بطور کلی چهار نوع Load shedding به شرح زیر موجود است. در مورد سیستمهای کوچک به دلیل اینرسی کم سیستمهای چرخان، سرعت عملکرد Load shedding جهت حفظ پایداری سیستم اهمیت می یابد. بنابراین سیستمهای مدرن PMS مورد استفاده در صنایع ، مجهز به Fast Load Shedding جهت پاسخگویی به نیازمندیهای سیستمهای قدرت جزیره ای می باشند.
Load Shedding function ensures the availability of electrical power to all critical and essential loads in the plant at all times. Load shedding is achieved by switching off non-essential loads when there is a shortage of power generation capacity in the electrical network of the plant..
There are four different types of load shedding:
Fast Load Shedding is based on electrical energy balance calculations. As soon as one or more electrical islands are detected (using network determination software), the system calculates if there is enough electrical power available in every individual island to power the loads. If not, any existing demand surplus is shed. The shedding process is dictated by priority tables, which are based on the operational conditions of the process.
Frequency Load Shedding (or back-up load shedding) uses a frequency drop as an input to activate load shedding. Activation of an actual shed command can be based on a frequency decay or by passing a frequency threshold. Frequency Load Shedding is usually used as an independent back-up system for fast load shedding
Slow Load Shedding is used when an overload has occurred. For example if a transformer is loaded at 120 percent, switching off some loads to bring the transformer back to its nominal load is by far the best solution. The system advises the operator which non-critical loads he can switch off. This manual effort must be done within a specified period of time otherwise the system will do it automatically.
Peak Shaving is another type of slow load shedding and occurs when the following situation arises: if in-house generation is maximized but it seems highly probable the 15 minutes sliding maximum power demand will exceed the contracted maximum value, then some of the low priority loads are shed. Manual Load Shedding is mainly used when one of the afore-mentioned conditions for Slow Load Shedding occurred but operations did not allow the system to shed automatically.
A load-shedding system requires accurate logic and control actions to achieve fast operation, particularly in islanded operation mode. Slow responses may lead to cascading outages and ultimately to total blackouts. Conventional, frequency-based schemes act more slowly because they depend on the frequency decaying to some threshold before they operate. In some operational scenarios, the system may not be stable or able to recover the nominal frequency due to the slow response. Accordingly, blackouts may occur.
In general, the speed of any implemented load-shedding system in islanded operation mode is the key design parameter because of two main factors: system inertia and generator operating points. Because the inertia of an islanded system is relatively low, compared to a utility, a system disturbance will have a greater impact on the system frequency.