Magnetic Force Analysis in a Gapped-Core Reactor Model Under Harmonic Magnetizations by Efficient Frequency-Domain Decomposition

The converter valve is one of the key equipments in the ultra-high voltage direct current (UHVDC) transmission systems. Shield case design is an important part in the process of converter valve design, appropriate shield case configuration can improve the electric field distribution of converter valve, extend the service life and enhance the long-term operation reliability of the UHVDC systems. In this paper, an electric field distribution equivalent calculation model of the double converter valve was discussed, and a 3-D transient calculation model of ±800 kV double converter valve was established by using a finite element method (FEM) analysis software ANSYS. The calculation results of potential and electric field distribution of the double converter valve were presented, the law of electric field distribution of the shield cases which run on different working conditions and locate in different positions of the double converter valve was revealed, the effect of electric field and potential distribution of different shield cases configuration schemes was discussed, and an optimal scheme was put forward. Verified by the computer simulation, the optimal scheme satisfied the external insulation and electromagnetic environment requirements of the double converter valve. The results provide an important basis on the design of the UHVDC double converter valve.

A nonlinear model is proposed representing air-gap magnetic flux density distribution in a synchronous machine with magnetic saturation in the pole tips. In order to explain the effect of magnetic saturation in a synchronous machine, the authors have measured air-gap flux in a specially designed synchronous machine, and recently proposed a model for the air-gap flux density distribution, concentrating on magnetic saturation in the field pole iron cores. In this paper, we propose an adaptation of our model for calculating air-gap flux, specifically addressing the magnetic saturation in the pole tips. By regarding the saturation as a consumer of m. m. f. similar to a virtual winding, we construct a magnetic circuit which represents the flow of magnetic flux inside the synchronous machine, and derive expressions which represent air-gap flux density distribution. Close agreement is obtained between the theoretically calculated results, and the results measured using search coils placed at the teeth of the armature.

High voltage direct current (HVDC) transmission offers several operation advantages over high voltage alternating current (HVAC) transmission systems. HVDC transmission lines have no frequency and hence minimal problems of harmonics, oscillations or transients; they actually damp out transients with effective controllers. Precise and fast power control in either direction is easier to achieve in HVDC than HVAC systems as dc power is minimally affected by reactance. This curtails system instability, cascading effect or voltage collapse. The ease of power flow control in HVDC lines makes it easy to achieve the maximum power and thermal capacity of transmission lines. It is also simple to control active power transfer at a predetermined level or even to modulate this to improve system damping. For weak AC grids, voltage source converter (VSC) based HVDC systems offer better controllability and grid flexibility for integrating intermittent renewable energy sources such as wind and solar-power. HVDC systems offer technical advantages: all angular stability problems disappear and even connection of systems at different frequency is feasible. The problems of cable charging current are eliminated. Thus cables can readily be used for underwater crossings. HVDC is the established technology of choice for long distance transmission and subsea electrical power transmission systems. In this paper, an investigation of voltage stability margin on Eskom high voltage eastern grid is carried out. Using the Newton Raphson iterative method, the impact of HVDC link controller and transformer tap changers on voltage stability were evaluated on Athene substation of Eskom eastern grid, an area of rapid load growth. Results obtained show that by introducing HVDC line, transmission capacity is enhanced, with lower line losses and improved voltage profile. Simulations were carried out using DigSILENT PowerFactory.

6 - High Voltage Direct Current (HVDC) electric power transmission systems

High voltage direct current (HVDC) transmission systems play an essential role in our modern power grids, not only providing bulk power transmission and asynchronous AC connection, but also enabling renewable energy integration into power grids. Renewable energy integration through HVDC transmission systems leads to the creation of multi-terminal HVDC (MT-HVDC) grids. Recently, the idea of electrifying remote communities by HVDC transmission systems has emerged, and multi-terminal small-scale HVDC grids have been investigated in such applications. Despite numerous applications and significant potential of HVDC transmission systems, various technical and economic challenges in utilizing these systems still persist. In this paper, an extensive literature review is conducted on HVDC transmission systems, particularly focusing on their role in renewable energy integration, and their potential application in remote communities. Future research directions are also recommended.

Recent developments in power semiconductor devices have made High Voltage Direct Current (HVDC) transmission systems the most crucial technology for future power systems. Today several HVDC projects have been commissioned around the world. The primary concern for such an advanced HVDC grid is to follow standard grid codes to maintain the safety and stability of the electrical equipment during transient and dynamic circumstances. Although the HVDC system has been recognized as the most prominent technology to transmit electric power, some issues must be addressed, such as selective and quick detection of faults. For selective fault detection, it is essential to look into the several fault detection strategies that fortify the HVDC system with time-frequency signal processing techniques. This research uses a time frequency-based method to identify the different HVDC transmission line faults. The most suitable mother wavelet for various transient states is selected by analyzing fault voltage and current signals based on variance, standard deviation, and range parameters. An experimental setup of a thyristor-based HVDC transmission system has been developed in the MATLAB/Simulink environment, and multiple fault scenarios have been analyzed. The simulation outcomes reveal that the most effective mother wavelet for detecting DC line, symmetrical and unsymmetrical faults occurs at the 6th level of fault signal decomposition. By examining current and voltage signals, our work has determined the best mother wavelet for various faults. Using more than one mother wavelet to detect faults is more reliable than using a single mother wavelet to detect all faults. Additionally, it has been noted that each fault's voltage and current signals have two distinct mother wavelets.

Aging characteristics of conductor surface of ultra-high voltage direct current (UHVDC) power transmission lines have a large influence on ground-level resultant electric field. A method to analyse the impact of the aged conductors on resultant electric field is introduced in this paper. Firstly, the surface profile and surface roughness of the new and aged conductors are measured by scanning electron microscopy and three dimensional phase shift Micro-Xam. Then, an indoor corona cage is set up to determine the roughness factor of the aged conductor. A time-domain finite volume method (FVM) is introduced to analyse the influence of aging characteristics of conductor surface on resultant electric field. Finally, the ground-level resultant electric field and ion current density of a typical ± 800kV UHVDC power transmission line are calculated and analysed. The result shows that the surface roughness factor of the aged conductor is much lower than that of the new conductor, and the groundlevel resultant electric field and ion current density are stronger than that of the new one.

High Voltage Direct Current (HVDC) system was often the link between major power grids. In order to ensure the safe operation of HVDC, fault diagnosis method was studied. The system structure of HVDC Light system was analyzed, and the normal and fault conditions of the transmission system were simulated in the Matlab platform. This method was very convenient and simple, and it could restore the real situation. The use of simulated waveform could effectively identify the fault types of actual situation, and then it was a flexible, efficient and practical method of fault diagnosis in HVDC Light system.

Direct Current (DC) transmission system is an important part of the interregional grid. However, the frequency impact of power grid caused by DC blocking is more serious with the continuous increase of scale of single DC transmission. The frequency coordination control system is applied to the receiving-end power grid in High Voltage Direct Current (HVDC) transmission system. In the cases of single-infeed HVDC blocking, multi-infeed HVDC blocking or fast power drop, the frequency coordination control system can reduce the frequency fluctuation of power grid by instantly promoting the transmission power of the rest normal DC systems and reducing power loss. The frequency coordinated control substation is an executive device of the system at the converter station. It keeps communicating with the coordinated control master station for real-time monitoring the operation of converter station, reports the fault and executes the remote power boost instructions. Taking the frequency coordinated control substation of 500 kV conventional DC systems as a research background, the load of converter station in a regional power grid is analyzed. The working mechanism of frequency coordination control system in the cases of current converter station fault or other station faults is analyzed. The actual effect of a frequency coordination control system on a converter station in East China power grid is investigated by testing the actual power, voltage and current variation of a conventional DC transmission system.

The present PhD thesis deals with transformer inrush current in offshore grids including offshore wind farms and High Voltage Direct Current (HVDC) transmission systems. The inrush phenomenon during transformers energization or recovery after the fault clearance is one of important concerns in offshore systems which can threaten the security and reliability of the HVDC grid operation as well as the wind farms function. Hence, the behaviour of wind turbines,Voltage Source Converters (VSC) and transformer under the normal operation and the inrush transient mode is analyzed. For inrush current reduction in the procedure of the offshore wind farms start-up and integration into the onshore AC grid, a technique based on Voltage Ramping Strategy (VRS) is proposed and its performance is compared with the operation of system without consideration of this approach. The new methodology which is simple, cost-effective ensures minimization of transformer inrush current in the offshore systems and the enhancement of power quality and the reliability of grid under the transformer energizing condition. The mentioned method can develop much lower inrush currents according to the slower voltage ramp slopes. Concerning the recovery inrush current, the operation of the offshore grid especially transformers is analyzed under the fault and the system restoration modes.The recovery inrush transient of transformers can cause tripping the HVDC and wind farms converters as well as disturbing the HVDC power transmission. A voltage control design based on VRS is proposed in HVDC converter to recover ali the transformers in offshore grid with lower inrush currents.The control system proposed can assure the correct performance of the converters in HVDC system and in wind farm and also the robust stability of the offshore grid. Esta tesis doctoral estudia las corrientes de energización de transformadores de parques eólicos marinos con aerogeneradores con convertidores en fuente de tensión (VSC) de plena potencia conectados a través de una conexión de Alta Tensión en Corriente Continua (HVDC). Las corrientes de energización pueden disminuir la fiabilidad de la transmisión eléctrica debido a disparos intempestivos de las protecciones durante la puesta en marcha o recuperación de una falta. Para la mitigación de las corrientes de energización durante la puesta en marcha del parque esta tesis propone una nueva estrategia basada en incrementar la tensión aplicada por el convertidor del parque eólico en forma de rampa (VRS). Este método persigue energizar el parque eólico con el menor coste y máxima fiabilidad. La tesis analiza diferentes escenarios y diferentes rampas. Otro momento en que las corrientes de energización pueden dar lugar a un disparo intempestivo de las protecciones es durante la recuperación de una falta en la red de alterna del parque eólico marino. Esta tesis extiende la estrategia VRS, utilizada durante la puesta en marcha del convertidor del parque, para los escenarios de recuperación de una falta.

This research is to regulate firing angle inverter-side of high voltage direct current (HVDC) system using ANFIS-based controller. Average-value model (AVM) is used to implement the HVDC systems because the AVM is needed in level-system simulation of power systems. The AVM neglects switching-device responses and time scale of the AVM is lower than detailed model. Controller based on ANFIS is implemented to control firing angle of inverter-side HVDC because this controller is simple and easy to be implemented. The HVDC system is constructed by using Matlab/Simulink programming. Initializing the variable and parameter of HVDC is loaded to the Matlab workspace using script programme. Some modules for converter, inverter and their controller are developed. Next, the ANFIS controller that consist of 7 inputs and a single output is applied to inverter module. Membership function (MF) that used for input and output are Gaussian MF and constant MF (Sugeno type 0), respectively. The ANFIS scheme control responses are compared to conventional controller responses to validate the ANFIS scheme controller.

A High Voltage Direct Current Transmission system (HVDC) requires of electronic converters by its characteristic to turn electrical AC energy in DC energy or vice versa . Basically exist two types of possible configurations of three-phase converters for this process of conversion: Current Source Converter (CSC) and Voltaje Source Converter (VSC). In the present paper will be analyzed threephase converters VSC with modulation PWM, the balance of active power and reactive power, the asynchronous interconnection of three-phase systems with VSC converters and the sizing of some elements of the part of power, using the SimPowerSystems of MatLab®.

The transfer of bulk electrical power over long distances can be done by high voltage alternating current (HV AC) transmission systems. These HV AC power transmission systems have their perfections and imperfections. To enhance the existing power transmission systems, high voltage direct current (HVDC) power transmission systems are a sustainable solution. The emergence of advanced HVDC systems is so rapid that they are replacing their counterpart HV AC systems in many parts of the world. In this paper, the prominence of modern HVDC power transmission systems is highlighted. The topologies, converter assortments, contrasts, and modeling methods are discussed. The renewable energy sources (RES) including solar photovoltaic (PV) parks and mainland/ onshore/ offshore wind farms can be assimilated into the existing AC grids via advanced HVDC converters. The perceptions of multi-terminal HVDC systems and DC grids are presented, whereas, the challenges concomitant with the future DC grids are also emphasized.

Appropriate planning of maintenance activities is a very useful method for reducing operating costs and enhance power system's performance. High Voltage Direct Current (HVDC) Transmission Systems contribute to the achievement of higher flexibility and increased transport capacity, their availability being therefore particularly important in today's global energy landscape. Given that HVDC schemes reached their expected lifetime quite recently, it is necessary to prioritize their subsystems with respect to performing maintenance activities, in order to minimize the unavailability time. This paper presents a hierarchical method based both on the assessment of the relative importance of the selected criteria against the established main objective and the considered subsystems on each criterion. Following Analytic Hierarchy Process (AHP) principle, an algorithm was developed in Matlab and applied to a HVDC transmission system, part of a case study. This led to obtaining the priority vector for its constituent subsystems regarding improved organization of the maintenance strategy.

Large electric power systems, such as high voltage direct current (HVDC) transmission systems, need dc reactors with large inductance and high transport current. Such systems experience a lot of electrical loss due to the resistance of their copper winding. Employing superconducting magnets for the reactors provides some advantages such as high current density, low electrical loss, and so on. This paper describes the design of a toroid-type high temperature superconductor (HTS) dc reactor magnet. The target inductance and the current level of the HTS dc reactor were 400 mH and 400 A, respectively. The toroid-type HTS magnet reduced leakage flux and increased the critical current of the reactors. The HTS dc reactors were designed using 2G HTS wires. The Finite Element Method was used for the design and analysis of the toroid-type HTS dc reactors.