1) Chromatographic analysis of dissolved gases in oil; 2) Measuring the DC resistance of the winding; 3) Measuring the winding insulation resistance, absorption ratio or polarization index; 4) Testing of tan δ of windings and capacitor bushings; 5) AC withstand voltage test; 6) Measuring the winding leakage current; 7) Measuring the insulation resistance of the piercing bolt, the iron jaw clamp, the iron core and the pressure ring; 8) Check the voltage ratio of all taps of the winding; 9) Check the polarity of the three-phase transformer group or single-phase transformer; 10) Measuring no-load current and no-load loss; 11) Measure short-circuit impedance and load loss; 12) Partial discharge measurement; 13) Check the operation of the on-load tap-changer; 14) Insulating oil test; 15) Transformer winding deformation test. I. Chromatographic Analysis of Dissolved Gases in Oil (1) Attention should be paid to the H2 and hydrocarbon gas content (volume fraction) in the operating equipment oil exceeding any of the following values (total hydrocarbons include CH4, C2H6, C2H4, and C2H2): The total hydrocarbon content is greater than 150 * 10-6; H2 content is greater than 150 * 10-6; C2H2 content is greater than 5 * 10-6 (500 kV transformer is 1 * 10-6). (2) The gas production rate of the sum of hydrocarbon gases is greater than 0.25 mL/h (open) and 0.5 (closed), or relative gas production rate greater than 10% / month, the device is considered abnormal. Note that equipment with a low total hydrocarbon content should not be judged by the relative gas production rate. (3) When the content of dissolved gas components has an increasing trend, it can be judged in combination with the gas production rate, and if necessary, shorten the cycle for tracking analysis. II. Measurement of Winding DC Resistance Measuring the DC resistance of a transformer winding is an important means of finding defects in the conductive loop. It can check whether the conductive circuit inside the transformer is in good contact, whether the welding quality inside the winding and the lead is reliable, and whether the tap changer and the bushing lead screw are tightened. 1. Measurement method There are many methods for measuring DC resistance, and the bridge method is often used for measurement. The mechanism and method of the bridge method can be found in the first section of Chapter 2. Here are three more practical ways to speed up the measurement. (1) Direct method. An additional resistor can be connected to the power supply side of the double-arm bridge. The principle wiring is shown in Figure 3-1. This method is called a resistance mutation method. In Figure 3-1, an additional resistor R is inserted in the current loop. The value is 4-6 times of the measured resistance. The closed switch S2 shorts the resistance. When S1 is turned on, all the voltage is applied to the measured resistance. The charging current has a large rising speed, and S2 is turned off until a predetermined charging current. The current quickly reaches a steady state and the DC resistance is measured. The method is generally used on transformers of 110-220 kV and 120 MVA; for 10-35 kV transformers; direct measurement by bridge. (2) Magnetic assist method. Due to the large charging time constant of the large capacity and low resistance transformers, in order to shorten the measurement time, the magnetic assist method can be used for measurement. When measuring the low-voltage winding resistance, the high-voltage side and low-voltage side current loops are strung together, and the current is kept in the same direction, so that the iron core is saturated as soon as possible, the inductance is reduced (i.e., the time constant is reduced), and rapid measurement is performed. Figure 3-2 shows the wiring diagram for this method. In Figure 3-2, the external power supply with the double-arm bridge is used to measure the magnetic flux through the high-voltage winding. At this time, the requirements are as follows: 1 The wiring is correct; 2 It is not suitable for long-time charging measurement to prevent the measurement error caused by the zero drift of the bridge; Power supply voltage, its value should be satisfied
2 I0 (Rx + R1) Where I0 - transformer no-load current; Rx - the resistance of the entire circuit, including the high voltage winding and the low voltage winding power pack; R1 - internal resistance of the instrument; Im - steady state current. The wiring diagram of the bridge assisted magnetic method is listed in Table 3-1 for on-site comparison. (3) Degaussing method. The degaussing method is opposite to the magnetically assisted method. The method strives to make the core flux zero, and the opposite current is used to cancel the magnetic flux in the two windings of the same core column, so that the measuring circuit reaches a linear circuit which is basically a pure resistance. The charging process is very short, eliminating the adverse factors affecting the measurement, making the measurement accurate and stable, simple and rapid. Figure 3-3 shows the wiring of this method. 2. Test requirements In accordance with the requirements of the "pre-regulation": (1) For transformers above 1.6MVA, the difference between the winding resistances of each phase should not be greater than 2% of the average value of the three phases. The windings with no neutral point should not differ by more than 1% of the average value of the three phases. (2) For transformers of 1.6 MVA and below, the phase difference is generally not more than 4% of the average value of the three phases, and the difference between the lines is generally not more than 2% of the average value of the three phases. (3) Compared with the measured values of the same parts in the past, the change should not exceed 2%. (4) Resistance values at different temperatures are converted according to equation (3-2)
R2=R1[(T+t1)/(T+t2)]
Where R1, R2 are the resistance values at temperatures t1 and t2, respectively; The constant for T-one calculation is 235 for copper wire and 225 for aluminum wire. III. Winding Insulation Resistance, Absorption Ratio or Polarization Index Measurement 1. Measurement Method In addition to the commonality of the insulation resistance of other equipment, the measurement of the insulation resistance of the transformer winding has its particularity. The first is that the measurement method is different. In the measurement, the insulation resistance between each winding and the other windings should be measured in turn. The lead ends of the tested windings should be short-circuited, and the other sub-side windings should be short-circuited to ground. In this way, the insulation between the grounded part and the different voltage parts of the tested winding can be measured, and the measurement error caused by the residual charge in each winding can be avoided. The measurement locations and sequence are as shown in Table 3-2. When measuring the insulation resistance, the oil-filled cycle should be allowed to stand for a while before measuring. For small transformers to stand for 6h, large transformers are allowed to stand for 24h. The measurement is carried out using a 2500V or 5000V megohmmeter. The test windings should be fully discharged before and after the measurement. The large transformer discharges for more than 5 minutes, and the small transformer discharges for more than 2 minutes. It should be measured as much as possible when the oil temperature is below 50 °C. 2. Judgment and analysis of test results According to the requirements of the "pre-regulation" and according to the actual situation, the test results should be judged and analyzed according to the following requirements: (1) The insulation resistance should be converted to the same temperature and should not change significantly compared with the previous test result, generally not lower than 70% of the previous value. The temperature conversion formula is:
R2=R1*15 (t1-t2)/10
Wherein R1 and R2 are insulation resistance values at t1 and t2, respectively. (2) The absorption ratio (in the range of 10 to 30 ° C) is not less than 1.3 or the polarization index is not less than 1.5, and neither of them is subjected to temperature conversion. Since the absorption ratio is uncertain in determining the insulation condition, especially for large transformers, the polarization index (PI) is used to judge the insulation condition, as shown in Table 3-3. (3) When the insulation resistance is 10000 MΩ, the absorption ratio should be not less than 1.1, or the polarization index should not be less than 1.3. (4) When the insulation resistance of the site is lower than 70% of the factory value, it should be considered whether the transformer oil is the same as the factory. If they are different, the effects of different insulating oils on the measurement results should be considered. IV. Leakage Current Test of the Winding The leakage current test of the winding is similar to the measurement of the insulation resistance. Due to the high DC voltage applied, it is usually found that some insulation resistance tests cannot find defects. The sequence and location of the test are the same as the measured insulation resistance (see Table 3-2). The test voltage standards are shown in Table 3-4. During the measurement, the negative voltage power supply is used to pressurize the test voltage, and the current value read after 1 minute is the measured leakage current value. In order to make the readings accurate, the micro-ampere should be connected to a high potential; the test should be fully discharged at the end of the test, and the oil temperature should be recorded at the same time. For transformers that are not oiled, the voltage applied to the transformer should be half of the values in Table 3-4 when measuring the leakage current. The test results should be unchanged from the previous test results. V. Winding tanδ Test 1. Measurement Method The tan δ of the winding can be measured with the QS1 type Xilin bridge, or with the M type medium tester. Here is mainly the sequence, location and method of measurement with Xilin Bridge. Since the fuel tank of the transformer is directly grounded, the anti-wiring method of the Xilin Bridge is often used for measurement, and the measurement parts are performed according to Table 3-5. According to Table 3-5, the tans and capacitance value C of the double-winding transformer are measured as shown in Figure 3-4. In the figure, A is connected to the QS1 type bridge A point. It can be seen from Figure 3-4 that the two ends of the winding to be tested are short-circuited during measurement, and the non-measured windings are short-circuited to avoid the winding inductance to bring errors to the measurement. Wherein, under each physical quantity, the angle mark H represents a high pressure; L represents a low pressure; H+L represents a high pressure and a low pressure. Solving the above equations in series, the capacitance value and dielectric loss tangent of each part are obtained.
C1=(C1-CH-GL+H)/2; C2=CL-C1; C3=CH-C2
VI. AC Withstand Voltage Test AC withstand voltage test transformer includes the applied frequency voltage withstand test, pressure test and the operation frequency wave induction voltage test. For fully insulated transformers of 66kV and below, when the site conditions are not available, only the external construction frequency withstand voltage test can be performed. The explanations are given below. (1) External construction frequency sealing test face The external construction frequency withstand voltage test plays a decisive role in assessing the main insulation of the transformer and inspecting local defects. It can effectively find that the main insulation is damp, cracked, or loosened, displaced, and attached to the winding insulation due to vibration during transportation. (1) The wiring method of the test must be correct. The correct wiring method should be short-circuit connection of all the bushings of the tested transformer, and the non-tested windings should also be short-circuited and reliably grounded. Figure 3-5 to Figure 3-7 show the correct and incorrect wiring. Figure 3-6 shows the wrong wiring with neither of the two windings shorted. Figure 3-7 shows the wrong wiring where both windings are shorted and the low voltage winding is not grounded. It can be seen from Fig. 3-6 that due to the influence of the distributed capacitance, there will be a capacitor current flowing through each winding, and the current flowing through the entire tested winding is not equal. The higher the potential near the X terminal due to the capacitance rise effect, the ratio is higher. The applied voltage is also high; and because the untested winding is open, the reactance of the tested winding is large, resulting in an increase in the potential of the X terminal, which may damage the insulation in severe cases. It can be seen from Fig. 3-7 that since the non-tested winding is not grounded and is in a suspended state, it is likely that the low-voltage side insulation is broken when the high-voltage side is under voltage. The floating voltage on the low voltage side is determined by the inter-winding capacitance and the capacitance to ground. (2) Measurement method of test voltage. Due to the effect of the capacitance rise of the capacitive load of the transformer, the test voltage must be measured on the high voltage side and subject to the peak value; if measured on the low voltage side or calculated by the capacitance rise, it is difficult to ensure accurate measurement. (3) Pay attention to the following items during the test: 1 Confirm that the tested transformer shell and the non-test winding have been reliably grounded; 2 After the oil-immersed transformer is filled with oil or after re-pressurization after the test breakdown, it should be allowed to stand for a while. 3) When correcting the ball gap or withstand voltage, the measured voltage should be divided by the peak value by A; 4 avoid cutting off the power supply under the test voltage to prevent overvoltage and damage the product. If there is discharge or breakdown, The power should be turned off immediately. (2) Frequency doubled induction withstand voltage test The frequency doubled induction withstand voltage test is to increase the power supply frequency and reduce the excitation current to achieve the purpose of increasing the applied voltage, and is suitable for the voltage withstand test of the graded insulated transformer. For the main insulation test of the graded insulated transformer, the general external high voltage method cannot be used. Only the induction withstand voltage test can be used. For example, if the tested transformer is properly wired, the main and vertical insulation can be tested at the same time. For fully insulated transformers, it can be used to check its longitudinal insulation (between winding layers, turns and inter-segments). Due to the increase of frequency, the test time is t=60 x100/f(s) (t is the pressurization time; f is the test power frequency) to determine, but not less than 15s. Test method (1) Induction withstand voltage test of fully insulated transformers. It can be tested by applying 2 times the rated voltage of 2 times and above according to the wiring shown in Figure 3-8. This type of wiring can only achieve the test voltage between the lines, and the external high voltage main insulation withstand voltage test is required for the neutral point and the winding. Whether the longitudinal insulation withstands the inductive withstand voltage needs to be judged based on the comparison of the no-load loss test values before and after the test. (2) Induction withstand voltage test of graded insulation transformer. For the main insulation of the graded insulation transformer, neither the external voltage test nor the three-phase induction withstand voltage can be used. The reason is: for graded insulation transformers, the phase and relative insulation levels are the same, such as 220kV grade products, the ground and phase test voltage is 400kV. It is impossible for both to meet the test voltage requirements at the same time, so it is only possible to test with single-phase induction withstand voltage. The various connections shown in Figure 3-9 can avoid excessive voltage between the ends of the wires. According to the structure of the transformer, the corresponding connection method is adopted to make the voltage between the ends of the winding wires and the ground reach the test requirements as much as possible. The scope of application is as follows: 1) When the neutral point insulation level of the high voltage winding can withstand at least 1/3u, the three connections of Figure 3-9 (a1), (a2), (a3) can be used, and the output of the double frequency generator is connected to the Test the transformer on the low voltage winding. If the three-phase transformer is a five-column or shell type, only the connection of Figure 3-9 (a1) can be used. 2) For three-phase five-column or shell-type transformers, the connection method of Figure 3-9(b) can also be used. If the transformer under test has a delta connection, it should be turned on. 3) The induction withstand voltage test of the autotransformer can use the connection method of Figure 3-9(c) to boost the neutral point voltage u1 of the transformer from the test with an auxiliary transformer. The rated voltages of the two self-disconnection windings are UN1 and UN2, and the corresponding test voltages are UN1 and UN2. The following relationship is available. This connection method can also be applied to a three-phase three-column graded insulation transformer, and the neutral point insulation level is less than 1/3 ut. 2. Test power supply The frequency of the transformer induction withstand voltage test power supply is generally 100~250Hz. The intermediate frequency synchronous generator can be used as the test power source, or the 100 Hz test power supply can be obtained by the wire-wound asynchronous motor reverse drag method, or the three-phase single-phase Software Router With 6 LAN, 6 GbE Firewall Software Router, Firewall Router For Home Shenzhen Innovative Cloud Computer Co., Ltd. , https://www.xcypc.com
Oil immersed power transformer test
According to the requirements of the "pre-regulation," the following tests should be carried out on oil-immersed power transformers: