Primary Injection Testing - Siemens 7SR242 Duobias Technical Manual

Chapter 6) 7SR242 Duobias Commissioning & Maintenance Guide
2.1.2

Primary Injection Testing

Primary injection is recommended to prove the relay connections, CT polarity and settings before putting the
protection scheme into service. Primary injection is essential to fully prove the connections of the Biased
Differential and REF protections. To provide a useful test the relay should have the final site specific settings
applied for primary injection tests.
WARNING!
It is important before carrying out any primary injection to ensure appropriate CTs are shorted to avoid operation
of mesh corner or busbar type unit protection. If the injected primary current is large enough, the bus zones
protection may operate.
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. This is difficult to achieve using high current primary injection
equipment due to the relatively high impedance of the transformer windings. An alternative method is to apply 415
LVAC to one side of the transformer with a short circuit applied to the other side. The external three-phase
primary short is usually applied to the HV side so that the LVAC supply is connected to the winding with lowest
impedance which will result in a higher current level. The test current that will be produced can be predicted
based on the impedance of the transformer and the applied test voltage. The primary test current is injected
through all of the biased differential CT's on the LV side.
Injection of 3 phase current in this way will simulate balanced load conditions, or through fault. During injection,
check that the W1 and W2 relay currents are in anti-phase by examination of the relay 'Differential Meters' in
'Instruments Mode'. Check each phase in turn, ensuring that the phase angle for 'W1 Relay' is in anti-phase with
'W2 Relay'.
When the transformer is eventually energised and carrying load current, the above examination of the W1 and W2
relay current phase angle should be re-checked for anti-phase to ensure that the correct ICT Connection settings
are applied to the differential protection.
It should be noted that checking of Vector Grouping by phase alignment between W1 and W2 by 3 phase primary
injection or on-load will highlight phase cross-over or connection polarity but will not show incorrect application of
zero sequence filters.
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