Research On The Key Techniques Of A Novel Unified Power Quality Conditioner With Fault Flexible Arc-suppression Ability

The national strategic carbon reduction goal of "carbon neutrality" and the "double high" characteristics of the future power system have brought new opportunities and challenges to the new generation of power grid.Speeding up the low-carbon transformation of the power system and building a "clean,low-carbon,safe and efficient" smart grid has become an important way for the development of the future power system.However,with the increasing complexity of the distribution network structure and the massive access of renewable energy sources,power quality problems have become increasingly prominent.In addition,due to the widespread use of power cables and power electronic equipment,the fault current increases sharply when grounding fault occurs in distribution network,and the fault characteristics are complex,which makes it more difficult to manage ground faults.To ensure the safe and reliable operation of the power grid,the problems of power quality treatment and grounding fault arc-suppression have become key issues that need to be solved in the current power system.Aiming at the above problems and requirements,a n ovel unified power quality conditioner(UPQC)with fault flexible arc suppression ability is proposed in this paper.It can not only realize the comprehensive treatment of power quality problems,but also effectively realize the flexible arc-suppression under grounding fault.The novel UPQC has flexible control and function integration,which greatly improves its utilization and practicability.The main work and innovations of this paper are as follows:(1)The working principle and operating characteristics of the novel UPQC are introduced.During the steady-state operation of the power grid,the novel UPQC works in the power quality control mode,which can realize the effective treatment of the power quality problem,and once the single-phase grounding fault occurs,it works in the fault flexible arc-suppression mode to achieve control of the ground fault.Firstly,the circuit topology and working principle of the proposed UPQC are introduced in detail.Secondly,in order to avoid the output voltage of the converter exceeding its maximum output capacity under load fluctuation,the operation state adjustment strategy suitable for the novel UPQC is given.Then,the influence of series coupling capacitance on the stability and filtering effect of the control system is analyzed,and the parameter design principle of series coupling capacitance is given.Finally,simulation and experiment have fully verified the correctness and feasibility of the proposed UPQC.(2)In the power quality control mode,an output capacity improvement method suitable for the novel UPQC is proposed.The method can control the load reactive power demand shared by the series and shunt sides,while improving the utilization of the series side,and reducing the active capacity and the dc-link voltage level of the UPQC,so as to improve the output capacity and utilization of the device.Firstly,the principle of output capacity improvement based on series-shunt coordinated operation is analyzed,and two working modes under this method are introduced in detail,including: power regulation mode under normal condition and voltage compensation mode under voltage sag condition.Then,the proposed method is compared with other schemes to show its advantages in improving the output capacity.Finally,simulation and experiment verify the effectiveness and feasibility of the proposed output capacity improvement method.(3)In the fault flexible arc-suppression mode,an active energy balance method suitable for the novel UPQC is proposed.The method effectively guarantees the energy balance of the dc-link of the UPQC during the arc-suppression period without additional charging equipment,so as to maintain constant dc-link voltage,and improves the practicability and cost performance of the device.Firstly,the principle of full compensation of fault current with energy loss is introduced in detail.Secondly,the power flow of UPQC system in the fault flexible arc-suppression mode is analyzed.In order to meet the long-term suppression of the arc suppression current demand,an active energy balance method is given.Finally,simulation and experiment verify the effectiveness and feasibility of the proposed active energy balance method.