Direct Current Distribution Research Articles

A Distributed Consensus-Based Algorithm for Optimal Power Flow in DC Distribution Grids

The optimal power flow problem over direct current (DC) grids is to minimize the cost of active power generation considering all power flow equations, the maximum capacitance of the lines, and voltage limited boundaries of all the buses. It is one of the famous non-convex problems in the power system area. In this paper, a new distributed consensus-based algorithm is proposed for solving optimal power flow for DC distribution systems. As this algorithm is based on semi-definite relaxation (SDR), a mathematical proof is provided to show the exactness of the SDR relaxation technique. Then, it is demonstrated that the proposed distributed method converges to the global optimal point while satisfying all system constraints. A detailed analysis of the proposed algorithm is provided by applying it to a modified version of the IEEE-30 bus system. Finally, this method is applied to different DC power flow case studies, such as 14, 30, 57, 118, 200-bus systems, to provide a strong performance assessment. The simulation results, when compared with the benchmark, are quite promising.

Effective Magnetic Component Design of Three-Phase Dual-Active-Bridge Converter for LVDC Distribution System

To improve the power conversion efficiency of a three-phase dual-active-bridge (3P-DAB) converter for low-voltage direct current distribution systems, a practical design methodology is proposed. Commonly, the 3P-DAB converter is designed by only specification, such as the rated power, input/output voltage, and frequency. However, using the conventional design method, the efficiency is low under heavy load conditions due to the high conduction loss. In this article, based on the practical power loss analysis, the effective design methodology of the coupling inductance in the 3P-DAB converter is proposed, focusing on power conversion efficiency. It can mitigate the drawbacks of the common design by reducing the peak and rms phase current and the insulated-gate bipolar transistors turn- off current. Experimental results verify the validity of the proposed design methodology and controller using a 25-kW prototype 3P-DAB converter.

Impact of brain atrophy on tDCS and HD-tDCS current flow: a modeling study in three variants of primary progressive aphasia.

During transcranial direct current stimulation (tDCS), the amount and distribution of current that reaches the brain depends on individual anatomy. Many progressive neurodegenerative diseases are associated with cortical atrophy, but the importance of individual brain atrophy during tDCS in patients with progressive atrophy, including primary progressive aphasia (PPA), remains unclear. In the present study, we addressed the question whether brain anatomy in patients with distinct cortical atrophy patterns would impact brain current intensity and distribution during tDCS over the left IFG. We developed state-of-the-art, gyri-precise models of three subjects, each representing a variant of primary progressive aphasia: non-fluent variant PPA (nfvPPA), semantic variant PPA (svPPA), and logopenic variant PPA (lvPPA). We considered two exemplary montages over the left inferior frontal gyrus (IFG): a conventional pad montage (anode over F7, cathode over the right cheek) and a 4 × 1 high-definition tDCS montage. We further considered whether local anatomical features, specifically distance of the cortex to skull, can directly predict local electric field intensity. We found that the differences in brain current flow across the three PPA variants fall within the distribution of anatomically typical adults. While clustering of electric fields was often around individual gyri or sulci, the minimal distance from the gyri/sulci to skull was not correlated with electric field intensity. Limited to the conditions and assumptions considered here, this argues against a specific need to adjust the tDCS montage for these patients any more than might be considered useful in anatomically typical adults. Therefore, local atrophy does not, in isolation, reliably predict local electric field. Rather, our results are consistent with holistic head anatomy influencing brain current flow, with tDCS producing diffuse and individualized brain current flow patterns and HD-tDCS producing targeted brain current flow across individuals.

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.

Multi-time Scale Optimal Power Flow Strategy for Medium-voltage DC Power Grid Considering Different Operation Modes

Direct current (DC) power grids based on flexible high-voltage DC technology have become a common solution of facilitating the large-scale integration of distributed energy resources (DERs) and the construction of advanced urban power grids. In this study, a typical topology analysis is performed for an advanced urban medium-voltage DC (MVDC) distribution network with DERs, including wind, photovoltaic, and electrical energy storage elements. Then, a multi-time scale optimal power flow (OPF) strategy is proposed for the MVDC network in different operation modes, including utility grid-connected and off-grid operation modes. In the utility grid-connected operation mode, the day-ahead optimization objective minimizes both the DER power curtailment and the network power loss. In addition, in the off-grid operation mode, the day-ahead optimization objective prioritizes the satisfaction of loads, and the DER power curtailment and the network power loss are minimized. A dynamic weighting method is employed to transform the multi-objective optimization problem into a quadratically constrained quadratic programming (QCQP) problem, which is solvable via standard methods. During intraday scheduling, the optimization objective gives priority to ensure minimum deviation between the actual and predicted values of the state of charge of the battery, and then seeks to minimize the DER power curtailment and the network power loss. Model predictive control (MPC) is used to correct deviations according to the results of ultra short-term load forecasting. Furthermore, an improved particle swarm optimization (PSO) algorithm is applied for global intraday optimization, which effectively increases the convergence rate to obtain solutions. MATLAB simulation results indicate that the proposed optimization strategy is effective and efficient.

Analysis of frequency tuning process of dual coupled optical microcavities

Different frequency detuning can excite different working mode in a dual coupled optical microcavities. Based on the nonlinear Schrödinger equations of dual coupled field, and by using the split-step Fourier method, the optical field evolution in the microcavities is analyzed under the condition of both positive and negative tuning, and various optical distributions are generated in the process of frequency tuning. Simulation results indicate that the field can develop into the bright soliton in the region of positive tuning. However, the region in which the bright soliton is maintained is small, and the field in the microcavities grows into direct current (DC) distribution because of the serious frequency detuning. In the region of negative tuning, the field of “turning pattern” with high power is generated. There is only chaos inside the microcavities without frequency detuning or the detuning parameters close to 0. In addition, under the condition of strong coupling, the bright soliton and the “turning pattern” cannot be excited. Even stronger coupling leads to optical field in the form of DC directly. After the bright soliton exciting in the microcavity, it can be preserved by selecting appropriate detuning parameters and pump power. Moreover, the bright soliton can be changed into “turning pattern” with low power by continuously changing the detuning parameter of the first microcavity. Theoretical analyses are significant for experimental research on the dual coupled microcavities.

Analysis of the change of direct current potential difference and diameter distribution of thin pure titanium wire subjected to tensile and fatigue loads for fracture prediction

Analysis of the change of direct current potential difference and diameter distribution of thin pure titanium wire subjected to tensile and fatigue loads for fracture prediction

Plug-in gate-loop compensators for series-connected IGBT drivers in solid-state fault current limiter

Solid-state fault current limiter (SSFCL) is the key protective equipment in direct current distribution network. To meet the high voltage requirement meanwhile reduce the cost, implementing a SSFCL based on series-connected insulated gate bipolar transistors (IGBTs) is a promising way. However, voltage unbalancing of IGBTs would be introduced if the gate-loops of the IGBTs are non-identical. In this paper, a plug-in gate-loop compensator with discrete gate voltage feedback and pulsewidth current compensation is proposed. The main merits are 1) with the plug-in structure, the extra current sources only provide small power to fine-tune the gate-loop without affecting the functions provided by the commercial IGBT gate driver; 2) the gate-emitter voltages of IGBTs are compared with the preset thresholds to obtain control criterions, and the pulsewidths of the current sources are controlled for gate-loop compensation, thus both analog-digital and digital-analog converters are avoided; 3) the control law is easy to be implemented in FPGA, and is robust to voltage variation of power-loop. With the proposed compensator, the voltage unbalancing is alleviated immediately at the present switching cycle, and further eliminated cycle-by-cycle during current limitation process. Experimental results verify the feasibility of proposed compensator.

Faulty feeder selection and segment location method for SPTG fault in radial MMC‐MVDC distribution grid

As a modular multilevel converter-based medium-voltage direct current (MMC-MVDC) distribution grid mostly utilises small current grounding modes, the weak fault current for single-pole-to-ground (SPTG) fault is very difficult to detect especially in the case of noises and fault resistance. Therefore, this study proposes a fast and accurate faulty feeder selection and segment location method. For SPTG fault, the transient current fault component is the charge-discharge current of distributed capacitance on the DC feeder. Analysis indicates that the waveforms of current fault component of positive and negative poles are negatively correlated for a faulty feeder; while they are positively correlated for a non-faulty feeder. According to these characteristics, a faulty feeder selection criterion using Pearson correlation coefficient (PCC) is constructed. Then, utilising the energies difference between the current fault components of two poles, the faulty pole can be verified. In addition, the faulty segment is located by the PCC of the current fault component on both sides of each segment. Finally, simulation results confirm the validity of the proposed faulty feeder selection and segment location method.

On Linear Analysis of the Power Flow Equations for DC and AC Grids With CPLs

This express brief presents an approximation of the power flow problem for alternating-current (ac) and directcurrent (dc) distribution networks by using a linear representation of the hyperbolic constraints i = p/v ↔ II* = ℤ/V related to the power balance at each constant power load node. Taylor's or Laurent's series expansion methods are not required to obtain an equivalent linear power flow model. The proposed linear method allows us to achieve a high quality approximation of the power flow modeling without iterative procedures. Our simulation results show the accurate estimation of the voltage profile in distribution networks by the proposed linear approach in comparison to existing methods in specialized literature for ac and dc networks, including linear estimators or classical numerical methods, such as Gauss-Seidel and Newton-Raphson approaches. Numerical implementation of those approaches is carried out in the MATLAB 2017a programming environment.

Transient High-Frequency Impedance Comparison-Based Protection for Flexible DC Distribution Systems

Flexible direct current (dc) distribution systems have emerged as the development trend for future distribution grids. However, these systems are vulnerable to dc faults, rapid fault identification and faulted line selection method are required to enhance the security of the entire system. A novel transient high-frequency impedance comparison-based dc protection for flexible dc distribution systems is proposed in this paper. The control-independent high-frequency impedance model of power converter is also investigated. Based on this model, the proposed method identified the faulted lines by comparing high-frequency impedance measurement differences. For dc bus with multiple branches, this technique minimizes the threshold calculation job, which is usually difficult to process for the transient value-based protections. Strict synchronization of data is also not required for this method. The simulation model of four-terminal flexible dc distribution networks is built in PSCAD/EMTDC to verify the effectiveness of the proposed protection model. Simulation results prove that the protection is robust to fault transition resistances and the measurement noise.

Optimal power flow solution in direct current grids using Sine-Cosine algorithm

In the next years, Colombian power system will have the connection of distributed generators and constant power loads; Therefore, it is necessary to propose the analysis methods that allow establishing the minimum requirements that have to be satisfied for the power system to guarantee an optimal power flow in order to preserve safe and reliable operation of it. For this purpose, in this paper presents an optimal power flow in direct current resistive grids with constant power loads and distributed generators. The optimal power flow problem is formulated as a master-slave optimization algorithm. The master stage covers the dispatching of all the distributed generators by using the Sine-Cosine algorithm. In the slave stage an efficient power flow method based on successive approximations is employed to determine the voltage variables and evaluate the objective function of the problem, which corresponds to the power loss minimization. A direct current distribution network composed by 21 nodes is used as test case by comparing its numerical performance with nonlinear optimization packages and two metaheuristic approaches named black-hole optimization and continuous genetic algorithm. All the simulations are conducted via MATLAB software.

Reliability evaluation method for AC/DC hybrid distribution power network considering cascaded multiport power electronic transformer

With the advantage of high-power supply capacity, low loss in power transmission and distribution process, strong power flow control ability, direct current (DC) distribution network has been one hotspot of the future distribution network. The cascaded multiport power electronic transformer (PET) has been proposed as key equipment for future distribution grid with multi-voltage level and alternating current (AC)/DC hybrid feature. Based on the typical topological structure and engineering reliability theory, a reliability evaluation model for PET considering the device redundancy is established. After that, the reliability evaluation method of the AC/DC hybrid distribution network with a cascaded multiport PET is proposed. Finally, case studies are provided in a ‘hand in hand’ AC/DC hybrid power distribution network structure. Case 1 analyses the PET reliability level under different design patterns and the redundancy level. Case 2 gives the reliability evaluation results of the AC/DC hybrid distribution network with PETs, which proves the correctness and validity of the proposed algorithm. Case 3 compares the reliability level of the AC and DC system under different scenarios and proposes the reliability promotion advice for the DC power distribution network.

Research on Access Mode of the Flexible DC Power Distribution System into AC System

The connection mode of the direct current (DC) power distribution system and the alternating current (AC) system is the foundation of system design, and it is also one of key technologies of the DC power distribution network. Based on the topology structure, grounding method, main equipment parameters, load parameters and system control protection strategy of the DC power distribution system, this paper establishes the system simulation model in the case of configuring the connection transformer and not configuring the connection transformer. Simulation results show that, when no connecting transformer is installed, the interaction between AC and DC systems will be great when faults occur, and the cost of converter valves and DC reactors will be increased. When connecting transformers are installed, the interaction between AC and DC systems can be effectively isolated, and the operation reliability of the system will be greatly improved while the cost is saved. Therefore, it is recommended to configure an independent connection transformer in the DC distribution system.

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.

Voltage-Adjustable Capacitor Isolated Solid-State Transformer

This article proposes a voltage-adjustable capacitor isolated solid-state transformer (ACISST) to link the medium-voltage direct current distribution and low-voltage direct current distribution. Compared with traditional isolated bidirectional dc-dc converter, the proposed ACISST does not contain high-frequency transformer in base modules (BMs). The galvanic isolation between the primary side and the secondary side of the ACISST is realized by the isolation capacitor as quasi-isolation. The elimination of the high-frequency transformer eliminates the core losses of magnetic elements, improves the conversion efficiency to a certain extent, and reduces the weight of the ACISST. The voltage-adjustable unit realizes the secondary voltage regulation of stable control when the primary-side voltage fluctuates in an acceptable range or load power varies. Bidirectional power flowing is one of the features of ACISST, which is based on the bidirectional power flowing characteristic of BM and simplified dual active bridge. Simulation and experimental results are presented to validate the theoretical analysis.

Unbalanced Voltage Suppression in a Bipolar DC Distribution Network Based on DC Electric Springs

In this paper, a method is proposed to mitigate unbalanced voltage and reduce power losses due to neutral line current in a bipolar direct current (DC) distribution network by introducing a DC electric spring (DC-ES). The coupling relationship between positive and negative pole voltages are investigated given the resistance of neutral line in the bipolar DC distribution network. The coupling matrixes of the positive and negative pole voltages and the DC-ES output voltages are deduced. And this matrixes demonstrate that the DC-ES control system is composed of double inputs of pole voltages and double outputs of DC-ES voltages. The small-signal model of the bipolar DC distribution network with DC-ESs is derived further. Subsequently, the control system with double inputs and outputs is transformed into two independent control systems by introducing the decoupling control variables, thereby increasing its rapidity and anti-interference performance significantly. The proposed unbalanced voltage suppression method and decoupling control of DC-ESs are validated by simulations and experiments.

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.

Sliding Mode Control of the Isolated Bridgeless SEPIC High Power Factor Rectifier Interfacing an AC Source with a LVDC Distribution Bus

This paper deals with the analysis and design of a sliding mode-based controller to obtain high power factor (HPF) in the bridgeless isolated version of the single ended primary inductor converter (SEPIC) operating as a single-phase rectifier. In the work reported here, the converter is used as a unidirectional isolated interface between an AC source and a low voltage direct current (LVDC) distribution bus. The sliding-mode control is used to ensure the tracking of a high quality current reference at the input side, which is obtained from a sine waveform generator synchronized with the grid. The feasibility of the proposal is validated using simulation and experimental results, both of them confirming a reliable operation and showing good static and dynamic performances.

Research on system dynamic performances of two‐/three‐level VSC–MVDC distribution systems with different capacitor arrangement schemes

With fewer power electronic devices, the two-/three-level voltage source converter (VSC) is more suitable for medium-voltage direct current (MVDC) distribution system than the modular multi-level converter (MMC). However, two important system dynamic performances, including voltage fluctuation during a normal power change and voltage collapse during a fault, are contradictory in two existing capacitor arrangement schemes. To balance both dynamic performances, a novel capacitor arrangement scheme is proposed, with the capacitors dispersed at the source side. First, the structure and basic working principle of the proposed scheme are presented. Then, dynamic performances of two-/three-level VSC-MVDC system with the proposed scheme are discussed, and detailed comparisons among two existing capacitor arrangement schemes and the proposed scheme are made. In the end, simulation models of 20 kV two-/three-level VSC-MVDC system with the three different capacitor arrangement schemes are established in the MATLAB/Simulink platform. Simulation results have proved that in the proposed scheme, voltage fluctuation during normal power change is minimum, and voltage collapse during a short-circuit fault is suppressed dramatically.