TRANSMISSION LINES PROTECTION BASED ON THE CURRENT EIGENVALUES DIFFERENTIAL CONCEPT V.F. Pires*, M. Guerreiro*, C. Fortunato* † , L.S. Martins* *Escola Superior Tecnologia Setúbal / Instituto Politécnico Setúbal, Setúbal , Portugal, emails: vpires@est.ips.pt; mgaspar@est.ips.pt; smartins@est.ips.pt † EDP – Energias de Portugal, Lisboa, Portugal, email: carlosf@est.ips.pt, Keywords: Current differential protection, eigenvector, eigenvalue, line protection, digital relay. Abstract This work presents a new approach for a current differential protection of the transmission lines. The proposed approach is based on the Clarke-Concordia transformation and principal component analysis. First the acquired current signals are transformed into “αβo” components by applying the Clarke- Concordia transformation. This allows obtaining typical patterns. To identify these patterns a principal component analysis is performed. Several tests under different fault conditions were performed. The obtained results allow verifying the effectiveness of the proposed approach. 1 Introduction The development of a computer relaying scheme allows improving power quality of modern power systems. One of the most important developments for the protection of transmission lines was the current differential relay. This protection was initially used for power transformers and generators [1,2,3,4]. However, with the development of communication and computer technology, this concept was also applied to transmission lines [5,6,7]. This allows improving speed clearing times for faults occurring at any point on a transmission line. However, the protection scheme is dependent of the available communications channel between the transmission line terminals. The communication channel is used to exchange information between each relay located at the transmission line terminal. However, when differential concept is applied to transmission lines, problems of sampling misalignment problem and communications channel delay make accurate current comparison difficult to achieve. To overcome the sampling misalignment problem and communications channel delay, current differential relay based on synchronized current measurement using Global Positioning system Satellite (GPS) is normally used [8]. However, GPS presents some problems. In fact, GPS is a sophisticated system that may suffer interruption, and it is not controlled by power system utilities. In this way a method based on a synchronous rotating frame has been proposed [9]. However, this method does not allow identifying the faulty phase. This work presents an investigation of a new approach for a current differential relay for transmission line protection. This approach is based on the obtained patterns of the Clarke- Concordia transformation and principal component analysis. So, instead of transmitting current samples, the obtained values of the principal component analysis are transmitted. This allows improving immunity to problems such as sampling misalignment and time delay of the communication channel. Several test results allow the validation of the proposed approach. 2 Proposed line current differential protection Current differential protection is one of the most effective power system protections. This concept is based on the kirchhoff’s current law. In this way current differential protection rests in the comparison of the sum of the incoming and outgoing currents. This protection has been used firstly for electrical equipments such as transformers and generators against internal faults. However, in the last years this protection has been used for the protection of transmission lines. However, for the protection of transmission lines a reliable communication channel is required. Fig. 1 shows an example for a transmission line current differential relaying scheme. This scheme consists of two relays at the terminations of the protected line with a communication channel between them. Fig. 1. Transmission line current differential relaying scheme. In traditional schemes each relay compares the currents of both sides of the line. In this proposed approach there are