Frequency-based control of islanded microgrid with renewable energy sources and energy storage

J. Mod. Power Syst. Clean Energy (2016) 4(1):54–62 DOI 10.1007/s40565-015-0178-z Frequency-based control of islanded microgrid with renewable energy sources and energy storage Konstantinos O. OUREILIDIS1, Emmanouil A. BAKIRTZIS1, Charis S. DEMOULIAS1 Abstract When a microgrid is mainly supplied by renewable energy sources (RESs), the frequency deviations may deteriorate significantly the power quality delivered to the loads. This paper proposes a frequency-based control strategy, ensuring the frequency among the strict limits imposed by the Standard EN 50160. The frequency of the microgrid common AC bus is determined by the energy storage converter, implementing a proposed droop curve among the state of charge (SoC) of the battery and the frequency. Therefore, the information of the SoC becomes known to every distributed energy resource (DER) of the microgrid and determines the active power injection of the converter-interfaced DERs. The active power injection of the rotating generators remains unaffected, while any mismatch among the power generation and consumption is absorbed by the energy storage system. Finally, in case of a solid short-circuit within the microgrid, the energy storage system detects the severe voltage decrease and injects a large current in order to clear the fault by activating the protection device closer to the fault. The proposed control methodology is applied in a microgrid with PVs, wind CrossCheck date: 15 October 2015 Received: 12 March 2015 / Accepted: 28 September 2015 / Published online: 16 January 2016 Ó The Author(s) 2016. This article is published with open access at Springerlink.com & Konstantinos O. OUREILIDIS Emmanouil A. BAKIRTZIS Charis S. DEMOULIAS 1 Department of Electrical and Computer Engineering, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece 123 generators and a battery, while its effectiveness is evaluated by detailed simulation tests. Keywords Microgrid, Frequency control, Renewable energy sources, Energy storage system, SoC control 1 Introduction As renewable energy sources (RESs) integration has considerably increased, the microgrid concept has been developed. According to the U.S. Department of Energy (DOE) [1], a microgrid is defined as a cluster of DERs and local loads connected to the utility grid, which can operate in parallel to the grid or isolated as an island. The microgrid concept also includes the integration and control of storage assets in order to ensure a high power quality [2, 3]. In the literature, the critical role of the energy storage system is focused on the regulation of the voltage and frequency [4] and on preserving the power balance due to the intermittent operation of the RESs [3, 4]. Furthermore, other ancillary functions of the energy storage may include the low-voltage ride-through (LVRT) capability, load leveling, peak shaving and operating reserve [3, 5]. When the microgrid is comprised of RESs and energy storage systems in island operation mode, the energy storage usually acts as grid-forming source and regulates the common AC bus appropriately, while the RESs are controlled to inject the available power to the microgrid [6]. However, this approach may lead the SoC to unsafe operation, provoking a damage in the energy storage. Furthermore, an active power imbalance among the generation and the consumption may deteriorate the microgrid frequency regulation [7]. Therefore, the control strategy should take into account the SoC control, ensuring a prolonged lifetime for the battery. Frequency-based control of islanded microgrid with renewable energy sources and energy storage Since in island operation mode, the frequency is no longer imposed by the utility grid, several control strategies propose the implementation of a secondary control for frequency regulation in order to ensure a frequency within a stipulated band [8]. This supervisory control level can be implemented either in a centralized or decentralized way [9, 10]. In case of implementing a centralized control method, a microgrid central controller (MCC) modifies the control of the DERs appropriately, by gathering measurements from local controllers [10]. In this control approach, the communication is considered necessary. However, the system reliability is reduced, since it is dependent on the operation of a physical communication system. On the other hand, the decentralized approach aims at providing the highest possible autonomy [11, 12]. Nevertheless, in many cases the communication is still considered a basic principle of the control. In [13, 14], frequency is used as a communication agent for the energy control in an islanded microgrid, with no need of further communication. The goal focuses on the adjustment of the conventional droop method, considering the frequency variation and the SoC of the energy storage. In [15], a decentralized energy management integrated in a microgrid with PVs and batteries is examined. The SoC of the battery determines the microgrid frequency, nevertheless additional control schemes are needed to achieve coordination with other kind of DERs. In [16], the frequency is also used as a communication parameter of the SoC of the energy storage system in a microgrid with converter-interfaced DERs. However, all the connected sources are considered as converter-interfaced DERs, while, due to the presence of secondary control and the associated communication network, only a limited frequency deviation is used in the control strategy implementation. Moreover, in [16], no fault-clearance methods are examined, and no voltage regulation is investigated. A power control strategy focusing on the control of the SoC of the energy storage is also proposed in [17]. The frequency is used again as a communication signal for sending the information of charging/discharging to all DERs. However, only converter-interfaced generation units are considered. This paper investigates the case of a microgrid in a small Greek island, which is currently supplied by conventional power sources. The conventional power sources are synchronous generators, driven by diesel engines; three generators, each one rated at 220 kVA, 400 V, 50 Hz and one generator rated at 90 kVA, 400 V, 50 Hz. The loads are concentrated, representing the small town consumption. According to measurements, the peak load is 350 kW (15min average power) during summer period, while it is reduced to 70 kW during the winter. The annual energy consumption corresponds to 1020 MWh. 55 Since the most abundant RESs in Greece are wind and solar power, this case study proposes the replacement of the conventional diesel-driven generators with a microgrid consisting of a 230 kWp PV installation and two asynchronous wind generators (WGs) of 275 kW each one. The energy production of the RESs cover in average 90 % of the total load energy consumption. Taking into account a projected 50 % increase of the load demand, the total annual energy consump (...truncated)