1 Overview
The three-phase voltage source PWM rectifier is highly valued for its ability to enable bi-directional energy flow, maintain sinusoidal grid currents, minimize input current harmonics, and achieve constant DC voltage control. Its compact filtering requirements and high power factor (close to unity) make it an excellent alternative to traditional rectifiers, which suffer from high input current harmonic distortion and poor power factor. As a result, it has found extensive applications in areas such as four-quadrant AC drives, active power filters, superconducting energy storage, and renewable energy generation.
There are numerous control strategies available for PWM rectifiers, with direct and indirect current control being the primary approaches. While these closed-loop methods offer sophisticated algorithms and modulation modules, the three-phase voltage-source PWM rectifier's direct power control (DPC) has garnered significant attention in recent years due to its simplicity, robust interference resistance, excellent dynamic performance, and ability to decouple active and reactive power control. This approach has sparked a wealth of research and advancements.
This paper provides an overview of the main circuit topology of the three-phase voltage-type PWM rectifier and explores the DPC-based control strategy. It compares and analyzes these approaches and outlines future prospects for the PWM rectifier’s control strategy.
2 Circuit Topology
Recent studies on the topology of three-phase voltage-type PWM rectifiers have focused on minimizing power switches in low-power applications and enhancing DC output performance. For high-power applications, the emphasis has shifted toward multi-level converters and soft-switching technologies. Among the more established topologies, two-level and three-level PWM rectifier structures dominate.
The two-level PWM rectifier represents the most fundamental topology. Due to its straightforward structure and mature control algorithms, it has been widely adopted. Despite having more switching devices and clamping diodes per bridge arm compared to the three-level rectifier, leading to a more intricate circuit and potential midpoint potential balancing challenges, it offers advantages such as greater conversion power and lower input current distortion rates.
3 Direct Power Control Method
The direct power control (DPC) system operates as a dual-closed-loop framework, with the DC-side voltage serving as the outer loop and instantaneous power control as the inner loop. By regulating the instantaneous active and reactive power flowing into the rectifier when the AC-side voltage remains constant, the system achieves the desired power factor and power flow direction.
3.1 Voltage-Based Direct Power Control (V-DPC)
Compared to earlier PWM rectifier control strategies, V-DPC stands out for several reasons:
1. It eliminates the need for a PWM modulation module and current closed-loop adjustments, instead directly controlling active and reactive power via a switch vector table, thus simplifying the control algorithm.
2. The system exhibits a faster dynamic response.
3. It achieves lower input current distortion.
4. It employs a predictive model without a voltage sensor, saving hardware costs.
However, it also presents some limitations:
1. The switching frequency is not fixed, complicating the selection of AC-side inductance.
2. It heavily relies on sensor conversion accuracy and system sampling frequency.
3.2 Direct Flux Control Based on Virtual Flux Linkage (VF-DPC)
In addition to the advantages of V-DPC, VF-DPC offers:
1. A lower sampling frequency.
2. Lower total current harmonic distortion (THD) when the input three-phase grid voltage is non-ideal.
Like V-DPC, VF-DPC struggles with an unfixed switching frequency.
3.3 Direct Power Control Based on Instantaneous Power Theory
Traditional theories define active and reactive power based on average values, limiting their applicability to sinusoidal voltages and currents. The concept of instantaneous power theory applies to both sinusoidal and non-sinusoidal conditions. The block diagram of a direct power control system based on instantaneous power theory shows a control principle akin to V-DPC. The active and reactive power, along with the power difference, are calculated and determined by the power hysteresis loop and the voltage vector sector.
While this approach requires an additional voltage sensor, it simplifies the algorithm and provides more precise active and reactive power transients. It also ensures a fast dynamic response and low input current distortion. However, it suffers from:
1. An unfixed switching frequency.
2. A requirement for a higher sampling frequency.
3.4 Space Vector-Based Direct Power Control (SVM-DPC)
Space vector-based direct power control (SVM-DPC) replaces traditional DPC's switch vector table and power hysteresis with space vector PWM modulation modules and PI links. This strategy offers:
1. No need for a nonlinear controller.
2. A fixed switching frequency, easing inductance parameter selection.
3. Reduced sampling frequency.
4. Ability to obtain voltage vectors in any direction without reactive offset regions.
5. Lower input current distortion rates.
Its drawbacks include:
1. A complex control algorithm.
2. Dependence on the current switching state for instantaneous power estimation.
3. Increased system debugging complexity due to multiple PI links.
Additionally, some scholars propose adding a grid-side voltage sensor to enhance instantaneous power calculations, but this approach performs poorly under three-phase input voltage asymmetry.
3.5 Direct Power Control Based on Power Prediction (P-DPC)
Power prediction-based DPC systems can operate at fixed or variable frequencies. Fixed-frequency control generally yields better results. The block diagram of a fixed-frequency DPC system shows how it acquires current instantaneous power through a prediction model and selects optimal voltage vector sequences and their corresponding action times. The power prediction formulas simplify parameter design but complicate the algorithm.
3.6 Direct Power Control Based on Power Decoupling
Since the three-phase voltage-type PWM rectifier is a hybrid nonlinear system, active and reactive powers are coupled, impacting system performance. Power decoupling control aims to isolate these powers, providing a more accurate control model. This approach enhances power and DC voltage tracking capabilities, improves static performance, and strengthens load disturbance resistance but faces challenges in algorithm complexity and parameter accuracy.
3.7 Direct Power Control Based on Dual Switch Tables
Traditional switch tables adjust both active and reactive powers simultaneously, often leading to suboptimal performance. Dual switch tables allow independent adjustments, reducing power coupling. This approach improves dynamic and static performance but increases algorithm complexity.
3.8 Direct Power Control Based on Output Regulation Subspace (ORS)
The ORS-based DPC strategy controls instantaneous active and reactive power to achieve unit power factor operation and DC voltage balancing. While it enhances system dynamics and handles unbalanced inputs, it significantly increases algorithm complexity.
3.9 Other Improved Direct Power Control Strategies
Fuzzy logic and neural networks have been incorporated into DPC strategies to improve performance. Setting dead zones in sector boundaries and using phase-locked loops to detect AC voltage phases are among other enhancements.
4 Future Prospects
With advancements in power electronics and control theory, DPC strategies will continue to evolve, focusing on reducing current distortion, DC ripple, and improving power factors. Future research includes stability theory-based DPC, unbalanced grid conditions, multi-level DPC, and intelligent control applications.
5 Conclusion
This paper introduces the application advantages of DPC in three-phase voltage-type PWM rectifiers, explains its control concepts, and discusses various topologies and control strategies. It concludes with a vision for the future development of DPC technology.
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