*3.1. Wind Energy*

Wind energy is the renewable energy used mostly in the world. The power of wind turbines today can reach up to 3 MW. However, the power generated is highly variable as is the wind speed [59]. A generic electrical scheme used in wind turbines is shown in Figure 14a. It consists of a variable speed wind turbine, an electric generator and a double power conversion stage with two power converters. There are different variants, but normally the converter connected between the generator and the intermediate continuous stage controls the turbine pitch and the generator torque (maximun power in function of the wind speed). On the other hand, the converter connected between the continuous stage and the power grid regulates the DC bus voltage and the reactive power exchanged with the power grid. Although the variation in the power generated can be reduced, it cannot be eliminated due to the dynamic response of the pitch control, which for fast speed variations is not capable of eliminating the fluctuation reflected in the output power. Another effect to take into account is the power fluctuation as a consequence of the tower shadow effect that causes a pulse in the torque and in turn a fluctuation in the output power.

Panhwar, I.H. [60] proposed a HESS (SC/lead-acid battery) to smooth the generated wind power. The wind energy converter consists of a wind turbine mimicking converter (wind turbine generator, rectifier and DC/DC converter), a SCs module, a charge controller and a battery as shown in Figure 14b. The first DC/DC converter charges the SCs and the charge controller control is designed to charge the battery from the energy stored in the SCs. The conclusion of the paper is that the proposed configuration allows smooth charging and extended discharging of the battery. On the other hand, the topology and the proposed mode of operation causes reduced stress on the generator and ancillary components of the circuit.

Liu, J. [61] also proposed a HESS (battery/SC) to smooth out the fluctuations of wind power. The low-frequency components of the generated power are absorbed by the battery and the high-frequency components by the SCs. That makes possible to reduce the charge/discharge times of the batteries and thus extend their useful life. A control strategy is proposed in which the SoC of the storage systems is taken into account to avoid deep discharges, overloads and allow them to work at the optimum efficiency points. Experimental results are presented in which a DC/AC converter controls the HESS to achieve bidirectional active and reactive power exchange. The proposed strategy is validated, demonstrating the optimal response of the HESS to improve power quality and enhance the stability of power systems.

(**b**)

**Figure 14.** (**a**) Simplified electrical diagram of a variable speed wind turbine with full-scale power converter; (**b**) Wind Energy Conversion System with proposed HESS adapted from [60].

### *3.2. Solar PV Energy*

Solar Photovoltaic (PV) panels are used to convert solar energy into electrical energy. The system that extracts energy from the sun is made up of a solar panel and a power conversion stage that connects the PV panel with the load or the electrical grid. The power conversion stage (power electronic converter) can be single stage (solar PV panel connected directly to the power inverter) or double stage (solar PV panel connected to a power converter with an intermediate direct voltage stage). Figure 15a shows a possible simplified electrical diagram of a double-stage solar PV plant connected to the electrical grid. The problems associated with solar energy are the irregular generation of power, maximum in periods of highest levels of solar irradiation or unregulated in partially cloudy days.

Cabrane, Z. [63] proposed SCs to compensate voltage drops during short periods of time in soalr PV systems connected to the grid. A coordinated control algorithm is described to compensate the effects caused by the variable solar irradiance in PCC and results obtained from simulations are shown. The algorithm applies a two-stage topology with the SCESSS connected to the intermediate DC stage through a DC/DC converter.

Ma, W. [64] proposed a HESS composed of batteries and SCs to smooth out the power fluctuations of solar PV systems. A multi-objective optimization model is established to split the required power between both ESS. The variables that are taken into account in the algorithm to decide how much power each one contributes are: power losses, lifetime aging, cost of batteries and the SoC of the SCs.

Cabrane, Z. [62] proposes an EMS for a HESS (battery/SCs) applied to PV energy in order to stabilize the DC voltage. Different topologies are compared (hybrid passive parallel, semi-active, and multiple converters) for the connection of SCs and batteries in photovoltaic energy systems. The advantages and disadvantages of each of them are listed. The control proposed by the authors applies to the configuration of multiple converters. For the distribution of the necessary energy between batteries and SCs, a low pass filter is applied. Proportional integral (PI) controllers are used to regulate the DC bus voltage. The output of these regulators is the ESS charge/discharge current reference. Finally, results obtained from simulations are shown and the SoC of batteries is compared for different filter constants.

**Figure 15.** (**a**) Simplified electrical diagramam of grid connected double conversion PV system; (**b**) Structure of a multiple converters configuration with maximum power point tracking (MPPT) adapted from [62].
