Basic Transformer Differential Protection Calculation

In this article, we will present a discussion on transformer differential protection calculation through an application example.

So far we have discussed about the principles of transformer differential protection. We started with the introduction into Percentage Biased differential were we talked briefly about operate and restraint current, and slope characteristic. Next we revisited the concept of transformer vector groups and identified the key take away which is “Master the Delta Connection and you’re on your way to mastering Vector Groups!”. Then we discussed about how CTs are actually connected to relays. Lastly, we talked about zero sequence and tap compensation and how they affect the operation of the transformer differential protection. Understanding these concepts are very important in carrying-out a transformer differential protection calculation

Application Example on Transformer Differential Protection Calculation

25/33MVA 69/23kV Delta-Wye Transformer on SEL-387A Relay

We will start the transformer differential protection calculation with the identification of the transformer vector group. For this example, we have a 25/33MVA 69/23kV Delta-Wye Transformer with vector group, Dyn1. We will apply wye connected CTs on both sides of the transformer. Based on our discussion on CT and relay connection, we apply CT compensation on the 23kV side using,

In SEL relays, this is represented by the setting CTC(1). SEL recommends compensating on the wye side. This setting also compensates for zero sequence currents as was discussed in our article on Zero Sequence Compensation. Next on the setting is TAP compensation which is based per unit concept.

The transformer MVA rating is 33MVA while the voltage rating is 23kV. Using a CT ratio of 240, the TAP setting value is 3.45Amps secondary for the wye side. Delta side TAP setting value can be calculated using a CT ratio of 80. The TAP setting compensation is automatically calculated in SEL relays by providing the appropriate transformer parameters and CT ratio. Other relay settings are listed below.

• E87 = Y
• W1CT = Y
• W2CT = Y
• CTR1 = 80
• CTR2 = 240
• MVA = 33
• ICOM = Y
• W1CTC = 0
• W2CTC = 1
• VWDG1 = 69
• VWDG2 = 23
• TAP1 = 3.45
• TAP 2 = 3.45

Next we define the percentage biased differential characteristic through the settings minimum operate pick-up, slope 1, slope 2, slope boundary, and unrestrained current pick-up. In our article, The Operate Quantity, we calculated the minimum operate pick-up, 0.28, based on the CT error and voltage ratio variation. In this application, we set the minimum operate pick-up to 0.3. The considerations in setting slope 1 and slope 2 were presented in our first article for transformer differential protection. The complete settings for the percentage biased differential characteristic is presented below. Take note that other settings not discussed here were left to their default relay values.

• O87P = 0.3
• SLP1 = 25
• SLP2 = 40
• IRS1 = 3.0
• U87P = 10
• PCT2 = 15
• PCT4 = 15
• PCT5 = 35
• TH5P = OFF
• DCRB = N
• HRSTR = N
• IHBL = N

Single Line-to-Ground Fault

Let us examine the response of our transformer differential protection on through fault conditions. Consider a single line-to-ground through fault as shown on the figure below.

The currents seen by the relay are affected by the CT ratio and connection hence the values of the secondary currents before compensation. By applying the phase and tap compensation setting, the currents values are as presented. These compensated current values are used in the calculation of the operate and restraint currents and are evaluated based on the percent biased differential characteristic.

The operate and restraint currents calculated are 0.01 and 4.28, respectively. If we take the slope of the operate and restraint currents, we will get a value very close to zero. This means that our transformer differential protection is well within the no operate zone. The combination of operate and restraint currents are reflected on the transformer differential characteristic curve as show below.

That’s it for our discussion on basic transformer differential protection calculation. Please share your thoughts in the comments section below.

Reference

SEL-387A Instruction Manual. Available at https://selinc.com

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