This article investigates the AC withstand voltage test of diesel generator sets, analyzes the existing problems, and demonstrates the feasibility of carrying out AC withstand voltage tests for low-voltage generator sets. The method, testing cycle, testing requirements, wiring method and key issues handling during the test process of the 400V low-voltage generator AC withstand voltage test are explained. The test results are evaluated, in order to further improve the AC withstand voltage test level of diesel generator sets. Provided basis. This article mainly discusses the AC withstand voltage test technology of diesel generators with a single unit capacity of 800kW and below and a rated generator voltage of 400V.
Problems in AC withstand voltage test
1. Lack of experimental basis
The basis for preventive testing of generators in my country for a long time is mainly the “Preventive Testing Procedures for Electric Power Equipment” (DL/T 596-1996) issued by the former Ministry of Electric Power Industry, which played an important role in production. However, there is a problem in this regulation, that is, the provisions related to the AC withstand voltage test of generators are mainly formulated for motors with an installed capacity of more than 6000kW and a rated voltage of more than 1000V. Diesel generators with a single unit capacity of 800kW and below and a rated voltage of 400V are not clearly defined and can only be implemented by reference, lacking suitability, pertinence and operability.
2. Lack of testing capabilities
For low-voltage power station users, due to factors such as scale, financial strength, technical strength, and testing equipment, there is a lack of testing capabilities. Especially for ordinary users with 800kW and below and a generator rated voltage of 400V, they rarely conduct testing. This test. According to the author’s investigation, at least 98% of low-voltage unit power stations have not carried out this test.
3. Lack of supervision mechanism
For a long time, low-voltage generating units have generally faced the problem of weak conceptual awareness, focusing on efficiency, neglecting safety, focusing on production, and neglecting prevention. As for whether preventive tests are generally carried out strictly, the company itself takes a chance that it can save money. It usually only conducts simple insulation tests during overhauls. Other tests including AC withstand voltage tests are not performed at all, and the test level is extremely low. Low.
Feasibility of AC withstand voltage test
The generator AC withstand voltage test is to apply an AC voltage higher than the overvoltage value that may be encountered during operation to the generator for a period of time to check the insulation level of the motor coil.
Since the test voltage of the AC withstand voltage test is consistent with the waveform and frequency of its operating voltage, during the test, the voltage distribution and breakdown performance inside the insulation are also consistent with that of the generator during operation. Therefore, the AC withstand voltage test is an insulation test that is closest to the actual operating conditions of the generator. It can better simulate the overvoltage that the generator will withstand in actual operation. In addition, the test voltage is much higher than the operating voltage, so It can best detect local defects in insulation and general deterioration of insulation. It is of decisive significance in judging whether the generator can be put into operation. Therefore, it is very necessary to carry out this test in low-voltage generator sets.
For small generators of 800kW and below, due to their simple structure, the test standards, test equipment, on-site environmental conditions, etc. are not as high and complex as the requirements for large motors. The test wiring can be carried out scientifically and reasonably under the principle requirements of the regulations. Simplification, the power station only needs to invest a small amount of money and purchase some test equipment based on the original conventional instruments to carry out this test. Therefore, it is feasible in economic and technical aspects.
Implementation of AC withstand voltage test
1. Determination of test voltage value
According to the regulations, different test voltages should be applied in different situations. In view of the characteristics of low-voltage generator sets such as small capacity, low voltage, and simple structure, appropriate adjustments can be made in actual use.
2. Test wiring method
The wiring for the AC withstand voltage test should be selected based on the capacity and voltage of the generator and the conditions of the on-site test equipment.
Things to note are as follows:
(1) Selection of test transformer:
The rated voltage of the high-voltage side of the test transformer shall not be lower than the maximum test voltage, and the rated current of the high-voltage side shall not be lower than the maximum capacitive current of the motor. When a voltage transformer is used as a test transformer, it is allowed to be overloaded 3 to 5 times within 3 minutes.
(2) Selection of voltage regulating equipment:
Since the capacity of the test variable is small, an auto-coupling voltage regulator should be used as the voltage regulating equipment, and its rated current carrying capacity is equivalent to the rated input current of the test variable.
(3) Selection of current limiting resistor R1
In order to limit the current when the motor breaks down, protect the test transformer and prevent the fault from expanding, a current-limiting resistor R should be added to the high-voltage side of the test transformer, and its value is generally 1Q/V. Current limiting resistors usually use water resistors.
(4) Selection of protective resistor Rz
In order to reduce the short-circuit current during the discharge of the overvoltage protection ball gap and prevent the protection ball gap from burning out, a protection resistor should be installed. Its resistance value is generally 1QV and water resistors are usually used.
(5) The ball gap discharge voltage is set to 110% of the test voltage.
For generators with a rated voltage of 400V, due to their small capacity and low voltage, the above wiring can be appropriately simplified and improved to facilitate on-site implementation.
Handling of key issues in the test
Since the test voltage of the AC withstand voltage test is generally much higher than the operating voltage, it is a destructive test for motors with poor insulation and is the last item to be carried out in the motor insulation test. Therefore, the following principles must be mastered during the test:
(1) Before conducting the withstand voltage test, the insulation resistance and absorption ratio should be tested first, and the condition of the insulation should be initially checked. If possible, DC leakage current and DC withstand voltage tests should also be done. If there are other defects, they must be studied and dealt with, and the decision whether to proceed can only be made after comprehensive analysis and judgment of the results of other non-destructive tests.
(2) When increasing the test voltage, it is forbidden to use mid-stroke closing to increase the voltage, and the method of increasing the voltage from zero should be used. During the voltage boosting process, the voltmeter should be monitored and the capacitor current value of the motor should be read every time the test voltage is increased by 2ONV. When simplifying the wiring, since there is no ammeter, you can stop briefly and observe whether there are any abnormalities in the motor coil. Generally, the time to increase the voltage from zero to the full test voltage should not exceed 30 seconds. When the voltage rises to the full test voltage, the timer starts and lasts for 1 minute. After the withstand voltage time is up, the voltage is reduced to zero and the test power is disconnected.
(3) During the test, if the following abnormal phenomena are found, the test should be stopped immediately and the reasons should be identified: the voltmeter pointer swings greatly, the ampere meter indication increases sharply, the generator insulation is burnt or smoking, the power generation The machine makes abnormal noises, etc.
(4) The rotor AC withstand voltage test method and wiring are similar to those of the stator.
Summarize:
The preventive test of a generator is one of the most important and effective means to ensure its safe and stable operation. It can promptly discover the hidden dangers of the generator in operation and prevent accidents or equipment damage. It is an important link in the operation and maintenance of the power station. . If no insulation flashover, discharge, breakdown, or overcurrent protection trip occurs during the test, the insulation resistance and absorption ratio of the stator winding measured after the withstand voltage test will remain basically unchanged compared to those measured before the withstand voltage test. If it is, it is considered normal, otherwise it is considered that there is a problem with the insulation. Preventive test projects such as AC withstand voltage test are an effective means for power stations to detect insulation defects and weak links as early as possible. It facilitates maintenance personnel to grasp the characteristics, current situation and changing patterns or trends of the generator, improve insulation maintenance, reduce insulation accidents, extend the life of the motor, and promote the safe and stable operation of the power station. Therefore, this work must be done actively, scientifically, and standardizedly.
