Grid-forming inverters to improve the power system’s stability

By Ms Nancy Van Nieuwenhove | 12 December 2020

Si Phu Me

1st Year PhD candidate, Faculty of Engineering/ Department of ECSE

Research interests: Grid-forming inverter, Fault Ride Through (FRT), Fault Recovery of Inverters, and application and control of grid-forming inverters in weak grid

Si Phu Me has always wanted to become an engineer. He holds a Bachelor of Electrical Engineering, specialised in Power Engineering and Power Electronics (Monash University, Australia). “After two years of studying in Vietnam, I moved to Australia where I completed a Bachelor course at Monash University in 2019. After participating in a few power engineering projects at Monash University and discovering the transformations the grid is undergoing to lower the dependency on fossil fuels, I decided to do a PhD in February 2020 to be part of the solution.”

Let’s imagine a future where traditional fossil-fuel synchronous generators are not connected to the grid. The power system is large and spreads across the country and rather than use fossil-fuels, we can instead use a system that replaces the role of these generators. Grid-forming inverters are able to create and define the voltage levels, and are able to be used instead of synchronous generators. Our power grid operates in an Alternative Current (AC) fashion; therefore, inverters are used to convert Direct Current (DC) voltage into AC voltage. More renewable energy leads to a higher penetration level of inverter-based generators in the power system. The voltage source inverter (VSI) interface between the resources and the main grid can also help improve the power system’s stability. We have two groups of inverters: the VSIs can be modified to play current sources (grid-following VSIs – this inverter will follow the voltage reference and the frequency of the grid) or voltage sources (grid-forming VSIs – this inverter will create the voltage reference and frequency) depending on their embedded control structure.

Si Phu Me’s PhD research, is supervised by Dr Behrooz Baharani (Senior Lecturer, Faculty of Engineering/ Department of ECSE and Director of Grid Innovation Hub, Faculty of Engineering) and Dr Sasan Zabihi (Principle Power System Scientist, Hitachi ABB Power Grids), and aims to contribute to the development of a new grid-forming inverter. This would be able to replicate the operation of a traditional generator in the power grid or overcome existing issues with traditional generator technology. “Being able to contribute to the work of replacing a technology (traditional generators) that has been existing for more than 100 years is an amazing opportunity. The grid-forming inverters (GFMIs), operate independently without voltage reference and show superior performances over the grid-following ones, especially in weak-grid cases. They allow a higher level of renewable penetration, improve the grid security and can operate in both grid-connected and microgrid conditions. Lot of companies and researchers are working on them, but at this stage, there are not as many commercialised products of GFMI on the market.” Si Phu is working on optimising those GFMIs in terms of voltage and frequency control, active and reactive sharing among VSIs, grid-supporting features, and renewable energy integration.

Si Phu is also working on Fault Ride Through (FRT) capability of GFMI in simulation models he built in PSCAD/EMTDC. He is working on protecting the device, making sure that recovery is smooth, and that the converter can still operate after the fault. “The fault current of GFMI’s must be limited to protect the GFMIs from the excessive current during network faults, otherwise overcurrent during faults would damage the inverters. There are two aspects to consider: current limiting effort and post-fault behaviour. Voltage spikes and power oscillations after the fault clearance might trigger the protection devices in the system and degrade the power quality. So, we proposed an adjustment for an existing current limiting method, by adding an adaptive virtual resistance to the current control loop at the instant when the fault is cleared. This virtual resistance helps dampen the post-fault voltage spike and the power oscillations in the system response instantly. Further tests are needed to make it more reliable”.

Si has also been working on the creation of an experimental platform at the Monash University Clayton campus to allow researchers to monitor measurements and internal variables in real-time, which is helpful in debugging the system and gaining more understanding of the system. “When we increase the volume of inverters in the power grid, there will be some issues, we expect to see more research outcomes on the solutions related to inverters integrations; like innovative FRT methods of GMI, GMI control design and tuning methods, solutions for inverter integration into weak grids.”

One of his early findings is using an Adaptive Virtual Resistor (VR) to dampen post-fault oscillations of GFMIs. In Table 1, the Short Circuit Ratio (SCR) is an index for evaluating the grid strength. Low SCR indicates a weak grid, which is easy to become destabilised.

Table 1. Post-fault power transient, with Virtual Resistor (VR) and Short Circuit Ratio (SCR).

When no VR is applied, high overshoot and long settling time are observed, which are not desirable. If a static VR is applied to dampen the post-fault oscillations, we lose robustness against changes in the network configurations. The static VR method is effective if SCR = 1.8, but when the network changes, (e.g. SCR increases to 4.5), the static VR might destabilise the operation of the GFMI. Hence, Si Phu proposed an adaptive VR model able to self-adjust the amount of virtual resistance when the gird condition changes to avoid destabilising the GFMI, yet provide enough damping for the post-fault oscillations.

In March 2020, Si Phu Me received a top-up scholarship from Monash University’s Grid Innovation Hub for his contribution towards Grid Stability. “Doing a PhD allows me to learn from key experts in this area and gives me opportunities to try different ideas with support from the University and its industry partners. The top-up scholarship is a chance to contribute more to energy-related research.

In 2021, Si Phu expects to see more research outcomes on solutions for low inertia and weak networks. “When we increase the volume of inverters in the power grid, there will be some issues, I expect to see more research outcomes on the solutions related to inverters integrations”.

Further information

• LinkedIn Profile