01-05-2012, 10:33 AM
Design, Modeling and Simulation of Dynamic Voltage Restorer for Voltage Swell/Sag Mitigation
Design, Modeling and Simulation of Dynamic Voltage Restorer for Voltage SwellSag ....pdf (Size: 443.12 KB / Downloads: 54)
Abstract:
While early FACTS devices consisted mainly of thyristor-controlled/switched RLC/transformer
components, the newer generation is based on the self-commutated voltage-source power converter. Different
shunt FACTS devices, namely Static VAR compensator (SVC) and Static Synchronous Compensator
(STATCOM), Dynamic voltage restorer (DVR) in transmission line using the actual line model. This paper
describes the problem of voltage sags and swells and its severe impact on non linear loads or sensitive loads.
The dynamic voltage restorer (DVR) has become popular as a cost effective solution for the protection of
sensitive loads from voltage sags and swells. The design procedure for various components of DVR is
presented. Finally Matlab/Simulink based model is developed and simulation results are presented.
Keywords: SVC; D-Statcom; DVR; voltage dip, voltage swell ;
1. Introduction
The voltage generated by power stations has a sinusoidal waveform with a constant frequency. Any
disturbances to voltage waveform can result in problems related with the operation of electrical and electronic
devices. Users need constant sine wave shape, constant frequency and symmetrical voltage with a constant rms
value to continue the production. This increasing interest to improve overall efficiency and eliminate variations
in the industry have resulted more complex instruments that are sensitive to voltage disturbances. The typical
power quality disturbances are voltage sags, voltage swells, interruptions, phase shifts, harmonics and transients.
Among the disturbances, voltage sag is considered the most severe since the sensitive loads are very susceptible
to temporary changes in the voltage. Voltage sag (dip) is a short duration reduction in voltage magnitude
between 10% to 90% compared to nominal voltage from half a cycle to a few seconds [1-4].
Voltage swells are not as important as voltage sags because they are less common in distribution
systems. Voltage sag and swell can cause sensitive equipment (such as found in semiconductor or chemical
plants) to fail, or shutdown, as well as create a large current unbalance that could blow fuses or trip breakers.
These effects can be very expensive for the customer, ranging from minor quality variations to production
downtime and equipment damage [5-7]. There are many different methods to mitigate voltage sags and swells,
but the use of a custom Power device is considered to be the most efficient method. Switching off a large
inductive load or Energizing a large capacitor bank is a typical system event that causes swells [1].
2. Dynamic Voltage Restorer (DVR)
The main function of a DVR is the protection of sensitive loads from voltage sags/swells coming from the
network. Therefore as shown in Figure 1 the DVR is located on approach of sensitive loads. If a fault occurs on
other lines, DVR inserts series voltage VDVR and compesates load voltage to pre fault value. The momentary
amplitudes of the three injected phase voltages are controlled such as to eliminate any detrimental effects of a
B.Vijayalakshmi et al. / International Journal of Engineering Science and Technology (IJEST)
ISSN : 0975-5462 Vol. 3 No. 6 June 2011 5026
bus fault to the load voltage VL. This means that any differential voltages caused by transient disturbances in the
ac feeder will be compensated by an equivalent voltage generated by the converter and injected on the medium
voltage level through the booster transformer.
Fig.1 Basic Configuration of DVR
The DVR works independently of the type of fault or any event that happens in the system, provided
that the whole system remains connected to the supply grid, i.e. the line breaker does not trip. For most
practical cases, a more economical design can be achieved by only compensating the positive and negative
sequence components of the voltage disturbance seen at the input of the DVR. This option is Reasonable
because for a typical distribution bus configuration, the zero sequence part of a disturbance will not pass through
the step down transformer because of infinite impedance for this component. The DVR has two modes of
operation which are: standby mode and boost mode. In standby mode (VDVR=0), the booster transformer’s low
voltage winding is shorted through the converter. No switching of semiconductors occurs in this mode of
operation, because the individual onverter legs are triggered such as to establish a short-circuit path for the
transformer connection. Therefore, only the comparatively low conduction losses of the semiconductors in this
current loop contribute to the losses. The DVR will be most of the time in this mode. In boost mode (VDVR>0),
the DVR is injecting a compensation voltage through the booster transformer due to a detection of a supply
voltage disturbance [4].