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.

Stairway to the Voltage Ledge

The preliminary steps to the Ledge are consistently recorded in major voltage instability events, and has the following progression:

High system demand conditions associated with a summer heat wave require large amounts of reactive power being imported into a load pocket.
Over time, the transient reactive supply provided by nearby or local large synchronous generators is withdrawn by the same generators (by operator action or through over-excitation limiters).
More and more reactive power is then drawn from farther away from the load pocket, increasing reactive transmission losses and resulting in a gradual drop in transmission voltages.
Step-down transformers between the transmission and distribution level voltages adjust to the drop in transmission voltage by automatically hanging taps. Thus distribution loads see “normal” voltage even as the transmission system voltage is dropping. As a consequence the demand remains constant and the load pocket’s reactive deficiency continues.
As transmission voltages continue to drop, the step-down transformers reach their tap limits. This would occur at about 0.9 p.u. voltage on the high side of the step-down transformers. The high side may be at a subtransmission voltage level between 34.5 to 138 kV. From higher voltages, this would appear as higher per unit values, up to 1.0 per unit at 345 kV and higher.
When the step-downs reach their tap limits, the 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.
As voltage drops, some motors may stall, drawing even more reactive power. But some motors may tripout, which would tend to give a slight increase in voltage. In addition, non-motor feeder load would decrease reactive demand at voltages below nominal resulting in an increase in feeder voltage.
So we have a state where there are factors seeking to drop voltage and others seeking to raise voltage.
In addition, if voltage recovers, some motors may restart, causing a drop in voltage. If the condition stays for several minutes, some loads start to show a self-restoring quality — these return their power demand to near normal levels even though terminal voltage is below nominal.
The combined effect of these various influences results in an equilibrium state, characterized by low voltage, relatively steady net demand and high line currents.

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. The voltage has held steady at the transmission level, making operators unwary of the goings on in the distribution system.

Countermeasures

Some actions intended to alleviate this state and their consequences include:

Grid operators may succeed in starting up emergency generators or obtain vars from a remote generator coming online. There are additional vars that reach the distribution system, but if the amount is not sufficient to relieve the Ledge state, it may just bring about a new equilibrium state, another voltage Ledge.
Operators may call for a voltage reduction which is a change in the setpoint of the step-downs to a lower voltage. The objective is to reduce demand. When the system is in the voltage Ledge, this does not produce an operating change, since the tap changers are already at their limit. If the voltage reduction order is made before the feeders reach a voltage Ledge, this might in fact hasten the entry into a voltage Ledge.

What actions help:

If the system can hold on until the heat wave diminishes, the lower demand will gradually ease the system out of a voltage Ledge.
If a large load is shed, this would also allow some reprieve.

The system may remain on the Voltage Ledge for an extended period, up to 2-3 hours as recorded in the 1999 East Coast event. If a contingency occurs, such a line outage or generator trip, this significantly reduces reactive supply, the system may enter into collapse. This time in earnest.

Case Samples

The following are voltage traces from previous voltage collapse and near voltage collapse events.

From July, 1999, near voltage collapse event in the PJM coastal regions, a trace of 500 kV voltages going below 1.0 per unit for 2-3 hours.
From August, 2003 for Northeast Blackout event, a trace of 500 kV voltages in PJM
From Tokyo Voltage Collapse event of August, 1987, trace of EHV voltages