Delta Star - Power Testing Flashcards

No Load Test

Connection:

Line leads going to X1, X2, and X3. Neutral grounded. HV open.

What:

Measures no-load loss and excitation current while the transformer is energized without carrying load.

Why:

No-load loss shows how much power the core consumes through hysteresis and eddy-current losses. Excitation current shows how much current is required to magnetize the core. Abnormally high results can indicate core damage, shorted laminations, incorrect assembly, or an overexcited core.

Hipot “Applied” Test

Connection:

HV shorted and connected to hipot output. X0-3 shorted and grounded.

What:

Applies high AC voltage to the winding and measures the leakage current flowing through the insulation to ground. The current should remain low and stable. A sudden increase, unstable current, or breakdown can indicate weak or damaged insulation.

Why:

Verifies that the transformer’s major insulation can withstand voltage stress between, winding and ground, winding to winding, and bushings, leads, and other grounded components

Induced (PD) Test

Connection:
Line leads connect through the smaller-wire spools to the LV bushings. X0 is grounded. Corona balls are installed on the HV bushings.

What:
Applies increased AC voltage through the LV winding, which induces a higher voltage across the transformer windings. The PD equipment measures small electrical discharge signals from the HV insulation, usually displayed in microvolts and converted to apparent charge in picocoulombs.

Why:
Checks insulation between turns, phases, windings, and ground while the transformer is under elevated voltage stress. Partial discharge can reveal voids, sharp points, contamination, loose connections, or damaged insulation that may eventually lead to dielectric failure.

Impulse

Connection:
Impulse is applied to one phase at a time. Other windings on its respective side are shorted to the current shunt. The opposite winding is shorted and grounded.

What:
The impulse tower supplies the surge, the voltage divider measures the applied voltage, and the chop gap creates the chopped wave. Applies reduced full-wave, full-wave, and chopped lightning impulses to the winding. Records the applied voltage waveform and the current response through the shunt, then compares the waveforms for changes.

Why:
Verifies that the winding insulation can withstand lightning-type surges. Differences between the reduced, full, and chopped waveforms can indicate turn-to-turn failure, winding-to-ground failure, internal arcing, or movement of the winding.

Load Loss

Connection:
Line leads connect to the HV winding. The LV winding is shorted with a bus bar, and X0 is grounded.

What:
A reduced voltage is applied until rated current flows. Measures the voltage, current, and power required to circulate rated current through the transformer while the secondary is shorted. The measured power is the total load loss.

Why:
Determines how much power is lost as heat while the transformer carries load. Load loss includes winding resistance loss and stray load losses caused by eddy currents in the windings, tank, clamps, leads, and other structural parts. High results can indicate incorrect conductor dimensions, poor joints, excessive stray flux, circulating currents, or assembly problems. This test helps us calculate impedance.

Temperature Rise

Connection:
HV connected to line leads. X1-3 shorted with bus bar. X0 grounded.

What:
Rated current is circulated through the transformer while top-oil, bottom-oil, and ambient temperatures are monitored. Winding resistance is measured before and after the heat run. Heats the transformer under simulated full-load conditions, then determines how much hotter the oil and windings become than the surrounding air. Oil temperature is measured directly, while winding temperature is calculated from the change in winding resistance.

Why:
Verifies that the transformer can carry its rated load without overheating. Excessive temperature rise can indicate inadequate cooling, blocked oil flow, poor connections, excessive losses, or a cooling-system problem.

Zero Sequence

HV open. X-3 shorted w/ a bus bar. A phase lead connects to the grounded neutral, and B phase lead connects to the shorted LV bushings. C phase lead stays hanging

What:
Applies single-phase voltage between the neutral and all three phases, then measures the current and voltage used to calculate zero-sequence impedance.

Why:
Shows how much the transformer opposes ground-fault current. The result is used for ground-fault studies, relay settings, and verifying the transformer’s neutral and zero-sequence current path.

Switching Surge

Connection:
For this A-phase test, the impulse is applied to H1. H3 is grounded and H2 is left open on the delta winding. X1 is left open, X2 and X3 are shorted together, and X0 is grounded.

What:
Applies a slow-front, long-duration switching surge to one phase and records the applied-voltage and winding-current waveforms. The waveforms are checked for sudden changes, distortion, or collapse that would indicate insulation breakdown.

Why:
Verifies that the winding insulation can withstand high-energy overvoltages caused by system switching. It stresses turn-to-turn, phase-to-phase, and phase-to-ground insulation differently than a lightning impulse.