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Glossary · Energy industry & energy transition

Cold Load Pickup

Cold load pickup restoration of supply power outage distribution grid load grid operation security of supply protection technology

Cold load pickup (CLPU, literally something like "picking up cold load") refers to the increased power and current demand that occurs when a network section is reconnected after a prolonged supply interruption. The term comes from network operation and protection engineering. It does not describe an energy source but a behaviour of the load: after restoration of supply, demand is usually higher than before the outage. It can even exceed anything previously measured on the same feeder. The word "cold" here means that during the interruption the load has fallen into a cooled-down or unsatisfied state. This does not necessarily have anything to do with winter.

Two components: inrush currents and loss of diversity

Technically, cold load pickup consists of two processes. The first is inrush currents. Transformers have to re-magnetise their iron core, motors start up, capacitors charge, and cold filaments have a low resistance. These currents can reach a multiple of the rated current, but usually decay within milliseconds to a few seconds. The second and longer-lasting part is the loss of load diversity. Many devices are controlled by temperature, pressure or fill level and, in normal operation, switch on and off independently of one another and at random times. Typical examples are heaters, refrigerators, air conditioners, pumps and water heaters. After a prolonged outage they all call for "demand" at the same time and draw power simultaneously when switched back on. This increased load can last many minutes to hours until the random cycling re-establishes itself.

An illustration: normally, in a street with a hundred refrigerators, only a portion of the compressors is running at any time. If the power is out for longer than a typical switching cycle, almost all of them start up at the same time once voltage returns. The IEEE report gives an example: devices that cycle about once every 30 minutes lose all of their diversity in an outage of more than 30 minutes. The effect is therefore closely related to the coincidence factor. Networks are not designed as if all consumers were running at full power all the time. It is precisely this assumption that temporarily no longer holds during cold load pickup.

To be distinguished from this is so-called hot load pickup: reconnection after very short interruptions, for example with automatic reclosing after a few seconds. In this case diversity has hardly changed, and inrush currents dominate. Inrush current and cold load pickup are therefore not the same thing. However, inrush current is one component of cold load pickup.

Orders of magnitude and influencing factors

How large the effect is depends strongly on the individual feeder. An IEEE report calculates a hypothetical winter feeder predominantly serving residential buildings after a two-hour outage. It arrives at about 4.8 times the normal current in the first instant, around 3 times after 0.2 seconds, and about 2.1 to 2.2 times from 2 seconds to 15 minutes. Older tests by a US utility in 1960 and 1962 gave similar results. After a 39-minute outage in winter, the inrush current was 3.4 times the current before the outage. After 80 minutes in summer, the peak was 6.7 times. In both cases diversity noticeably began to return after about 1000 seconds. In a nine-hour outage on a feeder with a lot of electric space heating, the current only returned to its pre-outage level after a good hour.

A technical report on distribution substation transformers in networks with a high share of electric heating cites a loading of about 200 percent with complete loss of diversity. For this, an outage of just 20 minutes at minus 40 degrees Celsius or 40 minutes at minus 10 degrees Celsius could suffice. Cold load pickup can become the limiting factor there for transformers and protection devices. These figures come from North American networks and partly from measurements decades old. They show orders of magnitude but cannot simply be transferred to European networks.

The technical literature names mainly the following influencing factors:

  • Outage duration: the longer the interruption, the more completely diversity is lost. Beyond a certain duration, what mainly grows is the time until the load normalises again.

  • Load mix: space heating, heat pumps with electric heating elements, air conditioners and small motors without contactors intensify the effect. Large industrial motors, by contrast, often drop out via contactors and do not restart by themselves.

  • Weather, time of day and load level: extreme cold or heat increases magnitude and duration. As a reference value, the IEEE report recommends the usual load at the time of reconnection on a comparable day, not the load at the moment of the outage.

  • Distributed generation: if generation plants disconnect from the grid during the disturbance, the upstream supply must, on reconnection, also pick up the load that was previously covered locally. Relative to the previously measured network load, cold load pickup is then higher.

  • Voltage and reactive power: starting motors draw strongly inductive current. This can lower the voltage and further increase the current demand. In extreme cases, voltage collapse or tripping of the feeder is a risk.

Significance for the distribution network and protection engineering

Cold load pickup is particularly relevant in the distribution network, because that is where the many small, temperature-controlled loads are connected. The core problem is overcurrent protection. The pickup threshold of a feeder protection is typically set at a multiple of the highest expected normal load. The cold load pickup current can exceed this threshold. The protection then interprets a healthy, fault-free feeder as a short circuit and trips it again. Protection engineering faces a trade-off here. High settings avoid spurious trips but reduce sensitivity to genuine faults. Low settings detect faults reliably but make restoration of supply more difficult. In addition, the settings must preserve selectivity with upstream and downstream protection devices.

Modern digital protection devices therefore often have a dedicated cold load pickup function. A timer detects that the circuit breaker was open for longer than a defined time. For reconnection, the pickup values are then temporarily raised or instantaneous stages briefly blocked. As soon as the current falls below the normal threshold or a maximum time expires, the device returns to its normal settings. In one variant, only the pickup threshold is raised while the trip characteristic remains the same. This preserves coordination with upstream devices. Older electromechanical relays do not allow such automatic switchovers. Here, cold load pickup must already be taken into account when determining the fixed settings. More functions at the same time mean more complexity and thus the risk of incorrect tripping or failure to trip.

The most widespread operational approach is section-by-section restoration. The feeder is divided into parts via sectionalising points. One section is energised first, and the next ones follow only once its current has decayed. Where remotely controllable or automatic switching devices are available, this staging can be automated. Some network operators also disable automatic reclosing before restoration. Others ask customers to switch off appliances during the outage and to switch them back on step by step afterwards.

Network restoration after major disturbances

Cold load pickup is not to be equated with black start. Black start describes how generation and the network are brought back up without external voltage. Cold load pickup describes the load side of this process. After large-scale outages, both occur together. In the EU, Regulation (EU) 2017/2196, the network code on emergency and restoration, obliges every transmission system operator to draw up a system restoration plan. In doing so it must take into account, among other things, the behaviour and capabilities of load and generation. During restoration, the transmission system operator, after coordination with the distribution system operators, determines how much net load is reconnected in the distribution networks. The distribution system operators comply with requirements on stepwise load reconnection and take into account the automatic reconnection of load and generation. The regulation does not mention cold load pickup explicitly. However, staged reconnection is precisely the means by which such load jumps can be managed with regard to frequency and network loading. How this is implemented in detail is governed by the national restoration plans and the operational requirements of the network operators.

Distinctions and common misunderstandings

  • Power rather than energy: what matters is the short-term power and current peak in kilowatts or megawatts, or amperes, not the amount of energy in kilowatt-hours. Part of the additional consumption is merely deferred demand, for example for reheating buildings.

  • Not normal peak load: peak load is the highest load in a regular observation period. Cold load pickup, by contrast, is a disturbance-related load peak with its own cause. It is central for protection settings and restoration plans, but says little about long-term electricity consumption.

  • Not a sign of a weak network: designing a network for full simultaneity would be expensive and mostly unnecessary. The task is to make rare but plausible reconnection situations operationally manageable. The IEEE report stresses that each feeder must be considered individually for this.

  • Collective phenomenon: a single device is insignificant. Only thousands of loads starting simultaneously noticeably change the load of a network section.

Electrification and outlook

With more heat pumps, charging facilities and electric process heat, the composition of the load is changing. The IEEE report particularly highlights heat pumps with additional electric heating elements. After an outage they can create additional load when the heating elements kick in. Whether electrification intensifies or weakens the effect cannot, however, be answered in general terms. Controllable devices could restart after an outage with a delay, in randomly staggered fashion or on a signal from the network operator. Uncontrolled devices, by contrast, start immediately. Decisive are device characteristics, communication capability, connection conditions and operational requirements. How network operators incorporate cold load pickup into their planning remains a trade-off between stronger equipment, better protection concepts and more automation and control.

In short: after a prolonged supply interruption, the returning load can be higher, more simultaneous and operationally more demanding than before the outage. Restoration of supply is therefore a controlled process and not a simple switching back to the previous state.

Sources

Related glossary entries

Edited by eCMS on 11.11.2026 17:47

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