[Abstract] This article summarizes and analyzes the problems existing in the traditional commissioning and debugging methods of capacitor zero-voltage and differential voltage protection, and proposes a scheme of applying voltage to the primary side of the capacitor discharge transformer to improve the reliability of capacitor zero-sequence voltage and differential voltage protection and verify the correctness of secondary circuit wiring, so as to ensure the safe and stable operation of the power system. [Keywords] Capacitor; Voltage; Protection; Test; Discussion (I) Introduction With the rapid development of the national economy, power users have increasingly higher requirements for the reliability of power supply and voltage quality. In order to improve the system power supply voltage and reduce equipment and line losses, various forms of reactive power compensation devices have been widely used in the power system. Therefore, it is crucial to conduct correct tests on the power capacitor protection of substations to ensure the normal and safe operation of capacitors. (II) Problems of the traditional differential voltage and zero-voltage protection test methods for power capacitor banks Since the output of the zero-voltage or differential voltage protection of capacitors is close to 0V during normal operation of the capacitor bank, there may be accidents such as voltage circuit open circuit protection failure to operate, or there may be hidden dangers of voltage circuit miswiring and protection maloperation. If the three-phase capacitors are configured in a balanced manner, it can improve voltage quality and stabilize the normal operation of the system. The failure of one (or several) capacitor fuses will cause a shift in the neutral point voltage of the capacitors. If this shift reaches the set value, the differential pressure or zero-pressure protection will trip the high-voltage switch. Therefore, before these two types of voltage protection are actually put into operation, the correct wiring of the secondary circuit of the voltage transformer needs to be verified by powering on. The method is as follows: 1. When testing new capacitors and protection under load, first conduct an impulse test on the capacitors and observe if it is normal. For capacitor modification tests, remove one (or several) capacitor fuses (hereinafter referred to as the "fuse removal test"), then power on again and test the zero-pressure or differential pressure to verify the correctness of the circuit and the configuration of the settings. Multiple operations on a single system pose safety risks and are time-consuming and inefficient. This testing method is only effective for traditional capacitor banks where the fuse is installed externally. However, for multi-unit capacitor banks, which have multiple fuses internally and cannot be individually removed during power outages (such as the parallel capacitor bank assembly produced by Shanghai Siyuan Electric Co., Ltd., model TBB35-1200/334-ACW), the installation between the capacitor and the connecting busbar is very compact, making it impossible to perform zero-voltage or differential-voltage tests to verify the protection. 2. Specialized division of labor leads to flaws in the testing method. Because high-voltage testers are not familiar with the secondary circuit of relay protection, they only focus on the electrical performance of individual primary equipment and do not pay enough attention to the correctness of the secondary circuit; while relay protection workers only carefully maintain the secondary circuit and pay less attention to the primary circuit, resulting in troublesome commissioning and operation of important protections such as differential voltage protection and zero-voltage protection, and high safety risks. (III) Improvement Measures How to verify the correctness of the differential voltage or zero-voltage protection circuit? Applying a test voltage from the primary side of the voltage transformer, allowing the zero-voltage and differential-voltage protection to reach the set value and trip, is a better option. The author believes that: 1. Theoretically feasible. The transformation ratios of 35kV and 10kV voltage transformers are not very large, and the setting values ​​for differential pressure protection and zero-voltage protection are not very high. This provides the prerequisite for testing the operating performance of protection by applying voltage to the primary side of the discharge transformer. For example, the transformation ratio of a 35kV discharge transformer is 35000/1.732/100 = 202.08/1, meaning that a voltage of 1000V can induce approximately 4.9V on the secondary side; for a 10kV discharge transformer, applying 1000V will induce approximately 17.3V on the secondary side. 1000V is not too high, making it possible to test differential pressure and zero-voltage protection by applying voltage to the primary side of the discharge transformer. 2. Requirements for safety, reliability, and efficiency in power system production. By applying a certain amount of voltage at once, the protection operation is achieved. This includes testing the correctness of the wiring of the primary and secondary voltage circuits of the discharge transformer and the setting values ​​of the zero-voltage and differential voltage protection. This avoids the cumbersome testing procedures of powering on, off, and removing capacitor fuses before powering on again, achieving safety and zero power outages. 3. Modern relay protection setting technology is mature enough. For equipment like capacitors, professional relay protection setting departments can guarantee the correctness of the settings and have successful operating experience, eliminating the need for methods like "fuse removal" to verify the protection settings. Therefore, the role of the "fuse removal" test is only to roughly verify the correctness of the differential voltage or zero-voltage protection circuit, including the correctness of the primary wiring of the discharge transformer. In other words, if a voltage test can be performed on the primary side of the discharge transformer to prove the correct operation of the differential voltage or zero-voltage protection, then the "fuse removal" test is unnecessary. (IV) Test Methods 1. Test Method for Zero-Voltage Protection: (See Figure 1) Explanation: The main equipment in the figure is a three-phase voltage regulating device, three test transformers SB1-3, and three discharge transformers YB1-3. These test transformers need to be custom-made, and the three transformers must have good consistency. The turns ratio is 1000V/57.74V, used as step-up transformers, intended to be compatible with the three-phase relay protection test equipment. This is mainly operated by relay protection personnel. Test Method: The test transformers and discharge transformers are each connected in a three-phase star configuration. A certain amount of positive phase sequence voltage is applied to the primary side of the discharge transformer. The open delta voltage (i.e., the voltage across the zero-voltage protection terminals) in the secondary circuit is checked to see if it is 0V. Changing the voltage of a certain phase to reach the set value (or changing the voltage phase sequence) triggers the protection operation. This directly checks and verifies the protection operation value and the correctness of the primary and secondary circuits of the discharge transformer. (See Figure 2) Please log in to: Power Transmission and Distribution Equipment Network for more information. 2. Test Method for Differential Voltage Protection: Test Wiring Diagram for Differential Voltage Protection: (See Figure 3) Explanation: The main equipment in the diagram is a three-phase voltage regulating device, two test transformers SB1~2, and three discharge transformers YB1~3. The diagram shows the test wiring diagram for a certain phase discharge transformer, such as phase A discharge transformer. Phases B and C are also connected and tested separately. Test Method: Apply a certain amount of in-phase sequence voltage from the high-voltage side of the discharge transformer. The differential voltage (i.e., the differential voltage protection operating voltage) detected in the secondary circuit is close to 0V. Change the voltage on one side until the differential voltage reaches the protection setting value, and the protection operates. This checks and verifies the correctness of the wiring of the primary and secondary circuits of the discharge transformer. (V) Test Procedure Step 1: Overhaul the capacitor bank; Step 2: Disconnect the discharge transformer from the capacitor bank; Step 3: According to the actual capacitor protection principle, use the corresponding test wiring for differential pressure protection or zero-voltage protection as shown in the diagram; Step 4: Apply pressure to verify the correctness of the differential pressure protection or zero-voltage protection. Due to the high test voltage, the discharge transformer and test transformer should be isolated with insulating tape to prevent electric shock. If necessary, please seek guidance from the high-voltage testing team. Step 5: Restore the wiring and check that it is correct and secure. Step 6: During the load test, it is only necessary to measure the unbalanced voltage at the protection installation point and ensure it is within the allowable range. It is not necessary to de-energize the capacitor bank and verify the correctness of the protection wiring by removing the capacitor fuse. (VI) Summary of Application Results In July 2007, during the commissioning test of capacitor banks #1 and #2 at Jiaxing Hecheng Substation after their renovation, the installation of capacitor banks from Shanghai Siyuan Power Co., Ltd., with fuses installed inside the capacitors, made the "fuse removal" test method unsuitable. Instead, a test method involving applying voltage from the primary side of the capacitor discharge transformer was used, which solved the problem easily, was simple and convenient, and ensured test safety. Because this method is indeed safe, simple, and effective, it also provides a better option for commissioning tests of capacitor banks with externally mounted fuses. Since this method is completed before the main equipment is energized, problems in the secondary circuit of the transformer can be detected and addressed in advance, reducing the risk of secondary power outages after problems are discovered during energization. This is a pre-control technical means. For newly commissioned substations, this test method can also be used before commissioning of the transformers to verify the correctness of the primary and secondary wiring of metering transformers, protection transformers, and open delta transformers. Combined with the load test of the secondary circuit after commissioning, this achieves full-process control, reduces work errors, greatly improves work efficiency, and ensures safe equipment operation. References [1] Wang Weijian. Principles and Applications of Relay Protection for Main Electrical Equipment. Second Edition. China Electric Power Press, 1996.1 [2] Tang Tao, Zhu Weinan, Yang Yisong, et al. Automation Technology and Application of Power Plants and Substations. China Electric Power Press, 2005.2