HVDC Power System Research Articles

Voltage Stability Constrained Economic Dispatch for Multi-Infeed HVDC Power Systems

Multi-infeed LCC-HVDC decreases voltage stability margins due to its current source nature. Moreover, the high penetration of variable renewable energy makes it difficult to reserve enough margins for all periods. Therefore, voltage stability constraints need to be considered in the scheduling stage. However, the challenge is that the voltage stability margin is not analytic with respect to state variables. We propose a sparse support vector machine to extract the nonlinear voltage stability rule. To embed the rule into economic dispatch (ED), we apply a convex reformulating technique to the rule and then formulate a voltage stability constrained ED (VSCED) model by semi-definite programming. The model is tested on a modified IEEE-14 system and a real-world system from Jiangsu Province, China; and it is compared with other typical data-driven-based models in the Jiangsu system. The numerical experiments suggest that the stability margins can be effectively reserved above the required threshold within an acceptable computation time by the proposed VSCED model, which outperforms the other models.

Synchronous Condenser Placement for Multiple HVDC Power Systems Considering Short-Circuit Ratio Requirements

Synchronous Condenser Placement for Multiple HVDC Power Systems Considering Short-Circuit Ratio Requirements

Determination of modular multilevel converters admittances and their impacts on HVDC power system stability

This paper deals with the determination of the complex admittances and trans-admittances of power converters in general and in particular of two modular multilevel converters (mmcs) connected by a high-voltage direct current (hvdc) link to two distinct ac systems. With the term trans-admittances we mean the cross admittance “seen” between the points of common coupling of the two mmcs in the two ac grids. These quantities show how the hvdc link couples the two seemingly separated ac systems. The accurate computation of admittances and trans-admittances requires careful consideration of several frequency harmonics and frequency up-conversion/down-conversion phenomena through the hvdc link. To determine them, we exploit the periodic small-signal (pac) analysis that we briefly introduce from a theoretical perspective. Results show how ac and dc instabilities can be induced by mmcs and how they propagate through the hvdc link.

High‐frequency resonance in HVDC and wind systems: Root causes and solutions

AbstractThis paper presents methods to model and solve high‐frequency resonance problems in HVDC and wind power systems. Control and digital PWM delays are identified as a common root cause for such resonances. A systematic method to include such delays in small‐signal sequence impedance models is presented. The developed models are fully validated and used to characterize the effects of delay on system stability. In addition to full models that can predict the associated impedance and negative damping behaviour accurately, simplified approximate models are also presented to gain insights. The method is applied to type‐III and type‐IV turbines as well as HVDC converters based on modular multilevel converters (MMC). In addition to resonance of MMC and wind turbines with overhead transmission lines, the paper also identifies several other modes of high‐frequency resonance, including resonance between wind turbines and MMC HVDC converters, among different wind turbines, and between MMC HVDC converters and cables. Different methods to damp high‐frequency resonances are also discussed and demonstrated.

MMC 기반 HVDC 전력변환기의 밸브 성능 시험회로

A new test circuit for an MMC-based valve HVDC power converter is proposed. The proposed scheme satisfies the required clauses from IEC-62501. The valve test current contains second harmonic component and DC offset as well as a fundamental component that is quite similar to the real operating arm current of MMC based HVDC power system. The structure of the proposed test circuit is simple compared to conventional test circuits. Furthermore, the power supply voltage rating of the proposed test circuit is reduced dramatically around 20% of the conventional scheme with the same current rating. The validity of the proposed test circuit is verified through simulation and experimental results.

Ion-Implanted Epoxy With Graded Surface Conductivity: An Approach to Improve Surface Flashover Voltage

The electric field distribution along the insulator surface plays a significant role in improving surface flashover characteristics of the insulation in an HVdc power system. Functionally graded material with conductivity distribution ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sigma $ </tex-math></inline-formula> -FGM) is a promising approach that enables the electric field to be more uniform, which leads to hinder the surface flashover. This article proposes a novel method for graded conductivity materials using surface modification via argon (Ar <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{+}$ </tex-math></inline-formula> ) ion implantation. Epoxy samples with homogeneous and graded surface conductivity are prepared considering different ion implantations to verify the effectiveness of the proposed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sigma $ </tex-math></inline-formula> -FGM method. Flashover tests of the virgin, homogeneously ion-implanted, and graded ion-implanted samples are conducted in air and vacuum at negative dc voltage. Moreover, finite-element method (FEM)-based simulations are carried out using COMSOL Multiphysics software to analyze the electric field relaxation before and after modification. The experimental findings indicate that the flashover characteristics of homogenously implanted samples are improved with the rise in implanted layer conductivity. In particular, the flashover voltage of samples with graded conductivity layers is improved by 27.71% in air and 28.90% in vacuum compared to the virgin samples. The experimental findings and theoretical analysis prove that ion-implantation-based graded conductivity material is a promising solution for the insulation of HVdc applications.

Experimental Study on the Current Limiting Characteristics of a Saturated Iron-Core Superconducting Fault Current Limiter in DC Power Systems

Several recent studies have proposed a conceptual design of a saturated iron-core superconducting fault current limiter (SI-SFCL) for DC power systems, but experimental studies to verify its operating characteristics and performance have not yet been conducted. In this paper, a laboratory-scale SI-SFCL was designed and tested to operate in a 500 V, 50 A DC power system. First, the detailed configuration, specifications, and hardware manufacturing contents of the SI-SFCL were presented, and then an experimental circuit was developed and installed for the performance test of the SI-SFCL in normal operation and failure conditions. A 3D finite element method model was constructed to compare the simulation results and experimental results for the operating function of the SI-SFCL. Without the SI-SFCL, the maximum fault current level was 500 A, as a result, the SI-SFCL has a large current limit function, which can reduce the magnitude of the fault current up to the design target value of 73.6%. The results obtained in this study can be effectively applied to large-scale SI-SFCL development studies for HVDC power systems.

High voltage DC power distribution: a panoramic view

"High voltage DC power systems (HVDC) provide essential technical advantages in relation with the AC classical distribution networks: increasing the capacity and the stability in a natural and efficient way, simultaneously with the decoupling of some local disturbances that may occur in their subsystems and the easy interconnection of power AC grids with different parameters. In addition, the increased share of renewable energy sources and the fact that their production takes place in concentrated areas, often away from the consumption areas, require the existence of distribution facilities in the field of GW power levels, with flexible connectivity possibilities. The paper presents general aspects regarding the configuration and the main components of a HVDC power system, implementation in several representative applications. "

Conceptual Design of a Saturated Iron-Core Superconducting Fault Current Limiter for a DC Power System

A superconducting fault current limiter can significantly reduce the stress on the DC circuit breaker in DC power systems due to its inherent physical characteristics that can quickly limit the fault current. This letter presented the conceptual design of a saturated iron-core superconducting fault current limiter (SI-SFCL) for DC power systems. First, the electrical characteristics of the SI-SFCL were investigated. Then, a detailed design process and corresponding configuration of the SI-SFCL for a 15 kV, 3 kA DC power system were presented. A PSCAD/EMTDC simulation model was built to analyze the operating and fault current limiting characteristics of the SI-SFCL. To demonstrate the effectiveness of the conceptual design parameters and configuration, a lab-scale SI-SFCL for a 500 V, 50 A DC power system was designed in detail. The fault current limiting capability of the lab-scale SI-SFCL was confirmed by the 3D finite element method. As a result, the lab-scale SI-SFCL has a high current limiting capability, which can reduce the magnitude of the fault current by up to 75%. This study results will be effectively applied to a large-scale SI-SFCL for HVDC power systems.

Coupling characteristic analysis and a fault Pole detection Scheme for single-circuit and double-circuit HVDC transmission lines

The mis-operation and protection issues associated with non-fault poles caused by electromagnetic coupling effects threaten the security and stability of HVDC power systems. Based on the distributed parameter model, the coupling characteristics of single-circuit HVDC transmission lines and double-circuit HVDC transmission lines are studied in this paper. The decoupling method of double-circuit HVDC transmission lines that are structurally unbalanced is discussed. Based on waveform and frequency characteristic analyses of electrical quantities considering coupling effects, a fault pole detection scheme is proposed. A rapid fault pole detection criterion is formulated based on the backward travelling wave, and an adaptive threshold selection method is proposed. A reliable fault pole detection criterion is formulated using the low-frequency component voltage, and a short-window fast-filtering method is proposed. The travelling wave-based fault pole detection method can be rapidly applied using only the electrical quantities of the protected line, and the sensitivity and reliability of the method are enhanced by using adaptive thresholds. Based on the frequency characteristics of the coupling effects, the fault pole detection method can be reliably applied using only the electrical quantities of the protected pole. Simulations demonstrate the feasibility and validity of the fault pole detection scheme.

Active power translation droop control method for converter station of MTDC system

The DC transmission system with wind farm has the characteristics of power and frequency follow-up, it is difficult to realize the stable control of power and voltage in the common control mode of converter station. In order to improve the steady-state characteristics of the active power control of the multi-terminal HVDC (MTDC) power system with wind farms, to eliminate the static deviation existing in the system and avoid the system overvoltage during the operation, the model for MTDC system with wind farms was established and analysed. And a control strategy for p-u sagging curve translation through power measurement is proposed in this paper. A four-terminal VSCMTDC simulation model in the Matlab/Simulink platform is built. And the proposed MTDC system control strategy is verified under the different simulation conditions. It can ensure that the system transmits active power within the rated capacity range and the DC voltage of the converter stations is stable and accurate.

A Novel Active Mechanical HVDC Breaker With Consecutive Interruption Capability for Fault Clearances in MMC-HVDC Systems

The lack of dc fault clearance capability is a major obstacle that restricts the development of high-voltage direct current (HVdc) systems. Employing an active mechanical dc circuit breaker (M-DCCB) is an effective method to handle the dc fault. However, the existing active M-DCCBs can interrupt the fault current once but have problems in making the second interruption instantly in case of reclosing a permanent dc fault. In this paper, a novel topology of the active M-DCCB is proposed by inserting a precharged capacitor into a bridge formed by four spark gap switches. The operating strategy for the proposed M-DCCB is discussed to handle the dc faults. Its parameters, including the oscillation inductor and the oscillation capacitor, are determined by inequations derived from the requirements of dc fault interruption. The selection standard for the spark gap switch is set based on its breakdown characteristics. Finally, simulations are conducted to investigate the performance of the proposed M-DCCB. Results show that the capacitor can accumulate a sufficiently large reverse charge voltage automatically charged by the fault current. The proposed active M-DCCB topology and the dc fault handling strategy can interrupt the dc fault in HVdc power systems consecutively.

Correlation between surface charge and DC surface flashover of plasma treated epoxy resin

Surface charge accumulation of polymer insulating materials is harmful to the safe operation of electrical equipment, especially under DC electric field excitation. In order to ensure the safety and stable operation of HVDC power system such as Gas-insulated transmission line, it is necessary to understand the influence of the surface charge on the flashover and improve the surface charge dynamic characteristics. In this paper, Epoxy resin (EP) is deposited by SiO x thin films with different plasma treatment conditions. The flashover voltages are measured with and without surface charges. The results show that the flashover occurs at lower voltage due to the accumulated charges on the EP surface. In particular, the surface charge with the same polarity of flashover discharge can reduce the surface flashover voltage significantly. Moreover, the charge decay rate is increased up to 90% by plasma treatment of 3 min, corresponding flashover voltage is increased. In addition, the flashover voltages of samples during the surface charge dissipation are obtained. Both static impact index I and dynamic impact index S are introduced to estimate effect of surface charge on dielectric surface flashover.

Subsynchronous Oscillations Caused by Open-Loop Modal Coupling Between VSC-Based HVDC Line and Power System

A closed-loop interconnected model of a power system with a voltage source converter (VSC)-based HVDC line (VSC-HVDC line) is established, wherein, a selected control system of the VSC-HVDC line and rest of the power system are modeled as two separate open-loop subsystems. Based on the established model, a special condition of open-loop modal coupling is studied, when a complex pole of the open-loop selected control subsystem of the VSC-HVDC line is close to a subsynchronous oscillation (SSO) mode of open-loop power subsystem on the complex plane. Theoretical analysis indicates that when the open-loop modal coupling occurs, subsynchronous interactions (SSIs) between the VSC-HVDC line and the rest of power system may become strong. It is very likely that the open-loop modal coupling may degrade the damping of power system SSOs. Hence, the mechanism that the VSC-HVDC line may cause the SSOs in the power system is revealed from the perspective of open-loop modal coupling. In this paper, study cases of example power systems with the VSC-HVDC line are presented to demonstrate and evaluate the analysis and conclusions made.

Design and Test of 10 kV/400 A Flux-Coupling-Type Superconducting Fault Current Limiting Module

The flux-coupling-type superconducting fault current limiter (FC-SFCL) is suggested to suppress the surge current in voltage sourced converter based HVdc (VSC-HVdc) power systems. One 10 kV/400 A FC-SFCL module with two branches is designed and tested. For the FC-SFCL module, the two branches could use the same or different YBCO tapes. Furthermore, the electromagnetic analysis and parameter calculation of the module are performed. In order to study the current-limiting performance of the FC-SFCL under the rapid change of fault current, a dynamic simulation model is established. Finally, the transient properties of FC-SFCL module are simulated and tested. According to the measurement results, the maximum impact current could be limited from 12 kA to 6264 A, and the largest resistance of the SFCL module is about 1.5 Ω when the dc impact voltage is 10 kV, which is close to the simulation results.

DC Conductivity Measurements of LDPE: Influence of Specimen Preparation Method and Polymer Morphology

&lt;p&gt;DC conductivity measurements are important for gaining fundamental understanding of conduction mechanisms of insulation materials, as well as in the development of HVDC power system components, such as extruded cable systems. In this study, the influence of sample processing on the morphology and DC conductivity of low-density polyethylene (LDPE) has been studied. Direct compression moulding of LDPE pellets is commonly used in research laboratories for obtaining plaque samples, whereas extrusion is an additional commonly used technique for dispersion of particles in nanocomposites prior to the compression moulding process. In this study LDPE plaques have been obtained by either compression moulding directly from pellets, or by extrusion followed by compression moulding. The morphology obtained in the first case consisted of banded spherulites, whereas the latter method yielded a morphology of small axialites. The difference in sample processing had also an impact on the DC conductivity. The DC conductivity at 22 ­°C and 3.3 kV/mm was of the order of 4E-18 S/m for the plaques obtained by extrusion and compression moulding whereas the plaques obtained by direct compression moulding exhibited a conductivity of 1E-16 S/m. In addition, the reproducibility of the performed DC conductivity measurements was also verified in a round robin test performed between the Royal Institute of Technology and &lt;br /&gt;Chalmers Technical University.&lt;/p&gt;

DC Conductivity Measurements of LDPE: Influence of Specimen Preparation Method and Polymer Morphology

DC conductivity measurements are important for gaining fundamental understanding of conduction mechanisms of insulation materials, as well as in the development of HVDC power system components, such as extruded cable systems. In this study, the influence of sample processing on the morphology and DC conductivity of low-density polyethylene (LDPE) has been studied. Direct compression moulding of LDPE pellets is commonly used in research laboratories for obtaining plaque samples, whereas extrusion is an additional commonly used technique for dispersion of particles in nanocomposites prior to the compression moulding process. In this study LDPE plaques have been obtained by either compression moulding directly from pellets, or by extrusion followed by compression moulding. The morphology obtained in the first case consisted of banded spherulites, whereas the latter method yielded a morphology of small axialites. The difference in sample processing had also an impact on the DC conductivity. The DC conductivity at 22 ­ºC and 3.3 kV/mm was of the order of 4xE-18 S/m for the plaques obtained by extrusion and compression moulding whereas the plaques obtained by direct compression moulding exhibited a conductivity of 1xE-16 S/m. In addition, the reproducibility of the performed DC conductivity measurements was also verified in a round robin test performed between the Royal Institute of Technology and Chalmers Technical University.

Offline Fault Localization Technique on HVDC Submarine Cable via Time–Frequency Domain Reflectometry

Fault localization is one of the most significant aspects in the maintenance of high-voltage direct current (HVdc) submarine cables that have unconventional installation characteristics, such as long cable lengths and underwater installation locations. In order to protect and diagnose the cable, an improved fault localization technique, that is, time–frequency domain reflectometry (TFDR) and tangent distance pattern recognition are proposed in this paper. The fault location information of the HVdc submarine cables can be obtained from the tangent distance, to support the results of TFDR. To verify the performance of the proposed method, a commercial HVdc submarine cable is used in the experiments. A test bed is constructed for creating a similar environment with that of the submarine cable and filled with sea water. Both low- and high-impedance faults are emulated in this experiment by local insulation faults with iron, sea water, and air. The theoretical concepts and experimental results of the proposed method are presented. It is expected that the proposed method can improve the reliability of real-world HVdc power systems.

Investigation on Economic and Reliable Operation of Meshed MTDC/AC Grid as Impacted by Offshore Wind Farms

The influence of complementarity balancing connected grid and control of offshore wind power generation on the economic and reliable operation of hybrid multiterminal HVDC (MTDC) and AC power system is investigated in the paper. The variances of distribution curve of both transmission loss of AC/DC system and voltage of critical nodes assessed by cumulant-based method are used to demonstrate how the different complementarity conditions, generation balancing strategies, MTDC grid topologies, and control schemes affect power system operation. In this paper, a study case of 16-generator 68-bus power system connected with three offshore wind farms is presented. The variance indices of transmission loss and voltage magnitude are calculated and then compared under different complementarity levels, active power balancing strategies, HVDC transmission systems, and controls, respectively. The analysis reveals that higher complementarity and multi-machine balancing strategy can effectively reduce system operational cost and enhance operational reliability. Compared to the point-to-point topology, the control pattern and parameter setting of multi-terminal topology are two key factors to determine its impact on system operation.

Wide-Area Robust Decentralized Coordinated Control of HVDC Power System Based on Polytopic System Theory

The present study proposes a hierarchical wide-area decentralized coordinated control framework for HVDC power system that is robust to multiple operating conditions. The upper level wide-area coordinated controller is designed in the form of dynamic output feedback control that coordinates the lower level HVDC supplementary controller, PSS, and SVC. In order to enhance the robustness of the designed controller under various operating conditions, the polytopic model is introduced such that the closed-loop control system can be operated under strong damping mode in virtue of the stability criterion based on damping ratio. Simulation results demonstrate that the proposed controller design algorithm is capable of enhancing the system damping over four different conditions.