Research On Operation Control Strategy Of VSC-MTDC Transmission System

As the technology of modular multi-level converter control continues to mature,the applications of VSC-HVDC system are increasingly widespread.Separate two-terminal system can not meet the requirements in the aspects of multiport wind farms integrated to grid,various forms of energy interconnecting with multiple load centers,multiple AC gird interconnection being composed of a DC grid and so on.Therefore,VSC-MTDC(Multi-terminal HVDC based on VSC)system has become one of research focuses in this field.The MTDC system has more control freedom compared with the double-ended system,therefore the operation mode is more flexible,but the control methods are relatively complicated.The control system is usually divided into several lays,including centralized control,station control,converter-control and valve-control layers.Centralized control and station control is of the top-level,which provids control mode,given values of power and voltage for each converter station.The tracking control is achieved in valve and converter control layers.The research contents of this paper are mainly on the strategies of converter and valve control.For the converter control,the foucus is coordination control strategy of MTDC and its stability.For the valve-control layer,the internal circulating current is detailedly analysised,because of MMC is usually used in VSC-HVDC system.The control functions of base frequency circulating current and energy balancing between arms are added besides second harmonic circulating.That can improve the control performance.The following aspects and innovation points are included in the paper.1.The application of conventional droop control method in MTDC is researched.The mathematic model of DC side in MTDC is established,and there are multiple DC voltage control stations in MTDC.The realationship between DC voltage and current is deduced based on the above model.The DC voltage-current droop controller is designed in DC voltage control station,so that the received / transmitted power of the DC voltage converter stations are assigned automatically in proportion to their own rated capacity,and it allowing multiple DC voltage control stations being in operation simultaneously.For a power control station,if DC voltage exceeds the maximum setting during transient,it would switch to DC voltage control station with DC voltage and active power droop characteristics,which can help stabilize the DC voltage of the system.4-terminal simulation model is built based on PSCAD for the verification of the aboved drooping control method,and the simulation results show that the power can be automatically distributed between multiple DC control stations in accordance with the set slope.In addition,the simulation results also show that,DC network voltage is also of deviation with its rated value,when the power is not equal to the rated operating point.For the deviation,the voltage secondary regulation method is referenced,and the DC voltage secondary control strategies are designed.The total active power error,between actual power of each converter station and its given value,is allocated among DC voltage control stations.And the DC voltage can restore to the rating value.The active power also can track its given value without error.2.If the conventional vector control strategy based on PI regulators is used,the overshoot of DC voltage maybe too high during the transient of DC voltage control stations being exit or restore operation dynamicly.The analysis shows that after the original DC voltage control station being out of operation,the power control station is switched to stabilize the DC voltage,but the response of DC voltage control loop is slow when switching,so the DC over voltage is serious.For this,the back-stepping is used for designing the DC voltage controller with droop character of power control station.The control law of asymptotic stability for DC voltage droop controller is derived.The improved droop controller can recude the duration time and value of DC over voltage in the switching process by simulation contrast.3.The control methods for circulating current of MMC are researched.Fundamental frequency circulating current control function is added and the conventional double fundamental frequency circulation suppression is reserved.So the voltage banlance control between upper and lower arm,also including capacitors voltage in one arm(Sorting capacitor voltage),is achieved.A combination of aboved balancing control method can improve system stability.Furthermore,the internal dynamic mathematical model of MMC is established,the voltage sum of upper and lower submodule capacitors voltage and circulating current being as state variables.Based on this,the stability of indirect and direct modulation method is analysised.And it is noted that the dynamic equation of voltage difference between upper and lower arm is of less damping characteristics.So,damping injection method is proposed,and the DC component of circulation is introduced as a feedback to increase the system damping.The simulation results show that the proposed method can eliminate the DC power fluctuations and improve system stability.4.The hybrid HVDC composed of MMC and the traditional line commutated converter(LCC)is researched,including system characteristics and control methods.Conventional LCC operates as a rectifier,using constant DC voltage control,and the MMC is as an inverter,controlling the transmission power.Simulation results show that,there is a low frequency power fluctuation in DC side,using the conventional vector control method for MMC.For this,DC side equivalent model of hybrid HVDC is established,and it is shown that the DC circuit is a LC resonant one.Moreover,there is a large smoothing reactor connected with the DC side of conventional LCC,so the rate of current change(di/dt)is low.It is contradictory with fast current inner-loop of conventional vector control strategy for MMC.Therefore,the power synchronization control method is proposed and the conventional inner current loop is cancelled.Active power tracking control can be achieved by adjusting the phase of modulation signal.Meanwhile transfer function between active power and phase angle of power synchronous control is derived.This transfer function is of less damping characteristics.The additional closed loop control for energy of arm capacitor is proposed.And it is equivalent to injection damping to the system,which can improve its stability.DC power is unidirectional in hybrid HVDC system,so,the topology of diode valve being series connecting in the DC circuit can be adopted to achieve DC fault isolation.The maximum anti-voltage on the diode valve during bipolar short circuit fault is analysed based on simulation and it can provide a reference for the actual engineering design.5.A small power three-terminal experimental platform is built.The basic and improved droop control algorithms including the DC voltage secondary adjustment and the backstepping control are verified based on this experimental platform.Futher more,the inverters in VSC-MTDC system's functions like isolated operation as the STATCOM(reactive compensation)and isolated power supply are also verified.That build up experimental basises for future more in-depth study of a multi-level MTDC system.Innovations of the paper are summarized in the following aspects1.The voltage secondary regulation method of AC power system is referenced,and the DC voltage secondary control strategies are designed for VSC-MTDC.The transmission power is distributed again,and system can run on the rated operating point.2.The DC voltage control strategy of VSC-MTDC system is deduced based on back-stepping method.It can reduce the overvoltage level during transient process(the power control station change to DC voltage control one)3.The internal dynamic mathematical model is built for MMC,and the under-damping characteristics of the mathematical model are analyzed.For this,the damping injection method is proposed,and it can eliminate the DC power fluctuations,improving system stability.4.The hybrid HVDC is researched based on LCC and VSC.The equivalent circuit of DC side is proposed and its less damping characteristics is shown.The additional closed loop control for energy of arm capacitors is proposed.And it is equivalent to injection damping to the system,which can improve its stability.