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Dielectric Type Tests Applied to MV/HV Switchgears

A switchgear must be voltage-resistant under normal and abnormal conditions and be able to carry varying load currents throughout its lifetime. In addition, it must be able to eliminate faults that occur due to unavoidable reasons and carry the fault current in the event of a direct fault. Considering all these, the switchgear must be safe for working personnel and other equipment in case of malfunction, as well as protecting the electrical system from serious damage.


Verification of the performance of a switchgear under normal and abnormal conditions should be carried out by various tests specified in National and International Standards, covering the largest possible number of practical scenarios. The equipment testing process begins with the planning phase, where the manufacturer evaluates all conceivable parameters to guarantee the long-term performance of the equipment.


These standards cover all applicable test criteria in most cases. However, in the context of needs and practices, customers and services may have varying requirements; This means that additional costs that were not initially anticipated may arise.


To be honest, it is very difficult to meet all public services or customer needs in one place. Many types of equipment are available, such as IEC 62271-100 and IEC 62271-200, for different voltage levels and specifications. However, customers are required to conduct a detailed inspection of the facility to understand testing requirements for optimum equipment performance.


Dielectric Type Tests

The power system occasionally undergoes temporary power frequency overvoltages, which occur during events such as load throw-off, incorrect transformer OLTC operation, inadequate shunt compensation, resonance, and others. Dielectric tests are conducted to confirm the rated insulation strength of switchgear, ensuring that when a circuit breaker is commissioned, it is designed to withstand overvoltages caused by the aforementioned factors, as well as those resulting from lightning and switching operations.


This has been verified as compliant with the Standards. The tests conducted within this technical article are outlined below.


 

Table of Contents:


 

1. One-Minute Dry-Power Frequency Voltage Withstand Test

This test is conducted to confirm the equipment's ability to endure the power frequency test voltage for one minute under dry conditions. The standards have specified the values of power frequency test voltage relative to the system voltage.



2. One-Minute Wet-Power Frequency Voltage Withstand Test

This test is equivalent to the one-minute dry power frequency voltage test, except it is performed with the equipment in a wet condition. It is applicable exclusively for outdoor installations.



3. Lightning Impulse Voltage Dry Withstand Test

The test is performed to confirm the switchgear's capability to endure overvoltages resulting from the peak value of the standard impulse (1.2/50 μs) caused by lightning.


Figure 1 – The application of impulse withstand voltage.


Application of the impulse withstand
Figure 1 – The application of impulse withstand voltage.

4. Switching Impulse Voltage Test (Optional)

This test is optional and is performed to ascertain if the switchgear can endure overvoltages caused by switching surges. It is particularly important for system voltages exceeding 300 kV.


Switching surges typically happen during the opening and closing of unloaded EHV lines or when interrupting inductive or capacitive loads.

These surges are relatively long-lasting (2500 μs), have a lower rate of rise, and are characterized by the standard switching impulse test wave of 250/2500 μs.



5. Partial Discharge Test (Optional)

The partial discharge test is a component-level test and it is not advisable to perform it on entire switchgear assemblies. This is because the switchgear design often includes a mix of standard components (such as CTs and VTs) that should be tested according to their specific standards.


However, for switchgear that utilizes organic insulating materials, this test is advisable. This is particularly true for integrated switchgear designs, especially Gas-Insulated Switchgear (GIS), where live parts and connections are encased in solid insulation.



6. Artificial Pollution Tests

These tests apply solely to outdoor installations and are conducted according to an agreement between the user and the manufacturer. The voltage values for these tests are defined in standards relative to the system voltage.



7. Switchgear Design Aspects

In the design of switchgear, components at varying electrical potentials must be isolated with insulation. This is crucial to protect the safety of personnel and to guarantee the reliability of operations by preventing phase-to-earth and phase-to-phase flashovers.


The insulation in switchgear has three primary functions: it acts as a barrier between live parts and the earthed metal enclosure, offers high dielectric strength to prevent breakdown at higher voltages, and ensures space efficiency in constrained environments.


  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.


A variety of materials are utilized to achieve the aforementioned insulation objectives. These materials may be in solid, liquid, or gaseous states. Depending on their physical properties and form, they serve to fill voids, support active components, and extinguish arcs.



7.1 Gaseous Air

Gaseous air, the most prevalent gaseous substance, consists of 80% nitrogen and 20% oxygen, giving it properties similar to nitrogen. The air clearances between phases and from phases to earth, relative to the system voltage, comply with the standards set forth in IEC 62271-100.


Recently, SF6 gas has been added to the array of gases used in switchgear enclosures, noted for its dielectric strength, which is roughly three times greater than that of air. It fulfills a dual role, functioning as both an insulating and an arc-quenching medium.


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


Figure 2 – Medium Voltage (MV) Switchgear insulated with SF6 gas.

Medium Voltage (MV) Switchgear insulated with SF6 gas.
Figure 2 – Medium Voltage (MV) Switchgear insulated with SF6 gas.

7.2 Fluids

Various fluids have been utilized as insulators in switchgear. Hydrocarbon oil, commonly known as 'transformer oil,' has been employed in bulk oil circuit breakers (CB). It offers the benefit of serving both as an insulator and an arc-quenching medium. Moreover, its dielectric strength is triple that of SF6 when 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!


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