Al-Balqa Applied University Power systems Protection course Dr Audih al-faoury Electrical Energy department 2018-2019 Department of Electrical Energy Engineering 1
Part 1 Power System protection Introduction Dr Audih alfaoury 2
Power System protection Introduction:- Protection is the art or science of continuously monitoring the power system, detecting the faults and abnormal operation and correct tripping of CB,s. The objectives of power system protection are to : 1.Limit the extent and duration of service interruption thought the protection zone. 2.Minimize damage to the system components Protection engineering concerned with the design and operation of "protection schemes. Protection schemes monitoring the power system, detecting faults or abnormal conditions and initiate correct action. Main idea is to remove faults as quickly as possible while leaving as much of the system intact as possible. 3
Fault sequence of events 1. Fault occurs somewhere on the system, changing the system currents and voltages parameters. 2. Current transformers (CTs) and potential transformers (PTs) sensors detect the change in currents/voltages 3. Relays use sensor input to determine whether a fault has occurred. 4. If fault occurs relays open circuit breakers to isolated the fault. 4
Protection systems must be designed with : primary protection and Backup protection In designing power system protection there are two main types of systems that need to be considered: 1. Radial: this is the easiest from a protection. Radial power system protection systems usually use inverse-time overcurrent relays. 2. Ring : power can flow in both direction, Protection is considered the Secondary systems in a Power systems 5
Protection against faults and abnormalities Types of Faults and Abnormalities a) Faults : The principal electrical system faults are: short circuits and overloads. Short circuits: failure of insulation, mechanical damage to electrical distribution equipment, failure of equipment as a result of overloading or other. Short circuits may occur between two-phase, all phases, one or more phase conductors and ground, open circuit or inter-turn faults in windings The short circuit may be solid or has relatively low impedance. 6
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b) Abnormalities: Under and over frequency. Power swings. Overload and over temperature. Overvoltage or under voltage. Under excitation of synchronous machines. Over fluxing of power transformers. Asynchronous operation of synchronous machines. Mechanical defects i.e. leaking oil, tap changer mechanism faults etc Typical Short-Circuit Type Distribution Single-Phase-Ground: 70-80 % Phase-Phase-Ground: 17-10 % Phase-Phase: 10-8 % Three-Phase: 3-2 % 8
Causes of Short-Circuit Faults: Insulation breakdown, Birds and animals,diggers for underground cables, poles collapsing, conductors breaking, vehicle impact,wind effect,incorrect operation by personnel..and so. Effects of Short-Circuit Type Faults 1) Large current can be involved. This cause equipment and generators would be damaged. Only be allowed to flow for a very short time as 10ms up to say 3 seconds. 2) Arcs, sparking and the heating effect can start fires 3) large mechanical forces which have potential to break or damage equipment. 4) Electric current linkage flow through paths where they could create a hazard to people 9
Planning system protection minimize the effects of faults and abnormalities occurring on the system. a) Quickly isolate faulted zone. b) Minimize the magnitude of short-circuit current and, minimize potential damage to the system. c) Provide alternate circuits, automatic transfers, or automatic reclosing devices, in order to minimize the duration outages. Performance Requirements of Protection System Discriminate between normal and abnormal conditions. Selectivity by coordinate with other protection systems. Fast enough to prevent damage and hazards - but not too fast. 10
No unprotected zones "blind spots.. High degree of reliability and availability. Secure against incorrect operation Should be easy to operate. What Info is Required to Apply Protection One-line diagram,impedances and connections of power system, system frequency, voltage level and phase sequence, existing schemes,maximum allowed clearance times,system fault studies, maximum load and system swing limits,cts and VTs locations, connections and ratios, future expansion and any special considerations for application 11
Basic Protection Scheme Components The primary protective equipment components are shown in the following figure: PR CB Equip CT VT DC Aux HMI PCL Tr Protection Relay Circuit Breaker Protected Item Current Transformer Voltage Transformer DC Auxiliary supply Man-machine interface Communications Link CB trip coil Mainly protection need are circuit breakers, and protective relays and transducers (CT and VT) 12