Transformer Installation & commissioning

INSTALLATION & COMMISSIONING

General:

This section Illustrates procedure for Installation & Commissioning of our transformer.

A format of “Commissioning Report” is included at the end of this section. Results of various precommissioning tests as well as confirmation of check points are to be recorded in this Report. This Report the would serve as a handy record for future reference.   

INSTALLATION

Location:

The transformer should be kept in a well ventilated place, free from excessive dust, corrosive fumes etc. Adequate ventilation is necessary for tank and Radiators so that they can dissipate heat. There should be clear space of about 1.25m on all sides of the transformers if it is enclosed in a room.

Foundation:

Foundation should be firm, horizontal and dry, where rollers are fitted, suitable rails should be provided.

Provisions for Oil Draining:

Necessary provisions for oil draining, in the event of fire, should be made by way of Oil Soke Pits. Fire separation walls should also be provided when necessary.

Assembly of Dismantled Components:

Various components dismantled for transportation should be duly assembled.

Main Tank:

 Keep the main tank in its permanent position of operation. Lock the rollers to prevent any accidental movement on rails. Draw an oil sample from the bottom of the tank and test it for Break down Voltage (BDV)

Note this value in “Commissioning Report.”

Bushing:

Clean the bushings and check that there are no hair-cracks or other damages. Test IR value of each bushing with a 500V Megger. It should be 100M-Ohms or greater. Note details of Bushings in the “Commissioning Report.” Mount all the bushings as described in 2.6. Ensure that the test tap is fully tightened, thus positively grounding the same. Adjust the Arcing Horn Gaps in accordance with the insulation Co-ordination. Note these values in the “Commissioning Report.”

Conservator & M.O.G.

Note details of M.O.G. in the “Commissioning Report.” If the M.O.G. is provided with a locking lever, it should be removed. Mount the conservator. When there is an OLTC its conservator is some times provided separately or by making a partition compartment in the main conservator. OLTC conservator, if separate, should also be mount.

Buchholz Relay:

 Note details of “Buchholz Relay” for the transformer and of “Oil Surge Relay” for the OLTC in the “Commissioning Report.” Buchholz Relay floats are tied to prevent transit damage. They should be released. Also if ‘Test’ lever is provided, it should be in the working position. Mount the “Buchholz Relay” and the shut off valves as described in  2.10. Similarly mount “Oil Surge relay.”

Breathers:

 Note details of Breathers, in the “Commissioning Report.” If OLTC is provided, it may have its own separate breather. Note details of these Breathers also in the “Commissioning Report.” Check that the color of Silica Gel in Main Breather is Blue. Remove the rubber cap closing the breather pipe and fit the breather. Fill oil in the oil cup and remove the seal which closes the breather opening. Similarly mount the OLTC breather.

Radiators:

Note the details of Radiators and Radiator Valves in the “Commissioning Report.” Mount the radiators as outlined below:

(i)                Radiators are to be assembled only one at time.

(ii)             Oil required to fill in the radiators is sent separately in drums. Test oil sample from each

Drum for BDV. Ensure it is more than minimum specified in I.S. 1866 for New Transformer.

(iii)           Fill the conservator full using an oil filter machine, if available.

(iv)            Clean one radiator externally. Remove blanking plates and clean the gaskets and radiator flanges. If gaskets are damaged, use spare gaskets.

(v)              If blanking plates are not in position and it is suspected that foreign material might have entered the radiators, clean them internally by flushing fresh and clean transformer oil.

(vi)           Oil might seep through the tank side radiator valves and be retained by the blanking plates. This oil should be collected in a clean container at the time of removing top and bottom blanking plates.

(vii)         Bring the radiator flanges close to the flanges on the tank. Ensure that tank gasket is in position. Fit by means of bolts, nuts, spring washers etc.

(viii)      Open the bottom radiator valve using the operating handle. Gradually unscrew the air release plug on the top of the radiator, until air starts escaping. Air release plug should not be removed from the engaging threads as it would be difficult to control oil coming out of it. Oil from main tank will now flow in the radiator, driving out air from air release plug. When oil comes out steadily from the air release plug and no more air comes through, close the air release plug. Open the top radiator valve. Oil level in the conservator would now have fallen. Check that there is no oil leakage from the radiator itself and the gasketted joints. Restore the oil level and assemble the next radiator in the same manner.

Assembly of radiators should be proceeded in only one direction-clockwise or anticlockwise. After filling last radiator, oil level in conservator should be restore only upto the filling mark.   

When OLTC compartment is provided in the main conservator, this compartment is to be separately filled upto its filling mark.

Marshalling Box:

Marshalling box should be fitted in position. Pockets of OTI and WTI should be filled with transformer oil. Bulbs of OTI and WTI should be fitted in position in their respective pockets.

Set the ‘Alarm’ & ‘Trip’ contacts of OTI & WTI at required values. OTI setting of A=85^oC & T=95^oC WTI setting of A=95^oC & T=105^oC are considered good. They can be worked out to suit local conditions.

Note details of OTI &WTI and their settings in the “Commissioning Report”

Cable Box:

When cable boxes are provided they should be mounted and cable terminations done. Oil filled cable boxes should be duly filled with oil. In case of “Bus-Duct” connections, transformer is provided with a flange to receive the bus-duct. Due connection and fitting should be done to over-head line by a terminal connector.

Other fittings:

Note the details of other important fittings like fans in the “Commissioning Report” and mount these fittings.

Oil Sampling & Filteration:

Taking usual precautions draw oil samples from Bottom and Top of main tank and from OLTC. They

Should be tested for BDV. If they meet the requirements of IS 1866 for new transformers, no further processing is required. However if they do not meet the requirements, oil needs to be filtered till the required BDV is obtained. Note the BDV values in the “Commissioning Report.”

Air Release:

Release air by slowly unscrewing Air Release Plugs provided at Bushings. Bushing Pockets, Buchholz Relay, Main Tank Cover, Oil Surge Relay etc. From plain porcelain bushing of 11, 22 & 33 KV, air can be release by loosening the nut pressing the rubber gasket and pressing the metal part down.

COMMISSIONING

Pre-Commissioning Tests

 Prior to energizing the transformer, several pre-commissioning tests are done. The objective of these tests is to confirm that the transformer has not suffered damage during transit and also to check any inadvertent slips in the factory tests, or supply.

Ratio Test

Ratio between all the three corresponding H.V. & L.V. phases is to be measured on all taps. It is desirable to do this test by a Ratio-meter. But if it is not available, a simple test of measuring voltages can also serve the purpose.

Referring to R&D plate, find out which terminals of H.V. & L.V. correspond to one phase e.g. for a vector Group of Dyn-11, H.V. Terminals 1U, 1V & L.V. terminals 2U, 2N correspond to U phase. Apply single phase, 415V or 240V, AC., 50Hz to H.V. side and measure voltage on the L.V. side. Measure this voltage on all taps and note them in “Commissioning Report.” Repeat for the other two phases. These observations should indicate a consistent trend of variation in line with the details given in R&D Plates. Numerical values should approximately check with the voltage ratio.

Vector Group Check Test:

Connect terminals 1U, 2U together. Apply 3-Phase, 415V, 50Hz. AC. To H.V. terminals 1U, 1V, 1W. Measure voltages between terminals 1V-2V, 1V-2W & 1W-2V, 1W-2W (suitable other voltages if required). Check that the measured voltages confirm the relative position of H.V. & L.V. vector group. For vector group DYn-11, the method is illustrated below.   

 H.V. & L.V. vectors are shown independently positions for DYn-11. For the condition 1U & 2U connected together, L.V. Vectors are redrawn as shown. With this configuration, Voltages 1V-2V & 1V-2W will equal while 1W-2V will be greater than 1W-2W.

The method can be extended to any other Vector Group. Note the observations in the “Commissioning Report.”

Magnetizing Current Test:

Apply 415 volts, 2-Phase, AC to the H.V. terminals, keeping the L.V. terminals open and tapping switch in the normal position. Measure the 3 line currents, if possible simultaneously, otherwise one after the other. Because the 3-Phases of the magnetic circuit of the core not similar, the 3 line currents will be approximately equal & V phase current around 80% of either on them. If H.V. is delta connected, V & W phase currents will be approximately equal and U phase current around 110%.

  In case of Power Transformers, such a test is done, some times, along with routine tests at the manufacturer works. When done at site and results compared with the factory test confirms that there are no transit damages to the core and windings. Note the observations in the “Commissioning report.”

Magnetic Balance Test:  

This is simple test to detect shorted turns in a winding. Its principle is that shorted turns oppose establishment of flux in that limb because of the current that circulates through the shorted turns. Apply single phase, 415V or 220V AC to such H.V. line terminals which would energizes U phase (Outer limb). Use an averaging instrument like a Multimeter. Measure the voltage induced in the V phase (Center limb) and W phase (other outer limb). Measure also the current drawn by the energized phase. In case the H.V. voltage is 66KV and above, the current drawn may be very small. In that case L.V. would, generally, be 11KV or more. This test can, then, be carried out on L.V. Side. Center limb being nearer to the energized limb, more flux passes through it and less flux in the outer limb. The division is around 70-30% & hence the measured voltage will be having approx this proportion. Next, energize W phase and measure the other two voltages as before. Results should be similar to previous ones. Then energize V phase 9Center limb). As both the outer limbs are symmetrically located w.r.t. center limb, flux will divide equally between them. Hence the voltages measured on outer phases will be approx. equal. Also, for reasons explained earlier, currents drawn when outer phases are excited, will be equal that center phase will be less (approx. 70%). Thus these observation will confirm the healthiness of the windings.

In case one the phase has shorted turns, it will draw a comparatively large current when it is energized. When other phases are energized, flux and hence voltage in the shorted phase will be significantly reduced. Hence all three observations will indicate the shorted phase. Note the observations in the “Commissioning Report.”

When the Magnetic Balance Test indicates a shorted phase, commissioning cannot be undertaken. Contact us immediately giving details test results.

Measurement of Insulation Resistance:   

Measure Insulation Resistance (IR) between windings and earth with a 2500/1000 V Megger, preferably motor driven, otherwise hand-driven. Before measuring I.R., thoroughly clean all the bushings with clean cotton cloth. If required using Carbon Tetrachloride. Also there should be no external connections to the transformer terminals. Check and adjust if required, the infinity setting of the Megger. Lead wires from the Megger to the transformer should run independently and be permanently clamped. They should not have any joints. It is known that IR readings continuous to increase initially and for comparison purpose, reading is to be taken at 1 min of energizing. It is also

Known that IR value is dependent on temperature. Hence temperature at the time of measurement should be noted. Compare the IR values measured, with the Factory result keeping in view the temperature at the time of measurement. Note the observations in “Commissioning Report”. Also measure and note the IR values of power the cables.

Short-Circuit Test:

For the HV side voltage and the % impedance, it would be possible to calculate the current which would flow in the HV side, with 415 V applied to it, while keeping LV side shorted. If the 415 V source can feed that current, a short circuit test can be carried out.

This test would confirm proper contact engagement at all tap positions. Apply 3 phase, 415 V, 50 Hz to HV side, keeping LV side shorted. Measure the 3 line current at all tap positions. If the switch is an OFF-CIRCUIT switch, supply has to be disconnected before charging tap. Note the observations in the “Commissioning Report.” Note also details of OLTC, if any.

A Consistent trend indicates healthiness. If short-circuit test is not possible due to limitation of source, carry out one tap changing operation over the entire range increasing as well as decreasing. Check the other modes of OLTC operation and note in “Commissioning Report”.

Parallel Operation:

Sometimes the transformer to be commissioned is required to run in parallel with an existing transformer. In this case, the following condition must be fulfilled by the incoming transformer:

(1)  Its voltage ratio is same as the existing transformer on all tappings.

(2)  Its % Impedance is +- 1% of value of existing transformer.

Note: Due to difference in % impedance, when one transformer reaches its rated load, the other would share less than its rated load. As a result, the combination can supply load less than the sum of the two KVAs.

(3)  Rated KVAs of the two transformers to be connected in parallel should not differ by more than 1:3 as otherwise only marginal increase will be obtained in the capacity of the combination.

(4)  Vector group is compatible. If the vector groups of the two are such that terminals to be paralleled have a phase different then they cannot be connected in parallel. Hence only certain Vector Groups are compatible with each other.

If possible, one should check zero voltage between the corresponding phases to be paralleled.

Other Relevant Details:

Other controlling and protecting equipments like CTs, Breakers, Relays etc. form a part of the transformer installation. Note their details in “Commissioning Report”.

Check Points:

After completing installation and precommissioning test, check that the various points mentioned in the “Commissioning report” have been complied with. Compliance notings should be made in the “Commissioning Report”.

Energising:

When all the pre-commissioning test are found satisfactory and all the check points are confirmed the transformer can be energized on No load. It is preferable to keep settings of all protective relays to the minimum. The transformer should be allowed to run on No-Load for about 3 hours. During this period, the transformer should be observed for any abnormality in the hum and gas collection in the Buchholz Relay. Sometimes during this period, trapped air is released and gets accumulated in Buchholz Relay.

At the end of this 3 hours period, transformer should be de-energized air released from bushings, Bushing pockets, Buchholz Relay, Main Tank cover, Radiators etc. The transformer should , then, be energized again and then relays set at their desired settings. Transformer should then be gradually loaded reaching full load in about 3 hours time. During the period and for further 3 hours,( i.e. total of 6 hours) transformer should be under observation. Check that all instruments, OTI & WTI, MOG are reading properly and that all radiators are uniformly hot. Check that there is no gas collection in Buchholz Relay. If everything is found in order, the transformer can continue working.

The “Commissioning Report” should be duly signed by the Authorized representatives of all the participating organization.