1. Introduction With the continuous advancement of high-voltage frequency converter technology and the tireless efforts of technical personnel in this field, medium and high-voltage frequency converters, which were previously only available through imports, are now increasingly being produced domestically, with numerous manufacturers. However, the testing methods for finished products from various manufacturers are mostly outdated, generally employing the method of running the motor under no-load conditions, which no longer meets the requirements for equipment testing. Foreign medium and high-voltage frequency converter manufacturers typically conduct real-load tests on their products after completion. This paper introduces a medium and high-voltage frequency converter testing platform, which allows for intelligent real-load performance testing of frequency converters with voltage levels ranging from 3kV to 10kV and power below 5000kW. 2. System Composition 2.1 Main Components of the System Power Transformer + High-Voltage Distribution Control Room + Frequency Converter + Demo Reversible Frequency Converter + Regen Reversible Frequency Converter + Control Unit + Motor Set + Spindle Torque Meter + Electrical Parameter Analyzer This system consists of the above seven separate modules. 2.2 The power transformer is a service transformer, consisting of two 2500kVA power transformers. Initially, one transformer will be put into operation. When the capacity exceeds this value, the second transformer will be put into operation in parallel. 2.3 The high-voltage distribution control room is equipped with all service switches, bus tie switches, line combination selection switches, circuit breakers, contactors, and other high-voltage operating cabinets. The Y/δ selection of the frequency converter transformer, motor selection, and Y/δ conversion connections are all performed in this control room. During testing, the main operations of the high-voltage cabinets are controlled by a PLC via commands from the host computer. 2.4 The frequency converter transformer is a multi-functional transformer specifically designed for primary-side Y/δ switching and secondary-side multi-winding, multi-tap combination selection for both the tested and inverter frequency converters. It has a maximum power of 6250kVA and can provide voltage levels such as 6-pulse 3kV, 6kV, 10kV, and 18-pulse 3kV, 6kV, etc., to provide different power supplies for load testing of different frequency converters. 2.5 Demo and Regen Inverters Two Rockwell Automation PowerFlex 7000 inverters were pre-configured at this test station. One inverter, with a power rating of 6kV and a capacity of 1000kW, is specifically designed to drive the 1000kW M1 motor in the motor unit. Its main purpose is to demonstrate load operation when the inverter under test is unavailable; this is called the demo inverter. The other inverter, with a power rating of 2000kW, is called the regen inverter. It is used for loading the demo inverter or the inverter under test when it is running under load. It is connected to the regenerative motor M3. When motors M1 or M2 are running, the regen inverter is put into a braking state, applying load to the spindle in a linear, square, or user-defined manner based on the motor speed. The energy generated by the motor is regenerated and inverted, feeding back to the power grid, thereby achieving energy savings. 2.6 Control Section The core of the control section of this system is the Rockwell Automation Control Logix series PLC controller. The host computer is an Advantech industrial control computer, and the human-machine interface uses Rockwell Automation's RS View SE configuration software. The system adopts a client/server architecture, allowing multiple client machines to access the server via Ethernet from different locations and control various controlled components for inverter testing. 2.7 The motor set currently has three motors installed, operating coaxially. One is a 6kV, 1000kW motor, used to connect to a demo inverter or a test inverter with a voltage rating of 6kV and a power rating below 1000kW. The second is a 10kV, 2000kW motor, used to connect to a test inverter with a voltage rating of 10kV and a power rating below 2000kW. The third is a load motor, connected to a Regen inverter, used to load either of the first two motors. Of course, M3 can also be used as a motor if necessary, as the Regen inverter connected to it has this capability. 2.8 Two torque meters are installed on the spindles between motors M1 and M2, and between M2 and M3, with ranges of 10000nm and 40000nm respectively. The torque meter has a local data processing and display unit and transmits the torque value to the PLC for acquisition through a 4-20mA current loop, and then sends it to the host computer for display. 2.9 Power Parameter Measurement and Analysis Instrument PowerMonitor II The power parameter measurement and analysis instrument is installed on each high-voltage bus or branch and is combined with the host computer RSPower32 software to monitor power parameters and display and analyze them. 3. Working Principle 3.1 Schematic diagram of the composition of the intelligent medium and high voltage frequency converter load test platform In the test station, seven separate modules form a test platform, as shown in Figure 1. 3.2 Operating principle of the intelligent medium and high voltage frequency converter load test platform (1) High voltage power supply introduction As shown in Figure 1, the 6kV high voltage power from the external power grid enters the high voltage distribution control room through two 2500kVA power transformers. The high voltage 6kV is sent to the primary side of the frequency converter transformer. There are two connection methods for the primary side: Y-connection and δ-connection. The two connection methods correspond to different power, different secondary voltage levels and different motors targeted by the frequency converter output. (2) Test Voltage Level Transformation Figure 1 Schematic diagram of test station principle Transformer connection When the primary side of the transformer is connected in a delta connection, the maximum power is 6250kVA. The voltage levels that can be output on the secondary side and the motors corresponding to the inverter output are: 3kV@m1, 6kV@m1, 18-pulse 6kV@m1, 6kV@m2, 10kV@m2, 18-pulse 6kV@m2; ​​When the primary side of the transformer is connected in a star connection, the maximum power is 3600kVA. The voltage levels that can be output on the secondary side and the motors corresponding to the inverter output are: 3kV@m1, 6kV@m1, 18-pulse 3kV@m1, 6kV@m2; ​​When the voltage level output to m1 is 3kV, it corresponds to the delta connection of the m1 motor; When the voltage level output to m1 is 6kV, it corresponds to the y connection of the m1 motor. When the voltage level output to m2 is 6kV, it corresponds to the δ connection of the m2 motor; when the voltage level output to m2 is 10kV, it corresponds to the y connection of the m2 motor. (3) Operation circuit control All high voltage switch actions are sent by the operator on the host computer by clicking the mouse, and the PLC controls the relay to drive the high voltage circuit breaker or contactor to complete the operation. After connecting the high voltage line according to the above correspondence, set the regen frequency converter to work in the braking state, select torque proportional to speed or proportional to the square of speed (the above automatic mode), or select to set the step torque (manual mode), set the maximum power not to exceed the output power of the frequency converter, and then send the frequency converter "enable". (4) Test data acquisition operation Start the test frequency converter or demo frequency converter. As the speed of motor m1 or m2 increases, the torque load will be added to the motor spindle. Two torque meters on the main shafts between motors M1 and M2, and between M2 and M3, transmit torque values ​​to the PLC for acquisition via a 4-20mA current loop, and then send the data to the host computer for display. 3.3 According to specific requirements, corresponding operations are performed. During actual operation of the frequency converter, four PowerMonitor II type electrical energy test and analysis instruments installed on the high-voltage cabinet measure and display the energy consumption of the high-voltage bus, the frequency converter power line, and the motor line, respectively. These instruments are connected to the host computer via Ethernet. The host computer uses RS Power32 software to display waveforms and analyze the electrical energy data. Simultaneously, this data is transmitted to RSViewSEE via a driver program, and the electrical energy data can be synchronously displayed on the human-machine interface, as shown in Figure 2. Figure 2. Screenshot of the high-voltage switch operation interface of the test station. In this system, Ethernet is used as the main control network medium. The server, clients, PLC, power analyzer and other equipment are interconnected through Ethernet. The PLC is connected to the Demo frequency converter, Regen frequency converter and the frequency converter under test through ControlNet. The PLC is connected to the power integrated protection device through DH+ network. The interconnection protocols mainly used are TCP/IP, Modbus, PPP, etc. Through the communication network, various analog and digital parameters are transmitted to the host computer in real time, and the operator also sends them to the PLC through the human-machine interface of the host computer for execution. 4. Core Technology Application 4.1 Control Logix PLC and Network The PLC controller of this system is the latest generation product of Rockwell Automation, the Control Logix series, and the controller selected is Control Logix 5550. This series of PLCs has the following features: (1) High-performance passive, multi-master bus; (2) The baseboard conforms to the producer-consumer protocol; (3) Multiple processors can be placed in the same frame and can run independently or in parallel; (4) Any module can be hot-swapped; (5) No slot limitation; (6) Memory can be increased or decreased according to the application scale, and can be expanded up to 2MB; (7) Distributed solution, CPU and I/O can be connected to the same ControlNet network; (8) Complete range of modules, including digital quantities 24, 48, 110VDC, TTL, 110, 220VAC; analog quantities voltage, current, RTD, thermocouple; pulse; communication, etc.; (9) Powerful communication capabilities: Supported communication networks and protocols include DH+, ControlNet, EtherNet, DeviceNet, RS232, etc. Due to the above-mentioned outstanding features, the author chose the ControlLogix controller as the core control unit of the system, utilizing its numerous advantages to serve the system, especially its superior network communication capabilities, which provided strong network support for the construction of the experimental platform. 4.2 rsview se The main differences between rsview se based on FactoryTalk directory and traditional HMI software: 4.2.1 Database design pattern Figure 3 is a schematic diagram of rsview operation. Figure 3 Schematic diagram of rsview operation (1) The database is still distributed in its original location (e.g., controller); (2) The client obtains the data definition by using the factorytalk directory (ftd), which is equivalent to a lookup table; similar to the principle of Internet DNS; (3) The data definition is both distributed and centralized; (4) A copy of the factorytalk directory is retained on each client; (5) Load balancing: The distributed database and data are distributed on each controller and are processed independently by each controller; (6) Faster and more direct access to data: Data does not need to be transferred from one database to another and then to the client, but is directly transferred from the controller to the client that needs the data; (7) Avoiding single point of failure: The database and data are distributed on each controller, and a copy of the factorytalk directory is retained on each client. 4.2.2 Tag Point Definition Mode (1) Use rslogix to define PLC points, and then save the project file; (2) Directly use the tag points created in the PLC in the HMI, without creating a new HMI tag database; (3) Reduce project development time and maintenance costs; (4) No need to input, output, and copy the tag database; (5) Avoid single points of failure that may occur in most databases; (6) Support the distribution of the tag database on multiple controllers on the network. 4.2.3 Screen Saving Method (1) The screen is saved on the HMI server, and the client does not have a screen; (2) Only simple modifications to the graphic display are needed, and the changes are effective throughout the entire system; (3) Can be modified both online and offline. 4.2.4 Advantages Not Available in Traditional Solutions (1) Significantly reduce project development time and maintenance costs; (2) Avoid inputting and copying graphic displays; (3) Support the distribution of projects on multiple computers on the network of HMI servers; (4) Convenient and simple screen updates in the automation system. 4.2.5 Reusability of the Control System (1) The hierarchical directory of Fatorytalk makes the component name (tag, alarm, screen) unique within the system (the area name determines the uniqueness of the tag name, that is, the same tag name can be used in different areas); (2) Users only need to copy the content in area1 to area2 to generate a new control, and the new information is immediately available; (3) The copied new project is exactly the same as the original project in terms of content and function, only the domain is different. 4.2.5 Configuration Method Figure 4 is a schematic diagram of rsview configuration and management. Figure 4 Schematic diagram of rsview configuration and management (1) Define the information source and information source group once, and it can be reused; (2) Develop the entire system offline on a laptop. Just copy the application to the required computer on site, and then change the computer name in the system to the computer name of the running software; (3) This process can also be reversed! That is, the entire application can be copied to the laptop on site, and then it can be modified and redeveloped. 4.3 Powerflex 7000 Inverter 4.3.1 Features of Powerflex 7000 Inverter The Powerflex 7000 inverter was chosen as the motor drive and load generation unit because its features are suitable for its needs. The Powerflex 7000 inverter has the following major features: (1) Simple and reliable power structure. The Powerflex™ 7000 uses 6.5kV 800A/1500A symmetrical gate commutator thyristors (SGCT), and the number of inverter switching devices used in the inverter is less than that of any other medium-voltage inverter technology. The power structure is fuse-free and has DC reactors to suppress current. The result is simpler, more robust and reliable. (2) Four-quadrant operation. The Powerflex™ 7000 has the ability to operate in four quadrants. The motor can operate in both directions as a motor or generator. When operating as a generator, energy can be fed back to the grid and the motor can be regeneratively braked. (3) Sensorless direct vector control. Sensorless direct vector control measures the motor flux by measurement rather than estimation. Like DC drive, the motor torque can be changed quickly without affecting the motor flux. This control method can produce better operating performance than standard V/Hz inverters. (4) SGCT - Ideal power switch for medium-voltage inverters. The gate drive integrated, high switching frequency, and double-sided cooled symmetrical gate commutated thyristor is the ideal power semiconductor switch for medium-voltage inverters. By optimizing the pulse width modulation (PWM) mode of controlling SGCT, the conduction and switching losses of the device are minimized, resulting in an extremely compact and efficient inverter design. Due to the unique performance of SGCT and the technical structure of PowerFlex 7000, this component can be used in PWM inverters and PWM rectifiers. 4.3.2 High reliability and four-quadrant operation capability of PowerFlex 7000 The high reliability and four-quadrant operation capability of PowerFlex 7000 make it the preferred inverter for this test platform. Using it, the mechanical energy generated by the actual load test can be regenerated by the motor and then fed back to the grid by the inverter. This greatly reduces the energy consumption of the test and saves the test cost. 5. Test items 5.1 This system can realize the following test items (1) Inverter load characteristic test; (2) Inverter actual load long-term assessment and aging test; (3) Inverter power device temperature rise test; (4) Inverter transformer temperature rise test; (5) Rated output voltage and voltage imbalance test; (6) Frequency accuracy test; (7) Efficiency test; (8) Harmonic analysis. 5.2 Inverter load characteristic test (1) The load characteristic of the inverter is one of the most important characteristics of the inverter. The quality of the torque characteristic of an inverter will directly affect the performance of the user's working conditions. On this test platform, light load, full load, overload and below characteristic load tests can be performed. The operation interface is shown in Figure 5. (2) Load characteristic test proportional to motor speed; (3) Load characteristic test proportional to the square of motor speed; (4) Step change load characteristic test; (5) User-defined load characteristic test. 5.3 Long-term load assessment and aging test of frequency converter According to different on-site working conditions, after designing the load characteristics, the frequency converter under test is allowed to run for a long time according to the user's specified time to assess the long-term reliability of the frequency converter under test. Figure 5 Screenshot of step load actual load curve 5.4 Temperature rise test of frequency converter power device Under actual load, the temperature change of the power device of the frequency converter under test is measured to provide the user with the necessary technical data for frequency converter design and development. 5.5 Temperature rise test of frequency converter transformer Under actual load, the temperature change of the transformer of the frequency converter under test is measured to provide the user with the necessary technical data for frequency converter transformer design. 6. Conclusion This test platform, jointly built by Harbin Jiuzhou Electric Co., Ltd. and global automation companies, including Rockwell Automation, a giant in medium-voltage frequency converters, is a testing ground for intelligent load-bearing performance testing of frequency converters at medium and high voltage levels. This platform has been used to conduct real-load tests on Jiuzhou Electric's PowerSmart series 10kV/1000kW and 6kV/1250kW, and Rockwell Automation's Powerflex 7000 series 6kV/1120kW and 18-pulse 6kV/3600kW, achieving excellent results. As a testing method before a product leaves the factory or is shipped to the user, it eliminates the need to wait until on-site installation to determine the overall load performance of the frequency converter. The flexibility and convenience, especially in frequency converter product development and performance testing, are unattainable through on-site operation or motor no-load testing.