Design, modeling and control of a 12.47 kV isolated three phase power factor correction rectifier

Tang, Hanning
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A novel three-phase rectifier with power factor correction feature is proposed for the medium voltage (MV) high power (HP) applications. A typical application is to use it as the front-end circuit to interface with power grid and supply the power to a customized load, including medium voltage variable frequency drive (MV-VFD), electric vehicle bus charger, cargo ship and renewable energy source. The proposed topology has numerous advantages over conventional systems in regarding of the system efficiency, reactive power consumption, power density and operating flexibility. On the other side, this system has some challenges in semiconductor selection, control logic development, current harmonics elimination, modular implementation and system protection strategy design.

The advanced silicon carbide (SiC) MV isolated three-phase power factor correction rectifier (MV-PFC) is targeting to the MV-VFD application. Chapter 1 is a system review of the industrial MV-VFD products in regarding of its major industrial applications, grid voltage and power ratings, motor control requirements, popular semiconductor devices and recognized circuit topologies. Following the chapter 1, chapter 2 reviews the popular topologies cited in both academic projects and industrial products. Each topology is analyzed and investigated thoroughly. Then, a table summarizes the pros and cons of each circuit in terms of the system flexibility, regeneration capability, galvanic isolation rating, system power density, operating redundancy, power module rating, switching frequency, modulation complexity, power quality and operating efficiency. Next, a novel MV three-phase PFC topology is proposed to boost up the system performance to the next level. In another word, this topology meets all the system operating demands with higher efficiency and better power density. Furthermore, it improves the system operating flexibility and the fault tolerance margin.

A silicon carbide metal-oxide semiconductor field-effect transistor (SiC MOSFET) module, rated at 12.5 kV and 375 A, is developed as the core component for the power circuit. Its internal chip layout is designed accordingly. Both the electric and thermal features of this power module are characterized to describe its performance envelope. Furthermore, the device mathematic model is implemented for system power loss and thermal energy distribution studies.

After finalizing the circuit architecture design, a novel control scheme including both modulation feedforward control and output feedback regulation is developed. The internal loop uses the power command reference, grid input and DC output to calculate the MOSFET firing angles for the next switching event. The outer loop generates the power command reference and evenly assigns it to all three phases based on the real-time load condition. Then, an application state machine, including I/O management, soft start-up strategy and system protection scheme, is designed to promote the overall design close to the industrial product. The soft start-up strategy effectively limits the inrush current and charges the output DC bus from zero to full energy level safely. For the sake of the functional validation, the system steady state study includes different loading conditions. Considering the long-term operating reliability, the case study covers the power grid oscillation situation and four different fault scenarios. The protection scheme is developed to accurately detect the fault location and recover the system from the fault when possible.

An issue is found from the system steady state study, which is the input grid current distortion at the ultra-light load condition. In order to resolve this problem, an additional hardware circuit including a separate inductor and bypass breaker is added, which increases the damping effect in the middle-stage circuit. The control scheme is modified to coordinate with the improved topology. As a result, the system can operate safely and reliably at the ultra-light condition with the minimum current harmonics.

As an alternative design approach for the integrated system structure, the modular dual-active-bridge (DAB) PFC rectifier is developed. The modular structure greatly decreases the device and component power stress and brings in some operating redundancy. In the meanwhile, the difference coming from module hardware arises the challenge to the inter-module power and voltage balancing control. A novel inter-module balance control layer is described in the chapter 6. As a result, the unbalance coefficient between modules is less than 1%. In addition, the protection strategy for the modular system is developed, which can cut off the defective power module and bring the rest of the system back to the 100% performance status within the half line cycle.