Abstract: A simple and practical capacitance tester is constructed using a frequency converter inverter power supply and a clamp meter, and its technical specifications and usage characteristics are introduced. Keywords: power; capacitor; tester; frequency converter; current-voltage method [b]0 Introduction[/b] In modern power systems, capacitors are widely used as devices for compensating negative sequence, filtering, and voltage extraction. To increase withstand voltage and capacity, capacitors are often connected in series and parallel. Once capacitors are connected in series (parallel), the total capacitance value is displayed externally, and ordinary capacitance measuring instruments cannot measure the value of a single capacitor online. To measure the capacitance of a specific capacitor, it must be removed from the parallel system, which is very complicated in practice. During restoration, fuse breakage and poor contact are very likely to occur, causing the differential pressure protection to trip after the capacitor system is put into operation. Based on this, a practical capacitor tester is introduced. [b]1. Principle of Capacitor Tester[/b] To achieve measurement without disconnection and simulating online operation, an AC voltage can be applied to the parallel capacitors on the bus (as shown in Figure 1, AI, AII). Then, a current clamp meter is used to measure the current of each capacitor under test, and the capacitive reactance of the capacitor is calculated using the formula X = U/I. The capacitor loss (R is the resistance of the capacitor, and q is the phase angle between the applied voltage and the capacitor current) can also be calculated using the formula Rx = Xc/tan(90°-q), thus identifying conditions such as insufficient oil, carbonization, and moisture in the capacitor. 1.1 Applied Voltage According to calculations, for the 4.51F capacitor currently used in the traction power supply system, to obtain a current of 1A, the applied power frequency voltage must be above 700V. To ensure the safety of the operator and the test instrument, it is necessary to reduce the applied voltage. A frequency conversion method is used in the design. For example, when using a 100Hz voltage, the applied voltage can be reduced by a factor of 2 for the same capacitor; when using a 200Hz voltage, it can be reduced by a factor of 4. To ensure the capacitor does not store charge after testing, the applied voltage should not be too high; 12V is selected. For a 1"F capacitor, the capacitive reactance at 50Hz is 3.18kJ/m². Using 12V, the clamp meter measures approximately 4mA. Therefore, the output accuracy when measuring 4mA current with a clamp meter is the same as the capacitance measurement accuracy. While it's easy for a regular ammeter to guarantee accuracy in measuring milliampere-level currents, it's very difficult for a clamp meter, as it's affected by many factors such as the size of the jaws, the closure gap, and the position of the wire within the jaws. More seriously, under the interference of harmonics in the traction power supply system, the no-load measurement value of a milliampere-level clamp meter can exceed 2A, making it unusable in the field. Improving the clamp meter's measurement accuracy to guarantee capacitance measurement accuracy is almost impossible. Field test results show that the clamp meter mainly measures the interference values ​​of the 2nd and 3rd harmonics. To eliminate the influence of external interference and improve measurement accuracy, the influence of harmonics must be avoided. Based on the above two considerations, a variable frequency sinusoidal inverter circuit is adopted, with an adjustable frequency of 30-2000Hz, a maximum current of 3A, and a maximum voltage of 12V AC RMS. 1.2 Current Measurement: The current flowing through the capacitor under test is measured using an AC clamp meter. Firstly, the inner diameter of the clamp meter must be slightly larger than the outer diameter of the capacitor's lead terminals. Secondly, when using a 200Hz voltage source, the measured current for a 1F capacitor is only 15mA; therefore, the clamp meter's measurement accuracy is also required to be high. Simultaneously, to coordinate with the microcontroller, the clamp meter's current ratio can be selected based on a primary current of 1mA and a secondary current of 100kΩ resistors connected in parallel to obtain a 1V voltage. 1.3 Phase Measurement A phase angle of 90° between the measured current and the applied voltage indicates that the measured capacitor has no losses. If the angle is not 90°, it is considered to be due to losses. In the design, a 10MHz (0.1/zs) pulse is used to measure the phase difference, achieving an accuracy of 0.00005° at a measured current of 50Hz. Based on this, the dielectric loss and power dissipation of the capacitor can be easily calculated. 1.4 On-site Correction To eliminate the influence of external electromagnetic interference on the capacitance measurement results, a built-in standard capacitor correction can be used. The standard capacitor should have suitable clamp meter terminals, allowing the measured value of the standard capacitor to correct the actual measured value in the field environment. 2 Tester Structure and Use Based on the principle of the capacitance tester, the tester is designed to consist of a computer system, a clamp meter sensor, a standard capacitor correction system, and a voltage output unit, as shown in Figure 2. The overall weight of the device is less than 3kg, making it suitable for on-site transport. When performing capacitance tests, first connect all leads, turn on the power switch, clamp the clamp meter onto the external output line of the standard capacitor, as shown by the dotted line in Figure 2, select on-site calibration in the instrument function menu, close the computer control switch K, collect data and complete automatic correction calculation (when performing other capacitance tests, the test results will directly eliminate the influence of external electromagnetic interference). Then connect the output voltage line to the busbar of the capacitor bank to be tested, connect the wires as shown in Figure 1, clamp the clamp meter onto one capacitor lead, press the counter button, and the device will automatically record the measured capacitance value. Clamp the clamp meter onto each capacitor one by one and press the counter button to complete the test of all capacitance values. When dielectric loss needs to be tested, select it in the function menu, and the capacitance value and dielectric loss tests will be performed simultaneously. After field testing, the device has a maximum capacitance test range of 1000ffF, an accuracy of 0.3, and a resolution of 0.01"F; the dielectric loss measurement range is tan≤15°, and the accuracy is 0.1. For a capacitor bank of 4 series and 8 parallel, the entire test time (including preparation time) does not exceed 15 minutes. Because the voltage is applied to the busbar, the current is obtained through the capacitor fuse during the test, which is equivalent to checking the integrity of the circuit (fuse and connectors), effectively preventing the differential pressure protection from tripping after the capacitor system is put into operation due to fuse breakage or poor contact. 3 Conclusion Portable capacitor tester can simulate the online operation of capacitor system. Without disconnecting the wires or fuses, it can conveniently measure the capacitance and dielectric loss of capacitors when they are energized. It is especially suitable for systems with multiple capacitors connected in parallel. It can also be used to check the electrical connection of capacitor system, the status of fuses, and the actual situation of capacitors being damp or lacking oil. It adopts a frequency conversion sinusoidal signal with low voltage and small current. After the measurement is completed, the capacitor does not store energy and does not need to be discharged. It poses no safety threat to the operator and is very suitable for field use. References [1] Li Yanji. Fault analysis and protection design of parallel capacitor compensation device EJI. Railway Standard Design, 2006 (4) E21 Beijing Railway Bureau. Q/BT143-96 Test Procedure for Traction Power Supply Equipment [s]