TEST-A-RELAY  Electrical Power Systems Protection & Metering Specialists





Spurious Motor Tripping on Start by Ron Slatem

 Effect of DC Transient component on High Speed Earth Fault Protection when Switching On an Inductive Circuit such as a Motor or Transformer.

 When energizing a 3-Phase inductive circuit, there is always a decaying transient dc component in at least two of the three phases.  The distribution of the dc component in the phases depends on the points on the supply voltage waves at which the circuit makes.  The duration of this dc transient is determined by the L/R ratio of the primary circuit and, in practical cases, itís amplitude will be negligible after 20 Ė 30mS.

Fig 1     Current transformers                                                      Fig 2   -  CT connection

The dc component causes the initial energisation current to be offset from zero for the first cycle or two, as shown in fig 1. The effect of this transient on the CTs feeding a protection relay is to cause the phase or phases with higher de component value to go into transient saturation as they cannot reproduce the decaying dc, which demands excessive flux densities. The remaining CTs can usually reproduce the required voltages to drive the required current ac and do components through the secondary circuit. 

 In a 3-phase relay with earth-fault protection, the connections are as shown in fig 2, with the EF relay in the 4th wire or residual circuit.

 To understand the problems that arise, consider the hypothetical extreme that the B phase CT is completely saturated by a de component of current so that it does not produce any ac voltage and the other two CT's have not saturated and can produce as much voltage as is required. The unsaturated CTs will drive the current through their internal resistance RCT and external load resistances REXT∑ (fig 2)∑ This will set up a voltage between the two neutrals will not exceed:

Vn-n  = Isb x (RCT + REXT)

(Isb  = Secondary current - B phase) 

The CT circuit during transient saturation of one phase can be replaced by an equivalent circuit as in fig 3 where the "dead" CT has been replaced by its resistance RCT only.

There are two possible return paths for the sum of the R and W phase currents:

      • Via the "dead" CT
      • Via the earth-fault relay.

The distribution of current between these two paths depends on their relative impedances (assumed resistive - as this is generally the case)

Ix   = Vn-n / (RCT + REXT) - Current returned via "dead" CT.

IEF = Vn-n / REF = Spurious EF current.

Fig 3  :   Equivalent circuit.

Care must be taken to ensure that the resulting "voltage" setting of the earth-fault relay does not exceed 50% of the knee point voltage of the CT and Primary current setting does not exceed the desired value for adequate protection of the motor or transformer. (The increased voltage for relay operation demands more exciting current from the CTs and this increases the effective primary setting above that of the (referred) relay current setting, sometimes the exciting current component predominates). Compliance with these criteria can be confirmed by calculation or by a simple secondary injection test.


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Last modified: 05/21/10