A transformer through-flow capacity adjusting method based on adaptive control

Abstract

The application discloses a transformer through-flow capacity adjusting method based on adaptive control, which comprises the following steps: collecting transformer operation parameters, environment parameters and power grid operation state parameters, and calculating a comprehensive state index; obtaining rated capacity and cooling capacity parameters, and calculating a target through-flow capacity value; judging whether the target through-flow capacity value meets a series resonance compensation starting condition; continuously monitoring a transformer thermal load state index during a series resonance compensation capacity promotion mode operation process; and triggering a switching control mode and capacity reduction protection when the thermal load state index exceeds a preset safety value or a resonance loop reaches a preset detuning condition. The application adjusts the through-flow capacity according to the thermal load state, effectively solves the safety hidden danger, and significantly improves the adaptability and control precision of capacity adjustment under different working conditions.

Description

Technical Field

[0001] This invention relates to the field of power system testing, and more specifically, to a method for adjusting transformer current carrying capacity based on adaptive control. Background Technology

[0002] After the construction or technical renovation of a substation is completed, in accordance with national standards such as the "Installation and Acceptance Specification for Relay Protection and Secondary Circuits" (GB / T 50976-2014), it is necessary to conduct load tests on primary equipment and secondary protection devices to verify the correctness of parameters such as polarity, phase sequence, and transformation ratio. In recent years, the industry has proposed off-grid load testing technology. This technology applies simulated current and voltage to the primary conductor using external testing equipment before the substation is put into operation, completing the vector relationship verification of CT and PT. It can effectively avoid the dependence of traditional grid-connected testing on the grid operation mode, thereby reducing the grid safety risks during the testing process.

[0003] In this off-grid load testing technology system, transformer current-carrying simulated load testing is a key component. Existing transformer current-carrying simulated load testing methods employ series resonance technology. By connecting a compensation capacitor in series with the transformer coil and bringing it close to resonance, a large test current can be generated in the transformer coil with relatively low input power. This significantly reduces the requirements for the test power supply capacity, enabling the test equipment to generate a large test current with a small power supply. However, existing technologies have the following shortcomings in practical applications: Existing current-carrying test methods using series resonance lack the ability to sense the real-time thermal state of the transformer. During the current-carrying test, the heat generated by the transformer coils can cause the temperature of hot spots in the windings to rise. When the test current is large or the duration is long, it may cause insulation overheating or even damage. However, existing methods rely solely on human experience to determine the upper limit of the test current, and cannot dynamically adjust the current-carrying capacity according to the actual thermal load of the transformer, posing a safety hazard. Summary of the Invention

[0004] To overcome the aforementioned deficiencies of the prior art, this invention provides a transformer current-carrying capacity regulation method based on adaptive control, which can significantly improve the adaptability and control accuracy of capacity regulation under different operating conditions.

[0005] This invention provides the following technical solution: a transformer current carrying capacity adjustment method based on adaptive control, comprising at least the following steps: S1. Collect transformer operating parameters, environmental parameters and power grid operating status parameters, and calculate the comprehensive status index based on multi-dimensional parameters that reflect at least the thermal state and electrical stress state of the transformer; S2. Obtain the rated capacity and cooling capacity parameters, and calculate the target flow capacity value based on the comprehensive state index; S3. Determine whether the series resonance compensation start-up condition is met based on the target current capacity value; S4. If satisfied, the series resonant compensation capacity enhancement mode is executed: by constructing a resonant circuit in series between the compensation capacitor and the transformer coil, and adjusting the resonant circuit to the series resonant state, the target current capacity value requirement is achieved. S5. If not satisfied, execute the normal capacity adjustment mode: based on the target current capacity value and coil current, execute capacity adjustment control and correct the capacity adjustment parameters in real time; S6. During the operation of the series resonant compensation capacity enhancement mode, the transformer thermal load status index is continuously monitored; when the thermal load status index exceeds the preset safety value or the resonant circuit reaches the preset detuning condition, the switching control mode and capacity reduction protection are triggered.

[0006] Furthermore, the process of collecting transformer operating parameters, environmental parameters, and power grid operating status parameters to calculate the comprehensive status index includes: S11. Collect transformer operating parameters and environmental parameters, and calculate the heat load state index; wherein, the transformer operating parameters include winding hot spot temperature, oil temperature, actual operating number and rated number of cooling fans; the environmental parameters include ambient temperature; S12. Collect power grid operating status parameters and load change parameters, and calculate the operating stress index; S13. Obtain the actual current capacity and rated capacity, calculate the average ratio of the actual current capacity to the rated capacity over the past N sampling periods, and obtain the historical capacity utilization rate. S14. Calculate the product of the thermal load state index and the operating stress index to obtain the coupling pressure value; select the larger value between the thermal load state index and the operating stress index as the dominant pressure value; add the coupling pressure value and the dominant pressure value and divide by two to obtain the base pressure value. S15. The basic pressure value and the historical capacity utilization rate are weighted and summed to obtain the comprehensive state index.

[0007] Furthermore, the calculation method for the heat load state index includes: The hot spot temperature index is obtained by calculating the ratio of the winding hot spot temperature to the maximum allowable temperature. Divide the difference between the oil temperature and the ambient temperature by the maximum design temperature rise to obtain the oil temperature load index; Calculate the cooling capacity correction factor based on the ratio of the actual number of cooling fans in operation to the rated number; The hot spot temperature index and the oil temperature load index are weighted and summed, and then multiplied by the cooling capacity correction factor to obtain the heat load state index.

[0008] Furthermore, the calculation method for the operating stress index includes: Calculate the ratio of the current coil current to the rated current to obtain the current load index; The voltage disturbance index is obtained by calculating the ratio of the voltage fluctuation amplitude to the upper limit of the allowable fluctuation. Calculate the absolute difference between the load power factor and the rated power factor, and normalize it to obtain the power deviation index; The operating stress index is obtained by weighting and combining the current load index, voltage disturbance index, and power deviation index.

[0009] Furthermore, the process of obtaining the rated capacity and cooling capacity parameters, and calculating the target flow capacity value based on the comprehensive state index, includes: S21. Obtain rated capacity and cooling capacity parameters, wherein the cooling capacity parameters include ambient temperature and design ambient temperature limit, calculate the ratio of the ambient temperature to the design ambient temperature limit, and obtain the environmental limitation factor. S22. Calculate the difference between the comprehensive state index and the preset safety threshold to obtain the capacity adjustment deviation value; S23. Multiply the capacity adjustment deviation value by a preset adjustment coefficient to obtain the capacity correction coefficient; S24. Multiply the difference between 1 and the capacity correction factor by the rated capacity to obtain the corrected capacity value; S25. Multiply the corrected capacity value by the environmental limiting factor to obtain the target flow capacity value.

[0010] Furthermore, the series resonance compensation activation conditions include any one or more of the following: The target current capacity exceeds the upper limit that can be safely output in normal capacity regulation mode under the rated active power of the current input power supply; The overall condition index is lower than the preset hot start threshold, and the current carrying capacity exceeds 1.2 times the rated current within a preset short period of time; Received an external test mode command indicating that a large current is output from a low-power supply to perform coil or insulation testing.

[0011] Furthermore, the series resonant compensation capacity enhancement mode includes: S41. Construct a resonant circuit in series between the compensation capacitor and the transformer coil, and apply an AC test voltage to the resonant circuit; S42. Collect the coil current, capacitor voltage, coil voltage, and input power parameters in the resonant circuit. S43. Calculate the resonant offset direction and residual reactive power based on the phase difference between the coil current and the voltage across the capacitor. S44. Adjust the compensation capacitor capacity or input frequency according to the resonant offset direction to adjust the resonant circuit to a series resonant state. S45. Adjust the input voltage according to the residual reactive power and the coil current so that the coil current reaches or exceeds the upper limit of the current corresponding to the target current capacity value, while maintaining the input power within the set range.

[0012] Furthermore, the conventional capacity adjustment mode includes: S51. Obtain the current coil current and the upper limit of the current corresponding to the target current capacity value, and compare the current coil current with the upper limit of the current corresponding to the target current capacity value. S52. When the coil current exceeds the upper limit of the current, a capacity reduction control command is generated and sent to the on-load tap changer or load distribution device. S53. When the coil current is lower than the upper limit of the current and the comprehensive state index is lower than the preset safety threshold, a capacity increase control command is generated. S54. Record the temperature and current changes after each capacity regulation control, obtain the recording results, and correct the capacity regulation parameters based on the recording results.

[0013] Furthermore, the adjustment of the compensation capacitor capacitance or input frequency according to the resonant offset direction includes: S441. When the resonant offset direction is inductive, reduce the compensation capacitor capacitance or increase the input frequency. S442. When the resonant offset direction is capacitive, increase the compensation capacitor capacitance or decrease the input frequency. S443. Gradually adjust the compensation parameters and monitor the rate of change of coil current and input power in real time; wherein, the compensation parameters refer to the compensation capacitor capacity and the input frequency; S444. When the absolute value of the phase difference is less than the set threshold and the power factor is greater than 0.95, it is determined that the series resonance state has been reached. S445. Lock the current compensation capacitor capacity and input frequency as the resonant operating point under the comprehensive state index, and store them in the parameter library; S446. When the change in the comprehensive status index exceeds the set range, S441 to S445 will be automatically re-executed.

[0014] Further, S6 includes: S61. During the operation of the series resonant compensation capacity enhancement mode, continuously monitor the thermal load state index, winding hot spot temperature and temperature rise rate, and resonant offset. S62. Real-time determination of whether any of the following triggering conditions are met: The heat load condition index exceeds the preset safety value; The rate of temperature rise of the winding hot spot exceeds the preset rate threshold. The resonance offset exceeds the preset detuning threshold, and two consecutive attempts to adjust it through step S44 fail to restore it to within the preset detuning threshold within the set time. S63. When any of the triggering conditions in S62 is met, the switching control mode and derating protection are immediately triggered; the derating protection includes at least: reducing the input voltage, disconnecting the resonant circuit, switching the control mode to the conventional capacity adjustment mode, and forcibly setting the derating protection value. S64. After implementing the derating protection, continue to monitor the thermal load state index and the resonant circuit state parameters. When the thermal load state index and the resonant circuit state parameters meet all preset release conditions, release the protection status.

[0015] Implementing this invention has the following beneficial effects: This invention provides a transformer current-carrying capacity adjustment method based on adaptive control. It calculates a thermal load state index by collecting data on winding hot spot temperature, oil temperature, number of operating cooling fans, and ambient temperature; simultaneously, it collects data on coil current, voltage fluctuation amplitude, load power factor, and rate of change to calculate an operating stress index; and finally, it weights and fuses these two data with historical capacity utilization to obtain a comprehensive state index. This comprehensive state index fully reflects the transformer's thermal safety margin, electrical stress level, and historical load trends, eliminating reliance on manual experience in capacity adjustment decisions and preventing the risk of insulation damage due to overheating from the outset.

[0016] This invention automatically determines whether the series resonant compensation start-up conditions are met based on the comprehensive state index and the target current capacity value, and intelligently selects either the conventional capacity adjustment mode or the series resonant compensation capacity enhancement mode. The conventional mode is suitable for the normal current range, with smooth and stable adjustment; the series resonant compensation mode achieves high current output with low power input, significantly reducing the requirements for the test power supply capacity. The seamless switching between the two modes ensures both economical operation under normal conditions and meets the high current requirements under special conditions.

[0017] To address the problems of existing series resonant circuit parameter adjustments relying on manual experience, being time-consuming, and struggling to accurately match the optimal resonant point, this invention automatically determines the direction of resonant offset (inductive or capacitive offset) by real-time acquisition of the phase difference between the coil current and the voltage across the capacitor. Based on this, it adjusts the compensation capacitor capacitance or input frequency, enabling the resonant circuit to quickly converge to a series resonant state. After locking the resonant operating point, it stores it in a parameter library. When the overall state index changes beyond a set range, it automatically re-optimizes, achieving automatic tracking and closed-loop control of the resonant parameters, significantly improving resonant efficiency and adjustment accuracy.

[0018] To address the problem that existing capacity regulation control parameters are fixed values ​​and cannot adapt to changes in operating conditions, this invention records the temperature changes (winding hotspot temperature changes and temperature rise rates) after each capacity regulation control in the conventional capacity regulation mode, and adjusts the capacity regulation parameters in real time based on the recorded results. This achieves online adaptive optimization of the capacity regulation parameters, significantly improving the adaptability and control accuracy of capacity regulation under different operating conditions. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, obtaining other drawings based on these drawings without creative effort still falls within the scope of the present invention. Figure 1 This is a schematic diagram of the main flow of an embodiment of a transformer current-carrying capacity adjustment method based on adaptive control provided by the present invention. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] like Figure 1 The diagram shows the main flow chart of an embodiment of a transformer current-carrying capacity regulation method based on adaptive control provided by the present invention. In this embodiment, the method includes at least the following steps: S1. Collect transformer operating parameters, environmental parameters and power grid operating status parameters, and calculate the comprehensive status index based on multi-dimensional parameters that reflect at least the thermal state and electrical stress state of the transformer; S2. Obtain the rated capacity and cooling capacity parameters, and calculate the target flow capacity value based on the comprehensive state index; S3. Determine whether the series resonance compensation start-up condition is met based on the target current capacity value; S4. If satisfied, the series resonant compensation capacity enhancement mode is executed: by constructing a resonant circuit in series between the compensation capacitor and the transformer coil, and adjusting the resonant circuit to the series resonant state, the target current capacity value requirement is achieved. S5. If not satisfied, execute the normal capacity adjustment mode: based on the target current capacity value and coil current, execute capacity adjustment control and correct the capacity adjustment parameters in real time; S6 continuously monitors the transformer thermal load status index during the operation of the series resonant compensation capacity enhancement mode; when the thermal load status index exceeds the preset safety value or the resonant circuit reaches the preset detuning condition, it triggers the switching control mode and capacity reduction protection.

[0022] The following will provide a detailed explanation of each step with specific examples.

[0023] Step S1: Collect transformer operating parameters, environmental parameters, and power grid operating status parameters, and calculate the comprehensive status index. Specifically, this includes: S11. Collect transformer operating parameters and environmental parameters, and calculate the heat load state index; wherein, the transformer operating parameters include winding hot spot temperature, oil temperature, actual operating number and rated number of cooling fans; the environmental parameters include ambient temperature, etc.

[0024] The calculation method for the heat load state index includes: The hot spot temperature index is obtained by calculating the ratio of the winding hot spot temperature to the design maximum allowable temperature; the oil temperature load index is obtained by dividing the difference between the oil temperature and the ambient temperature by the design maximum temperature rise; the cooling capacity correction coefficient is calculated based on the ratio of the actual number of cooling fans in operation to the rated number; the hot spot temperature index and the oil temperature load index are weighted and summed, and multiplied by the cooling capacity correction coefficient to obtain the heat load state index; the typical weight of the hot spot temperature index is 0.6, and the typical weight of the oil temperature load index is 0.4, which can be adjusted according to the actual situation. For every 6-8℃ increase in the winding hot spot temperature, the insulation life is significantly reduced, therefore the hot spot temperature index should be given a higher weight, and the sum of the weights is 1.

[0025] The maximum allowable design temperature is a fixed design parameter for the transformer. Specifically, it refers to the highest permissible hot spot temperature of the windings due to the transformer's insulation materials (e.g., Class A, Class B, or Class F insulation). For example, it is typically 105℃ for oil-immersed transformers. The maximum design temperature rise is also a fixed design parameter. Specifically, it refers to the maximum permissible increase in oil temperature relative to ambient temperature under rated load and design ambient temperature conditions (e.g., 55K or 65K).

[0026] S12. Collect grid operating status parameters and load change parameters, and calculate the operating stress index. The grid operating status parameters include the effective value of three-phase current / coil current, voltage fluctuation amplitude, and load power factor. The load change parameters include the current change rate and the power factor change rate. The effective value of the three-phase current can also be called the coil current (the coil current is usually taken as the maximum or average value of the three phases). The voltage fluctuation amplitude is specifically the absolute value of the difference between the measured voltage and the nominal voltage. The load power factor is calculated by measuring the phase difference between the voltage and current on the secondary side of the transformer using a power analyzer, or directly read by a power quality monitoring device, including portable power quality analyzers or online monitoring devices. The current change rate refers to the change in coil current per unit time. The power factor change rate refers to the change in load power factor per unit time.

[0027] The calculation method for the operating stress index includes: The current load index is obtained by calculating the ratio of the current coil current to the rated current; the voltage fluctuation index is obtained by calculating the ratio of the voltage fluctuation amplitude to the upper limit of the allowable fluctuation; the power deviation index is obtained by calculating the absolute difference between the load power factor and the rated power factor and normalizing it; the operating stress index is obtained by weighting the current load index, voltage fluctuation index and power deviation index; the typical weight of the current load index is 0.7, the typical weight of the voltage fluctuation index is 0.2, and the typical weight of the power deviation index is 0.1. The current load has the dominant effect, followed by the voltage fluctuation, and the power factor deviation has the least impact. The sum of their weights is 1.

[0028] The rated current is a design constant, specifically calculated from the rated capacity and rated voltage. The allowable fluctuation limit is a design / standard constant, such as ±10% or ±7% as specified in national standards. The rated power factor is a design constant, specifically derived from the nominal value on the transformer nameplate (e.g., 0.8, 0.9, etc.).

[0029] S13. Obtain the actual current carrying capacity and rated capacity, and calculate the average ratio of the actual current carrying capacity to the rated capacity over the past N sampling periods to obtain the historical capacity utilization rate. The actual current carrying capacity can be calculated from the effective value of the three-phase current / coil current and the rated voltage in S12. For a three-phase transformer, the calculation formula is: Actual current carrying capacity = × Rated voltage × Coil current; The formula for a single-phase transformer is: Actual current carrying capacity = Rated voltage × Coil current; Rated capacity is a transformer design parameter and is a fixed value, where N is a positive integer.

[0030] S14. Calculate the product of the heat load state index and the operating stress index to obtain the coupling pressure value; select the larger value between the heat load state index and the operating stress index as the dominant pressure value; obtain the base pressure value based on the coupling pressure value and the dominant pressure value, specifically: add the coupling pressure value and the dominant pressure value and divide by 2 to obtain the base pressure value; it should be noted that: when the difference between the two exceeds the threshold (typically 0.2), the dominant pressure value shall prevail.

[0031] S15. The base pressure value and the historical capacity utilization rate are weighted and summed to obtain the comprehensive state index. The typical weight of the base pressure value is 0.5, and the typical weight of the historical capacity utilization rate is 0.5. In this embodiment, the real-time state and the historical trend are each set to account for 50% to balance transient response and stable control.

[0032] Step S2: Obtain the rated capacity and cooling capacity parameters, and calculate the target flow capacity value based on the comprehensive state index. Specifically, this includes: S21. Obtain rated capacity and cooling capacity parameters, including ambient temperature, upper limit of design ambient temperature, ratio of actual to rated number of cooling fans (cooling capacity correction coefficient), etc. Calculate the ratio of the ambient temperature to the upper limit of design ambient temperature to obtain the environmental limitation factor; wherein, ambient temperature refers to the real-time air temperature at the current transformer installation location; upper limit of design ambient temperature refers to the highest allowable ambient temperature specified when the transformer is designed (usually 40℃).

[0033] S22. Calculate the difference between the comprehensive state index and the preset safety threshold to obtain the capacity regulation deviation value. The typical value of the preset safety threshold is 0.7. For oil-immersed transformers, the design hot spot temperature is 105℃. When the hot spot temperature reaches 85℃ (about 0.81 normalized), the alarm is triggered, corresponding to a comprehensive state index of about 0.70 to 0.75.

[0034] S23. Multiply the capacity adjustment deviation value by a preset adjustment coefficient to obtain the capacity correction coefficient; the typical value of the preset adjustment coefficient is 0.5, which is a balance value and can be adjusted in the range of 0.3-0.7. The preset adjustment coefficient controls the sensitivity of capacity adjustment.

[0035] S24. Multiply the difference between 1 and the capacity correction factor by the rated capacity to obtain the corrected capacity value.

[0036] S25. Multiply the corrected capacity value by the environmental limiting factor to obtain the target flow capacity value.

[0037] Step S3: Determine whether the series resonance compensation start-up condition is met based on the target current capacity value; if it is met, execute the series resonance compensation capacity enhancement mode in step S4; if it is not met, execute the conventional capacity adjustment mode in step S5.

[0038] The series resonance compensation activation conditions include any one or more of the following: If the target current capacity exceeds the upper limit of the safe output of the conventional capacity regulation mode under the rated active power of the current input power supply, it indicates that the conventional capacity regulation mode is insufficient. Here, the current input power supply is the grid power supply powered by the transformer or the field test power supply, and the rated active power is the rated output active power of the power supply (e.g., 100kW, 500kW). The formula for calculating the upper limit is: Upper limit = (rated active power × 0.85) / transformer short-circuit impedance percentage. The transformer short-circuit impedance percentage is a per-unit value, such as 0.06 representing 6%, which can be directly read from the transformer nameplate. The overall condition index is below the preset hot-start threshold (e.g., 0.6), and a short-term current carrying capacity exceeding 1.2 times the rated current is required. Here, "short-term" refers to a temporary current carrying capacity requirement lasting no more than 5 minutes (300 seconds). 1.2 times rated current = 1.2 × (rated capacity / ( ×Rated voltage); Upon receiving an external test mode command, requiring a high current output from a low-power power supply for coil or insulation testing, the system refers to a frequency-adjustable AC power supply with a rated active power significantly lower than the power required to directly drive the transformer to its rated capacity. Typically, the rated active power of the low-power supply is 10%-30% of the power required in the conventional mode. For example, for a 50MVA transformer, direct drive requires approximately 3MW of power, while using the series resonant compensation mode of this invention, only a 300kW-500kW low-power supply is needed to achieve the equivalent current capacity output. The "high output current" refers to the current value generated in the transformer coil under the series resonant compensation mode being significantly higher than the current achievable by direct drive with the same power supply. For example, using a 500kW-class frequency-adjustable AC power supply, through series resonant compensation, a high current of over 5000A can be generated in the coil of a 10kV transformer, meeting short-time current capacity requirements or on-site testing needs.

[0039] Step S4: If satisfied, execute the series resonant compensation capacity enhancement mode: This mode constructs a resonant circuit with the compensation capacitor connected in series with the transformer coil, and adjusts the resonant circuit to a series resonant state to achieve the target current capacity requirement.

[0040] The series resonant compensation capacity enhancement mode includes: S41. Construct a resonant circuit with the compensation capacitor connected in series with the transformer coil, and apply an AC test voltage to the resonant circuit. The construction process of the resonant circuit includes: selecting an adjustable compensation capacitor bank, the capacitance adjustment range of which needs to cover the expected resonant point, for example, 10μF~200μF; connecting the compensation capacitor in series with the transformer coil, and connecting an adjustable frequency AC power supply (frequency range typically 45Hz~65Hz) to both ends of the series circuit, ensuring that all connections use high-voltage test cables and that protective grounding is provided. The specific steps for applying the AC test voltage include: obtaining the transformer coil inductance L: estimating L based on the transformer's rated voltage U, rated capacity S, rated frequency f (50Hz or 60Hz), and short-circuit impedance percentage Z, L=(Z×U²) / (2πf×S); or determining it through a low-voltage measurement method; selecting the initial value C0 of the compensation capacitor (which can be directly taken as the midpoint of the capacitance adjustment range, or based on historical experience, the capacitance value used during the last successful resonance), and calculating the theoretical resonant frequency f0=1 / (2πf×S). Set the initial frequency of the adjustable frequency AC power supply to any value within the range of 0.95f0 to 1.05f0; adjust the power supply output voltage to zero, close the output switch, and start the power supply with 5% to 10% of the rated voltage as the starting voltage; observe the circuit current and phase difference. The phase difference refers to the phase difference between the coil current and the voltage across the capacitor. If there is no abnormality (current does not change abruptly, no overcurrent alarm), proceed to step S42; in the subsequent step S45, gradually increase the voltage to make the coil current reach the upper limit of the current corresponding to the target current capacity.

[0041] S42. Collect the coil current, capacitor voltage, coil voltage, and input power parameters in the resonant circuit. The input power parameters include the output active power, output reactive power, and power factor of the adjustable frequency AC power supply. Output active power refers to the active power delivered by the adjustable frequency AC power supply to the resonant circuit, which can be directly collected using a power analyzer, smart meter, or the power supply's built-in measurement module. Output reactive power refers to the reactive power delivered by the adjustable frequency AC power supply to the resonant circuit, and can be directly collected as well. The power factor specifically refers to the power factor at the output of the adjustable frequency AC power supply, reflecting the reactive power relationship between the resonant circuit and the power supply, and can be collected using a power analyzer or the power supply's built-in measurement module.

[0042] S43. Calculate the resonant offset direction and residual reactive power based on the phase difference between the coil current and the voltage across the capacitor. The steps for calculating the resonant offset direction and residual reactive power include: calculating the phase difference: measuring the time difference Δt between the zero-crossing point of the coil current and the zero-crossing point of the voltage across the capacitor, and converting it to an angle θ: θ = 360° × Δt / T, where T is the fundamental period; if the coil current lags behind the voltage across the capacitor, the resonant offset direction is determined to be inductive (positive angle), otherwise it is capacitive (negative angle); the resonant offset amount = / 90°, normalized, maximum value is 1; residual reactive power can be calculated as follows: calculate the apparent output power based on the effective value of the adjustable frequency AC power supply output voltage Uout and the effective value of the output current I (i.e., coil current). , = ×Uout×I (three-phase) or =Uout×I (single phase), based on the collected output active power With the output apparent power Calculate the residual reactive power Q, Q= Alternatively, the reactive power value can be directly read from the power analyzer as the residual reactive power. The above calculation results are verified using the power factor: the power factor should meet the following condition: calculated power factor value = output active power / output apparent power. If the deviation between the calculated value and the acquired value exceeds the set threshold (e.g., 0.02), the acquired data is determined to be abnormal, triggering data re-acquisition or an alarm.

[0043] S44. Adjust the compensation capacitor capacitance or input frequency according to the resonant offset direction to bring the resonant circuit to a series resonant state; wherein, the compensation capacitor capacitance refers to the actual capacitance of the compensation capacitor in the series circuit, and the input frequency refers to the output frequency of the adjustable AC power supply, which is typically continuously adjustable from 45 to 65 Hz. Specifically, this includes: S441. When the resonant offset direction is inductive, reduce the compensation capacitor capacitance or increase the input frequency. Reducing the compensation capacitor capacitance can be achieved by switching some capacitors in the adjustable compensation capacitor bank or by using an electronically adjustable capacitor to reduce the capacitance. Increasing the input frequency can be achieved by increasing the output frequency setting value on the adjustable frequency AC power supply (e.g., from 50Hz to 52Hz).

[0044] S442. When the resonant offset direction is capacitive, increase the compensation capacitor capacity or decrease the input frequency. Increasing the compensation capacitor capacity allows for the addition of more parallel capacitors. Decreasing the input frequency reduces the power supply output frequency setting (e.g., from 50Hz to 48Hz).

[0045] S443. Gradually adjust the compensation parameters and monitor the rate of change of coil current and input power in real time; wherein, the compensation parameters refer to the compensation capacitor capacity and the input frequency.

[0046] S444. When the absolute value of the phase difference is less than a set threshold (e.g., ±5°) and the power factor is greater than 0.95, the series resonance state is determined to be reached. The absolute value of the phase difference refers to the absolute value of the phase difference between the coil current and the voltage across the capacitor.

[0047] S445. Lock the current compensation capacitor capacity and input frequency as the resonant operating point under the comprehensive state index, and store them in the parameter library; the parameter library is a data structure (which can be a memory, database or configuration file) that stores historical resonant operating points, including: comprehensive state index (or thermal load state index, operating stress index), corresponding optimal compensation capacitor capacity and optimal input frequency, etc.

[0048] S446. When the overall status index changes beyond the set range, S441 to S445 are automatically re-executed. The typical value of the set range is ±0.1.

[0049] S45. Adjust the input voltage according to the residual reactive power and the coil current so that the coil current reaches or exceeds the upper limit of the current corresponding to the target current capacity value, while maintaining the input power within the set range; wherein, the input voltage refers to the effective value of the output voltage of the adjustable frequency AC power supply, and the input power refers to the output active power of the adjustable frequency AC power supply; the set range refers to the allowable operating range of the output active power of the adjustable frequency AC power supply, which consists of an upper power limit (main constraint) and a lower power limit (optional, used to avoid instability under light load, such as 0.05 to 0.1 times the rated power of the power supply; for conventional applications, the lower power limit can be omitted).

[0050] The specific implementation steps of S45 include: calculating the upper limit of the current It corresponding to the target current capacity value St, using the formula: It = St / ( ×U); compare the current coil current with the upper limit of the current; if the coil current < the upper limit of the current and the input power < the upper limit of the power (e.g., 95% of the rated power of the power supply), then increase the input voltage by a step size ΔU, where the step size ΔU adopts a variable step size strategy: ΔU=Kp×(It) The input voltage is increased by adding a step size ΔU to the current input voltage (coil current). The proportional coefficient Kp is pre-tuned or adaptively updated in step S46. The typical value of the proportional coefficient is 0.01-0.1. For small-capacity transformers (coil current <100A), Kp is 0.05-0.1. For large-capacity transformers (coil current >1000A), Kp is 0.01-0.05. If the coil current is greater than the current limit and the duration exceeds the overcurrent allowable time (e.g., 10 seconds), the input voltage is reduced. During the adjustment of the input voltage, the residual reactive power is monitored simultaneously. If the residual reactive power is greater than the preset threshold (the typical value of the preset threshold for residual reactive power is 5%-10% of the rated active power of the adjustable frequency AC power supply, indicating that the circuit deviates from the resonance state), the step size ΔU is appropriately reduced or the step size is multiplied by an attenuation factor (e.g., 0.5) to avoid mutual interference between voltage regulation and resonance regulation. If the input power has reached the power limit... If the current is below the upper limit of the current limit, but the coil current is still below the upper limit of the current limit, stop boosting the voltage and return to S44 to boost the coil current. Repeat the adjustment until the coil current stabilizes within the target range [upper limit of current × 0.95, upper limit of current × 1.05] and the stable time exceeds 1 second, and the target current capacity is determined to be reached. When the coil current has entered the target range and the input power has not exceeded the limit, maintain the current input voltage unchanged. When the input voltage has reached the upper limit of the rated output voltage of the adjustable frequency AC power supply, but the coil current is still below the lower limit of the target current, stop boosting the voltage and return to S44. If the coil current cannot be stabilized within the target range after a preset timeout time (e.g., 30 seconds) since the start of boosting the voltage, it is determined that the adjustment timeout has occurred, an alarm is triggered, and the system automatically switches to the normal capacity adjustment mode in step S5. When the coil current repeatedly crosses the target range (e.g., more than 3 times within 1 second, sampling period less than 0.2 seconds), it is determined that adjustment oscillation has occurred, and the proportional coefficient is automatically reduced by 50% to weaken the control response.

[0051] S46. Record the adjustment results of each series resonant compensation capacity enhancement mode, and correct the resonant adjustment parameters based on the adjustment results. The resonant adjustment parameters include a proportional coefficient, specifically including: S461. Record the time elapsed from the start of voltage boost (S45 start) to the time when the coil current first enters the target range and remains stable for more than 1 second. This time is taken as the actual response time. Statistically calculate the actual response time of the past F adjustments (F is typically 5-10). If the number of adjustments is less than F, use the average of the existing recorded actual response times as the statistical benchmark. The above data constitute the adjustment result.

[0052] S462. Calculate the average response time of the actual response time of the most recent F adjustments and compare it with the preset target response time. The typical range of the target response time is 2 to 8 seconds, which is set according to the transformer capacity and adjustment accuracy requirements. Small-capacity transformers take a smaller value, and large-capacity transformers take a larger value. When the average response time is greater than 1.2 times the target response time, it indicates that the current proportional coefficient is too small, resulting in a slow voltage adjustment speed and a slow rise in coil current. In this case, the new proportional coefficient is made equal to the current proportional coefficient multiplied by 1.1. When the average response time is less than 0.8 times the target response time, it indicates that the current proportional coefficient is too large. Although the response speed is fast, it may cause overshoot or oscillation. In this case, the new proportional coefficient is made equal to the current proportional coefficient multiplied by 0.9. When the average response time is between 0.8 and 1.2 times the target response time, it indicates that the response time is within the acceptable range, and the proportional coefficient remains unchanged. The updated proportional coefficient needs to be limited to a certain range, such as [0.01, 0.1]. The proportional coefficient is updated after each voltage adjustment in S45 (coil current stabilizes in the target range) is completed, without interrupting the current adjustment process. The updated proportional coefficient is stored in the parameter library and will take effect the next time the S45 voltage regulation is entered, replacing the currently used proportional coefficient.

[0053] Step S5: If the conditions are not met, then execute the normal capacity adjustment mode: This mode performs capacity adjustment control based on the target current capacity value and the coil current, and corrects the capacity adjustment parameters in real time.

[0054] The conventional capacity adjustment modes include: S51. Obtain the current coil current and the upper limit of the current corresponding to the target current capacity value, and compare the current coil current with the upper limit of the current corresponding to the target current capacity value.

[0055] S52. When the coil current exceeds the upper limit of the current, a capacity reduction control command is generated and sent to the on-load tap changer or load distribution device.

[0056] S53. When the coil current is lower than the upper limit of the current and the comprehensive state index is lower than the preset safety threshold, a capacity increase control command is generated and sent to the on-load tap changer or load distribution device.

[0057] Among them, the on-load tap changer (OLTC) is used to change the transformer ratio and adjust the output voltage when the transformer is under load, indirectly affecting the current carrying capacity; the load distribution device refers to the load switching switch or load management system, used to switch part of the load to other transformers or disconnect some unimportant loads, directly reducing the current coil current; the adjustment step size (number of downsizing / upsizing or load disconnection ratio) of the above-mentioned capacity increase / decrease control command is determined by the deviation, specifically: the adjustment step size of the capacity increase / decrease control command is determined by the ratio of the current deviation ΔI to the upper limit of the current, ΔI = coil current - upper limit of current; derating: when the ratio is ≤2%, it enters the adjustment dead zone and no adjustment command is sent; when 2% < When the ratio is ≤5%, the on-load tap changer should be adjusted to a step size of 1 level; when 5% < ratio ≤10%, the load distribution device should be adjusted to cut off 10%-20% (load distribution), or the on-load tap changer should be adjusted to a step size of 1-2 levels; when 10% < ratio ≤15%, the load distribution device should be adjusted to cut off 20%-30%, or the on-load tap changer should be adjusted to a step size of 2 levels; when the ratio is >15%, the load distribution device should be adjusted to cut off 30%-50% first, and then the on-load tap changer should be adjusted to a step size of 2 levels; for capacity increase: when the coil current is lower than the upper limit of the current and the comprehensive state index is lower than the preset safety threshold, the on-load tap changer should be adjusted to a step size of 1-2 levels.

[0058] S54. Record the temperature and current changes after each capacity adjustment control, obtain the recording results, and correct the capacity adjustment parameters based on the recording results. Temperature change refers to the change in winding hot spot temperature and the rate of temperature rise before and after each adjustment; the rate of temperature rise refers to the change in winding hot spot temperature per unit time. Current change refers to the change in coil current (ΔI) and response time before and after adjustment. The recording results include the adjustment timestamp, coil current value before / after adjustment, winding hot spot temperature value before / after adjustment, rate of temperature rise, and adjustment command type (increase / decrease). The capacity adjustment parameters include preset adjustment coefficients and preset safety thresholds.

[0059] The correction rule for the capacity adjustment parameter is as follows: When the temperature rise rate exceeds 1.5℃ / min, the preset adjustment coefficient is multiplied by 0.9; when the temperature rise rate is less than 0.5℃ / min and the coil current remains below 90% of the current upper limit for 3 seconds, the preset adjustment coefficient is multiplied by 1.05, so that the preset adjustment coefficient is within its corresponding constraint range, such as [0.3, 0.7]; in other cases, the preset adjustment coefficient remains unchanged. The peak value of the winding hot spot temperature in the past M adjustment records is statistically analyzed (M is typically 10; when the number of adjustments is less than M, the average value in the existing records is used); when the peak value of the winding hot spot temperature is >95℃, the preset safety threshold is multiplied by 0.98; when 75℃≤peak value of the winding hot spot temperature≤95℃, the preset safety threshold remains unchanged; when the peak value of the winding hot spot temperature is <75℃, the preset safety threshold is multiplied by 1.02 or remains unchanged; so that the preset safety threshold is within its corresponding constraint range, such as [0.65, 0.80].

[0060] Step S6: During operation in the series resonant compensation capacity enhancement mode, continuously monitor the transformer thermal load state index; when the thermal load state index exceeds the preset safety value or the resonant circuit reaches the preset detuning condition, trigger the switching control mode and derating protection. Specifically, this includes: S61. During the operation of the series resonant compensation capacity enhancement mode, continuously monitor the thermal load state index, winding hot spot temperature and temperature rise rate, and resonant offset.

[0061] S62. Real-time determination of whether any of the following triggering conditions are met: The heat load state index exceeds the preset safety value (e.g., preset safety value = 0.85). The rate of temperature rise of the winding hot spot exceeds a preset rate threshold (e.g., preset rate threshold = 2℃ / min). The resonant offset exceeds the preset detuning threshold, and two consecutive attempts to adjust it via step S44 fail to restore it to within the preset detuning threshold within a set time (e.g., 5 seconds). The typical value for the preset detuning threshold for the resonant offset is 0.17; when the resonant offset can be directly replaced by the phase difference, the typical value for the phase difference corresponding to the preset detuning threshold is 15°.

[0062] S63. When any of the triggering conditions in S62 is met, the switching control mode and derating protection are immediately triggered, and the protection actions are executed in the following order: S631. Send a protection event notification to the monitoring system and simultaneously trigger an audible and visual alarm device (such as a control cabinet buzzer or indicator light), with the warning continuing until the protection status is lifted; wherein, the protection event notification includes specific triggering conditions, the heat load status index at the time of triggering, etc. S632. Reduce the effective value of the output voltage of the adjustable frequency AC power supply to a safe voltage value (e.g., 30% of the rated voltage of the adjustable frequency AC power supply) at a preset voltage reduction rate (e.g., 50% of the current voltage per second), and the total duration of the voltage reduction process shall not exceed 2 seconds. S633. Disconnect the resonant circuit by controlling a contactor or solid-state switch to disconnect the series circuit between the compensation capacitor and the transformer coil, so that the transformer is out of the resonant compensation state. S634. Switch the control mode and transfer the control of the transformer current capacity from step S4 (series resonance compensation capacity enhancement mode) to step S5 (normal capacity adjustment mode). S635. In the normal capacity adjustment mode, the target current capacity of the current stage is forcibly set to the derating protection value. The derating protection value can be directly taken as 80% of the rated capacity.

[0063] S64. After implementing derating protection, continue to monitor the thermal load state index and resonant circuit state parameters. The resonant circuit state parameters include resonant offset state, power state, and electrical stress state. The resonant offset state includes the phase difference between the coil current and the voltage across the capacitor. The power state includes the output power factor of the adjustable frequency AC power supply. The electrical stress state includes the voltage across the capacitor. When the thermal load state index and resonant circuit state parameters meet all preset release conditions, the protection state is released.

[0064] The preset release conditions include: The heat load state index remains below the preset safety value for more than 30 seconds; The resonant offset state is normal, that is, the absolute value of the phase difference is ≤10° and it remains stable for more than 5 seconds; The power condition is normal, i.e., the power factor is ≥0.90; The electrical stress condition is normal, meaning the voltage across the capacitor does not exceed 90% of its rated voltage, and the voltage across the coil does not exceed the transformer insulation allowable value; the transformer insulation allowable value is usually 1.5-2.5 times the rated voltage (test voltage). After the protection is released, the start conditions of step S3 can be reassessed manually or automatically by the system to determine whether to re-enter the series resonant compensation capacity enhancement mode.

[0065] In summary, this invention provides a transformer current capacity adjustment method based on adaptive control. By collecting transformer operating parameters, environmental parameters, and power grid operating status parameters, a comprehensive status index is calculated, and the target current capacity value is dynamically determined based on this index. Then, the conventional capacity adjustment mode or the series resonant compensation capacity enhancement mode is intelligently selected. In both modes, adaptive parameter correction based on the adjustment results is achieved, and protection switching is automatically triggered when thermal load exceeds the limit or resonant detuning is detected.

[0066] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 Units that specify functions within one or more boxes.

[0067] The above description is merely a preferred embodiment of the present invention and should not be construed as limiting the scope of the invention. Therefore, any equivalent variations made in accordance with the claims of the present invention are still within the scope of the present invention.

Claims

1. A transformer current-carrying capacity regulation method based on adaptive control, characterized in that, It should include at least the following steps: S1. Collect transformer operating parameters, environmental parameters and power grid operating status parameters, and calculate the comprehensive status index based on multi-dimensional parameters that reflect at least the thermal state and electrical stress state of the transformer; S2. Obtain the rated capacity and cooling capacity parameters, and calculate the target flow capacity value based on the comprehensive state index; S3. Determine whether the series resonance compensation start-up condition is met based on the target current capacity value; S4. If satisfied, the series resonant compensation capacity enhancement mode is executed: by constructing a resonant circuit in series between the compensation capacitor and the transformer coil, and adjusting the resonant circuit to the series resonant state, the target current capacity value requirement is achieved. S5. If not satisfied, execute the normal capacity adjustment mode: based on the target current capacity value and coil current, execute capacity adjustment control and correct the capacity adjustment parameters in real time; S6. During the operation of the series resonant compensation capacity enhancement mode, the transformer thermal load status index is continuously monitored; when the thermal load status index exceeds the preset safety value or the resonant circuit reaches the preset detuning condition, the switching control mode and capacity reduction protection are triggered.

2. The method according to claim 1, characterized in that, The calculation of the comprehensive state index, based on the collected transformer operating parameters, environmental parameters, and power grid operating status parameters, includes: S11. Collect transformer operating parameters and environmental parameters, and calculate the heat load state index; wherein, the transformer operating parameters include winding hot spot temperature, oil temperature, actual operating number and rated number of cooling fans; the environmental parameters include ambient temperature; S12. Collect power grid operating status parameters and load change parameters, and calculate the operating stress index; S13. Obtain the actual current capacity and rated capacity, calculate the average ratio of the actual current capacity to the rated capacity over the past N sampling periods, and obtain the historical capacity utilization rate. S14. Calculate the product of the thermal load state index and the operating stress index to obtain the coupling pressure value; select the larger value between the thermal load state index and the operating stress index as the dominant pressure value; add the coupling pressure value and the dominant pressure value and divide by two to obtain the base pressure value. S15. The basic pressure value and the historical capacity utilization rate are weighted and summed to obtain the comprehensive state index.

3. The method according to claim 2, characterized in that, In step S11, the heat load state index is calculated in the following manner: The hot spot temperature index is obtained by calculating the ratio of the winding hot spot temperature to the maximum allowable temperature. Divide the difference between the oil temperature and the ambient temperature by the maximum design temperature rise to obtain the oil temperature load index; Calculate the cooling capacity correction factor based on the ratio of the actual number of cooling fans in operation to the rated number; The hot spot temperature index and the oil temperature load index are weighted and summed, and then multiplied by the cooling capacity correction factor to obtain the heat load state index.

4. The method according to claim 3, characterized in that, In step S12, the operating stress index is calculated in the following manner: Calculate the ratio of the current coil current to the rated current to obtain the current load index; The voltage disturbance index is obtained by calculating the ratio of the voltage fluctuation amplitude to the upper limit of the allowable fluctuation. Calculate the absolute difference between the load power factor and the rated power factor, and normalize it to obtain the power deviation index; The operating stress index is obtained by weighting and combining the current load index, voltage disturbance index, and power deviation index.

5. The method according to claim 4, characterized in that, In step S2, obtaining the rated capacity and cooling capacity parameters, and calculating the target flow capacity value based on the comprehensive state index, includes: S21. Obtain rated capacity and cooling capacity parameters, wherein the cooling capacity parameters include ambient temperature and design ambient temperature limit, calculate the ratio of the ambient temperature to the design ambient temperature limit, and obtain the environmental limitation factor. S22. Calculate the difference between the comprehensive state index and the preset safety threshold to obtain the capacity adjustment deviation value; S23. Multiply the capacity adjustment deviation value by a preset adjustment coefficient to obtain the capacity correction coefficient; S24. Multiply the difference between 1 and the capacity correction factor by the rated capacity to obtain the corrected capacity value; S25. Multiply the corrected capacity value by the environmental limiting factor to obtain the target flow capacity value.

6. The method according to claim 5, characterized in that, In step S3, the series resonance compensation activation conditions include any one or more of the following: The target current capacity exceeds the upper limit that can be safely output in normal capacity regulation mode under the rated active power of the current input power supply; The overall condition index is lower than the preset hot start threshold, and the current carrying capacity exceeds 1.2 times the rated current within a preset short period of time; Received an external test mode command indicating that a large current is output from a low-power supply to perform coil or insulation testing.

7. The method according to claim 6, characterized in that, The series resonant compensation capacity enhancement mode includes: S41. Construct a resonant circuit in series between the compensation capacitor and the transformer coil, and apply an AC test voltage to the resonant circuit; S42. Collect the coil current, capacitor voltage, coil voltage, and input power parameters in the resonant circuit. S43. Calculate the resonant offset direction and residual reactive power based on the phase difference between the coil current and the voltage across the capacitor. S44. Adjust the compensation capacitor capacity or input frequency according to the resonant offset direction to adjust the resonant circuit to a series resonant state. S45. Adjust the input voltage according to the residual reactive power and the coil current so that the coil current reaches or exceeds the upper limit of the current corresponding to the target current capacity value, while maintaining the input power within the set range. S46. Record the adjustment results of each series resonance compensation capacity enhancement mode, and correct the resonance adjustment parameters based on the adjustment results.

8. The method according to claim 7, characterized in that, In step S44, adjusting the compensation capacitor capacity or input frequency according to the resonant offset direction includes: S441. When the resonant offset direction is inductive, reduce the compensation capacitor capacitance or increase the input frequency. S442. When the resonant offset direction is capacitive, increase the compensation capacitor capacitance or decrease the input frequency. S443. Gradually adjust the compensation parameters and monitor the rate of change of coil current and input power in real time; wherein, the compensation parameters refer to the compensation capacitor capacity and the input frequency; S444. When the absolute value of the phase difference is less than the set threshold and the power factor is greater than 0.95, it is determined that the series resonance state has been reached. S445. Lock the current compensation capacitor capacity and input frequency as the resonant operating point under the comprehensive state index, and store them in the parameter library; S446. When the change in the comprehensive status index exceeds the set range, S441 to S445 will be automatically re-executed.

9. The method according to claim 8, characterized in that, The conventional capacity adjustment modes include: S51. Obtain the current coil current and the upper limit of the current corresponding to the target current capacity value, and compare the current coil current with the upper limit of the current corresponding to the target current capacity value. S52. When the coil current exceeds the upper limit of the current, a capacity reduction control command is generated and sent to the on-load tap changer or load distribution device. S53. When the coil current is lower than the upper limit of the current and the comprehensive state index is lower than the preset safety threshold, a capacity increase control command is generated. S54. Record the temperature and current changes after each capacity regulation control, obtain the recording results, and correct the capacity regulation parameters based on the recording results.

10. The method according to any one of claims 1 to 9, characterized in that, S6 includes: S61. During the operation of the series resonant compensation capacity enhancement mode, continuously monitor the thermal load state index, winding hot spot temperature and temperature rise rate, and resonant offset. S62. Real-time determination of whether any of the following triggering conditions are met: The heat load condition index exceeds the preset safety value; The rate of temperature rise of the winding hot spot exceeds the preset rate threshold. The resonance offset exceeds the preset detuning threshold, and two consecutive attempts to adjust it through step S44 fail to restore it to within the preset detuning threshold within the set time. S63. When any of the triggering conditions in S62 is met, the switching control mode and derating protection are immediately triggered; the derating protection includes at least: reducing the input voltage, disconnecting the resonant circuit, switching the control mode to the conventional capacity adjustment mode, and forcibly setting the derating protection value. S64. After implementing the derating protection, continue to monitor the thermal load state index and the resonant circuit state parameters. When the thermal load state index and the resonant circuit state parameters meet all preset release conditions, release the protection status.

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