A transformer partial discharge detection test method based on low frequency modification

Abstract

The application discloses a transformer partial discharge detection test method based on low-frequency transformation, and belongs to the technical field of partial discharge detection of transformer test, which comprises the following steps: transforming a power frequency AC power supply system to obtain a transformed low-frequency AC power supply system; supplying power to a transformer by using the transformed low-frequency AC power supply system; and performing partial discharge detection test on the bushing of the transformer supplied by the transformed low-frequency AC power supply system to obtain partial discharge detection test results. The single discharge energy is higher at low frequency, which is beneficial to stimulate deep insulation defects; the discharge pulse interval is increased, the pulse overlap probability is reduced, and misjudgment and missed detection are reduced; the method is more sensitive to slowly developing defects; and the discharge mode is closer to actual operation, and the insulation condition can be accurately evaluated. The application effectively solves the problem of traditional power frequency detection, meets multiple detection scenes and high standards, and has universality, generality and portability.

Description

Technical Field

[0001] This invention belongs to the field of partial discharge detection technology in transformer testing, specifically relating to a test method for partial discharge detection of transformers based on low-frequency modification. Background Technology

[0002] Partial discharge (PD) is a significant indicator of transformer insulation degradation. PD detection technology is crucial for transformer factory evaluation and health status monitoring. Current transformer testing techniques primarily employ methods for detecting partial discharge, including pulsed current testing, ultrasonic testing, ultra-high frequency (UHF) testing, high-frequency current transformer (HFCT) testing, and chemical analysis. Each method has its own characteristics and limitations; the pulsed current testing method is considered the most classic and standardized.

[0003] When partial discharge occurs, the electric field strength at the insulation defect (such as bubbles or cracks) exceeds the dielectric breakdown threshold, triggering a transient current pulse. This pulse generates a high-frequency current signal (typically 10kHz-1MHz) in the equipment grounding circuit. The traditional pulse current detection method under the IEC 60270 standard works by detecting this transient current pulse signal generated by partial discharge under power frequency (50Hz / 60Hz) AC power supply through a coupling capacitor or Rogowski coil, converting it into a voltage signal, and then quantifying the discharge quantity (pC, picocoulombs). The charge quantity Q is calculated by integrating the current pulse. The formula for calculating the discharge quantity Q is: Its detection system includes sensors, signal conditioning circuits, and data acquisition and analysis equipment, such as a partial discharge detector.

[0004] As mentioned above, classic partial discharge detection tests are conducted under power frequency (50Hz / 60Hz) AC power. However, tests under power frequency conditions inevitably have some drawbacks, making it impossible to detect partial discharge more comprehensively, efficiently, and stably: First, the energy of a single discharge is low, which may not be able to effectively excite deep insulation defects (such as delamination of oil paper or internal cracks) that require high energy triggering, leading to missed detections; Second, since the 50Hz period is only 20ms, the short discharge pulse interval makes it easy for pulse overlap to occur at high discharge rates, leading to misjudgment or missed detections by the detection system; In addition, the electric field changes rapidly and the charge migration rate is high under power frequency, making it difficult to form a continuous electric field distortion at slowly developing defects (such as moisture diffusion), resulting in insufficient sensitivity to slow-developing defects; Furthermore, the loss is high at 50Hz, which may cause an overall temperature rise in the insulation material, changing the dielectric properties of the material and potentially causing the discharge mode to deviate from the actual operating state. Summary of the Invention

[0005] The purpose of this invention is to overcome the defects of transformer partial discharge detection tests under power frequency conditions, and to propose a transformer partial discharge detection test method based on low frequency modification.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a method for detecting partial discharge in transformers based on low-frequency retrofitting, comprising the following steps: The power frequency AC power supply system is modified to a low frequency, resulting in a modified low frequency AC power supply system. The transformer is powered by the modified low-frequency AC power supply system; Partial discharge detection tests were conducted on the bushings of transformers powered by the modified low-frequency AC power supply system, and the results of the partial discharge detection tests were obtained.

[0007] Furthermore, the power frequency AC power supply system is modified to a low frequency, resulting in a modified low-frequency AC power supply system, including modifications to the motor and generator: To control the motor by adding a frequency point, the power frequency selection switch is replaced with a low-frequency to power frequency conversion switch, and the inverter's operating frequency and speed are changed to low frequency. The method to change the inverter's operating frequency to low frequency is to add a switch input point to the inverter. A forced air cooling system is added to the motor and generator. The forced air cooling system is connected to the electrical control cabinet of the forced air cooling equipment. An electrical interlock is set between the forced air cooling system and the motor main control system. The forced air cooling system is as follows: the base of the cooler is a sealed box, and fans are installed on both sides of the sealed box. Air is drawn in from the inside of the sealed box and air is delivered from the outside of the sealed box. The door of the cooler is sealed. The forced air cooling system is automatically controlled. The fans are connected to the electrical control cabinet of the cooling equipment. Each fan is equipped with individual overload protection and short circuit protection.

[0008] Furthermore, the electrical control cabinet for the cooling equipment includes intelligent control instruments, a frequency converter starter bus, a PLC, and a touch screen; The forced air cooling system employs automatic control, including: Temperature sensors in the motor and generator monitor and feed back temperature information to the intelligent control instrument, which then controls the motor of each fan based on the temperature information. The intelligent control instrument identifies the over-temperature protection action of the generator and motor, and controls the fan to continuously cool the internal operation of the generator and motor. The PLC and touch screen in the electrical control cabinet of the cooling equipment are used for intelligent alarm control of the brakes.

[0009] Furthermore, a circuit breaker is installed between the power supply of the wind turbine and the distribution cabinet.

[0010] Furthermore, the fan starts and operates at both power frequency and low frequency.

[0011] Furthermore, the rule for installing an electrical interlock between the forced air cooling system and the motor main control system is as follows: if the fan starts without fault, the motor unit will start; if the fan fails during the operation of the motor unit, the unit will trip and stop.

[0012] Furthermore, partial discharge detection tests were conducted on the bushings of the transformer powered by the modified low-frequency AC power supply system, and the results of the partial discharge detection tests are as follows: Partial discharge detection tests are performed on the bushings of the transformer. The partial discharge detection tests include: The transformer bushings were subjected to pre-pressurization, step-by-step voltage increase, withstand voltage test, and voltage reduction observation in sequence. Data was collected during the process, including discharge quantity, discharge number, phase, and pulse waveform. The collected data were corrected and analyzed to obtain the correction analysis results. The correction analysis results were evaluated to obtain the partial discharge detection test results. If the discharge quantity during the partial discharge test increases exponentially with time, the withstand voltage test should be terminated immediately.

[0013] Furthermore, the pre-pressurization is increased to a first voltage at a first rate; The step-by-step voltage increase is performed with the second voltage as the step size, and each stage is maintained for a certain period of time. The discharge amount and phase distribution at each voltage are recorded. The withstand voltage test is a routine test that maintains the voltage at the third voltage for a second period of time or a type test for a third period of time, and the discharge trend is monitored. The voltage was then reduced to the fourth voltage at the second rate, and the reversibility of the discharge was observed.

[0014] Furthermore, the calibration analysis converts the measured discharge quantity into a power frequency equivalent discharge quantity according to the calibration formula, which is shown below:

[0015] in, This is the power frequency equivalent discharge quantity. To measure the actual discharge quantity, This is the discharge quantity correction factor.

[0016] Secondly, the present invention provides a transformer partial discharge detection and testing system based on low-frequency modification, comprising: The low-frequency power supply module is used to supply power to the transformer using the modified low-frequency AC power supply system; The partial discharge detection module is used to perform partial discharge detection tests on the bushings of transformers powered by the modified low-frequency AC power supply system, and to obtain the partial discharge detection test results.

[0017] Compared with the prior art, the present invention has the following beneficial technical effects: This invention proposes a transformer partial discharge detection method based on low-frequency modification. After low-frequency modification, the single discharge energy is higher at low-frequency voltages compared to power frequency. This is because discharge energy is related to factors such as voltage and charge quantity. At low frequencies, charge accumulates for a longer period, resulting in a greater release of energy upon discharge. For deep insulation defects requiring high-energy triggering, higher discharge energy can more effectively excite these defects to produce discharge phenomena, thereby reducing missed detections and improving the detection capability for deep insulation defects. The longer period of low-frequency voltage increases the interval between discharge pulses, reducing the probability of pulse overlap. The detection system can more clearly distinguish each discharge pulse, thus reducing misjudgments or missed detections and improving the accuracy of detection results. At low-frequency voltages, the electric field changes slowly, and the charge migration rate is relatively low. This allows sufficient time for charge to accumulate at slowly developing defects, forming a continuous electric field distortion, making it easier to excite discharge phenomena. For slowly developing defects, low-frequency detection can more sensitively capture the discharge signals they cause, improving the detection capability for slow-developing defects. At low-frequency voltages, losses are relatively low, and the temperature rise of the insulation material is smaller. The dielectric properties of the insulation material are closer to the actual operating conditions, and the discharge mode more realistically reflects the transformer's actual operating status. The test results can more accurately assess the transformer's insulation condition, providing a more reliable basis for equipment maintenance and repair. Transformer partial discharge detection tests based on low-frequency retrofits effectively solve many problems existing in traditional power-frequency partial discharge detection tests by increasing discharge energy, widening pulse intervals, enhancing sensitivity to slow-moving defects, and maintaining the authenticity of the discharge mode. It can meet the increasingly diverse testing scenarios and gradually improving industry standards, making up for the deficiencies in the field of transformer partial discharge detection testing, and is fully versatile, universal, and portable. Attached Figure Description

[0018] The accompanying drawings described herein are for illustrative purposes only and are not intended to limit the scope of the invention in any way. Furthermore, the shapes and proportions of the components in the drawings are merely schematic to aid in understanding the invention and do not specifically limit the shapes and proportions of the components. In the drawings: Figure 1 This is a flowchart of a transformer partial discharge detection test method based on low-frequency modification according to the present invention.

[0019] Figure 2 This is a structural diagram of a transformer partial discharge detection test system based on low-frequency modification according to the present invention.

[0020] Figure 3The parameters of the 50Hz motor and generator equipment before modification are shown in this embodiment of the invention.

[0021] Figure 4 The parameters of the modified 20Hz motor and generator are shown in the embodiments of the present invention.

[0022] Figure 5 This is a detailed procedure for the partial discharge detection test under 20Hz power supply in an embodiment of the present invention.

[0023] Figure 6 This is a wiring diagram of a partial discharge detection test under 20Hz power supply in an embodiment of the present invention. Detailed Implementation

[0024] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments. 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 should fall within the scope of protection of the present invention.

[0025] It should be noted that when an element is referred to as being "set on" another element, it can be directly on the other element or there may be an intervening element. When an element is referred to as being "connected to" another element, it can be directly connected to the other element or there may be an intervening element. The terms "vertical," "horizontal," "left," "right," and similar expressions used herein are for illustrative purposes only and do not represent the only embodiments.

[0026] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0027] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0028] Example 1 See Figure 1 A method for detecting partial discharge in transformers based on low-frequency retrofitting includes the following steps: The power frequency AC power supply system is modified to a low frequency, resulting in a modified low frequency AC power supply system. The transformer is powered by the modified low-frequency AC power supply system; Partial discharge detection tests were conducted on the bushings of transformers powered by the modified low-frequency AC power supply system, and the results of the partial discharge detection tests were obtained.

[0029] Under low-frequency conditions, the voltage stress on the transformer insulation is relatively low, thus reducing the required test voltage for partial discharge detection. This not only reduces the capacity requirements of the testing equipment but also lowers the safety risks during the test, making the test easier to implement. Due to the reduced test voltage, the specifications and costs of the required equipment (such as high-voltage power supplies and measuring instruments) are also reduced. Under low-frequency conditions, the propagation and attenuation characteristics of the electromagnetic signals generated by partial discharge change within the transformer, making these signals easier for detection instruments to capture. This helps improve the accuracy of partial discharge identification and reduces the possibility of false positives and false negatives. A large amount of electromagnetic interference exists in power frequency AC power supply systems, which may mask partial discharge signals and affect the accuracy of the detection results. Low-frequency modification can reduce the impact of this interference and improve the reliability of the detection results. Performing partial discharge detection tests under low-frequency conditions can reduce damage to the transformer insulation. Regular low-frequency partial discharge detection tests can promptly detect potential faults and insulation defects inside the transformer. For large transformers, due to their complex structure and high insulation requirements, traditional power frequency partial discharge detection tests may be difficult to implement. The testing methods based on low-frequency modification can reduce the difficulty and cost of testing, making it possible to detect partial discharge in large transformers.

[0030] The industrial frequency AC power supply system is retrofitted to a low-frequency AC power supply system, resulting in a modified low-frequency AC power supply system, including the modification of the motor and generator: To control the motor by adding a frequency point, the power frequency selection switch is replaced with a low-frequency to power frequency conversion switch, and the inverter's operating frequency and speed are changed to low frequency. The method to change the inverter's operating frequency to low frequency is to add a switch input point to the inverter. By increasing the number of operating frequency points to control the motor, replacing the mains frequency selection switch with a low-frequency-to-mains frequency conversion switch, and changing the inverter's operating frequency and speed to low-frequency parameters, the motor can operate at a low frequency. This provides suitable low-frequency power supply conditions for transformer partial discharge detection tests based on low-frequency modifications, meeting the specific frequency requirements of certain test scenarios. The low-frequency-to-mains frequency conversion switch facilitates flexible switching between low-frequency testing and normal mains frequency operation, improving the system's applicability and flexibility, and allowing for rapid adjustment of the motor's operating status according to different needs. Adding switch input points to the inverter to change the operating frequency to low frequency enables more precise control of the motor's operating frequency, ensuring the stability and accuracy of low-frequency operation and providing reliable power supply for testing.

[0031] A forced air cooling system is added to the motor and generator. The forced air cooling system is connected to the electrical control cabinet of the forced air cooling equipment. An electrical interlock is set between the forced air cooling system and the motor main control system. The forced air cooling system is as follows: the base of the cooler is a sealed box, and fans are installed on both sides of the sealed box. Air is drawn in from the inside of the sealed box and air is delivered from the outside of the sealed box. The door of the cooler is sealed. The forced air cooling system is automatically controlled. The fans are connected to the electrical control cabinet of the cooling equipment. Each fan is equipped with individual overload protection and short circuit protection.

[0032] When electric motors and generators operate at low frequencies, their cooling conditions may worsen due to reduced speed. Adding a forced-air cooling system, using fans to force airflow, effectively removes heat generated by the motor, ensuring it operates at a suitable temperature and preventing performance degradation and insulation damage caused by overheating, thus guaranteeing normal equipment operation. The cooler base uses a sealed box design with fans installed on both sides. Air is drawn in from the inside of the box and supplied from the outside, while the door is sealed. This design creates an effective air circulation channel, improving heat dissipation efficiency and allowing the heat generated by the motor to dissipate more quickly. An electrical interlock is installed between the forced-air cooling system and the motor's main control system. The forced-air cooling system automatically starts when the motor starts and stops when the motor stops. This interlock mechanism ensures effective cooling of the motor throughout operation, preventing overheating damage due to cooling system failure and enhancing system safety. Each fan is equipped with individual overload and short-circuit protection, enabling timely detection and handling of faults during fan operation, preventing the fault from escalating and affecting the normal operation of the entire cooling system and even the motor system, further improving system reliability and safety. The forced air cooling system adopts automatic control. The fan is connected to the electrical control cabinet of the cooling equipment, which can automatically adjust the start-stop and operation status of the fan according to the motor's operating status, reducing the tediousness and error of manual operation and improving the system's automation level and operating efficiency.

[0033] The electrical control cabinet for the cooling equipment includes intelligent control instruments, a frequency converter starter bus, a PLC, and a touch screen; The forced air cooling system employs automatic control, including: Temperature sensors in the motor and generator monitor and feed back temperature information to the intelligent control instrument, which then controls the motor of each fan based on the temperature information. The intelligent control instrument identifies the over-temperature protection action of the generator and motor, and controls the fan to continuously cool the internal operation of the generator and motor. The PLC and touch screen in the electrical control cabinet of the cooling equipment are used for intelligent alarm control of the brakes.

[0034] The intelligent control instrument accurately receives temperature information from the temperature sensor and precisely controls the motor of each fan according to preset algorithms and logic. The inverter starting bus, used in conjunction with the inverter, allows for flexible adjustment of the fan voltage and frequency according to actual needs, thus achieving precise adjustment of the fan speed to meet cooling requirements under different operating conditions. During fan startup, the inverter starting bus and inverter work together to achieve soft starting, effectively reducing starting current, minimizing impact on the power grid, and extending the service life of the fan and related electrical equipment. The PLC has powerful programming capabilities, allowing for flexible programming of control programs based on the actual operating conditions and cooling requirements of the motor and generator. The touchscreen provides an intuitive human-machine interface, allowing operators to easily set cooling system parameters such as temperature thresholds and fan operating modes. Temperature sensors monitor the temperature of the motor and generator in real time and promptly feed this information back to the intelligent control instrument. Based on the feedback temperature information, the intelligent control instrument quickly adjusts the fan's operating status to ensure the equipment always operates within a suitable temperature range.

[0035] A circuit breaker is installed between the wind turbine's power supply and the distribution cabinet. When a wind turbine malfunctions, the circuit breaker automatically trips and cuts off the power. Maintenance personnel can quickly determine the location of the fault based on the circuit breaker's tripping status, narrowing down the troubleshooting scope. During maintenance, the circuit breaker disconnects the power, providing a safe operating environment and preventing maintenance operations while the circuit is energized, thus reducing maintenance risks. The circuit breaker effectively isolates the faulty wind turbine, protecting other equipment from the fault and minimizing economic losses due to equipment damage. The circuit breaker has remote control capabilities and can be integrated with the distribution cabinet's automated control system. Through this system, operators can remotely control the circuit breaker's opening and closing operations, enabling remote start and stop of the wind turbine. The circuit breaker works in conjunction with other protection devices in the distribution cabinet to form a complete protection system.

[0036] The fan starts and runs at both power frequency and low frequency.

[0037] The rule for installing an electrical interlock between the forced air cooling system and the motor main control system is as follows: if the fan starts without fault, the motor unit will start; if the fan fails during the operation of the motor unit, the unit will trip and stop.

[0038] Partial discharge detection tests were conducted on the bushings of the transformer powered by the modified low-frequency AC power supply system. The results of the partial discharge detection tests are as follows: Partial discharge detection tests are performed on the transformer bushings. These tests include: The transformer bushings were subjected to pre-pressurization, step-by-step voltage increase, withstand voltage test, and voltage reduction observation in sequence. Data was collected during the process, including discharge quantity, discharge number, phase, and pulse waveform. The collected data were corrected and analyzed to obtain the correction analysis results. The correction analysis results were evaluated to obtain the partial discharge detection test results. If the discharge quantity during the partial discharge test increases exponentially with time, the withstand voltage test should be terminated immediately.

[0039] Correcting and analyzing the collected data can eliminate various errors and interference factors during the measurement process. Evaluating the correction and analysis results can determine whether the partial discharge of the bushing is within the normal range and whether there are any potential safety hazards. An exponential increase in discharge indicates that the partial discharge condition of the bushing is rapidly deteriorating, potentially indicating serious insulation defects. Continuing the test may lead to bushing breakdown or even more serious safety accidents. Timely termination of the test can avoid equipment damage and personnel injury, ensuring the safe conduct of the test. Regularly conducting partial discharge detection tests to promptly identify and address bushing insulation problems can prevent further deterioration of insulation performance and extend the service life of the transformer.

[0040] Pre-pressurization raises the voltage to the first level at the first rate, enabling the transformer bushings to gradually adapt to voltage changes and eliminating potential residual charge and uneven internal stress.

[0041] The stepped voltage ramping method gradually increases the voltage step by step with the second voltage as the step size, and maintains each step for a certain period of time, recording the discharge quantity and phase distribution at each voltage. This method can accurately locate the position and extent of bushing insulation defects, and the changes in discharge quantity and phase distribution at different voltages can reflect the performance changes of the insulation material under different electric field strengths.

[0042] The withstand voltage test is a routine test that maintains the third voltage for a second period of time or a type test for a third period of time, and monitors the discharge trend; it can verify whether the insulation strength of the bushing meets the requirements under the specified voltage. Through the withstand voltage test, it can be ensured that the bushing can withstand the specified voltage in actual operation without breakdown or flashover.

[0043] The voltage is then reduced to the fourth voltage at a second rate, and the reversibility of the discharge is observed. Slowly reducing the voltage and observing the discharge can avoid excessive voltage changes and stress inside the bushing caused by sudden voltage reduction, thus reducing the risk of equipment damage. If the discharge is reversible, meaning the discharge phenomenon disappears or significantly weakens after voltage reduction, it may indicate that the defect is caused by some temporary factors, such as surface contamination or localized moisture. Cleaning and drying may restore the bushing's insulation performance. If the discharge is irreversible, meaning the discharge phenomenon persists or does not significantly improve after voltage reduction, it may mean that the bushing has serious insulation damage, requiring further inspection and repair.

[0044] The calibration analysis converts the measured discharge quantity into an equivalent power frequency discharge quantity according to the calibration formula, which is shown in the following formula:

[0045] in, This is the power frequency equivalent discharge quantity. To measure the actual discharge quantity, This is the discharge quantity correction factor.

[0046] Example 2 See Figure 2 A transformer partial discharge detection and testing system based on low-frequency modification, comprising: The low-frequency power supply module is used to supply power to the transformer using the modified low-frequency AC power supply system; The partial discharge detection module is used to perform partial discharge detection tests on the bushings of transformers powered by the modified low-frequency AC power supply system, and to obtain the partial discharge detection test results.

[0047] The system adopts a modular design with low-frequency power supply and partial discharge detection modules. Each module has a clearly defined function, making system installation, debugging, and maintenance easier for operators. When a module malfunctions, only that module needs repair or replacement, reducing maintenance costs and complexity. The modular design also facilitates integration and expansion with other related equipment or systems. For example, more detection modules can be added or data can be exchanged with other monitoring systems to achieve more comprehensive monitoring and analysis of transformer partial discharge. Through the coordinated operation of the low-frequency power supply and partial discharge detection modules, the system can automatically complete power supply and partial discharge detection tests on the transformer and directly obtain the test results. This significantly improves detection efficiency, reduces manual operation time and errors, and achieves automation and intelligence in the detection process.

[0048] Example 3 This embodiment provides a transformer partial discharge detection test method based on low-frequency modification. In this embodiment, the low-frequency modification transforms a 50Hz AC power supply system into a 20Hz AC power supply system. The modified 20Hz AC power supply system is then used for transformer partial discharge detection. The specific implementation is as follows: The parameters of the motor and generator before and after the modification are as follows: Figure 3 , Figure 4 As shown.

[0049] Upgrades to the 5000kW motor and generator unit (motor) in the ultra-high voltage power supply room: The existing high-voltage frequency converter is programmed to control the motor by adding operating frequency points. The original 50 / 60Hz selector switch is replaced with a 20-50-60Hz changeover switch. The frequency converter is given additional switch input points so that the frequency converter operates at 20Hz, corresponding to a speed of 300r / min.

[0050] Upgrades to a 30000kVA motor and generator unit: Add a forced air cooling system. Since both the motor and generator use built-in air circulation, the motor rotor and fan blades generate airflow based on the motor's rotation. This is only sufficient to cool the motor under load when operating at rated conditions: frequency: 50 / 60Hz, speed: 750 / 900 r / min. If the motor and generator are required to operate at 20Hz and 300 r / min, insufficient air cooling under load will result in overheating of the generator set. Therefore, forced air cooling is added by installing air supply and suction devices at the bottom of both the motor and generator to ensure compensatory cooling under low-frequency load conditions.

[0051] The specific plan for adding a forced air cooling system is as follows: replace the original cooler base frame with a sealed box, and install two sets of fans on each side of the sealed box to create an airflow circulation between the inside and outside of the box, resulting in air intake on the inside and air delivery on the outside. The fan motors are two-pole motors. The forced air cooling system added to the electric motor and generator is the same, only the fan power is different. At the same time, the cooler chamber door will be reinforced with a stronger seal to minimize air leakage to the outside.

[0052] Add an electrical control cabinet for the forced-air cooling equipment (cooling fans). The electrical control cabinet includes intelligent control instruments, a frequency converter starter bus, and a PLC + touchscreen. Automatic control of the cooling fans is implemented, utilizing the backup RTDs (thermal resistance temperature detectors) on the motors and generators. RTDs are a type of temperature sensor used to monitor the thermal state of the equipment, providing temperature feedback to help the control system regulate temperature or protect the equipment. Intelligent control instruments intelligently control the motors of the assigned fans, adjusting the temperature in real time based on the temperature information monitored and fed back by the temperature sensors (RTDs). When the temperature is too high, the fan speed is increased; when the temperature decreases, the fan speed is decreased to maintain the equipment within a suitable temperature range. The intelligent control instruments are also integrated into the generator and motor protection system to effectively prevent the generator set from automatically shutting off power in case of overheating. The protection action refers to the automatic power cut-off when the generator set overheats. However, a large amount of heat still accumulates inside the equipment, so the fans need to continue operating for cooling. Here, the intelligent control instruments identify the entire system's protection measures, thus preventing the generator set's protection action from cutting off power to the fans simultaneously. A frequency converter starter bus is installed to start all fans sequentially. Each fan is equipped with individual overload and short circuit protection. The control cabinet uses a PLC and touch screen for intelligent braking alarm control.

[0053] The control cabinet and the motor bottom cooler compartment should be installed in a suitable location nearby. The control cabinet consists of multiple cabinets.

[0054] Design calculations were performed for the fans and airflow. Based on the current cooler power and airflow parameters, design calculations were conducted for the parameters and airflow of the new fans to determine the appropriate size and installation location of the fans. The new fans should simultaneously meet the operating requirements of 20-50-60Hz. The preliminary design calculations are as follows: Generator: 4 x 25kW cooling fans; Motor: 4 x 20kW cooling fans. Total power: 180kW.

[0055] Configure the power supply for the wind turbine. The original distribution cabinet is located in the generator room and is used by the generator set. Select a suitable location in the original distribution cabinet and install a molded case circuit breaker with a rated current of not less than 400A. Connect the circuit breaker to the main power supply cabinet for this wind turbine via cables and cable trays.

[0056] Instructions for using the fan: After the modification, the cooling fan must be started and running regardless of the unit's operating frequency. For safety, electrical interlocks are installed on the fan system and the motor main control system: if the fan starts without fault, the unit can be started; if the fan fails during unit operation, the unit will trip and shut down.

[0057] The 50Hz AC power supply system was upgraded to a 20Hz AC power supply system, resulting in the upgraded 20Hz AC power supply system.

[0058] The key equipment involved in partial discharge detection of transformers powered by the modified 20Hz AC power supply system mainly includes: the modified 20Hz AC power supply system, coupling capacitors, detection impedance, signal lines, standard pulse generator, partial discharge detector, voltage transformer, and auxiliary equipment. Optional auxiliary equipment includes an infrared thermal imager and an ambient temperature and humidity sensor. The specific steps are as follows: Preliminary test preparation: Use a standard pulse generator to calibrate the detection system (optional, the standard pulse generator is 100pC) and verify the linearity of the voltage transformer at 20Hz, with an error of <±1%.

[0059] The voltage ramp-up process involves increasing the voltage at a rate of 1 kV / s to 0.8 times the rated voltage Ur during the pre-pressurization phase and maintaining it for 10 minutes to eliminate residual charge. During the step-up phase, the voltage is gradually increased in increments of 0.2 times the rated voltage Ur, with each increment maintained for 5 minutes, and the discharge quantity Q and phase distribution are recorded at each voltage. During the withstand voltage test phase, a conventional test is conducted at 1.3 times the rated voltage Ur for 30 minutes, or a type test for 60 minutes, while monitoring the discharge trend. During the voltage reduction observation phase, the voltage is reduced to 0.5 times the rated voltage Ur at a rate of 0.5 kV / s, and the reversibility of the discharge is observed.

[0060] Data acquisition was performed to meet the requirement of 20Hz pulse broadening, with a sampling rate of 50MS / s, analyzing the bandwidth in the range of 10kHz-2MHz. During the process, discharge quantity Q, discharge count N, phase φ, pulse waveform, and other data were recorded.

[0061] Furthermore, the collected data is analyzed to assess the discharge quantity. The measured discharge quantity is converted to a power frequency equivalent discharge quantity using a correction formula, and then compared with the IEC 60270 standard (where ≤10pC is considered acceptable) to evaluate the power frequency equivalent discharge quantity. The correction formula is: , in, This is the power frequency equivalent discharge quantity. To measure the actual discharge quantity, This is the discharge quantity correction factor. .

[0062] During the test, attention should be paid to trend judgment. If the discharge quantity increases exponentially with time at 1.3 times the rated voltage Ur, such as an increase of >20% every 10 minutes, it should be regarded as a dangerous sign and the test should be terminated immediately.

[0063] The detailed procedure for partial discharge detection under 20Hz power supply is as follows: Figure 5 As shown, the wiring diagram is as follows: Figure 6 As shown.

[0064] This embodiment uses the 5000kW motor and generator set in the ultra-high voltage generator room involved in the test as a blueprint, and modifies it to provide a generator set capable of supplying 20Hz AC power, providing a basis for conducting transformer partial discharge tests under 20Hz AC power supply. This modification fully utilizes the operating characteristics of the original generator set and the test requirements to achieve low-frequency modification with maximum efficiency. This embodiment of the modification has sufficient safety, robust installation, and reliable intelligent control.

[0065] This embodiment aims to conduct transformer partial discharge tests under a 20Hz AC power supply, which has the following application effects compared with traditional power frequency tests: At 20Hz, the energy of a single discharge is 2.5 times that at power frequency (formula: W∝1 / f), making it easier to detect insulation defects that are sensitive to high energy, such as delamination of oil paper and deep cracks. These defects may often be missed at power frequency due to insufficient discharge energy. In the actual operation of low-frequency power systems, this embodiment can play a specific role, such as being applied to transformer tests in low-frequency power supply scenarios such as rail transit (16.7Hz) and ship power systems (20-25Hz), making the results more consistent with actual operating conditions, thereby avoiding the problems of "over-design" or "under-testing" during power frequency tests. This embodiment can reduce signal overlap and improve detection resolution. At 20Hz, the period is increased from 20ms to 50ms, the pulse interval is extended, and the number of discharges is only 40% of that at power frequency, thereby reducing the probability of signal overlap and making it easier to separate individual discharge events. In particular, it enhances the ability to capture weak discharge signals (such as <10pC) and is suitable for field tests in high-noise environments. As can be seen from the dielectric loss power P∝f, the loss at 20Hz is only 40% of that at the power frequency, which makes the overall temperature rise of the insulation during the test smaller, close to the long-term low-load operation state of the transformer, and the results can better reflect the actual aging characteristics. Under low-frequency electric fields, the migration speed of charges inside the insulation slows down, causing charges to accumulate at defects (such as bubbles and impurities), forming stronger local electric field distortions, which makes it easier to trigger slowly developing defect discharges, i.e., it is more sensitive to slow-developing defects.

[0066] The following explanation of the invention is based on insulation diagnosis of a 110kV oil-immersed transformer: The system operates at a frequency of 16.7Hz. It is easy to see that the existing 50Hz power frequency test system cannot effectively detect insulation defects under low-frequency operation. In order to match the test power supply frequency with the actual operating conditions, a 20Hz test system is constructed.

[0067] The test equipment was set up. The key equipment used mainly included: a 20Hz AC power supply system modified from a 50Hz mains frequency, coupling capacitors (Haefely C8000, 1000pF / 200kV, frequency response 20Hz-10MHz), a partial discharge detector (OmicronMPD 600, bandwidth 10kHz-30MHz, sampling rate 200MS / s), a voltage divider (Tettex 2965, voltage division ratio 10000:1, accuracy 0.1 grade), and a standard pulse generator (Haefely TD610), etc.

[0068] Conduct preliminary testing. Use a standard pulse generator to inject a 50pC, 20Hz standard pulse to verify the linearity of the detection system, with an error of ±3%; calibrate the voltage divider under 20Hz conditions to ensure that the voltage measurement error is <±0.5%.

[0069] The voltage was increased. In the pre-pressurization stage, the voltage was increased to 88kV (0.8Ur) and held for 10 minutes to eliminate residual charge and monitor baseline noise. In the step-increase stage, the voltage was increased step by step in the order of 22-44-66-88, with each step lasting for 5 minutes. Q, N, and φ were recorded, and the QV curve was plotted. In the withstand voltage test stage, the voltage was increased to 143kV (1.3Ur) and held for 60 minutes, with the discharge trend recorded every 5 minutes. In the voltage reduction observation stage, the voltage was reduced to 55kV and held for 15 minutes to observe the reversibility of the discharge.

[0070] Data acquisition settings. The sampling rate was set to 200MS / s to meet the resolution requirements of a pulse width of 10ms at 20Hz, and to analyze the bandwidth of 30kHz-2MHz. During the experiment, the discharge quantity Q, the number of discharges N, and the average discharge power were recorded.

[0071] Recording and analysis of typical data. During the test, at the 143kV withstand voltage stage, the measured discharge quantity was 18.5pC, which was converted to a corrected discharge quantity of 29.2pC, exceeding the standard limit by 10pC. Combined with the PRPD spectrum, which showed a bimodal distribution, it was determined that this discharge was likely caused by insulation delamination defects due to insufficient impregnation of the oil paper during the production process. After disassembling the bushing, it was confirmed that there was a 0.5mm oil gap inside, which was consistent with the discharge location result.

[0072] Compared to the test results at 50Hz, this test improved the detection rate of oil paper delamination defects by 300%; furthermore, by modifying the power supply with a frequency converter, significant equipment investment was saved; finally, the test data, after conversion and correction, conformed to the IEC 60270 standard. IEC 60270:2000 "High-voltage test techniques - Partial discharge measurements".

[0073] This embodiment demonstrates the practical feasibility and significant value of transformer partial discharge testing under 20Hz AC power supply.

[0074] This embodiment provides a transformer partial discharge detection test method based on low-frequency retrofitting, and a low-frequency 20Hz power supply retrofitting scheme. Using a 5000kW motor and generator set in an ultra-high voltage power room as a model, it fully utilizes the original generator set's operating characteristics and testing requirements. The existing high-voltage frequency converter is programmed to add operating frequency points, replace the transfer switch, and add switching input points, achieving a conversion from power frequency to 20Hz, thus maximizing low-frequency retrofitting efficiency. This retrofitting method possesses sufficient safety, robust installation, and reliable intelligent control. Furthermore, the transformer partial discharge detection process at 20Hz provided in this embodiment has a targeted advantage in detecting defects such as delamination, cracks, or moisture intrusion. It also holds a significant advantage in low-frequency system verification in applications such as rail transit and shipbuilding, effectively compensating for the shortcomings of traditional power frequency testing and possessing irreplaceable value in special scenarios and in-depth testing.

[0075] Many embodiments and applications beyond the examples provided will be apparent to those skilled in the art upon reading the foregoing description. Therefore, the scope of this teaching should not be determined by reference to the foregoing description, but rather by reference to the foregoing claims and the full scope of their equivalents. For purposes of completeness, all articles and references, including patent applications and publications, are incorporated herein by reference. The omission of any aspect of the subject matter disclosed herein in the foregoing claims is not intended as a waiver of that subject matter, nor should it be construed as an indication that the applicant has not considered that subject matter as part of the disclosed inventive subject matter.

[0076] The above content provides a further detailed description of the present invention. It should not be construed that the specific embodiments of the present invention are limited to this. For those skilled in the art, several simple deductions or substitutions can be made without departing from the concept of the present invention, and all such deductions or substitutions should be considered to fall within the scope of protection defined by the present invention.

Claims

1. A test method for detecting partial discharge in transformers based on low-frequency retrofitting, characterized in that, Includes the following steps: The power frequency AC power supply system is modified to a low frequency, resulting in a modified low frequency AC power supply system. The transformer is powered by the modified low-frequency AC power supply system; Partial discharge detection tests were conducted on the bushings of transformers powered by the modified low-frequency AC power supply system, and the results of the partial discharge detection tests were obtained. The process of modifying the power frequency AC power supply system to a low frequency version results in a modified low-frequency AC power supply system, including modifications to the motor and generator. To control the motor by adding a frequency point, the power frequency selection switch is replaced with a low frequency to power frequency conversion switch, and the frequency converter's operating frequency and speed are changed to low frequency. The method of changing the frequency converter's operating frequency to low frequency is to add a switch input point to the frequency converter. A forced air cooling system is added to the motor and generator. The forced air cooling system is connected to the electrical control cabinet of the forced air cooling equipment. An electrical interlock is set between the forced air cooling system and the motor main control system. Specifically, the forced air cooling system is as follows: the base frame of the cooler is a sealed box, and fans are installed on both sides of the sealed box. Air is drawn in from the inside of the sealed box and air is delivered from the outside of the sealed box. The door of the cooler is sealed. The forced air cooling system is automatically controlled. The fans are connected to the electrical control cabinet of the cooling equipment. Each fan is equipped with individual overload protection and short circuit protection. The fan can start and operate at both power frequency and low frequency. The rule for installing an electrical interlock between the forced air cooling system and the motor main control system is as follows: if the fan starts without fault, the motor unit will start; if the fan fails during the operation of the motor unit, the unit will trip and stop.

2. The method for detecting partial discharge in a transformer based on low-frequency retrofitting according to claim 1, characterized in that, The electrical control cabinet of the cooling equipment includes intelligent control instruments, frequency converter start bus, PLC and touch screen; The forced air cooling system employs automatic control, including: Temperature sensors in the motor and generator monitor and feed back temperature information to the intelligent control instrument, which then controls the motor of each fan based on the temperature information. The intelligent control instrument identifies the over-temperature protection action of the generator and motor, and controls the fan to continuously cool the internal operation of the generator and motor. The PLC and touch screen in the electrical control cabinet of the cooling equipment are used for intelligent alarm control of the brakes.

3. The method for detecting partial discharge in a transformer based on low-frequency retrofitting according to claim 1, characterized in that, A circuit breaker is installed between the power supply of the fan and the distribution cabinet.

4. The method for detecting partial discharge in a transformer based on low-frequency retrofitting according to claim 1, characterized in that, The partial discharge detection test was conducted on the bushings of the transformer powered by the modified low-frequency AC power supply system, and the partial discharge detection test results were obtained as follows: Partial discharge detection tests are performed on the bushings of the transformer. The partial discharge detection tests include: The transformer bushings were subjected to pre-pressurization, step-by-step voltage increase, withstand voltage test, and voltage reduction observation in sequence. Data was collected during the process, including discharge quantity, discharge number, phase, and pulse waveform. The collected data were corrected and analyzed to obtain the correction analysis results. The correction analysis results were evaluated to obtain the partial discharge detection test results. If the discharge quantity during the partial discharge test increases exponentially with time, the withstand voltage test should be terminated immediately.

5. The method for detecting partial discharge in a transformer based on low-frequency retrofitting according to claim 4, characterized in that, The pre-pressurization is increased to a first voltage at a first rate; The stepped voltage increase is gradually increased with the second voltage as the step size, and each stage is maintained for a certain period of time. The discharge amount and phase distribution at each voltage are recorded. The withstand voltage test is a conventional test that maintains the voltage at the third voltage for a second period of time or a type test for a third period of time, and the discharge trend is monitored. The voltage reduction observation involves reducing the voltage to a fourth voltage at a second rate and observing whether the discharge is reversible.

6. The method for detecting partial discharge in a transformer based on low-frequency retrofitting according to claim 4, characterized in that, The correction analysis converts the measured discharge quantity into a power frequency equivalent discharge quantity according to the correction formula, which is shown in the following formula: in, This is the power frequency equivalent discharge quantity. To measure the actual discharge quantity, This is the discharge quantity correction factor.

7. A transformer partial discharge detection and testing system based on low-frequency retrofitting, characterized in that, include: The low-frequency power supply module is used to supply power to the transformer using the modified low-frequency AC power supply system; The partial discharge detection module is used to perform partial discharge detection tests on the bushings of transformers powered by the modified low-frequency AC power supply system, and to obtain the partial discharge detection test results. The process of modifying the power frequency AC power supply system to a low frequency version results in a modified low-frequency AC power supply system, including modifications to the motor and generator. To control the motor by adding a frequency point, the power frequency selection switch is replaced with a low frequency to power frequency conversion switch, and the frequency converter's operating frequency and speed are changed to low frequency. The method of changing the frequency converter's operating frequency to low frequency is to add a switch input point to the frequency converter. A forced air cooling system is added to the motor and generator. The forced air cooling system is connected to the electrical control cabinet of the forced air cooling equipment. An electrical interlock is set between the forced air cooling system and the motor main control system. Specifically, the forced air cooling system is as follows: the base frame of the cooler is a sealed box, and fans are installed on both sides of the sealed box. Air is drawn in from the inside of the sealed box and air is delivered from the outside of the sealed box. The door of the cooler is sealed. The forced air cooling system is automatically controlled. The fans are connected to the electrical control cabinet of the cooling equipment. Each fan is equipped with individual overload protection and short circuit protection. The fan can start and operate at both power frequency and low frequency. The rule for installing an electrical interlock between the forced air cooling system and the motor main control system is as follows: if the fan starts without fault, the motor unit will start; if the fan fails during the operation of the motor unit, the unit will trip and stop.

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