ABSTRACT
Voltage frequency is the most important figure in a power network. First of all, the speed of rotation of electric motors, and therefore the performance of the machines and mechanisms, depends directly on frequency. Second, generators in power stations are designed for work at fixed frequencies.
Deviations from these fixed frequencies, in either direction, by 5 to 10% can lead to a sharp intensification of vibration in a large-tonnage rotor (Figure 14.1), to premature failure of the generator, to sharp productivity slowdowns in the following: powerful pumps that deliver water to the boiler, fans of the air injection systems, cooling systems pumps and many other important systems of power plants. Third, if there are several generators in the power network, their work must be
synchronized by frequency with a high degree of accuracy. Fourth, any decrease of voltage frequency caused by an overload of generators is
inadmissible in itself. Even a slight excess of power consumption over generator power can lead to a significant voltage frequency drop in the power system (Figure 14.2). When there is such a frequency drop below a certain critical level, usually some of the customers, or even a line and whole subsystem, is automatically disabled in order to maintain serviceability of the generators and the network. Frequency decreases because of power system overload, while a frequency increase is
evidence of a power excess. Power excess occurs in the system when one or several hard loaded lines are suddenly disabled. Surplus power is directed to other lines, causing dangerous power flows that can lead to a power system breakdown. Such an accident, followed by a frequency excess of up to 63 Hz, took place on August 14, 2003 during the biggest power system breakdown in the U.S.A. That is why it is so important to control voltage frequency. Like all other parameters of electric circuits, frequency too is controlled by special
relays.
