1. Introduction In 2007, with energy conservation, emission reduction, and environmental protection as key national priorities, high-voltage frequency converters (VFDs), the most effective energy-saving product, are increasingly widely used. However, the full-scale load performance testing of VFDs before they leave the factory is still limited to testing with an unloaded motor or some simple loads. This cannot simulate the different load conditions in the field and cannot test various performance indicators of the VFDs. Therefore, it is essential to build a test system that can comprehensively test the full-scale load performance of different types and power VFDs. Rockwell's vector control 4-quadrant DTD VFD makes it possible to establish a comprehensive test system for VFD performance. The Harbin Jiuzhou Electric Co., Ltd. high-voltage VFD test station is currently the largest multi-functional test station for testing high-voltage VFDs in Asia. It was jointly built with Rockwell Automation and is a test site for intelligent load performance testing of VFDs at medium and high voltage levels. It can test VFDs with voltage levels from 3kV to 10kV and a maximum power range of 5000kW. 2. Main Configuration and Functions of the Electrical System 2.1 Power Transformer Considering future expansion of the test station, the incoming transformer consists of two 2500kVA power transformers. Initially, one will be put into operation. When the capacity exceeds this value, the second transformer will be put into operation in parallel. 2.2 Frequency Converter A multi-functional transformer specifically designed for the tested inverter frequency converter, featuring primary-side star/delta switching and secondary-side multi-winding multi-tap combination selection, is selected based on the maximum installed capacity. It has a maximum power of 6250kVA and can be configured to provide 6-pulse 3kV, 6kV, and 10kV, and 18-pulse 3kV and 6kV voltage levels, providing different power supplies for load testing of different frequency converters. 2.3 High-Voltage Control Cabinet This cabinet contains all incoming switches, bus tie switches, line combination selection switches, circuit breakers, disconnectors, etc. The transformer star/delta switching, motor selection, and star/delta connection are all handled in this control cabinet. 2.4 The demo inverter and inverter system is equipped with two Rockwell Automation PowerFlex 7000 inverters. One is a 1000kW inverter specifically designed to drive the 1000kW M1 motor in the motor unit. Its main purpose is to perform load demonstrations and motor maintenance tests when the inverter under test is unavailable. The other inverter, a 2000kW inverter called the Regen inverter, is used to load the inverter or the inverter under test. It is connected to the regenerative motor M3. When either M1 or M2 is running, the Regen inverter is put into braking mode, applying a linear or quadratic speed load to the spindle and inverting the energy back to the grid, thus saving energy. 2.5 Control Section The core of the control section of this system is the Rockwell Automation ControlLogix series PLC controller. The frequency converter testing system's control core is the Rockwell Automation ControlLogix series PLC controller, which consists of a CPU module, an Ethernet communication module, a Concholnet communication module, and I/O modules. It controls various interlocks of the high-voltage switchgear, star/delta switching of the frequency converter transformer, and output group switching. It controls the inverter frequency converter to load the spindle at a linear or square-proportional speed ratio and receives commands from the host computer to complete various specified operations. 2.6 Host Computer The host computer uses an Advantech industrial computer. The system adopts a client/server architecture, allowing multiple client computers to access the server via Ethernet from different locations and control various controlled components to perform frequency converter testing. 3. Functional Implementation 3.1 System Design The 10kV incoming line side consists of a voltage transformer cabinet, a power receiving cabinet, a No. 1 transformer cabinet, a No. 2 transformer cabinet, and a metering cabinet. The 10kV grid voltage is stepped down to 6.3kV through two incoming line transformers. The 6.3kV voltage is connected to two busbars through two power receiving cabinets. The two 6.3kV busbars are connected by a circuit breaker isolating switch. The primary side of the inverter transformer is connected to the 6kV busbar through the inverter transformer control cabinet. The secondary side is connected to the frequency converter under test through 10kV feeder cabinet, 6kV feeder cabinet, 3kV feeder cabinet, and 18-pulse feeder cabinet. The output voltage of the inverter transformer (10kV, 6kV, 3kV) can be selected according to the rated voltage of the frequency converter under test, and the motor power can be selected according to the power of the frequency converter under test. Motors M1 and M2 are used as load motors, and motor M3 is used as a regenerative generator connected to the inverter frequency converter. This allows the inverter frequency converter to load the spindle linearly or quadratically, receiving commands from the host computer to complete various specified operations and achieve different testing objectives. The G15 switchgear in the diagram is for future expansion to a 5000kW load. The primary system diagram is shown in Figure 1. Figure 1 Primary System Diagram 3.2 Host Computer Configuration For ease of operation and safety, all high-voltage operations are performed remotely via the host computer; therefore, the following interface needs to be designed. 3.2.1 Bus Tie Switch Status Interface Functionality: Displays the switch status of the output receiving cabinets of the #1 and #2 incoming transformers, the status of the 6kV bus switch, and the switch's location (see Figure 2). Figure 2 Bus tie switch status interface 3.2.2 Switch operation interface functions (1) Display the status of the frequency converter transformer incoming switch, m1 motor control contactor, m2 motor control contactor, and m3 motor control contactor; (2) Display the 6kV bus voltage, current, active power, reactive power, and power factor; (3) Control the opening and closing of the frequency converter transformer incoming circuit breaker g2 by clicking the "start, stop, button" button; (4) Frequency converter transformer star/delta conversion key to control the primary star or delta connection of the frequency converter transformer; (5) Frequency converter transformer output conversion key to select different conversion keys according to the type of frequency converter being tested. The interface settings in Figure 3 ensure the safety and reliability of high-voltage operation and prevent misoperation. Figure 3 Switch operation interface 3.2.3 Motor monitoring interface functions to monitor in real time: (1) Detect the bearing temperature and stator temperature of each motor; (2) Torque monitoring; (3) Output power monitoring to ensure normal operation of the motor. Figure 4 Motor monitoring interface 3.2.4 Regen inverter control interface Function: This interface controls the Regen inverter to work according to the preset program (see Figure 5). Figure 5 Regen inverter control interface 3.2.5 Step load setting interface Function: By setting the load torque percentage and the working time of each torque, the Regen inverter is controlled to output according to the set torque and time, so that the inverter under test works according to the step load (see Figure 6). Figure 6 Step load setting interface 3.2.6 PLC programming (1) Use one input module and one output module to collect the high voltage switch status of each high voltage control cabinet, as well as analog quantities such as motor torque and temperature, control the opening and closing of each high voltage switch, and the interlock between switches, and upload them to the host computer. (2) Regen inverter given command programming: According to the test requirements, the Regen inverter given command programming is used to simulate different working conditions to achieve the purpose of testing the inverter. (3) Communicate with the host computer, receive the host computer control command, execute it, upload the equipment status, and display it on the host computer. 3.3 Regen Function Implementation The Regen inverter uses a Rockwell Powerflex 7000 series DTD inverter. The input and output use SGCT power devices in series. A resistor is connected between the neutral point of the filter capacitors on the input side and the motor side to eliminate common-mode voltage. This eliminates the need for an isolation transformer on the input side. Because the DC link of the current-source inverter is a reactor rather than a capacitor, and symmetrical power devices are used, the DC voltage direction can be changed. Therefore, motor energy can be fed back to the grid, achieving four-quadrant operation. See Figure 7. Figure 7: Current-source inverter achieving four-quadrant operation. 4. Testable Functions of this System 4.1 Inverter Load Characteristic Test The load characteristic of an inverter is one of its most important characteristics. The quality of an inverter's torque characteristics directly affects the performance of the user's operating conditions. On this test platform, the following tests can be performed: (1) load characteristic test proportional to motor speed; (2) load characteristic test proportional to the square of motor speed; (3) stepped simulation load characteristic test, as shown in Figure 8; Figure 8 Stepped simulation load characteristic test (4) User-defined load characteristic test. 4.2 Long-term load assessment and aging test of frequency converter After designing the load characteristics according to different on-site working conditions, 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. 4.3 Temperature rise test of power device of frequency converter 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 the design and development of frequency converter. 4.4 Temperature rise test of transformer of frequency converter 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 the design of transformer of frequency converter. 5. Conclusion This high-voltage frequency converter testing system has been validated in practical applications (JZE and Rockwell frequency converters have undergone full testing by this system before leaving the factory). The system is simple to operate, safe, and reliable, achieving satisfactory results in all tests of the tested frequency converters. Because the Regen frequency converter feeds the power from the regenerative generator to the grid, it saves energy. In particular, tests for special load characteristics can be implemented simply through PLC software programming, which is extremely convenient. However, due to its complex design and high cost, it requires a company with considerable resources to build.