Study of the construction of the current relay
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STUDY OF THE CONSTRUCTION OF THE CURRENT RELAY Protection system The protection system is designed and managed to deliver the energy to the utilization points with: Reliability Economy Expensive equipment safety like: Generator Transformer Transmission lines Bus bars FAULT IN POWER SYSTEM Fault is defined as any abnormal condition that effects the basic requirements of normal power system. When any fault occurs in any power system, then: – Current increases – Voltage decreases – Frequency varies – Phase difference created IMPORTANT CONSIDERATIONS WHEN DESIGN PROTECTION SYSTEM Types of fault and abnormal Conditions to be protected against Quantities available for measurement Types of protection available Speed Fault position discrimination Dependability / reliability Security / stability Basic Components of Power System Protection Voltage transformers and current transformers. To monitor and give accurate feedback about healthiness of a system. Relays to accurately sense all types of faults and issue trip commands to circuit breaker to disconnect the faulty system. Type of protection Over current. Directional over current. Distance. Over voltage. Differential. Reverse power. Other. RELAY PROTECTION DISCRIMINATION BY TIME. In this system, an adequate time setting is provided to each of the protection relays controlling the power circuit breakers in an electrical power system to make sure that the circuit breaker nearest to the fault location opens first. A fundamental radial distribution electrical system is presented in Figure 1, to demonstrate the operational logic.
Relay protection discrimination by current Relay protection discrimination by current is based on the fact that the short circuit current changes with the location of the fault because of the difference in impedance figures between the source and the short circuit. Therefore, usually, the protection relays controlling the different power circuit breakers are programmed to trip at appropriately tapered values of current such that only the protection relay closest to the fault operates its breaker. Figure 2 presents the method. For a fault at location F1, the electrical system fault current is expressed as: where: ZS - source impedance = =485 Ω ZL1 = cable impedance between C and B = 0.24 Ω Therefore =880 A Therefore, a protection relay controlling the power circuit breaker at location C and programmed to trip at a short circuit current of 8800A would in theory save the whole of the underground cable section between locations C and B. Nevertheless, there are two critical practical points that impact this co-ordination procedure: It is not efficient to differentiate between a fault at location F1 and a fault at location F2, since the separation between these locations may be only a few meters, corresponding to a variation in short circuit current of roughly 0.1%. In practice, there would be variations in the source short circuit level, usually from 250MVA to 130MVA. At this lower short circuit level the short circuit current would not surpass 6800A, even for an underground cable short circuit near to location C. A protection relay set at 8800A would not save any part of the underground cable section concerned. Relay protection discrimination by current is hence not a practical suggestion for correct grading between the power circuit breakers at locations C and B. Nevertheless, the issue changes appreciably when there is major impedance between the two circuit breakers concerned. Note the grading needed between the power circuit breakers at locations C and A in Figure 2. Presuming a short circuit at location F4, the short-circuit current is presented as: Where ZS - source impedance = =485 Ω ZL1 = cable impedance between C and B = 0.24 Ω ZL2 – cable impedance between location B and 4 MVA transformer =0.04 Ω Zr – transformer impedance =0.07 2.12 Ω Yüklə 66,07 Kb. Dostları ilə paylaş:
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