Study of self-starting mode parameters of complex load electric motors on a simulation model taken into account when selecting relay protection settings of high-voltage transmission lines

Background. When selecting the settings for the backup stages of distance protection of the electric power transmission lines, we should consider the parameters of the self-starting mode of load electric motors, characterized by the K_str coefficient of current increase in the protected line and the j_str angle of the load impedance. The available methods for selecting the settings of the backup stages of the line distance protection, as a rule, consider only the approximate values of the K_str coefficient. The j_str angle is often assumed to be equal to the j_oper angle of impedance at the “terminals” of protection in the maximum operating mode without considering the self-starting of the load motors. Errors caused by the inaccuracy of determining the parameters K_str and j_str may be the reason for a decrease in the selectivity and sensitivity of the backup stages of the line distance protection, i.e. the effectiveness of remote backup. To clarify the available methods for selecting the settings of the line distance protection, an urgent task is to study the dependencies of the parameters of the self-starting mode K_str and j_str on the proportion and type of motor load as a part of the complex load.

Materials and methods. To study the dependencies of the parameters K_str and j_str of the self-starting mode on the proportion and composition of the motor load in the complex load the MATLAB software package is used with the SimPowerSystems package, designed for modeling electrical devices and systems. The model of the object under study includes a 110(220) kV power supply, a 110(220) kV line with a 110(220)/6(10) kV step-down transformer at the end of the line, a 6(10) kV network section with cable lines for supplying a group of high-voltage asynchronous electric motors (AMHV) and a 6(10) kV step-down transformer 6(10)/0,4 kV for supplying a group of low voltage asynchronous electric motors (AMLV). The parameterization of the model has been carried out considering the real characteristics of its elements and methods for calculating the parameters of equivalent circuits.

Results. The authors have proposed an approach to approximate calculation of self-starting parameters based on data about the typical consumers of a complex load node with load division into three categories: AMHV, AMLV and static load. It is shown that the main factors influencing the parameters K_str and j_str of the self-starting mode on 110–220 kV transmission lines are the proportions of AMHV K_AMHV, AMLV K_AMLV and their ratio in the complex load. The dependences of the parameters K_str and j_str of the self-starting mode on the proportion of the motor load in the complex load, as well as the dependence j_str = f(K_str), are obtained.

Conclusions. The obtained dependences K_str = f (K_AMHV, K_AMLV), j_str = f(K_AMHV, K_AMLV) and j_str = f(К_str) can be used to clarify the methods to calculate the settings of backup stages of 110–220 kV line distance protection. It will increase the efficiency of remote backup in high-voltage electric networks.