Decentralized Estimation of Two-Port Equivalents for Power Transmission Lines

Abstract

Two-port equivalent model of the system around a transmission line is commonly used for various line protection related analyses. The main aim of the paper is to present methods that can be adopted in a decentralized manner at the substation level for estimating such an equivalent model. Firstly, the paper contributes to methods for estimation of simple two-source equivalents based on events such as fault on the line or shunt element switching at its terminals. The paper extends a discussion towards limitations of such models in the presence of a transfer path across the line terminals and proposes an extension of the estimation techniques to overcome the issue. Lastly, the paper proposes methods for updating the two-port equivalent model when network topology in the vicinity of the line of concern changes. This contribution differs in the aspect that it only seeks to update the two-port equivalent model, unlike the previous contributions wherein the aim is to estimate the equivalent model without assuming the availability of an initial solution. The methods for updating utilize the equivalent model parameters in a base network scenario and the measurements excited from a topology event. The proposed methods use only limited measurements of bus voltages and line currents, and model parameters of the line of concern and other neighboring apparatus (such as other incident lines or shunt reactors/capacitors). This is an advantage as the availability of the complete network model at any substation is always a challenge. Therefore, the methods can be deployed directly at substations in a decentralized manner. The proposed solutions are computationally easy and amenable for implementation in the framework of digital substations with advanced communication infrastructure. Testing and analysis are done using a simple two source transmission system and the IEEE 39 bus test system.