Circuit for Converter Group Selection
Circuit for converter Group Selection:
Fig: Converter group selection in non-circulating current scheme
- The load current is allowed to flow through the P-converter or the N-converter through suitable logic.
- D is the delay during which period the firing pulses to both the converters are inhibited.
- The delays are not introduced in some control schemes, where a small circulating current is permitted during the cross-over instants of the fundamental current only.
- The scheme is still recognized as the non-circulating current operation since during a major portion of the output cycle, it operates in the non-circulating current mode.
- The circuit is an essential part of the control scheme of a cyclo-converter with the non-circulating mode of operation. The function is to ensure that only one converter operates at a time depending upon the polarity of the current.
- The positive converter is operated, when the load current is positive, and the negative converter is operated, when the load current is negative.
- The converter group selection is not straight forward primarily due to non-ideal nature of the output current waveform. Since the actual load voltage waveform itself is far from sinusoidal, the load current is also non-sinusoidal.
- Depending upon load circuit parameters and converter pulse number, the load current may become zero before the fundamental half-period.
- If the group selection and blanking circuit were to operate at each current zero instant, it may cause erratic switching of converters. The result of this is to further distort the output voltage.
- One possible solution to this problem is to see that the blanking circuit operates at the zero crossing of the fundamental current.
- The fundamental component of the load current is extracted, and the converter bank selection is made to occur at the zero crossings of this fundamental component of the current.
- There are some operational difficulties, in the design of filter components specially, when the cyclo-converter is required to operate over a range of the output frequency, and with variable load. The filter, which operates satisfactorily over the desired range of frequency, will have to be used.
- Thus, the envelope distortion of the output current and the output voltage are reduced, and the possible steady state discontinuous conduction within the fundamental period does not cause any erratic switching of converters.
- Because of filters, certain amount of phase shift may be introduced between the zero crossings of the fundamental output current and actual load current.
- In order to eliminate the waveform distortion, certain amount of circulating current may be allowed to flow during this short overlap period in some control schemes.
- The presence of circulating current is a must in such a design. However, if no circulating current is permitted to flow, some distortion in the output voltage is to be tolerated. This distortion arises due to the delays introduced at the zero crossings of the load current to ensure turn-off of thyristors in the outgoing group before the thyristors in the incoming group are turned on.