Abstract: The successful development of a lightweight power compressor test bench is fundamental to the research and development of our institute's ×× type engine. One of the key challenges was overcoming the development of the test bench's drive system (which also has speed control capabilities). Through joint research with Hekang Yisheng Company, a domestically produced high-voltage frequency converter was applied to the compressor test bench for the first time, achieving a "one-to-two" mode where one high-voltage frequency converter (via a high-voltage motor) drives two compressor test benches. This paper, starting from the compressor load characteristics, demonstrates the feasibility of using a high-voltage variable frequency speed control system for drive, and briefly introduces the application of Hekang Yisheng's high-voltage frequency converter on this compressor test bench, paving a new path for the development of similar test bench drive systems.
Keywords: Light-duty power; Compressor test bench; High-voltage frequency converter; Vector control
I. Introduction
Lightweight propulsion generally refers to lightweight gas turbines with a thrust of less than 1000 kgf or a power of less than 1 MWe. These are typically used in aircraft propulsion, mobile power sources, or distributed energy supply, and are of great significance in national defense and energy strategy. The development of lightweight propulsion is a iterative process of design, manufacturing, testing, modification, remanufacturing, and retesting—a process that gradually approaches perfection. Among these processes, testing plays a crucial role. Taking aero-engines as an example, before formal design finalization, they must undergo numerous tests, both large and small, from the ground to the air.
The testing of light-duty engines is generally divided into component testing and engine testing. Testing of major components such as the compressor, fuel chamber, and turbine plays a crucial role in the development of light-duty engines. Among these, the compressor operates under complex conditions (wide range of air parameter variations and large power variations) and has stringent technical requirements. It is the most expensive and technically complex component in engine testing, and its testing tasks involve many aspects: ① measuring compressor characteristic parameters (airflow rate, pressure ratio, efficiency, etc.), ② determining stable operating boundaries, ③ conducting flow loss tests, and ④ checking the reliability of the compressor regulation system.
The compressor test platform (equipment) developed in this paper is the material basis for conducting the above-mentioned compressor tests.
II. Overview of the Compressor Test Bench
(I) Composition of the compressor test bench
The compressor test bench consists of an intake and exhaust system, a drive system (power unit), a speed increaser, a tester, a lubrication system, a testing system, and a safety monitoring system (Figure 1).
- Intake and exhaust system: includes flow pipe, inlet throttling device, pressure regulator, exhaust volute and exhaust valve, quick-release valve, silencer, etc. Adjusting the inlet throttling device can change the compressor inlet pressure and compressor power consumption, while adjusting the exhaust valve can change the compressor airflow.
- Drive system (power unit): Provides power to the test bench and enables precise speed control. For example, the drive system of this test bench was developed by Hekang Yisheng Company;
- Speed increaser and test apparatus: To accommodate compressor tests with different power and speed, speed increasers are selected between the drive system and the test apparatus. For example, this test bench is designed in a "one-to-two" mode, that is, one power unit drives two different test apparatuses through two speed increasers;
- Lubricating oil system: including oil tank, oil filter, oil supply and return pump, radiator, electric heating and automatic control device, used for lubrication of bearings and speed increaser of the testing machine;
- Testing system: measures the compressor's operating parameters, including pressure, temperature, flow rate, speed, vibration, etc.
- Safety monitoring system: Monitors abnormalities that occur during the operation of the test compressor and other equipment, identifies faults, and takes appropriate measures.
(II) Performance Indicators of the Compressor Test Bench
The test bench drive system has a maximum power of 800kW and can perform low-speed and high-speed compressor tests. The specific details of the two test chambers are as follows.
Table 1 Design performance parameters of the compressor test bench
Name | low-speed compressor platform | High-speed compressor platform |
Drive system power | 800kW | 650kW |
Maximum speed | 32000rpm | 65000rpm |
Speed increaser ratio | 12.0 : 1 | 21.6 : 1 |
Speed control accuracy | ±0.02% | ±0.02% |
Turn | positive | Reverse |
airflow | 0~3kg/s | |
Minimum intake pressure | 0.02MPa | |
Maximum exhaust pressure | 0.60MPa | |
Maximum exhaust temperature | 620K | |
The energy parameters are shown in Table 1.
This test bench is primarily designed for light-powered compressors. As shown in Table 1, the power of the drive system is not high, but the speed control accuracy is relatively high (0.02%). Furthermore, the test bench's dynamic performance requirements stipulate that when the compressor torque suddenly changes by 10%, the drive system must recover to the original set speed within 1 second, and the speed fluctuation during the transition should not exceed ±0.2%. Therefore, these stringent technical requirements make the drive system one of the most challenging aspects of developing the compressor test bench.
III. Compressor Test Bench Drive System
(a) Power (or torque) of the compressor test
During compressor testing, the performance parameters of the compressor at different relative equivalent speeds (0.60, 0.70, 0.80, 0.90 and 1.0) are mainly measured to obtain the relationship between pressure ratio, flow rate and speed, determine the surge or stall point, and obtain the stable operating boundary of the compressor. The characteristic curve is shown in Figure 2 (with the lined variable dimensionless).
(ii) Drag system type
Existing compressor test benches can be categorized into those driven by gas turbines (or aero-engines) and those driven by electric motors. Gas turbine drives are suitable for large compressor tests, providing significant power. However, the high speed of the gas turbine's power turbine necessitates a relatively small speed increase from the speed increaser (or no speed increaser at all). Consequently, gas turbine drive systems exhibit poor speed control accuracy and dynamic performance. Electric motor drives are further divided into DC motor drives and asynchronous AC motor drives.
In the past, given the shortcomings of frequency converter technology in speed control, DC speed-regulating drive systems were the first thought for any power system involving precise speed control. Therefore, almost all existing compressor test benches that selected electromagnetic drives used DC speed-regulating motors.
With the development of high-voltage variable frequency technology and the application of vector control technology, the speed and torque control accuracy of AC asynchronous motors has reached the level of DC speed-regulating motors. Compared with DC speed regulation systems, it can achieve wide-range, efficient, and continuous speed regulation control, perform high-frequency start-stop operation, and easily achieve forward and reverse switching of the motor. In addition, variable frequency speed regulation systems also have the advantages of low cost, simple structure, and convenient maintenance.
(III) High-voltage variable frequency drive system
The compressor test bench developed in this paper was driven by a high-pressure variable frequency speed control system after comprehensive analysis of factors such as power, speed, speed control accuracy, and engineering cost. As the load of the drive system, the torque-speed characteristics of the test compressor are a key factor to consider when determining the control method and scheme of the speed control system.
The maximum torque at different speeds of the test compressor shown in Figure 3 was extracted, and the compressor's speed-torque curve was plotted, as shown in Figure 4. It can be seen that this curve exhibits load characteristics similar to those of fans and pumps, i.e., 2. Of course, the speed-torque characteristics may differ for different test compressors, but the characteristic of maximum torque corresponding to maximum speed will not change.
Therefore, the load characteristics of the frequency converter and the motor maintain constant power characteristics near the rated speed and constant torque characteristics in the low-speed range. Appropriate control methods and schemes can improve system operating quality, reduce costs, and easily achieve the speed regulation performance indicators of the test bench.
Figure 4 Torque curve of the drive system
IV. Hekang Yisheng High Voltage Variable Frequency Speed Control System
(I) Composition of Variable Frequency Speed Control System
Hekang Yisheng's variable frequency speed control system includes: high voltage switch, transformer cabinet, power unit cabinet, control cabinet, high voltage variable frequency motor and shaft encoder, etc.
The transformer cabinet contains a phase-shifting transformer that provides three-phase power to the power units. The secondary winding wiring areas are located on the front right and rear left sides of the transformer, connecting to the corresponding three-phase reactor input cables. A dry-type transformer temperature controller is located on the cabinet door, providing temperature alarms and overheat protection. A safety switch is installed inside the front door, triggering an alarm when the door is opened.
The power units and three-phase input reactors installed in the power unit cabinet are divided into three groups, each group connected in series to form one phase. Each phase has 6 power units and 6 reactors connected in series, arranged in a front-to-back pattern. After being connected in series, the three power units A1, B1, and C1 are star-connected. The outputs of the last three units, A6, B6, and C6, are connected to the copper busbar in the high-voltage output compartment. Hall current sensors are run through the star-connected cables to detect the output current.
The control cabinet, from top to bottom, consists of the controller, I/O interface board, control power system, and human-machine interface system. The controller comprises three fiber optic boards, one signal board, one main control board, and one power supply board, all connected via a busbar base plate. The I/O interface board handles the logic processing of internal switching signals, field operation signals, and status signals, enhancing the flexibility of the inverter's field applications. The control power system provides DC power to the control system. The human-machine interface offers a user-friendly Chinese interface, handling information processing and external communication; optional supervisory control and data acquisition (SCADA) monitoring enables networked control of the inverter.
a) Compressor test bench (high-voltage motor)
Figure 6 Block diagram of the main circuit of the high-voltage frequency converter
The Hekang Yisheng series high-voltage frequency converter adopts a direct AC-DC-AC high-voltage method, with IGBTs as the main circuit switching element. Due to the voltage withstand limitations of IGBTs, direct inverter output of 6kV or 10kV is not possible, and direct series connection is impossible due to technical challenges such as high switching frequency and difficulty in voltage equalization. This high-voltage frequency converter uses power unit series connection and superimposed voltage boosting, fully utilizing the mature technology of ordinary voltage frequency converters, thus exhibiting high reliability. Figure 6 shows the main circuit block diagram of the frequency converter.
(ii) Vector control
Based on the performance requirements of the compressor test bench for the variable frequency speed control system, Hekang Yisheng decided to choose vector control to directly control the motor torque, ensuring rapid speed response without overshoot and guaranteeing system acceleration/deceleration and starting performance. The basic principle of vector control is to measure and control the stator current vector of the asynchronous motor, and then control the excitation current and torque current of the asynchronous motor according to the principle of field orientation, thereby achieving the purpose of controlling the asynchronous motor torque. Vector control includes vector control based on slip frequency control, sensorless control, and control with a speed sensor.
The HIVERT-TVF series frequency converter from Hekang Yisheng utilizes a decoupling vector control system. The speed error is obtained by subtracting the speed setpoint (frequency setpoint) from the speed feedback. This speed error is then adjusted by PID control to output the torque current setpoint. The excitation current setpoint is adjusted according to the dynamic needs of the system, based on empirical values derived from different motors and loads. The motor's three-phase current feedback, (where is the inverse of its sum), is sampled by sensors and then subjected to Clarke transformation based on the rotor position electrical angle. The transformed values are and , which are then subjected to Park transformation to output . The error between and is calculated by comparing these values with the setpoints and , and after PID control, is output . The rotor position electrical angle is then subjected to inverse Park transformation to output , . Finally, and are subjected to inverse Clarke transformation to output the motor stator three-phase voltages , , and . These three-phase voltages are used as comparison values for PWM (Pulse Width Modulation) to output the PWM waveform, which is then sent to the inverter to drive the motor. (See Figure 7.)
Figure 7. Vector control block diagram with speed sensor
(III) Electromagnetic compatibility of high-voltage frequency converters
Electromagnetic compatibility (EMC) of variable frequency speed control systems is a crucial issue that cannot be ignored, as it largely determines the reliability of the system. The overall EMC design mainly includes wiring, grounding, shielding, filtering, and device layout design.
The power cables and control cables of the variable frequency speed control system are laid out in separate layers. Armored shielded cables are used for the power cables, while shielded twisted-pair or shielded wires are used for the control cables. The power cables and control cables are grounded separately, connected to "power system ground" and "measurement system ground" respectively. The shielding layer of the motor speed measuring shaft encoder shielded cable is grounded on the motor accessory box side. The device layout design ensures independent power supplies for the frequency converter, PLC, and host computer, cutting off harmonic currents. The separate cabinet installation layout for strong and weak current control components minimizes system noise current.
V. Conclusion
The compressor test bench adopts a high-pressure variable frequency speed control system. After no-load and load testing, all technical indicators of the test bench have met or exceeded the design targets. The main achievements are as follows:
(1) For the first time, the domestically produced high-voltage frequency converter was used as the speed control device for the compressor test bench, and it was successful;
(2) The motor shaft encoder serves as the speed reference for the high-voltage variable frequency speed control system, ensuring that the speed control accuracy of the test bench is better than ±0.02%.
(3) The high-voltage variable frequency speed control system has a wide speed regulation range, which ensures that performance tests can be carried out at lower speeds (low operating conditions).
References
[1] Zhang Baocheng. Aero-engine Testing and Experimentation Technology [M]. Beijing: Beijing University of Aeronautics and Astronautics Press, 2005.
[2] Jiang Hao, Qin Qin. Detailed Explanation of Aero-engine Testing Process—An Interview with Academician Liu Daxing, a Renowned Aero-engine Expert in my country [J]. Ordnance Technology, 2008.11, 12~19
[3] Ping Hua, Zeng Yuelan, He Ping. Development and application of variable frequency speed control system in a compressor test apparatus. 6th Annual Meeting of the Chinese Society of Aeronautics and Astronautics, 2006, CSAA06-P-142
[4] Ping Hua, Ding Yi, Tan Xuefeng. Single-frequency multi-drive technology in the development of power unit for compressor test equipment. The 8th Symposium on Engine Testing and Measurement, 2006, CSAA06-PE-015
[5] You Qunzhi, Cao Zhiguo, Liao Jie. Summary of the development of a compressor experimental device. The 7th Annual Conference on Engine Testing Technology of the Power Branch of the Chinese Society of Aeronautics and Astronautics, 2005, CSAA05-PET-037
[6] Meng Xiaofang, Li Ce, Wang Jue, et al. Siemens Series Frequency Inverters and Their Engineering Applications [M]. Beijing: Machinery Industry Press, 2008.
[7] Zhang Hao, Xu Mingjin, Yang Mei. High-voltage high-power AC variable frequency speed control technology. Beijing: Machinery Industry Press, 2006.
Author Biography: Fang Aibing (1977-), male, from Botou, Hebei Province, holds a PhD. Tel: 010-82543087, E-mail: [email protected]
