A start-stop control method for main transformer cooler

Abstract

A method for starting and stopping main transformer coolers is disclosed. A main transformer 1B is equipped with six coolers 1BC to 6BC. Each main transformer cooler corresponds to an "automatic / manual" switching handle, a "start" button, and a "stop" button. Main transformer cooler 1BC corresponds to an "automatic / manual" switching handle 21SA, a "start" button 11S, and a "stop" button 12S. Main transformer cooler 2BC corresponds to an "automatic / manual" switching handle 22SA, a "start" button 21S, and a "stop" button 22S. The same applies to main transformer cooler 3BC and so on, in numerical order. Position nodes of the "automatic / manual" switching handles 21SA-26SA, the "start" buttons 11S-61S, and the "stop" buttons 12S-62S are respectively connected in series to a power supply circuit of main transformer coolers 1BC-6BC via a fifth set of control cables, thereby implementing start and stop control of main transformer coolers 1BC-6BC. The present invention adopts a "dual control mode" of a monitoring system and a programmable controller to achieve precise control of the main transformer cooler. The programmable controller formulates different start and stop control logics according to different main transformer operating modes.

Description

Technical Field

[0001] The present invention belongs to the technical field of main transformer control, and in particular relates to a start-stop control method for a main transformer cooler. Background Art

[0002] Large-capacity main transformers generate significant heat during operation, requiring multiple coolers to dissipate the heat and ensure safe operation at appropriate temperatures. Therefore, the coolers must be activated promptly after the large-capacity main transformer is commissioned. If the transformer cooler is started and stopped without considering the transformer's operating mode, or if the operating mode is not fully considered, or if the operating mode is determined based on inaccurate criteria, operational problems may arise.

[0003] Most power plants consider the operating mode of the main transformer, but often only connect to the position node of the circuit breaker on one side, rather than the position nodes on both the high- and low-voltage sides. For example, when a generator is connected to the main transformer and the plant high-voltage transformer via a generator-side circuit breaker, only the position node of the generator-side circuit breaker (i.e., the breaker on the low-voltage side of the main transformer) is connected. The position node of the circuit breaker on the high-voltage side of the main transformer is not connected. This makes it impossible to distinguish whether the main transformer is operating with system load or plant load. However, the heat generated by the main transformer operating with plant load is significantly lower than that generated with system load. If this distinction is not made, the same number of coolers would be required when operating with plant load as when operating with system load, resulting in waste of plant power and cooling water. Some power plants use the transformer's high-voltage side current as an auxiliary criterion. However, in hydropower plants, when the main transformer is operating with plant load, the high-voltage side current is very low, and this auxiliary criterion cannot distinguish between an outage state and a plant load state. In addition, if only the circuit breaker node is connected but not the isolating switch node, then when the isolating switch is disconnected and the circuit breaker opening and closing test is performed, the main transformer may be mistakenly considered to be in operation after the circuit breaker is closed, resulting in the incorrect start-up of the cooler.

[0004] In existing technologies, monitoring systems are generally connected to circuit breaker and isolating switch position nodes, but the main transformer cooler control system only uses a programmable controller to determine the main transformer operating mode and control the start and stop of the cooler. In order to determine the main transformer operating mode, additional auxiliary cables are required to repeatedly connect the circuit breaker position node to the programmable controller, resulting in a waste of resources.

[0005] Existing technology uses a regular rotation system to ensure that the automatic operating time of each group of coolers is as close as possible. However, if the system shuts down, this regular rotation is interrupted, and then restarted from the beginning upon system startup. Over time, the operating time of the first few groups of coolers often exceeds that of the last few groups, making failure more likely. Moreover, this regular rotation system cannot account for the time a cooler group has been out of operation if it fails or is manually shut down.

[0006] In the existing technology, when the transformer load increases or decreases, usually only two fixed values ​​are selected to increase or decrease the number of coolers put into operation. The influence of the cooler water temperature is not considered. The load can only be controlled according to the long-term allowable load at the highest cooling water temperature, which is not accurate enough. Summary of the Invention

[0007] To address the aforementioned technical issues, the present invention provides a main transformer cooler control system and start-stop control method. This invention utilizes a dual-control approach, combining a monitoring system and a programmable controller, to precisely control the main transformer cooler. The monitoring system utilizes comprehensive collected information to accurately determine the main transformer's operating mode and issue instructions, while the programmable controller develops different start-stop control logic for different main transformer operating modes. Furthermore, a method for controlling the number of operating coolers based on load, cooling water temperature, main transformer oil temperature, and winding temperature, as well as a method for controlling the start, stop, and rotation of coolers using a balanced timing approach, are provided. These methods address existing issues such as low cooler control automation, inaccurate control quantity, uneven control timing, high energy consumption, and malfunctions.

[0008] The technical solution adopted by the present invention is:

[0009] A main transformer cooler control system, the system includes: a generator 1F, a main transformer 1B, a plant high-voltage transformer 1CB, a monitoring system 1MS, and a main transformer cooler control cabinet 1CC;

[0010] Generator 1F is connected to the main transformer low-voltage side circuit breaker 801, which is connected to the main transformer low-voltage side isolation switch 8011, which is connected to the main transformer 1B and the plant high-voltage transformer 1CB respectively;

[0011] The main transformer 1B is connected to the main transformer high-voltage side circuit breaker 851, and the main transformer high-voltage side circuit breaker 851 is connected to the main transformer high-voltage side isolation switch 8511;

[0012] The plant high-voltage transformer 1CB is connected to the plant high-voltage transformer circuit breaker 101;

[0013] The main transformer 1B is equipped with a cooler and a current transformer 1CT on the high-voltage side of the main transformer;

[0014] The monitoring system 1MS is connected to the main transformer high-voltage side isolation switch 8511, the main transformer high-voltage side circuit breaker 851, the plant high-voltage transformer circuit breaker 101, the main transformer low-voltage side isolation switch 8011, and the main transformer low-voltage side circuit breaker 801 respectively;

[0015] The monitoring system 1MS is connected to the programmable controller 1PLC in the main transformer cooler control cabinet 1CC, and the programmable controller 1PLC is respectively connected to the current transformer 1CT on the high-voltage side of the main transformer and the temperature sensor installed in the main transformer 1B;

[0016] The programmable controller 1PLC is respectively connected to a touch screen 1CMP, buttons and a switching handle arranged in a main transformer cooler control cabinet 1CC.

[0017] The main transformer cooler control cabinet 1CC is equipped with a "remote / local" switching handle 1SA, an "automatic / manual" switching handle, a "start" button, and a "stop" button.

[0018] The monitoring system 1MS is connected to the main transformer high-voltage side disconnector 8511, the main transformer high-voltage side circuit breaker 851, the plant high-voltage transformer circuit breaker 101, the main transformer low-voltage side disconnector 8011, and the main transformer low-voltage side circuit breaker 801 via a first set of control cables. Signals such as the position nodes of these components are collected via the first set of control cables.

[0019] The monitoring system 1MS is connected to the programmable controller 1PLC via a communication cable to collect status signals of the programmable controller 1PLC.

[0020] The monitoring system 1MS is connected to the programmable controller 1PLC via a second set of control cables, and sends "turn on the cooler with system" and "turn on the cooler with factory power" instructions to the programmable controller 1PLC;

[0021] The programmable controller 1PLC is connected to the main transformer high-voltage side current transformer 1CT, the oil temperature and winding temperature sensors set on the main transformer 1B, the "remote / local" switching handle 1SA, the "automatic / manual" switching handle, and the water temperature sensor set on the main transformer cooler 1B through the third set of control cables. It is used to collect the current of the main transformer high-voltage side current transformer 1CT, the oil temperature and winding temperature of the main transformer 1B, the position node of the "remote / local" switching handle 1SA, the position node of the "automatic / manual" switching handles 21SA-26SA, and the cooling water temperature status information of the main transformer cooler.

[0022] The programmable controller 1PLC is connected to the touch screen 1CMP via a network cable or a data cable, sends the collected information to the touch screen 1CMP, and receives instructions from the touch screen 1CMP.

[0023] The programmable controller 1PLC is connected to the power supply circuit of the main transformer cooler through the output relay and the fourth group of control cables to realize the start and stop control of the main transformer cooler.

[0024] A method for controlling the start and stop of a main transformer cooler is disclosed. A main transformer 1B is equipped with six coolers, 1BC, 2BC, 3BC, 4BC, 5BC, and 6BC. Each main transformer cooler corresponds to an "auto / manual" switching handle, a "start" button, and a "stop" button. Main transformer cooler 1BC corresponds to an "auto / manual" switching handle 21SA, a "start" button 11S, and a "stop" button 12S. Main transformer cooler 2BC corresponds to an "auto / manual" switching handle 22SA, a "start" button 21S, and a "stop" button 22S. The same applies to main transformer cooler 3BC, etc., in numerical order.

[0025] The position nodes of the "auto / manual" switching handle 21SA-26SA, the "start" button 11S-61S and the "stop" button 12S-62S are respectively connected in series to the power supply circuit of the main transformer cooler 1BC-6BC through the fifth set of control cables to realize the start and stop control of the main transformer cooler 1BC-6BC.

[0026] The start and stop control methods of the main transformer cooler include remote automatic control, local automatic control, and local manual control. Each control mode is distinguished by the "remote / local" switching handle 1SA and "automatic / manual" switching handle 21SA-26SA of the main transformer cooler control cabinet.

[0027] The present invention provides a main transformer cooler control system and a start-stop control method, and the technical effects are as follows:

[0028] 1) A dual-control mode of monitoring system and programmable controller is used to construct the main transformer cooler control system. The monitoring system determines the operating mode of the main transformer, while the programmable controller does not. This can avoid the traditional method of repeatedly installing auxiliary cables to the programmable controller.

[0029] 2) The main transformer operating mode is determined by using all circuit breaker and disconnector position nodes, which provides comprehensive signals and accurate judgment. This avoids the traditional method of using only circuit breaker position nodes, which may cause misjudgment during circuit breaker opening and closing tests after the disconnector is disconnected.

[0030] 3) After the monitoring system predicts the main transformer operating mode in advance, the instructions to the programmable controller are refined, avoiding the traditional method where the programmable controller cannot distinguish or is difficult to accurately distinguish between the "main transformer with system operation mode" and the "main transformer with plant power operation mode". The number of coolers started under the "main transformer with plant power operation mode" is reduced, and coolers with limited lifespans can operate for a longer time, which also helps to reduce the consumption of plant electricity and cooling water.

[0031] 4) The programmable controller continuously records the total operating time of each cooler, whether it is the initial startup, the rotation startup after a period of operation, or the restart and shutdown of the main transformer cooler due to changes in the main transformer load, main transformer oil temperature, and winding temperature. It always prioritizes the startup of one or several main transformer coolers with the shortest total operating time, which makes the operating life management of the cooler more accurate and effectively avoids the imbalance of the total operating time of the cooler caused by the interruption of regular rotation of the cooler or the manual operation of the cooler.

[0032] 5) The current setting on the high-voltage side of the main transformer can be adjusted based on the long-term load allowed by the main transformer at different cooling water temperatures, providing a wider range of settings with greater accuracy. As the main transformer load changes, the number of coolers required for operation can be more accurately controlled, effectively reducing the number of coolers required for operation at low water temperatures. Coolers with limited lifespans can be operated for longer periods of time, which also helps reduce plant power and cooling water consumption. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The present invention will be further described below with reference to the accompanying drawings and examples:

[0034] Figure 1 Schematic diagram of the main transformer cooler control system structure.

[0035] Figure 2 This is the start-up control flow chart of the main transformer coolers 1BC-6BC using remote automatic control.

[0036] Figure 3 This is the logic diagram for determining the main transformer operating mode.

[0037] Figure 4 This is the "Logic for turning on the cooler with system" diagram implemented by the programmable controller 1PLC.

[0038] Figure 5 This is the "Logic diagram for turning on the cooler with factory power" executed by the programmable controller 1PLC.

[0039] Figure 6 This is the layout diagram of the main transformer cooler control cabinet. DETAILED DESCRIPTION

[0040] Taking the typical hydropower station generator-transformer-plant high-voltage transformer group as the object, the main transformer cooler control system is constructed. The system structure includes: main transformer 1B, plant high-voltage transformer 1CB, main transformer high-voltage side circuit breaker 851, main transformer high-voltage side isolation switch 8511, main transformer low-voltage side circuit breaker 801, low-voltage side isolation switch 8011, plant high-voltage transformer circuit breaker 101, main transformer high-voltage side current transformer 1CT, main transformer cooler 1BC-6BC, monitoring system 1MS, main transformer cooler control cabinet 1CC, programmable logic controller 1PLC, touch screen 1CMP, "remote / local" switching handle 1SA, "automatic / manual" switching handle 21SA-26SA, "start" button 11S-61S, "stop" button 12S-62S and other major equipment.

[0041] The monitoring system 1MS adopts the HS9000 system, which consists of the local control unit LCU, plant-level server, touch screen, etc., and usually realizes the monitoring and control of the electrical and mechanical equipment of the entire power station.

[0042] like Figure 1 Figure 2 shows a schematic diagram of a main transformer cooler control system based on the typical generator-transformer-high-voltage transformer group connection scheme at a hydropower station. Generator 1F supplies power to main transformer 1B and high-voltage transformer 1CB via main transformer low-voltage side circuit breaker 801 and main transformer low-voltage side disconnector 8011. After main transformer 1B steps up the voltage, it supplies power to the system load via main transformer low-voltage side circuit breaker 851 and main transformer low-voltage side disconnector 8511. High-voltage transformer 1CB steps down the voltage before supplying power to the plant load via high-voltage transformer circuit breaker 101. Main transformer 1B is equipped with six coolers: 1BC, 2BC, 3BC, 4BC, 5BC, and 6BC.

[0043] like Figure 6 As shown, the programmable controller 1PLC, touch screen 1CMP, "remote / local" switching handle 1SA, "auto / manual" switching handles 21SA-26SA, "start" buttons 11S-61S, "stop" buttons 12S-62S and other devices and their circuits are arranged in the main transformer cooler control cabinet 1CC.

[0044] The monitoring system 1MS collects signals such as the position nodes of the main transformer high-voltage side isolation switch 8511, the main transformer high-voltage side circuit breaker 851, the plant high-voltage transformer circuit breaker 101, the main transformer low-voltage side isolation switch 8011, and the main transformer low-voltage side circuit breaker 801 through the first set of control cables;

[0045] The monitoring system 1MS is connected to the programmable controller 1PLC via a communication cable to collect status signals of the programmable controller 1PLC.

[0046] Monitoring system 1MS can also connect to programmable controller 1PLC via a second set of control cables, issuing commands to it to "activate cooler with system" and "activate cooler with auxiliary power." Through a third set of control cables, programmable controller 1PLC collects status information such as the current in main transformer high-voltage-side current transformer 1CT, the oil and winding temperatures of main transformer 1B, the position node of the "remote / local" switch handle 1SA, the position nodes of the "auto / manual" switch handles 21SA-26SA, and the cooling water temperature of main transformer coolers 1BC-6BC.

[0047] The programmable controller 1PLC is connected to the touch screen 1CMP via a network cable or a data cable, sends the collected information to the touch screen 1CMP, and receives instructions from the touch screen 1CMP.

[0048] The programmable controller is connected to the power supply circuits of the main transformer coolers 1BC-6BC through the output relay and the fourth group of control cables to realize the start and stop control of the main transformer coolers.

[0049] Each main transformer cooler is equipped with an automatic / manual selector lever, a start button, and a stop button. For example, main transformer cooler 1BC has an automatic / manual selector lever 21SA, a start button 11S, and a stop button 12S. Main transformer cooler 2BC has an automatic / manual selector lever 22SA, a start button 21S, and a stop button 22S. The same applies to main transformer cooler 3BC, and so on. The position nodes of automatic / manual selector levers 21SA-26SA, start buttons 11S-61S, and stop buttons 12S-62S are connected to the power circuits of main transformer coolers 1BC-6BC via a fifth set of control cables, enabling start and stop control of the main transformer coolers.

[0050] Main transformer cooler start and stop control methods include remote automatic control, local automatic control, and local manual control. Each control mode is distinguished by the "remote / local" switch handle 1SA and the "automatic / manual" switch handles 21SA-26SA on the main transformer cooler control cabinet.

[0051] (I): The remote automatic control mode is applicable to the operation of the main transformer cooler when the relevant equipment is normal. Only when the "remote / local" handle 1SA is switched to the "remote" position, all coolers 1BC-6BC with the "automatic / manual" handles 21SA-26SA switched to the "automatic" position are controlled, and the operator issues the "open cooler" or "close all coolers" command on the monitoring system 1MS: After the "open cooler" is issued, the monitoring system 1MS determines whether the transformer 1B is operating in the system mode, the factory power mode or the shutdown mode, and sends the command to the programmable controller 1PLC to open the cooler with the system or with the factory power. The programmable controller 1PLC starts different numbers of coolers according to the corresponding logic based on different start-up instructions from the monitoring system 1MS. The number of coolers started is determined by the operating mode of the main transformer and the cooling water temperature. After the coolers are started, they are rotated on and off according to the balanced timing control method. At the same time, the number of coolers put into operation is increased or decreased according to the changes in the main transformer load, main transformer oil temperature and winding temperature. After the "all coolers closed" instruction is issued, the monitoring system 1MS sends a all cooler close signal to the programmable controller 1PLC, and the programmable controller 1PLC shuts down the running coolers in turn.

[0052] Remote automatic control mode for the main transformer cooler 1BC-6BC start control as follows Figure 2 As shown, the steps include:

[0053] Step 1: The operator performs remote automatic start of the main transformer cooler in the monitoring system 1MS;

[0054] Step 2: The monitoring system 1MS collects position signals of the main transformer high-voltage side circuit breaker 851, the main transformer high-voltage side isolation switch 8511, the main transformer low-voltage side circuit breaker 801, the main transformer low-voltage side isolation switch 8011, and the plant high-voltage transformer circuit breaker 101 related to the main transformer 1B;

[0055] Step 3: The monitoring system 1MS executes the main transformer operation mode discrimination logic to determine the main transformer operation mode, including operation with system load, operation with plant power load, and main transformer shutdown.

[0056] In the above step 3, the main transformer operation mode judgment logic is as follows Figure 3 As shown, the details are as follows:

[0057] ①: When the main transformer high-voltage side circuit breaker 851, the main transformer high-voltage side isolation switch 8511, the main transformer low-voltage side circuit breaker 801, the main transformer low-voltage side isolation switch 8011, and the plant high-voltage transformer circuit breaker 101 are all in the closed position, the monitoring system 1MS determines that the main transformer 1B is operating in the system load mode.

[0058] ②: When the main transformer high-voltage side circuit breaker 851, the main transformer high-voltage side disconnector 8511, and the plant high-voltage transformer circuit breaker 101 are all in the closed position, and the position node of the main transformer low-voltage side circuit breaker 801 or the main transformer low-voltage side disconnector 8011 is in the open position, the monitoring system 1MS determines that the main transformer 1B is operating in the main transformer with plant power load.

[0059] ③: When the main transformer high-voltage side circuit breaker 851 or the main transformer high-voltage side isolation switch 8511 is in the open position, the main transformer is in the main transformer shutdown mode.

[0060] Step 4: The monitoring system 1MS issues a corresponding cooler start instruction based on the current operating mode of the main transformer 1B. After the operator confirms, the monitoring system 1MS issues a corresponding cooler start instruction to the programmable controller 1PLC in the main transformer cooler control cabinet.

[0061] In the above step 4, when the monitoring system 1MS determines that the main transformer 1B is in the main transformer belt system load mode, it issues a "belt system turn on cooler" prompt. After the operator confirms, the monitoring system 1MS issues a "belt system turn on cooler" instruction to the programmable controller 1PLC.

[0062] In the above step 4, when the monitoring system 1MS determines that the main transformer 1B is in the main transformer load mode with plant power, it issues a prompt "Turn on cooler with plant power". After the operator confirms, the monitoring system 1MS sends a "Turn on cooler with plant power" instruction to the programmable controller 1PLC.

[0063] In step 4 above, when monitoring system 1MS determines that main transformer 1B is in shutdown mode, it simultaneously displays the prompts "Enable cooler with system," "Enable cooler with auxiliary power," and "Main transformer shutdown, cancel operation." If the operator selects "Enable cooler with system," monitoring system 1MS issues the "Enable cooler with system" command to programmable controller 1PLC. If the operator selects "Enable cooler with auxiliary power," monitoring system 1MS issues the "Enable cooler with auxiliary power" command to programmable controller 1PLC. If the operator selects "Enable cooler with auxiliary power," monitoring system 1MS issues the "Enable cooler with auxiliary power" command to programmable controller 1PLC. If the operator selects "Enable cooler with auxiliary power," monitoring system 1MS does not issue the command, and the operation terminates.

[0064] Step 5: Programmable controller 1PLC receives the command from monitoring system 1MS and the position of remote / local switch handle 1SA. Based on the position of remote / local switch handle 1SA, programmable controller 1PLC determines whether the cooler control system is in remote or local control mode.

[0065] In the above step 5, when the "remote / local" handle 1SA is in the "remote" position, the current control mode is the remote control mode, allowing the monitoring system 1MS to remotely control the cooler through the programmable controller 1PLC.

[0066] In the above step 5, when the "remote / local" handle 1SA is in the "local" position, the current control mode is the local control mode, and all control instructions sent to the programmable controller 1PLC by the "local" position node locking monitoring system 1MS are terminated.

[0067] Step 6: Based on the above steps 4 and 5, when the programmable controller 1PLC receives the "Start cooler with system" command from the monitoring system 1MS and determines that the current control mode is remote control, it will execute according to the "Start cooler with system" logic, automatically start the corresponding cooler, and automatically start and stop the cooler according to relevant criteria. When the programmable controller 1PLC receives the "Start cooler with plant power" command from the monitoring system 1MS and determines that the current cooler control mode is remote control, it will execute according to the "Start cooler with plant power" logic, automatically start the corresponding cooler, and automatically start and stop the cooler according to relevant criteria.

[0068] In step 6 above, the programmable controller 1PLC executes the "with system open cooler logic" as follows Figure 4 As shown, the details are as follows:

[0069] Step 6.1: The programmable controller 1PLC receives the "start the cooler with the system" instruction from the monitoring system 1MS and executes the logic of starting the cooler with the system;

[0070] Step 6.2: The programmable controller 1PLC reads the initial load current I0. The initial load current I0 can be set as a cooler control parameter via the touch screen 1CMP. The value of the initial load current I0 can be selected based on the most common load conditions after the main transformer is put into operation. For example, set I0 = 0.5Ie, where Ie is the rated current of the main transformer high-voltage side.

[0071] Step 6.3: The programmable controller 1 (PLC) collects the inlet and outlet temperatures of each cooler to determine the current cooling water temperature, Ts. This example uses a forced oil circulation water-cooled cooler. To prevent blockage in the cooler pipes, the cooling water flow is typically rerouted regularly. Therefore, a water temperature sensor is installed at each end of each cooler, for a total of 12 water temperature sensors. The current cooling water temperature, Ts, is equal to the average of the temperatures collected by these 12 water temperature sensors.

[0072] Step 6.4: The programmable controller 1PLC determines the number of coolers to be turned on, n, based on the "initial load current I0" and "cooling water temperature Ts," and by referring to the cooling efficiency comparison table. The cooling efficiency comparison table is usually provided by the main transformer cooler manufacturer based on relevant tests.

[0073] For example, Table 1 is a cooling efficiency comparison table for a main transformer with a rated capacity of 880 MVA. If the initial load current I0 = 0.5Ie, then the initial load is S0 = 0.5Sn = 0.5*880 = 440 MVA. If the current cooling water temperature Ts is calculated to be 22°C, referencing the table below, first determine if the current cooling water temperature is between 20°C and 25°C. To ensure safety, use the column with the larger value of 25°C as a reference for the main transformer's allowable long-term load. The current initial load of 440 MVA is between 380 MVA and 635 MVA. To ensure safety, use the row corresponding to the larger value of 635 MVA, thus determining the number of coolers to be activated, n = 3.

[0074] Table 1 Comparison of cooling efficiency of a main transformer with a rated capacity of 880MVA

[0075]

[0076] Step 6.5: The programmable controller 1PLC collects the status of each cooler and selects the n coolers with the least total operating time from the coolers with "auto / manual" switch handles 21SA-26SA in the "auto" state and no faults as the first group of coolers to be put into operation. Based on the above steps, the programmable controller 1PLC needs to continuously record the total operating time of the six coolers. For example, the total operating time of 1BC, 2BC, 3BC, 4BC, 5BC, and 6BC is 16800 hours, 16700 hours, 16850 hours, 16900 hours, 16600 hours, and 16780 hours, respectively. Of the six coolers, 1BC, 2BC, 3BC, 4BC, and 5BC are in the automatic state, and 6BC is in the manual state. The number of coolers in operation, n=3, then the three coolers with the least total operating time are selected as the first group of coolers to be put into operation, including 1BC, 2BC, and 5BC.

[0077] Step 6.6: PLC 1 sends start signals to the first group of coolers, 1BC, 2BC, and 5BC, in ascending order of cooler number. This signals are then switched on at the corresponding output nodes of PLC 1. This signaling is separated by a time interval of T1 to prevent simultaneous start-up of multiple coolers from causing abnormal oil flow in the main transformer and erroneous signal transmission. T1 can be set as a cooler control parameter using the touch screen 1CMP; a typical setting is 10 seconds.

[0078] Step 6.7: The programmable controller 1PLC output nodes are turned on in sequence, so that the power circuits of the first group of coolers: 1BC, 2BC and 5BC are turned on in sequence, and coolers 1BC, 2BC and 5BC are started in sequence.

[0079] Step 6.8: After the first group of coolers 1BC, 2BC, and 5BC are started, the programmable controller 1PLC will shut down one active cooler and start a standby cooler every T2 hours, using a balanced timing control method. T2 is the main cooler rotation period and can be set as a cooler control parameter via the touch screen 1CMP. It is typically set to one week, with T2 = 168 hours. After 168 hours, the total operating hours of 1BC, 2BC, 3BC, 4BC, 5BC, and 6BC will be 16968 hours, 16868 hours, 16850 hours, 16900 hours, 16768 hours, and 16780 hours, respectively. If 6BC returns to automatic mode, it should also be included in the comparison. The three coolers with the least total operating hours are 3BC, 5BC, and 6BC. Then, return to Step 6.6 and start coolers 3BC, 5BC, and 6BC in sequence.

[0080] Step 6.9: Based on step 6.7, after the first group of coolers is started, the programmable controller 1PLC continuously collects the main transformer high-voltage side current I1 provided by the main transformer high-voltage side current transformer 1CT.

[0081] Step 6.10: Based on step 6.9, according to the change of the high-voltage side current I1 of the main transformer, when the current set value is reached, the programmable controller 1PLC determines the number of coolers to be opened m2 based on the "high-voltage side current I1 of the main transformer" and the "cooling water temperature Ts", referring to the cooling efficiency comparison table. The current set value can be set as a control parameter through the touch screen 1CMP. Referring to the main transformer cooling efficiency comparison table, the current set value can be adjusted according to the allowable long-term load of the main transformer corresponding to different cooling water temperatures. Here, the cooling water temperature is 22°C, and the allowable long-term loads of the two main transformers corresponding to the cooling water temperature of 25°C are adjusted to 760MVA and 790MVA, that is:

[0082] Is1=760 / 880*Ie=0.86Ie;

[0083] Is2=790 / 880*Ie=0.89Ie;

[0084] When the current I1 on the high-voltage side of the main transformer rises and exceeds Is1, m2=4; when I1 continues to rise and exceeds Is2, m2=5.

[0085] When the high-voltage side current I1 of the main transformer drops from 0.89Ie to below Is2, m2=4; when I1 continues to drop below Is1, m2=3.

[0086] If the cooling water temperature is only 9°C and the initial load current is 0.5Ie, that is, the initial load is 440MVA, then the cooling water temperature can be 10°C as a reference, n=2, and as the current I1 on the high-voltage side of the main transformer increases, the three main transformers will have three allowable long-term loads of 720MVA, 810MVA, and 890MVA, which is more accurate than traditional control methods.

[0087] Step 6.11: Based on step 6.10, as the high-voltage side current I1 of the main transformer increases, delay T3 to set n=m2. As current I1 decreases, delay T4 to set n=m2. Then, return to step 6.6 and sequentially start the n coolers with the least total operating time. Delays T3 and T4 can be set as control parameters via the touch screen 1CMP. Typically, T3 = 60 seconds and T4 = 600 seconds are used to prevent frequent cooler starts and stops due to load fluctuations.

[0088] Step 6.12: Based on step 6.7, after the first group of coolers is started, the programmable controller 1PLC continuously collects the main transformer oil temperature Ty and winding temperature Tr.

[0089] Step 6.13: Based on Step 6.12, and based on the changes in the main transformer oil and winding temperatures, when the set temperature values ​​are reached, the programmable controller 1PLC determines the number of coolers required for operation as m1. The set temperature values ​​can be set as control parameters via the touch screen 1CMP. The set temperature values ​​corresponding to the main transformer oil temperature are typically Ty1 = 55°C and Ty2 = 60°C. The set temperature values ​​corresponding to the main transformer winding temperature are typically Tr1 = 70°C and Tr2 = 75°C. When the main transformer oil temperature Ty rises from below Ty1 to exceed Ty1, m1 = n+1; if Ty continues to rise and exceeds Ty2, m1 = n+2. When the main transformer oil temperature Ty drops from above Ty2 to exceed Ty2, m1 = n-1; if Ty continues to drop and exceeds Ty1, m1 = n-2. When the main transformer winding temperature Tr rises from below Tr1 to exceed Tr1, m1 = n+1; if Tr continues to rise and exceeds Tr2, m1 = n+2. When the main transformer winding temperature Tr drops from greater than Tr2 to exceed Tr2, m1 = n-1; when Tr continues to drop and exceeds Tr1, m1 = n-2. The value of m1 is not greater than the total number of coolers, 6.

[0090] Step 6.14: Based on the changes in the main transformer oil temperature and winding temperature in Step 6.13, set a delay of T5 to set n = m1. Then, return to Step 6.6 and sequentially start the n coolers with the least total operating time. The delay T5 can be set as a control parameter via the touchscreen 1CMP. Typically, T5 = 10 seconds is used to prevent frequent cooler starts and stops due to temperature fluctuations.

[0091] In step 6 above, the logic of "starting the cooler with factory power" executed by the programmable controller 1PLC is as follows: Figure 5 As shown, the details are as follows:

[0092] Step 6-1: The programmable controller 1PLC receives the "turn on the cooler with the system" instruction from the monitoring system 1MS and executes the logic of turning on the cooler with the factory power;

[0093] Step 6-2: The programmable controller 1 (PLC) collects the inlet and outlet temperatures of each cooler to determine the current cooling water temperature, Ts. This example uses a forced oil circulation water-cooled cooler. To prevent blockage in the cooler pipes, the cooling water flow is typically rerouted regularly. Therefore, a water temperature sensor is installed at each end of each cooler, for a total of 12 water temperature sensors. The current cooling water temperature, Ts, is the average of the temperatures collected by these 12 water temperature sensors.

[0094] Step 6-3: The programmable controller 1PLC determines the number of coolers to be activated, n, based on the "auxiliary power load current Ic" and "cooling water temperature Ts," referring to the cooling efficiency comparison table. The cooling efficiency comparison table is usually provided by the main transformer cooler manufacturer based on relevant tests.

[0095] Usually, the power load of a hydropower station is very low, far less than the 190MVA corresponding to a cooling water temperature of 25°C, so the number of coolers to be opened can be determined to be n=1.

[0096] Step 6-4: The programmable controller 1PLC collects the status of each cooler and selects the cooler with the least total operating time from the coolers with "auto / manual" switch handles 21SA-26SA in the "auto" state and no faults as the first group of coolers to be put into operation. Based on the above steps, the programmable controller 1PLC needs to continuously record the total operating time of the six coolers. For example, the total operating time of 1BC, 2BC, 3BC, 4BC, 5BC, and 6BC is 16800 hours, 16700 hours, 16850 hours, 16900 hours, 16600 hours, and 16780 hours, respectively. Among the six coolers, 1BC, 2BC, 3BC, 4BC, and 5BC are in the automatic state, and 6BC is in the manual state. The number of coolers turned on, n=1, then the cooler with the least total operating time is selected as the first group of coolers to be put into operation, that is, 5BC.

[0097] Step 6-5: The PLC 1 sends start signals to the first group of coolers (5BC) in ascending order of cooler number. This signals are then switched on at the corresponding output nodes of the PLC 1. This signaling is separated by a time interval of T1 to prevent simultaneous start-up of multiple coolers from causing abnormal oil flow in the main transformer and erroneous signal transmission. T1 can be set as a cooler control parameter via the touch screen 1CMP, typically 10 seconds. When n = 1, no delay is required and the system starts immediately.

[0098] Step 6-6: The programmable controller 1PLC turns on the output nodes in sequence, turning on the power supply circuits of the first group of coolers 5BC in sequence, and the coolers 5BC start;

[0099] Step 6-7: After the first group of coolers 5BC starts, the programmable controller 1PLC rotates the coolers every T2 hours using a balanced timing control method. T2 is the main transformer cooler rotation cycle and can be set as a cooler control parameter via the touch screen 1CMP. It is typically set to one week, with T2 = 168 hours. After 168 hours, the total operating hours of 1BC, 2BC, 3BC, 4BC, 5BC, and 6BC will become 16,800 hours, 16,700 hours, 16,850 hours, 16,900 hours, 16,768 hours, and 16,780 hours, respectively. At this point, 1BC-6BC will all return to automatic mode, with 5BC having the least total operating time. The process then returns to step 6-6, starting cooler 5BC in sequence. Since cooler 5BC is already operating, it only needs to maintain operation. Step 6-8: Based on step 6-7, after the first group of coolers starts, the programmable controller 1PLC continuously collects the main transformer oil temperature Ty and winding temperature Tr.

[0100] Step 6-9: Based on step 6-8 and the changes in the main transformer oil and winding temperatures, when the set temperature values ​​are reached, the programmable controller 1PLC determines the number of coolers required for operation as m1. The set temperature values ​​can be set as control parameters via the touch screen 1CMP. The set temperature values ​​corresponding to the main transformer oil temperature are typically Ty1 = 55°C and Ty2 = 60°C. The set temperature values ​​corresponding to the main transformer winding temperature are typically Tr1 = 70°C and Tr2 = 75°C. When the main transformer oil temperature Ty rises from below Ty1 to exceed Ty1, m1 = n+1; if Ty continues to rise and exceeds Ty2, m1 = n+2. When the main transformer oil temperature Ty drops from above Ty2 to exceed Ty2, m1 = n-1; if Ty continues to drop and exceeds Ty1, m1 = n-2. When the main transformer winding temperature Tr rises from below Tr1 to exceed Tr1, m1 = n+1; if Tr continues to rise and exceeds Tr2, m1 = n+2. When the main transformer winding temperature Tr drops from greater than Tr2 to exceed Tr2, m1 = n-1; when Tr continues to drop and exceeds Tr1, m1 = n-2. The value of m1 is not greater than the total number of coolers, 6.

[0101] Step 6-10: Based on the changes in the main transformer oil temperature and winding temperature in step 6-9, set a delay of T5 to set n = m1. Then, return to step 6-6 and sequentially start the n coolers with the least total operating time. The delay T5 can be set as a control parameter via the touch screen 1CMP. Typically, T5 = 10 seconds is used to prevent frequent cooler starts and stops due to temperature fluctuations.

[0102] The remote automatic control method for fully closing the main transformer cooler includes the following steps:

[0103] S1: The operator issues the "close all coolers" command in the monitoring system 1MS;

[0104] S2: The monitoring system 1MS sends a "full cooler off" signal to the programmable controller 1PLC;

[0105] S3: The programmable controller (1PLC) identifies the running coolers and issues stop commands to each cooler in ascending order: 1BC, 2BC, 3BC, 4BC, 5BC, and 6BC. These commands are separated by a time interval of Tt to prevent simultaneous stop of multiple coolers, which could cause excessive changes in the main transformer's oil flow and cause erroneous signaling. Tt can be set as a control parameter via the touch screen (1CMP); typically, Tt = 10 seconds.

[0106] (II) The local automatic control mode is applicable to the operation of the main transformer cooler in special circumstances such as abnormal communication between the monitoring system and the programmable controller or local testing. Only when the "remote / local" handle 1SA is switched to the "local" position, all coolers 1BC-6BC with the "automatic / manual" handles 21SA-26SA switched to the "automatic" position are controlled. The monitoring system 1MS command is not accepted. The operator issues the "turn on cooler with system", "turn on cooler with factory power", or "turn off coolers" command to the programmable controller 1PLC through the touch screen 1CMP. The programmable controller executes according to the same logic as the remote automatic control mode.

[0107] The local automatic control method to open the cooler includes the following steps:

[0108] A1: The operator switches the "remote / local" handle 1SA to the "local" position;

[0109] A2: The operator determines whether the main transformer is currently operating in system load mode or auxiliary power load mode, and issues a command on the touch screen 1CMP to start the cooler with system load or auxiliary power load.

[0110] A3: The touch screen 1CMP instruction is sent to the programmable controller 1PLC via communication;

[0111] A4: After the programmable controller 1PLC receives the command to start the cooler with the system, it executes the logic of starting the cooler with the system. This logic is the same as the logic of starting the cooler with the system in the remote automatic control mode, such as Figure 4 As shown, the corresponding cooler is turned on; after the programmable controller receives the factory power start cooler instruction, it executes the factory power start cooler logic, which is the same as the factory power start cooler logic of the remote automatic control mode, such as Figure 5 As shown, turn on the corresponding cooler.

[0112] The local automatic control method to fully shut down the cooler includes the following steps:

[0113] B1: The operator switches the "remote / local" handle 1SA to the "local" position;

[0114] B2: The operator issues a command to completely shut down the main transformer cooler on the touch screen 1CMP;

[0115] B3: Touch screen 1CMP sends the cooler full-close signal to programmable controller 1PLC;

[0116] B4: The programmable controller (1PLC) identifies the running coolers and issues stop commands to each cooler in ascending order of cooler number. These commands are separated by a time interval of Tt to prevent simultaneous stop of multiple coolers, which could cause excessive changes in the main transformer's oil flow and cause erroneous signaling. Tt can be set as a control parameter via the touch screen (1CMP); typically, Tt = 10 seconds.

[0117] (III) On-site manual control is applicable to the operation of the main transformer cooler in extreme special circumstances such as PLC failure, cooler failure, on-site testing, etc. Only after the "Auto / Manual" handle 21SA-26SA is switched to the "Manual" position, the operator can start or stop the cooler in manual state.

[0118] The on-site manual control method to open the cooler includes the following steps:

[0119] a1: The operator determines the cooler to be turned on and switches the "Auto / Manual" handle of the cooler to the "Manual" position. For example, to turn on cooler 1BC, the operator switches the "Auto / Manual" handle 21SA of cooler 1BC to the "Manual" position;

[0120] a2: The operator presses the "start" button 11S of the cooler 1BC;

[0121] a3: The normally open node of the "start" button 11S connected in series in the power supply circuit of the cooler 1BC is turned on, and the cooler 1BC is powered on.

[0122] Stopping the cooler by manual control on site includes the following steps:

[0123] b1: The operator determines the cooler to be stopped and switches the "Auto / Manual" handle of the cooler to the "Manual" position. For example, to stop cooler 1BC, switch the "Auto / Manual" handle 21SA of cooler 1BC to the "Manual" position;

[0124] b2: The operator presses the "stop" button 12S of the cooler 1BC;

[0125] b3: The normally closed node of the "stop" button 12S connected in series in the power supply circuit of cooler 1BC is disconnected, and cooler 1BC loses power and stops operating.

[0126] The present invention provides a main transformer cooler control system with the following features:

[0127] (1): A control system is constructed using a dual-control mode of a monitoring system and a programmable controller. The monitoring system determines the operating mode of the main transformer and sends instructions to the programmable controller to start and stop the cooler. The programmable controller does not participate in the determination of the main transformer operating mode, but only determines the operating status of the main transformer and its cooler. Based on the instructions of the monitoring system and the operating status of the main transformer and its cooler, it starts and stops the cooler according to a certain control logic, thus achieving precise control of the cooler under different main transformer operating modes.

[0128] (2): The monitoring system remotely controls the main transformer cooler based on the main transformer operating mode. During remote control, the monitoring system determines whether the main transformer is "operating with system load", "operating with plant power load" or "exiting operation" based on the positions of the circuit breakers and their isolation switches on each side of the main transformer and the plant high-voltage transformer. Then, the monitoring system sends corresponding instructions to the programmable controller of the main transformer cooler control system - "start cooler with system load", "start cooler with plant power" or "stop cooler". According to different instructions, the programmable controller executes "start cooler with system load", "start cooler with plant power" and "stop cooler" logic respectively, starting the corresponding number of the first group of coolers or stopping all coolers.

[0129] (3): Referring to the cooling efficiency comparison table of the main transformer cooler, the programmable controller determines the initial number of coolers to be started first according to the monitoring system instructions and the cooling water temperature. If the monitoring system instruction is "turn on the cooler with the system", it corresponds to the main transformer operating with the system load. At this time, the corresponding load is the "initial load". Referring to the cooling efficiency comparison table of the main transformer cooler, the number of coolers required for the long-term operation of the main transformer corresponding to the "initial load" and the current cooling water temperature is taken as the initial number of coolers for the first group of coolers. If the monitoring system instruction is "turn on the cooler with the plant power", it corresponds to the main transformer operating with the plant power load. At this time, the corresponding load is the "plant power load". The "plant power load" is usually very small. Referring to the cooling efficiency comparison table of the main transformer cooler, the number of coolers required for the long-term operation of the main transformer corresponding to the "plant power load" and the current cooling water temperature is taken as the initial number of coolers for the first group of coolers. This method optimizes the number of operating main transformer coolers to achieve the purpose of energy saving and consumption reduction.

[0130] (4): The operating status of the main transformer and its cooler identified by the programmable controller includes the main transformer load size, main transformer oil temperature, main transformer winding temperature, operating cooler, out-of-service cooler, faulty cooler, cooling water temperature, etc. When "operating with system load", the "system control logic" is executed according to the monitoring system instructions. First, the first group of initial number of coolers are put into operation. Then, when the main transformer load increases to a certain value, the main transformer oil temperature increases to a certain value, or the main transformer winding temperature increases to a certain value, the number of coolers to be put into operation is changed. When "operating with plant power load", the "plant power control logic" is executed according to the monitoring system instructions. First, the first group of initial number of coolers are put into operation. Regardless of the main transformer load size, only the influence of the main transformer oil temperature and winding temperature is considered. The number of coolers to be put into operation is changed as the oil temperature and winding temperature increase or decrease, so as to achieve precise control of the cooler under different main transformer working conditions.

[0131] (5) The current setting value on the high-voltage side of the main transformer can be adjusted according to the long-term load allowed by the main transformer corresponding to different cooling water temperatures. The number of setting values ​​is larger, and the control of the number of coolers put into operation due to the change of main transformer load is more accurate. At low water temperatures, the setting value has more steps, which can effectively reduce the number of coolers put into operation and improve control efficiency.

[0132] (6): The programmable controller implements the start and stop and rotation control of the coolers in a balanced timing manner. The programmable controller continuously records the total operating time of each cooler. Under the premise of ensuring that the number of operating coolers meets the requirements of the main transformer operating load, oil temperature, winding temperature, etc., after a certain period of operating time, one or several main transformer coolers with the least total operating time are started first, which can effectively ensure that the total operating time of each cooler is similar.

Claims

1. Remote automatic control method for starting the main transformer cooler, characterized by The following steps are involved: Step 1: The operator performs remote automatic start of the main transformer cooler in the monitoring system 1MS; Step 2: The monitoring system 1MS collects position signals of the main transformer high-voltage side circuit breaker 851, the main transformer high-voltage side isolation switch 8511, the main transformer low-voltage side circuit breaker 801, the main transformer low-voltage side isolation switch 8011, and the plant high-voltage transformer circuit breaker 101 related to the main transformer 1B; Step 3: The monitoring system 1MS executes the main transformer operation mode discrimination logic to determine the main transformer operation mode, including operation with system load, operation with plant power load, and main transformer shutdown. Step 4: The monitoring system 1MS issues a corresponding cooler start instruction based on the current operating mode of the main transformer 1B. After the operator confirms, the monitoring system 1MS issues a corresponding cooler start instruction to the programmable controller 1PLC in the main transformer cooler control cabinet. Step 5: The programmable controller 1PLC receives the command from the monitoring system 1MS and the position status of the "remote / local" switch handle 1SA. The programmable controller 1PLC determines whether the control mode of the cooler control system is remote control or local control based on the position of the "remote / local" switch handle 1SA. Step 6: Based on the above steps 4 and 5, when the programmable controller 1PLC receives the "turn on the cooler with the system" instruction from the monitoring system 1MS and determines that the current control mode is the remote control mode, it executes according to the "turn on the cooler with the system" logic, automatically starts the corresponding cooler, and automatically starts and stops the cooler according to relevant criteria; when the programmable controller 1PLC receives the "turn on the cooler with the plant power" instruction from the monitoring system 1MS and determines that the current cooler control mode is the remote control mode, it executes according to the "turn on the cooler with the plant power" logic, automatically starts the corresponding cooler, and automatically starts and stops the cooler according to relevant criteria.

2. The remote automatic control method for starting a main transformer cooler according to claim 1 is characterized in that: In step 3, the logic for determining the main transformer operating mode is as follows: ①: When the main transformer high-voltage side circuit breaker 851, the main transformer high-voltage side isolation switch 8511, the main transformer low-voltage side circuit breaker 801, the main transformer low-voltage side isolation switch 8011, and the plant high-voltage transformer circuit breaker 101 are all in the closed position, the monitoring system 1MS determines that the main transformer 1B is operating with system load; ②: When the main transformer high-voltage side circuit breaker 851, the main transformer high-voltage side isolation switch 8511, and the auxiliary transformer high-voltage side circuit breaker 101 are all in the closed position, and the main transformer low-voltage side circuit breaker 801 or the main transformer low-voltage side isolation switch 8011 is in the open position, the monitoring system 1MS determines that the main transformer 1B is operating in the auxiliary power load mode; ③: When the main transformer high-voltage side circuit breaker 851 or the main transformer high-voltage side isolation switch 8511 is in the open position, the main transformer is in the main transformer shutdown mode.

3. The remote automatic control method for starting a main transformer cooler according to claim 1 is characterized in that: In step 4, When the monitoring system 1MS determines that the main transformer 1B is in the main transformer with system load mode, it issues a "with system to turn on the cooler" prompt. After the operator confirms, the monitoring system 1MS sends a "with system to turn on the cooler" command to the programmable controller 1PLC. When the monitoring system 1MS determines that the main transformer 1B is in the auxiliary power load mode, it issues a "turn on cooler with auxiliary power" prompt. After the operator confirms, the monitoring system 1MS sends a "turn on cooler with auxiliary power" command to the programmable controller 1PLC. When the monitoring system 1MS determines that the main transformer 1B is in the main transformer shutdown mode, it simultaneously gives the prompts of "turn on the cooler with the system", "turn on the cooler with the auxiliary power" and "the main transformer is shut down, cancel the operation"; if the operator chooses "turn on the cooler with the system", the monitoring system 1MS sends the "turn on the cooler with the system" instruction to the programmable controller 1PLC; if the operator chooses "turn on the cooler with the auxiliary power", the monitoring system 1MS sends the "turn on the cooler with the auxiliary power" instruction to the programmable controller 1PLC; if the operator chooses "turn on the cooler with the auxiliary power", the monitoring system 1MS sends the "turn on the cooler with the auxiliary power" instruction to the programmable controller 1PLC; if the operator chooses "turn on the cooler with the auxiliary power", the monitoring system 1MS does not send the instruction and the operation is terminated.

4. The remote automatic control method for starting a main transformer cooler according to claim 1 is characterized in that: In step 5, When the "remote / local" handle 1SA is in the "remote" position, the current control mode is remote control mode, allowing the monitoring system 1MS to remotely control the cooler through the programmable controller 1PLC; When the "remote / local" handle 1SA is in the "local" position, the current control mode is the local control mode. All control instructions sent to the programmable controller 1PLC by the monitoring system 1MS are locked through the "local" position node of the handle, and the programmable controller operation is terminated.

5. The remote automatic control method for starting a main transformer cooler according to claim 1 is characterized in that: Step 6 includes the following steps: Step 6.1: The programmable controller 1PLC receives the "Start the cooler with the system" command from the monitoring system 1MS and executes the logic for starting the cooler with the system; Step 6.2: The programmable controller 1PLC reads the initial load current I0; Step 6.3: The programmable controller 1PLC collects the inlet and outlet temperatures of the cooling water of each cooler and determines the current cooling water temperature Ts; Step 6.4: The programmable controller 1PLC determines the number of coolers to be turned on n based on the "initial load current I0" and "cooling water temperature Ts" and the cooling efficiency comparison table; Step 6.5: The programmable controller 1PLC collects the status of each cooler and selects the n coolers with the least total operating time from the coolers with "Auto / Manual" switch handles 21SA-26SA in the "Auto" state and no faults as the first group of coolers to be put into operation; Step 6.6: The programmable controller 1PLC sends an open signal to the first group of coolers in ascending order of the cooler numbers, that is, the corresponding programmable controller 1PLC output nodes are turned on, and the interval between the signals is T1; Step 6.7: The output nodes of the programmable controller 1PLC are turned on in sequence, so that the power circuits of the first group of coolers are turned on in sequence, and the first group of coolers are started in sequence; Step 6.8: After the first group of coolers starts, the programmable controller 1PLC uses a balanced timing control method to shut down one operating cooler and start one standby cooler every time it runs for T2. T2 is the main transformer cooler rotation cycle, and the three coolers with the least total running time are selected. Then, return to step 6.6 and start the coolers in sequence. Step 6.9: Based on step 6.7, after the first group of coolers is started, the programmable controller 1PLC continuously collects the main transformer high-voltage side current I1 provided by the main transformer high-voltage side current transformer 1CT; Step 6.10: Based on step 6.9, when the current I1 on the high-voltage side of the main transformer reaches its set value, the programmable controller 1 PLC determines the number of coolers to be opened (m2) based on the "main transformer high-voltage side current I1" and the "cooling water temperature Ts" and the cooling efficiency comparison table. Step 6.11: Based on step 6.10, during the rising process of the high-voltage side current I1 of the main transformer, delay T3 to make n=m2; During the falling process of current I1, delay T4 to make n=m2; Then return to step 6.6 and start the n coolers with the least total running time in sequence; Step 6.12: Based on step 6.7, after the first group of coolers is started, the programmable controller 1PLC continuously collects the main transformer oil temperature Ty and winding temperature Tr; Step 6.13: Based on step 6.12, according to the changes in the main transformer oil temperature and the winding temperature, when the temperature reaches the set value, the programmable controller 1PLC determines that the number of coolers to be put into operation is m1; When the main transformer oil temperature Ty rises from less than Ty1 to exceed Ty1, m1=n+1; when Ty continues to rise and exceeds Ty2, m1=n+2; when the main transformer oil temperature Ty drops from greater than Ty2 to exceed Ty2, m1=n-1; when Ty continues to drop and exceed Ty1, m1=n-2; when the main transformer winding temperature Tr rises from less than Tr1 to exceed Tr1, m1=n+1; when Tr continues to rise and exceed Tr2, m1=n+2; when the main transformer winding temperature Tr drops from greater than Tr2 to exceed Tr2, m1=n-1; when Tr continues to drop and exceed Tr1, m1=n-2; the value of m1 shall not be greater than the total number of coolers, 6; Step 6.14: Based on the changes in the main transformer oil temperature and winding temperature in step 6.13, delay T5 to make n=m1; Then return to step 6.6 and start the n coolers with the shortest total operating time in sequence.

6. The method of opening / closing the cooler in an on-site automatic control mode is characterized by: The local automatic control method to open the cooler includes the following steps: A1: Switch the "remote / local" handle 1SA to the "local" position; A2: Determine whether the main transformer is currently operating in system load mode or auxiliary power load mode, and issue a command on the touch screen 1CMP to start the cooler with the system or with auxiliary power; A3: The touch screen 1CMP instruction is sent to the programmable controller 1PLC via communication; A4: After receiving the command to start the cooler with the system, the PLC 1 executes the logic for starting the cooler with the system and turns on the corresponding cooler. After receiving the command to start the cooler with the auxiliary power supply, the PLC 1 executes the logic for starting the cooler with the auxiliary power supply and turns on the corresponding cooler. The local automatic control method to fully shut down the cooler includes the following steps: B1: Switch the "remote / local" handle 1SA to the "local" position; B2: Issue the command to fully shut down the main transformer cooler on the touch screen 1CMP; B3: Touch screen 1CMP sends the cooler full-close signal to programmable controller 1PLC; B4: The programmable controller 1PLC identifies the running coolers and issues stop commands to the coolers in ascending order of cooler numbers. The commands are issued at intervals of Tt.

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