DVRs, or dynamic voltage restorers, are a relatively new static var device that has seen applications in a variety of distribution and subtransmission applications. DVRs are series compensation devices that protect electric load against voltage sags, swells, unbalance and distortion. Though these devices may provide good solutions for customers subject to poor power quality, we caution regarding their application in systems that are subject to prolonged reactive power deficiencies (resulting in low voltage conditions) and in systems that are susceptible to voltage collapse.
The reason for the caution is that in many instances the main protection of networks against voltage collapse is the natural response of load to decrease demand when voltage drop. The implementation of DVRs would act to maintain demand even when incipient voltage conditions are present thus reducing inherent ability to halt a collapse and increasing the risk of cascading outage and blackout.
The first DVR was installed in North America in 1996, a 12.47 kV system located in Anderson, South Carolina. Since then, DVRs of capacities up to 50 MVA have seen applications to critical loads in food processing, semiconductor and utility supply. Cost and installation constraints limit these to where there is clear need for constant voltage supply.
DVRs are power electronic controllers that use voltage source converters (VSC). They inject independent phase voltages to the distribution feeder to regulate voltage seen by the critical load. In various publications, the voltage injection have been termed, “the missing volts.” A typical DVRs design is shown at right. The source of the injected volts is the commutation process for reactive power demand and an energy source for real power demand. The energy source may vary according to the design and manufacturer of the DVR; some examples of energy sources applied are DC capacitors, batteries and drawn from the line.
The capacity of DVRs are determined primarily by theinverter current capability. Bypass protection would trigger once the current capability is exceeded.
During normal conditions, the DVR operates in stand-by mode. Since the device is connected in series, there are conduction losses, which can be minimized by usingIntegrated Gate-Commutated Thyristor (IGCT) technology in the inverters.
Note that there is a similarity in the technical approach to DVRs to that of providing low voltage ride-through capability in wind turbine generators. The dynamic response characteristics, especially for line supplied DVRs are thus similar to LVRT-mitigated turbines.
When a fault occurs on a distribution feeder, the voltage sags in neighboring feeders as well in the portion of the feeder itself which remains supplied through a power source. After the fault is cleared, the voltage recovers in a manner influenced by the number of induction motors connected to the feeder. In general, the more motors there are, theslower and more oscillatory is the voltage recovery. This is true whether the fault is the more common single phase fault to any of the variations of fault types, including two-phase, three-phase and open phase faults.
Studies (by other researchers) have found that DVRs havesuccessfully provided protection against voltage sags to as low as 0.5 p.u. for durations of up to 0.1 seconds. However, there is considerablevariation in conditions when response is simulated. For example, in tests where there is no phase-shift component to the voltage deviation, the DVR performs well over large dips with prolonged durations. In tests where significant phase-shift is present in the study or assumed system, the size and type of the energy source has a significant impact. DC capacitor sources tend not to hold voltage well under phase-shifts. Battery sources are a bit better. Line-connected sources through a rectifier provided the best phase-shifted voltage deviation response in terms of maintaining voltage at the load.
From the transmission viewpoint, a DVR would extend the voltage range when load behaves as constant power load. The combination of on-load tap-changing distribution transformers, voltage-switched capacitor banks and direct-connected DVRs lead to more current drawn from the transmission system during periods of reactive deficiency and low voltages.
A Loss of Relief
As noted in earlier Techblogs, during periods of extremely high demand such as during summer heat storms, there is a natural relief system that tends to arrest a declining voltage in the distribution system as reactive power deficiency in the transmission system spreads. Devices such as the DVR tend to mask distribution load from the drop in voltage in the supply side. Hence, instead of a drop in demand as voltage drops, demand remains at the nominal level. This can hasten the onset of voltage collapse by removing the mechanism that helps systems reach a Voltage Ledge.
Direct-connected DVRs bring the added concern that as voltage drops and induction motors load require a phase-shifted supply, the DVRs increase demand from the transmission system. This type of response can accelerate a voltage collapse.
Just a Warning
When implementing DVRs, it is important to take into the account the nature of the loadwhose voltage supply is being secured, as well as the the transmission system which must tolerate the change in voltage-response of the load. In certain applications, it may be necessary to provide local fast reactive supply sources in order to secure the system, with the DVR added, from voltage collapse and cascading outages. A careful simulation study which includes the transmission system is highly recommended.
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