Advance in Electronic and Electric Engineering.
ISSN 2231-1297, Volume 4, Number 6 (2014), pp. 645-654
© Research India Publications
http://www.ripublication.com/aeee.htm
Voltage Sag Compensation Using Dynamic Voltage Restorer
Mayank Paliwal, Rohit Chandra Verma and Shaurya Rastogi
Department of Electronics, Electrical & Communication Engineering
Galgotias University.
Abstract
This paper presents the modeling and simulation of a dynamic voltage
restorer as a voltage sag mitigation device in electrical power
distribution networks. The dynamic voltage restorer with its excellent
dynamic capabilities, when installed between the supply and a critical
load feeder, can compensate for voltage sags/swells, restoring line
voltage to its nominal value within few milliseconds and hence
avoiding any power disruption to the load. In this paper the technical
aspect feasibility related to the use dynamic voltage restorer (DVR) of
traditional DC storage systems are evaluated. This topology would
ensure a constant DC voltage across the DC link during the process of
voltage compensation. The modeling of dynamic voltage restorer is
carried out component wise and their performances are analyzed using
MATLAB software. The simulation results shows that the control
technique is very effective and yields excellent compensation for
voltage sag Mitigation.
Keywords: Power Quality, Dynamic Voltage Restorer (DVR),
Operating States.
1. Introduction
In the early days of power transmission voltage deviation during load changes, power
transfer limitation was observed due to reactive power unbalances. Modern power
systems are complex networks, where hundreds of generating stations and thousand of
load centers are interconnected through long power transmission and distribution
networks. The main concern of customer is the quality and reliability of power supply
at various load centers. Even though power generation in most well-developed
countries is fairly reliable, the quality of supply is not. Power distribution system
should ideally provide their customers an uninterrupted flow of energy with smooth
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Mayank Paliwal et al
sinusoidal voltage at the contracted magnitude and frequency. However, in practice
power system especially the distribution system, have numerous non linear loads,
which are significantly affect the quality of power supply. As a result, the purity of
waveform of supply lost. This ends up producing many power quality problems.
To improve power quality, custom power devices are used. In 1995 the concept of
custom power is first explained by Hingorani[1]. The thought of custom power (CP)
identifies with the utilization of electronic controllers for power system network. There
are number of custom power units which are given below, Distribution Statcom (DSTATCOM), Dynamic Voltage Restorer (DVR), Unified power quality conditioner
(UPQC), Active Power Filters, Battery Systems (BESS), Distribution Series
Capacitors (DSC), Surge Arresters (SA), Un-interruptible Power Supplies (UPS), Solid
State Fault Current Limiter (SSFCL), Solid-State Transfer Switches (SSTS), and Static
Electronic Tap Changers (SETC)[2]. Dynamic Voltage Restoration (DVR) is a method
and apparatus used to sustain, or restore, an operational electric load during sags, or
spikes, in voltage supply. DVRs are a class of custom power devices for providing
reliable distribution power quality. They employ a series of voltage boost technology
using solid state switches for compensating sags/swells.
2. Power Circuits of DVR
The power circuit of the DVR shown in Fig. 1 can be divided into four parts:
a) Voltage source inverter (VSR) b) Three single-phase injection transformers c)
Passive filters d) Energy storage.
2.1 Voltage Source Inverter (VSR)
This may be 3-phase 3-wire VSI or 3-phase 4-wire VSI. The inverter rating is
relatively low in voltage and high in current due to the use of step-up injection
transformers. There is generally no need for extensive multilevel structures seen in
Flexible AC Transmission system (FACTS). The most common inverter connections
use either a three-level inverter or a conventional three-phase Graetz bridge inverter.
2.2 Injection Transformers
The three single-phase transformers can be connected to the distribution line with
star/open star winding or delta/open star winding. The star/open star winding allows
the injection of positive, negative and zero sequence voltages whereas the delta/open
winding only allows the injection of positive and negative sequence voltages.
However, the delta/open star winding maximizes the use of dc link voltage when
compared to the star-open winding. The choice of injection transformer winding
greatly depends on the manner in which the distribution line 132kV- 11 kV step down
transformer is connected. If a A-Y with the neutral grounded is used, zero-sequence
voltages and 11kV/O.415kV harmonics will not propagate through the transformer
when unbalanced faults occur on the high voltage network. Therefore, there is no need
to compensate for zero sequence voltages. However, this is not the case when a Y-Y
with the neutral grounded connection is used. Hence, the choice of injection
Voltage Sag Compensation Using Dynamic Voltage Restorer
647
transformer winding and the control algorithm adopted are dependent on the step-down
transformer winding in the distribution system. Moreover, to avoid saturation under all
conditions, the injection transformer must be sized to handle at least twice the normal
steady-state flux requirement at maximum rms. Injection voltage, without saturation.
Fig. 1: a) Equivalent electrical circuit
diagram of the DVR
Fig. 1: b) Block Diagram of General DVR
2.3 Passive Filters
The filtering scheme in the dynamic voltage restorer can be placed either on the highvoltage-side or the inverter side of the series injection transformer. The advantage of
the inverter-side filter is that it is on the low-voltage side of the series transformer and
is closer to the harmonic source. Using this scheme, the high-order harmonic currents
will be prevented from penetrating into the series transformer thus reducing the voltage
stress on the transformer. However, when the DVR acts as a source the introduction of
the filter inductor L causes voltage drop and a phase-angle shift in the fundamental
component of the inverter output. This can affect the control scheme of the DVR. As
the filter is located on the high-voltage side of the series transformer, high-order
harmonic currents will penetrate into the series transformer, thus necessitating a higher
rating of the transformer. However, the leakage reactance of the transformer can be
used to aid the filtering characteristic. A common problem facing these two filtering
schemes is that the filter capacitor will cause an increase in the inverter rating.
2.4 Energy Storage
Energy storage is required to provide real power to the load when large voltage sags
take place. Examples of energy storages are lead-acid batteries, flywheel, SMES, etc.
The energy storage device used for this study is lead-acid batteries. Energy storage
batteries are devices in which electric energy is stored in electrochemical form by
creating electrically charged ions. Batteries provide rapid response for either charge or
discharge, but the discharge rate is limited by chemical reaction rates so that the
available energy depends on the discharge rate. Lead-acid battery technology is also a
mature technology.
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3. Control of the DVR
The main considerations for the control system of a DVR include: detection of the start
and finish of the sag, voltage reference generation, transient and steady-state control of
the injected voltage, and protection of the system. The control system was used to
control the DVR with a sampling and switching frequency. It requires measurement of
four parameter groups.
 Three phase-voltages before the DVR to detect voltage sag and for feedforward control of the output voltage.
 Three-phase voltages after the DVR for feed-back control of the output voltage.
 Three currents in the converter to protect the DVR by both saturation control
and over current.
 The dc-link voltage for dc voltage compensation (to decouple the controller
from variations in the dc-link voltage), for converter protection, and to provide
energy storage information.
With the grid voltage in its normal level the DVR is held in a null state to keep the
losses to a minimum [3]. Once voltage sag is detected the DVR converts into active
mode to react as fast as possible and inject the required ac voltage to the grid.
Table 1: Table showing System quantities/ parameter.
S. No
1.
System Quantities
Inverter Specification
2.
3.
Transmission Line Parameter
PI Controller
Standards
IGBT based, 3 arms, 6 Pulse, Carrier
frequency =1080 Hz, Sample Time= 5 μs
R=0.001 ohms, L=0.005H
KP=0.5, Ki=50, Sample time=50 μs
Fig. 2: Single line diagram of test system for DVR.
Voltage Sag Compensation Using Dynamic Voltage Restorer
649
4. Simulation Results
System performance is analyzed for compensating voltage sag with different DC
storage capacity so as to achieve rated voltage at a given load. Various cases of
different load condition are considered to study [4] the impact DC storage on sag
compensation.
Case 1: A three-phase fault is created at point X via a resistance of 0.66 Ω which
results in a voltage sag of 17.02 %. Transition time for the fault is considered from 0.4
sec to 0.6 DVR performance in presence of capacitor rating of 750×10-6 F with energy
storage devices via 3.1KV.
Fig. 3: a) Three Phase, Phase to Phase
Voltage without DVR Energy Storage
Fig. 3: b) Voltage p.u. at the Load
Point without DVR System
Fig. 3: c) Voltage p.u. at the Load Point with
DC storage of 3.1 kV
Case 2: A three-phase fault is created at point X via a resistance of 0.60 Ω which
results in a voltage sag of 19 %. Transition time for the fault is considered from 0.4 s to
0.6 s DVR performances in presence of capacitor rating of 750×10-6 F with energy
storage devices via. 3.3 kV.
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Mayank Paliwal et al
Fig. 4: a) Three Phase, Phase to Phase
Voltage without DVR Energy Storage
Fig. 4: b) Voltage p.u. at the Load
Point without DVR System
Fig. 4: c) Voltage p.u. at the Load Point with DC storage of 3.3 kV
Case 3: A three-phase fault is created at point X via a resistance of 0.50 Ω which
results in a voltage sag of 23 %. Transition time for the fault is considered from 0.4 s to
0.6 s. DVR performance in presence of capacitor rating of 750×10-6 F with energy
storage devices via. 3.5 kV.
Fig. 5: a) Three Phase, Phase to Phase
Point without DVR System
Fig. 5: b) Voltage p.u. at the Load
Point without DVR System
Voltage Sag Compensation Using Dynamic Voltage Restorer
651
Fig. 5: c) Voltage p.u. at the Load Point with DC storage of 3.5 kV
Case 4: A three-phase fault is created at point X via a resistance of 0.45 Ω which
results in a voltage sag of 26 %. Transition time for the fault is considered from 0.4 s to
0.6 s. DVR performance in presence of capacitor rating of 750×10-6 F with different
energy storage devices via. 3.7KV.
Fig. 6: a) Three Phase, Phase to Phase
Point without DVR System
Fig. 6: b) Voltage p.u. at the Load
Point without DVR System
Fig. 6: c) Voltage p.u. at the Load Point with DC storage of 3.7 kV
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Mayank Paliwal et al
Case 5: A three-phase fault is created at point X via a resistance of 0.40 Ω which
results in a voltage sag of 29 %. Transition time for the fault is considered from 0.4 s to
0.6 s. DVR performance in presence of capacitor rating of 750×10-6 F with different
energy storage devices viz. above 3.7KV.
Fig. 7: a) Three Phase, Phase to Phase
Voltage without DVR Energy Storage
Fig. 7: b) Voltage p.u. at the Load
Point without DVR System
Fig. 7: c) Voltage p.u. at the Load Point with DC storage of 4.3 kV
Table 2: Comparison of desired % voltage sag at DC voltage
S. No.
1.
2.
3.
4.
5.
% Voltage Sag
17.02
19
23
26
29
Required DC Voltage (KV)
3.1
3.3
3.5
3.7
Above 3.7
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653
In the above table it is shown that required DC storage values are not same for
different voltage sag conditions [5]. When the load is fixed on 11 KV feeders, the
amount of DC energy storage is increased with increase in the percentage voltage sag.
It is observed that when percentage voltage sag is increased above 28%. The per unit
voltage fall below 1 per unit value and it is continuously decreases with increase in
percentage voltage sag for 11 kV feeder [6].
5. Conclusion
Fig. 8 shows the variation of DC storage voltage with the increase percentage voltage
sag. DC storage value can be estimated from the following equation:
= 0.0012 − 0.008 + 1.8 − 10
Where Y= DC Voltage (kV) and X= % Voltage sag
Voltage sag values are major factors in estimating the DC storage value. The
effectiveness of a DVR system mainly depends upon the rating of DC storage rating
and the percentage voltage sag. In the test system it is observed that after a particular
amount of voltage sag, the voltage level at the load terminal decreases.
Fig. 8: % voltage sag verses DC storage (KV)
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