Is there a grid-forming battery in your future? There likely is, and probably not just one. But will every single battery energy storage system (BESS) be equipped with grid-forming functionality in the future? Let’s look at grid forming from three angles: system stability requirements, technical capabilities of advanced BESSs, and market designs for stability services. We’ll take the UK market as a practical example, but the general approach could be applied for any electricity market on the way to 100% renewable energy supply.
Ensuring system stability together with the ramping up of renewable shares needs new approaches to system planning and operation. With renewable shares exceeding about 40% of annual energy production, multiple challenges come up: renewable generation curtailment, transmission system constraints, and challenges to system stability. While we focus on stability here, we should be aware that advanced BESSs help solve all three challenges (energy shifting reduces curtailment, and stressed transmission line corridors can be relieved more efficiently).
Renewable Generation Technology Was Not Designed to Form the Grid Alone
Power systems have historically had grid forming done by synchronous generators, effectively proven by the very first interconnections of multiple generation plants more than a century ago. With synchronous generators not being part of renewable generation plants, new approaches in system planning are necessary to determine requirements regarding stability. New stability needs arise for additional inertia, additional short-circuit power, and small-signal voltage strength or system strength. Inertia needs can be identified in frequency stability studies for future system conditions, where study cases of “system split” conditions are usually the cases that drive the need for additional inertia. The outcome is a wide-area need for additional inertia, such as what Scotland is experiencing now. For short-circuit power and system strength, the requirements would be determined for each major substation.
Which Plant Types Can Be Equipped with Grid Forming?
We should be aware of two major constraints. First, short-term power and energy reserves: inertia reaction means the provision of instantaneous active power. Naturally, this is possible with batteries, but with photovoltaics (PV) and wind turbines it is difficult or impossible. In PV plants, the energy stored in the DC-link is insufficient for “reasonable” support. And while wind turbines do have kinetic energy stored in the rotor, mechanical peak loads and control interactions introduce additional complications and risk.
The second constraint is suitable voltage levels. In low- and medium-voltage grids the requirement for active islanding detection and prevention of undesired islands is incompatible with grid forming, which ensures continued islanded operation.
Due to these constraints, the focus now and in the medium term is on high-voltage connected BESS, stand-alone or as part of hybrid PV-battery or wind-battery plants.
Stability Service: Inertia and Short-Circuit Level
With grid-forming capabilities, batteries can provide inertia response and short-circuit level. The term “stability service” describes a certain amount of inertia response (in megawatt-seconds) combined with a certain short-circuit level (in megavolt-amperes), according to a given technical and service specification.
- Stability service is stacked on top of the battery’s base use case. That is, the battery will continue to provide energy shifting or frequency control services at the same time.
- Inertia constant and damping can be adjusted and adapted to system needs in real time. A guaranteed power margin is reserved to enable a firm inertia response, independent of the base loading of the BESS. That is, the response quality is independent of energy shifting or frequency control services.
- Short-circuit level is a design parameter. Multiples of the continuous rating can be supplied as fault level if the plant is designed accordingly.
- Amounts of base power, firm inertia reserve, and short-circuit rating can be adapted over time.
Providing an extended fault level requires additional inverter capacity, for which efficient plant sizing can easily be found. In technology-agnostic market-based procurement of short-circuit level, the BESS would compete against synchronous condensers. Since batteries operate with stacked services, often the techno-economical optimum would be in favor of the BESS and enable lowest cost of energy.
To give incentives for adding stability service capability, a suitable market design is required. Given that the power reserves for inertia and the potentially increased short-circuit level will change the business case both in capital expense and operating expense, longer-term contract intervals are beneficial.
UK Stability Pathfinder as an Example
The UK Stability Pathfinder program is an innovation stream that addresses the stability needs of the UK power system. In Phase 2 the focus is on Scotland, where National Grid ESO (UK) has identified a regional need for additional inertia of 6.7 GWs (gigawatt-seconds) and the need for additional short-circuit level at eight major substations (total 11.5 GVA). Based on new technical specifications (now part of The Grid Code, keyword “GBGF”) the “quality” of inertia and short-circuit current from grid-forming units (inverters or synchronous machines) has been defined. In a service contract draft, the stability service is defined regarding “quantity,” i.e., MWs of inertia and MVA of short-circuit level, availability, and compensation. A service interval of 10 years was chosen to enable investments, while tendering for the lowest cost solutions.
The tender Phase 2 has a lot of attention in industry, and a large number of participants submitted their solution (quantity, location, price). National Grid ESO used an optimization algorithm to identify the cost-optimal combination of projects, and 10 projects were awarded to become stability service providers from 2024 onwards. Five projects are based on advanced BESS. Figure 1 shows the relative shares of inertia, short-circuit level, and overall service cost attributed to the five BESS projects and five synchronous condenser projects. BESSs provide the majority of inertia (65%) and a share of 21% of short-circuit level (SCL) at a share of 19% of annual cost. The specific cost of both inertia and SCL is lower for BESSs than synchronous condensers. See the press release by developer Zenobe on Project Blackhillock for details on one of the projects in execution.
Figure 1: UK Stability Pathfinder Phase 2 tender results: (based on published tender results from National Grid ESO) (source: SMA)
The Stability Pathfinder Phase 2 tender is the first market-based procurement of stability services, and it might be a blueprint for other markets. The important ingredients of a successful market-based stability planning process could be adapted for other markets:
- Technical specifications
- Power system planning to declare need for additional inertia and short-circuit level
- Market designs that give incentives for stability services
- Commitment to innovation by power system operators, utilities, investors, project developers, system integrators, OEMs, and legislative and regulatory bodies.
System planning will identify the need for stability contributions, and market-based procurement ensures that the right amount of stability service is active in the right location in time. The combination of energy shifting and stability service from advanced BESS is a key contribution to a cost-efficient transition to 100% renewable electricity.
SMA Solar Technology AG