Generators: Gen 1 132kV, Z = 0+j12.5% on 25MVA Gen 2 132kV, Z= 0+j12.5% on 25 MV
ID: 2249450 • Letter: G
Question
Generators:
Gen 1 132kV, Z = 0+j12.5% on 25MVA
Gen 2 132kV, Z= 0+j12.5% on 25 MVA
Gen 3 20kV, Z = 0+j10% on 25 MVA - connected windings
Transformers:
Tx 1 20/66kV, Z = 0+j10% on 25MVA Earthed Y windings (both sides)
Tx 2 132/66kV, Z = 0+j12% on 50MVA Earthed Y windings (both sides)
Tx 3 66/11kV, Z = 0+j10% on 25MVA Windings –connected 66kV side; earthed Y on the 11 kV side.
Transmission Lines:
Line 1 132kV, Z = 0+j1 ohm/phase
Line 2 66kV, Z = 0+j1 /phase
Line 3 66kV, Z = 0+j2 /phase
Line 4 11kV, Z = 0+j1 /phase.
(a) Calculate the steady-state, three-phase short-circuit current in Amperes that will flow if a fault is applied at point ‘F’.
(b) Calculate the steady-state, single-phase to earth current in Amperes that will flow if a single-phase to earth fault is applied at point ‘F’.
Ignore resistances in your calculations. Generator and transformer impedances are shown in per cent, on each machine’s own rating as power base. Line impedances are shown in ohms. Assume zero sequence impedances of all lines are 2.5 x the positive sequence values.
Explanation / Answer
In electrical engineering, a protective relay is a relay device designed to trip a circuit breaker when a fault is detected. The first protective relays were electromagnetic devices, relying on coils operating on moving parts to provide detection of abnormal operating conditions such as over-current, over-voltage, reverse power flow, over-frequency, and under-frequency.
Microprocessor-based digital protection relays now emulate the original devices, as well as providing types of protection and supervision impractical with electromechanical relays. Electromechanical relays provide only rudimentary indications of involved phase and zone targets.[3] In many cases a single microprocessor relay provides functions that would take two or more electromechanical devices. By combining several functions in one case, numerical relays also save capital cost and maintenance cost over electromechanical relays.However, due to their very long life span, tens of thousands of these "silent sentinels" are still protecting transmission lines and electrical apparatus all over the world. Important transmission lines and generators have cubicles dedicated to protection, with many individual electromechanical devices, or one or two microprocessor relays.
The theory and application of these protective devices is an important part of the education of a power engineer who specializes in power system protection. The need to act quickly to protect circuits and equipment as well as the general public often requires protective relays to respond and trip a breaker within a few thousandths of a second. In some instances these clearance times are prescribed in legislation or operating rules. A maintenance or testing program is used to determine the performance and availability of protection systems.
Based on the end application and applicable legislation, various standards such as ANSI C37.90, IEC255-4, IEC60255-3, and IAC govern the response time of the relay to the fault conditions that may occur.
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