The percentage of penetration of wind power generation in grid-tied microgrids (MGs) is growing [1-3]. The type of generation system implemented by wind turbines and the fluctuating nature of the wind can have a negative effect on the dynamic security and the power quality of the MG [4-7].

To mitigate these problems, new energy storage devices arise as an alternative with the purpose of compensating the power fluctuations. These energy storage systems can swiftly provide the MG with both reactive and active power simultaneously, and consequently serve to enhance the MG dynamic security and power quality [8]. There are numerous storage devices, some of which will be more convenient than others, depending on the application [7-11].

Among modern energy storage devices, Flywheel Energy Storage System (FESS) and Vanadium Redox Flow Battery (VRFB) are some of the most promising technologies. Both storage devices have quick response time, high energy efficiency, and long lifetimes. FESS has power rating in the order of MWs, and discharge time in the order of minutes. FESS stand out for their low capital cost per unit power (around 700 US$/kW); nevertheless, it has a high capital cost per unit energy (around 2000 US$/kWh) [10-13]. On the other hand, VRFB has power rating in the order of MWs, and discharge time in the order of hours. In contrast to FESS, VRFB is remarkable for its low capital cost per unit energy (approximately 600 US$/kWh) and its high capital cost per unit power (2500 US$/kW) [10,11,13].

Many solutions using FESS or VRFB devices have been proposed in the bibliography either to smooth wind power fluctuations or to provide ancillary services [14-22]. Reference [14] proposes a novel control strategy using flywheel energy storage to smooth the power injected by wind generators. In [15], a direct torque control for an induction machine used in flywheel storage is proposed to compensate the power fluctuations of a variable-speed wind generator. Reference [16] proposes a multi-level control for a controller composed by a DSTATCOM and a FESS in order to enhance the integration of wind power generation in electrical networks. In [17], control strategies using a fuzzy logic supervisor with FESS are developed in order to smooth the rapid fluctuations of wind power and to provide ancillary services. Reference [18] presents a control strategy for the frequency regulation in a wind-diesel powered MG using flywheels and fuel cells. In [19], a VRFB is proposed to improve the power quality and the stability of the power system that include wind power generation. Reference [20] highlights the alternative to spilling wind to provide frequency response capability using wind farm with VRFB. Reference [21] proposes a methodology for the economic optimization of the sizing of a VRFB whilst enhancing the participation of wind power plants in network primary frequency control support. In [22], a thyristor-based flexible ac transmission system is proposed and developed for controlling the exchange of active power flow between the VRFB and the grid in order to compensate the wind power variations generated by DFIG turbines. Although the papers mentioned present practical solutions to improve the integration of wind power generation using FESS or VRFB devices, they do not use both devices combined taking advantage of the better characteristics of each device. Nor do they perform a combined active power control in order to level the power oscillations of wind generation and provide ancillary services so as to participate in frequency control.

Based on the previous discussion, this work proposes the use of an integrated controller constituted by a FESS and a VRFB for better integration of wind power systems in microgrids. This integrated controller also includes a power conditioning system (PCS) in order to connect the FESS-VRFB to the ac power system. To implement the PCS/FESS-VRFB controller, requires administration of the energy stored in the FESS and VRFB devices so that these devices always have the capacity to deliver or store energy. In this manner, a function that can efficiently manage the stored energy is essential. This function, carried out via a control strategy for the active power of the PCS/FESS-VRFB, has to accomplish two aims simultaneously: smooth the wind power fluctuations injected into the MG effecting a suitable use of the stored energy, and assist the frequency control in case of perturbations in the MG.

The objective of this work is to elaborate an effective control methodology of the controller proposed both to level the power fluctuations injected by wind generators in the MG and to contribute to the frequency control of the system in case of perturbations in the MG. For this purpose, while considering the characteristics and costs of each storage device, a control is proposed so that the FESS works applying frequency control when important faults occur in the MG (the storage device requires low energy and much power); and the VRFB works effecting power levelling (the storage device requires much energy and low power). The control method proposed includes two control modes of the PCS/FESS-VRFB controller, namely: power levelling (carried out by the VRFB), and frequency control (carried out by the FESS). For both proposed control modes, the charge status of each storage system is considered. The dynamic performance of the developed control method and its effect on the operation of the MG is validated by computer simulation the MATLAB/Simulink, by using SimPowerSystems and C++ codes.

A model of the PCS/FESS-VRFB controller proposed for this study consists mainly of the PCS, the FESS and the VRFB. The PCS is the power electronic interface between the FESS-VRFB and the ac electric utility grid, pointing at reaching three aims: one is to convert electric power from the FESS to the ac grid, another is to convert the dc voltage of the VRFB to the ac grid, and the final one is to efficiently load/unload the FESS and VRFB storages. The diagram of the proposed modelling of the PCS/FESS-VRFB controller is shown in Fig. 1.

Figure 1 Source: Authors

The PCS proposed is integrated by a back-to-back ac/dc/ac converter (composed of two voltage source inverters). The proposed inverter structure to the ac/dc/ac converter is based on a two-level six-pulse topology. Moreover, it uses sinusoidal pulse width modulation techniques to control the output voltage control of the VSI. The chopper consists of a buck-boost converter, which is commonly used when the magnitude of two dc bus voltages is close to each other and the voltage ratio is smaller or greater than unity and it does not vary significantly. The power exchange between the flywheel and the ac/dc/ac converter is carried out through a permanent magnet synchronous machine (PMSM). The flywheel is modelled as an additional mass coupled to the rotor shaft of the PMSM [23]. The FESS model with its PCS is comprehensively analysed in [16]. The VRFB model is composed of two parts: the stack model (electrochemical model), and the mechanical model. In the stack model, the terminal VRFB current ( ^_IVRFB ) determines the stack current ( ^_Istack ) and consequently, the terminal stack voltage ( ^_Ustack ) and the state of charge (soc) of the electrolyte. In the mechanical model, the external control system sets the flow rate that produces a pressure drop in the hydraulic system. The VRFB model with its PCS is comprehensively analysed in [22].

The control proposed for the PCS/FESS-VRFB controller is divided into three: the Chopper control, the machine-side VSI control, and the grid-side VSI control. For each control, a multi-level scheme is suggested [24,25]. The multi-level control scheme of the chopper is divided into two levels: internal and middle levels; and the multi-level schemes of the machine-side VSI and the grid-side VSI are divided into three levels: internal, middle and external levels. The control system of the PCS/FESS-VRFB controller is shown in Fig. 2. The control algorithm for the flywheel energy storage is fully developed in [16], and for the VRFB and chopper in [22]. Only the external level of the grid-side VSI control (bottom, right-hand corner of Fig. 2) is presented below which is the main contribution of this work.

Figure 2 Source: Authors

The external level control of the grid-side VSI is in charge for establishing the power exchange between the grid-side VSI and the electric power system [16,24]. The external level control has the following modes of operation of the PCS/FESS-VRFB: power levelling (PL), frequency control (FC), and voltage control (VC). The PCS/FESS-VRFB controller constantly works in VC and PL modes with the VRFB connected and the FESS in standby; enhancing the power quality generated by wind power generators. When significant frequency deviations take place, the FC mode is active, helping to recuperate the rated system frequency and thus improving the grid security. In the FC mode, the FESS is activated. In this work, only the PL and FC modes are developed, which are the core of the contribution.

The test power system employed to validate the proposed control methodology of the PCS/FESS-VRFB controller for enhancing the incorporation of wind power generators in MGs is shown in Fig. 5. This electrical network implements a MG connected through a 20 km transmission line and a transformer to a bulk power system represented by a single machine-infinite bus type system. The MG consists of different distributed energy resource units (distributed generation based on renewable and fossil fuels and the advanced PCS/FESS-VRFB controller) and loads.

Figure 5 Source: Authors

The distributed generation based on renewable resources consists of a wind farm integrated by three rows of wind turbines with three turbines per row. The technology used by each wind turbine corresponds to a variable speed wind turbine with full-scale frequency converter. The rated power of the wind farm is 9MW (each wind turbine has 1 MW rated power). The distributed generation based on fossil fuel (50 MVA rated power) consists of a dispatchable unit powered by multi-stage tandem compound steam turbine and includes a speed governor and a voltage regulator. All loads are grouped at bus 5. The proposed controller is placed at bus 2, representing the point of common coupling of the wind farm (PCC-WF) to the rest of the ac microgrid, and includes a ±10 MVA/750 V dc bus PCS, a 6 MW/60 MJ FESS system, and a 2 MW/3600 MJ VRFB system. Full test system data are included in [16,17,22].

The testing of the control proposed for the PCS/FESS-VRFB controller in grid-tied operation is done through two case studies that demand a high dynamic response of the controller. The first case analyses the behaviour of the control of the PCS/FESS-VRFB when a real profile of wind speed is established to the wind turbines so that the PCS/FESS-VRFB controller operates continuously by delivering and storing energy. In this case, the behaviour of the PL control mode and the operation of the VRFB is mainly shown. In the second case, the PCS/FESS-VRFB controller is tested when external perturbations are imposed on the bulk power system, so that the microgrid is changed to islanded operation mode and major frequency shifts occur. In this case, the behaviour of the FC control mode and the operation of the FESS are mainly observed.

This work presents control aspects of a controller composed by a power conditioning system (PCS), a Flywheel Energy Storage System (FESS) and Vanadium Redox Flow Battery (VRFB) to enhance the integration of wind power generation into ac microgrids. A control method of the PCS/FESS-VRFB controller is proposed using two control modes, power levelling (PL) and frequency control (FC). The PL control mode uses a low-pass filter with a particular function to level the power fluctuations injected by wind generators into an electric power system. The FC control mode is employed to assist to the frequency control when significant faults occur in the system.

From the results achieved, authors concluded that the developed control methods operate satisfactorily. The PCS/FESS-VRFB controller, with the operation of the VRFB, effectively countervails the wind power fluctuations. The WF plus PCS/FESS-VRFB generates a more levelled power than that of the wind farm alone without the PCS/FESS-VRFB. Moreover, the WF plus PCS/FESS-VRFB, now with the operation of the FESS, also contributes to the recovery of the frequency when important faults take place in the system. In reference to the PL control mode, it is possible to conclude that the active power control of the VRFB using a regulator reach an outstanding handling of the stored energy, by correcting the fluctuations of wind power and avoiding the VRFB from becoming discharged or overcharged. With PL mode, the power quality of wind farms is enhanced and a suitable utilization of the energy stored for the VRFB is reached. In the FC mode, when the microgrid is isolated due to faults, causing the frequency rises or falls over a given value; the PCS/FESS-VRFB changes from PL to FC control mode conclusively, thereby activating the FESS device, and successfully contributing to recovering the microgrid frequency at its rated value. With the FC mode, excellent handling of the stored energy for the FESS is also notable, having available energy to store or generate to deal with power imbalances properly when the microgrid is isolated. In the FC mode, the wind farm with the PCS/FESS-VRFB controller provide an ancillary service to enhance the system security of the microgrid. Finally, it is clearly distinguished that optimal usage of energy storage devices is achieved with the PCS/FESS-VRFB and the control proposed. The best characteristics of the energy storage devices have been considered depending on the application. For load levelling, the VRFB is used contemplating their high discharge times, low capital cost per unit energy and high capital cost per unit power. And for frequency control, the FESS is used primarily considering their fine values of power rating, the low capital cost per unit power and high capital cost per unit energy. Hence, the incorporation of wind power generation in microgrids is efficiently enhanced with the compensation of the PCS/FESS-VRFB controller.