Research On Fault Characteristics And Protection Theory Of Distribution Network With Inverter-based Distributed Generation

In recent years,inverter-based distributed generation technologies represented by wind power and photovoltaic solar power have been developed in China.But,a large number of inverter-based distributed generation grid-connected also brings the following new problems to the distribution network:(1)The fault output characteristics of the inverter-based distributed generation are different from those of a traditional grid power.Affected by its control strategy,the grid-connected inverter-based distributed generation changes the fault characteristics of the distribution network.(2)When a large number of inverter-based distributed generation are connected to the distribution network,the distribution network changes from the traditional single-supply radial network to a complex network with multiple power supplies,and current protection used in the traditional distribution network may occur incorrectly,which affects the safety and reliability of the distribution network operation.For this reason,the main research of this paper is as followsFirst of all,this paper analyzes the working principle and grid-connected control strategy of the inverter-based distributed generation under normal conditions.Based on this,there is a mutual coupling relationship between the positive and negative sequence components in the asymmetrical three-phase voltage of the power grid.The grid-connected control strategy of inverter-based distributed generation in the case of distribution network faults is studied: double-current loop control based on positive and negative dq rotating coordinate system.Based on this control strategy,the system can eliminate the negative sequence component of the inverter-based distributed generation output current when the grid voltage is unbalanced and improve the waveform of its output current.In addition,in order to meet the requirements of grid-connected regulations,this paper studies a low-voltage ride-through operation control strategy of inverter-based distributed generation.The above principle is verified by Matlab/Simulink.The simulation results show that the proposed inverter-based distributed generation with low voltage ride-through has only positive sequence components in the output current during the symmetrical or asymmetric drop of the grid voltage,and can provide sufficient reactive power support to the grid to meet the grid requirements.Secondly,this paper analyzes the problems of the traditional grid fault analysis method when it is applied to the distribution network connected by inverter-based distributed generation.For the proposed low-voltage ride-through control strategy,the inverter-based distributed generation is equivalent to a voltage-controlled current source model.Based on this,a grid fault analysis method including this kind of inverter-based distributed generation is established.The feasibility of the fault analysis method proposed in this paper is verified by simulation experiments.Finally,the influence of the inverter-based distributed generation connected to the current protection of the distribution network is analyzed in detail,and according to this,the distribution network is divided into three regions according to the different positions of the distributed generation connect(the upstream areas and downstream areas of the feed line where the distributed power source is located and the adjacent feeder area)then individually protected.In the downstream area,the existing protection setting method was redesigned considering the maximum short-circuit current provided by the inverter-based distributed generation,and a corresponding sensitivity check is performed for the new setting method.According to the difference between inverter-based distributed generation and traditional power supply,a new power direction element adapted to inverter-based distributed generation is designed,and a directional element and cascade blocking are used to protect it.Finally,the feasibility of the proposed protection principle is verified by simulation in this paper.