Conventional Droop Control Scheme Research Articles

Droop control method for load share and voltage regulation in high-voltage microgrids

When the line impedance is considered in the microgrid, the accuracy of load sharing will decrease. In this paper, the impact of line impedance on the accuracy of load sharing is analyzed. A robust droop control for a high-voltage microgrid is proposed based on the signal detection on the high-voltage side of the coupling transformer. For a high-voltage microgrid, the equivalent impedance of coupling transformer connecting distributed generator with the grid is usually the dominate factor. Compared with the conventional droop control strategy, the proposed control method in this paper detects the feedback signal from the high-voltage side of the coupling transformer. The impact of line impedance on the load sharing accuracy can be mitigated significantly. The proposed droop control only changes the detection point of the feedback signal, thus it is easy to be implemented. The PSCAD/EMTDC simulation results show the effectiveness of the proposed robust droop control concept in load sharing and voltage regulation with highly accuracy.

Frequency-Coordinating Virtual Impedance for Autonomous Power Management of DC Microgrid

In this paper, the concept of frequency-coordinating virtual impedance is proposed for the autonomous control of a dc microgrid. This concept introduces another degree of freedom in the conventional droop control scheme, to enable both time-scale and power-scale coordination in a distributed microgrid system. As an example, the proposed technique is applied to the coordinating regulation of a hybrid energy storage system composed of batteries and supercapacitors. With an effective frequency-domain shaping of the virtual output impedances, the battery and supercapacitor converters are designed to absorb low-frequency and high-frequency power fluctuations, respectively. In this way, their complementary advantages in energy and power density can be effectively exploited. Furthermore, the proposed concept can be integrated into a mode-adaptive power management framework with autonomous mode transitions. The entire solution features highly versatile functions based on fully decentralized control. Therefore, both flexibility and reliability can be enhanced. The effectiveness of the presented solution is verified by experimental results.

Mode-Adaptive Decentralized Control for Renewable DC Microgrid With Enhanced Reliability and Flexibility

A mode-adaptive decentralized control strategy is proposed for the power management of a dc microgrid with multiple renewable distributed generators and energy storage systems. In the presented solution, the dc bus voltage signal is used not only to enable power sharing among different sources, but also to designate microgrid operation modes and facilitate seamless mode transitions. With this mode-adaptive operation mechanism, a greater control freedom can be achieved than the conventional dc voltage droop control scheme. More importantly, this approach features fully self-disciplined regulation of distributed converters without an extra control center or communication link. Therefore, both reliability and flexibility can be enhanced. Meanwhile, a novel mode definition criterion is also provided to highlight the special characteristics of the dc microgrid which is different from an ac one. Three typical operation conditions are summarized according to which type of sources are dominating the power balance. Finally, the effectiveness of the proposed technique is verified experimentally based on a composite dc microgrid test system.

Robust Droop Controller for Accurate Proportional Load Sharing Among Inverters Operated in Parallel

In this paper, the inherent limitations of the conventional droop control scheme are revealed. It has been proven that parallel-operated inverters should have the same per-unit impedance in order for them to share the load accurately in proportion to their power ratings when the conventional droop control scheme is adopted. The droop controllers should also generate the same voltage set-point for the inverters. Both conditions are difficult to meet in practice, which results in errors in proportional load sharing. An improved droop controller is then proposed to achieve accurate proportional load sharing without meeting these two requirements and to reduce the load voltage drop due to the load effect and the droop effect. The load voltage can be maintained within the desired range around the rated value. The strategy is robust against numerical errors, disturbances, noises, feeder impedance, parameter drifts and component mismatches. The only sharing error, which is quantified in this paper, comes from the error in measuring the load voltage. When there are errors in the voltage measured, a fundamental tradeoff between the voltage drop and the sharing accuracy appears. It has also been explained that, in order to avoid errors in power sharing, the global settings of the rated voltage and frequency should be accurate. Experimental results are provided to verify the analysis and design.

Control of Parallel-connected Inverters to Achieve Proportional Load Sharing

AbstractIn this paper, a completely new concept has been brought to the controller design for inverters. The capacitor C of the inverter LC filter is regarded as a part of the load and, hence, the control plant for the inverter controller is just the inductor L. This reduces the order of the control plant to be 1, reduces the variables to be measured for feedback to two, and considerably simplifies the design and analysis of the controller. The inherent limitations of the conventional droop control scheme are then revealed and it is proved that parallel-connected inverters should have the same per-unit impedance in order for them to share the load accurately in proportional to their power ratings if the conventional droop control scheme is adopted. The droop controllers should also generate the same voltage set-point for the inverters. Both of these two are impractical and difficult to meet, which results in errors in proportional load sharing. An improved droop controller is then proposed to achieve accurate proportional load sharing. It is robust against parameter drifts and component mismatches. The strategy also reduces the output voltage drop due to the effect of loading and droop control so that the output voltage can be maintained within the desired range around the rated value. Experimental results are provided to verify the analysis and design. Copyright © 2011 IFAC.