Contributions to secondary control of voltage and reactive power for large area islanded microgrids with power-electronic-based distributed energy resources

Abstract

Global concerns on natural resource depletion and environmental degradation have initiated the power grid evolution from one based on large-scale rotating generators to one with many small-scale distributed energy resources. Smart distribution microgrids can combine the advantages offered by the prevalence of power-electronic-interfaced distributed energy resources (DERs) into providing significant controllability and flexibility to end users through the embedded control system. However, the lack of large-scale, high-inertia rotating generators may lead to instability in the events of fast voltage and/or frequency deviation. For microgrids operating in the islanded mode, it is also crucial to govern the dynamics among the DERs whilst maintaining the grid voltage and frequency within the desired specification. Control system of microgrids typically appears in the form of hierarchical structure consisting of primary, secondary, and tertiary control layers, which are distinguished by control bandwidth and technical functionalities. Through the primary layer’s decentralised droop control scheme, the active and reactive load powers can be shared autonomously among the DERs in an islanded microgrid. However, it is established that the reactive load power cannot be accurately shared through standard droop control alone. This is because of the voltage discrepancies at the points of DERs connection. Furthermore, it is also well established that voltage and frequency deviations are inevitable in droop mechanism.This has motivated this research thesis to design, develop and implement novel secondary control strategies to improve the voltage regulation and reactive power sharing among the powerelectronic-interfaced DERs for large-area droop-controlled islanded microgrids. Their performance is substantiated by a series of detailed and in-depth simulation studies and analyses. It is hoped that he work in this thesis can contribute to the evolution of conventional power grid towards future mart grid. A practical DIgSILENT-PowerFactory-Python co-simulation platform is established and exploited for control strategies verification. A variety of advanced droop control schemes targeting reactive power sharing improvement reported to date will be categorised and explained further in this thesis. An assessment is carried out on two groups of reactive power focused droop-based schemes, standard droop control with dispatch and virtual-output-impedance (VOI)-based droop control, to establish their merits and demerits. It is found that the latter, which is a relatively new technique made possible by the embedded control system of power-electronic-interfaced DERs, has controller gains to be tuned and that the resulted voltage deviation is slightly higher for the scheme with static-dynamic impedance components. Owing to sparse communication requirement, distributed control structure is highly relevant to large-area microgrids as DERs and loads are sparsely connected across the network. Through the previous assessment, it is found that the standard droop output voltages of all participating DERs converged upon proportional reactive power sharing through VOI-droop scheme. This has prompted its utilisation in distributed consensus control algorithm in conjunction VOI-droop scheme. To begin with, the state-of-the-art consensus VOI-droop control with reactive power is improved by nullifying the static component of virtual output impedance which, in turn, improves the voltage profile. The relationship between DER’s reactive power output and virtual output resistance/reactance is established. An operating-point-dependent consensus control tuning guideline is also presented. It is found that virtual-output-reactance-based VOI-droop scheme results in faster correction and dynamic-only, single component (virtual output resistance/reactance) VOI variant is sufficient to realize proportional reactive power sharing with improved voltage profile. These findings, however, raise new research questions on why there is never a need to regulate both virtual output impedance components, and why the virtual output reactance leads to a faster correction dynamic. Accordingly, a novel droop equivalent impedance concept is introduced as an attempt to answer the above questions. From the droop equivalent impedance equation, it is revealed that the reactance component has a more direct impact on reactive power, which explains the faster correction dynamic by the reactance-type control scheme. In addition, it is discovered that upon the convergence of droop equivalent reactance (resistance) through consensus control, the droop equivalent resistance (reactance) converges too. This hints that as far as reactive power sharing correction is concerned, there is never a need to simultaneously regulate both virtual impedance components. In due course, a novel droop-equivalent-reactance-based consensus adaptive VOI-droop control is developed. A systematic tuning guideline is introduced by first establishing the relationship between the droop equivalent reactance and the virtual output reactance. A well-tuned distributed controller with less cascaded PI controllers can certainly improve the practicality and robustness of the secondary control scheme while reducing the tuning effort especially in future large-area microgrids. In the previous standard droop scheme and the reactive-power-focused improved variants of droop control schemes, it is concluded that network bus voltages will deviate due to intrinsic trade off between DERs droop output voltage and reactive output power. Therefore, a computational friendly, power-flow-embedded, centralised secondary optimal control strategy is subsequently designed to address the multi-objective control problem. In this thesis, secondary voltage regulation is categorised into those that regulate DER-buses and those that regulate load-buses. While voltage regulation at DER-buses is widely attempted, voltage regulation at load-buses is considerably challenging. This is due to the fact that information of load-buses (located remotely) is not directly available to the DER control system. However, not only does the voltage regulation in distributed islanded microgrids involves high number of DERs in a much shorter control timeframe, but classical power flow algorithms are also not directly applicable due to the inexistence of slack bus in droop controlled islanded microgrids and the dependence of active/reactive powers on frequency/voltage droop. A non-iterative Decoupled Linearised Power Flow (DLPF), formerly proposed for large-scale power system planning and operation, is exploited as the linear approximation that fits well with the droop behaviour of islanded microgrids. In order to address the conflicting control objectives of voltage and reactive power regulation, standard droop control is preferred over the VOI-droop scheme as its aggregation with DLPF is rather straightforward. The centralised secondary optimal control strategy is proven capable of realising single (optimal reactive power sharing or single loadbus voltage regulation) and multi-objective (optimal reactive power sharing with single/multiple load-buses voltage regulation) controls. In addition, the effectiveness of the optimal control strategy in respecting the practical network constraints is successfully demonstrated. Apart from the reliability issue, it is expected that the computational complexity of a centralised optimal control scheme will increase exponentially as the microgrid scales up. Therefore, a semi-distributed multi-objective secondary optimal control targeting very-large-area droop controlled microgrids is relevant. It is proposed that the very-large-area microgrid’s secondary control layer is first segregated into multiple sub-microgrid clusters. The intention is to segregate the large-scale optimisation control problem into multiple sub-problems so that the computational burden can be distributed across multiple control entities. By embedding the consensus control concept into the design, a semi-distributed optimal control strategy is developed. The intra- and intercluster controls enable voltage and reactive power regulation within, respectively, each microgrid cluster and among multiple microgrid clusters. The viability of the semi-distributed control scheme is fully verified using MATLAB simulation. Through extensive simulation proofs, the effectiveness of the semi-distributed control strategy in collectively managing voltage and reactive power tradeoff is fully demonstrated. The control scheme is expected to have a low risk of single-point failure while being highly scalable as the computational effort is distributed across multiple secondary ontrollers. In this thesis, advanced secondary control strategies are designed and implemented for voltage regulation and/or reactive power sharing improvement among power-electronic-interfaced DERs in large-area droop-controlled islanded microgrids. All control strategies are fully verified via extensive theoretical derivation and simulation means. It is hope that the control schemes can benefit the development and motivate the adoption of future smart microgrids.

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ORCID for Andrew Cruden: orcid.org/0000-0003-3236-2535

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