Abstract: To meet the operational requirements of AC drive diesel locomotives, a traction variable frequency speed control system with two operating modes was designed: a dual-vehicle parallel operating mode and a single-vehicle independent operating mode. The main circuit and control system of the variable frequency speed control system were designed and analyzed. Finally, the system was experimentally tested and found to meet the expected performance requirements.
Keywords: Variable frequency speed control system, torque, inverter, slip frequency
Design of Diesel locomotive VVVF Traction Control System
LIU Guang-peng, QI Xiang-dond
(School of Electronic Information, Taiyuan University of Science and Technology, Taiyuan 030024, China)
ZHAO Rui-ling
(Anyang County the first vocational high schools, Anyang 455112, China)
Abstract: According to demand specifically for AC drive Diesel locomotive. Having designed a VVVF Traction Control System that is provided with two kinds of operating modes, which are the double car parallel operation mode and individual car independent operation mode. And then design and analyze Frequency Control System's main circuit and Control System. Finally, the system is Carried out experiments testing, and the test result proves that it has reached the expected performance requirements.
Keywords: Frequency Control System, Torque, Inverter, Slip frequency
1 Introduction
To meet the needs of railway speed increases, the development of AC electric drive engineering locomotives is essential. These locomotives should be designed to operate in two modes: high-speed, long-distance traction and ultra-low-speed stable operation. Currently, engineering locomotives used in China neither employ AC electric drive nor possess the necessary performance characteristics.
2. Characteristics of traction motors
Figure 1. Torque/speed characteristics of an asynchronous motor at a certain frequency/voltage.
As known from the principles of motors, the typical torque-speed characteristic of an asynchronous motor is shown in Figure 1. When the motor rotor is at synchronous speed, the torque is 0. When the slip is very small, the torque changes almost linearly with decreasing speed, i.e., increasing slip. When the slip S is positive, it represents the motoring torque; when the slip is negative, it represents the braking (generating) torque. The slip is the ratio of the slip frequency (rotor current frequency) to the stator current frequency f1.
S = Δf / f1 (1)
If the rotational frequency f2 of the asynchronous motor rotor can be measured and calculated, and based on the load's torque requirements and the motor's control characteristics, its corresponding slip frequency Δf can be found. Then, the stator current frequency f1 output by the frequency converter is:
f1 = f2 ±Δf (2)
In formula (2), (+) corresponds to the electric traction state, that is, the frequency f1 of the stator current is greater than the frequency f2 of the rotor rotation; (-) corresponds to the generator braking state, at which time the frequency f2 of the rotor rotation is greater than the frequency f1 of the stator current. [4]
Figure 2. Rated slip frequency response curve of variable frequency traction asynchronous motor
Figure 2 shows the characteristic curve of the rated slip frequency Δf of the variable frequency traction asynchronous motor. This curve can be obtained from the design parameters of the traction asynchronous motor. This characteristic curve serves as the original basis for torque setting (slip frequency Δf). It is corrected during the inverter-traction motor matching experiment, and the speed is adjusted appropriately as needed during on-site commissioning of the engineering locomotive. [1]
3. Characteristics of Traction Variable Frequency Speed Control System
3.1 Control Method of Variable Frequency Traction Speed Regulation System
Since the locomotive itself and the trailer it tows are heavy, they are generally large inertial loads, and their start/stop times are long, and their torque response time does not require rapid response. Therefore, variable frequency traction speed regulation adopts slip frequency control to realize torque setpoint control and speed slip closed-loop control, which can fully meet the various requirements of traction control. [2]
3.2 Operating Modes of the Traction Variable Frequency Speed Control System
Engineering work vehicles operate on tracks, typically in a "double-unit" configuration. This configuration increases equipment reliability and adapts to varying trailer loads and the demands of long slopes, high-speed, and long-distance operations.
In view of the design and use requirements of AC drive diesel locomotives, the traction variable frequency speed control system should be designed according to the following working mode: [2]
(1) Dual-vehicle parallel operation mode:
It operates in torque-based control mode (slip frequency Δf control);
High-speed, long-distance (heavy-load, long-slope, etc.) traction operation.
(2) Independent working mode of a single vehicle:
It operates in either closed-loop speed-slip control or open-loop V/F frequency control mode;
Stable operation at low speed.
3.2.1 Dual-vehicle parallel operation mode
Figure 3. Schematic diagram of the control principle of dual-vehicle parallel operation and torque setting method.
The principle of the parallel operation of the two vehicles and the torque setting method is shown in Figure 3. The control system calculates the actual operating frequency f2 of the traction electric rotor during rotation. If the rotor of the motor only needs to run with the locomotive at this time, the rotor operating frequency f2 is used as the output frequency f1 of the traction inverter (the frequency applied on the stator winding), i.e., f1 = f2. When a certain traction force (electric torque) needs to be applied, the control system only needs to add the slip frequency Δf′ (Δf′/Δf = actual torque/rated torque) corresponding to a certain frequency f2 of the motor rotor at this time to f2, i.e., f1 = f2 + Δf′, so that the motor outputs the corresponding torque. Through the mechanical transmission mechanism, the locomotive obtains the corresponding traction force. In order to give the diesel generator set a certain adjustment time, the application of traction force/braking force needs to be buffered by a given ramp time. With torque control in this way, the traction variable frequency speed regulation system will be very stable. [3]
According to Figure 2, if the torque is controlled in (1 to 15) levels (the levels are proportional), there are 15 slip frequency characteristic curves available for the user to choose from.
3.2.2 Single-vehicle independent working mode
Figure 4. Schematic diagram of the control principle of single-vehicle independent speed closed-loop/V/F open-loop frequency method
After understanding the working principle of torque setting mode, we will discuss the working principle of speed slip closed loop. As shown in Figure 4, the input signal f2 in the PWM calculation block is the same as f2 in torque setting control mode, which will not be repeated here. The speed regulator PI calculates the difference between Vg and Vf and performs PI operation. The output Δf value is limited by the Δf data. That is, when the Δf value is within the rated value (Δf), it outputs its actual value. When it exceeds the rated Δf, it is limited to the Δf value corresponding to the f2 frequency. That is, unlike the torque setting control mode which only has 15 curves, Δf has countless curves in the traction/braking working range. [3]
V/F open-loop frequency (speed) control uses the speed signal directly as the output frequency signal f1 of the traction inverter. However, V/F open-loop frequency control must take into account the characteristics required for traction control, which is far beyond the capabilities of general-purpose inverters.
4. Main Circuit Design of Traction Variable Frequency Speed Control System
4.1 Special requirements for traction variable frequency speed control systems
Given the specific application of this variable frequency speed control system, the main circuit design needs to consider the following factors:
(1) The prominent low immunity of the diesel generator set power supply system;
(2) The special requirements of the power supply for traction motors;
(3) The necessity of maintenance-free requirements for traction converters. [5]
4.2 Analysis of the main circuit of the traction variable frequency speed control system
Figure 5. Block diagram of the main circuit principle and operation control principle of the traction variable frequency speed control system
The circuit principle and operation control principle block diagram of the main circuit of the traction variable frequency speed control system are shown in Figure 5. As can be seen from the figure, the main circuit consists of: input circuit, rectifier, pre-charge circuit, filter, kinetic braking and inverter, etc. The functions of each component are briefly described as follows: [6]
(1) Incoming circuit
The incoming line circuit consists of a knife switch K, an incoming line reactor Lp, and a fuse RD. The functions of each electrical component are as follows:
Knife switch K: Isolates the traction variable frequency speed control system (device) from the locomotive diesel-generator power supply system.
Incoming line reactor Lp : Makes the AC input and output current of the rectifier continuous and smooth, reducing the interference of the rectifier circuit on the diesel generator set power grid.
Fuse RD : Provides protection for diesel generator sets.
(2) Rectifier
A three-phase bridge semi-controlled rectifier circuit is composed of three thyristors and three rectifier diodes. Here, the thyristors are not phase-controlled but level-triggered, meaning that when a thyristor is working, it is equivalent to a rectifier diode.
(3) Pre-charging circuit
The traction variable frequency speed control system is an "AC-DC-AC voltage type frequency converter". The intermediate DC voltage link is composed of multiple large-capacity electrolytic capacitors in series/parallel to protect the rectifier and capacitors from damage due to large charging current.
The pre-charge circuit consists of a three-phase bridge rectifier ZL and charging current-limiting resistors 2R1 and 2.
(4) Filter
The filter is mainly composed of multiple large-capacity electrolytic capacitors connected in series/parallel. Voltage equalization resistors 1R1 and 2 are used to force voltage equalization so that the voltage across the series capacitors is nearly uniform.
The filter capacitor here has three functions: 1. to smooth out the ripple of the current voltage; 2. to provide reactive current for the asynchronous motor; 3. to provide a low-impedance path for kinetic braking and the commutation of fully controlled power electronic switching elements in the inverter. [3]
(5) Kinetic braking circuit
When the traction motor is in regenerative braking mode, its output voltage and current are in opposite directions. In the inverter, the time the IGBT transistor carries current is less than the time it carries current through the freewheeling diode within one output frequency cycle. With the help of the diode, the inverter sends the electrical energy converted from the load's kinetic energy to the filter capacitor. This energy cannot be returned to the AC grid through the rectifier, resulting in a continuous increase in energy and voltage on the capacitor. When the voltage reaches a certain value (e.g., 700V), the control system turns on the lower IGBT transistor. This connects the positive terminal of the DC power supply to the negative terminal through the discharge braking resistor RB and the turned-on transistor, allowing current IB = Vdc / RB to flow. This releases the stored energy in the capacitor. The inverter continuously feeds back the electrical energy generated by the motor, thus providing a certain braking force to the locomotive.
(6) Inverter
The inverter consists of six IGBT transistor switches and six fast recovery freewheeling diodes connected in anti-parallel to the IGBTs to enable bidirectional current flow. This circuit structure uses a two-level control method, employing PWM control technology to achieve coordinated voltage/frequency (V/F) control (VVVF control).
When the traction asynchronous motor is running in electric traction mode, the voltage/current at the stator winding of the AC motor is in the same direction (with a phase angle difference). The inverter converts the DC power supply into AC power to supply power to the motor. When the traction asynchronous motor is running in braking mode, the voltage/current at the stator winding of the AC motor is in opposite directions (with a phase difference). The inverter rectifies the AC power generated by the motor into DC power and sends it back to the filter capacitor. The principle of AC to DC rectification conversion is as described in (5) Kinetic braking. [2]
In addition to the main circuit components, Figure 5 also includes an auxiliary power control circuit, signal detection elements, and a low-voltage control unit for the control system.
5 Control System Design
The control system uses an INTEL 16-bit microcontroller as the main control chip and adopts a space voltage vector wave control method to realize a fully digital slip frequency control mode suitable for single-machine operation and multi-machine linkage, [2] as shown in the figure below:
Figure 6 Locomotive traction control block diagram
The control system, through the combination of software and hardware, has the following characteristics:
(1) Low frequency (starting) high torque;
(2) 200% overload capacity and software inverse time limit characteristics;
(3) Automatic stall control is added to prevent dynamic overvoltage and overcurrent;
(4) Software-selectable flexible PWM energy-saving braking;
(5) Automatic voltage control (AVC) to overcome grid fluctuations and maintain output voltage;
(6) Anti-blocking and limiting characteristics (excavator characteristics);
(7) Zero braking function suitable for automatic switching between electric parking on slopes or emergency electric braking;
(8) Dynamic speed tracking ensures that the system can be powered while the vehicle is running;
(9) Overload protection, peak overcurrent protection, overheat protection, undervoltage protection, phase loss protection, and fault memory and fault self-recovery function;
(10) An S-shaped acceleration curve suitable for heavy-load vehicle traction.
6. Conclusion and Analysis
This system belongs to the drive control level. The main rectifier adopts a three-phase bridge semi-controlled rectifier circuit; the motor control strategy is vector control, which is a high-performance asynchronous motor control technology with fast dynamic response and excellent steady-state performance. It also features low harmonic content.
The AC drive control system for diesel locomotives, as an independent whole, can fully meet the performance requirements of mining locomotives and is the direction to meet the needs of domestic and international markets, with good application prospects.
In recent years, with the increasing use of industrial and mining locomotives, locomotive manufacturers have intensified their research and development of engineering vehicles. Cooperation with locomotive manufacturers has accumulated considerable technology and experience in the design, manufacturing, and application of traction variable frequency speed control systems. It is believed that with further cooperation, even higher-performance traction variable frequency speed control systems will be developed in the future.
References
[1] Cao Guorui. Electric Transmission for Diesel Locomotives [M]. Beijing: China Railway Publishing House, 1990.
[2] Huang Jirong. Electric Traction AC Drive and Control [M]. Beijing: Machinery Industry Press, 1998.
[3] Li Huade. AC Speed Control System [M]. Beijing: Electronic Industry Press, 2004.
[4] Chen Boshi. Automatic Control System for Electric Drive [M]. Beijing: Machinery Industry Press, 2003.
[5] Jin Biao. Research on Electric Transmission System Scheme for High-Power AC Drive Internal Combustion Locomotives [D]. Southwest Jiaotong University, 2002, 38-48.
[6] Diao Lijun, Liu Zhigang, Shen Maosheng, Lei Jun. Overview of AC drive test system schemes [J]. Electrical Drive, 2007, 9: 33-34
First Author : Liu Guangpeng; Year of Birth: 1980; Gender: Male; Place of Origin: Liuzhuang Village, Mailing Town, Xiangcheng County, Xuchang City, Henan Province; Postgraduate Student, Class of 2007, School of Electronic Information, Taiyuan University of Science and Technology; Major: Power Electronics and Electric Drives
Research Interests : Computer Control
Contact number : 13834568337
Contact Address: Graduate Students, Class of 2007, School of Electronic Information, Taiyuan University of Science and Technology, Taiyuan, Shanxi Province, China. Postcode: 030024
Email: [email protected]
