**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

<sup>19</sup> Original equipment manufacturers.

**Figure 6.** *Target states in the greenstart energization sequence of an HVDC-connected offshore WPP; reproduced from [18].*

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.

#### *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

<sup>20</sup> Uninterruptible power supply.

<sup>21</sup> Supervisory control and data acquisition.

<sup>22</sup> To avoid collisions with ships and aeroplanes.

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

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&M<sup>23</sup> 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 VSMbased 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].

<sup>23</sup> Operational and maintenance.

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 grid<sup>24</sup> which can help compensate for the extra developmental cost of GFM-WTG technology and any additional energy storage needs, the offshore network of interarray 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-

<sup>24</sup> Like inertia, voltage and frequency control and support, blackstart and islanding capabilities.

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

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 smallsignal 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 unenhanced 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 pickup, 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].
