22-08-2012, 02:24 PM
A New Approach to Multifunctional Dynamic Voltage Restorer Implementation for Emergency Control in Distribution Systems
1Multifunctional Dynamic Voltage.pdf (Size: 1.02 MB / Downloads: 92)
INTRODUCTION
VOLTAGEsag and voltage swell are two of the most important
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 transformers
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 improve
the PQ and compensate the load voltage [6]–[13].
Previous works have been done on different aspects of
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].
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.
1Multifunctional Dynamic Voltage.pdf (Size: 1.02 MB / Downloads: 92)
INTRODUCTION
VOLTAGEsag and voltage swell are two of the most important
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 transformers
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 improve
the PQ and compensate the load voltage [6]–[13].
Previous works have been done on different aspects of
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].
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.