# PhD defence by Qiao Peng on Stability and Control of Grid-Friendly PV Systems

## Time

08.12.2020 kl. 13.00 - 16.00

## Description

Qiao Peng, Department of Energy Technology, will defend the thesis "Stability and Control of Grid-Friendly PV Systems"

### TITLE

Stability and Control of Grid-Friendly PV Systems

### PHD DEFENDANT

Qiao Peng

### SUPERVISOR

Associate Professor Yongheng Yang

### CO-SUPERVISOR

Professor Frede Blaabjerg

Professor Huai Wang

### MODERATOR

Professor Francesco Iannuzzo

### OPPONENTS

Associate Professor Amjad Anvari-Moghaddam, Dept. of Energy Technology, Aalborg University (Chairman)

Professor Gabriela Hug-Glanzmann, Swiss Federal Institute of Technology (ETH)

Lecturer/Head of Department Martin Hill, Cork Institute of Technology (CIT)

### ABSTRACT

To develop a more environment-friendly power grid, the traditional fossil fuel is being replaced by renewable energy sources, e.g., wind farms and solar photovoltaic (PV) systems. To integrate renewable energy into the power grid, more and more power electronics are demanded than ever before. It makes the power grid more flexible and controllable. However, it also brings challenges to the modern power grid. One of the crucial challenges is the frequency instability due to the reduction of mechanical inertia, which is conventionally provided by the rotors of synchronous generators (SG). When the SGs are replaced by renewable energy sources, the alternative inertia should be provided. Otherwise, the normal operation of SGs will be challenged, which further threatens the stability of the entire power grid. To deal with this issue, inertia characteristic analysis and inertia enhancement strategy should be addressed. The former requires a general analysis tool for various power generating units in the grid, while the latter demands flexible control of power converters and renewable energy sources.

For the stability evaluation as well as the inertia characteristic analysis, a universal modeling of power converters is proposed in this project. The main concept is to model the power converters with the same external interfaces, i.e., the power exchanged with the grid and the internal voltage, referring to as the power-internal voltage (PIV) characteristic. The PIV model is in analogy to the model of SGs, which can thus reflect the inertia characteristics of power converters, and then, the inertia analysis can be realized. Additionally, the PIV model is obtained in open-loop, which is independent of grid parameters.

In this way, the model becomes more general and applicable for the grids consisting of multiple power converters. The proposed model is validated by simulations, and it is verified to be effective for stability analysis.

Besides the inertia characteristic analysis, the effective inertia enhancement strategy is required as well. The virtual inertia provision from DC-link capacitors is proven to be a viable and general solution for this issue. However, the proper design of the virtual inertia control (VIC) of DC-link capacitors with adequately considering the system stability has been rarely discussed. Aiming at this, the maximum virtual inertia analysis method is proposed in this project. The foundation of the method is the multi-timescale PIV model, which is constructed by several submodels at different timescales. First, the system stability at concerned timescale is investigated based on the submodel at the corresponding timescale, where the VIC of DC-link capacitors can be properly designed.

Then, the effective virtual inertia is identified in the submodel at the rotor motion timescale (RMT), which can effectively regulate the frequency as the SG rotors do. The simulation and experimental results show that the proposed method can identify the maximum virtual inertia that ensures system stability.

Notably, more vigorous inertia provision is demanded to help the power grid ride through severe incidents. In this case, the inertia emulation of PV systems requires exploration. The challenge is that the conventional maximum power point tracking (MPPT) strategy cannot ensure full-range frequency regulation, as the PV system is unable to release much power when the grid requires. The cost-effective solution is to reserve a certain amount of PV power in advance. Accordingly, a cost-effective and computation-efficient power reserve control (PRC) of PV systems is developed in this project. It executes the MPPT algorithm periodically to measure the real-time maximum available power (MAP), and then the power reserve reference can be determined accurately. A transient power impulse damping control (TPDC) is designed to buffer the transient power generated by the MPPT execution. Then, the PV output power to the grid is smoothed. Based on the PRC, the VIC of PV systems is achieved, where an event-triggering signal is designed to organize the control loops. The simulation results show that the proposed PRC can effectively achieve power reserve for PV systems, and the grid frequency can be adequately supported by the PRC-based VIC.

Apart from the inertia emulation, the frequency damping control (FDC) of PV systems is also explored in this project. The coordination strategy of VIC and FDC of PV systems is developed to adaptively employ the power reserve for optimal frequency support. The impacts of inertia and damping on frequency dynamic are analyzed at first. Then, considering the grid codes on frequency quality, the grid requirements on the VIC and FDC are obtained. Specifically, the large inertia is required at the early stage of the frequency incident to reduce the rate of change of frequency (RoCoF). Then, the large damping is desired to decrease the frequency deviation. The VIC and FDC coordination strategy of PV systems is designed accordingly. Comparison of different frequency regulation strategies of PV systems is carried out. The simulation results reveal that the proposed strategy can optimally utilize the power reserve, which performs effectively in supporting the grid frequency.

### THE DEFENCE IN ENGLISH - all are welcome.

Streaming info TBD.

### Host

Department of Energy Technology