Reles Modernos

Application of Modern Relays to Dual-Breaker Line Terminals
Bogdan Kasztenny GE Multilin Ilia Voloh GE Multilin

1. Introduction
Standard practice today with respect to protecting dual-breaker line terminals – breaker-and-a-half or ring-bus – is to sum the two breaker currents externally and feed a single-input line relay (distance, line current differential or phase comparison) with thetotal cur-rent flowing into the protected line. Breaker failure protection requires monitoring the two breakers and currents separately, and is typically implemented as a stand-alone device out-side of the main protection relay. Reclosing and synchrocheck control functions require monitoring and controlling both breakers as well as measuring two pairs of voltages for the purpose of synchrocheck,are also – in majority of cases – implemented outside of the main line protection device. This paper discusses several protection and control aspects in relation to dual-breaker line terminals. First, breaker failure and reclosing functions are discussed as applied to dual-breaker configurations. Second, the paper talks about protection security as related to saturation of Current Transformers(CTs) under fault currents flowing locally through the two breakers. When the two currents are summated externally, the CT errors – if significant – can override the potentially low actual line current, and cause stability problems for the main line protection. Not only sensitive ground overcurrent functions are jeopardized, but also distance, current differential, or phase comparison. The paperexplains the problem and presents solutions. Stub bus protection is also discussed as pertaining to dual-breaker configurations. The paper looks at the above applications from the point of view of a modern micro-processor-based relay. New generation of line relays support dual CT inputs to monitor both breakers individually, and three voltage points to provide for the main line protection, andsynchrocheck across both breakers. These relays often include two breaker failure, two synchrocheck, and dual-breaker autoreclose functions. This allows integrating protection, breaker failure and reclose functions into a single relay. The paper points to advantages and disadvantages of such integration, and provides some guidance regarding dual-breaker line applications.

2. Capabilities of Modern LineIEDs
Modern microprocessor-based line protection relays (or Intelligent Electronic Devices, IEDs) allow for protection and control of the dual-breaker line arrangement from a single device. Application of separate breaker failure and/or synchrocheck relays is no longer dictated by limitations of the main protection relay, but driven by the user’s protection philosophy to either combine therequired functions, mostly for the for cost benefit, or to keep them separate for security, retaining present testing and maintenance practices, avoiding re-training the personnel, etc. With reference to Figure 1, a modern IED capable of the dualbreaker application supports two three-phase current inputs in order to measure both the currents individually for the breaker failure protection (50BF),backup overcurrent protection (51P), and associated metering functions. The two currents are added internally in the relay’s software to become the input for the distance (21) or high-set overcurrent (50) functions. When properly implemented, the line current differential (87L) and phase comparison (87PC) functions use individual currents for stability under through fault conditions.

Figure 1.Modern dual-breaker line IEDs.

Application of Modern Relays to Dual-Breaker Line Terminals

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Such modern IED typically support one three-phase voltage input required for the main (distance) or backup protection (distance backup on line current differential or phase comparison relays), and at least two single-phase voltage inputs in order to facilitate synchrocheck (25) across both the...