EXCITATION SYSTEM CEILING VOLTAGE OF SYNCHRONOUS GENERATORS

Fast control of excitation current is needed to preserve SG transient stability and control its voltage. Higher ceiling excitation voltage, corroborated with low electrical time constants in the excitation system, provides for fast excitation current control.

Today’s ceiling voltages are in the range of 1.6 to 3.0 P.U. There is a limit here dictated by the effect of magnetic saturation, which makes ceiling voltages above 1.6 to 2.0 P.U. hardly practical.

This is more so as higher ceiling voltage means sizing the insulation system of the exciter or the rating of the static exciter voltage for maximum ceiling voltage at notably larger exciter costs.

The debate over which is best — the alternating current (AC) brushless exciter or static exciter (which is specified also with a negative ceiling voltage of –1.2 to 1.5 P.U.) is still not over.

A response time of 50 msec in “producing” the maximum ceiling voltage is today fulfilled by the AC brushless exciters, but faster response times are feasible with static exciters.

However, during system faults, the AC brushless exciter is not notably disturbed, as it draws its input from the kinetic energy of the turbine-generator unit.

In contrast, the static exciter is fed from the exciter transformer which is connected, in general, at SG terminals, and seldom to a fully independent power source.

Consequently, during faults, when the generator terminal voltage decreases, to secure fast, undisturbed excitation current response, a higher voltage ceiling ratio is required.

Also, existing static exciters transmit all power through the brush slipring mechanical system, with all the limitations and maintenance incumbent problems.