Analysis of Negative Sequence Current Control for MMC HVDC Transmission Systems

Abstract

High Voltage Direct Current (HVDC) transmission system technology has recently attracted a lot of attention within the power system community. Modular Multilevel Converter (MMC) HVDC is the state of the art HVDC transmission technology, which contains hundreds and even thousands of submodules. It is the modularity, scalability and efficiency that are the most striking advantages of the MMC HVDC next to the reduced harmonic content of the output currents and the smaller AC filter size. The Electro Magnetic Transient (EMT) type simulation of the MMC HVDC transmission system faces the challenge of large computation time. In tackling that problem, the efficient model offers great improvements in terms of simulation time duration without compromising the accuracy. One of the grid code requirements which is under discussion and is being proposed nowadays involves the control of the negative sequence current components by means of Voltage Source Converter (VSC) HVDC systems during asymmetrical faults. The VSC HVDC system should be able to withstand the negative phase loading during asymmetrical faults without tripping the converter. This negative sequence component can be controlled with the enhanced control scheme which is known as Negative Sequence Current Control (NSCC). It allows negative sequence currents in VSC HVDC either to be suppressed or injected depending on the choice of Transmission System Operators (TSO). Suppression or injection of the negative sequence current demonstrates a different response to the power system performance during asymmetrical faults. This MSc thesis discusses the NSCC implementation using the efficient model of MMC HVDC transmission in the PSCAD / EMTDC. This thesis shows that the suppression of the negative sequence current (it is commonly applied by vendors) by the MMC HVDC converter leads to reduced negative sequence current components. With only the positive sequence current present, three phase currents appear as balanced during asymmetrical voltage conditions and so the converter is able to withstand the asymmetrical current fault. However, although this is beneficial from the power electronic component perspective, it has the drawbacks firstly of the double frequency power oscillations in the DC link, and secondly of the increased overvoltage which occurs because of the rise in the negative sequence voltages. Furthermore, the fault currents at the converter terminals appear to be very low in the steady state values range. As an alternative approach, the injection of negative sequence current is studied here. The motivation for the negative sequence current injection is either the enhancement of AC system protection schemes or the suppression of double frequency harmonics in the output power during faults. These are the different ways and methods that are evaluated in this work. The adequate injection of the negative sequence current during asymmetrical faults shows improvements in the DC voltage and power ripples in different case studies. Both the NSCC suppression and the NSSC injection in relation to the MMC HVDC are tested in the point to point MMC HVDC, 3 terminal MTDC and 9 bus AC grid connected Western System Coordinating Council (WSCC) with point to point MMC HVDC connection. In the 3 Terminal MMC HVDC applications, NSCC implementation shows optimal effects in the terminal where the single line to ground fault is applied. In the point to point MMC HVDC interconnection with 9 bus AC system, NSCC shows its effect in every section of the AC system. Moreover, the effect of NSCC is strongest near the MMC HVDC terminal in which NSCC is activated.