Page 247 - Power Electronics Handbook

Phase-controlled cycloconverters 237

level and to switch to the group-blocking mode when the current increases.
By adopting this technique the circulating-current reactor can be made
small, since it is not required to be effective on heavy load currents and can
therefore saturate.
The circulating-current suppression method of control requires a more
complex thyristor-firing circuit, although generally circulating-current
systems are a natural extension of ordinary phaseantrolled rectifiers and
ensure the correct thyristor conduction for rectification or inversion. It
gives the simplest and smoothest control method. The one disadvantage of
circulating-current operation is that a relatively large current flows
between the two converter groups, which results in increased device rating
and reduced efficiency. Non-circulating-current converters prevent this by
blocking one of the converters at all times, so that there can be no
interchange of power between the two groups. The price to be paid is
greater circuit complexity and higher load voltage harmonics, although this
is only important when the ratio of input to output frequency is low.
Other factors affect the load harmonics as well, and these are directly
related to the pulse number of the converter, the ratio of input to output
frequency, the level of output voltage, the load power factor and the
technique used to control the load voltage. For instance, the load voltage is
varied by adjusting the oscillation of the thyristor delay angle by about 90”.
This oscillation should follow a cosine law to give a load voltage closely
resembling a sine wave, but imperfections in the control electronics often
mean that this perfect law cannot be followed and the load voltage is
distorted. If a,,, is the minimum delay angle (the maximum delay being
180-a,) then the r.m.s. load voltage V,, is given by equation (10.2).
lr
v,, = ~(2) v f sin p cos a, (10.2)

This is directly comparable to the equation obtained with phase-
controlled rectifiers described in Chapter 9. In common with these
converters, the load voltage waveform from a cycloconverter is affected by
phase overlap caused by source reactance. This is illustrated in Figure
10.19, which shows the shift in the load voltage harmonic current. Source

Figure 10.19 Load-voltage waveforms for a cycloconverter: (a) with no source reactance;
(b) with source reactance