Page 185 - Power Electronics Handbook

Synchronous tap changer 177

the usual phase-control systems described previously, it is now necessary to
sense the load current as well as the zero voltage cross-over points and to
inhibit the firing pulses if danger of short circuit exists.
Clearly, the system of Figure 8.20 can be extended to a greater number
of taps if these were available on the output transformer. Generally,
synchronous tap changing is used if an output transformer is available and
only a limited range of control is required. Phase control is the most
commonly used system and the simplest. Where load power being
controlled is large and radio frequency interference requirements
stringent, as for domestic heating applications, integral half-cycle control is
preferable, provided the load has a relatively large inertia. A.C. chopper
control finds application primarily on special systems where speed of
response and minimisation of harmonics are essential.