Grid-Forming Inverter Successfully Delivers Critical Short-Circuit Current, But Long-Term Grid Stability Questions Remain

Blackhillock, scotland – In a landmark presentation of grid-forming inverter technology, a system deployed by Zenobē at the Blackhillock substation in Scotland successfully delivered a 140-millisecond high-current pulse required by National Grid Electricity System Operator (NESO) during a simulated grid fault. This marks a significant step forward in the potential for power electronics to replace customary synchronous generators in providing essential grid stability services, but concerns about long-term reliability and system response persist.

The Challenge: Replacing Mechanical Stability with Electronics

Historically,large rotating generators – notably synchronous generators – have been the backbone of grid stability.These machines inherently respond to fluctuations in grid frequency and voltage.When demand increases, the generator naturally slows slightly, releasing more steam or water to maintain speed and deliver increased current. This inherent inertia and responsiveness provide a crucial buffer against disruptions.

Though, the increasing penetration of renewable energy sources like solar and wind, which rely on power electronic inverters to connect to the grid, presents a challenge. Unlike synchronous generators, inverters don’t possess this natural inertia. Traditional inverters follow the grid frequency and voltage; they don’t form them.

Grid-forming inverters are designed to overcome this limitation.They actively create their own voltage and frequency, delivering whatever current is needed to maintain stability.While conceptually similar to a synchronous generatorS response – increasing current output when voltage dips – inverters face a critical physical limitation.

High currents generate significant heat within the transistors that comprise the inverter. Synchronous generators can handle current increases of up to 700% during a fault, while conventional inverters are typically limited to 10-20% above their rated current to avoid overheating and failure. This disparity has been a major obstacle to widespread inverter adoption for grid stabilization.

A Novel Solution: Short-Burst Capacity

SMA Solar Technology, the German inverter manufacturer supplying Zenobē’s Blackhillock system, addressed this challenge by exploiting the short duration of fault events. Rather than redesigning the entire inverter for continuous high-current operation,they programmed the system to briefly exceed it’s nominal current capacity.

According to Aaron Gerdemann,a business development manager at SMA,the Blackhillock inverter is capable of delivering 250% above its normal current for the 140-millisecond pulse required by NESO.Following the pulse, the inverter reduces output to allow components to cool. This approach offers a cost-effective alternative to adding more expensive transistors to increase continuous current capacity.

The Future of Grid Stability: Batteries vs. Synchronous Condensers

Zenobē believes grid-forming batteries are poised to become the dominant technology for grid stability services.Their global director of network infrastructure, Semih Oztreves, highlights the “multifunctionality” of these systems.Unlike synchronous condensers, which primarily remain idle until a fault occurs, grid-forming batteries can simultaneously provide services like energy arbitrage – buying power when its cheap and selling when demand is high – generating revenue daily.

Though,the technology remains largely untested in real-world,large-scale grid disturbances. A key concern revolves around how traditional transmission relays, designed to respond to the characteristics of synchronous generators, will react to the digitally-defined current surges produced by inverters.

A recent report commissioned by Australian grid operator Transgrid cautioned against over-reliance on grid-forming inverters for short-circuit current, citing “high to very high risk.” Transgrid subsequently announced a plan to deploy a mix of both synchronous condensers and grid-forming batteries to enhance grid resilience.

“It might not be the cost-optimal solution, but it may be the wise solution,” says Hoke, reflecting the current cautious approach.

While the Blackhillock demonstration represents a significant milestone, further testing and refinement are crucial to fully understand the capabilities and limitations of grid-forming inverters and their role in securing a stable and reliable grid powered by renewable energy. The triumphant delivery of short-circuit current is a promising sign, but the long-term integration of this technology requires careful consideration and a balanced approach.