Research On The Grid Friendly Control Of VSC-MTDC System

The last decade has witnessed a remarkable transformation in power transmission system.The rapid development of the renewable energy requires a cost-efficient and flexible way to transmit the large amount of power over long distances.The progress of the voltage source converter(VSC)prompts the development of the high voltage direct current(HVDC)transmission system.Nowadays,the VSC based multi-terminal HVDC(VSC-MTDC)system is one of the most attractive technology to integrate the renewable energy and to connect the non-synchronous ac grids.VSC-MTDC systems have several superior advantages,such as the decoupling control of active and reactive power,independent power supply ability to isolated grids and black start,and so on.VSC-MTDC systems can significantly improve the renewable energy utilization,strengthen the network structure.However,the increasing renewable energy and VSC-MTDC systems rise a number of challenges for power system stability,such as inertia reduction and the risk of oscillations.The hybrid AC/DC power systems including the renewable energy and VSC-MTDC systems have a complex dynamic phenomenon.Multiple components with different dynamic time constant can interact with each other,which could further lead to stability issues in multi-frequency.For example,the inertia of power systems could be reduced by the high penetration of electronics,and the frequency stability may loom.Moreover,the uncertainties introduced by the renewable energy make it hard to predict the operating state of power system,and the properties of oscillation modes could also change.Thus,to secure the stability of hybrid AC/DC power systems and improve the efficient utilization of renewable energy,appropriate control strategies of VSC-MTDC systems can be designed based on its flexible power control ability.Based on the support of support of the National Natural Science Foundation of China under Grant 51477046,the frequency stability and low frequency oscillation damping problem of hybrid AC/DC power systems are investigated and the gridfriendly control of VSC-MTDC systems are designed,which includes the decentralized and distributed control strategies of VSC-MTDC systems for primary frequency support,the virtual synchronous generator-based low frequency oscillation damping method of VSC-MTDC systems and the probabilistic small signal stability evaluation method of hybrid AC/DC power systems.The main results are summarized as follows:(1)An adaptive droop control strategy is proposed for VSC-MTDC systems to contribute frequency support for onshore AC grids.The objective of the proposed control scheme is to minimize the frequency deviation according to adaptive power sharing ability.To derive this ability,the power allocation mechanism of VSC-MTDC systems under the control of basic droop control is investigated based on a typical dc grid topology for the integration of offshore wind farms.Then the virtual inertia concept is used to design the adaptive droop characteristic,which can adjust the output power of each voltage source converter(VSC)station according to the dc voltage deviation in the dc side and frequency deviation in the ac side.According to the adaptive power sharing,the power mismatch can be shared by all the AC grids connected to the VSC-MTDC system.The proposed control strategy is a decentralized method because the adaptive droop characteristic has no requirement for the communication between VSC stations,which ensures the response speed of the control action.It is established that the proposed control strategy could be effective in providing fast power support and reducing the frequency deviation.(2)A distributed cooperative control strategy based on the consensus protocol is proposed for VSC-MTDC to provide frequency support.The distributed cooperative control consists of a local P-V droop controller and a cooperative agent network for cooperation control.Each agent controller is composed of a frequency regulation subagent for frequency support and a load ratio subagent for balancing the power sharing amongst the stations connected to the same AC grid.Moreover,to reduce the impact of the communication link failure,a topology evaluating method is proposed to select the suitable topology of the communication network via graph theory.The small signal stability analysis and the steady-state analysis are performed to investigate the stability and the consensus performance of the VSC-MTDC systems with DCC.Compared with decentralized control,the DCC can simultaneously achieve better frequency support and power sharing without leading to a large DC voltage deviation.(3)A virtual synchronous generator-based damping controller is proposed for VSC-MTDC systems to improve the damping performance to low frequency oscillations.The virtual synchronous generator has been widely applied for mimicking the dynamic performance of a synchronous generator by power-electronic interfaced power sources.When the virtual synchronous generator is implemented in VSC-MTDC systems,the impact of VSG on both ac-and dc-grid dynamics,i.e.,the low frequency oscillatory modes and dc voltage stability should be taken into consideration.This paper investigates the participation of the virtual synchronous generator controller in low frequency oscillatory modes and the impact on dc voltage stability.The influence of VSG parameters on the system dynamics is investigated by the Singular Value Decomposition analysis and modal analysis.A parameter alternating virtual synchronous generator controller is proposed to improve the damping performance to LFO modes and mitigating the negative impact of VSG on dc voltage stability.The effectiveness of the proposed AVSG controller is validated according to the nonlinear simulations on two test systems.(4)A probabilistic small signal stability analysis method for the hybrid AC/DC power systems are proposed.Owing to the stochastic states of power systems with large-scale renewable generation,the impact of VSC-MTDC systems on the stability of power systems should be examined in a probabilistic manner.A probabilistic small signal stability assessment methodology to select the best locations for VSC-MTDC systems in AC grids is proposed first.The deterministic sequence,i.e.,Sobol sequence technique is used to generate the stochastic operation scenarios of the original power system with the consideration of several stochastic factors.The statistical controllability index is used to identify the best locations of VSC-stations for improving the damping of the oscillation modes.Then the hybrid AC/DC power systems is evaluated by the statistical small stability index.The proposed method can effectively evaluate the influence of different locations and control strategies on the system stability using a small size of power system operating samples.