Differential Protective Relay is a protective relay that functions on a percentage or phase angle or other quantitative difference of two currents.
Differential relaying provides selectivity by providing a zone of protection with a circuit of interconnected CT’s. CT’s having the same ratio are installed in all the connections to the component(Example Transformer) to be protected and the secondary of the CT’s are connected in parallel to the relay.
As long as the current flow through the protected component is unchanged in magnitude and phase, the relay does not pick up. Such a condition would occur for a short-circuit fault outside the zone of relay protection. Should a fault occur inside the zone of relay protection (that is between the CT’s), the differential relay would receive current in the operating coil.
The example differential protection relay shown in the below figure is a transformer differential relay. The following protection functions are used in this relay.
Differential Relay Protection Functions:
Protection Functions used in this relay are
- 87 Differential Protection
- 50G Ground Fault Protection
Sensitive Earth Fault protection is provided by using NCT
- 64H Restricted Earth Fault
- 74Trip circuit Supervision
With 2 Binary inputs/With 1 Binary input
- Inrush Detector
In the following figure the relay is configured and LEDs are assigned as follows.
- Differential Pickup
Relay is Senses the differential current and Picks up
- Differential Trip
This indicates Differential trip occurs due to any In-zone fault.
- SEF Pickup
When any earth fault occurs in the out-zone Sensitive Earth Fault element picks up.
- SEF trip
This indicates SEF tripping occurred due to any earth fault in the out- zone.
- REF Pickup
When any fault occurs in the restricted zone REF element picks up.
- REF Trip
This indicates REF tripping occurred due to any fault in the restricted zone.
- Trip Circuit Unhealthy
This indicates problem in the trip circuit.
Explanation of Differential Protection:
Differential Protection comprises both biased differential and high-set differential functions.
Before going through bias differential settings, below settings are needed to specify when there are no interposing CTs used in the circuit.
- ICT Multiplier needs to specify.
- ICT Vectors needs to specify.
1.Internal Current Multiplier:
Line CTs are used to measure fundamental frequency currents. These line currents need to vector corrected before applying to the differential elements.
The multiplier is used to correct any CT ratio mismatch so that ideally nominal current is applied to the biased differential algorithm.
2. Internal Interposing Current Vectors:
ICT Vector setting applies the correct vector compensation to the current applied to the differential algorithm. Means the phase change introduced due to transformer connections is nullified using this setting.
Bias Differential Protection:
Reasons for Bias Differential Protection:-
- Effect of Tap Changer
- Effect of Inrush Magnetizing Current
- Requirement of 2nd Harmonic restrain.
87 Bias Differential Settings:
Magnitude restraint bias is used to ensure the relay is stable when the transformer is carrying load current and during the passage of through (out of zone) fault current.
- 87BD Initial Setting:
This is the minimum level of differential current at which the relay will operate. This setting is used at low levels of load to prevent operation of differential relay due to OLTC tap positions.
Typically this setting is chosen to match the on load tap-change range. For example if the tap change range is +10% to –20%, a setting of 0.3*nominal current is selected.
- 87BD 1st Bias Slope Setting:
With increasing load the differential current required for operation should increase to ensure stability.
In case of through fault or increased loads, the unbalance currents due to OLTC tap position, relay tolerance and CT measurement errors also increase. To prevent operation of the relay due to these unbalance currents the bias setting need to increase.
This setting is expressed in the slope form. The bias slope expresses the current to operate the relay relative to the biasing (restraint) current.
The Bias slope setting chosen must be greater than the maximum unbalance, it is selected to ensure stability when through fault or heavy load current flows in the transformer and the tap changer is in its extreme position.
- 87BD 1st Bias Slope Limit Setting:
This setting is primarily needed to prevent operation of relay in case of through fault results saturation of CTs causes a transient differential current to be detected by the relay.
The bias slope limit is chosen to ensure the biased differential function is stable for high through fault currents coincident with CT saturation.
- 87BD 2nd Bias Slope Setting:
Bias Slope Type is either Line or Curve.
These settings are chosen to ensure the biased differential function is stable for further high through fault currents coincident with CT saturation.
87BD Inrush Action:
Harmonic bias is used to prevent relay operation during magnetizing inrush current into one winding when the transformer is first energized.
The magnetizing Inrush detector operates when the second harmonic current exceeds a set percentage of the fundamental current.
87BD Overfluxing Action:
This setting can be used to prevent operation of the 87BD elements in the presence of allowable over-fluxing conditions.
Overfluxing function works by detecting the 5th harmonic content.
Differential Protection Relay-Highset (87HS) Settings:
The 87HS element is generally applied as an unrestrained differential element to provide fast tripping for heavy internal faults.
Restricted Earth Fault Protection:
REF is more sensitive than overall biased differential protection (87BD) to earth faults it can protect against faults for a greater portion of the transformer windings or where the impedance in the earth fault path is relatively high.
For a solidly earthed star winding, the REF function is roughly twice as sensitive in detecting a winding earth fault, than biased differential protection.
Restricted Earth Fault Protection in detail explained in previous post.
Sensitive Earth Fault Protection:
Unlike REF protection SEF acts for out zone faults.
Need of SEF:
If the earth is dry it is difficult to make an effective short circuit to earth. The resulting earth fault current limited to very low levels. So in this case SEF protection is needed.
As the input impedance of the energizing circuit of the earth fault relay is extremely low it can be used with CBCT core balance current transformer or Zero sequence CT (Neutral CT) to detect sensitive earth faults.