Research On The Control Strategies Of Voltage Source Converter Based HVDC

As the next generation of DC transmission technology, VSC-HVDC has made rapid progress due to its unique technical merits and it has been put into commercial applications such as the connection of island networks and renewable sources to a main grid, asynchronous connection of two grids for power market, reactive power compensation, etc. However research of the VSC-HVDC technology is still in its infancy in China. In this dissertation, the mathematical modeling and control strategy are studied, and the main contents are as below.1. Per-unit based mathematical model of VSC-HVDC is built up. The operating principle and the steady-state power characteristic of VSC station are analyzed, and then the influence of main circuit parameters on the operating characters of VSC station is made clear. On this basis, the detailed design principles of the phase reactors, DC capacitors and the DC voltage under the limit of power are obtained. Frequency domain model of VSC station is also built up to carry out the research of AC filter so as to gain its design principles.2. Based on the transient-state mathematical model of VSC-HVDC, the direct-current control dead-beat based discrete-time controller in dq synchronous rotation coordinate system is proposed. VSC voltage limiter, anti-windup controller, Smith predictor are selected to solve the corresponding problems of discrete-time controller. The detail design principles of inner current loop controller and the outer power loop controller are deduced.3. The power character of VSC station which takes both the negative and positive components into account is analyzed. On this basis, the system side control is adopted which takes the power on the L and R into account so as to cut off the spread of 2nd order DC voltage fluctuation from one station to the other through the DC cables. A novel anti-SSM (Sequence Separation Method) based dq-PLL with frequency adaptive character is proposed to gain better performance in unbalanced conditions.4. LVRT (Low Voltage Ride Through) methods of VSC-HVDC system are studied. Over-current phenomena of VSC valves in AC low voltage fault conditions are analyzed and then a novel reference current limiter for DVCC controller is proposed. Two LVRT methods are compared in this dissertation. One is to use VSC valve with larger current capacity, and the other is to use reference current limiter in control system. On this basis, a LVRT method with the novel reference current limiter is proposed, which is to switch the station function when in reference current limiting condition so as to ride through the faults without over-current phenomena.5. A transformer-isolating multi-module topology is proposed which is aiming to decrease the device number in series and its operating switching frequency and at the same time to simplify its topology. Then a novel control strategy for VSC-HVDC with this topology is proposed which consists of two parts, one is the independent control in module level and the other is the coordinated control in system level. Furthermore, an active power component adjusting method is proposed to balance DC voltages of different modules for sending-end station with this topology.