Inam Ullah Nutkani

Droop Scheme With Consideration of Operating Costs

Although many droop schemes have been proposed for distributed generator (DG) control in a microgrid, they mainly focus on achieving proportional power sharing based on the DG kVA ratings. Other factors like generation costs, efficiencies, and emission penalties at different loads have not been considered even though they are different for different types of DGs. This letter thus proposes an alternative droop scheme, which can better distinguish the different operating characteristics and objectives of the DGs grouped together in a weighted droop expression. The power sharing arrived in the steady state will meet the intended objectives, as demonstrated in the experiment with a defined objective of minimizing the total microgrid operating cost.

Cost-Prioritized Droop Schemes for Autonomous AC Microgrids

This paper presents two cost-prioritized droop sche- mes for distributed generators (DGs) in a rural or islanded microgrid. Dispatch prioritization of the schemes allows autonomous identification of the appropriate DGs for generation, in accordance to the overall load conditions of the microgrid. The result is a lower total generation cost for the microgrid when compared to other droop schemes. An experimental system has been implemented and tested with results showing a higher saving achieved by the proposed schemes.

Autonomous droop scheme with reduced generation cost

Droop scheme has been widely applied to the control of Distributed Generators (DGs) in microgrids for proportional power sharing based on their ratings. For standalone microgrid, where centralized management system is not viable, the proportional power sharing based droop might not suit well since DGs are usually of different types unlike synchronous generators. This paper presents an autonomous droop scheme that takes into consideration the operating cost, efficiency and emission penalty of each DG since all these factors directly or indirectly contributes to the Total Generation Cost (TGC) of the overall microgrid. Comparing it with the traditional scheme, the proposed scheme has retained its simplicity, which certainly is a feature preferred by the industry. The overall performance of the proposed scheme has been verified through simulation and experiment.

Distributed Operation of Interlinked AC Microgrids with Dynamic Active and Reactive Power Tuning

Microgrids are small grids formed by clustering modern generating sources, storage systems, and loads together. Being independent, the formed microgrids can, in principle, operate at their own preferred voltages and frequencies. Tying them to the mains grid or another microgrid would therefore require some interlinking power converters, whose control should preferably be autonomous without depending on fast communication links. Contributing to this theme of research, a distributed power management scheme has been proposed in this paper for interlinking two or more independent microgrids operating at different voltages and frequencies. The proposed scheme allows sources in the microgrids to concentrate more on active power harnessing, while the interlinking converters focus more on meeting the load reactive demand. If necessary, backup active power from an underloaded microgrid can also be transferred to an overloaded microgrid, allowing it to supply loads in excess of its rated capacity. The performance of the proposed scheme has already been tested in experiment.

Decentralized Economic Dispatch Scheme With Online Power Reserve for Microgrids

Decentralized economic operation schemes have several advantages when compared with the traditional centralized management system for microgrids. Specifically, decentralized schemes are more flexible, less computationally intensive, and easier to implement without relying on communication infrastructure. Economic operation of existing decentralized schemes is also usually achieved by either tuning the droop characteristics of distributed generators (DGs) or prioritizing their dispatch order. For the latter, an earlier scheme has tried to prioritize the DG dispatch based on their no-load generation costs. Although the prioritization works well with some generation costs saved, its reliance on only the DG no-load generation costs does not allow it to operate well under certain conditions. This paper thus presents a more comprehensive economic dispatch scheme, which considers the DG generation costs, their power ratings, and other necessary constraints, before deciding the DG dispatch priorities and droop characteristics. The proposed scheme also allows online power reserve to be set and regulated within the microgrid. This, together with the generation cost saved, has been verified experimentally under different reserve, load, and DG conditions.

Linear Decentralized Power Sharing Schemes for Economic Operation of AC Microgrids

This paper presents two linear power sharing schemes for distributed generators (DGs). The purpose is to reduce the total generation cost (TGC) of the considered autonomous microgrid. The saving in cost is realized by tuning the DG droop gradients in accordance to their respective maximum or mean generation costs. Compared with the existing nonlinear cost-based schemes, the proposed schemes use linear droop functions, which are easier to tune and implement in practice for a greater reduction in TGC. The proposed schemes have been tested experimentally with an example microgrid powered first by dispatchable DGs only, and then by a combination of dispatchable and nondispatchable DGs. Experimental results obtained have demonstrated the anticipated outcomes under different operating conditions.

Cost-based droop scheme for DC microgrid

DC microgrids are gaining interest due to higher efficiencies of DC distribution compared with AC. The benefits of DC systems have been widely researched for data centers, IT facilities and residential applications. The research focus, however, has been more on system architecture and optimal voltage level, less on optimized operation and control of generation sources. The latter theme is perused in this paper, where cost-based droop scheme is proposed for distributed generators (DGs) in DC microgrids. Unlike traditional proportional power sharing based droop scheme, the proposed scheme considers the generation costs of DGs and dynamically tunes their droop gradients to produce more power from less costly DGs and vice versa. The proposed scheme is fully autonomous, simple to implement in dispatchable and non-dispatchable sources coupled with storage, support islanded and grid-connected operation. Most importantly, the proposed scheme can reduce overall total generation cost in DC microgrids without centralized controller and communication links. The performance of the proposed scheme has been verified under different load conditions.

Cost-prioritized droop schemes for autonomous microgrids

Droop schemes for parallel source control usually aims for proportional power sharing tuned in accordance to the source ratings. This works fine for sources with close similarity, but not really for an autonomous microgrid where different types of Distributed Generators (DGs) may be present without centralized optimal dispatch control. To better adapt to this non-uniformity, an alternative viewpoint based on reducing the Total Generation Cost (TGC including fuel cost, emission penalty and other operational concerns) of the microgrid is discussed, from which two new cost-prioritized droop schemes are developed. The schemes operate by tuning the dispatch priorities of the DGs and curve shapes of their resulting active power versus frequency plots. Their effective reduction of TGC has been verified through simulation.

Autonomous economic operation of grid connected DC microgrid

This paper presents an autonomous power sharing scheme for economic operation of grid-connected DC microgrid. Autonomous economic operation approach has already been tested for standalone AC microgrids to reduce the overall generation cost and proven a simple and easier to realize compared with the centralized management approach. In this paper, the same concept has been extended to grid-connected DC microgrid. The proposed economic droop scheme takes into consideration the power generation cost of Distributed Generators (DGs) and utility grid tariff and adaptively tunes their respective droop curves. The scheme can be realized with the local information (terminal voltages and currents) without centralized controller and high bandwidth communication links. Microgrid voltage can be maintained within the define limit (±5%), which, if required, can be restored back to the nominal value through the proposed secondary control. The performance of the proposed scheme has been verified for the example grid-connected DC microgrid.

Power flow control of interlinked hybrid microgrids

Microgrids are small reliable grids formed by clustering distributed sources and loads together. They can, in principle, operate at different voltages and frequencies like 50 Hz, 60 Hz, 400 Hz or even dc. Tying them either to the mains grid or among themselves would certainly require some interlinking power converters, whose active and reactive power flow control should preferably be done autonomously without demanding communication links. This theme of investigation is now pursued, which upon performed appropriately, will result in more robust hybrid microgrids with many advantages like higher efficiency and lower reserve requirement. The theoretical findings concluded have already been verified through simulation and a scaled-down experimental system.