Multi-terminal System Research Articles

Analysis of Fault Waveforms of Key Electrical Parameters in Hybrid Multi-terminal DC System

LCC-VSC hybrid multi terminal HVDC system combines the advantages of traditional HVDC and flexible HVDC technology, and becomes one of the new directions of HVDC development. In this paper, taking the Baihetan Jiangsu HVDC transmission project as an example, under the appropriate simplified model, the critical electrical stresses of the DC side faults of a typical LCC-VSC hybrid multi-terminal DC system are analyzed. The simulation results verify the correctness of the simplified analysis in this paper.

Unit commitment for multi-terminal VSC-connected AC systems including BESS facilities with energy time-shifting strategy

This paper proposes a novel modelling framework for Unit Commitment (UC) studies in multi-terminal VSC-connected AC grids. Battery Energy Storage Systems (BESS) are also considered with an energy time-shifting strategy whose charge and discharge modes are defined within the 24-hour planning horizon to promote a high competition level of energy trading in the power grid. The entire transmission network is formulated by shift factors and power losses of AC/DC branches and VSC units are included by piecewise linear functions. Thus, this novel UC approach retains a mixed-integer linear programming model with a high modelling versatility for arbitrary hybrid power grids accommodating any number of VSC stations and BESS units.The method described in this paper enables the optimal generation scheduling combined with the charge and discharge BESS modes. Its applicability is confirmed using two case studies, one featuring a four-terminal VSC-HVDC system with two BESS, and another characterised by seven VSC-connected AC systems with five BESS. To validate the proposed method, results are compared against those furnished by the classic UC representing the network through nodal equations. It is concluded that both methods favourably agree with each other as the errors are inferior to 2%. The usefulness of the proposed method to the real-time operation of AC/DC grids with BESS facilities is valuable.

Application of Multi-terminal Pilot Protection Algorithm in Network Information Big Data for New Feeder Automation Terminal

With the development of big data and Internet of Things technology, society has put forward higher requirements for the safety and reliability of the power grid. In order to improve the overall power supply quality, the State Grid Corporation changed the previous concept of focusing on power transmission and light distribution, and increased its investment in the field of distribution network, which ushered in its own spring for the entire power distribution industry. This article aims to study the application of big data-based multi-terminal longitudinal directional protection algorithm in the new feeder automation terminal. Based on the introduction of the development status of feeder automation, the existing problems and the workflow of the multi-terminal longitudinal protection system, the multi-terminal longitudinal protection system is analyzed. The application of the directional protection algorithm in the feeder automation terminal. Finally, the treatment of abnormal situations is studied. The research results show that the new type of feeder automation terminal only needs to exchange the comprehensive information of the fault to independently detect and isolate the fault. This multi-terminal protection the structure has high flexibility and can quickly isolate faults, which can meet the protection requirements of the new feeder automation terminal.

Correction to: Adaptive coordinated control strategy for multi-terminal flexible DC transmission systems with deviation control

Correction to: Adaptive coordinated control strategy for multi-terminal flexible DC transmission systems with deviation control

Reliability Assessment of a Multi-State HVDC System by Combining Markov and Matrix-Based Methods

The purpose of this paper is to highlight that, in order to assess the availability of different HVDC cable transmission systems, a more detailed characterization of the cable management significantly affects the availability estimation since the cable represents one of the most critical elements of such systems. The analyzed case study consists of a multi-terminal direct current system based on both line commutated converter and voltage source converter technologies in different configurations, whose availability is computed for different transmitted power capacities. For these analyses, the matrix-based reliability estimation method is exploited together with the Monte Carlo approach and the Markov state space one. This paper shows how reliability analysis requires a deep knowledge of the real installation conditions. The impact of these conditions on the reliability evaluation and the involved benefits are also presented.

Research on DC Protection Strategy in Multi-Terminal Hybrid HVDC System

Multi-terminal hybrid high-voltage direct current (HVDC) systems have been developed quickly in recent years in power transmission area. However, for voltage-source converter (VSC) stations in hybrid HVDC systems, no direct current (DC) filters are required. In addition, the DC reactor is also not installed at the line end because the DC fault can be limited by the converter itself. This means that the boundary element at the line end is absent, and the single-ended protections used in line commutated converter (LCC) based HVDC (LCC-HVDC) systems or VSC-HVDC systems cannot distinguish the fault line in multi-terminal hybrid HVDC systems. This paper proposes a novel single-ended DC protection strategy suitable for the multi-terminal hybrid HVDC system, which mainly applies the transient information and active injection concept to detect and distinguish the fault line. Compared with the single-ended protections used in LCC-HVDC and VSC-HVDC systems, the proposed protection strategy is not dependent on the line boundary element and is thus suitable for the multi-terminal hybrid HVDC system. The corresponding simulation cases based on power systems computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC) are carried out to verify the superiority of the proposed protection.

Fault detection method using high-pass filtering in VSC based multi-terminal DC system

With the rapid discharge of the dc-link capacitors during the fault occurrence, the dc line current grows rapidly in the dc grid, which interfaces the ac sources through the voltage source converter. Therefore, it is essential to develop a dc fault detection method, which can detect the dc fault with fast response speed and high reliability. In this study, a novel fault detection method is proposed based on high-pass filtering. By selecting the peak value of the filtered signal as the detection parameter, the fault features could be emphasized and the steady-state characteristics is suppressed. Its validity is tested with a seven bus multi-terminal dc (MTDC) system in the OPAL-RT simulator and a four bus MTDC experimental dc system. In the test, a response delay within 1 ms and 100 % reliability under different conditions are achieved. The proposed method is compared with four existing methods based on the wavelet transform, the short time Fourier transform, the S transform or the Hilbert-Huang transform to underline its integral performance.

Sequential network‐flow based power‐flow method for hybrid power systems

This paper presents a sequential network-flow graph-based method for a steady-state power flow solution in hybrid multi-terminal power systems. The proposed method is a unique and novel one, which differs from other established methods that involve the use of modified versions of classical power flow methods. The proposed method formulates a power flow problem as a maximum network-flow problem and solves it using a push-relabel max-flow algorithm. The solution procedure solves AC and DC parts sequentially, while accounting for voltage source converter losses using a generalised converter model. The proposed flow-Augmenting method solves the power flow problem using matrix vector multiplication in its most abstract form, and it is independent of system parameters and network configuration. The proposed formulation is computationally efficient, as it is based on matrix vector multiplication, and is scalable, because the formulation works as a graph-based method, which, inherently, allows for parallel computation for added computational speed. Further, unlike previously reported methods, the proposed method does not rely on Jacobian matrix formulation or any matrix inversion. This proves to be a strong advantage for the proposed method, as a significant reduction in computational time is observed, as a result. The proposed method is validated on 5-bus hybrid system and CIGRE B4 DC system.

Research on the Coordinated control Strategy of fault crossing capability of offshore wind power multi-terminal flexible DC system

To improve the ride-through capability (RTC) of the multi-terminal offshore wind power flexible DC system in response to faults, this paper is based on the basic structure and control strategy of the voltage source converter (VSC) station of the multi-terminal offshore wind power flexible DC transmission system (MTDC), and aims at the AC grid connected to the system with three-phase short circuits and wind turbine output changes According to the situation, the mechanism of voltage drop caused by the fault is analyzed, and the DC chopper energy bleeder circuit is connected to the DC side to cooperate with the comprehensive control scheme for fault ride-through of multi-terminal offshore wind power flexible DC collection system. Through theoretical analysis and building a simulation model, the design of the control strategy of the flexible collection system and the simulation study of fault ride-through is carried out on the Shandong Peninsula Beihai Wind Farm. The results show that: the VSC station controller of the offshore wind power multi-terminal system can make the multi-terminal system wind power distribution and consumption reasonable, and realize the stable operation of the system.

Interoperability of modular VSC topologies in Multi-converter multiterminal DC systems

The ongoing transformation of power systems driven by the need for more interconnected networks will eventually lead multiterminal dc (MTDC) grids to become more commonplace in future electricity networks. Addressing interoperability challenges associated with multiple converter topologies for future MTDC implementations is a paramount requirement. This paper introduces a multi-converter MTDC (MC-MTDC) system combining both state-of-the-art and emerging modular voltage source converter (VSC) topologies. The developed MC-MTDC system, (i) demonstrates operation of the modular multilevel converter (MMC) and the alternate arm converter (AAC) in different MTDC configurations, and (ii) sets the basis for further development of MTDC configurations with diverse modular VSC topologies. The developed MC-MTDC system will also facilitate accurate testing and validation of multi-converter interoperability using real-time electromagnetic transient (EMT) models. The design parameters of the AAC-based terminals are determined ensuring compliance with existing MMC-based dc power systems. Detailed analysis and a complete set of results based on real-time implementation for different operating scenarios including steady-state, transient, and contingencies, verify multi-converter interoperability and associated control functions in different MTDC configurations.

Directional Correction of Over-Current Relay Using Voltage Slope in Multi Terminal DC System With SFCL

In the DC system, a larger fault current flows due to capacitor discharging than in an AC system. In the event of a fault in the MTDC (multi-terminal DC), although the magnitude of the current flowing through each DCCB (DC circuit breaker) varies depending on the fault location, all capacitors in MTDC may discharge and all DCCB may interrupt. In this paper, the trigger type SFCL (superconducting fault current limiter) were constructed to limit large fault current, and DC OCR (overcurrent relay) was designed to prevent interrupting malfunction according to the impact of SFCL. When the fault current was small by operation of SFCL, the DCCB did not interrupt or trip time is delayed. These problems cause malfunction even when DC OCR is applied. Therefore, DC OCR requires an additional correction method. In this paper, a new correction method was proposed in consideration of the current direction and voltage slope. The simulation was conducted according to four cases, and the proposed OCR correction was verified to eliminate malfunctions through case studies.

Multiterminal DC Fault Identification for MMC-HVDC Systems Based on Modal Analysis—A Localized Protection Scheme

For the purpose of dc fault identification in multiterminal modular multilevel converter (MMC) high-voltage dc (HVDC) systems, the proposed work addresses the issues of localized protection schemes. The proposed work is focused on selectivity issues, such as differentiating between forward external and internal faults, and the classification of the type of fault contingency, i.e., pole to pole (PTP) or pole to ground (PTG) in the system. The scheme uses an equivalent network of multiterminal MMC-HVDC systems for dc fault identification. Modal transformation is used to analyze line-mode and zero-mode voltages across the current-limiting reactor (CLR) for different possible contingencies in the system. A maloperation region has been defined to address the issue of selectivity of internal HIFs and external low-impedance faults (LIFs). The rate of change of dc voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$dU_{\mathrm{ dc}}$ </tex-math></inline-formula> <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">/dt</i> ) is used as the mitigation technique to differentiate a HIF as high as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1000 \Omega $ </tex-math></inline-formula> . Furthermore, a sensitivity function is formulated, which gives the variation of decisive parameters i.e., line-mode voltage and zero-mode voltage with respect to change in fault resistance and value of CLR. PSCAD-/EMTDC-based simulations are used to validate the performance of the proposed scheme. The scheme is validated in terms of detection time, robustness to WGN in measurement, security for ac faults, dc circuit breaker (DCCB) operation, and power reversal (load transients).

Switching valley filtered current directions in multiterminal graphene systems

Valley filtering processes have been explored in different graphene-based configurations and scenarios to control transport responses. Here we propose graphene multi-terminal set-ups properly designed to obtain valley filtered currents in a broad range of energy, besides the possibility of controlling their directions. We explore graphene systems with extended mechanical fold-like deformations as an opportunity to enhance valley filtered transmission. The mixing between the electronic confinement effects due to a magnetic field and strain results in a selective drive of the current components in the quantum Hall regime. We adopt the mode-matching method within the Green's function formalism, allowing the direct analysis of the strain effect on each valley transmission. We estimate a threshold map of confinement parameters, characterized by the magnetic, deformation, and set-up lengths, to optimize valley filter transport processes and the proper switch of the valley polarized current directions.

A Full-Newton AC-DC Power Flow Methodology for HVDC Multi-Terminal Systems and Generic DC Network Representation

The use of Direct Current (DC) transmission links in power systems is increasing continuously. Thus, it is important to develop new techniques to model the inclusion of these devices in network analysis, in order to allow studies of the operation and expansion planning of large-scale electric power systems. In this context, the main objective of this paper is to present a new methodology for a simultaneous AC-DC power flow for a multi-terminal High Voltage Direct Current (HVDC) system with a generic representation of the DC network. The proposed methodology is based on a full Newton formulation for solving the AC-DC power flow problem. Equations representing the converters and steady-state control strategies are included in a power flow problem formulation, resulting in an expanded Jacobian matrix of the Newton method. Some results are presented based on HVDC test systems to confirm the effectiveness of the proposed approach.

A brief review of thermal transport in mesoscopic systems from nonequilibrium Green’s function approach

With the rapidly increasing integration density and power density in nanoscale electronic devices, the thermal management concerning heat generation and energy harvesting becomes quite crucial. Since phonon is the major heat carrier in semiconductors, thermal transport due to phonons in mesoscopic systems has attracted much attention. In quantum transport studies, the nonequilibrium Green’s function (NEGF) method is a versatile and powerful tool that has been developed for several decades. In this review, we will discuss theoretical investigations of thermal transport using the NEGF approach from two aspects. For the aspect of phonon transport, the phonon NEGF method is briefly introduced and its applications on thermal transport in mesoscopic systems including one-dimensional atomic chains, multi-terminal systems, and transient phonon transport are discussed. For the aspect of thermoelectric transport, the caloritronic effects in which the charge, spin, and valley degrees of freedom are manipulated by the temperature gradient are discussed. The time-dependent thermoelectric behavior is also presented in the transient regime within the partitioned scheme based on the NEGF method.

Solution to Fault of Multi-Terminal DC Transmission Systems Based on High Temperature Superconducting DC Cables

In this paper, we developed the small-signal state-space (SS) model of hybrid multi-terminal high-voltage direct-current (HVDC) systems and fault localization method in a failure situation. The multi-terminal HVDC (MTDC) system is composed of two wind farm side voltage-source converters (VSCs) and two grid side line-commutated converters (LCCs). To utilize relative advantages of the conventional line-commutated converter (LCC) and the voltage source converter (VSC) technologies, hybrid multi-terminal high-voltage direct-current (MTDC) technologies have been highlighted in recent years. For the models, grid side LCCs adopt distinct two control methods: master–slave control mode and voltage droop control mode. By utilizing root-locus analysis of the SS models for the hybrid MTDC system, we compare stability and responses of the target system according to control method. Furthermore, the proposed SS models are utilized in time-domain simulation to illustrate difference between master–slave control method and voltage droop control method. However, basic modeling method for hybrid MTDC system considering superconducting DC cables has not been proposed. In addition, when a failure occurs in MTDC system, conventional fault localization method cannot detect the fault location because the MTDC system is a complex form including a branch point. For coping with a failure situation, we propose a fault localization method for MTDC system including branch points. We model the MTDC system based on the actual experimental results and simulate a variety of failure scenarios. We propose the fault localization topology on a branch cable system using reflectometry method. Through the simulation results, we verify the performance of fault localization. In conclusion, guidelines to select control method in implementing hybrid MTDC systems for integrating offshore wind farms and to cope with failure method are provided in this paper.

Multi-objective optimal power flow considering the multi-terminal direct current

Introduction. In recent years, transmission systems comprise more direct current structures; their effects on alternating current power system may become significant and important. Also, multi-terminal direct current is favorable to the integration of large wind and solar power plants with a very beneficial ecological effect. The novelty of the proposed work consists in the effects of the aforementioned modern devices on transient stability, thus turn out to be an interesting research issue. In our view, they constitute a new challenge and an additional complexity for studying the dynamic behavior of modern electrical systems. Purpose. We sought a resolution to the problem of the transient stability constrained optimal power flow in the alternating current / direct current meshed networks. Convergence to security optimal power flow has been globally achieved. Methods. The solution of the problem was carried out in MATLAB environment, by an iterative combinatorial approach between optimized power flow computation and dynamic simulation. Results. A new transient stability constrained optimal power flow approach considering multi-terminal direct current systems can improve the transient stability after a contingency occurrence and operate the system economically within the system physical bounds. Practical value. The effectiveness and robustness of the proposed method is tested on the modified IEEE 14-bus test system with multi-objective optimization problem that reflect active power generation cost minimization and stability of the networks. It should be mentioned that active power losses are small in meshed networks relative to the standard network. The meshed networks led to a gain up to 46,214 % from the base case.

Lyapunov theory-based control strategy for multi-terminal MMC-HVDC systems

This paper presents a coordinated control strategy based on direct Lyapunov theory to handle the consistency of AC grids in a multi-terminal (MT) modular multilevel converter (MMC)-HVDC systems during varying both loads and DC link voltage. As the first contribution, a set of dynamic equations is proposed based on separating the dynamics of MMCs upper and lower arms state variables. The dynamics consists of only their related upper/lower arms state variables leading to more effective components for the steady state terms of proposed control technique. To develop the dynamic parts of the controller, the global asymptotical stability of MT MMC-HVDC system is assessed by direct Lyapunov method. As another advantage of the separated dynamic equations, the Lyapunov theory is able to exploit very simple decoupled components for the dynamic parts of proposed control functions. Moreover, in order to specify the variation trend of Lyapunov coefficients, further stability analysis contributes to demonstrating the effects of Lyapunov coefficients on the MMCs state variable errors and its dynamic. As another main contribution of this paper, two independent capability curves based on the power injection capability of the MMCs upper and lower arms, are obtained which will be assessed through changing the input and output voltages as well as MMC parameters. Finally, simulation results in MATLAB software are utilized to verify the validity of proposed control strategy.

Ultra-Dense LEO Satellite Based Formation Flying

In this article, we consider a downlink ultra-dense LEO-based multi-terminal satellite system where multiple satellites fly in formation to serve a number of ground terminal stations for data transmission. Benefited from the dense satellite constellation, high channel capacity can be achieved via the massive virtual antenna array formed by multiple satellites. We aim at exploring how the satellite distribution and formation size influence the channel capacity in this case. We first derive the upper bound of the channel capacity in the general multi-antenna satellite networks. The single-antenna satellite case is considered where we present the specific forms of the derived capacity bounds to prove that the capacity first increases linearly then increases more and more slowly with the satellite formation size, and there exists an optimal LEO satellite distribution to achieve the maximum capacity of the system. Simulation results verify our theoretical analysis and show that such statements also hold for the general multi-antenna satellite case.

Adaptive coordinated control strategy for multi-terminal flexible DC transmission systems with deviation control

Due to the advantages of multiple power supply points and multiple power receiving points, the multi-terminal direct current (MTDC) technology has gradually become the main trend in the future development of DC power grids. The coordinated control of a MTDC system is one of the key technologies to realize the stable operation of power systems. Droop control is an effective coordinated control method to maintain the power and voltage balance of a DC transmission system. However, improper selection of the droop coefficient leads to an imbalance of the power and voltage distribution. An improved coordinated control strategy based on an adaptive droop coefficient is proposed to realize a faster dynamic response and a more reasonable power distribution, which is applicable to all of the topologies of MTDC systems. The power and voltage deviation controls are added to prevent the output power and voltage from exceeding the range that a converter can bear. In addition, a lagging link is added in the current loop to compensate for the delay caused by the current loop and to improve the response speed of the system. Taking the MMC topology as an example, the dynamic characteristics of the proposed control strategy are analyzed, and a four-terminal MTDC simulation model based on the MATLAB/Simulink simulation platform has been established to verify the superiority of the proposed strategy under different disturbances.