Advances in Power Electronics

Volume 2017, Article ID 8158964, 10 pages

https://doi.org/10.1155/2017/8158964

## Experimental Verification of a Battery Energy Storage System for Integration with Photovoltaic Generators

^{1}Department of Electrical Power Engineering, Universiti Tenaga Nasional, Selangor, Malaysia^{2}Power Electronics and Renewable Energy Research Laboratory (PEARL), Department of Electrical Engineering, University of Malaya, Kuala Lumpur, Malaysia

Correspondence should be addressed to Nadia M. L. Tan; ym.ude.netinu@aidan

Received 26 June 2016; Revised 20 September 2016; Accepted 22 December 2016; Published 24 January 2017

Academic Editor: Antonio J. Marques Cardoso

Copyright © 2017 Rajkiran Singh et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

This paper presents the experimental verification of a 2 kW battery energy storage system (BESS). The BESS comprises a full-bridge bidirectional isolated dc-dc converter and a PWM converter that is intended for integration with a photovoltaic (PV) generator, resulting in leveling of the intermittent output power from the PV generator at the utility side. A phase-shift controller is also employed to manage the charging and discharging operations of the BESS based on PV output power and battery voltage. Moreover, a current controller that uses the - synchronous reference frame is proposed to regulate the dc voltage at the high-voltage side (HVS) to ensure that the voltage ratio of the HVS with low-voltage side (LVS) is equivalent to the transformer turns ratio. The proposed controllers allow fast response to changes in real power requirements and results in unity power factor current injection at the utility side. In addition, the efficient active power injection is achieved as the switching losses are minimized. The peak efficiency of the bidirectional isolated dc-dc converter is measured up to 95.4% during battery charging and 95.1% for battery discharging.

#### 1. Introduction

Current grid codes for low-voltage grid-connected PV systems consider a low PV penetration and in many countries stipulate the regulation for anti-islanding and total harmonic distortion of injected current to be less than 5%. A high penetration of PV systems will require strict adherence to the grid codes because a large amount of varying active power will result in frequency variations and eventually instability of the power grid. Therefore, the integration of battery energy storage system, a type of high energy density storage device, is needed to level the output power from PV generators. Moreover, future grid codes are expected to include the fault ride-through (FRT) capability and reactive power injection of the PV systems [1]. Battery energy storage systems could be employed to absorb active power from PV during the FRT conditions.

A bidirectional isolated dc-dc converter with high-frequency galvanic isolation is one of the technologies that enables the integration of energy storage devices such as batteries and electric double-layer capacitors to the utility grid [2–6]. The bidirectional operation of the converter easily charges and discharges energy storage devices. Moreover, the high-frequency galvanic isolation increases the power density and the reliability of the energy storage system. The efficiency of the bidirectional isolated dc-dc converter has improved since the converter was introduced in 1991 [7]. At that time, the first generation of IGBT suffered from high switching losses. The performance of the latest-generation IGBT improved in device switching losses. Furthermore, superjunction MOSFET has a low on-state resistance that further reduces conduction loss of the switching device [2].

A 6 kW, a 10 kW, and a 100 kW bidirectional isolated dc-dc converter achieved maximum efficiency of 96.9% [3], 97.4% [4], and 98.7% [5], respectively. The authors of [5] showed that, with the usage of silicon carbide MOSFETs, the performance of the bidirectional isolated dc-dc converter is greatly improved as compared to the dc-dc converter using silicon-based switching devices. The dc-dc converter operation is optimum only when the voltage ratio of the HVS and LVS is equal to the turn ratio of the transformer. Otherwise, the circulating current will be high, leading to an increase in switching loss due to high peak switching current and high turn-off overvoltage across the semiconductor switches. The circulating current should be minimized so that the efficiency of the converter is maintained high in applications with a broad range of operating voltage. Therefore, a reliable control system is required to adjust the voltage of HVS with respect to the voltage of LVS of the converter so that the voltage ratio of the HVS with LVS is close to the transformer turns ratio.

This paper describes a battery energy storage system (BESS) that consists of a battery unit, a bidirectional isolated dc-dc converter, and a PWM converter that can be applied to regulate the output power of a PV system. This paper verifies the feasibility of operating a 2 kW BESS that responds the changes of a varying PV output power. In order to achieve this purpose, a current controller that uses - synchronous reference and also a phase-shift controller are employed. The main function of the current controller is to regulate the voltage at HVS to maintain the voltage ratio of the HVS with LVS equal to the transformer turns ratio. The phase-shift controller is also employed to control the charging and discharging modes of the battery based on PV output power and battery voltage. Consequently, the operation of the both control systems maintains the grid-injected power at a constant value. Reference [8] has shown only the simulation results of the proposed system. This paper aims to validate the proposed system by the construction of a laboratory prototype and to show that the controllers have fast response and are feasible to be integrated with a PV system.

#### 2. Experimental System

Figures 1 and 2 present the experimental circuit of the charging and discharging modes of the proposed BESS, respectively. The system consists of a three-phase PWM converter, a 2 kW bidirectional isolated dc-dc converter, and a three-phase resistive load bank. The PWM converter is connected at ac side with a 150 V, 50 Hz power supply through ac-link inductors and filter capacitors . The proposed operating voltage of the battery, , varies between 50 V and 60 V, and at the HVS, is regulated between 300 V and 360 V to adjust the ratio HVS and LVS close to the transformer turns ratio. In Figure 1, a single-phase resistive load bank is used to represent the battery bank that is being charged. In Figure 2, a three-phase resistive load is connected in delta configuration at the ac side to verify the BESS in the discharging mode. Table 1 summarizes the system parameters.