**1. Introduction**

In solar power plants, active power transfers from the photo voltaic (PV) array to the grid during daytime and the array loses its power generating capability during nighttime or when the solar irradiation is weak. To also supply power to the grid during nighttime, energy storage is required. Since the power requirements during nighttime are usually much lower than those during the daytime, energy storage with 25% of the PV array rated power may be selected for 24-h operation. A block diagram of a grid energy storage system in a solar PV power plant is shown in Figure 1.

**Figure 1.** Generalized block diagram of a grid energy storage system in photo voltaic applications.

The different modes of operation of the above system are explained below:


With such systems, it is also possible to charge the batteries from the grid during non-peak load hours and the batteries along with PV array feed power to the grid during peak load hours [1]. Since the system is connected to three different power sources i.e., PV array, battery storage system and the grid, these three power sources need to be isolated to ensure the safety of the equipment. Existing energy storage systems for PV applications using a buck-boost chopper-based battery charger are briefly explained below.

In the configuration presented in [2], a DC-DC converter is connected between the PV array and PV inverter and the battery is connected across the DC link as shown in Figure 2a. In such systems, the DC/DC converter needs to be designed for the maximum capacity of the PV array even though the battery capacity is much less when the system operates in Mode3, the inverter should act like an active rectifier to charge the batteries and there is no isolation between the PV array and the batteries. In the configuration shown in Figure 2b, the PV array and PV inverter are connected to the DC link and the battery is connected to the DC link through a buck-boost chopper. In this case, the charger needs to be rated only for the rating of the battery. In the configuration presented in [3,4], independent DC-DC converters are required to connect the battery and PV array to the DC link as shown in Figure 2c.

An optimized operation of a dual active bridge (DAB) converter feeding a PV inverter connected to the grid is presented in [5,6]. Isolation between the grid and DC side is provided through a high-frequency transformer used in the DAB as shown in Figure 2d. In such a configuration, the DAB needs to be designed for the full capacity of the PV array. Since the design of a DAB is complex for high power ratings, this configuration is more suitable for low power applications. There is also no isolation between the DC link and the power bank with this configuration.

**Figure 2.** Buck-boost chopper-based energy storage system configurations for photo voltaic applications (**a**); two stage conversion with battery directly connected to DC Link (**b**); single stage conversion with chopper based Battery charger (**c**); two stage conversion with chopper based battery charger (**d**); dual active bridge based photo voltaic inverter with chopper based battery charger.

From the above discussions, it is observed that buck-boost chopper-based ESS cannot provide complete isolation. In this paper, a DAB-based energy storage system (ESS) for PV applications is proposed which can mitigate the drawbacks of buck-boost chopper-based systems. In the DAB-based ESS configuration, PV array and the PV inverter are directly connected to the DC link and the battery is connected to the DC link through a DAB-based bi-directional battery charger as shown in Figure 3. A high-frequency transformer in the DAB provides isolation between the DC link and the power bank. A transformer connected between the inverter and grid provides isolation between the DC sources and AC grid.

**Figure 3.** Dual active bridge-based energy storage system for a grid connected PV system.

The following technical features are the main advantages of the proposed system:


In this paper, a power balancing control for the DAB-based energy storage system is proposed and validated through real-time simulations. Detailed discussions on the proposed system, design calculations, and the control structure are presented in Section 2. The proposed power balancing control algorithm is explained in Section 3. In Section 4, a hardware-in-the-loop (HIL) simulation setup to validate the control algorithm is explained. HIL results are presented in Section 5.
