[Abstract] This article mainly describes how the Delta C-2000 inverter and AFE2000 together form a four-quadrant inverter control mode. This control mode demonstrates Delta's breakthrough in energy feedback technology, truly realizing a green inverter and effectively saving energy consumption.
Since the late 1980s, when variable frequency drive (VFD) technology entered the industrial transmission arena, it has become the most influential industrial automation speed control technology due to its advantages such as wide speed range, high speed accuracy, high efficiency, flexible control, and ease of use. VFDs developed based on this technology have consistently used uncontrolled or semi-controlled devices for grid-side rectification. This mode means the VFD can only operate in motoring mode and cannot achieve true braking; therefore, these VFDs are called two-quadrant VFDs. The weakness of two-quadrant VFDs lies in their inability to achieve braking feedback, leading to wasted energy; furthermore, their low power factor prevents the current on the DCBUS from forming a true sine wave, indirectly causing unnecessary energy waste.
The biggest problem with two-quadrant frequency converters is that the rectifier-side devices cannot be fully controlled, making energy feedback impossible. Therefore, high-frequency PWM rectification technology was developed. High-frequency PWM rectification technology is divided into two types: direct current-controlled PWM rectification and indirect current-controlled PWM rectification. Indirect PWM rectification is a control method based on the steady-state voltage balance relationship of the PWM rectifier, possessing good static characteristics and simple and convenient control. However, it suffers from slow dynamic response and poor steady-state performance due to the lack of input AC current detection. Therefore, in practical designs, a voltage outer loop is often added to the indirect current PWM rectification to form a double closed-loop structure, ensuring dynamic response.
1. Working principle of three-phase PWM rectifier
1.1 Main circuit operating mode
The main circuit of the three-phase voltage-source PWM rectifier is shown in Figure 1.
Figure 1. Main circuit of a three-phase voltage-source PWM rectifier
When the rectifier enters steady-state operation, the output DC voltage is constant, and the three-phase arms of the rectifier bridge are driven according to a sinusoidal pulse width modulation (PWM) law. When the PWM rectifier is in rectification mode, the three-phase AC power supply will be rectified to the DC terminal through IGBTs or diodes. When the PWM rectifier is in inverter mode, i.e., when energy feedback is required, the DC terminal current will be fed back to the grid through IGBTs or the rectifier.
To discuss the rectification and inversion process of a three-phase PWM rectifier, the space voltage vector shown in Figure 2 is used to describe the switching state of the three-phase bridge arm.
Figure 2 Space voltage vector
Figure 2 shows that after one cycle of the grid voltage signal, the space voltage vector has rotated one full cycle from U1-U5-U4-U6-U2-U3-U1, and each state transition includes two states, U0 and U7. Based on Figure 2, the three-phase current space coordinates are defined as shown in Figure 3.
Figure 3 Spatial Current Coordinates
We define U1-U5 as region I, U5-U4 as region II, U4-U6 as region III, U6-U2 as region IV, U2-U3 as region V, and U3-U1 as region VI. The changes in the space vector of the current in each region collectively cause the rotation of the composite magnetomotive force, thus forming a sinusoidal current. The results are shown in Figure 4.
Figure 4. One-cycle state of the composite magnetic potential
Taking region I as an example, the conduction and current flow of the three-phase bridge arms are further described in conjunction with a three-phase voltage-source PWM rectifier, as shown in Figure 5.
Figure 5 Current variation and IGBT conduction status in Region I
In state U1, V4, V6, and V5 are on, and current flows through VD4, VD6, and VD5 respectively. In state U2, V1, V6, and V5 are on, and current flows through V1, VD6, and VD5 respectively. Other states can be analyzed in the same way. Therefore, it can be seen that even if the IGBTs of a three-phase PWM rectifier are on, current may not necessarily flow due to the voltage difference, while the parallel diodes can cooperate to facilitate current flow. This is the most significant characteristic of IGBT operation in a three-phase PWM rectifier.
1.2 Control Algorithm Principle
Analysis of the main circuit drive status of the three-phase PWM rectifier reveals that at high switching frequencies, the harmonic currents generated by higher harmonic voltages are very small due to the filtering effect of the inductor. Considering only the fundamental frequencies of current and voltage, the rectifier bridge can be regarded as an ideal three-phase AC voltage source. Appropriate adjustment of the magnitude and phase of the control quantity can control the phase of the input current, thereby changing the power factor. Controlling the magnitude of the input current controls the energy input to the rectifier, thus controlling the DC-side voltage. Therefore, the control objectives of the PWM rectifier are the input current and the output voltage, with the control of the input current being the key to rectifier control. The control objective of the input current is to make the current waveform a sine wave and in phase with the input voltage.
The specific control concept of the three-phase PWM rectifier is to control the lead angle through SVPWM to regulate the power factor. Here, the lead angle is defined as , therefore the power factor can be controlled within a certain range. The DC-BUS side DC voltage can be controlled within a certain range through modulation depth. For the PWM control circuit, the modulation depth and controller angle can be arbitrarily set. Its control principle diagram is shown in Figure 6.
The rectifier in the diagram uses SVPWM control. By adjusting the phase difference and modulation depth, the power factor and DC voltage can be independently controlled. The yellow and green dotted lines in the diagram represent the phase control loop and the voltage control loop, respectively. Using the phase control loop is sufficient to operate the PWM rectifier, while using the DC voltage control loop ensures a constant DC-BUS voltage, thereby achieving energy feedback in overvoltage conditions and guaranteeing voltage stability. The phase and voltage control processes will be analyzed in more detail below.
(1) Phase control
Phase control, also known as power factor control, essentially adjusts the power factor to ensure that the current and voltage are in phase. The phase control loop detects the fundamental phase of the phase current, obtains the phase angle after low-pass filtering, compares it with the command, and then uses the result to adjust the phase difference of the PWM modulation, allowing the system to operate at any power factor angle. The accuracy of phase detection significantly affects the control characteristics; therefore, a stable fundamental current phase detection circuit is required. The output signal level of the LPF determines the control performance, and a limiting circuit is generally added to keep it within acceptable limits.
Figure 6 Control System Block Diagram
(2) Voltage control
The output DC voltage of a PWM rectifier is essentially dependent on the AC line voltage and the modulation depth, and is generally inversely proportional to the modulation depth. Therefore, it can be controlled independently of the power factor. From a control characteristic perspective, when stable DC voltage control is required, a voltage control loop must be used. Since the DC voltage is inversely proportional to the power factor, the control circuit should ideally have a linear relationship with the voltage control signal. Furthermore, in closed-loop control, it is best to add a modulation depth limiting circuit to ensure that the modulation depth is not less than a certain value.
1.3 Active and Reactive Power Decomposition and Control
After understanding the control principle of a three-phase PWM rectifier, we proceed with in-depth research on its algorithm. Through understanding the control principle, we can see that the control objectives of a PWM rectifier are input current and output voltage, with input current control being the key to rectifier control. The objective of input current control is to ensure that the current waveform is sinusoidal and in phase with the input voltage.
In the PWM rectifier control method, the three-phase AC current is transformed into a dq coordinate system, thereby enabling separate control of the d and q components of the current. This makes it very simple to adjust the active and reactive power separately.
The three-phase control voltage equations of the PWM rectifier can be listed from Figure 1:
(2-1)
Using spatial coordinate transformation, the above equations are transformed into stationary coordinates. The transformation matrix is as follows:
(2-2)
The transformation equation is:
(2-3)
The transformation equation is:
(2-5)
After the above transformation, the equation of the PWM rectifier in the synchronous rotating coordinate system is:
(2-6)
In the above formula, the power supply voltage is in the coordinate system, and the control voltage at the midpoint of the bridge arm is in the coordinate system. Returning to the three-phase stationary coordinate system, let's take the three-phase input voltage.
(2-7)
(2-8)
Substituting (2-8) into (2-6) gives:
(2-9)
As can be seen from equation (2-9), there is coupling between the components. There are generally two types of decoupling control: voltage feedforward decoupling control and current feedback decoupling control. While the former is a completely linear decoupling control scheme, its real-time performance is not ideal. This paper adopts the current feedback decoupling control method, which is convenient to implement and has a simple control circuit.
In practical applications, when the sampling frequency of the voltage loop is much higher than the frequency of the grid voltage, the mutual coupling in the equation has little impact on the performance of the current regulator. This factor can be ignored. The current control command is then compared with the feedback current, and the error is adjusted by a PI controller to obtain the voltage command signal, i.e.:
(2-10)
Integrating the ideas from equations (2-1) to (2-10), the control block diagram is shown in Figure 7:
Figure 7. Vector Conversion Control Diagram of PWM Rectifier
2 Delta AFE2000 Energy Feedback Unit
The AFE2000 is an energy feedback device proposed by Delta IABU for use with frequency converters. The AFE2000 employs active and reactive current decoupling control, achieving this through Clark-Park conversion, thereby realizing power factor adjustment and energy feedback control. Its appearance is shown in Figure 8.
Figure 8. AFE-2000 exterior view
The AFE2000 feeds back regenerated energy to the grid via IGBTs, overcoming the drawbacks of traditional RTDs, such as wasted heat energy and difficult maintenance. A comparison of the two modes is shown in Figure 9.
Figure 9. Energy consumption method of regeneration
As shown in Figure 9, energy feedback is divided into two types: one is energy feedback only, but it cannot improve the power factor; the other is energy feedback and power factor improvement at the same time. AFE2000 is the highest level, which can perform both energy feedback and power factor improvement, and the control parameters are simple and easy to adjust.
3. Conclusion
Three-phase PWM rectifiers are increasingly favored by engineering professionals due to their power factor improvement and energy feedback functions. Delta's AFE2000 is an energy feedback unit developed based on a three-phase PWM rectifier. It features a simple control mode, powerful functions, and the ability to simultaneously adjust both power factor and DCBUS voltage. When connected to a frequency converter, it truly realizes the concept of green frequency regulation, abandoning traditional energy-intensive heat dissipation methods and fully utilizing regenerated energy, aligning with the principles of energy conservation, environmental protection, and care for the planet.
