Voltage dip mitigation in distribution system by using d statcom

Journal of Energy Technologies and Policy www.iiste.org
ISSN 2224-3232 (Paper) ISSN 2225-0573 (Online)
Vol.1, No.1, 2011

Voltage Dip mitigation in Distribution System by Using D-

Statcom

Sambugari Anil Kumar1*, D. vanurrappa2
1. Department of Electrical and Electronics Engineering, G.Pulla Reddy Engineering College,
Kurnool-518007, Andhra Pradesh. India
2. Department of Electrical and Electronics Engineering, Brindavan Institute of Technology &
Science,Kurnool -518218, Andhra Pradesh, India
* E-mail: sanil.0202@gmail.com

Abstract
A Power quality problem is an occurrence manifested as a nonstandard voltage, current or
frequency that results in a failure or a mis-operation of end user equipments. Utility distribution networks,
sensitive industrial loads and critical commercial operations suffer from various types of outages and
service interruptions which can cost significant financial losses. With the restructuring of power systems
and with shifting trend towards distributed and dispersed generation, the issue of power quality is going to
take newer dimensions. In developing countries like India, where the variation of power frequency and
many such other determinants of power quality are themselves a serious question, it is very vital to take
positive steps in this direction .The present work is to identify the prominent concerns in this area and
hence the measures that can enhance the quality of the power are recommended.
This work describes the techniques of correcting the supply voltage sag, swell and interruption in
a distributed system. At present, a wide range of very flexible controllers, which capitalize on newly
available power electronics components, are emerging for custom power applications. Among these, the
distribution static compensator and the dynamic voltage restorer are most effective devices, both of them
based on the VSC principle. A D-STATCOM injects a current into the system to correct the voltage sag,
swell and interruption. Comprehensive results are presented to assess the performance of each device as a
potential custom power solution
The STATCOM is applied to regulate transmission voltage to allow greater power flow in a voltage
limited transmission network, in the same manner as a static var compensator (SVC), the STATCOM has
further potential by giving an inherently faster response and greater output to a system with depressed
voltage and offers improved quality of supply. The main applications of the STATCOM are; Distribution
STATCOM (D-STATCOM) exhibits high speed control of reactive power to provide voltage stabilization
and other type of system control. The DSTATCOM protects the utility transmission or distribution system
from voltage sag and /or flicker caused by rapidly varying reactive current demand. During the transient
conditions the D-STATCOM provides leading or lagging reactive power to active system stability, power
factor correction and load balancing
Keywords: Distribution Static Synchronous Compensator (D-STATCOM), Voltage Dip, Distribution
System

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1. Introduction
One of the most common power quality problems today is voltage dips. A voltage dip is a short time (10 ms
to 1 minute) event during which a reduction in r.m.s voltage magnitude occurs. It is often set only by two
parameters, depth/magnitude and duration. The voltage dip magnitude is ranged from 10% to 90% of
nominal voltage (which corresponds to 90% to 10% remaining voltage) and with a duration from half a
cycle to 1 min. In a three-phase system a voltage dip is by nature a three-phase phenomenon, which affects
both the phase-to-ground and phase-to-phase voltages. A voltage dip is caused by a fault in the utility
system, a fault within the customer’s facility or a large increase of the load current, like starting a motor or
transformer energizing. Typical faults are single-phase or multiple-phase short circuits, which leads to high
currents. The high current results in a voltage drop over the network impedance. At the fault location the
voltage in the faulted phases drops close to zero, whereas in the non-faulted phases it remains more or less
unchanged.
Electric power distribution network becomes more increasingly important and plays an essential
role in power system planning. This type of power systems has a major function to serve distributed
customer loads along a feeder line; therefore under competitive environment of electricity market eservice
of electric energy transfer must not be interrupted and at the same time there must provide reliable, stable
and high quality of electric power. To complete this challenge, it requires careful design for power network
Planning. There exist many different ways to do so. However, one might consider an additional device to be
installed somewhere in the network. Such devices are one of capacitor bank, shunt reactor, series reactors,
and automatic voltage regulators and/or recently developed dynamic voltage restorers, distribution
STATCOM or combination of them.
Most industries and companies prefer electrical energy with high quality. If delivered energy to
these loads has poor quality, products and equipment of these loads such as microcontrollers, computers,
motor drives etc are damaged. Hurt of this phenomenon in companies that dealing with information
technology systems is serious. According to a study in U.S., total damage by voltage sag amounts to 400
Billion Dollars .For these reasons power quality mitigation in power systems is necessary. Nowadays,
Custom Power equipments are used for this purpose. DSTATCOM is one of these equipments which can be
installed in parallel with. Sensitive loads. This device mitigates the load voltage by Injecting necessary
current to the system

2. Structure of Statcom
Basically, STATCOM is comprised of three main parts ,a voltage source inverter (VSI), a step-up coupling
transformer, and a controller. In a very-high-voltage system, the leakage inductances of the step-up power
transformers can function as coupling reactors. The main purpose of the coupling inductors is to filter out
the current harmonic components that are generated mainly by the pulsating output voltage of the power
converters.
2.1Voltage Source Converter (VSC)
A voltage-source converter is a power electronic device, which can generate a sinusoidal voltage with any
required magnitude, frequency and phase angle. Voltage source converters are widely used in adjustable-
speed drives, but can also be used to mitigate voltage dips. The VSC is used to either completely replace
the voltage or to inject the ‘missing voltage’. The ‘missing voltage’ is the difference between the nominal
voltage and the actual. The converter is normally based on some kind of energy storage, which will supply
the converter with a DC voltage. The solid-state electronics in the converter is then switched to get the
desired output voltage. Normally the VSC is not only used for voltage dip mitigation, but also for other
power quality issues, e.g. flicker and harmonics.

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2.2 A Controller
The aim of the control scheme is to maintain constant voltage magnitude at the point where a sensitive load
is connected, under system disturbances. The control system only measures the r.m.s voltage at the load
point, i.e., no reactive power measurements are required. The VSC switching strategy is based on a
sinusoidal PWM technique which offers simplicity and good response. Since custom power is a relatively
low-power application, PWM methods offer a more flexible option than the Fundamental Frequency
Switching (FFS) methods favored in FACTS applications. Besides, high switching frequencies can be used
to improve on the efficiency of the converter, without incurring significant switching losses.
The controller input is an error signal obtained from the reference voltage and the value rms of the terminal
voltage measured. Such error is processed by a PI controller the output is the angle δ, which is provided to
the PWM signal generator. It is important to note that in this case, indirectly controlled converter, there is
active and reactive power exchange with the network simultaneously: an error signal is obtained by
comparing the reference voltage with the rms voltage measured at the load point. The PI controller process
the error signal generates the required angle to drive the error to zero, i.e., the load rms voltage is brought
back to the reference.

COUPLING
TRANSFORME
R
Q P
v
Qref
v=vmsin(t −ϕ)
ω
AC
controller
ϕ Pref

P

enery storage

Fig.1Block diagram representation of STATCOM

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Fig.2 Single-line Diagram of a STATCOM and Its Control System Block Diagram

The control system consists of:

• A phase-locked loop (PLL) which synchronizes on the positive-sequence component of the three-
phase primary voltage V1. The output of the PLL (angle Θ=ωt) is used to compute the direct-axis
and quadrature-axis components of the AC three-phase voltage and currents (labeled as Vd, Vq or
Id, Iq on the diagram).
• Measurement systems measuring the d and q components of AC positive-sequence voltage and
currents to be controlled as well as the DC voltage Vdc.
• An outer regulation loop consisting of an AC voltage regulator and a DC voltage regulator. The
output of the AC voltage regulator is the reference current Iqref for the current regulator (Iq =
current in quadrature with voltage which controls reactive power flow). The output of the DC
voltage regulator is the reference current Idref for the current regulator (Id = current in phase with
voltage which controls active power flow).
• An inner current regulation loop consisting of a current regulator. The current regulator controls
the magnitude and phase of the voltage generated by the PWM converter (V2d V2q) from the
Idref and Iqref reference currents produced respectively by the DC voltage regulator and the AC
voltage regulator (in voltage control mode). The current regulator is assisted by a feed forward
type regulator which predicts the V2 voltage output (V2d V2q) from the V1 measurement (V1d
V1q) and the transformer leakage reactance.

3. Principle of Operation
The D-STATCOM is a three phase and shunt connected power electronics based reactive power
Compensation equipment, which generates and /or absorbs the reactive power whose output can be varied
so as to maintain control of specific parameters of the electric power system.

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The AC voltage difference across the leakage reactance power exchange between the D-STATCOM
and the Power system, such that the AC voltages at the busbar can be regulated to improve the voltage
profile of the power system, which is primary duty of the D-STATCOM.The D-STATCOM employs an
inverter to convert the DC link voltage Vdc on the capacitor to a voltage source of adjustable magnitude
and phase. Therefore the D-STATCOM can be treated as a voltage controlled source. The D-STATCOM
can also be seen as a current controlled source. The basic objective of a VSI is to produce a sinusoidal AC
voltage with minimal harmonic distortion from a DC voltage. The operation of the D-STATCOM is as
follows: The voltage is compared with the AC bus voltage system (Vs).

• When the AC bus voltage magnitude is above that of the VSI magnitude (Vc); the AC
system sees the D-STATCOM as inductance connected to its terminals.
• Otherwise if the VSI voltage magnitude is above that of the AC bus voltage magnitude, the
AC system sees the D-STATCOM as capacitance to its terminals.
• If the voltage magnitudes are equal, the reactive power exchange is zero.

If the D-STATCOM has a DC source or energy storage device on its DC side, it can supply real
power to the power system. This can be achieved by adjusting the phase angle of the D-STATCOM
terminals and the phase angle of the AC power system. When phase angle of the AC power system leads
the VSI phase angle, the DSTATCOM absorbs the real power from the AC system, if the phase angle of the
AC power system lags the VSI phase angle, the D-STATCOM supplies real power to AC system

Fig.3 Operating Principle of the STATCOM

4. Compensation schemes
4.1 General Compensation Scheme
A shunt-connected solid-state synchronous voltage source, composed of a six-pulse/five level, voltage-
sourced inverter and a dc energy storage device, is shown schematically in Figure 7. As explained in the
previous section, it can be considered as a perfect sinusoidal synchronous voltage source behind a coupling

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reactance provided by the leakage inductance of the coupling transformer. If the energy storage is of
suitable rating, the STATCOM can exchange both reactive and real power with the ac system. The reactive
and real power, generated or absorbed by the STATCOM, can be controlled independently of each other,
and any combination of real power generation/absorption With var generation/absorption is possible, as
illustrated in Figure 7b. The real power that the STATCOM exchanges at its ac terminals with the ac system
must, of course, be supplied to, or absorbed from, its dc terminals by the energy storage device. By
contrast, the reactive power exchanged is internally generated by the STATCOM, without the dc energy
storage device playing any significant part in it.

Fig 4: a) shunt connected synchronous voltage source and b) its possible operating modes for real and
reactive power generation

4.2 Reactive Power Compensation Scheme
If the D-STATCOM is used only for reactive shunt compensation, like a conventional static var
compensator, then the dc energy storage device can be replaced by a relatively small dc capacitor. In this
case, the steady-state power exchange between the STATCOM and the ac system can only be reactive.
When the STATCOM is used for reactive power generation, the inverter itself can keep the capacitor
charged to the required voltage level. This is accomplished by making the output voltages of the inverter

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lag the system voltages by a small angle. In this way the inverter absorbs a small amount of real power
from the ac system to replenish its internal losses and keep the capacitor voltage at the desired level. The
same control mechanism can be used to increase or decrease dc capacitor voltage, and thereby the
amplitude of the output voltage of the inverter, for the purpose of controlling the var generation or
absorption

Fig 5.a) synchronous voltage source operated as the static condenser b) its possible operating mode for
reactive power generation

5. Simulation Block Diagram of D-Statcom

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6. Simulation Results
6.1 Without D-Statcom
Here initially the D-STATCOM was not connected to the system and the load of three phase RLC load

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of 3MW, 0.2 MVAR is applied on the system in the time interval of 0.1sec to 0.5 sec. the voltage got
dipped from 1.12 p.u to 0.98p.u for voltage across B1 and 1.06 p.u to 0.94 p.u for voltage across B3.

6.2 With D-Statcom
Here the D-STATCOM was connected to the system and the load of three phase RLC load of 3MW,
0.2 MVAR is applied on the system in the time interval of 0.1sec to 0.5 sec. the voltage got dipped from
1.09 p.u to 1.01p.u for voltage across B1 and 1.02 p.u to 0.96 p.u for voltage across B3

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7. Conclusion
Voltage dip and voltage flickering are the two major power quality problems which are frequently seen in
the distribution systems. These power quality problems in 25KV, 100 MVA distribution system are
investigated in this paper. The analysis and simulation of a DSTATCOM application for the mitigation of
power quality problems are presented and discussed. The Mat lab Power System Block set simulation
results shows that the mitigation of the power quality problems (voltage dip and the voltage flickering)
done effectively with D-STATCOM. The voltage got dipped from 1.12 p.u to 0.98p.u for voltage across B1
and 1.06 p.u to 0.94 p.u for voltage across B3 for without D-Statcom and 1.09 p.u to 1.01p.u for voltage

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across B1 and 1.02 p.u to 0.96 p.u for voltage across B3 with D-Statcom.

References

G.Yaleinkaya, M.H.J. Bollen, P.A. Crossley (1999), “Characterization of voltage sags in industrial
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Hague, M.H (2001), “Compensation of distribution system voltage sag by DVR and D-STATCOM”,
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pp. 644-655, Nov.
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S.Anil Kumar was born in India in 1986. He received his B.Tech in Electrical & Electronics Engineering
from Jawaharlal Nehru Technological University, Hyderabad in 2007. He is pursuing his Master of
Technology in Electrical power systems from the Jawaharlal Nehru Technological University, Anantapur.
He is currently working as an Assistant Professor in Electrical and Electronics Engineering Department at
G.Pulla Reddy Engineering College, Kurnool. His Research interests include electric power distribution
systems, HVDC, FACTS and Power system operation and control.

D.Vanurrappa was born in India in 1982. He received his B.Tech in Electrical & Electronics Engineering
from Jawaharlal Nehru Technological University, Hyderabad in 2007. He is pursuing his Master of
Technology in Electrical power systems from the Jawaharlal Nehru Technological University, Anantapur.
He is currently working as an Assistant Professor in Electrical and Electronics Engineering Department at
Brindavan Institute of Technology & Sciences, Kurnool. His Research interests include electric power
distribution systems, HVDC, FACTS and Power system operation and control.

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