26-05-2012, 01:44 PM
Dynamic Voltage Restorer Based on Load Condition
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INTRODUCTION
Power quality means maintaining nearly sinusoidal voltage
at frequency 50/60 Hz. Voltage sag is broadly considered as a
short duration voltage variation and method of
characterization involves both magnitude and duration. The
duration of voltage sag varies between five cycles to a minute.
To prevent sensitive load from sag interruption in the source
side, a series connected custom power device is used. SSSC
(static synchronous series compensator) and DVR both are
presently used for series voltage sag compensation.
Operating principle and functioning of these devices differ
significantly as the SSSC injects a balance voltage in series
whereas the DVR compensates the unbalance in supply
voltage of different phases. The DVR supplies the active
power with help of DC energy storage and required reactive
power is generated internally without any means dc storage.
DVR can compensate voltage at both transmission and
distribution sides. Usually a DVR is installed on a critical
load feeder. During the normal operating condition (without
sag condition) DVR operates in a low loss standby mode [1].
During this condition the DVR is said to be in steady state.
When a disturbance occurs (abnormal condition) and supply
voltage deviates from nominal value, DVR supplies voltage
for compensation of sag and is said to be in transient state.
The DVR is connected in series between the load and the
supply voltage [2].
PARAMETERS OF DVR TEST SYSTEM
Electrical circuit model of DVR test system is shown in
Fig.3. System parameters are listed in Table 1. Voltage sag is
created at load terminals via a three-phase fault as shown in
Fig.3. Load voltage is sensed and passed through a
sequence analyzer. The magnitude is compared with
reference voltage (Vref ).
SIMULATION RESULTS
Detailed simulations are performed on the DVR test
system using MATLAB SIMULINK. System performance is
analyzed for compensating voltage sag with different DC
storage capacity so as to achieve rated voltage at a given load.
Various cases of different load condition are considered to
study the impact DC storage on sag compensation. These
various cases are discussed below:
CONCLUSION
Based on the analysis of test system, it is suggested that
load amount are major factors in estimating the DC storage
value. Investigations were carried out for various cases of
load at 11kV feeder. The effectiveness of a DVR system
mainly depends upon the rating of DC storage rating and the
loads. In the test system it is observed that after a particular
amount of load increases on 11 kV feeders, the voltage levels
at the load terminal decreases.