30-07-2013, 03:41 PM
A New Approach to Multifunctional Dynamic Voltage Restorer Implementation for Emergency Control in Distribution Systems
A New Approach to Multifunctional .pdf (Size: 1.02 MB / Downloads: 59)
Abstract
The dynamic voltage restorer (DVR) is one of the
modern devices used in distribution systems to protect consumers
against sudden changes in voltage amplitude. In this paper, emer-
gency control in distribution systems is discussed by using the
proposed multifunctional DVR control strategy. Also, the multi-
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 a virtual impedance with
the main aim of protecting the PCC voltage during downstream
fault without any problem in real power injection into the DVR.
Simulation results show the capability of the DVR to control the
emergency conditions of the distribution systems.
INTRODUCTION
OLTAGE sag and voltage swell are two of the most impor-
tant power-quality (PQ) problems that encompass almost
80% of the distribution system PQ problems [1]. According to
the IEEE 1959–1995 standard, voltage sag is the decrease of 0.1
to 0.9 p.u. in the rms voltage level at system frequency and with
the duration of half a cycle to 1 min [2]. Short circuits, starting
large motors, sudden changes of load, and energization of trans-
formers are the main causes of voltage sags [3].
According to the definition and nature of voltage sag, it can
be found that this is a transient phenomenon whose causes are
classified as low- or medium-frequency transient events [2]. In
recent years, considering the use of sensitive devices in modern
industries, different methods of compensation of voltage sags
have been used. One of these methods is using the DVR to im-
prove the PQ and compensate the load voltage [6]–[13].
DVR COMPONENTS AND ITS BASIC
OPERATIONAL PRINCIPLE
DVR Components
A typical DVR-connected distribution system is shown in
Fig. 1, where the DVR consists of essentially a series-connected
injection transformer, a voltage-source inverter, an inverter
output filter, and an energy storage device that is connected to
the dc link. Before injecting the inverter output to the system, it
must be filtered so that harmonics due to switching function in
the inverter are eliminated. It should be noted that when using
the DVR in real situations, the injection transformer will be
connected in parallel with a bypass switch (Fig. 1). When there
is no disturbances in voltage, the injection transformer (hence,
the DVR) will be short circuited by this switch to minimize
losses and maximize cost effectiveness. Also, this switch can
be in the form of two parallel thyristors, as they have high on
and off speed [21]. A financial assessment of voltage sag events
and use of flexible ac transmission systems (FACTS) devices,
such as DVR, to mitigate them is provided in [22]. It is obvious
that the flexibility of the DVR output depends on the switching
accuracy of the pulsewidth modulation (PWM) scheme and
the control method. The PWM generates sinusoidal signals by
comparing a sinusoidal wave with a sawtooth wave and sending
appropriate signals to the inverter switches. A further detailed
description about this scheme can be found in [23].
Under Study Test System
In this paper, the IEEE standard 13-bus balanced industrial
system will be used as the test system. The one-line diagram of
this system is shown in Fig. 9.
The test system is modeled in PSCAD/EMTDC software.
Control methods of Figs. 5 and 8 were applied to control the
DVR, and the voltage, current, flux, and charge errors were
included as the figures show. Also, the DVR was modeled
by its components (instead of its transfer functions) in the
PSCAD/EMTDC software to make more real simulation re-
sults. A 12-pulse inverter was used so that each phase could
be controlled separately. Detailed specifications of the DVR
components are provided in the Appendix.
Starting the Induction Motor
A large induction motor is started on bus “03:MILL-1.”
The motor specifications are provided in Appendix C. The
large motor starting current will cause the PCC voltage (bus
“03:MILL-1” voltage) to drop. The simulation results in the
case of using the DVR are shown in Fig. 11. In this simulation,
the motor is started at
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.
During this period, the PCC bus is under voltage sag. From
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 (bus “05:FDR F”
voltage) at the normal condition. Also, as can be seen in the
enlarged version of Fig. 11, the DVR has succeeded in restoring
the load voltage in half a cycle from the instant of the motor
starting.
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 a variable impedance. The problem of absorbed active
power is solved by entering an 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.