Gao, Xian

PhD student Xian Gao

PROJECT TITLE: Voltage and Frequency Issues of Distributed Grids with Highly-Penetrated Photovoltaic Systems

PhD period: 2021.02.01 – 2024.01.31.
Section: Power Electronic Systems
Research Programme: Photovoltaic Systems
Supervisor: Frede Blaabjerg
Co-Supervisors: Dao Zhou, Amjad Anvari-Moghaddam
Contact Information

Funding: Self-financing.


In recent decades, due to the still declining price of photovoltaic (PV) modules, growing numbers of people are installing PV systems at their homes or businesses. PV systems will have an even more significant role in the future power production. However, the power they inject into distribution lines depends on its environmental conditions, being of high randomness and volatility, so in the case of strong irradiance, a large amount of roof-top PV systems may lead to reverse power flow in the feeder, and in turn, the voltage will rise.

Moreover, PV systems are connected to the power grid through power electronics converters which have a fast response speed and provide almost no moment of inertia for the grid. Today’s power grids rely on generators to produce mechanical inertia. These generators are very large and rigidly synchronized with each other, so only major disturbances can affect the grid’s frequency. But distributed PV systems don’t have synchronous generators, and the inverters they use to connect to the grid are designed to simply lock onto the grid’s frequency and follow it. With the increasing penetration of PV systems, many large central power plants are being retired. Eventually, the grid will lack the inertia it has today to maintain a stable voltage and frequency in the event of a large disturbance.

In light of this, the Ph.D. project will focus on the voltage and frequency issues of distributed grids with highly-penetrated PV systems, where the smart power inverters design, and more importantly, advanced control strategies will be developed.  First, to solve the problems of voltage rise, centralized online optimization and local rapid control should be combined to achieve quick control and centralized decision-making. An optimal quality control strategy considering power losses, voltage deviations and imbalance issue through active participation of PV inverters will be proposed. Then, in order to compensate for low inertia in distributed grids with highly-penetrated PV systems, this project adopts the technology of virtual synchronous generator (VSG). And a self-optimizing technology of the moment of inertia and damping coefficient for the power system with multi-parallel VSGs and PV systems will be proposed. Finally, an optimal power distribution strategy will be proposed to achieve coordinated control among VSGs.


Publications in journals and conference papers may be found at VBN.