Toward Self-Reliant Wind Farms

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.

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].

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].

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 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 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 FACTS¹ 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 MIGRATE² 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 RoCoF³ 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 farms⁴ 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)⁵ 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 capabilities and opening up new markets that facilitate non-traditional technologies⁶ 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].

Figure 2.

Evolution of yearly average newly installed capacity of offshore wind turbines and farms; reproduced from [14]. Today the world’s largest offshore wind farm is 1.2 GW Hornsea-1 (UK) and offshore wind turbines are already reaching ratings upto 15 MW, as of 2021.

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 transport⁸ 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].

2. Looking into the future

As discussed above, large offshore wind power plants (WPPs) are deemed suitable candidates to take up the responsibility of maintaining power system stability and security, potentially even participating in early state network restoration. Since the fast-growing capacity of the overall site and the individual turbines is pushing offshore WPPs further away from the shore and into deeper waters, as indicated in Figure 3, HVDC transmission is more suited to export the power to the onshore grid. Although more expensive, the fully controllable VSC interface allows HVDC to provide various dynamic grid support services that enhance system stability and resilience [5].

Figure 3.

Average water depth and distance to shore of bottom-fixed offshore wind farms: the overall site capacity is indicated by bubble size; reproduced from [14].

However, large offshore WPPs today consist of upto 100s of WTGs connected in a large inter-array network of upto 70 km of subsea cables, with long HVAC or HVDC transmission corridor that transports the bulk power onshore, requiring either special reactive power compensation or large converter substations both offshore and onshore to manage the power flow efficiently. This makes the offshore WPP an aggregated unit with a converter-dominated environment and a very rich resonance spectrum that must be first operated in a stable and robust manner before providing onshore grid services [17].

2.1 Toward next-generation wind farms

Traditionally, the first-generation grid-connected PEC-interfaced grid-feeding sources only supplied set-point based real and reactive power with basic survivability over a certain voltage and frequency range, while the second-generation devices now are required to provide more support to the grid, especially in areas of high RES energy penetration, hence classified as grid-supporting units [6], shown in Figure 4. Today wind turbines and wind farms already contribute to system stability through provision of ancillary services such as fast power reduction as frequency response to create a spinning reserve margin for primary frequency regulation, steady-state and dynamic reactive power control for voltage support, and fault ride through characteristics that involve reactive current injection for improving voltage stability during faults. In countries of high RES penetration, more active participation of wind power in voltage and frequency stability is required. Thus, local vocal control in weak grid scenario and fast frequency response is being increasingly demanded by the new grid codes. Furthermore, latest requirements include power oscillation damping and synthetic inertia provision that mimics the exchange of kinetic energy from a synchronous rotating mass by injecting active power in proportion to calculated RoCoF [19].

Figure 4.

Working philosophy of (a) grid feeding, (b) grid supporting and (c) grid forming units. Together (a) and (b) are referred to as grid following [18].

However, while synthetic inertia is an important service to compensate for reduced inertia in the power system, it requires measurement-based activation unlike conventional inertia that is based on inherent physical characteristics of synchronous machines [20]. This makes it an insufficient replacement for inertia as true synchronous inertia-like response can be achieved only by grid-forming units [21]. Today wind turbines are grid-following (GFL) in that they rely on an external grid voltage⁹ to which the control latches using a PLL¹⁰ for stable operation, effectively making the WTG behave as a current source injecting controlled power. Such wind turbines are currently exempt from network restoration services as not only can they not create their own voltage and lack controlled islanding capabilities but also connection in early stages of blackstart process can result in a recurrence of blackout due to the grid not being strong enough for large wind farms [17].

Now as the penetration of PEC increases, assumptions valid for stronger, traditional grids may no longer hold resulting in improper PLL behavior causing a negative impact of the controller power sharing and system stability [22]. This necessitates the next generation of converter-based units, called grid-forming (GFM) and shown in Figure 4, to be capable of proactively supporting the grid in all states, especially emergency and blackout without having to rely on services from synchronous generators. Thus, such GFM units must be able to take the lead in creating system voltage to control instead of just supporting its amplitude and frequency, prevent adverse control interactions, counter harmonics and unbalances and support system survival while contributing to short circuit power and system inertia—limited by the boundaries of energy storage capacity and available power rating [6]. These are key to ensure flexible, efficient and reliable operation of future decentralized converter-rich grids.

2.2 Some potential advantages

Contrary to conventional WTGs, a GFM wind turbine behaves as a voltage source, thus not only allowing outward energization without having to wait for completion of network reconstruction, but also potentially participating in sectionalizing strategy for defense against blackouts by ensuring continuity of power supply in a regional island or at the very least switching to trip-to-houseload operation that reduces restoration time compared with cold startup and facilitates bottom-up grid recovery [19].

Moreover, GFM-WTGs can potentially minimize dependence on the offshore auxiliary diesel generator that is associated with capital and operational costs¹¹, not to mention the high emissions—especially when the diesel generator is operating at full-load during unscheduled outages that can last upto 4–6 months. Since a GFM-WTG can produce produce power to keep itself warm avoiding the risk to its health¹² as long as the wind blows, replacing the diesel genset with few of these can yield significant economic and reliability benefits over the project’s lifetime [17]. The CO₂ displacement can also contribute to reducing carbon-footprint taxes and ensure a smoother/faster granting of permits in the future.

Additionally, GFM-WTGs will also come in handy in the future to supply charging stations offshore essential for maritime vessel electrification, thus displacing a significant amount of marine fuel with green electricity and ultimately playing a key role in achieving our climate goals. Thus, blackstart and islanding capabilities unlocked by GFM-WTGs are an essential feature of self-reliant WPPs that not only make them more actively participate in advanced voltage and frequency control but also enable them to take up the responsibility of ensuring stable and robust grid operation without relying on synchronous generation, thus accelerating net-zero transition.

3. Wind-powered virtual synchronous generators

With the displacement of conventional generation, the most cost-effective way of ensuring reliable and secure operation of the power system while continuing with the decarbonization strategies is to make converters integrated into the grid behave like synchronous machines as it is impractical to now change the entire philosophy of operation and control of the entire power system without incurring significant losses to economy and livelihood all over the world. Thus, as mentioned before, GFM converter units are key to ensure flexible, efficient and reliable operation of future decentralized converter-rich grids. This concept is, however, not new having existed as different names in different converter applications. A simple representation of a GFM converter is shown in Figure 4, acting as a controlled AC voltage source behind an impendace. In principle, this is to some extent functionally similar to electronic oscillators used, for example, in clock generators in microelectronics, and has been researched upon so far in the power system context mainly for microgrids and to a limited extent in FACTS applications where it is traditionally called voltage injection.

Today GFM is a hot topic in power system research, especially in the context of converter-rich networks that are expected in the future due to high integration of RES and IBRs. The simplest way to mimic system-level functionalities of grid-connected synchronous generators such as self-regulation capability and communication-less power sharing is by employing traditional droop-based power controllers [23]. Further complexities can be added to fully utilize the controllability of PEC interface and emulate inertia and damping characteristics of synchronous machines—of course limited by the the stored energy capacity and available power rating that is connected at the converter backend. In this scope, the Virtual Synchronous Machine (VSM)-based concept was introduced using a detailed implementation of synchronous machine dynamics in its power control loop [24]. This power-based synchronization inspired from the swing equation that acts as a self-synchronizing block presents a more stable solution than the traditionally used voltage-based PLL, which requires enhancements to ensure stability under unbalanced and distorted voltage conditions such as voltage sag, weak grids or off-grid operation [25].

Since a converter can be made to behave as needed by modifying its controls, different schemes of the VSM family have been developed to emulate synchronous generator characteristics with varying degree of details, as reviewed in [26], resulting in a range of dynamic and transient stability performance needs such as independently adjustable inertia, damping and steady-state droop or highly non-linear behavior during grid faults and connection-disconnection processes [25]. Another synchronous machine-inspired control with different implementations and enhancements is the Synchronverter, a detailed review of which is given in [27]. Finally, non-linear GFM control strategies relying on the duality between PECs and synchronous machines have been recently developed, such as Machine-Matching and Virtual Oscillator Control, which have demonstrated robust steady-state droop-like behavior with a faster and better damped response during transients, albeit for low-power (microgrid) applications [28].

3.1 From grid following to grid forming…

Today the experience of power system operators with GFM-PEC is currently limited to battery facilities in South Australia where the Dalrymple Battery Energy Storage System (BESS) has successfully demonstrated some of the most immediately sought-after benefits of GFM converter-based resources such as virtual inertia. This was seen during the system separation event on November 16, 2019, when it provided almost instantaneous power injection proportional to RoCoF due to a slip/difference between its internal virtual rotor frequency and the grid frequency exactly mimicking the mechanism of inertial response from a synchronous spinning mass. Contrary to FFR, this does not require any measurement or frequency detection to start responding. The high-power GFM control additionally allows the BESS to behave closely to a synchronous generator during both steady-state and transient conditions, enabling advanced performance in stand-alone operation, when paralleling with other voltage and/or current sources or when grid-connected [29]. In addition, a secondary control housing the main automation and functional logic allows the provision of reliability and flexibility services such as very low SCR¹³ operation, seamless islanding transition and live-live grid resynchronization, support to non-synchronous system strength via short-term fault current injection¹⁴, controlled islanded operation¹⁵, blackstart capability with soft-start for limiting transformer inrush, and fast active power injection as part of SIPS¹⁶ [29].

Advanced system support GFM functionalities can also be obtained from solar PV and wind energy. However, not all players are equally predestinated for GFM as the cost of development for each differ. While making a GFM-BESS is relatively straightforward since the backend is simply a voltage source, solar PV and wind require maximum power point tracking control for its backend resource capture. This adds some complexities as the backend RES control must now be integrated into the DC link controller. In this regard, making a GFM WTG is likely to require highest efforts, since the mechanical rotating mass puts limitations on the power and energy buffer that is needed to provide transient performance, for example, during a phase-jump event on the grid side.

Thus changes are needed mainly in control (software) and some in hardware to transform a GFL-WTG for operation as a GFM unit. For reference, a typical WTG electrical model used in simulations to study electro-magnetic transients can be seen in Figure 5, which consists of Grid Side Converter (GSC) and Rotor Side Converter (RSC) with their respective controls in different levels of detail depending on the study needs, along with the simplified generator electro-mechanical model and the turbine controller, for example, in [30]. In conventional GFL-WTG today, voltage is provided by an external grid, and the WTG connects with the aim to normally supply maximum power extracted from wind. This is achieved by controlling the generator speed to operate at optimal tip-speed ratio for each wind speed¹⁷. For this, the RSC uses standard vector-based (or field-oriented) torque control to extract electrical power from the generator based on a reference (torque/power) obtained from the turbine controller, which ensures maximum power point tracking. Additionally, pitch control is present in the turbine controller to limit the power captured that is essential to avoid over-speeding of the rotor at high/above-rated wind speeds or for intentional de-rated operation (using set-point control) [31]. The GSC then is tasked with controlling the DC link voltage by transporting all the power extracted to the grid. However, this requires a stiff grid point which can absorb the WTG power output without excessive voltage/frequency rise.

Figure 5.

Commonly used detailed electrical model for WTG to study electro-magnetic transients in simulation. Average models for the converter (i.e. approximating its behavior as a voltage or current source mainly considering only the control scheme) can also be used when switching transients are not of concern based on study needs.

The primary difference between a GFM and GFL WTG’s operation is that the former generates its own voltage, and so there is no need for an external grid voltage. This can be achieved in a relatively straightforward manner by implementing GFM control in the GSC. However, since power flow is now set by the load (when in islanded mode) or by set-point control (in grid-connected mode or parallel sharing), RSC must extract equal electrical power from the generator to regulate the DC link voltage. Thus, the torque or power reference now (needed by RSC) comes from a DC link controller rather than the turbine controller, as stated previously for GFL-WTGs. The turbine controller’s main task now is to regulate the speed at rated and prevent over-speeding by using the pitch controller when necessary [32]. Alternatively, the generator speed can be controlled for sub-optimal tip-speed ratio which requires a speed controller that receives reference from DC link control to feed the generator torque control [33]. It is likely that pitch control would be operated more in GFM operation and so along with alternate revenue streams to make up for the wasted wind power, the impact on mechanical loading and in turn the lifetime of the turbine must also be investigated since GFM controls can result in a different power ripple spectra than traditional GFL WTGs potentially leading to higher levels of vibrations at frequencies close to the natural resonant modes of the generator shaft, rotor and tower [34].

3.2 But it is not so easy…

Recently, for the first time worldwide, Scottish Power Renewables in collaboration with Siemens-Gamesa Renewable Energy has successfully demonstrated the ability of onshore GFM-WTGs to operate in island condition supplying local loads while supporting conventional GFL-WTGs and ultimately energizing the upstream grid transmission network [35]. While GFM control allows WTG to provide frequency stability services, notably phase-step power injection in response to phase jumps in the grid and inertial response proportional to RoCoF to arrest frequency events autonomously and immediately, there are certain limitations that must be overcome for robust operation [34]. Since an individual WTG can find it difficult or impossible even with a high inertia setting to extract the inertial response from the background power ramps due to wind speed fluctuations, farm-level aggregation must be taken into account. Additionally, at low/zero power, only a small power/energy response is possible from the WTG DC link capacitance as the rotor does not have sufficient energy yet. Although an extra energy storage device¹⁸ can allow a more guaranteed response over a wider range of operating conditions but adding significant additional cost due to high energy required for more extreme events (more than 1 Hz/s). In absence of this, there is a risk of large reduction in rotor speed drawing the WTG into recovery (resulting in a second power output dip), or worse below cut-out speed if wind is low enough. This is the opposite of what is desired and can cause further grid instabilities and even lead to blackouts due to system separation if many WTGs are involved [34]. While dynamic inertia, curtailment and deliberate sub-optimal operation are technical solutions to be considered, challenges of grid code compliance must be overcome and alternate revenue streams be opened.

3.3 Non-existent markets

Recently, UK’s energy regulator OFGEM has approved the first ever technical specification GC-0137 of GFM control from PEC integrated into the grid, proposed by National Grid ESO [36]. Although non-mandatory, it marks a step as signficant as RES integration itself in the net-zero transition because of providing essential clarity for describing synchronous coupling with power grid in a technology neutral manner, which will enable any connecting power module utilizing PEC technology (e.g. wind, solar, HVDC) to offer grid stability services more actively. Despite a long way yet to go with testing and coordination coming next, such a specification already breaks the circular problem faced by manufacturers and system operators, fed by lack of widely available functionalities from IBRs today due to unclear specifications or demand leading to operational constraints making it even less attractive for them to develop resulting in shrinking market volumes for OEMs¹⁹.

The full potential of GFM power generators is however unlocked through the provision of blackstart and islanding capabilities along with voltage and frequency control that are essential for ensuring stability, reliability and security in future converter-rich grids without relying on synchronous generators, while compensating for the cost of developing such functionalities in RES such as wind and solar. The Dalrymple BESS in Australia current relies on only a few revenue streams compared with its technical capabilities, as mentioned before, namely inertial response for frequency stability, islanding to reduce unserved energy, frequency control ancillary services and energy arbitrage [29]. However, more services are possible such as blackstart, short-term fault current provision, voltage regulation and pre-emptive response for SIPS, but these are not yet monitized due to lack of any mechanism to do so under current market and regulatory frameworks [29]. While GFM-related capabilities are relatively more straightforward to implement in individual PEC-interfaced units such as BESS, solar-PV or WTGs, there are many challenges to ensure robust and reliable operation of GFM units aggregated into parks such as large offshore WPPs, despite the numerous advantages available from them such as reduced LCoE, increased energy production due to steadier wind conditions at sea, no inland space and noise constraints and higher reliability by combining electrical resource and maintenance facilities, justifying the cost of grid connection.

3.4 Green-starting wind farms

Since a large offshore WPP is considered as an aggregated unit consisting of many active components such as WTG converters, offshore HVDC converter in case of HVDC-connected WPPs that can have adverse control interactions in certain operating conditions and are a source of harmonics which can trigger resonances due to the presence of long high-voltage inter-array and export cables, transformers and filters, especially in the offshore network. Furthermore the offshore grid has low damping in the network provided mainly by auxiliary load that is limited to 1% for WTG and 0.1% for offshore substation, which can create situations in certain scenarios such as energization where transient and harmonic stability can be a challenge to ensure. Thus, stable and robust operation of the offshore network and export link must be ensured before the WPP can actively participate in not only supporting the grid with voltage and frequency ancillary services but also provide essential stability and reliability services through GFM control functionalities, like the ones mentioned before.

The energization and stable operation of a large offshore WPP upto the transmission interface point where it connects to the onshore grid, as shown in Figure 6, have recently been referred to as greenstart to distinguish from the commonly used blackstart of the power grid on a larger scale since it is of more concern to the wind farm developer. However, to better understand the range of technical challenges associated with it, the entire sequence can be divided into different target states just like traditional power system restoration being comprised of different stages, namely preparation and defensive actions, system build-up by blackstart units and transmission backbone energization followed by load restoration and meshing for resilience. These target states as highlighted in Figure 6 start with initial energization of WTG3 auxiliary load by a backup supply (TS-1) followed by houseload operation when the rotor is oriented to the wind (TS-2). Then multiple GFM and GFL WTGs must synchronize for operating in parallel (TS-3) to emulate a voltage source strong enough to energize the offshore network (TS-4) while ensuring stable and robust islanded operation of the HVDC link (TS-5) before finally connecting to the onshore grid for block load pickup or re-synchronization (TS-6) [17]. The challenges associated with each stage are discussed below.

Figure 6.

Target states in the greenstart energization sequence of an HVDC-connected offshore WPP; reproduced from [18].

3.4.1 Self-start and sustain

At the individual WTG level, an industrial grade UPS²⁰ as auxiliary power supply is currently used to keep energized the central control units for braking, yaw and pitch, dehumidifiers and heating units, grease lubrication system, fire protection, relays, hub computers, distribution boards and the SCADA²¹ interfaces and positioning and warning lighting system²²—all of which are essential to ensure safe and reliable operation of the WTG offshore. However, a UPS can provide idling mode energy sufficient only for a grid outage upto few days after which the WTG enters shutdown, which is not good for its health due to potential vulnerability to damage from moisture, icing up of electronics, bearing deformation, standstill marks and vibrations due to unfavorable yaw-axis orientation—all of which impact the lifetime and efficiency, contributing to high O&M²³ costs.

Since a GFM-WTG can produce power to sustain its own houseload as long as wind blows, not only relieving any dependence on external supplies but also re-charging the UPS, it can more importantly provide outward energizing power to inter-array cables, transformers and filters, for supplying auxiliary loads of other GFL-WTGs in the network and even the offshore substation [35]. However, additional energy storage is necessary to support the WTG in dealing with the demanding power (MW) transients during energization (even if soft-start is considered) and network configuration changes (such as connection-disconnection of WTGs) by avoiding severe mechanical stree, but also ensuring enough energy capacity (MW h) in the system for the entire duration to avoid speed recovery related insecurities, especially at the start when the rotor has insufficient energy. While research is happening in integrating batteries, supercapacitors and flywheels as high-power-density sources, high-energy-density alternatives to diesel such as hydrogen are gaining momentum for long-term energy management [17]. The not-so-insignificant costs of such storage systems are expeceted to be compensated by future upcoming markets that utilize enhanced services from GFM converters such as blackstart, islanding and voltage/frequency control.

3.4.2 Multi-unit grid forming

As mentioned before, since an aggregated unit such as an offshore WPP consists of many (order of upto 100) WTGs, their synchronized parallel operation is essential to allow any energization capabilities of enhanced stability services onshore. Consequently, many questions must be answered to ensure cost-effective self-reliant operation. Firstly, since GFM-WTGs add to the capital cost of the project, it is essential to fine-tune the number of GFM-WTGs required. This must take into account numerous factors, especially the application under consideration, which could be auxiliary power needs for which reliability comparisons and carbon emissions must be taken into account, or for providing onshore services for which relevant markets and regulatory frameworks must exist to provide a sound business case. From a technical point of view, the most important is the choice of GFM control to be implemented in the WTG-GSC and while a single GFM unit is relatively straightforward to operate maintaining stability in grid-connected and islanded modes, the challenge is to optimally tune the parameters for operating many units in parallel that maintain synchronism in the face of large network transients and configuration changes.

Recent studies have shown that while different GFM control strategies are able to deal with the transients such as energization in a controlled manner maintaining stability of voltage and frequency at the offshore terminal, there transient behavior exhibits differences and some are prone to more oscillations than others such as VSM-based control due to reduced system damping owing to lower control bandwidths that push the system closer to instability, while Direct Power Control–based strategy exhibits more stiff control over the voltage and frequency resulting in superior performance [37].

Thus, for the entire offshore WPP to behave as a strong enough GFM source without any loss of synchronism between the multiple parallel units, extensive system-level studies are required for an optimal tuning of all the different levels of control loops, which ensures transient stability across different operational scenarios and can help reduce costs too. For example, while it is suggested that a ratio of 3:1 (GFM:GFL) is safe to use, especially for a mix of turbines from different manufacturers, an optimized set of control parameters can allow a single GFM-WTG to support upto 20 GFL-WTGs providing robust operation at least in small load steps [35]. This is however valid only for onshore WPPs since offshore WPPs tend to be much larger in capacity with longer and higher cross section of inter-array cables thus lead to more demanding transient and dynamic requirements for GFM WTGs.

That said, ensuring stability for high-power converters can be quite a challenge since the higher rating that puts a limit on the switching frequency of the semiconductor devices due to loss considerations and so the controller bandwidths allowed are lesser, ultimately translating into lower stability margins [37]. Furthermore to complicate matters, improving one oscillation mode can trigger another and the tuning strategy used for a single converter unit might not be applicable directly to multiple units operating in parallel [38]. This necessitates case-specific enhancements for active damping including but not limited to virtual impedance, cross feed-forward compensations and lead lag controllers. It is important to note here that solutions such as master-slave approach cannot be used for numerous assets spread across several kilometers as in large offshore networks because high-bandwidth communication links needed to ensure reliable, robust, low-power and secure operation not only become increasingly costly but also the additional delays are undesirable as they can trigger control instability [23].

3.4.3 Stable, robust and safe islanding

In order for a large offshore WPP to supply essential GFM services to the onshore grid²⁴ which can help compensate for the extra developmental cost of GFM-WTG technology and any additional energy storage needs, the offshore network of inter-array cables, transformers, filters and WTG converters must be controlled in a stable and robust manner with the ability to deal with contingencies and maintain high security and reliability. This is however quite challenging to achieve since the offshore network is a converter-dominated environment which makes its dynamics very different from traditional onshore grids now, which are also expected to exhibit similar characteristics in the decarbonized future.

The large share of high-voltage cables, transformers and filters offshore provides a resonance-rich spectrum which is not static due to various configurations of cables and WTGs in service, especially during contingencies. These resonances are prone to be excited by the harmonic injection of converters, especially when resonant frequencies corresponding to longer cable lengths (and thus larger capacitance) are in range of controller bandwidths, making harmonic stability critical to assess. Additionally, reduced online generation and loading in the early stages of energization lead to lesser system damping resulting in sharp resonant points which can be triggered by slight changes in the network configuration. This must be avoided as sustained over-voltages cause accelerated aging, insulation degradation and component failure due to dielectric and thermal stress on the equipment [39].

A completely new regime of challenges is introduced due to unexpected interactions between controllers and filters of nearby converters present in the system since cross-coupling between electro-mechanical dynamics and electro-magnetic transients due to the wide-ranging control timescales of PEC can lead to negative damping in the control output admittance. This makes it far more complicated to tune parameters and ensure stability and robustness of operation in different scenarios, especially with changing network configurations during large load steps, WTG connection/disconnection and energization sequence involving long cable switchings. Impedance and eigenvalue-based methods are commonly used for system-level stability analysis and while reduced order models can reveal great insight, detailed models are becoming increasingly important to get a more holistic view since assumptions valid for small-signal models and traditional strong grids do not hold for large transients and the offshore weak grid case.

Furthermore, since the offshore network formed by GFM-WTGs represents a relatively weak grid compared with today’s offshore HVDC-VSC-based grid that is backed a strong onshore grid, transient stability of PLL-connected GFL-WTGs proves to be a challenge, which can be attributed to the well-known problems of an un-enhanced PLL in weak grid operation [22], especially during large reactive power steps when large cable switchings are involved. This further affects the choice GFM control strategy and its tuning since instabilities can be highly sensitive to certain parameters making it difficult to maintain synchronism and cause maloperation of protection, posing a risk of disconnection of WTGs triggering a re-blackout, especially in the early stages of energization or during low-power operation when less generation and load are connected [18].

Last but not the least, resilience to faults in the offshore grid and HVDC transmission is essential to allow robust operation and reduce the risk of a re-blackout. Although the Dalrymple BESS in Australia can provide short-term overload current for clearing faults, the normal protection settings based on high fault current in-feeds (over-current and earth fault) are insufficient to protect the network in island or blackstart mode since the injection from the WTGs is too low to trigger the relay pick-up, especially at low numbers of WTGs. Thus, a special set of settings along with voltage protection is required to protect the network over the full range of planned operating scenarios. However, several fault scenarios may still not be picked up and a re-design of the protection scheme may be needed [40]. GFM-WTGs can potentially help by actively limiting the current causing the fault to automatically extinguish [41].

4. Summary

The extensive integration of renewable energy and HVDC that is yet to grow in the coming years is already changing the dynamics of the grid and pushing it to its limits, which necessitates the advent of a new class of converter-based power modules, namely grid forming. It is foreseen that such modifications will not only enable them to participate in supporting grid stability but also allow them to contribute more actively in providing essential services to ensure reliable and secure operation of future decarbonized electric network. Offshore wind power plants are deemed to play a key role in achieving this reality, but there are many obstacles yet to overcome. However, a dialog between system operators, developers and manufacturers to facilitate the development of required technology and market for a faster uptake of the responsibilities conventionally targeted to large thermal power plants by renewable sources and aggregated non-traditional technologies is already gaining momentum.

Recent studies and demonstrations have shown that grid forming wind turbines can not only provide blackstart and islanding capabilities but also support conventional grid following wind turbines while maintaining complete control over voltage and frequency with a desired response to transient events that contributes to stability of the grid. However, there are limits to the capabilities both at the individual turbine level and the aggregated system/farm level. In addition to the need of additional energy storage and new revenue streams along with potential re-design of the protection scheme, adverse control interactions in the resonance-rich offshore network make harmonic and transient stability critical to assess for ensuring reliable and secure operation. Thus, there is still a long way to go to make self-reliant wind farms a reality, but their potential to yield significant operational cost benefits while also reducing the carbon footprint over the project’s lifetime makes them an unavoidable player in helping meet our climate goals while ensuring high reliability and resilience of electricity supply with the most cost-effective and efficient usage of grid infrastructure.

Acknowledgments

The author would like to thank the InnoDC project that has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement number 765585.

Conflict of interest

The authors declare no conflict of interest.

Thanks

The author would like to thank his Ph.D. study supervisors, Prof. Dr. Nicolaos A. Cutululis and Dr. Jayachandra N. Sakamuri, discussions with whom contributed significantly to this work. The author would also like to thank the Department of Wind Energy at the Technical University of Denmark (DTU) and Vattenfall Vindkraft A/S (Denmark) for hosting the author’s Ph.D. study and start of his career in the renewable energy industry.

Abbreviations

BESS	battery energy storage system
GFL	grid following
GFM	grid forming
HVAC	high-voltage alternating current
HVDC	high-voltage direct current
IBR	inverter-based resources
PEC	power electronics converter
RES	renewable energy sources
VSM	virtual synchronous machine
VSC	voltage source converters
WPP	wind power plant
WTG	wind turbine generator