TPterra's TechBlog: Independent Opinion Articles on Power Technology Subjects
Application of DVRs in Networks Subject to Reactive DeficienciesMarch 2007 Pterra ConsultingDVRs, 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. About DVRsThe 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.
The capacity of DVRs are determined primarily by the inverter 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 using Integrated 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. Characteristic ResponseWhen 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, the slower 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.
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
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 WarningWhen implementing DVRs, it is important to take into the account the nature of the load whose 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. References:
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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.
Studies
(by other researchers) have found that DVRs have successfully
provided protection against voltage sags to as low as 0.5 p.u. for
durations of up to 0.1 seconds. However, there is considerable
variation 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.
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
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