Multi-terminal HVDC Transmission Research Articles

A Protection Principle of LCC–VSC Three-Terminal HVdc System Based on Instantaneous Boundary Impedance

In the field of hybrid multi-terminal HVdc (high voltage direct current) transmission, an inevitable problem is the short availability of fault information. Namely, only 1-ms complete fault information can be used to identify faults. To overcome this limitation, this study introduces the concept of instantaneous boundary impedance and proposes a dc line protection method. First, a dc transmission line is divided into three types of zones, namely internal, forward-external, and backward-external zones. Then, the instantaneous boundary impedance is analyzed and compared under normal operation and different types of faults. The results indicate that under normal operation, the instantaneous boundary impedance is zero; under a dc line fault, the instantaneous boundary impedance is less than zero; finally, under an external fault, the instantaneous boundary impedance is greater than zero. The proposed principle is validated using Wudongde LCC (line-commuted converter)-VSC (voltage source converter) three-terminal hybrid HVdc RTDS (real-time digital simulation) test system. The experimental results demonstrate that the proposed protection method is effective under different types, locations, and impedances of faults and signal-to-noise ratios within a period of 1 ms. Compared to the existing methods, the proposed protection method can achieve better performance in withstanding fault impedance (600 Ω) and has better robustness against noise interference (10 dB).

Research on influencing factors of emergency power support for voltage source converter-based multi-terminal high-voltage direct current transmission system

Research on influencing factors of emergency power support for voltage source converter-based multi-terminal high-voltage direct current transmission system

Pilot protection for hybrid multi-terminal HVDC transmission lines based on Euclidean distance between differential current and distributed capacitance current

Pilot protection for hybrid multi-terminal HVDC transmission lines based on Euclidean distance between differential current and distributed capacitance current

A pilot protection based on Zero-Sequence voltage ratio for AC lines in hybrid Multi-Terminal HVDC transmission system

The reliability of protection is the precondition for the security of hybrid multi-terminal high-voltage direct-current (HVDC) transmission system. For AC system at the receiving terminal, due to the infeed of DC system, the conventional AC line protection may malfunction. In this context, a new protection utilizing zero-sequence voltage ratio is proposed in this article. Based on the fault analysis of zero-sequence voltage distribution, the comparative voltage is defined, and find that under an internal fault, for two sides of line, the ratios of zero-sequence voltages to their corresponding comparative voltages are less than 1. Under an external fault, for the distance faulty end, the voltage ratio is less than 1, however, near faulty side, the voltage ratio is larger than 1. Thereby, this difference is utilized to construct the fault identification criterion. Simulations show that the proposed protection can correctly identify internal and external faults. Meanwhile, it is immune to DC commutation failure, and has a good tolerance to high-resistance faults.

An Improved Power-Sharing Method for a Multi-Terminal HVDC Transmission System Based on Adaptive Voltage Droop Control

The prerequisite for the normal operation of a flexible high-voltage direct current (HVDC) transmission system is the maintenance of the stability of the direct current (DC)-side voltage, and droop control has a good dynamic regulation capability. In this paper, we first study the operating characteristics of droop control and derive its equivalent circuit, as well as the power distribution equation for droop control with a four-terminal system as an example. Then, based on this, an improved droop control method is proposed so that the droop factor can be adaptively adjusted according to the power change and provide corresponding characteristics under different operating conditions to enhance the power regulation capability of the controller under high power fluctuations. Finally, a power systems computer-aided design (PSCAD) electromagnetic transient model of the four-terminal, flexible, high-voltage DC transmission system was established and verified by simulation results.

Novel hybrid fault current limiter with hybrid resonant breaker in multi-terminal HVDC transmission system

Novel hybrid fault current limiter with hybrid resonant breaker in multi-terminal HVDC transmission system

A Novel Protection Based on Current Correlation for DC Lines in Hybrid Cascaded Multi-Terminal HVDC Transmission System

This article proposes an innovative DC line protection utilizing current correlation coefficient for hybrid cascaded high-voltage direct-current (HVDC) system. First, the equivalent discharge circuit after DC line fault is inferred, and then fault characteristic analysis on the superimposed network reveals that for the transmission lines connecting line commutated converter (LCC) and modular multilevel converters (MMCs), high-frequency currents of faulty line and non-fault line are negatively correlated, their correlation coefficient is theoretically -1. Whereas, high-frequency currents of two non-fault lines show a strong positive correlation, their correlation coefficient equals theoretically 1. Then, cosine correlation coefficient is adopted to describe current correlations and construct the identification criterion. Simulation demonstrates that protection could differentiate the faulty line accurately. Additionally, it has high sensitivity for high-resistance fault. And it is simple and has low computational complexity, and is insensitive to the line boundary element.

A novel voltage projection component-based protection scheme for hybrid multi-terminal HVDC transmission lines

Combining the technical advantages of line commutated converter (LCC) and modular multilevel converter (MMC), hybrid multi-terminal HVDC (Hybrid-MTDC) system provides larger capacity, more economical and flexible transmission mode, which has become an important development trend of dc technology in the future. However, high fault resistance, complex topology and large transmission network pose great challenges to the protection of Hybrid-MTDC transmission lines. In this paper, the variation trajectories of internal and external fault voltage vectors with fault resistance are elaborated, and a projection axis through the origin is constructed on the vertical direction of the external metallic fault voltage vector. Combining the voltage amplitude and phase characteristics, the projection component amplifies the characteristic difference between internal and external high-resistance faults. On this basis, a voltage projection component-based protection scheme for Hybrid-MTDC transmission lines is proposed, which can sensitively and reliably identify the line faults under high fault resistance. Finally, the proposed method is thoroughly validated in the PSCAD/EMTDC simulation model of Kun-Liu-Long hybrid three-terminal HVDC system, and the results demonstrate that the proposed protection scheme is practicable, highly sensitive and reliable.

Selection of the Parameters of Reactive Power Compensation and Harmonic Filtering Devices for an HVDC Transmission System Based on Voltage Source Converters

This article describes a method for selecting the parameters of AC and DC filters for a multi-terminal HVDC transmission system based on voltage source converters (VSC-HVDC). The equivalent circuit model of a multi-terminal VSC-HVDC system with the parameters ±500 kV and 500 MW was developed in the Matlab Simulink software package and presented in d-q coordinate axes. The system voltage source converters in the model were controlled using a three-level pulse-width modulation (PWM) to reduce the level of current and voltage higher harmonic components and decrease the capacity of AC and DC filters. To select the most efficient filter for use as part of a multi-terminal VSC-HVDC system, high-pass filters (HPF) and low-pass filters (LPF) were considered.

Design and Study of the Key Characteristics of a New DC Vacuum Interrupter With a High Peak and Low Residual Magnetic Field

With the rapid development of multiterminal high-voltage direct current (HVdc) transmission systems, the research and development of high-speed and stable high-voltage dc circuit breakers have received attention. Among them, mechanical dc circuit breakers based on manual zero-crossing dc breaking technology have been applied in practical projects, owing to their small size, low on-state loss, and low price. However, this method adds a high-frequency current, which deteriorates the magnetic field between the poles of the interrupter when the arc is broken. Therefore, the development of HVdc circuit breakers that maintain high peak magnetic fields and low residual magnetic fields at high frequencies has become the key to the development of multiterminal HVdc transmission. To solve this problem, this article proposes a new interrupter for a vacuum dc circuit breaker based on artificial zero-crossing technology and studies its key characteristics, such as temperature rise, electromagnetic field, and arc column plasma distribution characteristics. The results reveal that the new interrupter proposed in this article has a low-temperature rise, uniformly distributed electric field, high peak value and low remanence of the longitudinal magnetic field, and uniformly distributed plasma in the arc column, and exhibits arc diffusion. Its ability to cope with high-frequency current is significantly higher than that of the traditional cup-shaped axial magnetic field (AMF) contact system interrupter, which is conducive to the high-speed and stable breaking of the interrupter.

Protection scheme for multi-terminal HVDC system with superconducting cables based on artificial intelligence algorithms

This paper presents the development of a novel data-driven fault detection and classification scheme for DC faults in multi-terminal HVDC transmission system which incorporates superconducting cables and modular multi-level converters. As the deployment of superconducting cables for bulk power transmission from remote renewable generation is progressively increasing in the future energy grids, many fault-related challenges have been raised (i.e., fault detection, protection sensitivity/stability). In this context, the applications of Artificial Intelligence techniques have started to be considered as a powerful tool for the development of robust fault management solutions. The proposed artificial intelligence-based method utilizes local current and voltage measurements to detect and classify all types of faults on the DC cables and DC buses, without the requirement of measurements exchange among different DC substations. The performance of the proposed scheme has been assessed through detailed transient simulation analysis and the results confirmed its effectiveness against a wide range of fault conditions (i.e., various fault types, fault locations and fault resistances). Furthermore, the feasibility of the developed scheme for real-time implementation has been validated using real-time software in the loop testing. The results revealed that the proposed algorithm can correctly, and within a very short period of time (i.e. less than 2 ms) detect and classify the faults within the protected zone and concurrently remain stable during external faults. Additionally, the generalization capability of the algorithm has been verified against influencing factors such as the addition of noise, highlighting the robustness of the presented scheme.

A travelling wave based primary and backup protection for MMC-MTDC transmission system using morphological un-decimated wavelet scheme

The primary protection speed requirement for the HVDC grid is much faster as compared to the conventional HVAC grid. This research article proposes a travelling wave-based primary and backup protection scheme for the multi terminal HVDC transmission system using the morphological un-decimated wavelet decomposition signal processing technique. Depending on the polarity and arrival time information of the first incident travelling wave at both DC transmission line terminals, the fault is identified to be either in the internal zone, forward external zone, or backward external zone of the DC transmission line. Each DC transmission line is covered by its own internal zone as well as forward & backward external zone of its neighbouring line. The internal zone fault identification of the protection scheme sends the primary relay trip signals to associated hybrid DC circuit breakers. But if the primary protection relay fails, then the neighbouring DC substation protection system identifies the fault either in forward external or backward external zone. Then it sends a remote backup trip signal to the DC circuit breaker in the forward or backward external direction with a coordinated time delay.

DC fault current approximation and fault clearing methods for hybrid LCC-VSC HVDC networks

The DC-side fault analysis of Line commutated converter based high voltage direct current (LCC-HVDC) and Voltage source converter based high voltage direct current (VSC-HVDC) networks has been studied by most researchers and achieved fruitful results. However, the traditional fault analysis method does not consider in detail the change process of the trigger angle of the LCC converter station and the AC current feeding of the VSC converter station caused by the control strategy. Therefore, this paper proposes a new DC side fault transient analysis method suitable for hybrid multi-terminal HVDC transmission networks. In addition, considering the deficiency that the active fault current suppression method only relying on the control strategy cannot fundamentally clear the DC fault, this paper proposes a new Z-source current-limiting circuit (ZCLC) with reference to the topology of the Z-source dc circuit breaker (DCCB). Then, ZCLC will cooperate with DCCB to improve the fault clearing capacity of the whole system. Firstly, this paper establishes the equivalent Laplace transform model of the three-terminal LCC-VSC-VSC, and analyzes the short-circuit current expression on both sides. Then, the change process of trigger angle is approximately linearized by the least square method, which obtains a new expression of fault transient current on the LCC side. Similarly, the composition of AC current fed to DC side is deduced by using the commonly used inner-loop PI controller model, which also obtains a new expression of fault transient current on the VSC side. Secondly, referring to the traditional crossed-type Z-source DCCB, series-type Z-source DCCB, and parallel-type Z-source DCCB, this paper also proposes a new ZCLC and analyzes its working principle and parameter design. Then, the newly proposed ZCLC is combined with DCCB to be applied to the hybrid multi-terminal HVDC network. Finally, the simulation results based on Matlab/Simulink verify the effectiveness of the fault analysis and passive fault clearing methods, and the semi-physical experimental platform based on PLECS/RTbox verifies the superiority of the new ZCLC.

Study of 160 kV/1 kA R-SFCL Design and Application in Nanao MMC–MTDC Project

The use of a resistive-type superconducting fault current limiter (R-SFCL) in the nanao modular multilevel converter based multiterminal high-voltage direct current transmission project (MMC–MTdc) is the first of its kind over the world. There were no relevant development and operational experience to support the construction of this project. In response to the difficulties in implementation, the corresponding methods were straightened out in this article. These contributions include 1) proposal of an iterative optimization calculation method for the time-varying quenching resistance of the R-SFCL; 2) the establishment of a simulation platform based on PSCAD/EMTDC for MMC–MTdc system with a nonlinear R-SFCL model; 3) two-group with three-outlet-terminal topology scheme for R-SFCL device; 4) dc-side short-current calculation and short-current suppression rate analyses for MMC–MTdc with the developed R-SFCL; and 5) steady-state operation, insulation performance, and power switching tests of the developed R-SFCL. Conclusively, the feasibility of the R-SFCL prototype is verified with the simulation and engineering practice of pole-to-ground test in the nanao MMC–MTdc system. In conclusion, the presentation of these research contents and conclusions illustrates the future value of industrial experience in the design and operation of R-SFCL MMC–MTdc systems.

Fault Analysis and Traveling Wave Protection Based on Phase Characteristics for Hybrid Multiterminal HVDC Systems

By fully combining the advantages of line commutated converter (LCC) high-voltage direct current (HVDC) and modular multilevel converter (MMC)-HVDC systems, hybrid multiterminal HVDC (hybrid-MTDC) transmission technology has become a new technical means to overcome the shortcomings of conventional two-terminal HVDC systems. However, the conditions of high fault resistances and more sophisticated network topologies have brought considerable challenges to the existing traveling wave (TW) protection principles. In this article, a parallel hybrid-MTDC system is taken as the research object, and its mathematical analysis model is established. Based on the mathematical model, the TW characteristics that the phase of the high-frequency voltage TW during the occurrence of an internal and external fault approaches approximately 90° and 0°, respectively, are clarified. Therefore, considering the influences of reflection and refraction of TWs and line propagation, the phase-characteristics-based criterion is constructed. Using the proposed criterion, a protection scheme for hybrid-MTDC systems is proposed, which realizes the identification of line faults with high fault resistance. The proposed method is verified in detail with the electromagnetic transient simulation model using PSCAD/EMTDC, and the test results show that the proposed protection scheme is feasible and has high sensitivity, reliability, and adaptability to different operation modes.

Short-term generation capacity expansion planning considering multi-terminal VSC[sbnd]HVDC links using a linear programming framework improved by shift factors

This paper describes the modelling of short-term generation capacity expansion planning (GCEP) for multi-terminal VSC-based HVDC transmission systems. The GCEP formulation is based on a mixed-integer linear programming (MILP) framework where transmission losses are considered by piecewise linearisation. As opposed to existing methods, the present GCEP formulation is based on shift factors which enable the representation of the hybrid AC/DC grids including VSC stations. This way new power generation investments can be efficiently determined at any of the VSC-connected AC systems. This new method lies in sharp contrast with the so-called classic GCEP method where the system nodal power balances are formulated using the voltage phase angles as decision variables thus increasing the model complexity. Indeed, to determine the GCEP investments, the present model aimed to multi-terminal HVDC links only uses AC/DC transmission line losses, power generations and projected generating power plants as decision variables. The new GCEP formulation is compared to the classic method using two compelling multi-terminal VSCHVDC test systems for validity and applicability purposes. Results show that the proposed formulation reduces the simulation running time by more than 80%, compared to the standard model, accompanied by acceptable accuracy of results since errors inferior to 5% are obtained.

Single terminal coordination-protection scheme for hybrid three-terminal DC line based on station terminal current characteristics

Grounding electrode is an essential component in hybrid multi-terminal HVDC transmission system, By connecting converter station to the bus bar as a main station, this work analyzes the fault current component characteristics of grounding electrode line under various fault conditions for a hybrid three terminal DC transmission system. It is concluded that under single pole line grounding fault, current at the measuring ends of grounding, fault and sound electrodes satisfies the Kirchhoff current theorem, while considering converter station as a node. However, for reverse fault of line protection, current at above mentioned electrodes doesn’t follow the Kirchhoff law. Under various DC line faults, fault line is selected according to the mutation direction of current at the back-side of bus bar. Essentially, the current obtained by grounding electrode’s measuring point is a zero mode current, hence, it can sense the fault current precisely and has directivity. A comprehensive single terminal coordination-protection scheme for DC lines in hybrid multi-terminal transmission system is constructed by using currents’ characteristics of station terminal. The proposed method does not rely on boundary elements, and has an ability to quickly identify internal and external faults, select the fault pole, and withstand high resistance. The developed algorithm for protection scheme is simple, requires low sampling rate, and ensures high reliability for protection scheme.

Transient overvoltage characteristics in submarine cables of multi-terminal flexible HVDC transmission system

The overvoltage transient characteristics of submarine cables are one of the key technologies for submarine power transmission. This article takes the submarine cable of Zhoushan Multi-terminal Flexible HVDC Transmission Project as the research object. The MMC-HVDC simulation models at both ends of converter stations are built in PSCAD. The parameters and equivalent models of the flexible HVDC cables are determined, and the submarine DC cable operating overvoltage is calculated by using statistical methods. The simulation results show that the maximum overvoltage of the submarine cable, that is, the maximum overvoltage of the conductor to sheath, the sheath to the armour, and the armour to the ground are 400 kV, 7.27 kV and 0.45kV respectively. The maximum overvoltage when it occurs faults on grid-side or valve-side appears in the middle of the submarine cable. Maximum overvoltage of HVDC single pole grounding fault appears at the connection point of land cable and submarine cable. The simulation results and the discovery of features in this article provide reference in the design, selection, and testing of submarine power transmission equipment.

Research on coordinated control strategy based on hybrid multi-terminal HVDC transmission network

Most of the control strategies of hybrid multi-terminal HVDC transmission technology consider frequency support, voltage control and power control, which ignore the influence of external parameter disturbance and power change. Aiming at the deficiencies of the above control strategies, this paper proposes a coordinated control strategy that can resist the external parameters to be disturbed and suppress the bus voltage to be fluctuated caused by the power mutation. Firstly, the control law function with passive characteristics is derived by using the system topology, and the optimal injection damping parameters can be selected by the bode diagram of the transfer function of the state variable, which proves that the traditional passivity-based control (PBC) strategy has certain anti-parameter disturbance ability. Secondly, considering that the traditional PBC strategy does not eliminate the influence of sudden external power changes on the AC bus voltage, the nonlinear disturbance observer (NDO) is applied to the PBC strategy to realize real-time monitoring of disturbance variables. Meanwhile, the selection of appropriate observer gain can shorten the time of voltage fluctuation and improve its stability. Finally, simulation results based on Matlab/Simulink verify the practicability and effectiveness of the control method proposed in the hybrid multi-terminal HVDC grid.

Quench recovery time of an optical fiber encapsulated resistance-type DC superconducting fault current limiter coil determined by a Raman-based distributed temperature sensing system

The superconducting fault current limiter (SFCL), as a very important power facility for high voltage DC system, can consume fault current energy and make DC breakers cut off fault transmission lines successfully. For the moment of reclosing DC breakers, the quench recovery time of the SFCL must be determined in advance. In this article, according to the design requirement of a 160 kV/1 kA resistance type DC SFCL in the Nan’ao multi-terminal flexible HVDC transmission demonstration project, the quench recovery time of a SCFL coil wound by two optical fiber encapsulated high temperature superconducting (OFE-HTS) tapes was checked by our optimized Raman-based distributed temperature sensing (Rm-DTS) system. The experimental results show that the used OFE-HTS tapes can withstand a 1500 A/100 ms AC short circuit current impact and are qualified with the SFCL. Furthermore, the quench recovery time of the SFCL coil measured by the Rm-DTS was 2930 ms. During the quench, the highest temperature zone occurred near the center of the SFCL coil.