Research On Control Strategy For VSC-HVDC Under Unbalanced Conditions

Based on fully controlled semiconductors and pulse width modulation technology, voltage source converter based high voltage direct current (VSC-HVDC) transmission is new and developing DC transmission over the past few decades. Compared with line-commutated current source converter based high voltage direct current (LCC-HVDC) transmission,it has its own merits.These features make VSC-HVDC transmission attractive for applications such as passive or weak ac systems, urban power networks, multi-terminal HVDC transmission and renewable sources to a main grid.In actual operation process, voltage unbalance and fault conditions in AC system are inevitable.The control strategy for VSC-HVDC system under unbalanced conditions is directly related to its safe and stable operation, so it has a great importance for the engineering.In this paper, the control of two-terminal VSC-HVDC transmission system is investigated under unbalanced conditions.The main contents of this paper are as follows:1. Transient mathematics models of VSC-HVDC system under balanced and unbalanced conditions are built respectively. Then the paper mainly analyzes operation characteristics of VSC-HVDC system under unbalanced conditions.The results show that under ac system asymmetrical conditions large negative sequence current will appear in the ac side,and dc voltage and active power will exist the double frequency fluctuation. Subsequently,two control methods for VSC-HVDC system under unbalanced conditions are briefly discussed.In order to restrain negative sequence current, based on Hilbert transform and instantaneous symmetrical components, a control strategy of negative sequence voltage compensation is proposed in this paper.2. The definition, attribute and significance of Hilbert transform are described in detail. Then this paper introduces the construction process of analytic signal and the relationship between analytic signal and rotary vector.By means of finite impulse response digital filter,the implementation principle and method of Hilbert transform are discussed.At last, with remez fuction provided by MATLAB, a digital Hilbert transformer is designed.3. Based on instantaneous symmetrical components,mathematical expressions of decomposing three-phase voltage rotary vectors in time region are derived. Feasibility and correctness of the method is validated by digital simulation based on PSCAD/EMTDC.4. In order to restrain the negative sequence current, the paper presents an unbalanced control strategy based on negative voltage compensation for VSC-HVDC system, and design its inner and outer controller. the control method is fulfilled by adding limited control blocks into traditional current vector current control, so it simplifies the structure of the control system. At last,feasibility and correctness of the control method is validated by digital simulation based on PSCAD/EMTDC.