An Improved Droop Control Strategy for Parallel Inverters in Microgrid


This paper proposes an improved droop control strategy for parallel invert er s in micro grid. It employs a double closed loop control method based on measured voltage feedback aiming at the voltage sags problem caused by the introduction of virtual impedance, for invert er control in the micro grid. Firstly, the frequency response character of the closed loop transfer function with virtual impedance and the inductive variation was analyzed in the frequency domain, indicating that the improved droop control method is necessary.


an improved droop control strategy based on inductive virtual impedance with measured voltage feedback was proposed. Lastly, the Mat lab/Sim u link simulation results show that the improved droop control strategy can not only solve the output voltage sags of the invert er, but also improve the accuracy of power allocation of droop control, maintain the system voltage and frequency stability. It is proved that the improved droop control strategy is effective.

  1. Micro grid
  2. Invert er
  3. Droop control
  4. Virtual impedance
  5. Voltage sags




Fig. 1 Block Diagram of Droop Control Based on Inductive Virtual Impedance



Fig. 2 Operation Characteristics of Independent Micro grid in the Case of Casting or Cutting Load

Fig. 3 Operation Characteristics of D G 1 Independent Micro grid in D G 2 Casting or Cutting State



Aiming at the shortage of the traditional droop control strategy, this paper proposes an improved Q-V droop control strategy based on the inductive virtual impedance. Firstly, the frequency response curves of the closed loop transfer function of invert er control system based on inductive virtual impedance and the inductive virtual impedance variation on closed loop transfer function are analyzed in the frequency domain, indicating that the improved Q-V droop control method is necessary .


Then, the simulation experiments are built in parallel invert er s operation model of two distributed generations, simulation results of two kinds of operating conditions show that the proposed improved Q-V droop control strategy can eliminate the problem of voltage sags caused by the introduction of the inductive virtual impedance, improve the power allocation accuracy of droop control, and maintain the stability of the system voltage and frequency, so as to ensure the power supply quality of the independence micro grid system. Simulation results show the effectiveness of the improved Q-V droop control strategy.


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Design of an Efficient Dynamic Voltage Restorer for Compensating Voltage Sags, Swells, and Phase Jumps


This paper presents a novel design of a dynamic voltage restorer (DVR) which mitigate voltage sags, swell and phase jumps by injecting minimum active power in system and provides the constant power at load side without any disturbance. The design of this compensating device presented here includes the combination of P WM-based control scheme, d q 0 transformation and PI controller in control part of its circuitry, which enables it to minimize the power rating and to response promptly to voltage quality problems faced by today’s electrical power industries.


immense knowledge of power electronics was applied in order to design and model of a complete test system solely for analyzing and studying the response of this efficient DVR. In order to realize this control scheme of DVR MAT LAB/SIM U LINK atmosphere was used. The results of proposed design of DVR’s control scheme are compared with the results of existing classical DVR which clearly demonstrate the successful compensation of voltage quality problems by injecting minimum active power.



 Fig.1. Block Diagram of DVR



Fig.2.Source Voltage with Sag of 0.5 p.u.

Fig.3.Load Voltage after Compensation through proposed DVR

Fig.4. Load Voltage after Compensation through classical DVR

Fig.5. Voltage injected by proposed DVR as response of Sag

Fig.6.Source Voltage with Swell of 1.5 p.u.

Fig.7. Load Voltage after compensation through proposed DVR

Fig.8. Load Voltage after Compensation through classical DVR

Fig.9. Voltage injected by DVR as response of Swell

Fig.10. .Load Voltage after Compensation of Phase jump

Fig.11. d q 0 form of difference voltage obtained by proposed DVR

Fig.12. d q 0 form of difference voltage obtained by classical DVR


As the world is moving towards modernization, the most essential need that it has is of an efficient and reliable power of excellent quality. Nowadays, more and more sophisticated devices are being introduced, and their sensitivity is  dependent upon the quality of input power. Because even a slight disturbance in power quality, such as Voltage sags, voltage swells, and harmonics, which lasts in tens of milliseconds, can result in a huge loss because of the failure of end use equipment s. For catering such voltage quality problems an efficient DVR is proposed in this paper with the capability of mitigating voltage sags, swells, and phase jumps by injecting minimum active power hence decreasing the VA rating of DVR.


[1] K u m  a r, R. A nil, G. Siva K u mar, B. K a l y an K u mar, and Ma he sh K. Mi s h  R a. “Compensation of voltage sags and harmonics with phase jumps through DVR with minimum VA rating using Particle Swarm Optimization.” In Nature & Biologically Inspired Computing, 2009. NaBIC 2009. World Congress on, pp. 1361-1366. IEEE, 2009.
[2] Song song, Chen, Wang Jian wei, Ga o Wei, and Hu Xiaoguang. “Research and design of dynamic voltage restorer.” In Industrial Informatics (INDIN), 2012 10th IEEE International Conference on, pp. 408-412. IEEE, 2012.
[3] A. Bendre, D. Divan, W. Kranz, and W. E. Brumsickle, “Are Voltage Sags Destroying Equipment?,” IEEE Industry Applications Magazine, vol. 12, pp. 12-21, July-August 2006.


Modeling And Simulation For Voltage Sags/Swells Mitigation Using Dynamic Voltage Restorer (Dvr)


 This project describes the problem of voltage sags and swells and its severe impact on non linear loads or sensitive loads. The dynamic voltage restorer (DVR) has become popular as a cost effective solution for the protection of sensitive loads from voltage sags and swells. The control of the compensation voltages in DVR based on dqo algorithm is discussed. It first analyzes the power circuit of a DVR system in order to come up with appropriate control limitations and control targets for the compensation voltage control. The proposed control scheme is simple to design. Simulation results carried out by Matlab/Simulink verify the performance of the proposed method .


  1. DVR
  2. Voltage sags
  3. Voltage swells
  4. Sensitive load





Figure 1: Schematic diagram of DVR




Fig.2 Flow Chart Of Feed forward Control Technique For DVR Based Ob DQO Transformation

Three-phase voltages sag:

Figure 3. Three-phase voltages sag: (a)-Source voltage,(b)-Injected voltage, (c)-Load voltage

Single-phase voltage sag

Figure.4. Single-phase voltage sag: (a)-Source voltage, (b)-Injected voltage, (c)-Load voltage

Three-phase voltages swell

Figure.5.Three-phase voltages swell: (a)-Source voltage, (b)-Injected voltage, (c)-Load voltage

Two-phase voltages swell

Figure. 6. Two-phase voltages swell: (a)-Source voltage, (b)-Injected voltage, (c)-Load voltage



 The modeling and simulation of a DVR using MATLAB/SIMULINK has been presented. A control system based on dqo technique which is a scaled error of the between source side of the DVR and its reference for sags/swell correction has been presented. The simulation shows that the DVR performance is satisfactory in mitigating voltage sags/swells.



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  • Modeling And Simulation For Voltage Sags/Swells Mitigation Using Dynamic Voltage Restorer (Dvr)