Tranformer Turn Ratio Test Method

• The simplest way is to ring the manufacturer with the trnasformer serial number and ask.
• It might be in the manufacturer's information pack originally supplied with it.
• Another possibility is to divide the no-load output voltage into the supply voltage.

Your best bet if you need to have this done in the field (such as commissioning tests at transformer installation time) is to hire a testing contractor who has the proper equipment and experience to do the testing you need. Check with NETA (InterNational Electrical Testing Association) for one of their member companies near you (www.netaworld.org). That way you know it's done right.

If you only have to do it this one time, it probably won't be economical for you to buy the test equipment needed - the testing contractor will cost you less, and will back up their testing with good reporting and knowledge.

many thanks peter & pwslack for your valuable comments.Can it done by normal meggering meter ??I am interested to know the procedure>>>>We have normal megger ....But what is the philosphy & formulae behind this .

If by "meggering meter" you mean an insulation resistance tester, no, you cannot. Transformer turns ratio has nothing to do with the transformer's insulation, and the megger (or insulation resistance) test has nothing to do with the turns ratio.

If you mean a test meter produced by the Megger(tm) corporation, it depends on what meter it is. Megger does produce several different transformer turns ratio testers, but it is not the common insulation tester most people call a "megger."

A TTR test set applies a low voltage (usually 40-80 VAC) to the transformer's higher voltage winding, and measures the voltage produced on the other winding. What is shown to the operator is the calculated ratio.

At site it is generally aceptable to carry out ratio test by connecting the high voltage side to a low mains supply say 220 or 415 volts and taking readings by an acurate voltmeter at both primary and secondary. Do take into account the setting of tap changer. Factory test certificates provided by the manufacturer are depended upon.

For accurate reading and to ensure that the permissible tolerence is withen + 5% of the declared ratio proper method is to use a ratiometer transformer apparatus and an ammeter connected between the two windings to detect circulating current, however this is out of your reach.

Tan delta test is a Type catagory test, not recommended for site i.e. not a Routine catagory test.

You seem to be confusing Oil Dielectric strength test with tan delta test. For dielectric strength test send sample of transformer oil to any of the transformer manufacturer or any of the testing labarotary near you.

Actually, Tan delta (or dissipation factor or insulation power factor) testing is a widely used field test method for evaluating the insulation of electrical apparatus, including transformers, HV circuit breakers, generators, etc. One of the best known suppliers is Doble Engineering, whose Doble(tm) insulation power factor test equipment is used worldwide. It is routinely used in maintenance testing of transformers, to give additional information not available from other tests such as TTR, DC insulation resistance (hipot or megger), oil analysis, etc.

If I were to have to choose the minimum set of field tests that would give me the most information about the general condition of a power transformer with the least testing, I would recommend the insulation power factor/tan delta test and a full oil test battery including dissolved combustible gas analysis.

Selection of Routine and Type test catagories are not based on personal openion.

So long as the codes and standards do not incorporate tan delta test as a Routine catagory test the engineer at site is free from all liabilities. For Insulation the admissible Rutine tests are done by Meggar.

Not just opinion. Insulation power factor is accepted and included as routine test in various standards. For example:

http://cr4.globalspec.com/comment/586250

I'm not meaning to say other tests are worthless. Just from experience in evaluating transformer condition, I gave the tests that provided the most diagnostic information.

You did not check the table properly, please go to it again, with reference to Measurement of Insulation Power Factor, requirement is given as "N".

Keeping aside this argument please provide the standard reference test values, OP has asked for it.

If you know the winding wire gauge then DC resistance can give clear idea of the total wire length used. It still does not give accurate idea of number of turns, however you can reach very close to the figure.

I did find that some transformers were bound with less or more turns than they are supposed to be and there was this problem in center tap transformers giving different voltages in two sides.

Inductance measurement may be more accurate if one knows about the core before hand.

I think it is an excellent problem for electrical engineers and all must explore it out.

Hey guys

You can just use Megger "TTR100"

Hai,

If you have TTR meter with you follow these steps (before check TX Isolated from power supply or not)

The purpose of the measurement is to check that the deviation of the voltage ration does not exceed the limit of the transformer standard (generally 0,5%). The vector group is also checked at the same time.

The voltage ratio measurements are carried out by means of a measuring bridge. The accuracy of the bridge is ±0,1%.

The voltage supply used for the bridge is 400/230 (380/220) V, 50 Hz.

The function of the bridge is shown in Fig.. The voltages of the transformer under test are compared to the corresponding voltages of a regulating inductive divider, which is placed inside the bridge and equipped with a decade display. When the zero indicator is equilibrated, the voltage ration of the inductive divider is the same as that of the transformer under test. The result of deviations is shown directly on the display of the bridge.

Because the bridge measuring device works on the single-phase principle the voltage ratio is measured phase by phase between two windings mounted on the same leg. The indication on the bridge display depends on the vector group of the transformers main voltages

At the same time with the voltage ratio measurement the vector group symbol of the transformer is also checked. When the measuring conductors of the transformer are connected to the bridge according to Fig.and Vector group (Dyn11 or Ynd5 etc )the bridge can be balanced only if the vector group is correct.

The above shows checking each pase separatly. If meter having 3ph facility u can connect & measure directly all phases.

If you don't have TTR meter follow Mr. Mountk (# 5) suggetions like apply 230 / 415V at HV side and measuare voltage & Current at HV & LV side by voltmeter & Milli ammeter. Cross check the values with Actual voltage or previous data.

Example: TX-60 MVA, 132 / 33 KV

Applied voltage on HV (normal tap): 415 V

Measured voltage on LV: 103.75 V

Cross check with main volatge : (415 x 33K) / 132K = 103.75V (+/- 5% allowable as per IS & IEC)

Like this check all taps.

I hope this helps some extent.

Dear Friends,

Many thanks for sharing your views .Thanks particularly to Mr. Hanumanthu,Peter and others.

Ok ...facing another problem now.PRD actauted after few mins of operaton.TTR & oil analysis revealed nothing.what could be the reason behind PRD actuation...sure sudden surge of pressure ..but what could be the reason.Buccholz didt operate at that time....strange...>Any comments.

Can you give me some clarity abt PRD actuation?

I think pressure builded in tank but why?.

Buchhloz is operating for gases.

U r doing any oil filtration process?

Pls check at conservator Breather whether expantion & contraction happning?

Wt is the temperature of oil at that time?

Yes some oil came out of it.Our conservator consist of three parts two for diverter switch & one for main tank.we did replace diverter switch.But never touch main conservator valve going to tank.

Pls check the diverter switch properly connected and taps top (first) and Bottom (second) moving properly. Sparking occur in contacts, If any mall function in taps. It possible to incresing of sudden pressure in Tapswitch tank.

Dear Mr. Beth,

Taps of diverter switch working pefectly fine.I am thinking that if main valve going from conservator to tank closed than this could happen???But still not corroborating evidences to prove this.Moreover if there is any malfunction in diverter taps than it could also occur again.

Rgds,

Hai Power,

Yes ur right, either diverter switch or tank-conservator valve.

Did u observed the valve opend / closed after PRV actuation?

A question on the oil analysis you did after this event - did you have the oil analyzed for dissolved combustible gases? This will tell you if you had an arc in the tank.

If not, then suspicion is that the the work you did on the transformer left some air somewhere in the tank. It is vital with a conservator to properly bleed off any air that might have entered, and also to have all valves properly set. Otherwise that air can expand, migrate, and cause false activation of PRD, Buchholz, etc.

Dear Mr. Peter,

Many thanks to all you guys for sharing info.Well I feel diverter tanks oil filling was done & to their respective chambers in conservator tank.Air was properly released as well.I am still working out with the current plots during those period.If i will get something I will share with you all.

Regarding oil analysis....yes we immediately done but the results are perfectly ok.TTR also ok.

Regards,

DESCRIPTION

http://www.trs-rentelco.com/Specification/MGR_550503_Spec.pdf

The Three-Phase Automatic TTR is designed to measure

the turns ratio of power, instrument, and distribution

transformers in a substation or manufacturing environment.

At 16.5 lbs (7.5 kg), it weighs less than any other

commercially available instrument. A rugged and robust

design makes this TTR well suited for use in a variety of

harsh environments. The TTR is also particularly suited for

testing in power transformer manufacturing environments

where testing of complex, three-phase substation power

transformers can be performed quickly while minimizing

the possibility of errors.

This new TTR measures the highest turns ratio range in the

industry (10,000:1) and also provides the highest accuracy

(0.1%). No other instrument's performance is comparable

that is commercially available today.

Another excellent feature of this new TTR is the ability to

measure phase deviation (in minutes or centiradians) of

the transformer primary versus secondary. This will

quickly indicate problems in a transformer such as partial

shorted turns and core faults. This measurement is also

useful in verifying phase errors of all types of PTs and CTs.

The TTR also features special software capabilities. The

TTR comes equipped with sufficient onboard memory to

store up to 200 test results in the field for later retrieval in

the office. Test results can be printed on an optional serial

printer whenever a hard copy is desired, or the data can be

downloaded to a PC. Identification of individual test

readings is also easily done. The system software allows

entry of the transformer alphanumeric serial number,

transformer type and tap information for each test

performed.

This new TTR also comes with a unique optional, remote

control software program. This Windows® based program

permits control and operation from a PC keyboard,

download of test data from the TTR to a PC, print out a

test results report and assist in the preparation of management

and/or analysis reports using either Excel® or Access®.

Realizing the extreme environments in which the TTR must

operate, special attention has been paid to making it extra

rugged (with a hard, shock resistant case), yet incredibly

light weight (16.5 lbs). It features a high contrast LCD

screen which can be seen in bright or ambient light and

comes equipped with specially designed leads which

provide the necessary flexibility needed in cold weather

conditions.

APPLICATIONS

The proper operation of a transformer relies almost

entirely on the electrical properties of its windings. To

ensure continued proper operation, transformers are tested

to verify that their electrical properties have not changed

from design specifications.

A TTR is an extremely useful instrument for testing

transformer windings, because it can locate several types

of problems within a transformer. It is also ideal to use for

testing in Meter shops for the upcoming inspection of CTs

and PTs. It can determine the no load accuracy of all CTs

and PTs and also determine the need to further test faulty

CTs and PTs.

The above is for ur perusal. U can also visit the site as spesified above.

Dickson.