Abstract
The Modular multilevel converters (MMCs) have excellent performance when operated with high modulation index such as the case of gird-connected converters in high voltage direct current (HVDC) system. Reliable and fault tolerant operation (FTO) of the system is very important in HVDC. A novel FTO algorithm under sub module failure without using any additional redundant SMs is proposed. In the proposed method, the direct current bus voltage control and on-load tap changer (OLTC) control strategy are combined for interruption free operation of the system without any additional voltage stress on switching devices and performance degradation of the system during fault. The analytical details of the normal as well as FTO operation are given. The proposed FTO strategy is verified through simulation and also experimentally using a prototype. The simulation and experimental results are presented.
Highlights
T HE large power system interconnection using high voltage direct current (HVDC) system has brought significant social and economic benefits due to reduced pollution, large power transmission, low cost and better efficiency
The proposed fault tolerant operation (FTO) algorithm allows HVDC to work with low performance degradation under sub module (SM) failure
In the proposed approach a 10% reduction in DC bus voltage will not have any significant impact on the fundamental component, THD, and waveform quality of the converter output voltages
Summary
T HE large power system interconnection using high voltage direct current (HVDC) system has brought significant social and economic benefits due to reduced pollution, large power transmission, low cost and better efficiency. In [17], the author has proposed an optimized method of redundant SMs, which reduces SM capacitor voltage, decreases switching frequency in normal operation, and fault restoration is very fast. In [24] an adaptive carrier phase disposition pulse width modulation (PDPWM) method based on the centralized capacitor voltage balancing is proposed This method needs, only one voltage reference and one carrier for each arm, which reduces the computational burden and able to continue the operation of MMCs when certain SM fails. If we consider higher order harmonics in MMC output voltage and currents, higher order harmonic components can be seen in the arm power but their magnitude will be small, compared to fundamental and second harmonic component [28]. PROPOSED FTO UNDER SM FAILURE Two novel FTO algorithms, namely (i) FTO through VDC control and (ii) FTO using OLTC combined with VDC control are proposed in this paper
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