Electrical Power Equipment Maintenance and Testing Practice Questions

A set of practice flashcards covering electrical power equipment testing including transformers, bushings, circuit breakers, and insulating oil analysis.

What is the relationship between the test procedure for three-winding transformers and two-winding transformers?

The test technique for three-winding transformers is an extension of the two-winding transformers.

In performing PF tests on a three-phase autotransformer, which seven bushings must be shorted together?

H1, H2, H3, X1, X2, X3, and H0, X0.

Why are PF tests routinely performed only on the primary winding of Potential Transformers (PTs)?

Because of the low voltage (LV) rating of the PTs.

At what voltage rating do transformer bushings transition from having PF test taps to capacitance taps?

Bushings 23 to $69\,kV$ have PF test taps, whereas bushings rated at $69\,kV$ and above have capacitance taps.

In a condenser-type bushing, what does the $C1$ insulation represent?

The main insulation from the conductor to the tap electrode or shield layer.

What is the standard acceptance limit for bushing capacitance compared to the nameplate value during Test No. 1?

$\pm 10\%$ of nameplate.

What is the purpose of the hot-collar test performed on noncondenser-type bushings?

To detect moisture, deterioration, or low levels of oil or compound in the early stages.

What kind of reading pattern is expected when performing a transformer excitation current test on a three-phase unit?

A pattern of two high and one low, or two low and one high reading, depending on the winding connections.

How is the Tank Loss Index (TLI) calculated for Oil Circuit Breakers (OCBs)?

TLI = Closed breaker loss $-$ sum of open breaker losses.

What is the Power Factor (PF) range for an OCB interrupter assembly to be considered 'good'?

Between $0\%$ and $35\%.$

What is 'PF tip-up' as defined in the context of rotating machinery insulation?

The increase in the PF as the voltage is increased from an initial test value to the maximum test value.

How should PF tests be performed on unshielded or unsheathed cables if a definite ground return path cannot be identified?

By using another phase conductor of the same circuit as a leakage current return path and testing in UST mode.

What are the four categories for evaluating insulation serviceability?

1. Good, 2. Deteriorated, 3. Marginal, and 4. Bad.

To what base temperature should all PF and DF test results be converted for periodic comparison?

$$20^\circ C$$

What is the expected PF of good, new transformer oil at $20^\circ C$?

$0.05\%$ or less.

What does the Neutralization Number (acidity test) represent?

The milligrams (mg) of potassium hydroxide (KOH) required to neutralize the acid contained in $1\,g$ of transformer liquid.

According to the IFT test, what value almost invariably indicates that sludging is occurring in transformer oil?

An IFT of less than $15\,dyn/cm$

According to Table 4.1, what is the maximum allowable moisture in oil for a system rated at $240\,kV$?

$$15\,ppm$$

Which key gas is predominantly liberated when high-intensity arcing ($700^\circ C$-$1800^\circ C$) occurs inside a transformer tank?

Acetylene ($C_2H_2$)

In the DGA method, what does 'Condition 4' status signify regarding the Total Dissolved Combustible Gas (TDCG)?

Excessive decomposition where continued operation could result in transformer failure (TDCG $> 4630\,ppm$).

What is the effect of a $10^\circ C$ ($50^\circ F$) increase in temperature on the rate of oil oxidation?

It generally doubles the rate of oxidation.