Page 246 - Power Electronics Handbook

236 Direct a.c. frequency converters

3 phase
double star
a.c. supply

point

Fiapre 10.18 Push-pull six-pulse cycloconverter with circulating current limiting reactor

greater extent. If p is the pulse number of the converter system, V the
r.m.s. supply voltage to the cycloconverter, and X the effective source
reactance, then the circulating current (i,) is limited to the value given by
equation (10.1).

(10.1)
An alternative method for suppressing the flow of circulating current is
to ensure that only one converter conducts at any time. This is by far the
most satisfactory arrangement, since the circulating currents can often
become extremely large, but it does result in a much more complex
thyristor control system in which the load current must be sensed and used
to block one or other group of converter, depending on the current
direction. Another disadvantage of the non-circulating current mode is
that the load voltage distortions can become much higher on light loads,
when the current becomes discontinuous. Since discontinuous operation is
only serious on light loads, it is feasible to run a cycloconverter in a
circulating-current mode when the load current falls below a certain critical