Living on the Ledge – Operating Distribution Systems at Low Voltage

(A serialized, expanded and updated version of this article can be
found at the Pterra Blog site:

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

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
  • 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    local large synchronous generators is withdrawn by the same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 changing 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.


Some actions intended to alleviate this state and their consequences

  • Grid operators may succeed in starting up emergency generators
    or obtain vars from a remote generator coming online.  There
    additional vars that reach the distribution system, but if
    the amoutn 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.

  1. 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
  2. From August, 2003 for Northeast Blackout event, a trace
    of 500 kV voltages in PJM

  3. From Tokyo Voltage Collapse event of August, 1987, trace
    of EHV voltages


  1. Report
    of the U.S. Department of Energy’s Power Outage Study Team, Findings
    and Recommendations to Enhance Reliability from the Summer of 1999
    Washington, DC, March 2000.
  2. Blackout
    August 14, 2003 Final Report
    ,” New York Independent System
    Operator,  February, 2005.
  3. Final
    Report on the August 14, 2003 Blackout in the United States and
    Canada: Causes and Recommendations
    ,” U.S.-Canada Power System
    Outage Task Force, April 2004.
  4. Technical
    Conference on August 14 Blackout
    ,” North American Electric
    Reliability Council, presentation by Shinicho Imai, Tokyo Electric
    Power, January, 2004.

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