Why have grid-forming inverters become a hot topic?
High penetration of renewable energy, mainly from wind and photovoltaics (PV), has been associated with a major shift in power source connections. The shift has been from power sources being connected to the power systems directly using synchronous generators, to connections via solid state inverters, introducing inverter-based resources (IBRs) (which in Europe are called power electronic interfaced power sources). Further background on this may be found at . While instantaneous penetration of IBRs remains below about 60-70% of the total generation, a steady evolution of improved contribution by IBRs to voltage and frequency control has been successful in substituting critical system stability services provided previously by synchronous generators. A summary of these IBR capabilities can be found in the report of the Brussels workshop, “High Penetration of Power Electronic Interfaced Power Sources and the Potential Contribution of Grid Forming Converters,” held on 24th January 2020 at .
Analysis and experience have demonstrated that going beyond two-thirds penetration of IBR at any time introduces a step change in challenges to maintain system stability. Preparing to be ready to approach penetrations of 100% IBR, operating occasionally without support of synchronous generators, is needed in an increasing number of countries. In Europe, the Brussels report details seven new challenges related to system stability in high-penetration power systems. The associated mitigation capabilities are described as Class 3, going beyond the advanced requirements of Class 2. This report introduces one possible solution to these challenges, namely grid-forming inverters (GFIs), in Europe referred to as grid-forming converters (GFCs). These inverters provide a fundamental change to the control strategy of the power electronics associated with the IBRs. The associated working group (Technical Group High Penetration) combines expertise from transmission providers, inverter manufacturers (wind, PV, and HVDC), as well as academia, to discuss these seven challenges and the potential mitigation contributions that could be made by GFI control of IBRs. The presentations by the Technical Group High Penetration at the report launch workshop in Brussels can also be accessed via .
Could IBRs with GFI deliver what is needed to allow further expansion of renewable energy?
Initially, in the absence of IBRs providing answers to the seven challenges, several transmission system operators (including the Australian Electricity Market Operator, Energinet in Denmark, and the Electric Reliability Council of Texas) have introduced synchronous compensators to deliver system strength in the absence of synchronous generators, in the future with added flywheels for an enhanced inertia contribution. Before applying synchronous compensators, some transmission system operators have constrained IBRs to a limit, e.g., in Ireland at 65%. In Great Britain, it has been suggested that 10 GVA of synchronous compensators would be needed to support an occasional 100% IBR penetration while avoiding excessive curtailment of renewable energy. Clearly, this would come at a substantial cost.
IBRs equipped with GFI could also deliver system strength, reducing or eliminating the need to add rotating plant like synchronous compensators. GFI controls of IBRs could deliver capabilities to respond to the seven stability challenges, including the widely discussed lack of system inertia. GFI-controlled IBRs can make an adequate contribution to secure voltage, frequency, and angular stability. Presentations at the Brussels workshop by manufacturers of wind, PV, battery, and HVDC equipment indicate that it can be done. However, the discussion suggested that making GFIs ready for the mass market is a large task. Maturity of GFI products similar to the level of existing grid-following converter products is needed, including a new compliance process. Wind Europe suggests that five years is required for this, with a reasonably settled specification as a starting point, preferably avoiding a different specification for each system or country. Part of the wind industry further suggests that the volume of GFIs needed for onshore wind in Europe may be too small to justify the additional development cost, and evidence is needed of willingness from their customers to pay for the additional capabilities. The latter should be forthcoming as some of the earliest affected transmission system operators suggest that large increases in ancillary services costs (up to five times in Ireland) are happening or expected, covering wider stability challenges as renewable energy penetration increases. A recent example is one new service in Great Britain financed at £328M under Stability Pathfinder shown in . This is related to Great Britain’s commitment to be ready for the first carbon-free system operation by 2025 (zero coal and gas).
Where do the need cases stand and what obstacles need to be overcome?
The need case for added system strength has spread from small island systems to larger systems like Ireland, Australia, Great Britain, and Texas. In Great Britain, a formal Grid Code Working Group is being established to draft the initial GFI performance requirements. The most controversial aspect of an initial draft has been a requirement to deliver short-term services beyond rated capacity, say, for 0.5s. This aspect is under review, with strong industry pressure to apply GFI within rated capacity and allowing use of headroom (natural or created). In Great Britain, this part of the Grid Code is proposed as a non-mandatory requirement, which defines capability that can be offered as a new service in the ancillary service market (under the Stability Pathfinder project). In continental Europe, which is a single synchronous area, some individual countries, such as Germany, are approaching possible 100% IBR penetration, although the continental Europe synchronous area as a whole is not. Here, considerations to ensure system survival under extremely rare system splits are being pursued, raising the need for introduction of GFI-type capabilities.
A high proportion of renewable energy resources and hence IBRs are connected at the distribution level rather than directly to the transmission systems. Evaluation of the effectiveness of the contribution to the required minimum system strength from deeply embedded IBRs is needed. If these become part of the way forward, then a solution to a Loss of Mains islanded operation protection challenge would be needed. Applying GFI-type control deeply embedded is envisaged to be a problem for cases of system splits where small islands which are not viable (or unsafe, i.e., inadequately grounded) are formed. The island could be brought into MW balance by GFI action and hence stay close to target frequency. This would make Loss of Mains protections such as those based on Rate of Change of Frequency or on Vector Shift ineffective. GFI, when applied deep in networks, may therefore need to be associated with new types of Loss of Mains protection systems which are a better fit for this purpose. The University of Strathclyde (Glasgow, Scotland) has developed one such solution based on satellite communication of reference vectors, see .
Ideal way forward through grid codes, mandatory or not
There is an urgent need for an international high-level specification supported widely enough to ensure a forward market which is large enough to encourage manufacturers to get on with delivering GFI solutions. In Europe, the European Network of Transmission System Operators for Electricity (ENTSO-E) is well placed to take the lead in this, building upon initial grid code proposals from individual countries (e.g., Great Britain). This would result in updates to the Connection Network Codes for generation and for HVDC. Collaboration beyond Europe would be beneficial to quickly create an adequate volume of GFI equipment orders.
The emerging network codes/grid codes could be non-mandatory or mandatory. To secure an initial volume of GFI-based IBRs, one way may be to require something on the order of 25% of total installed capacity of IBRs to be GFIs for an initial period of time. The manufacturing industry is suggesting this could even be achieved retroactively (e.g., an existing 23-turbine wind farm in Scotland was updated to GFI control as a trial in 2019). This could be achieved through non-mandatory means, combined with appropriate financial incentives, possibly via ancillary services markets. The sense of urgency to move forward with a decision is building as the level of IBRs continues to increase.
Helge Urdal, FIET, CEng
Urdal Power Solutions, Great Britain
Visiting Professor, Electronic & Electrical Engineering, University of Strathclyde, Glasgow.
 IEEE Power & Energy Magazine Nov-Dec 2019 p. 89-97
 Xinyao Li and Adam Dysko, University of Strathclyde, Glasgow, Technical Report Dec 2014 “Satellite enabled LOM protection scheme design and initial testing without considering communication latency”. SAC/SLP/TR/2014-1.