HVAC-based hierarchical energy management system for microgrids

With the high penetration of renewable energy into the grid, power fluctuations and supply-demand power mismatch are becoming more prominent, which pose a great challenge for the power system to eliminate negative effects through demand side management (DSM). The flexible load, such as heating, ventilation, air conditioning (HVAC) system, has a great potential to provide demand response services in the electricity grids. In this thesis, a comprehensive framework based on a forecasting-management optimization approach is proposed to coordinate multiple HVAC systems to deal with uncertainties from renewable energy resources and maximize the energy efficiency. In the forecasting stage, a hybrid model based on Multiple Aggregation Prediction Algorithm with exogenous variables (MAPAx)-Principal Components Analysis (PCA) is proposed to predict changes of local solar radiance, vy using the local observation dataset and real-time meteorological indexes acquired from the weather forecast spot. The forecast result is then compared with the statistical benchmark models and assessed by performance evaluation indexes. In the management stage, a novel distributed algorithm is developed to coordinate power consumption of HVAC systems by varying the compressors’ frequency to maintain the supply-demand balance. It demonstrates that the cost and capacity of energy storage systems can be curtailed, since HVACs can absorb excessive power generation. More importantly, the method addresses a consensus problem under a switching communication topology by using Lyapunov argument, which relaxes the communication requirement. In the optimization stage, a price-comfort optimization model regarding HVAC’s end users is formulated and a proportional-integral-derivative (PID)-based distributed algorithm is thus developed to minimize the customer’s total cost, whilst alleviating the global power imbalance. The end users are motivated to participate in energy trade through DSM scheme. Furthermore, the coordination scheme can be extended to accommodate battery energy storage systems (BESSs) and a hybrid BESS-HVAC system with increasing storage capacity is proved as a promising solution to enhance its selfregulation ability in a microgrid. Extensive case studies have been undertaken with the respective control strategies to investigate effectiveness of the algorithms under various scenarios. The techniques developed in this thesis has helped the partnership company of this project to develop their smart immersion heaters for the customers with minimum energy cost and maximum photovoltaic efficiency

Distributed Control Strategies for Microgrids: An Overview

There is an increasing interest and research effort focused on the analysis, design and implementation of distributed control systems for AC, DC and hybrid AC/DC microgrids. It is claimed that distributed controllers have several advantages over centralised control schemes, e.g., improved reliability, flexibility, controllability, black start operation, robustness to failure in the communication links, etc. In this work, an overview of the state-of-the-art of distributed cooperative control systems for isolated microgrids is presented. Protocols for cooperative control such as linear consensus, heterogeneous consensus and finite-time consensus are discussed and reviewed in this paper. Distributed cooperative algorithms for primary and secondary control systems, including (among others issues) virtual impedance, synthetic inertia, droop-free control, stability analysis, imbalance sharing, total harmonic distortion regulation, are also reviewed and discussed in this survey. Tertiary control systems, e.g., for economic dispatch of electric energy, based on cooperative control approaches, are also addressed in this work. This review also highlights existing issues, research challenges and future trends in distributed cooperative control of microgrids and their future applications

Gather-and-broadcast frequency control in power systems

We propose a novel frequency control approach in between centralized and distributed architectures, that is a continuous-time feedback control version of the dual decomposition optimization method. Specifically, a convex combination of the frequency measurements is centrally aggregated, followed by an integral control and a broadcast signal, which is then optimally allocated at local generation units. We show that our gather-and-broadcast control architecture comprises many previously proposed strategies as special cases. We prove local asymptotic stability of the closed-loop equilibria of the considered power system model, which is a nonlinear differential-algebraic system that includes traditional generators, frequency-responsive devices, as well as passive loads, where the sources are already equipped with primary droop control. Our feedback control is designed such that the closed-loop equilibria of the power system solve the optimal economic dispatch problem

An Assessment of PIER Electric Grid Research 2003-2014 White Paper

This white paper describes the circumstances in California around the turn of the 21st century that led the California Energy Commission (CEC) to direct additional Public Interest Energy Research funds to address critical electric grid issues, especially those arising from integrating high penetrations of variable renewable generation with the electric grid. It contains an assessment of the beneficial science and technology advances of the resultant portfolio of electric grid research projects administered under the direction of the CEC by a competitively selected contractor, the University of California’s California Institute for Energy and the Environment, from 2003-2014

독립형 혼합 AC/DC 마이크로그리드의 전역 전력 균형을 위한 非중앙집중형 제어 방안 연구

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 전기·정보공학부, 2021. 2. 문승일.Recently, islanded microgrids (IMGs) have been considered the most attractive solution to supply electricity to rural and remote areas where electricity is hard to be provided. An IMG is designed to actively supply and manage the power and energy itself by various distributed generators (DGs) using neighboring natural sources. On the other hand, according to the growth of DC technology, an islanded hybrid AC/DC microgrid (IHMG), which consists of AC and DC MGs that are connected by an interlinking converter (IC), has been implemented to exploit its characteristics of higher efficiency and better compatibility. Generally, in MG areas, maintaining the desired power sharing ratio between the DGs is one of the crucial issues. Maintaining power sharing between DGs allows securing more reserve for DGs, reducing stress on DGs, and minimizing the generation costs of DGs. In each AC or DC MG, DGs usually adopt droop control as conventional generators to maintain the power sharing ratio between the DGs. From droop control, the power sharing ratio between the DGs is maintained based on the frequency and common DC bus voltage in each AC MG and DC MG. For global power sharing (GPS) between all DGs in an IHMG beyond individual MGs, IC plays a key role to achieve GPS by adjusting their outputs. As the power sharing ratio between the DGs in each AC and DC MGs is based on the frequency and common DC bus voltage, respectively, GPS can be achieved by equalizing the normalized frequency and the common DC bus voltage through active power control of ICs. On the other hand, to improve the reliability of the entire IHMG system and increase the power transfer ability between the AC and DC MGs, multiple ICs can be interconnected in an IHMG system. In this case, similar to the reasons for DGs, the desired power sharing ratio between ICs must be maintained. Thus, ICs should be controlled to achieve GPS and power sharing between ICs. This is called global power balancing (GPB) of an IHMG in this dissertation. Control strategies to achieve GPB through ICs can be categorized as centralized and non-centralized controls. In a centralized control strategy, a central control unit receives information about the IHMG system and provides control signals to individual ICs via a centralized communication network. However, as centralized control strategy is exposed to single point failure (SPF) risks, non-centralized control strategies, such as local control strategy and distributed control strategy, have attracted research attention. Without central control units, only local information is used for local control strategy, or information from distributed communication among only ICs is used for the distributed control strategy. By adopting a local control strategy, system reliability can be improved without using communication; however, the control objective cannot be achieved perfectly due to insufficient information. By contrast, by implementing a distributed control strategy, control objectives can be obtained accurately with sufficient information about an IHMG system; however, the use of communication can degrade system reliability. To implement non-centralized control, the operator usually chooses between local and distributed control strategies considering what the system needs most. Various studies have been conducted on non-centralized control methods of an IC to achieve GPB in an IHMG. By adopting a local control strategy, the normalized droop control method of ICs has been most widely employed for local control method of ICs. However, this control method causes steady-state error of GPS, and if the gains of the controller are adjusted to improve the GPS accuracy, it may cause system instability, consequently making it difficult to reduce the error. On the other hand, under a distributed control strategy, distributed control methods of ICs have been proposed. The most advanced distributed control method is based on additional proportional-integral controllers, virtual leader-followers, and distributed communication. This method makes the IHMG system achieve perfect GPS; however, it may cause the system to collapse when unexpected changes, such as communication delays and the failure of DGs, occur in the system. Therefore, in this dissertation, novel non-centralized control methods for ICs are proposed to overcome the limitations of conventional non-centralized control methods. A new local control method for ICs is proposed that drastically decrease the error of GPS accuracy without any instability issues. Based on the required active power of ICs to achieve GPS, the active power reference of each IC is determined, and a damping factor is introduced to overcome issues regarding power sharing between ICs. In addition, guidelines for designing the damping factor are presented based on analyses of stability and GPS accuracy. Furthermore, a new distributed control method for ICs is proposed, which improves the robustness against unexpected changes in the system, such as communication delay and failure of ICs and DGs. Based on the required active power of ICs to achieve GPS, the active power reference of each IC is determined and an estimator to obtain the required data for control is proposed with a dynamic consensus algorithm and distributed communication between ICs. In addition, a design method for the parameters of the estimator is proposed based on stability analysis. To demonstrate the effectiveness of the proposed control methods, stability analyses based on the small-signal model and Simulink-based simulations are performed for each proposed control method. Furthermore, the proposed control methods are implemented as hardware-in-the-loop experiments through real-time simulators and real controllers of ICs and case studies are performed. The effectiveness of the proposed methods is observed under an actual hardware experimental environment. Owing to the proposed control methods, GPB in an IHMG can be achieved more accurately and robustly and can ultimately help improve the reliability of the system. Finally, we hope that this study will make a significant contribution to achieve stable power supply to rural and remote areas, where people frequently face power shortages and blackouts.최근들어 도서지역 및 미전화지역 등 고립된 소규모 지역에 대한 전력공급의 주요 해결책으로 다양한 분산전원을 기반으로 한 독립형 마이크로그리드가 큰 관심을 받고 있다. 동시에 직류 기술의 발전으로 AC와 DC 마이크로그리드가 연계컨버터를 통해 연결되는 구조를 가진 독립형 혼합 AC/DC 마이크로그리드가 빠르게 확산되고 있다. 마이크로그리드 내에서 분산전원간 출력 분배율을 원하는 대로 유지하는 것은 매우 중요한 이슈 중 하나로, 이를 통해 예비력의 안정적인 확보와 분산전원의 수명증진 및 발전비용을 최소화시킬 수 있다. 각 AC 및 DC 마이크로그리드 내 분산전원들은 기성 발전원과 유사하게 드룹제어를 기반으로 동작하며, 이를 통해 분산전원 간 출력 분배율이 주파수와 공통 DC 모선 전압을 기반으로 원하는 비율로 유지된다. 개별 마이크로그리드를 넘어 독립형 혼합 AC/DC 마이크로그리드 전역의 분산전원 출력 분배율을 유지하기 위해서는 연계컨버터의 추가적인 제어가 필요하다. 이를 위해 연계컨버터는 일반적으로 유효전력 출력 제어를 수행하여 주파수와 공통 DC 모선 전압의 정규화된 값을 일치시키며, 이를 통해 독립형 혼합 AC/DC 마이크로그리드 전역의 모든 분산전원 출력 분배율을 유지시킬 수 있다. 한편, 연계컨버터 탈락에 대비하고, AC와 DC 마이크로그리드 간 더 많은 전력을 전송하기 위해 다기의 연계컨버터가 병렬로 구축되는 사례가 증가하고 있다. 이 경우, 연계컨버터간 출력 분배율도 분산전원의 경우와 유사한 이유로 원하는 대로 유지될 필요가 있다. 따라서 연계컨버터는 모든 분산전원의 출력 분배율과 연계컨버터간 출력 분배율을 원하는 대로 유지하도록 제어를 수행해야 하며, 이를 본 논문에서는 독립형 혼합 AC/DC 마이크로그리드의 전역 전력 균형 달성이라 정의한다. 전역 전력 균형의 달성을 위한 연계컨버터의 제어 전략으로는 중앙집중형 제어 전략과 비중앙집중형 제어 전략이 선택될 수 있다. 중앙집중형 제어 전략에서는 중앙 통신 네트워크를 통해 중앙 제어기가 계통 정보를 받아 각 연계컨버터에게 제어 신호를 제공한다. 하지만 중앙집중형 제어 전략은 중앙 제어기 탈락과 같은 단일 지점 탈락에 대한 큰 위험성을 가지고 있어, 비통신 제어 전략과 분산형 제어 전략과 같이 중앙 제어기를 이용하지 않는 비중앙집중형 제어 전략에 대해 최근 많은 연구가 이뤄지고 있다. 중앙 제어기 없이, 비통신 제어 전략의 경우는 주변 측정 정보만을 이용하여 제어하며 분산형 제어 전략의 경우는 연계컨버터간의 분산 통신 정보만을 추가적으로 이용하여 제어를 수행하게 된다. 비통신 제어 전략은 통신을 이용하지 않기에 높은 신뢰성을 가지나 정보의 한계로 완벽한 제어가 불가능할 수 있다. 한편, 분산형 제어 전략의 경우 충분한 정보를 확보하여 완벽한 제어를 가능하게 하나 통신을 이용하였다는 점에서 계통 신뢰성을 하락시킬 수 있다. 따라서 연계컨버터의 비중앙집중형 제어방안으로 시스템의 상황에 따라 비통신 및 분산형 제어 전략 중 한 가지가 선택되어 적용될 수 있다. 전역 전력 균형 달성을 위한 연계컨버터의 비중앙집중형 제어에 대하여 다양한 선행연구가 수행된 바 있다. 먼저, 비통신 제어 전략을 적용한 연계컨버터 제어 방안으로는 정규화 드룹 기반의 방식이 가장 널리 이용된다. 하지만, 이 제어 방안은 전역 전력 균형에 대한 정상상태 오차를 야기하고 만약 전역 전력 균형의 정확도를 높이기 위해 제어 이득값을 변경하게 되면 시스템의 불안정성을 야기할 수 있어 오차를 크게 줄이기 어렵다. 한편, 분산형 제어 전략 하에서도 댜앙한 연계컨버터의 제어 방안이 제안되었다. 가장 고도화되었다고 여겨지는 분산형 제어 방식은 추가 PI 제어기들과 가상 리더-추종자, 그리고 분산형 통신을 이용한 것이다. 이 방안은 독립형 혼합 AC/DC 마이크로그리드 시스템이 완벽한 전역 전력 균형을 달성하도록 하지만, 통신 지연과 분산전원 탈락과 같은 예기치 못한 변화에 대해 시스템 붕괴를 야기할 수 있다. 이에 본 논문에서는 기존 제어 방안들의 한계를 극복할 수 있는 연계컨버터의 새로운 비통신 및 분산형 제어 방안들이 제안되었다. 먼저, 기존 비통신 기반 정규화 드룹제어 방안으로는 크게 줄이기 어려웠던 전역 전력 균형 오차를 크게 개선하기 위해 새로운 비통신 기반 제어 방안이 제안되었다. 전역 전력 균형을 위해 필요한 연계컨버터의 유효전력량 산정을 통해 제어기가 도출되며, 연계컨버터간 출력 분배율 이슈를 극복하기 위해 댐핑 파라미터가 도입되었다. 그리고 안정도와 전역 전력 균형의 정확도 분석을 통해 댐핑 파라미터의 설계 가이드라인이 제시되었다. 또한, 병렬 연계컨버터 구축 시 기존 분산형 통신 기반 제어 방안의 한계인 예기치 못한 계통 변화에 대한 취약성을 극복하기 위해 새로운 분산형 제어 방안이 제안되었다. 전역 전력 균형을 위해 필요한 연계컨버터의 유효전력량 산정을 통해 제어기가 도출되었으며, 연계컨버터끼리의 분산형 통신과 동적 합의 알고리즘을 기반으로 제어에 필요한 파라미터를 추정하는 추정기를 구축하였다. 그리고 전체 통합 시스템의 안정도 분석을 통해 제어기와 추정기 파라미터 설계 방안이 제시되었다. 제안된 방안들의 성능을 확인하기 위해 각 제안된 제어기 마다 소신호 모델 기반의 안정도 분석과 Simulink를 통한 시뮬레이션이 수행되었다. 마지막으로 제안된 방안들은 실시간 시뮬레이터와 실제 제어기를 통해 Hardware-in-the-loop 실험으로 구현되었고 이에 대한 사례연구를 수행하여 제안된 방안들의 효과를 실제 하드웨어 실험 환경하에서 확인하였다. 제안된 방안을 활용하면 독립형 혼합 AC/DC 마이크로그리드 내 전역 전력 균형이 보다 안정적이며 정확하게 수행되어 계통의 신뢰성 향상에 도움이 될 것으로 여겨진다. 최종적으로는, 본 연구가 도서지역 및 미전화지역으로의 원활한 전력공급에 큰 기여를 할 수 있길 기대한다.Chapter 1 Introduction 1 1.1 Motivations and purposes 1 1.2 Highlights and contributions 5 1.3 Dissertation organization 7 Chapter 2 Islanded Hybrid AC/DC Microgrid 8 2.1 Concept of islanded hybrid AC/DC microgrid 8 2.2 Control strategy of DGs in AC microgrid 11 2.3 Control strategy of DGs in DC microgrid 20 2.4 Technical issues on control of interlinking converter 26 Chapter 3 Local Control Method of Interlinking Converters 35 3.1 Review of conventional local control method for ICs 36 3.2 Required active power of interlinking converters for GPS 38 3.2.1 Equivalent model for the IHMG 38 3.2.2 Required active power of the ideal IC 39 3.3 Proposed local control method of single IC 41 3.4 Proposed local control method of multiple ICs 43 3.4.1 Active power reference for each IC 43 3.4.2 Proposed local control method of each IC with damping factor 43 3.5 System configuration of test system 45 3.6 Effectiveness of damping factor 47 3.6.1 Simplified small-signal state space model 47 3.6.2 Stability analysis with damping factor 50 3.6.3 GPS error due to damping factor 52 3.6.4 Guideline to determine the damping factor 55 3.7 Simulation results 57 3.7.1 Case I: load variation scenario with single IC 58 3.7.2 Case II: DG failure with single IC 60 3.7.3 Case III: multiple ICs of the proposed method without damping factor 62 3.7.4 Case IV: multiple ICs of the proposed method with damping factor 64 3.7.5 Case V: comparison with the conventional method 68 Chapter 4 Distributed Control Method of Interlinking Converters 71 4.1 Review of conventional distributed control method for ICs 73 4.2 Proposed distributed control method of ICs 76 4.2.1 Active power reference for each IC 76 4.2.2 Dynamic consensus-based estimator for PICs and ρk 77 4.2.3 Calculation method for the parameters from local measurement 78 4.2.4 Structure of the entire proposed controller 79 4.3 System configuration 80 4.4 Design method of the consensus-based estimator 82 4.4.1 Simplified small-signal state space model 82 4.4.2 Parameter determination method for the estimator 85 4.5 Examining improved robustness with stability analysis 87 4.5.1 Stability with respect to communication delay 87 4.5.2 Stability with respect to DG status 88 4.6 Simulation results 90 4.6.1 Case I: IC failure 91 4.6.2 Case II: varying communication delay 92 4.6.3 Case III: DG failure 94 4.6.4 Case IV: GPS for cost minimization 98 4.6.5 Case V: power sharing between ICs to improve efficiency 101 Chapter 5 Hardware-in-the-loop (HIL) Experiments 105 5.1 HIL setup 107 5.2 HIL experimental results 108 5.2.1 Implementation for the proposed local control method of single IC 108 5.2.2 Implementation for the proposed local control method of multiple ICs 110 5.2.3 Implementation for the proposed distributed control method of ICs 112 Chapter 6 Conclusions and Future Extensions 119 6.1 Conclusions 119 6.2 Future extensions 121 Bibliographies 122 Appendix A. Design Method of the Normalized Droop Constants Considering GPS Error 133 초록 134Docto

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Presents an overview and analysis of the possible approaches to bringing an active demand side into electricity markets. Part of a series of research reports that examines energy issues facing California

Resilience-driven planning and operation of networked microgrids featuring decentralisation and flexibility

High-impact and low-probability extreme events including both man-made events and natural weather events can cause severe damage to power systems. These events are typically rare but featured in long duration and large scale. Many research efforts have been conducted on the resilience enhancement of modern power systems. In recent years, microgrids (MGs) with distributed energy resources (DERs) including both conventional generation resources and renewable energy sources provide a viable solution for the resilience enhancement of such multi-energy systems during extreme events. More specifically, several islanded MGs after extreme events can be connected with each other as a cluster, which has the advantage of significantly reducing load shedding through energy sharing among them. On the other hand, mobile power sources (MPSs) such as mobile energy storage systems (MESSs), electric vehicles (EVs), and mobile emergency generators (MEGs) have been gradually deployed in current energy systems for resilience enhancement due to their significant advantages on mobility and flexibility. Given such a context, a literature review on resilience-driven planning and operation problems featuring MGs is presented in detail, while research limitations are summarised briefly. Then, this thesis investigates how to develop appropriate planning and operation models for the resilience enhancement of networked MGs via different types of DERs (e.g., MGs, ESSs, EVs, MESSs, etc.). This research is conducted in the following application scenarios: 1. This thesis proposes novel operation strategies for hybrid AC/DC MGs and networked MGs towards resilience enhancement. Three modelling approaches including centralised control, hierarchical control, and distributed control have been applied to formulate the proposed operation problems. A detailed non-linear AC OPF algorithm is employed to model each MG capturing all the network and technical constraints relating to stability properties (e.g., voltage limits, active and reactive power flow limits, and power losses), while uncertainties associated with renewable energy sources and load profiles are incorporated into the proposed models via stochastic programming. Impacts of limited generation resources, load distinction intro critical and non-critical, and severe contingencies (e.g., multiple line outages) are appropriately captured to mimic a realistic scenario. 2. This thesis introduces MPSs (e.g., EVs and MESSs) into the suggested networked MGs against the severe contingencies caused by extreme events. Specifically, time-coupled routing and scheduling characteristics of MPSs inside each MG are modelled to reduce load shedding when large damage is caused to each MG during extreme events. Both transportation networks and power networks are considered in the proposed models, while transporting time of MPSs between different transportation nodes is also appropriately captured. 3. This thesis focuses on developing realistic planning models for the optimal sizing problem of networked MGs capturing a trade-off between resilience and cost, while both internal uncertainties and external contingencies are considered in the suggested three-level planning model. Additionally, a resilience-driven planning model is developed to solve the coupled optimal sizing and pre-positioning problem of MESSs in the context of decentralised networked MGs. Internal uncertainties are captured in the model via stochastic programming, while external contingencies are included through the three-level structure. 4. This thesis investigates the application of artificial intelligence techniques to power system operations. Specifically, a model-free multi-agent reinforcement learning (MARL) approach is proposed for the coordinated routing and scheduling problem of multiple MESSs towards resilience enhancement. The parameterized double deep Q-network method (P-DDQN) is employed to capture a hybrid policy including both discrete and continuous actions. A coupled power-transportation network featuring a linearised AC OPF algorithm is realised as the environment, while uncertainties associated with renewable energy sources, load profiles, line outages, and traffic volumes are incorporated into the proposed data-driven approach through the learning procedure.Open Acces