Toward Self-Reliant Wind Farms

*Anubhav Jain*

### **Abstract**

Large-scale integration of renewable energy generators, inverter-based resources and network interconnections into the grid brings forth a massive penetration of power electronic converters. This results in a highly dynamic environment that poses a risk to stability of system voltage and frequency and can ultimately trigger wide-area blackouts. Since conventional synchronous generation is being phased out, alternate sources must be included to provide support through ancillary services in future power networks. In a completely decarbonized system, they must also take the lead in ensuring stability and security by participating in blackout defense and network restoration. Offshore wind power plants are deemed suitable candidates due to their capability of providing large amounts of power with fast startup times and advanced control functionalities. However, a change in control philosophy to *grid forming* is required to enable a more active participation from the next-generation wind turbines. Such changes also have the potential to minimize dependence on auxiliary diesel gensets for a greener carbon footprint. This chapter aims to give insight into the forthcoming challenges and highlight potential solutions to make wind farms more self-reliant resulting in wind energy as cornerstone of the future electricity supply.

**Keywords:** wind, power electronics, converters, grid forming, greenstart, islanding, transient, stability

### **1. Introduction**

It is evident that the undeniable rise of global warming owing to global greenhouse gas emissions from worldwide energy consumption that is not showing any signs of slowing down must be curbed to avoid its irreversible impact. Fossil fuels accounted for nearly 70% of the growth in energy demand in 2018 despite solar and wind growing at a double-digit pace since renewables were not able to catch up, with the power sector accounting for nearly two-thirds of emission growth [1]. Thus, green energy transition is of paramount importance, and the highest levels of ambition and effort on a global scale are needed to achieve the 1.5°C Paris climate goal, as highlighted in **Figure 1**. It is clear that the energy system of the carbon-neutral world of the future will have electricity as its backbone being responsible for almost half of the increase in total energy demand in 2018. However, a threefold expansion of power generation is required for electricity to assert itself as the *fuel of the future*, with its total share exceeding 60% by 2050 compared with 20% today [2].

The integration of renewable energy sources (RESs) on a large scale into power grids all around the world is currently the most efficient, cleanest and cost-effective way of

**Figure 1.**

*Annual net CO2 emissions (in Gt/yr) from 2021 to 2050: it is clear that current planned policies will yield only stabilization of global emissions by 2050 but a 27% baseline rise is likely if not fully implemented; reproduced from [2].*

electrifying the world. Out of 170 countries in the world that have set up ambitious targets for decarbonization, 30 are already set to achieve net zero in the coming decades with strategic action plans [2]. A recent example of a significant milestone in a country's energy system can be seen in Denmark, where 50% of the electricity consumption in 2019 was supplied by wind and solar—with the former contributing a staggering 47% [3]. Overall strong renewables growth is expected beyond 2022 when the global installed capacity of coal-fired plants is set to peak before starting to decline in the following years and be overtaken by solar and wind energy in 2025 [4].

Since conventional thermal generation is being replaced by RES distributed across different time zones and climates, located far from consumers, cross-border interconnections over long distances have an undeniable role to play in the unified electric network of the future. They allow cost-effective grid expansion without significantly upgrading the current transmission grid infrastructure, thus ensuring efficient, flexible and resilient flow of clean energy. Although high-voltage alternating current (HVAC) connections are currently more common, its requirement of special reactive power compensation to prevent capacity drop-off becomes costly at longer distances. This makes high-voltage direct current (HVDC) connections a more economic alternative for efficient long-range bulk power transmission between countries, islands and offshore resources, and additionally their controllability due to voltage source converters (VSC) allows advanced functionalities to enhance stable grid operation [5].

#### **1.1 The changing power grid**

The aforementioned steps to electrify the globe, namely large-scale integration of renewables and dense interconnections via high-voltage corridors, are the key to

#### *Toward Self-Reliant Wind Farms DOI: http://dx.doi.org/10.5772/intechopen.103681*

achieving the climate neutrality target. However, such a transformation in the grid infrastructure introduces a massive amount of power electronic converters (PECs) that is essential to manage the variability of wind and solar energy sites. Additionally, the widely scattered distribution of resources necessitates larger power transits, which must be coped with expanding the transmission network either through capacity boosting using FACTS<sup>1</sup> or new HVDC corridors that both rely on PEC. Such a change in operational philosophy is paramount for efficient grid usage [6].

This paradigm shift in generation, transmission and demand naturally results in future power grids being very different from the current one, mainly owing to PEC introducing control interactions with faster time constants although needed for faster decision-making and advanced control functionalities. This creates a highly dynamic environment and poses a risk to power system stability, which has been investigated in detail in the MIGRATE<sup>2</sup> project [7]. Firstly, the PEC interface leads to inertial decoupling of rotating machines such as wind turbine generators (WTG) leading to a reduction in total inertia, which causes *frequency stability* issues due to to higher RoCoF<sup>3</sup> and dynamic frequency nadirs or peaks during power imbalance. Secondly, reduced fault-current contribution due to limited overloading capability of semiconductors makes *fault detection* harder in a converter-dominated environment. Morevoer, overburdened reactive power reserves due to increasing distance between load centres and generation, coupled with limited voltage control capabilities in the transmission grid, can lead to local/regional *voltage stability* issues and a reduced *transient stability* margin, especially during system contingencies.

The declining strength of the network and increasing threat to stability make it challenging to contain voltage and frequency excursions due to faults exposing a greater proportion of PEC-interfaced units to sudden under-voltage trips, which ultimately can trigger wide-area blackouts if large generation such as offshore wind farms<sup>4</sup> is involved, as has already been seen, for example, in South Australia (2016) and around London in the United Kingdom (2019) [8, 9].

#### **1.2 Offshore wind as a cornerstone**

The massive penetration of PEC in the grid due to the prevalence of renewable generation and inverter-based resources (IBR)<sup>5</sup> has increased the risk to power system stability and reliability, which translates to more frequent blackouts, especially in areas with high volume of RES [10]. Thermal generation plants that are conventionally responsible for maintaining power system stability and security are now being phased out in favor of renewables and non-traditional technologies due to societal decarbonization aims, rising fuel costs coupled with aging assets and decreasing load factors. Since this increases the cost of ancillary services and of warming-up the generators (cold start) to provide blackstart services, maintaining the status quo is not an option. Thus, considerable changes are required in developing technological

<sup>1</sup> Flexible AC transmission system.

<sup>2</sup> Massive integration of power electronic devices @ www.h2020-migrate.eu.

<sup>3</sup> Rate-of-change of frequency.

<sup>4</sup> Interchangeably referred to as offshore wind power plants.

<sup>5</sup> Encompassing FACTS, batteries, HVDC links and PEC-regulated loads such as electric vehicle battery chargers and variable speed motor drives.

capabilities and opening up new markets that facilitate non-traditional technologies<sup>6</sup> to support the system, adding more resilience against dependence on a single technology and alleviating reliance on specific transmission routes [11, 12].

Offshore wind is one of the fastest growing RESs in the world, and its rise has been possible thanks to technological innovations and strong policy support despite higher capital and operational costs to cope with the rough sea conditions. Contrary to space constraints for onshore wind, higher capacity factors and full load hours due to steady wind conditions together present a good business case for offshore wind, and coupled with economies of scale, its LCoE7 is expected to drop to about 6–7 = €c/kWh by 2025, becoming competitive with onshore wind prices, which is the cheapest generation source in majority of places in the world [13]. Moreover, wind power has been shown to have CO2 emissions about four times lower than solar and with offshore wind turbines and farms getting enormous in size, as highlighted in **Figure 2**, their carbon footprint could beat even the original large-scale zero-carbon source nuclear power [15].

Thus, offshore wind power has a significant role to play as an electricity generation source in the future power system. However, only decarbonization of the grid is not sufficient to meet our climate goals since sectors such as heavy industry and transport<sup>8</sup> are *difficult-to-electrify*. This highlights the need for alternate energy vectors that can be obtained from renewable sourced electricity, referred to by the umbrella term *Power-to-X* (P2X). Recently, offshore wind has gained attention to generate hydrogen for sector coupling as P2X can reduce curtailment needs since excess wind output can be transformed into hydrogen as energy storage also, thus enabling flexible demand and conferring grid benefits. Thus, *green hydrogen* has the potential to transfer the benefits of renewables beyond the electricity sector by facilitating decarbonization of all sectors of the economy, where currently no climate-neutral alternatives exist [16].
