Online coordinated voltage control in distribution systems subjected to structural changes and DG availability

The responses of multiple DG units and voltage regulating devices such as tap changers and capacitor banks for correcting the voltage may lead to operational conflicts and oscillatory transients, where distribution systems are subjected to network reconfiguration and changes in availability of the DG units. Therefore, coordinated voltage control is required to minimize control interactions while accounting for the impact of structural changes associated with the network. This paper proposes a strategy for coordinating the operation of multiple voltage regulating devices and DG units in medium voltage (MV) distribution systems, under structural changes and DG availability, for effective voltage control. The proposed strategy aids to minimize the operational conflicts by allowing the farthest voltage regulating device to operate first on a priority scheme designed based on the electrical-distance between voltage regulating devices and DG units, while maximizing the voltage support by the DG units. The proposed coordination scheme is designed to enact with an aid of a substation centered distribution management system (DMS) for online voltage control. The control actions of proposed coordination strategy are tested on a MV distribution system, derived from the state of New South Wales, Australia, through simulations, and results are reported

Multi-objective optimisation method for coordinating battery storage systems, photovoltaic inverters and tap changers

The many well-established advantages of distributed generation (DG) make their usage in active distribution networks prevalent. However, uncontrolled operation of DG units can negatively interfere with the performance of other equipment, such as tap-changers, in addition to resulting in sub-optimal usage of their potential. Thus, adequate scheduling/control of DG units is critical for operators of the distribution system to avoid those adverse effects. A linearised model of a multi-objective method for coordinating the operation of photovoltaics, battery storage systems, and tap-changers is proposed. Three objective functions are defined for simultaneously enhancing voltage profile, minimising power losses, and reducing peak load power. The formulated multi-objective problem is solved by means of the epsilon-constraint technique. A novel decision-making methodology is offered to find the Pareto optimality and select the preferred solution. To assess to proposed model's performance, it is tested using 33-bus IEEE test system. Consequently, tap-changers suffer lessened stress, the batteries state-of-charge is kept within adequate limits, and the DG units operation is at higher efficiency. The obtained results verify the effectiveness of this approach.fi=vertaisarvioitu|en=peerReviewed

Coordinated Voltage Control in Modern Distribution Systems

Modern distribution systems, especially with the presence of distributed generation (DG) and distribution automation are evolving as smart distribution systems. Distribution management systems (DMSs) with communication infrastructure and associated software and hardware developments are integral parts of the smart distribution systems. With such advancement in distribution systems, distribution system voltage and reactive power control are dominant by Volt/VAr (voltage and reactive power) optimisation and utilisation of DG for system Volt/VAr support. It is to be noted that the respective controls and optimisation formulations are typically adopted from primary, secondary and tertiary voltage and reactive power controls at upstream system level. However, the characteristics of modern distribution systems embedded with high penetration of DG are different from transmission systems and the former distribution systems with uni-directional power flow. Also, coordinated control of multiple Volt/VAr support DG units with other voltage control devices such as on-load tap changer (OLTC), line voltage regulators (VRs) and capacitor banks (CBs) is one of the challenging tasks. It is mainly because reverse power flow, caused predominantly by DG units, can influence the operation of conventional voltage control devices. Some of the adverse effects include control interactions, operational conflicts, voltage drop and rise cases at different buses in a network, and oscillatory transients. This research project aimed to carry out in-depth study on coordinated voltage control in modern MV distribution systems utilising DG for system Volt/VAr support. In the initial phase of the research project, an in-depth literature review is conducted and the specific research gaps are identified. The design considerations of the proposed coordinated voltage control, which also uses the concept of virtual time delay, are identified through comprehensive investigations. It emphasises on examining and analysing both steady-state and dynamic phenomena associated with the control interactions among multiple Volt/VAr support DG units and voltage control devices. It would be essential for ensuring effective coordinated voltage control in modern distribution systems. In this thesis, the interactions among multiple DG units and voltage control devices are identified using their simultaneous and non-simultaneous responses for voltage control through time domain simulations. For this task, an analytical technique is proposed and small signal modelling studies have also been conducted. The proposed methodology could be beneficial to distribution network planners and operators to ensure seamless network operation from voltage control perspective with increasing penetration of DG units. Notably, it has been found that the significant interactions among multiple DG units and voltage control devices are possible under conventional standalone, rule-based, and analytics based control strategies as well as with real-time optimal control under certain system conditions. In the second phase of the research project, the proposed coordinated voltage control strategy is elaborated. The control design considerations are fundamentally based on eliminating the adverse effects, which can distinctly be caused by the simultaneous and non-simultaneous responses of multiple Volt/VAr support DG units and voltage control devices. First, the concept of virtual time delay is applied for dynamically managing the control variables of Volt/VAr support DG units and voltage control devices through the proposed control parameter tuning algorithm. Because it has been found that the conventional time-graded operation cannot eliminate the adverse effects of DG-voltage control device interactions under certain system conditions. Secondly, the distinct control strategies are designed and tested for effectively and efficiently coordinating the operation of multiple Volt/VAr support DG units and voltage control devices in real-time. The test results have demonstrated that the proposed coordinated voltage control strategy for modern MV distribution systems can effectively be implemented in real-time using advanced substation centred DMS. The proposed coordinated voltage control strategy presented in this thesis may trigger paradigm shift in the context of voltage control in smart distribution systems. In the final phase of the research project, short-term and/or long-term oscillations which can be possible for a MV distribution system operation embedded with Volt/VAr support DG are discussed. Typically, the short-term oscillations are occurred due to interactions among different DG units and their controllers (i.e., inter-unit electro-mechanical oscillations in synchronous machine based DG units) while the long-term oscillations occurred due to DG-voltage control device interactions. Also, sustained oscillations may occur due to tap changer limit cycle phenomenon. The concept of alert-state voltage control is introduced for mitigating the sustained oscillations subjected to OLTC limit cycles in the presence of high penetration of DG. The investigative studies in this thesis further emphasise the requirements of supplementary control and other mitigating strategies for damping the oscillations in modern active MV distribution systems. The proposed research will pave the way for managing increasing penetration of DG units, with different types, technologies and operational modes, from distribution system voltage control perspective

Multilevel Monte Carlo approach for estimating reliability of electric distribution systems

Most of the power outages experienced by the customers are due to the failures in the electric distribution systems. However, the ultimate goal of a distribution system is to meet customer electricity demand by maintaining a satisfactory level of reliability with less interruption frequency and duration as well as less outage costs. Quantitative evaluation of reliability is, therefore, a significant aspect of the decision-making process in planning and designing for future expansion of network or reinforcement. Simulation approach of reliability evaluation is generally based on sequential Monte Carlo (MC) method which can consider the random nature of system components. Use of MC method for obtaining accurate estimates of the reliability can be computationally costly particularly when dealing with rare events (i.e. when high accuracy is required). This thesis proposes a simple and effective methodology for accelerating MC simulation in distribution systems reliability evaluation. The proposed method is based on a novel Multilevel Monte Carlo (MLMC) simulation approach. MLMC approach is a variance reduction technique for MC simulation which can reduce the computational burden of the MC method dramatically while both sampling and discretisation errors are considered for converging to a controllable accuracy level. The idea of MLMC is to consider a hierarchy of computational meshes (levels) instead of using single time discretisation level in MC method. Most of the computational effort in MLMC method is transferred from the finest level to the coarsest one, leading to substantial computational saving. As the simulations are conducted using multiple approximations, therefore the less accurate estimate on the preceding coarse level can be sequentially corrected by averages of the differences of the estimations of two consecutive levels in the hierarchy. In this dissertation, we will find the answers to the following questions: can MLMC method be used for reliability evaluation? If so, how MLMC estimators for reliability evaluation are constructed? Finally, how much computational savings can we expect through MLMC method over MC method? MLMC approach is implemented through solving the stochastic differential equations of random variables related to the reliability indices. The differential equations are solved using different discretisation schemes. In this work, the performance of two different discretisation schemes, Euler-Maruyama and Milstein are investigated for this purpose. We use the benchmark Roy Billinton Test System as the test system. Based on the proposed MLMC method, a number of reliability studies of distribution systems have been carried out in this thesis including customer interruption frequency and duration based reliability assessment, cost/benefits estimation, reliability evaluation incorporating different time-varying factors such as weather-dependent failure rate and restoration time of components, time-varying load and cost models of supply points. The numerical results that demonstrate the computational performances of the proposed method are presented. The performances of the MLMC and MC methods are compared. The results prove that MLMC method is computationally efficient compared to those derived from standard MC method and it can retain an acceptable level of accuracy. The novel computational tool including examples presented in this thesis will help system planners and utility managers to provide useful information of reliability of distribution networks. With the help of such tool they can take necessary steps to speed up the decision-making process of reliability improvement.Thesis (Ph.D.) -- University of Adelaide, School of Electrical and Electronic Engineering, 201