Research On Hybrid Multi-terminal HVDC Operation Characteristics

LCC-HVDC and VSC-HVDC have their own technical advantages and different application occasions.Due to large capacity and low power losses,LCC-HVDC is suitable for large-capacity and long-distance transmission.As for VSC-HVDC,its control flexibility and capability of forming a multi-terminal HVDC make it suitable for renewable energy integration and power supply to weak AC systems.Hybrid multi-terminal HVDC(Hybrid-MTDC)can combine the advantages of LCC-HVDC and VSC-HVDC to achieve multi-power supply,multi-point receiving which is a more flexible and efficient way of transmission to obtain the greatest economic and technical benefits.At present,the power industry is moving in the direction of increasing the use of clean energy and large-scale grid interconnection.Hybrid-MTDC will play an important role in the development of power system,and some existing projects already have hybrid HVDC prototype.As a result,it is necessary to study the operation characteristics of the Hybrid-MTDC to embrace the practical application of the Hybrid-MTDC.The main research work of this paper and the corresponding results are shown as follows:(1)The consensus algorithm based decentralized autonomous control for the Hybrid-MTDC.In this control method,each terminal only communicates with the adjacent terminals to obtain global information,which reduces the performance and reliability requirements for the communication system.Each terminal can not only operate independently but also cooperate with each other to complete the system optimization goal.Also,it can not only realize the fine adjustment of the operation point of each terminal,but also the group control of terminals according to the actual situation.Compared to centralized control,this control method does not require the central control unit,will have more application prospects in the growing DC power grid.(2)The key factors influencing the power losses of the Hybrid-MTDC system.In this paper,it is pointed out that the Hybrid-MTDC power losses has to be optimized between two contradictory targets: on the one hand,the DC voltage is needed to be as high as possible and the power is needed to be delivered as much as possible by the LCC.On the other hand,when the current is large,the LCC will turn to constant ? control,this time the VSC needs to correspondingly reduce the DC voltage,otherwise,more DC current will flow through the LCC inverter,which will lead to commutation failure,but the DC voltage will lead to increasing power losses.Based on this,the consensus algorithm based decentralized autonomous control is applied in the Hybrid-MTDC power losses optimization,so that each terminal can cooperate with each other to achieve the optimization goal.For different operation modes,this paper also compares the optimization results under different global information integrity and different number of terminals participating in the optimization control.It is concluded that the higher integrity of the global information and the more terminals that participate in the optimization control will result in smaller system power losses.(3)The control strategy for the VSC connected to the wind farm in the Hybrid-MTDC system.Due to the stochastic and intermittent of wind energy,the wind farm should be in island operation and its AC voltage should be provided by the VSC,so the model predictive control is designed for the VSCs in the Hybrid-MTDC in this paper.The control strategy establishes the prediction model for the AC and DC side of the VSC and does not contain any PI loop.The optimal solution of the modulation waveform in several sampling periods can be directly obtained,which avoids the interaction with the PI loop in the doubly-fed induction generator controller.Besides,the anti-disturbance capability and fault recovery performance are better when the wind speed fluctuates and the AC and DC fault occurs.(4)The AC background harmonic transfer through MMC,the resonance problem of the DC network and the harmonic mitigation control strategy.First,the impact of AC background harmonics on the MMC phase-legs is studied theoretically,and the mechanism of the AC background harmonics transferred to AC side and remote AC system through MMC is obtained.Then,taking a 4-terminal MMC HVDC system as an example,the frequency-impedance characteristics of the DC network is analyzed to reveal the reason of harmonic amplification.Based on this,a harmonic mitigation control strategy using the energy storage capability is proposed in this paper to eliminate the DC network resonance caused by AC background harmonics and the harmonic propagation.The simulation in PSCAD/EMTDC shows that the control strategy proposed in this paper can effectively block the propagation of the background harmonics to the DC network,and avoid the DC network resonance due to the background harmonics.In the Hybrid-MTDC composed of MMC and LCC,there still will be several resonant frequencies in the DC network,the harmonic mitigation control strategy proposed in this paper can be also applied in eliminating the harmonic propagation of the Hybrid-MTDC system.