Hitachi Relion 670 Series Applications Manual page 186

Section 8
Current protection
The tank is grounded at a certain location by a conductor where a CT measures the tank to ground
current as shown in Figure 70.
The transformer tank protection function measures the current in the tank earth conductor and give
an instantaneous trip signal in case of an insulation break down between a winding or terminal and
the tank.
IEC15000105 V2 EN-US
Figure 70:
Transformer tank protection function is used to sense the tank leakage current and operates if the
current is above the set level.
In the case of grounded neutral systems, protection can be provided by insulating the transformer
tank from ground apart from a connection to ground through a CT whose secondary energizes an
overcurrent relay. Such an arrangement gives sensitive protection for arc-overs to the tank or to the
core, but it will not respond to turn faults or to faults in the leads to the transformer.
The protection scheme should not initiate a trip in case of an external phase to ground fault. Figure
71 illustrates the different elements that are involved in that case.
The neutral displacement voltage that appears in case of an external phase to ground fault is the
source voltage for current flowing through the protection circuit. The amount of this current is
proportional to the ratio given by the ground contact resistance R
the tank as explained in Figure 71.
IEC15000106 V2 EN-US
Figure 71:
180
Transformer tank and grounding arrangement
Behaviour of ground fault protection during an external fault
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