Transformer Differential Protection Calculations P642 Relay

Dear All,

Differential protection is the main protection for any transformer.  Differential protection is also called as unit protection scheme, it mean that it will operate in case of in zone fault and at through faults differential protection should not operate.

A practical transformers and CTs pose some challenge to Differential Protection. They are as follows:

• The primary of transformer will carry no load current even when the secondary is open circuited. This will lead to differential current on which the protection scheme should not operate.

• It is not possible to exactly match the CT ratio as per equation. This would also lead to differential currents under healthy conditions.

• If the transformer is used with an off nominal tap, then differential currents will arise as the CT ratio calculated for a particular Tap (Nominal Tap) will be different for different Tap, even under healthy conditions.

To prevent the Differential Protection scheme from picking up under such conditions, a Percentage Differential Protection scheme is used. It improves security at the cost of sensitivity.

In Percentage Differential Protection, we provide a slope feature to the Differential Protection Relay. In modern Numerical Differential protection Relay two slopes are provided.

In this course, we will perform the setting calculation of 100MVA transformer with following data:

Transformer rating: 100 MVA

HV side voltage:  132 kV

LV side voltage: 33 kV

Vector group: YNd1

Impedance: 12.5 %

HV side current: 437.4 A

LV side current : 1749.5 A

Tap range :  ±10%

Primary CT ratio :  500/1 A

Secondary CT ratio : 1800/1 A

Further we will calculate:

• Full load amperes

• Short circuit MVA

• Short circuit current at HV & LV sides

• relationship between short circuit MVA and percentage impedance of transformer

• Magnitude and phase angle error and its compensation

• CT error and its compensation

• Operating and restraining currents

• Pickup current calculations

• Slope 1 & 2 calculations

• Harmonics blocking calculations

• Through fault stability calculations

Best Regards