It is necessary to realise that the value of if to be

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Unformatted text preview: n Equation 15.4 is the complete function of the fault current and the spill current IR through the relay, in the limiting condition, will be of the same form. If the relay requires more time to operate than the effective duration of the d.c. transient component, or has been designed with special features to block the d.c. component, then this factor can be ignored and only the symmetrical value of the fault current need be entered in Equation 15.4. If the relay setting voltage, Vs, is made equal to Vf, that is, If (RL + RCT), an inherent safety factor of the order of two will exist. 15.8.2 Effective Setting or Primary Operating Current In the case of a faster relay, capable of operating in one cycle and with no special features to block the d.c. component, it is the r.m.s. value of the first offset wave that is significant. This value, for a fully offset waveform _ with no d.c. decrement, is √3If. If settings are then chosen in terms of_ the symmetrical component of the fault current, the √3 factor which has been ignored will take up most of the basic safety factor, leaving only a very small margin. where: Finally, if a truly instantaneous relay were used, the relevant value of If would be the maximum offset peak. In this case, the factor has become less than unity, possibly as low as 0.7. It is therefore possible to rewrite Equation 15.4 as: I SL = K × VS R L + R CT …Equation 15.5 where: ISL = stability of scheme VS The minimum primary operating current is a further criterion of the design of a differential system. The secondary effective setting is the sum of the relay minimum operating current and the excitation losses in all parallel connected current transformers, whether carrying primary current or not. This summation should strictly speaking be vectorial, but is usually done arithmetically. It can be expressed as: IR = IS +nIeS ...Equation 15.6 IR = effective setting IS = relay circuit setting current IeS = CT excitation current at relay setting voltage n = number of parallel - connected CT’s Having established the relay setting voltage from stability considerations, as shown in Section 15.8.1, and knowing the excitation characteristic of the current transformers, the effective setting can be computed. The secondary setting is converted to the primary operating current by multiplying by the turns ratio of the current transformers. The operating current so determined should be considered in terms of the conditions of the application. For a phase and earth fault scheme the setting can be based on the fault current to be expected for minimum plant and maximum system outage conditions. However, it should be remembered that: = relay circuit voltage setting • 242 • Network Protection & Automation Guide This will not happen to any large degree if the fault current is a larger multiple of setting; for example, if the fault current is five times the scheme primary operating current and the CT knee-point e.m.f. is three times the relay setting voltage, the additional delay is unlikely to exceed one cycle. a. phase-phase faults give only 86% of the threephase fault current b. fault arc resistance and earth path resistance reduce fault currents somewhat c. a reasonable margin should be allowed to ensure that relays operate quickly and decisively The primary operating current is sometimes designed to exceed the maximum expected circuit load in order to reduce the possibility of false operation under load current as a result of a broken CT lead. Desirable as this safeguard may be, it will be seen that it is better not to increase the effective current setting too much, as this will sacrifice some speed; the check feature in any case, maintains stability. It is desirable that the primary effective setting should not exceed 30% of the prospective minimum fault current. In the case of a scheme exclusively for earth fault protection, the minimum earth fault current should be considered, taking into account any earthing impedance that might be present as well. Furthermore, in the event of a double phase to earth fault, regardless of the interphase currents, only 50% of the system e.m.f. is available in the earth path, causing a further reduction in the earth fault current. The primary operating current must therefore be not greater than 30% of the minimum single-phase earth fault current. In order to achieve high-speed operation, it is desirable that settings should be still lower, particularly in the case of the solidly earthed power system. The transient component of the fault current in conjunction with unfavourable residual flux in the CT can cause a high degree of saturation and loss of output, possibly leading to a delay of several cycles additional to the natural operating time of the element. An overall earth fault scheme for a large distribution board may be difficult to design because of the large number of current transformers paralleled together, which may lead to an excessive setting. It may be advantageous in such a case to provide a three-elem...
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This note was uploaded on 02/18/2013 for the course EE 45 taught by Professor Kjald during the Spring '13 term at Aachen University of Applied Sciences.

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