Design and development of Thyristor based MLCR CSC
Thesis DisciplineElectrical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The new concept of Multi-Level Current Reinjection (MLCR) combines the advantages of DC ripple reinjection, multi-level conversion and soft-switching technique. Taking advantage of the soft-switching technique which uses zero current switching for the main bridge switches, thyristor based MLCR current source converter (CSC) is proposed. This concept adds self-commutation capability to thyristors and produces high quality line current waveforms. Various thyristor based MLCR CSC topologies have been simulated extensively using PSCAD/EMTDC in this thesis and their performance characteristics investigated.
Questions have been raised about the ability to force the main thyristors off using the reinjection bridge in a real-world implementation, where there are inevitable stray capacitances and inductances which may influence the thyristor turn-off; and simulation switching models may not represent the switching characteristics fully or accurately. For this proof of concept, a small scale prototype has been built in the laboratory. The 3-level MLCR CSC, which increases the pulse number from 12 to 24, is chosen to verify the concept. The experimental investigation of the 3-level MLCR CSC, under steady-state conditions, verified the following:
• The reinjection current allows the main bridge thyristors to be switched at negative firing angles.
• This current reinjection technique allows self-commutation capability in a practical system despite the finite turn-off times of the thyristor.
• This current reinjection technique improves the harmonic characteristics of the thyristor based converter.
• It is observed that the deviation of the actual waveforms from the theoretical waveforms is mainly due to the snubber across the reinjection switch, and a trade-off in the choice of snubber components is required.