Dynamic Voltage Restorer (DVR)

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.