*3.5. Related AC Microgrids*

WDIMs are remote microgrids [70,71] that operate in an autonomous mode [72], so WDIMs can be related to other isolated microgrid studies. Reference [73] shows a BESS included in an isolated microgrid that provides frequency support and uninterrupted power supply of critical loads; this study is applicable to the ESS in a WDIM in the case of the generators being out of order. Reference [74] deals with a ship with a DG power plant that includes a BESS; the BESS smooths the ship power variation, and this study is applicable to a WDIM working in DO mode. In [75], new DG controllers are proposed to enable the operation of a DG in a microgrid that includes inverter-based sources avoiding circulating reactive power and frequency oscillations caused by standard DG controllers, so this study is applicable to the DGs included in WDIMs. Reference [76] presents graphs of the active and reactive power sharing between a DG and an inverter-based generator in a microgrid when frequency/voltage droop control is used. This study is applicable to a WDIM as ESSs and WT–PMSGs are always connected with an inverter to the grid.

In [77], a BESS is used to counteract the voltage variations caused by renewable power source power fluctuations; [78] presents a standalone microgrid with a BESS–CESS combination and with a fuzzy logic control system to stabilize frequency and voltage; in [79], a load shedding optimal control is carried out to reduce the fuel consumption during the operation time of a DG included in a microgrid with renewable power generation. The utilization of fuel cells to provide controllable active power has also be considered in WDIM and microgrid simulations. In such cases, the microgrid usually includes a water electrolyzer that consumes renewable power excess to produce hydrogen for the fuel cell. Reference [80] models and simulates a WDIM consisting of a DG, four pitch-controlled WTGs, load, a flywheel as short-term ESS and a fuel cell electrolyzer as a long-term ESS, and both ESSs are used to support the WDIM frequency regulation. Reference [81] simulates several cases of a microgrid that includes three WTGs, one DG, a photovoltaic system, two

fuel cells and flywheel and battery ESSs. The isolated microgrid in [82] includes wind and photovoltaic generators and fuel cells but does not include a DG. Reference [82] combines a BESS and a CESS to control the frequency of a microgrid. Reference [83] shows an optimal sizing of a BESS used for frequency support in a microgrid with a DG, microturbine, fuel cell and photovoltaic generation.

DLs are also used to balance active power in isolated hydropower systems with no power regulation in the hydro turbine generator (HTG), and thus the DL is used to regulate system frequency [84]. This DL use is the same as that in WO mode in no-ESS WDIMs [6]. Additionally, WO mode simulations are also presented in the wind hydro isolated microgrid of [85], where a simulation of the transition from WO mode to wind– hydro mode (the mode where both the HTG and WTG are supplying) is also shown.

DGs and WTGs are also combined with other renewable power generators, such as in the power plant of El Hierro Island in Spain, which combines wind, hydro and diesel power sources. The diesel-off mode of this wind–hydro–diesel isolated grid has been simulated in several studies. Reference [86] shows how a rotating no-flow Pelton turbine can supply power in less than 10 s when the WTG-produced power is not enough to cover the load demand. In [87], the regulation of the system frequency in WO mode is performed by using the fixed- and variable-speed pumps which belong to the hydropower pumped-storage to balance active power. Reference [88] shows and compares different control schemes for frequency regulation in WO mode in the El Hierro isolated power system.

A broader study about different configurations and control of microgrids can be seen in [89].

#### **4. WDIM Dynamic Modeling and Simulation Example**

By using the Matlab–Simulink software, this section models a no-storage WDIM which comprises one 300 kVA DG, one 275 kW fixed-pitch constant-speed WTG, consumer load and a 446 kW discrete DL. After the modeling, the WDIM is simulated, and the system response to wind speed and load variations is shown. The Simulink–SimPowerSystem schematic of the simulated WDIM is presented in Figure 6, and the dynamic modeling of the different components of the WDIM is presented next.

**Figure 6.** WDIM Simulink schematics.

#### *4.1. Modeling of the WDIM Components*

The DG is built with the blocks of Figure 6: diesel engine, 300 kVA SM and voltage regulator. The sixth-order model of the SM and the IEEE type 1 voltage regulator model are available in the Simscape Electrical library [14]. The diesel engine block includes the models of the DE and its speed governor based on [90]. The DE model consists of a gain and a delay, and the speed governor consists of a speed regulator and actuator modeled as a second-order system. The speed regulator is a Proportional-Integral-Derivative (PID) type, and the PID integral part makes the WDIM frequency be the rated one in steady state.

The constant-speed WTG is represented in Figure 6 by the 275 kW induction generator block and the WT block that models the fixed pitch WT. The fourth-order model of the IG is available in the Simscape Electrical library [14]. The WT block is based on the wind turbine power curves [28] and uses a look-up table to calculate the produced mechanical WT shaft power as a function of the wind speed and the WT speed. The WT used has a fixed pitch, so there is no possibility of regulating its produced power, which mainly varies with the cube of the wind speed; therefore, the WT–SCIG generating power is uncontrolled. A 35 kVA capacitor bank for reactive power support of the SCIG is included.

The 446 kW DL consists of eight three-phase resistors (R0–R7), each one in series with an electronic switch (S0–S7). By closing/opening the switch SJ, the corresponding three-phase resistor RJ is connected/disconnected to/from the isolated grid, so the DL active power consumption can be controlled. The rated powers of resistors R0–R7 follow a binary progression and have the values P0, 2·P0, <sup>2</sup>2·P0, ... , <sup>2</sup>7·P0. The DL-consumed power ranges from 0 (all the switches OFF) to 255·P0 (all the switches ON), and the DL variation is discrete in steps of P0 kW. For the present application P0 = 1.75 kW, and so PD-NOM = 446 kW. The DL model is based on [28]. The DL is used in these simulations to avoid DG reverse power as explained in Section 2. For this aim, an integral action controls the DL-consumed power in such a way that the DG-produced power never goes below 12 kW in steady state. When the *PT* > *PL* condition happens, the DL is ordered to consume power *PD* to allow the DG final power *PG* to be within a 12–18 kW minimum power interval (4–6% DG rated power) so that the set DE + speed governor can control WDIM frequency.
