Ramachandra Rao Kolluri

Power Sharing in Angle Droop Controlled Microgrids

Component mismatches and parameters drifts drastically affect stability and long-term operation of droop-controlled inverter-based microgrids. This paper analyzes and illustrates the impact of design variations and parameter drifts between angle droop controlled inverter-interfaced sources in a microgrid. It is shown that microgrid stability is very sensitive to parameter drifts, especially in frequency. A coordination control scheme that uses internode communications is proposed for improving the stability margin and ensuring the desired power sharing. Conditions for stability are derived and simulation results are presented to validate the performance of the proposal.

On making energy demand and network constraints compatible in the last mile of the power grid

Abstract In the classical electricity grid power demand is nearly instantaneously matched by power supply. In this paradigm, the changes in power demand in a low voltage distribution grid are essentially nothing but a disturbance that is compensated for by control at the generators. The disadvantage of this methodology is that it necessarily leads to a transmission and distribution network that must cater for peak demand. So-called smart meters and smart grid technologies provide an opportunity to change this paradigm by using demand side energy storage to moderate instantaneous power demand so as to facilitate the supply-demand match within network limitations. A receding horizon model predictive control method can be used to implement this idea. In this paradigm demand is matched with supply, such that the required customer energy needs are met but power demand is moderated, while ensuring that power flow in the grid is maintained within the safe operating region, and in particular peak demand is limited. This enables a much higher utilisation of the available grid infrastructure, as it reduces the peak-to-base demand ratio as compared to the classical control methodology of power supply following power demand. This paper investigates this approach for matching energy demand to generation in the last mile of the power grid while maintaining all network constraints through a number of case studies involving the charging of electric vehicles in a typical suburban low voltage distribution network in Melbourne, Australia.

Power sharing correction in angle droop controlled inverter interfaced microgrids

Power sharing between angle-droop controlled inverter-based sources largely depends on the line impedances and choice of droop coefficients. Simple power correction methods such as set-point correction and droop coefficient modification work satisfactorily for specific network topologies and require only limited amount of communication. However, their performance can be inadequate for microgrids with different topologies. In this work, we propose a topology-independent power sharing correction technique based on inter-node communications in order to eliminate the power sharing errors between inverters. We analyse stability and convergence of the proposed solution and present simulation results. Finally, we study the robustness of frequency-droop and angle-droop controlled systems with respect to unknown impedances and parameter uncertainties.

On the effect of component mismatches in inverter interfaced microgrids

In droop-controlled inverter-based microgrids, component mismatches and parameter drifts may have a significant effect on system stability. Analyzing this practical and important problem, it is shown that droop control is sensitive to such drifts and certain conditions have to be satisfied to maintain stability. Furthermore, these mismatches lead to a deviation in power sharing between the droop-controlled inverters even in the stable case. A central supervisory control that uses very low bandwidth communication is proposed to correct the resulting deviations. Simulation results of a simple microgrid example demonstrate the problem and the proposed solution.

Consensus only control in DC microgrids

Voltage droop control for DC microgrids has limited power/current sharing capabilities. Although droop complemented with a consensus control loop can achieve desired power/current sharing, the resulting voltage deviation may need correction. We propose a consensus only control that achieves proportional current sharing and simultaneously avoids large voltage deviations. The proposed technique is evaluated against several others using a constant impedance network model. The impacts of communication delays and power-sharing performances are also briefly discussed.

Scheduling Fast Local Rule-Based Controllers for Optimal Operation of Energy Storage

In most applications involving energy storage, multiple different opportunities to generate revenue or reduce cost must be stacked to optimise economic return. This requires repeated scheduling of the energy storage charging schedule in advance, across a finite future horizon. When forecasts and degradation models are incorporated, optimal solutions can become expensive to compute. Local conditions, however, can change instantly, and require fast response by local controllers to truly maximise the energy storage systems returned value. This work proposes a method for optimally operating energy storage in response to multiple value streams that finds the optimal solution over a future horizon, and then breaks this down into a schedule for a small set of simple, local, rule-based controllers. Each local controller is set up to maximise one particular value stream, and can respond to changing conditions on a timescale of milliseconds. The method is specifically designed for implementation on real systems, and successful at ensuring a near-optimal value return by the energy storage system.

Stability and active power sharing in droop controlled inverter interfaced microgrids: Effect of clock mismatches

Stability and power sharing properties of droop controlled inverter-based microgrid systems depend on various design factors. Little explored is the effect of component mismatches and parameters drifts on the stability, steady state behaviour and power sharing properties of these systems. In this paper, the behaviour of frequency droop controlled inverter based microgrid systems in the presence of non-identical clocks is analysed. It is shown that power sharing between converters in a microgrid can be sensitive to clock mismatches. Our proposal shows that a coordination control that uses sparse inter-node communications is useful in ensuring desired active power sharing. Conditions are derived to ensure stability in the presence of the proposed controller and simulation results are presented.