Module 3 — Site checks and energization

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A transformer that passed its factory tests can be damaged, wet, or misconnected on the way to site. Module 2 is the factory record. This module is the comparison of this serial number with that record, and the order of work before the first voltage. IEEE C57.152 is the IEEE guide for diagnostic field testing. IEEE C57.93 covers installation and maintenance practice. IEC practice uses the same measurements, with IEC 60076-1 and IEC 60076-18 as the reference for ratio, resistance, and frequency response. Name those documents in the procedure so the acceptance limits are the ones already agreed.

Do not apply voltage to see whether the unit is fine and then treat a successful energization as a substitute for the tests. A unit can pick up for a short time with a wrong tap, a reversed current transformer, or a wet bushing. The damage shows up as a trip, a circulating current, or a failure on the first through-fault.

Identity and the mechanical setup come first

Confirm the serial number against the routine-test report and the nameplate from Module 1. Confirm the impact-recorder record and the transport locks. A recorder event above the limit the manufacturer set is a reason to stop and inspect, not a reason to hurry the electrical tests. Confirm the liquid level, the breather, and that the valves between the tank, the conservator, and the coolers are in the position the drawing requires. A closed valve at the gas-operated relay leaves the unit unprotected while the rest of the checks look normal.

Confirm that fans and pumps, where they are fitted, rotate in the marked direction and that their alarms function. Confirm the earthing of the tank, the core if a separate core earth is brought out, and the neutral connection the protection study requires. A core earth left open, or a second accidental core earth, is checked by the core-to-tank insulation test the manufacturer specified. Repeat it if the manual says it is a site test. Do not invent a megohm limit for it. The factory figure is the comparison.

Liquid, then the electrical fingerprint

Fill and process the liquid to the contract before the dielectric conclusions are drawn. Unused mineral oil is specified in IEC 60296. Natural ester is IEC 62770. Synthetic ester is IEC 61099. After filling, let the unit stand for the time the manufacturer requires so the sample represents the liquid in the tank rather than the bottom of a hose. Breakdown voltage is measured by IEC 60156. IEC 60422 and IEEE C57.106 give acceptance guidance by voltage class. Use the row for this Um. A limit copied from a distribution unit is the wrong row for an extra-high-voltage unit. An ester is not mineral oil. Do not paste a mineral-oil limit onto an ester report.

Then measure voltage ratio on the tap the unit will be energized on, and on the other taps the procedure lists. Compare with the factory values for this serial number. IEC 60076-1 tolerances are the reference unless the site procedure tightens them. Measure winding resistance and compare the pattern, corrected to the same temperature as the factory sheet, or compare phase to phase. A single high phase can be a joint or a tap-changer contact. Confirm the vector group the same way the factory did. Terminal marking errors are still found at this step on units that were correct in the factory, because bushing leads are removed for transport.

Insulation resistance and polarization index are comparisons with the factory figure and between phases, corrected for temperature as far as the procedure allows. There is no single megohm value that means pass for every rating and every temperature. IEEE C57.152 discusses how to use the measurement. A value that collapses against the factory figure is a reason to investigate moisture, a bushing, or contamination before voltage is applied.

Bushings and the frequency-response baseline

Bushing capacitance and dissipation factor, sometimes called power factor, are compared with the bushing nameplate and with the factory reading. IEC 60137 covers bushings. The IEEE field context is IEEE C57.152. A large change in capacitance after transport points at that bushing, often a shorted layer in a condenser bushing. Do not add the bushing result to the winding result and call the sum a winding test. They are separate insulation systems. Investigate a change against the limit in the procedure. Do not substitute a percent remembered from a different apparatus.

Repeat the frequency-response measurement on the same taps and the same connections as the factory baseline taken under IEC 60076-18. Compare the traces with IEEE C57.149 and with the manufacturer's comparison. IEC 60076-18 defines the measurement so that someone else can repeat it: connections, taps, frequency range, and the record of the setup. The useful configurations include end-to-end open-circuit, end-to-end short-circuit, and the inter-winding measurements the standard names. A different connection from the baseline makes a difference that looks like damage and is only a setup error.

There is no single decibel limit that means the winding has moved on every design. The guides associate parts of the trace with the core, the windings, and the connections. Read the band in the guide you named. Oil level, tap position, and whether the unit is filled all change the trace. A comparison between a drained factory trace and an oil-filled site trace, without the allowance in IEC 60076-18, is not valid. A trace with no factory baseline is still worth keeping. It cannot prove that the road did nothing. Ratio can stay inside tolerance when a winding has moved, because ratio depends on turns. A normal ratio does not clear a bad frequency-response comparison, and a normal frequency-response comparison does not clear a wrong ratio. Do the set.

Protection before voltage

Prove the gas-operated relay, the pressure-relief device, the winding and liquid temperature indicators, and the sudden-pressure device. A device that the drawing says should trip has to trip the breaker it is wired to. A device that should only alarm must not be found to trip. Prove current-transformer ratio and polarity into the relay, not only at the marshalling box. A reversed current transformer on a differential scheme trips on load or, worse, restrains during a fault. Operate the on-load tap changer through its range with the transformer isolated, and confirm the position indication matches the mechanism. These checks use the factory results from Module 2 as the comparison, not as a reason to skip the site check. Leads are disturbed in transport.

The first energization

Apply voltage with the secondary open unless the system study says otherwise. The usual case is no-load from one side, for long enough to listen, to watch the current, and to confirm the voltage with the system voltages. Magnetizing inrush on the first closure can be several times rated current and can decay over seconds. Differential protection has to be able to ride through that inrush by the method in the protection scheme, usually a harmonic restraint or an equivalent setting. Inrush is not, by itself, a failure. A large unbalance that does not decay, or a loud internal noise, is a reason to switch off.

No-load current should be in the order of the factory no-load current, allowing for the actual system voltage. Load is applied after the no-load check, the protection check, and the cooler check. Record the tap, the liquid temperature, and the time. That record is the start of the operating history the dissolved-gas samples in Module 4 will be compared with. File the ratio sheet, the resistance sheet, the bushing readings, the frequency-response overlay, the liquid results, and the protection-trip checks with the factory report before the first load.

Check your reading

Why is a successful first energization not a substitute for the site tests?
What is the frequency-response baseline compared against?
Why is a normal ratio not enough to accept the unit after transport?
What has to be proved about a current transformer before the unit is energized?
What should you expect the current to do at the moment of first closure?

Answers. A unit can pick up with a wrong tap, a reversed current transformer, or a wet bushing and still run briefly. The frequency-response comparison is this serial number against its own IEC 60076-18 factory trace, taken with the same connections and the same tap. Ratio checks turns. Winding movement can change the frequency-response trace while the ratio stays inside tolerance, so both results are required. Current-transformer ratio and polarity have to be proved at the relay, and the trip path of each protective device has to match the drawing. The first closure can draw a magnetizing inrush of several times rated current, decaying over seconds. A current that stays unbalanced is a different event.