24-08-2012, 03:02 PM
Cold Load Pick-Up – Theory and Industry Practices
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Abstract:
Cold Load Pick-Up (CLPU) is an issue that has been recognized for many years, at least back
into the 1940s. The term “Cold Load Pick-Up” (CLPU) refers to the increased currents
encountered when re-energizing a circuit after an extended outage. The short term increase in
currents, due to loss of load diversity and feeder inrush, can require accommodation in the
protective relay settings, or affect performance of the protection scheme. CLPU also has been
handled differently by different power companies. It may cause challenges over time as circuits
are reconfigured, or if protection settings are not reviewed routinely as system expansions
occur. The different approaches and lessons learned in protection and control have provided an
opportunity to review the concepts of cold load pick-up, and investigate how CLPU is generally
dealt with in protection and automation schemes. This paper discusses the concepts of cold
load pick-up, the causes and effects, and investigates how CLPU generally is handled in
protection schemes. The paper reviews past studies and an informal survey of a number of
power companies conducted for the paper.
What is Cold Load Pick-Up?
Cold load pick-up is the loading imposed on a distribution feeder after service restoration in
which some loss of load diversity has occurred. The characteristics of cold load, described by
some empirical relationship of load as a function of time, i = f(t), will vary according to the types
of load of a particular distribution feeder. Some of the common load classifications include
fossil-heating, thermostatically controlled devices such as residential ventilation systems, light
industrial, and small commercial loads, each with its own CLPU characteristic. Feeder load
transfer and its possible impact on CLPU is also an important consideration during service
restoration.
Cold Load Pick-Up Modeling
The measurable parameters that determine the shape of the CLPU characteristics for different
types of customer loads have been studied by many researchers, manufacturers, and power
system companies. Mathematical models have been developed to aid in identifying the
magnitude and duration of the CLPU for homogeneous as well as diversified feeder loads. The
authors of these papers have described computer-aided modeling techniques [1, 2, 3, 4, 5],
heuristic methods [6], knowledge-based approaches [7], and a network flow approach [8]. A
selection of papers about system modeling for distribution circuits also can be found in [2]. All of
these articles address the issue of CLPU, transformer thermal behavior, and search for the best
switching strategy to maximize the number of sections restored and to minimize the time to
supply power to the unaffected areas.
Load Behavior After a Circuit Interruption
Individual loads on residential feeders can be categorized into two different groups;
thermostatically-controlled and manually-controlled. In general, thermostatically-controlled
devices such as air-conditioners, heaters, and heat pumps provide the largest contribution to
the total load in a typical home. Feeder loads that are predominantly homes with low R factors
(as defined in Figure 2), such as mobile homes, the low R value results in very high loads after
an extended interruption.
Manually-controlled loads are switched on and off by occupants of the house in undetermined
patterns, but are part of the cause of a transient delayed peak that can occur roughly a minute
after restoration. During normal conditions, diversity among loads is present; therefore, the
aggregate load of a number of homes is less than the connected load. When an abnormal
condition such as an extended interruption occurs in a distribution system, some or all
thermostatically-controlled devices will be on as soon as the power is restored. Similarly, the
aggregate load of manually-controlled devices will be higher than normal upon restoration, but
after a short delay. Many people may be in a darkened house waiting to turn on the washer or
dryer or cook dinner. As soon as power is restored, these normal activities resume, but after a
short delay while the customers rejoice at the return of power.