Direct Current Lines Research Articles

Grid interaction of multi‐VSC systems for renewable energy integration

AbstractIntegration of converter‐interfaced renewable energy sources (RESs) into the power system and the transfer of power from RESs to remote load centres over high‐voltage direct current (HVDC) lines may require connecting multiple voltage‐sourced converters (VSCs) to a common alternating current (AC) system. Because of this connection, control loops of various converters will interact through the AC grid, leading to instability and an undesirable transient response. This paper focuses on the system‐level integration of multi‐VSC systems for the integration of RESs. μ analysis is used to determine under which control modes the independently stabilized VSCs connected to a common AC system ensure the multi‐VSC system stability. Furthermore, a sufficient criterion is proposed for the design of the converters' outer control loops independently to ensure the stability of the interconnected multi‐VSC system. For cases of severe interactions, where the interconnected multi‐VSC system may become unstable even if individual VSCs are stable, a joint controller design for converters is proposed to stabilize the multi‐VSC system. The interaction analysis indicates that employing AC voltage control mode by all the converters causes the highest interaction level, and having more converters in reactive power control mode reduces the impact of interactions on the interconnected system stability.

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

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

Influences of Wind Deviation on the Ground Resultant Electric Field under Parallel Operation of 750‐kV Ultra‐High‐Voltage Alternating Current and ±800‐kV Extra‐High‐Voltage Direct Current Transmission Lines Passing through Residential Areas in a High‐Altitude Area

The formation mechanism of the resultant electric field is complex under parallel operation of extra‐high‐voltage alternating current (AC)/direct current (DC) transmission lines. However, existing methods are computationally expensive and fail to guarantee the stability and convergency of results. To solve this problem, a calculation method with nonuniform grids and varying time steps was proposed on the basis of the finite‐volume and finite‐element method for calculating the ground resultant electric field of parallel AC/DC transmission lines. The selection of the initial value of space charges and the setting of the surface charge density of conductors were also improved, which greatly enhanced the computational stability and efficiency. Then, the improved method was adopted to calculate the ground resultant electric field under parallel operation of a 750‐kV AC transmission line and a ±800‐kV DC transmission line when considering and not considering wind deviation. Results show that, when ignoring wind deviation, the normalized values of the ground resultant electric field within 6 m from the side‐phase conductor of the AC line are always less than 1 under conditions of the minimum clearances to earth of the AC and DC lines passing through a residential area stipulated in the national standards, which meets codified requirements. When considering wind deviation, instead of an increase, the calculated ground resultant electric field of parallel AC/DC lines is significantly reduced, and the weighted value is much less than the limiting value.

Revealing interactions between HVDC cross-area flows and frequency stability with explainable AI

The transition to renewable energy sources challenges the operation and stability of the electric power system. Wind and solar power generation are volatile and uncertain, and energy sources may be located far away from the centers of the load. High Voltage Direct Current (HVDC) lines enable long-distance power transmission at low losses, both within and between different synchronous power grids. HVDC interconnectors between different synchronous areas can be used to balance volatile generation by leveraging their fast control behavior, but rapid switching may also disturb the power balance. In this article, we analyze the interaction of HVDC interconnector operation and load-frequency control in different European power grids from operational data. We use explainable machine learning to disentangle the various influences affecting the two systems, identify the key influences, and quantify the interrelations in a consistent way. Our results reveal two different types of interaction: Market-based HVDC flows introduce deterministic frequency deviations and thus increase control needs. Control-based HVDC flows mitigate frequency deviations on one side as desired but generally disturb frequency on the other side. The analysis further provides quantitative estimates for the control laws and operation strategies of individual HVDC links, for which there is little public information. Furthermore, we quantify the importance of HVDC links for the frequency dynamics, which is particularly large in the British grid.

Dynamic physical characteristics of DC arc on arcing horn for HVDC grounding electrode line

The dynamic physical characteristics of a DC arc on an arcing horn for a high voltage direct current (HVDC) grounding electrode line are significantly different from those of the switching device arc, secondary arc, AC fault arc and pantograph-catenary arc. In this work, an experimental platform for the DC arc on the arcing horn was built, and mechanisms of the arc column short circuit and arc root movement were studied. This work further analyzes the characteristics and mechanisms of the arc motion when wind speed and direction, magnetic field and the expansion angle of the electrode are varied. Arc root movement is more likely to occur at the upper electrode. There is a competitive relationship between arc expansion and the transferring effect. The effect of wind on the arc column is greater than the effect on the arc root. The magnetic field has a significant driving effect on both the arc column and the arc root. The research results provide a comprehensive experimental basis for further probing the method of DC arc suppression, and the improvement of the arcing horn.

Coordinated Planning of HVDCs and Power-to-Hydrogen Supply Chains for Interregional Renewable Energy Utilization

With the urgent requirement of the utilization of renewable energy (RE) for the decarbonization of energy systems, power transmission via high-voltage direct current (HVDC) lines shows limits in the face of fluctuations of RE generation. This paper addresses the coordinated planning of HVDCs and power-to-hydrogen supply chains (P2HSCs) for RE utilization. First, the lumped cluster model of the large-scale power-to-hydrogen is established based on the derivation of the maximum production points. On this basis, P2HSC is modeled as a constrained power load. Then, a multi-objective multi-stage stochastic planning model of HVDCs and P2HSCs is proposed to fully consider RE fluctuation and uncertainty characteristics. Furthermore, a decomposition algorithm based on the dynamic programming method is designed to solve this problem. Finally, the proposed model and algorithm are applied in real Inner Mongolia-Shandong case studies. The effectiveness and robustness of the proposed model are quantitatively verified. Furthermore, the investment interplay of HVDCs and P2HSCs on the planning timeline and the operational interplay of the two sectors as complementary power loads are verified to create the maximum social benefit in renewable energy systems.

Catenary-Powered Electric Traction Network Modeling: A Data-Driven Analysis for Trolleybus System Simulation

In the context of smart cities, direct current overhead contact lines, usually adopted to power urban transportation systems such as trolleybuses, tramways, metros, and railways, can serve as a backbone to connect different modern emerging technologies. Among these, in-motion charging (IMC) trolleybuses with on-board batteries are expected to be very impactful on the DC network’s power flow and may require specific voltage and current control. These factors motivate the development of a simulation tool able to emulate these devices’ absorption and their effect on the supply infrastructure. The main innovative value of the work is to improve a simulation model of a trolleybus grid through a data-driven approach by using measurements of voltage and current output from a traction substation. The measurements are essential for understanding the behavior of vehicle weight variation throughout the day. Thanks to this information, a characterization of the current draw by conventional trolleybuses and IMC trolleybuses is then provided for each trolleybus route in a specific power section of the Bologna trolleybus system. By integrating the variation in vehicle weight within the model, a simulation of a possible daily operation of a trolleybus feeding section has been performed, obtaining a 7% error between the daily energy calculated from the simulation and that obtained through measurements. This analysis demonstrates the feasibility of the adopted simulation tool, which can also be used to evaluate additional hypothetical trolleybus operation scenarios. One of these possible scenarios considers IMC vehicles, and it is also evaluated in this paper.

Simulation study on the pollution accumulation of the porcelain insulator in a wind tunnel under direct-current composite electric field

A direct-current (DC) composite electric field is formed because of corona discharge on transmission lines. This field facilitates particle charging in the atmosphere. The particles charged by such a field are influenced by DC electric force, which affects their motion toward and deposition on insulators and eventually results in variations in the pollution characteristics of DC line insulators. Simulating the effect of a composite electric field near a conductor on the insulator pollution of a DC line is crucial for preventing pollution. In this paper, a three-dimensional simulation model was developed for simulating the pollutant accumulation on an insulator string under a DC composite electric field. This model was also used to simulate the ion flow, particle charging, fluid dynamics, and kinetics and collision–adhesion properties of charged particles under a DC composite electric field. In the electrical submodel, the space charge density and ionic current density on the outer surface of insulator pins and caps were used to calculate metals' charge and their floating potential. A charging submodel was developed that embodied the contributions of the electric field intensity and the space charge density of the composite field to the pollution particles’ charging in motion. The pollution accumulated on the insulator strings was simulated at different values of various factors, such as conductor voltage, ion flow intensity, and airflow velocity. The simulation results and analyses of the relationship between the factors above and insulator pollution characteristics indicated that a specific composite electric field resulted in the highest pollutant accumulation on the insulator string. The pollutant accumulation on insulator sheds’ lower surface increased and decreased with the weakening of the ion flow field when the nominal electric field near the insulator string was excessive and most favourable for polluting, respectively.

Impact of Li-Ion Battery on System's Overall Impedance and Received Signal Strength for Power Line Communication (PLC).

In anticipation of the hybrid utilisation of the radio frequency (RF) wireless transceiver technology embedded in future smart Li-ion battery cells to deliver hybrid links based on power line communication (PLC) and wireless connections, herein we present an empirical high-frequency investigation of the direct current (DC) bus. The focus is to determine, via statistical tools including correlation coefficient (CC), root mean squared error (RMSE) and feature selective validation (FSV) method, the impedance and signal change impact on a possible communication link when fully charged cells are present or completely missing from the bus. Moreover, to establish if technological differences may be accounted for during the empirical experiments, Li-ion cells from two different manufacturers were selected and connected via three subsequent capacitive couplings of 1 µF, 1 nF and 1 pF. According to a methodical comparison by employing CC, RMSE, and FSV over the measured impedance and signal attenuation, this study has shown that the physical DC network is the dominant impedance at high frequencies and that the signal attenuation on the DC line supports communication in the investigated spectrum. The reported findings are critical for in situ hybrid PLC and wireless communication implementation of BMS for Li-ion systems supported through only one RF transceiver.

Assessing Potential Scenarios for Achieving New and Renewable Energy Targets in Java-Bali Power System, Indonesia

Geographic circumstances, government policies, and power system characteristics face many countries struggling to achieve their new and renewable energy (NRE). In addition, one characteristic of renewable energy (RE) which cannot be moved is a severe problem for archipelagic countries like Indonesia in achieving their NRE targets. Therefore, this research creates a long-term open-source generation expansion planning (GEP) model that considers renewable energy integration between islands, government policies, and power system characteristics of Indonesia. The model proposes a high voltage direct current (HVDC) line to facilitate abundant energy transfer between islands. The research also included multiple scenario analyses based on the potential strategies that could realistically be applied. Based on the long-term GEP model results, possible alternative routes to achieving NRE targets are mapped and assessed by considering power system characteristics and national energy policies. Specifically, the Java-Bali system of Indonesia is employed as a case study to demonstrate the performance of the proposed long-term GEP model. The optimum planning to achieve the targets produces the generation cost of 7.05 cents USD/kWh and the CO2 emission reduction of 2,297 million tons of CO2.

Study on AGC strategy involving the frequency control of HVDC power regulation

The asynchronous interconnection between the Southwest China power grid (SCPG) and Central China power grid (CCPG) reduces the inertia of rotation of SCPG to 1/5, which leads to a prominent risk of frequency stability. Moreover, in order to suppress the ultra-low frequency oscillations, the parameters of hydropower governors were optimized, which further worsen the frequency regulation capability of the system. Therefore, the frequency control (FC) of high voltage direct current (HVDC) line along with automatic generation control (AGC) are designed to improve the frequency restoration after a large power disturbance. However, the characteristic of FC regulation is contradictory to the subsequent AGC regulation. In order to improve the post-contingency frequency response of power system, an improved AGC strategy is developed. The proposed strategy involves the FC regulation power and distribute the control power according to the characteristics of different generator units. Case study on the actual model of SCPG proves the effectiveness of the strategy.

Deep Learning Based Hurricane Resilient Coplanning of Transmission Lines, Battery Energy Storages, and Wind Farms

In this article, a multistage model for expansion coplanning of transmission lines, battery energy storages, and wind farms (WFs) is presented considering resilience against extreme weather events. In addition to high-voltage alternating current lines, multiterminal voltage source converter based high-voltage direct current lines are planned to reduce the impact of high-risk events. To evaluate the system resilience against hurricanes, probable hurricane speed scenarios are generated using Monte Carlo simulation. The fragility curve concept is utilized for calculating the failure probability of lines due to extreme hurricanes. Based on each hurricane damage, the probable scenarios are incorporated in the proposed model. Renewable portfolio standard policy is modeled to integrate high penetration of WFs. To deal with the wind power and load demand uncertainties, a chronological time-period clustering algorithm is introduced for extracting representative hours in each planning stage. A deep learning approach based on bidirectional long short-term memory networks is presented to forecast the yearly peak loads. The mixed-integer linear programming formulation of the proposed model is solved using a Benders decomposition algorithm. A modified IEEE RTS test system is used to evaluate the proposed model effectiveness.

Siting and sizing of synchronous compensator based on electromagnetic transient simulation

With the increasing load of power grid and the access of new energy sources and direct current lines, power grid voltage stability becomes increasingly prominent. Installing synchronous compensator is an option to supplement reactive power and improve voltage stability. Compared with other reactive power compensation equipment (SVC, SVG, etc.), synchronous compensators have advantages in service life, size, easy to maintenance and etc. Siting and sizing of synchronous compensator are critical for investment and voltage stability adjustment performance of the equipment. Considering that the power grid contains new energy and HVDC equipment, it is necessary to use electromagnetic transient (EMT) simulation to evaluate transient stability of a power system when the siting and sizing of a synchronous compensator was determined. In this paper, a method of siting and sizing the synchronous compensator based on voltage sensitivity index (VSI), severity index (SI) and EMT simulation is proposed. VSI of different bus in the system is calculated by injecting reactive power into each bus, and the suitable candidate bus location for compensator is obtained. Furthermore, the calculation of power grid fault severity index is designed, and the synchronous compensator sizing method with the SI as optimization objective is designed. An actual AC–DC hybrid power grid system with 1343 buses was used to test the proposed methods and testing results demonstrate the performance of the proposed method.

Stability Improvement of The Iraqi Super Grid (400kV) using High Voltage Direct Current (HVDC) Transmission

This research analyzes the level of the short circuit effect of the Iraqi super network and decides the suitable location for the High Voltage Direct Current (HVDC) connections in order to obtain the best short circuit reduction of the total currents of the buses in the network. The proposed method depends on choosing the transmission lines for Alternating current (AC) system that suffers from high Short Circuit Levels (SCLs) in order to reduce its impact on the transmission system and on the lines adjacent to it and this after replacing the alternating current (AC) line by direct current (DC) line. In this paper, Power System Simulator for Engineering (PSS/E) is used to model two types of HVDC lines in an effective region of Iraqi networks and to perform comparative studies to test the location of Short Circuit Levels (SCLs) between an actual AC and AC/DC case study in a portion of the Iraqi national network. The results proved the effectiveness of this method in eliminating severe faults and unwanted short currents, and the results showed that the bipolar type is better in reducing Short Circuit Levels of the Iraqi network.

Research on Improved Fault Current Analysis Method for Flexible Direct Current Power Grid Considering Alternating Current Feed

Flexible direct current (DC) grid can realize large-scale renewable energy, wide-area coordinated complementation, and reliable power transmission. It is an important development that can be used to support high-voltage and large-capacity flexible DC transmission in the future. The short-circuit current of the DC line is one of the important bases for the selection of key main equipment parameters such as converter valves, DC circuit breakers, and reactors in the flexible DC grid. In this paper, a flexible DC grid equivalent circuit network model with alternating current (AC) feed-in is established. Aiming at the monopolar ground fault of the flexible DC grid grounded through the metal loop, an optimized traditional matrix calculation method is proposed to obtain the accurate line fault current value. On this basis, with an actual engineering background, the equivalent circuit model of the four-terminal bipolar flexible DC power grid is established, and the influence of grounding position, grounding parameters, and current-limiting reactor on the fault current of the DC line is analyzed. Finally, simulation using the PSCAD software verifies the effectiveness and accuracy of the proposed method. The method proposed in this paper can provide the necessary bases and references for the selection of flexible DC grid equipment.

Distributed frequency emergency control with coordinated edge intelligence

Developing effective strategies to arrest grid frequency drop in case of severe contingencies is an important requirement. While distributed responsive loads for primary frequency control have been studied in the literature, the coordination of the distributed loads relies on high-speed communication or centralized frequency threshold setting. They might be cost-prohibitive or not sufficiently fast in severe contingencies, like the fault of ultrahigh voltage direct current (UHVDC) lines in China. In this paper, a local power loss ∆P estimation approach for frequency emergency control is proposed, and the coordination of the distributed loads is achieved by the edge intelligence and Internet-of-Things (IoT) technologies. The proposed approach can enable the distributed loads to provide fast and accurate frequency support to power grids. The parameter-setting in the control center and decision-making at the edge level for the proposed approach are described in detail. Based on a typical IEEE test system and a real bulk power system in China, numerical simulations and hardware experiments are conducted to verify the frequency support performance of the proposed approach. Further, a direct load frequency control system named Grid Sense implemented in China is explicitly described, and the real-world issues for implementing the proposed approach are analyzed.

Voltage control and virtual synchronous generator control of HVDC interconnection line for improving stability of power system including large‐scale wind farm

Large-scale integration of renewable energy sources (RESs) such as wind power can impact on the existing power system's stability. For example, there is a possibility that the wind farm (WF) cannot be maintained stable and then it is disconnected from the grid when the instantaneous voltage sag caused by a grid disturbance occurs. Moreover, the increasing integration of RESs in the power system can reduce the number of conventional synchronous generators (SGs) connected to the grid, and lead to a reduction of system inertia in the grid. The system inertia reduction can cause larger and faster frequency fluctuations during the transient period. Hence, this paper proposes a new control strategy composed of the voltage control for suppressing the voltage sag and the virtual synchronous generator (VSG) control based on the proportional-integral-differential (PID) fuzzy logic control (FLC) for maintaining the frequency stability which is performed by high voltage direct current (HVDC) interconnection line. The aim of the new control strategy is to improve the stability of the entire power system with large-scale integration of RESs. The effectiveness of the proposed strategy is confirmed through the simulation analyses by PSCAD/EMTDC.

Voltage distribution indices method to analyse the performance of various structures of stray current collectors in direct current transit lines

The stray current produced by a direct current (DC) transit system leads to the corrosion of rails and metallic structures buried under and around the metro line. One practical approach to diminish current injection into buried structures is to implement stray current collectors underneath the rail. An analytical method is proposed herein using voltage distribution indices (VDIs) to calculate the voltage and current collection level of each collector located under the rail soil, and the solutions are validated by comparing with modelling results. The proposed method helps to achieve the voltage of different points under the ground and the current reaching the collectors using a circuit model, without facing the limitations of practical methods, as a supplementary study for experimental tests. Moreover, the effect of different arrangements of collectors under running rails is modelled. The analytical method is deployed considering two objectives, that is stray current collection by the conductors and a reduction in the installation cost of the conductors. Various arrangements of stray current collectors are compared and evaluated. Using integral and matrix equations, current density and potential at different points under the soil are calculated and the obtained results are compared with European (EN) standards.

Hybrid Method of FEM and Divergence Theorem to Analyze Ion Flow Field Including Dielectric Film’s Accumulation Charges

Due to the dielectric film with charges from corona discharge of high-voltage direct current (HVDC) overhead lines, the electric field near the greenhouses covered by the dielectric film can be distorted. The electric field at some position on the ground level can be measured under the dielectric film. However, the distribution characteristics of the electric field cannot be obtained. In this article, based on the measurement results of some defined points, an algorithm to calculate the ion flow field with the presence of a dielectric film is proposed. Poisson's equation and the current continuity equation are iteratively solved by the finite-element method (FEM) and the divergence theorem, respectively. The experiment validates the validity of the hybrid method. The research is helpful to understand the influence of charged dielectric film fully.

Identification of Environmental and Experimental Factors Influencing Human Perception of DC and AC Electric Fields.

As part of the energy transition in Germany, high-voltage direct current (HVDC) lines producing DC electric fields (EF) are in planning. Since the human perception of DC EF was rarely investigated in the past, we aimed to identify environmental and experimental factors influencing the human perception of direct current (DC) EF, alternating current (AC) EF, and the co-exposure of DC EF and AC EF (hybrid EF) under whole-body exposure. Additionally, first estimates of DC EF and AC EF perception thresholdsas well as differences in human perception of DC EF and AC EF concerning the type of sensation experienced and the affected body part were evaluated. A highly sophisticated exposure lab was built to expose participants to various EF strengths and ask for their assessment concerning the presence of an EF. To estimate the individual perception thresholds of 11 participants, the signal detection theory as well as the single-interval-adjustment matrix procedure were applied. Relative humidity could be identified as an environmental factor influencing the perception of AC EF and DC EF in different ways. An appropriate ramp slope and an exposure duration for future studies could be elaborated. Additionally, perception thresholds were lower under hybrid EF exposure than under DC EF or AC EF exposure alone. Cutaneous sensations evoked under DC EF and AC EF exposure were individually different and attributed to various parts of the body. Several environmental and experimental factors influencing the human perception of EF could be identified and provide an essential basis for a large-scale study. © 2021 Bioelectromagnetics Society.