توضیحات
From an operational point of view, power systems are affected by factors that change each second, minute, hour, day, season, and year. In each time frame of operation, it is necessary that balance be kept between the load on the system and the available supply in order for system frequency to remain in an acceptable range. In the very short time frames (seconds-to-minutes), bulk power system reliability is almost entirely maintained by natural inertia and automatic equipment and control systems, such as generators governor system and the power system’s automatic generation control (AGC). In the intermediate to longer time frames, economic dispatch mechanisms, system operators’ action and operational planners’ edict become the primary factors to maintaining system reliability.
Generally, frequency regulation is one of the significant challenges for system operators. The system frequency must be maintained within a narrow range for three reasons:
1) Customers, particularly those managing industrial processes, are highly sensitive to frequency deviations from the nominal level;
2) Generating units are designed to operate most efficiently at their nominal speed and subject to damage if operated beyond certain frequency bounds; and
3) Automatic protection systems will disconnect load and generation if the system frequency drops below a certain value.
Therefore, in order to keep a balance between generation and consumption at all times, the load is forecasted and sufficient generation is committed and dispatched to maintain supply/demand balance. However, since the demand forecast is never 100% accurate and since unpredictable contingencies (forced outages) like generator and network trips may occur in power systems, the generation and consumption will experience mismatch on regular basis. This unbalance can cause reduction or increase of system frequency. As a result, the system frequency frequently deviates from its nominal value jeopardizing the reliability of power system. Consequently, the system operator needs suitable resources to maintain system frequency and guarantee reliability of power system operation. These resources that are called reserves should be available at the right time in order to keep power system frequency in an acceptable range. In other words, frequency control requires that a certain amount of active power to be kept in reserve to be able to regain the balance between demand and generation at all times. Generally, reserve consists of upward and downward reserves that both of them are required for maintenance of stability and frequency. When the amount of generation is more than demand, the downward reserve is utilized to retain frequency by automatically reducing generation. Upward reserve refers to available generation capacity (or voluntary and controlled load reduction) that is added to the power system in order to maintain the frequency when system suffers from frequency drop (load exceeds generation for any reason).
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