EHV Transmission Lines Research Articles

Estimation of lightning shielding failures and mid-span back-flashovers based on the performance of EHV double circuit transmission lines

In Japan, many unbalanced installation double-circuit EHV transmission lines were constructed to prevent simultaneous lightning faults on both the circuits. A one-line fault on the higher insulated circuit is either a shielding failure or a mid-span back-flashover. To discriminate shielding failures and mid-span back-flashovers among all the faults, the author analyzed the lightning performance of these unbalanced insulation lines. Both the unbalanced insulation lines and the balanced insulation lines are similar in the probability of causing a shielding failure and a midspan back-flashover, since both are the same in the configuration of shielding wires and phase conductors independent of the insulation mode. As a result of analysis, the author estimated that about 25/spl sim/40% of all the lightning faults on the double-circuit balanced insulation lines are shielding failures, and that few mid-span back-flashovers happen there.

A nonpilot phase selector based on superimposed components for protection of double circuit lines

This paper gives details of a new phase-selection scheme for simultaneous phase-ground faults on both circuits of double circuit EHV transmission lines. Fast phase selection (within half a cycle) can be achieved by using the information contained within the fault transients. The theory is presented along with results obtained through simulations using EMTP. The scheme enables single pole tripping to be employed for simultaneous faults. The proposed scheme does not require communication between the two ends of the line. Limitations due to the low bandwidth of CVTs have not caused problems and the method has shown immunity to system noise.

96/03167 - Determining EHV transmission line magnetic fields by applying the vector magnetic potential concept

96/03167 - Determining EHV transmission line magnetic fields by applying the vector magnetic potential concept

Artificial neural-network-based protection scheme for controllable series-compensated EHV transmission lines

Controllable series compensation (CSC) is one of the main flexible AC transmission systems (FACTS) devices, which have the ability to improve the utilisation of existing transmission systems. However, the implementation of this technology changes the apparent line impedance, which is controlled by the firing angle of thyristors, and is accentuated by other factors. This poses problems for conventional protection schemes. The paper proposes an adaptive protection scheme, based on neural networks, with special emphasis on the analysis of the zone-1 performance. The paper describes, in detail, the feature extraction, sampling rate, data window length and training of the designed artificial neural networks (ANNs). The main idea of the protection scheme is to employ an artificial neural network (ANN) to make a decision based on extracting useful features in the desired spectra within a certain frequency range under fault conditions. System simulation and test results are presented and analysed in this paper to indicate the feasibility of using an ANN-based protection scheme in CSC transmission systems.

Nonunit protection technique for EHV transmission systems based on fault-generated noise. Part 3: Engineering and HV laboratory testing

The paper describes the various stages involved in the prototype hardware development and implementation of a new protection technique for EHV transmission lines; the fundamental principles, CAD design and performance evaluation using simulated results were described in Parts 1 and 2. The paper includes a description of the experimental setup at the National Grid Company, High Voltage Laboratories, Leatherhead and illustrates the performance of the prototype hardware when subjected to fault-generated high frequency signals obtained under arcing faults on the experimental rig.

Non-unit protection technique for EHV transmission systems based on fault-generated noise. Part 2: signal processing

The paper describes the signal processing techniques developed for a new protection technique for EHV transmission lines. The signal processing unit, which forms a major part of the protection scheme, utilises fault-generated high-frequency signals derived from the signal measurement circuit to determine whether the fault is internal or external to the protected zone. The protection scheme has been designed using computer-aided techniques which closely emulate the hardware currently under development and which is being implemented using present-generation hardware.

EFFECTS OF VOLTAGE SENSITIVE LOADS ON THE VOLTAGE STABILITY OF COMPENSATED EHV-TRANSMISSION LINES

ABSTRACT In this paper, effects of voltage sensitive loads, namely, constant power, constant current and constant impedance type loads on the voltage collapse phenomenon of EHV transmission lines with different types of compensation schemes are studied and discussed. Exponential modelling is used for the three types of loads and exact long line equations are used in the analysis. It is seen that the power transfer capability of a long line for any type of compensation scheme is the highest for a constant impedance load, and the lowest for a constant power load. The power transfer capability for a constant current load has an intermediate value compared to those for constant impedance and constant power loads respectively. Further, the transformer tap control has an adverse effect on the voltage stability for constant current and constant impedance type loads. Improved power factor improves the voltage stability condition for all types of compensation schemes. In all compensation schemes, an increased degr...

Voltage collapse at the load end of a series compensated EHV transmission line

In this paper, the effect of the operating conditions and dynamic performance on the voltage collapse phenomenon of EHV lines with series compensation is discussed. Exponential modelling of loads and exact long line equations are used in the analysis. It is shown that when series compensation is provided, the changes of voltage collapse occuring are less. The effect of switched series compensation on the voltage collapse phenomenon is also investigated. The random loading conditions of the system and random load parameter variations are also considered for the first time in analysing the voltage collapse phenomenon.

Impact of HV and EHV transmission on generator protection

HV and EHV transmission lines impose hazards to generators connected to these lines that can be potentially more severe than similar hazards imposed to generators connected to lower voltage transmission lines. This paper reviews a number of these hazards and discusses some of the protection schemes and/or equipment techniques employed in the industry to detect and/or minimize the hazards.

The stochastic modeling of fault-induced transients

A study on the stochastic characterization of fault-induced transients in HV and EHV transmission lines is performed, considering time dependence for both variance and autocorrelation. The aim is the definition of noise models for the design and the evaluation of digital relaying algorithms. A number of realistic network structures are considered for 400 and 220 kV lines, including shunt reactors and capacitors. Frequency-dependent models for machine windings are applied and proved to be important for the long-term behavior of the noise. The capacity of plants and network sparsity near the sending bus are investigated and shown to have a major role in the transients magnitude. Analog prefiltering is also assessed on a stochastic basis. The sampled variance and autocorrelation provide the basis for general conclusions on the modal dependence of the noise on network structures and for state modeling. >

Development of a 500 kV transmission line arrester and its characteristics

A 275 kV gapped type arrester was developed and tested on an EHV transmission line which is liable to be extensively affected once a line fault occurs. The study was followed by the development of a 500 kV gapped type arrester, and its field test on an existing line was started in September 1990. The main characteristics of the 500 kV gapped type arrester are discussed, and evidence to corroborate its applicability to 500 kV transmission lines is presented. The series gap length was reduced by adopting an optimal electrode configuration and a reasonable concurrence probability of the arrester unit and a switching impulse, in order to design the practicable gapped type arrester for existing 500 kV transmission towers. It was demonstrated that the 500 kV gapped type arrester can be depended upon to cut off the follow current even when it is contaminated. Satisfactory insulation coordination with the existing insulator assemblies was demonstrated.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Theoretical concept and digital simulation of the Pramod scheme for UHS protection of EHV transmission lines

The theoretical concept of the Pramod scheme for ultra-high-speed (UHS) protection of EHV transmission lines is described. The principle is based on detection of the fault-induced high-frequency signals of the order of kilohertz. A technique for the digital simulation of the transmission line and the high-frequency detecting circuit is explained, and the results are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Digitally simulated test performance of the Pramod scheme for UHS protection of EHV transmission lines

The author describes a digital simulation technique for the signal processing of the Pramod scheme and studies the digitally simulated test performance of the scheme for satisfactory operation under different fault conditions. The Pramod scheme is based on the detection of high-frequency voltage signals through stack tuners. It is assumed that the digitally simulated stack tuner output signals are available for the purpose of the signal processing under different fault conditions. The Pramod scheme has been investigated for the discrimination feature of reverse fault, faults at different points of the transmission line, and different types of faults, and the performance has been found satisfactory.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Evaluation of lightning fault rate of EHV transmission line based on lightning parameters derived from electromagnetic observation

Cloud-to-ground lightning flashes in the Niigata area in Japan have been observed for a few years by a magnetic direction-finder network. Data obtained so far are analyzed carefully taking into account the operating characteristics of the system. As a result, frequency distributions of return-stroke current for each polarity are produced, and their seasonal variations and the occurrence probabilities of high current exceeding 200 kA are clarified. Based on the estimated current distributions, back-flashover rates of 500-kV 2 cct transmission lines are calculated. It is known that the back-flashover caused by downward flashes can account for most of the line outages in the summer for both single- and multiple-line faults. However, this mechanism cannot explain the high outage rate in the winter experienced in the winter thunderstorm area in the late 1970s. In spite of the low lightning flash density in the winter, the multiple-line fault rate in the winter caused by back-flashovers is predicted to be comparable to that in the summer, because of the seasonal variation of the frequency distributions of return-stroke current.

A reliable digital directional relay based on compensated voltage comparison for EHV transmission lines

A digital directional relay based on compensated voltage comparison has been implemented with a 16 bit microcontroller. The authors describe the digital implementation of the phase-angle comparators which make use of fundamental frequency components of the compensated voltages. A simple hybrid hardware structure and intelligent software routines have been developed to ensure high reliability. Using an adaptive sampling interval, the relay can maintain its overall performance even for faults that occur during power frequency disturbances. Dynamic tests on a 500 kV, 290 km transmission line analog model confirmed that the relay can provide correct operation for various faults during different operating conditions of the protected line.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Probabilistic analysis of subsynchronous resonance in series-compensated EHV transmission lines

The stability limit curves in the R-X c parameteric plane have been determined for EHV lines with series-compensation raking into consideration the nondeterministic nature of the generating system, mechanical system, and transmission system parameters. Studies have shown that the effect of random system parameters is to increase the area of the unstable zone in the parametric plane. First, the border line between the stable and unstable zones has been determined considering a deterministic model of the system. The value of the critical frequency at which the self-excited oscillations including the effect of hunting are set up and the system critical resistance necessary to damp out the oscillations are obtained using a modified D-partition technique. The modification involves the determination of the critical frequency by the Newton method. The nature of the random system parameters is then considered using the results of the deterministic study to calculate the variances of the critical frequency and resistance, and the stability limit curves.

Multistory transmission tower model for lightning surge analysis

A multistory transmission tower model to be used in multiconductor analysis by EMTP (Electromagnetic Transients Program) is proposed. The circuit parameters of the model are determined based on the measurement of voltages across the insulator strings on an actual 500 kV transmission tower. The multiconductor lightning surge analysis considered enables one to predict the waveform of each insulator voltage, which is useful in determining the phase and the instant of a back flashover. The new tower model is recommended in Japan to be used in the new multiconductor analysis of EHV and UHV class double circuit transmission lines. The new tower model and conventional tower models are compared in the analysis of minimum back flashover lightning currents at a UHV transmission line. The new model gives about 20% smaller current. >

Artificially triggered lightning experiments to an EHV transmission line

In a triggered-lightning experiment carried out on a mountain in Japan, thirteen strikes were obtained for the winter seasons of 1986 and 1987. The lightning occurred on the conductor and the tower of a 275 kV test transmission line, on the ground, etc. The authors outline the triggering method and the experimental results obtained from electrical and optical observations. Comparatively intense lightning with a current greater than 50 kA or a charge greater than 200 C was obtained. >

An Investigation into a Method of Detecting the Fault Induced High Frequency Voltage Signals of EHV Transmission lines for Protection Applications

An Investigation into a Method of Detecting the Fault Induced High Frequency Voltage Signals of EHV Transmission lines for Protection Applications

Performance of the Pramod Scheme for UHS Protection of EHV Transmission Lines Under Arcing Fault Conditions

Performance of the Pramod Scheme for UHS Protection of EHV Transmission Lines Under Arcing Fault Conditions