Medium Voltage Microgrid Research Articles

Frequency Stability Constrained BESS Sizing Model for Microgrids

The grid integration of renewable energy sources necessitates using energy storage systems (ESSs) to provide more flexibility and controllability. This paper proposes a frequency stability-constrained battery energy storage system (BESS) sizing model for microgrids formulated as a mixed-integer linear programming (MILP) problem and decomposed using Benders decomposition. The optimal size of BESS is determined as a trade-off between minimizing the operating costs or maximizing the benefits and the high investment costs of BESS. Both the grid-connected and stand-alone operating modes are modeled for the microgrid along with the corresponding generation contingencies. The microgrid scheduling optimization model is built for cost-benefit analysis considering unit commitment and frequency stability (FS) constraints. The transient frequency dynamics are considered using an accurate time-domain model based on the discretized swing equation. The proposed method ensures frequency stability criteria, such as frequency nadir/overshoot, rate of change of frequency (RoCoF), and steady-state frequency, are within their allowable ranges following generation contingencies. Simulation results based on a medium-voltage microgrid test case with historical data of load, PV generation, and market price verify the effectiveness of the proposed method.

Development and Analysis of Optimization Algorithm for Demand-Side Management Considering Optimal Generation Scheduling and Power Flow in Grid-Connected AC/DC Microgrid

The world energy sector is experiencing many challenges, such as maintaining a demand–supply balance with continuous increases in demand, reliability issues, and environmental concerns. Distributed energy resources (DERs) that use renewable energy sources (RESs) have become more prevalent due to environmental challenges and the depletion of fossil fuel reserves. An increased penetration of RESs in a microgrid system facilitates the establishment of a local independent system. However, these systems, due to the uncertainties of RESs, still encounter major issues, like increased operating costs or operating constraint violations, optimal power management, etc. To solve these issues, this paper proposes a stochastic programming model to minimize the total operating cost and emissions and improve the operational reliability with the help of a generalized normal distribution optimization (GNDO). A day-ahead demand response is scheduled, aiming to shift loads to enhance RES utilization efficiency. Demand-side management (DSM) with RESs is utilized, and battery energy storage systems in low-voltage and medium-voltage microgrids are shown. Mathematical formulations of each element in the microgrids were performed. Optimal and consumer-friendly solutions were found for all the cases. Environmental concerns based on the amount of harmful emissions were also analyzed. The importance of demand response is demonstrated vividly. The aim is to optimize energy consumption and achieve optimum cost of operation via DSM, considering several security constraints. A comparative analysis of operating costs, emission values, and the voltage deviation was carried out to prove and justify their potential to solve the optimal scheduling and power flow problem in AC/DC microgrids.

Single-phase grounding line selection method for closed-loop distribution network adapted to medium voltage microgrid access

Aim: The access of the medium-voltage (MV) microgrid changes the radial structure of traditional distribution networks, causing the distribution network to operate in a closed-loop manner. The closed-loop operation complicates the fault characteristics of the distribution network, and the traditional single-phase grounding (SPG) line selection method is no longer applicable. Therefore, it is necessary to propose a new targeted SPG line selection method. Methods: This paper analyzes the network structure of MV microgrids and MV distribution networks and establishes a multi-level closed-loop operation distribution network model with MV microgrid access. Then, the SPG fault characteristics of the closed-loop operation distribution system under the small current grounding mode are analyzed from the perspective of two-level bus nodes. Furthermore, based on the analysis of fault characteristics, a closed-loop operation distribution network line selection method suitable for microgrid access is proposed, and a detailed design is carried out in terms of equipment configuration and communication schemes, forming a relatively complete automatic line selection system. At the same time, the proposed line selection scheme is based on the two-level bus nodes, which can be analogously applied to the multi-level closed-loop operation distribution network. Results: The simulation results show that the proposed line selection scheme can be effectively applied to the closed-loop operation distribution network with MV microgrid access. Conclusion: The traditional SPG line selection method will have misjudgment problems after the MV microgrid is integrated into the distribution network, but the line selection method proposed in this paper has good adaptability.

Cuckoo search firefly aided optimization framework for design of medium voltage micro-grid connected soft switching solid-state transformer (MS4T)

A single phase Soft Switching-Solid State Transformer (SS-SST) design is proposed with H-bridge topology as an alternative solution to fulfil the demand of low (or) medium grid power applications. A medium/low frequency transformers fed with H-bridge circuit are incorporate without DC-voltage link, and it’s provided sinusoidal output voltage into the grid. An optimization of Cuckoo Search Firefly (CSF) algorithm was proposed in this research to find optimum switching angle and duty cycle in bridge circuit unit. At present optimum grid power is achieved a maximum efficiency of medium/low power frequency with the help of proposed SS-SST (MS4T) model. For proposed design is used to electric aircraft, ship power systems, battery energy storage systems (BESS) and fast charging electric vehicles (EV). Which are appealing the networks of medium-voltage DC (MVDC). Proposed MS4T design is based on soft-switching transformer with low conduction loss, low EMI and high efficiency via H-bridge converter circuit. The capacitor voltage balancing control between cascade module and design of the component including a medium level voltage frequency transformer that is implement a 1 kV to 0.25 kV MS4T described. Therefore, the efficacy of the present investigations are established with MATLAB platform. The medium voltage Micro Grid (MG) output is estimated under different operation load conditions. A simulation result of the grid power is measured minimum harmonics level by using optimum switching angle, switching frequency and duty cycle arrangements.

A Supplemental Control for Dynamic Voltage Restorers to Improve the Primary Frequency Response of Microgrids

Distribution utilities around the world have begun deploying dynamic voltage restorers (DVRs) to improve the power quality for end-use consumers. While these devices normally operate to improve power quality, the sensing and control they use can be leveraged to provide additional operation benefits. This paper describes a supplemental control for DVRs that enables them to improve the primary frequency response of microgrids through regulating voltage-dependent loads. The control strategy has been tested on a commercially available DVR to demonstrate the practicality. Furthermore, to examine the impacts on full-size, medium-voltage microgrids that typically have thousands of nodes, an electromechanical model of the DVR has been developed and compared with experimental results. Simulation in a modified, islanded IEEE 8500 Node Test Feeder with 15 DVRs shows that the supplemental control can effectively improve the primary frequency response of microgrids. From the industry application perspective, the proposed supplemental control expands the functionality of DVRs from only improving power quality to including frequency regulation. Therefore, the DVRs deployed in microgrids not only can compensate for voltage disturbances, but also can stabilize the system frequency.

A Distributed Dual-Optimization Framework for Ancillary-Service Coordination Between MV Microgrids and LV Distribution Networks

This article aims to develop an efficient distributed real-time strategy for internal device scheduling and ancillary service coordination in the interconnected system of a medium voltage microgrid and a low voltage distribution network (LVDN). A distributed agent-based dual engine is proposed for coordinated ancillary service optimization, thereby reducing communication and computational burden. The strategy enables the microgrid operator and the LVDN aggregator, which are selfish entities, to perform ancillary service power exchanges by coordinating for both the active/reactive power as well as the compensation rates. The proposed strategy is numerically validated on a 30-node microgrid-LVDN test system consisting of solar PVs, conventional and renewable distributed generators, loads, community battery, and electric vehicles (EVs). This is shown to be beneficial than simply tracking a power profile by the LVDN or considering fixed compensation rates. Microgrid reduces its total operating cost significantly by approximately 31.78%, while maintaining the voltage and state of charges of EVs and community battery within the operation limit. Further, ancillary service coordination for residential and commercial LVDNs with a car park is analyzed, and the study is extended to consider fast charging of EVs with other related constraints.

A control strategy for a grid-connected virtual synchronous generator with virtual impedance

Virtual synchronous generator (VSG) can simulate synchronous generator turning and damping characteristics, which can further improve the penetration rate of renewable energy in the grid. However, the output impedance of virtual synchronous generator in medium and low voltage microgrids is dominated by resistance, which leads to the coupling of active/reactive power and affects the grid integration control effect, and the phase of VSG and grid voltage will be biased in the absence of a pre-synchronization link. For this purpose, a strategy of grid-connected control of VSG with virtual impedance is proposed. Firstly, the VSG mathematical model is established and virtual impedance is introduced into the VSG electrical portion to improve the grid-connected inverter output characteristics. Secondly, a secondary frequency and voltage regulation strategy to improve VSG control is proposed to restore the frequency and voltage of the VSG, and the SOGI phase-locked loop collects the grid voltage phase to complete the grid-connected pre-synchronization design. Finally, the effectiveness and correctness of the proposed strategy were verified by the simulation results based on MATLAB/ Simulink.

A novel solid state transformer based control topology for interconnected MV and LV hybrid microgrids

Hybrid microgrid continues to be an attraction for researchers as it brings the advantages of both ac and dc systems under one roof. The interlinking converter (ILC) of this system regulates the ac frequency and dc voltage while maintaining bidirectional power. This paper proposes a novel solid-state transformer (SST) based topology for interconnected medium voltage (MV) and low voltage (LV) hybrid microgrids. Based on the topology, various control modes are identified and correspondingly, an autonomous power sharing scheme is presented with dc voltage and ac frequency regulation. A linear voltage-frequency control is discussed for ac/dc and dc/ac converters of SST which provides a secondary control mechanism. The dual active bridge (DAB) of the SST operates at voltage control mode while providing electrical isolation and stable system operation. The proposed topology and control scheme is significant in the sense that it allows integration of ac and dc grids with different voltage and frequency levels. In addition, the secondary dc voltage or ac frequency regulation control at any hybrid microgrid automatically regulates the ac frequency and dc voltage in the system which reduces system cost. Various simulated scenarios in PSCAD with load changes highlight the optimal performance of the proposed scheme.

Scale-Free Cooperative Control of Inverter-Based Microgrids With General Time-Varying Communication Graphs

This paper presents a method for controlling the voltage of inverter-based Microgrids by proposing a new scale-free distributed cooperative controller. The main contribution of this paper is that the proposed distributed cooperative controller is scale-free where is independent of any information about the communication system and the number of distributed generators, as such it works for any Microgrids with any size. Moreover, the communication network is modeled by a general time-varying graph which enhances the resilience of the proposed protocol against communication link failure, data packet loss, and arbitrarily fast plug and play operation in the presence of arbitrarily finite communication delays as the protocol does not require the knowledge of the upper bound on the delay. The stability analysis of the proposed protocol is provided. The proposed method is simulated on the CIGRE medium voltage Microgrid test system. The simulation results demonstrate the feasibility of the proposed scale-free distributed nonlinear protocol for regulating voltage of Microgrids in the presence of communication failures, data packet loss, noise, and degradation.

Adaptive Virtual Impedance Controller for Parallel and Radial Microgrids With Varying X/R Ratios

In this paper, an adaptive virtual impedance-based universal voltage source converter (VSC) control scheme is proposed to improve the stability, and power-sharing performance of VSC interfaced distributed energy resources in hybrid AC/DC microgrids with different feeder characteristics. The proposed control scheme modulates the VSC output impedance by tracking the active and reactive power transfer difference between the inverter terminal and the point of connection to the microgrid. Firstly, the stability limits of microgrid with resistive and inductive microgrid feeders are investigated by theoretical analysis. Subsequently, influence of the feeder parameters on the stability of microgrid is evaluated. From the theoretical analysis, the proposed control scheme shows resistive inverter output impedance at low frequency, while it shows more inductive behavior at system frequency for both the low and medium voltage microgrid feeders. The effectiveness of the proposed control strategy is demonstrated through a range of scenarios for two different microgrid types. The results show that the proposed controller can autonomously vary the output impedance of the VSC and provides significantly improved damping and power-sharing performance of the microgrid.

Frequency Stability Constrained Microgrid Scheduling Considering Seamless Islanding

The frequency stability of microgrids is of paramount importance due to their low inertia and high share of renewable energy sources. In this paper, a frequency stability-constrained microgrid scheduling (FSC-MS) model is developed based on a time-dependent discretized frequency response model considering seamless unintentional islanding events. The optimal dispatch of controllable units, as well as optimal upward and downward primary reserves, are determined with a minimum operating cost while ensuring frequency stability criteria are maintained in their safe ranges following an unintentional islanding event, such as rate of change of frequency (RoCoF) and frequency nadir and overshoot. The limitation of generators with respect to primary frequency response is formulated in the form of primary reserve capacity and primary ramp rate limits. A cost-based model is presented to describe the available primary reserve of synchronous generators. The proposed FSC-MS model is formulated as a mixed-integer linear programming (MILP) problem using Benders decomposition. Simulation results on a medium voltage microgrid test system demonstrate the effectiveness of the proposed model to meet the frequency stability constraints while minimizing the total operating cost.

Inrush Current Management During Medium Voltage Microgrid Black Start With Battery Energy Storage System

This paper addresses the black start of medium voltage distribution networks (MV-DNs) by a battery energy storage system (BESS). The BESS consists of a two-level voltage source inverter interfacing MV-DN which has limited overcurrent capability. On the other hand, MV-DN normally includes several step-up and step-down transformers that are drawing sympathetic inrush current in energizing stage. Therefore, the major difficulty to perform black start by BESS during MV-DN island operation lies in the fact that the inverter has to simultaneous control the network voltage and its output current. This paper presents two control approaches for a BESS in order to control the inrush current during MV-DN black start. The proposed control schemes consist of droop, voltage and current loops in the stationary reference frame. The droop loop is used to generate the voltage reference. An intermediate voltage and inner current loops are designed for output voltage regulation, current reference generation as well as current tracking, respectively. New reference modifiers are included into droop and voltage loops to limit inrush current. The proposed control schemes are experimentally tested for energizing Ingå MV-DN in Finland by 1 MVA BESS, and their performance is compared in terms of inrush current value and voltage quality. The obtained results prove that both methods are able to feed the load with a fixed voltage in the steady-state as well as to limit the transformer inrush current considering the inverter overcurrent limit.

Sizing Algorithm for a Photovoltaic System along an Urban Railway Network towards Net Zero Emission

A reliable transportation system is essential for the development of a community. Especially in urban transportation, rail transportation is a faster, more comfortable way to travel for the commuters. These benefits can be valued further when the rail transportation system is with zero emissions. Electric trains can be considered a zero-emission transportation method. However, a rail transportation system operates with net-zero emissions when electricity is generated from zero-emission-based sources. Photovoltaic systems have already been integrated into railway stations and spare land owned by railways to achieve net-zero emissions. Furthermore, medium-voltage DC network and microgrid concepts have been proposed to incorporate more renewable energy sources into railway electrification systems. However, the energy generated from those systems is not enough to realise net-zero emissions, as the power requirements of an urban railway electrification system are high. Accordingly, this article investigates the possibility of implementing a photovoltaic system along the railway tracks to meet the energy demands of an urban railway electrification system so that net-zero emissions can be achieved. Other significant advantages of the proposed photovoltaic system are lower feeder losses due to distributed photovoltaic systems integrated into the railway electrification system, lower conversion losses due to the direct integration of the photovoltaic system into the railway electrification system, and the nonrequirement of additional space to install the photovoltaic system. In this paper, a photovoltaic system capacity sizing algorithm is proposed and presented by considering a railway electrification system, the daily schedule of trains, and historical photovoltaic weather data. This proposed photovoltaic system capacity sizing algorithm was evaluated considering a section of the urban railway network of Sri Lanka and a three-year, 2017-2020, photovoltaic weather data. The results indicated that the potential for photovoltaic generation by installing photovoltaic systems along a railway track is much higher than the requirement, and it is possible to meet the required train scheduling options with proper sizing. Furthermore, in the three-year analysis, it is possible to achieve 90% of the energy required for the railway electrification system with effective train scheduling methods.

Protection of converter dominated MV microgrid using changes in current's phase angle

Converter interfaced distributed generations in a microgrid feed the modulated current of limited magnitude during fault conditions. The protection design and its operation are thus challenging due to limited fault current which is further reduced by Petersen coil grounding in medium-voltage (MV) level. This paper aims to address this challenge by developing a current-only directional relay algorithm for the protection of converter dominated MV microgrid with Petersen coil grounding. The relay's operating principle is based on the sign of the change in phase angle of the fault current with respect to the prefault which indicates the direction of fault. The negative and positive changes in current's phase angle determine the fault in forward and reverse direction, respectively. The tripping decision is derived by comparing the binary output of the relay at both ends of the line segment under protection. This requires a simple, flexible and low bandwidth communication channel. Both theoretical analyses and simulation studies have been performed on a typical distribution grid intended to be operated as microgrid. The proposed protection method is suitable for microgrid having the converters with and without reactive power support. Various operating conditions are evaluated, including bidirectional power flow, high resistance fault, different fault types, loading conditions and signals with noise.

An Intelligent Data Mining-Based Fault Detection and Classification Strategy for Microgrid

The specific characteristics and operations of microgrid cause protection problems due to high penetration of distributed energy resources. To resolve these issues, the proposed scheme employs the Hilbert transform and data mining approach to protect the microgrid. First, the Hilbert transform is used to preprocess the faulted voltage and current signals to extract the sensitive fault features. Then, the obtained data set of the extracted features is input to the logistic regression classifier for fault detection. Later, fault classification is done by training the AdaBoost classifier. In the proposed scheme, the simulation results for feature extractions are evaluated on a standard International Electrotechnical Commission (IEC) medium voltage microgrid, compatible with MATLAB/SIMULINK software environment, whereas, Python is used for training and testing of data mining model. The results are evaluated under grid-connected and islanded modes for both looped and radial configurations by simulating various fault and no-fault cases. The results show that the accuracy of the proposed logistic regression and AdaBoost classifier is higher when compared to decision tree, support vector machine, and random forest methods. The results further validate the robustness of the proposed method against the measurement noise.

Fault Protection in Microgrid Using Wavelet Multiresolution Analysis and Data Mining

The protection problems in microgrid effect the reliability of the power system caused due to high distributed generator penetrations. Therefore, fault protection in microgrid is extremely important and needs to be resolved to enhance the robustness of the power system. This manuscript proposes a combined signal processing and data mining-based approach for microgrid fault protection. In this study, first the multiresolution decomposition of wavelet transform is employed to preprocess the voltage and current signals to compute the total harmonic distortion of the voltage and current. Then, the statistical indices of standard deviation, mean, and median of the total harmonic distortion and the negative sequence components of active and reactive power are used to collect the input data. After that, all the available data is provided to the random forest-based classifier to evaluate the efficiency of the proposed scheme in terms of the detection, identification, and classification of faults. This study used different aspects for data collection by simulating various fault and no-fault cases for both looped and radial configurations under grid-connected and islanded modes of operation. The simulations were performed on a standard medium voltage microgrid using MATLAB/SIMULINK, whereas the analysis for testing and training of the random forest were conducted in Python. It is recognized that the proposed method performed better than support vector machines and decision tree that are reported in the literature. The results further demonstrate that the proposed method can also detect simultaneous faults, and it is also effective against measurement noise.

Influence of load dynamic response on the stability of microgrids during islanding transition

Microgrids (MGs) are characterized by faster dynamics than conventional grid-connected distributions systems. This is in part due to the lack of inertia of inverter-interfaced distributed energy resources such as photovoltaic and energy storage systems. In this context, load dynamics considerably affects the transient stability performance of MGs, especially if disconnected from the main grid. The aim of the paper is to analyze the influence of the load modeling and the load composition in the transient stability assessment of a medium voltage MG during islanding transition. The MG we analyze consists of a photovoltaic power plant, two battery energy storage systems, a synchronous generator and different classes and types of load, which we consider representative for the problem of interest. The system is implemented in the EMTP-RV simulation environment. The importance of appropriate load modeling for the adequate analysis of the transient response of an islanding microgrid is also addressed and discussed.

Distributed online learning and dynamic robust standby dispatch for networked microgrids

Appropriate distributed dynamic standby dispatch schemes are of great importance to manage distributed energy systems and integrate large-scale renewables without the guidance of central intelligence or day-ahead forecasting. This study focuses on systems based on networked microgrids (MGs), which include self-contained medium-voltage MGs—consisting of renewable or dispatchable generators, energy storage systems (ESS), and flexible loads. An online dynamic dispatch scheme is proposed to support autonomous operation or to serve as a standby dispatch scheme in an emergency when a dispatch center is unavailable or day-ahead planning is infeasible. First, the multi-MG energy management problem is modeled into several distributed coupled subproblems. Afterwards, taking Lyapunov drift and online learning of future costs into consideration, the distributed problem is transformed into a robust long-term optimization. Moreover, a Lyapunov-based dynamic algorithm is employed to solve the problem in a fully distributed fashion. Finally, taking an actual system as an example, the superiority and feasibility of the proposed strategy are verified by simulation. The proposed distributed online standby dispatch has good performance in the long-term economy optimization and queue stability without any day-ahead forecasting and central decision-making compared with existing frameworks.

Microgrid Protection Strategy Based on the Autocorrelation of Current Envelopes Using the Squaring and Low-Pass Filtering Method

To resolve the protection issues caused by high penetration of distributed energy resources, this paper proposes an efficient protection scheme for microgrids based on the autocorrelation of three-phase current envelopes. The proposed strategy uses a squaring and low-pass filtering approach for evaluating the envelope of the current signal. Then, the variance of the autocorrelation function is used to extract the hidden information of the distorted envelope to detect the fault signatures in the microgrid. Furthermore, the reactive power is used for determining the fault direction. The performance of the proposed protection scheme was verified on a standard medium-voltage microgrid by performing simulations in the MATLAB/Simulink environment (Version: R2017b). The proposed scheme was shown to be easy to implement and have good performance under looped and radial configuration for both grid-connected and islanded operation modes. The simulation results showed that the scheme could not only detect, locate, classify, and isolate various types of short-circuit faults effectively but also provide backup protection in case of primary protection failure.

A parallel multi-period optimal scheduling algorithm in microgrids with energy storage systems using decomposed inter-temporal constraints

Because microgrids have relatively high share of renewable energy sources and energy storage systems (ESSs) compared with existing large-scale power systems, the inter-temporal constraints such as the generators’ ramp-rates and the state-of-charge of the ESSs have a much greater impact on system operation. Therefore, in this paper, the optimization of the microgrid operation, the commitment of generators and the charging/discharging of ESSs, is formulated as a mixed-integer nonlinear programming (MINLP) problem with inter-temporal constraints. In order to find the optimal solution to the problem effectively, we propose a parallel computation method based on the generalized Bender’s decomposition and the optimality condition decomposition. The method has the structure which is suitable for parallel computation and the convergence to the optimal solution is greatly improved compared with conventional sequential optimization methods. The proposed method is applied to the CIGRE medium-voltage microgrid benchmark system and the simulation results show that the proposed method has a potential for facilitating full-scale parallel computation ability and the application to the real-time operation of microgrid system.