| With the construction of new power systems,the proportion of power electronic equipment in the power system that is connected to the grid has exceeded previous expectations.my country has been committed to supporting the structural transformation of the energy supply side and vigorously supporting the development of new energy sources such as wind power and photovoltaics.At the same time,the construction of High Voltage Direct Current Transmission(HVDC)systems is also increasing,and the power system is currently developing towards the "double high" characteristics of a high proportion of renewable energy and a high proportion of power electronic equipment densely connected to the grid.As a common problem in HVDC transmission system,commutation failure often leads to many problems such as large voltage and power flow fluctuations in the system,which leads to a series of other failures in the system and increases the risk of blackouts.At present,there is still a lack of relevant research on the probability inference of faults after commutation failures in grids with multiple DC feeds.In order to ensure the safe and stable operation of the power system,it is of great practical significance to analyze the influence of continuous commutation failure on system instability and to propose a probability inference method for other system failures caused by continuous commutation failure.This paper will carry out the following research work on the above-mentioned related issues:(1)Establish a multi-DC feed-in power grid model.The equivalent model is established based on the actual power grid in a certain region in the south,including modules such as ±500k V HVDC transmission,±800k V UHV DC transmission,offshore wind power grid-connected,and traditional thermal power generation grid-connected modules.After being connected to the grid,a ring network topology is formed.According to the basic equivalence principle of modeling,the equivalent simulation model of this typical area is established on PSCAD simulation software.(2)Based on the typical simulation model of multiple DC feeding power grids,this paper starts from the mechanism analysis of single commutation failure,and analyzes the causes of single commutation failure.The essential reason is the insufficient turn-off area.The influencing factors of continuous commutation failure are studied.Continuous commutation failure has new characteristics different from single commutation failure.The root cause of continuous commutation failure is that the commutation conditions fail continuously or intermittently in the time domain,and the cause of continuous commutation failure is the response of the HVDC control system.This paper firstly analyzes the influence of different harmonics on commutation failure by using the voltage time area method,and simulates the equivalent model established in PSCAD simulation software to verify the influence of AC line phase-to-phase fault on commutation failure..The results show that the phase-to-phase faults and harmonic factors in the multi-DC feed-in power grid are the important reasons for the failure of single commutation and then the failure of continuous commutation.Aiming at a series of systemic problems after continuous commutation failures,this paper analyzes the correlation of system failures caused by continuous commutation failures from the perspective of large fluctuations in voltage and system power flow.(3)Based on the influence of continuous commutation failure on the system stability in the multi-DC feeding grid,this paper adopts the methods of theoretical analysis and practical experience to determine and identify the risk factors,and introduces the fault level FL,that is,the failure of the AC system at the outlet side.Severity index,conduct research on the scenario of a large number of wind power and HVDC transmission grid-connected,when the transmission power of the wind turbine and the voltage on the outlet side of the wind turbine are constant,the comprehensive evaluation index to define the commutation failure is obtained through the change of the RMS voltage.cf.For different fault levels,different short-circuit ratios of AC systems,and different coupling impedances,the influencing factors of continuous commutation failure with multiple DC feeds are studied.At the same time,combined with the on-site wave recording,the failure analysis of the converter’s continuous commutation failure and blocking is carried out.The analysis and research provide an effective guarantee for the safe and stable operation of the power system,and the research results have strong objectivity. |
