Decarbonization of Electric Power Networks: 100% Inverter-Based Renewable Generation

We studied the synchronization and stability of power grids with heterogeneous inertia and damping factors, and demonstrated power feasibility of operating a system consisting of only renewable generation technologies with enhanced stability.

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The decarbonization of the electric power sector, one of the five major economic sectors that contribute to carbon dioxide emissions from fossil fuel combustion, is an essential keystone to achieve a sustainable energy future. It will require the large-scale deployment of carbon-free technologies, such as variable renewable power generation, of which capacities are expected to increase greatly over the coming decade. Among the suite of variable renewable power generation technologies, solar photovoltaics (PV) and wind power plants are now cost competitive with conventional generation in most locations. These favorable economics are driving a significant deployment of these wind and solar PV renewable power plants.

Power grids are massive, critical infrastructure systems whose reliability is paramount to the economic welfare and national security of the served population. Thus, assuring stability and resilience in this new landscape with large instantaneous and annual shares of variable renewable energy sources is pivotal. Although the decarbonization of power systems is underway and accelerating quickly, the annual and instantaneous penetration levels of renewable energies remain relatively low in medium and larger system (see Fig. 1). This leaves a broad range of associated technical challenges; those experienced today in smaller systems (at the scale of small islands), while on the horizon for larger systems (at the scale of continental interconnections), that must be solved to ensure the reliable operation of power systems.

  Fig 1: Annual and instantaneous penetrations of renewable power plants by system size in 2018 - Source: B. Kroposki, NREL 

In almost all conventional power plants, electricity is generated using synchronous generators. These generators transform mechanical power – derived from disparate fuel sets such as coal, natural gas, nuclear fission, hydro, etc. -  directly into alternating current electric power (see Fig. 2). Over the past century, synchronous generators have matured and presently offer a reliable performance for the bulk generation of electricity. Renewable energy resources are most often integrated into power grids with a power electronic interface known as an inverter (see Fig. 3); these are inverter-based resources. Inverter-based resources can be broadly categorized by the primary control mode as either (i) grid-following or (ii) grid-forming, with both classes of inverters have similar hardware components and the structure differing chiefly in the control scheme. The primary control mode has  a considerable impact on the device interaction with the electric power grid. Grid-following inverters, the most common class of inverters, rely on a phase-locked loop to estimate and track the frequency at the point of interconnection; a present and stable voltage waveform is required. Typically, the control objective of these devices is to maintain a set power export regardless of varying grid conditions, but active power and reactive power can be adjusted according to desired relationships to frequency and voltage deviations for grid support. In contrast, grid-forming inverters, an emerging and promising technology as applied to the parallel operation in bulk power systems, adjust the frequency and voltage according to power injection deviations at the point of interconnection. In other words, these devices support the grid by generating a local voltage phasor (constituting phase and magnitude) independent of the grid operating conditions. A distinguishing constraint in grid-forming control is the necessity of available positive headroom to perform these control objectives.

Fig. 2: Conventional electricity generation by synchronous generators
Fig. 2: Conventional electricity generation by synchronous generators
Fig. 3: Emerging electricity generation by variable energy resources

The synchronization of parallel connected generation sources is paramount to a reliable power grid operation; the lack thereof may result in any disturbance yielding sustained oscillations and a loss of stability. Synchronization in a power network can be interpreted as a stable state when the pace of evolution of the electric angle of all generators across a network  are equal. The pace of evolution of electric angle is device frequency, which is the network coupling variable; when synchronized and in steady state, frequency will be consistent across the network. Each generator that establishes a local voltage waveform exhibits a frequency that evolves according to power export; this relationship is commonly known as frequency response.

The power network operator is concerned with continuously maintaining a stable and homogeneous system frequency across the network (60Hz in North America and 50Hz in the rest of the world), which is a direct reflection of the balance between electric power generation and consumption. In real-world instances, this system frequency is continuously fluctuating within a small, stable equilibria due to slight, countless perturbations, but it can be approximated as a quasi-steady state system frequency due to the small boundedness of the deviations. Any perturbation can induce transient oscillations, while in a stable system these oscillations are damped out within a relatively short time (see Fig. 4 as an example).


Fig. 4. Frequency response of synchronous grid of Continental Europe

One of the main challenges pertaining to the decarbonization of power grid is that synchronous generators are being gradually supplanted by inverter-based resources. The current literature has linked the deterioration of frequency response in power grids to the reduced available mechanical inertia because of the paucity of synchronous generators, creating a field of low-inertia power systems, which yields the necessity for further research to find a counterbalancing solution to cope with “reduced inertia” as the perceived issue.

Our paper fundamentally challenges the hypothesis that low-inertia is the foremost concern and found no analytic or computational evidence to support it as a fundamental challenge associated with high shares of inverter-based resources. Instead, we establish the benefits that reduced mechanical inertia offers to operate a power grid with more agility and efficiency; our findings highlight the crucial importance of damping. We leverage the formalism developed in this paper and present an analytical model that allows the quantification of the criticality of damping in achieving a stable frequency synchronization, regardless of the inertia level. We demonstrate that the increased heterogeneity of constituting parameters across emerging networks with high shares of grid-forming inverters can achieve reliance with up to 100% variable energy resources as a result of the adaptive inertia and damping parameters that grid-forming inverters are capable of offering.

Fig. 5. Our results establish the benefits that reduced inertia offers in power grids and highlight the crucial importance of damping in achieving a stable frequency response

We are optimistic that the findings in this paper provide a new perspective on power networks with high shares of inverter-based resources. This perspective may help scholars versed in electric power systems shift their attention from a reactionary approach to the challenges of reduced inertia towards the wholistic benefits inverter-based resources offer, given the capabilities of grid-forming inverters to stabilize the system with the addition of a substantial damping component.

Our paper is available at https://www.nature.com/articles/s41467-022-30164-3

 

Amirhossein Sajadi

Research Affiliate, University of Colorado Boulder