Line Resistances Research Articles

Enhanced Frequency Droop Method for Decentralized Power Sharing Control in dc Microgrids

This article proposes two novel approaches to improve the superimposed frequency droop scheme for the control of dc microgrids (MGs). Conventional voltage-based control strategies suffer from issues such as undesirable voltage regulations, poor power sharing among the sources, and negative effects of line resistances on the equivalent droop characteristics. To overcome these challenges, a superimposed frequency droop scheme has been introduced. However, this method suffers from three major issues: 1) instability in terms of load variation, which is due to the location of system dominant poles; 2) limitation in system loading due to the limitation in the transferred reactive power; and 3) poor voltage quality caused by injection of the ac voltage. In this article, two methods are presented to stabilize the system and enhance its loading condition, consequently improving its viability for control of the dc MG. Furthermore, the system voltage quality is improved by limiting the amplitude of the injected ac voltage. The effectiveness of the proposed schemes is shown by different simulations and is further validated by experiments.

Optimal power flow in multi‐terminal HVDC networks with embedded high‐power DC–DC converters for voltage matching and flexible DC transmission

High-power DC–DC converters are integrated into multi-terminal high-voltage DC (MTDC) networks, typically for voltage matching or flexible DC transmission. This study addresses droop-controlled radial and mesh MTDC networks with transmission loss minimisation considering the impact of integrating a high-power DC–DC converter. DC voltage droop control can achieve minimum power loss in radial MTDC transmission network by controlling the power-sharing among the receiving nodes inversely proportional to their transmission line resistances. Updating the droop characteristics by an optimisation algorithm guarantees optimal power flow (OPF) during renewable source power generation fluctuation. Meanwhile, the mesh network DC voltage references can be updated by the OPF for transmission loss minimisation. This study presents the impact of incorporating a high-power DC–DC converter at different system access lines of a radial MTDC network while targeting efficient power transfer and DC transmission loss minimisation. The effect of introducing a high-power DC–DC converter on the droop characteristic settings during steady-state operation in a radial MTDC network is investigated. Moreover, the impact of a high-power DC–DC converter in mesh MTDC network with OPF operation is explored. To validate the presented concepts, the CIGRE B4 MTDC network is used as a benchmark system with a metaheuristic optimisation technique.

Geomagnetically Induced Current Model Validation From New Zealand's South Island

AbstractGeomagnetically induced currents (GICs) during a space weather event have previously caused transformer damage in New Zealand. During the 2015 St. Patrick's Day Storm, Transpower NZ Ltd has reliable GIC measurements at 23 different transformers across New Zealand's South Island. These observed GICs show large variability, spatially and within a substation. We compare these GICs with those calculated from a modeled geolectric field using a network model of the transmission network with industry‐provided line, earthing, and transformer resistances. We calculate the modeled geoelectric field from the spectra of magnetic field variations interpolated from measurements during this storm and ground conductance using a thin‐sheet model. Modeled and observed GIC spectra are similar, and coherence exceeds the 95% confidence threshold, for most valid frequencies at 18 of the 23 transformers. Sensitivity analysis shows that modeled GICs are most sensitive to variation in magnetic field input, followed by the variation in land conductivity. The assumption that transmission lines follow straight lines or getting the network resistances exactly right is less significant. Comparing modeled and measured GIC time series highlights that this modeling approach is useful for reconstructing the timing, duration, and relative magnitude of GIC peaks during sudden commencement and substorms. However, the model significantly underestimates the magnitude of these peaks, even for a transformer with good spectral match. This is because of the limited range of frequencies for which the thin‐sheet model is valid and severely limits the usefulness of this modeling approach for accurate prediction of peak GICs.

DG-side earth fault tolerance enhancement based on topology improvement and common and differential mode control strategy

Parallel distributed generation (DG) can extract power from a renewable energy source and supply it to the grid, but the exposure of DG components, such as photovoltaic panels, to the natural environment results in DG-side earth faults and common earth circulating current (CECC) problems. This study proposes an improved topology and a common and differential (C&D) mode control strategy to enhance the earth fault tolerance in parallel DG systems. First, the effects of line resistances and input voltages on CECC are analyzed through a mathematical model, and the disadvantages of the conventional buck converter are identified. Second, an additional switch in a negative pole is designed for the application of the C&D control strategy. Specifically, common-mode algorithm control is used to inhibit CECC, and differential-mode control is applied to ensure current sharing accuracy. Third, an inertial element is added to the reference voltage to restore the bus voltage produced by the differential-mode control strategy. Lastly, an experiment is conducted to verify that the proposed control outperforms conventional control in various conditions.

Application of linearised load flow method for droop‐controlled DCMGs

In this study, the authors present a linear power flow method for droop-controlled DC microgrids (MGs). Traditional power flow techniques do not apply to droop-controlled DCMGs owing to the power and voltage flexibility capability of the droop-controlled dispatchable generation units. Therefore, new power flow techniques embodying the droop-control operation philosophy were developed by the researchers. The existing methods are time-consuming since they involve iterative solutions of a set of non-linear equations. The proposed linear power flow method is accurate, simple, and gives faster results in comparison to the existing methods. The power flow model applies to all load types (constant power, constant current, and constant resistance) and works for both radial and non-radial system configurations. The linear power flow can be applied for convex optimisation, optimal power flow, and network reconfiguration problems. The proposed method is validated on five test systems. A comparison with a modified Newton–Raphson power flow demonstrates that the proposed linear power flow technique provides accurate results. The maximum error in voltage calculation is not more than 0.0812%. The performance of the proposed method is also acceptable for heavy loads and high line resistances. The computational time reduces significantly with the proposed method.

A frequency-based economical-sharing strategy for low-voltage DC microgrids

This paper introduces a frequency-based control strategy to minimize the total generation cost in low-voltage dc microgrids. In dc microgrids, although the voltage is the only variable for controlling the outputs of sources, it is not a common variable for all sources due to the different line resistances. Hence, applying the conventional voltage-based droop control will not be able to share the load among the sources accurately. Within the context alluded above, to realize an accurate load sharing in dc microgrids, the communication-based approaches are employed. However, any link failure in these approaches may impair the controller functionality and consequently, render the whole system inoperable. To cope with the associated problems of the communication-based approaches, the frequency-based control strategy has been introduced for accurate load sharing in dc microgrids which operates without any communication network. This study proposes a frequency-based economical-sharing control strategy that has the capability to minimize the total generation cost and regulate the voltage of sources. The main advantage of this strategy over the previously addressed frequency-based strategies is providing an optimal combination of dispatchable sources to supply the required load on the grid. Through the employed control strategy, the incremental costs of all dispatchable sources will be equal in the steady state which causes the optimal operation of the dc microgrid. The small-signal modeling of the proposed strategy is also carried out to analyze the system stability and parameter sensitivity. A complete set of simulation studies is provided in PLECS software to verify the effectiveness of the proposed control strategy.

On the Impact of Internal Cross-Linking and Connection Properties on the Current Distribution in Lithium-Ion Battery Modules

The often-observed current distribution between parallel-connected lithium-ion cells within battery modules is probably evoked by the properties of the connection, inhomogeneous contact and power line resistances, the impedance behavior of single cells and of the DoD. The extent to which each of the contributors and the interaction between them affects the current distribution within the battery module is crucial to improve the system’s efficiency, which is investigated here via various electrically cross-linked, physicochemical-thermal simulations with variable system terminal (ST). Consequently, cross-connectors balance the system and reduce DoD shifts between cells. Furthermore, if one compares ST-side and ST-cross, the position of the ST is negligible for topologies that incorporate at least two cells in serial and parallel. Depending on the ST configuration, point and axis symmetry patterns appear for the current distribution. Compared to welding seam and cross-connector resistances, the string connector resistance dominates the current distribution. Like the behavior of a single cell, the system’s rate capability shows a non-linear decrease with increasing C-rate under constant current discharge. As a recommendation for the assembly of battery modules using multiple lithium-ion cells, the position of the ST is of minor importance compared to the presence of cross-connectors and low-resistance string connectors.

Adaptive Droop Control Method for Suppressing Circulating Currents in DC Microgrids

DC microgrids are introduced to reduce the conversion stages needed for connection of DC sources to the DC loads. They employ the droop control algorithm for managing the power flow from sources to the loads. However, the droop control functionality is affected by circuit parameters, especially line resistances. As a consequence, load sharing as the primary objective of the droop controller lacks accuracy. Parallel-connected converters have mismatched output voltages, resulting in circulating currents. This paper proposes an adaptive droop control algorithm for suppressing circulating currents in a low voltage DC microgrid. Line resistances are estimated through mathematical calculations and droop parameters are adjusted accordingly. Moreover, a distributed secondary controller is proposed to improve the load sharing accuracy and eliminate the effect of line resistances. The secondary controller shifts the droop controller voltage setpoint according to the converter current. Both of the proposed methods result in an accurate load sharing; Each of the participating converters has the rated current and consequently circulating current is suppressed. The effectiveness of the proposed method is verified through simulation and hardware-in-the-loop (HIL) setup.

8T SRAM Cell as a Multibit Dot-Product Engine for Beyond Von Neumann Computing

Large-scale digital computing almost exclusively relies on the von Neumann architecture, which comprises separate units for storage and computations. The energy-expensive transfer of data from the memory units to the computing cores results in the well-known von Neumann bottleneck. Various approaches aimed toward bypassing the von Neumann bottleneck are being extensively explored in the literature. These include in-memory computing based on CMOS and beyond CMOS technologies, wherein by making modifications to the memory array, vector computations can be carried out as close to the memory units as possible. Interestingly, in-memory techniques based on CMOS technology are of special importance due to the ubiquitous presence of field-effect transistors and the resultant ease of large-scale manufacturing and commercialization. On the other hand, perhaps the most important computation required for applications such as machine learning, etc., comprises the dot-product operation. Emerging nonvolatile memristive technologies have been shown to be very efficient in computing analog dot products in an in situ fashion. The memristive analog computation of the dot product results in much faster operation as opposed to digital vector in-memory bitwise Boolean computations. However, challenges with respect to large-scale manufacturing coupled with the limited endurance of memristors have hindered rapid commercialization of memristive-based computing solutions. In this paper, we show that the standard 8 transistor (8T) digital SRAM array can be configured as an analoglike in-memory multibit dot-product engine (DPE). By applying appropriate analog voltages to the read ports of the 8T SRAM array and sensing the output current, an approximate analog–digital DPE can be implemented. We present two different configurations for enabling multibit dot-product computations in the 8T SRAM cell array, without modifying the standard bit-cell structure. We also demonstrate the robustness of the present proposal in presence of nonidealities such as the effect of line resistances and transistor threshold voltage variations. Since our proposal preserves the standard 8T-SRAM array structure, it can be used as a storage element with standard read–write instructions and also as an on-demand analoglike dot-product accelerator.

Generalized high‐isolation n‐way Gysel power divider with arbitrary power ratio and different real terminated impedances

An exact closed-form design approach for a generalized high-power n-way Gysel power divider is proposed. The power divider could be designed to achieve an arbitrary power ratio with the flexible multiway application, arbitrary real terminated impedance, excellent isolation, and easy fabrication through both planar and three-dimensional structures. Moreover, this improved power divider could maintain high power processing capacity through the coaxial cavity transmission line and grounding resistances. The exact analytical solutions related to ideal port matching and high isolation are obtained based on the circuit and transmission-line theory. To verify the proposed approach, a compact 3-way coaxial power divider with a pre-designed power ratio of 1:1.5:2 and four different real terminated impedances of 50, 55, 60, and 65 Ω is designed and fabricated. Excellent agreement is achieved between the simulated and measured results. Measurements from 4.7 to 5.7 GHz show that the return losses of all input and output ports are better than 15 dB. The maximum insertion loss is 0.5 dB, and the phase imbalance is approximately less than 6.1°. In addition, the isolation between any two output ports is better than 23 dB from 4.5 to 6 GHz. Meanwhile, the power handling capability can reach the maximum power of the commercial 50 Ω SMA connectors (2.098 kW).

Power Sharing in Hybrid Microgrids Using a Harmonic-Based Multidimensional Droop

Droop-based control strategies are generally used in hybrid microgrids (HMGs) for voltage and frequency regulation in the dc and ac microgrids (MGs). The control of interlinking converters (ICs) that connect ac and dc MGs plays a vital role in the stability and efficiency of HMGs. However, conventional P-V droops for the dc side may not be effective due to line resistances and local nature of voltage. Also, conventional Q-V droops for the ac side may result in circulating power and current among parallel ICs diminishing their performance. In this paper, dc voltage harmonic frequencies are employed to create a global variable in the dc side of HMGs and to introduce an efficient droop control strategy for the dc MG without deteriorating power quality. Accordingly, accurate active power sharing is achieved and circulating power is eliminated. In addition, a three-dimensional (3-D) droop is suggested for voltage control at the ac MG using virtual impedances. Thus, proper reactive power sharing is accomplished among parallel ICs, while circulating currents are suppressed as well. The optimal gains of the 3-D droop are determined via an optimization problem. The efficacy of the proposed method is evaluated by examining it on a typical HMG in different situations.

Frequency superimposed energy bifurcation technology for a hybrid microgrid

A conventional microgrid comprising both ac and dc components undergoes a significant power loss during the multistage conversion process. To overcome the problem of reduced efficiency in a conventional microgrid, a significant attention has been drawn towards the structure of the hybrid ac/dc microgrid. The precise power management in a hybrid microgrid is relatively complicated due to the presence of the dc sub-grids in the architecture. In dc sub-grids, the converter power can only be controlled by regulating the output voltage. However, the computation of the reference voltage turns out to be a challenging task due to the system resistance. The complexity further increases as the line resistances associated with the interlinking converter differs. Addressing this issue, to implement an efficient power management strategy in a dc sub-grid, a superimposed virtual frequency based control variable is proposed in this work. The proposed control variable remains unaffected by the system impedance which helps to attain an efficient power bifurcation. The study extensively evaluates the maximum exchanged power, storage utilisation factor, and circulating power among the multiple interlinking converters. The proposed strategy efficiently manages the bidirectional power flow between the ac and dc sub-grids.

Modeling and Analysis of Circulating Currents Among Input-Parallel Output-Parallel Nonisolated Converters

Input-parallel output-parallel (IPOP) nonisolated converters, including dc/dc and dc/ac converters, are effective solutions to increase the power rating and to improve the reliability of the system. For the safe and steady operation of these IPOP nonisolated converters, the circulating currents are the main challenge, whose detailed mathematic model and corresponding characteristics are not well studied. This paper focuses on the modeling and analysis of circulating currents among these IPOP converters by deriving the detailed mathematic model. Through these models, the complicated characteristics of the circulating currents are presented, which shows that there are various types of circulating currents including circulating currents within the single converter and circulating currents among the multiple converters. It is first demonstrated that the circulating currents will cause port degradation of the converter, that is, the positive and negative currents of the converter are unequal, which makes some port-based control methods ineffective and influences the relay protection. Furthermore, the corresponding influence factors, including line resistances, filters, and so on, are analyzed in detail, especially the asymmetry of line resistances will cause large circulating currents even resulting in instability. It is also found that the traditional control methods cannot completely eliminate these circulating currents because of the strong coupling and complexity of IPOP nonisolated converters. All the theoretical analyses are verified by the real-time hardware-in-loop (HIL) tests.

Distributed Control for DC Microgrid Based on Optimized Droop Parameters

ABSTRACTThe droop control approach is widely used in case of parallel operation of DC sources. The conventional droop control method is realized by linearly reducing the load voltage as the load current increases. This droop control is unable to achieve effective equal load sharing as well as the bus voltage regulation. The load sharing error is enhanced when cable line parameters of parallel-connected DC sources are not equal. The droop gain and nominal voltage reference of source converters are the key parameters to achieve the bus voltage deviation within the permissible limits and effective load sharing. In this paper, the droop parameters of the proposed distributed control scheme are optimized with the help of the Particle Swarm Optimization technique for the aforementioned control objectives of the DC microgrid for unequal cable line resistances. The performance of the proposed control scheme based on optimal droop parameters is verified through MATLAB/Simulink environment and validated by a hardware-in-the-loop real-time simulator based on the dSPACE 1202 platform.

A distributed control strategy based on droop control and low-bandwidth communication in DC microgrids with increased accuracy of load sharing

A microgrid is obtained from the integration of several distributed energy resources such as fuel cells and storages. Droop control is a method for load sharing between sources or storages in a microgrid. In the conventional droop control method, one can only achieve one of the low voltage drops and accurate load sharing requirements by considering the line resistances. This paper proposes a new non-centralized control approach based on low-bandwidth communication (LBC) and the droop control method for the DC microgrid (DC-MG). The droop gain is calculated with respect to the output currents of the distributed generation (DG) units. Factors like different line resistances, different DG capacities, and the presence of local loads do not affect the controller performance. All calculations and control operations are performed locally and the LBC is used solely for transferring current information—this ensures high reliability. Simulations and experimental results show that the use of the proposed control method decreases voltage deviations and the effects of constant power loads (CPLs), while it increases system efficiency and the accuracy of load sharing.

Droop Control Methods for PV-Based Mini Grids with Different Line Resistances and Impedances

Different droop control methods for PV-based communal grid networks (minigrids and microgrids) with different line resistances (R) and impedances (X) are modelled and simulated in MATLAB to determine the most efficient control method for a given network. Results show that active power-frequency (P-f) droop control method is the most efficient for low voltage transmission networks with low X/R ratios while reactive power-voltage (Q-V) droop control method is the most efficient for systems with high X/R ratios. For systems with complex line resistances and impedances, i.e. near unity X/R ratios, P-f or Q-V droop methods cannot individually efficiently regulate line voltage and frequency. For such systems, P-Q-f droop control method, where both active and reactive power could be used to control PCC voltage via shunt-connected inverters, is determined to be the most efficient control method. Results also show that shunt-connection of inverters leads to improved power flow control of interconnected communal grids by allowing feeder voltage regulation, load reactive power support, reactive power management between feeders, and improved overall system performance against dynamic disturbances.

Impact of Line Resistance Combined with Device Variability on Resistive RAM Memories

In this paper, the performance and reliability of oxide-based Resistive RAM (ReRAM) memory is investigated in a 28nm FDSOI technology versus interconnects resistivity combined with device variability. Indeed, common problems with ReRAM are related to high variability in operating conditions and low yield. At a cell level ReRAMs suffer from variability. At an array level, ReRAMs suffer from different voltage drops seen across the cells due to line resistances. Although research has taken steps to resolve these issues, variability combined with resistive paths remain an important characteristic for ReRAMs. In this context, a deeper understanding of the impact of these characteristics on ReRAM performances is needed to propose variability tolerant designs to ensure the robustness of the technology. The presented study addresses the memory cell, the memory word up to the memory matrix.

Autonomous Operation of a Hybrid AC/DC Microgrid With Multiple Interlinking Converters

Applying conventional dc-voltage-based droop approaches for hybrid ac/dc microgrids interconnected by a single interlinking converter (IC) can properly manage the power flow among ac and dc subgrids. However, due to the effect of line resistances, these approaches may create a circulating power as well as overstressing the ICs in the case of employing multiple ICs for interconnecting the ac and dc subgrids. This paper proposes an autonomous power sharing approach for hybrid microgrids interconnected through multiple ICs by introducing a superimposed frequency in the dc subgrid. Hence, a suitable droop approach is presented to manage the power among the dc and ac sources as well as ICs. The outcomes are proportional power sharing, preventing circulating power and overstressing the ICs as well as acceptable dc voltage regulation. Furthermore, the maximum transferred power by the ICs can be improved by employing the proposed approach. The effectiveness of the proposed approach is evaluated by simulation.

Synchronverter-Enabled DC Power Sharing Approach for LVDC Microgrids

In a classical ac microgrid (MG), a common frequency exists for coordinating active power sharing among droop-controlled sources. Like the frequency-droop method, a voltage-based droop approach has been employed to control the converters in low voltage direct current (LVDC) MGs. However, voltage variation due to the droop gains and line resistances causes poor power sharing and voltage regulation in dc MG, which in most cases are solved by a secondary controller by using a communication network. To avoid such an infrastructure and its accompanied complications, this paper proposes a new droop scheme to control dc sources by introducing a small ac voltage superimposed onto the output dc voltage of converters. Therefore, dc sources can be coordinated together with the frequency of the ac voltage, without any communication network like synchronous generators (SGs) in conventional power systems. Small signal stability analysis, as well as mathematical calculations, is presented to demonstrate the analogy between the proposed strategy and frequency-based droop approach of the SGs. The effectiveness of the proposed control system is evaluated by simulations and verified by experiments.

On Secondary Control Approaches for Voltage Regulation in DC Microgrids

Centralized or decentralized secondary controller is commonly employed to regulate the voltage drop raised by the primary controller. However, in the case of high capacity microgrids (MGs) and long feeders with much voltage drop on the line resistances, the conventional methods may not guarantee the voltage regulation on the load busses within a suitable range. Therefore, in addition to compensate the voltage drop of the primary controller, it is necessary to regulate the voltage of critical loads. In this paper, a new voltage regulation strategy is proposed to regulate the voltage of MG by employing the average voltage of identified critical busses, which are determined by the proposed modal analysis. Numerical steady-state analysis and preliminary simulation results validate effectiveness of the proposed scheme. Furthermore, experimental results are performed to demonstrate the viability of the proposed approach.