Bc4101312317

Salava V Satyanarayana et al Int. Journal of Engineering Research and Applications www.ijera.com
ISSN : 2248-9622, Vol. 4, Issue 1( Version 1), January 2014, pp.312-317

RESEARCH ARTICLE

OPEN ACCESS

Emergency Control Using Multifunctional DVR for Distribution
Systems
Salava V Satyanarayana1, A Tejasri M.Tech2
M.TECH Scholar, Dept of EEE, GIET College Rajahmundry, AP
Assistant Professor, Dept. of EEE, GIET College Rajahmundry, AP India

Abstract
This paper discusses a new multifunctional dynamic voltage restorer (DVR) and closed loop controller using the
Posicast and P+Resonant controllers. The closed loop controller using the Posicast and P+Resonant controllers
is proposed in order to improve the transient response and eliminate the steady-state error in DVR response,
respectively. The proposed algorithm is applied to some disturbances in load voltage caused by induction
motors starting, and a three-phase short circuit fault. Also, the capability of the proposed DVR has been tested to
limit the downstream fault current. The current limitation will restore the point of common coupling (PCC) (the
bus to which all feeders under study are connected) voltage and protect the DVR itself. The innovation here is
that the DVR acts as virtual impedance with the main aim of protecting the PCC voltage during downstream
fault without any problem in real power injection into the DVR. Results of the simulation studies in the
MATLAB/SIMULINK software environment indicate that the proposed control scheme with faults and
induction motor under multifunctional DVR.

I.

Introduction

Electric power quality is capacity of an
electric power system to supply electric energy of a
load in an acceptable quality. Power distribution
system should provide with an uninterrupted flow of
energy at smooth sinusoidal voltage at the contracted
magnitude level and frequency to their customers.
Power systems especially distribution systems, have
numerous non linear loads, which significantly affect
the quality of power. Apart from non linear loads like
capacitor switching, motor starting and unusual faults
could also inflict power quality problems. Many
problems can result from poor power quality (PQ),
especially in today’s complex power system, such as
the false operation of modern control systems.
Voltage sag is an important PQ problem because of
sensitive loads growth. Worldwide experience has
showed that short circuit faults are the main origin of
voltage sag; therefore there is a loss of voltage quality
[1]-[2].
Voltage sag is defined as a sudden reduction
in supply voltage to between 90% and 10% of the
nominal value, followed by a recovery after a short
interval (the standard duration of sag is between 10
milliseconds and 1 minute). The most common
compensator for voltage sag is the dynamic voltage
restorer (DVR). The basic operation of the DVR is
based on injection of a compensation voltage with
required magnitude, phase angle and frequency in
series with the sensitive electric distribution feeder.
Previous works have been done on different
aspects of DVR performance, and different control
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strategies have been found. These methods mostly
depend on the purpose of using DVR. In some
methods, the main purpose is to detect and
compensate for the voltage sag with minimum DVR
active power injection [4], [5]. Also, the in-phase
compensation method can be used for sag and swell
mitigation [6]. The multiline DVR can be used for
eliminating the battery in the DVR structure and
controlling more than one line [7], [14]. Moreover,
research has been made on using the DVR in medium
level voltage [8]. Harmonic mitigation [9] and control
of DVR under frequency variation [10] are also in the
area of research. The closed-loop control with load
voltage and current feedback is introduced as a simple
method to control the DVR in [15]. Also, Posicast and
P+Resonant controllers can be used to improve the
transient response and eliminate the steady-state error
in DVR. The Posicast controller is a kind of step
function with two parts and is used to improve the
damping of the transient oscillations initiated at the
start instant from the voltage sag. The P+Resonant
controller consists of a proportional function plus a
resonant function and it eliminates the steady-state
voltage tracking error [6]. The state feed forward and
feedback methods [7], symmetrical components
estimation [8], robust control [10], and wavelet
transform [12] have also been proposed as different
methods of controlling the DVR.
The basis of the proposed control strategy in
this paper is that when the fault current does not pass
through the DVR, an outer feedback loop of the load
voltage with an inner feedback loop of the filter
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capacitor current will be used. Also, a feed forward
loop will be used to improve the dynamic response of
the load voltage. Moreover, to improve the transient
response, the Posicast controller and to eliminate the
steady-state error, the P+Resonant controller are used.
But in case the fault current passes through the DVR,
using the flux control algorithm [11], the series
voltage is injected in the opposite direction and,
therefore, the DVR acts like series variable
impedance.

II.

disturbance compensation process. In particular, if
capacitors are used as energy storage, the DC-link
voltage will decrease with the dwindling storage
energy during compensation.

Dynamic Voltage Restorer

Dynamic voltage restorer was originally
proposed to compensate for voltage disturbances on
distribution systems. A typical DVR scheme is shown
in Fig. 1. The restoration is based on injecting AC
voltages in series with the incoming three-phase
network, the purpose of which is to improve voltage
quality by adjustment in voltage magnitude, waveshape, and phase shift. As shown in Fig. 1, the DVR
essentially consists of a series-connected injection
transformer Ti, a voltage-source inverter (VSI), a
harmonic filter, and an energy storage device [4],
[13]. Meanwhile, a parallel switch is used to bypass
and protect the DVR, when a downstream fault is
detected [14], [15]. As shown in Fig. 1, the line-side
harmonic filter topology [11] consists of the leakage
inductance of the injection transformer and the filter
capacitor. Meanwhile, denotes the dc-link capacitor.
The series injected voltage of the DVR,
Vdvr, is synthesized by modulating pulse widths of
the inverter-bridge switches. The injection of an
appropriate Vdvr in the face of an up-stream voltage
disturbance requires a certain amount of real and
reactive power supply from the DVR. The reactive
power requirement is generated by the inverter.

Fig. 2; Vector Diagram of Voltage Injection Method
The
corresponding
phasor
diagram
describing the electrical conditions during voltage sag
is depicted, where only the affected phase is shown
for clarity. Let the voltage quantities Il, φ, δ and α
represent the load current, load power factor angle,
supply voltage phase angle and load voltage advance
angle respectively. Although there is a phase
advancement of α in the load voltage with respect to
the pre-sag voltage in Fig. 2, only in-phase
compensation where the injected voltage is in phase
with the supply voltage (α = δ) is considered.
By mounting the battery in DC link of the
centralized inverter, the DC/DC converter for battery
can be saved as shown in Figure 1(b) [1], [2].
Different from the centralized inverters in Figure 1(a)
and (b), the separate grid inverters are used by the two
PV systems in Figure 1(c). Thus, the power rating of
each inverter becomes smaller, and the inverter is
intended to be modular design as well as mass
production.

III.

Fig. 1: schematic diagram of the DVR with line side
harmonic filter

Proposed Multifunctional DVR

In addition to the aforementioned
capabilities of DVR, it can be used in the mediumvoltage level as in Fig. 2 to protect a group of
consumers when the cause of disturbance is in the
downstream of the DVR’s feeder and the large fault
current passes through the DVR itself. In this case,
the equipment can limit the fault current and protect
the loads in parallel feeders until the breaker works
and disconnects the faulted feeder.

Widely used in present DVR control is the
so-called in phase voltage injection technique where
the load voltage V2 is assumed to be in-phase with
the pre-sag voltage. As the DVR is required to inject
active power into the distribution line during the
period of compensation, the capacity of the energy
storage unit can become a limiting factor in the
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Fig. 2: A multifunctional DVR connected in a
medium-voltage level power system.
The large fault current will cause the PCC
voltage to drop and the loads on the other feeders
connected to this bus will be affected. Furthermore, if
not controlled properly, the DVR might also
contribute to this PCC voltage sag in the process of
compensating the missing voltage, hence further
worsening the fault situation [11].
To limit the fault current, a flux-charge
model has been proposed and used to make DVR act
like a pure virtual inductance which does not take any
real power from the external system and, therefore,
protects the dc-link capacitor and battery as shown in
Fig. 1 [11]. But in this model, the value of the virtual
inductance of DVR is a fixed one and the reference of
the control loop is the flux of the injection
transformer winding, and the PCC voltage is not
mentioned in the control loop. In this paper, the PCC
voltage is used as the main reference signal and the
DVR acts like variable impedance. For this reason,
the absorption of real power is harmful for the battery
and dc-link capacitor. To solve this problem,
impedance including a resistance and an inductance
will be connected in parallel with the dc-link
capacitor. This capacitor will be separated from the
circuit, and the battery will be connected in series
with a diode just when the downstream fault occurs so
that the power does not enter the battery and the dclink capacitor. It should be noted here that the
inductance is used mainly to prevent large oscillations
in the current. The active power mentioned is,
therefore, absorbed by the impedance.
3.1 Proposed control scheme for using Flux charge
model
In this part, an algorithm is proposed for the
DVR to restore the PCC voltage, limit the fault
current, and, therefore, protect the DVR components.
The flux-charge model here is used in a way so that
the DVR acts as a virtual inductance with a variable
value in series with the distribution feeder. To do this,
the DVR must be controlled in a way to inject a
proper voltage having the opposite polarity with
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respect to usual cases. It should be noted that over
current tripping is not possible in this case, unless
additional communication between the DVR and the
downstream side over current circuit breaker (CB) is
available. If it is necessary operate the over current
CB at PCC, communication between the DVR and the
PCC breaker might have to be made and this can be
easily done by sending a signal to the breaker when
the DVR is in the fault-current limiting mode as the
DVR is just located after PCC [11]. The proposed
DVR control method is illustrated in Fig. 8. It should
also be noted that the reference flux (𝜙 𝑟𝑒𝑓 ) is derived
by integration of the subtraction of the PCC reference
voltage (𝑉 ∗ ) and the DVR load-side voltage. This
𝑃𝐶𝐶
control strategy, the control variable used for the
outer flux model is the inverter-filtered terminal flux
defined as:
(1)
Where 𝑉 𝑜𝑑𝑣𝑟 is the filter capacitor voltage of the DVR
(at the DVR power converter side of the injection
transformer). The flux error is then fed to the flux
regulator, which is a P+Resonant controller, with a
transfer function given in (1). On the other hand, it
can be shown that a single flux-model would not
damp out the resonant peak of the LC filter connected
to the output of the inverter.

Fig. 3: Proposed method.
To stabilize the system, an inner charge
model is therefore considered. In this loop, the filter
inductor charge, which is derived by integration of its
current, tracks the reference charge output of the flux
regulator. The calculated charge error is then fed to
the charge regulator with the transfer function

This is actually a practical form of the
derivative controller. In this transfer function, the
regulator gain is limited to N at high frequencies to
prevent noise amplification. The derivative term in
𝑆 + 𝑆 neutralizes the effects of voltage and
1
𝑁
current integrations at the inputs of the flux-charge
model, resulting in the proposed algorithm having the
same regulation performance as the multiloop
voltage-current feedback control, with the only
difference being the presence of an additional low–
pass filter in the flux control loop in the form of
1 + 𝑆 . The bandwidth of this low–pass filter is
𝑆
𝑁
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tuned (through varying N) with consideration for
measurement noise attenuation, DVR LC-filter
transient resonance attenuation, and system stability
margins.

4.1 Fault Current Limiting

Fig. 4: Multi-loop control using the Posicast and
P+Resonant controller
Theoretically, the resonant controller
compensates by introducing an infinite gain at the
resonant frequency of 50 Hz to force the steady-state
voltage error to zero. The ideal resonant controller,
however, acts like a network with an infinite quality
factor, which is not realizable in practice. A more
practical (nonideal) compensator is therefore used
here, and is expressed as

Where 𝑤 𝑐𝑢𝑡 is the compensator cutoff frequency
which is 1 rad/s in this application.

Fig. 5: Open loop control using the Posicast controller
To improve the damping, as shown in Fig. 4,
the Posicast controller can be used just before
transferring the signal to the PWM inverter of the
DVR. The transfer function of the controller can be
described as follows:

Where 𝛿 and 𝑇 𝑑 are the step response overshoot and
the period of damped response signal, respectively. It
should be noted that the Posicast controller has
limited high-frequency gain; hence, low sensitivity to
noise.

IV.

Fig. 6: SIMULINK Model of Proposed a
Multifunctional DVR under Three Phase Fault
condition.
MATLAB/simulink model of proposed
multifunctional DVR under three phase fault in
figure.6; The last simulation is run for a symmetrical
downstream fault, and the capability of the DVR to
reduce the fault current and restore the PCC voltage is
tested.
For the simulation with DVR compensation,
the three-phase fault is applied at t = 205 ms and then
removed after 0.1 s. Also, a breaker will remove the
faulted bus from the entire system at t = 300 ms. Fig.
13 shows the DVR operation during the fault. As can
be seen, the rms load bus voltage reaches zero during
the fault, and as the enlarged figure shows, in about
half a cycle, the
DVR has succeeded in restoring the PCC
voltage wave shape to the normal condition. It should
be noted that the amount and shape of the oscillations
depend on the time of applying the fault. As Fig. 13
shows, at this time, the voltage value of phase B is
nearly zero; this

MATLAB/Simulink modelling and
results

In this section, the proposed DVR topology
and control algorithm will be used for emergency
control during the voltage sag. The three-phase short
circuit and the start of a three-phase large induction
motor will be considered as the cause of distortion in
the simulations.

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Fig. 8: SIMULINK model of proposed a
multifunctional DVR with Induction Motor.
During this period, the PCC bus is under
voltage sag. From t = 1.4 s, as the speed approaches
nominal, the voltage also approaches the normal
condition. However, during all of these events, the
DVR keeps the load bus voltage at the normal
condition. The DVR has succeeded in restoring the
load voltage in half a cycle from the instant of the
motor starting.

Fig. 7: Fault current limiting by DVR. (a) Threephase PCC voltages. (b) Three-phase load voltages.
(c) RMS voltages of the PCC and load. (d) Threephase currents.
4.2 Induction Motor
The large motor starting current will cause
the PCC voltage to drop. The simulation results in the
case of using the DVR are shown in Fig. 9. In this
simulation, the motor is started at t = 405 ms. As can
be seen in Fig. 11, at this time, the PCC rms voltage
drops to about 0.8 p.u. The motor speed reaches the
nominal value in about 1 s. MATLAB/simlink model
of proposed multifunctional DVR with induction
motor in figure8;

Fig. 9: Starting of an induction motor and the DVR
compensation. (a) Three phase PCC voltages. (b)
Three-phase load voltages. (c) RMS voltages of PCC
and load.

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V.

CONCLUSION

In this paper, a multifunctional DVR is
proposed, and a closed-loop control system is used for
its control to improve the damping of the DVR
response. Also, for further improving the transient
response and eliminating the steady-state error, the
Posicast and P+Resonant controllers are used. As the
second function of this DVR, using the flux-charge
model, the equipment is controlled so that it limits the
downstream fault currents and protects the PCC
voltage during these faults by acting as variable
impedance. The problem of absorbed active power is
solved by entering impedance just at the start of this
kind of fault in parallel with the dc-link capacitor and
the battery being connected in series with a diode so
that the power does not enter it. The simulation
results verify the effectiveness and capability of the
proposed DVR in compensating for the voltage sags
caused by short circuits and the large induction motor
starting and limiting the downstream fault currents
and protecting the PCC voltage.

[8]

[9]

[10]

[11]

[12]

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