Specifications

7SR242 Duobias Applications Guide
Section 5: Supervision Functions
5.1 Inrush Detector (81HBL2)
87 Inrush Element (Enable, Disable)
When a transformer is energized transient magnetizing inrush currents flow in each phase of the energised
winding. Inrush currents only flow into one transformer winding and the resulting unbalance can be sufficient to
cause mal-operation of the biased differential elements. To prevent the relay operating for this non-fault condition,
the presence of even harmonics in the current is used to distinguish between inrush currents and short circuit
faults.
The inrush restraint detector can be used to block the operation of selected elements during transformer
magnetising inrush conditions.
The 81HBL2 Bias setting allows the user to select between, Phase, Sum and Cross methods of measurement.
Each of the three selections has a specific application.
Phase – The even harmonic content in each phase is measured independently and compared to the total operate
current in its own phase i.e. each phase of the biased differential elements is blocked by even harmonic content in
its own phase only.
This method is used exclusively where large transformers are manufactured with three separate phase tanks
each containing a phase core. This construction facilitates transportation. Each of the phase cores is not
magnetically affected by the flux in the other phase cores.
These large single phase transformers are often auto-transformers used on EHV transmission systems. A typical
setting level for this application is 18% of Id.
Cross – Each phase is monitored and if the even harmonic present in any phase exceeds the setting then all
three phases are blocked. This method is used for the majority of applications of the relay to power transformers.
Generally the default setting of 0.20 x Id provides stable operation.
Sum – The level of even harmonic current (2nd and 4th) in the differential signal for each phase is measured. The
square root is taken of each of these even harmonic currents and these three values summated. This single
current level is then divided by the Inrush Setting to arrive at the Harmonic Sum with which each of the phase
currents are compared.
If the operate current in any phase is greater than this Harmonic Sum then its differential element will operate.
The advantage of this method is it allows fast operation of the biased differential element if the transformer is
switched onto an internal phase to earth fault. The cross method may suffer from slowed operation for this
situation, as healthy phase inrush may block all three phases (including the one feeding the fault current) from
operating. Where REF is used to protect the winding, the slowed operation is not critical as the REF will operate
very fast, typically in about 20ms for this rare condition.
The Sum method is not slowed down when switching onto an in zone earth fault, as the Harmonic Sum is reduced
by the presence of the fault current and therefore allows relay operation.
Typically the Sum method will allow the biased differential elements to operate in the normal time of about 30ms,
if a transformer earth fault occurs when it is energised.
This setting is recommended if REF is not used to protect the windings for earth faults on effectively earthed
power systems. The recommended setting that offers a good compromise between stability for typical inrush
currents and fast operation for internal faults is 0.15 x Id.
87 Inrush Setting (0.1 to 0.5 x Id)
This defines the levels of inrush used in each of the above methods.
The setting applied will determine the level of even harmonic (second and fourth) content in the relay operating
current that will cause operation of the relay to be inhibited. The lowest setting of 0.1 x Id therefore represents the
setting that provides the most stability under magnetising inrush conditions.
©2010 Siemens Protection Devices Limited Chapter 7 Page 42 of 56