REF Protection of Transformer

REF protection is one type of electrical protection that is provided to power transformers against earth faults. Full form of REF protection is restricted earth fault protection.

This protection works for a limited zone between the installation of CTs that are installed on secondary side and neutral of transformer. That's why it is called restricted earth fault protection. It will only sense earth fault in this zone. Earth faults out of this zone will not be sensed by REF protection.

In normal condition without any earth fault in this zone,  vector sum of currents that is being induced in three CTs of secondary side and one CT of neutral is zero. 

On incident of earth fault in this zone the vector sum of currents flowing in relay from CTs of secondary side of transformer and neutral connection will not be zero.

REF protection measures this vector sum of currents and if it goes above the setting value, REF will operate and will trip the breaker of transformer.

CTs on the secondary side of transformer are generally being installed in the electrical panel so this protection covers power cable up-to incomer of this panel in addition to secondary side winding of transformer.

Now we will discuss that despite of having differential protection and simple earth fault protection in transformer why we are using REF protection in addition.

The neutral connection of transformer is a point which is common point without any potential or very low potential. Due to very low potential,  the faults near to neutral connection of transformer will have small quantities of currents. These type of low currents can not be sensed by protection like differential and simple earth fault protection. This is the main purpose of REF protection. 

Turns ratio or Voltage ratio test of Transformer

Note: Kindly follow necessary safety rules for testing of transformer.

Transformer testing are mainly two types, one is type test and second is routine test. Transformer voltage ratio or turns ratio test is one type of routine test for transformers. Reference standard for this test is IS-2026.

Purpose:

Transformers have specified voltage rating on primary side and secondary side. We are conducting this test to verify voltage ratio of the transformer. If value of voltage transformer ratio does not come within limits, we can know about high resistance in leads of winding and tap changer connection. Shorting between turns of winding can also be detected by this test.

Voltage ratio equation of transformer:

Transformer voltage ratio= (Primary side voltage / Secondary side voltage)

Voltage ratio error:

The voltage ratio error should be within 0.5 %. Equation for error in voltage ratio is as below.

Error in voltage ratio= ((Calculated voltage ratio – Measured voltage ratio)/Measured voltage ratio) X  100 %

                                               

Test:

In this test we apply low AC voltage to primary side and measure the secondary side output voltage. AC low voltage is injected at high voltage side of transformer so that LV voltage will not be high for measuring and safety purpose. This test is done at no load position of transformer.

Suppose the transformer voltage ratio is 6.6 KV/ 433 V. The voltage ratio for this transformer as per division will come 15.24 for phase to phase value. Now suppose we are applying 415 V to primary side and we are getting 27.23 V from secondary output side then voltage ratio test is ok as 415/27.23 = 15.24.

For testing we are considering Delta to Star connected transformer with 5 taps, whose details are as below.

Tap number	HV voltage (Phase- Phase)	LV voltage(Phase- Phase)	Calculated Ratio (Phase- Phase)
1	6930	433	16.00
2	6765		15.62
3	6600 (Neutral tap)		15.24
4	6435		14.86
5	6270		14.48

Test steps:

1. This test is carried out on no load so first isolate the transformer from HV and LV side.
2. Put the transformer at first tap or last tap so that you will get voltage ratio values in ascending or descending order.
3. Connect the cables to HV and LV side of transformer for applying and measuring voltages. Make the cable connection without looseness.
4. Apply low AC voltage to HV side of transformer and measure the LV side voltage for phase to phase and phase to neutral. Here primary is delta connected and secondary is star connected so phase to neutral voltage will be 1/√3 of the phase to phase voltage for secondary side. 
5. Repeat the test for next tap up to last tap and record the values.

The values of voltage ratio of above transformer will nearly come as below. The voltage ratio error should be within 0.5 %.

Tap number	Applied HV voltage (Phase- Phase)	LV voltage(Phase- Phase)	LV voltage (Phase- Neutral)	Measured Ratio (Phase- Phase)
1	433	27.06	15.64	16.00
2	433	27.72	16.02	15.62
3	433 (Neutral tap)	28.41	16.42	15.24
4	433	29.14	16.84	14.86
5	433	29.90	17.28	14.48

Now a days, one electronic kit called as TTR (Transformer Turns Ratio) kit is available. By carrying out the test with this kit, the kit will automatically show the turns ratio or voltage ratio. So you don’t need to measure voltage either on primary or secondary voltage and calculation of it.

Differential protection of transformer

How differential relay not operate during transformer no load charging or inrush current

During the no load charging of transformer, inrush current phenomena takes place. High current up to 10 times of normal rated current flows into transformer. During no load charging, transformer is connected to power supply at one side keeping other side open for no load charging. One type of overfluxing phenomena takes place inside transformer. During no load charging, there is heavy flow of harmonics current inside transformer for few minutes. Modern relays are built such that it identifies this 2nd and 5th harmonic current. Relays are set such that it not generates any trip signal during presence of this harmonic current and due to this transformers don't trip during inrush current of no load charging.