Power Sharing Correction in Angle Droop Controlled Inverter Interfaced Microgrids Ramachandra Rao Kolluri * , Iven Mareels Tansu Alpcan and Marcus Brazil Department of Electrical and Electronic Engineering The University of Melbourne Julian de Hoog and Doreen Thomas Department of Mechanical Engineering The University of Melbourne Abstract—Power sharing between angle-droop controlled inverter-based sources largely depends on the line impedances and choice of droop coefficients. Simple power correction meth- ods such as set-point correction and droop coefficient modifi- cation 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 com- munications 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. Index—Angle droop control, frequency droop control, power sharing error, consensus protocol, parameter mismatches, power converters, microgrids. I. I NTRODUCTION As distributed and renewable power generation become an increasingly common phenomenon, parts of the electricity net- work with high supply to demand ratio have the opportunity to operate in isolation from the main grid. Such isolated parts of the electricity networks are generally known as microgrids [1]. Since most of the renewable energy sources require an inverter interface to connect to the AC grid, microgrids based on renewable energy can be considered as low inertia networks. Achieving synchronization, voltage and frequency stability in such low-inertia systems is a challenging task. Inverter-interfaced microgrid design should be able to com- pensate for the loss of grid (scarcity in inertia) and also provide reliable power quality. Ideally, all the sources in the microgrid should be able to compensate for the loss of grid, i.e. regulate the voltage (V ) and frequency (f ) within permissible levels and avoid a single point-of-failure. Additionally, due to intermittent generation profiles of renewable energy sources, limited energy storage, and constrained inverter capabilities, power sharing between inverters is a desirable property. Motivated by synchronous generators, power sharing be- tween inverter based sources through droop control was ini- tially proposed in [2]. Although droop control is usually * Corresponding author email: rkolluri@student.unimelb.edu.au This work has been partly funded by a Linkage Grant supported by the Australian Research Council and Senergy Australia. implemented on frequency and real power (P - f droop), various modifications of droop control have been studied. One well known variant of droop control is the angle-droop first proposed in [3]. Implemented only on real power (P - for highly inductive networks), this scheme is motivated by the fact that small angle differences will cause a change in the power sharing between the sources. Therefore, each inverter is controlled to change its phase angle (δ) according to its real power (P ) output. Depending on the network characteristics the angle-droop is also modified to control the real or reactive power flow in networks [4]. Although the implementation is challenging, angle-droop controlled inverter systems provide better stability margin [5], [6]. Unlike frequency-droop, angle- droop causes zero frequency deviation. However, power shar- ing between angle-droop controlled inverters is affected by network impedance distribution [4], [5]. Contributions: In this work, we analyse the power sharing properties of angle-droop controlled systems under different network topologies. We identify the limitations of imple- mentation and power sharing correction techniques discussed in the literature [4], [5]. Inspired by consensus-based fre- quency restoration [7] and consensus-based droop control techniques [8], we propose a topology-independent inter-node communications based power sharing correction technique to eliminate the power sharing errors. We also perform a conver- gence analysis to demonstrate the stability and performance of the proposed technique. Finally, we analytically compare the performance of frequency-droop and angle-droop controlled systems, particularly with respect to parameter uncertainties. Paper Organization: The fundamentals of power sharing and angle-droop control are discussed in Section II. A consen- sus based power sharing technique is proposed and discussed in Section III. The frequency-droop and angle-droop controlled systems are compared in Section IV. Simulations results are presented in Section V followed by conclusion and discussions on future directions in Section VI. II. ANGLE- DROOP CONTROL Phasor power flow between the source V 1 ∠δ 1 and load V 0 ∠δ 0 shown in Figure 1 is given in (1).  S 10 = P 10 + jQ 10 ==  V 1  I 10 * =  V 1   V 1 -  V 0  Z 10  * (1)