Power system fault currents

By magnitude In different parts of power system fault currents of different magnitude can flow. 

Mcmy remote mine sites ire coimected to relatively weak diesel generator power supply systems (cis compared to more stable utihty coimected systems) for their electrical power needs md overall system fault current levels ire usually low. These systems do not handle large, real (or reactive) power swings well without significant power quahty issues occurring, such as... 

Care should be taken in the design of systems such that critical circuits are not used with series-rated systems. In the case of our 200,000-AIC main, if there were a high current fault at a downstream sight, the 100,000-AIC circuit breaker would interrupt and it would also cause the 200,000-AIC main circuit breaker to interrupt simultaneously, which would terminate power to the entire distribution system. If this were used in, say, a hospital, you would not want a short circuit in a panel board or a downstream distribution switchboard to also cause the main circuit breaker to trip, taking aU power out of the hospital. In those types of critical situations, the minimum circuit breaker AIC rating should be no less than the available system fault current delivered by the utility. 

A power circuit is basically an R-L circuit. In the event of a fault, the system voltage (V , sin ft))) may occur somewhere between V = 0 and V = on its voltage wave. This will cause a shift in the zero axis of the fault current, 7sc> and give rise to a d.c. component. The fault current will generally assume an asymmetrical waveform as illustrated in Figure 13.27. 

Table 13.5 RMS values of fault currents under different conditions of fault in a power system...

A fault current on a power system is normally asymmetrical as discussed next, and is composed of a symmetrical a.c. component /sar.m.s.) nd an asymmetrical sub-transient d.c. component (Figure 14.5). The forces arising out of /jc aie referred to as electromagnetic and... 

A directional G/F relay basically is a power-measuring device, and is operated by the residual voltage of the system in conjunction with the residual current detected by the three CTs used for non-directional protection, as shown in Figure 21.19. To provide directional protection, therefore, a residual VT is also essential, in addition to the three residual CTs. The voltage phasor is used as a reference to establish the relative displacement of the fault current. In healthy conditions, i.e. when the current flows in the right direction. = 0. (refer to. Section 15.4.3 for details), and the relay remains inoperative. The relay operates only when the current flows in the reverse direction. 

Since a switchyard is normally connected to more than one supply system, the ground fault current in a power station is contributed by the power plant as well as by the switchyard and the transmission networks. The following possibilities may arise ... 

A fault current-limiter is a component which protects power transmission and distribution systems from surges caused by, for example, a lightning strike, fulfilling a function similar to that of a varistor (see Section 4.3.1). The limiter should be capable of reducing the fault current to a fraction of its peak value in less than a cycle. Because for this application the requirement is for low Jc, fault current limiters are already a commercial product. In the case of the lead shown in Fig. 4.58(b) the fault current is limited to a safe value within 5 ms of the arrival of the current spike . 

In power supply networks, fault currents cause arcs with the release of a considerable amount of energy. An AC system with period T, peak current Iol... 

The fault current effects have been described above for the making duty. However, some further points are appropriate for the breaking duty. The breaking duty root-mean-square duty is usually specified to take place after a number of cycles of fault current have passed. (It may also be expressed as a peak value of current, although this is less common.) This is usually taken to be the time given by the manufacturer for the circuit breaker to open and clear the fault. This is typically 5 to 8 cycles of the fundamental current. The engineer should specify the requirements for the particular power system and the manufacturer should then confirm whether the equipment offered could meet the requirement. Each power system should be considered on its own merits in this regard. 

Historically early designs began to fail nntil it was realised that the prospective fault currents in typical power systems had gradually increased. This was due to the natural development and expansion of those systems. Reference 1 gives a good description of the Pt characteristic. 

The method adopted below is based upon the principles set out in IEC60363 and 1EC60909, both of which describe how to calculate sub-transient and transient fault currents, and are well suited to oil industry power systems. The method will use the per-unit system of parameters and variables. Choose the base MVA to be Nbase-... 

Many power system networks can be reduced to a simple series-connected circuit containing a resistance R and an inductance L, for the purpose of calculating the transient fault current. Furthermore a... 

As a project moves into the detail design phase it acquires more precise data for all aspects of the work. It is then possible to calculate the fault currents more accurately. However, it should be noted that the tolerances on most of the data are seldom better than plus or minus 15%, and so increasing the quantity of data will not necessarily improve the results significantly. During the detail design phase the power system tends to be modified and additional switchboards added. It is then necessary to calcnlate the fault currents at least at the busbars of each switchboard, and this can become a laborious task if hand calcnlations are attempted. 

During a fault condition, the load side of the power system can contribute currents to the fault. The origin of such contribution is motors, which can be either induction or synchronous machines. 

Reactors are indnctance coils and the name reactor is used to imply their use for limiting fault current. Current limiting is often achieved by adding reactance into part of the power system. Reactors perform this fnnction economically. 

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