The most common type tests required for medium and high voltage switchgear

Switchgear Type Tests

Let me put the definition of switchgear type tests this way. A switchgear MUST withstand voltage in normal and abnormal conditions and to carry varying load currents during its lifetime. A switchgear MUST be able to clear faults occurring due to various unavoidable reasons and to carry fault current in case of through-faults. Type tests are the proof that a switchgear can fulfill all these requirements.

As a result of all this, the switchgear MUST safeguard power system from severe damage while also being safe for operating workers and other equipment in the case of a breakdown in the switchgear panel.

The performance of a switchgear in normal and abnormal situations MUST be confirmed by subjecting it to various types of tests, the parameters for which are outlined in National and International standards in order to cover as many practical scenarios as feasible. Equipment testing begins with the idea stage, when the maker examines all conceivable parameters to ensure the equipment’s long-term performance.

The truth is that it’s pretty tough to establish all of the needs for all utilities or customers in one location. They can now choose between numerous types of equipment for different voltage levels and specifications, such as IEC 62271-100 and IEC 62271-200. However, the customer must inspect the site to determine how the test requirements will differ for the optimal equipment performance.

The number of test samples provided in IEC 62271-100 should be used for all type tests.

Switchgear Type Tests:

1. Dielectric Tests
2. Radio Interference Test
3. Measurement of the Resistance of Main Circuit
4. Temperature Rise Test
5. Short-Time Current Withstand Test
6. Verification of the Degree of Protection Test
7. Electromagnetic Compatibility (EMC) Test
8. Mechanical Operation Tests at Ambient Temperature
9. Short-Circuit Duties Test
10. Capacitive Current Switching Tests
11. Environmental Test
    1. Normal Service Conditions
    2. Abnormal Service Conditions
12. Critical Current Test
13. Short Line Fault Test
14. Out-of-Phase Making and Breaking Tests

Switchgear Type Tests

1. Dielectric Tests

The power system experiences occasional temporary power frequency overvoltages, which arises during load throw, wrong transformer OLTC operation, insufficient shunt compensation, resonance, etc. Dielectric tests are performed to verify the rated insulation strength of the switchgear to ascertain that when a circuit breaker is put into service, its design is capable of withstanding overvoltages occurring due to above reasons and due to lightning, switching operations, etc.

This is verified in compliance with the Standards. The tests which are performed under this category are discussed below.

1.1 One-Minute Dry-Power Frequency Voltage Withstand Test

This test is carried out to verify the capability of the equipment to withstand the power frequency test voltage for one minute in dry condition. The values of power frequency test voltage with respect to the system voltage have been identified in the standards.

1.2 One-Minute Wet-Power Frequency Voltage Withstand Test

This is the same as the one minute dry power frequency voltage test, but is conducted with equipment in wet condition. These are applicable for outdoor installations only.

1.3 Lightning Impulse Voltage Dry Withstand Test

This test is conducted to verify whether the switchgear is able to withstand overvoltage due to the peak value of standard impulse (1.2/50 μs) during lightning.

Figure 1 – Application of the impulse withstand

1.4 Switching Impulse Voltage Test (Optional)

This test is optional. It’s conducted to verify whether the switchgear is able to withstand overvoltages due to switching surges. This test is significant for system voltages above 300 kV.

Switching surges are  comparatively of a longer duration (2500 μs), lower rate of rise and are represented by standard switching impulse test wave of 250/2500 μs.

1.5 Partial Discharge Test (Optional)

Partial discharge test is a component test and is not recommended to be carried out on complete switchgear, wherein the design of the switchgear consists of a combination of conventional compose. (e.g. CTs and VTs) which can be tested in accordance with their respective Standards.

But in the case of switchgear wherein organic insulating material is used, then this test is recommended, such as in the case of integrated switchgear design, specially GIS, where live parts and connections are embedded in solid insulation.

Suggested reading – Why is continuous on-line monitoring of partial discharge in the switchgear necessary?

Why is continuous on-line monitoring of partial discharge in the switchgear necessary?

1.6 Artificial Pollution Tests

These tests are applicable for outdoor installations only and are carried out on the basis of an agreement between the user and the manufacturer. The voltage values for the above tests are specified in standards against the system voltage.

1.7 Switchgear Design Aspects

While designing a switchgear, the parts located at different electric potential MUST be separated by insulation in order to ensure the safety of personnel working on it and the reliability of its operations, to prevent phase-to-earth, or phase-to-phase flashover.

The insulation in switchgear serves three main purposes. It provides insulation:

1. Between current-carrying live parts and earth;
2. In contact gap during ‘breaker open’ condition; and
3. Between current-carrying live parts of different phases.

1.7.1 Gaseous Air

Gaseous air is the most commonly used gaseous material, which is composed of 80% nitrogen and 20% oxygen. Its properties are therefore close to nitrogen. Air clearances between phases and between phases to earth against the system voltage are as mentioned to IEC 62271-100.

Among gases, air is the only insulating material which can be used effectively at atmospheric pressure.

Figure 2 – Medium voltage (SF6 gas insulated) switchgear

1.7.2 Fluids

A range of fluids has been used for insulation in switchgear. Hydrogen carbon oil often referred to as ‘transformer oil’ was being used in bulk oil CB. It has the advantage of fulfilling the dual role of both an insulation as well as an arc-extinguishing medium. Its dielectric strength is three times that of SF6 at atmospheric pressure.

However, it is now coming under increasing scrutiny from the safety point of view because of its inflammable property. So, it’s not an option any more!

1.7.3 Vacuum

Another medium used for insulation is vacuum. This is very expensive, that’s why its use in distribution switchgear is restricted only to circuit interruption devices.

Figure 3 – MCSet 17.5 kV vacuum switchgear

1.7.4 Solid Materials

Solid insulating materials include many of natural origin such as mica, asbestos, slate or those derived from natural material such as porcelain and shellac varnish. The principal use of solid materials in switchgear is to insulate conductors where they pass through walls of metal enclosures apart from supporting the conductors.

Most of the support insulators are made by resin cast technique. One major advantage of the rests casting technique is the almost limitless range of shapes that it offers. The resin is usually epoxied.

The reasons for the failure of various insulation media in switchgear are:

Reason #1 – Tracking of Solid Insulation

This takes place due to degradation of the surface glaze by sparking, presence of conductor film on surface due to moisture, etc. In order to prevent this eventuality, the insulation should be clean and undamaged.

Reason #2 – Breakdown by Leakage in Solid Insulating Material

Surface discharge occurs when the insulator is subjected to higher electric stress, which is greater than the permitted value. Cavilie, poor design, presence of moisture, etc. also cause discharge within the insulator. In order to avoid this during the manufacturing process of the insulator, care should be taken to make it void-free with maximum tracking index of that material.

Figure 4 – Examples of surface and disruptive discharge

Reason #3 – Thermal Breakdown

Heat is generated in the electrically stressed insulating parts due to dielectric loss. Heat is imparted to the insulation by neighbouring current-carrying parts, which is lost by it through conduction, convection and radiation. The resistivity of some dielectrics reduces with an increase in temperature.

Reason #4 – Failure of Chemical Insulation

Moisture causes deterioration of transform, oil and other chemical insulating materials. Moisture gets condensed on the insulator surface and causes flash-over. This can be prevented if ingress of the dust particles and moisture, is restricted.

Reason #5 – Effects of Oxygen and Humidity

Some organic/inorganic materials oxidize in the presence of oxygen when exposed to light. Rubber oxidizes and cracks when exposed to light. Epoxy insulators are not suitable for outdoor applications.

Reason #6 – Incompatibility of Dielectric Material

Some dielectric materials are not suitable for use in certain assemblies due to their incompatibility with the surrounding substance. For example, all the materials are not suitable with SF6 media and care should be taken to select compatible insulating material.

Reason #7 – Electrochemical Deterioration

In some insulating materials, the impurities get dissociated under electric stress during ionization of the material and consequently the material deteriorates.

Reason #8 – Presence of Arc

The insulating gas/oil gets decomposed in the presence of arc. Although the products of decomposition re-combine after arc extinction, some remainders are there, which is why the insulating properties of dielectric get affected.

Figure 5 – Partial melting of busbars after short time stationary arcing at the insulation barriers

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2. Radio Interference Test

Radio interference test applies only to switchgear and controlgear having a rated voltage of 123 kV and above, and should be conducted when specified in the relevant standard.

The test voltage should be applied as per IEC 62271-1 under following conditions:

1. In closed position between one terminal and the earthed frame, and
2. In open position between one terminal and the other terminal connected to the earth frame and then with the connection reversed if the switching device is not symmetrical.

The switchgear and controlgear should be dry and clean at approximately the same temperature of the room in which the test is carried out.

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3. Measurement of the Resistance of Main Circuit

The function of switchgear is to carry its rated current through the current-carrying conductor. They have many joints to meet the design parameter. Each joint will have joining resistance.

Care should be taken to minimize the total resistance of the circuit, i.e. the total watt loss should be minimum for a low rise in temperature The measured value will set the limiting parameter during the routine test The measurements are taken as a DC voltage drop in circuit. The minimum applied current will be 100 A and the maximum up to the rated current.

Suggested reading – Measurement of insulation resistance (IR)

Measurement of insulation resistance (IR)

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4. Temperature Rise Test

Temperature rise test determines the highest normal current that the switchgear can carry without exceeding the maximum permitted temperature at any point. Since the current carried affects the temperature rise, the current rating is dependent upon the maximum ambient temperature. Sometimes one temperature limit prevails in determining the current rating, and different countries have different limits for similar materials.

For example, the ANSI standard states that any part that can be handled by an operator in the normal course of his duties must not exceed 500°C which could become a limitation if solar heating is taken into account.

IEC standard gives 1000°C as the highest permitted temperature for metallic parts in contact with oil, whereas the ANSI document limits this to 90°C.

When AC current is passed through a breaker, heat is generated due to I²R loss. The generated heat is dissipated by conduction, convection and radiation. In conduction, heat travels through the solid body, convection uses liquid media and radiation implies transmission through goes.

In order to maintain the temperature rise within specified limits, l²Rt loss should be reduced by increasing the conductor cross-section using suitable low resistivity material and by improving heat transfer through conduction, convection and radiation of heat.

Figure 6 – Temperature rise test performed on medium voltage switchgear

4.1 Design Aspects

Copper and aluminium are the most suitable materials for making current-carrying conductors. Copper has lower resistivity and also resists oxidation better than aluminium, particularly at temperatures below about 80°C. Aluminium has a much smaller specific gravity and though larger cross-sections of conductor are needed to give the same resistance as the equivalent of copper, the weight and therefore the cot of aluminium is found advantageous.

Following table shows differences between relative properties of copper and aluminium.

Table 1 – Differences between Relative Properties of Copper and Aluminium

For equal conductivity, aluminium is lighter in weight than copper. In many switchboard applications however space considerations are more important than weight and copper is generally preferred over aluminium.

The most important characteristic of the materials used as conductors in switchgear is that they should have a low specific resistance. Excessive temperature rise due to high resistance conductors in a small enclosure will Iced to a reduction to the life of the associated insulation materials. The next important characteristic is that the material mot resist corrosion.