Abstract: The ZN28A vacuum circuit breaker is specifically designed to replace the SN10 oil-minimum circuit breaker, making replacement very convenient. Its frame structure and installation dimensions are identical to the SN10 oil-minimum circuit breaker, and the positions of the upper and lower outgoing line aluminum busbars are also compatible. This circuit breaker also boasts advantages such as long service life, simple maintenance, no explosion hazard, no dirt accumulation, and low noise, and is suitable for demanding working conditions such as frequent operation. Keywords : Wanyao Power Station switchgear technical renovation 1 Introduction The Wanyao Power Station in Jiangshan City was put into operation in May 1997, using a two-machine, one-transformer wiring configuration. The station has three SN10 oil-minimum circuit breakers. For several years after commissioning, serious potential faults have existed, manifested as overheating and oil leakage in one phase of the conductive circuit. Although considerable effort and time have been spent each year addressing these problems, they have not been fundamentally resolved. In October 2001, we decided to upgrade three switchgear units, replacing the oil circuit breakers with ZN28A vacuum circuit breakers. 2. Problems with Oil Circuit Breakers 2.1 Severe Overheating of the Conductive Circuit of the Oil Circuit Breaker The three oil circuit breakers at this station were manufactured during a period when counterfeit and substandard products flooded the market, resulting in poor product quality. The 35kV outgoing line switch was identified as a substandard product during commissioning by the power department and ordered to be replaced within a specified period. After commissioning, severe overheating was found in phase B of switch number one and phase C of switch number two, with the insulating oil turning black. After a power outage, the insulating cylinder was also found to be hot, but disassembly revealed that the contact burning was not severe. Measurements of contact travel, three-phase synchronicity, and conductive circuit contact resistance were all within acceptable ranges. Despite multiple treatments and oil changes, the overheating situation did not improve. To monitor operating temperature, two alcohol thermometers were attached to the outside of the insulation cylinders of two phases of each oil circuit breaker. It was observed that the temperature of the normal phase was only slightly higher than room temperature, while the temperature of the heating phase was more than 10 degrees Celsius higher. Under these operating conditions, the biggest concern is that a fault could cause the circuit breaker to trip under load, seriously threatening operational safety. 2.2 Severe Oil Leakage Another common problem with low-oil circuit breakers is oil leakage. The crank arm, drain screw, and oil level indicator are difficult points to stop leakage. The most difficult to handle is the oil leakage at the junction of the insulation cylinder and the upper and lower outgoing line seats. Epoxy resin sealing, paint, and sealant were all ineffective; the circuit breaker couldn't be disassembled. The result was that the problem persisted year after year, polluting the operating environment, creating safety hazards, and affecting the standardized management of the power station. Considering the problems existing in the oil circuit breakers of our station's switchgear, in October 2001, we began the oil-free transformation of the switchgear. 3. Upgrade Plan Considering the upgrade costs, site availability, and workload, a "core replacement" approach was adopted. This involves retaining the original switchgear cabinet and replacing the oil circuit breakers with vacuum circuit breakers. After consulting with multiple switchgear manufacturers, replacing oil circuit breakers with vacuum circuit breakers is technically feasible. 3.1 Vacuum Circuit Breaker Selection The 35kV switchgear model at this station is GBC-35-212, and the 6kV switchgear model is GG-1AF7-DA. Based on the switchgear manufacturer's information, the ZN28A type vacuum circuit breaker is the most suitable. The ZN28A type vacuum circuit breaker is a dedicated circuit breaker for fixed switchgear. Its main body does not have an operating mechanism; it is generally used in conjunction with a CD10 type electromagnetic operating mechanism or a CT10 spring operating mechanism. The CD10 type operating mechanism of the 6KV switchgear in this station is a good fit. However, the original CD type operating mechanism of the 35KV handcart-type switchgear was not suitable for the ZN28A-35 type vacuum circuit breaker, so the operating mechanism was replaced with a CT10 type spring operating mechanism. The original oil circuit breaker had a rated current of 1000A, while the selected vacuum circuit breaker has a rated current of 1250A, increasing both its rated current and short-circuit breaking current, thus improving operational reliability. 3.2 Structural Features The ZN28A type vacuum circuit breaker is specifically designed to replace the SN10 type oil-minimum circuit breaker, making replacement very convenient. Its frame structure and installation dimensions are exactly the same as the SN10 type oil-minimum circuit breaker, and the positions of the upper and lower outgoing line aluminum busbars also match. This circuit breaker also has advantages such as long service life, simple maintenance, no explosion hazard, no dirt, and low noise, and is suitable for harsh working conditions such as frequent operation. 3.3 Arc-extinguishing principle of ZN28A vacuum circuit breaker The ZN28A vacuum circuit breaker is equipped with a center-sealed longitudinal magnetic field vacuum interrupter, and its arc-extinguishing principle differs significantly from that of oil circuit breakers. When the moving and stationary contacts are energized and opened under the action of the operating mechanism, the arc burns in the vacuum between the contacts. Due to the special structure of the contacts, an appropriate longitudinal magnetic field is generated in the contact gap during arcing. This magnetic field allows the arc to be evenly distributed on the contact surface, maintaining a low arc voltage. It also enables the vacuum interrupter to have a high post-arc dielectric strength recovery rate, low arc energy, and low electro-corrosion rate, thereby improving the circuit breaker's ability to interrupt short-circuit current and the contact electrical life. 4. Switchgear Modification Implementation Process 4.1 Pre-installation Inspection of Vacuum Circuit Breakers After unpacking the vacuum circuit breaker upon arrival at the site, the following items should be carefully inspected and maintained: 1) Check the arc-extinguishing chamber for cracks and leaks, and for internal oxidation; 2) Check for damage to components; 3) Check for loose fasteners and electrical connections, and implement anti-loosening measures; 4) Check for clearly marked grounding points; 5) Check that moving parts are flexible and free from jamming, and apply a small amount of lubricating oil to moving components and friction surfaces; 6) Clean dust and dirt from the overall surface; 7) Conduct a power frequency withstand voltage test. 4.2 Removing the Oil Circuit Breaker Disassemble the upper and lower conductive busbars, disconnect the vertical connecting rod and fixing bolts between the oil circuit breaker and the operating mechanism, and remove the oil circuit breaker from the switchgear. 4.3 Installing the Vacuum Circuit Breaker Carefully move the vacuum circuit breaker into the switchgear, adjust its position on the frame, and fix it to the frame by passing four M16 bolts through the frame mounting holes. Visually inspect or use a level to check that the circuit breaker is level. With the operating mechanism in the open position, and using the closed position of its auxiliary switch's delayed normally closed contact as a reference, connect the mechanism's transmission system to the portion of the main shaft extending from the frame side plate using tapered pins. Manually test opening and closing the circuit breaker. After the opening and closing status is generally normal, connect the upper and lower conductive busbars (sometimes requiring modification and machining). Note that when installing the busbars, ensure good contact at the overlapping surfaces (apply conductive grease), and that the busbars should not exert pushing, pulling, or torsional forces on the circuit breaker body. 4.4 Adjustment Test The adjustment test includes: manually opening and closing the vacuum circuit breaker, measuring the contact distance and contact stroke, measuring the opening and closing speeds, measuring the three-phase asynchrony, and the contact closing bounce time, etc. 4.4.1 Adjusting the Contact Distance Technical requirements for contact distance: 11 ±1mm for 6kV circuit breakers, and [missing information] for 35kV circuit breakers. If these requirements are not met, adjust the buffer by adding or removing adjustment pieces. 4.4.2 Adjusting the Contact Travel: After closing the circuit breaker, measure the contact travel, then open it and adjust the contact travel. Technical requirements for contact travel: 4 ±1mm for 6kV circuit breakers, and [missing information - likely a specific value] for 35kV circuit breakers. If these requirements are not met, adjust the adjusting bolt at the insulating tie rod. The bolt pitch is 1125mm, so turning the bolt out half a turn will increase the contact travel by 0.625mm, and vice versa. Simultaneous changes in the three-phase contact travel can be achieved by adjusting the vertical guide rod of the operating mechanism; this adjustment will not change the contact opening distance. 4.4.3 Adjusting the Three-Phase Asynchronous Contact Travel: After adjustment, operate the circuit breaker with power on and measure the asynchronous opening and closing of the three phases using a light oscilloscope or characteristic tester. The technical requirement for the asynchronous opening and closing of the three phases is ≤2ms. Actual measurements show 0.45ms for 6kV circuit breakers and 0.8ms for 35kV circuit breakers. If these requirements are not met, adjust the contact travel of each phase contact separately. During adjustment, ensure that the threaded portion of the insulating tie rod and vertical guide rod is screwed into the nut to a depth of at least 15mm. 4.4.4 Measure the average opening and closing speeds using a light-emitting oscilloscope or a switch dynamic characteristic tester. Technical requirements: For 6kV circuit breakers, the closing speed is 0.4–0.8 mPs, and the opening speed is 1 ± 0.3 mPs; for 35kV circuit breakers, the closing speed is 0.8 ± 0.2 mPs, and the opening speed is 2.0 ± 0.3 mPs. If these requirements are not met, the length of the opening spring can be adjusted. Note that tightening the opening spring increases the opening speed and shortens the opening time, but simultaneously reduces the closing speed and prolongs the closing time, and vice versa. 4.4.5 Measure the contact bounce time: Use a switch dynamic characteristic tester to measure the circuit breaker's closing bounce time. For 6kV circuit breakers, it should be ≤2ms; for 35kV circuit breakers, it should be ≤3ms. 4.4.6 Measure the DC resistance of the conductive circuit: Measure the DC resistance of the conductive circuit. Technical requirement: ≤50μΩ for both 6kV and 35kV circuit breakers. 4.4.7 Adjust the secondary circuit: After replacing the circuit breaker, the original secondary circuit will have some changes, and the position of the auxiliary switch will also change. Adjust the linkage and crank arm on the auxiliary switch to ensure the circuit breaker's opening, closing, and signal indication circuits work normally and accurately. Note that there should be no dead points in the linkage and crank arm during adjustment. 4.4.8 Adjust the arc contact fingers: The arc contact fingers connecting the trolley-type 35kV circuit breaker to the cabinet's conductive busbar will have significant changes after the circuit breaker replacement. Careful adjustment is required to ensure that all six arc contact fingers of the three phases are accurately positioned. 4.4.9 Conduct power frequency withstand voltage tests: Perform phase-to-phase and phase-to-ground withstand voltage tests in the closed state, and perform inter-break AC withstand voltage tests in the open state. Technical requirements: 42kV P/min for 6kV circuit breakers; 95kV P/min for 35kV circuit breakers. 4.5 Operational Tests: 1) High-voltage operational tests (5 times): Closing 110%Ue, Opening 120%Ue; 2) Low-voltage operational tests (5 times): Closing 85%Ue, Opening 65%Ue; 3) Rated voltage manual and electric operation (5 times each); 4) Manual slow opening and slow closing; check the operation of all components of the circuit breaker to ensure there is no jamming; Electric opening and closing should be flexible and reliable. 5 Commissioning and Maintenance Once the vacuum circuit breaker has passed installation and commissioning, it can be put into normal operation. Although normally operating vacuum circuit breakers are stable and reliable, they should still undergo necessary maintenance inspections or tests annually. The items include: 1) Thorough cleaning of the exterior and lubrication of transmission components; 2) Checking for loose screws in all parts of the body and operating mechanism; 3) Checking and adjusting the closing and operating circuits for accuracy; 4) Measuring mechanical characteristics: contact stroke, contact distance, synchronicity, opening and closing speed, and opening and closing time; 5) Measuring the DC resistance of the conductive circuit; 6) Measuring the insulation resistance of the primary and secondary circuits; 7) Conducting an AC withstand voltage test at power frequency to verify the vacuum level of the vacuum interrupter; 8) Checking the contact burn-out thickness; 9) Performing a complete opening and closing operation test. 6. Conclusion This switchgear technical upgrade, due to thorough argumentation, meticulous work, and comprehensive preparation, cost only over 80,000 yuan for the three switches, and the construction period was only 3 days, with significant upgrade results. For more than two years since the switchgear was upgraded, the vacuum circuit breaker has been operating safely and reliably, greatly reducing the workload of maintenance personnel and significantly improving the equipment environment.