Frequency Control at Different Timescales [2, 3]

The maintenance of frequency involves a response from generators over several timescales ranging from seconds to days. This response is not only required to follow the demand’ s fast variability from second to second and its slow variability over the day, but also sudden substantial mismatches between generation and demand, for example during system faults.

Figure 3.10 illustrates a typical system frequency trajectory plotted on a nonlinear time scale. From the origin of the graph until about 8 seconds, the frequency exhibits the usual noise associated with minor mismatch between the continuously varying demand and the efforts of generation to match it through governor action. At 8 seconds something unpredicted and serious takes place. It could be that a large power station trips because the overhead line connecting it to the transmission system suffers a mechanical failure due to high winds and accumulated snow. This contingency results in an instantaneous large shortfall of generation. The trace describes a typical time history of the frequency and the measures taken to constrain the frequency excursion within the statutory and operational limits. Such measures are taken by all utilities but are given a variety of labels. Here the labels shown in Figure 3.10 are the ones adopted in the UK.

A power system has at its disposal a number of generators with diverse characteristics. These are arranged in a hierarchy of plant appropriate for operation at different timescales as described below.

A continuous or frequency response is provided by generators equipped with appropriate governing systems that control their outputs to counteract the frequency fluctuations that arise from relatively modest changes in demand or generation [2] , Large generators on the grid

Frequency Control at Different Timescales [2, 3]

are selected on technical and economic merits and instructed by the system operator to operate in frequency-sensitive mode (i. e. under active governor control) to provide this service. For this to be achieved, some generators are held below maximum output.

An occasional service or reserve is available to contain significant and abnormal frequency excursions caused by sudden mismatches in the generation/demand balance (e. g. loss of generation) [2] . Part-loaded large synchronized generators as well as deferrable loads fitted with frequency sensitive relays provide such services. The reserves are subdivided into primary and secondary categories. In Figure 3.10 the initial rate at which the frequency drops after the incident at 8 seconds is controlled and limited instantly by the inertial energy release from all the decelerating generators (and consumer drives) on the system. This provides a breathing time before the fast governors of some generators begin to act. Massive steam valves have to be opened hydraulically and increased steam flow has to be transported from the boiler to the turbines. It therefore takes a finite time for the substantial stored energy in the boiler to be exploited.

Primary reserves require the most rapid generator response. The key requirement for gen­erators allocated this task is that they should be capable of increasing their active power output within 10 seconds of predefined system frequency excursions and be capable of main­taining this response for a further 20 seconds [2].

Secondary reserves require a slower initial response but maintained for longer periods of time. This requires the capability of increasing the active power output within 30 seconds and maintaining the response for a further 30 minutes [2] . A fast response capacity is also provided by partly loaded hydro or pumped storage (when available), which are not bedev­illed by the constraints imposed on thermal plant. Water driven plant can respond in a few minutes and be started up automatically when the frequency falls below a critical value.

A fundamental feature of generators providing frequency response and reserves, collec­tively known as the operating margin. is the requirement for generators to have headroom in order to increase output. The operating margin is the difference between available genera­tion and actual demand. Generators providing such services will therefore be part-loaded.

Besides hydro and pumped storage plant (if available) the main providers of such services are flexible large coal fired power plant.

Beyond primary and secondary reserves, power systems have further robust tertiary defences known as standing reserves. These are sourced from unsynchronized standby gen­erators capable of mains connection and generation of the instructed level of output within 20 minutes [3]. Typically, standing reserve is provided from generators driven by open cycle gas turbines and reciprocating internal combustion engines.

High frequency response services are required in the event of excessive system frequency events when large loads are suddenly lost. Such services happen rarely and are initiated through governor action requiring either to reduce output or to cease generation altogether.

Finally, utilities have some control over the demand by implementing voltage control. System voltage, like system frequency is rarely exactly at its nominal value but is allowed to vary within controlled limits. One response to a loss of generation, which may occur due to manual intervention or automatically, is a reduction of system voltage. Total system load will fall with voltage depending on the nature of the loads. Voltage and frequency reductions can cope with serious credible demand or generation changes although only in exceptional circumstances would both be allowed to reach their minimum or maximum values.

Updated: September 26, 2015 — 5:49 pm