Power system protection
Power system protection Lecture 1 1 / 15 Definition of the Power system protection Definition of the Power system protection Components of Protection System Function of Protection Relay RELIABILITY SELECTIVITY SPEED Speed ECONOMICS PRINCIPLES OF RELAY APPLICATION Protection Zones Observation PROTECTIO ZONES PROTECTION ZONES EXAMPLE, Branch of electrical power engineering deals with the protection of electrical power systems from faults through the isolation of faulted parts from the rest of the electrical network. The protective relays act only after an abnormal or intolerable condition has occurred, with sufficient indication to permit their operation. Protection does not mean prevention, but rather, minimizing the duration of the trouble and limiting the damage, outage time, and related problems that may result otherwise. 2 / 15 Components of Protection System Definition of the Power system protection Components of Protection System Function of Protection Relay RELIABILITY SELECTIVITY SPEED Speed ECONOMICS PRINCIPLES OF RELAY APPLICATION Protection Zones Observation PROTECTIO ZONES PROTECTION ZONES EXAMPLE, ■ ■ ■ ■ ■ Current and voltage transformers to step down the high voltages and currents of the electrical power system to convenient levels for the relays to deal with Protective relays to sense the fault and initiate a trip, or disconnection, order; Circuit breakers to open/close the system based on relay and autorecloser commands; Batteries to provide power in case of power disconnection in the system. Communication channels to allow analysis of current and voltage at remote terminals of a line and to allow remote tripping of equipment. 3 / 15 Function of Protection Relay Definition of the Power system protection Components of Protection System Function of Protection Relay RELIABILITY SELECTIVITY SPEED Speed ECONOMICS PRINCIPLES OF RELAY APPLICATION Protection Zones Observation PROTECTIO ZONES PROTECTION ZONES EXAMPLE, 1. Reliability: assurance that the protection will perform correctly. 2. Selectivity: maximum continuity of service with minimum system disconnection. 3. Speed of operation: minimum fault duration and consequent equipment damage and system instability. 4. Simplicity: minimum protective equipment and associated circuitry to achieve the protection objectives. 5. Economics: maximum protection at minimal total cost. 4 / 15 RELIABILITY Definition of the Power system protection Components of Protection System Function of Protection Relay RELIABILITY SELECTIVITY SPEED Speed ECONOMICS PRINCIPLES OF RELAY APPLICATION Protection Zones Observation PROTECTIO ZONES PROTECTION ZONES EXAMPLE, Reliability has two aspects, dependability and security. Dependability is defined as the degree of certainty that a relay or relay system will operate correctly. Security relates to the degree of certainty that a relay or relay system will not operate incorrectly. 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. 5 / 15 SELECTIVITY Definition of the Power system protection Components of Protection System Function of Protection Relay RELIABILITY SELECTIVITY SPEED Speed ECONOMICS PRINCIPLES OF RELAY APPLICATION Protection Zones Observation PROTECTIO ZONES PROTECTION ZONES EXAMPLE, Selectivity is the ability that a relay has to only open those breakers that isolate the faulted element. Selectivity discrimination can be achieved by time grading or by unit protection. Selectivity by time grading means that different zones of operation are graded by time and that in the occurrence of a fault, although a number of protections equipment respond, only those relevant to the faulty zone complete the tripping function. Selectivity by unit protection means that the relay will only operate under certain fault conditions occurring within a clearly defined zone. 6 / 15 SPEED Definition of the Power system protection Components of Protection System Function of Protection Relay RELIABILITY SELECTIVITY SPEED Speed ECONOMICS PRINCIPLES OF RELAY APPLICATION Protection Zones Observation PROTECTIO ZONES PROTECTION ZONES EXAMPLE, The protection must isolates a trouble zone as rapidly as possible. ■ In applications where selectivity is involved, faster operation can be accomplished by more complex and a higher-cost protection. Note: The faster the operation, the higher the probability of incorrect operation. ■ Time, remains as one of the best means of distinguishing between tolerable and intolerable transients. A high-speed relay is that operates in less than 50ms = 3cycles (IEEE 100). ■ 7 / 15 Speed Definition of the Power system protection Components of Protection System Function of Protection Relay RELIABILITY SELECTIVITY SPEED Speed ECONOMICS PRINCIPLES OF RELAY APPLICATION Protection Zones Observation PROTECTIO ZONES PROTECTION ZONES EXAMPLE, Modern high-speed circuit breakers operate in the range of 17 − 50ms, 1 − 3cysles; others operate at less than 83ms = 5cycles. Thus, the total clearing time (relays plus breaker) typically ranges from approximately 35 − 130ms = 2 − 8cycles 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. 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. 8 / 15 ECONOMICS Definition of the Power system protection Components of Protection System Function of Protection Relay RELIABILITY SELECTIVITY SPEED Speed ECONOMICS PRINCIPLES OF RELAY APPLICATION Protection Zones Observation PROTECTIO ZONES PROTECTION ZONES EXAMPLE, It is fundamental to obtain the maximum protection for the minimum cost, and cost is always a major factor. 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. 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. 9 / 15 PRINCIPLES OF RELAY APPLICATION Definition of the Power system protection Components of Protection System Function of Protection Relay RELIABILITY SELECTIVITY SPEED Speed ECONOMICS PRINCIPLES OF RELAY APPLICATION Protection Zones Observation PROTECTIO ZONES PROTECTION ZONES EXAMPLE, The power system is divided into protection zones defined by the equipment and the available circuit breakers. Six categories of protection zones are possible in each power system: 1. 2. 3. 4. 5. 6. generators and generator-transformer units, transformers, buses, lines (transmission, subtransmission, and distribution) utilization equipment (motors, static loads, or other), capacitor or reactor banks (when separately protected). 10 / 15 Protection Zones Definition of the Power system protection Components of Protection System Function of Protection Relay RELIABILITY SELECTIVITY SPEED Speed ECONOMICS PRINCIPLES OF RELAY APPLICATION Protection Zones Observation PROTECTIO ZONES PROTECTION ZONES EXAMPLE, Ideally, zones of protection should overlap, so that no part of the power system is left unprotected. graphics[scale=1.35]fig/zones01.png 11 / 15 Observation Definition of the Power system protection Components of Protection System Function of Protection Relay RELIABILITY SELECTIVITY SPEED Speed ECONOMICS PRINCIPLES OF RELAY APPLICATION Protection Zones Observation PROTECTIO ZONES PROTECTION ZONES EXAMPLE, 1. To disconnect only a faulty element, C.B’s are located in the connections to each power element. 2. A separate zone of protection is around each system element. The significance of this is that any failure occurring within a given zone will cause the tripping only those breakers. 3. Failures within the region where two adjacent protective zones overlap, more B tripped than the minimum necessary to disconnect faulty. 4. But, if there were no overlap, no breakers would be tripped. (overlap is the lesser of the two evils) 12 / 15 PROTECTIO ZONES Definition of the Power system protection Components of Protection System Function of Protection Relay RELIABILITY SELECTIVITY SPEED Speed ECONOMICS PRINCIPLES OF RELAY APPLICATION Protection Zones Observation PROTECTIO ZONES PROTECTION ZONES EXAMPLE, A S.C at X, the breakers of zone B, including breaker C, will be tripped; ■ A S.C outside zone A, the relaying equipment of zone B must also trip certain breakers in zone A if is necessary to interrupt the flow of short circuit current from zone A to the fault. ■ A fault at X, the same breakers in zone A will be tripped unnecessarily for other faults in zone B to the right of breaker C. ■ 13 / 15 PROTECTION ZONES Definition of the Power system protection Components of Protection System Function of Protection Relay RELIABILITY SELECTIVITY SPEED Speed ECONOMICS PRINCIPLES OF RELAY APPLICATION Protection Zones Observation PROTECTIO ZONES PROTECTION ZONES EXAMPLE, a typical arrangement of overlapping zones. 14 / 15 EXAMPLE, Definition of the Power system protection Components of Protection System Function of Protection Relay RELIABILITY SELECTIVITY SPEED Speed ECONOMICS PRINCIPLES OF RELAY APPLICATION Protection Zones Observation PROTECTIO ZONES PROTECTION ZONES EXAMPLE, What breakers should operate for a fault at, F 1 , F 2 , F 3 , F 4 , F 5 , F 6 , F 7 F8 , 15 / 15
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