Protection grounding faults

RTD protection Over-current protection Ground fault protection Gas protection Differential protection Restricted ground fault protection... 

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. 

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...

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...

Restricted ground fault protection Differential protection... 

Duration This will depend upon the ground fault protection scheme adopted and may be considered as follows ... 

Important parameters for selecting a ground fault protection scheme 20/669... 

Below we briefly discuss the criteria and theory of selecting a grounding system to achieve a desired level of fault current to suit a predetermined ground fault protection scheme, i.e. type of grounding and grounding impedance to suit the system voltage, type of installation, and location of installation. 

When some extra impedance R, Xq, Xi or a combination of these is introduced into the ground circuit it will become possible to alter the magnitude and the characteristic of the ground circuit current, /g, to suit an already designed ground fault protection scheme as discussed below. 

This is to achieve a higher level of fault current to obtain a quicker tripping on fault. It is obtained when the system has a ground fault factor not exceeding 1.4 (Vg < O.SVf), as noted above. A solidly grounded system will provide effective grounding. This system will reduce the transient oscillations and allow a current sufficient to select a ground fault protection. It is normally applicable to an LT system. 

A ground fault protection scheme that is easy to handle, clear the fault quickly and prevent it from spreading. 

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