The broadcasting company has two SFP 7-370000/220 type main transformers manufactured by Baoding Transformer Factory. The two transformers were put into operation in October 1999 and February 2000, respectively. After commissioning, both transformers were found to have severe fluctuations in the pointers of the oil flow relays. No other abnormalities were observed during the first two or three months of operation. However, as the operating time increased, the contacts of the oil flow relays gradually developed poor contact, causing frequent switching on and off of the standby cooler, which could easily lead to burnout of the submersible pump motor and fan motor. More seriously, in March 2000, all the parts inside the oil flow relay oil pipe of the third cooler of main transformer No. 1 detached. Four M6×20 screws and their anti-loosening elastic washers were washed away by the oil flow along the oil pipe and disappeared. Although main transformer No. 1 is still in operation, these screws and washers pose a significant safety hazard to its operation. This phenomenon served as a warning, and a thorough inspection of all oil flow relays in both main transformers was immediately conducted. The inspection results were astonishing: all the relay baffles' shafts and bearings were severely worn, with the most severely worn shaft having a quarter of its length worn away. The bearings were worn into oval holes, with the ratio of their major axis to minor axis being 1.5 to 2. The bearings were made of copper or aluminum alloy, and the metal powder from the wear had been washed into the transformer, posing a significant threat to the safe operation of the transformer. Why do the pointers of the oil flow relays on these two main transformers jitter during operation, while other transformers do not exhibit this phenomenon? The power plant conducted the following investigation and analysis: The submersible oil pumps used on the two main transformers are 4B135-7/4V disc-type transformer submersible oil pumps manufactured by Hunan Yuejin Machinery Factory, with a power of 4kW, a speed of 1380r/min, a head of 7m, and a flow rate of 135m³/h. The oil flow relays used are YJ-150/135 manufactured by Shenyang Special Relay Factory, with a nominal pipe diameter of 150mm, a flow rate of 135m³/h, an operating oil flow rate of 101±10%, and a return oil flow rate of 75±10%. The rated flow rates of the submersible oil pumps and the flow rates of the oil flow relays are matched. Inspection of the oil flow relays revealed no abnormalities other than wear, and the relay wear was caused by the vibration of the relay's moving plate. There's no reason to doubt the nameplate values ​​from the two manufacturers, so where does the problem lie? The nameplate values ​​from the relay and oil pump manufacturers should have been tested. The diameter of the connecting pipe provided by the transformer manufacturer is also 150mm. We learned from the transformer manufacturer that the cooler and transformer body were not connected for trial operation when the transformer left the factory. Could it be a problem with the piping? The actual installation situation on site is shown in Figure 1. Figure 1. On-site installation diagram of the connecting pipe. Pump outlet cross-section: 100mm × 100mm = 10000mm², Connecting pipe cross-sectional area: LR² = 3.14 × (150/2)² = 3.14 × 75² = 17662.5mm². Pump outlet velocity (V) = Flow rate (Q) ÷ Cross-section (S) = 135m³/h/1000mm² = 3.75m/s, Connecting pipe velocity (V) = Flow rate (Q) ÷ Cross-section (S) = m³/h/(3600s × 17662.5mm² = 2.1m/s). The oil velocity at the pump outlet is 1.78 times greater than the velocity in the connecting pipe, while the pump outlet is only 300mm from the moving plate of the oil flow relay. The pump outlet velocity cannot immediately decrease to the theoretical velocity calculated in the connecting pipe. The force on the moving plate of the oil flow relay is only related to the velocity, not the flow rate. The actual velocity at the location of the relay's moving plate is greater than the rated velocity required by the oil flow relay. This is the main reason for the vibration during operation of the oil flow relay. How to solve this? Since the transformer and cooler are already installed and positioned, it's impossible to increase the distance between the pump outlet and the oil flow relay. Therefore, the only solution is to work on the oil flow relay itself. Its working principle is as follows: After the oil pump starts, oil flow is generated in the connecting pipe. When the oil flow rate reaches a certain value (operating oil flow rate), the moving plate inside the relay rotates, causing the indicator to rotate synchronously through the magnetic couple force, closing the signal contacts and sending a normal signal. When the oil flow rate decreases to a certain value (return oil flow rate), the moving plate returns, sending a fault signal. The relay moving plate can only be fixed in one position (i.e., the pointer is stationary) when the impact force of the oil on the moving plate and the reaction force of the damping spring are balanced. Let's analyze the working condition of the oil flow relay at a flow rate higher than the rated flow rate, as shown in Figure 2. Figure 2 shows a schematic diagram of the force analysis on the moving plate. When the relay starts, the moving plate is acted upon by the oil flow, driving the rotating shaft. When the relay rotates to the working position, the moving plate, being relatively thin, is essentially unaffected by the oil flow and can therefore be omitted. At this point, only the rudder plate is affected by the oil flow. The force on it equals the oil flow velocity multiplied by the rudder plate area. If the force on the rudder plate is less than the damping on the rotating shaft, the moving plate will rotate downwards; if the force on the rudder plate is greater than the damping on the rotating shaft, the moving plate will rotate upwards. Only when the force on the rudder plate equals the resistance on the rotating shaft can the moving plate stabilize. As the previous analysis shows, when the oil flow velocity at the moving plate is greater than the relay's rated flow velocity, the moving plate will inevitably deflect upwards. When the moving plate deflects to a certain angle φ with the oil flow direction, the moving plate is again impacted by the oil flow, causing it to rotate downwards. This cycle repeats continuously, causing the relay to vibrate and malfunction. In summary, adjusting the damper or reducing the area of ​​the rudder plate in this situation might stabilize the moving plate, but even with the damper adjusted to its maximum, the moving plate still cannot function properly. The rudder plate's function is to stabilize the moving plate and ensure sufficient pressure on the electrical contacts driven by it; therefore, it cannot be eliminated, only reduced. How much reduction is appropriate? Since the flow velocity at the moving plate cannot be accurately measured on-site, a "blindfolded" approach was used to gradually reduce the rudder plate area. After multiple tests, it was confirmed that reducing the rudder plate area to 2/3 of its original size satisfied both the electrical contact pressure and ensured the moving plate remained stable without vibration. The power plant performed the same treatment on all 10 oil flow relays of the two main transformers, and all of them have operated very stably. To date, no relays have experienced any problems, fully achieving the predetermined goals. The plant has also claimed 10 replacement oil flow relays from the manufacturer, making a significant contribution to the safe operation of the two main transformers of the power company.