Keywords: transformer; electrical test
Chinese Classification Number TP206 Literature Identification Code A Article No. 1673-9671-(2012)072-0105-01
Power transformer is one of the core electrical components of the electric power system, and the analysis of the electrical test in the process of its installation is a key link in controlling the quality of the installation of equipment. Electrical test must strictly follow the requirements of the technical specifications, carefully analyze the test data, for exceeding the standard data must find out the reasons, and timely elimination of hidden dangers, in order to ensure the safety of the power system.
1 electrical test items and analysis
Large power transformers due to the excitation mode, voltage level, structural type, etc., its electrical test items and judgment standards are also different, this paper analyzes a 2 × 600 MW supercritical power generation project # 1 main transformer as an example. The project main transformer model DFP-240000/500, outdoor, double winding, forced oil circulation air-cooled single-phase power transformer group, non-excited regulator, tap ratio of 525/±2×2.5%/22 kV.
1.1 Electrical test items during installation
1) core and clip-on insulation test: the test needs to be measured separately two sets of data: Core / clip, shell, clip / core, shell, test voltage of 2500 V, time 1 minute, the requirement of the absorption ratio of not less than 1.6.
2) casing CT test: the test must be completed before the installation, mainly through the test to ensure that the CT parameters indicators meet the design requirements. The test **** there are five items: variable current ratio test, polarity test, volt-ampere characteristic test, insulation test, secondary DC resistance test. Variable ratio test non-tested winding must be shorted to ground, otherwise it will induce high voltage injury to personal safety.
3) Casing test: mainly to determine whether the casing is harmed during transportation, and whether the casing is damp. The test includes four items: the main circuit / shell insulation, the main circuit / shell of the dielectric loss, the end of the screen / shell insulation, the end of the screen / shell of the dielectric loss. The insulation of the main circuit should be tested with 5000 V, and the insulation of the final screen should be tested with 2500 V, requiring an absorption ratio of not less than 1.3. For the dielectric loss test, a positive connection method is preferred. Because the dielectric loss test is very susceptible to external factors, the vase must be wiped clean before the test, the casing inclination should not be less than 45 °, the air humidity should be controlled below 60%. The main circuit test voltage is 10 kV, the end screen test voltage is 2.5 kV. require test capacitance value should be in the factory value (1±5%), dielectric loss angle tangent value tanδ (%) is not greater than 0.5.
4) low voltage casing and winding connection contact resistance test: the test focuses on checking the reliability of the electrical connection, the project requires the test value of ≤ 2 μΩ.
5) power transformer installation and installation, the casing should not be less than 45°, air humidity should be controlled below 60%. p>5) power transformer installation is over, but also need to measure the core again, the insulation resistance of the clamps: to test the installation process whether there are remains in the transformer.
Transformer installation is completed, the need for vacuum oil injection, oil filtration, until the oil samples are qualified, and at the same time stand still for 72 hours before other test items.
1.2 Electrical test items after the completion of installation
1) Insulation test: according to the regulations of the required items for testing, recording 15 s, 60 s, 600 s insulation data, calculate the absorption ratio and polarization index. Requirements for absorption ratio and polarization index at room temperature, not less than 1.3.
2) transformer windings with casing DC resistance test: the main transformer high-voltage side of the measurement of the DC resistance of each tap position, the requirements of each phase of the maximum difference between the direct resistance to be less than 2% of the average value, and the factory report and compared to the conversion to the same temperature, the corresponding change can not be greater than 2%. Due to the large time constant of the low-voltage side winding, it takes a long time, and it is recommended to use the assistant
magnetic method.
3) the tap position of the transformer voltage ratio test and connection group check: compared with the manufacturer's nameplate data should not be significantly different from the permissible error of its voltage ratio in the rated tap position of ± 0.5%; transformer connection group must meet the design requirements.
4) Measurement of core, clip insulation resistance: test methods and requirements as above.
5)Measurement of winding with casing dielectric loss angle tangent value tanδ: the test includes three test items: winding/shell, high-voltage winding/low-voltage, shell, low-voltage winding/high-voltage, shell, using the inverse connection method, the test voltage selection of 10 kV. the requirements of the measured winding tanδ value should not be greater than 130% of the factory test value.
6) Measure the DC leakage current of the winding together with the casing: the test voltage of this project is 60 kV for the high-voltage winding, and the test voltage of the low-voltage winding is 20 kV. the requirement is that the leakage current of the high-voltage side at room temperature should not be greater than 30 μA, and the leakage current of the low-voltage side should not be greater than 50 μA.
7) Transformer Winding Deformation Test: the main test is to check if the transformer is harmed in the process of transportation, and to check whether the transformer is harmed in the process of transportation. For 66 kV and above voltage level transformer, it is appropriate to use the frequency response method to measure the winding characteristics of the map.
8) low voltage side winding AC withstand voltage test: in this project using frequency resonance withstand voltage device, through the tuning effectively reduce the test current, improve the test efficiency.
9) the winding together with the casing of the long-time induced voltage test with partial discharge measurement: require the test voltage waveform as close as possible to sinusoidal, the test voltage value for the measurement of the peak voltage divided by, the test should be monitored at the high-voltage end. External application of AC voltage test voltage frequency should be 45 Hz ~ 65 Hz, full voltage withstand time of 60 s.
10) other test items: at the rated voltage of the transformer's impact closing test, carried out five times, each interval is appropriate for 5 min; transformer phase checking, should be the same as the design phase; voltage level of the 500 kV transformer, but also in the rated voltage and rated frequency measurement of noise. Voltage level of 500 kV transformer, should also be measured in the rated voltage and rated frequency noise, the noise value should not be greater than 80 dB (A); transformer insulation oil test.
2 Problems encountered and solutions
1) In the transformer C-phase high-voltage side of the casing test, measured several times the end of the screen / shell of the dielectric loss of 3.5%, nearly 10 times the factory value. But the insulation is qualified, so that the end screen is not insulation damage, there must be and shell distance is too close to the point where the discharge occurs. Considering the structure of the end screen, the point is most likely to exist in the flange combined, according to work experience, suspected that the problem in the end of the screen small casing lead line, this position is easier to close to the shell. Will casing flat check, there is a small section of the lead line from the shell of the nearest distance of 3 mm, the discharge occurred, after treatment, the test data meet the requirements.
2) in the transformer B-phase winding with the casing of the dielectric loss test, found that the low-voltage side of the winding of the dielectric loss value of 1.6%, obviously large. Through the analysis of the first excluded the transformer oil problem, because if it is the oil problem then the high-voltage winding data should not be qualified. Is the low-voltage winding and shell insulation defects? We then again on the low-voltage windings and core clamps between the insulation test, found that the insulation is good. The problem is limited to the dilemma, and then associated with the Lianyungang project main transformer high-voltage side of a similar situation, at that time is due to the casing of the end of the screen grounding caused by bad. So check the end of the screen, and indeed found that the grounding nut loose, resulting in poor grounding. After treatment, the test results meet the requirements.
3 Conclusion
Electrical test is a very technical work, do a good job of electrical testing focus on theoretical learning, through the theory to guide practice, but also should be good at summarizing the accumulation of practical work experience.
References
[1]Chinese People's Republic of China *** and the Ministry of Construction. Electrical installation project electrical equipment handover test standard [J]. China Planning Press,2006.
[2]Chen Tianxiang,Wang Yinzhong. Electrical test[J]. China Electric Power Press,2005.
Author Introduction
Du Hongbo (1982-), female, Han nationality, Jining, Shandong Province, assistant engineer, engaged in electrical test and relay protection work.
Zhao Dezhong (1982-), male, Han nationality, from Zhoukou, Henan Province, Bachelor of Engineering, Engineer, mainly engaged in power plant electrical professional and technical management.
Zhao Dezhong (1982-)