Generator Controls

Whenever a prime mover and generator are operated in parallel on the utility-derived power system, it is necessary to provide controls to assure that equipment is operated within its ratings. The role of the governor is to maintain the frequency within some specified range in the steady state, as load on the prime mover varies. Modern isochronous governors operate prime movers in response to load demand with no droop in frequency. Prime mover governors and controls are described in Chapter 13.

Generators are designed to produce a specific voltage under fixed conditions of field excitation, speed, and load. Any change in the connected load without a change in the alternator's excitation will cause the generator's voltage to change.

Voltage regulation is defined as the difference between the steady-state voltage under no load and the steady-state voltage at full-load output, expressed as a percentage of the full-load voltage. Most alternators control field strength to regulate voltage and have the ability to compensate for factors that affect voltage. Figure 25-8 shows a typical digital voltage regulator.

There are two types of voltage regulated generators:

Fig. 25-7 Delta Connection.

Wye and delta configurations are used to provide a range of output voltage combinations. Typical generator terminal voltages for 60 Hz service are shown in Table 25-1.

Fig. 25-8 Typical Digital Voltage Regulator. Source: Marathon Electric

Table 25-1 Typical Generator Voltages for 60 Hz Service.

Fig. 25-8 Typical Digital Voltage Regulator. Source: Marathon Electric externally regulated and self- (or inherently) regulated. Externally regulated generators include devices that sense the output voltage and adjust the exciter output to maintain output voltage at the required set point. Self-voltage-regulated generators are simpler devices that use current transformers to compensate excitation based on load current.

In addition to load, the major factors affecting voltage regulation are speed and winding temperature. Rotational speed depends on the prime mover and its speed regulating system. Generators are inclined to vary voltage directly with the speed at which they are rotated and this can be problematic with self-voltage-regulated alternators. Generators equipped with external voltage regulators will provide better control due to their accuracy in compensating for speed variations.

Winding temperature depends on load current. The increased resistance from no load (with cold windings) to full load causes the output voltage to drop gradually as the windings warm up. This drop in voltage is referred to as temperature drift. Externally regulated units can compensate for this drift.

Load and power factor control are required for parallel operation with the utility grid. Load controllers output a proportional signal to the prime mover governor based on generator power output. A VAR/PF (or power factor) controller provides input to the voltage regulator to meet load requirements.

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