Data-Driven Chance-Constrained Design of Voltage Droop Control for Distribution Networks

This paper addresses the design of local control methods for voltage control in distribution networks with high levels of distributed energy resources (DERs). The designed control methods modulate the active and reactive power output of DERs proportional to the deviation of the local measured voltage magnitudes from a reference voltage, which is referred to as droop control; thus, the design focuses on determining the droop characteristics that satisfy network-wide voltage magnitude constraints. The uncertainty and variability of DERs renders the design of optimal droop controls very challenging; hence, this paper proposes chance constraints to limit the risk from intermittent DERs by designing droop control coefficients that guarantee the satisfaction of network operational constraints with a specific probability. In addition, the proposed approach relies entirely on historical data rather than assuming knowledge of the probability distributions that characterize the uncertainty of DERs. The efficacy of the proposed method is demonstrated on a 37-bus distribution feeder

A Local Control Approach to Voltage Regulation in Distribution Networks

This paper address the problem of voltage regulation in power distribution networks with deep penetration of distributed energy resources (DERs) without any explicit communication between the buses in the network. We cast the problem as an optimization problem with the objective of minimizing the distance between the bus voltage magnitudes and some reference voltage profile. We present an iterative algorithm where each bus updates the reactive power injection provided by their DER. The update at a bus only depends on the voltage magnitude at that bus, and for this reason, we call the algorithm a local control algorithm. We provide sufficient conditions that guarantee the convergence of the algorithm and these conditions can be checked a priori for a set of feasible power injections. We also provide necessary conditions establishing that longer and more heavily loaded networks are inherently more difficult to control. We illustrate the operation of the algorithm through case studies involving 8-,34- and 123-bus test distribution systems.Comment: shorter version submitted to NAPS 201

Compliance of Distribution System Reactive Flows with Transmission System Requirements

Transmission system operators (TSOs) often set requirements to distribution system operators (DSOs) regarding the exchange of reactive power on the interface between the two parts of the system they operate, typically High Voltage and Medium Voltage. The presence of increasing amounts of Distributed Energy Resources (DERs) at the distribution networks complicates the problem, but provides control opportunities in order to keep the exchange within the prescribed limits. Typical DER control methods, such as constant cosϕ or Q/V functions, cannot adequately address these limits, while power electronics interfaced DERs provide to DSOs reactive power control capabilities for complying more effectively with TSO requirements. This paper proposes an optimisation method to provide power set-points to DERs in order to control the hourly reactive power exchanges with the transmission network. The method is tested via simulations using real data from the distribution substation at the Sundom Smart Grid, in Finland, using the operating guidelines imposed by the Finnish TSO. Results show the advantages of the proposed method compared to traditional methods for reactive power compensation from DERs. The application of more advanced Model Predictive Control techniques is further explored.©2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).Part of this work was carried out in the SolarX research project with financial support provided by Business Finland, 2019–2021 (grant No. 6844/31/2018).fi=vertaisarvioitu|en=peerReviewed

Deep Reinforcement Learning for Control of Microgrids: A Review

A microgrid is widely accepted as a prominent solution to enhance resilience and performance in distributed power systems. Microgrids are flexible for adding distributed energy resources in the ecosystem of the electrical networks. Control techniques are used to synchronize distributed energy resources (DERs) due to their turbulent nature. DERs including alternating current, direct current and hybrid load with storage systems have been used in microgrids quite frequently due to which controlling the flow of energy in microgrids have been complex task with traditional control approaches. Distributed as well central approach to apply control algorithms is well-known methods to regulate frequency and voltage in microgrids. Recently techniques based of artificial intelligence are being applied for the problems that arise in operation and control of latest generation microgrids and smart grids. Such techniques are categorized in machine learning and deep learning in broader terms. The objective of this research is to survey the latest strategies of control in microgrids using the deep reinforcement learning approach (DRL). Other techniques of artificial intelligence had already been reviewed extensively but the use of DRL has increased in the past couple of years. To bridge the gap for the researchers, this survey paper is being presented with a focus on only Microgrids control DRL techniques for voltage control and frequency regulation with distributed, cooperative and multi agent approaches are presented in this research

Decentralized optimization approach for power distribution network and microgrid controls

The smart grid vision has led to the development of advanced control and management frameworks using distributed generation (DG) and storage resources, commonly referred to together as distributed energy resources (DERs). Albeit environment-friendly, these DERs in distribution networks including microgrids (MGs) could greatly challenge the operational goal of maintaining adequate power system reliability standards because of their high intermittency, uncertainty, and lack of physical inertia. Meanwhile, these networks are inherently unbalanced and lack high-quality communications to a centralized entity as compared to the bulk transmission grid. Both aspects contribute to the challenge of designing voltage and frequency control frameworks therein. To tackle these problems, we propose decentralized control strategies, which account for cyber-physical network interactions automatically and dynamically while being either cognizant of various communication scenarios or resilient to malicious cyber intrusions. By treating the transmission grid as an infinity bus, voltage stability is the main concern in distribution networks where more DERs are being installed in the near future. Thanks to advances in power electronics, DERs can also be excellent sources of reactive power (VAR), a quantity that is known to have a significant impact on the network voltage level. Accordingly, we first formulate the local VAR-based voltage control design by minimizing a weighted quadratic voltage mismatch error objective using gradient-projection (GP) updates. The step-size design under both static and dynamic settings is further analyzed for practical implementation purposes. Nonetheless, such local design suffers degraded performance due to lack of information exchanges, especially under limited VAR resources. To address this issue, we develop the distributed voltage control (DVC) design based on the alternating direction method of multipliers (ADMM) algorithm. The DVC design has simple node-to-node communication architecture while seamlessly adapting to dynamically varying system operating conditions and being robust against random communication link failures. To further reduce communication complexity and enhance robustness to imperfect communications, especially under the worst-case scenarios of a total communication outage, we integrate both local and distributed control designs to a hybrid voltage control (HVC) scheme that can achieve the dual objectives in terms of flexible adaptivity to variable rate of communications and global optimality of voltage regulation performance. Such an innovative design aims to unify the separated framework of either local or distributed control design. Numerical tests using realistic feeders and real time-series data have been demonstrated for the voltage control designs. The aforementioned decentralized voltage control designs can improve the power system stability while distribution feeders are interconnecting to the bulk transmission grids. With a high penetration of DERs in the networks, it is possible to build a discrete energy system, namely, a microgrid (MG), that is capable of operating in parallel with, or independently from, the transmission grids. Henceforth, MGs are likely to emerge as a means to advance power and cyber physical resiliency in future grid systems. As MGs may operate independently, these mostly power electronics-interfaced DERs exhibiting low-inertia characteristic have raised significant concern over the frequency stability issues. To tackle this problem, we introduce the concept of virtual inertia of DERs and cast the secondary frequency control design for isolated MGs as a consensus optimization problem. We solve it distributively by adopting the partial primal-dual (PPD) algorithm. Interestingly, parts of our specially designed control algorithm turn out to mimic the dynamics of network power flow and virtual synchronous generator-based inverter. Thus, such dynamics is seamlessly governed by the physical system itself. Given a proper control parameter choice, the convergence of the consensus is guaranteed without assuming the time-scale separation of the hierarchical control design methodologies. By extending this work to a practical industrial MG network that follows the IEC 61850 communication protocol, similar frequency regulation objective is introduced and solved by a decentralized ADMM-based algorithm. The countermeasures for malicious attacks on the communication network for both PPD- and ADMM-based control designs are also investigated. Specifically, we analyze two types of malicious attacks on the communication network, namely, the link and node attacks. Meanwhile, anomaly detection and localization strategies are developed based on the metrics of optimization-related variables. We showcase the microgrid frequency regulation operation to demonstrate the effectiveness of the proposed frequency control designs under a real-time simulation environment

Coordinated Control of Distributed Energy Resources in Islanded Microgrids

As the penetration of the distributed energy resources (DERs) in the power grid increases,new challenges are revealed, including: stability issues, frequency fluctuations, voltage control, protection system coordination, etc. A systematic approach for dealing with those issues is to view the DERs and associated loads as a subsystem or a microgrid (MG). MGs can operate either in the grid connected or islanded modes. As opposed to the grid connected mode, the voltage and frequency regulation and load/generation balancing during islanded mode is solely dependent on the local generation units. Therefore, stable and reliable operation of islanded MGs requires a real time coordinated control scheme. Conventionally, such coordination is achieved by means of the active power-frequency and reactive powervoltage droop control schemes. The conventional droop method, which is based on P-f droop concept in power systems, lacks compatibility with the resistive nature of networks as well as the low inertia of electronically interfaced DER units in MGs. As a result, it features a slow dynamic response but also a low power quality due to frequency and voltage fluctuations. This PhD research proposes a novel droop concept based on the global positioning system (GPS) and voltage-current (V-I) droop characteristics for coordination of inverter-based DER units in islanded MGs. The concept of V-I droop control is introduced in Chapter 2. In this control approach, each DER is equipped with a GPS receiver, which produces a pulse at frequency of 1Hz (1PPS). Since all GPS receivers are locked to atomic clocks of the GPS satellites, the 1PPS signal can be utilized to synchronize the time reference of the DER units. Using the common time reference and fixing the frequency at the nominal value, all of the units can share a common synchronous rotating reference frame (SRRF). Furthermore, proportional load sharing is achieved by drooping the d and q axis components of the reference voltage with respect to the d and q axis components of current, respectively. The proposed scheme not only circumvents the issue of frequency fluctuations but also is in accordance with the fast dynamics of inverter-based DER units and resistive nature of the networks in islanded MGs. The V-I droop scheme, in its basic form, relies on availability of GPS signals at each of the DER units. With the intention of improving the MG robustness with respect to GPS signal failure, a new control strategy based on V-I droop concept is presented Chapter 3. In this method, an adaptive reactive power-frequency droop scheme is used as a backup for the V-I droop controller to ensure synchronization in case of a GPS signal failure. Droop control schemes in general, and the proposed V-I droop strategy in particular are characterized by non-ideal sharing of current among the DER units due to the variations of voltage along the MGs. In order to improve the sharing accuracy of the V-I droop scheme iv while regulating the average voltage at the nominal value, a new distributed secondary control method based on consensus protocol is proposed in Chapter 4. In this method, the daxis droop characteristics is altered so as to regulate the average microgrid voltage to the rated value but also guarantee proper sharing of active power among the DERs. Additionally, the q-axis component of voltage is adjusted to perform proper sharing of current. Generally, DERs might be supplied from different energy sources, including renewables and storage systems. The intermittency of renewable energy resources on one hand and the limited capacity of the energy storage systems on the other hand, necessitate modification of droop characteristics based on an energy management plan. In Chapter 5, a novel distributed secondary control strategy is introduced for power management of integrated photovoltaicbattery DER units in islanded MGs. The distributed secondary controllers are coordinated based on a leader-follower framework, where the leader restores the MG voltage to the rated value and the followers pursue energy management. Unbalanced and nonlinear loads, which are quite common in MGs, adversely affect the power quality and sharing accuracy. In order to mitigate those issues, two new solutions are proposed in this thesis. In the first approach (Chapter 6), a new supplementary droop control scheme is added to the V-I droop controller to reduce the voltage unbalance while preventing current and power overload under unbalanced loading conditions. In the second approach (Chapter 7), a hierarchical control scheme, consisting of primary (modified V-I droop) and distributed secondary control levels is introduced to mitigate harmonic distortions and prevent overcurrent stresses under nonlinear and unbalanced loading conditions. Finally, the conclusions and possible future work are addressed in Chapter 8

A hybrid approach for planning and operating active distribution grids

This paper investigates the planning and operational processes of modern distribution networks (DNs) hosting Distributed Energy Resources (DERs). While in the past the two aspects have been distinct, a methodology is proposed in this paper to co-optimize the two phases by considering the operational flexibility offered by DERs already in the planning phase. By employing AC Optimal Power Flow (OPF) to analyse the worst-case forecasts for the load and distributed generator (DG) injection, the optimal set-points for the DERs are determined such that the network's security is ensured. From these results, the optimized individual characteristic curves are then extracted for each DER which are used in the operational phase for the local control of the devices. The optimized controls use only local measurements to address system-wide issues and emulate the OPF solution without any communication. Finally, the proposed methodology is tested on the Cigre LV benchmark grid confirming that it is successful in mitigating with acceptable violations over- and under-voltage problems, as well as congestion issues. Its performance is compared against the OPF-based approach and currently employed local control schemes