CN116106659A - A distribution transformer energy efficiency high-precision testing system and its application - Google Patents

Abstract

The invention discloses a high-precision testing system for energy efficiency of a distribution transformer and application thereof, comprising a test control module, a testing unit, a switching line unit, a high-precision multichannel measuring module and an output unit; a plurality of test modules of the test unit are connected with the switching line unit, and the switching line unit is connected with the output unit through the high-precision multichannel measurement module; the test control module is connected with the test module and the switching line unit; the test control module controls the test unit and the switching module to work; the testing module tests different energy efficiency testing modes of the testing transformer; the switching line unit switches the test module, and switches the control circuit and the circuit in different energy efficiency test modes; the high-precision multichannel measuring module collects voltage and current of each port in real time, and converts different test result values according to different test modes. According to the scheme, test of different energy efficiency test projects is realized, a plurality of different test modules are highly integrated, the equipment volume is reduced to the greatest extent, and the field test portability is improved.

Description

A distribution transformer energy efficiency high-precision testing system and its application

technical field

The invention relates to testing equipment, in particular to a distribution transformer energy efficiency high-precision testing system and its application.

Background technique

The transformer is one of the most important equipment in the power system, and it is the basis for ensuring the reliability of power supply. As the core equipment of the power grid, the loss of the distribution transformer accounts for about 40%-50% of the power loss of transmission and distribution. With the rapid development of the entire economy, the demand for transformers will continue to increase. However, with the increase in the installed capacity of power transformers, the energy consumed by them is also increasing.

With the official implementation of "Power Transformer Energy Efficiency Limits and Energy Efficiency Grades" (GB-20052-2020), high-efficiency and energy-saving transformers have become a hot spot in the market, and the trend of large-scale procurement growth is obvious. In order to ensure that each distribution transformer put into operation meets the requirements of energy efficiency grades, It is particularly important to carry out energy efficiency test and detection of transformers. It is the general trend to carry out transformer energy efficiency testing in batches on site. However, in the traditional mode, the transformer tester has a single function, and the test equipment and test wiring need to be changed frequently during the test process. The test data is manually recorded during the test process. There are generally deficiencies such as high work intensity, low operation efficiency, high safety risks, and inaccurate test results.

Facing the increasing trend of large-scale procurement of high-efficiency and energy-saving transformers and the urgent need for field tests, it is urgent to develop a high-precision distribution transformer energy efficiency tester to realize the "safe, standardized, efficient, and controlled" energy efficiency test of high-efficiency and energy-saving distribution transformers. ", which can directly meet the needs of high-efficiency distribution transformer energy efficiency verification on-site and in batches, and at the same time promote the reduction of warehouse staff and increase efficiency, lower the technical threshold of on-site test personnel, and provide strong support for building a modern, green, and smart supply chain.

Contents of the invention

Purpose of the invention: The purpose of the present invention is to provide a distribution transformer energy efficiency high-precision testing system and its application. This solution can realize the testing of different energy efficiency test items, and highly integrate multiple different test modules to minimize the size of the equipment and improve Field trial portability.

Technical solution: A distribution transformer energy efficiency high-precision test system of the present invention includes a test control module, a test unit, a switching line unit, a high-precision multi-channel measurement module, and an output unit; wherein, the test unit includes several test modules, Several test modules are electrically connected to the switch line unit, and the switch line unit is electrically connected to the output unit through a high-precision multi-channel measurement module; the test control module is respectively connected to a number of test modules of the test unit and the switch line unit; the test control Several test modules and switch modules of the module control test unit work according to the test process; different test modules in the test unit test the different energy efficiency test modes of the test transformer respectively; the switch line unit tests the test modules of the test unit Switching is performed to switch control circuits and circuits in different energy efficiency test modes; the high-precision multi-channel measurement module collects the voltage and current of each port in real time, and converts different test result values according to different test modes.

The test unit includes a high-power variable-frequency power supply, a high-current DC source, a power analysis module, and a direct resistance measurement module; The switching line unit is electrically connected to form a control loop; the high-current DC source, power analysis module, and direct resistance test module are all electrically connected to the switching line unit, and the switching line is completed under the control of the test control module.

The output unit includes a DC resistance measurement output module, a no-load test measurement output module, and a load test measurement output module. The three are respectively connected to the high-precision multi-channel measurement module, and are switched to different measurement output modules according to the energy efficiency test mode.

The switching line unit includes a test and measurement module switch, a low-voltage short-circuit device, a line switch, a distribution transformer winding connection socket, a capacitive compensation switch, and an intelligent compensation capacitor; wherein the test and measurement module switch is connected to several test modules, Complete the switching of the test modules required for the test according to different test items; the line switching switch adopts a high-voltage vacuum circuit breaker, which is connected to the wiring socket of the distribution transformer winding to complete the line switching of different measurement parts; the capacitive compensation switch and the intelligent compensation capacitor connected to complete the switching of the intelligent compensation capacitor; the low-voltage short-circuit device is used to realize the short-circuit of the low-voltage side winding during the short-circuit impedance and load loss measurement process; the intelligent compensation capacitor compensates the load inductive current during the no-load test process.

The present invention also includes the application of a distribution transformer energy efficiency high-precision testing system, using the distribution transformer energy efficiency high-precision testing system to measure the winding resistance, no-load loss and no-load current measurement, and the 90% and 110% rated voltage No-load loss and no-load current measurement, short circuit impedance and load loss measurement.

The testing method of described winding resistance measurement, comprises the following steps:

S101: Generating a stable current through a large current DC source to magnetize the winding coil until stable;

S102: Select the test current gear according to the measured transformer capacity and rated current;

S103: Test on the line segments of each winding respectively, and all other non-tested line ends are open-circuited, and when the three-phase transformer winding is Y-connected and has no neutral point lead out, test its line resistance;

S104: During the winding resistance test, record the winding temperature, and discharge the test circuit after each test;

S105: Carry out unified conversion for the test winding, and satisfy the following conditions:

In the formula, R ₁ represents the resistance value at temperature t ₁ ; R ₂ represents the resistance value at temperature t ₂ ; T represents a constant.

Described no-load loss and no-load current measurement, concrete test method comprises the following steps:

S201: Start and connect the power supply, pressurize the low-voltage side of the distribution transformer to the rated voltage;

S202: Test the three-phase output current, the voltage of the test product port and the power loss at the test product end;

S203: The no-load current takes the average value of the three-phase current and converts it into a percentage of the rated current. The calculation formula is as follows:

In the formula, I _0a , I _0b , I _0c represent the measured values of a, b, and c three-phase no-load current; I _r represents the rated current of the excitation winding;

S204: Calculate the no-load loss, the calculation formula is as follows:

P _m =P _m -P _wv -P _s

In the formula, P _m represents the measured loss of the transformer; P _wv represents the loss of the instrument; P _s represents the loss of the measurement cable;

S205: Calculate the correction value of no-load loss, the calculation formula is as follows:

P ₀ =P _m (1+d)

d=(U-U)/

In the formula, d represents a negative number; P _m represents the measured loss; U represents the average voltmeter reading; U represents the root mean square value voltmeter reading.

The no-load loss and no-load current measurement at 90% and 110% of the rated voltage, the specific test method includes the following steps:

S301: Complete the line switching by the line switching unit;

S302: Boost the variable frequency power supply to 90% of the rated voltage on the low-voltage side of the distribution transformer, and then boost the 90% rated voltage to 110%;

S303: Sampling and recording the voltage and current respectively through the power analysis module;

S304: Record the voltage, current, and power of each phase, and finally measure the corresponding no-load loss and no-load current.

Described short-circuit impedance and load loss measurement, concrete test method comprises the following steps:

S401: In the case of a good short-circuit on the low-voltage side, start the power supply, and increase the current on the high-voltage side to not less than 50% of the rated current;

S402: Test three-phase output current, test product port voltage and test product terminal loss power;

S403: Convert the loss and short-circuit impedance at the ambient temperature to the reference temperature, and the loss is converted according to the following formula:

In the formula, P _k represents the load loss at the reference temperature; P _kt represents the load loss at t°C; ∑I ² R represents the resistance loss of a pair of windings at the test temperature; _t represents the resistance temperature conversion coefficient;

S404. Calculating the short-circuit impedance value and the converted value of the short-circuit impedance:

In the formula, Z _kt represents the short-circuit impedance of the winding at t°C; U _kt represents the impedance voltage of the winding through the test current I _k when the winding temperature is t°C; I _k represents the applied current of the test; U _r represents the rated voltage of the applied voltage side; I _r Indicates the rated current on the applied voltage side; Z _k indicates the short-circuit impedance at the reference temperature; P _kt indicates the load loss at t°C; P _k indicates the load loss at the reference temperature; S _r indicates the rated capacity.

Beneficial effects: Compared with the prior art, the technical solution of the present invention has the beneficial effects of: (1) The device is highly integrated with test modules such as DC resistance and power analysis, which minimizes the size of the equipment, improves the portability of field tests, and can be intelligently , Efficiently complete the on-site energy efficiency detection test of distribution transformers; (2) Develop intelligent switching matrix modules for different detection items, and cooperate with special test cables to complete the switching and insulation isolation between various energy efficiency test functions and control circuits and output wiring wait.

Description of drawings

Fig. 1 is a system block diagram of an embodiment of the present invention;

Fig. 2 is the schematic diagram of the DC resistance test wiring of the embodiment of the present invention;

Fig. 3 is the direct resistance test flowchart of the embodiment of the present invention;

Fig. 4 is the no-load loss and no-load current test flowchart of the embodiment of the present invention;

Fig. 5 is the flow chart of no-load loss and no-load current test under 90% and 110% rated voltage of the embodiment of the present invention;

Fig. 6 is the flow chart of short-circuit impedance and load loss test of the embodiment of the present invention;

FIG. 7 is a schematic diagram of a switching line switch system according to an embodiment of the present invention;

FIG. 8 is a schematic diagram of a switching line unit according to an embodiment of the present invention;

Fig. 9 is a schematic diagram of an intelligent control system according to an embodiment of the present invention.

Detailed ways

The technical solutions of the present invention will be described in detail below in combination with specific implementation methods and accompanying drawings.

Example 1

As shown in Figure 1, the test system of the present invention includes a power input, a main switch, an emergency stop protection, a test control module, a test unit, a switching line unit, a high-precision multi-channel measurement module, and an output unit. The test unit includes several test modules, specifically high-power frequency conversion power supply, high-current DC source, power analysis module, and direct resistance measurement module; the test control module is connected with high-power frequency conversion power supply, high-current DC source, power analysis module, and direct resistance measurement module. The module and the switching line unit are electrically connected to form a control loop. The high-power frequency conversion power supply, high-current DC source, power analysis module, and direct resistance measurement module are all electrically connected to the switching line unit, and the switching line is completed under the control of the test control module. The switching line unit is electrically connected with the output unit through a high-precision multi-channel measurement module. The output unit includes a DC resistance measurement output module, a no-load test measurement output module and a load test measurement output module. The three are respectively connected to the high-precision multi-channel measurement module, and are switched to different measurement output modules according to the energy efficiency test mode. The test control module controls several test modules and switching modules of the test unit to work according to the test process. The test control module includes a self-check module, an automatic test module, an automatic record module, and an output module; Different energy efficiency test modes are tested. The switch line unit switches each test module of the test unit, thereby switching control circuits and circuits in different energy efficiency test modes. The high-precision multi-channel measurement module collects the voltage and current of each port in real time, and converts different test result values according to different test modes.

Table 1 Key performance indicators of distribution transformer energy efficiency high precision tester

This device is composed of high-power frequency conversion power supply, high-current DC power supply, switching line module, high-precision multi-channel measurement module, and test control module. No-load loss and no-load current measurement under voltage, short-circuit impedance and load loss measurement, a total of 4 types of test capabilities. Among them, the high-power frequency conversion power supply can provide test power for no-load loss and no-load current measurement, no-load loss and no-load current measurement at 90% and 110% of rated voltage, short-circuit impedance and load loss measurement; high-current DC power supply can Provide test power for winding resistance measurement; switch line unit distributes test power to corresponding ports in each step of the test; multi-channel measurement module collects voltage and current of each port in real time, and converts different test results according to different test modes. The measurement modes of each test are as follows:

(1) The test method of the winding resistance measurement of the direct resistance measurement module comprises the following steps:

S101: Winding resistance measurement A stable current is generated through a large current DC source to magnetize the winding coil until it is stable;

S102: Select the test current gear of the tester according to the measured transformer capacity and rated current;

S103: The measured current shall not be greater than 10% of the rated current of the tested winding, and usually 3%-10% of the rated current of the tested winding can be used as the current value for measuring DC resistance. As shown in Figure 2, the wiring schematic diagram is measured on the line segments of each winding, and all other non-tested line ends are open. When the three-phase transformer winding is Y-connected and has no neutral point lead, its line resistance should be measured, for example AB, BC, CA; when the winding is connected by D, the phase resistance should be measured for the windings that are led out from both the head and the end, and the line resistance should be measured for the closed triangle test product;

S104: When measuring the winding resistance, the winding temperature must be accurately recorded. After each measurement, the instrument will automatically discharge the measurement circuit;

S105: The instrument automatically converts the test winding in a unified manner, wherein, the temperature t ₁ of the test winding and the resistance R ₁ , the DC resistance tested at different temperatures can be converted to the same temperature t ₂ according to the following resistance conversion formula;

In the formula, R ₁ represents the resistance value at temperature t ₁ ; R ₂ represents the resistance value at temperature t ₂ ; T represents a constant, among which, the copper wire is 235, and the aluminum wire is 225.

When the energy efficiency tester measures the winding resistance, it first completes the switching line through the switching line unit; then outputs the test current through the large DC power supply; then uses the direct resistance measurement module to sample and record the voltage and current; wait until the instrument is magnetized. Stabilize the voltage and current values, and finally convert them into resistance values.

(2) No-load loss and no-load current measurement, the no-load loss and no-load current measurement is carried out through a high-power frequency conversion power supply and a multi-channel measurement module. The specific test method includes the following steps:

S201: Start and connect the power supply, pressurize the low-voltage side of the distribution transformer to the rated voltage;

S202: Measure the three-phase output current, the voltage at the test product port and the power loss at the test product end;

S203: The no-load current takes the average value of the three-phase current and converts it into a percentage of the rated current, namely:

In the formula, I _0a , I _0b , I _0c represent the measured values of a, b, and c three-phase no-load current; I _r represents the rated current of the excitation winding;

S204: Calculate the no-load loss, the calculation formula is as follows:

P _m =P _m -P _wv -P _s

In the formula, P _m represents the measured loss of the transformer; P _wv represents the loss of the instrument; P _s represents the loss of the measurement cable;

S205: Calculate the correction value of no-load loss, the calculation formula is as follows:

P ₀ =P _m (1+d)

d=(U-U)/(d is usually negative)

In the formula, d represents a negative number; P _m represents the measured loss; U represents the average voltmeter reading; U represents the root mean square value voltmeter reading.

When testing no-load loss and no-load current, first switch the line through the switch line module; then boost the variable frequency power supply to the rated voltage; then use the power analysis module to sample and record the voltage and current; and record the voltage and current of each phase , power; finally get the rated no-load loss and no-load current.

(3) Measurement of no-load loss and no-load current at 90% and 110% of rated voltage, the specific test method includes the following steps:

S301: Switching the line through the switching line module;

S302: Boost the variable frequency power supply to 90% and 110% of the rated voltage, which is a continuous boosting process;

S303: Sampling and recording the voltage and current through the power analysis module;

S304: Record the voltage, current, and power of each phase; finally measure the corresponding no-load loss and no-load current.

(4) The measurement of short-circuit impedance and load loss is carried out through a high-power variable frequency power supply and a multi-channel measurement module. The specific test method includes the following steps:

S401: In the case of a good short-circuit on the low-voltage side, start the power supply, and increase the current on the high-voltage side to not less than 50% of the rated current;

S402: Measure the three-phase output current, the voltage of the test product port and the power loss at the test product end;

S403: Convert the loss and short-circuit impedance at the ambient temperature to the reference temperature, and convert the load loss to:

In the formula, P _k represents the load loss at the reference temperature; P _kt represents the load loss at t°C; ∑I ² R represents the resistance loss of a pair of windings at the test temperature; _t represents the resistance temperature conversion coefficient;

S404. Calculating the short-circuit impedance value and the converted value of the short-circuit impedance:

In the formula, Z _kt represents the short-circuit impedance of the winding at t°C; U _kt represents the impedance voltage of the winding through the test current I _k when the winding temperature is t°C; I _k represents the applied current of the test; U _r represents the rated voltage of the applied voltage side; I _r Indicates the rated current on the applied voltage side; Z _k indicates the short-circuit impedance at the reference temperature; P _kt indicates the load loss at t°C; P _k indicates the load loss at the reference temperature; S _r indicates the rated capacity.

When performing short-circuit impedance and load loss tests, first complete the switching line through the switching line module; then step up the variable frequency power supply to the rated current not less than 50% of the high voltage side; use the power analysis module to sample and record the voltage and current; and record each Phase voltage, current, power; finally get the rated short-circuit impedance and load loss.

The switching line unit provides the switching work of the test line between the test module and the test object of the whole test project. As shown in Figure 8, the switching line unit includes a test measurement module switching switch, a low-voltage short-circuit device, a line switching switch, a distribution transformer winding wiring socket, a capacitive compensation switch and an intelligent compensation capacitor. Among them, the switching switch of the test measurement module is connected with several test modules, and the switching of the test modules required for the test is completed according to different test items. The low-voltage short-circuit device provides the short-circuit function of the low-voltage side winding during the measurement of short-circuit impedance and load loss; the capacitive compensation switch is connected to the intelligent compensation capacitor to complete the switching of the intelligent compensation capacitor. The intelligent compensation capacitor compensates the load inductive current during the no-load test and improves the equivalent test power supply capacity.

The switching line unit also includes a line switching switch, and the switching line unit is connected with the wiring socket of the distribution transformer winding to complete the switching lines of different measurement parts. The switch is the core component of the intelligent switch line device. At present, the multiplexer switches in the measurement field mostly use electronic switches and are designed as integrated circuit boards, which have the advantages of fast speed, good response, and easy control, but the switch channel has low withstand voltage level and small flow capacity. In the test process of power equipment such as distribution transformers, the main circuit needs to pass through large current and high voltage, so the electronic transfer switch of integrated circuit cannot be used. The line transfer switch of this scheme adopts high-voltage vacuum circuit breaker with series-parallel connection and intelligent control The system realizes the flexible conversion of the test wiring mode, and its principle is shown in Figure 7.

Example 2

On the basis of Embodiment 1, this solution also includes an intelligent control system. The intelligent control system is a central processing system that controls and regulates the entire test loop. Its structural principle is shown in FIG. 9 . The status detection module is responsible for monitoring the test status information such as the status of the test loop, the status of the instrument and equipment; the parameter input module is responsible for recording the parameter information of the test product, the test item information and various parameter configuration information, and is responsible for receiving the control information of the master control system on the test process. It is also responsible for human-computer interaction and remote communication, so as to realize the control and remote control of the test process by the test personnel in a timely and convenient manner; the alarm processing module is responsible for preprocessing the detected alarm information; the test master control system is responsible for comprehensive status information, Parameter configuration, remote control information and alarm information formulate the test control strategy; the switch control module is responsible for formulating the corresponding switch control strategy according to the test control strategy; the control output simulation is responsible for converting the completed switch control strategy into a control signal and sending it to each automatic The transfer switch is performed.

Claims (9)

1. A distribution transformer energy efficiency high accuracy test system which characterized in that: the device comprises a test control module, a test unit, a switching line unit, a high-precision multichannel measurement module and an output unit; the test unit comprises a plurality of test modules, the test modules are electrically connected with the switching line unit, and the switching line unit is electrically connected with the output unit through the high-precision multichannel measurement module; the test control module is respectively connected with a plurality of test modules of the test unit and the switching line unit;

the test control module controls a plurality of test modules and switching modules of the test unit to work according to a test flow;

different test modules in the test unit respectively test different energy efficiency test modes of the test transformer;

the switching line unit switches a plurality of test modules of the test unit, so as to switch control circuits and circuits in different energy efficiency test modes;

the high-precision multichannel measuring module collects the voltage and the current of each port in real time, and converts different test result values according to different test modes.

2. The energy efficiency high precision test system for a distribution transformer of claim 1, wherein: the testing unit comprises a high-power variable-frequency power supply, a high-current direct-current source, a power analysis module and a direct resistance measurement module;

the test control module is respectively and electrically connected with the high-power variable-frequency power supply, the high-current direct-current source, the power analysis module, the direct resistance measurement module and the switching line unit to form a control loop.

3. The energy efficiency high precision test system for a distribution transformer of claim 1, wherein: the output unit comprises a direct resistance measurement output module, an idle test measurement output module and a load test measurement output module, wherein the direct resistance measurement output module, the idle test measurement output module and the load test measurement output module are respectively connected with the high-precision multichannel measurement module, and the direct resistance measurement output module, the idle test measurement output module and the load test measurement output module are switched to different measurement output modules according to an energy efficiency test mode.

4. The energy efficiency high precision test system for a distribution transformer of claim 1, wherein: the switching line unit comprises a test measurement module switching switch, a low-voltage short circuit device, a line switching switch, a distribution transformer winding wiring socket, a capacitive compensation switch and an intelligent compensation capacitor; the test measurement module change-over switch is connected with the plurality of test modules, and the test modules required by the test are switched according to different test items; the circuit switching switch adopts a high-voltage-resistant vacuum circuit breaker, is connected with a wiring socket of the distribution transformer winding to finish circuit switching of different measuring positions, and the capacitive compensation switch is connected with the intelligent compensation capacitor to finish switching of the intelligent compensation capacitor;

the low-voltage short-circuit device is used for realizing short-circuit of a low-voltage side winding in the short-circuit impedance and load loss measurement process;

the intelligent compensation capacitor compensates load inductive current in the empty load test process.

5. Use of a high-precision power efficiency test system for a distribution transformer according to claim 1, characterized in that winding resistance measurements, no-load losses and no-load currents at 90% and 110% rated voltages, short-circuit impedance and load losses are measured using the high-precision power efficiency test system for a distribution transformer.

6. The use of a high precision power distribution transformer energy efficiency test system according to claim 5, wherein the winding resistance measurement test method comprises the steps of:

s101: magnetizing the winding coil to be stable by generating stable current through a high-current direct current source;

s102: selecting a test current gear according to the capacity and rated current of the tested transformer;

s103: testing on the line segments of each winding respectively, wherein all other non-tested line ends are in open circuit, and testing the line resistance of the three-phase transformer winding when the three-phase transformer winding is Y-connected and no neutral point is led out;

s104: when the winding resistance is tested, the temperature of the winding is recorded, and after each test is finished, the discharge operation is carried out on the test loop;

s105: the test windings are subjected to unified conversion, and the following conditions are satisfied:

wherein R is ₁ Indicating the temperature t ₁ Resistance value at the time; r is R ₂ Indicating the temperature t ₂ Resistance value at the time; t represents a constant.

7. The use of a high precision power distribution transformer energy efficiency test system according to claim 5, wherein the no-load loss and no-load current measurements, the specific test method comprising the steps of:

s201: starting to switch on a power supply, and pressurizing the low-voltage side of the distribution transformer to rated voltage;

s202: testing three-phase output current, test port voltage and test port loss power;

s203: the no-load current is an average value of three-phase current and converted into percentage of rated current, and the calculation formula is as follows:

wherein I is _0a 、I _0b 、I _0c A, b and c three-phase idle current actual measurement values are shown; i _r Indicating the rated current of the exciting winding;

s204: the no-load loss is calculated as follows:

P _m ＝P _m -P _wv -P _s

wherein P is _m Representing the actual measured loss of the transformer; p (P) _wv Indicating instrument loss; p (P) _s Indicating a measured cable loss;

s205: the correction value of the no-load loss is calculated as follows:

P ₀ ＝P _m (1+d)

d＝(U-U)/

wherein d represents a negative number; p (P) _m Representing measured loss; u represents the average voltmeter reading; u represents the square root voltmeter reading.

8. The use of a high precision power distribution transformer energy efficiency test system according to claim 5, wherein said no-load loss and no-load current measurements at 90% and 110% rated voltage, the specific test method comprising the steps of:

s301: completing a switching line through a switching line unit;

s302: boosting the variable frequency power supply to 90% rated voltage at the low-voltage side of the distribution transformer, and boosting the 90% rated voltage to 110%;

s303: the voltage and the current are sampled and recorded respectively through a power analysis module;

s304: recording the voltage, current and power of each phase, and finally measuring the corresponding no-load loss and no-load current.

9. The use of a high precision power distribution transformer energy efficiency test system according to claim 5, wherein the short circuit impedance and load loss measurements, the specific test method comprising the steps of:

s401: under the condition of good short circuit of the low-voltage side, starting to switch on a power supply, and enabling the high-voltage side to flow up to not less than 50% of rated current;

s402: testing three-phase output current, test port voltage and test port loss power;

s403: converting the loss and short circuit impedance at ambient temperature to a reference temperature, the loss being converted according to the following equation:

wherein P is _k Representing load loss at a reference temperature; p (P) _kt Representing load loss at t ℃; Σi ² R represents the resistance loss of a pair of windings at the test temperature; _t representing a temperature conversion coefficient of the resistor;

s404, calculating a short-circuit impedance value and a short-circuit impedance calculation value:

wherein Z is _kt Representing the short-circuit impedance of the winding at t ℃; u (U) _kt Indicating the current I through the winding at t DEG C _k Impedance voltage of (a); i _k Indicating the applied current of the test; u (U) _r A rated voltage on the applied voltage side; i _r A rated current on the side of the applied voltage; z is Z _k Representing the short circuit impedance at the reference temperature; p (P) _kt Representing load loss at t ℃; p (P) _k Representing load loss at a reference temperature; s is S _r Indicating rated capacity.

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