**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

<sup>11</sup> Diesel genset on the offshore platform not only occupies extremely costly space but also requires annual refueling, special fire protection, personnel safety protocols and maintenance. In addition, a backup diesel generator is present to combat startup issues.

<sup>12</sup> due to moisture damage, icing up of electronics and equipment, bearing deformation, standstill marks and vibrations due to unfavorable yaw-axis orientation

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, nonlinear 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

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

#### **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.*

reliability and flexibility services such as very low SCR<sup>13</sup> 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

<sup>13</sup> Short circuit ratio; very low means ≪1.5.

<sup>14</sup> Overloading capability of 2 pu for 2 s.

<sup>15</sup> Including wind farm power dispatch/curtailment for reducing unserved energy and distributed energy resources curtailment to avoid conditions due to uncontrolled local generation such as rooftop solar PV.

<sup>16</sup> System Integrity Protection Scheme, which is a sectionalizing strategy to protect against complete area blackouts.

<sup>17</sup> That varies, and hence, this is called maximum power point tracking.

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.

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 device<sup>18</sup> 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

<sup>18</sup> Integrated within the DC bus of the WTG or connected as a separate unit through converter interface.

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.
