Fault protection

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. 

Lythall, R.T., AC Motor Control (on earth fault protection and thermistor protection). 

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. 

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

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... 

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. 

Restricted ground fault protection Differential protection... 

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

Grounding Theory and Ground Fault Protection Schemes... 

Grounding theory and ground fault protection schemes 21/679... 

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 21.12 Ground fault protection through a single CT...

When it becomes essential to discriminate between a fault within the circuit to be protected from one outside the circuit, this scheme may be adopted. While doing so, it must be ensured that adequate ground fault protection is available to the remaining feeders, if connected on the same system. 

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