Advances in Power Electronics

Volume 2016, Article ID 8518769, 11 pages

http://dx.doi.org/10.1155/2016/8518769

## A Control Method for Balancing the SoC of Distributed Batteries in Islanded Converter-Interfaced Microgrids

Department of Electrical and Computer Engineering, Aristotle University, 54124 Thessaloniki, Greece

Received 27 March 2016; Accepted 16 May 2016

Academic Editor: Francesco Profumo

Copyright © 2016 Spyros I. Gkavanoudis 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

In a low-voltage islanded microgrid powered by renewable energy sources, the energy storage systems (ESSs) are considered necessary, in order to maintain the power balance. Since a microgrid can be composed of several distributed ESSs (DESSs), a coordinated control of their state-of-charge (SoC) should be implemented, ensuring the prolonged lifespan. This paper proposes a new decentralized control method for balancing the SoC of DESSs in islanded microgrids, without physical communication. Each DESS injects a current distortion at 175 Hz, when its SoC changes by 10%. This distortion is recognized by every DESS, through a phase-locked loop (PLL). In order to distinguish the origin of the distortion, each DESS injects a distortion of different time duration. This intermediate frequency has been selected in order to avoid the concurrence with the usual harmonics. The DESSs take advantage of this information and inject a current proportional to the SoC. Implementing this strategy, a comparable number of charging/discharging cycles for each DESS are achieved. Furthermore, an active filter operation, implemented in the rotating frame for each individual harmonic, is integrated in the control of the distributed generation units, supplying nonlinear loads with high-quality voltage. The effectiveness of this method is verified by detailed simulation results.

#### 1. Introduction

The tremendous increase in renewable generation penetration level in the electrical grids has favored the development of the microgrid concept, which is defined as a cluster of interconnected distributed generation units (DGUs), loads, and ESSs [1]. A microgrid is able to operate in an uninterruptable way under grid-connected or islanded mode.

Different approaches concerning the effective energy management of such microgrids have been proposed in the literature. Most of them suggest a communication-based management system with central and local controllers [2, 3]. However, if the communication is lost, the controllers may generate undesirable control commands. Therefore, dependence on communication reduces the reliability of the control strategy. On the other hand, decentralized methods without communication rely on the power sharing among the micro-sources by taking into account only local measurements. A control method without communication is the droop control method, where the DGUs emulate the operation of parallel synchronous generators [4].

In the technical literature, the critical role of the energy storage in microgrids is focused on the regulation of the voltage and frequency [5] and on preserving the power balance due to the intermittent operation of the renewable energy sources (RESs) [6]. Furthermore, other ancillary functions of the energy storage [7] may include the low-voltage ride-through (LVRT) capability, load leveling, peak shaving, and operating reserve. Integration of ESSs in microgrids allows flexibility but adds further complexity in the energy management and control. A crucial parameter to be considered when designing the energy management control is the SoC of the ESS. The SoC is the ratio of the remaining capacity to its nominal one.

The conventional control methods do not take into account the different capacities of the DESSs, leading to a significant differentiation in their lifespans. Therefore, the applied power sharing methodology in an islanded microgrid should consider the SoC of each DESS as a crucial parameter. The target is to maintain a comparable number of charging/discharging cycles among all energy storage devices. Most of the technical references on SoC balancing refer to DC microgrids with distributed batteries [8, 9]. In [10] a power sharing control for an autonomous AC microgrid with batteries is presented. Although the sharing is based on the battery SoC, extensive communication is proposed, using central and local controllers.

Another important issue of the islanded microgrid concerns the voltage quality. Since the common loads feature nonlinearity, the control of DGUs should incorporate strategies for the attenuation of the most common harmonics. Assuming only three-phase symmetrical loads, the harmonics of third order are absent. The literature deals with the harmonic problem by placing extra converters parallel to the nonlinear loads [11, 12] or incorporating the active filter operation in the internal control of the DGUs [13, 14]. On the one hand, the placement of extra devices raises the initial investment of the microgrid, while it limits the further addition of nonlinear loads. On the other hand, the active filter control seems more effective. In [15], current harmonic loops are added in the current control, while in [16] the authors propose the distorted current waveform for the harmonics in phase with the voltage at the terminals of the DGU. In [17], an enhanced virtual impedance control on selected harmonics is implemented; however low-bandwidth communication is regarded. Other methodologies propose the use of resistive virtual impedances [18, 19]. Nevertheless, the voltage is severely distorted. In [20], the control strategy employs negative virtual harmonic impedance, in order to compensate the effect of line impedance on harmonic power delivery. The harmonic attenuation methods proposed in the literature are implemented in either or *αβ* stationary frame. In first case, independent control systems for each phase are required, provoking a more complex control scheme. In case the *αβ* stationary frame is used, the resonant frequency of the proportional-resonant (PR) controllers should be updated for every frequency change.

This paper proposes a new decentralized method for controlling the SoC of DESSs in an islanded AC microgrid with linear and nonlinear loads. A primary aim of the control is to balance the number of charging/discharging cycles of the DESSs, by adjusting their power sharing. For this reason, a power sharing strategy according to the SoC of each DESS is implemented. This method is based on the superstition of a distortion at 175 Hz in the injected output current of each DESS, when the SoC changes by 10%. In order to recognize the origin of the distortion, different time durations are used for each DESS. In this way, each DESS adapts the power injection according the SoC in respect to the SoC of the other DESSs. An intermediate frequency at 175 Hz has been selected in order to avoid the concurrence with harmonics of the basic frequency of 50 Hz. Moreover, unlike the high frequency (>3 kHz) power line communication (PLC) methods [21], where the signals might be attenuated by the low-pass filters, a signal at 175 Hz can effectively be propagated in the microgrid bus. The magnitude of the injected distortion has been selected in compliance with the permissible limits for harmonic distortion in the LV grids. The proposed method is integrated in the control of each DESS, without requiring any additional communication infrastructure leading in reduced installation cost. A second target of the proposed control is to deal with the nonlinear loads. A nonlinear method for attenuating the 5th, 7th, 11th, and 13th harmonic of the voltage is applied. Unlike the presented methodologies in the literature, the proposed method is implemented in the rotating frame for each individual harmonic order. Its effectiveness is highlighted in islanded microgrids, due to the operation with variable frequency around the preassigned limits, without needing any further control update.

The rest of the paper is structured as follows. In Section 2, the microgrid decentralized control strategy for power sharing and harmonic attenuation is presented in detail. In Section 3, the proposed control strategy is described, while in Section 4 analytical simulation tests validate the methodology in an islanded microgrid with linear and nonlinear loads, being fed by PVs and batteries.

#### 2. Microgrid Decentralized Control Strategy

##### 2.1. Droop Control Method

The microgrid can be consisted of different type of DGUs, which are interfaced to the microgrid via DC/AC or AC/DC/AC converters. In order to avoid extra communication infrastructure, a decentralized strategy, based on the droop control method, is applied for the power sharing among the DGUs. Each DGU determines the frequency and the voltage magnitude of the common AC bus according to the injected active and reactive power, respectively. In this way, the DGUs emulate the parallel operation of synchronous generators. Considering inductive line impedances, the microgrid frequency is determined by the active power , while the node voltage is determined by the reactive power , as it is described by the following equations: where and are the output voltage frequency and magnitude at no load, and the droop coefficients, and the average active and reactive power, and and the derivative droop coefficients, respectively. The active and reactive output power of the inverter can be adjusted by means of the droop coefficients, while the derivative terms absorb the abrupt variations, preserving the power sharing dynamic stability [22].

When the microgrid consists of RESs, the droop coefficients are adjusted in accordance with the available power . Thus, the droop coefficients are calculated by the following equations: where and are the maximum deviation of the frequency of the magnitude of the voltage as it is defined by the Standard EN 50160.

However, in low-voltage microgrids the line impedance rarely is purely inductive. In most cases it consists of both resistive and inductive part [23]. With complex impedances, an accurate power sharing cannot be achieved, due to the coupled active and reactive power characteristic of the system. As a result, reactive currents are circulating among the DERs. To overcome this problem, the droop control method is modified with the virtual impedance control, as it is well stated in the literature [24]. The control scheme of each DGU is presented in Figure 1.