New approach to distance relays with quadrilateral polar characteristic for e.h.v.-line protection

A new technique is presented using principles of multisignal relaying for the synthesis of a universal-type quadrilateral polar characteristic. The modus operandi consists in the determination of the phase sequence of a set of voltage phasors and the provision of a trip signal for one sequence while blocking for the other. Two versions, one using ferrite-core logic and another using transistor logic, are described in detail. The former version has the merit of simplicity and has the added advantage of not requiring any d.c. supply. The unit is flexible, as it permits independent control of the characteristic along the resistance and reactance axis through suitable adjustments of replica impedance angles. The maximum operating time is about 20ms for all switching angles, and with faults within 95% of the protected section. The maximum transient overreach is about 8%.

Analysis of Natural Half-Wave-Length Power Transmission Lines

This paper provides additional theoretical information on half-wave-length power transmission. The analysis is rendered more general by consideration of a natural half-wave line instead of a short line tuned to half-wave. The effects of line loading and its power factor on the voltage and current profiles of the line and ganerator excitation have been included. Some of the operating problems such as charging of the line and synchronization of the half-wave system are also discussed. The inevitability of power-frequency overvoltages during faults is established. Stability studies have indicated that the use of switching stations is not beneficial. Typical swing curves are also presented.

Developments in solid-state under-frequency relays

Simpler circuits for frequency-sensitive relays responding to change and rate of change of system frequency have been developed employing phase-locked loops. A new relay responding to time intergral of the fall in system frequency has also been developed and its performance has been compared with those responding to change and rate of change of system frequency. The relays have been tested and calibrated with the help of a specially designed calibration kit.

Effectiveness of Series Capacitors in Long Distance Transmission Lines

The effectiveness of series capacitors used with long distance transmission lines in improving system stability is analyzed. Compensation efficiency is defined as the effectiveness of series capacitors. The influence of various factors on compensation efficiency such as capacitor location, line length, and degree of series compensation is investigated. Proper use of shunt reactors with series capacitors, in addition to limiting power frequency over- voltages, increases the maximum power transfer. Analytical expressions are included to aid in the calculation of compensation efficiency for a few typical cases. Curves are also presented indicating the critical value of shunt Mvar required for various degrees of series compensation and line lengths.

The radiation resistance of off-set series slots in microstrip line

The radiation resistance of off-set series slots has been calculated for microstrip lines using the method proposed by Breithaupt for strip lines. A suitable transformation is made to allow for the difference in structure. Curves relating the slot resistance to the microstrip length, width and off-set distance have been obtained. Microstrip slot antenna arrays are becoming important in applications where size and weight are of significance. The radiation resistance is a very significant parameter is the design of such arrays. Oliner first calculated the radiation conductance of centered series slots in strip transmission lines and that analysis was extended by Breithaupt to the off-set series slots in stripline. The radiation resistance of off-set series slots in microstrip lines is calculated in this paper and data are obtained for different slot lengths, slot widths and off-set values. An example of the use of these data in array antenna design in shown.

Aids in the design of microstrip components I: Getsinger's chart

Charts relating the capacitance to the width, spacing, thickness and height above the ground plane of coupled microstrips have been obtained. These are used to design hairpin line and hybrid hairpin line filters as well as multiplexers using microstrip comb line filters. The experimental results agree reasonably well with the design specifications. Getsinger's original charts for parallel coupled bars between parallel plates have been formulated for the microstrip case. Corresponding charts relating the capacitances to the width, spacing, thickness and height above the ground plane of coupled microstrips have been obtained. Examples of the use of these charts are shown in the design of hairpin lines and hybrid hairpin line filters as well as multiplexers using comb line filters. The hairpin line/hybrid hairpin line filters were designed to operate at a central frequency of 9÷5 GHz with 11 per cent bandwidth and 0÷5 dB ripple. The three filters constituting the comb line filters have center frequencies of 2÷4, 3÷0 and 3÷6 GHz. The components so designed were fabricated and tested. The dielectric used for the microstrip was teflon. Experimental curves for the attenuation (insertion loss) and VSWR are given. The design specifications arc satisfied quite well.

Approximate synthesis formulas for microstrip line with aperture in ground plane

Approximate closed-form expressions for the propagation characteristics of a microstrip line with a symmetrical aperture in its ground plane are reported in this article. Well-known expressions for the characteristic impedance of a regular microstrip line have been modified to incorporate the effect of this aperture. The accuracy of these expressions for various values of substrate thickness, permittivity and line width has been studied in detail by fullwave simulations. This has been further verified by measurements. These expressions are easier to compute and find immense use in the design of broadband filters, tight couplers, power dividers, transformers, delay lines, and matching circuits. A broadband filter with aperture in ground plane is demonstrated in this article. (c) 2011 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2012.

BER Analysis of Uplink OFDMA in the Presence of Carrier Frequency and Timing Offsets on Rician Fading Channels

In orthogonal frequency-division multiple access (OFDMA) on the uplink, the carrier frequency offsets (CFOs) and/or timing offsets (TOs) of other users with respect to a desired user can cause multiuser interference (MUI). Analytically evaluating the effect of these CFO/TO-induced MUI on the bit error rate (BER) performance is of interest. In this paper, we analyze the BER performance of uplink OFDMA in the presence of CFOs and TOs on Rician fading channels. A multicluster multipath channel model that is typical in indoor/ultrawideband and underwater acoustic channels is considered. Analytical BER expressions that quantify the degradation in BER due to the combined effect of both CFOs and TOs in uplink OFDMA with M-state quadrature amplitude modulation (QAM) are derived. Analytical and simulation BER results are shown to match very well. The derived BER expressions are shown to accurately quantify the performance degradation due to nonzero CFOs and TOs, which can serve as a useful tool in OFDMA system design.

Estimation of Series Capacitance of a Transformer Winding Based on Frequency-Response Data: An Indirect Measurement Approach

A novel procedure to determine the series capacitance of a transformer winding, based on frequency-response measurements, is reported. It is based on converting the measured driving-point impedance magnitude response into a rational function and thereafter exploiting the ratio of a specific coefficient in the numerator and denominator polynomial, which leads to the direct estimation of series capacitance. The theoretical formulations are derived for a mutually coupled ladder-network model, followed by sample calculations. The results obtained are accurate and its feasibility is demonstrated by experiments on model-coil and on actual, single, isolated transformer windings (layered, continuous disc, and interleaved disc). The authors believe that the proposed method is the closest one can get to indirectly measuring series capacitance.

Passive vibration isolation of reaction wheel disturbances using a low frequency flexible space platform

Reaction wheel assemblies (RWAs) are momentum exchange devices used in fine pointing control of spacecrafts. Even though the spinning rotor of the reaction wheel is precisely balanced to minimize emitted vibration due to static and dynamic imbalances, precision instrument payloads placed in the neighborhood can always be severely impacted by residual vibration forces emitted by reaction wheel assemblies. The reduction of the vibration level at sensitive payloads can be achieved by placing the RWA on appropriate mountings. A low frequency flexible space platform consisting of folded continuous beams has been designed to serve as a mount for isolating a disturbance source in precision payloads equipped spacecrafts. Analytical and experimental investigations have been carried out to test the usefulness of the low frequency flexible platform as a vibration isolator for RWAs. Measurements and tests have been conducted at varying wheel speeds, to quantify and characterize the amount of isolation obtained from the reaction wheel generated vibration. These tests are further extended to other variants of similar design in order to bring out the best isolation for given disturbance loads. Both time and frequency domain analysis of test data show that the flexible beam platform as a mount for reaction wheels is quite effective and can be used in spacecrafts for passive vibration control. (C) 2011 Elsevier Ltd. All rights reserved.

Frequency response analysis of layered ground in Ahmedabad during Bhuj earthquake

This paper presents the results of seismic response analysis of layered ground in Ahmedabad City during the earthquake in Bhuj on 26(th) January 2001. An attempt has been made to understand the reasons for the failure of multistoreyed buildings founded on soft alluvial deposits in Ahmedabad. Standard Penetration test at a site very close to the Sabarmati river belt was carried out for geotechnical investigations. The program SHAKE91, widely used in the field of earthquake engineering for computing the seismic response of horizontally layered soil deposits, was used to analyse the soil profile at the selected site considering the ground as one dimensional layered elastic system. The ground accelerations recorded at the ground floor of the Regional Passport Staff Quarters building, which is very close to the investigated site, was used as input motion. Also, Finite Element Analysis was carried out for different configurations of multistorey building frames for evaluating their natural frequencies and is compared with the predominant frequency of the layered soil system. The results reveal that the varying degree of damage to multistorey buildings in the close proximity of Sabarmati river area was essentially due to the large amplification of the ground and the near resonance condition.

Adaptive window zero-crossing-based instantaneous frequency estimation

We address the problem of estimating instantaneous frequency (IF) of a real-valued constant amplitude time-varying sinusoid. Estimation of polynomial IF is formulated using the zero-crossings of the signal. We propose an algorithm to estimate nonpolynomial IF by local approximation using a low-order polynomial, over a short segment of the signal. This involves the choice of window length to minimize the mean square error (MSE). The optimal window length found by directly minimizing the MSE is a function of the higher-order derivatives of the IF which are not available a priori. However, an optimum solution is formulated using an adaptive window technique based on the concept of intersection of confidence intervals. The adaptive algorithm enables minimum MSE-IF (MMSE-IF) estimation without requiring a priori information about the IF. Simulation results show that the adaptive window zero-crossing-based IF estimation method is superior to fixed window methods and is also better than adaptive spectrogram and adaptive Wigner-Ville distribution (WVD)-based IF estimators for different signal-to-noise ratio (SNR).

Beat frequency oscillations in the output voltage of high voltage DC power supplies under pulsed loading

High voltage power supplies for radar applications are investigated, which are subjected to pulsed load (125 kHz and 10% duty cycle) with stringent specifications (<0.01% regulation, efficiency>85%, droop<0.5 V/micro-sec.). As good regulation and stable operation requires the converter to be switched at much higher frequency than the pulse load frequency, transformer poses serious problems of insulation failure and higher losses. Few converter topologies are proposed to tackle these problems. A study is made regarding the beat frequency oscillations that may exist with pulsed loading. It is illustrated with respect to the proposed converter topologies. Methods are proposed to eliminate or minimize these oscillations.

Effect of power line filter on MOV clamping voltage

Metal oxide varistors (MOV) are popularly used to protect offline electronic equipment against power line transients. The offline switched mode power supplies (SMPS) use power line filters and MOVs in the front-end. The power line filter is used to reduce the conducted noise emission into the power line and the MOVs connected before this line filter and the MOVs connected before this line filter to clamp line transients to safer levels thereby protecting the SMPS. Because of the presence of 'X' capacitors at the input of line filter the MOV clamping voltage is increased. This paper presents one such case and gives theoretical and experimental results. An approximate method to predetermine the magnitude of such clamping voltages is also presented.