VOLTAGE HARMONICS MITIGATION USING
DVR
A Project Synopsis Submitted
In Partial Fulfillment of the Requirements
for The Degree Of
BACHELOR OF TECHNOLOGY
in
Electrical & Electronics Engineering
by
Udgam Tiwari(Roll No-1130921038)
Aniket Naugariya(Roll No-1130921011)
Shivam Pandey(Roll No-1130921040)
Under the Guidance of:
Mr. Priyank Agarwal
Raj Kumar Goel Engineering College, Pilkhuwa
to the
Faculty of (Electrical & Electronics)
UTTAR PRADESH TECHNICAL UNIVERSITY
LUCKNOW
August, 2014
INTRODUTION
POWER quality (PQ) problems in the distribution system and their solutions have received
much attention in the recent years. The incorporation of the large numbers of nonlinear loads
for improved efficiency and their better controlled use, the nonconventional power
production technologies such as solar and wind power, and the various power electronics
devices used in the system introduce new problems, like additional harmonic voltage/current
distortion, particularly higher order harmonics Under the generic name of custom power
devices , a new group of devices is developed and used for improving the PQ in the
distribution system. As per the standard such as the IEEE 519,a number of custom power
devices are installed and used at the consumer premises to protect the critical loads. The
dynamic voltage restorer (DVR), one of the aforementioned devices, is used for improving
the PQ of the load terminal voltages against voltage sags, swells, transients, and harmonic
distortions in the source voltages. A DVR is a voltage-source converter (VSC)-based power
electronics device connected in series between the supply and the critical loads, which are to
be protected from the supply side voltage quality problems, other than outages, by injecting
the required compensating voltage through DVR into the distribution line. A DVR can restore
a balanced sinusoidal load voltage of desired amplitude even when the source voltage is
unbalanced and/or distorted. The voltage injected by selfsupported DVR is in quadrature with
the feeder current; hence, it does not need any active power during steady state. However, its
disadvantage is that, in case of the voltage sag/swell, the restored voltage may not be in phase
with the presag/preswell voltage. The self-supported DVR is used when the phase jump,
caused by the quadrature voltage injection, is affordable. The DVR is an important controller
in the custom power park. The analysis, design, and voltage injection schemes of the self-
supported DVR are discussed and the different control strategies for the DVR have been
developed in. Control techniques based on synchronous reference frame (SRF) theory
(SRFT), Adaline-based fundamental extraction instantaneous symmetrical component theory,
energy optimized control, PQR instantaneous power theory, symmetrical component
estimation, etc., for the DVR are reported in the literature. Different topologies of the DVR
are discussed in. The DVR supported by a capacitor has become popular as a costeffective
solution for the protection of sensitive loads from the supply-side voltage quality problems.
Currently, most of the research is on DVR dealing with the protection of balanced linear load;
however, there are a few which are related to the protection of unbalanced and nonlinear
loads . In modern industries, power electronics-based drives such as the current-source-
inverter-fed synchronous motor drive, thyristor converter-based dc motor drive, VSC-based
induction motor drive, etc., are increasingly used whose performance and control largely
depend upon the supply voltage quality. Moreover, these nonlinear industrial loads give rise
to additional harmonic distortion in the supply voltage at the point of common coupling
(PCC) due to the harmonic voltage drop into the feeder impedance, particularly when the
feeder impedance is large. In this paper, a simple generalized control algorithm for the self-
supported DVR is developed based on the basic SRFT. This novel algorithm makes use of the
fundamental positivesequence phase voltages extracted by sensing only two unbalanced
and/or distorted line voltages. The algorithm is general enough to handle linear as well as
nonlinear loads. The selfsupported DVR maintains balanced sinusoidal load voltage with
desired magnitude against any supply voltage quality problem even when the load is
unbalanced and nonlinear in nature. The algorithm based on instantaneous symmetrical
components along with the complex Fourier transform to protect unbalanced and nonlinear
load discussed in [17] is computationally demanding and requires huge memory space. The
approach discussed here is comparatively simple as it needs only the extraction of the
fundamental positive-sequence phase terminal voltages, thus making it computationally
simpler with the least memory requirement. The proposed fundamental positive-sequence
extractor requires the sensing of only two line voltages of supply. This reduces the analog-to-
digital converter (ADC) requirements of a digital controller and corresponding sensing
element. Moreover, it is able to extract three fundamental positive-sequence phase voltages
irrespective of the distribution system configuration such as three-phase, fourwire or three-
phase, three-wire system where the neutral is not available for sensing phase voltages. In this
paper, a hybrid structure of the self-supported DVR is considered in which a shunt capacitor
filter is used to provide the low impedance path for higher order harmonics of the load
currents [17]. The DVR is realized by three single-phase H-bridge VSCs with a constant
switching frequency hysteresis band voltage controller [18]. The proposed control algorithm
is validated through extensive simulation and real-time experimental studies performed using
an OPAL-RT real-time digital simulator and a dSPACE DS1103 digital signal processor.
Fig.1. Schematic diagram of DVR (ideal Voltage sources) connected power system
Nowadays, modern industrial devices are mostly based on electronic devices such as
programmable logic controllers and electronic drives. The electronic devices are very
sensitive to disturbances and become less tolerant to power quality problems such as voltage
sags, swells and harmonics. Voltage dips are considered to be one of the most severe
disturbances to the
industrial equipments. Voltage support at a load can be achieved by reactive power injection
at the load point of common coupling. The common method for this is to install mechanically
switched shunt capacitors in the primary terminal of the distribution transformer. The
mechanical switching may be on a schedule, via signals from a supervisory control and data
acquisition (SCADA) system, with some timing schedule, or with no switching at all. The
disadvantage is that, high speed transients cannot be compensated. Some sags are not
corrected within the limited time frame of mechanical switching devices. Transformer taps
may be used, but tap changing under load is costly. Another power electronic solution to the
voltage regulation is the use of a dynamic voltage restorer (DVR). 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 voltage sags/swells.
The DVR applications are mainly for sensitive loads that may be drastically affected by
fluctuations in system voltage. Reliability and power quality are two important issues in
power systems. Reliability refers to the continuity of power supply, while power quality
relates to power disturbances in terms of voltage, current and frequency variations. The
power electronics loads in modern industries are susceptible to disturbances such as short
interruptions, voltage sags, harmonics, etc., that historically were not cause of major concern.
Revitalizing of industry with more micro-electronic devices and more energy efficient
equipment pushes power quality issues into even more prominent position. As a result, there
is an increasing interest in power quality and the problems of power quality are challenging
every participant in the chain of electricity supply. Power quality is a collection of various
subjects in terms of voltage quality, current quality, supply quality and consumption quality
[1]. It can be defined as: Any power problem manifested in voltage, current, or frequency
deviations that result in failure or malfunction of customer equipment [2]. Power quality is a
customer-driven issue in general.
DVR Control strategy
The main aim of the DVR is to inject a required amount of compensating voltage in series
with the supply to regulate the load terminal voltage. In this section, the proposed control
algorithm is discussed with an ideal DVR model. The schematic diagram of DVR (ideal
voltage sources) connected distribution feeder is shown in Fig. 1. A three-phase supply is
represented by the star-connected three single-phase voltage sources (Vsa, Vsb, Vsc) along
with their series source impedances (Za, Zb, Zc). To regulate the load voltages (VLa, VLb,
VLc) to be balanced and sinusoidal against various PQ problems in the terminal voltages
(Vta, Vtb, Vtc), DVR injects the required compensating voltages (VCa, VCb, VCc) in each
phase. The practical implementation of a DVR using three single phase H-bridge VSCs along
with a common dc capacitor i. discussed later. The energy storage device is a capacitor, so the
following condition is stipulated on the DVR. The DVR should not supply any real power in
steady state. This implies that, in steady state, the phase difference between instantaneous
DVR voltages and instantaneous line currents must be 90.
According to that there are three dynamic strategies:-
1. DVR Operation Under Balanced Sinusoidal Supply
Condition with Balanced Linear Load
2. DVR Operation Under Unbalanced and/or Distorted
Supply Condition With Unbalanced Linear Load
3. Fundamental Positive-Sequence Extractor
DVR POWER CIRCUIT
In the previous section, a response of DVR in different PQ problems with the developed
control algorithm assuming DVR realized by ideal voltage sources is demonstrated. In this
section, the power circuit of DVR is discussed. Practically, the DVR is realized by three
single-phase H-bridge VSCs along with a common dc capacitor (Cdc) as shown in Fig. 5.
The three H-bridge VSCs are connected to each phase of the distribution feeder through the
improved structure ripple filter (Lr, Cr,Rr) and an injection transformer. The injection
transformer not only reduces the voltage requirement of the converter but also provides
isolation between the converter and
the distribution feeder. The shunt capacitor filter Cf is used to provide a low impedance path
to higher order harmonics of load currents when the load current is nonlinear. The operation
of
practical DVR with nonlinear load current is discussed in the next section. To track the
reference compensating voltages, an improved filter structure constant switching frequency
hysteresis band controller is used in this work. The main advantages of the band controller
are unconditional stability, faster response and easy implementation compared to other
controllers like carrier-based controllers, dead-beat control , state feedback control, combined
feedforward and feedback control, etc., which are based on complex mathematical
computations and need much information about system parameters. Despite these advantages,
the main disadvantage of the band controller compared to carrier-based controllers is variable
switching frequency which may cause stress in the switches of the VSC, resulting in the
deterioration of its life. The band controller has other drawbacks also like poor
controllability, heavy filter currents, parabolic band voltage response, and frequent band
violations due to the use of a conventional LC filter which has a second-order characteristic
equation. The constant switching frequency hysteresis band controller with improved filter
structure discussed in preserves all the advantages of the band controller and also overcomes
its drawbacks by improved filter structure and adaptive hysteresis band which gives constant
switching frequency. The single-phase equivalent circuit of the DVR-connected system in
Fig. 2 is shown in Fig. 3 to explain the basic principle of the hysteresis band controller. The
reference compensating voltage for the DVR is calculated using the proposed algorithm. To
inject this voltage in series with the distribution feeder, appropriate switching pulses for VSC
are generated using the hysteresis band controller with hysteresis band h. The VSC output
voltage is made to track the reference voltages within upper and lower boundaries vref C + h
and vref C h, respectively. When the DVR voltage VC goes below the lower boundary, the
positive dc voltage is applied across the ac filter combination (Cr,Rr) by turning switches S1
and S2 on. If DVR voltage VC goes above the upper boundary, the negative dc voltage is
applied by turning switches S3 and S4 on. The switching logic thus can be stated as given in
Table I.
Fig.2. DVR power ckt
Fig.2. Complete block diagram of proposed control algorithm
Table 1. SWITCHING LOGIC OF VSC WITH HYSTERESIS BAND CONTROLLER
CONCLUSION
A simple generalized algorithm has been developed for the generation of instantaneous
reference compensating voltages for controlling self-supported DVR based on basic SRFT to
protect unbalanced and nonlinear loads. A fundamental positive-sequence extractor has been
proposed, which extracts three fundamental positive-sequence phase voltages by sensing only
two unbalanced and/or distorted line voltages. It has been shown that, when load is nonlinear,
only DVR, without a shunt capacitor filter, is not able to suppress the voltage spikes. The
performance of DVR with the proposed control algorithm has been evaluated for both the
harmonic current source and the harmonic voltage source type of nonlinear loads with
different supply voltage quality problems. The performance of the DVR has been observed to
be satisfactory in all the cases. Moreover, it is shown that DVR is capable of providing self-
support to its dc bus by taking active power from the ac line at fundamental frequency. In this
paper, performance of a DVR in mitigating voltage sags/swells is demonstrated with the help
of MATLAB. A forced commutated voltage sources converter is considered in the DVR
along with energy storage to maintain the capacitor voltage. The DVR handles both balanced
and unbalanced situation without any difficulties and injects the appropriate voltage
component to correct any anomaly in the supply voltage to keep the load voltage balanced
and constant at the nominal value. In the case of a voltage sag, which is a condition of a
temporary reduction in supply voltage, the DVR injects an equal positive voltage component
in all three phases w hich are in phase with the supply voltage to correct it. On the other hand,
for a voltage swell case, which is a condition of a temporary increase in supply voltage, the
DVR injects an equal negative voltage in all three phases which are anti-phase with the
supply voltage. For unbalanced conditions, the DVR injects an appropriate unbalanced three-
phase voltage component Time domain simulation of the DVR under different conditions
including voltage sag caused by a three-phase to ground fault, voltage swell and distorted
supply voltage has been carried out to verify the ability of DVR to maint the load voltage
around nominal value. Simulation results shows the DVR abilities in harmonic elimination as
well as voltage mitigation. It is concluded that DVR successfully mitigated long duration
voltage sags/swells and perfectly restored the load voltage almost 1 p.u. study on rated power
of injection transformer shows, although larger transformer has a better performance in
sags/swells mitigation, it increases THD of load voltage when the supply voltage distorted, so
there is a trade-off between rms value and THD of the load voltage.
REFERENCES
1. IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 49, NO. 5
2. Thammasat Int. J. Sc.Tech.,Vol. 11
3. Sci.Int.(Lahore),26(2),627-631,2014
4. NIIT,Rourkela
