Ground fault

FIG. 29-8 Typical high-voltage ac motor starter illiistrating several protective schemes fuses, overload relays, ground-fault relays, and differential relays with the associated current transformer that act as fault-current sensors. In practice, the differential protection current transformers are located at the motor, hut the relays are part of the starter. 

Both differential and ground relaying detect ground faults. Ground-fault protection is located at the starter and protects the cable and the motor differential CTs are located at the motor and protect the motor only. Economic priorities indicate ground-fault protec tion first, adding differential protection when justified by potential savings in downtime and repair costs. 

Instantaneous ground fault by providing a zero sequence relay in the residual circuit of the CTs. 

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. 

Since the ground fault currents in generators can be higher (Section 20.10.1) than the sub-transient state current, special care need be taken while grounding a generator to limit the ground fault current. Section 20.10.1 covers this aspect also. 

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... 

Ground fault on one phase System neutral grounded... 

Consider a ground fault on phase R. The voltage across this phase will become zero and the phasor diagram will be as illustrated in Figure 15.5(b). The other two phasors will remain the same as in a healthy system and add to give the residual voltage F, i.e. 

Figure 15.5(b) An RVT under ground fault on a 3-p four-wire grounded neutral system... 

Figure 15.5(c) Ground fault on a 3-p, three-wire delta or 3-p. four-wire ungrounded star system... 

To detect a ground fault or operate a directional ground fault relay (Section 21.7.4). 

The current element of a relay is wound for a wide range of current settings in terms of the rated secondary current of the CT, such as 10-80% for a ground fault protection, 50-200% for an overcurrent and 300-800% for a short-circuit protection. At lower current settings, while the VA requirement for the operation of the relay will remain the same, the VA capacity of the CT will decrease in a square proportion of the current. A CT of a correspondingly higher VA level would therefore be necessary to obtain the reduced VA level. 

Where three CTs for unrestricted or four CTs for restricted ground fault or combined O/C and G/F protections are employed in the protective circuit, the VA burden of the relay is shared by all the CTs in parallel and a normal VA CT may generally suffice. Such is the case in most of the protective schemes discussed in Sections 21.6 and 15.6.6(1), except for those employing only one CT to detect a ground fault condition, such as for a generator protection with a solidly grounded neutral (Figure 21.12). 

These are protection CTs lor special applications such as biased differential protection, restricted ground fault protection and distance protection schemes, where it is not possible to easily identify the elass of accuracy, the accuracy limit factor and the rated burden of the CTs and where a full primary fault current is required to be transformed to the secondary without saturation, to accurately monitor the level of fault and/or unbalance. The type of application tind the relay being used determine the knee point voltage. The knee point voltage and the excitation current of the CTs now form the basic design parameters for such CTs. They are classified as class PS CTs and can be identified by the following characteristics ... 

Figure 15.22 A circulating current scheme to provide a phase and a ground fault differential protection...

Figure 15.24 Equivalent control circuit diagram for a differential ground fault protection scheme of Figure 15.22...

As a rule and as recommended by lEC 60255-6, the POC may be chosen within 30% of the minimum estimated ground fault eurrent. When the. scheme is required to deteet only a ground fault, a single-pole relay is conneeted between all the CTs shorted ends (Figure 15.29). All the CTs now fall in parallel. 

When the scheme is required to detect the ground fault as well as the phase faults, a triple-pole relay is used, eaeh pole of which is connected between the shorted terminals of the two same phase CTs and the neutral formed by shorting the other terminals of all the CTs, as shown in Figure 15.22. The setting of all the poles is kept the same. In other words, the sensitivity level remains the same for all types of faults. 

Relay - High impedance single element ground fault differential protection relay... 

Ground fault protection of a machine and setting of the relay. The following example illustrates the procedure to select class PS CTs for a typical G/F scheme. In practice, this scheme would be more appropriate for phase and ground fault protections, as illustrated in Figure 15.22. 

Figure 15.32 Phase and ground fault differential protection scheme for a transformer and feeder bus protection...

The scheme Is suitable to detect both a ground fault and a phase fault. 

Ammeter selector switch 32. Over current ground fault... 

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