Research On Coordination Control And Steady State Analysis Method Of VSC-HVDC Transmission System

Voltage source converter based high voltage dc (VSC-HVDC) transmission technology is a kind of new dc transmission based on voltage source converter, full controlled power electronics device and pulse width modulation. VSC-HVDC technology has lots of advantages, such as fast independent control of active and reactive power, dynamic reactive power compensation ability, power energy quality improving ability, passive network power supply capacity, unaltered voltage polarity when flow is turned, easy to form multi-terminal dc (MTDC) network, etc. Therefore, it’s dramatically adapted to renewable generating integration, power supply of urban grids and island networks, asynchronous interconnection of ac system, electricity market trading, etc., which has a broad application prospect and business value. What’s more, it is paid close attention by many scholars and engineers and its engineering practice is in full swing within the whole world scope. Control strategy is the key of VSC-HVDC transmission system. On one side, effective and reliable local dc voltage controllers need to be designed and the coordination control between converter stations should be researched, for ensuring dc system voltage control and power balance under all operation conditions. On the other one, only with the local dc voltage controller is difficult to guarantee the VSC-MTDC system safety and economy under various operation conditions, so the upper dispatching control based on remote communication needs to be explored.Based on the above, the key of this paper is fault control of two-terminal VSC-HVDC system, coordination control and steady-state analysis method of VSC-MTDC system. The specific research works are as follows:(1) A hysteresis loop control based mode switching control strategy VSC-HVDC transmission system.Mode switching control strategy under ac system fault of DC voltage control side is analyzed. A mode switching control based on hysteresis loop and local DC voltage detection is proposed and implementation of this method is given. Then, relationship between DC power and voltages of both sides under steady state is derived and a method for calculating DC voltage working range of active power control converter is given. Based on the above, a method for determining DC voltage thresholds and references in the mode switching control strategy is put forward. Finally, simulations are undertaken in PSCAD/EMTDC to verify the validity of the mode switching control strategy under various faults and operating state, and results show that this determination method can provide mode switching control strategy of VSC-HVDC system with a reliable and accurate reference.(2) An advanced active power control strategy based on additional signal for VSC-HVDC transmission system.Power characteristics of the advanced controller are given. The outer-loop active power controller is designed and calculation formula of active power correction is derived. Operating principle of the controller is analyzed while VSC-HVDC system operates in both normal and fault states. Simulation studies are undertaken in PSCAD/EMTDC to verify that the proposed control strategy can guarantee DC voltage within safe range when AC voltage sags or the leading converter breaks down suddenly. Results show that the control strategy can improve system security and reliability.(3) Coordination control strategy of Four-terminal VSC-MTDC transmission system.Taking a typical four-terminal VSC-MTDC transmission system applied to wind power integration for example, dc power flow distribution is analyzed, dc flow calculation model is derived and active power control of assistant converter station with dc voltage-active power characteristic is proposed. A fault ride through method with damping circuit is given when master or assistant converter station outages. Simulations in PSCAD/EMTDC are conducted to verify the validity of coordination dc voltage control method under various operating state and MATLAB programming are undertaken to verify the accuracy of dc power flow model.(4) Coordination control strategy of Five-terminal VSC-MTDC transmission system.Using a typical five-terminal VSC-MTDC transmission system applied to wind power integration as an example, a coordination control strategy based on local controller among converter stations is proposed, improved control strategy for assistant and APC converter station are proposed respectively based on dc voltage-active power characteristic, and working modes of the two converter stations are analyzed. The parameter selection method of assistant and APC converter station is presented via dc power flow distribution and maximum/minimum operation mode. Simulations in PSCAD/EMTDC are conducted to validate the coordination dc voltage control method under normal, master converter station fault and assistant converter station fault conditions respectively.(5) DC operation characteristic analysis and steady-state point calculation of VSC-MTDC transmission system.VSC-MTDC transmission system is of various operating mode in practice: because of different converter numbers, converter control mode and orders. As the change of power grid operation conditions, control mode of VSC-MTDC transmission system is changed accordingly. Thus, operation center in VSC-MTDC system needs determine converter station control modes and state variables quickly based on converter operating characteristic, power grid conditions and system parameters, which is of great significance to fast dispatching and operation security. This paper analyses dc voltage-current characteristic of the master converter, assistant converter, active-power-control converter and wind-farm-side converter, while characteristic equations of all converters in all control modes are derived and electric-parameter range under different control mode is given. Steady-state-point calculation method is proposed and the correction method of converter station control mode is completed. Taking a typical fiver-terminal dc transmission system for example, MATLAB programming is conducted to verify the validity of dc characteristic analysis and steady-state-point calculation method. Results show that the proposed method can get the steady state operation point quickly and accurately.(6) An N-1principle based steady-state control scheme of VSC-MTDC transmission system. The requirements for maintaining VSC-MTDC transmission system safe and stable are analyzed. An N-1principle based control strategy is proposed for guaranteeing steady-state safe. Basing on continuous dc power flow calculation method and considering dc voltage limit and real power limit, control strategy is designed in detail which copes with one-converter-lost fault. A method of optimizing real power references is illustrated when there is no solution in the steady-state control strategy. An evaluation index is defined for searching the best real power references after optimization. Finally, a typical five-terminal dc system is introduced to verify feasibility and accuracy of the proposed strategy using MATLAB programming. Results show that this method can provide VSC-MTDC transmission system with reliable and safe references. The proposed steady-state control scheme can keep dc voltage safe, balance dc power and realize optimization of system operation mode under normal and N-1fault conditions, which will supply a reliable reference for dc system operation and dispatching.