Dvr Based Power Quality Improvement In Distribution System

International Journal of Engineering Science Invention
ISSN (Online): 2319 – 6734, ISSN (Print): 2319 – 6726
www.ijesi.org ||Volume 4 Issue 5 || May 2015 || PP.32-39
www.ijesi.org 32 | Page
Dvr Based Power Quality Improvement In Distribution System
Priyanka R1
, Selvamathi R2
Electrical and Electronics Engineering, VTU, BELAGAVI
ABSTRACT: In this paper, the dynamic voltage restorer (DVR) with reduced rating VSC is used to improve the
power quality by eliminating the harmonics to reduce voltage sags and swells observed in the distribution
system. Also different voltage injection schemes is been discussed. A new control technique to ensure power
quality is being proposed here by controlling the capacitor supported DVR. Unit vectors are used to estimate
the load voltage. Synchronous reference frame theory is used to convert the voltages from the rotating vectors to
the stationary frame. Also the DVR with battery energy storage system is also demonstrated to eliminate the
power quality problems stated above.
INDEX TERMS: Dynamic voltage restorer (DVR), voltage sag, voltage swell, voltage harmonics, unit vector,
voltage source converter (VSC), PI controller, Park’s transformation.
I. INTRODUCTION
Power quality became the major concern in power systems since 1990s. Here we are concentrating on
distribution system. Power quality problems such as voltage sags, swells, transients, harmonics and
interruptions are observed in the supply voltage. These problems are caused due to the use of sensitive and
critical equipments such as communication networks, process industries precise manufacturing industries and
other modern loads in distribution side. And also affect the sensitive loads due to the variations in supply
voltage. In order to avoid this kind of power quality problems in distribution side we are using custom power
devices. Custom power devices are of three categories viz., series connected compensators, shut connected
compensators and series and shunt connected compensators.
Here, in this paper we are using a series connected compensator called DVR (Dynamic Voltage Restorer).
DVR is a series connected compensator with the capability of regulating the voltage from problems such as
sags, swells and harmonics. So to enhance the quality of power on distribution side we prefer to use DVR and
ensure that, DVR will protect the loads from tripping and avoids losses due to it.
DVR is one of the most effective and efficient custom power devices [2,3,4] with the advantages of fast
response, lower cost and smaller in size. It consists of a control unit to calculate the amount of voltage to be
added or removed in order to maintain the constant voltage. The controlling of DVR is done by a Proportional
Integral (PI) Controller and a PWM Generator. PI controller is a type of feedback controller which operates the
system to be controlled with a weighted sum of error. It generates the desired signal for the PWM generator to
trigger the PWM inverter. The Phase lock loop (PLL) and dq0 transformation are also the basic components of
DVR. Synchronous reference frame theory is used for the conversion of voltages from rotating vectors to
stationary frame.
This paper, investigates the performance of DVR in improving the quality of power under three phase faults
at two different time periods. Also, the reduced rating VSC is used to improve the power quality with improved
efficiency compared to the reference base paper [1].
II. OPERATION OF DVR
The schematic representation of DVR connected system is as shown in Fig. 1(a). The supply voltage Vs
will be varying and distorted due to the disturbances in the system. So the injected voltage Vinj is inserted to the
system by DVR to maintain the load voltage Vload constant in its magnitude and undistorted. The phasor diagram
of different voltage injection scheme for DVR is shown in Fig. 1(b). VL(pre-sag) is the voltage across the critical
load prior to the voltage sag condition. During the voltage sag, the voltage is reduced to Vs with a phase lag
angle of θ. Now, the DVR injects a voltage such that the load voltage magnitude is maintained at the pre-sag
condition. According to the phase angle of the load voltage, the injection of voltages can be in four ways [9].

Dvr Based Power Quality…
Fig. 1(a): schematic representation of DVR connected system. (b) Phasor diagram of different voltage injection
schemes of DVR.
Vinj1 represents the voltage injected in phase with the supply voltage. With the injection of Vinj2, the load
magnitude remains same but it leads Vsby a small angle. In Vinj3, the load voltage retains the same phases that of
the pre sag condition, which may be an optimum angle considering the energy source[10]. Vinj4 is the condition
where the injected voltage is in quadrature with the current and this case is suitable for a capacitor supported
DVR as this injection involves no active power[7]. However, a minimum possible rating of the converter is
achieved by Vinj1. The DVR is operated in this scheme with a battery energy storage system(BESS) and also a
capacitor supported DVR is operated.
Fig.2 (a) Schematic diagram of a DVR connected system with BESS.
Fig.2 (a) shows the schematic of a three phase DVR connected to restore the voltage of a three-phase critical
load. A three phase supply is connected to a critical load with the help of a three phase series injection
transformer. The equivalent voltages of the supply for phase A is VMa and is connected to the point of common
coupling(PCC), VSathrough short-circuit impedance ZSa. The DVR injects the voltage VCain phase A, such that
the load voltage VLa is maintained at rated magnitude and is undistorted. A three-phase DVR is connected to the
line in series using three single phase transformers Tr to inject a voltage. Lr and Crrepresent the filter
components. These are used to filter the ripples in the injected voltage. A three leg VSC with 6 insulated gate
bipolar transistors (IGBTs) is used as a DVR, along which a BESS is connected to its DC bus.
III. CONTROL OF DVR SYSTEM
The compensation of power quality problems like voltage sag using a DVR can be done by injecting the
real power or reactive power [7]. When the injected voltage is in quadrature with the current at the fundamental
frequency, the compensation is made by injecting reactive power; the DVR is with a self-supported dc bus[3,4].
And when the injected voltage is in phase with the current, the DVR injects real power. Hence a battery is
required to store the backup power at the DC bus of the VSC. The limitations like voltage injection capability
(transformer and converter ratings) and the optimization of energy storage element size are to be considered by
the control techniques used for DVR control.
A. Control of DVR with BESS
Fig.2. (b) control block of the DVR which uses SRF method of control for BESS

Fig. 2 (b) represents the control block of the DVR with the SRF theory used for the estimation of reference
signal. The voltages at the PCC(VS) and at the load terminal (VL) are sensed and then the IGBTs’ gate signals
are drawn. The reference load voltage VL
*
is taken using the derived unit vector [13]. Load voltages
(VLa,VLb,VLc) are converted to the rotating reference frame using abc-dqo conversion using Park’s
transformation with unit vectors(sinθ, cosθ) and are derived using a phase locked loop as
(1)
Similarly, reference load voltages and voltages at PCC VS are also converted to the rotating
reference frame. Then, the DVR voltages are obtained in the rotating reference frame as
(2)
(3)
The reference DVR voltages are obtained in the rotating reference frame as
(4)
(5)
The error between the reference and actual DVR voltages in the rotating reference frame is regulated using two
proportional-integral (PI) controllers.
By taking from equation (4), from equation (5) and as zero, we obtain reference DVR voltages in
the abc frame from a reverse Park’s transformation as follows:
Reference DVR voltages and actual DVR voltages are used in a pulse
width modulated (PWM) controller to generate gating pulses to a VSC of the DVR. The PWM controller is
operated with a switching frequency of 10kHz.
B. Control of self-supported DVR (capacitor supported DVR)
Fig.3. (a) Schematic diagram of a capacitor supported DVR connected system. (b) Control block of the DVR
which uses SRF method of control for capacitor connected DVR

The schematic of a capacitor supported DVR connected to three phase critical load is as shown in fig.
3(a) and its control block using SRF theory is shown in fig.3(b). The voltages at the PCC (VS) are converted to
the rotating reference frame using Park’s transformation(abc-dqo conversion). The harmonics in the voltage are
eliminated using the low pass filters (LPFs) [13]. The components of voltages in the d-axis and q-axis
respectively are
Vd= Vddc+ Vdac (7)
Vq= Vqdc+ Vqac (8)
The compensating strategy for compensation of voltage quality problems considers that the load terminal
voltage should be of rated magnitude and undistorted.
In order to maintain the dc bus voltage of the self-supported capacitor, a PI controller is used at the dc bus
voltage of the DVR and the output is considered as a voltage for meeting its losses
(9)
Where is the error between the reference and sensed dc voltage at the
sampling instant. and are the proportional and the integral gains of the dc bus voltage PI controller.
The reference d-axis load voltage is therefore expressed as follows:
(10)
The amplitude of load terminal voltage VL is controlled to its reference voltage using another PI controller.
The output of the PI controller is considered as the reactive component of voltage for voltage regulation of
the load terminal voltage. The amplitude of load voltage VL at the PCC is calculated from the ac voltages
as (11)
Then, a PI controller is used to regulate this to a reference value as
(12)
Where denotes the error between the reference and the actual load terminal voltage
amplitudes at the nth
sampling instant. and are the proportional and integral gains of the dc bus voltage PI
controller.
The reference load quadrature axis voltage is expressed as follows:
(13)
Reference load voltages in the abc frame are obtained from a reverse Park’s ransformation as in
(6). The error between the sensed load voltages and reference load voltage is used over a
controller to generate gating pulses to the VSC of the DVR.
IV. MODELING AND SIMULATION
The DVR connected system consisting of a three-phase supply,three phase critical loads [11], and the
series injection transformers shown in fig.2 (a) i.e., with BESS is modelled in MATLAB/Simulink
environment.The sim power system toolbox is used in modelling this system. The simulated model of fig.2 (a)
without controls for DVR is shown in fig.4 (a) and the resulting voltages without compensations for voltage sag
is shown in fig. 4(b) and that for swell is shown in fig. 4(c)and the outputs with controls using SRF theory for
themcompensating the sags and swells respectively is shown in figs 5 (a)and 5 (b) respectively. Similarly, the
capacitor supported DVR system in fig. 3(a) is simulated as per controls shown in fig. 3(b) and its results for sag
and swell compensationare respectively as shown in fig. 6(a) and fig. 6(b). An equivalent load considered is
10kVA, 0.8-pf lag linear load. The parameters of the considered system for the simulation study are given in the
appendix. The reference DVR voltages are derived from sensed PCC voltages and load voltages
A PWM controller is used over the reference and sensed DVR voltages to generate the gating
signals for the IGBTs of the VSC of the DVR.
Fig. 4(a) simulated model of DVR connected system with BESS without controls.

fig. 4(b) voltage sags observed at PCC voltage and load voltage
fig. 4(c) voltage swells observed at PCC voltage and load voltage
fig. 5(a) voltage sags compensated at PCC voltage andload voltage
fig. 5(b) voltage swells compensated at PCC voltage and load voltage

fig. 6(a) voltage sags compensated at PCC voltage and load voltage
fig. 6(b) voltage swells compensated at PCC voltage and load voltage
V. PERFORMANCE OF THE DVR SYSTEM
The performance of the DVR is demonstrated for different supply voltage disturbances such as voltage
sag and swell in [1.] Fig. 5(a) to 6(c) shows the transient performance of the system under voltage sag and swell
conditions as explained in modelling part. At 0.2s-0.25s, a sag in supply voltage is created for six cycles, and at
0.6s-0.65s, a swell in the supply voltages is created for 1cycles. It is observed that the load voltage is regulated
to maintain constant amplitude under both sag and swell conditions. PCC voltages load voltages DVR
voltages amplitude of load voltage VL and PCC voltage Vs, source currents iS, reference load voltages and dc
bus voltage are also shown in output results obtained in fig.5 and fig.6. The total harmonic distortions (THDs) of
the voltage at the load voltage for BESS and capacitor supported systems are shown in fig 7(a) and (b)
respectively.
Fig 7(a).input signal and its THD analysis for DVR connected system with BESS without control
Fig 7(b).input signal and its THD analysis for DVR connected system with BESS

Fig 7(c).input signal and its THD analysis for capacitor supported DVR connected system
It is observed that the load voltage THD is reduced to a level of 0.02% from the PCC voltage of 18.12%.in
BESS system. and is reduced to a level of 0.09% in capacitor supported DVR system. so we can notice the
improved efficiency compared with the performance of system in[1].
The magnitudes of the voltage injected by the DVR for mitigating the same kinds of sag in the supply with
different angles of injection are observedin[1].The injected voltage, series current and kVA ratings of the DVR
for the four injection schemes are given in table-1. In scheme-1 in table-1, the in-phase injected voltage is Vinj1
in the phasor diagram in fig.1 and only this scheme is executed with the improved efficiency in this paper. In
scheme-2, a DVR voltage is injection at a small angle of 30degree, and in scheme-3, the DVR voltage is
injected at an angle of 45deg. The injection of voltage in quadrature with the line current is in scheme-4. The
required rating of compensation of the same using scheme-1 is much less than that of scheme-4. Hence, only
scheme-1 is used in improving the efficiency.
TABLE-1: SELECTING DVR RATINGS FOR MITIGATING THE SAG
Scheme
-1
Scheme
-2
Scheme
-3
Scheme
-4
Phase voltage 90 100 121 135
Phase current 13 13 13 13
VA per phase 1170 1300 1573 1755
kVA (% of
load)
37.5% 41.67% 50.42% 56.25%
VI. CONCLUSION
The operation of a DVR has been demonstrated with a new control technique using various methods
for voltage injection scheme-1 of table-1. A comparison of the performance of the DVR with scheme-1for
existing one in [1] and one performed with areduced- rating VSC for BESS and capacitor-supported DVR
shows that the efficiency is higher in this paper compared to that in [1]. The reference load voltage has been
estimated using the method of unit vectors and the control of DVR has been achieved as per the controls in [1],
with improved efficiency for both BESS and capacitor supported DVR. The same SRF theory has been used for
estimating the reference DVR voltages as in[1]. It is concluded that the voltage injection in-phase with the PCC
voltage results in minimum rating of DVR but at the cost of an energy source at its dc bus along with better
efficiency in mitigating the power quality problems.
APPENDIX
AC line voltage: 415V, 50Hz
Line impedance: Ls= 3.0mH, Rs= 0.01Ω
Linear loads: 10-kVA 0.80-pf lag
Ripple filter: Cf = 10μF, Rf= 4.8Ω
Series transformers: three phase transformer of rating 10kVA, 200V/300V
DVR with BESS:
Dc voltage of DVR: 200V
Ac inductor: 0.005H
Gains of the d-axis PI controller:kp=0.63 kd=0.0504

Gains of the q-axis PI controller:kp=0.63 kd=0.0504
PWM switching frequency:10kHz
DVR with capacitor:
Dc voltage of DVR: 200V
Ac inductor: 0.005H
Gains of the d-axis PI controller:kp=0.63 kd=0.0504
Gains of the q-axis PI controller:kp=0.63 kd=0.0504
PWM switching frequency:10kHz
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