Inverters to integrate renewables into weak grids
In a future without fossil fuel synchronous generators, we need something else to supply power with desired voltage levels into our grid. In this first instalment, we will look at the use of inverters and management strategies used to control stability in weak grids. With a focus on energy transmission and distribution – and tools to build a stronger energy system. With Sajjad Hadavi, Milad Zarif Mansour and Si Phu Me.
Inverters convert the Direct Current (DC) voltage into Alternating Current (AC) voltage
If we want to increase the renewable energy generation share in our grid, we need to figure out a way to do so without causing instability. More renewable energy leads to a higher penetration level of inverter-based generators in the power system. The Voltage Source Inverters (VSIs), synchronisation units inside the inverters, can help to provide system stability. Depending on their embedded control structures, VSIs can be current sources like the grid-following inverters (GFLIs) or voltage sources like the grid-forming inverters (GFMIs). GFLIs follow the voltage reference and the frequency of the grid, while GFMIs create the voltage reference and frequency of the grid.
Renewable energies introduce new challenges for power systems, such as low inertia and weak grids
Sajjad Hadavi (2nd Year PhD candidate, Faculty of Engineering/ Department of ECSE), supervised by Dr Behrooz Bahrani, is investigating the integration of renewable energy in weak grids and is working on enhancing the strength of the grid with new assets and control structures. He is looking at Static Compensators and Synchronous Condensers (SynCons) to help the grid stabilise itself in weaker areas. He used a convex-optimisation for optimal allocation and sizing of SynCons in a large power system and proposed an optimal design of a SynCon exciter controller. “The increasing number of weak-grid-connected renewable energy resources in power systems has created various challenges in recent years. Some examples include undamped voltage oscillations in the ERCOT power system (in texas, America) and subsynchronous resonance in the North-China power grid. Several solutions for these challenges have been proposed, such as SynCons. However, their installation/operation costs and lead-time bring their own problems. Additionally, the interaction of new assets and the existing renewable energy farms can cause instability in a weak grid. We need to operate the strengthening assets optimally, which means optimal allocation, sizing, and optimal design of their controller”.
Sajjad is also studying the interaction of grid-following inverters, grid-forming inverters, and how these interactions might create system instability. “I focus on the big picture, the equipment in the system and the interactions. If my framework works, we can reshape the controller and investigate the instability margin, then when we add any wind or solar to the system without any instability.”
With one in five Australian homes owning solar panels, there are millions of small (around 1.5 kilowatts) grid-following inverters at the distribution level. At a transmission level, there are fewer of them, each rated at several megawatts.
Grid-following inverters follow the voltage with Phase Locked Loop (PLL). However, PLL has difficulty performing in weak grids and is likely to make the system unstable.
Milad Zarif Mansour (2nd Year PhD candidate, Faculty of Engineering/ Department of ECSE) supervised by Dr Behrooz Bahrani, is studying the stability of grid-following inverters (GFLIs) and is trying to optimise their efficiency in weak grids. He focuses on the stability of grid-connected renewable resources while connected to a weak-grid via Voltage Source Inverters (VSIs). “The GFLIs are not 100% perfect, but I have implemented some of them in special microgrids and if we have the required inertia and system strength, I think they can work properly. The VSIs control loop, particularly its PLL, is playing a key role in allowing the system to work properly with a weak grid.”
Figure 1. The simulation results of a compensated VSI (M. Z. Mansour et al., 2020).
Figure 2. The simulation results of a conventional VSI (M. Z. Mansour et al., 2020).
Milad has been working on devising a Feedback Linearisation stabiliser for the PLL and the outcomes show that this controller is working effectively and adds stability to the system. Figure 1 shows the simulation results of a traditional GFLI when it is integrated into a weak grid, whilst Figure 2 presents the simulation results of the compensated system connected to the same grid. “The duty of the PLL is to capture the phase of the grid voltage and based on its active or inactive power references, will synchronise its voltage with the grid voltage, so that it can eject the desired active or reactive power into the grid. The PLL performance will degrade as the grid strength reduces in weak grids. The effect of the grid impedance on the PLL can cause system instability. If the grid impedance decreases, it can contribute to the stability in the system, and the stability margin will be wider (upcoming publication). I am also planning to use a hybrid structure for the VSIs control that has the benefits of both grid-forming and grid-following structures.”
Milad also found a stability region using the nonlinear model of the system. And he has already used his results towards real-world applications, proposing a new Grid Strength Index.
Grid-forming inverters are currently not deployed at the distribution level and a few exist at the transmission level. There is a lot of discussion taking place in both academia and industry to explore ways in which the share of grid forming inverters in the system can be increased.
When the load is changing the grid, it is up to the grid operator to bring the frequency back to 50 Hertz as quickly as possible. There are some mechanisms to do this and one of them would be the grid-forming inverters (GFMIs), as they can provide ’virtual inertia’ for the system. Those GFMIs are not as susceptible to problems of weak grids, because they form their own voltage at the point of connection, and by changing their amplitude and frequency, they can inject power into the grid. Another technology that can create inertia in weak grids is synchronous condensers (SC). These old pieces of technology do not give you energy but rotate in the system to enhance the grid performance by maintaining its stability and controlling its frequency.
Si Phu Me (1st Year PhD candidate, Faculty of Engineering/ Department of ECSE) supervised by Dr Behrooz Bahrani and Dr Sasan Zabihi (Principle Power System Scientist, Hitachi ABB Power Grids) is looking into the control of GFMIs and how to improve their impact on the grid. GFMIs could replicate the operation of a traditional generator in the grid or overcome existing issues with traditional generators. “Being able to contribute to the work of replacing a technology that has been existing for more than 100 years is an amazing opportunity. The GFMIs operate independently without voltage reference and show superior performances over the GFLIs, especially in weak-grid cases. They allow a higher level of renewable penetration, improve grid security and can operate in both grid-connected and microgrid conditions. A lot of companies and researchers are working on them, but at this stage, there are not as many commercialised products of GFMIs 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 also proposes some mechanisms to smooth the transient after fault, so that GFMIs do not exhibit any oscillatory behaviours. Si Phu has been working on Fault Ride Through (FRT) capability of GFMI in simulation models he built in PSCAD/EMTDC. He aims to protect the device, making sure that recovery is smooth, and that the converter is still able to operate after a fault. “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 (VR) 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 (Table 1). Further tests are needed to make it more reliable”.
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. When no Virtual Resistance (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 VR when the gird condition changes to avoid destabilising the GFMI, yet provide enough damping for the post-fault oscillations.
Table 1. Post-fault power transient, with Virtual Resistance (VR) and Short Circuit Ratio (SCR).
| Cases/ Methods | Overshoot (SCR1.8, SCR4.5) | Setting time (SCR1.8, SCR4.5) |
|---|---|---|
| w/o VR | 42%, 41% | 2.23s, 0.64 s |
| Static VR | 16%, unstable | 0.54 s, unstable |
| Proposed Adaptive VR | 15%, 20% | 0.31 s, 0.30 s |
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 helps debug the system and gain more understanding about it. “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.”
For further information
M. Z. Mansour, S. Hadavi and B. Bahrani, "Stability Analysis and Nonlinear Control of Phase Locked Loop of a Weak-grid Connected Voltage Source Converter," 2020 Fifteenth International Conference on Ecological Vehicles and Renewable Energies (EVER), Monte-Carlo, Monaco, 2020, pp. 1-6, doi: 10.1109/EVER48776.2020.9243004.