I. Introduction The ZN28A-12 indoor high-voltage vacuum circuit breaker is a domestically designed product. This product is small in size, simple and practical in structure, highly standardized and serialized, requires little maintenance, is easy to repair, has a long service life, and a large breaking current capacity, with a maximum current capacity of 5000A and a short-circuit breaking capacity of 63kA. This product is mainly suitable for fixed switchgear and is currently the most widely used vacuum circuit breaker in China. The ZN28A-12 vacuum circuit breaker can be used with many domestically produced fixed switchgear, such as GG-1A and XGN2-10. Since this circuit breaker has the same installation dimensions and position as the SN10 oil-minimum switch, the operating mechanism, current transformer, secondary circuit, and protection circuit do not need to be modified during retrofitting. Therefore, this product is particularly suitable for the oil-free retrofitting of previously manufactured GG-1A and XGN2-10 switchgear with SN10 oil-minimum switch. II. Modification Process Step 1: Disassemble the oil-minimum circuit breaker and install the ZN28A-12 vacuum circuit breaker. First, disassemble the original oil-minimum circuit breaker SN10-10, and then install the ZN28A-12 vacuum circuit breaker. Since the SN10-10 oil-minimum circuit breaker and the ZN28A-12 vacuum circuit breaker have the same installation dimensions and positions, the ZN28A-12 vacuum circuit breaker can be directly installed. If a different model of vacuum circuit breaker is used, the mounting bracket must be re-welded and the installation position of the current transformer inside the cabinet must be changed. Furthermore, due to their larger size, other models of vacuum circuit breakers generally cannot be directly installed through the front or rear door of the cabinet; the circuit breaker must be disassembled, installed inside the cabinet, and then reassembled. Step 2: Remove the original electromagnetic mechanism CD10 and install the CT19B operating mechanism. After removing the original electromagnetic mechanism CD10, install the CT19B operating mechanism in the original position where the electromagnetic mechanism CD10 was installed. Because the CT19B and CD10 are structurally completely different, new mounting holes and secondary wiring holes must be drilled. Due to the requirements of the CT19B spring mechanism's energy storage circuit, one intermediate relay, one energy storage signal indicator and its switching switch, and one double-pole protection air switch for the energy storage circuit power supply must be installed in the instrument room and on the instrument door. The intermediate relay and air switch are installed in the instrument room; the energy storage signal indicator and its switching switch are installed on the instrument door. Since the mounting holes for the signal indicator and switching switch are Φ25mm round holes, a special drilling tool is required to drill these holes on the instrument door to ensure a clean and aesthetically pleasing surface. Step 3: Secondary Wiring After completing all the above installation steps, the next step is to perform secondary wiring. Because the CT19B spring mechanism differs from the CD10 electromagnetic mechanism, and this mechanism has a spring energy storage circuit, the secondary circuit needs to be modified. The specific secondary circuit schematic diagram is as follows: Wiring of the opening and closing circuits and auxiliary nodes follows the substation wiring diagram. The power supply for the spring energy storage circuit uses the original closing power supply of the CD10 electromagnetic mechanism. The closing circuit fuse RL1-60/35A, originally installed inside the electric door under the operating mechanism, will no longer be usable. There are two reasons for this: First, due to the small power of the energy storage motor and its insufficient energy storage current (less than 1A), a 35A fuse is insufficient for protection. Second, after this modification, the electric door of the switchgear will be sealed off by the CT19B operating mechanism. If fuse RL1-60/35A blows, replacing the fuse core will require de-energizing the equipment before entering the cabinet. Therefore, a double-pole air switch GM32M-2308R will be installed in the instrument room to replace its protective function. Fourth step: Busbar fabrication and installation. After installing the vacuum circuit breaker, while the secondary wiring personnel are performing secondary wiring, the primary assembly technicians will fabricate the three-phase busbars according to the actual situation (facilitating all busbars in the same cabinet simultaneously), and apply different colored heat-shrinkable insulation material to phases A, B, and C. Then, the primary busbars can be installed, with fabrication and installation strictly following the process requirements. Step 5: Connecting the Transmission Part and Crank Arms. After completing all the above work, the next step is to connect the transmission part and crank arms. The most crucial step in this work is determining the angles of the two crank arms on the horizontal transmission shaft (the mechanism power output crank arm and the transmission shaft drive crank arm). If the angles are incorrect, it will cause great difficulties in the subsequent debugging work, and may even make it impossible to debug successfully. The work on the linkage part is also the debugging work. The goal of the debugging work is to ensure that the opening distance between the moving and stationary contacts of the circuit breaker is 11±1mm, and the spring compression stroke (also known as the contact stroke) is 4±1mm. Step 6: Cleaning and Sweeping the Site. After the installation and replacement are completed, and after five manual and electric no-load operations without errors, remove any metal and other debris left on the site, and conduct a final inspection. Step 7: Assisting the User in Acceptance According to Standards. After completing all the above modification work, the user's repair and testing department will strictly conduct acceptance according to standards. The main acceptance items include: power frequency withstand voltage test (circuit breaker contact and ground power frequency test), contact resistance, and switch action characteristic test, etc. After acceptance, the final work begins: installing the aluminum supports for the vacuum switch and the insulation protection boxes at the busbar connections. This completes the oil-free conversion of one switchgear unit. III. Conversion Time for ZN28A-12 Vacuum Circuit Breaker Generally, the entire conversion process for one unit requires 4 hours for low current and 5 hours for high current (calculated based on 4 people converting one unit simultaneously). IV. Conclusion From the above analysis, it can be seen that using the ZN28A-12 vacuum circuit breaker to convert the GG-1A switchgear, compared to using other models of vacuum circuit breakers, saves conversion costs, shortens power outage time, and has significant economic benefits. Furthermore, the converted cabinet is aesthetically pleasing, and the surface coating is not damaged due to welding of the mounting brackets. Therefore, this product is currently the most ideal product for oil-free conversion.