Flexible Power control in Large Power Current Source Conversion
Thesis DisciplineElectrical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This thesis describes a new concept, applicable to high-power current-sourced conversion (CSC), where a controllable ﬁring-angle shift is introduced between series and parallel converters to enable independent active and reactive power control. The ﬁring-shift concept solves a difficult problem, by giving thyristor based CSCs the control ﬂexibility of pulse-width modulated (PWM) converters, but without a loss in efficiency or rating. Several conﬁgurations are developed, based on the ﬁring-shift concept, and provide ﬂexible, eﬃcient solutions for both very high power HVDC transmission, and very high current industrial processes.
HVDC transmission conﬁgurations are ﬁrst developed for 4-quadrant high-pulse operation, based on the series connected multi-level current reinjection (MLCR) topology. Independent reactive power control between two ends of an HVDC link are proven under ﬁring-shift control, with high-pulse operation, and without on-load tap changing (OLTC) transformers. This is followed by application of ﬁring-shift control to a bi-directional back-to-back HVDC link connecting two weak systems to highlight the added dc voltage control ﬂexibility of the concept.
The fault recovery capability of an MLCR based ultra-HVDC (UHVDC) long distance transmis-sion scheme is also proven under ﬁring-shift control. The scheme responds favourably to both ac disturbances and hard dc faults, without the risk of commutation failures and instability experienced during fault recovery of line-commutated conversion.
The two-quadrant capability of very high current rectiﬁcation is also proven with conﬁgurations based on phase-shifted 12-pulse and MLCR parallel CSCs. The elimination of the electro-mechanical OLTC/satruable reactor voltage control, the high-current CSC’s biggest shortcoming, greatly improves controllability and with ﬁring-shift control, ensures high power-factor for all load conditions. This reduces the reactive power demands on the transmission system, which results in more eﬃcient power delivery