Ratio of input frequency to output frequency

If the ratio of input frequency to output frequency is not integer, the short circuit current flows through input side.
The firing angle of the P – converter and N – converter should be adjusted in order to stop generation of short circuit current.
If the load current is discontinuous, it is complicated to adjust firing angle of the converters.
The circulating current system is used in order to control firing angle of the converters.
If the SCRs of the P – converter operates at firing angle α_p, the firing angle of SCRs of the N – converter should be such that

α_p + α_n = 180^o

Therefore, the P – converter operates in the rectifier mode and N – converter operates in inverter mode.
The average output voltage of the rectifier group is equal to the back emf of the inverter group and this will prevent circulation current flows between converters.
However, the harmonic current flows between two converters due to instantaneous voltage of the both converters are not equal.
The harmonic current is limited by external reactor or impedance.

Function of Intergroup Reactor

The harmonic current is reduced by using inter group reactor as shown in the figure A.
The load is connected at one end at the centre point of inter group reactor.
The value of reactance should be 4nX_L in order to reduce low frequency output current.

N = Harmonic order number and

X_L = Half reactance of reactor

The SCRs of the rectifier group should be blocked for selected time in order to reduce circulating current between two converters or gate pulse of SCRs of rectifier group should be remove for selected time period.
There is a current sensing device connected at the output terminal of the Cycloconverter and it detects the direction of current.
The current sensing device generates control signal according to direction of current.
The gate pulse of SCRs is removed in case of overload and fault condition in order protect the SCRs.

Advantages of Circulating Current Scheme

The advantages of circulating current scheme are as given follows.
The load current flows in any direction ( Either from load to Cyclo converter or Cyclo converter to load ).
The effect of continuous current or discontinuous current does not depends upon firing angle of SCRs and output voltage.