Withstand Voltage Test/Partial Discharge Test
What Is a Withstand Voltage Test?
The withstand voltage test also called the dielectric strength test or hi-pot test, is a test to evaluate whether the insulation of electrical products and parts has sufficient dielectric strength against the voltage. A higher-than-normal voltage is applied to the insulation for a specified period of time to determine whether dielectric breakdowns occur.
What Is a Withstand Voltage ?
A withstand voltage is the voltage at which dielectric breakdown occurs when a DC or AC voltage is applied to an insulating material such as resin. A very high voltage (up to the range at which dielectric breakdown is likely to occur) is applied to determine the withstand voltage. The voltage is gradually increased and the point at which dielectric breakdown occurs is called the withstand voltage (limit voltage).
The Purpose of Withstand Voltage Tests
All electrical products must be safe and not endanger people’s lives or cause property damage. The purpose of voltage resistance testing is to verify whether electrical products have sufficient dielectric strength to protect consumers and their property from electric shock, fire, and other hazards.
More About the Withstand Voltage Testing
The figure on the left illustrates the withstand voltage test. It shows a test connection diagram for the primary circuit of electrical equipment. The output cable (HIGH side) of the withstand voltage tester is connected to the AC input terminal of the test object where the live (L) and neutral (N) are connected to the shorted part. The other output cable (LOW side) is connected to a conductive part (chassis) which can be touched by a person.
A high AC (alternating current) voltage is applied between HIGH and LOW. The leakage current flowing between the AC line and the chassis is measured to check the insulation. Z1 and Z2 between the AC line and the chassis contain Y capacitors, which have the characteristics of passing AC and are mainly used for noise removal. The withstand voltage tester detects the overall leakage current, including the current flowing through the capacitors, and checks whether it exceeds the set limit value.
What Is Partial Discharge Testing?
Partial discharge is a phenomenon in which a part of an insulator undergoes partial dielectric breakdown. It is an air discharge that occurs mainly in insulator voids. Usually, voids are made of gases such as air. They are very small, and so is their electrostatic capacitance.
When voltage is applied between the electrodes, a large voltage flows to the part of the void where the capacitance is low. The electric field is concentrated in the void, causing partial discharge.
When a withstand voltage test is conducted between points A and B, a voltage nearly equal to the test voltage is applied to the CV capacitance of the void inside the insulator. Then, the void with a short insulation distance discharges at a lower voltage.
However, even when a discharge occurs in the void, because of the insulator, it does not lead to a discharge that short-circuits the electrodes. As shown in the above example, a partial discharge is a state in which a discharge occurs in a void inside an insulator but does not lead to a discharge between the electrodes. Partial discharge testing is used to measure the charge transfer and discharge pulse at that time.
Key points of partial discharge testing:
- The capacitance of voids is extremely small.
- A large voltage is applied to voids.
- Voids with short insulation distances discharge at low voltages.
- This discharge does not result in a short-circuit discharge between the electrodes. However, a transfer of electric charge occurs (partial discharge).
*Does not lead to the breakdown of the load.
Benefits of Partial Discharge Testing
Partial discharge testing detects conditions prior to dielectric breakdown, which allows the detection of latent defects and manufacturing variations that could not be identified with conventional withstand voltage testing. Below, you can find a list of advantages over conventional methods.
- Optimized insulation design
- Reduced manufacturing variations
- Material variations can be identified
- Improved safety
- Contribution to the analysis of defects caused by unknown factors
- If defects can be detected nondestructively, faulty areas can be investigated.
- Long-term reliability of insulation performance can be evaluated.
Relation Between Partial Discharge and Dielectric Breakdown
The typical relation between the voltage applied between the electrodes and the discharge is shown in the figure above. When the voltage applied to the test object is gradually increased, first, a partial discharge begins (A), and then sparks are generated (D). Next, as the test object transitions to arc discharge (E), it undergoes dielectric breakdown. If additional power is supplied after the transition to arc discharge, the test object, whose insulation has been damaged, burns due to the heat of the arc discharge.
Looking more closely at the partial discharge graph, after the partial discharge starts in (A), there is a stable area between (B) and (C). Here, partial discharge occurs in multiple voids in the insulator. Then, after passing through the partial discharge surge part between (C) and (D), the insulation between each void is destroyed, and the arc discharge occurs.
What Is the Difference Between Withstand Voltage Testing and Partial Discharge Testing?
In withstand voltage test, a high AC (or DC) voltage is applied to a test object. The leakage current that flows when the object breaks down is detected in order to determine whether the test was passed or failed. In other words, dielectric breakdown equals an increase in electric current. In most cases, a test object that fails is damaged beyond reparation.
If there are no breakdowns (increases in current) during the withstand voltage test, the product is presumed to be good, even if there are voids or other defects that can cause insulation failure. Partial discharge test, on the other hand, captures the increase in electric charge before dielectric breakdown occurs. Therefore, in most cases, the test object is not destroyed. Since the test determines the state of the product before dielectric breakdown occurs, it is possible to detect potential defects and manufacturing variations that could not be detected by conventional withstand voltage testing.
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