On the definition of power factor and apparent power in unbalancedpolyphase circuits with sinusoidal voltage and currents

Definition for the apparent power S and for the power factor PF in unbalanced polyphase circuits with sinusoidal waveforms are presented. It is proved that the definition S²=(V_a²+V _b²+V_c²) (I _a²+I_b²+I _c²) has a definite physical meaning, leading itself to a convenient resolution in positive, negative and zero sequence, nonactive and active power. It is suggested that the power factor be represented with the help of the ratio P⁺/ S, where P⁺ is the positive sequence active powers     

Contribution to the discussion of `1987 ampacitytables-demystifying the myths'

The author discusses J. Nasilowski's remarks (see ibid., vol.26, no.4, p.605, 1990) on a paper by D.W. Zipse (see ibid., vol.25, no.5, pp.910-17, 1989). Nasilowski claimed that in M=(hS/R)^1/2, where M is the current coefficient of heating, h is the heat transfer coefficient, including both convection and radiation, S is the surface area of the conductor, and R is the resistance, M is constant for a specific material for a range of currents and for temperature rises up to about 60 K. The author argues that M is approximately constant only for a specific conductor and for restricted ranges of temperature rise and ambient temperature. In his reply, Nasilowski stresses the utility of the assumption that M =constant for calculations of steady state temperature rise of the conductors in a quiet air     

Destruction of living cells by pulsed high-voltage application

Destruction of living cells in liquid has been formed by pulsed high-voltage application to the liquid. S. cerevsaie (yeast cell) or Bacillus natto, dispersed in deionized water and one- and three-percent NaCl solution, were used in this experiment. Four different electrodes (plate-plate, needle-plate wire-cylinder, and rod-rod electrode) were tested. The survival rate of cells was measured against peak electric field E_p pulsewidth T _W, and pulse application number N. The experimental results indicate that the survivability roughly follows Weilbull distribution. Yeast cells dispersed in deionized water could be almost completely destroyed when the wire-cylinder electrode was used with E_p=20 kV/cm, T_W=100 μs, and N=200. The energy input to a unit volume of the liquid to complete the cell destruction, however, differed significantly with the electrode type. The wire-cylinder electrode required above 10-30 cal/cm ³ to destroy the yeast cell in deionized water to 10^-6 survivability. This value was less than that required more than 70 cal/cm³. Using the rod-rod electrode contained in a pressure vessel, an arc discharge was generated to produce an intensive shock wave, which also destroyed the cells by its mechanical force. In this case, 5-10 cal/cm³ of energy was required to decrease the survivability of yeast cells in deionized water to 10^-6. Though further studies are necessary, this results indicate a possibility of the cell destruction by pulsed high voltage to be used as an energy-efficient sterilization process     

Mode ordering algorithms for sparse vector method improvement

Three node-ordering algorithms that enhance sparse vector methods without losing sparse matrix method efficiency are presented. The performance of the algorithms is compared with that of the minimum-degree algorithm for eight test systems up to 661 nodes. For the factorization of a sparse matrix into U^tU, the new algorithms give a similar or even better sparsity for U, while clearly improving the sparsity of U^-1      

A W-matrix methodology for solving sparse networkequations on multiprocessor computers

The authors describe a methodology for solving efficiently sparse network equations on multiprocessor computers. The methodology is based on the matrix inverse factors (W-matrix) approach to the direct solution phase of Ax=b systems. A partitioning scheme of the W-matrix, based on the leaf-nodes of the factorization path tree, is proposed. The methodology allows the performance of all the updating operations on vector b in parallel, within each partition, using a row-oriented processing. The approach takes advantage of the processing power of the individual processors. Performance results are presented and discussed     

Theory for finite-phase traveling-wave boundary-guided transport oftriboelectrified particles

Numerical simulations are used to picture synchronous and asynchronous domains of traveling-wave pumping of charge-conserving particles (having mass m, charge q, radius a, and mobility b) in terms of the dimensionless frequency Ω=(ω/k)/(bE₀), mass M=(m/q) (kb) (bE₀), and gravitational acceleration G=mg/qE₀, where k and ω are the wavenumber and angular frequency of the imposed wave and E₀=kV, where V is the peak voltage. The effects of having a finite number of phases consisting of discrete electrodes covered by a semi-insulating layer are highlighted. The time-average velocity in the direction of wave travel is found to be synchronous (have velocity ω/k) for 0<Ω<Ω*<1, where Ω* is reduced by having finite phases. Because the discrete electrodes result in hops of higher magnitude, they tend to result in a conversion to asynchronous hopping and `certain' modes at a lower frequency than with a sinusoidal wave. At low M, they can also result in the stalling of particle pumping as the frequency is raised. Predicted effects of image forces, dielectric layer thickness, bulk conductivity, and surface conductivity as well as particle sticking and slipping are discussed     

A reformulation of the Nyquist criterion for discrete systems

The Nyquist criterion is a useful method in checking the closed-loop stability of linear systems. However, in its existing formulation, it is cumbersome to apply when the open-loop system has poles on the unit cycle of the z plane. An alternative formulation of the Nyquist criterion that is easier to apply than the existing one is presented. Instead of having to determine the images of the detours around the unit circle poles and then determine the number of encirclements about the (-1,0) point in the open-loop transfer function G(z) plane as required by the usual Nyquist criterion, the angle swept by the vector pointing from the (-1,0) point to the polar plot of G(z) is checked in order to determine the closed-loop stability     

A stator flux-oriented voltage source variable-speed drive based onDC link measurement

The authors present a new control strategy for variable speed drives, which is aimed at improving or even replacing existing volts per hertz (V/f) open loop variable-speed drives. This strategy uses only the DC link current and voltage, which are readily available in a V/f drive for protection purposes, to implement closed loop flux and torque control. Stator flux and electromagnetic torque feedback signals are derived from the DC link voltage, the DC link current, and inverter switching states. Digital implementation is seen to be possible with slight modification on existing V/f drives. Practical considerations in implementing such a system are emphasized, and experimental results are presented to show the feasibility of the scheme     

A performance comparison of full- and half-bridge series resonantinverters in high-frequency high-power applications

Load-commutated full- and half-bridge series resonant inverters are dual at lower operating frequencies where the di/dt inductance L_s has negligible effect on their performance. At higher frequencies, however, the effect of inductor L_s dominates the ratings of components, and their performance is significantly different. The difference in their performance is a function of the operating frequency and is more pronounced at higher frequencies. To study the behavior of inverters at higher frequencies, a detailed analysis is presented which includes the effect of the di/dt inductance. Finally, the performance of two inverters is compared in terms of inverter kVA rating, and the kVA rating of the compensating capacitor     

Powers in nonsinusoidal situations-a review of definitions andphysical meaning

The author expands on the physical meaning of the reactive power in nonsinusoidal situations. The sinusoidal waveform case is surveyed, viewing the active current as a component of identical waveform with the voltage. This approach, when extended to nonsinusoidal waveforms, supports Fryze's model for apparent power, S²=p ²+Q²_F. It is proved that the total reactive power Q_F is composed from four distinctive types of elementary reactive powers. Each of the basic reactive powers is identified as the amplitude of an oscillation of instantaneous power. The separation of Q_F in Q ₁, the reactive power at the system frequency, and in Q_H, the reactive power at harmonic frequencies, is recommended as an effective mean for monitoring filter efficacy and power-factor compensation. Two major recommendations are supported by the results of this study: abolish the power model using distortion power, and measure the active power of the system frequency separately from the active power of the harmonics     

Investigation of ramped air arcs near current zero in dual-flownozzle system

Ramped air arcs with a dI/dt= 6 A/μs were investigated in dual-flow nozzle system. The nozzle pressure ratio was varied to study the arc under different flow Mach number gradients. The dI/dt, dV/dt, and postzero resistance and current measurements were taken for an electrode separation of 5.5 cm and a nozzle gap spacing of 1.23 cm. The adverse shock wave effects were quantitatively observed for nozzle pressure ratios with separated flow in the nozzle. The arc power, which is equal to the total radiation losses and heat transferred from the plasma, was determined near current zero for subsonic and supersonic flow fields. It was shown that the nozzle pressure ratio is a very important parameter in the circuit breaker performance. A comparison of this work with Brown Boveri data was made. It was concluded that the convection stabilized ramped arcs near current zero are laminar and become unstable in the dielectric regime     

A heuristic and algorithmic approach to VAR planning

The authors present a knowledge- and algorithm-based approach to voltampere-reactive (VAr) planning in a transmission system. The VAr planning problem involves the determination of locations and sizes of new compensators considering contingencies and voltage collapse problems in a power system. An expert system model analyzes the operating conditions of a power system and suggests one or more of the P-V, Q-V and S-V (S denotes the system complex power) curves for use in assessing the voltage collapse problem. A second expert system module suggests control actions with the existing VAr controllers and sizes and locations for the installation of new compensators. The IEEE 30-bus system was used to demonstrate the capability of the proposed approach     

The onset of electrothermal convection in nonpolar liquids on thebasis of a dissociation-injection conductivity model

The onset of convection for a layer of nonpolar liquid subjected to an adverse thermal gradient and intense electric field is examined. The electrical conductivity is determined via a recent physiochemical model based on dissociation and injection charge generation. Convective instabilities and overstabilities are predicted as a function of strength of injection C and residual conduction C₀. Physical and mathematical arguments show that for values of C bigger (smaller) than 2√2C₀ , the regime is approximately injection dominated (conduction dominated). It is also shown that dissociation always plays a stabilizing role, which is contrary to the injection which can have either a stabilizing or a destabilizing effect due to its indirect coupling with thermal perturbations     

Influence of an insulating spacer on the V-tcharacteristics of a coaxial gas-insulated cable. II. Tests on EHV buses

For pt.I see ibid., vol.3, no.1, p.16-25 (1988). The V- t characteristics of EHV buses were evaluated for two configurations with rated voltages of 550 and 800 kV. The bus dimensions were 6 m in length and 17.8/53.3 cm and 20.3-61 cm in diameter, respectively. They were each fitted with two tripod-type epoxy spacers. The results show a pronounced influence of the supporting spacers on the V-t characteristic, corresponding to a reduced withstand voltage and less dispersion in the measured breakdown voltages. A statistical method is presented for deriving the V- t characteristics of a gas-insulated system from those of the individual components     

Basic mechanisms of generation and flow of harmonic signals inbalanced and unbalanced three-phase power systems

The basic mechanisms of generation and flow of harmonic signals in electric power networks are discussed. With regard to generation, the unbalanced operating condition of a six-pulse converter is studied. Guidelines which are applicable in the general case are obtained. With regard to flow of harmonic signals, a new relationship between distortion voltamperes and total harmonic distortion is derived. This relationship is very similar to the P/δ and Q/| V| relations which are well known in electric power flow studies     

Teaching electromagnetism in terms of the potentials instead of the`Maxwell' equations

A questionnaire circulated in Britain has documented the decreasing time spent there in teaching electromagnetic theory, and underlines the need for a reappraisal of what is taught. The advantages of a change of approach are argued. The field is defined by the electrical potential φ and magnetic vector A as computed in most numerical field software packages used as teaching aids, and E and B are viewed as no more than symbols denoting differentials. The energy and momentum are defined as properties of the charges, instead of the fields, and the usual Maxwell equations are not needed     

Potential distributions of metal oxide surge arresters undervarious environmental conditions

A semi-analytical approach is proposed for potential distribution of metal-oxide surge arresters under normal condition. Conventional approaches like the finite-element method are also applicable. The authors suggest that the proposed method is more appropriate for a general approach; an analysis in combination with lumped constants of R, L and C is possible. Environmental conditions like pollution of snow-covering on the surface of insulators may disturb the potential distribution at normal condition. Potential distributions were experimentally measured in an artificial pollution test, naturally polluted and snow-covering conditions, whose major characteristics are shown     

Fully digital, vector-controlled PWM VSI-fed AC drives with aninverter dead-time compensation strategy

A dead-time compensation method in vector-controlled pulse width modulator (PWM) voltage source inverters (VSIs) is proposed. The method is based on a feedforward approach that produces compensating signals obtained from the I_d-I_q current and inverter output angular frequency references in the rotating reference (d-q) frame. It provides excellent inverter output voltage distortion correction for both fundamental and harmonic components. The correction is not affected by the magnitude of the inverter output voltage or current distortions. Since this dead-time compensation method allows current loop calculations in the d- q frame at a slower sampling rate, with a conventional microprocessor than calculations in the stationary reference frame, a fully digital. vector-controlled speed regulator with just a component current control loop is realized for PWM VSIs. Simulations and test results obtained for the compensation method are also described     

The relationship between virtual time and quality factor forheavy-current test circuits

The use of deviations in virtual time for plotting the time-current characteristics and evaluating the thermal effect of the DC component of a fault current is discussed. The relationship between the virtual-time deviation, the initial EMF phase making angle and the Q factor is used as the basis of a newly developed method for evaluation of the Q factor. This method is particularly useful in the range of higher values of Q, where other methods are not as efficient. For example, the most common DC component damping method is very vulnerable to even small errors in the estimation of the value of this component. In the method presented here, the main contributing factor is that the evaluation of Q is based on measuring the constant component of the virtual-time deviation, which in turn is directly proportional to the time constant of the circuit. The only major shortcoming of the method is the need to maintain the test current until the quasi-steady state is reached     

High harmonic currents in fluorescent lamp circuits

Some formulae associated with the percentage of high harmonics in the current in fluorescent lamp fixtures are derived for the cases of L and LC ballasts. The third harmonic is analyzed most carefully, and the theoretically predicted values are compared with those found experimentally. The agreement appears to be quite satisfactory. To meet the requirement of a standard, the power factor of the system, which appears to be a nontrivial point, was taken into account