Friday, 23 September 2016

Sensitive Earth Fault Protection

Sensitive Earth fault Protection scheme is used for the detection of earth fault. Protection against phase to ground faults can be a difficult problem since ground fault currents vary within a large range, becoming almost negligible in some situations. The ground fault current magnitude depends on the power system grounding which can vary from solidly grounded to ungrounded neutral. The sensitive earth fault protection is usually used in alternators and transformers with high resistance grounding.  High resistance grounding restricts the earth fault current to a very less value and permits the operation of equipment. 

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As we know that, the earth fault current magnitude is given by,

IF = 3VLN / (Z0 + Z1 + Z2 + 3ZE)

Where Z0 = Zero sequence impedance

Z1 = Positive sequence impedance

Z2 = Negative sequence impedance

ZE = Impedance of earth fault

ZE represents the impedance of the ground return circuit including the fault arc, the grounding circuit, and the intentional neutral impedance, if present. If we consider a solidly grounded system, then ZE = 0 and if Z0 = Z1 = Z2 then earth fault current will be given as shown below.

IF = VLN / Z1

The Sensitive Earth Fault protection scheme works by measuring the residual current across the three phases in a system. Measurement of three phase residual current is done either by using Core Balance Current Transformer (CBCT) or three CTs connected in parallel.   In the ideal condition, the residual current will be zero as all the currents flow through the three phases. Here Residual current means current flowing through neutral or zero sequence current.

As we know that

IN = 3I0 = IR + IY +IB

But during normal operation,

IR + IY +IB = 0 therefore, no residual current will flow.

The most important part in SEF or Sensitive Earth Fault Protection is to make proper setting of the Relay. The protection setting should take into consideration that the three CTs do not have identical characteristics and will perform differently for heavy phase-to-phase faults or for initial asymmetrical motor starting currents. This can produce false residual currents. The setting should also be above the line maximum unbalance current.

The above conditions must be satisfied to avoid nuisance tripping. In addition, the ground fault protection must be sensitive to minimum ground fault current at the end of the line. Sensitive earth Fault protection scheme is very sensitive to detection of earth fault in the sense that its setting can be as low as 0.2%.

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