Research Team

High-voltage DC (HVDC) and/or medium-voltage DC (MVDC) may become the means to integrate photovoltaic (PV) systems due to higher efficiency and lower control complexity. High-power high-voltage interface DC-DC converters are a major bottleneck of HVDC or MVDC PV systems. Hence, there is a significant value to explore the topologies, control schemes, and reliability of the interface DC-DC converters. Dual-active bridge (DAB) DC-DC converter has a wide application in PV systems, and compared with traditional two-level DAB converters, multilevel converters can withstand higher voltage and realize higher step-up ratio, thus reduce the number of cascaded converters and lower the hardware cost. However, the research on multilevel DAB converters is still in the infant stage. The aim of this Ph.D. project is thus to explore the performances of multilevel DAB converters for the integration of PV systems into the MVDC grid on the following aspects: (a) Basic characteristics including static and dynamic characteristics, such as dead-time effects and DC bias. (b) Efficient modulation schemes. (c) Bi-oriented design considering efficiency and reliability. (d) Modulation of cascaded DAB converters.

PV inverters are moving toward multilevel configurations in order to accommodate the 1500-V DC-links. In this case, the design and control of such systems may be retrofitted completely in order to maintain high efficiency and high reliability. This Ph.D. project will focus on the design and control of 1500-V PV power converters for high efficiency and reliability. The project is dedicated to address the challenges in three major aspects: 1) Optimal design of 1500-V power converters, 2) Control strategies to enable grid-friendly integration of PV energy, and 3) Novel power converters to accommodate the high DC-link. Topological innovations and also advanced control strategies are expected in this project to improve the efficiency and reliability of PV inverters, and thus, reduce the cost of energy.

The growing installation of distributed generation (DG) units has brought structural changes of modern power distribution systems. Conventional solutions, where multiple DG units are connected in parallel, are not very cost-effective because of the two-stage power conversion configuration. To simplify the system structure, reduce cost, and improve efficiency, the series-connected converters have been widely investigated to meet the requirements of high-voltage PV and battery applications, and recently been explored for PV-battery hybrid systems. However, there are still certain limitations in present applications and investigations on the series-connected converter systems, such as less redundancy, higher communication burden (as the number of cells increase), and so on. To address these issues and enhance the performance of the series-connected converter systems, this Ph.D. project is initiated to address but not limited to the following aspects in high-voltage power converters for PV-battery systems: 1) Modifications on modulation schemes, 2) Stable operation boundary expansion methods, 3) Fault identification and fault-ride-through methods, and 4) Harmonic management methods.

Performance of Photovoltaic (PV) systems is greatly affected by mismatch incidents. Therefore, bypass diodes are traditionally used to reduce the effect of mismatching. However, doing so does not solve this problem completely, and therefore, power electronic-based solutions can be used. Power electronic-based converters, e.g., Differential Power Converters (DPPs), are employed to mitigate the effect of mismatching to maximize the energy yield. However, reliability, efficiency, and complexity along with the cost of converters still remain an important factor. This project will mainly focus on the mismatch fault, inducing performance degradation and damage to the PV panels. Special attention will be put on the detection and mitigation of mismatch faults by introducing novel power electronic topologies.

Impedance source converters (ISCs) have attracted much attention. Although the ISCs have shown many advantages over the conventional two-stage power electronic systems, there are still certain issues needed to be solved. With more stringent constraints brought by renewable energy systems like photovoltaic (PV) systems, how to model and control the ISCs is still an important task. This project thus focuses on the modelling and control of ISCs for PV systems. With a comprehensive small signal modelling of ISCs, a detailed design guideline of impedance source networks for optimized size and configuration will be explored. Moreover, advanced control systems and improved modulation methods will be proposed to fulfill the load/grid demands in terms of power quality, efficiency, reliability, and thereby cost of energy.

With the revolution of renewable energies, the power system is becoming more complicated and also integrated with more and more power electronics, which is referred to as “power electronized” or power electronic-based power system. It may consist of wind farms, photovoltaic (PV) systems, synchronous generators (SGs), induction motors, etc. Due to intermittency, uncertainty, and interaction, the system dynamic becomes more unpredictable. However, the dynamic characteristics and operational mechanism of power systems are increasingly important for further improvement. Unfortunately, the research of the highly aggregated power electronic-based power system does not match the advancement. The aim of this PhD project is thus to investigate the dynamics of highly aggregated power electronic-based PV systems to explore potential stability issues, where a large-scale of PV systems are installed. In this way, the entire system planning and operation can be improved.

The development of power electronics has been enhancing the penetration of renewable energy in modern power systems, where the interactions among multiple converters could lead to issues on stability and reliability. Attempts have been made to model and evaluate the two performances in microgrids, with different controllers thereby developed to enhance them. However, the validation of stability and reliability in microgrids is also a practical concern, which is closely related to the characteristics of microgrids and has not been fully addressed so far. The main target of this Ph. D. project is to develop general guidelines or systematic methodologies for validating both stability and reliability in microgrid systems. Technical overview on modelling and evaluation of stability and reliability in microgrids will be carried out, to seek for solutions to practical issues in the validations. The methodologies will then be derived on benchmark models, considering different architectures, mission profiles and control strategies in microgrids. A microgrid system will be built in the laboratory for performing the tests, and the validation methodologies will be tested from component level to system level. Relevant software tools are also supposed to be developed.

The power systems are becoming power electronics-dominant with an ever increasing penetration level of renewable energies, like wind and solar photovoltaic. In this regard, new methods must be developed to model and assess such systems in terms of reliability—which is of paramount importance in the design, planning, and operation of power systems. On the other hand, conventional reliability metrics in power system analysis do not take into account wear out of power components as well as the complicated structure of a power converter when doing such analysis. The hypothesis is that physics of failure analysis can be applied to each component and converter, and then aggregated up to an assessment of the power system, where also dependency on the operating conditions should be included. The modelling and evaluation methods should take into account the uncertainty of operation and climate conditions. The assessment method should further be able to quantify the demands to the individual components/converter before they are used e.g., in the design phase. In other words, the overall goal is to develop new methods for ensuring reliable and safe operations of power electronic based power systems. An integrated reliability assessment tool, which can incorporate both the physical characteristics of hardware components and control software optimisation at the system level, will be developed for predicting the lifetime of power electronic based power systems. New insights into the mutual interactions of power electronic converters at multiple time scales will be revealed to leverage the control effects of power electronics for power grid stabilisation and protection.

The overall goal of this Ph.D. project is to develop new methods for which safe and reliable operation of power systems where power electronics are highly integrated. A reliability assessment tool which integrates both the physical characteristics of the hardware components and control algorithm optimization at system level is needed. An effort to gain an understanding of the mutual interactions of power electronic converters, will be made while considering different mission profiles and configurations of power converters. The key components which has a major contribution to the overall lifetime degradation will be extracted in order to obtain an understanding of the lifetime characteristics of both single and multiple converters. Having extracted the key components will result in a modelling and analysis simplification of complex power electronic converter systems moreover, it will serve as a functional insight in which ways the power electronic based power system can be improved and extended in terms of reliability.