accep.test rep.good trs

8/14/2019 Accep.test Rep.good Trs.

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G.A.Narayana Swamy B.E. Ph :( 080) 23490692,(M)9448351455,Retd. Engineer No..1240, 12th B cross, 10th main, 2nd

stage,Mahalakshmipuram, Bangalore 86. Date: Sunday, March 01,

2009

TESTING OF REPAIRED-GOOD DISTRIBUTION TRANSFORMERS.

Here it should be noted that the innovations can only be brought out after proper

assimilation of the existing technologies. Actually in field activities no proper understanding of

the technology is available and a lot of handling of equipment is done mechanically.Electrical technology is a very abstract subject compared to say civil or mechanical

engineering. In the latter fields grasping is easy. Besides visual models can be easily set up. In

electricals a detailed explanations often starting from fundamentals is required. This all the more

necessary for supervisory staff coming up from non-engineering cadres ie. who are not

engineering diploma \ degree holders.

The number of distribution transformers in India is supposed to exceed 2.5 million. More

than 50% of them are reported to be failing earlier than their lifetime period.I headed for a period of 8 years (from 1989-1996); transformer sub division in the then

M. T. Division, Bangalore of the erstwhile KEB, now unbundled into several companies.

In view of large numbers, the repair works were outsourced a few months earlier. One of

the assignments of the transformer subdivision was evolving and later actually adopting aprocedure fortesting the repaired good distribution transformers & certification, based on which

the repaired good transformers were later accepted .All the transformer repairs of entire KEB

were handled here only.

Before drawing up the standards for testing we contemplated on the existing proceduresand observed a number of failed transformers after de-tanking the active parts (core &coil

assembly). About 60% of the parts were reusable. The replacement by new ones was to theextent of transil oil & H.V. windings & L.V. windings (Many a time the LV windings were re-

insulated only) and giving a new coat of metallic paint. The transil oil was also not new oil but

filtered good oil. The cost of repairs was about 25% of the cost of a new transformer of similar

KVA capacity.We also examined the various tests conducted by transformer manufacturers on new

transformers. In view of usage of a large number of old parts, the costs involved, the years

through which they have served already, the balance years of their expected service & opinionsof the repairers, we evolved tests and standards which were different from those for newly

manufactured transformers. The same were proposed to the then Chief Engineer MM &P(Material Management & Purchase) & approval obtained. At the chief engineers office Iexplained at length to the connected engineers etc the reasoning behind the tests. In certain tests

there was a dilution of standards. The logic behind the same was also explained. Later a format

was got printed. Subsequently, all the details of the tests like nature of tests, purpose of tests,electrical theory behind each test, expected standards etc. were explained by me to my

subordinate assistant engineers and junior engineers who were the actual engineers deputed for

testing.


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But over a period all the testing engineers stand transferred and found that the knowledge

was not transferred from one to the other. A sort of mechanical handling of events had come to

stay. For the purpose of giving knowledge to the present testing engineers & to create apermanent record of all the details of testing procedures, the tests on repaired good distribution

transformers are narrated as below.

ACCEPTANCE TESTS FOR REPAIRED GOOD DISTRIBUTION TRANSFORMERS.

At the outset for the purpose of identifying each transformer & to differentiate one from the other

the name plate details of each transformer shall be noted. In cases where name plates are missing

suitable name plates may be provided with his own serial number etc.1. Megger or Insulation Resistance test: Megger is an acronym for mega-ohm meter. Mega is

for a million and ohms stand for resistance in ohms. In other words megger test is to meter

the millions of ohms ofinsulation resistance offered for the electricity or electrons to escape

from its metal route to ground. It is the ability to prevent the electrons from leaking out. It is

the first & foremost test of an electrical engineer to ascertain the healthiness of an electricalgadget from the point of confining electricity to its metallic parts. The quality & strength of

the insulation provided decides the mega ohms.

The strength of a chain lies in its weakest link. This proverb hold good here. The insulationprovided at the weakest point decides the strength of the insulation & hence the mega ohms.

The megger values of a winding are actually much more at its dried condition. ie.

immediately after ovening (A process for driving out all the moisture). After adequateovening, the winding is put into transil oil which occupies the vacuum points created while

drying out the moisture. At this stage the megger values actually decreases but the values

remains sustained at constant values for a long period. The values of course get affected due

to age & other factors like over & impulse voltages.

While actually measuring the megger values it is necessary to involve all theindependent metallic items into consideration. In a distribution transformer the metallic items

available are HV winding, LV winding and tank ( magnetic core is bolted to the tank andhence can be taken as part of tank). Since tank is going to be earthed when the transformer is

put to service, HV winding megger results can be between HV winding and tank. When

actually measuring it is important to interconnect LV winding and tank as shown in thesketch below. Similarly LV winding megger results is between LV winding and tank inter

connected with HV winding.


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Item meggered Mega ohms Temperature

HV to Ground

LV to ground

HV to LV

Below are the megger results of a typical repaired good 63 and 100 kva ,1100 \ 433

transformers.

Item meggered Megger used Mega ohms

HV to Ground 2.5 KV 5000

HV to LV 2.5 KV 5000

LV to ground 500 volts 5000

Tests on transformer efficiency: These tests decide how effectively the repairs have been

carried out & to what extent the transformer could be used after the repairs. For the purpose

one will have to measure the losses, both when the transformer is on No-load condition as

well as Full-load condition. Hence the losses are termed as No load loss & load lossrespectively.

Fortunately the losses can be easily measured by subjecting the transformer to what are

called open circuit test & short circuit tests.

HV LV

MEGGE

TANK

Transil oil


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2. Open Circuit test: Here the test consists of applying the No-load rated voltage of adistribution transformer ie. 3 x 433 volts to the L.V. winding of the transformer, leaving the

H.V. winding open. Here one must note that the voltage applied must be 433 volts and not

400 volts since all distribution transformers are designed for 400 volts at full load and 433volts at no load. The circuitry is setup to measure the currents as well as watts.

The LV currents are normally measured as above. Some times tong tester can also be used.The watts are measured by two wattmeter method. The results, including the applied volts are

entered into a table as shown below. Below are the open circuit test results of a typical

repaired good 63 and 100 kva ,1100 \ 433 transformers

Particulars ofTransformer

L.V. Voltagein Voltsbetween

Current inamperes

No Load Losses

2U-

2V

2V-

2W

2W-

2U

2U 2V 2W W1 W2 W1 W2

Transformer63KVA

433 433 433 0.

8

0.

9

0.8 80 40 40 watts.

Transformer 100

433 433 433 1.

6

1.

6

1.5 32

0

16

0

160 watts.

Transformer No.3

433 433 433

The watts measured here pertain to what occurs when the transformer is in idle service ie

without any load. These losses occur always ie 24hrs a day & 365 days of the year. They are

called iron losses. The iron losses measured should not exceed a pre-determined value

(standard set by CEE MM&P) for a given KVA of the transformer.3. Short circuit test: Here the test consist of applying the same 3 x 433(or 400) volts to L.V winding of the transformer with H.V. winding shortcircuited. Here current in amperes is more important than the appliedvolts.

Leave HV open

2V2U

w2

A

1

A

2

A

3

Supplyfrom 3phaseDimmerstat

Ammeter

Wattmeter

w1

2W

1U 1V1W


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The LV currents are normally measured as above and the HV currents by a tong tester. Sometimes tong tester can be used for measuring both the currents.

The watts are measured by two wattmeter method. The results are entered into a table as shown

below. Below are the short circuit test results of a typical repaired good 63 and 100 kva ,1100 \

433 transformers

Perticulars

Of Trans

Applied volts. Current in amperes Watts

recorded.

Load losses

. 1U 1V 1W 2U 2V 2W W1 W2 W1-W2

Trans-100KVA 430 5 5.1 5 128 127 128 1520 240 1280 watts

Trans-63 KVA 430 3 3.1 2.9 79 80 80 800 160 640 watts.

Trans-3

The watts measured here are called load losses, (at 433 volts in lieu of 11000 volts) because thetest circuitry resembles the full load condition of a transformer. Here one should understand what

is meant by loading a transformer & what is meant by No load & full load. No load of course is a

status when the transformer is put on the rated voltage but no supplies are extracted from theother winding.

What is meant by loading? Suppose the transformer is 50KVA distribution transformer. The full

load current would be 66 amperes.Then what is meant by fully loading a transformer? Decreasing the resistance R to values so that

I = 66 amps. By short circuiting the secondary & applying the rated L.V. volts ie 3 x 433 volts

to the primary we are providing for maximum current flow at 433 volts. Since the winding is

designed for 11000, shot circuiting will not result in any damages. The watts obtained isextrapolated to the full load temperatures of transformer when the transformer is heated to

around 75C which is room temperature plus designed allowable temperature rise in transil oi ie

45C. So watt loss at 30 + 45 = 75C is more than watt loss at 30C (normal room temperatureassumed).

In other words, the load loss at 75C is arrived by adding figures (empirical ie based on

experience) depending upon the total KVA capacity of transformers. Please note that thisextrapolation is only for short circuit test & not to open circuit test or No load test, since the

temperature almost remains the room temperature in the latter case. The watts measured as in the

case of iron loss should not exceed predetermined or standard value for a given KVA of thetransformer. The losses can be computed to losses at full load currents by employing a formula

given in part-1 of I.S. 2026 of 77. Computed loss at rated current = measured loss at test current

x (rated current) (test current)

L.V. short circuited.w

2

A

1

A

2

A

3

Supplyfrom 3phaseDimmerstat

Ammeter

Wattmeter

w1


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3. Winding resistance measurement:The total winding resistance is ameasure of total number of turns provided & cross section of the wiresused in the construction of the full winding. Why DC in lieu of AC? WhenAC is used the winding reactance also comes into play. Hence the same is

eliminated by applying only direct current volts to the winding. Nowadaysthere are meters which show the resistance readings directly & the sameare recorded as below.

Below are the results of a typical repaired good 63 and 100 kva ,1100 \ 433 transformers

ParticularsofTransformer

D.C. resistance in ohms Remar ks

1U-1V 1V-

1W

1W-1U 2U-2V 2V-2W 2W-2U

Transforme

r 100 kva

18.9

ohms

18.9

ohms

18.9

ohms.

24.2

Mili-

ohms

24.2

Mili-

ohms

24.1

Mili-

ohms

Transforme

r 100 kva

20.1

ohms

20.1

ohms

20.1

ohms

25.1

Mili-

ohms

25.1

Mili-

ohms

25.2

Mili-

ohms

Transforme

r 63 kva

34.6

ohms

34.6

ohms

34.6

ohms

25.7

Mili

ohms

25.7

Mili

ohms

25.6

Mili

ohms

4. Dielectric tests

4 a)separate source voltage withstand test. This is tocheck the adequacy of the insulations to earth andbetween windings. Here the HV & LV windings are treatedseparately since each is insulated to different voltagelevels. A little concession in the case of repairedtransformers is made here compared to newlymanufactured transformers. So, the HV & LV windings aresubjected 21 KV & 2.25 (in lieu of 28 and 3 KV as per IS)respectively for a duration of 60 seconds and observe forany irregularities.. Here one should understand what ismeant by say 21KV. It means applying a potential

difference between the metal part of winding & earth at21,000 volts.


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HV LV

Apply 21 KVfor 60seconds

TANK

Transil oil

Earthing bolt

HV LV

Apply 2.25 KVfor 60seconds

TANK

Transil oil

Earthing bolt


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5. Induced over voltage test :This test is popularly called double voltage double frequency test

This is actually a high voltage test on the transformer as in test 5. Here the LV winding of thetransformer is applied with twice the rated voltage 3 x 866 volts for a duration of 60 seconds &

observing for any irregularities in transformer functioning. But the core which provides paths forthe magnetic lines created is designed for 433 volts of LV & 11000 volts of H.V ( plus with asmall safety as per I.S ). When the voltage is increased to twice, the number of lines also

increases by two fold & the core designed for 11000/433 volts will not accommodate or provide

a path to the increased magnetic lines. So the core gets saturated at a voltage less than 22KV. Inother words we will not impress the 22 KV to core and LV winding.

Here one phenomenon of the magnetic lines created with reference to frequency of voltage is

used. The magnetic density in a core is inversely proportional to the frequency of voltageapplied. Namely the number of lines created is directly proportional to the frequency. In other

words a core designed for say 10,000 lines at 11,000/433 volts at 50 cycles can accommodate or

provide a path to 20,000 lines of 22,000/866 volts at 100 cycles frequency & 30,000 lines of

33,000/1299 volts at 150 cycles frequency.Thus the frequency of double voltage test is raised to double frequency namely 100

cycles / second so that all the three elements namely the HV & LV windings and core are

subjected to twice their normal voltages

Ratio test: This test is to ensure the correctness of voltage transformation ratio of a transformer

namely 11,000/433 volts. Incase the transformer is provided with off load tap changer one mustcarry out this test on all taps. (Please note that the transformation ratio changes from tap to tap).

This test also ensures proper engagement without any loose contactness at all contacts of the off

load tap changer in the interiors because these are several instances of the transformers failing

HV LVTANK

Transil oil

Apply 866V, 100 c\s supply for 60


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while in service after one or two days of tap changing because of loose contacts. Ratio bridges or

ratio meters giving a direct reading of the ratios can also be used.

Below are the results of a typical repaired good 63 and 100 kva 1100 \ 433 transformers

6. Vector group test: This is to check up polarity and to ensure that all the transformers testedbelong to the same vector group. Almost all the distribution transformers are manufactured to

Dyn11 vector group. In other words, the phase displacement of LV voltage from HV voltage

is as in a clock showing 110 clock. At 11o clock the angle between hour arm & minutesarm would be 330 ( measured in clock wise direction only) ie the power supply in LV

winding lags behind that of HV winding by 330 electrical degrees. Suppose in the field a

situation arises for putting two or three transformers in parallel. If the transformers do notbelong to the same vector group, parrelling of the transformers would lead to short circuiting

and would be dangerous. Hence we must ensure that all the transformers belong to the same

vector group. This is ensured by carrying out this vector group verification test. The same isdone by short circuiting 1U & 2N ( This is one type; out of three to four types

available).Before such shorting the transformer vectors of a DY11 vector group would be as

below

Applied

voltage

1U 1V 1V 1W 1W 1U

430Volts 430Volts 430Volts

Output

voltage

2U 2V 2V 2W 2W 2U 2U 2n 2V 2n 2W 2n

16.1 Volts 16.1 Volts 16.1 Volts 9.1 Volts 9.1 Volts 9.1 Volts

1U

1V

1W

2W

2U

2N 2V


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The vector diagram after shorting would be as below. Apply 3 phase 400 volts to the three

terminals of primary & measuring the output voltages as shown in the table.

If the polarity between HV and LV is same and vector group is Dy11, then the following

THREE observations should be ensured

.

1. The voltage 1V-2U should be equal to the sum of voltages 1V-1U & 1U-2U.2. The voltage 1U-1W should be equal to the sum of voltages 1W-2W & 2W-1U.3. The voltages 1V-2W and 1V-2V should be equal.

Below are the results of a typical repaired good 63 and 100 kva 1100 \ 433 transformers

1U

2W

2N 2V

Applied

voltage

1V 1U 1U 1W 1W 1U

426Volts 426Volts 426Volts

Output

voltage

1V 2U 1V 1U 1U 2U 1U 1W 1W 2W 2W 1U

435 Volts 426Volts 9Volts 426 Volts 418 Volts 8 Volts

Outputvoltage

1V 2W 1V 2U

405Volts 405Volts

1W 1V

2U


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So the transformer windings are correctly inter connected to correct polarities and DYN

vector group.

Painting :- This is recently introduced measure for identification of different capacities

of transformers. One of the conditions of repairing a transformer is giving a fresh coat of

paint with a red oxide base coat. Instead of retaining the existing colour the transformersare given new colours so that one can identify the capacity of the transformer by looking

at the body paint alone. The colour codes are sky blue, silver grey and green for 25, 63and 100 KVA transformers respectively.

In all the above tests standards are to be fixed to serve as a yard stick for passing the transformer

as good. This will have to be decided by a committee of engineers including expert repairers.

-------- G. A. Narayana Swamy.

accep.test rep.good trs

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