the advantages of using fuel cell:

The unit is lighter and smaller and require little m

aintenance

They cause little pollution and little noise

Very efficient

They are a renewable source of energy.

4.3FU

EL

CE

LL

BA

SED

DV

R

The majority of pow

er quality problems are due to different fault

conditions. These

conditions cause

voltage sag,

swell

and distortions.

Dynam

ic voltage restorer (DV

R) can provide the cost effective solution to

mitigate pow

er quality problem by establishing the appropriate voltage

quality level. This mitigation of voltage is provided w

ith the help of a fuel

cell. Fuel cell coupled with the supercapacitor is used as the energy storage

device for the DV

R in this research w

ork as shown in the Figure 4.2. The D

C

57

voltage from the fuel cell is stored in a super capacitor. Super capacitors are

new generation energy storage devices which store energy via charge

separation at the electrode-electrolyte interface, and they can withstand a

large number of charge/discharge cycles without degradation.

Figure 4.2 Block diagram of Fuel cell based DVR

The major advantages of super capacitors include higher

capacitance density, higher charge-discharge cycles, reliable, long life, and

maintenance-free operation, environmentally safe, wide range of operating

temperature, high power density and good energy density, so they are a good

alternative. The dc voltage is converted using a impedance source inverter.

The proposed Z-source inverter has the unique feature that it can boost/buck

the output voltage by introducing shoot through operation mode, which

is forbidden in traditional voltage source inverters. With this unique feature,

the Z-source inverter provides a cheaper, simpler, buck-boost inversion by

single power conversion stage, strong EMI immunity and low harmonic

distortion.

58

4.4 MATHEMATICAL FORMULATIONS

4.4.1 PI Controller

Sag occurs when there is increase in load or during the occurrence

of load while a swell occurs when there is a sudden removal of load or due to

addition of capacitor banks. This sag or swell in load voltage is sensed and its

magnitude is compared with a reference voltage and the error signal is given

to the PI controller. The output of error detector is

–ref inV V (4.1)

where Vref is the reference voltage

Vin is the load voltage

The difference between load voltage Vin and reference voltage Vref

is supplied to the PI controller. The PI controller voltage is taken as feedback.

The IGBT inverter is triggered from the pulse generated by the PWM

generator. The IGBTs are triggered depending upon the firing angle which

introduces additional lag or lead in the voltage.

= ( + + ) (4.2)

= (4.3)

The supply side voltages Va, Vb and Vc are transformed into d–q

values of positive sequence.

C=1

21

2

0 32

32

(4.4)

( ) = cos sinsin cos (4.5)

59

4.4.2 Synchronous Frame Theory

Adda et al (2013) discussed the synchronous reference frame

theory for nanogrid applications. Whenever the system detects a voltage

sag/swell, the DVR should react as fast as possible and inject an ac voltage

into the grid. It can be implemented using the synchronous reference frame

(SRF) technique based on the instantaneous values of the supply voltage. The

control algorithm produces a three phase reference voltage to the PWM

inverter to maintain the load voltage at its reference value. The voltage

sag/swell is detected by measuring the error between the supply voltage and

the reference value. The reference component is set to a rated voltage. The

SRF method can be used to compensate all type of voltage disturbances,

voltage sag/swell, voltage unbalance and harmonic voltage. The difference

between the reference voltage and the supply voltage is applied to the ZSI to

produce the load rated voltage, with the help of pulse width modulation

(PWM) through the PI controller.

d a b cV = 2 3[V sin( t)+ V sin( t-2 / 3) V sin( t+2 / 3)] (4.6)

q a b cV = 2 3[V cos ( t)+ V cos( t-2 / 3) V cos ( t+2 / 3)] (4.7)

0 a b cV =1 3[V + V V ] (4.8)

where w= rotation speed (rad/s) of the rotating frame.

a d q 0V =[ V sin ( t) V cos( t) V ] (4.9)

b d q 0V =[ V sin ( t-2 / 3) V cos( t-2 / 3) V ] (4.10)

c d q 0V =[ V sin ( t+2 / 3) V cos( t+2 / 3) V ] (4.11)

60

4.4.3 Fuzzy Logic Controller

The PI controller is the most frequently used controller in the

DVRs. The disadvantage of the PI Controller is its inability to work well

under a wider range of operating conditions. Hence fuzzy controller is

proposed. In this control method, PLL for each phase tracks the phase of

network voltage phasor and generates a reference signal with magnitude of

unit to supply frequency for each phase. The supply voltage for each phase is

converted to p.u. and error is obtained from the difference of reference PLL

generated signal and actual supply voltage converted to p.u. Error and error

rate are the inputs for the Fuzzy Logic Controller (FLC). The output of the

FLC is fed to the PWM generator to produce switching pulses for ZSI. Jurado

et al (2003) described the voltage sag correction by DVR using FLC.

Ashari et al (2007) discussed the fuzzy controller for DVR. The

desired response from DVR-PLL system is quite different from other

applications. It is because, the phase of the supply voltage prior to the sag is

generally preferred and if the PLL reacts quickly to changes in the phase

during sag, the post-sag phase may be used. Therefore the DVR would not be

able to compensate for the phase jump. Conventionally, once sag is detected,

the target phase of the voltage reference is fixed to the pre-sag phase to ensure

that if the reference is correctly tracked, then the load voltage phase will

remain unaffected. Through a suitable choice of the time constant of the PLL,

the DVR restores the instantaneous voltage waveform in the sensitive load

side to the same phase and magnitude as the initial pre-sag voltage. The fuzzy

member function is shown in the Figure 4.3 and the fuzzy matrix is shown in

the Table 4.2.

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Figure 4.3 Fuzzy Member Functions

Table 4.2 Fuzzy Matrix

4.5 SIMULATION STUDIES

The Fuel cell based DVR is simulated with three controllers for sag

and swell compensation using matlab/simulink platform.

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4.5.1 Sag Compensation Using PI, SRF and Fuzzy Controller

The Simulink model for sag compensation is shown in the

Figure 4.4. The Fuel cell, Fuzzy controller and SRF subsystems are shown in

the Figure 4.5 to Figure 4.7.

Figure 4.4 Simulink model for sag compensation

Figure 4.5 Fuel cell subsystem

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Figure 4.6 Fuzzy Controller Subsystem

Figure 4.7 SRF Subsystem

PI Controller

The sag compensation using PI controller is shown in the

Figure 4.8. The source supplies a nominal voltage of 415 V and nominal

frequency of 50Hz. The linear load considered is an R-L load (R=250 ,

L=31e-5 H). Additional load is added during the period of 0.05 sec to 0.2 sec,

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so sag occurs and the voltage in all the 3 phases drops to 249V. DVR is

activated at 0.05 sec and it provides the compensating voltage of 141V to

phase A, 137V to phase B and 120V to phase C. The load voltage is not

compensated to 415V.

Figure 4.8 Sag compensation using PI Controller

SRF Controller

The sag compensation using SRF controller is shown in the




so sag occurs and the voltage in all the 3 phases drops to 249V. DVR is


phase A, 143V to phase B and 126V to phase C. The load voltage is not


65

Figure 4.9 Sag compensation using SRF controller

Fuzzy Logic Controller

The sag compensation using Fuzzy Logic controller is shown in the




so sag occurs and the voltage drops to 249V. DVR is activated at 0.05 sec and

it provides the compensating voltage of 166V. The load voltage is


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Figure 4.10 Sag Compensation using Fuzzy Logic Controller

Table 4.3 Comparison of Distorted voltage, Injected voltage and Load

voltage for different controllers (Sag compensation)

Controllers

Distorted voltage (volts)

Injected voltage (volts)

Load voltage (volts)

Phase A

Phase B

Phase C

Phase A

Phase B

Phase C

Phase A

Phase B

Phase C

PI 249 249 249 141 137 120 390 386 369

SRF 249 249 249 146 143 126 395 392 375

Fuzzy 249 249 249 166 166 166 415 415 415

4.5.2 Swell Compensation using PI, SRF and Fuzzy Controller

(Sudden Removal of Load)

The simulink model for swell compensation (sudden removal of

load) is shown in the Figure 4.11.

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Figure 4.11 Simulink model for swell compensation (sudden removal of

load)

PI Controller

The swell compensation using PI controller is shown in the


frequency of 50Hz.The linear load considered is an R-L load (R=250 ,

L=31e-5 H). A sudden removal of load at 0.05 sec causes voltage swell to

456.5V(10% Swell).DVR is activated at 0.05 sec and injects negative voltage

of 64.5V to phase A, 86.5V to phase B and 74.5V to phase C. The Load

voltage is not compensated to 415V.

Figure 4.12 Swell compensation using PI Controller

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SRF Controller

The swell compensation using SRF controller is shown in the



L=31e-5 H). A sudden removal of load at 0.05 sec causes voltage swell to

456.5V (10% Swell). DVR is activated at 0.05 sec and injects negative

voltage of 53.5V to Phase A, 81.5V to phase B and 66.5V to phase C. The

Load voltage is not compensated to 415V.

Figure 4.13 Swell compensation using SRF controller

Fuzzy Logic Controller

The swell compensation using FLC controller is shown in the



L=31e-5 H). A sudden removal of load at 0.05 sec causes voltage swell

to 456.5V (10% Swell). DVR is activated at 0.05 sec and injects

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negative voltage of 41.5V up to 0.2 second. The Load voltage is compensated

to 415V.

Figure 4.14 Swell compensation using Fuzzy Logic Controller

Table 4.4 Comparison of Distorted voltage, Injected voltage and Load

voltage for different controllers (Swell compensation)

Controllers




Phase A

Phase B

Phase C

Phase A

Phase B

Phase C

Phase A

Phase B

Phase C

PI 456.5 456.5 456.5 64.5 86.5 74.5 392 370 382

SRF 456.5 456.5 456.5 53.5 81.5 66.5 403 385 390

Fuzzy 456.5 456.5 456.5 41.5 41.5 41.5 415 415 415

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4.5.3 Sag Compensation for LG, LL, LLG and Three Phase Fault

using PI, SRF and Fuzzy Controller

The simulink model for sag compensation (Fault) is shown in the

Figure 4.15.

Figure 4.15 Simulink model for Fault Compensation

PI Controller

Line to Ground fault (LG)

The sag compensation for LG fault using PI controller is shown in

the Figure 4.16. The source supplies a nominal voltage of 415 V and nominal


L=31e-5 H). Single line to ground fault occurs during the period of 0.05 sec to

0.2 sc, so sag occurs and the voltage drops to 225V in phase A and 390V in

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phase C. DVR is activated at 0.05 sec and it provides the compensating

voltage of 85V to phase A and 23V to phase C. The load voltage is not


Figure 4.16 Sag compensation for LG fault using PI Controller

Line to Line fault (LL)

The sag compensation for LL fault using PI controller is shown in



L=31e-5 H). A line to line fault occurs during the period of 0.05 sec to 0.2

sec, so sag occurs and the voltage drops to 413V in phase A and 340V in

phase B and 330V in Phase C. DVR is activated at 0.05 sec and it provides

the compensating voltage of 2V to phase A and 63V to phase B and 72V to

phase C. The load voltage is not compensated to 415V.

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Figure 4.17 Sag compensation for LL fault using PI Controller

Double Line to Ground fault (LLG)

The sag compensation for LLG fault using PI controller is shown in



L=31e-5 H). A double line to ground fault occurs during the period of 0.05

sec to 0.2 sec, so sag occurs and the voltage drops to 410V in phase A and

310V in phase B and 320V in Phase C. DVR is activated at 0.05 sec and it

provides the compensating voltage of 5V to phase A and 90V to phase B and

70V to phase C. The load voltage is not compensated to 415V.

73

Figure 4.18 Sag compensation for LLG fault using PI Controller

Three Phase Fault

The sag compensation for three phase fault using PI controller is

shown in the Figure 4.19. The source supplies a nominal voltage of 415 V and

nominal frequency of 50Hz.The linear load considered is an R-L load (R=250

, L=31e-5 H). A Three phase fault occurs during the period of 0.05 sec to

0.2 sec, so sag occurs and the voltage drops to 210V in all the phases. DVR is


phase A and 205V to phase B and 170V to phase C. The load voltage is not


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Figure 4.19 Sag compensation for Three Phase fault using PI Controller

Synchronous Reference Frame Controller


The sag compensation for LG fault using SRF controller is shown

in the Figure 4.20. The source supplies a nominal voltage of 415 V and

nominal frequency of 50Hz. The linear load considered is an R-L load

(R=250 , L=31e-5 H). Single line to ground fault occurs during the period

of 0.05 sec to 0.2 sec, so sag occurs and the voltage drops to 225V in phase A

and 390V in phase C. DVR is activated at 0.05 sec and it provides the

compensating voltage of 188V to phase A and 20V to phase C. The load

voltage is not compensated to 415V.

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Figure 4.20 Sag compensation for LG fault using SRF controller


The sag compensation for LL fault using SRF controller is shown



(R=250 , L=31e-5 H). A line to line fault occurs during the period of 0.05

sec to 0.2 sec so sag occurs and the voltage drops to 413V in phase A and



80V to phase C. The load voltage is not compensated to 415V.

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Figure 4.21 Sag compensation for LL fault using SRF controller

Double Line to fault (LLG)

The sag compensation for LLG fault using SRF controller is shown



(R=250 , L=31e-5 H). A Double line to ground fault occurs during the

period of 0.05 sec to 0.2 sec so sag occurs and the voltage drops to 410V

in phase A and 310V in phase B and 320V in Phase C. DVR is activated

at 0.05 sec and it provides the compensating voltage of 5V to phase A and

101V to phase B and 93V to phase C. The load voltage is not compensated

to 415V.

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Figure 4.22 Sag compensation for LLG fault using SRF controller

Three Phase Fault

The sag compensation for three phase fault using SRF controller is


nominal frequency of 50Hz.The linear load considered is an R-L load (R=250

, L=31e-5 H).A Three phase fault occurs during the period of 0.05 sec to 0.2

sec so sag occurs and the voltage drops to 210V in all the phases. DVR is


phase A and 210V to phase B and 170V to phase C. The load voltage is not


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Figure 4.23 Sag compensation for Three Phase fault using SRF controller

Fuzzy Logic Controller (FLC)


The sag compensation for LG fault using FLC controller is shown



(R=250 , L=31e-5 H). Single line to ground fault occurs during the period

of 0.05 sec to 0.2 sec, so sag occurs and the voltage drops to 225V in phase A

and 390V in phase C. DVR is activated at 0.05 sec and it provides the

compensating voltage of 190V to phase A and 25V to phase C. Thus the load

voltage is compensated to 415V.

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Figure 4.24 Sag compensation for LG fault using Fuzzy logic controller


The sag compensation for LL fault using FLC controller is shown



(R=250 , L=31e-5 H). A line to line fault occurs during the period of 0.05

sec to 0.2 sec, so sag occurs and the voltage drops to 413V in phase A and



85V to phase C. The load voltage is compensated to 415V.

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Figure 4.25 Sag compensation for LL fault using Fuzzy logic controller

Double Line to Ground fault (LLG)

The sag compensation for LLG fault using FLC controller is shown



(R=250 , L=31e-5 H).A Double line to ground fault occurs during the

period of 0.05 sec to 0.2 sec, so sag occurs and the voltage drops to 410V in

phase A and 310V in phase B and 320V in Phase C. DVR is activated at 0.05

sec and it provides the compensating voltage of 5V to phase A and 105V to

phase B and 95V to phase C. The load voltage is compensated to 415V.

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Figure 4.26 Sag compensation for LLG fault using Fuzzy logic controller

Three Phase Fault

The sag compensation for three phase fault using FLC controller is



(R=250 , L=31e-5 H). A Three phase fault occurs during the period of 0.05

sec to 0.2 sec, so sag occurs and the voltage drops to 210V in all the phases.

DVR is activated at 0.05 sec and it provides the compensating voltage of

215V to phase A and 215V to phase B and 215V to phase C. The load voltage

is compensated to 415V

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Figure 4.27 Sag compensation for Three Phase fault using Fuzzy logic controller

Table 4.5 Comparison of Distorted voltage, Injected voltage and Load voltage for different controllers (Fault compensation)

Controllers Faults




PhaseA

PhaseB

PhaseC

PhaseA

PhaseB

PhaseC

PhaseA

PhaseB

PhaseC

PI

LG 225 415 390 85 0 23 310 415 413

LL 413 340 330 2 63 72 415 403 402

LLG 410 310 320 5 90 70 415 400 390

3 phase 210 210 210 215 205 170 425 405 380

SRF

LG 225 415 390 188 0 20 413 415 410

LL 413 340 330 2 72 80 415 412 410

LLG 410 310 320 5 101 93 415 411 413

3 phase 210 210 210 215 210 170 425 420 380

Fuzzy

LG 225 415 390 190 0 25 415 415 415

LL 413 340 330 2 75 85 415 415 415

LLG 410 310 320 5 105 95 415 415 415

3 phase 210 210 210 215 215 215 415 415 415

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4.5.4 Comparative Analysis of Various Controllers for Sag and Swell

Compensation

The comparative analysis of various control methods for sag

compensation is shown in the Figure 4.28.

Figure 4.28 Comparative analysis for sag compensation

Additional load is added during the period of 0.05 sec to 0.2

sec, so sag occurs and the voltage drops.

DVR is activated at 0.05 sec and it provides the compensating

voltage.

From the three control methods, it is evident that the Fuzzy is

superior than the other methods.

With the Fuzzy controller, voltage restoration is maximum.

Phase A Phase B Phase C

Conventional 390 386 370SRF 395 392 375Fuzzy 415 415 415

340

350

360

370

380

390

400

410

420

VOLT

AG

EV

84

The comparative analysis of various control methods for swell

compensation is shown in the Figure 4.29.

Figure 4.29 Comparative analysis for swell compensation (sudden

removal of load)

A sudden removal of load at 0.05 sec causes voltage swell.

(10% Swell).

DVR is activated at 0.05 sec and injects negative voltage up to

0.2 sec.






340

350

360

370

380

390

400

410

420VO

LTA

GE

V

85


compensation (LG fault) is shown in the Figure 4.30.

Figure 4.30 Comparative analysis for LG fault sag compensation

Single line to ground fault occurs during the period of 0.05 sec

to 0.2 sec, so sag occurs and the voltage drops in phase A and

phase C.


voltage.






300

320

340

360

380

400

420

440

VOLT

AGE

V

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compensation (LL fault)is shown in the Figure 4.31.

Figure 4.31 Comparative analysis for LL fault sag compensation

A line to line fault occurs during the period of 0.05 sec to 0.2

sec, so sag occurs and the voltage drops in phase A, phase B

and phase C.


voltage.






395

400

405

410

415

420VO

LTAG

EV

87


compensation (LLG fault)is shown in the Figure 4.32.

Figure 4.32 Comparative analysis for LLG fault sag compensation

A Double line to ground fault occurs during the period of 0.05

sec to 0.2 sec, so sag occurs and the voltage drops in phase A,

phase B and phase C.


voltage.






375

380

385

390

395

400

405

410

415

420VO

LTAG

EV

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compensation (Three phase fault)is shown in the Figure 4.33.

Figure 4.33 Comparative analysis for three phase fault sag compensation

A Three phase fault occurs during the period of 0.05 sec to 0.2

sec, so sag occurs and the voltage drops in all the phases.


voltage.






360

370

380

390

400

410

420VO

LTAG

EV

89

Simulation parameters

Nominal Frequency : 50Hz

Three phase Peak Amplitude : 415V

Line Resistance : 0.1

Line Inductance : 10e-3

Active power : 10e3 W

Resistance : 250

Inductance : 31e-5

4.6 CONCLUSION

This chapter explained the fuel cell based dynamic voltage restorer

for voltage sag and swell compensation. The DVR is based on a shunt

capacitor fed series Z source inverter through dc-to-dc step up converter. The

three different voltage controllers were designed for DVR voltage regulation

such as PI Controller, SRF controller and Fuzzy logic controller. The

comparative analysis for three controllers has been done. The detailed

simulation analysis found that fuzzy logic controller is the best method. Fuel

cell coupled with the supercapacitor is used as the energy storage device for

the DVR. Band controlled DVR is designed and further validated by

simulation results.

CHAPTER 4 MITIGATION OF VOLTAGE SAG / SWELL USING FUEL...

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