Voltage Stability: The Voltage Ledge Seen Close-Up
Conventional wisdom says that the more motors connected to a feeder, the faster voltage will collapse when there is a reactive deficiency. This is true to the extent that voltages do drop faster, but the voltage may not fall all the way — so a voltage collapse does notoccur. A different and more common state is reached when the feeder is in a quasi-equilibrium state at a low per unit voltage. This is the Voltage Ledge.
Rising Out of the Trench: Insight from the Voltage Response Curve
The Voltage Response Curve (or for purposes of this article, the “VRC“) is what you get when you plot the voltage at or near a system node just before, during and immediately after an event involving a fault and subsequent clearing. The VRC is a record of the dynamic response of the system. It can be obtained from computer simulation, a well-isolated system test or a disturbance recorder. In much the same way that planning and operations personnel look at swing curves (Note 1) to determine if a synchronous system is angularly stable, much can be deduced about the underlying system and its voltage stability by studying the VRC.
Observability and Controllability in Highly Compensated Systems
Shunt compensation, in the form of capacitor banks and static var devices (SVD), are commonly used to provide voltage support in heavily loaded systems. Shunt devices offer a relatively cheap andeasy-to-implement solution to providing reactive power to load pockets or remote load areas of the grid. In concept, one can add a combination of switched and controlled shunt compensation toincrease import capacity up to the thermal limit of the transmission system. The savings from deferred investment in new transmission or congestion costs can justify the implementation of large shunt devices. (The largest existing SVDs are a +500/-150 MVAR behemoth in the Allegheny Power service territory in Pennsylvania, USA, and the Chamouchouane SVC (actually two SVCs at one site) rated +330/-330 MVAR in Quebec, Canada.)
The Voltage Ledge
Ricardo Austria, “The Voltage Ledge,” California ISO Monthly Technology Seminar, April 27, 2007, Folsom, CA
Application of DVRs in Networks Subject to Reactive Deficiencies
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.
Living on the Ledge – Operating Distribution Systems at Low Voltage
Conventional wisdom says that the more motors connected to a feeder, the faster voltage will collapse when there is a reactive deficiency. This is true to the extent that voltages do drop faster, but the voltage may not fall all the way — so a voltage collapse does not occur. A different, and perhaps more problematic state is reached, when the feeder is in equilibrium point at a low per unit voltage. This is operation on the Voltage Ledge.
The In-Between Voltage State
Among steady-state techniques, one of the most common methods for evaluating voltage stability is through the venerable P-V curve, also known as the “knee” curve. This method is used to identify the real power, or megawatt, margin to the point when the transmission grid is no longer able to support voltages, a state the industry has referred to as “voltage collapse.” Many analysts have long suspected that the P-V curve at best provided an approximate indicator, and that there was a lot more it was not capturing. Recent indications from near voltage collapse events seem to confirm that there is indeed a lot more, which may perhaps require a re-thinking of the whole process of using steady-state methods for voltage analysis.