Three-phase Fault

A three-phase fault somewhere in the bus system, without reactive compensation and ignoring the line impedance, can reach a level of... 

Where the upstream source is another transformer, or a utility connection, the calculation of the three-phase fault current is straightforward and it will not usually vary significantly with the operating configuration of the upstream network. 

Short circuits that do not involve earth, and which are within the length of the cable, can be detected by setting the instantaneous elements of the overcnrrent relays to a value of current calculated at the receiving end of the cable that flows into a zero-impedance fault. Customarily this fault is a three-phase fault for which the calculations are straightforward. If the fault is beyond the cable for example in a consumer then the fault current will be less and shonld be cleared by the consnmer protective device. The feeder cable relays will then act as a back np to the consumer relays. 

If the feeder cable is protected by fuses then these shonld be chosen to rapidly clear an internal line-to-line or three-phase fault. They should be supplemented with a (51) relay to provide overcnrrent protection. 

In a multi-machine network the generators and motors should be considered in relation to the source impedance to which they are connected. This impedance will also be dependent upon the location and type of disturbance e.g. near to a generator, remote from a generator, three-phase fault, line-to-ground fault, change in the state of the load such as starting a large motor direct-on-line. 

Single phase fault current in percent of the three phase fault current <1% Variable, can be 100 % or greater 5 a 20%... 

The distance between bus bars and switchboard walls connected to ground was 14 cm therefore, it must be at least an overvoltage of 160 kV to break down the air dielectric strength. Then, another arc flash occurred in the un-faulted phases, after it there was produced a three-phase fault at bottom of the bus bar. Due to three-phase fault the copper material was fused in bars until it was detected by the network protections. 

The system logs indicate that the failure of PT began 5 30" before the three-phase fault occurred on the switchgear, see Fig. 4.3.1.2. 

Due to the open delta connection of PT s, it follows that the switchgear fire was started by the failure phase (between phases 2 and 3) of PT. According to the records of the fault currents, it is estimated that this event had duration of about 2 cycles (see Fig. 25). As shown in the figure, because the fault is a fault-phase, unbalanced residual current is generated through the common point of the current transformers. From the oscillograms it is concluded that the evolution of biphasic failure to three-phase fault was about 30 to 40 ms. 

Three-phase faults (external to the targeted major section)... 

When only power-frequency components are considered, SLG faults and three-phase faults are studied using theoretical formulas. Some utilities study SLG faults and three-phase faults using EMTP in order to consider transient components of the sheath voltage. Switching surges rarely become an issue for the sheath overvoltage. 

Therefore, the level of phase-to-phase asymmetrical faults will he generally of the same order as the three-phase symmetrical faults. The ground faults, however, will he higher than the symmetrical faults. Special care therefore needs he taken while grounding a generator, when they are solidly grounded, particularly to limit the ground fault currents See also Section 20.10.1. 

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. 

When this balance is disturbed, due to either an unbalance in the loads or due to a ground fault, a residual or zero phase sequence voltage in the neutral circuit will appear. When one of the phases in the secondary of a three-phase transformer is open circuited and a three-phase supply is applied to its primary windings, there will appear... 

If there are N number of CTs connected in parallel, the magnetizing current will flow through all of them. In a CiF protection scheme all the three CTs of all the feeders being protected together will fall in parallel, while in case of a combined GF and phase fault protection scheme, only one third of these CTs will fall in parallel. The CT in the faulty circuit must be able to draw enough current to feed the magnetizing losses of all the CTs falling in parallel and the relay pickup current, The sensitivity of the differential scheme can therefore be expressed more appropriately as... 

Differential proteetion (Relay Code 87) To detect a stator phase-lo-phase fault by a three-pole differential protection relay, current setting 10 0%. For scheme diagrams, refer to Section 15.6.6(1). 

This is not a method of providing an artificial neutral, as in the previous case, but to detect an unbalance or residual voltage (zero sequence voltage) in a three-phase three-wire or a three-phase four-wire ungrounded system. The residual or zero sequence voltage that may appear across the open delta will be the reflection of an unbalance or a ground fault in the system (Figure 20.10). Also refer to. Section 15.4.1 for more details. 

The instant (sub-transient) fault current, /jjgf, through a generator in a symmetrical three-phase system, irrespective of the condition of neutral as defined in Table 13.9 will be... 

The theory of operation of such a protection scheme is based on the prineiple that in a balanced cireuit the phasor sum of currents in the three healthy phases is zero, as illustrated in Figure 21.7, and the current through the grounded neutral is zero. In the event of a ground fault, i.e. when one of the phases becomes grounded, this balance is upset and the out-of-balance current flows through the grounded neutral. A healthy three-phase circuit, how ever. 

A scheme for a ground fault protection will depend upon the type of system and its grounding conditions, i.e. whether the system is three-phase three-wire or three-phase four-wire. A three-wire system will require an artificial grounding while for a four-wire system the type of grounding must be known, i.e. whether it is effectively (solidly) grounded or non-effeclively (impedance) grounded. 

Figure 21.14 Scheme for a three-phase three-wire restricted ground fault protection for an FIT system... 

Figure 21.17 Scheme for restricted G/F protection for a three-phase four-wire system. Fault occurring outside the protected zone... 

The CT provided in the ground circuit is now removed and the same scheme becomes suitable for an unrestricted G/F or a combined G/F and phase fault protections. It is true for a three-phase three-wire or a three-phase four-wire system. This scheme may also be arranged for a combined 0/C and G/F protections as illustrated in Figures 21.5(a) and 21.5(b). 

Consider the same LT system of 415 V, as of Example 23.8, three-phase 50 Hz, and having a fault level of 36 MVA and employing capacitor banks of 360 kVAr. 

Single-or three-phase Single-phase reactors are used in the neutral circuit either to limit the ground fault currents or as arc-suppression coils (Section 20.5). Similarly, three-phase reactors are used for three-pha.se applications. 

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