Long-distance Power Transmission Research Articles

Sub-Synchronous Oscillation Caused by HVDC and the Damping Characteristic Analysis

Because of China's energy and load imbalance, respectively, large-scale long-distance power transmission becomes inevitable. Under this background , HVDC develops rapidly, SSO (Sub-synchronous oscillations) caused by HVDC is becoming increasingly apparent. The sub-synchronous electrical damping within the frequency range is difficult to calculate. Based on small-signal test method-complex torque coefficient method in the time-domain simulation , the paper calculated the sub-synchronous electrical damping characteristics of the system and analyzed the impact of the unit interaction, DC power and the generator output on sub-synchronous electrical damping characteristics.

Distributed Voltage and Current Control of Multi-Terminal High-Voltage Direct Current Transmission Systems

High-voltage direct current (HVDC) is a commonly used technology for long-distance power transmission, due to its low resistive losses and low costs. In this paper, a novel distributed controller for multi-terminal HVDC (MTDC) systems is proposed. Under certain conditions on the controller gains, it is shown to stabilize the MTDC system. The controller is shown to always keep the voltages close to the nominal voltage, while assuring that the injected power is shared fairly among the converters. The theoretical results are validated by simulations, where the affect of communication time-delays is also studied.

Voltage stability in a long-distance power transmission system impacted by the geoelectric field due to a geomagnetic disturbance

The voltage stability problem in long-distance transmission systems can be exacerbated by geomagnetic disturbance (GMD) even in middle and low latitude areas where the effects of GMDs are considered to be mild compared to auroral areas. With the high voltage and the long-distance transmission lines, power system in China has to face the voltage instability risk. To clarify and measure the risk from GMD represented by geoelectric field, method for analysis of relationship between voltage stability of the long-distance transmission system and the size and direction of geoelectric field is provided. On the basis of calculation for geomagnetically induced currents (GICs) of power system and the additional reactive power losses of transformer due to GIC, the model of long-distance transmission line affected by geoelectric field is established. To measure the impact on the voltage stability of power system and the sensitivity of voltage to geoelectric field, the voltage stability index and the set of voltage limit violation nodes are proposed and calculated considering different geoelectric field and different initial operation conditions of power system. By taking the Northwest 750 kV power system in China as an example, voltage stability with geoelectric field magnitude from 1 V/km to 10 V/km, direction (0°) from north to south (180°) is analyzed and the voltage stability index is calculated, and the set of voltage limit violation nodes are summarized as well. The results show that the method is feasible and the index can reflect actually the relationship between the long-distance transmission system voltage stability and the geoelectric field, and the set of the voltage limit violation nodes can indicate which nodes are most susceptible to GMD.

Prospects for novel deformation processed Al/Ca composite conductors for overhead high voltage direct current (HVDC) power transmission

In the quest for more efficient long-distance power transmission, High Voltage Direct Current (HVDC) transmission offers a reliable, cost-effective alternative to HVAC for long distance overhead transmission. Development of stronger, more conductive, and lighter conductor materials could lower HVDC construction costs, reduce energy losses, and increase reliability. In this paper, we describe a new deformation processed Al/Ca composite conductor now in development that has the potential to offer substantially lower density, higher strength, and higher conductivity for HVDC than today's conductor materials. This Al/Ca composite has shown promising early corrosion and elevated temperature performance test results, but additional research is needed to determine its fatigue and creep strengths, as well as its susceptibility to electrotransport phenomena and other factors. The design process, production methods, and physical properties of the new composite conductor are discussed. An economic and thermal analysis of this conductor material is also presented.

The Ionized Fields and the Ion Current on a Human Model Under $\pm$800-kV HVDC Transmission Lines

The ultra-high-voltage direct-current (UHVDC) system has many advantages in long-distance power transmission. This paper is aimed at analyzing the distributions of the ionized fields and the ion current on the human model under ±800-kV UHVDC transmission lines; therefore, a numerical method which combines the 3-D charge simulation method and Deutsch's assumption is used. And the validity of the presented method is demonstrated through a test in the laboratory. For ±800-kV UHVDC transmission lines, the ionized fields and the ion current on the human model are calculated, and numerical results show that the ionized fields and the ion current density on the top of the human model increase evidently when the human model stands under ±800-kV UHVDC transmission lines. And the total ion current flowing on the human model is far lower than the permitted limit.

Numerical Simulation Analysis of High-Voltage Transmission Tower

With the rapid development of Chinese economy construction, hv transmission line has become the main channel for long-distance power transmission and also the most basic conveyance systems and infrastructure for industrial and mining enterprises. The major coal mine fields in China have hv transmission lines passing above. Coal mining subsidence has an important influence on the line tower. So it is important to study the High-voltage transmission line tower to ensure its safety. In this paper, the model is a High-voltage transmission tower. The mechanical analyses for the tower under different boundary conditions are made by finite element simulation. The results can be used as reference for the design of high-voltage transmission line tower in mining area.

The Unwinding of a Transformer

The usefulness of passive radio-frequency identification (RFID) and general passive sensor technology is currently limited by relatively low field strengths due to regulations, giving a notoriously short reader-to-tag link range. The energizing and thus communication strength from receiver to detector depends on the amount of dc voltage that a chip can rectify from an interrogating electromagnetic field. Voltage-amplification/rectification methods employ multi-stage charge pumps, using nonlinear circuit components that incur significant losses, with diminishing returns for increasing stages. In RFID/sensor technology, where field strength is limited by regulations, technology, or distance, there is an opportunity for an alternate approach to RF-generated dc voltage. The proposed solution uses the Ferranti effect, where, for certain lengths and distributed parameter values, the voltage amplitude at the receiving end of a transmission line is greater than that of the sending end. As the Ferranti effect is typically an issue in high-voltage, low-frequency, long-distance power transmission, this problem poses significant design puzzles at RFID frequencies, transmission-line lengths, and voltage scales. By extending the formalism of the relevant transmission-line theory to the domain of common microstrip structures, microstrip transmission lines have been investigated and tested to demonstrate the effect. Because significant voltage gains (between two and five times the input amplitude) have been observed at a number of frequencies in the practical or RFID application range, it is concluded that the Ferranti effect is indeed a viable means of voltage amplification for RFID and related passive sensor applications.

Energy transmission modes based on Tabu search and particle swarm hybrid optimization algorithm

In China, economic centers are far from energy storage bases, so it is significant to select a proper energy transferring mode to improve the efficiency of energy usage. To solve this problem, an optimal allocation model based on energy transfer mode was proposed after objective function for optimizing energy using efficiency was established, and then, a new Tabu search and particle swarm hybrid optimizing algorithm was proposed to find solutions. While actual data of energy demand and distribution in China were selected for analysis, the economic critical value in comparison between the long-distance coal transfer and electric power transmission was gained. Based on the above discussion, some proposals were put forward for optimal allocation of energy transfer modes in China. By comparing other three traditional methods that are based on regional price differences, freight rates and annual cost with the proposed method, the result indicates that the economic efficiency of the energy transfer can be enhanced by 3.14%, 5.78% and 6.01%, respectively.

Modeling cascading failures in the North American power grid

The North American power grid is one of the most complex technological networks, and its interconnectivity allows both for long-distance power transmission and for the propagation of disturbances. We model the power grid using its actual topology and plausible assumptions about the load and overload of transmission substations. Our results indicate that the loss of a single substation can result in up to $25\%$ loss of transmission efficiency by triggering an overload cascade in the network. The actual transmission loss depends on the overload tolerance of the network and the connectivity of the failed substation. We systematically study the damage inflicted by the loss of single nodes, and find three universal behaviors, suggesting that $40\%$ of the transmission substations lead to cascading failures when disrupted. While the loss of a single node can inflict substantial damage, subsequent removals have only incremental effects, in agreement with the topological resilience to less than $1\%$ node loss.

Gas insulated transmission lines

Gas-insulated transmission lines (GIL) are a further development of the tubular conductor intended for long distances and are especially suitable for connecting conurbations to centers of power supply. They are also as invisible as ordinary power cables as well as having a number of other advantages. The first mixed-gas-insulated power transmission line in the world successfully completed its field trials with an endurance test in early 1999. The most important aim in the further development of tubular conductors for long-distance power transmission was to reduce costs so that they can be used economically over distances of up to several kilometers. This has now been achieved. The system costs of the gas-insulated transmission lines developed by Siemens are now only a factor of 8 or 10 above those of overhead power lines, down from a factor of 30. Now the GIL has reached the order of magnitude to be accepted by utilities if an overhead line must be brought underground. The basis of this reduction in costs was: adaptation of installation techniques similar to those used in laying pipelines; simplification and standardization of individual components; and use of a sulfur hexafluoride (SF/sub 6/) and nitrogen (N/sub 2/) gas mixture. In the past, pure SF/sub 6/ has been the most widely used gas for tubular conductors, most of which have been incorporated into substations.

Development of 500-kV XLPE cables and accessories for long-distance underground transmission lines. IV. Electrical properties of 500-kV extrusion molded joints

Joints for 500-kV XLPE cable are now under development in Japan to enable the long-distance transmission of high-capacity power to urban areas. Basic research was carried out from 1989 to 1992 that determined joint type and design values. This paper reports on the electric performance of the fabricated joints. Various tests were conducted to evaluate the performance and the design values. As a result of this evaluation it was found that it was possible to realize joints for 500-kV XLPE cable. Field tests are conducted as the final stage to confirm the long-term performance of the system.

The evolution of the synchronous machine

The beginnings of the synchronous machine are to be found in the closing decades of the last century. Since 1891, when the first long-distance transmission of high-voltage three-phase power was demonstrated at the International Exposition at Frankfurt, Germany, using a machine with an effective power output of 210 kW, the synchronous machine has undergone continuous development. Turbogenerators are now built with ratings of over 1700 MVA.

Improvement in reliability of extrusion type molded joint for 275 kV XLPE cable

In the construction of a 275 kV long-distance power transmission line, the reliability of the extrusion-type molded joint (EMJ) is of utmost importance. To improve the reliability of EMJs, a study was undertaken to establish the control value and margin by computer simulation; verify the quality tolerance, variability, and control margin by ascertaining the characteristics of numerous samples; and confirm the properties of EMJs assembled under simulated typical trouble conditions obtained by determining assumed trouble patterns, through failure mode analysis. It was found that improved quality control measures based on the quality control data obtained during assembly permitted jointing work to be performed with greater reliability. >

Fundamental analysis of dynamic stability in superconductive power systems

The features of instability in a long-distance bulk power transmission model system with superconductive power transmission lines or with superconductive synchronous generators are analyzed numerically. It is shown that self-excited oscillation is easily induced in superconductive AC power transmission systems. Torsional oscillation and electromechanical power swing in a model power system with a superconductive turbine generator power plant are analyzed for various system conditions, and it is confirmed in a digital simulation study that adequate energy control of the field winding of a superconductive generator can stabilize the power system.

Representation of series capacitors in electric power system stability studies

Series capacitor compensation applied to long-distance power transmission systems is addressed. Series capacitors reduce the effective transmission line inductive reactance, resulting in a larger capacity for stable power transfer. It is important to mention the effect of series capacitors on increased short-circuit current levels, as compared with the corresponding currents for noncompensated systems. These currents may reach very high values, especially for faults electrically near capacitor bank terminals, depending on their location, on network electric characteristics, and also on how series compensation is distributed. The authors analyze the representation of bypassed capacitors in stability studies of one-phase-to-ground short circuit, during and after short-circuit occurrence. The short-circuit and bypass periods are also discussed, leading to the adoption of a conservative modeling. >

Power transmission along biological membranes

Hypothesis on long-distance power transmission along extended energy-transducing membranes (Skulachev, 1969, 1971, 1980), has been experimentally proven in four different systems, namely, (i) trichomes of filamentous cyanobacterium Phormidium uncinatum; (ii) filamentous mitochondria and mitochondrial network in fibroblasts; (iii) clusters of roundish heart muscle mitochondria interconnected with mitochondrial junctions; (iv) mixed animal cell cultures interconnected with gap junctions. In all cases, energy was shown to be transmitted in the form of a transmembrane electric potential difference. The transmission occurred for distances as long as several tens of micrometers. Since the (a) delta-muH-bearing cytoplasmic membrane of cyanobacteria and the inner mitochondrial membrane and (b) delta-muNa-bearing outer animal cell membrane were found to be competent in such an effect, one may assume that the power transmission is a fundamental function of extended membrane systems. This mechanism can be used at the intracellular level (mitochondrial) as well as at the supracellular level (cytoplasmic and outer cell membranes). Studies on the possible involvement of membranes in lateral transport of oxygen, ions, fatty acids and membrane proteins seem to hold good promise.

Co-operative research into the engineering and design of a new digital directional comparison scheme

The paper reviews the salient features of a major co-operative research, development and site trial validation programme which was initiated in 1980 with the primary objective of accomplishing a major advance in power transmission system protection. Details of the project are given, together with the research/development methodology found necessary to transform a new approach to directional comparison protection into a next-generation product to meet stringent operational design objectives. The basic principles of the new scheme are outlined, together with details of the early computer-aided design studies which formed the basis of first prototype digital measuring equipments. Details are also given of the site-trial tests which were performed on the British 400 kV supergrid system, together with the results of investigations into the performance in long-distance power transmission applications. The paper concludes with brief details of preproduction prototype equipment and its performance.

Controllable Static Reactive-Power Compensators in Electrical Supply System

Long-distance transmission of electrical power through extra-high voltage overhead lines and power distribution through cable system call for controllable reactive-power compensating circuits on an increasing scale. The transmission losses are thereby reduced, voltage deviations kept within permissible limits and the system stability is improved The achievements in the field of thyristor techniques made it possible to build steplessly controllable compensating reactors which are combined with a three-phase thyristor power controller. Selection of optimum voltages, current and reactor reactance of the static compensator is dealt with and the current and voltage ratings of the thyristors are explained. The problem of harmonics, the influence of unbalanced conditions, and the protective and monitoring arrangements are also referred to.

A study of short-time ac flashover voltage of contaminated insulators and a consideration of its application to transmission line insulation design

Insulator contamination is the decisive factor in making insulation design of most transmission lines in Japan. This paper describes the performance of contaminated insulators against a short-time ac overvoltage in the order of several cycles, taking into consideration comparatively higher overvoltage due to the one-phase-to-ground fault of recent long-distance bulk power transmission lines with series capacitors. It has been made clear by this study that, in the case of a fault clearance within 0.07 sec, e.g., the flashover voltage increases by about 15% more than in a continuous energization. This implies that the insulation of series-capacitor-compensated lines can be met by the conventional insulation design levels.

The present state of R & D for superconducting transmission in Japan

Abstract The advantages of superconducting cables for large-scale, long-distance power transmission have been widely recognised. However, there is a need for much costly research into the scientific and engineering problems before superconducting cables can be incorporated in the power network. This paper outlines the current state of R & D in Japan on superconducting transmission. The immediate aim is a 10 GW cable that can handle 500 kV ac or ± 200 kV dc.