Mathematical Problems in Engineering

Volume 2015, Article ID 608520, 17 pages

http://dx.doi.org/10.1155/2015/608520

## A Novel Algorithm for Power Flow Transferring Identification Based on WAMS

^{1}State Key Laboratory of Alternate Electrical Power System with Renewable Energy Source, North China Electric Power University, Baoding 071000, China^{2}School of Electrical and Electronic Engineering, North China Electric Power University, Baoding 071000, China

Received 4 May 2015; Accepted 14 September 2015

Academic Editor: Yang Tang

Copyright © 2015 Xu Yan et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

After a faulted transmission line is removed, power flow on it will be transferred to other lines in the network. If those lines are heavily loaded beforehand, the transferred flow may cause the nonfault overload and the incorrect operation of far-ranging backup relays, which are considered as the key factors leading to cascading trips. In this paper, a novel algorithm for power flow transferring identification based on wide area measurement system (WAMS) is proposed, through which the possible incorrect tripping of backup relays will be blocked in time. A new concept of Transferred Flow Characteristic Ratio (TFCR) is presented and is applied to the identification criteria. Mathematical derivation of TFCR is carried out in detail by utilization of power system short circuit fault modeling. The feasibility and effectiveness of the proposed algorithm to prevent the malfunction of backup relays are demonstrated by a large number of simulations.

#### 1. Introduction

With the interconnection of power networks, transmission lines are operating close to their limit. Power system relay protection is of great importance to the security and the stability of power system. Recently, many well-known blackouts occurred worldwide [1–5]. Related researches show that the incorrect operation of far-ranging backup relays is relevant to most of them. After the faulted transmission line is tripped by protective relays, the flow on it will be transferred to other lines, which will result in overloads on them if they have been heavily loaded before. Although the backup relays such as zone 3 distance relays will remove these lines according to their setting principles, they will deteriorate the system status and will promote the process of system collapse. If the transferred power flow is identified, the backup relays tripping will be blocked in time and some measures to eliminate the overload will be taken to prevent cascading trips. Unfortunately, because of the only utilization of local data to make judgment, the existing traditional backup protection cannot distinguish whether the malfunction is caused by internal fault or by transferred power flow.

The rise of wide area measurement system (WAMS) has opened a new gate for power system wide area backup protection [6–9]. The system operating parameters, such as three-phase voltages and currents, power angle, and active and reactive power flows, can be updated every 20–50 milliseconds; thus it is allowed to acquire and to deal with the synchronous data of the whole system thanks to the time delay of backup protections. Although the substitution of supervisory control and data acquisition (SCADA) system by WAMS is limited by the current technical and economic conditions [10, 11], the complete observability of power system can be fully guaranteed according to the optimal placement methods of the limited-number GPS-based synchronized phasor measurement units (PMUs). Therefore, the realization of real-time tracking of the complicated network topology makes it possible to carry out researches on backup protection scheme taking the whole system safe, stable, and reliable operation as targets [12, 13].

The past decades have witnessed some categories of methods provided by a growing number of investigators at home and abroad to prevent cascading trips, such as multipoint measurement information based differential backup protection [14, 15], adaptive scheme for distance protection [16], expert decision system for wide area backup protection of transmission networks [17, 18], and fault directional comparison principle [19]. However, these references focus on the optimal procedure of power system fault elimination and how to avoid the incorrect operation of protective relays by internal fault, not taking prevention of backup relays tripping caused by nonfault overload into consideration. On the other hand, the flow transferring relativity factor in the paper [20] and the DC flow based transfer power flow sensitivity factors in the paper [21] are, respectively, presented to estimate the postfault power flow distribution (the branch current and the active power flow distribution). By comparing with the measured flow by WAMS, whether the overload is caused by the transferred flow or not can be identified. However, these methods may identify incorrectly because the variation in reactive power injection caused by fault elimination is ignored during the estimation of postfault flow distribution. Besides, with the number of grid nodes becoming larger, the computational burden of these methods is too heavy to estimate the flow distribution in the allowed delay of the backup protection.

In this paper, a novel algorithm for power flow transferring identification based on WAMS is proposed, through which transmission line overload caused by the transferred power flow will be identified exactly, and the possible incorrect tripping of backup relays will be blocked in time. The original property of the distance protection that protective relays operate as soon as the delay time is out will not be changed. The novel identification criteria of the algorithm employ a newly proposed concept of Transferred Flow Characteristic Ratio (TFCR) composed of 3 ratios which are calculated after zone 3 distance protection starts. Because the variation in reactive power infection, the load type, the different fault locations, and transition resistances do not influence the identification accuracy, the proposed algorithm owns a high reliability. Besides, the settings of the identification criteria can be determined in terms of the analysis in this paper as well as the on-the-spot conditions, making the proposed algorithm highly flexible. Moreover, as the algorithm is simple in principle and the electrical quantities required for the TFCR can be directly measured by PMUs, the burdens of computation and communication are lightened to improve the performance of traditional backup protection.

The rest of this paper is structured as follows. In Section 2, two ratios of TFCR are derived mathematically by utilization of the fault current model. Two current criteria to identify internal fault or nonfault overload caused by the transferred power flow are proposed. Section 3 solves another ratio of TFCR using the fault voltage model and proposes the voltage criterion. Section 4 states the logical relationship of the 3 criteria and provides the scheme and its implementation of the novel algorithm for power flow transferring identification. A number of simulation results in Section 5 validate the effectiveness of the proposed algorithm. The paper is concluded in Section 6.

#### 2. Mathematical Derivation of Criteria Based on Fault Current Model

If the overload is caused by the transferred power flow, three-phase currents on transmission lines will stay symmetric and the three-phase current increments will have equal amplitudes. Moreover, zero-sequence current will not exist. However, if the overload resulted from an asymmetric short circuit fault, the three-phase current increments which are called fault components of 3 phase currents will not be equal. Furthermore, if it is an earth fault, there will appear the zero-sequence current. In order to establish the criteria to identify the asymmetric short circuit fault, two characteristic ratios of a transmission line calculated by fault components of 3 phase currents and zero-sequence current are derived mathematically via current distribution factor theory and asymmetric short circuit fault modeling.

##### 2.1. Theory of Current Distribution Factor

Current distribution factor represents the proportion of fault current supplied by each power source in the network. The definition of current distribution factors and their usage in solving fault components of three phase currents are explained in the following paragraph.

As is shown in Figure 1, assume that a short circuit fault occurs at point . Distance Protection 1 is installed near Bus , of which zone 3 is considered as the far-ranging backup protection of Distance Protection 2 installed near Bus . We select phase as the reference phase; that is to say, we use sequence components of phase to solve three-phase voltage phasors and current phasors during a fault. The -phase current flowing out of fault point is represented by and the fault component of -phase current on Line 1 is represented by .