How to test a transformer

What testing proves

Three things fail in a transformer: its windings, its insulation and, in a liquid-filled unit, its oil. Every test below answers one of three questions.

Are the windings intact?

Turns ratio and winding resistance. Shorted turns change the ratio; a loose connection or a broken strand raises the resistance of one phase.

Does the insulation still hold?

Insulation resistance and power factor. Moisture, carbon and age let current leak through the insulation, between the windings and to the tank or core.

Is the oil sound, and what has it recorded?

Dielectric strength and moisture say whether the oil still insulates; dissolved gas analysis says whether anything inside has been overheating or arcing.

Together they take a crew about half a day on a distribution transformer, with the unit de-energized, plus a few days at the oil lab. A clean set of results, dated, is what separates a used transformer you can energize from one you’re guessing about.

Before any test: inspection and safety

Every electrical test here is done with the transformer de-energized, isolated from every source, locked out and tagged, and with its terminals grounded until the moment of each test. Only qualified people do this work: the tests put up to 5,000 V DC into the windings, and a transformer holds a charge after a DC test until it’s discharged.

Before the leads go on, look:

• The nameplate. Record the kVA, voltages, connection, impedance, taps, fluid type and year. Every test result is compared with what the nameplate promises.
• Bushings and terminations. Cracks, chips, tracking marks or oil weeping at a bushing fail the unit before any test is run.
• Tank, radiators and gaskets. Leaks, rust-through, a bulged tank (a sign of an internal fault) and the condition of the paint, which tells you how it was stored.
• Gauges. Oil level, oil temperature, the pressure-vacuum gauge (a sealed tank that reads zero after years outdoors may be leaking) and the drag hand on the temperature gauge, which records the hottest it has run.
• Tap changer. That the de-energized tap changer moves through every position and seats on each, and that it’s left on the tap the nameplate and the tests assume.
• Inside a pad-mount’s compartments. Elbows, parking stands, the bayonet fuses and their holders, and signs of water, insects or rodents.

Insulation resistance (megger test)

An insulation resistance test puts a DC voltage across the insulation and measures how little current leaks through. The instrument is a megohmmeter, a megger, and the result is in megohms: the higher, the better.

Three measurements cover a two-winding transformer, each held for a minute, and for ten for a polarization index:

1. High-voltage winding to the low-voltage winding, with the tank grounded.
2. High-voltage winding to ground, with the low-voltage winding grounded.
3. Low-voltage winding to ground, with the high-voltage winding grounded.

The test voltage follows the winding’s rating: 1,000 V DC for windings up to 600 V, 2,500 V from 601 V to 5 kV, and 5,000 V above that. The readings are corrected to 20 °C before they’re compared, because insulation resistance roughly halves for every 10 °C the windings are warmer: a cold transformer reads far higher than the same unit in July.

ANSI/NETA ATS Table 100.5, for liquid-filled and dry-type transformers. A reading well above the minimum is normal; the trend against the factory test and past tests tells you more than the number.

The polarization index is the ten-minute reading divided by the one-minute reading. Clean, dry insulation keeps absorbing charge and the reading keeps rising, so the index runs above 2. Wet or dirty insulation flattens out near 1, and an index below 1 means the insulation is getting worse as the test goes on, which is a stop sign.

Turns ratio (TTR)

The turns-ratio test proves the windings are what the nameplate says. A TTR set puts a low AC voltage (typically 8 to 100 V) on one winding, measures what comes out of the other, and reports the ratio and the phase angle between them, one phase at a time.

The ratio is compared with the nameplate ratio on every tap. On a 12,470 V to 480 V transformer the ratio on the rated tap is 25.98, and on the tap 2.5% above it, 26.63. The tolerance is ±0.5% (IEEE C57.12.00), and the three phases should agree with each other within the same band.

Off by more than 0.5% on every tap

Shorted turns in a winding, or the wrong tap recorded. A single shorted turn in a winding of thousands can show up only as a ratio error, with the unit otherwise looking fine.

Right on some taps, wrong on others

A tap changer contact that isn’t making, or an open tap lead.

One phase unlike the other two

A fault in that phase, or on a wye winding, a bad neutral connection.

Excitation current far higher on one phase

The TTR set also reports the magnetizing current. A shorted turn or a core fault pushes it up on the affected phase. (On a three-legged core the two outer phases read a little higher than the center one, which is normal.)

The test also confirms the polarity and the vector group (the 30-degree shift on a delta-wye unit), which has to match anything the transformer will be paralleled with.

Winding resistance

A winding-resistance test passes a DC current through each winding and measures its resistance to a few decimal places, with a micro-ohmmeter or a transformer ohmmeter that can hold current through a large inductance until the reading settles.

It catches what the turns ratio can’t: a loose or corroded connection, a broken strand in a conductor, a tap changer contact with high resistance. Any of these heats under load and ends in a failure. The three phases of a winding should agree with each other and with the factory values within about 1 to 2% once corrected to the same temperature (copper’s resistance rises about 0.4% per °C), and every tap position is measured on the tapped winding.

The test leaves the core magnetized, so it’s done after the turns-ratio test, and the core is demagnetized afterward where the instrument allows it.

Power factor (dissipation factor)

The power factor test, often called a Doble test after the maker of the test sets, measures the insulation with AC rather than DC. It applies up to 10 kV and compares the watts lost in the insulation with the volt-amperes supplied. Good insulation is almost purely capacitive, so the loss, and the power factor, is tiny; moisture, carbon and aging raise it.

The same three connections as the insulation resistance test are measured (high to low, high to ground, low to ground), and the test set reports each one’s capacitance too, which is compared with the factory value and past tests: a change in capacitance means something has moved or shorted inside.

Doble’s guidance for oil-filled units. Dry-type transformers read higher and vary by construction, so they’re judged against their own factory values.

Bushings rated 25 kV and above have a test tap and get their own power factor and capacitance readings (C1 and C2), since a wet bushing fails before the winding does.

Oil tests: dielectric strength, moisture and dissolved gas

A liquid-filled transformer is also tested through its oil. A sample drawn from the bottom valve goes to a lab for two kinds of test. The first is a quality screen: can the oil still insulate and cool? The second is dissolved gas analysis (DGA): what has been happening inside the tank?

IEEE C57.106, rounded. The oil guide has the full set, and what each fluid type changes.

Dissolved gas analysis measures the gases a fault leaves behind in the oil: hydrogen from partial discharge, methane and ethane from mild overheating, ethylene from a serious hot spot, acetylene from arcing, and carbon monoxide and dioxide from overheated paper. On a used transformer it’s the single most telling test, because it reports faults every other test misses: a hot connection or arcing that only happens under load. Our guide to transformer oil and DGA covers what each gas means and the limits.

Testing a transformer with a multimeter

A multimeter can tell you whether a small transformer’s windings are open or shorted, and whether it puts out the right voltage. It can’t test insulation (its ohms range uses a few volts, against the thousands a megger applies), and it can’t measure a ratio to half a percent. For a control transformer, a doorbell transformer or a small single-phase unit, it’s the right first check. For anything you’d call a distribution transformer, use it only to confirm the leads before the real tests.

1. Isolate it

   Switch off and lock out the supply, and disconnect both windings from everything else, so you’re measuring the transformer and not the circuit around it.

2. Measure each winding’s resistance

   On the ohms range, across the primary leads and then across the secondary leads. Each should read a steady, low resistance: a few ohms to a few hundred ohms on a high-voltage winding, under an ohm on a large low-voltage winding. An open winding reads infinite (OL); a winding reading near zero where it should read tens of ohms is shorted.

3. Measure between the windings

   From a primary lead to a secondary lead. It should read open (OL). Any reading means the windings are shorted together.

4. Measure to the core or frame

   From each winding to the core, frame or tank. Open (OL) is right. Any continuity means a winding is grounded.

5. Check the output voltage

   Only on a low-voltage unit, with the secondary unloaded and the meter on AC volts: energize it and measure the secondary. It should be within about 5% of the nameplate, a little above it with no load. A secondary far below the nameplate, with the primary voltage right, points to shorted turns.

A reading of a few ohms on both windings of a 12,470 V to 480 V pad-mount tells you only that the leads are connected. The questions that matter on that unit, whether its insulation will hold 12,470 V and whether its ratio is still 25.98, need a megger and a TTR set.

Dry-type transformers

A dry-type transformer gets the same electrical tests without the oil work: insulation resistance, turns ratio on every tap, winding resistance and power factor, judged against its own factory values, since dry insulation reads differently from oil. What’s added is an inspection you can actually do, because the coils are in the open: dust between the coils and in the cooling ducts (a dry-type is cleaned before it’s tested), cracks or discoloration in cast-resin coils, loose coil blocking, the fans and their controls, and the tightness of every connection, since a dry-type’s joints see more thermal cycling than an oil-filled unit’s.

Our guide to the types of dry-type transformer covers what each construction is sensitive to.

What the test report should show

Ask any seller for the test report before you buy a used transformer, and read it for these things:

• The date, the ambient and oil temperatures, and the instruments used. Results without temperatures can’t be corrected or compared.
• Turns ratio for every tap and every phase, with the nameplate ratio beside each and the percent error.
• Insulation resistance for all three connections at one and ten minutes, corrected to 20 °C, with the polarization index.
• Winding resistance per phase and per tap, with the temperature.
• Power factor and capacitance for all three connections, corrected to 20 °C.
• The oil screen and a dissolved gas analysis from a named lab, dated within the last few months, and on a unit from before about 1990, a PCB result.

That’s the set every unit we sell gets before it ships: turns ratio on every tap, insulation resistance, winding resistance, power factor and capacitance, and an oil sample for dielectric strength and dissolved gases, run by a test crew and written up so you can read it before you commit. A listing shows its tested date once the unit has been through it.