Electrical Engineering ⇒ Topic : Voltage Regulation

  The voltage regulation of an alternator is defined as the variation in terminal voltage from full-load to no-load when the full-load is thrown off, with thefield current and the speed of the alternator kept unaltered. Usually the alternators are designed to maintain a constant terminal voltage when delivering full-load at a specified power factor (0.8 lagging). For example, consider the alternator regulation characteristics shown in Figure (a). Let OA be the full load current at unity power factor. The corresponding terminal voltage is OB. When the load on the alternator is suddenly removed, i.e. when load current becomes zero, the terminal voltage will rise from OB to OC. The change in the terminal voltage BC is the regulation. Then, the percentage voltage regulation (R) is given by

                Change in the terminal voltage when the

                                                                                 ........... (1)

                                                      FIGURE (a) Regulation of an alternator at different power factor

  There are various methods by which the regulation of an alternator can be determined. Only a few important methods are discussed in this section,because in power systems the importance of regulation is reduced by employing automatic voltage regulators to keep the system voltage constant at all loads.

VOLTAGE REGULATION

Voltage regulation of a synchronous machine is defined as the change in terminal voltage expressed as a percentage of rated voltage when load at a given power factor is removed. The speed and field current remains constant

                                             ............. (1)

                                         .............. (2)

where E_f is no-load excitation voltage and V is full-load terminal voltage.

Voltage regulation depends on pf. of the load. For unity p.f. and lagging pf., E_f is greater than V. So voltage regulation is always positive. For leading pf., voltage regulation may be positive or negative due to decrease of E_f.

The importance of voltage regulation is as follows:

• Voltage rise must be known during throwing off the load because the winding insulation must be able to withstand the voltage.
• The usage of automatic voltage control is determined by voltage regulation.
• Voltage regulation affects the steady state short-circuit conditions and stability.
• Voltage regulation also affects the parallel operation of one alternator with the other alternators