A dual control strategy for power sharing improvement in islanded mode of AC microgrid
© The Author(s) 2018
Received: 17 August 2017
Accepted: 28 March 2018
Published: 20 April 2018
Parallel operation of inverter modules is the solution to increase the reliability, efficiency, and redundancy of inverters in microgrids. Load sharing among inverters in distributed generators (DGs) is a key issue. This study investigates the feasibility of power-sharing among parallel DGs using a dual control strategy in islanded mode of a microgrid. PQ control and droop control techniques are established to control the microgrid operation. P-f and Q-E droop control is used to attain real and reactive power sharing. The frequency variation caused by load change is an issue in droop control strategy whereas the tracking error of inverter power in PQ control is also a challenge. To address these issues, two DGs are interfaced with two parallel inverters in an islanded AC microgrid. PQ control is investigated for controlling the output real and reactive power of the DGs by assigning their references. The inverter under enhanced droop control implements power reallocation to restore the frequency among the distributed generators with predefined droop characteristics. A dual control strategy is proposed for the AC microgrid under islanded operation without communication link. Simulation studies are carried out using MATLAB/SIMULINK and the results show the validity and effective power-sharing performance of the system while maintaining a stable operation when the microgrid is in islanding mode.
The key issues related to microgrids is the parallel operation of different generations in islanded mode . Distributed generators (DGs) during islanding operation of the microgrid are commonly coupled via inverters to an AC distributed system. Various techniques have been introduced for controlling the inverter parallel operation or power-sharing . A microgrid system requires the operation, control structure, power and voltage regulation and energy management . The basic structure of a microgrid contains one or several renewable energy sources (RES), different types of load and energy backup systems integrated together . The actual performance observed when the microgrid operates in islanded mode was presented in . Microgrid control and operation, as well as switching among the different operation modes are the main challenges. For increasing the reliability and decreasing the transmission losses the whole microgrid system need be designed to operate under grid connected or islanded mode .
In grid connected operation, the microgrid supplies energy to the grid or desired load, charging backup, etc. Hence, the inverter acts as voltage supporter and the distributed power is handled through real power reference which is linked to the generated energy. However, the power flow structure becomes an important aspect .
Moreover, in islanded mode, a decentralized droop control method is commonly used with wireless control operation. It is suitable when a number of distributed generators are placed far from each other with no communication connection among them . However, droop control has numerous limitations and challenges, such as voltage drop during load change and frequency variation resulting from the droop principles and is sensitive to distribution line impedance [9, 10].
Furthermore, as the inverters under PQ droop control can operate independently without communication, system redundancy is improved. In case of an inverter failure, the others can continue operating under normal conditions without being affected. The key purpose of this control methodology is to control the whole system in which the DGs are responsible for power delivery .
This study considers that all the DGs work as the inverter interface. The losses of the inverters and harmonics are negligible, and two different control strategies for the islanded AC microgrid are presented. The PQ control goal is to adjust the power tracking convergence and to achieve a fast dynamic response. Moreover, the P-f/Q-E droop controller delivers the real and reactive power and provides voltage regulation. A decentralized enhanced droop control method is proposed with frequency restoration scheme (FRS) to restore the frequency and provide the exact real and reactive power.
The rest of the paper is organized as follows. Section II describes the significance of the presented microgrid system, and Section III analyzes PQ control of the inverter and its capability in grid connected operation. Section IV presents the droop method with frequency restoration scheme. In order to achieve power-sharing in a fully decentralized way, Section V presents the combined PQ and droop control strategies and its operation for improving power-sharing in islanding operation of the AC microgrid. Section VI discusses the simulation results while Section VII draws the conclusion.
1.1 Significance of the presented system
When the microgrid inverters reach the PWM saturation limit and rated power, reactive power demand is fulfilled by the grid-connected droop-controlled inverter. In case of increment in active power, the frequency of the microgrid will drop so the real power of DG1 will increase according to its droop characteristic and at the same time, the reactive power of the parallel DGs will be rearranged.
2.1 PQ controlled inverter
In PQ control, the inverter is used to deliver the required real and reactive power according to their set-points. The controller consists of current and power control loops. The inner current loop can rapidly respond to disturbances including input voltage fluctuation, converter dead time and inductance parameter variation. Therefore, the performance of the system is significantly improved . A phase-locked loop (PLL) is used to synchronize the inverter to the microgrid. The RES provides constant real power and reactive power to the grid by PQ control. The operation of PQ control based on DQ reference frame which defines the components of the d-axis and q-axis AC currents.
When the microgrid is in grid-connected mode, the inverter is in current control mode so the references of the frequency and voltage are both measured by the PLL which also provides the orientation of the synchronous rotating coordinate system.
2.2 Droop controlled inverter
Equations (13) and (14) show that the power angle is dependent on real power and the voltage difference depends on reactive power. Thus, the angle can be controlled by regulating the real power while the inverter voltage is controlled by the reactive power.
The reactive power control loop is linked to voltage amplitude and the reactive power-sharing is realized with the Q-E droop control and is affected by voltage drop and load condition. Thus, (16) becomes
The voltage drop occurs at the connecting point because the generated reactive power of DG1 is higher than that of DG2. Figure 6 shows the control diagram of the enhanced droop control. The output voltage is fed to the inverter through the synchronous reference frame. The output power of the DG is measured and filtered through the low pass filter . The filtered power is given to the droop controller to create voltage magnitude E and the frequency ω. The reference voltage V ref = sinθ is produced in the synchronous reference frame by ω and E. Finally, V ref is applied to the PWM modulator to generate the PWM signal for the inverter.
In the droop control strategy, the change in load is managed by the distributed generators in a prearranged way and decentralized control of parallel inverters is designed based on the use of the system frequency as a communication link within the microgrid. This method has two problems, i.e. power coupling and slop selection. However, power coupling can be avoided through some improve strategies such as virtual power frame transformation or virtual output impedance, though the swapping between the power-sharing and voltage, amplitude, and frequency deviation depends on the selection of the droop coefficient m and n.
3 Dual control performing criteria of MG
The study is conducted to observe the behavior of the microgrid in islanded operation under different control structures of the parallel inverters. The parallel inverters under combined dual control strategy are unable to perform their task coordinately. To address this issue an enhanced droop controller is proposed to synchronize the inverters at the time when they are connected. In the islanding operation, both voltage and frequency depend on the load. In droop control method the supervisory droop ensures adequate load sharing. However, this results in voltage and frequency variation which may cause undesirable operation of the microgrid. In this section, real and reactive load-sharing issues are discussed for three-phase parallel inverters in the islanded mode of AC microgrid with combined PQ and droop control strategy.
AC Bus Voltage
P-f and Q-E Slope Droop
m = 0.0015, n = 0.0001
Resistance of LC Filter
DG output power
Inner current loop
KP = 2 Ki = 0.1
KP = 0.3 Ki = 50
T = 0.3 s
4 Results and discussion
In this paper, the enhanced droop and PQ control strategies for controlling parallel DGs in islanding mode of AC micro-grids were investigated to achieve flexible power regulation. The main advantage of this dual control strategy is to enable operation without any communication between the parallel DGs. The power tracking error for PQ control based inverters was investigated and the enhanced droop control implemented with predefined droop characteristics for power reallocation was proposed. To improve and restore the frequency, a frequency restoration scheme (FRS) implemented among the distributed generators was developed. The proposed droop controller provides stable operating under different control strategies in islanded operation and the DG voltage can quickly respond to the required voltage demand. The PQ controller can effectively track the active and reactive power and the droop control provides voltage control in islanded mode. The simulation results obtained from MATLAB/SIMULINK verified the stability of the load voltage and frequency.
This work was supported in part by the National Natural Science Foundation of China under Grant 51477098 and National Key R&D Program of China (2016YFB0900504).
SH performed the simulation and wrote the draft, GL corresponding, engaged in modifying the paper and submited it to the PCMP. KW checked the grammar and writing of the paper. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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