Dielectric losses and Capacitance test

Question 1

Purpose behind Dielectric testing?

Answer 1

Measure fundamental AC electrical characteristics of the insulation system for the apparatus under test.

Question 2

First thing to setup test for Dielectric testing

Answer 2

Short HV bushings and LV bushings together.
Why? Because then entire XFMR are at the same potential, therefore excluding all other systems in the XFMR from the test circuit.

Question 3

Define “Insulation”

Answer 3

A material or combination of suitable non-conducting materials that provides isolation of two parts at different voltages. Properties relating to.. it’s ability to prevent the flow of current, expressed by RESISTIVITY.

Question 4

Define “Dielectric”

Answer 4

A medium in which it is possible to establish and maintain an electric field with little or no supply of energy.

Energy required to establish electric field is recoverable by whole or in part.

Question 5

How is a dielectric test performed? (3 steps)

Answer 5

1. Apply a signal of AC voltage at power frequency
2. Measure a response at same frequency as signal. Referenced to applied voltage.
3. Calculate desired parameters: Measure the fundamental AC electrical characteristics of the insulation system.

Question 6

How is the measured current separated?

Answer 6

Into it’s 2 main components:
1. Reactive component (capactiance in picoFarads and capacitive current in microAmps)
2. Real component (Dielectric Loss via watts loss and Dissipation Factor or % Power Factor)

Question 7

Resistance is related to what kind of power?

Answer 7

Real Power (watts) voltage and current are in-phase. Resistance dissipates energy via heat.

Question 8

Capacitance is related to what kind of power?

Answer 8

Reactive power (VARs).

This is voltage dependent.
Current leads voltage?
Energy Storing (electrostatic field)

Question 9

Inductance is related to what kind of power?

Answer 9

Reactive power (VARs)

Current dependent.
Current lags voltage.
Energy storing (magnetic field)

Question 10

What is Capacitance equal to in a parallel plate capacitor? (give formula)

Answer 10

([A]Area of conductors * [e]Dielectric constant)/[d]distance between conductors.

Capacitance permits the storage of an electrical charge, when a potential difference exists between the conductors.

Question 11

What does the evaluation of the reactive (capacitive) component from our dielectric test tell us?

Answer 11

shows physical changes in the insulation system geometry.

We know this because all its variables are physical properties. Area, Dielectric constant, and Distance.

Question 12

What can cause a physical change to the insulation system?

Answer 12

3 things:

1. XFMR - movement of core/coils
2. Bushings - shorting of capacitive layers or loss of oil
3. Arresters - broken elements

Question 13

What are suggested capacitance limits as +/- % change from nameplate/benchmark?

Answer 13

Less than 5% GOOD
Between 5-10% INVESTIGATE
Greater than 10% BAD - DO NOT PUT BACK IN SERVICE UNTIL ISSUE RESOLVED.

Question 14

Is it good if capacitance goes down over time instead of up?

Answer 14

No, any change of capacitance up or down means there has been a physical change in the insulation system.

Question 15

What does the evaluation of the Real Component in Dielectric Loss test tell us?

Answer 15

the SEVERITY of contamination and/or deterioration of the insulation system.

Question 16

What are watts a measure of? Give the equation for Watts and explain the correlation between watts loss and the quality of the insulation system.

Answer 16

Watts loss (dielectric loss) are measure of the heat losses within the insulation.

Equation for watts loss is:
(W) Watts = (V) voltage x (I) current

The lower the watts loss, the better quality of the insulation system.

Question 17

Dissipation Factor (Power Factor) is the relationship between what?

Answer 17

Total Current, Capacitive current, and Resistive current.

Question 18

In a perfect insulation system, what would be the resistive current value? What would the capacitive current therefore be?

Answer 18

resistive current would be 0 and the capacitive current would equal the total current. The close the dissipation factor is to 0%, the more efficient the insulation system.

Question 19

Power Factor is equal to what? (formula)

Answer 19

Real power divided by Apparent power

Question 20

Why use Power Factor instead of Watts loss?

Answer 20

Power Factor is Independent of specimen size. The relationship between resistive current (I-R) and measured current (theta) remains similar.

Question 21

All insulative materials change with what variable?

Answer 21

Temperature

Question 22

What is a general limit for Power Factor?

Answer 22

Typically 0.5% or less but there are many exceptions.

Compare to previous results, compare to similar/like units, look at trend of this unit.

Question 23

When do you NOT use Power Factor as an evaluation tool?

Answer 23

When the measured current is less than 300 microAmps (0.300 mA).

Why don’t we use PF below 0.300 mA?

Because then resistive current is a disproportionally large percentage of the total current and will skew results.

Question 24

What does a “Tip-up test” look for?

Answer 24

A tip up test compares two power factors as a function of voltage. One at Line to Ground voltage (L-G) and one at 25% of Line to Ground voltage.

Question 25

What are the six(6) components of a simplified test current?

Answer 25

1. Power Source
2. Measurement current
3. High Voltage test cable
4. Low Voltage test leads
5. Insulation System under test (XFMR, bushing, etc.)
6. Test ground

Question 26

Define ‘Grounded Specimen Test’

Answer 26

A test that measures current flowing to ground via the measurement circuit.

Question 27

Define ‘Ungrounded Specimen Test’

Answer 27

A test that measures current flowing to an ungrounded (floating) measurement circuit

Question 28

Define ‘Kirchhoffs voltage law’

Answer 28

Kirchhoffs voltage law states that the directed sum of the potential differences (voltages) around any closed loop is zero.

Question 29

Define ‘Kirchhoffs current law’

Answer 29

Kirchhoffs current law states the algebraic sum of currents in a network of conductors meeting at a point is zero.

OR

Kirchhoffs current law states that for any node(junction) in an electrical circuit, the sum of currents flowing into that node is equal to the sum of currents flowing out of that node.

Question 30

How does a negative Power Factor occur?

Answer 30

Technically there is no such thing as a negative power factor.

Caused by the presence of a highly resistive leakage path to ground in the circuit (dirty bushing) AND the return mode of that current to the test set (UST)

Question 31

Does a decreased power factor show that insulation has electrically ‘healed’?

Answer 31

No. Likely a dirty bushing. Similar to the negative power factor scenario, a change from 0.3 to 0.1 PF means there is a highly resistive leakage path to ground.