Frequency-based control of islanded microgrid with renewable energy sources and energy storage
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 generators and a battery, while its effectiveness is evaluated by detailed simulation tests.
KeywordsMicrogrid Frequency control Renewable energy sources Energy storage system SoC control
As renewable energy sources (RESs) integration has considerably increased, the microgrid concept has been developed. According to the U.S. Department of Energy (DOE) , 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  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 . 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 . Therefore, the control strategy should take into account the SoC control, ensuring a prolonged lifetime for the battery.
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 . 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 . 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 , 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 , 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 , 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 . 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 (15-min average power) during summer period, while it is reduced to 70 kW during the winter. The annual energy consumption corresponds to 1020 MWh.
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 consumption yields 1530 MWh. The WGs cover approximately 80 % of the projected annual energy consumption (1224 MWh), while the rest 20 % (306 MWh) is covered by the PVs.
In order to ensure power balance among the power production and consumption without implementing any physical communication between the DERs in this small Greek island, this paper proposes a decentralized control strategy based only on local measurements for the effective frequency control in a microgrid with an energy storage system. The energy storage is considered as the converter-interfaced battery. According to the proposed methodology, the battery converter adjusts the microgrid frequency by considering its SoC and implementing a proposed droop curve. Thus, the information of the SoC is transferred indirectly through the frequency to all DERs, which determine their power output appropriately. In this way, the frequency always remains within the preassigned limits, while the SoC control ensures a prolonged battery lifetime. Finally, the case of a short-circuit is examined. The battery converter identifies the fault due to the deep voltage decrease and modifies its control in order to inject a large current. The fault is cleared within a few seconds.
The paper is organized as follows: Section 2 describes the microgrid modelling. Extended simulation results are demonstrated and discussed in Section 3. Finally, the control strategy advantages are concluded in Section 4.
2 System description and modelling
2.1 Battery energy storage system model
The bidirectional DC/AC three-phase converter is placed at the output of the filter. This converter is formed by a sinusoidal pulse width modulation (SPWM) and its purpose in steady-state mode is to control the magnitude and frequency of the AC bus, according to the proposed control strategy. For this reason, it can be characterized as a grid-forming converter . Furthermore, taking measurements from the battery bank about its SoC, the converter undertakes the charge and discharge operation of the battery.
The control strategy is based on forming an AC microgrid with frequency proportional to the SoC of the battery. Therefore, each DER measures the SoC of the battery indirectly, by measuring the frequency at their nodes. By implementing the proposed control strategy, no further communication signal is needed to be transferred, while the frequency of the main bus remains within the preassigned limits imposed by  for islanded grids.
The internal control of the converter is also presented in Fig. 3. In order to maintain the line voltage of the AC bus at 400 Vrms independently of the active and reactive power deviations among the DERs and the loads, a feedback control compares the magnitude of the measured voltage with the desired of 400 V. Since the nominal open-circuit voltage of the discharged battery is sufficiently high, the converter operates constantly at the linear region.
By implementing the proposed control strategy, the converter operates at four quadrants, allowing the charging and discharging of the battery bank. Finally, the converter absorbs or injects the appropriate amount of reactive power to maintain the voltage at 400 V.
2.2 PV system model
Therefore, the converter calculates the active power from the measured voltage and current components at the output filter and compares with the injected active power. The error signal is driven to a PI controller in order to determine the active power current component, ip. The deterioration of the injected power is implemented by shifting away from its MPP.
2.3 Wind generators
Number of poles
Moment of inertia
3 Simulation results
Transformer 0.4/15 kV parameters 
Three different simulation cases are examined, by using PSIM software. For simulation purposes, the nominal battery electrical charge is considered equal to 8100 A·s.
3.1 Case 1: constant wind speed
When the SoC of the battery reaches the value of 100 % and frequency is set to 51 Hz, at t = 1.5 s, the over/under frequency relay of the wind generators (R14) is activated and disconnects them from the microgrid, resulting in a new power balance. The active power deficit is covered by the battery, which changes its operation mode to discharge mode. As a result, the frequency decreases linearly and the converter of the PV increases the injected active power. At t = 6.6 s, the frequency becomes equal to 49.67 Hz (i.e. SoC = 60 %), and the wind generators reconnect to the microgrid, forming again a new combination of active power among the DERs. Since the microgrid load consumption can be fulfilled and there is a surplus of active power, the battery control changes to charging operation mode, as is shown in Fig. 7. The SoC of the battery increases, causing a respective increase in the common frequency. When the frequency exceeds 50.67 Hz, the PV converter decreases linearly its injected active power. When the frequency reaches 51 Hz (at t = 13.7 s), the PV zeroes its output, while the WGs disconnect from the microgrid, in order to protect the battery from overcharging.
According to the presented simulation results, the proposed control strategy ensures that the frequency remains inside the strict limits of , meanwhile, the SoC of the battery is controlled effectively. Thus, the load is supplied with a high power quality.
3.2 Case 2: variable wind speed
The second simulation test deals with the effect of the wind speed variation. The load is still considered equal to 400 kW, pf = 0.8, while the PV MPPT 100 kW. The battery SoC equals to 49.7 % forming an AC voltage of 49.32 Hz frequency. At t = 1 s, the WGs operate at 8 m/s and inject 300 kW active power to the microgrid. At t = 1.5 s, a wind speed oscillation with magnitude equal to 4 m/s and frequency of 1 Hz takes place. At this range of wind speed, the mechanical power of the WGs follows the wind power.
The simulation results presented in this case show that the proposed control strategy can absorb any large wind speed variations, while the frequency and the voltage still remains among the preassigned limits.
3.3 Case 3: short-circuit at the load
Subsequently, the battery converter returns to voltage-source control mode and the RESs reconnect to the microgrid. The microgrid returns to steady-state operation after a small transient effect. Since L2 is disconnected from the microgrid, the aggregated load current becomes lower than in the pre-fault operation. As a result, the total active power balance has changed and the surplus of active power is absorbed by the battery. The simulation results are presented in Fig. 9, proving the effectiveness of the proposed control strategy. As is figured, the battery discharges during the fault and its corresponding SoC is decreasing. After the fault clearance, the battery switches to charge mode and its SoC is increasing.
A new frequency-based control strategy for a microgrid with an energy storage is proposed. The frequency of the main AC bus is determined by the battery SoC in terms of implementing a proposed droop curve. The microgrid could consist of both converter-interfaced and rotating generators. In the examined microgrid, only RES have been considered, showing the effectiveness of the proposed methodology even under intermittency of the primary source. The aim of the proposed control strategy is to ensure a frequency deviation among the strict limits of the Standard EN 50160 for providing a high quality power to the connected loads. At the same time, the battery SoC is controlled in order to avoid overcharging or deep discharging. Finally, in case of a solid short-circuit within the microgrid, the battery undertakes to feed the fault with a high current and clear it. The proposed methodology is confirmed by detailed simulation results representing a microgrid in a Greek island. The microgrid contains a battery as an energy storage and is mainly fed by PVs and WGs.
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