Fault level

When there is no dedicated transformer and these circuits are connected on the system bus directly a large inductor will be essential at the incoming of the static circuits, sufficient to absorb the trapped charge within the transformer and the interconnecting cables up to the converter unit. The size of the inductor can be calculated depending on the size (kVA) of the distribution transformer, its fault level and the characteristics of its current limiting protective device. An inductor sufficient to absorb //, L of the transformer and the cables may be provided at the incoming of the sialic circuits. 

Yes because of lower capacity of motor, cables and switchgears and a low fault level... 

Similar cost reduction possible but all requirements to be suitable for slightly higher fault level because of high /s,... 

Rated short-time current rating or fault level of a system (breaking current for an interrupting device) Duration of fault... 

Rated short-time current rating or fault level of a system... 

To increase the impedance of the network, a series resistor or reactor is sometimes used to contain the fault level of a system within a desirable limit. This may be required to make the selection of the interrupting device easy, and from the available range, without an extra cost for a new design as well as an economical selection of the interconnecting conductors and cables. Such a situation may arise on HV >66 kV or EFIV > 132. kV transmission networks, when they are being fed by two or more power sources, which may raise the fault level of the system to an unacceptable level. The cost of the interrupting device for such a fault level may become disproportionately high, and sometimes even pose a problem in availability. 

Below, we analyse the likely fault levels of a system under different circuits and fault conditions for an easy understanding of the subject. It is a prerequisite to decide the level of fault, to select and design the right type of equipment, devices and components and the protective scheme for a particular network. 

As discussed above, it is usual practice to assume the highest fault level of a network by considering the least possible impedance of the faulty circuit such as the... 

Therefore the level of three-phase symmetrical faults will he the highest compared to a phase-to-phase or a ground fault and the system design may he based on the symmetrical fault level. 

A transformer is not a source of supply (it only transforms one voltage to another) but it is considered so, in terms of fault level calculations. In fact, it provides a means to add to the impedance of a circuit on the lower voltage side, and limits the fault level of the network to which it is connected. One will appreciate that the capacity of the actual source of supply, on the higher voltage side, will be much larger. On the LV side it is controlled by the impedance of the transformer. It is customary to consider this impedance to determine the fault level on the LV side. The fault level is measured as the dead... 

To establish the minimum fault level, impedances of the feeding lines from the source of supply up to a selected point, at which the fault level is to be determined, must be added. For a step-by-step calculation to arrive at such a fault level refer to lEC 60909 and the literature on the subject as well as the references at the end of this chapter. 

The prescribed system standard fault level nearest to if, as per Table 13.7 = 35 kA... 

Table 13.7 For an LT system typical fault levels on the LV side of a transformer...

This is a simple calculation to determine the maximum symmetrical fault level of a system, to select the type of equipment, devices and bus system etc. But to decide on a realistic protective scheme, the asymmetrical value of the fault current must be estimated by including all the likely impedances of the circuit. 

Fault level of a system under cumulative influence of two power sources... 

Short-lime rating of the tap-offs It is possible that in certain installations as a result of system needs it m iy not be possible to limit the fault level of the system within a desirable limit. A power-generatina station, connected to an external grid, to... 

Through the tap-offs of the bus, the unit auxiliary transformers (UATs) are connected to feed the station auxiliary services. For more clarity we have taken out the portion of the tap-offs from Figure 13.21 and redrawn it in Figure 13.18 to illustrate the above system and its interconnections. The tap-offs are now subject to the cumulative inOuence of the two supply sources. In the event of a fault on this section, both the sources would feed the same and the fault current through the tap-offs would add up. The tap-offs should thus be designed for the cumulative effect of both fault levels. For the sake of an easy reference, Table 13.8 suggests a few typical values of fault currents, worked out on the basis of data considered for the G and GT. One such example is also worked out in Example 13.3. 

But Ihe GT is connected to a power grid and the grid to a transmission system. If we consider the fault level of the transmission system as 40 kA, as in Table 13.10, then Ihe maximum fault that can occur on the LV side of Ihe GT will be governed by Ihe fault level of the transmission system (40 kA) and not the GT (84 kA). 

Accordingly, the fault level for which the tap-offs should be designed,... 

Switchgear and controlgear assemblies 13/353 Table 13.8 Typical parameters of a power generating station and Its likely fault levels... 

A power system is connected to a number of power supply machines that determine the fault level of that. system (e.g. generators and transformers). The impedances of all such equipment and the impedances of the interconnecting cables and overhead lines etc. are the parameters that limit the fault level of the system. For ease of calculation, when determining the fault level of such a system it is essential to consider any one major component as the base and convert the relevant parameters of the other equipment to that base, for a quicker calculation, to establish the required fault level. Below we provide a few common formulae for the calculation of faults on a p.u. basis. For more details refer to a textbook in the references. 

We illustrate a typical powerhouse generation and transmission system layout in Figure 13.21, and reproduce in Table 13.10 the typical fault levels of different transmission and distribution networks in practice for different voltage systems. 

We have mentioned two systems, I second and 3 second. A choice of any of them would depend upon the location and the application of the equipment and criticality of the installation. Generally speaking, it is only the one-second system that is in practice. The three-second system may sometimes be used for low fault level networks, where V3 / c, would fall within the capability of the available interrupting devices and at reasonable cost. 

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