Research on the Source Tracing Method of Three-Phase Unbalance Based on Equivalent Circuit

The three-phase imbalance phenomenon of power system under normal operation has aroused the increasing concern of power supply and consumption departments, which will have a certain impact on the power quality of the grid. It is urgent to determine the source of imbalance in the complex distribution system, and then take appropriate governance measures to solve the problem of three-phase imbalance in power system. How to determine the location of the unbalanced disturbance source is particularly important for taking further control measures. However, the existing methods require that the power quality testing instrument used to measure the phase requires high accuracy and may cause error in judgment when the negative sequence current is less than a certain range of the measuring instrument. Therefore, following the modeling idea of harmonic source load equivalent circuit, this paper equates the unbalanced load as the parallel connection of negative sequence current source and impedance. By comparing the three-phase unbalanced voltage contribution level of the system side and load side at the measuring point, the location of the main three-phase unbalanced source can be distinguished. The method is simple and effective and has certain advantages in practical engineering applications.

I. Introduction

The problem of three - phase imbalance in power system has been paid more and more attention by power supply and consumption departments. The voltage imbalance under normal operation is generally caused by the imbalance between the power supply link and the power consumption link [1]–[4]. The power supply link is composed of four links: power generation, transmission, power transformation and distribution. The three-phase components involved in each link will lead to voltage imbalance. The unbalance of power consumption link is mainly caused by various unbalanced loads of users, such as electric iron, electric arc furnace, single-phase household load, etc. [5]. Three-phase imbalance is one of the most common phenomenon in power system, it will affect the power quality of the grid. To determine the root cause of unbalance in complex power distribution system and take corresponding measures to avoid the phenomenon of three-phase unbalance is the key point at present. Firstly, the theoretical analysis method of tracking unbalance source is given, and the theoretical analysis results are verified by simulation experiment. Then, this theory is applied to the three-phase imbalance analysis of the actual distribution system to determine the root cause of the three-phase imbalance of the power grid [6]–[7]. The different distribution of single-phase load in each phase and the commutation of asymmetric transmission lines are the two main factors that cause voltage unbalance in power system.