Black-Start Using Renewable Energy Resources
Written by Sam Salem
Achieving 100% Renewable Energy Grid will require wind, solar, and energy storage systems to help restart electric grids after a blackout. This will be a necessary change of the role for inherently intermittent renewable energy sources, which are usually viewed as contributing to grid instability.
In 2017, wind farm shutdowns were linked to a statewide blackout across South Australia. Similarly, in 2019, Hornsea One offshore wind farm in the UK was partly to blame for a power outage that affected around a million British customers. One consideration for operating inverter-dominated ac power systems is the need to start grids once they have gone down. To accomplish this, the generation on the system needs to be able to both act as a voltage source and to provide adequate power to start electrical equipment with high in-rush currents, such as transformers and motors. Inverter-dominated systems will need to be able to provide sufficient starting current, or the loads must be segregated in such a manner as to enable controlled repowering of the grid.
The process of restoring an electric power station or a part of an electric grid to operation without relying on the external electric power transmission network to recover from a total or partial shutdown is called a "black Start". The increasing penetration levels of inverter-based resources (IBRs), such as wind, photovoltaics (PV), and battery energy storage systems (BESS), have created a need to assess the technical capabilities and costs of using these IBR resources to provide black-start support. The use BESS to black-start conventional generators has been demonstrated. The ability of a voltage source converter-based high voltage DC system to black-start large inductive loads has also been tested. In addition, grid-forming inverter control with virtual oscillator has demonstrated potential black-start capability with grid-forming IBRs. These demonstrations provided some evidence regarding the ability of IBRs, particularly BESS, to provide black-start support. However, other important aspects of black-starting with IBRs require further study.
Historically, a 5MW grid-scale battery park in Germany was the first to utilize energy storage for quick restarting in the event of a blackout in 2016. A utility in Southern California had successfully demonstrated the use of a battery energy storage system to provide a ‘black start’, firing up a combined cycle gas turbine from an idle state in 2017. In 2020, the 69 MW Dersalloch wind farm black-started part of the Scotland grid using virtual synchronous machines. According to ScottishPower, the use of a wind farm as part of a black-start restoration process was a world first. "Grid-forming” technology, or virtual synchronous machine (VSM), was used to regulate the frequency and voltage of the power from the turbines.
Distributed ReStart is a world-first initiative to explore how distributed energy resources can be used to restore power to the transmission network in a blackout event. Distributed ReStart was launched in January 2019 and is set to run to March 2022. The project's main objectives are to review the capability of renewable energy technologies to provide Black Start and investigate the challenges around power system strength and stability, specifically in relation to power islands with high penetrations of converter‐based technology. The third objective is to deliver a sophisticated planning tool designed to simulate distributions for the reliable output of wind over different periods of time. The first project report was released in June 2019. The report outlined the existing Black Start technical requirements set out by the National Grid ESO. The report acknowledged the likelihood that the renewable technologies investigated will never be capable of providing the full set of Black Start requirements such as is offered presently from large synchronous power generation. Barriers were highlighted and mitigations were proposed. Roadmaps were introduced to highlight the key developmental requirements to overcome specific barriers.
The report highlighted the different roles of grid‐forming and grid-following converters during a Black Start and restoration. A grid‐forming converter will initiate outward energization from its own site onto the local and wider network to start creating a Power Island. Grid‐following converters will latch onto the voltage signal created by the grid‐forming converter(s) in the Power Island. Which will then grow as more sites are able to join. The converter technologies also play a role in improving ability of a site to deliver reactive power while not producing any active power. This is particularly useful during a Black Start to provide improved voltage regulation and stability in the early stages of restoration.
Renewable energy technologies cannot meet self-starting capability requirement on a large enough scale at present. Solar PV and battery storage are able to self-start, but they are limited by resource availability. During the hours of darkness, solar could not self‐start. Battery storage may shutdown with insufficient charge. To increase the certainty of the self‐start capability of a site, back‐up generation with sufficient capacity to meet the Black Start requirements can be installed. Co-locating different types of distributed energy, most likely battery storage with wind or solar, is another option to increase resource capability. One generation type can be used to start the other.
- Himanshu Jain, Gab-Su Seo, Eric Lockhart, Vahan Gevorgian, and Benjamin Kroposki, "Blackstart of Power Grids with Inverter-Based Resources", presented at the IEEE Power and Energy Society General Meeting, 2020.
- Benjamin Kroposki, Brian Johnson, Yingchen Zhang, Vahan Gevorgian, Paul Denholm, Bri-Mathias Hodge, Bryan Hannegan, "Achieving a 100% Renewable Grid", IEEE Power & Energy Magazine, March/April 2017.
- "ScottishPower in 'pioneering world first' after wind farm black-out boost". www.scotsman.com. Reid, Scott, 3 November 2020.
- National Grid ESO, "Black Start from Non‐Traditional Generation Technologies: Technology Capability and Readiness for Distributed Restoration", June 2019.
This article edited by Ali Nabavi