If transformer should be considered to be protected only with conventional overcurrent and earth-fault
protection, operating time should be set to delayed tripping characteristics coordinated to the low
voltage side relays due to the fact that transformer normal condition energizing and short circuit supply
to high/low voltage side shall be directly seen on both sides of the transformer and overcurrent in
instant operation would cause timing coordination problems or sensitivity problems if the instant
protection should be set on high current starting criteria. This is not considered a big problem in
smaller transformers in which the installation and maintenance of differential protection is considered
more expensive than possibly not full coverage of protection.
Also, differential protection is very sensitive and internally scaled to the loading/fault current flowing
through the transformer. When considering interturn faults in the transformer windings, overcurrent
relay not necessarily even pick up for the fault which could have been already tripped with differential
relay in rst power cycle. Same goes for the transformer internal earth faults which with conventional
earth fault protection in some cases are impossible to be noticed before the fault evolves so that it will
cause heavier fault currents e.g. in cases where the fault location is close to the neutral inside the star
winding.
These are the main arguments for using differential protection, sensitive and fast operation in internal
in-zone faults and high stability on the out zone faults guarantee minimum unwanted power outages
and minimized and reduced damage to the transformer itself.
On the other hand, differential protections negative properties are that it is not the easiest to set up to
operate correctly and second set of current transformers are required thus increasing the installation
cost. In bigger scale power transformers this still is marginal cost.
In following chapter, the principles of the transformer and how to set the differential protection correctly
for an example application are presented.
Transformer properties and basic concepts for differential protection
To set correct differential protection parameters at least transformer nominal data needs to be known.
At least this data should be known:
Transformer nominal power
HV and LV side nominal voltages
Transformer special properties like tap changer and auxiliary windings
Transformer vector group (for matching the transformer per unitized vectors)
HV and LV side current transformer ratios and properties.
In this chapter the setting and principle of transformer differential protection are shown step by step.
Figure. 5.4.10. - 96. Transformer and its components forming the “differential zone”.
AQ-T216
Instruction manual
Version: 2.00
© Arcteq Relays Ltd
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