TRANSFORMER COMMISSIONING

Insulation Resistance

All know that the resistance of a pure insulation is infinity. But to no extend we can make a pure insulation. So we have a small leakage current to flow through insulation depending upon the voltage applied. Thus as the result we get a resistance value in the range of Mega Ohms(MΩ) or Giga Ohms(GΩ).  Here the applied voltage is DC. We have separate kit for injecting the DC voltage. Usually that equipment is known as megger.

The two windings are connected as in the diagram above and Megger is used to measure the insulation resistance. It applies a high voltage normally 500V or 1kV for time period of minimum 30 seconds to One minute and the respective resistance are noted down. Usually for a new transformer, result value will be in Giga Ohms(GΩ). During the ageing, deposition of dust, moisture and other unwanted particles can cause the megger value to reduce. So to gain the resistance back to normal, the dust deposited should be removed, moisture should be reduced using heaters and other necessary steps must be taken in account.

 

      Tan-delta Test

Also known as Loss Angle Test or Dissipation Factor Testing do the same process what Insulation Resistance do. The difference here is instead of DC we inject AC voltage. It’s principle of working is that all insulators are pure capacitive in nature because one end is energised while the other is connected to ground. So for a pure capacitor the current will lead 90 degree to voltage. Since there is no pure capacitor we will get some difference in the mentioned angle. This difference is known as loss angle. With this test we measure the loss in angle measure due to ageing of transformer oil, windings and bushing, etc.

       

All insulators will have a capacitive and resistive nature. So they consume both capacitive current and resistive current. Initial in the commissioning stage the angle delta(δ) will be as low as possible. Due to ageing there will develop a resistive current which increases the angle measurement and beyond a particular value the equipment cannot be put in service. There is no such method to increase the tan-delta value during maintenance.

 

      Ratio Test

As the name implies the ratio test is done to note whether the transformation ratio of a transformer is as specified in the name plate. This test is carried out in all the taps of transformer. Supply is three phase and is measurement is recorded in phase to phase values and phase to neutral values. Now connection is established as in the figure shown below.

Now all the three phases will be charged instantaneously and the secondary voltage (phase-phase and phase-neutral) in all respect will be recorded. Fist it is done on the lower tap and carried out in the continual taps. Recorded values will be finally compared with the theoretical values then obtained.

 

      Core Balance Test

Test conducted both on high voltage and low voltage winding. At first we inject voltage to terminals R and N. After that voltage is measured from terminal Y-N and B-N. The same process id done in all three cores and the other core values are recorded. The same is repeated on the second winding too.

Above diagram shows the arrangement of windings in a transformer core. Consider this windings are star connected at primary and secondary. Now if we energise the R phase terminal and the star point, we observe an induced emf in the other two terminals of the same windings. And measurement will be in the ratio, Y=75% and B= 25% of the injected value of voltage. Summation of Y phase and B phase will be equal to R phase. Then voltage is applied on Y and star point. Now voltage developed in R phase and B phase are same and is 50% each. So Y phase is the sum of both R and B phase. The same procedure is repeated on B phase. Now obtained measurement in R phase is 25% and Y phase is 75%. The same process is repeated in secondary winding also and measurement is noted down.

 

      Magnetisation Test

It’s simple test to find out how much current the transformer consumes for initializing the magnetic flux over the core. Three phase supply is given to the HV winding and the LV is marked open. Now the current consumed is measured and recorded. It is said that magnetisation current should be limited to One(1%) percentage of the FLA( full load amperes).

 

      Vector Grouping Test

It’s a way of labelling the HV and LV winding of a three phase transformer. Take an example YNd11, the first two letter indicates the primary winding usually in capital letters and the second alphabet denotes the secondary winding and the number denoting the phase shift between HV and LV winding. Here (YNd11) we can say that primary is a star connection with neutral and secondary is a delta connected with a phase shit of 30degrees lagging with primary. The figure above is an example of testing vector grouping in a Y-Y(YNyn0) transformer. A temporary shorting is given to the primary neutral (N) and secondary U-phase. Three phase voltage is applied on the primary side and the voltages are measure likewise in the figure.

 

      Short-Circuit Test

This test is mainly done to find whether there is any manufacturing defects in the OLTC windings and connections. As shown in the diagram, the Low Voltage (LV) side of the transformer is short circuited and a small three phase voltage is applied on the High Voltage (HV) side through a moving iron ammeter. The ammeter is to be selected to measure the maximum rated current in the primary. BCT or the Bushing Current Transformer which supplies the input for the winding temperature measurement is also checked at the normal tap of the transformer and confirms the BCT ratio is as specified by manufacturer. Three phase supply is switched on and from lower tap to higher the current is measured and noted. In the meantime of tap changing the three ammeter currents are visually checked to ensure that there is no fluctuation in any of the meter during the change over. This ensures that a secure connection in the diverter switch of OLTC mechanism.

 

      REF Stability Test

A method of protecting both winding of a transformer for earth-fault. It’s basically a unit protection. REF should only act for in ZONE faults. Testing procedure includes injecting primary current through R-phase CT and NCT in series so there will be no spill in relay. Note that both CT should have the P2 side towards transformer. If there is any spill then the secondary connection of any one of the CT’s is inversed. (This can be discussed in later posts)

 

      Differential Stability Check

Foremost protection in transformer is differential protection. It protects almost 60% to 70% of the total winding for particularly phase to phase faults. Rest of the windings is protected by REF relays. A differential relay operates on two quantities, one is the operating or differential current and the second is restraining or bias current. So during commissioning it’s important to check all the connections such as jumpers, CT connections in Primary and it’s secondary leads are in proper terminations. The inputs of differential relay is from both HV and LV CT’s.  So testing procedure includes, applying # phase supply on the primary end including CTs while the secondary side will be shorted and earthed after the LV CTs. So differential current inside the relay will be zero and biasing current will be high. For the next step of testing the input supply will be given to transformer by bypassing any one CT at one END. So that CT will not read the SC current while the other CT’s read their respective primary values. This unbalance creates a differential current to rise and initiates a trip signal when the operating current moves up beyond the restraining value.

 

 

      Circuit Breaker Tripping test on Faults

So finally, the protective relays will be tested via a secondary injection test setup and confirms no false tripping occur while it trips for every faults. Now all transformer trouble alarms and trips will be checked from yard. Trouble alarms include Buccholz Trip/Alarm, Pressure Relief Valve, Winding Temperature Trip/Alarm, Oil Temperature Trip/Alarm, Oil Surge Relay, Magnetic Oil Gauge, etc.