HVDC Technology Research Articles

HVDC technology and power quality issues in Slovenian transmission system – technical study

HVDC technology and power quality issues in Slovenian transmission system – technical study

Control of HVDC systems based on diode rectifier for offshore wind farm applications

The transfer of energy from offshore wind farms using HVDC technology is highly dependant on the converter technology used. The conversion through HVDC has conventionally been done by Line Commutated Converters (LCC) or Voltage Source Converters (VSC), by using thyristor rectifiers or IGBTs, respectively. However, 12-pulse diode rectifier at the offshore converter station can be a promising option due to lower conduction losses, reduced installation costs and converter size, and higher reliability. The main drawback is that the overall control strategy needs to adapt to the uncontrollable diode rectifier. This paper describes the system topology and control solutions proposed with the use of diode rectifier. In addition the paper proposes a shifted voltage and frequency droop control strategy related to the Phase Locked Loop (PLL) to improve the control system as an original contribution. The control strategies are validated in Matlab/Simulink.

IMPROVEMENT OF POWER SYSTEM QUALITY USING VSC-BASED HVDC TRANSMISSION

The HVDC technology can be represented by the combination of a Direct Current (DC) circuit with two power electronics converters, each one at a link terminal, for AC/DC and DC/AC conversion The principal characteristic of VSC-HVDC transmission is its ability to independently control the reactive and real power flow at each of the AC systems via the Point of Common Coupling (PCC). The active and reactive power is related to the power angle and the magnitude of voltage in the reference -frame selected such that the quadrature component will result in the ratio between the maximum fundamental peak phase voltage and the DC total voltagehttp://dx.doi.org/10.4314/njt.v36i3.31

Interface tailoring for charge injection control in polyethylene

The insulating materials used to develop HVDC technologies suffer from a major drawback, which is the accumulation of electrical charges forming internal space charge with possibly two major consequences: (i)-the out-of-control of the internal electric field distribution initiating current runaway and (ii)-cumulated molecular level damages extending or creating defects and leading ultimately to breakdown. To prevent space charge accumulation, one possible route, not examined in depth by the scientific community to date is to control the charge injection at the interfaces between the insulating material and the electrodes (metallic or semi-conducting). Different routes were followed in this work for tailoring the interface between electrode and polyethylene material, based on chemical modification of the insulation or layer intercalation. Depending on the process, charge injection control is achieved either for negative charges or for charges of both polarities. The process of charge injection control is discussed with reference to the chemical/physical modifications brought about by the different treatments. The results provide indication towards a strategy to control the injection in power cables and other electrical components.

A Review on VSC-HVDC Reliability Modeling and Evaluation Techniques

With the fast development of power electronics, voltage-source converter (VSC) HVDC technology presents cost-effective ways for bulk power transmission. An increasing number of VSC-HVDC projects has been installed worldwide. Their reliability affects the profitability of the system and therefore has a major impact on the potential investors. In this paper, an overview of the recent advances in the area of reliability evaluation for VSC-HVDC systems is provided. Taken into account the latest multi-level converter topology, the VSC-HVDC system is categorized into several sub-systems and the reliability data for the key components is discussed based on sources with academic and industrial backgrounds. The development of reliability evaluation methodologies is reviewed and the issues surrounding the different computation approaches are briefly analysed. A general VSC-HVDC reliability evaluation procedure is illustrated in this paper.

The Development of the ±80kV 60MW HVDC System in Korea

HVDC transmission systems can be configured in many ways to take into account cost, flexibility and operational requirements. [1] For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses. For underwater power cables, HVDC avoids the heavy currents required to charge and discharge the cable capacitance of each cycle. For shorter distances, the higher cost of DC conversion equipment compared to an AC system may still be warranted, due to other benefits of direct current links. HVDC allows power transmission between unsynchronized AC transmission systems. Since the power flow through an HVDC link can be controlled independently of the phase angle between the source and the load, it can stabilize a network against disturbances due to rapid changes in power. HVDC also allows the transfer of power between grid systems running at different frequencies, such as 50 Hz and 60 Hz. This improves the stability and economy of each grid, by allowing the exchange of power between incompatible networks. This paper proposed to establish Korean HVDC technology through a cooperative agreement between KEPCO and LSIS in 2010. During the first stage (2012), a design of the ±80kV 60MW HVDC bipole system was created by both KEPCO and LSIS. The HVDC system was constructed and an operation test was completed in December 2012. During the second stage, the pole#2 system was fully replaced with components that LSIS had recently developed. LSIS also successfully completed the operation test. (2014.3)

A feasible coordination protection strategy for MMC-MTDC systems under DC faults

Modular multilevel converter based multi-terminal HVDC (MMC-MTDC) technology is increasingly adopted as a promising option in the power transmission and distribution fields. However, how to isolate dc fault currents and ensure the continuous operation of the healthy parts of the MMC-MTDC system under dc faults presents a huge challenge. Based on a comprehensive analysis on the dynamic behavior of the dc fault current of a MMC, along with the operation characteristics of hybrid dc circuit breakers (CBs), a protection strategy will be developed. It coordinates the system parameters to ensure the hybrid dc CBs cut off the dc fault current before triggering MMC blocking in bipolar dc faults. The mathematical approximation of the dc fault current without converter blocking is achieved through theoretical analysis and numerical calculation, and the smoothing inductance at the dc side of MMC station has shown a major influence on the dynamic behavior of the dc fault currents. The smoothing inductance can be determined based on the mathematical expression of the dc fault current, as well as the breaking capacity of hybrid dc CBs. Data analysis and simulation are performed based on a three-terminal dc test system to validate the effectiveness of the coordination protection strategy.

Optimization of wavelet and thresholding for partial discharge detection under HVDC

With the rapid development of HVDC technology, the detection and analysis of partial discharge (PD) under HVDC are new challenges to ensure reliable operation of the related power apparatus. The wavelet technique has been proposed for analyzing PD pulses under HVAC and ultra-high frequency signal, but its application for PD under HVDC has not been discussed. This paper dealt with the selection of the optimal wavelet and thresholding for PD pulses in order to apply the wavelet technique to PD detection under HVDC. Four electrode systems, namely protrusion on conductor, protrusion on enclosure, free particle, and crack inside spacer were fabricated to simulate typical defects in a gas insulated switchgear. The detected PD pulses were decomposed by multiresolution analysis. The correlation coefficient and dynamic time warping methods were used to select the optimal wavelet. The optimal threshold and thresholding function were chosen from various combinations with the simulated pulses. The results revealed that processing PD pulses with the mother wavelet of bior2.6, automatic threshold, and intermediate thresholding function presented the best performance.

Analysis of dynamic behavior of the back-to-back High Voltage Direct Current link model as part of electrical power system

One of the main directions of the development of electric power systems is the introduction of technologies based on high-power semiconductor switches, such as FACTS devices and HVDC technologies. These systems effectively solve a number of urgent tasks of EPS, connected with asynchronous connection of EPS, transmission of electricity, improve local and systemic flexibility and reliability of EPS, increasing the capacity of network elements that contains a “weak” connection. However, the implementation and operation of mentioned technologies in the EPS determines the need for a wide range of analysis and research that can only be done with the help of mathematical modeling.

A novel and comprehensive single terminal ANN based decision support for relaying of VSC based HVDC links

HVDC technology is increasingly important for long distance bulk power transmission, but existing protection relaying techniques for such a system are subject to limitations. This paper presents a novel Artificial Neural Network (ANN) based on an algorithm for fault detection, location and classification in VSC-HVDC systems. Taking advantage of the ability of ANNs to identify and classify patterns, the proposed algorithm is able to detect and correctly classify a fault occurring at either the rectifier substation on the DC line or at the inverter substation. Therefore, such a scheme can be used as a decision support tool or as a backup protection. Only local signals are used at the rectifier substation and no communication link is necessary, thus improving the system’s protection reliability and reducing the overall cost of the hardware implementation. A detailed VSC-HVDC system is described and used to simulate a number of fault scenarios in the system. Using the resulting fault waveforms, a comprehensive decision support scheme is developed and described, paying particular attention to the signal processing chain and design of the specific ANNs for each relaying task. Finally, a detailed analysis of the influence of key fault parameters on the limits of the algorithm’s performance is carried out.

Research on fault diagnosis for MMC-HVDC Systems

With the development of flexible HVDC technology, the fault diagnosis of MMC-HVDC becomes a new research direction. Based on the fault diagnosis theory, this paper proposes a robust fault diagnosis method to study the fault diagnosis problem of MMC-HVDC systems. By optimizing the gain matrix in the fault observer, fault detection with good sensitivity and robustness to disturbance is achieved. In the MMC-HVDC system, because of the inherently uncertain system and the presence of various random disturbances, the study of robust fault diagnosis method is particularly important. Simulation studies during various AC faults have been carried out based on a 61-level MMC-HVDC mathematical model. The results validate the feasibility and effectiveness of the proposed fault diagnosis method. So this fault diagnosis method can be further applied to the actual project, to quickly achieve system fault diagnosis and accurately complete fault identification.

An Overview on VSC-HVDC Power Transmission Systems

The Voltage Source Converter based High Voltage Direct Current(VSC-HVDC) is a new generation HVDC technology. Because of the technology of the PWM and the turn- on and turn-off electronic apparatus, VSC-HVDC could make up for the defective of the traditional HVDC. And the development is rapid. This paper introduces some operation principle and summarizes instances. Firstly, the paper introduces the structure and operation principle of VSC-HVDC. Then the paper states the technology characteristic of VSC-HVDC and the application field. In the end of the paper, the paper discusses the situations of the VSC-HVDC in China and other countries.

Elimination of Commutation Failures of LCC HVDC System with Controllable Capacitors

This paper presents a novel hybrid converter configuration for conventional Line-Commutated Converter (LCC) HVDC technology aiming to eliminate commutation failures under serious faults. Dynamic series insertion of capacitors during commutation is utilized to increase the effective commutation voltage. The operating principles are presented followed by detailed mathematical analysis for both zero impedance single-phase and three-phase faults in order to select the required capacitor size and its voltage level. The performance of the proposed method is validated by simulation results in Real Time Digital Simulator (RTDS) and the results show that the proposed converter configuration is able to eliminate commutation failures under both fault cases. Consequently partial power transferring capability during single-phase fault and fast fault recovery from three-phase fault can be achieved. Further simulation results show that the harmonic content of inverter AC voltage and current are not significantly increased and the voltage stress of the thyristor valve is comparable to that of the original benchmark system.

A Novel Strategy on Smooth Connection of an Offline MMC Station Into MTDC Systems

Themodular multilevel converter-based multiterminal HVDC (MMC-MTDC) technology has been applied to the Nanao project, which was commissioned successfully in China, and has shown many advantages. However, how to connect an offline MMC station into an operating MTDC system smoothly without expensive dc circuit breakers presents a huge challenge. Making full use of the structural characteristics of MMC, the proposed strategy implements the smooth connection by sequential operations of the dc disconnectors (DSs) at the positive and negative polarities, without the need for changing system parameters or hardware. The charging circuits of MMC are studied fully after every operation of DSs, and a four-order state equation is built to explore the potential maximum surge current after closing the DSs. Case study and simulation are performed based on the PSCAD/EMTDC model of the Nanao three-terminal MMC-HVDC system to validate the effectiveness of the proposed strategy. Besides easy operation and high reliability, the strategy neither influences the operating system nor increases any cost.

Research on DC Field Wiring Scheme of Hybrid HVDC VSC-Converter Station

LCC-HVDC transmission system has numbers applications in world. But the inherent technical characteristics of LCC-HVDC transmission system may cause some problems, such as the failure of commutation of converter station, unable to supply power to weak AC system. Hybrid DC transmission technology has the advantages of both two kinds of HVDC technology. It has the high flexibility control method and low cost, and has a good prospect in the energy convergence of renewable energy, asynchronous grid interconnection and long-distance transmission. In this paper, the DC field wiring scheme of the VSC-HVDC converter in hybrid HVDC system is proposed and optimized according to its characteristics. The performance of the system is improved and the investment cost is saved. And it has wide application prospect. for the reconstruction of LCC-HVDC converter station to VSC-HVDC converter station.

Multivariable control with grid objectives of an HVDC link embedded in a large-scale AC grid

Abstract The HVDC links are increasingly used not only to interconnect asynchronous AC systems but are also embedded into a same meshed AC power system. Thanks to its speed and flexibility, the HVDC technology is able to provide transmission system advantages as transfer capacity enhancement and power flow control. In addition, studies have shown that the way of controlling the HVDC converters impacts the stability of the AC system. This can be particularly exploited to enhance the dynamic power system performances during transients. In this paper a robust multivariable control design for HVDC link converters is proposed. It is based on the coordination of the control actions of the HVDC converters and the use of a control model which takes into account the dynamics that mostly impact stability of the neighbor zone of the HVDC link. This new methodology was used to synthesize the controller for an actual grid 1000 MW HVDC link reinforcement project called “Midi-Provence” in the southern part of the French grid. The synthesis, implementation and validation processes are presented in detail. The new controller is tested in comparison with the standard vector control. A large-scale dynamic model of the whole European power system, currently used and updated by the European TSO’s for the interconnection studies has been used with Eurostag simulation software.

System reliability enhancement in a metropolitan area using HVDC technology

Recently, due to the increase of massed demand areas, in terms of system reliability has been raised several issues such as voltage, frequency problems and fault current. In Korean power systems, more than 40% of the total loads are concentrated in metropolitan areas, however most large plants are located in non-metropolitan areas. In these long-distance transmission systems, the transmission capacity is limited not by thermal capacity constraint, but by stability constraint. To solve these problems, the study of enhancing power system stability and increasing transmission capacity has been progressed. In this paper, we analyse domestic power systems in order to discuss the best solutions for current reduction using PSS/E simulation. Fault current problems can be resolved through gird segmentation which adopts Voltage Sourced Converter Back-to-Back High Voltage Direct Current (VSC BTB HVDC). Furthermore, grid segmentation can enhance the interface flow margin with the independent reactive power control in...

Tracking/Erosion Resistance Analysis of Nano-Al(OH)3Filled Silicone Rubber Insulating Materials for High Voltage DC Applications

HVDC technology has become popular as an economic mode of bulk power transmission over very long distances. Polymeric insulators in HVDC power transmission lines are affected by surface tracking and erosion problems due to contamination deposit, which pose a greater challenge in maintaining the reliability of the HVDC system. In addition, polymeric insulators are also naturally affected by aging due to various environmental stresses, which in turn accelerates the surface tracking and erosion problems. Research works towards the improvement of tracking and erosion resistance of polymeric insulators by adding nano-sized fillers in the base material are being carried out worldwide. However, surface tracking and erosion performance of nano-filled aged polymeric insulators for HVDC applications are not well reported. Hence, in the present work, tracking and erosion resistance of the nano Al(OH)3 filled silicone rubber insulation material has been evaluated under DC voltages at different filler concentrations and aged conditions, as per IEC 60587 test procedures. Leakage current and contact angle measurements were carried out to understand the surface hydrophobicity. Moving average technique was used to analyze the trend followed by leakage current. Water aged specimen shows less tracking resistance when compared with thermal aged specimen. It is observed that nano- filler concentration of 5% is even sufficient to get better tracking/erosion resistance under DC voltages.

Oscillation analysis of a DFIG-based wind farm interfaced with LCC-HVDC

With the increasing capacity of wind farm, HVDC technology has become a promising transmission scheme for long distance transportation of large-scale wind power. However oscillation caused by this system will have a great influence on the security and stability of power system operation. In this paper, the oscillation of a doubly-fed induction generator (DFIG)-based wind farm interfaced with line commutated converter (LCC) based HVDC is discussed. Low-frequency oscillation and subsynchronous oscillation (SSO) are studied since these two oscillations are the particularly concerned oscillations in the stability study of power system in recent years. The model of a DFIG-based wind farm interconnected with LCC-HVDC is developed. The impact of drive train model’s structure and parameters on the oscillation characteristics is analyzed. Eigenvalue and participation factor analysis are used to identify the three main modes, which include controller mode, electromechanical mode, and shaft mode. The effects of DFIG controller’s parameters, wind speed and operating conditions of HVDC on those modes are studied. Electromagnetic transient simulations are performed to verify the results of the eigenvalue analysis.

Synthesis of a hybrid model of the VSC FACTS devices and HVDC technologies

The motivation of the presented research is based on the need for development of new methods and tools for adequate simulation of FACTS devices and HVDC systems as part of real electric power systems (EPS). The Research object: An alternative hybrid approach for synthesizing VSC-FACTS and -HVDC hybrid model is proposed. The results: the VSC- FACTS and -HVDC hybrid model is designed in accordance with the presented concepts of hybrid simulation. The developed model allows us to carry out adequate simulation in real time of all the processes in HVDC, FACTS devices and EPS as a whole without any decomposition and limitation on their duration, and also use the developed tool for effective solution of a design, operational and research tasks of EPS containing such devices.