Influence of an insulating spacer on the V-tcharacteristics of a coaxial gas-insulated cable. I. Study on areduced-scale coaxial conductor

Two different spacer designs, a disc spacer and a composite-profile cone, were evaluated in a coaxial conductor 2.5/7 cm in diameter to study their influence on the V-t characteristic of the conductor when it is subjected to repeated applications of impulse voltages of constant waveshape and increasing magnitude. The results show that an insulating spacer can reduce the critical withstand voltage and yield smaller dispersion in the breakdown voltages. These effects can be minimized by adopting a design that favors breakdown in the gas rather than along the spacer interface     

Xerographic properties of a-Se:Te photoconductors

It is shown that the dark decay of the electrostatic surface potential on a corona-charged a-Se:Te alloy photoreceptor occurs via electric field-enhanced xerographic depletion discharge (FEXDD) in which Poole-Frenkel-assisted thermal emission of holes from deep mobility-gap states and their subsequent sweep out generates a negative bulk space charge. The theoretical model development is applied to explain the observed experimental dark discharge data over a wide range of charging (initial) voltages. It is shown that although the time required for the surface potential to decay to its half value t_1/2 initially increases with the charging voltage V₀ at the highest charging voltage, t_1/2 actually decreases with V₀. Results obtained from cycled-up xerographic experiments on single and double-layer photoreceptors are also reviewed and discussed in conjunction with transient photoconductivity experiments     

Single-stage AC electrostatic precipitation

Measurements are reported on the collection efficiency of a laboratory-scale single-stage electrostatic precipitator (3 cm plate spacing) excited by AC voltages (V_rms~8 kV, f =20→500 Hz) biased to give no time-average. With f=60 Hz, an effective migration velocity of 3.5 cm/s is observed. Previously published works on the charging and migration of particles in AC coronas and on the effects of finite turbulent diffusivity on the precipitation process are combined to predict the experimentally observed dependence of the efficiency on the magnitude and frequency of the applied voltage. When applied to the experimental situation reported here, the model reduces to one of complete mixing, with the effective particle migration velocity being predicted in terms of the velocity that is achieved with DC charging and a charging time constant that is either estimated or obtained from charging experiments     

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     

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 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     

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     

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     

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     

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     

Transformer modelling for distribution system studies. II. Additionof models to YBUS and ZBUS

For pt.I see ibid., vol.7, no.2, p.567-74 (1992). A distribution transformer modeling procedure is discussed which represents the distribution transformer with a minimum of input data for network, load, and fault studies thereby allowing the transformer to be routinely included as part of the distribution network. How these transformer methods are used in the formation of Y_BUS and Z _BUS system models is described     

Electrical breakdown of SF6 at small values of theproduct pd

The breakdown of SF₆ when static and impulse voltages are applied at small values (from 10^-4 bar-mm to 1 bar-mm) of the product pd was investigated. It was found that the prevailing breakdown mechanism up to 8×10^-3 bar-mm was the Townsend mechanism. Between 8×10^-3 and 5.5 bar-mm the Townsend and streamers mechanisms combined, whereas from 5.5 bar-mm on the streamers mechanism took over. The Paschen law was found to hold for the points lying to the right of the minimum when static voltage was applied, whereas edge-type breakdown occurred on the points lying to the left of the minimum. When impulse voltage was applied, the Paschen law was valid only for the points to the right of the 1 bar-mm point. The electrode material affected the static and impulse breakdowns through the values of its work function     

A Z-transform model of transformers for the study ofelectromagnetic transients in power systems

A Z-transform model, which combines transformer frequency-dependent short-circuit impedances with gain functions, has been developed. It sets up a relationship between transient voltages and currents on both sides of a transformer winding pair. The model can be used to calculate impulse responses of the transformers with open-circuit secondary winding as well as those connected to other networks. It could be incorporated into EMT (electromagnetic transient) programs for calculating the EMTs in power systems in which the distributed characteristics of transformer windings are to be considered     

Split-phase induction motor operation from PWM voltage sourceinverter

The operation of split-phase induction motors from pulse-width modulated (PWM) voltage source inverters is examined. Splitting the phase windings leads to reduced voltage ratings for the inverter switches. As compared with seven positions for the space phasor of voltage in three-phase machines, 48 different locations bounded by a 12-sided polygon are possible in the split-phase machine. Based on space phasor modulation, a three-phase inverter can give a maximum peak fundamental of 0.577 V_DC. In comparison, the split-phase configuration can provide an equivalent three-phase maximum peak fundamental of 0.643 V_DC with a DC bus voltage of 0.5 V_DC/cos 15     

A reduced model for power system observability: analysis andrestoration

An algorithm for observability analysis and restoration in power system state estimation is presented. The problem of P-δ, Q-V and complete observability is addressed. In the proposed algorithm, groups of buses are represented by supernodes and the problem of observability analysis and restoration is examined with a reduced network. The proposed algorithm is easily implemented and it is independent of the state estimation solution algorithm. In case of unobservability, the proposed methodology identifies directly the maximal islands and determines the number and the placement of the required pseudomeasurements for observability restoration. Results from several test cases are presented     

Heaviside's operational calculus applied to electrical circuitproblems

Heaviside's operational calculus approach is compared with the one-sided Laplace transform method augmented with distribution theory analysis as applied to electrical circuit problems. It is seen that there is a high degree of commonality to these methods, but also several significant problem-solving and pedagogical advantages to Heaviside's method. In particular, the Heaviside operational calculus is better physically motivated, conceptually parallel to the well-established complex sinusoidal circuit analysis (the jw method), and less mathematically intricate, and may with ease also handle initial value problems. Moreover, the mathematical theory of the impulse or δ( t) function, defined as the convolution identity and developed using Heaviside's operational approach, compares very favorably with the quite extensive mathematical machinery of the theory of distributions     

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     

An approach for the regularization of a power flow solution aroundthe maximum loading point

A method which is capable of greatly widening the range where the power flow solution is regular is presented. This is achieved by giving some modifications to the conventional power flow solution method and eliminating the singular point or shifting it to region where the voltage is lower than that of the maximum loading point. Then, the continuous execution of V-P curves including the maximum loading point is realized. The efficiency and effectiveness of the method were tested in a practical 598-node system are compared with the conventional method     

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     

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