DIFFERENTIAL PROTECTION 8
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Transcript of DIFFERENTIAL PROTECTION 8
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DIFFERENTIAL PROTECTION
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Differential protection
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Differential protection application
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Differential Relay – Operating Principle
PLANT TO BE PROTECTED
I1 I2
I1 = I2 Healthy; no trip| I1 - I2 | > Limit Fault; trip
“Balanced beam” type of relay – difference current trips beam; through current restrains it.Modern relays do the comparison by microprocessor
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Differential Relay – Operating Principle .. 2
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Differential Relay Coil Connection
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Differential Relay – Transformer Application
• Transformation ratio has to be taken into account. Trip if |aI1 – I2| > Limit, where a is trf. ratio (VP/VS)
• Three-phase delta-star or star-delta. The phase shift and 3 have to be taken into account
• Allow for inrush currents – suppress tripping (2nd harmonic restraint).
• Allow for ratio changes due to tap-changing (may be 10-15%)
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Differential Relay – Transformer Application .. 2
HV and LV CT ratios have to be in the same ratio as the transformer
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Transformer Inrush Current-Full DC Offset
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Differential Relay Set-up for Delta-Star Transformer
CTs in delta
CTs in star
DeltaStar
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Transformer Differential Protection Example
• 33/11 kV, star-star transformer. Rating = 25 MVA
• Standard CT ratios available are 200:5, 400:5, 600:5, 800:5, 1000:5, 1600:5, 1800:5
• Allow for 15-20% unbalance
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Transformer Differential Protection Example .. 2
• Full–load current = 437 A/ph (33kV side), =1312 A/ph (11kV side)• Allow 150% overload. Choose 600:5 CT for 33kV. • Transformation ratio = 3. Choose 1800:5 CT for 11kV
side• Set HV normal current In = 450 A say• Set bias to 20%.• If delta-star transformer, allow for 3 (in LV CT ratio for
old relays – Idelta = Iline/ 3; taken care of in setting for modern relays)
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Busbar Differential Protection Scheme
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Problems with CT
Saturation in B-bar Prot’n
Schemes CT 1 is unsaturated and CT2 saturated. Note the distorted CT 2 current i2. This can create a net difference current and false trip the relay.
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CT Saturation Detector
The cct. detects saturation and reduces the width of the output square-wave pulses to the extent the relay won’t work. Explain how the cct. works!
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Limitations with OC Protection in Transformers
• A simple overcurrent and earth fault relay will not provide adequate protection for winding earth faults.
• Need some earth fault protection.• Degree of earth fault protection is very
much improved by the application of unit differential or restricted earth fault systems.
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Restricted Earth Fault Protection
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Restricted Earth Fault - How it Works
• On the HV side, the residual current of the 3 line CT’s is balanced against the output current of the CT in the neutral conductor.
• For the LV side, earth faults occurring on the delta winding may also result in a level of fault current of less than full load. HV overcurrent relays will not provide adequate protection.
• A relay connected to monitor residual current will provide restricted earth fault protection since the delta winding cannot supply zero-sequence current to the system.
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Restricted Earth Fault - How it Works .. 2
• Both windings of the transformer can thus be protected separately with restricted earth fault.
• Provide high speed protection against earth faults over the whole of the transformer windings.
• Relay used is an instantaneous high impedance type.
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High Impedance Differential Scheme
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High Impedance Scheme - How it Works
The CTs generate currents that create a high voltage across the relay (which has a high burden).
CTs must generate 2 times the trip voltage to work on an internal fault
If one only CT saturates for through fault, not enough voltage is generated to trip relay.
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High Impedance Scheme - Requirements
• CTs must have identical turns ratios, knee-point voltages 2 times relay operating voltage, high accuracy, low reactance (‘PX’ class)
• High internal faults can generate high voltage across relay - some form of voltage dependent resistor (“metrosil”) is required to avoid this.
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High Impedance Scheme - Relay Circuit
Refer worked tutorial example on how the stabilising resistor value Rstab is calculated.
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High Impedance Scheme -
Example … 1
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High Impedance Scheme - Example … 2
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Feeder Differential Protection (Balanced Voltage)
“Translay” protection
Top - external fault, CT current balance, no net voltage across relays
Bottom - internal fault, CT currents add, net voltage across both relays.
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