High-voltage transformers are some of the most important (and expensive) pieces of equipment required for operating a power system. The purchase, preparation, assembly, operation and maintenance of transformers represent a large expense to the power  system.

When transformers are received from the factory or reallocated  from another location it is necessary to verify that each transformer is dry, no damage has occurred during shipping,  internal connections have not been loosened, the transformer’s ratio, polarity, and impedance agree with its nameplate, its major insulation structure is intact, wiring  insulation has not been bridged, and the transformer is ready for service.

Physical size, voltage class, and kVA rating are the major factors that dictate the amount of preparation required to put transformers in service. Size and kVA rating also dictate the kind and number of auxiliary devices a transformer will require. All of these factors affect the amount of testing necessary to certify that a transformer is ready to be
energized and placed in service.

There are a multitude of checks and tests performed as a transformer is being assembled at a substation.

Some tests and procedures may be performed by specialists during the assembly phase. Special tests, other than those listed, may also be required. Many require special equipment and expertise that construction electricians do not have and are not expected to provide. Some tests are performed by an assembly crew, while other tests are done by
the person(s) making the final electrical tests on the transformers.

 The following information is not intended to describe, or include, the details for performing the entire array of tests needed to prepare transformers for service, only the tests that may be performed by field personnel. Even though details have been limited, descriptions should allow field personnel to perform, or assist in performing, the
basic tests they may be asked to do. Procedures and tests are described somewhat generically, but apply to most transformers in one way or another. Also, the following test descriptions provide an anchor point from which to ask for help when needed.

 The following items are discussed or described:

Before proceding with transformer measurements the test engineer will become familiar with the safety rules of Section

THESE RULES MUST BE FOLLOWED FOR ALL TEST PROCEDURES.

Following is an approximate sequence for transformer testing:

1. Inspect transformer and parts for shipping damage and moisture.
2. Check nameplate and prints for proper voltages and external phasing
connection to the line or bus.
3. Check calibration of all thermal gauges and hot-spot heater, bridge RTDs and
associated alarm contacts. Contact settings should be similar to the following:

4. Check and Megger all wiring point to point: Fans, pumps, alarms, heaters, tap
changers, and all other devices on the transformer and interconnecting cables
5. All banks above 150 MVA should be vacuum dried. Do not apply test voltages
to the winding during the vacuum drying process. Make certain the terminals
are shorted and grounded during oil circulation because of the large amount of static charge that can build up on the winding.
6. After the tank has been filled with oil, confirm that an oil sample was sent to
the Chemical Lab and that its results are entered in the bank test reports. Note
the oil level and temperature at completion of filling.
7. Power operate to verify proper rotation of pumps and fans and correct operation of the under load (UL) tap changer, when provided. Also, check heater, alarms and all other devices for proper operation. 

12. Following are the winding tests to be performed:

13. Load CT circuits overall and flash for polarity. 

14. Before energization, trip-check bank protection schemes and make sure the gas-collection relay is free of gas.
15. When energizing a bank or picking up load, monitor bank currents and voltages,
including UL tap-changer operation.
16. Check proper phasing and voltage of the bank to the system before load is
picked up. When possible, large transformers (>1 MVA) should remain
energized for eight hours before carrying load.
17. Make in-service checks on meters and relays.
18. Turn in revised prints and test reports, which should include the following:

Collecting nameplate data is not testing, but it must be done for all equipment. This data is recorded by the person(s) performing the equipment tests. The act of recording the nameplate data also helps test personnel familiarize themselves with the unit to be tested.
For a transformer, much of the needed information can be obtained from the main nameplate.
(typical on large transformers). Bushings, fuses, fan and pump motors, lightning
arrestors, and disconnect switches will also have individual nameplates. An attempt  should be made to fill in all pertinent spaces on the data sheet.

Terminal marking of power transformers is determined by ANSI standards. Two-
winding transformers have terminals designated by H and X (e.g. H1, H2, X1,X2,)

where H is the higher voltage-rated winding and X is the lower voltage winding. As viewed from the high-voltage side, H1 bushing terminal will be located on the right. Three-or-more-winding transformers will have winding designation H, X, Y and Z, where H is the
high-voltage winding (or, the highest kVA-rated winding in case windings have the same voltage rating) and X, Y, and Z are for decreasing winding voltage ratings.

Most hand-crank Meggers have output voltages from 250 to 500 volts DC. All wiring on transformers should be Meggered at 250 or 500 VDC. Meggering transformer wiring is emphasized because of the numerous small terminal
boxes mounted on large power transformers. Conduit connecting them together can have moisture accumulation or water leaks. In addition, when wiring is pulled through the metal conduit on a transformer, occasionally the insulation is scraped down to the bare wire. Also note that any box mounted on a vertical surface should have a small drain hole drilled at the bottom in case water leaks in from a loose conduit joint. Larger boxes or cabinets usually have resistive heaters and air-vent holes covered by screens to prevent moisture accumulation.Terminal boxes mounted on horizontal surfaces must have good
weather seals for their covers. Any gasket with questionable ability to provide a
watertight seal should be replaced.

Transformer bushing CTs should be tested using the Current Ratio test method before the transformer has been completely assembled. CTs should be tested before they are mounted on the transformer. In some cases, CTs may have to be tested by connecting
test leads to both ends of an installed bushing. This can be difficult! If the CTs are already mounted in the transformer, large (high-capacity) current-testing leads can be pulled through the CT centers before bushings have been inserted.
Occasionally it is not possible to perform a Current Ratio test. CT tap ratios can be
verified by applying a voltage across the full CT winding – a Tap Voltage Ratio test -- then measuring the voltage drop across each individual tap. This is a simple test to perform, and voltage ratios will be directly proportional to the CT turns ratio between taps.
This Tap Voltage Ratio test, however, should not be chosen as a substitute for a Current Ratio test. The voltage method should be regarded as the last alternative. Testing the equipment at rated current offers more assurance that it will perform as expected when placed in service. The Current Ratio method reflects this philosophy.The Tap Voltage method cannot establish true orientation
(polarity) of the installed CT, or test the primary to secondary current ratio, and leaves some points unverified.
In addition to Tap Voltage Ratio, a secondary Tap Current Ratio test can be performed.
For this test, rated or less current is injected through a tap input and the output current of the full CT winding is measured by transformer action. It is equivalent to the procedure used for performing a Short-Circuit Impedance test on an autotransformer.

It is still necessary to verify CT polarity. One method used to establish CT polarity in power transformers is commonly referred to as "Flashing the CTs." This test can be performed by applying 6-to-12 volts DC to the transformer bushings, using a hot stick to make and break the test circuit. An automobile battery is often most convenient because
work vehicles are usually available at the job site, but a lantern battery will work as well. The transformer winding resistance is usually enough to limit the current flow from a 12-volt car battery, but adding series (current-limiting) resistance (a load box) to the test circuit is advisable in any test circuit with an automotive battery.
Be aware that the DC test circuit will generate a voltage kick when disconnected. Take precautions to prevent electric shock. If performing this test directly on CTs, always
include a current-limiting resistance (a load box) in the flash lead connections. Lantern batteries have high internal resistance and don’t need an extra series resistor. Arc flash on a power transformer can be limited if the transformer windings are short circuited on the side opposite those being flashed through.

Ratio, polarity, and impedance measurements are compared with nameplate data to verify their correctness and to ensure that there is no hidden shipping damage, that the transformer field assembly is correct, and that the transformer is ready for service. In addition, these test data reports become a valuable tool when compared with later diagnostic tests used to assess transformer condition.
Single-phase test procedures can be used to measure the ratio and impedance of two-winding transformers, three-winding transformers, autotransformers, and
three-phase transformers. Moreover, in the case of three-phase transformers (with a Wye connection) and grounding banks, zero-sequence impedance measurements are made with the single-phase procedure. Comparisons between measurements are useful when single-phase tests are made on three identical transformers or on each phase of a? three-phase transformer, as it is unlikely that each single-phase unit or each phase of a three-phase transformer would have sustained the same damage.

The polarity designation of each transformer winding is determined by the relative direction of instantaneous current or voltage as seen at the transformer terminals. For example, primary and secondary leads are said to have the same polarity when, at a given
instant, current enters the primary lead in question, the instantaneous induced voltage in the secondary is increasing when the impressed voltage on the primary is increasing, or conversely, if they are both decreasing at the same instant.
Transformer polarity relates to how winding leads are brought out to bushing terminals.
These connections are determined by transformer design, winding directions and internal-lead clearance requirements. Transformer polarity is either subtractive or additive. If the instantaneous polarity (as defined above) of adjacent terminals is the same, transformer polarity is subtractive (See Fig. A). If diagonally opposite terminals of a transformer
have the same instantaneous polarity, transformer polarity is additive (See Fig.B). Transformer winding polarity locations are important when identical windings on a transformer are to be paralleled, when paralleling transformers with identical ratios and voltage ratings, when determining three-phase connections of transformers, and to establish the correct connections for three-phase transformers that operate in parallel with the power system. Polarity between transformer windings may be determined either by comparison with a transformer of known polarity, DC flashing or the AC method.

The DC Flashing test uses 1.5-V to 6-V dry-cell batteries and a DC voltmeter with 1.5-V to 10-V scales. Meter deflection directions will be more discernible if the meter’s mechanical zero position can be adjusted in an upscale direction to allow deflecting in both directions. As shown in Fig. the test is conducted by connecting the voltmeter to the transformer low-voltage terminals and connecting the battery intermittently to the high-voltage winding.
transformer polarity is subtractive if the meter shows an upscale kick when test connection SW is closed and a downscale kick when test connection SW is opened.
In this case bushings “A and B” would be labeled “X1 and X2,” respectively as polarity terminals are adjacent. Transformer polarity would be additive if the meter shows a downscale kick when test connection SW is closed and upscale kick when test connection SW is opened. In this case bushings “A and B” would be labeled “X2 and X1” respectively as polarity terminals are diagonally opposite.

Adequate meter deflection will require a dry-cell battery that is in good condition, low-resistance connections to bushing terminals, and positive make-and-break connections to terminal H1. The usual procedure is for the person operating the battery connections to say, “Make” when connecting the positive battery terminal to the H1 bushing and “Break” when opening the connection. Then, observation of meter deflections determines transformer-winding polarity. Also, verify meter terminal markings by measuring a dry cell prior to testing.

TAP CHANGERS:

Tap changers allow transformer input and output voltages to be matched with the rest of the power system or to adjust voltage levels between power sources and customers. They allow transformers to be applied in situations where voltage control would be verydifficult without utilizing additional expensive equipment..
Tap changers are either “no load” (NL) or “under load” (UL) control devices. No load means that the transformer must be de-energized before an operator may change tap positions. Under load means that tap positions can be changed while the transformer is energized and carrying power. UL tap changers allow transformers to be used as voltage
regulating devices; however, the other method for voltage regulation on the power system is through the use of shunt capacitors and reactors using circuit switchers and/or breakers to switch them on and off line While Voltage Ratio, Impedance, Resistance, and TTR tests are being performed, the tapchanger
mechanism must be operated to get test data under all available tap settings.
When the Ratio test is performed, all possible tap positions should be checked for both NL and UL tap changers. This is especially important for a new transformer. Limited assistance may be available in the manufacturer's instruction books for personnel who are not familiar with these mechanisms.

When a NL tap changer is tested, a small amount of current is applied to excite the tap winding, and a meter monitoring the output winding is carefully observed to determine where windings are dropped out and picked up during tap changes. This check should be used to verify that tap-changer contacts are properly centered, and that they actually do
open and make up at the correct point between tap changes. All tap changers must have their contacts firmly and correctly placed when in a tap position. On some tap changers there is some spring pressure felt when manually changing from tap to tap. This pressure should be consistent on manually operated (NL) tap changers when moving from one position to another, either up or down. Note that NL tap changers actually create an open circuit during tap change transitions. If they do not appear to operate correctly, investigate!  When testing UL tap changers, excitation current should be monitored continuously to verify that the winding is NEVER open-circuited.
Both local and remote operation and indication of UL tap changers must be verified.
Remote operation from the control house should be tested completely. The limits of the control should be checked to see that the tap changer will not go beyond its stops. Check SCADA control over the entire operating range. Make sure that the local and remote position readings match. Check for operation with the supervisory cutoff switch in both
the "on" and "off" positions.

Source:Transformer testing pdf