Protection of Power Systems 2 System Protection Components

Protection of Power Systems 2. System Protection Components

n Protection systems have three basic components: 1. Instrument transformers 2. Relays 3. Circuit breakers

Fig. 1: Overcurrent Protection Schematic

n Fig. 1 shows a simple overcurrent protection schematic with: 1. one type of instrument transformer—the current transformer (CT), 2. an overcurrent relay (OC), and 3. a circuit breaker (CB) for a single-phase line.

Current Transformer (CT) n The function of the CT is to reproduce in its secondary winding a current I that is proportional to the primary current I. n The CT converts primary currents in the kiloamp range to secondary currents in the 0– 5 ampere range for convenience of measurement.

CT Advantages n The CT provides the following advantages: n Safety: Instrument transformers provide electrical isolation from the power system so that personnel working with relays will work in a safer environment. n Economy: Lower-level relay inputs enable relays to be smaller, simpler, and less expensive. n Accuracy: Instrument transformers accurately reproduce power system currents and voltages over wide operating ranges.

Relay n

Circuit Breaker (CB) n When the relay contacts close, the trip coil of the circuit breaker is energized, which then causes the circuit breaker to open. n Note that the circuit breaker does not open until its operating coil is energized, either manually or by relay operation.

Typical Power Circuit Breakers n Protective relays provide the ‘‘brains’’ to sense trouble, but as low-energy devices, they are not able to open and isolate the problem area of the power system. n Circuit breakers and various types of circuit interrupters, including motor contactors and motor controllers, are used for this and provide the ‘‘muscle’’ for fault isolation.

n Thus, protective relays and circuit breaker interrupting devices work together; both are necessary for the prompt isolation of a trouble area or damaged equipment. n A protective relay without a circuit breaker has no basic value except possibly for alarm. n Similarly, a circuit breaker without relays has minimum value, that being for manually energizing or de-energizing a circuit or equipment.

Dead-Tank Circuit Breaker

Live-Tank Circuit Breaker

n In dead-tank breakers the tank or breaker housing is at ground potential. n In live-tank breakers the interrupting mechanisms and housing are at the highvoltage level and insulated from ground through porcelain columns.

n Dead-tank breaker usually have a single trip coil that initiates simultaneous opening of all three-phase breaker poles. n Live-tank types generally have a trip coil and mechanism for operating each pole or phase independently.

n For these types the relays must energize all three trip coils to open the three phase power circuit. n It is possible to connect the three trip coils in parallel or in series for tripping all three poles. n Three trip coils in series are preferred. n This arrangement permits easier monitoring of circuit continuity and requires less trip current.

n The practice for many years has been to open all three phase for all types of faults, even though one or two of the phases may not be involved in the fault. n Because most transmission-line faults are transient single-line-to-ground type, opening only the faulted phase would clear it.

n With a transient fault, such as that resulting from lightning-induced overvoltage, immediate reclosing of the open, faulted phase would restore three-phase service. n Known as single-pole tripping, this tends to reduce the shock on the power system.

n At the lower voltages, the circuit breaker (interrupter) and relays frequently are combined into a single-operating unit. n The circuit breaker switches commonly installed in the service entrance cabinet in modern residential homes and commercial buildings are typical examples. n In general, this type of arrangement is used up through 480– 600 V. n Primarily, the protection is overcurrent, although overvoltage may be included.

n Based on information from instrument transformers, a decision is made and ‘‘relayed’’ to the trip coil of the breaker, which actually opens the power circuit—hence the name relay.

Design Criteria n System-protection components have the following design criteria: 1. 2. 3. 4. 5. Reliability Selectivity Speed Economy Simplicity

1. Reliability n Reliability has two aspects, dependability and security. n Dependability is defined as ‘‘the degree of certainty that a relay or relay system will operate correctly’’. n Security ‘‘relates to the degree of certainty that a relay or relay system will not operate incorrectly’’.

n In other words, dependability indicates the ability of the protection system to perform correctly when required, whereas security is its ability to avoid unnecessary operation during normal day-after-day operation, and faults and problems outside the designated zone of operation. n As a generality, enhancing security tends to decrease the dependability, and vice versa.

2. Selectivity n Relays have an assigned area known as the primary protection zone, but they may properly operate in response to conditions outside this zone. n In these instances, they provide backup protection for the area outside their primary zone. n This is designated as the backup or overreached zone.

n Selectivity (also known as relay coordination) is the process of applying and setting the protective relays that overreach other relays such that they operate as fast as possible within their primary zone, but have delayed operation in their backup zone. n This is necessary to permit the primary relays assigned to this backup or overreached area time to operate.

n Otherwise, both sets of relays may operate for faults in this overreached area; the assigned primary relays for the area and the backup relays. n Operation of the backup protection is incorrect and undesirable unless the primary protection of that area fails to clear the fault. n Consequently, selectivity or relay coordination is important to assure maximum service continuity with minimum system disconnection.

3. Speed n Obviously, it is desirable that the protection isolates a trouble zone as rapidly as possible. n In some applications this is not difficult, but in others, particularly where selectivity is involved, faster operation can be accomplished by more complex and a highercost protection. n Zero-time or very high speed protection, although inherently desirable, may result in an increased number of undesired operations.

n As a broad generality, the faster the operation, the higher the probability of incorrect operation. n Time, generally of a very small amount, remains as one of the best means of distinguishing between tolerable and intolerable transients.

n A high-speed relay is one that operates in less than 50 msec (three cycles on a 60 Hz basis). n The term instantaneous is defined to indicate that no (time) delay is purposely introduced in the action of the device. n In practice, the terms instantaneous and highspeed are used interchangeably to describe protective relays that operate in 50 msec or less.

n Modern high-speed circuit breakers operate in the range of 17– 50 msec (one to three cycles at 60 Hz); others operate at less than 83 msec (five cycles at 60 Hz). n Thus, the total clearing time (relays plus breaker) typically ranges from approximately 35– 130 msec (two to eight cycles at 60 Hz).

n In the lower-voltage systems, in which time- coordination is required between protective relays, relay-operating times generally will be slower; typically on the order of 0. 2– 1. 5 sec for the primary zone. n Primary-zone relay time longer than 1. 5– 2. 0 sec are unusual for faults in this zone, but they are possible and do exist.

n Thus, speed is important, but it is not always absolutely required, nor is it always practical to obtain high speed without additional cost and complexity, which may not be justified. n Relay speed is especially important when the protected facility exists in a stability sensitive area of the power system network. n Faster fault clearing reduces the amount that generators can accelerate during the fault and, therefore, improves stability margins.

4. Simplicity n A protective relay system should be kept as simple and straightforward as possible while still accomplishing its intended goals. n Each added unit or component, which may offer enhancement of the protection, but is not necessarily basic to the protection requirements, should be considered very carefully. n Each addition provides a potential source of trouble and added maintenance.

n As has been emphasized, incorrect operation or unavailability of the protection can result in catastrophic problems in a power system. n Problems in the protective system can greatly impinge on the system-in general, probably more than any other power system component.

n The increasing use of solid-state and digital technologies in protective relaying provides many convenient possibilities for increased sophistication. n Some will enhance the protection; others add components that are desirable to have. n All adjuncts should be evaluated carefully to assure that they really, and significantly, contribute to improve the system protection.

5. Economics n It is fundamental to obtain the maximum protection for the minimum cost, and cost is always a major factor. n The lowest-priced, initial-cost-protective system may not be the most reliable one; furthermore, it may involve greater difficulties in installation and operation, as well as higher maintenance costs.

n Protection costs are considered high when considered alone, but they should be evaluated in the light of the higher cost of the equipment they are protecting, and the cost of an outage or loss of the protected equipment through improper protection. n Saving to reduce the first costs can result in spending many more times of this saving to repair or replace equipment damaged or lost because of inadequate or improper protection.