DC Network Research Articles

Integrated RES With Intelligent MPPT For Efficient Power Generation & Wireless Transmission for PV Applications

The increasing global energy demand and the urgent need to shift to sustainable practices have made the integration of renewable energy sources (RESs) into distribution power systems imperative. This project's main objective is to implement rooftop photovoltaic (PV) systems as an essential part of low-voltage (LV) DC networks' building-integrated centralized generation Maximum Power Point Tracking (MPPT) using Artificial Neural Networks (ANNs) is used to maximize power extraction from photovoltaic (PV) panels and boost power generation efficiency, particularly in partially shaded circumstances. After that, the PV system's generated power is sent to a Luo converter, which effectively tracks and optimizes the power production. For wireless power transmission, the Luo converter's output is then fed into a high-frequency converter. This wireless power transmission improves the system's adaptability and scalability by facilitating smooth energy transfer without the requirement for physical connections. An isolation transformer is attached to the high-frequency converter's output in order to guarantee the system's dependability and safety. The high-frequency converter is isolated from the downstream components by the isolation transformer, which also offers protection against electrical risks. Finally, a battery made especially for use in electric vehicle (EV) applications receives the transformer's isolated output, which is also supplied to DC loads. Finally, we will use the MATLAB 2021a / Simulink program to do a number of numerical simulations in order to validate the suggested controls. The output voltage of 130v is stored in the battery if a voltage of 65 v is taken as output from the photovoltaic system.

Assessment of Quasi-Stationary Power Quality Phenomena in DC Power Systems

The adoption of dc architectures for electricity distribution has been increasingly considered to reduce the number of energy conversions and to exploit their inherent ability to cope with peaks in energy supply and demand. Furthermore, dc systems have been commonly and widely adopted for high-voltage transmission systems and public transport for many years. Despite this, the Power Quality (PQ) assessment in dc power networks is still unstandardized. In the scientific literature, various indices have been proposed for the quantification of dc PQ. However, not all the aspects encountered in practical applications are adequately accounted for, and their effects regarding the impact on the measurement results are not fully analysed. This paper focuses on signal processing for the analysis of PQ of dc networks in stationary or quasi-stationary conditions aimed at compatibility level assessment. The influence quantities that affect the measurement results of the most commonly adopted procedures for determining the PQ index determination are identified and modelled, and their impact is analytically calculated. Advanced signal processing methods, suitable to be adopted in standards for dc PQ compatibility level surveys, are proposed to improve the accuracy and reliability of results. The rationality and effectiveness of the proposed procedures are shown with analytical calculations and in real measurement situations.

An AC Z-bus-based distribution factors for contingency analysis of AC–DC networks

The AC–DC networks are susceptible to deliberate and unforeseen contingencies, much like any other traditional power network. Due to the complex form of the unified impedance matrix derived for the provided network, the classic Z-bus-based contingency analysis approach cannot comprehend the AC–DC network. The distribution factors thus obtained from a unified complex Z-bus lead to an indication of the flow of complex current for DC lines. Therefore, an alternative method to discover post-contingency consequences is looked for. A streamlined approach for visualizing an equivalent AC network for the given AC–DC network has been proposed in this work. The AC-equivalent network takes the original AC–DC network and recreates it using two parallel AC networks to get the requisite distribution factors. The ultimate current/power distributions of the original AC–DC network can be determined by applying the superposition rule to the parallel networks, by the linear characteristics of the suggested technique. With this, the standard AC-contingency analysis method to complete the steady-state contingency analysis for the AC–DC network can be utilized. The proposed approach has been validated by the encouraging outcomes achieved for AC–DC networks with 10, 13, and 66 buses.

Comprehensive case study on the technical feasibility of Green hydrogen production from photovoltaic and battery energy storage systems

AbstractThe growing demand for alternative energy sources to alleviate environmental impacts highlights the need to move from fossil fuels to renewable energy. This study demonstrated the technical feasibility of using a solar photovoltaic (PV) system for the production of green hydrogen. This research examined electrical and power data from a PV plant in Irecê, Bahia, using open data sources to provide insights into the production of green hydrogen from renewable sources. The system mainly depends on the use of a renewable source, PV solar energy, integrated with batteries, electrolyzers, and hydrogen tanks. Electrolyzer, battery, and hydrogen tank sizing analysis for optimal hydrogen production was effectively conducted using HOMER Energy software. The predicted system topology prioritizes a local DC network, optimizing efficiency for electrolyzers that have inherently low efficiency. The electrolyzer simulation involves initial Python‐based sizing and comprehensive sizing with HOMER Energy software, ensuring accuracy within a 10% discrepancy limit. This highlights the importance of analytical calculations and optimization software for sizing more complex systems.

Optimal power flow in hybrid AC-DC systems considering N-k security constraints in the preventive-corrective control stage

The optimal power flow methods for AC-DC systems containing VSC-HVDC generally only consider the economy during normal operation, overlooking the distribution of line transmission power in fault conditions. As a result, lines that continue to operate after a fault may experience overloading or operate at full capacity. Thus, a method for optimal power flow calculation is proposed that incorporates N-k security constraints in the preventive-corrective control stage for secure and economic operation of hybrid AC-DC systems. This method ensures that the line transmission power in the system meets the limits in the normal, short-term fault, and long-term fault states. In addition to the optimal power flow in the normal state, the method incorporates the system's imbalance as an indicator to evaluate system resilience. It combines this indicator with the economic, network loss, and performance metrics of the system, forming a two-stage bi-level multi-objective optimization model. Furthermore, to address the curse of dimensionality in anticipating system fault sets, a method for generating the anticipated fault set using non-sequential Monte Carlo simulation is proposed, along with a fault scenario search approach based on robust thinking to identify the most severe faults. Finally, the traditional IEEE 30-bus system was improved, and simulation verification was conducted using examples of an AC/DC system with a three-terminal DC network and a wind-solar-storage hybrid AC/DC system with a three-terminal DC network. The simulation results indicate that the proposed optimal power flow method considering the preventive-corrective control stage with N-k security constraints can effectively enhance system resilience. Furthermore, it improves the economic efficiency while ensuring the secure operation of the system.

DC near‐area voltage stability constrained renewable energy integration for regional power grids

AbstractWith the increasing deployment of renewable energy resources, the scale of DC networks and renewable capacity continues to grow. System security is challenged by the decrease in inertia from traditional synchronous generators. In order to accommodate high‐penetration renewable energy, voltage stability should be considered in the renewable energy integration planning. Herein, first, a voltage stability‐constrained minimum startup index and algorithm for conventional thermal power plants are proposed. Then, based on time series production simulation, a renewable energy integration capacity analysis algorithm is designed considering voltage stability and peak shaving constraints. Finally, based on the boundary conditions of the “Fifteen‐Five Plan”, the renewable energy capacity considering voltage stability and peak shaving constraints for Northwest and East China power grids are analysed to verify the effectiveness and engineering practicality of the proposed methodology. The results demonstrate that the proposed method can maintain the renewable energy integration goal outlined in the “Fifteen‐Five Plan” while maintaining the voltage stability in these regions.

Optimal allocation of distributed generation on DC Networks

A consistent increase in the installation of distributed generation (DG) in response to rising energy cost and demand throughout the world, especially for the last two decades, primarily aims to obviate upgrading investments for transmission and distribution lines on the existing power grid while ensuring sustainable, resilient, and reliable service to end users. Random installation of DG, on the other hand, can invalidate the existing grid’s designated objectives, and operational boundaries due to the changes in network topology resulting in unforeseen load flow. Appropriate integration of DG units on DC networks should be remedy to high transmission/distribution losses in lines, instabilities in voltage profiles, issues in power quality parameters, and derogation in the protection schemes. Based on the behavior of load profiles, e.g., 24 h or longer period, operational and technical constraints, this study proposes a framework that minimizes transmission/distribution losses on a DC network, in which discrete variables stand for the location of DGs. Optimal power flow (OPF) problem formulation underpins the proposed optimization framework. Overall, this computationally challenging planning problem refers to a mixed-integer nonlinear program (MINLP) including discrete variables with non-convex power balance/flow representations. To deal with the non-convexities and discrete form of the formulation, a mixed-integer second-order cone programming (MISOCP) relaxation, and branch-and-bound search are assigned. The validation of the introduced approach is carried out on the IEEE modified benchmark systems. With the help of proposed method, optimal allocation of DG units on 9-bus, 14-bus, and 30-bus systems, respectively, lead up to 99.93%, 94.38%, and 79.09% total power losses reduction.

Improved assembly DC circuit breaker based on resonant current injection.

DC circuit breakers (DCCBs) with high breaking capacity and low cost are necessary for quick fault clearance in DC networks. The assembly DC circuit breakers (ADCCBs) have a main breaking section (MBS) and a sub-breaking sections (SBS) for each line, which greatly reduce the cost. But in conventional operation, it bears high voltage for a long time when there is a main switch grounding process in any line fault action. To address this problem, a multiport assembly circuit breaker based on current injection (CI-MPACB) is proposed, which is able to generate a resonant current with increasing amplitude by controlling the duty cycle of Integrated Gate-Commutated Thyristors (IGCTs). Then the resonant current is injected into the SBS to generate current zero crossing and arc extinction. A complex frequency domain circuit analysis is performed on the MBS to describe the action logic as well as the commutation characteristics. In addition, the parameters of each component of the MBS are subject to multiple constraints and reasonable design to ensure the fault current could be cut off quickly and reliably. The cost of existing design is greatly reduced due to the design idea of resonant current injection device parameter selection. Finally, a PSCAD/EMTDC simulation confirms the opening viability of CI-MPACB and the accuracy of the parameter design. The test results show that the designed CI-MPACB can cut off DC fault lines.

Energy management in gyms microgrid: Nonlinear control of stationary bikes and treadmills with parallel rectifiers and AC/DC conversion

This article presents the complete design of a nonlinear control system for multiple stationary bikes connected to a mechanical energy conversion system equipped with squirrel cage induction generators (SIGs) linked to AC/DC converters. The primary objective of this study is to investigate the feasibility of powering multiple treadmills using the DC bus via DC/AC converters. The novelty of our approach lies in the integration of a new strategy of control system to achieve multiple control objectives within a gym microgrid environment, and an energy management algorithm to ensure the energy flow between intermittent generation and the considered, somewhat random, demand. The system is composed of several subsystems: i) stationary bikes connected to the DC bus via inverters acting as intermittent power sources; ii) a Li-ion battery-based energy storage system interfaced through a Buck-Boost converter; iii) electromechanical loads, including treadmills, powered by DC/AC converters, in addition to other DC loads. The main control objectives are as follows: a) each stationary bike extracts energy from the DC network by regulating the torque applied by the athlete, ensuring that the torque follows the reference torque; b) the treadmill’s speed must follow the reference generated based on the athlete’s condition; c) ensuring the protection of the energy storage system by monitoring its current and voltage; d) all mentioned objectives are achieved while maintaining the DC bus voltage at a reference value. To achieve these objectives, a nonlinear control approach based on Lyapunov theory is used to design the controllers. A formal analysis is conducted to assess the performance of the studied system. The system’s performance is demonstrated using the MATLAB/Simulink environment. Numerous simulations demonstrate that every control objective is achieved. Specifically, the system maintained the DC bus voltage at 500 V with a maximum deviation of 1 V, and the treadmill speed followed the reference within a margin of 0.1 m/s.

Study on the Design of Valve Base Insulation and Power Supply Transformer for High-Voltage DC Circuit Breaker

Abstract High-voltage DC circuit breaker is a key piece of equipment to building DC power grids. DC circuit breakers need to support and isolate from high voltage to ground through valve-based equipment and use valve-based power supply transformer to continuously supply energy to high potential components, the electrical performance of these two equipment determines whether DC circuit breakers and DC networks can be operated safely and reliably. For this reason, this paper carries out the research on the insulation capability of the valve base and the performance of the power supply transformer, such as the design and simulation of electric and thermal fields, as well as the test program. Firstly, the insulation design and simulation study of the valve base as a whole is carried out, and the design optimization of the equipotential scheme of the top homogeneous tubular bus, diagonal pull insulator and hydroelectric pole is completed. Secondly, the design and simulation verification of the thermal field distribution of the energy-supply transformer, the overvoltage between the ends of the output windings and the optimization scheme, and the short-circuit impedance were carried out. Finally, the insulation performance of the valve base, the temperature rise of the energy supply transformer, the overvoltage simulation of the output winding and the short-circuit characteristics were tested. The results show that: after optimization, the maximum field strength of the homogenizer is 2.3kV/mm, and the overall maximum field strength of the valve base is 2.61 kV/mm on the diagonal insulator, and there is no pressure difference between the water circuit and the fittings using the three-pin vertically distributed hydropower electrode scheme; the temperature rise of the supply transformer in the simulation and the actual measurement is 27K and less than 30K; by means of the parallel connection of the MOV, the overvoltage of the output winding of the power supply transformer under voltage impulse is effectively reduced to -1.8kV; the measured short-circuit impedance of the power supply transformer is 69.5%. The optimized design of DC circuit breaker valve base and power supply transformer can operate safely, and effectively support the construction of DC circuit breaker and DC power network.

ПУТИ ПОВЫШЕНИЯ ЭФФЕКТИВНОСТИ ЭКСПЛУАТАЦИИ ТРАДИЦИОН-НЫХ И ГИБРИДНЫХ РАСПРЕДЕЛИТЕЛЬНЫХ СЕТЕЙ

The main goals, development methodology and existing problems in distribution networks are con-sidered. It has been established that a promising direction for the development of electrical networks is the introduction of hybrid DC-AC networks, which combine the advantages of AC and DC net-works. Hybrid networks are promising, and already at the design stage, “smart” networks can be cre-ated on their basis, which can facilitate the work of the power system dispatcher, reduce personnel and maintenance costs

A faulty feeder selection method for SLG faults based on active injection approach in non-effectively grounded DC distribution networks

Identifying single-line-to-ground faults in non-effectively grounded DC distribution networks is challenging because the fault current is fleeting in the transient stage and negligible in the fault steady stage. First, an active signal injection method is proposed to inject signals into the DC network after the fault occurs. And the fault characteristics of the zero-mode fault circuit including the injected signal are analyzed. Second, a zero-mode parameter identification algorithm is proposed to calculate the length of each feeder based on the injected signal in the fault steady stage. It is found that the calculated length of the healthy feeder corresponds to its actual length, but the calculated result of the faulty feeder is the opposite of the total length of all healthy feeders. Finally, a faulty feeder selection method based on the polarity of the calculated length is proposed. The simulation results shown that the method can select faulty feeders under conditions of up to 5000 Ω transient resistance and 20 dB noise.

Understanding how junction resistances impact the conduction mechanism in nano-networks

Networks of nanowires, nanotubes, and nanosheets are important for many applications in printed electronics. However, the network conductivity and mobility are usually limited by the resistance between the particles, often referred to as the junction resistance. Minimising the junction resistance has proven to be challenging, partly because it is difficult to measure. Here, we develop a simple model for electrical conduction in networks of 1D or 2D nanomaterials that allows us to extract junction and nanoparticle resistances from particle-size-dependent DC network resistivity data. We find junction resistances in porous networks to scale with nanoparticle resistivity and vary from 5 Ω for silver nanosheets to 24 GΩ for WS2 nanosheets. Moreover, our model allows junction and nanoparticle resistances to be obtained simultaneously from AC impedance spectra of semiconducting nanosheet networks. Through our model, we use the impedance data to directly link the high mobility of aligned networks of electrochemically exfoliated MoS2 nanosheets (≈ 7 cm2 V−1 s−1) to low junction resistances of ∼2.3 MΩ. Temperature-dependent impedance measurements also allow us to comprehensively investigate transport mechanisms within the network and quantitatively differentiate intra-nanosheet phonon-limited bandlike transport from inter-nanosheet hopping.

FEM-Circuit co-simulation of superconducting synchronous wind generators connected to a DC network using the homogenized J–A formulation of the Maxwell equations

High-temperature superconductors (HTS) are greatly appealing for the development of high efficient, and high energy density power devices. They are particularly relevant for applications requiring light and compact machines such as wind power generation. In this context, to ensure the proper design of the superconducting machines and their reliable operation in power systems, it is then important to develop models that can accurately include their physics but also can describe properly their interaction with the system. To achieve such a goal, one approach is the co-simulation. This numerical technique can bring fine geometrical and physical details of the machines through a finite element model (FEM) meanwhile dealing with the operation of the whole system that incorporates the machine and a subset of the power grid represented by an external electrical circuit. The goal of the present work is to put to use this numerical technique when superconducting components are involved. Here, a case study is proposed involving a 15 MW hybrid superconducting synchronous generator (HTS rotor and conventional stator) coupled to a direct current network via a rectifier and its associated filter. The case study related to wind power application allows grasping the technical issues when employing co-simulation dealing with HTS machines. The FEM of the generator is done in the commercial software COMSOL Multiphysics, which interacts with the circuit simulator Simulink through the built-in Functional Mock-up Unit. For the present study, a new version of the latest J–A formulation combined with homogenization technique is introduced allowing an even faster computation time compared to the T–A formulation. Distributed variables and global variables such as current density, magnetic flux density, and local losses for the former and voltage, current, electromagnetic torque, and power quality for the latter are estimated and compared for both formulations. The idea is to find the best-suited combination FEM-circuit under criteria of computational speed, accuracy, and numerical stability. Thus, it is shown that all formulations generate an error of less than 5% on the machine parameters and that the J–A formulation with first order elements stands out with a significant 4-fold reduction in computational costs.

Integration of Superconducting Fault Current Limiter and DC Circuit Breaker for Electric Aircraft DC Network

Integration of Superconducting Fault Current Limiter and DC Circuit Breaker for Electric Aircraft DC Network

Pole-to-Pole Fault Management for Electric Aircraft DC Network With HTS Cables

Pole-to-Pole Fault Management for Electric Aircraft DC Network With HTS Cables

A decentralized coordinated control scheme for the DC network with a multiport converter

For complex networks with multiple subgrids formed by solid-state transformers, existing studies primarily focus on coordination within the networks in islanded mode, overlooking cooperation in grid-connected mode. To address this gap, this paper proposes a coordinated control scheme for the triple active bridge (TAB) converter in the grid-connected DC network. By establishing a droop relationship between the input power and the voltage difference, the TAB converter facilitates coordination between the utility grid and the DC network. The energy exchange with the utility grid is not only regulated by the TAB converter but also adjusted according to the utility grid state. So, the proposed scheme effectively facilitates coordination between the higher- and lower-level networks, strengthening system stability. Besides, the proposed scheme employs a unified control structure, which has power sharing and voltage regulation capabilities in grid-connected mode, as well as operating in islanded mode. This control structure enables the TAB converter to seamlessly transition across different operation modes, avoiding mode switching process and reducing the control complexity. Moreover, the proposed scheme is decentralized which negates the necessity for communication links, bolstering the system reliability. Lastly, the feasibility of the proposed scheme is analyzed in theory and validated by the hardware-in-loop tests.

Experimental evaluation of virtual synchronous generators with high regulation performance to enhance AC–DC grid interconnections

In the recent years there has been a significant increase in the interest surrounding the interconnection of AC and DC networks due to its potential to improve the power systems reliability. Despite the potential advantages, however, the AC and DC grid coexistence can pose challenges if not appropriately managed. In this scenario, this paper presents an enhanced control scheme enabling the use of Virtual Synchronous Generators (VSGs) as elements of interconnection between AC and DC networks. The proposed solution enables VSG operation with both fast dynamics for DC voltage regulation and slow dynamics for AC inertia emulation. This approach accommodates the contrasting requirements arising from the distinct mechanisms of DC voltage regulation and AC frequency regulation. Simulation and experimental results obtained with a 8 kVA three-phase converter prototype are provided to validate the effectiveness of the proposed technique and prove its better performances with respect to the solutions currently presented in literature.

Modular Circuit Synthesis Oriented Modelling Approach for Non-Isolated DC-DC Converters in DCM

The continued and envisioned large-scale integration of renewable energy sources as a reaction to rising global temperatures and climate change will need a readily available DC grid to increase commissioning and operating efficiency. The effective operation of these grids is predicated on the correct control of its main control points. A plethora of DC-DC converters that find use in DC microgrids act as the main control points. DC-DC converters are non-linear and can operate in different modes with completely unique characteristics. To utilise classical control techniques, laborious equivalent linear models are derived for DC-DC converters using averaging modelling schemes. The application and limitations of these modelling techniques are well captured in the available literature. The most common limitation of the available modelling schemes is that more focus is dedicated to converter attributes like order, functionality and operating mode, even when optimal power flow and voltage regulation within the DC network are of more interest. Structure-based modelling techniques like the use of basic building blocks nullify converter attributes in the modelling process which translates to modelling efficiency. In light of the merits seen with the use of basic building blocks when modelling converters in CCM, the current study extends these merits to converters operation in DCM. Similar to modelling converters in CCM, modelling techniques that are available in the literature continue to consider converter attributes in the modelling process for DCM operation. Moreover, the two modes of operation are treated as unique entities and often modelled in a non-unified manner, which compromise modelling efficiency since the same converter can operate in a different state solely based on loading. The aim is to increase modelling efficiency but also nullify operating mode in the modelling process. The same basic building blocks are now modelled as two-port networks for DCM operation and adopted based on the exact configuration of a specified converter to compute its steady-state and dynamic models. All the advantages seen when modelling converters in CCM using basic building blocks are retained and augmented when considering DCM operation. Thus, any converter with well-defined basic building blocks can be easily modelled solely based on the connection of constituent basic building blocks.

Equivalent Reduced DC Network Models With Nonlinear Load Functions: A Data-Driven Approach

Nonconvex AC power flow models feature larger computation burden and lack optimality guarantees to be considered in industry expansion and operational planning activities. Additionally, the consideration of full network models, in general, leads to undesirable levels of computational burden, even under the standard DC approximations. Notwithstanding, current reduction methods fail in accurately reproducing real system responses for a wide range of operating points. In this context, we propose a novel data-driven network reduction method to generate equivalent reduced DC power flow models with nonlinear load functions. Our method aims at providing a high-quality representation of the internal system by minimizing the mismatch between the response of the equivalent DC model and the complete AC power flow (or real measurements) under multiple operating-point data. Thus, the approach can be interpreted as a physics-informed machine-learning method as it trains (or estimates) some parameters of the reduced DC power-flow model to fit pre-calculated data. The method determines: 1) a reduced external equivalent network model (topology and parameters), 2) the optimal allocation of external power injections to the boundary buses, and 3) the coefficients determining artificial nonlinear load functions of the operating-point data. Out-of-sample evaluation tests corroborate the performance of our model against modified Ward reduction benchmark models.