The difference between high voltage AC withstand voltage test and DC withstand voltage test
1. Principle of AC withstand voltage test
AC withstand voltage test refers to the use of AC power supply to test the high voltage equipment under the specified frequency and amplitude. The main purpose of this experiment is to check the insulation performance and anti-electric shock ability of the equipment under the action of AC electric field. The principle of AC withstand voltage test can be analyzed from the following aspects:
Electric field distribution and insulation characteristics:
The characteristics of AC electric field are that the direction and magnitude of the electric field change periodically over time. Therefore, the insulation material of the equipment needs to maintain good insulation performance under the action of frequently changing electric fields. When high-voltage equipment is subjected to AC voltage, its insulation performance will be affected by the change of electric field. The loss of dielectric in AC electric field is more obvious, and this loss is manifested as an increase in dielectric loss. If the insulation material of the equipment is of good quality, then its dielectric loss under AC electric field is small and its voltage bearing capacity is strong.
Periodic changes in current:
The current in the AC withstand voltage test changes in a sinusoidal waveform within a cycle, and the direction and amplitude of the current will change over time. Since the AC current changes periodically, the dielectric in the device will repeatedly experience the reverse action of the electric field, and this repeated action can easily cause aging and breakdown of the dielectric. Therefore, the AC withstand voltage test can better simulate the voltage variability of the device in the actual working environment.
Withstand voltage conditions:
The voltage amplitude of the AC withstand voltage test is generally required to be between 1.5 and 2 times the normal operating voltage of the equipment, and the test time is usually 1 to 10 minutes. In order to reduce the risk of electrical breakdown and insulation damage, the voltage needs to be gradually increased during the test until the set withstand voltage value is reached, and the voltage is maintained for a period of time for testing.
Breakdown and discharge phenomenon:
In AC withstand voltage test, if the insulation of the equipment is poor or the design is improper, when the electric field strength reaches a certain threshold, the insulation medium inside the equipment may break down, resulting in discharge phenomenon. This breakdown phenomenon is usually manifested as a sudden increase in current or sparks, further damaging the equipment.
2. Principle of DC withstand voltage test
DC withstand voltage test refers to the use of DC power supply to test the withstand voltage of high-voltage equipment. Unlike AC withstand voltage test, the DC voltage is constant and has no periodic changes. The DC withstand voltage test mainly tests the insulation capacity of the equipment under constant voltage and its stability when it withstands high voltage. The principle of DC withstand voltage test can be analyzed from the following aspects:
Stability of the electric field direction:
The direction of the DC electric field is constant and the voltage remains unchanged throughout the test. This is different from the periodic changes of the AC electric field, so the insulation material of the equipment only needs to maintain the stability of the electric field in one direction and does not need to reverse the electric field as frequently as in AC tests. Therefore, the insulation material under DC voltage is more likely to withstand higher electric field strengths.
Current response:
In a DC withstand voltage test, since the direction of the electric field does not change, the current response is relatively simple and usually appears as a steady-state value. The magnitude of the current reflects the leakage current of the device's insulating material. If the device is well insulated, the leakage current should be small; if the device has internal defects or the insulating material is aged, the leakage current will increase. The leakage current in a DC withstand voltage test is usually low, but if the leakage current is too large, it means that the device has an insulation problem, which may cause damage to the device.
Withstand voltage conditions:
The voltage amplitude of the DC withstand voltage test is usually required to be higher than that of the AC withstand voltage test. The test voltage is generally 2 to 3 times the rated working voltage of the equipment. The test time is also 1 to 10 minutes. Since DC voltage does not cause dielectric loss, its test results can better reflect the stability of the equipment insulation.
Breakdown phenomenon:
In the DC withstand voltage test, if the insulation material of the equipment is defective or aged, the leakage current may increase sharply, and eventually lead to the breakdown of the insulating medium. Due to the unidirectional effect of the DC electric field, the breakdown process is usually more violent and difficult to self-heal compared to the AC electric field. Therefore, the DC withstand voltage test can more clearly determine the absolute stability of the equipment under higher voltage conditions.
3. The difference between AC and DC withstand voltage test
Electric field properties:
The electric field in the AC withstand voltage test is a periodically changing electric field, while the electric field in the DC withstand voltage test is constant. This makes the electric field characteristics of the equipment in the two tests completely different. The reverse action of the electric field in the AC test is more frequent, which may cause greater pressure on the insulation material of the equipment.
Withstand voltage:
Due to the frequent changes in the AC electric field, the insulation performance of the equipment under the AC electric field is usually weaker than that under the DC electric field. Therefore, the voltage of the AC withstand voltage test is usually set at 1.5 to 2 times the rated working voltage of the equipment, while the voltage of the DC withstand voltage test is usually set at 2 to 3 times.
Current response:
The current in the AC withstand voltage test changes periodically, which is closely related to the direction and amplitude of the electric field. In the DC withstand voltage test, the current is constant, reflecting the leakage current of the device. The current response of the DC test is usually relatively stable, while the current fluctuation of the AC test is large.
Difficulty and risks of the test:
AC withstand voltage test may cause greater damage to the insulation materials of the equipment due to repeated changes in voltage, and the risk during the experiment is higher; while DC withstand voltage test is easier to control due to constant voltage, and the risk during the experiment is lower.
Different destructiveness:
AC withstand voltage test: Since the test current is capacitive current, it is relatively destructive to insulation.
DC withstand voltage test: Since insulation basically does not produce dielectric loss under DC voltage, it is less destructive to insulation.
Scope of application and limitations:
AC withstand voltage test: Applicable to checking the insulation performance of power equipment, communication equipment and household appliances.DC withstand voltage test: Applicable to the insulation performance test of high-voltage equipment, power cables, capacitors and other fields.
AC withstand voltage test and DC withstand voltage test have their own characteristics and different application scenarios in the insulation performance test of high-voltage equipment. AC withstand voltage test can better simulate the voltage fluctuation of the equipment in actual work, so it is more reliable for the performance test of power equipment in alternating electric field. DC withstand voltage test is more used to detect the insulation stability of equipment under constant voltage conditions, which can clearly reflect the leakage and insulation degradation of the equipment. The combination of the two can comprehensively evaluate the insulation performance and anti-breakdown capability of the equipment, and provide guarantee for the safe operation of the equipment.
