voltage tip mitigation in distribution system

8/10/2019 voltage tip mitigation in distribution system

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Title of the Project :

VOLTAGE DIP MITIGATION IN

DISTRIBUTION SYSTEM WITH

D-STATCOM

Under the guidance of

Sri S.S.Tulasiram Das Garu

Professor in EEE Dept

byM.Dorababu

Roll No. 11012D4302


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The most common power quality problems are voltage sags ,

harmonic distortion and low power factor

Voltage sags is caused by fault in the Distribution system

Harmonic currents in the distribution system will cause harmonic

distortion , low power factor & additional losses as well as

heating of electrical equipment.

Different ways to enhance power quality problems in transmission

and distribution systems.

D-Statcom is the most effective device used for enhancement of

power quality in transmission & Distribution systems

PWM based control scheme is used for control of electronic

valves in the D-STATCOM

The D-Statcom injects a current in to the system to mitigate thevoltage sag.

LCL Passive Filter is added to D-STATCOM to improve harmonic

distortion and low power factor


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The objective is

Voltage sags are mitigated by inserting D- STATCOM in the

Distribution System

By adding LCL passive filter to D-STATCOM the harmonicdistortion is reduced & power factor increase close to unity.

Statcom requires some amount of energy storage to accommodateharmonic power and AC system unbalances

Statcom equivalent circuit


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D-Statcom :

IL = Load current

Is = Source currentVth = Thevenin Voltage

VL = Load voltage

Zth = impedance

Iout= IL- IS= IL- (Vth-VL) / Zth


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The output current ( I out) from the D-STATACOM will correctthe voltage sags by adjusting the voltage drop across the systemimpedance ( Zth= R+ j X)

Voltage sag correction with D- Statcom depends on:

The value of impedance : ( Zth = R+ j X )

The fault level of load bus

The shunt injected current Ish corrects the voltage sag by adjustingthe voltage drop across the system impedance Zth.

The value of Ish

can be controlled by adjusting the output voltage ofthe converter

The switching frequency is set at 475 Hz


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D STATACOM( Distribution Static Compensator):

The D-STATCOM is the voltage-sourced inverter that converts aninput dc voltage into a three-phase output voltage at fundamentalfrequency.

It consist of

i) Voltage source converter (VSC)

ii)DC energy storage device

iii) Controller

iv) Coupling Transformer

Voltage source converter (VSC):a) It is a power electronic device which can generate a

sinusoidal voltage with any required magnitude, frequency &phase angle.

b) The VSC is used to either completely replace the voltage or toinject the missing voltage.

c) The missing voltage is the difference between the nominalvoltage and the actual.


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d) It converts the DC voltage across the storage device in to a setof

three phase ac out put voltages.

e) These voltages are in phase and coupled with the ac system

through the reactance of the coupling transformer.

f) Suitable adjustment of the phase and magnitude of the D-

STATCOM output voltages allows effective control of active and

reactive power exchanges between the D-STATCOM and the ac

system

g) This configuration allows the device to absorb or generate

controllable active and reactive power.

The VSC connected in shunt with the ac system which can beused for up to three quite distinct purposes:

1. Voltage regulation and compensation of reactive power;

2. Correction of power factor; and

3. Elimination of current harmonics.


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Operation principle of the voltage source rectifier:

The voltage source rectifier operates by keeping the dc linkvoltage at a desired reference value, using a feedback control loop

To accomplish this task, the dc link voltage is measured andcompared with a reference VREF

The error signal generated from this comparison is used to switchthe six valves of the rectifier ON and OFF

In this way, power can come or return to the ac source accordingto dc link voltage requirements

Voltage VDis measured at capacitor CD


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When the current IDis positive (rectifier operation), the capacitor CDis

discharged, and the error signal ask the Control Block for more

power from the ac supply.

The Control Block takes the power from the supply by generating theappropriate PWM signals for the six valves.

In this way, more current flows from the ac to the dc side, and the

capacitor voltage is recovered.

Inversely, when ID becomes negative (inverter operation), the

capacitor CDis overcharged, and the error signal asks the control to

discharge the capacitor and return power to the ac mains.

The PWM control not only can manage the active power, but also

reactive power, allowing this type of rectifier to correct power factor.

The ac current waveforms can be maintained as almost sinusoidal,

which reduces harmonic contamination to the mains supply.


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Vi: effective output voltage of the D-STATCOM & :power angle &Vs: System voltage.

The D-STACOM employs an inverter to convert the DC link

voltage Vdc on the capacitor to a voltage source of adjustable

magnitude and phase.

The inductance(L) and resistance(R)represents the equivalent

circuit elements of the step-down transformer and the inverter is

the main component of the D-STATCOM


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The controller of the D STATCOM is used to operate the inverter in the

phase angle between the inverter voltage and the line voltage is dynamically

adjusted so that the D-STATCOM generates or absorbs the desired VAR at

the point of connection.

If Vi is equal to Vs, the reactive power is zero and the D-STATCOM does not

generate or absorb reactive power

No-load mode (Vs = Vi)

When Vi is greater than Vs, the DSTATCOM shows an inductive reactance

connected at its terminal. The current, I, flows through the transformer

reactance from the D-STATCOM to the ac system, and the device generates

capacitive reactive power


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If Vs is greater than Vi, the D-STATCOM shows the system as a

capacitive reactance. Then the current flows from the ac system to

the D-STATCOM, resulting in the device absorbing inductivereactive power


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Phase-Modulation of the control signal

The sinusoidal signal Vcontrol is phase-modulated by means of the angle

VA = Sin (t+)VB= Sin(t+-2/3)VC = Sin (t++2/3)


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Proportional-integral controller (PI Controller) is a feedback controllerwhich drives the system to be controlled with a weighted sum of theerror signal (difference between the output and desired set point) andthe integral of that value

PI controller will process the error signal to zero

The load r.m.s voltage is brought back to the reference voltage bycomparing the reference voltage with the r.m.s voltages that had beenmeasured at the load point

PWM generator will generates the Sinusoidal PWM waveform orsignal.

It produces the desired synchronizing signal that required The modulated signal is compared against a triangle signal in order to

generate the switching signals for VSC valves

Energy Storage Circuit:

DC source is connected in parallel with the DC capacitor

It carries the input ripple current of the converter and it is the mainreactive energy storage element

DC capacitor could be charged by a battery source or could berecharged by the converter itself.


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Start


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Start

Design distribution system using MATLAB SIMULINK

Create distortion by inserting different types of faults

Analyze the result from scope

Vary the value of fault resistance

Is the voltage

sag>0.9p.u

PF>0.9

Is the THD

below 5%

Run the simulation between 0-1s

end

Inject D-STATCOM

into distribution

system

Add LCL passive

filter

Flow Chart

Yes

Yes

No

No


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SIMULINK DIAGRAM WITHOUT D-STATCOM

Different Types of faults :

i)Three phase to ground fault (TPG)

ii) Double line to ground fault (DLG)

iii) Line to line fault (LL)

iv) Single line to ground fault (SLG)


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Fault resistance(Rf) :0.66 Three phase to ground fault

(TPG)

Fault resistance(Rf) :0.66 Double line to ground fault

(DLG)

Time in

sec

Vpu

Vpu

Time in

sec


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Fault resistance(Rf) :0.66 Line to line fault (LL)

Fault resistance(Rf) :0.66 Single Line to Ground fault

(SLG)

Vpu

Vpu

Time in

sec

Time in

sec


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Results of voltage sags for different types of fault without D-

Statcom

FaultResistance

(Rf)

Voltage sagfor TPG

fault (p.u)

Voltage sagfor DLG

fault (p.u)

Voltage sagfor LL fault

(p.u)

Voltage sagfor SLG

fault (p.u)

0.66 0.6600 0.7070 0.7587 0.8259

0.76 0.7107 0.7487 0.7918 0.8486

0.86 0.7515 0.7833 0.8210 0.8679


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SIMULINK DIAGRAM WITH D-STATCOM

Fault resistance(Rf) :0 66 Three phase to ground fault (TPG)


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Fault resistance(Rf) :0.66 Three phase to ground fault (TPG)

Fault resistance(Rf) :0.66 Double line to ground fault (DLG)

Vpu

Vpu

Time in

sec

Time in


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HARMONIC SPECTRUM OF OUTPUT CURRENT WITH OUT LCL PASSIVE FILTER

OUTPUT CURRENT WITHOUT LCL FILTER

ave orm o spec rum s or on ou pu curren w ou pass ve er or ree ase o rounf lt


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fault

The above table shows the current harmonic for different types of faults


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Number ofharmonic

spectrum

Harmonicdistortion of

TPG fault%


DPG

fault%


LL fault%


SLG fault%

1st 100.00 100.00 100.00 100.00

3rd 61.57 86.09 42.52 47.225th 1.98 1.84 1.55 1.73

7th 13.43 17.38 10.27 8.28

9th 5.98 8.53 5.00 3.56

11th 1.21 2.95 2.10 0.95

13th 5.29 6.92 3.55 3.36

15th 0.84 0.74 0.62 0.61

17th 1.75 2.84 1.71 1.14

19th 1.90 2.54 1.33 1.24

THD 63.63 88.68 44.52 48.27

Power factor 0.84 0.75 0.91 0.90

The above table shows the current harmonic for different types of faults

without filter The percentage of THD shows that , it is not within the IEEE STD

519-1992. The percentage of power factor is low in the range of 0.75 to 0.91

lagging.

Results of current harmonic for different types of faults without LCLassive filter


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HARMONIC SPECTRUM WITH LCL FILTER

OUTPUT CURRENT WITH LCL FILTER


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The below table shows that with LCL passive filter the percentage of


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Number of

harmonic

spectrum

Harmonic

distortion of

TPG fault%

Harmonic

distortion of

DLG fault%

Harmonic

distortion of

LL fault%

Harmonic

distortion of

SLG fault%

1st 100.00 100.00 100.00 100.00

3rd 0.80 0.81 0.43 0.83

5th 0.41 0.42 0.16 0.42

7th

0.29 0.30 0.13 0.329th 0.23 0.24 0.13 0.28

11th 0.19 0.19 0.08 0.20

13th 0.16 0.16 0.07 0.17

15th 0.14 0.14 0.05 0.14

17

th

0.12 0.12 0.05 0.1219th 0.11 0.11 0.04 0.11

THD 1.11 1.12 0.65 1.15

Power factor 0.99 0.99 0.99 0.99

The below table shows that with LCL passive filter the percentage of

THD has reduced . Now the THD is within IEEE STD 519-1992. The

percentage of power factor increases close to unity.

Results of current harmonic for different types of faults with LCL

passive filter


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From the above results it is concluded that the voltage sags are

mitigated by inserting D-STATCOM in to the distribution system. Byadding LCL passive filter to D-STATCOM, the total harmonic distortion

(THD) is reduced within the IEEE STD 519-512. The power factor also

increases close to unity. Hence it is concluded that by adding D-

STATCOM with LCL filter the power quality is improved.


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voltage tip mitigation in distribution system

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