UNIT 5

HIGH VOLTAGE TESTING  AND INSULATION CO-ORDINATION

PART-A

1. What is Salt Fog Test?

In this test, the maximum normal withstand voltage is applied on the insulator and then artificial salt fog  is created around the insulator by jets of salt water and compressed air.

2. What are the necessity of high voltage testing?

• To check whether they are as per the design and as per specifications & standards.
• To ensure that the HV equipment is able to withstand over voltages produced naturally or within the system.

3. Define Flashover voltage, Creepage distance, withstand voltage, Disruptive Discharge Voltage impulse voltage

Flashover voltage

The voltage that causes a flashover at each or its application under specified conditions when applied to test object.

Creepage distance

It is the shortest distance on the contour of the external surface of the insulator unit or between metal fittings on the insulator.

Withstand voltage

The voltage which has to be applied to a test object under specified condition in a withstand test is called withstand voltage.

Disruptive Discharge Voltage

It is defined as the voltage which produces the loss of dielectric strength of an insulation. It is that voltage at which the electrical stress in the insulation causes a failure which includes the collapse of voltage and passage of current.

 Impulse voltage

Impulse voltages are characterized by polarity, peak value, time to front (t_f) and time to half the peak value after the peak (t_t).

4. What are type test and routine test? (or) What is the difference between type and routine tests?

5. What are the different tests conducted on insulators (or) high voltage insulators?

The different tests are conducted on high voltage insulator

• Power frequency tests

• Dry and Wet Flashover Tests
• Wet and Dry Withstand Tests (one minute)
  • Impulse Tests

• Impulse Withstand Voltage Test
• Impulse Flashover Test
• Pollution Testing

6. Define partial discharges.

The weak points in an insulation like voids, cracks and other imperfections lead to internal discharges in the insulation. These imperfections were considered as power loss. An electrical discharge that partially the dielectric or insulating medium between two conductors is called as partial discharge.

7. What is the significance of impulse tests?

To determine the ability of the insulation of the transformers to withstand the transient voltages due to lightning.

These tests apply for equipment at voltages above 300 KV.

8. What are the significance of power factor tests?

In this test, the bushing is set up as in service or immersed in oil. It is connected that the line conductor goes to the high voltage side and tank or earth portion goes to the detector side of the high voltage schering-bridge. Voltage is applied upto line value in increasing steps and then reduced. The capacitance and power factor are recorded at each step. This is a normal routine test but sometimes may be conducted on percentage basis.

9. Find and locate the fault during impulse testing of transformer?

The fault in a transformer insulation is located in impulse tests by any one of the following methods

1. General Observations: If fault can be located by general observations like noise in the tank or smoke or bubbles in breather.
2. Transformer surge current method:  In this method, the voltage across a resistive shunt connected between the LV winding and ground is used for fault location. A short high-frequency discharge oscillation is capacitively transferred at the event of failure and is recorded.

10. Define insulation. (or) What is insulation co-ordination? (or) What is meant by insulation co-ordination?

The selection of suitable values for the insulation levels of the various components in any electrical system and their arrangement in a rational manner is called insulation coordination

11. What is Basic Impulse Level (BIL)?

The basic insulation levels are reference levels fixed by standards for each voltage levels. Basic impulse levels are reference levels expressed in terms of impulse crest voltage(Vp) with a standard lightning impuse voltage(1.2/50 micro seconds wave) for any apparatus the insulation level as demonstated by suitable tests should be greater than or equal to the BIL

12. Give the insulation coordination design procedure of EHV and UHV systems (or) stations?

• Stations have transformers and other valuable equipment that have non-self restoring insulation
• The protective levels for lightning surges and switching surges are almost equal and even overlap.

13. State the importance of insulation co-ordination in power system.

Insulation co-ordination is the grading of the insulation level of

(1) Various equipments in a power system

(2) Various parts of the equipments

(3) Protection devices in such a way that, in the event of a serious over voltage, less vital, less important, less costlier, easy to repair equipment/part of equipment breaks down first and thereby avoiding major breakdown & interruption to consumers, cost of replacement etc.

Give the atmospheric condition factor for high voltage testing as per Indian standard and British standard specifications

14. Mention the advantages and disadvantages of synthetic test in circuit breaker.

Advantages

• Circuit breaker is tested under actual conditions like those that occur in a given network.
• To assess the thermal and dynamic effects of short circuit current.
• To revise the performance test procedures.

Disadvantages

• Circuit breaker can be tested at only a given rated voltage and network capacity.
• Necessity to interrupt the normal services.
• To test only at light load condition.

15. What are the different tests conducted on bushings (or) high voltage bushings?

The different tests are conducted on high voltage bushings

• Power Frequency Tests

• Power Factor – Voltage Test
• Internal or Partial Discharge Test
• Momentary Withstand Test at Power Frequency
• One Minute Wet Withstand Test at Power Frequency
• Visible Discharge Test at Power Frequency

• Impulse Voltage Tests

• Full Wave Withstand Test
• Chopped Wave Withstand and Switching Surge Tests

• Thermal Tests

16. What are the different tests conducted on circuit breakers and isolator?

The different tests are conducted on high voltage circuit breakers and isolator

• Dielectric Tests or Overvoltages Tests
• Temperature Rise Tests
• Mechanical Tests
• Short Circuit Tests

17. What are the different tests conducted on cables (or) high voltage cables?

The different tests are conducted on high voltage cables

• Dielectric Power Factor Tests
• Partial Discharge Tests
• Impulse Withstand Voltage Tests
• Power Frequency Withstand Voltage Tests
• Mechanical Test
• Thermal Duty Test
• Life Expectancy Test

18. What are the different tests conducted on Transformers?

The different tests are conducted on high voltage transformer

• Induced Overvoltage Test
• Partial Discharge Tests
• Impulse Testing

19. What are the different tests conducted on surge arresters?

The different tests are conducted on high voltage insulator

• Power Frequency Sparkover Test
• Hundred Percent Standard Impulse Sparkover Test
• Front of Wave Sparkover Test
• Residual Voltage Test

PART-B

1. Explain the various tests conducted on high voltage insulators. (16 m)
2. Discuss the different high voltage tests conducted on bushings. [or]

Explain the power frequency and impulse voltage test conducted on bushings.

[ 16 m]

3. Explain the various tests conducted on high voltage cables. [or]

Discuss the dielectric power factor test and partial discharge test conducted on high voltage cables. (16 m)

4. Explain the various tests conducted on high voltage transformers. (16 m)

Explain the method of impulse testing of high voltage transformer. What is the procedure adopted for locating the failure? (8 m)

5. What are the tests conducted on isolators and circuit breaker explain in detail?

(16 m)

Explain with a neat diagram of synthetic testing of cicuit breakers. Why is synthetic testing advantages over direct method for short circuit test (16 m)

6. Explain the different aspects of insulation design and insulation co-ordination adopted for EHV systems. (16 m)
7. Explain the various tests conducted on high voltage surge arrester. (16 m)
8. Explain the high-current impulse test on surge arresters. ( 8 m)
9. Explain the following terms

1. Withstand voltage b) Flashover voltage c) Disruptive discharge voltage d) Impulse voltages