Читать книгу Handbook of Large Hydro Generators - Geoff Klempner - Страница 35

When dealing with three‐phase balanced circuits, electrical engineers use the one‐line or single‐line representation. This simplification is allowed because in three‐phase balanced circuits, all currents and voltages, as well as circuit elements are symmetrical. Thus, by “showing” only one phase, it is possible to represent the three‐phase system, as long as care is taken in using the proper factors. For instance, the three‐phase balanced system of Figure 1.4-1 or Figure 1.4-2 can be represented as shown in Figure 1.7-5. Hereinafter, when describing a three‐phase generator by an electrical diagram, the one‐line method will be used.

The most convenient way to determine the performance characteristics of synchronous machines is by means of equivalent circuits. These equivalent circuits can become very elaborate when saturation, armature reaction, harmonic reactance, and other nonlinear effects are introduced. However, the simplified circuit in Figure 1.7-6 is conducive to obtaining the basic performance characteristics of the machine under steady‐state conditions.

In Figure 1.7-7, the reactance X_a, represents the magnetizing or demagnetizing effect of the stator windings on the rotor. It is also called the magnetizing reactance or armature reaction reactance. R_a represents the effective resistance of the stator. The reactance X_l represents the stator leakage reactance. The sum of X_a and X_l is used to represent the total reactance of the machine and is called the synchronous reactance (X_s). Z_s is the synchronous impedance of the machine. The internal generated voltage is represented by E_o, the airgap voltage by E_r, and finally the terminal voltage of the machine by E_t. It is important to remember that the equivalent circuit described in Figure 1.7-7 represents the machine only under steady‐state conditions.

Figure 1.7-6 One‐line representation of the circuit shown in Figures 1.4-1 and 1.4-2.

Figure 1.7-7 Steady‐state equivalent circuit of a synchronous machine.

The simple equivalent circuits of Figure 1.7-8 suffices to determine the steady‐state performance parameters of the synchronous machine connected to a power grid. These parameters include voltages, currents, power factor, and power angle. The regulation of the machine can be easily found from the equivalent circuit for different load conditions by using the regulation formula:

For a detailed review of the performance characteristics of the synchronous machine, in particular the hydro generator, the reader is referred to Chapter 4.

Note: Regulation in a generator indicates how the terminal voltage of the machine varies with changes in load. When the generator is connected to an infinite bus (i.e. a bus that does not allow the terminal voltage to change), a change in load will affect the machine's output in a number of ways. (See Chapter 4 for a discussion of this topic.)