Low Voltage Direct Current Distribution Network Research Articles

Protection scheme for LVDC distribution network based on stepwise blocking of power electronic transformer

AbstractCascaded H‐bridge power electronic transformers (CHB‐PET) can efficiently connect AC/DC distribution networks, realizing power transmission and electric energy conversion. Once a pole‐to‐pole DC fault occurs on the low‐voltage direct current (LVDC) side of CHB‐PET, the fault current has a fast rise and high amplitude, which easily leads to the rapid blocking of power electronic devices and makes it difficult for DC line protection to obtain fault information. A stepwise blocking scheme of PET is proposed for LVDC network protection. When the fault on the LVDC side of PET is detected, cascaded H‐bridge and low‐voltage H‐bridge of dual‐active‐bridge are blocked first, and then high‐voltage H‐bridge of dual‐active‐bridge is blocked after a 5 ms delay. The stepwise blocking can provide fault information for DC line protection. On this basis, wavelet packet transform is employed to extract different frequency components of fault current. The energy of the specific frequency band corresponding to the switching frequency of DAB and the total energy of all frequency bands contained in the fault current is calculated; then their ratio can be utilized to construct the protection scheme. Finally, the PSCAD/EMTDC simulation results verify the feasibility and effectiveness of proposed protection scheme based on stepwise blocking of CHB‐PET.

A Low-Voltage AC, Low-Voltage DC, and High-Voltage DC Power Distribution System with Grid: Design and Analysis

Low-voltage (LV) and high-voltage (HV) DC distribution systems are being investigated as alternatives due to the growth of DC distribution energy resources (DER), DC loads such as solar and wind power systems, and energy storage sources (ESSs). Furthermore, an HV/LV DC distribution system offers various advantages, including lower conversion losses, an easier connecting strategy for DC DERs, and less complex power management techniques. As renewable energy sources are increasingly incorporated into the electrical grid, it is important to create novel, effective approaches for connecting such sources and loads. It would hence be effective to merge DC distribution with AC distribution to fulfill the energy demands of both DC and AC consumers. To this end, this study proposes a multizone design with four buses: low-voltage direct current (LVDC), high-voltage direct current (HVDC), low-voltage alternating current (LVAC), and an electrical grid. A model of this system that covers crucial elements, including power systems, DER systems, and power electronic devices, to serve as a foundation for the analysis and design of this architecture is proposed. MATLAB/Simulink is used to conduct a simulation study to verify the performance of the proposed design. In this study, a hybrid electrical grid with an LVDC, HVDC, and LVAC distribution network test is used and implemented. Additionally, a transient and steady-state characteristic analysis of the test system is performed.

Active detection fault diagnosis and fault location technology for LVDC distribution networks

Under the rapid development of direct current (DC) transmission technology, the interconnection between converter stations in the DC network has been further improved, and a looped network topology has been gradually formed. Practical applications have shown that loop troubleshooting takes a long time to perform and requires substantial field experience, which has considerably reduced the efficiency of troubleshooting and the safety factor of low-voltage direct current (LVDC) distribution networks. The complex, transient characteristics and short duration of power electronic system faults limit the application of passive detection and protection technology, raising difficulties in DC line fault detection and location. To adapt to the development of power systems, this paper presents an active detection fault diagnosis and fault location technology for LVDC distribution networks based on a converter. After a line fault occurs, the fault type is identified by injecting detection signals into the fault line through the converter, and the fault location is determined by calculating harmonic impedance. The method can detect the fault of the system in time to reduce loss when the system on the converter side is faulty but can still run. It effectively utilizes the converter equipment in power systems and does not require the installation of additional detection devices, which is largely unaffected by fault resistance in fault location, and has accurate detection results and fast speed.

Fault analysis of photovoltaic based DC microgrid using deep learning randomized neural network

The implementation of solar photovoltaic (PV) based Direct Current (DC) microgrid is limited due to lack of well-defined protection standards. These low voltage DC distribution networks are facing implementation challenges especially under multi-distributed generations subjected to various events such as PV arc faults, load switching faults, PV partial shading, change in PV irradiance and DC cable faults, etc. Thus a new deep learning neural network model is proposed for the accurate fault classification and distance calculation for an effective monitoring and protection coordination of photovoltaic based DC microgrid. The proposed model is the combination of an Adaptive variational mode decomposition (AVMD) and deep minimum variance random vector functional link network (DRVFLN). The optimal parameters of AVMD are selected using chaotic sine–cosine firefly algorithm (CSCFA) and efficient weighted kurtosis index (EWKU). Further, a novel non-iterative DRVFLN network is applied for accurate fault classification and location. In the DRVFLN direct connections are present from preceding layers to the forward layers of the network like random vector functional link network. These connections help to reduce the model complexity and also regularize the randomization. Further, the denoising criterion is also introduced in this network where the uncorrupted input can be recovered from the corrupted versions by using the autoencoder to obtained better results than the traditional networks. The performances of various models are evaluated using some performance index such as overall accuracy and sensitivity. Results conclude that the proposed AVMD-DRVFLN model classified the faults with an accuracy and sensitivity of 100% for all the events. The model performance is also tested and validated against the unwanted noise by considering different signal-to-noise ratio to ensure the robustness of the proposed model. Results conclude that the proposed model correctly classified the faults against such incorporation of noise. Similarly, the performances of various models for the estimation of fault distance are validated by computing the relative error. Results conclude that the proposed AVMD-DRVFLN model produces promising result in terms of relative error(which is less than 5%) for all the events. Comparisons with some existing models like extreme learning machine, support vector machine, random vector functional link network and deep extreme learning machine are included to validate the efficacy of the proposed AVMD-DRVFLN model. Results conclude that proposed method outperforms all the other methods. The effectiveness of the proposed AVMD-DRVFLN model for fault classification and distance estimation is established through rigorous case studies in MATLAB augmented by some real time test results.

Small-Signal Stability Analysis for Multi-Terminal LVDC Distribution Network Based on Distributed Secondary Control Strategy

This paper presents a detailed and accurate small-signal analysis model for a four-terminal low-voltage direct current (LVDC) distribution network with distributed secondary control strategy. To tackle the contradiction between power sharing accuracy and average voltage recovery ability of voltage source converters (VSCs), a distributed secondary control strategy is adopted, which is based on average consensus algorithm and local voting protocol. Furthermore, to analyze the stability of LVDC distribution network based on the proposed distributed secondary control strategy, a detailed and accurate small-signal analysis model is formulated which is derived from various non-linear state-space sub-models. The time-domain simulation and electromagnetic simulation are carried out to verify the validity and accuracy of the proposed model. Based on this model, the influence rules of main eigenvalues are summarized with the variation of PI control and DC line parameters. Time-domain simulations conducted in PSCAD are used to validate the operational limits of the secondary controllers and DC line obtained from the small-signal stability analysis.

A Novel Fault Let-Through Energy Based Fault Location for LVDC Distribution Networks

Low Voltage Direct Current (LVDC) distribution systems have recently been considered as an alternative approach to electrical system infrastructure as they provide the additional flexibility and controllability required to facilitate the integration of more low carbon technologies (LCTs). However, DC protection systems and, more specifically high accuracy DC fault location, have been recognised as a key challenge to facilitating post-fault network maintenance. Most of the existing fault location techniques rely on current derivative or communications-based methods that are either very sensitive to noise or require a high level of data synchronisation. Fault energy has been recognized as a reliable indicator of more accurate fault location estimations. Therefore, this paper develops a mathematical model for describing fault energy during the transient period of DC faults. The method subsequently proposes a new fault let-through energy based DC fault location working strategy to facilitate post-fault network maintenance. The proposed method does not require data synchronisation regardless of the voltage, current, and the size of the converters connected to the LVDC feeder. The capabilities of the proposed fault location strategy are validated against different faults applied on an LVDC test network in PSCAD/EMTDC and shown to be more reliable and accurate than existing methods.

Simulation Test of a DC Fault Current Limiter for Fault Ride-Through Problem of Low-Voltage DC Distribution

The low voltage direct current (LVDC) distribution networks are connected with too many kinds of loads and sources, which makes them prone to failure. Due to the small damping value in the DC lines, the fault signal propagates so fast that the impact current with the wave front of millisecond and the transient voltage pose great challenges for fault detection. Even worse, some faults with small currents are difficult to detect and the communication is out of sync, resulting in protection misoperation. These problems have severely affected the new energy utilization. In view of this, a DC fault current limiter (FCL) composed of inductance, resistance, and power electronic switch was designed in this paper. The rising speed of fault current can be decreased by the series inductance and the peak value of the fault current can be limited by series impedance, thus in this way the running time can be gained for fault detection and protection. For distributed energy access, by deducing the short circuit fault characteristic expression of LVDC distribution network, the feasibility of FCL was verified. Based on the structure of the bridge-type alternating current (AC) current limiter, the structure and parameters of the DC FCL were determined according to the fault ride-through target. Then, a low voltage ride-through strategy based on DC FCL was proposed for the bipolar short-circuit fault of LVDC distribution network. Finally, MATLAB/Simulink simulation was used to verify the rationality of the proposed FCL and its ride-through strategy.

Novel Monitoring System for Low-Voltage DC Distribution Network Using Deep-Learning- Based Disaggregation

The deployment rate of distributed energy resources (DER) based on renewable energy has recently been increasing worldwide. Direct current (DC) power distribution has been proposed as an efficient approach for operating digital loads and DC-based renewable energy. DC distribution systems with DERs, however, are not commonly used in the real world. From the viewpoint of distribution system operators, it is important to identify the operation status of DERs for effective grid operation. In this article, a novel monitoring methodology for low-voltage DC (LVDC) distribution systems with DERs is proposed based on frequency-domain analyses. A deep-learning technology is applied to model the frequency characteristics of individual DERs. A case study considering two approaches was conducted using a photovoltaic generator, wind turbine, diesel generator, and energy-storage system installed in an LVDC testbed operated by KEPCO in Gochang, Jeolla-do, Korea. In the first approach, monitoring is performed with sensors installed near individual DERs. In the second approach, monitoring is performed with a single sensor in the distribution line, and the signal is disaggregated to identify the status of the individual DERs. The results show that the proposed methodology tracks the status of DERs with an accuracy of 98% and 95%, respectively, demonstrating the validity of the proposed methodologies.

Data-Driven Control of LVDC Network Converters: Active Load Stabilization

This paper addresses the (model-free) data-driven control of power converters, acting as distributed generators, in low voltage direct current (LVDC) networks (e.g., DC microgrids, DC distributed power systems, DC buses wit multiple sources and loads, etc.). Since traditional stand-alone control design, cannot guarantee stability when converters are connected to a network, it is proposed a deterministic solution that does not require the network model–an approach purely based on measurement data. This is a suitable way to overcome common issues when using a model-based approach, e.g., the use of an excessive number of variables and equations, the presence of un-modeled dynamics, unknown parameters and/or the lack of first principle model equations. To corroborate the advantages of the proposed approach, the present work addresses an extreme but also realistic scenario: weak networks with active loads, such as constant power loads (CPLs). It is also shown that the proposed scheme guarantees stability in a rigorous deterministic way–using a Lyapunov approach based on coefficient matrices directly constructed from data. Simulation results using a multi-bus LVDC distribution network, based on PSCAD/EMTDC, are provided as proof of concept.

Improved voltage‐based protection scheme for an LVDC distribution network interfaced by a solid state smart transformer

The increasing electrification of transport and heat will place increasing demand on low voltage (LV) networks with the potential to overload medium voltage (MV)/LV transformers and LV cables. Deployment of a solid-state transformer (SST) at MV/LV substations and using LV direct current (LVDC) distribution systems offer great potential to address such challenges. However, the SST deployment in addition to the introduction of LVDC will fundamentally change LV fault behaviour and protection requirements due to the limited short-circuit capabilities of such technologies. The SST will deliver limited fault currents, making current-based protection (widely used in LV networks) less reliable. Therefore, this study presents an advanced communication-less protection scheme which can effectively detect and locate DC faults even with reduced fault levels. The developed protection scheme overcomes the selectivity limitations in LVDC voltage-based protection solutions by using a combination of DC voltage magnitude, voltage concavity (sign of d 2 v /d t 2 ) and the sign of the rate of change of current (d i /d t ) regardless of the current magnitudes. The credibility of the developed protection algorithm is tested against different fault scenarios applied on an active LVDC network model built in PSCAD/EMTDC. Noise signals have been included in the simulation to appraise the resilience of the developed scheme.

Fault location in low‐voltage direct current network based on auxiliary injection unit

Fault location plays an important role in accelerating fault clearance and grid recovery, which greatly improves the reliability and robustness of low-voltage direct current distribution network. A fault location method based on auxiliary injection unit (AIU) is proposed in this study with respect to its basic structure and principle. With single terminal information, the proposed method allows low requirement of sampling frequency and enhances the tolerance to fault resistance by utilising the characteristic of AIU. Furthermore, design criteria of components in AIU are discussed in detail. Optimised parameters contribute to better performance in estimating fault distance. Numerous simulations demonstrate that the proposed method can realise accurate fault location with fault distance and fault resistance varying in a wide range.

Protection Scheme of a Last Mile Active LVDC Distribution Network with Reclosing Option

A low voltage direct current (LVDC) distribution network is a promising technology to meet the standards of future energy demands for smart loads. An LVDC distribution network can not only supply efficient, smooth and clean energy, but also makes the integration of renewable energy resources in the distribution system easy. A major obstacle in the implementation of the LVDC distribution network is the protection of the network during abnormal grid conditions, such as transients and faults. This paper analyzes DC fault characteristics considering an LVDC distribution network, highlights the worst case scenario during a fault and protection related issues and proposes the protection schemes for the LVDC network. In the proposed protection scheme, a fault is detected and located through superimposed components. To minimize the effect of the DC fault on the distribution network, distributed fault current limiters are introduced and the final decision to disconnect or reconnect the affected line is made on the basis of the type of fault. In addition, a reclosing scheme for a temporary fault is proposed to avoid high inrush currents and false tripping, which eventually increases the reliability. A fast communication-based backup protection is also suggested, and to reduce dependency, a secondary backup is used in the case of communication delay or failure. The proposed scheme is verified using the modified IEEE 13 node test system, which is implemented in ATPDraw. The results show that the proposed scheme can successfully detect, locate and limit a DC fault in an LVDC distribution network with different fault resistances or locations. Moreover, the network is restored successfully in the case of temporary faults.

Validation of Fast and Selective Protection Scheme for an LVDC Distribution Network

Low-voltage direct-current (LVDC) distribution systems potentially enable more efficient power distribution and wider uptake of distributed renewables and energy storage. They do, however, present significant fault protection and safety challenges. To address these, the use of advanced protection techniques or significant system redesign is required. This paper reviews these protection key challenges, and presents experimental results of a prototype advanced protection scheme designed to help enable LVDC distribution networks for utility applications. The developed scheme is DC current direction-based and uses multiple intelligent electronic devices relays in combination with controllable solid-state circuit breakers to detect and locate DC faults. This scheme provides selective protection tripping within submillisecond timescales. A scaled laboratory demonstrator that emulates an LVDC distribution network is used as a test platform. It allows for the characterization of transient behavior for various fault conditions and locations. The developed protection algorithm is implemented in LabVIEW, and its performance against such fault conditions is tested within this environment.

Comparison between Underground Cable and Overhead Line for a Low-Voltage Direct Current Distribution Network Serving Communication Repeater

This paper compares the differences in economic feasibility and dynamic characteristics between underground (U/G) cable and overhead (O/H) line for low-voltage direct current (LVDC) distribution. Numerous low loaded long-distance distribution networks served by medium-voltage alternative current (MVAC) distribution lines exist in the Korean distribution network. This is an unavoidable choice to compensate voltage drop, therefore, excessive cost is expended for the amount of electrical power load. The Korean Electric Power Corporation (KEPCO) is consequently seeking a solution to replace the MVAC distribution line with a LVDC distribution line, reducing costs and providing better quality direct current (DC) electricity. A LVDC distribution network can be installed with U/G cables or O/H lines. In this paper, a realistic MVAC distribution network in a mountainous area was selected as the target model to replace with LVDC. A 30 year net present value (NPV) analysis of the economic feasibility was conducted to compare the cost of the two types of distribution line. A simulation study compared the results of the DC line fault with the power system computer aided design/electro-magnetic transient direct current (PSCAD/EMTDC). The economic feasibility evaluation and simulation study results will be used to select the applicable type of LVDC distribution network.

Observer-Based Output Voltage Control for DC Power Distribution Purposes

Observer-based output voltage control is proposed for single-phase full-bridge inverter, that is used to produce the voltage for a customer in a low-voltage direct current distribution network. To avoid difficulties to realize a feasible-output-voltage-tracking control for sinusoidal references, the control system is realized in a virtual dq-frame. The virtual frame is created using a second-order generalized integrator. In the proposed control system, two different observer structures are applied. Using observer-based control, also provides means to reduce the number of measurements down to one. In this paper, different realizations of an estimator-based control system for a single-phase inverter are investigated. Specifically, it is considered how many measurements are needed and which variables should be measured and which estimator structure is to be applied in order to achieve the best overall performance.