1. Introduction
Hydraulic couplings are core transmission components in power generation, metallurgy, and large heavy-duty equipment. Their torque transmission, speed characteristics, and ultimate load-bearing capacity directly determine the operational reliability of complete trains.
Traditional coupling test benches generally adopt water eddy current dynamometers or resistive energy-consuming load schemes, which have three major shortcomings. First, all electrical energy is converted into heat throughout the entire test process. A single full-load test of large capacity consumes a huge amount of electricity, resulting in high long-term test operating costs. Second, the linearity of resistance torque regulation is poor, and torque control accuracy in the low-speed range is insufficient, making it impossible to accurately acquire the torque-speed characteristic curves under different scoop tube openings. Third, dynamometer equipment is bulky with complex cooling systems, requiring extensive operation and maintenance, and is difficult to meet the demand for continuous loading over a wide speed range.
To address these issues, the newly built coupling test bench in this project adopts a dual-motor back-to-back closed-loop energy recovery architecture. The test bench is equipped with two 8200 kW medium voltage asynchronous motors. One side is powered by a two-quadrant drive medium voltage drive as the input power source; the other side is equipped with a four-quadrant loading medium voltage drive as a controllable load.
Nancal Electric supplied the four-quadrant medium voltage drive for this project. The grid side adopts a fully controlled IGBT rectifier to achieve bidirectional energy flow. The load side can operate either in motoring mode or in generating mode, feeding energy back to the grid. The drive meets the requirements of different test conditions and solves the industry challenges of traditional test benches, including high energy consumption, insufficient control accuracy, and incomplete operating condition coverage.
Figure 1: Hydraulic Coupling Test Bench Site
2. System Solution
As shown in Figure 2, QF11 is the incoming switch of the drive-side medium voltage drive, and QF21 is the incoming switch of the load-side medium voltage drive. The load side is equipped with an 8200 kW medium voltage asynchronous motor, rigidly connected to the output end of the tested coupling through a gearbox. A motor of the same specification on the drive side is connected via the coupling. The two motors form a back-to-back closed-loop system.
The Nancal Electric medium voltage drive consists of five parts: pre-charging cabinet, phase-shifting transformer cabinet, power cell cabinet, control cabinet, and outgoing cabinet, arranged side by side to suit indoor layout. The system adopts a cell series multi-level four-quadrant topology, with 8 power cells connected in series per phase, producing a 17-level output phase voltage and a 33-level line voltage. The IGBT bidirectional rectifier topology enables bidirectional energy flow.
In the motoring condition of the motor, the grid supplies electrical energy to the motor. In the generating condition, the electrical energy from the load motor is fed back to the medium voltage busbar through the power cells and phase-shifting transformer, achieving a closed-loop energy cycle.
Figure 2: Single Line Diagram
The test bench operates in two modes: uncoupled no-load condition and back-to-back loading condition, meeting factory inspection and type test requirements:
① Uncoupled no-load condition (overspeed, etc.)
The load motor is completely disconnected from the coupling. Only the two-quadrant drive on the drive side operates, completing basic verification such as overspeed rotation of the coupling.
② Back-to-back loading condition (loaded and full-load tests)
The drive motor drives the coupling. The coupling drives the load motor through the gearbox. The load motor enters generating mode, and the four-quadrant drive absorbs the generated energy and feeds it back to the grid. At the same time, it outputs reverse resistance torque in torque closed-loop mode to simulate the actual working load of the coupling, performing verification of ultimate load-bearing capacity, efficiency, heat generation, and other parameters.
3. Performance Advantages of Nancal Electric Four-Quadrant Medium Voltage Drives
3.1 Key Performance
① Optimized design for power quality
The grid side adopts 48-pulse phase-shifting rectification. The total harmonic distortion (THD) of input current is < 2%, fully meeting the IEEE 519 and GB/T 14549 grid harmonic standards. The input power factor is > 0.95 over the 30%–100% load range. The output du/dt is < 1000 V/μs, eliminating the need for an additional medium voltage output filter.
② Control platform
The drive uses a DSP+FPGA+ARM triple-core integrated main control system with optical encoder speed sensor vector control. The torque closed-loop response is fast, meeting the requirements for dynamic loading and precise torque regulation during coupling type tests across the speed regulation range.
③ Reliability and overload capacity
The drive has a 120% rated current overload capacity for 1 minute, suitable for short-term impact loading tests.
④ Comprehensive protection system
The drive provides power cell-level protection (overvoltage, undervoltage, IGBT fault, cell overtemperature, communication interruption), transformer-level protection (overtemperature, secondary short circuit), and motor-side protection (overload, overspeed). It also integrates safety interlocks such as medium voltage cabinet door interlock, emergency stop, and pre-charging failure.
3.2 Advantages Compared with Traditional Test Solutions
① Outstanding energy-saving benefits
The closed-loop energy feedback architecture allows most electrical energy to circulate within the system under full-load conditions. The grid only supplements system losses (transformer, fans, cells, etc.). Compared with a water eddy current dynamometer, which consumes all energy at full load, the electricity saved in a single full-load test is considerable, and long-term testing can significantly reduce electricity costs.
② Precise loading across the full speed range
Torque closed-loop independent control is not affected by motor speed. It is linearly adjustable across the full speed range and can stably output a wide range of resistance torque. It accurately acquires coupling performance curves and meets the precision requirements of third-party testing.
③ Simple operation and maintenance, optimized footprint
The modular drawer-type power cells are equipped with dedicated cell replacement carts, allowing rapid replacement of faulty cells. The cabinets integrate medium voltage pre-charging, phase-shifting transformer, power cells, and control system. Air cooling eliminates the need for an additional cooling water pool or water circulation system, resulting in high plant space utilization.
Figure 3: Four-Quadrant Medium Voltage Drive Room
4. Project Application and Promotion Value
After commissioning, the test bench has completed type tests for high-power nuclear power hydraulic couplings, forming a mature standardized megawatt-class back-to-back testing solution. Compared with water eddy current and resistive energy-consuming dynamometer equipment, it combines the two core advantages of energy saving and high precision in high-power test scenarios. It is suitable for factory inspection and type testing of various heavy-duty transmission components in thermal power, mining, marine, and nuclear power fields.
The cell series multi-level four-quadrant medium voltage drive topology adopted in this project has strong versatility. In addition to coupling test benches, it can also be extended to motor back-to-back performance testing, high-power powertrain regenerative test platforms, and other applications.
5. Conclusion
The Nancal Electric four-quadrant medium voltage drive has been successfully commissioned in the hydraulic coupling test bench project. Based on core technologies such as cell series multi-level topology, high-performance vector control, and energy feedback, it fully meets the stringent requirements of complex test conditions for wide speed range, high torque, and high-precision loading. It also delivers multiple benefits including stable operation, energy saving, and convenient operation and maintenance.
From solution design, equipment manufacturing, and on-site commissioning to full lifecycle service, the project forms a complete closed loop. It verifies the reliability and advanced nature of localized four-quadrant medium voltage drives in the field of high-power industrial test equipment, providing an excellent engineering example for the localization and green upgrade of domestic test benches for large transmission component performance testing.