Fault current Doubling

The fault current on a double star can therefore be calculated on the same basis as for a single-star capacitor bank. 

Consider Example 25.1 for 7200 kVAr units arranged in a single star and 14 400 kVAr units arranged in a double star. The computation of overvoltages and fault currents in the event of failure of a single unit in the two configurations is in a tabulated form as follows ... 

Application of the Doubling Factor to Fault Current found in 11.6... 

The method of sub-section 11.8.2 may be used, but an allowance for fault current decrement needs to be made (because the X-to-R ratio is known). Table H.lb gives the appropriate doubling factor for the situation at one-quarter of a cycle for a known X-to-R ratio. 

The choice of current or voltage setting for the relay will depend upon the design value of earth fault current that will pass in the NER during the specihed time e.g. 20 amps for 10 seconds. If the setting is too low the relay may respond to stray and harmonic currents in the neutral circuit. The maximum expected third plus triplen currents should be determined and the relay set at say double their combined level, or higher. 

Safety barrier types There are two types of safety barrier DC and AC type. The DC type is shown in Figs. X/3.7.1-1 and X/3.7.3-1A where it is seen that Zener diodes are in one configuration only. In this type, polarity is an important factor and if it is connected in reverse it will not function. On the other hand, to overcome the polarity problem, the AC type (double) is used, especially for thermocouples and RTDs. An AC double-type barrier is shown in Fig. X/3.7.1-3. Here it is seen that two sets of Zener diodes are in back-to-back connection to accept any polarity of connections. Because in this connection both fault current directions are accepted, it is an AC-type barrier. 

Use a set of multiplying factors to modify the precalculated value of the rms symmetrical subtransient current 1. Apply the factor at the given fault clearance time. (This factor functions in a manner similar to the doubling factor described in sub-sections 11.6 and 11.6.1.3.) Suitable values of the factor are given in clause 12.2.1.3 of IEC60909, equation (47) and Figure 16 therein. 

The previous fault scenario considers an open circuit fault in phase line a which can be viewed as a structural change causing a change of the number of state variables. In the following, the effect of an abrupt increase of a load resistance is studied. It is assumed that at t = t2 = 0.04 s load resistance doubles. That is, the inverter is faultless, the number of state variables remains invariant but a parametric fault occurs in the load. According to the FSM in Table 8.3, this fault can be detected but not isolated. A deviation of ARR residual r from zero could also be due to a fault in the inductance La. Figure8.29a shows that an increase of load resistance Ria reduces the phase line current ia-... 

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