Spurious Motor Tripping on Start
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, it’s amplitude will be negligible after 20 – 30mS.
Fig 1
Current transformers
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 can- not 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 CTs 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equivalent circuit as in fig 3 where the "dead" CT has been replaced by its resistance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.
Fig 2
CT connection
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.
For static or electronic relays the earth-fault element resistance is of the order of 20-30
milli-ohms whereas RCT + REXT will be of the order of 1 or 2 ohms. Clearly the
majority of the "return current" will flow through the earth-fault relay and for motor
starting or transformer energisation this current will often be
above relay setting of the earth-fault element causing it to "mal-operate" on the
spurious residual current resulting from ·the transient CT saturation.
For older electromechanical·relays, the earth-fault element resistance would generally
have been much greater than that of the "dead" CT path, so the bulk of the "return
current” would have been forced back through this CT and the probability of an earthfault maloperation during switch-on greatly reduced,
The solution, for all relays is, however, simple and inexpensive. By connecting a
"stabilising" resistor in series with the fast earth-fault relay element, the resistance of its
path can be increased artificially to force the major portion of return current back
through the dead CT and reduce the earfh-fault relay's current to below its setting thus
preventing mal-operation, This resistor, when correctly chosen, has negligible effect on
the earth-fault relay sensitivity. The resistance value and wattage rating of the
stabilising resistor required·can be
calculated if the following are known:


CT internal resistances RCT , knee point voltage Vk
and corresponding exciting current I,)

CT circuit external resistance (burden) per phase REXT

The earth-fault relay current setting (secondary amps)

The maximum primary r.m,s, current that can occur during switch-on of a
transformer or motor starting.
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.
Contact Test-A-Relay on Tel : +27 (012) 665-0545.