Direct Current Grids Research Articles

A Fault Current Limiting Hybrid DC Circuit Breaker

Due to the low impedance characteristic of the high voltage direct current (HVDC) grid, the fault current rises extremely fast after a DC-side fault occurs, and this phenomenon seriously endangers the safety of the HVDC grid. In order to suppress the rising speed of the fault current and reduce the current interruption requirements of the main breaker (MB), a fault current limiting hybrid DC circuit breaker (FCL-HCB) has been proposed in this paper, and it has the capability of bidirectional fault current limiting and fault current interruption. After the occurrence of the overcurrent in the HVDC grid, the current limiting circuit (CLC) of FCL-HCB is put into operation immediately, and whether the protected line is cut off or resumed to normal operation is decided according to the fault detection result. Compared with the traditional hybrid DC circuit breaker (HCB), the required number of semiconductor switches and the peak value of fault current after fault occurs are greatly reduced by adopting the proposed device. Extensive simulations also verify the effectiveness of the proposed FCL-HCB.

Power Grid Reliability Estimation via Adaptive Importance Sampling

Electricity production currently generates approximately 25% of greenhouse gas emissions in the USA. Thus, increasing the amount of renewable energy is a key step to carbon neutrality. However, integrating a large amount of fluctuating renewable generation is a significant challenge for power grid operating and planning. Grid reliability, i.e., an ability to meet operational constraints under power fluctuations, is probably the most important of them. In this letter, we propose computationally efficient and accurate methods to estimate the probability of line overflow, i.e., reliability constraints violation, under a known distribution of renewable energy generation. To this end, we investigate an importance sampling approach, a flexible extension of Monte-Carlo methods, which adaptively changes the sampling distribution to generate more samples near the reliability boundary. The approach allows to estimate overload probability in real-time based only on a few dozens of random samples, compared to thousands required by the plain Monte-Carlo. Our study focuses on high voltage direct current power transmission grids with linear reliability constraints on power injections and line currents. We propose a novel theoretically justified physics-informed adaptive importance sampling algorithm and compare its performance to state-of-the-art methods on multiple IEEE power grid test cases.

An Architecture for a Multi-Vendor VSC-HVDC Station With Partially Open Control and Protection

High voltage direct current (HVDC) grids are envisioned for large-scale grid integration of renewable energy sources. Upon realization, components from multiple vendors have to be coordinated and interoperability problems can occur. To address these problems, a multi-vendor HVDC system can benefit from a partially open control and protection system. Unwanted interactions can be investigated and solved more easily in partially open software compared to when applying black-boxed and vendor-specific software. Although a partially open approach offers these advantages, practical aspects, such as the implementation in a real station architecture, have to be addressed carefully. This paper covers this important topic, first by reviewing the required control and protection functions and second by discussing the choice for certain open and closed software parts, their implementation in physical units as well as the required communication and interfaces. The result from this discussion is a first proposal of a station architecture for a multi-vendor HVDC system using partially open control and protection. This architecture will be a helpful starting point to industry and academia working with research and harmonization on this topic as ad-hoc solutions in terms of practical aspects can be avoided.

Estimating the environmental impacts of HVDC and UHVDC lines for large-scale wind power transmission considering height-dependent wind and atmospheric stability

Due to the imbalance of China’s resources allocation, inter-provincial high voltage direct current (HVDC) transmission grid is considered to be the promising solution to solve to the large-scale wind power transmission, which could tackle the reverse regional distribution problem of energy resource and electricity load. In this contribution, a power law profile model of wind speed on embedded atmospheric stability is developed for calculating the height-dependent wind speed. The Monin-Obukhov length is introduced to determine the atmospheric stability. The ion flow environment model considering height-dependent wind speed is proposed to estimate the influence of height-dependent wind on ground-level electric field environment of DC transmission line and the iterative flux tracing method-finite difference is proposed to solve the model. It is observed that the stable, neutral and unstable atmospheric conditions have a large influence on transverse wind speed and then indirectly influence the electric field and ion current environment of DC lines. For the electromagnetic environment of the practical DC line used for the wind power transmission, the proposed model provides a reasonable agreement with measurements from a perspective of practical engineering application.

Photovoltaic Power Plant Collection and Connection to HVDC Grid with High Voltage DC/DC Converter

Photovoltaic (PV) power plant collection and connection to a high voltage direct current (HVDC) grid has many advantages. Compared with the traditional AC collection and grid-connection scheme, it can reduce the power conversion links and improve the system efficiency. As one of the most important devices in the application of a PV HVDC collection and grid-connection system, a high voltage, large capacity, high step-up ratio DC/DC converter is the critical technology. A DC/DC converter scheme based on a boost full bridge isolated power module cascaded in input parallel output series (IPOS) structure is proposed to meet the technical requirements of PV power conversion with high voltage, large capacity, and high step-up ratio. The operation mode of the power module is analyzed, the soft switching method is proposed, and the constraint to realize the soft switching of the power module is deduced. Aiming to resolve the problem of multi-module voltage and current equalization in a cascaded DC/DC converter, a distributed module equalization control strategy is proposed to realize the reliable operation of a power module and converter. A 5 kV/80 kW standard power module is developed, the DC/DC converter experimental platform is built, and the proposed system scheme and control strategy are verified by experiments. Based on a 14-power module input parallel output series connection, a ±30 kV/1 MW PV DC/DC converter is developed, a ±30 kV PV HVDC collection and grid-connection demonstration system is established, and the experimental test is completed to realize the stable operation of the system.

Technical Assessment of Hybrid HVDC Circuit Breaker Components under M-HVDC Faults

One of the technical challenges that needs to be addressed for the future of the multi-terminal high voltage direct current (M-HVDC) grid is DC fault isolation. In this regard, HVDC circuit breakers (DCCBs), particularly hybrid circuit breakers (H-DCCBs), are paramount. The H-DCCB, proposed by the ABB, has the potential to ensure a reliable and safer grid operation, mainly due to its millisecond-level current interruption capability and lower on-state losses as compared to electromechanical and solid-state based DCCBs. This paper aims to study and evaluate the operational parameters, e.g., electrical, and thermal stresses on the IGBT valves and energy absorbed by the surge arrestors within H-DCCB during different DC fault scenarios. A comprehensive set of modeling requirements matching with operational conditions are developed. A meshed four-terminal HVDC test bench consisting of twelve H-DCCBs is designed in PSCAD/EMTDC to study the impacts of the M-HVDC grid on the operational parameters of H-DCCB. Thus, the system under study is tested for different current interruption scenarios under a (i) low impedance fault current and (ii) high impedance fault current. Both grid-level and self-level protection strategies are implemented for each type of DC fault.

Voltage control of common capacitor and self-faults analysis of DC current flow controllers for meshed multi-terminal HVDC grids

In this paper, the control methodology of the common capacitor voltage and the DC current flow controller (DC CFC) under self-fault conditions are investigated with the following two contributions. (1) A selection method of the common capacitor voltage based on various specific control requirements with explicit control margins of the DC line currents is proposed with analytic derivations of two-line/three-line DC CFCs in the meshed three-terminal/four-terminal high-voltage direct current grids (HVDC), respectively. (2) The impact of different self-faults of the DC CFC on the control performance of the DC line currents is analyzed with a simplified topology with unidirectional current flow presented. Case studies are conducted in the time-domain simulation environment PSCAD/EMTDC and the results justify the effectiveness of the theoretical analysis. The proposed common capacitor voltage control method and self-fault analysis would be useful for practical applications of the DC CFC under both steady and transient states.

Power quality mitigation and transient analysis in AC/DC hybrid microgrid for electric vehicle charging

The usage of electric vehicles (EV) increased in recent years as the vehicles design and performances are nearly similar to petrol vehicles. The main source of energy for EV is taken from the grid for charging. So, the penetration of EVs in alternating current (AC) grid creates more power quality issues like voltage sag, swell and harmonics in the current. This energy can also be produced from the renewable energy resources like photovoltaic (PV) power generation. This PV energy can also be used as direct current (DC) grid. The electric vehicle chargers which come with intelligent grid operation is considered as load in this paper. This paper is an attempt to discuss the penetration of EVs in AC/DC hybrid micro grid which causes the power quality problems, and the power quality problem is mitigated by using the unified power quality conditioner (UPQC). The results are analyzed for three cases and four scenarios which is based on the function of UPQC and the action of smart charger in grid connected as well as autonomous mode operation of the AC/DC micro grid when the load is considered as dynamic load. The simulation is carried out in MATLAB2017b environment

Active injection protection scheme for flexible HVDC grids based on amplitude of input impedance

High-voltage direct current (HVDC) grids require fast and reliable protection of the DC lines. The performance of traditional protection schemes is easily impaired by the limitations of the boundary condition and nonlinearity from the control of converters. One of the key technologies for flexible HVDC grids is the half-bridge modular multilevel converter (HB-MMC). Considering the high controllability of HB-MMC, this study proposes an active injection protection scheme to improve the reliability and sensitivity of the HVDC grid protection. The HB-MMC is used to inject a sinusoidal characteristic signal, at the specified frequency, into the DC lines. Then, the voltage and current at the specified frequency are extracted using the Prony algorithm to calculate the input impedance, which is used for the identification of internal and external faults. The active injection protection scheme was simulated for various cases in the simulation software Power Systems Computer Aided Design. The simulation results indicate that the proposed protection scheme is highly reliable and can overcome transition resistance.

Heuristic remedial actions in the reliability assessment of high voltage direct current networks

<p>Planning of high voltage direct current (HVDC) grids requires inclusion of reliability assessment of alternatives under study. This paper proposes a methodology to evaluate the adequacy of voltage source converter/VSC-HVDC networks. The methodology analyses the performance of the system using N-1 and N-2 contingencies in order to detect weaknesses in the DC network and evaluates two types of remedial actions to keep the entire system under the acceptable operating limits. The remedial actions are applied when a violation of these limits on the DC system occurs; those include topology changes in the network and adjustments of power settings of VSC converter stations. The CIGRE B4 DC grid test system is used for evaluating the reliability/adequacy performance by means of the proposed methodology in this paper. The proposed remedial actions are effective for all contingencies; then, numerical results are as expected. This work is useful for planning and operation of grids based on VSC-HVDC technology.</p>

DC Communities: Transformative Building Blocks of the Emerging Energy Infrastructure

Serious environmental concerns call for revolutionary solutions to cope with the harmful effects of the conventional energy landscape. Therefore, residential and commercial customers require cleaner and more reliable energy sources as they become more dependent on energy for daily and critical needs. In this case, transitioning to a cleaner energy economy is of paramount importance for both the environment and the utilities as well as the end-users. The desired transformation will require the deployment of massive amounts of clean energy sources. Many of these resources, such as solar photovoltaic (PV), provide electricity in the form of direct current (DC) that enables the return of DC grids to the electric power arena. The electric system has slowly transitioned to DC, mainly on the demand side. In recent years, modern electronic devices, lighting systems, and an increased number of appliances (≈22% of the residential and commercial loads) have adopted DC systems. Studies suggest that DC loads would account for more than 50% of the available loads in the next few years. Furthermore, the growing proliferation of electric vehicles influx is another example of a successful DC application. From this perspective, the viability of returning to the DC distribution system in the form of DC community grids is explored. We start by defining the DC community grid, which is followed by introducing the benefits of adopting DC at the distribution level. Finally, a summarizing outlook of successful pilot cases, projections of DC community deployment, barriers and concerns, strategies to address barriers and concerns, and suggestions for future research directions are presented. This perspective could shed new light on the building blocks of the transformed energy landscape for various stakeholders.

Application of smart DC-Grid for efficient use of solar photovoltaic system in driving separately excited DC motor: Dynamic performance and techno-economic assessments

Solar energy is one of the solutions to the global threats - climate change, environmental pollution, fossil fuel depletion and high cost of energy - associated with increased energy demand. In this research, the techno-economic feasibility and dynamic performance assessment of a smart solar Photovoltaic (PV) Direct Current grid (DC-grid) powered Separately Excited Direct Current Motor (SEDCM) is presented. The energy mapping was done at Agbani City (6°18.3′N, 7°33.8′E), in Enugu State, Nigeria. Agbani city was identified to have a good amount of solar energy with an annual average solar radiation of 4.67 kWh/m 2 /d. The dynamic performance assessment of a smart solar PV DC-grid powered SEDCM was conducted with the help of MATLAB/SIMULINK software. The result show that a change in sun intensity affects the photon-generated current and it has effect on the open-circuit voltage of the solar panel. This change in the intensity can be compensated for by the use of a boost converter. The whole system was optimized in HOMER Pro simulation software to assess the techno-economic feasibility of the system. From the techno-economic study, it was observed that the proposed DC-grid PV system is feasible for the generation of electricity to power the SEDCM. The cost of energy was calculated and the categorized result from HOMER Pro shows that the capacity of the DC-grid PV system (35.67 kW) with 120 Surrette S-260 batteries gives the cheapest configuration. This configuration has a cost of energy of 0.11 $/kWh and a Net Present Cost (NPC) value of 0.048 million dollars and was used as optimum system configuration. Comparing the system with the national grid, the proposed DC-grid PV configuration has a cost saving of 35% with 1.679 gCO 2 ~eq/kWh greenhouse gas emission, while the national grid emits a significant amount of greenhouse gas up to 15,104 kg/y.

Comparison of Star and String Offshore DC Collector Grid Topologies on the Aspect of Stability—An Impedance Approach

Offshore Direct Current (DC) collector grids are a promising technology for decreasing the installation and operation costs of offshore wind parks. Nevertheless, the stability properties and hence the design of such DC collector grids is not common or standardised. Hence, this paper describes an attempt to fill these gaps by analysing the stability of two different types of DC collector grids—star and string—by considering identical operating conditions. The approach follows a non-parametric formulation of the impedance based Nyquist Stability Criterion. The hyperbolic Π equivalent formulation of the telegraph equation is adopted for modelling the submarine cable due to high capacitance that is distributed and thus the conventional 50 Hz Π-model is not sufficient anymore. Furthermore, the paper shows how to integrate the complex dynamics of wind turbines into the overall stability assessment through an impedance building algorithm. Finally, it is shown how to stabilise the collector grids by means of active control parameter changes and it has been observed that the star configuration of wind turbines is more favourable on account of stability and controllability.

HVDC grids stability improvement by direct current power system stabilizer

High-voltage direct current breaker is among the essential components of high-voltage direct current grids. Such a breaker generally needs a direct current reactor to reduce the fault currents rate. However, direct current reactors have destructive effects on the multi-terminal high-voltage direct current grid dynamic stability, and in such a system, despite the variety of controllers, the system dynamics are highly sensitive to the operating point. Therefore, additional damping control will be needed. This paper proposes a modification to be applied to the traditional droop controller of high-voltage direct current grids to cope with the influence of these large reactors, improving the direct voltage stability and decreasing power variations in the transient events by introducing a direct current power system stabilizer. The proposed method for direct voltage control has been investigated through the analytical model of the system. Stability improvement has been studied following the application of the proposed method by investigating zeros, poles, and frequency response analysis. Moreover, a method is proposed for optimal design and optimal placement of direct current power system stabilizer. The system analysis and time-domain simulations demonstrate a decent damping improvement attained by the proposed method. All simulations and analytical studies are conducted on Cigré DCS3 test high-voltage direct current grid in MATLAB/Simulink.

Single-Ended Fast Protection for Transient Power of Multi-Terminal Flexible and Direct Power Grids Containing Renewable Energy

With the proposal of the carbon peak goal, a multi-terminal flexible DC grid containing various renewable energy becomes the one of main ways of renewable energy power transmission. Thus, it is increasingly important to improve its transmission reliability and stability. This paper proposes a single-ended protection principle based on the analysis method of transient power change for the “mesh structure” ring-shaped flexible direct current (DC) grid. Based on the propagation characteristics of voltage, current, and anti-traveling waves during fault periods, the transient power variation coefficient based on SOD transformation is introduced to distinguish the internal and external faults. Besides, simulation based on the PSCAD/EMTDC platform within the Zhangbei four-port network verifies the effectiveness of the proposed protection principle under various fault conditions. Meanwhile, the protection criterion is designed according to the difference in the change coefficient of the transient power SOD within and outside the fault time zone. Moreover, the setting value of the start of protection is obtained by analyzing the simulation results, thus a set of fast and reliable single-ended protections for the same-side short-time transient power window of an enhanced flexible straight power grid is developed via combining it with the starting criterion of a sudden voltage change.

A Systematic Review of Key Challenges in Hybrid HVAC–HVDC Grids

The concept of hybrid high-voltage alternating current (HVAC) and high-voltage direct current (HVDC) grid systems brings a massive advantage to reduce AC line loading, increased utilization of network infrastructure, and lower operational costs. However, it comes with issues, such as integration challenges, control strategies, optimization control, and security. The combined objectives in hybrid HVAC–HVDC grids are to achieve the fast regulation of DC voltage and frequency, optimal power flow, and stable operation during normal and abnormal conditions. The rise in hybrid HVAC–HVDC grids and associated issues are reviewed in this study along with state-of-the-art literature and developments that focus on modeling robust droop control, load frequency control, and DC voltage regulation techniques. The definitions, characteristics, and classifications of key issues are introduced. The paper summaries the key insights of hybrid HVAC–HVDC grids, current developments, and future research directions and prospects, which have led to the evolution of this field. Therefore, the motivation, novelty, and the main contribution of the survey is to comprehensively analyze the integration challenges, implemented control algorithms, employed optimization algorithms, and major security challenges of hybrid HVAC–HVDC systems. Moreover, future research prospects are identified, such as security algorithms’ constraints, dynamic contingency modeling, and cost-effective and reliable operation.

A reverse travelling wave differential protection scheme for DC lines in MMC–HVDC system with metallic return

With the increase of voltage level and transmission capacity, the symmetric bipolar modular multilevel converter based high voltage direct current (MMC–HVDC) grid has become popular in actual projects. Due to the DC fault must be cleared within the timeframe of milliseconds, fast and reliable identification of faulty line is a key issue. The grounding method of HVDC grid and the frequency dependent characteristics of transmission line influence the fault characteristics are rarely taken consideration. To make up for the insufficiency, a reverse travelling wave differential protection scheme is proposed in this paper. Based on the grounding mode of metallic return, the compound sequence boundary conditions under different DC faults are derived first. Then, the travelling wave theory based on Marti frequency dependent model is analysed, and the reverse travelling wave differential value is calculated. Based on the analysis, the protection strategy of different DC faults is established. The proposed protection scheme can meet the rapidity requirements, with low sampling frequency and easy setting. The effectiveness is verified by PSCAD/EMTDC, and extensive simulation results are carried out to verify the advantages in fault resistance tolerance and anti-interference capability.

Fault detection and protection in a meshed MMC[sbnd]HVDC grid based on bus-voltage change rate and fault component current

Fault protection for a modular multi-level converter (MMC)-based high voltage direct current (HVDC) grid with dc circuit breakers (DCCBs) is a great challenge, because faults need to be isolated fast to prevent the collapse of the grid and overcurrent damage to the equipment. Before clearing faults, faults must be detected as soon as possible. However, the fast rising speed of fault currents and the emerging fault between the DCCB and line reactor increase the difficulty of fault protection. A fast and reliable fault detection and protection method relying on bus-voltage change rate and fault component current is proposed in this paper. It requires the measurements with local sensors and makes use of a lower number of voltage sensors. The proposed algorithm is applied to a four-terminal MMCHVDC grid. Results show that the proposed method can identify the fault type and discriminate bus faults, line faults, and wall bushing faults. It can detect faults quickly and does not generate erroneous and unnecessary trip signals.

Optimal sparse control of interval networks: Voltage source converter based DC grid study

Decentralized optimal control of interval state space systems involves significant complications due to the required sparsity, robustness, and convergence rate. By adding an l1 term to the standard linear quadratic optimization, the obtained composite index, accommodating the sparsity, contains both differentiable and non-differentiable terms. Knowing that the choice of optimization techniques are problem-dependent, in this paper, several smart provisions such as “Active set”, “Coordinate descent”, and separately managing the differentiable part of the cost function are put together to form a relatively fast algorithm. In addition, a rule is introduced to adjust automatically the degree of sparsity and a constraint to confine the interval closed-loop system eigenvalues to the desired s plane region. As a result, a robust sparse decentralized control (RSDC) approach for interval networks is established. The RSDC method is utilized for the control of a multi-terminal VSC (Voltage Source Converter ) -based DC (Direct Current) grid with parametric uncertainty under various external random perturbations. The test results show that the proposed method with just 20 feedback links (out of available 100) performs superiorly to the alternative ones in terms of sparsity, convergence rate, and time-domain performances, verified through extensive simulations.

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.