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.

The steps that lead up to this state may have the following progression:

  1. There is a reactive deficiency in the bulk power transmission system leading to a drop in voltages.
  2. Step-down transformers between the transmission and distribution level voltagesadjust to the drop in transmission voltage by automatically changing taps. Distribution loads continue to see “normal” voltage while the taps are not at their limit.
  3. The bulk system reactive deficiency deepens pushing transmission voltages lower. The step-down transformers reach their tap limits. This might occur at about 0.9 per unit (pu) voltage on the high side of the step-down transformers.
  4. As the reactive deficiency deepens even further, while the step-downs have already reached their tap limits, loads on the distribution network begin to see a dropping voltage. As noted at the start of this article, when there are more motors on a feeder, the voltage drop is much faster. Motors, primarily induction type air conditioning units, draw increasing reactive power as voltage drops.
  5. As voltage drops, some motors may stall, drawing even more reactive power.
  6. But some motors may tripout, which would reduce both the reactive and real power demand. In addition, non-motor feeder load would decrease reactive demand at voltages below nominal resulting in an increase in feeder voltage.
  7. An equilibrium state may be reached where there is just enough reactive supply for the combination of stalled, tripped and online motors, and non-motor loads. (See diagram below.)

Picture1The Voltage Ledge is characterized by an unusual resilience. An increase in reactive supply from say switching of capacitor banks or through some mechanism in the bulk system may lead some motors to restart, causing a drop in voltage, and thus leading other motors to trip out. If the condition stays for several minutes, some loads (both motor and non-motor) start to show a self-restoring quality — increasing their power demands to compensate for the low voltages.

This is life on the Ledge

Loads, such as computers and electronic devices, are exposed to a continuous shift in voltage leading to insulation and other failures. Transformers, bushings and insulation are exposed to high current that is peaky and filled with harmonics. Feeders may operate on the Ledge for minutes to hours. It would require a large inrush of reactive supply to start all the stalled motors, and the capability to absorb a sudden drop in reactive demand once these motors reach rated speed – something that a static var device might be able to handle, but clearly not a capacitor bank. A simpler method for escaping the Ledge is to re-start the feeder; i.e., switch it off for a few and then re-energize.

In later blogs, we’ll focus on the transmission system for conditions that may lead to the Ledge, assessment methods for voltage stability and countermeasures that can be used to prevent or minimize voltage stability. Please subscribe to this blog.

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