LVDC Microgrid Research Articles

An improved decentralized control strategy for a PV hybrid energy storage system in an LVDC microgrid

An improved decentralized control strategy for a PV hybrid energy storage system in an LVDC microgrid

Supervisory Power Management Scheme of a Laboratory Scale Wind-PV Based LVDC Microgrid Integrated With Hybrid Energy Storage System

Supervisory Power Management Scheme of a Laboratory Scale Wind-PV Based LVDC Microgrid Integrated With Hybrid Energy Storage System

Moving towards a methodology for conductor sizing and protection in LVDC microgrids

Future power systems will be increasingly evaluated on their overall efficiency and sustainable use of raw materials. From this point of view, low-voltage DC networks have distinct advantages over AC networks. A large research interest is noted where these benefits are highlighted through case studies and demonstrators worldwide. However, the protection of DC networks is very challenging due to the absence of natural zero-crossings and fast network dynamics, and a general design methodology is missing in literature. This paper presents a general design methodology for the dimensioning of conductors and design of their protection in LVDC grids, in order to work towards a set of universal design methodologies for the development of LVDC grids based upon standards, best practices and experience, as is already a common resource in AC. The methodology is verified through a case study, where the differences with AC are highlighted and recommendations are made for standardization committees.

Fault detection technique using time-varying filter-EMD and differential-CUSUM for LVDC microgrid system

Fault detection technique using time-varying filter-EMD and differential-CUSUM for LVDC microgrid system

On-Line Fault Identification, Location, and Seamless Service Restoration using Transfer Learning-Based Convolution Neural Network for Low-Voltage DC Microgrid

DC microgrid over the last decade has gained a global paradigm in the power system field. Through the effective integration of distributed energy resources, significant researchers have improved its advantages over conventional power systems. The new state-of-the-art infrastructure despite its numerous advantages possess challenges in implementing an appropriate protection system. Impact of selecting a definite threshold for voltage and current compromises with the accuracy and speed of detection in conventional fault detection methods. Although many machine learning methods are successful in fault detection and classification for DC microgrid they still suffer from overfitting problems and exhaustively time-consuming. This article intends to provide an Online fault protection method for a LVDC microgrid system based on a transfer learning-based convolution neural network (TCNN). With the help of transient voltages and currents at different buses, image data for faults at different buses serves as input to the convolution neural network layer. First, the pre-trained Alex-Net CNN initializes the weights and biases for the targeted offline CNN’s. Secondly, the transferred layers from the offline CNN’s, initializes the online convolution neural network for real-time fault detection and classification. This work aims to accurately identify and locate the faults without complex dataset and multiple thresholds while improving accuracy of fault detection and classification. To ensure reliability of the system the recognized faulty bus reconnects to the healthy bus via sectionalizing circuit breakers through the detected signals. The proposed TCNN framework has an accuracy of 99.78%. The proposed method results when compared with state-of-the-art machine learning techniques such as SVM, LSTM, RNN, multilayer perceptron, and wavelet-based ANN showed better results in terms of accuracy and has significantly reduced data abundance.

A New Proposal for Power Sharing in LVDC Energy Community Microgrids

In recent years the development of LVDC distribution networks is under consideration. DC electrical distributions offer several advantages compared to AC ones in many applications, in particular in the presence of energy storage systems and distributed generation like high efficacy, flexibility and simple integration of renewables. The DC distribution allows to integrate in a more efficient “microgrid” different sources with DC/DC converters. The paper proposes an innovative model of microgrid configuration for aggregations of end-users able to share the power produced by common generators and energy services named by the authors Power Sharing Model (PSM) using a DC bus that connects in a one way approach, the common generators to the end-users. The paper investigates on the different suggested configurations of the PSM, with the converter characteristics and controls. A simplified case study is analyzed to test the performance of the sharing model and the stability of the control in different scenarios. The paper compares the PSM based on a LVDC grid with existing approaches of virtual aggregations, and it highlights the main differences between the currently existing methods and our new LVDC microgrid approach. The suggested PSM appears more efficient, convenient and flexible than the existing virtual models, because users physically self-consume and share the energy locally generated.

Circulating Current Control of LVDC Microgrid Based On Optimal Offset Voltage Decentralized Control Scheme with CPLs

Circulating Current Control of LVDC Microgrid Based On Optimal Offset Voltage Decentralized Control Scheme with CPLs

Reliability analysis of buck converters in a household-sized low-voltage DC microgrid

Household-sized low-voltage (LV) DC microgrids (MGs) are mostly interconnected to a platform that is called a local energy community (LEC). Each household-sized DC MG commonly consists of battery energy storage systems (BESSs), solar energy generating systems (SEGSs), AC loads, and LV DC appliances. The different AC loads operate through DC/AC power inverters which are connected to the high-voltage DC (HVDC) common bus in the LEC, while the LV DC appliances operate through DC–DC buck converters (DCBKCs) that are also linked to the same system. The DCBKCs can reduce the operating voltage to a certain level to be adapted to household-sized DC loads. To demonstrate the importance of BKCs in the household-sized LVDC MG, this paper presents the reliability analysis of BKCs by considering all dynamic and faulted operations during their used time. In general, the performance reliability of BKCs could be reduced by several factors including high short-circuit currents, dynamic voltage ripples, power losses, and their used time, which can cause a significant decrease in the power-supply reliability for LVDC loads. For the reliability analysis of BKCs, the primary objective is to formulate the rippled-voltage-dependent failure rate (RVDFR) and the short-circuit-current-dependent failure rate (SCCDFR) in the system. Secondly, a combination of the aforementioned failure rates (FRs) with the operating-time-dependent failure rate (OTDFR) is proposed to determine the system-level reliability of the BKC through a Markov-state transition diagram. Consequently, experiment results of reliability evaluation of the BKC in the DC microgrid show that i) capacitor and inductor components are vulnerable to multiple dynamic cases; ii) the diode is the least affected component of the BKC from random dynamic operations; and iii) faults at the BKC’s output have a significant impact on the working reliability of inductors, switching elements, and capacitors. Influenced by all dynamic and faulted operations during the used time, the BKC’s reliability could decrease to 50% within 6.9 years of usage, while the mean-time-between-failures (MTBF) value rapidly decreases within the first 5 working years.

MPC-based control for a stand-alone LVDC microgrid for rural electrification

Electricity access in developing countries, where the availability of public distribution grids is still poor, is considered a key factor for improvement of people life conditions. In these situations, the lack of a reliable grid can be mitigated by the introduction of stand-alone DC microgrids, including small Photovoltaic (PV) generators and storage devices. This paper focuses on optimal energy management and power supply reliability of such a microgrid. In particular, a Model-Predictive-Control (MPC) - based control system is introduced to optimally manage storage devices and coordinate load shedding actions. Additionally, an Artificial-Neural-Network (ANN) - based predictor is introduced to manage unpredictable solar irradiance and temperature variations. The availability of reliable adaptive forecasts provided by the ANN-based predictor increases the efficiency of the optimization performed by the MPC-based control over the prediction horizon, avoiding the well-known issues related to optimization performed on unreliable forecast. In this paper, the proposed control approach is detailed for a specific case study and its advantages with respect to traditional controller algorithms are highlighted by comprehensive numerical simulations. The presented results highlight that the proposed MPC controller provides a substantial increment in power supply reliability with respect to standard controls, especially for priority loads. This is obtained at the expense of an increased battery stress, which is acceptable for electricity access applications where power supply reliability is usually the foremost need.

Residual-based Short-Circuit fault detection and isolation in LVDC microgrid

Designing protection for a DC microgrid is challenging due to its DC nature and heavily capacitor-dominated DC bus that induces high amplitude current spike during short-circuit faults. Moreover, for small-scale applications such as residential DC microgrids, low line-resistance cables increase the fault detection time constraint. Thus, this paper proposes a fast short-circuit fault detection and isolation method based on current residual computation for a standalone residential DC microgrid. The proposed method continuously compares the computed residual from sensed branch currents to their respective defined thresholds. These thresholds are predetermined by analyzing the microgrid operation in healthy mode, considering both steady-state and transient operations. The protection device isolating the faulty branch is then activated once the corresponding residual exceeds its threshold. The protection scheme is validated through a processor-in-the-Loop simulation approach, during which the power stage of the system remains simulated on the host computer using PSIM software, while its control algorithm is executed from a Texas Instrument TMS320F28335 hardware target. The proposed protection algorithm is able to discriminate transient perturbations due to load connection from short-circuit faults. Furthermore, the algorithm implementation is not limited by the size of the DC microgrid nor by the type of the connected loads.

Static state power smoothing and transient power quality enhancement of a DC microgrid based on multi-function SMES/battery distributed hybrid energy storage system

Using hybrid energy storage system (HESS) to solve the issues of power regulation and fault protection of DC microgrids has attracted extensive attention. To realize multiple functions of power flow control and power quality enhancement of the DC microgrid, a concept of distributed HESS based on superconducting magnetic energy storage (SMES) and battery energy storage (BES) is introduced in this paper. A SMES-based energy-storage-composited DC transformer (ESDCT) is designed to interconnect the low-voltage DC microgrid into a medium-voltage DC power system. It is with the functions of DC voltage transformation, and grid fault isolation. The BES is selected to regulate the unstable output power of the LVDC microgrid caused by renewable energy generators and load variations. The modeling of the SMES-based ESDCT and the cooperative control of the BES and SMES is analyzed. Finally, the performance of the SMES/BES distributed HESS is verified via a simulation, and the feasibility of the proposed SMES-based ESDCT is demonstrated via a scaled-down experiment.

Reliability evaluation of an aggregate power conversion unit in the off-grid PV-battery-based DC microgrid from local energy communities under dynamic and transient operation

In a rural/local energy community (LEC), a PV-battery-based DC microgrid (MG) normally operates at the off-grid/islanded mode. At this off-grid operation mode, the reliability of power converters in both the PV system and the battery energy storage system (BESS) could be reduced by dynamic operation scenarios of the DC microgrid which are frequently repeated, such as the intermittency of PV-system output power, the random fluctuation of load power. Indeed, the dynamic operation of the PV-generating system and the load system in the off-grid DC microgrid could lead to a certain decrease in the reliability of DC–DC bidirectional converters of the BESS, boost converters of the PV system, and buck converters for low-voltage DC loads because of withstanding different power flows to provide a source-load power balance in the DC microgrid. Moreover, transient operation scenarios of the off-grid DC microgrid can significantly impact the reliability of the mentioned power converters. To make the above assumptions more clear, reliability analysis of an aggregate power conversion unit (APCU) in the off-grid PV-battery-based DC microgrid under dynamic and transient operation in the local energy community is presented in this paper. The APCU contains the boost converter of the PV system, the bidirectional converter of the BESS, and the buck converter of the low-voltage DC load system. The main objective of the paper is to shed light on calculations of the dynamic-voltage-dependent failure rate (DVDFR) and fault-current-dependent failure rate (FCDFR) of the APCU from dynamic and transient operation conditions respectively in the off-grid LVDC microgrid. Then, a novel combination of the useful-time-dependent failure rate (UTDFR), the DVDFR, and the FCDFR is proposed to evaluate the system-level and component-level reliability of the APCU in the off-grid DC microgrid. Markov-state transition diagrams are applied for the APCU’s reliability assessment. Experimental results show that the reliability of the bidirectional power converter is more affected by the dynamic and transient operation than that of the boost or buck converters in the APCU. In addition, the DVDFR of the APCU is almost smaller than its FCDFR, but the system-level reliability of the APCU could be significantly reduced by dynamic cases in the islanded DC microgrid. Last but not least, the MTTF and MTBF of the APCU could be dramatically decreased by the dynamic and transient operation of the DC microgrid.

A stepwise methodology for the design and evaluation of protection strategies in LVDC microgrids

Over the past years, there has been an increasing scientific interest in Low Voltage DC grids as an alternative to traditional Low Voltage AC grids, driven by the energy transition. However, fault detection and protection of LVDC microgrids still poses an important challenge for their breakthrough on a large scale. Due to the required speed and reliability of LVDC microgrid protection, an increasing amount of research is focussing on local, measurement-based protection algorithms. While several solutions for such a local, measurement-based fault protection algorithm have been proposed in literature, the design process of these algorithms is often unclear. As a result, it is often not straightforward to an LVDC grid designer how to tackle the design of an LVDC microgrid protection strategy. Furthermore, the framework within which the proposed algorithms are designed is often not discussed, as well as how well the algorithm will perform when the fault conditions deviate from this reference framework. Therefore, this paper presents a stepwise methodology for the design of fault protection strategies in LVDC microgrids with the aim of making this design process more straightforward and transparent. Furthermore, the proposed methodology allows to evaluate the robustness and boundaries of the designed protection strategy easily. The different steps of the methodology are discussed in detail and applied to a case study, followed by a sensitivity analysis of the developed protection algorithm to investigate its boundaries and improve its robustness.

Residual-Based Fault Detection and Isolation in Lvdc Microgrid

Residual-Based Fault Detection and Isolation in Lvdc Microgrid

Current-Source Solid-State DC Transformer Integrating LVDC Microgrid, Energy Storage, and Renewable Energy Into MVDC Grid

Solid-state dc transformer to integrate low-voltage dc (LVdc) microgrid, wind turbine (WT) generator, photovoltaic (PV), and energy storage (ES) into medium-voltage (MV) direct-current (MVdc) distribution grids is attractive. This article proposes current-source dc solid-state transformer (SST) for MVdc collection system in WT, PV, and ES farms or as an interface between the MVdc grid and the LVdc microgrid. Compared to conventional current-source converter (CSC) based SSTs, a switch reduction scheme on reverse-blocking device bridges is proposed to reduce device count and the number of devices on the dc-link current path. Importantly, the proposed switch reduction scheme is generic and can be applied to the dc ports of dc–ac, ac–dc, or dc–dc hard-switching or soft-switching CSC-based SSTs. Based on this scheme, the proposed current-source dc SSTs are derived, which have reduced electrolytic-capacitor-less dc-link. The proposed dc SSTs also achieve single-stage isolated dc–dc or dc–ac conversion, full-range zero-voltage switching (ZVS) for main switches, zero-current switching (ZCS) for resonant switches, and controlled <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ dv/dt $</tex-math></inline-formula> . The proposed dc SSTs, operating principles, predictive control method, the ZVS, and the controlled <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ dv/dt $</tex-math></inline-formula> under voltage buck-boost ranges are verified with MV simulations and an experimental prototype based on SiC <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfets</small> , diodes, and a nanocrystalline transformer.

Precise current sharing and decentralized power management schemes based on virtual frequency droop method for LVDC microgrids

One of the challenging issues in DC microgrids attracted by many researchers is the current sharing enhancement among distributed generations (DGs) with different rated currents and/or lines impedances. In this paper, DG topology is considered comprising photovoltaic (PV) system as power resource and battery energy storage system (BESS). By inspecting the frequency droop control of synchronous generators, a unique corrective sinusoidal term is added to the output voltage in DC-DC converters of each DG with purpose of current sharing control between DC DGs based on their nominal capacities. This virtual decentralized current sharing scheme named as the frequency droop method (FDM) can accurately regulate the point of common coupling (PCC) voltage without any communication link. Moreover, a decentralized power management scheme with different operating modes for PV system and BESS is proposed in this paper based on the state of charge of BESS and the frequency obtained from FDM scheme. The performance of FDM and power management schemes is investigated by time domain off-line and real-time simulations in MATLAB/SIMULINK environment. Seven case studies are implemented considering the effects of load and solar irradiance variations under equality and inequality of lines impedances as well as rated currents, mode transition, and plug-and-play ability. By applying the proposed control schemes, the purposes of precise current sharing and DC-link voltage regulation are achieved smoothly with an acceptable transient compared to an existing power sharing method. Also, FDM scheme is more reliable than available methods due to its independence on line impedance.

LVDC Microgrid를 위한 웨이브릿 변환 기반 지락고장 검출

LVDC Microgrid is a small-scale power grid that can increase the stability and efficiency of power supply in a small area, enabling independent operation and increasing the acceptability of DERs (Distributed Energy Resources). However, this is an ungrounded IT (Isolated Terre) grounding system, and even when a ground fault occurs, there is no fault current path and no significant difference compared to the steady state current, so fault detection is very difficult. WT (Wavelet Transform), an easy tool for transient analysis and signal identification, is to be applied to the detection of ground faults in LVDC Microgrid.<BR>In this paper, we propose a Wavelet transform-based ground fault detection method for an ungrounded LVDC Microgrid. By changing the level and order of various MW (Mother Wavelets) using the current collected in the existing pulse injection method, the optimal MW for ground fault detection method is derived. First, an ungrounded LVDC Microgrid is modeled using PSCAD s/w, and ground faults are simulated by varying the fault point and fault resistance. We implement the selected optimal Wavelet transform-based ground fault detection method using Matlab s/w. Finally, performance evaluation is performed through simulation using various collected signals.

A new adaptive instantaneous average current sharing technique for circulating current minimization among parallel converters in a LV DC-microgrid

This paper proposes a new adaptive instantaneous average current sharing technique for load current sharing and minimizing circulating current among parallel-connected converters in a LV islanded DC microgrid. This control technique is common for load current sharing among parallel inverters in AC systems, however, its use in DC networks has not been reported in the open literature. Unfortunately, the conventional controller does not come up with the expected improvements in terms of regulating the load voltage and minimizing circulating current among parallel DC-DC converters. Therefore, adaptive instantaneous average current sharing technique, based on the steepest descent method, is proposed to achieve this goal. A mathematical model is derived for n-parallel converters with the proposed controller. In addition, stability analysis of closed-loop controllers is carried out by studying locations of poles using the root locus plot. The analysis and simulation of the proposed system demonstrate that the proposed controller shows better performance for regulating the load voltage and achieving very low circulating current level. Matlab/Simulink model is implemented to verify the proposed controller performance under different operating conditions.

Protection of LVDC Microgrids in Grid-Connected and Islanded Modes Using Bifurcation Theory

Low-voltage dc (LVdc) microgrids facilitate the integration of renewable energy systems and modern loads. However, they suffer from the lack of a sensitive, selective, reliable, and fast protection strategy. The low fault current of high-resistance faults makes fault detection and faulty zone identification challenging tasks for protection engineers. This article proposes a protection scheme that is effective for both bolted and high-resistance faults at the LVdc feeders. The developed strategy relies on two intelligent electronic devices at both ends of the protected zone, calculating the normalized superimposed component of the line current. By adding this component to the input of the periodically forced harmonic oscillator (PFHO), both fault occurrence and fault direction can be simultaneously determined based on the oscillator state variable with only two possible values during the stable and unstable modes. It eliminates the need for the selection of various protection thresholds for different LVdc microgrid structures/topologies. The proposed protection strategy can classify pole-to-ground and pole-to-pole faults and is effective for both grid-connected and islanded LVdc microgrids. The reliable performance of the developed PFHO-based protection is assessed on the simulation model of a ±750 V meshed LVdc microgrid with the TN-S grounding system.

VMD-Enabled Current-Based Fast Fault Detection Scheme for DC Microgrid

The utilization of low-voltage direct current (LVdc) distribution system is gaining popularity in recent years due to its numerous advantages. However, the development of efficient fault detection scheme for an LVdc microgrid is a challenging problem. This article proposes a fast and effective variational mode decomposition (VMD)-based fault detection technique for the LVdc distribution system with penetration of renewable sources using the local end current measurements only. This scheme processes the rate of change of current signal of the distribution lines through the VMD algorithm that decomposes it to different modes. The cumulative energy of the most significant mode is being computed and used as a fault detection index. The proposed scheme is tested for pole to pole and pole to ground fault with broad variation in fault location, fault resistance, and distributed generations penetration in grid-connected and islanding mode of microgrid operation. The performance of the proposed technique is compared with the existing differential current and overcurrent relaying scheme as well as other existing schemes. The testing has been done in MATLAB/Simulink and Typhoon hardware-in-loop testbed. The proposed scheme detects all types of fault with wide variation in fault and operating conditions of the microgrid with faster response time. The test results indicate that the proposed protection scheme is a potential candidate for providing effective protection measure for LVdc microgrid.