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10.1 Biased Differential Protection
Sufficient primary current to prove the connections and settings is required so that a minimum secondary current
of about 10mA rms circulates in the relay inputs.
An external three-phase primary short is required on one side of the transformer, ideally the HV side. Apply 415
LVAC to the other side ensuring the primary current is injected through all of the biased differential CT's. The
following procedure should be followed to check the a.c. scheme and settings are correct.
i
Use Reydisp Evolution software to trigger a Waveform Record of the currents.
ii
Retrieve the waveform record from the relay.
iii
View the Waveform Record in Reydisp Evolution.
iv
Check the W1 and W2 Relay Currents are in anti-phase by placing the cursors on the peak relay
currents. Check each phase in turn.
If the current transformers associated with the protection are located in power transformer bushings it may not be
possible to apply test connections between the current transformer and the power transformer windings. Primary
injection is needed however to verify the secondary connection of a neutral CT relative to the phase CTs and the
relay. In these circumstances primary current must be injected through the associated power transformer winding.
It may be necessary to short-circuit another winding in order to allow sufficient current to flow.
If difficulty is experience due to physical restraints, the differential may be proven using load current.
10.2 Restricted Earth Fault
The CT polarities forming the Restricted Earth Fault (REF) protection must be proved, and the recommended way
to achieve this is by primary injection. Inject single phase or three phase current from a suitable primary test set
through earth and primary conductors. The results of these tests may be recorded in Table
During these primary injection tests the injected current may be limited due to the impedance of the neutral
connection. Temporary shorts must be added to allow a definite result to be established. Place a temporary short
across the setting resistor to allow the secondary current to be measured.
Insert the test block with necessary shorts across from CT to relay side. Inject primary current sufficient to allow
measurement. Measure and record the REF spill current displayed by the appropriate relay Instrument. This spill
current should be very small e.g. a few mA's.
Reverse the connections to the secondary winding of the neutral CT the spill current should become larger.
Repeat for other phases if necessary.
Re-connect the auxiliary d.c. supplies for trip and alarm operations and insert the Trip and InterTrip links.
The differential protection should be operated by secondary injection to check that correct tripping (or
intertripping) and indication occurs.
Simulate the operation of each external contact that initiates a Duobias M status input. This can be done by
temporarily shorting across the operating contact. In each case check the appropriate LED illuminates and that
the correct tripping, intertripping and alarm initiation occurs.
Disconnect the d.c. power supply to the Duobias-M relay and check the correct PROTECTION UNHEALTHY
alarm contact operates. If this alarm is wired to a remote indication point e.g. a control centre, the operation of the
alarm at this point should also be checked.
Operate the differential protection and the REF protections in turn by primary or secondary injection and check
that the correct tripping and indication occurs.
11
Tests using load currents
Re – insert all the d.c. fuse and links for all supply, trip and alarm functions.
Connect the laptop and check communications is established with the relay by downloading all settings. Ensure
that the Duobias-M relay is set with the correct setting for the specific application of the relay. Select
INSTRUMENTS MODE.
Under steady load conditions record in to Table 15 the readings displayed on the instruments. If the secondary
connections and the matching of the differential protection to the transformer ratio and vector group connections
are correct the readings "OPERATE A, B, C" should be negligible for all three phases.
©2010 Siemens Protection Devices Limited
7SG14 Duobias-M-200 Commissioning
Chapter 7 Page 12 of 20
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