*3.3. Modes of Operation*

The literature about WDIM operation mode simulations deals with WDIMs of different architectures. The following papers present WD mode simulations in no-storage WDIMs. Reference [22] deals with a WDIM with one DG and one WTG and shows several simulations considering pitch or stall regulation WTs and WT–SCIG types or WTs driving SGs directly connected to the grid. In [23], the responses of a two-DG–three-WTG WDIM to sudden WTG disconnection and load demand variations are simulated. In the WDIM of [26], modifications to the DE speed governors are proposed to share active power among DGs in order to reduce the total fuel consumption. Reference [57] presents simulations of a WDIM whose fixed-pitch WT drives a dynamic slip-controlled wound rotor induction generator [58]. Reference [57] shows that by varying the rotor WRIG resistance as a function of the grid frequency by means of a variable external resistor, this WT–WRIG provides frequency support to the minigrid. ESSs have also be considered in WD mode simulations. Reference [24] simulates a WDIM with one WTG, one DG, a DL and a FESS based on hydrostatic transmission. In [25], a WDIM includes a high-speed flywheel driven by a switched reluctance electrical machine, and simulations with the WDIM response to WTG power and load increases are presented. Reference [59] models a WDIM with WT–SCIG, DG, BESS, load and power lines connecting all the elements, and it simulates the WDIM in response to a wind gus<sup>t</sup> disturbance.

Several papers deal with simulations in WO mode. In the following papers, an SM provides the isolated grid AC voltage. Reference [29] models an isolated wind power system that consists of a WT–SCIG, SM, load and BESS. The simulation in [29] shows how the BESS is used to regulate frequency by consuming/supplying the WTG active power excess/deficit. Reference [54] deals with a similar architecture, except that the BESS is substituted by a low-speed flywheel driven by an induction electrical machine, and shows how the FESS is used to regulate the isolated system frequency. No-storage WDIMs have also been simulated in WO mode, such as in [6], which shows WDIM frequency regulation by using a DL which consumes the WTG power excess. An architecture similar to that of [6] is presented in [60], but [60] uses a different DL control for frequency regulation. In [61], the regulation of frequency is performed by a BESS and a DL that are coordinated to share the WTG power excess.

In other WO simulations, the grid converter of a WT–PMSG type generates the AC isolated grid voltage, and the frequency regulation is performed by regulating the DC link voltage of the WT–PMSG double power converter. Examples of this case are [34], where a DL and BESS combination regulates the DC link voltage; [35], where a BESS–CESS combination is used for the DC regulation; and [36], only a DL is used to balance the WTG active power generation excess.

The transitions between the different WDIM modes of operation have also been simulated in the literature. Reference [27] simulates a DO-to-WD mode transition by connecting a WT–SCIG to a DG isolated grid. Reference [62] shows a much smoother DO-to-WD transition than [27] as the WT–SCIG is equipped with a soft starter. In [7,49,63], the simulated high-penetration WDIM has a DG with a friction clutch. By means of the clutch, the DE can be locked/disengaged to/from the SM. This clutch-type DG is shown in Figure 5. When the clutch is locked, the WDIM mode is DO–WD; when the clutch is disengaged, the WDIM mode is WO and the SM provides the AC grid voltage. The WO-to-WD transition in the clutch-type DG is faster than in the standard no-clutch DG [64] shown in Figure 1.

**Figure 5.** Clutch-type diesel generator (DG).

In the clutch-type DG, the clutch is ordered to engage when the DE and SM have quite similar speeds ( *ωD* ≈ *ωS* in Figure 5) [7]. The no-clutch DG type shown in Figure 1 needs extra time to perform the synchronization of the SM voltage with the grid voltage before connecting the DG circuit breaker IG of Figure 1. The WO-to-WD transition simulation in [49] is done by locking the DE to the SM so that the DE produces in WD mode the power supplied previously in WO by the BESS. The WD-to-WO transition simulation of [63] is done by disengaging the DE from the SM when the WTG power generation exceeds the load consumption. Reference [7] simulates the mandatory WO-to-WD transition when power generation is below the power consumption. During this WO-to-WD transition case simulated in [7], there is a frequency falling due to the active power deficit until the clutch is locked. After the locking of the clutch, the DE supplies power and the active power equilibrium can be restored.

#### *3.4. Power Quality and Stability Issues*

As previously mentioned, WDIMs are low-inertia isolated power systems, where it is quite difficult to balance power generation with load consumption due to the uncontrolled power generated by the WTGs and consumed by the loads. These unbalances provoke significant system voltage and frequency deviations. Several papers deal with power quality and system stability studies in no-storage WDIMs: [27] studies how the variations of WTG power and load influence WDIM power quality, [65] shows the stabilization of the WDIM voltage by using the combination of a static reactive compensator and the SM voltage regulator, and [26] proposes modifications in the SM voltage regulators to reduce WDIM voltage variations. In [66], the DE speed governor is supported in the frequency regulation by controlling distributed heating loads. In [67], the control of the WTG couples the kinetic energy stored in a WT–DFIG with the rate of change of frequency to emulate inertia, increasing the total inertia of the WDIM and improving the frequency transients.

When a short-term ESS is included in a WDIM, it provides several benefits such as frequency and voltage stabilization and improved stability and reliability [8]. These benefits are more effective in WDIMs than in big power systems which have larger inertia. Reference [51] shows the stabilization of the WDIM frequency by using a capacitive ESS. Reference [68] uses a BESS with a redox flow battery type for frequency stabilization of a WDIM. In [40], the WDIM includes a Ni–MH BESS, and the study compares by simulation the responses for the BESS and no-BESS cases, showing much better transients in voltage and frequency in the BESS case. Similarly, [59] shows by comparing the BESS and no-BESS cases that the DG active and reactive power transients are greatly improved in the BESS case, as the BESS tracks and compensates for the WTG power changes. Reference [69] deals with a WDIM with DL and FESS and shows that the only-FESS case responses present better power quality than the only-DL case when they are used in WD mode to control an active power excess scenario. In [19], the simulated WDIM includes a BESS which performs a peak shaving strategy in order to avoid a DG overload scenario, improving the WDIM reliability. In [4], the WDIM control uses a BESS to increase the power consumption to avoid DG reverse power, so that the DE speed governor can continue controlling system frequency by regulating the DE fuel injection and the WDIM reliability is augmented. Reference [62] also shows how WDIM reliability is increased by ordering a generic ESS to load/supply when needed.
