The purpose and test classification of power and electrical equipment for withstand voltage test

Author: Noyafa–CCTV Tester

The purpose and test classification of power and electrical equipment for withstand voltage test Under normal circumstances, the voltage waveform in the power system is a sine wave. During the operation of the power system, due to reasons such as lightning strikes, operation, faults or improper parameter matching of electrical equipment, the purpose and test classification of a certain power and electrical equipment in the system are caused by the withstand voltage test. Under normal circumstances, the voltage waveform in the power system is a sine wave . During the operation of the power system, due to lightning strikes, operation, faults or improper parameter matching of electrical equipment, the voltage of some parts of the system suddenly rises and greatly exceeds its rated voltage, which is overvoltage.

Overvoltage can be divided into two categories according to its causes. One is the overvoltage caused by direct lightning strike or lightning induction, which is called external overvoltage. The magnitude of lightning impulse current and impulse voltage are large, and the duration is very short, which is extremely destructive. However, because the overhead lines of 3-10kV and below in general industrial enterprises are protected by the shielding of workshops or tall buildings, the probability of being directly struck by lightning is very small, which is relatively safe.

The other type is caused by the energy conversion or parameter changes inside the power system, such as fitting the no-load line, cutting off the no-load transformer, and single-phase arc grounding in the system, which is called internal overvoltage. Internal overvoltage is the main basis for determining the normal insulation level of various electrical equipment in the power system. That is to say, the design of the insulation structure of the product should consider not only the rated voltage but also the internal overvoltage of the product use environment.

The withstand voltage test is to detect whether the insulation structure of the product can withstand the internal overvoltage of the power system. It can also be used to check the insulation manufacturing or maintenance quality of electrical equipment; eliminate damage to insulation caused by raw materials, processing or transportation, and reduce the early failure rate of products; check the electrical clearance and creepage distance of insulation. Withstand voltage test, also called electrical strength test.

What is a withstand voltage test, which is to apply a high voltage several times the rated voltage between the live parts of the device under test and the casing, and this voltage must last for a specified period of time to verify whether the live parts of the device under test are grounded or not. Breakdown, if there is no insulation breakdown at the end, this test is considered to be passed. If any part fails due to the processing process, components or materials, breakdown will occur; in addition, if the electrical clearance is too small, it may not fail under normal operating voltage, after a period of use, due to dust accumulation, humidity, etc. The electric gap is broken down, causing electric shock hazard. Withstand voltage test can detect this hidden danger in advance.

Withstand voltage test is one of the main methods to verify the ability of electrical appliances, electrical equipment, electrical installations, electrical circuits and electrical safety appliances to withstand overvoltage. The explanation given by the electronic component technology network is: withstand voltage test or high voltage test (HIPOT test), which is used to verify the quality and electrical safety characteristics of products (such as JSI, CSA, BSI, UL, IEC, TUV and other international safety agencies A 100% production line test of required standards) and the most well known and frequently performed production line safety test. The HIPOT test is a non-destructive test to determine that electrical insulating materials are sufficiently resistant to transient high voltages, and is a high-voltage test that is applicable to all equipment to ensure that the insulating material is adequate.

Classification of withstand voltage test The withstand voltage test is divided into two types: power frequency AC withstand voltage test and DC withstand voltage test. The test voltage of the power frequency AC withstand voltage test is more than one to several times the rated voltage of the tested equipment, not less than 1000V. The pressurization time: 1 minute for equipment with porcelain and liquid as the main insulation, 5 minutes for equipment with organic solids as the main insulation, 3 minutes for voltage transformers, and 10 minutes for oil-immersed power cables.

The DC withstand voltage test can pass the value of the leakage current at different test voltages, draw the leakage current—Voltage characteristic curve. Usually the AC withstand voltage test is easier to be accepted by the safety agency than the DC withstand voltage test. The main reason is that most items under test will operate under AC voltage, and the AC withstand voltage test provides the advantage of alternating two poles to stress the insulation, which is closer to the stress that the product will encounter in actual use.

Since the AC test does not charge the capacitive load, the current reading remains the same from the start of the voltage application to the end of the test. Therefore, there is no need to ramp up the voltage since there are no stabilization issues required to monitor current readings. This means that unless the product under test senses a suddenly applied voltage, the operator can immediately apply full voltage and read the current without waiting.

Since the AC voltage does not charge the load, there is no need to discharge the device under test after the test. When testing capacitive loads, the total current consists of reactive current and leakage current. When the amount of reactive current is much larger than the true leakage current, it may be difficult to detect products with excessive leakage current.

When testing large capacitive loads, the total current required is much greater than the leakage current itself. This can be a greater hazard due to the higher current the operator is exposed to. DC withstand voltage test: If the measured object is highly capacitive, the product will require an AC withstand voltage test with a very high output current capacity due to capacitive reactance, but at this time, the higher current capacity may bring risks to the operator , at this time, the DC withstand voltage test can be used, and the safety hazard is much smaller.

The DC test will cause the capacitor to generate an initial charging current, which exponentially decreases to 0. Once the device under test is fully charged, the current flowing through is the real leakage current, which enables the DC withstand voltage test to clearly display the true nature of the product under test. Leakage situation; at the same time, because only the instantaneous charging current is applied, the output power AC test is small. So what is the disadvantage of the DC withstand voltage test? Unless the device under test has no capacitance, the test must gradually increase the voltage from 0 to the test voltage; after the test, the DUT must be discharged. A good rule of thumb is to conduct and apply a high voltage ground at the same time.