Direct Current Grids Research Articles

Small-Signal Modeling and Analysis of Converter Interactivity in 48 V DC Grid

Due to the flexibility of realization in decentralized form, environmentally genial, and the easy integration with renewable energy sources, storage units, and loads, the direct current (DC) microgrid has shaped as a subject matter worldwide. In a DC microgrid structure, the concurrent operation among multiple DC-DC converters, connected in parallel fashion across the common DC bus, realizes substantially with the average current information-based droop control technique. However, such control logic requires two minimal controllers with an inter-reliant design process to implement the overall control action. Considering the coupling effect during converter operation, this paper analyses the implementation of a mixed-signal peak current information-based droop control logic for the parallel connected converter system. A thorough small signal and switch-based modeling process for the interconnected systems are carried out with consideration of resistive and constant power loads. Adaptability to modeling with a first-order approximation-based approach makes the methodology easy for the design of the controller and stability analysis process in a parallel connected converter system. The veracity of the designed model is verified using simulation studies. A hardware prototype with two boost converters operating in parallel and feeding power to a resistive load and a buck converter (behaving as a constant power load) is fabricated to study the performance of the designed controller, along with the effect of different transient events on overall system performance.

Predictive source management for low power domestic direct current grids

The decrease in the price of solar photo voltaic (PV) panels has led to the widespread adoption of the solar power as a renewable energy source, not only at the grid level but also on the roof tops of the residential buildings. The solar PV panels produce direct current (DC) and can be readily used to drive DC powered loads or charge batteries. Direct powering of the loads from the solar panels is hindered by the highly variant nature of solar power generation which depends on a number of external as well as internal factors. The paper proposes a prediction based direct connection between PV panel and DC loads, only when the panel will be able to supply the required power to the load. The loads have been categorized based on how they need to be powered and their priority levels. The data specific to the rooftop was collected over two years using which the prediction of the panel output was carried out. Combining the load categorization and the power prediction a smart management system was designed which was able to decide on how the loads need to be powered and hence were connected to the appropriate source.

Integration of E-core DC reactor based on thyristors with hybrid HVDC breaker

In general, the fault current in high-voltage direct current (HVDC) grids has two characteristics: the rapid increase and the lack of a zero-crossing point. Therefore, a large reactor is necessary to fulfill DC fault current limiting. But this can have disadvantages on the dynamics and stability of the HVDC grid. In this paper, the integration of an E-core DC reactor based on thyristors with a hybrid HVDC breaker (RTHCB) is suggested as a solution. The proposed model reduces the rising rate and peak of the fault current and protects the LCS through the primary inductance of the reactor. In RTHCB, several capacitors and thyristors are placed in parallel with the secondary winding of the reactor to control the gradual and soft placement of the large inductance in the path of the fault current. This integration reduces the electrical stresses caused by the sudden disconnection of conventional HCBs. Due to the presence of the capacitors, the energy absorbed by the arrester decreases. In addition, it decreases semiconductor switches and absorbs fault energy in a short time. After theoretical analysis, the RTHCB is modeled on the 3D space of the COMSOL Multiphysics. Finally, an MMC-HVDC grid has been simulated in the PSCAD/EMTDC to validate the performance of RTHCB.

A Fault Detection Algorithm Based on Artificial Neural Network Threshold Selection in Multi-Terminal DC Grids

The DC fault is one of the critical challenges in the High Voltage Multi-Terminal Direct Current (HV-MTDC) grid. The fixed value threshold used in some implementations may be affected by different grid conditions and thus causes errors in the fault detection procedure. This paper offers a two-stage fault detection algorithm based on the integrated Artificial Neural Network (ANN) and Discrete Wavelet Transform (DWT) theory. The main innovations of the proposed method are: 1) Increasing the reliability of the fault detection operation by solving the fixed value threshold problems; 2) Offering high robustness against the wide range of fault resistances; 3) Providing bus protection capability and 4) Setting relays locally, and avoiding the requirement to high-speed communications links. Also, it offers DC fault detection in a short period of time. The performance of the proposed protection scheme has been ascertained via simulations in the MATLAB/Simulink environment. The results prove the robustness of the proposed method against noise disturbance, high resistance fault, and grid parameters variations.

A multiport hybrid DC circuit breaker with high economic efficiency and adaptive reclosing capability

In a high voltage direct current (HVDC) grid, hybrid DC circuit breakers (HCBs) are one of the effective solutions to interrupt fault currents. The conventional two-port HCB features high economic costs since the expensive insulated gate bipolar transistors (IGBTs) are needed. Each line in the multiterminal HVDC grid must be equipped with the HCB, which is extremely costly to implement. In addition, the likelihood of temporary faults is considerably raised by the fact that the HVDC grid transfers power across overhead lines (OHLs). By reclosing operations after temporary faults, the HVDC grid can resume normal operation. However, it is vital to discern between temporary faults and permanent faults before reclosing operation to prevent secondary shocks to the HVDC grid caused by blindly reclosing on permanent faults. For the problems of high cost and blind reclosing of HCBs, this article proposes a multiport HCB (MHCB) with high economy and adaptive reclosing capability. All ports of the MHCB share a single main breaker (MB) for fault isolation. The MHCB utilizes the capacitor voltage to lower the operating voltage of the metal oxide varistor (MOV), which reduces the number of IGBTs connected together at both ends of the MOV. With the above design, the expense of the MHCB is significantly decreased. Additionally, the MHCB achieves adaptive reclosing by using the difference of current amplitudes in the capacitor branch under permanent and temporary faults.

From pole-to-ground fault current return paths in a meshed HVDC network to a grounding modelling simplification for protection studies

Meshed bipolar High Voltage Direct Current grids are considered as one of the preferable solutions for integration of renewable energy sources and increasing the security of power systems on a continental scale. In this context, several fault current studies are proposed in the literature, considering different grounding methods for modular multilevel converter neutral points. But these studies often focus on fault current paths to the fault location, and none of them analyzes the return paths of fault current from the fault location. This article deals with fault currents return paths in case of pole-to-ground fault in a grounding configuration using surge arresters in all stations except one, which is solidly grounded. The influence of this solidly grounded point location on the return paths of fault currents is evaluated. With these results, a modelling simplification is considered for HVDC protection studies. Specifically, the discussion investigates whether all MMC neutral points can be solidly grounded.

HVDC grids stability enhancement through the integration of battery energy storage systems

AbstractIn high voltage direct current (HVDC) grids, the reference values of power controllers are normally adjusted based on the DC‐link voltage as the DC voltage is regulated by the droop controllers. As a matter of fact, except for the DC slack bus, there is a trade‐off between power‐sharing and voltage regulation of all the remaining HVDC buses. The main issue with this control strategy is that in case of contingencies, significant voltage and power oscillations throughout HVDC grids can be expected. To address such issues, the integration of a battery energy storage system into HVDC grids through a multi‐port DC/DC power converter is investigated in this paper. The DC/DC converter used in this paper consists of three ports: (1) two ports are connected in cascade with the intended DC transmission line, formerly named cascaded power flow controller, and (2) the third port is for the integration of the battery energy storage system to form a multi‐functional device. Therefore, the proposed framework can potentially achieve small‐signal stability enhancement and regulate/adjust the power throughout the HVDC grids.

Simple Chargers for a Small DC Micro-Grid for a Home Emergency Power System

Recently, the danger of a long blackout is discussed in Europe. Blackouts can be caused by failures in the energy distribution, errors in large power plants or even cyber-attacks. This can lead to a chain reaction and a disintegration of the mains. Longer blackouts have an extreme impact on the economy as a whole and on local households. Therefore, a small local grid at home which can supply the most important loads over some time has garnered increasing interest. With a small direct current (DC) grid, critical loads such as for deep freezers and refrigerators can be supplied, and some LED lights can be used in the evening or at night. Solar generators (panels) can be used to charge energy storage devices, e.g., batteries. A DC grid can not only be used in the case of an emergency, but can also be used to reduce energy consumption out of the public mains and reduce energy bills. The architecture of the household emergency DC grid is discussed; suggestions for batteries are given; two simple chargers, based on DC-DC-converters like the Buck (step-down) and on the Boost (step-up) converters, are shown; dimensioning suggestions are given; and simple, robust controllers, a P-controller with disturbance feedforward and a hysteresis controller, are treated and tested via simulations. The goal of the paper is to show a simple autonomous home energy system without an external fieldbus, LAN or internet connection with special focus on simple charger topologies.

High-Frequency Oscillation Suppression Strategy for Flexible DC Transmission Systems Based on Additional Joint Damping

Flexible DC transmission systems experience negative damping characteristics in the high-frequency range due to the delay in the system control link. This can lead to interactions between the impedance of modular multilevel converters (MMCs) and AC transmission lines, resulting in high-frequency oscillation (HFO) issues. To resolve this problem, a simplified MMC control model was developed, taking into account the current inner loop control link. The Sobol method was employed to analyze the factors influencing the impedance characteristics of MMC. Additionally, a combined approach involving a virtual passive filtering method and an equivalent current sampling method was proposed to provide additional damping. The study compared and analyzed the impact of additional damping control on the impedance characteristics and system stability of MMCs. Lastly, a flexible direct current grid connection simulation model was constructed using PSCAD to validate the effectiveness of the proposed additional damping control strategy.

Research on circuit breaker failure protection and the secondary accelerated fault isolation scheme of VSC-HVDC grids

When the direct current circuit breaker (DCCB) fails to operate, the fault range will further expand, endangering the safe operation of the power gird. Therefore, this study proposes a DCCB failure protection method and a secondary accelerated fault isolation scheme of voltage source converter-based high-voltage direct current (VSC-HVDC) grids. On the basis of considering the influence of DCCB disconnection, characteristics of fault current after the failure of the DCCB are analyzed, and a circuit breaker failure protection method based on “increase current” discrimination is proposed. By analyzing the logical relationship among failed breaker re-tripping, adjacent DCCB action, blocking of the converter station, and AC circuit breaker action, a secondary accelerated fault isolation scheme is proposed to minimize the fault isolation area. The scheme achieves the accelerated isolation of DC faults after the failure of the DCCB and avoids the trip of the AC circuit breaker effectively. Simulation results verify the effectiveness of the proposed scheme.

Harnessing solar and wind for sustainable cross-border electricity trade in the Greater Mekong Subregion

Sustainable, low-emission electricity generation options are needed in the Greater Mekong Subregion, including for cross-border electricity trade. Large-scale investment in solar and wind power, together with off-river pumped hydro energy storage, is identified as a promising way forward. The GMS has many potential off-river pumped hydro sites. Actionable recommendations include greater use of bilateral power purchase agreements for cross-border solar and wind power supply, and potential development of a high-voltage direct current grid. Institutional prioritization and ongoing evaluation are required to ensure desired social, environmental, and economic outcomes from the transition.

An elevated gain DC-DC converter with active switched inductor for PV application

Innovative power electronic converters have emerged because of renewable energy expansion in the past two decades. Direct current DC grids can operate independently or connected to power grids. A DC grid has distributed generation (DG) units, such as solar panels and, fuel cells. It is necessary to have a DC-DC converter in DC grids to raise output voltage. Boost converters have limited voltage gain, and their switching stress is often equal to their output voltage. This work proposes a converter which gives high gain and less stress when compared to other recent converters. At 0.8 duty ratio, this converter produces an elevated gain of 59. An experimental prototype is built for the proposed converter and the results are presented.

Impedance Phase Based Active Injection Protection for Radial VSC-HVDC Grid

Effective DC fault identification is very important for a half-bridge modular multi-level converter (HB-MMC) based high voltage direct current (HVDC) grid. The performance of the traditional protection principles lacks reliability and sensitivity, and there is a further limitation under the current limiting strategies. To address the problem, a scheme coordinating the control and protection is proposed, which actively creates the known feature for fault identification and limits the fault current at the same time. In this paper, the characteristics of the input impedance phase under different faults are analyzed in various frequency bands. Based on the analysis of the input impedance phase, the converter switches the control mode and generates a characteristic signal at a particular frequency after fault detection. Then, the Prony algorithm is employed to acquire the phase information at the injection frequency. Finally, the faulty zone can be identified by the value of the input impedance phase. This active injection protection scheme is applicable to identify different faults, verified in PSCAD/EMTDC, and tested by hardware-in-the-loop simulation in RTDS.

Diode-Assisted Gate-Commutated Thyristor for Hybrid MVDC Circuit Breaker Applications

This paper presents a Hybrid Circuit Breaker for Medium Voltage Direct Current grids, consisting of an ultra-fast mechanical breaker and a parallel Diode-Assisted Gate-Commutated Thyristor. This device is a GCT based devices, especially developed for this application. It has a higher current interruption capability than commercial IGCTs and a more robust, power-efficient and faster gate drive unit. We designed a stray inductance optimized package with an integrated gate drive unit for the Diode-Assisted Gate-Commutated Thyristor and included diodes with a very high forward voltage drop, which were radiated especially for use in a Diode-Assisted Gate-Commutated Thyristor, to increase its current turn-off capability. The build prototype can interrupt fault currents of 10 kA. An analysis of the stray elements in the package showed a possible increase of the current interruption capability to xy. We analysed the boundary conditions for potential use in an Hybrid Circuit Breaker by investigating the turn-on and turn-off characteristics of the device embedded in a diode full bridge and including a snubber capacitor.

A protection scheme for MMC–HVDC grid based on blocking time of converter station

This paper proposes a local protection scheme for the high voltage direct current grid based on the modular multilevel converter (MMC-HVDC), which determines the protection action time by blocking time of converter station and cuts the fault line by the coordination of action time. In view of the problem that the first protection action will accelerate the blocking of other converter stations, the protection action time margin is set in order to make the protection act in advance to remove the fault before any converter is blocked. By defining the virtual blocking time of the inverter station, the protection criterion of all the converter stations is unified and the selectivity of protection is guaranteed. The proposed scheme fully considers the coordination between the protection action and the blocking of converter station, and can selectively cut off fault lines according to the severity of overcurrent of the bridge arm of the converter. Simulation results verify the effectiveness of the proposed protection scheme.

Research on the energy consumption mechanism and characteristics of the gallium indium tin liquid metal arcing process

For high-voltage direct current (HVDC) power grid transmission with higher voltages, the energy-consuming branch of the DC circuit breaker is required to dissipate huge energies of more than megajoules in a short time in the case of a fault and short circuit. The requirements for huge volume and weight are difficult to meet with energy-consuming equipment based on ZnO. In this paper, a new energy consumption method is proposed based on gallium indium tin (GaInSn) liquid metal in the arcing process, and a test platform with adjustable short-circuit current is built. The mechanism triggering GaInSn liquid metal arcing energy consumption is studied. It is found that short-circuit current and channel aperture are the key parameters affecting the energy consumption of liquid metal arcing. The characteristics of GaInSn liquid metal energy consumption are investigated, and four stages of liquid metal energy consumption are found: oscillatory shrinkage, arc breakdown, arc burning phase change and arc extinction. The influence of short-circuit current and channel aperture on the energy consumption characteristics of GaInSn liquid metal is investigated. To further explore the physical mechanism of the above phenomena, a magneto-hydrodynamic model of energy consumption in the GaInSn liquid metal arcing process is established. The influence of short-circuit current and channel aperture on the temperature distribution of the liquid metal arc is analyzed. The mechanism of the effect of short-circuit current and channel aperture on peak arc temperature and the temperature diffusion rate is clarified. The research results provide theoretical support for this new liquid metal energy consumption mode DC circuit breaker.

Moth‐flame‐optimisation based parameter estimation for model‐predictive‐controlled superconducting magnetic energy storage‐battery hybrid energy storage system

AbstractSuperconducting magnetic energy storage‐battery hybrid energy storage system (HESS) has a broad application prospect in balancing direct current (DC) power grid voltage due to its fast dynamic response ability under low‐frequency/high‐frequency disturbances. Model‐predictive‐control (MPC) with characteristics such as high sampling rate and wide applicability could be applied to HESS. However, considering that the relevant circuit parameters would change with ambient temperature, interference and ageing, the effect of MPC may deteriorate inevitably. This article proposes an improved MPC strategy for SMES‐Battery HESS, taking moth‐flame‐optimisation (MFO) algorithm to calculate the circuit parameters in real time. The actual parameters are updated by MFO and then sent to model predictive controller to minimise the model mismatches. The advantages of high accuracy and fast convergence speed is verified by comparison with grey wolf optimisation and particle swarm optimisation. The simulation shows that by taking the proposed scheme, DC bus voltage are more stable and the superconducting magnetic energy storage can maintain more than 95% capacity utilisation and avoid over‐discharge even if the model parameters are inconsistent with the actual ones under circumstances of alternating current grid fault and fluctuation of new energy output.

A systematic review of Intelligent Fault-Tolerant Protection Scheme for Multi-terminal HVDC Grids


 
 
 Due to the advancement of power electronics devices and control techniques, the modular multilevel converter (MMC) has become the most attractive converter for multiterminal direct current (MTDC) grids thanks to its most relevant features, such as modularity and scalability. Despite their advantages, conventional MMCs face a major challenge with: i) fault-tolerant operation strategy; i) energy losses in conversion; iii) lack of DC fault handling capability. This paper provides a systematic review to identify the gaps in the literature about Intelligent Fault-Tolerant Protection Schemes for multi-terminal HVDC grids. Through the bibliometric analysis, it was possible to identify topics still to be developed within the four main clusters (Offshore wind farms, Wind turbines, Voltage Source Converters, and Wind power). The research topic opens three research paths: the first is the analysis of failures in HVDC (High Voltage Direct Current) grid equipment by the FDD (Fault Detection and Diagnosis) method; the second is failure analysis by the IFDD (Inverse Fault Detection and Diagnosis) method and the third is the possibility of interconnecting the different energy generation zones with different frequencies.
 
 

Design and implementation of dual‐leg generic converter for DC/AC grid integration

SummaryA newly designed generic photo‐voltaic (PV) DC‐DC/DC‐AC converter is for direct current (DC) grid or single‐phase alternating current (AC) grid integration. Main concept of the proposed converter is universal power conversion, and the same converter is used for DC‐DC/DC‐AC applications which also aiming for minimum redundancy because the proposed converter can be able to produce DC and AC output from the fixed/variable DC source. The proposed converter is designed with single‐stage dual leg topology, with a designed filter, and protection circuits are connected in output/grid side. The proposed circuit is compared with existing topologies, and comparative analyses are made in both chopper (DC‐DC) and inverter (DC‐AC) modes for universal or generic operation. Real‐time implementation of the proposed model is prepared for the power rating of 3.5 KW during inverter mode and 4 KW when same circuit working in chopper mode. Hardware results are obtained from the model from both chopper and inverter modes. Finally, correct applications, advantages, and future work are concluded in the last section.

Multisegmented Intelligent Solution for MT-HVDC Grid Protection

Fault detection continues to be a relevant and ongoing topic in multiterminal High Voltage Direct Current (MT-HVDC) grid protection. In MT-HVDC grids, however, high DC-fault currents result from a failure of a complex protective threshold in traditional protection schemes, making Voltage Source Converter (VSC) vulnerable to such potent transient currents. In this innovative single-ended DC protection scheme, multiple time window segments are used to consider the effects of the transient period across limiting inductors at each end of the link. Multiple segments of 0–0.8, 0.8–1.5, and 1.5–3.0 ms reduce relay failure and improve the sensitivity to high fault impedance while requiring minimal computational effort. It employs feature extraction tools such as Stationary Wavelet Transform and Random Search (RS)-based Artificial Neural Networks (ANNs) for detecting transmission line faults within DC networks. Its goal is to improve the accuracy and reliability of protective relays as a result of various fault events. Simulations showed that the proposed algorithms could effectively identify any input data segment and detect DC transmission faults up to 500 ohms. Accuracy for the first segment is 100% for fault impedance up to 200 ohms, whereas the second and third segments show 100% accuracy for high impedance faults up to 400 ohms. In addition, they maintain 100% stability even under external disturbances.