Electromechanical differential relay setting (87TS)

The ratios of CTs used for the electromechanical differential relay, the connections of the CTs, currents provided by the CTs and the relay currents are listed in Table 19.

Table 19  CT and relay data for electromechanical differential relay

Winding 1 Winding 2 Winding 3

CT Ratio 2000/5 4000/5 4000/5

CT connection Delta Delta Wye

CT secondary current at 400 MVA (A) 1.67 2.45 *8.36

CT secondary current at 675 MVA (A) 2.82 4.13 *14.05

Relay current @ 400 MVA (A) 2.89 4.24 *8.36

Relay current @ 675 MVA (A) 4.89 7.15 *14.05

[1.67A@ 80 MVA] *Though the tertiary winding is rated for 80 MVA, relay currents are calculated at MVA rating of the higher rated windings, to check for relay stability for external faults.

9.2.2.1 Criteria for CT ratio selection and relay tap setting

 CT secondary current at Maximum Transformer MVA (675 MVA in this example) does not exceed the thermal rating of the CT secondary winding.

 The relay current at Maximum Transformer MVA should not exceed the continuous

thermal rating of the relay. It is also desirable that:

 The CT ratios are selected such that the relay currents under maximum internal fault conditions do not exceed the relay short time thermal ratings to prevent damage.

 The relay taps are chosen such that the error due to mismatch is less than 5 percent.

CAUTION

If a transformer is loaded beyond the nameplate rating due to an emergency, the CT or relay, or both are likely to be damaged if their thermal ratings are exceeded.

The relay current at self-cooled rating of 400 MVA of this transformer should not exceed the TAP value. This is to prevent the operation of the unrestrained high set unit (typically 8 times the tap value) due to inrush current.

The ratios of currents from CTs (as a fraction of the current from winding 1), selected tap settings and ratios of tap settings (as a fraction of the tap on winding 1) are listed in Table 20. The percentage errors and total mismatch are also listed in this table.

Table 20  Current ratios, relay taps, tap ratios and mismatch errors

Winding 1 Winding 2 Winding 3

Selected relay tap 2.9 4.2 8.7

Current ratio 1 1.46 2.89 Tap ratio 1 1.45 3.0 Percentage error 1.46 1.45 100 0.7% 1.45    2.89 3.0 100 4% 3.0   

Mismatch error Less than 5%

9.2.2.2 Minimum pick up

In many electromechanical relays, the user cannot set the minimum pick up current of the relay. It is a fixed percentage of the tap setting and it varies with the manufacturer. Typical values are 30 % and 35 %.

9.2.2.3 Harmonic Blocking/ Restraint setting

This is not a user-defined setting. The percentage varies with the manufacturer –value of one manufacturer‟s relay is 20 % where as the value of another manufacturer‟s relay is 15%.

9.2.2.4 High set unit current setting

9.2.2.5 Other Settings

Mismatch due to no-load taps is 5 % and the maximum ratio error mismatch is 4 %. The total mismatch error is, therefore, 9 % approximately.

The available slope settings are 15, 25 and 40 percent. Slope setting of 25 % could be selected.

9.2.2.6 Verification of the thermal rating of the relay coils

Consider that the fault currents at this location are as listed in Table 21. Currents for faults on the 34.5 kV side are less than the currents for faults on the 118 kV side.

Table 21  Currents for faults in the transformer zone

Fault Fault currents (A) from

Currents (A)

to electromechanical relay from Type Location 345 kV side 118 kV side 345 kV side 118 kV side Total

3 Phase 345 kV 26,500 5,500 115 12 127

3 Phase 118 kV 5,530 34,169 24 74 98

Total current in the electromechanical relay is less than 220A, the maximum allowed in this case. Relay currents due to single-phase to ground and other shunt faults should also be checked to verify that the currents in the relay will not exceed the rating of the relay. In this example it is assumed that relay currents during other types of shunt faults are less than those listed in Table 21

9.2.2.7 CT burden verification

The burden on the CTs should be checked, as described in IEEE Std C37.110™, Guide for Application of Current Transformers Used for Protective Relaying Purposes to ascertain the CTs do not saturate for faults in the transformer zone and in the networks it is connected to.

9.2.2.8 Transformer overload capability

Continuous current rating of the relay coils vary with the manufacturer. One of them specifies the continuous rating as 2 times the tap setting, whereas another manufacturer has a minimum rating of 8A with a maximum varying with the tap setting. Consider that the relay used in this case is for the first type.

The tap setting on the 345 kV side is 2.9. This is 2.89 100 59%

4.89  of current at 675 MVA.

The tap setting on the 118 kV side is 4.2. This is 4.24 100 59%

7.15  of current at 675 MVA

The tap setting on the 34.5 kV side is 8.7. This is 167% of current at 80 MVA

The transformer can be overloaded up to 118 % (259) of 675 MVA rating on 345 and 118 kV

windings without exceeding the thermal limit of the relay. If the transformer needs to be loaded beyond 118% of its rating, CTs of higher ratio should be used and then auxiliary CTs should be used to match the working of the differential relay. This would allow higher tap settings to be used on the 345 kV and 118 kV sides.

If the second type of relay is used, the 2.9 tap setting has 8A continuous rating and 4.2 A tap has a rating of 13A. The transformer could be loaded up to 160 % without exceeding the continuous rating of the relay inputs.