Transformer partial discharge detection test method based on low-frequency transformation

Abstract

The invention discloses a transformer partial discharge detection test method based on low-frequency transformation, and belongs to the technical field of transformer test partial discharge detection, and the method comprises the steps: carrying out the low-frequency transformation of a power frequency AC power supply system, and obtaining a transformed low-frequency AC power supply system; the transformed low-frequency alternating-current power supply system is used for supplying power to the transformer; and performing a partial discharge detection test on the bushing of the transformer powered by the transformed low-frequency alternating-current power supply system to obtain a partial discharge detection test result. According to the invention, the single discharge energy is higher at low frequency, which is beneficial to excitation of deep insulation defects; the discharge pulse interval is increased, the pulse overlapping probability is reduced, and misjudgment and missing detection are reduced; the method is more sensitive to slow development defects; and the discharge mode is closer to actual operation, and the insulation condition can be accurately evaluated. According to the method, the problem of traditional power frequency detection is effectively solved, multiple detection scenes and high standards are met, and universality, universality and transportability are achieved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of partial discharge detection of transformer test, and particularly relates to a partial discharge detection test method for transformer based on low-frequency modification. BACKGROUND

[0002] Partial discharge (PD) is an important sign of deterioration of transformer insulation quality, and the partial discharge detection technology of transformer is particularly important for factory evaluation and health state detection of transformer. In the existing transformer test technology, the mainstream detection methods of partial discharge include pulse current method, ultrasonic method, ultra high frequency (UHF) method, high frequency current transformer (HFCT) method and chemical analysis method. Each method has its own characteristics and limitations, and the pulse current method is relatively classical and standardized.

[0003] When partial discharge occurs, the electric field intensity at the insulation defect (such as bubble, crack) exceeds the dielectric breakdown threshold, and a transient current pulse is triggered. The pulse will form a high-frequency current signal (typical frequency band is 10 kHz-1 MHz) in the equipment grounding loop, and the pulse current detection method under the traditional IEC 60270 standard basically principle is to detect the transient current pulse signal generated by partial discharge under power frequency (50Hz / 60Hz) alternating current power supply through coupling capacitor or Rogowski coil, and convert it into voltage signal, and then quantify the discharge amount (pC, picocoulomb). The charge quantity is calculated by measuring the integral of the current pulse, and the discharge amount Q calculation formula is: The detection system includes a sensor, a signal conditioning circuit and a data acquisition and analysis device, such as a partial discharge detector.

[0004] As described above, the classical partial discharge test is carried out under power frequency (50Hz / 60Hz) alternating current power supply, however, the test under power frequency condition inevitably has some defects, which cannot more comprehensively, efficiently and stably detect partial discharge: first, the single discharge energy is low, which may not effectively trigger the deep insulation defect (such as oil-paper delamination, internal crack) needing high energy trigger, resulting in missed detection; secondly, due to the 50Hz cycle of only 20ms, the discharge pulse interval is short, and under high discharge rate, pulse overlap easily occurs, resulting in false judgment or missed detection of the detection system; in addition, the electric field changes fast under power frequency, and the charge migration rate is high, which is difficult to form a sustained electric field distortion at the slowly developing defect (such as moisture diffusion), and the sensitivity to slow defect is insufficient; furthermore, the loss is high at 50Hz, which may cause the overall temperature rise of the insulation material, change the dielectric properties of the material, and may cause the discharge mode to deviate from the real operating state. SUMMARY

[0005] The present application aims to overcome the problem of the defect of transformer partial discharge detection test under power frequency conditions, and proposes a transformer partial discharge detection test method based on low frequency modification.

[0006] In order to achieve the above-mentioned purpose, the present application adopts the following technical scheme: In the first aspect, the present application provides a transformer partial discharge detection test method based on low frequency modification, comprising the following steps: Low frequency modification is made to the power frequency AC power supply system to obtain the modified low frequency AC power supply system; The modified low frequency AC power supply system is used to supply power to the transformer; The partial discharge detection test is carried out on the bushing of the transformer supplied by the modified low frequency AC power supply system to obtain the partial discharge detection test result.

[0007] Further, low frequency modification is made to the power frequency AC power supply system to obtain the modified low frequency AC power supply system, including modification of the motor and the generator: The motor is increased to run at a frequency point, the power frequency selection switch is replaced with a low frequency-power frequency conversion switch, the frequency converter running frequency is changed to low frequency, and the speed is changed to low frequency speed, and the frequency converter running frequency is changed to low frequency, which is achieved by increasing the switch input point of the frequency converter; A forced air cooling system is added to the motor and the generator, the forced air cooling system is connected to the electrical control cabinet of the forced air cooling device, an electrical interlock is provided between the forced air cooling system and the motor main control system, and the forced air cooling system is specifically: the bottom frame of the cooler adopts a closed box, and fans are installed on both sides of the closed box, the inside of the closed box is air suction, the outside of the closed box is air supply, the door of the cooler is sealed, the forced air cooling system adopts automatic control, the fan is connected to the electrical control cabinet of the cooling device, and each fan is provided with separate overload protection and short circuit protection.

[0008] Further, the electrical control cabinet of the cooling device includes an intelligent control instrument, a frequency converter starting bus, a PLC and a touch screen; The automatic control of the forced air cooling system includes: The temperature sensor of the motor and the generator monitors and feeds back temperature information to the intelligent control instrument, and the intelligent control instrument controls the motor of each fan according to the temperature information; The intelligent control instrument identifies the over-temperature protection action of the generator and the motor, and controls the fan to continuously cool the internal operation of the generator and the motor; The PLC and the touch screen of the electrical control cabinet of the cooling device are used for braking intelligent alarm control.

[0009] Further, a circuit breaker is installed between the power supply of the fan and the power distribution cabinet.

[0010] Further, the fan is started and runs at the power frequency and the low frequency.

[0011] Further, the electrical interlocking rule between the forced air cooling system and the motor master control system is that if the fan is started without failure, the motor set is started; if the fan fails during the operation of the motor set, the motor set is tripped and stopped.

[0012] Further, the partial discharge detection test is performed on the bushing of the transformer powered by the transformed low-frequency AC power supply system, and the partial discharge detection test result is obtained, which is specifically: The partial discharge detection test is performed on the bushing of the transformer, and the partial discharge detection test includes: The bushing of the transformer is sequentially subjected to pre-voltage, step-voltage, withstand voltage test, and voltage reduction observation, data is collected in the process, the data includes discharge quantity, discharge times, phase, and pulse waveform, the collected data is corrected and analyzed to obtain the correction and analysis result, the correction and analysis result is evaluated to obtain the partial discharge detection test result; If the discharge quantity of the withstand voltage test increases exponentially with time during the partial discharge detection, the withstand voltage test is immediately terminated.

[0013] Further, the pre-voltage is increased to the first voltage at the first rate; The step-voltage is gradually increased by the second voltage as a step size, each step is maintained for the first period of time, and the discharge quantity and phase distribution under each voltage are recorded; The withstand voltage test is a conventional test at the third voltage for the second period of time or a type test for the third period of time, and the discharge trend is monitored; The voltage reduction observation is reduced to the fourth voltage at the second rate, and whether the discharge is reversible is observed.

[0014] Further, the correction and analysis convert the measured discharge quantity into the power frequency equivalent discharge quantity according to the correction formula, and the correction formula is as shown in the following formula:

[0015] wherein, the power frequency equivalent discharge quantity is Q, the measured discharge quantity is Qm, and the discharge quantity correction coefficient is K.

[0016] In a second aspect, the present application provides a partial discharge detection test system for a transformer based on low-frequency transformation, comprising: A low-frequency power supply module for powering the transformer by using the transformed low-frequency AC power supply system; A partial discharge detection module for performing a partial discharge detection test on the bushing of the transformer powered by the transformed low-frequency AC power supply system, and obtaining a partial discharge detection test result.

[0017] Compared with the prior art, the present application has the following beneficial technical effects: The transformer partial discharge detection test method based on low-frequency modification provided by the present application has the following advantages: after the low-frequency modification, the single discharge energy is relatively higher at low-frequency voltage than at power frequency. This is because the discharge energy is related to factors such as voltage and charge quantity. At low frequency, the charge has a longer time to accumulate, and when discharge occurs, the released energy is greater. For deep insulation defects that require high-energy triggering, higher discharge energy can more effectively stimulate these defects to produce discharge phenomena, thereby reducing missed detection and improving the detection capability for deep insulation defects. The cycle of low-frequency voltage is longer, and the longer cycle increases the interval between discharge pulses, reducing the probability of pulse overlap. The detection system can more clearly distinguish each discharge pulse, thereby reducing the occurrence of misjudgment or missed detection and improving the accuracy of the detection results. At low-frequency voltage, the electric field changes slowly, and the charge migration rate is relatively low. This allows the charge to have enough time to accumulate at slowly developing defects, forming a sustained electric field distortion, thereby more easily stimulating discharge phenomena. For slowly developing defects, low-frequency detection can more sensitively capture the discharge signals caused by the defects, improving the detection capability for slow defects. At low-frequency voltage, the loss is relatively low, and the temperature rise of the insulation material is small. The dielectric properties of the insulation material are closer to the actual operating state, and the discharge mode more realistically reflects the situation of the transformer in actual operation. The detection results can more accurately evaluate the insulation condition of the transformer, providing a more reliable basis for the maintenance and repair of the equipment. The transformer partial discharge detection test based on low-frequency modification effectively solves many problems existing in traditional power-frequency partial discharge detection tests by increasing discharge energy, increasing pulse interval, enhancing sensitivity to slow defects, and maintaining the authenticity of the discharge mode. It can meet the increasingly diverse detection scenarios and gradually improving industry standards, make up for the defects in the field of transformer partial discharge detection tests, and fully have universality, universality and portability. BRIEF DESCRIPTION OF DRAWINGS

[0018] The drawings described herein are for illustrative purposes only and are not intended to limit the scope of the present disclosure in any way. In addition, the shapes and scale dimensions of the components in the figures are only illustrative and are used to help understand the present application and are not specific limitations on the shapes and scale dimensions of the components. In the drawings: Figure 1 A flowchart of the transformer partial discharge detection test method based on low-frequency modification of the present application.

[0019] Figure 2 A structure diagram of the transformer partial discharge detection test system based on low-frequency modification of the present application.

[0020] Figure 3The 50Hz motor and generator equipment parameters before modification in the embodiment of the present application.

[0021] Figure 4 The 20Hz motor and generator equipment parameters after modification in the embodiment of the present application.

[0022] Figure 5 The detailed flow of the partial discharge detection test under 20Hz power supply in the embodiment of the present application.

[0023] Figure 6 The wiring schematic diagram of the partial discharge detection test under 20Hz power supply in the embodiment of the present application. DETAILED DESCRIPTION

[0024] In order to make the personnel in the art better understand the present application scheme, the technical scheme in the embodiment of the present application will be described clearly and completely below in combination with the drawings in the embodiment of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by the person of ordinary skill in the art without creative labor should belong to the scope of protection of the present application.

[0025] It should be noted that when an element is referred to as being "on" another element, it can be directly on the other element or there can be one or more intervening elements. When an element is referred to as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element or intervening elements can be present. As used herein the terms "vertical", "horizontal", "left", "right" and the like are merely used for the purpose of illustration and are not intended to be limiting.

[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 application belongs. The terminology used in the description of the application herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0027] It is to be understood that the terminology "first", "second" and the like used in the specification and the claims of the application as well as the foregoing drawings is merely intended to distinguish between similar objects and not necessarily to describe a particular sequential or chronological order. It is to be understood that the use of the terms data, information, etc. can be interchanged where appropriate to refer to the same or similar concepts in different embodiments of the application described herein. Furthermore, the terms "comprise", "comprising", "include", "including", and their syntactic combinations, are intended to be inclusive or open-ended and not exclude additional, unrecited elements or methods. It is to be understood that the use of the singular herein, like "a", "said", "the", etc., includes the plural or plural iteration unless expressly contradicted by the context.

[0028] Embodiment one Referring to Figure 1 A transformer partial discharge detection test method based on low-frequency modification, comprising the following steps: The power frequency alternating current power supply system is modified to obtain a modified low-frequency alternating current power supply system; The transformer is powered by the modified low-frequency alternating current power supply system; The partial discharge detection test is performed on the bushing of the transformer powered by the modified low-frequency alternating current power supply system, and the partial discharge detection test result is obtained.

[0029] Under low-frequency conditions, the voltage stress on the insulation of the transformer is relatively low, so the required test voltage for the partial discharge detection test is also correspondingly reduced. This not only reduces the capacity requirement of the test equipment, but also reduces the safety risk during the test, making the test easier to implement. Due to the reduction of the test voltage, the specifications and costs of the equipment required for the test (such as high-voltage power supply, measuring instruments, etc.) are also correspondingly reduced. Under low-frequency conditions, the propagation and attenuation characteristics of the electromagnetic signals generated by partial discharge inside the transformer change, making it easier for detection instruments to capture these signals. This helps to improve the identification accuracy of partial discharge and reduce the possibility of misjudgment and missed judgment. There are a large number of electromagnetic interferences in the power frequency alternating current power supply system, which may mask the partial discharge signals and affect the accuracy of the detection results. By low-frequency modification, the influence of these interferences can be reduced, and the reliability of the detection results can be improved. Performing a partial discharge detection test under low-frequency conditions can reduce damage to the insulation of the transformer. Through regular low-frequency partial discharge detection tests, potential faults and insulation defects inside the transformer can be found in a timely manner. 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 detection test method based on low-frequency modification can reduce the difficulty and cost of the test, making it possible to detect the partial discharge of large transformers.

[0030] The low-frequency modification is performed on the power frequency AC power supply system to obtain a modified low-frequency AC power supply system, including modified motor and modified generator: In the motor, the operating frequency point control motor is added, the power frequency selection switch is replaced with a low-frequency-power frequency conversion switch, the frequency converter operating frequency is changed to low frequency, and the speed is changed to low frequency speed. By adding operating frequency point control motor, replacing the power frequency selection switch with a low-frequency-power frequency conversion switch, and changing the frequency converter operating frequency and speed to low frequency parameters, the motor can operate in a low-frequency state, thereby providing suitable low-frequency power supply conditions for transformer partial discharge detection tests based on low-frequency modification, and meeting the special requirements of specific test scenarios for power frequency. The setting of the low-frequency-power frequency conversion switch facilitates flexible switching between low-frequency tests and normal power frequency operation, improving the applicability and flexibility of the system, and quickly adjusting the operating state of the motor according to different needs. Adding the switching quantity input point of the frequency converter to change the operating frequency to low frequency can more accurately control the operating frequency of the motor, ensuring the stability and accuracy of low-frequency operation and providing reliable power supply for tests.

[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 device, an electrical interlock is provided between the forced air cooling system and the motor main control system, and the forced air cooling system specifically comprises: a closed box is used for the bottom frame of the cooler, and fans are installed on both sides of the closed box; the inside of the closed box is air suction, and the outside of the closed box is air supply; 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 device, and each fan is provided with separate overload protection and short circuit protection.

[0032] When the motor and generator are running at low frequency, the heat dissipation condition may become poor due to the reduced rotating speed. The forced air cooling system can timely take away the heat generated by the motor by forced ventilation through the fan, ensure the motor to run at a suitable temperature, avoid performance degradation, insulation damage and other problems caused by overheating, and protect the normal operation of the equipment. The cooler chassis adopts a sealed box design, and fans are installed on both sides. The inside of the box absorbs air, and the outside blows air. At the same time, the room door is sealed. This design can form an effective air circulation channel, improve the heat dissipation efficiency, and make the heat generated by the motor dissipate more quickly. The forced air cooling system and the motor main control system are electrically interlocked. When the motor starts, the forced air cooling system automatically starts. When the motor stops, the forced air cooling system also stops. This interlocking mechanism can ensure that the motor is always effectively cooled during operation, avoid overheating damage caused by cooling system failure, and enhance the safety of the system. Each fan is provided with separate overload protection and short circuit protection, which can timely find and handle the faults occurring during the operation of the fan, prevent the faults from expanding to affect the normal operation of the entire cooling system or even the motor system, and further improve the reliability and safety of the system. The forced air cooling system adopts automatic control, and the fan is connected to the electrical control cabinet of the cooling equipment. It can automatically adjust the start-stop and running state of the fan according to the running state of the motor, reduce the tediousness and error of manual operation, and improve the automation level and running efficiency of the system.

[0033] The electrical control cabinet of the cooling equipment includes an intelligent control instrument, a frequency converter starting bus, a PLC, and a touch screen. The automatic control of the forced air cooling system includes: The temperature sensor of the motor and generator monitors and feeds back temperature information to the intelligent control instrument, which controls the motor of each fan according to the temperature information through the intelligent control instrument; 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 of the electrical control cabinet of the cooling equipment are used for braking intelligent alarm control.

[0034] The intelligent control instrument accurately receives temperature information from temperature sensors and precisely controls each fan motor based on pre-set algorithms and logic. The VFD starting busbar, used in conjunction with the VFD, allows flexible adjustment of fan voltage and frequency based on actual needs, enabling precise adjustment of fan speed to meet cooling requirements under varying operating conditions. During fan startup, the VFD starting busbar and VFD work together to achieve soft start, effectively reducing starting current, minimizing impact on the power grid, and extending the service life of the fan and associated electrical equipment. The PLC offers powerful programming capabilities, enabling flexible control programming tailored to the actual operating conditions and cooling requirements of the motor and generator. The touchscreen provides an intuitive human-machine interface, allowing operators to easily configure cooling system parameters such as temperature thresholds and fan operating modes. Temperature sensors monitor the motor and generator temperatures in real time and provide timely feedback to the intelligent control instrument. Based on this temperature information, the intelligent control instrument rapidly adjusts the fan's operating status to ensure the equipment always operates within the appropriate temperature range.

[0035] A circuit breaker is installed between the fan's power supply and the power distribution cabinet. When a fan fault occurs, the circuit breaker automatically trips and cuts off the power supply. Based on the circuit breaker's trip status, maintenance personnel can quickly determine whether the fault may lie within the fan or its associated wiring, narrowing the scope of troubleshooting. During maintenance, the circuit breaker can cut off the power supply, providing a safe operating environment for maintenance personnel and avoiding maintenance operations while energized, thereby reducing maintenance risks. The installation of a circuit breaker effectively isolates the faulty fan, protecting other equipment from the fault and minimizing economic losses caused by equipment damage. The circuit breaker has remote control capabilities and can be integrated with the power distribution cabinet's automated control system. Through the automated control system, operators can remotely control the circuit breaker's opening and closing operations, enabling remote starting and stopping of the fan. The circuit breaker works in conjunction with other protective devices in the power distribution cabinet to form a complete protection system.

[0036] The fan starts and runs at both industrial 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: if there is no fault after the fan is started, the motor unit will be started; if the fan fails during the operation of the motor unit, the unit will trip and shut down.

[0038] A partial discharge test was conducted on the bushing of a transformer powered by the modified low-frequency AC power supply system. The test results are as follows: Conduct partial discharge test on transformer bushing. The partial discharge test includes: The bushing of the transformer is sequentially subjected to pre-pressurization, step-by-step voltage increase, withstand voltage test, voltage decrease observation, data is collected in the process, the data includes discharge quantity, discharge times, phase, pulse waveform, the collected data is corrected and analyzed to obtain correction analysis results, the correction analysis results are evaluated to obtain partial discharge detection test results; If the discharge quantity of the withstand voltage test during the partial discharge detection process increases exponentially with time, the withstand voltage test is immediately terminated.

[0039] The collected data is corrected and analyzed, various errors and interference factors in the measurement process can be eliminated, the correction analysis results are evaluated, whether the partial discharge of the bushing is within the normal range and whether there is a potential safety hazard can be determined. Exponential growth of discharge quantity indicates that the partial discharge of the bushing is rapidly deteriorating, there may be serious insulation defects, and continuing the test may cause the bushing to break down, or even cause more serious safety accidents. Timely termination of the test can avoid equipment damage and personnel injury, and ensure the safe performance of the test. Regular partial discharge detection tests can timely detect and handle insulation problems of the bushing, preventing further deterioration of insulation performance and prolonging the service life of the transformer.

[0040] The pre-pressurization is increased to the first voltage at a first rate; it can make the bushing of the transformer gradually adapt to the voltage change, eliminate the possible residual charge and internal stress unevenness.

[0041] The step-by-step voltage increase gradually increases the voltage by a second voltage as a step, each level is maintained for a first period of time, and the discharge quantity and phase distribution under each voltage are recorded; it can accurately locate the position and degree of insulation defects of the bushing, and the change of discharge quantity and phase distribution under different voltages can reflect the performance change of insulation materials under different electric field strengths.

[0042] The withstand voltage test is a conventional test maintained at a third voltage for a second period of time or a type test maintained at a third voltage for a third period of time, and the discharge trend is monitored; it can verify whether the insulation strength of the bushing at the specified voltage meets the requirements, and through the withstand voltage test, it can ensure that the bushing can withstand the specified voltage without breakdown or flashover failure in actual operation.

[0043] The voltage decrease observation decreases the voltage to a fourth voltage at a second rate, and observes whether the discharge is reversible. Slowly decreasing the voltage and observing the discharge condition can avoid the generation of excessive voltage change and stress in the bushing due to sudden voltage decrease, thereby reducing the risk of equipment damage. If the discharge is reversible, i.e., the discharge phenomenon disappears or significantly weakens after the voltage decrease, it may indicate that the defect is caused by some temporary factors, such as surface contamination, local dampness, etc., and the insulation performance of the bushing may be restored through cleaning, drying, etc. If the discharge is irreversible, i.e., the discharge phenomenon still exists or does not significantly improve after the voltage decrease, it may mean that the bushing has a relatively serious insulation damage, which needs to be further checked and repaired.

[0044] The correction analysis converts the measured discharge quantity into the power frequency equivalent discharge quantity according to the correction formula, and the correction formula is as shown in the following formula:

[0045] wherein, is the power frequency equivalent discharge quantity, is the measured discharge quantity, is the discharge quantity correction coefficient.

[0046] Example two Referring to Figure 2 , a transformer partial discharge detection test system based on low-frequency modification, comprising: A low-frequency power supply module for supplying power to the transformer using the modified low-frequency AC power supply system; A partial discharge detection module for performing partial discharge detection tests on the bushings of the transformer supplied by the modified low-frequency AC power supply system to obtain partial discharge detection test results.

[0047] The system adopts modular design of low-frequency power supply module and partial discharge detection module, and each module has clear function, so that the operator can more conveniently install, debug and maintain the system. When a module fails, only the module needs to be repaired or replaced, reducing the maintenance cost and difficulty. Modular design makes the system easier to integrate and expand with other related devices or systems. For example, more detection modules can be added or data interaction with other monitoring systems according to actual needs, to realize more comprehensive monitoring and analysis of transformer partial discharge. Through the cooperative work of the low-frequency power supply module and the partial discharge detection module, the system can automatically complete the power supply and partial discharge detection test of the transformer and directly obtain the detection results. This greatly improves the detection efficiency, reduces the time and error of manual operation, and realizes the automation and intelligentization of the detection process.

[0048] Example three The present embodiment provides a transformer partial discharge detection test method based on low-frequency modification. The low-frequency modification of the present embodiment modifies the power frequency 50Hz AC power supply system into a 20Hz AC power supply system, and uses the modified 20Hz AC power supply system to detect the transformer partial discharge. The specific implementation is as follows: The parameters of the motor and generator equipment before and after modification are shown in Figure 3 , Figure 4 respectively.

[0049] The 5000kW motor and generator unit (motor) in the ultra-high voltage machine room is modified: The existing high voltage frequency converter is programmed, the running frequency point is increased to control the motor, the original 50 / 60Hz selection switch is replaced by a 20-50-60Hz switch, the frequency converter increases the switch input point, and the frequency converter runs at 20Hz, corresponding to the speed of 300r / min.

[0050] The 30000kVA motor and generator set is reformed: The forced air cooling system is increased. Since the motor and the generator both use built-in air circulation, the motor rotor adds wind blades to rely on the rotation of the motor to generate air circulation, which can meet the cooling of the motor under the rated condition: frequency: 50 / 60Hz, speed: 750 / 900r / min. If the motor and the generator are required to run at a frequency of 20Hz and a speed of 300r / min, it will cause insufficient air cooling effect of the motor under load, causing the motor set to heat up. Therefore, forced air cooling is increased, and air supply and air suction equipment is added at the bottom of the motor and the generator to ensure the compensation cooling of the motor under low frequency load.

[0051] The specific scheme of the forced air cooling system is to replace the original cooler bottom frame with a sealed box, and two sets of fans are arranged on both sides of the sealed box to form an air circulation path inside and outside the box, so that air is sucked inside and air is supplied outside. The fan motor is a two-pole motor. The forced air cooling system of the motor and the generator is the same, only the power of the fan is different. At the same time, the cooler room door is sealed to minimize air leakage.

[0052] The electrical control cabinet of the forced air cooling equipment (cooling fan) is increased. The electrical control cabinet includes intelligent control instruments, frequency converter starting bus, PLC+touch screen; the cooling fan is automatically controlled, the motor and the generator are provided with a standby thermal resistance, the thermal resistance is a temperature sensor, which is used to monitor the heat state of the equipment, feedback temperature information, help the control system to regulate and control temperature or protect the equipment, the intelligent control instrument is used to intelligently control the motor of the fan, the temperature information of the equipment monitored and fed back by the temperature sensor (thermal resistance) is real-time regulated and controlled, when the temperature is too high, the fan is strengthened, when the temperature is reduced, the fan is weakened, to maintain the equipment in a suitable temperature range; and the intelligent control instrument is included in the generator and motor protection system to effectively prevent the motor set from over-temperature failure. The protection action refers to the automatic power-off of the motor set when the temperature is too high. However, a large amount of temperature still accumulates in the equipment, so the fan needs to continue to operate to cool down. Here, the intelligent control instrument identifies a complete set of system protection measures, so as to prevent the phenomenon that the protection action of the generator set is cut off together with the fan. A set of frequency converter starting bus is set to start the fans one by one. Each fan is provided with separate overload and short circuit protection. The control cabinet is controlled by a set of PLC+touch screen.

[0053] The control cabinet and the motor bottom cooler room are installed at a suitable location nearby. The control cabinet is composed of multiple cabinets.

[0054] The fan and the air path are designed and calculated. According to the current cooler power, air path parameters, etc., the newly added fan parameters and air path are designed and calculated to determine the appropriate size and installation position of the fan. The newly added fan should also meet the use requirements of 20-50-60Hz. The preliminary design calculation is as follows: 4 sets of 25kW cooling fans for generators; 4 sets of 20kW cooling fans for motors. The total power is 180kW.

[0055] The fan power supply is set. The original distribution cabinet is set in the generator room for use by the generator set. A plastic shell circuit breaker with a rated current not less than 400A is installed at a suitable position in the original distribution cabinet. The total power supply cabinet of the fan is introduced through cables and bridge.

[0056] The use of the fan is explained: after the transformation, the cooling fan must be started regardless of the frequency at which the unit is running. In order to be safe, the fan system and the motor main control system are equipped with electrical interlocking: if the fan starts without failure, the unit can be started; if the fan fails during unit operation, the unit trips and stops.

[0057] The transformation from the power frequency 50Hz AC power supply system to the 20Hz AC power supply system is completed, and the transformed 20Hz AC power supply system is obtained.

[0058] The transformer partial discharge detection using the transformed 20Hz AC power supply system involves the following key equipment: the transformed 20Hz AC power supply system, the coupling capacitor, the detection impedance, the signal line, the standard pulse generator, the partial discharge detector, the voltage transformer, and the auxiliary equipment, which can be selected from an infrared thermal imager and an environmental temperature and humidity sensor. The specific steps are as follows: Pre-test preparation, using a standard pulse generator to calibrate the detection system, and the standard pulse generator can be selected as 100pC. The linearity of the voltage transformer at 20Hz is verified, and the error is <±1%.

[0059] Boosting, in the pre-voltage stage, it is raised to 0.8 times the rated voltage Ur at a rate of 1kV / s, and maintained for 10min to eliminate residual charge; in the step-up stage, it is gradually raised by 0.2 times the rated voltage Ur as a step, each step is maintained for 5min, and the discharge quantity Q and phase distribution at each voltage are recorded; in the withstand voltage test stage, it is maintained at 1.3 times the rated voltage Ur for 30min of conventional test or 60min of type test, and the discharge trend is monitored during this process; in the voltage observation stage, it is reduced to 0.5 times the rated voltage Ur at a rate of 0.5kV / s, and it is observed whether the discharge is reversible.

[0060] Data acquisition, meet the demand of 20Hz pulse broadening, set the sampling rate of 50MS / s, analyze the bandwidth of 10kHz-2MHz. And record the discharge Q, discharge N, phase φ, pulse waveform and other content in the process.

[0061] Further, the collected data is analyzed and the discharge amount is evaluated. The measured discharge amount is converted into power frequency equivalent discharge amount according to the correction formula, and the power frequency equivalent discharge amount is evaluated according to the IEC 60270 standard (such as ≤10pC is qualified). The correction formula is: , Among them, is the power frequency equivalent discharge amount, is the measured discharge amount, is the discharge correction coefficient, .

[0062] During the test, attention should be paid to trend judgment. Under 1.3 times rated voltage Ur, if the discharge amount increases exponentially with time, such as increasing by >20% every 10min, it should be considered as a dangerous sign and the test should be terminated immediately.

[0063] The detailed process of the partial discharge detection test under 20Hz power supply is shown in Figure 5 , and the wiring diagram is shown in Figure 6 .

[0064] This embodiment takes the 5000kW motor and generator set in the ultra-high voltage room involved in the test as the blueprint, and modifies the generator set that can supply 20Hz AC power supply, which provides the basis for the transformer partial discharge test under 20Hz AC power supply. This modification makes full use of the operating characteristics of the original generator set and the test requirements, and realizes the maximum efficiency of the low-frequency modification. This modification has sufficient safety, firmness of installation and reliability of intelligent control.

[0065] This embodiment aims to set the transformer partial discharge test under 20Hz AC power supply, which has the following application effects compared with the traditional power frequency test: At 20Hz, the single discharge energy is 2.5 times that at power frequency (formula: W∝1 / f), so it is easier to detect insulation defects sensitive to high energy, such as oil-paper delamination and deep cracks, which may be missed at power frequency due to insufficient discharge energy; In the actual operation of low-frequency power system, this embodiment can play a specific role, such as transformer test in low-frequency power supply scenarios such as rail transit (16.7Hz) and ship power system (20-25Hz), so that the results are more consistent with the actual working condition, thereby avoiding the problem of "overdesign" or "underdetection" in power frequency test; The embodiment can reduce signal overlap and improve detection resolution. At 20 Hz, the period increases from 20 ms to 50 ms, the pulse interval is lengthened, the discharge frequency is only 40% of the power frequency, the signal overlap probability is reduced, the single discharge event is separated, the capture ability of weak discharge signals (such as <10 pC) is enhanced, and the on-site test in a high-noise environment is suitable; It is known from the dielectric loss power P∝f that the loss at 20 Hz is only 40% of the power frequency, so that the overall insulation temperature rise during the test is smaller, close to the long-term low-load operation state of the transformer, and the result can better reflect the actual aging characteristics; Under a low-frequency electric field, the charge migration speed in the insulation is slowed down, the charge is easy to accumulate at defects (such as bubbles and impurities), a stronger local electric field distortion is formed, and the slowly developing defect discharge is more likely to be triggered, that is, the sensitivity to slow defects is stronger.

[0066] The following describes the insulation diagnosis of a certain 110 kV oil-immersed transformer, and the application is further explained and described: the system operating frequency is 16.7 Hz, and it is easy to know that the existing 50 Hz power frequency test system cannot effectively detect the insulation defects under low-frequency operation. In order to match the test power frequency with the actual working condition, a 20 Hz test system is constructed.

[0067] The test equipment is set. The key equipment used mainly includes: a 20 Hz alternating current power supply system modified from a 50 Hz power frequency, a coupling capacitor (Haefely C8000, 1000 pF / 200 kV, frequency response 20 Hz-10 MHz), a partial discharge detector (Omicron MPD 600, bandwidth 10 kHz-30 MHz, sampling rate 200 MS / s), a voltage divider (Tettex 2965, voltage division ratio 10000:1, accuracy 0.1 level), a standard pulse generator (Haefely TD610), and the like.

[0068] Pre-test preparation is performed. The standard pulse generator is used to inject 50 pC, 20 Hz standard pulses, the linearity of the detection system is verified, and the error is ±3%; the voltage divider is calibrated under the condition of 20 Hz to ensure that the voltage measurement error is <±0.5%.

[0069] The voltage is raised. In the pre-voltage raising stage, the voltage is raised to 88 kV (0.8 Ur) and maintained for 10 min to eliminate residual charge and monitor the baseline noise; in the step-voltage raising stage, the voltage is raised in the order of 22-44-66-88, each step is maintained for 5 min, Q, N and φ are recorded, and a Q-V curve is drawn; in the withstand voltage test stage, the voltage is raised to 143 kV (1.3 Ur) and maintained for 60 min, and the discharge trend is recorded every 5 min; in the voltage lowering observation stage, the voltage is lowered to 55 kV and maintained for 15 min, and the discharge reversibility is observed.

[0070] Data acquisition settings. Set the sampling rate to 200MS / s to meet the resolution requirements of the pulse width of 10ms at 20Hz, analyze the 30kHz-2MHz bandwidth; during the test, record the discharge quantity Q, discharge times N and average discharge power.

[0071] Typical data recording and analysis. During the test, the measured discharge quantity at 143kV withstand voltage stage is 18.5pC, and the converted discharge quantity is 29.2pC, which exceeds the standard limit of 10pC. Combined with the PRPD spectrum, it shows a double peak distribution, and it is judged that this discharge may be caused by the insulation layering defect of the oil paper not fully immersed in the production process. After disassembling the bushing, it is confirmed that there is a 0.5mm oil gap inside, and it is consistent with the discharge positioning result.

[0072] Compared with the test results at 50Hz, the detection degree of the oil paper layering defect in this test is improved by 300%; and by modifying the power supply through the frequency converter, a large amount of equipment investment is saved; finally, the test data after conversion and correction meets the IEC 60270 standard. IEC 60270:2000 High-voltage test techniques-Partial discharge measurements.

[0073] This embodiment shows that the transformer partial discharge test under 20Hz alternating power supply has practical feasibility and important value.

[0074] The transformer partial discharge detection test method provided in this embodiment is a low-frequency 20Hz power supply modification scheme, which takes the 5000kW motor and generator set in the ultra-high voltage room involved in the test as the blueprint, fully utilizes the operation characteristics and test requirements of the original generator set, programs the existing high-voltage frequency converter, increases the operation frequency point, replaces the switching switch, and increases the switching amount input point, realizes the conversion from power frequency to 20Hz, realizes the maximum efficiency of low-frequency modification. This modification has sufficient safety, firmness of installation and reliability of intelligent control. In addition, the transformer partial discharge detection process under 20Hz provided in this embodiment has a targeted advantage for layering, cracking or moisture intrusion defects, and also has an absolute advantage in low-frequency system verification such as rail transit, ship, etc., which can effectively make up for the shortcomings of traditional power frequency test, and has irreplaceable value in special scenes and deep detection.

[0075] Many embodiments and many applications other than those described herein will be apparent to those skilled in the art from consideration of the specification and practice of the teachings herein. Therefore, the scope of the present teachings should be determined by the appended claims and equivalents thereof, rather than by the description alone. All articles and references, including patent applications and publications, are incorporated herein by reference for all that they contain. Any aspect of the subject matter disclosed herein that is not recited in the claims is hereby abandoned. Nothing herein is to be construed as an admission that the subject matter disclosed herein is not entitled to antedate any prior art disclaimer.

[0076] The above description is further detailed of the present application, and cannot be considered as limiting the specific embodiments of the present application, and for those skilled in the art, without departing from the concept of the present application, a number of simple deductions or replacements can be made, which should be considered as belonging to the present application.

Claims

1. A transformer partial discharge detection test method based on low-frequency transformation, characterized in that: The following steps are involved: Performing low-frequency transformation on the industrial frequency AC power supply system to obtain a transformed low-frequency AC power supply system; The transformer is powered by the modified low-frequency AC power supply system; A partial discharge detection test is performed on the bushing of a transformer powered by the modified low-frequency AC power supply system, and the partial discharge detection test results are obtained.

2. A transformer partial discharge detection test method based on low-frequency transformation according to claim 1, characterized in that: The low-frequency transformation of the industrial frequency AC power supply system to obtain the transformed low-frequency AC power supply system includes transforming the motor and the generator: To control the motor at an increased operating frequency point, replace the power frequency selection switch with a low-frequency-power frequency conversion switch, change the inverter operating frequency to a low frequency, and change the speed to a low-frequency speed. The method of changing the inverter operating frequency to a low frequency is to increase the switch input point of the inverter. A forced air cooling system is added to the motor and generator, and 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 specifically as follows: the chassis of the cooler adopts a closed box, and fans are installed on both sides of the closed box. The inside of the closed box sucks air and the outside of the closed box supplies air. The door of the cooler is sealed. The forced air cooling system adopts automatic control. The fans are connected to the electrical control cabinet of the cooling equipment. Each fan is provided with separate overload protection and short-circuit protection.

3. A transformer partial discharge detection test method based on low-frequency transformation according to claim 2, characterized in that: The cooling equipment electrical control cabinet includes an intelligent control instrument, a frequency converter starting bus, a PLC and a touch screen; The forced air cooling system adopts automatic control including: The temperature sensors of 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 of the cooling equipment electrical control cabinet are used for intelligent braking alarm control.

4. The transformer partial discharge detection test method based on low-frequency transformation according to claim 2 is characterized in that: A circuit breaker is installed between the power supply of the fan and the power distribution cabinet.

5. The transformer partial discharge detection test method based on low-frequency transformation according to claim 2 is characterized in that: The fan is started and operated at both the power frequency and the low frequency.

6. The transformer partial discharge detection test method based on low-frequency transformation according to claim 2 is characterized in that: The rule for installing an electrical interlock between the forced air cooling system and the motor main control system is: if there is no fault after the fan is started, the motor unit is started; if the fan fails during the operation of the motor unit, the unit is tripped and shut down.

7. The transformer partial discharge detection test method based on low-frequency transformation according to claim 1 is characterized in that: The partial discharge detection test is performed on the bushing of the transformer powered by the modified low-frequency AC power supply system, and the partial discharge detection test results are specifically as follows: Conduct partial discharge test on transformer bushing. The partial discharge test includes: The transformer bushing is subjected to pre-pressurization, step-up voltage, withstand voltage test, and voltage reduction observation in sequence. Data is collected during the process, including discharge amount, number of discharges, phase, and pulse waveform. The collected data is calibrated and analyzed to obtain calibration analysis results, which are then evaluated to obtain partial discharge detection test results. If the discharge amount of the withstand voltage test increases exponentially with time during the partial discharge detection process, the withstand voltage test shall be terminated immediately.

8. The transformer partial discharge detection test method based on low-frequency transformation according to claim 7 is characterized in that: The pre-voltage is increased to a first voltage at a first rate; The step-by-step voltage boosting stepwise increases the voltage with the second voltage as a step length, each step is maintained for the first period of time, and the discharge amount and phase distribution at each voltage are recorded; The withstand voltage test is performed at the third voltage for the second period of time as a conventional test or the third period of time as a type test, and the discharge trend is monitored; The voltage reduction observation is to reduce the voltage to a fourth voltage at a second rate, and observe whether the discharge is reversible.

9. The transformer partial discharge detection test method based on low-frequency transformation according to claim 7 is characterized in that: The correction analysis converts the measured discharge amount into the power frequency equivalent discharge amount according to the correction formula, and the correction formula is shown as follows: in, is the power frequency equivalent discharge capacity, is the measured discharge capacity, is the discharge capacity correction factor.

10. A transformer partial discharge detection test system based on low-frequency transformation, characterized in that: include: A 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 a partial discharge detection test on the bushing of a transformer powered by a modified low-frequency AC power supply system to obtain a partial discharge detection test result.

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