Power System Load

the total electric power consumed by all users connected to the distribution network of a system, and also the power used to compensate for losses in all parts of the network (transformers, converters, and transmission lines).

The change in the power system load over time—that is, the change in the power consumed or the current in the network as a function of time—is called the load curve. A distinction is made between individual and group load curves (for individual users and groups of users, respectively). Loads determined by the rated power of the users are random quantities that may assume various values with a certain probability. Consumers usually do not operate simultaneously and are not all at their full rated power; therefore, the actual power system loads are always less than the sum of the rated powers of all individual users. The ratio of the maximum consumed power to the connected power is called the coincidence factor, and the ratio of the maximum load of a given group of consumers to their rated power is called the demand factor.

In determining system loads, a distinction is made between the mean load and the mean square load. The mean load is defined as the ratio of the amount of power produced or consumed during a certain period of time to the duration of the period (in hours). For the mean square load, the time period is a day, month, quarter, or year. The active or reactive system load is understood to mean the total active or reactive power of all consumers, including network losses. The active power P of an individual load, a load group, or the entire system is defined as P = S cos ɸ, where S = UI is the total power (U is the voltage, and I is the current), cos ɸ is the power factor, and ɸ = arc tan (Q/P), where Q is the reactive power of the load.

Power system loads with drastically or abruptly changing load curves are called fluctuating loads. Transient processes can arise in power system loads as a result of changes in operating conditions or disturbances of operational schedules (changes in voltage, frequency, transmission parameters, network configuration, and so on). In studying such processes it is customary to consider not the individual loads but rather load groups (load centers) connected to a powerful substation, a high-voltage distribution network, or a transmission line. Load centers may also include synchronous compensators, low-power generators (with power ratings significantly lower than the load), or small power plants. The composition of users connected to a load center may vary within wide limits, depending on the locality (city, industrial or rural area, and so on). On the average, an urban load is characterized by the following distribution: asynchronous electric motors, 50–70 percent; lighting equipment, 20–30 percent; rectifiers, inverters, furnaces, and heating equipment, 5–10 percent; synchronous electric motors, 3–10 percent; network losses, 5–8 percent.

The processes taking place in the load centers influence the operation of the power system as a whole. The degree of influence depends on the characteristics of the load; these characteristics usually include the voltage and frequency dependence of the active and reactive power consumed in the load centers, as well as the torque or current. There are two kinds of load characteristics, static and dynamic. A static characteristic shows the dependence of power, torque, or current on voltage or frequency for slow changes in the load. The static characteristic is represented by the curves P = ɸ₁(U); Q = ɸ₂(U); and P = ɸ₁(f) and Q = ɸ₂(f). The same characteristics determined for rapid changes in the power system load are called dynamic characteristics. In a power system, reliability of operation under given conditions depends markedly on the ratio of the load for the conditions to the limiting maximum load.