Voltage
variations are a nightmare for the industries like Semiconductor fabrication
plants, Paper mills, Food processing plants and, automotive assembly plants
etc. Fast variation in source voltage can affect the overall performance of the
industry and also cause damage to critical loads in these industries resulting
in great financial loss. The most common disturbances that occur in the source
voltage are voltage sags or swells that can be due to disturbances in the transmission
line or system, adjacent feeder faults and Fuse or breaker operation. Voltage
sags of even 10% lasting for 5-10 cycles can result in costly damage in
critical loads and heavy machineries in an industry. The voltage sags can be
symmetrical faults in which negative and zero sequence components are also
present.
Presence of
uncompensated nonlinear loads in the distribution system can cause harmonic
components in the supply voltages which are extremely dangerous. To mitigate
the problems caused by poor quality of power supply, series connected
compensators are used. These compensators are called Dynamic Voltage
Restorer (DVR), as their primary application is to compensate for voltage
sags and swells. Their configuration is similar to that of SSSC (Static
Synchronous Series Compensator). However, the control techniques are different.
A DVR is expected to respond fast (less than ¼ cycles) and thus employs PWM
converts (Pulse-width modulation converts) using IGBT (Insulated gate bipolar transistor) or IGCT (Integrated
Gate-Commutated Thyristor) device.
A DVR has to
supply energy to the load during the voltage sags. If a DVR has to supply
active power over longer periods, it is convenient to provide a shunt converter
that is connected to the DVR on the DC side. As a matter of fact one could
envisage a combination of DSTATCOM (Distribution Static Compensator) and DVR
connected on the DC side to compensate for both load and supply voltage
variations. The combined compensator is
termed as Unified Power Quality Conditioner (UPQC) which has similar to UPFC (Unified Power Flow Controller). However
the control objectives are different.
The DVR was first
installed on Duke Power System in USA in August 1996, with a rating of 2MVA
with 660 kJ of energy storage capacity, it is capable of compensating 50%
voltage sag for a period of 0.5 second (30 cycles). It was installed to protect
an automated yarn manufacturing and rug weaving facility. Since then, several
DVRs have been installed to protect microprocessor fabrication plants, paper
mills etc. typically, DVRs are made of modular design with a module rating of 2
MVA or 5 MVA. They have been installed in substations of voltage rating from 11
kV to 66 kV.
