Relay System, Protective

a set of devices (or an individual device) that includes relays and is able to react to a short circuit in various elements of an electrical system by automatically identifying and isolating the faulted section. In a number of cases, protective relays can react to other disturbances of the normal operating conditions of the system—for example, over-voltages or overcurrents—by actuating a signal system or, less often, by isolating the corresponding element of the system. A short circuit is the principal type of fault in electrical systems in frequency of occurrence and scale of adverse consequences. When a short circuit occurs, the voltage decreases abruptly and nonuniformly in the system, and the current in individual elements of the system increases substantially. This situation can eventually cause a power failure for consumers and damage to equipment. The use of protective relay systems helps minimize the harmful effects of a short circuit.

Protective relays operate when specified electrical quantities vary. The type most frequently encountered reacts to an over-current. Sometimes voltage is used as the actuating quantity. There also exist protective relays that respond to a decrease in the voltage-to-current ratio, which is proportional to the distance from the point of the short circuit to the location of the protective relay; in this case, we speak of distance protection. Ordinarily the protective relays are isolated from the system; they receive information regarding the electrical quantities from current or voltage instrument transformers or other measuring transducers.

Each element of an electrical system—for example, a generator, transformer, or transmission line—is usually equipped with separate protective relays. The system as a whole is protected by an integrated selective protective relay system. When a failure occurs, the faulted element is isolated entirely by a specific protective relay; the other protective relays do not operate when they receive the information about the short circuit. Such a protective relay should operate when short circuits occur within its protected element; it should not operate when short circuits occur outside the protected element or when no short circuit occurs.

The selectivity of a protective relay can be characterized by the extent of the zone protected by the relay: when a short circuit occurs within this zone, the protective relay systems operate with a given speed. The selectivity can also be characterized by the types of operating conditions of the electrical system for which the relay should not operate. A common classification of protective relays depends on the level of selectivity for external short circuits: absolutely selective relays do not operate when any external short circuits occur; relatively selective relays are designed to operate when external short circuits occur only if the protective relay circuit breaker of an adjacent faulted element fails; nonselective relays are permitted to operate—for the sake of simplicity—when external short circuits occur within the limits of some zone. The relatively selective types are the most common. All protective relays must satisfy the requirements of stability and reliability of operation. Stability is characterized by the efficiency of the relay’s methods of identifying the electrical system’s operating conditions. Reliability is determined primarily by the absence of failures in the operation of the protective relays.

One of the simplest methods of achieving selectivity of protective relaying is usually employed in overcurrent and distance protection. This method involves the use of a relay in which there is a definite time delay between the moment when the relay is required to operate and the moment when the operation of the relay is completed.

Figure 1 shows a diagram of a section of a radial electrical system, in which current flows to the location of a short circuit from one direction. The system is equipped with relatively selective protective relays, and the corresponding time delays are also shown in Figure 1. The protective relays 1 and 2 each have three stages, and each stage is adjusted for certain values of the input signal so that the time delay of the relays is a step function of the distance to the location of the short circuit. The extent of the zones that are protected by individual stages and the corresponding delay times are chosen so that the protective relays in the faulted sections of the system operate sooner than the other relays. The first protective-relay stage has no special time delay. The zone of this stage should be somewhat smaller than the protected section because relay 1, for example, cannot distinguish between short circuits at points S₁ and S₂. The last stages (the third stages in the protective relaying shown in Figure 1) are backup stages and often have no clearly defined zones.

Figure 1. Diagram of a section of a radial electrical system, wherein power is supplied from one direction; the section is equipped with relatively selective protective relays. The lower part of the figure shows the time delays of the relays; the distance along the line is plotted along the axis of abscissas. (A), (B), and (C) substation buses, (CB₁) and (CB₂) circuit breakers, (P) power source, (CT₁) and (CT₂) current transformers; (1) and (2) protective relays, (S₁) and (S₂) locations of short circuit, (t) time delay.

In systems where current can flow to a short circuit from two directions (from different power sources or through a bypass connection) the relatively selective protective relays are directional relays, which operate only when the short-circuit power is transmitted through the protected elements in a certain direction from the buses in the nearest substation. Thus, for a short circuit at the point S (Figure 2), only relays 1, 3, 4, and 6 can operate. In order to ensure selectivity in this case, relays 1 and 3 (4 and 6) are coordinated with each other with respect to their zones and time delays.

Figure 2. Diagram of protective relaying for a system with power supplied from two directions. (A), (B), (C), and (D) substation buses, (P) power sources, (1), (2), (3), (4), (5), and (6) protective relays, (S) location of short circuit.

In many cases, such as in high-power generators and transformers and in power lines at voltages of 110 kilovolts or higher, comparatively complicated absolutely selective protective relays are used to provide high-speed protective relaying. The most common approach here is longitudinal protection, wherein information is supplied to a protective relay from opposite ends of an element in order to identify a short circuit at the end of the relay’s “own” section and at the beginning of the adjacent section. Thus, a longitudinal differential system for current protection responds to the vector difference between the current vectors at the ends of an element. For an external short circuit, this difference is theoretically equal to zero; for an internal short circuit, it is equal to the current at the point of fault. Other types of protective relaying are also used. In phase comparison relaying, the phase angles of the current vectors are balanced. In directional comparison relaying, the directions of the power flow at the ends of an element are compared. Longitudinal protective relays for electrical machines and lines of up to 10 km in length receive information on changes in the electrical quantities directly through the conductors proper of the line or machine. For longer lines, such information is usually transmitted through high-frequency carrier channels along the conductors of the line; microwave channels and radio-relay links, however, are also used.