Load characteristics Several definitions are used to quantify load characteristics at a given location on a circuit: 1. Demand 2. Load factor 3. Coincident factor 4. Diversity factor 5. Responsibility factor Demand The load average over a specified time period, often 15, 20, or 30 minutes. Peak demand over some period of time is the most common way utilities quantify a circuit’s load. In substations, it is common to track the current demand. Load factor The ratio of the average load over the peak load. Peak load is normally the maximum demand but may be the instantaneous peak. The load factor is between zero and one. A load factor close to 1.0 indicates that the load runs almost constantly. A low load factor indicates a more widely varying load. From the utility point of view, it is better to have high load-factor loads. Load factor is normally found from the total energy used (kilowatt-hours) as: where: LF = load factor kWh = energy use in kilowatt-hours dkW = peak demand in kilowatts h = number of hours during the time period Coincident factor The ratio of the peak demand of a whole system to the sum of the individual peak demands within that system. The peak demand of the whole system is referred to as the peak diversified demand or as the peak coincident demand. The individual peak demands are the noncoincident demands. The coincident factor is less than or equal to one. Normally, the coincident factor is much less than one because each of the individual loads do not hit their peak at the same time (they are not coincident). Diversity factor The ratio of the sum of the individual peak demands in a system to the peak demand of the whole system. The diversity factor is greater than or equal to one and is the reciprocal of the coincident factor. Responsibility factor The ratio of a load’s demand at the time of the system peak to its peak demand. A load with a responsibility factor of one peaks at the same time as the overall system. The loads of certain customer classes tend to vary in similar patterns. Commercial loads are highest from 8 a.m. to 6 p.m. Residential loads peak in the evening. Weather significantly changes loading levels. On hot summer days, air conditioning increases the demand and reduces the diversity among loads. At the transformer level, load factors of 0.4 to 0.6 are typical (Gangel and Propst, 1965). Several groups have evaluated coincidence factors as a function of the number of customers. Nickel and Braunstein (1981) determined that one curve fell roughly in the middle of several curves evaluated. Used by Arkansas Power and Light, this curve fits the following: where n is the number of customers
Posted on: Thu, 04 Jul 2013 04:47:43 +0000
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