Abstract: The Rockwell PowerFlex 7000 inverter is used as a converter inverter in the testing system of Jiuzhou Medium Voltage Variable Frequency Experimental Station. Because it is a current source type inverter, it has a four-quadrant operation function and can realize energy feedback and The braking of the motor and the advanced vector control technology can realize various complicated and varied load characteristics. It plays an irreplaceable role in simulating the motor load, and realizes the full load test before the inverter is shipped from the factory.
Keywords: PowerFlex7000, current source, vector control, energy feedback
ABSTRACT: A Rockwell PowerFlex7000 Variable Frequency Drive is used in JiuZHou Electric medium voltage drive test bay as the converter. Because of it belong to the current source drive, it has the function of the four quadrant, it can achieve the energy feedback and make the Motor brake, and also has the advanced vector control technology, so it can achieve all kinds of load test, there are no other drives can replace it in simulate motor load. PowerFlex7000 drive has ability to achieve the full load test before the drive leave factory .
Key words: PowerFlex7000, Current Source Type, Control by vector, energy feedback
1 Introduction
With the rapid development of the economy, energy issues have increasingly become the most concerned issue. When energy is divided into renewable resources and non-renewable resources, when non-renewable resources are increasingly in short supply, how to save energy is the current The problem that needs to be solved. At present, the cost of industrial electricity continues to increase. In order to reduce costs, speed control equipment has been widely used in factories. With the continuous maturation of technology, frequency conversion speed control equipment has become more popular in the industry, resulting in the inverter market The competition is increasingly fierce, especially in the medium-voltage inverter market competition is more brutal. If anyone can take a certain advantage in market competition, then anyone can laugh until the end.
Harbin Jiuzhou Electric Co., Ltd. and Rockwell Co., Ltd. jointly established Asia’s largest medium-voltage variable frequency experimental base, the Harbin Jiuzhou Medium Voltage Frequency Conversion Experimental Station, which can fully realize the full-load test of medium-voltage frequency converters before they leave the factory and meet different users. The actual needs. Harbin Jiuzhou Inverter Experimental Station adopts the PowerFlex 7000 medium voltage inverter manufactured by Rockwell Automation as the main body of the experimental station, and the medium voltage inverter is fully loaded and tested at the simulation site before leaving the factory. Effectively ensure the quality of the inverter.
2 Structure and Features of PowerFlex 7000 Inverter
2.1 PowerFlex 7000 Inverter Circuit Structure
The PowerFlex 7000 is Rockwell Automation's third-generation medium voltage drive. Its PowerFlex series inverters incorporate advanced technology, embed communication functions, and have great versatility in multi-platform, network, operator interface programming and hardware. The PowerFlex 7000 is a universal, stand-alone, medium voltage drive that controls the speed, torque, direction, start, and stop of standard synchronous or asynchronous AC motors. It is suitable for a wide range of standard and special applications such as fans, pumps, compressors, mixers, conveyors, kilns, fan pumps and test stands. The main industries for these applications include petrochemicals, cement, mining and smelting, power generation and water/wastewater treatment.
The PowerFlex 7000 inverter is different from the medium voltage inverters produced by other manufacturers in the world. It is the only current source medium voltage inverter in the world. According to the technical characteristics of the current source inverter, it is easy to implement four-quadrant operation. Voltage source inverter control is simpler and more reliable. The motor can be operated in the forward and reverse direction with a motor or a generator. When the motor is operated in a generator mode, energy can be fed back to the grid and regenerative braking. This provides the possibility for a full-load test experiment at the Jiuzhou Medium Voltage Inverter Experimental Station.
The PowerFlex 7000 inverter is a current-mode medium-voltage inverter. It has a simple structure, a small number of components, four-quadrant operation, and a vector control function. Figure 1 shows the circuit topology of a current transformer with no input transformer in the PWM control mode.
Fig. 1 Circuit topology of current-source inverter
The circuit structure is AC-DC-AC type, and the power device adopts symmetrical gate commutated thyristors (SGCT). For 6KV inverters, each bridge arm uses 3 SGCTs with a withstand voltage of 6500V in series, with all IGBTs. Advantages, and can block the voltage in both directions, reduce the number of devices used, so that the drive and the absorption circuit is further simplified, the performance is more superior.
2.2 How Does the PowerFlex 7000 Frequency Converter Work?
PowerFlex 7000 inverter circuit part is divided into high-voltage main loop part and low-voltage control loop part. The high-voltage main circuit power unit is also divided into two parts, the Rectifiers and the INVERTER. The three-phase 6000V line power is first passed through the AC LINE REACTOR. The line side AC reactor can provide additional filtering and current limiting to prevent line short-circuit faults. Then enter the rectification side. The rectification side is a rectifier consisting of SGCT or SCR (for 18 pulses) that converts AC to DC, passes DC LINK to the inverter side, and converts DC into DC through pulse width modulation. AC, and then filtered by the filter capacitor on the motor side, the nearly perfect sinusoidal current voltage waveform is output to the motor. The low voltage control loop part includes the display operation interface, low voltage circuit, various control circuit boards, uninterrupted power supply (UPS) and so on.
The PowerFlex 7000 inverter controls the motor speed by controlling the torque of the motor. It calculates and measures the motor speed and adjusts the torque as required to make the measured speed equal to the speed command. The motor and load determine the stator frequency and synchronize the frequency converter with the motor. This is in contrast to most V/F controlled AC frequency converters. The V/F controlled frequency converter is where the frequency converter determines the stator frequency and cannot make the output and the motor. Synchronize. The PowerFlex 7000's control method is direct vector control. The term vector control indicates that the position of the stator current vector is controlled relative to the motor flux vector. Direct vector control means that the motor flux can be measured, which is different from most commonly used indirect vector control, in which the flux of the motor is estimated. In both control algorithms, the stator current () is decomposed into an independently controllable flux component and an orthogonal torque component. The purpose of vector control is to obtain similar performance as a DC motor, which can quickly change the torque without affecting the magnetic flux. . Figure 2 is a functional block diagram of the powerflex7000 control system.
Fig. 2 Function block diagram of powerflex7000 system
3 Rockwell inverter implements load test principle
3.1 Implementation of load testing
There are three test motors in Jiuzhou Medium Voltage Frequency Conversion Test Station, one is 1000KW, and the other is 2000KW. The voltage levels are 6000V and 10000V respectively. In the future, a 5000KW motor will be introduced to achieve a load test of 5000KW. Three motors are connected to a coaxial motor by a coupling. Two of them are used as the drive to be tested. The other is connected to an AB PowerFlex 7000 inverter as a load motor. Since the DC link of the current inverter is a reactor instead of Capacitance, and the use of a symmetrical power device, the direction of the voltage on the DC side can be changed, so the electrical energy of the motor can be fed back to the grid to achieve four-quadrant operation. During operation, the load motor is used as a regenerative motor, and the energy generated by the AB inverter is fed back to the power grid, which saves a part of energy and does not cause pollution to the power grid. Therefore, it is called a regenerative inverter. The main circuit principle of Jiuzhou Converter Experimental Station is shown in Figure 3.
Figure 3 Schematic diagram of the main circuit of the experimental station
We know that the asynchronous motor runs as a generator and must be excited by the current in the phase lead voltage, and the PowerFlex 7000 inverter can precisely meet this requirement. It can provide exciting current to the motor, which is better solved. Generator excitation problem. When the frequency converter drags the M1 or M2 motor to rotate, the M3 motor is coaxial with M1 and M2, so the rotor of the M3 motor also rotates at the same time. When the Rockwell inverter is started, the stator three-phase winding is immediately energized. The current causes the stator to generate a static magnetic field. When the M3 rotor is rotated by the rotor of M1 or M2, its conductor cuts the magnetic field to induce an electric current and generate an electromagnetic brake torque opposite to that of the rotor to achieve braking, so that the stored kinetic energy is converted into electric energy.
When the energy is fed back, the controller of the PowerFlex 7000 inverter shifts the trigger angle of the thyristor backwards, and the rectifier enters an active inverter state and returns energy to the grid. The current source inverter uses a more precise vector control PWM rectification control algorithm to increase the network side power factor and reduce the network side harmonics. This process is achieved through the PowerFlex 7000 current control block function.
The main function of the current control block is to determine the size of the trigger angle of the inverter at the input and the inverter at the output. Its input is the torque component and flux component in the DC bus current command values ​​from the speed control block and the flux control block, and the DC bus current and input voltage from the line rectifier feedback. The square root of the squared sum of the command value and the command value is the command value of the DC bus current, and the measured DC current feedback value is subtracted from the DC current reference value, which is the DC current error, which is also the input signal of the current regulator. The current regulator's response is determined by the parameters current regulator bandwidth, DC bus inductance, and DC bus time constant. The trigger angle of the incoming line rectifier is obtained by inverting the cosine of the DC voltage reference value. The trigger angle of the driver side inverter is obtained by taking the ratio of the command value and the command value in inverse tangent. Figure 4 shows the current control model.
Figure 4 Current Control
Powerflex7000's PWM rectification control algorithm calculates the three-phase current of the grid-side inverter after the grid-side DC current calculated by the current control block and the grid-side AC current set value, which is generally set to zero, are inverse-synchronized. Given the value and output directly to the grid through the PWM generator, the grid current is precisely controlled. Ideally, the power factor at the grid side can be increased to about 1 and the network-side harmonics can be minimized.
The PowerFlex 7000 (DTD) frequency converter uses SGCT rectification, which is fully controlled rectification. The input power detection board can detect input voltage and current phases in real time and adjust the rectification part of the SGCT through the CIB board (user interface control board). Trigger angle, so as to achieve the function of correcting waveform and compensating the power factor of the power grid, the harmonic generated by the PowerFlex 7000 inverter itself is very small and will not affect the power grid.
When the motor runs from normal operation to braking operation, the output torque of the motor changes from the same direction as the rotation speed to the opposite direction of the rotation speed. The motor-side inverter transfers the active power from the motor to the DC inductor, causing the DC inductor current to flow. When the inductor current PI regulator detects this rising current, it will trigger θ to move backwards, so that the grid-side inverter enters the energy feedback state. Adjust the trigger angle of the control thyristor by adjusting the PI regulator of the PowerFlex 7000 inverter to adjust the output torque of the motor. Since the M3 motor is dragged by the M1 or M2 motor, increase the reverse torque of M3 to the M3. The motor rotor is braked so that it can brake the M1 or M2. The M1 or M2 motor is driven by the tested inverter. Increasing the braking torque for the M1 and M2 motor rotors is equivalent to adding M1. With the load of the M2 motor, the load test of the test inverter is realized. According to the rated power of the motor of Jiuzhou Converter Experimental Station, the frequency conversion experiment station can test the inverter with a rated power of 3000KW.
From the above, we can see that the M3 motor and PowerFlex 7000 inverter constitute an analog load. PowerFlex 7000 inverters use direct torque control to output torque according to the set value without speed limitation. Torque is measured using a torque sensor. Torque control is accomplished through the PID function block in the AB company's RSLogix5000 PLC. The electricity generated when the simulated load motor M3 operates in the power generation state is rectified by the SGCT and fed back to the DC bus. PowerFlex 7000 inverter itself has VSBL (incoming voltage feedback board) used to detect and feedback the voltage, phase, power factor and other parameters of the incoming grid voltage. The CIB board of the PowerFlex 7000 frequency converter can control the SGCT's duty cycle and frequency according to the detected grid parameters, and invert the DC power into the power grid and the AC power grid itself with the same phase and the same frequency as the AC power.
3.2 Experiment of Communication and Load Characteristics of PowerFlex 7000 Inverter
The PowerFlex 7000 inverter uses the CONTROLNET communication method to communicate with other devices. The configuration software uses the RSView SE Client configuration software. Each client can perform real-time status monitoring and control of the PowerFlex 7000 drive and download and save the resulting data. Figure 5 shows the system schematic diagram of Jiuzhou Medium Voltage Converter Experimental Station.
Fig. 5 System principle diagram of frequency conversion experiment station
Since the PowerFlex 7000 inverter uses an advanced vector control system, the PowerFlex 7000 inverter can be used to perform various load characteristic experiments, such as load characteristic test proportional to the motor speed, through computer software and PLC programs; proportional to the motor speed squared. The load characteristics test; ladder simulation load characteristics test; user-defined load characteristics test and so on. Figure 6 shows the Rockwell inverter's step load control interface to the motor.
Figure 6 Step Load Simulation Interface
4 Conclusion
PowerFlex 7000 inverter as a regenerative inverter not only realizes the load test of the tested inverter, but also feeds back the generated power to the power grid, which saves a part of energy. The most fundamental is the real detection of the load of the factory inverter. Ability and various operational indicators, thus verifying the stability and reliability of the product, and can perform simulation tests on various complex working conditions that may occur in the field to test the product's ability to react to and adapt to the complex and ever-changing field environment. . In a sense, load testing lays a solid foundation for improving the quality of frequency converters.
references:
[1] Rockwell POWER FLEX7000 Technical Manual, Rockwell, Inc. 2004
[2] Yi Peng High Voltage High Power Inverter Technology Principle and Application, People's Posts and Telecommunications Press 2008.2
[3] Ye Shuang, Guo Yaohua Application of Frequency Converter in Motor Testing Platform Motor and Control Applications 2007, 07
About the Author:
Yu Mingtao (1982-), male, undergraduate, engineer, is now engaged in commissioning and testing of Jiuzhou POWERSMART voltage source inverters and Rockwell POWERFLEX7000 current source inverters at Jiuzhou Electric Medium Voltage Inverter Experimental Station.