Thévenin Equivalent of Voltage-Source Converters for DC Fault Studies

PROJECT TITLE :

Thévenin Equivalent of Voltage-Source Converters for DC Fault Studies

ABSTRACT:

This paper presents a simple modulation index-dependent Thévenin equivalent circuit model of the voltage-source converters (VSCs) and also the associated anti-parallel diode rectifier. The model derivation uses orthogonal transformation and a transformer analogy to transfer impedances to the dc aspect of the converter. The paper proposes a switched equivalent representation of a 3-phase diode rectifier shaped due to the antiparallel diodes when VSC switching is inhibited. The proposed model is implemented in Alternate Transients Program-Electromagnetic Transients Program and simulation results are compared with the switched model. The proposed model is shown to below predict the fault currents for pole-ground (PG) faults and to produce a nearly precise match for pole-pole faults. The rectifier equivalent provides increased accuracy within the transient region with the inclusion of the supply inductance. A multiterminal dc (MTDC) system model is employed to check effects of aggregation which offers increased accuracy of the equivalent circuit. It is shown that the MTDC system simulation with the proposed Thévenin equivalent is regarding 3 times faster than full-switched circuit simulation for a time step of five μs. The MTDC system simulation with the Thévenin equivalent circuit remains accurate and 126 times faster when the time step is increased to 750 μs.

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