Analysis of tripping problems during AC withstand voltage test of electrical equipment in waste incineration power plants

introduction

In recent years, the construction of municipal solid waste incineration power plants has developed rapidly in my country. As one of the ways to clean waste treatment, waste incineration power generation has become the preferred way to treat municipal solid waste in my country due to its advantages of harmlessness, reduction, high degree of resource utilization, fast processing speed, small environmental impact, and waste heat can be converted into energy [1]. Waste incineration power generation has gradually become an important means and measure for the construction of new urbanization and urban ecological civilization, and plays an important role in urban ecological environment protection, comprehensive resource utilization and renewable energy development. As of the beginning of 2018, there were 360 ​​municipal solid waste incineration power plants in operation across the country, with an annual waste treatment capacity of more than 105 million tons, a total installed capacity of more than 6.8 million kW, and an annual power generation of more than 35 billion kW h. Comprehensively improving the safe operation and maintenance level of waste incineration power plants in my country and promoting the healthy and sustainable development of the waste incineration power generation industry has become one of the important development directions of the waste incineration power generation industry. At present, the safety, stability, economy and emission compliance of the equipment system of waste incineration power plants in my country have received great attention. Among them, operational safety is the primary assessment indicator for the industry's performance appraisal of waste incineration power plants.

1Necessity of AC withstand voltage test for waste incineration power plants

1.1 Principle of AC withstand voltage test

During the operation of power equipment, its insulation gradually deteriorates due to the influence of electric field, temperature and mechanical vibration, including overall degradation and partial degradation, and then forms defects. AC withstand voltage test is the most effective and direct method to identify the insulation strength of power equipment, and is an important part of preventive testing [2]. In addition, since the AC withstand voltage test voltage is generally higher than the operating voltage, the equipment has a large safety margin after passing the test. Therefore, AC withstand voltage test is an important means to ensure the safe operation of power equipment. The AC withstand voltage test system equipment is generally composed of a voltage regulator, a test transformer, a current and voltage relay, a protective resistor, a discharge tube and corresponding measuring equipment. It is usually a complete set of equipment, including control and voltage regulating devices. The AC power supply is connected through a switch and is increased to the test voltage through the voltage regulator and the test transformer to detect the insulation level of the equipment under test.

1.2 Necessity of AC withstand voltage test

According to relevant national regulations and industry standards, newly installed electrical equipment and electrical equipment after overhaul must be tested in accordance with regulations, including AC withstand voltage test, especially in the power industry, such as power plants. Practice has proved that it is an indispensable link to conduct preventive detection and testing of power and electrical equipment before the electrical equipment of a power plant is put into use. At present, many newly built waste incineration power plants are put into operation in my country every year. Unlike conventional thermal power plants, waste incineration power plants have a small installed capacity, and the total installed capacity of a single plant is mostly 25~60Mw, generally less than 100Mw. Although waste incineration power plants do not generate much electricity compared with thermal power plants, they play an important role in the harmless treatment of urban domestic waste. Waste incineration power plants have a one-month shutdown period every year. Once the furnace is shut down, it will affect the disposal of domestic waste in the service area. At this time, the domestic waste will be sent to the landfill for emergency disposal. However, the storage capacity of many landfills in major cities across the country is under great pressure, so it is necessary to ensure that the waste incineration power plants can operate safely and stably after being shut down for overhaul, so that the disposal of domestic waste in the service areas can get back on track as soon as possible.

There are many electrical equipment in waste incineration power plants, such as high and low voltage distribution cabinets, frequency conversion cabinets, transformers, etc. After the electrical equipment is overhauled, relevant electrical equipment tests must be carried out in accordance with regulations. The AC withstand voltage test is the most effective and direct method to identify the insulation strength of power equipment, and is an important part of preventive testing. The AC withstand voltage test can effectively find the local free defects and insulation aging defects of electrical equipment, especially the inspection of concentrated insulation defects is more effective. It can identify the withstand voltage strength of electrical equipment, and then timely grasp the insulation status of electrical equipment, and carry out targeted maintenance and repair of electrical equipment to eliminate the above defects, so as to improve the reliability of the operation of electrical equipment in waste incineration power plants. Therefore, it is very necessary to carry out AC withstand voltage test on the installation and commissioning of waste incineration power generation units and after the overhaul of electrical equipment.

2 Requirements for test equipment for AC withstand voltage test

2.1 Requirements for voltage regulating equipment

The voltage regulator should be able to smoothly adjust the voltage to meet the test requirements. Commonly used voltage regulators include auto-coupling regulators (suitable for small capacity equipment), coil-shifting regulators, and induction regulators (large no-load current). The voltage regulator should output a sinusoidal waveform, and the capacity should also meet the test requirements, usually the same as the capacity of the test transformer.

2.2 Requirements for high voltage test transformers

High-voltage test transformers have the characteristics of high voltage, small capacity, short continuous working time, thick insulation layer, and one end of the high-voltage winding is grounded. When selecting a suitable high-voltage test transformer, it is necessary to consider whether the low-voltage winding of the high-voltage test transformer matches the on-site power supply and voltage regulator, the best matching value of the high-voltage test transformer capacitance and voltage and current, and the high-voltage winding current of the test transformer.

2.3 Requirements for high voltage circuit resistance

Since the capacitive reactance of the device under test and the leakage reactance of the test transformer are connected in series, when the circuit generates series resonance, the device under test will generate a voltage much higher than the test voltage. Usually, the leakage reactance of the voltage regulator and the test transformer is not large, while the capacitive reactance of the test product is very large, so generally no series resonance overvoltage will be generated, but in some special cases, third harmonic resonance will be generated. In order to avoid resonance, an LC series circuit can be connected in parallel to the low-voltage winding of the test transformer or the line voltage can be used. When the device under test breaks down, overvoltage will also be generated between the turns and layers of the test transformer due to the internal electromagnetic oscillation of the winding of the test transformer. Therefore, it is required to connect a protective resistor Rl in series in the high-voltage circuit to limit the overcurrent within the allowable range of the device under test. However, there are certain requirements for the selection of the protective resistor, and neither too large nor too small does not meet the requirements. Another function of the resistor is to prevent excessive electric force from being generated on the high-voltage side of the test transformer when the device under test is broken down.

3 Analysis of tripping causes during AC withstand voltage test

3.1 AC withstand voltage test wiring principle

The wiring schematic diagram of the AC withstand voltage test is shown in Figure 1. Among them, Cx is the capacitive reactance of the tested equipment, Rb/Rq is the protection resistor, G is the protection ball gap, Ic is the capacitive current of the tested equipment, T is the test transformer, A is the ammeter, and V is the voltmeter.

3.2 Main reasons that may cause tripping during the test

The AC withstand voltage test of garbage incineration power plants is generally outsourced. In actual work, some outsourced electrical test units have repeatedly encountered the situation that the electrical equipment of the generator set tripped during the AC withstand voltage test. The main reasons for the tripping during the test are:

(1) The protection setting is unreasonable and the setting value is too small.

(2) The insulation of the equipment under test is insufficient, the leakage current is too large or there is a short circuit.

(3) The gap between the protective balls is unreasonable.

(4) The selection of test transformer parameters is unreasonable.

4. Analysis of test tripping examples

Take the AC withstand voltage test conducted during the installation of electrical equipment in a waste incineration power plant as an example. From 2003 to 2014, there were several cases of small generator sets tripping after being installed during the power frequency AC withstand voltage test. After analysis, it was found that the cause of these trips was that the test personnel did not correctly select the test transformer parameters. Although the rated capacity and rated voltage of the test transformer met the test requirements, the test personnel ignored whether the rated current of the test transformer met the capacitive current of the tested equipment under the test voltage and the possible leakage current problem.