voltage tip mitigation in distribution system
TRANSCRIPT
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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%
Harmonicdistortion of
DPG
fault%
Harmonicdistortion of
LL fault%
Harmonicdistortion of
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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