A.P.S. Meliopoulos

Power system harmonic state estimation

A power system state estimation based on (a) multiphase model, (b) voltage and current waveform measurements, (c) synchronized measurements and (d) multifrequency model (i.e. the approach accounts for waveform distortion or harmonics) is formulated. The paper focuses on the following: (a) modeling, (b) implementation, (c) observability and (d) performance. Sensitivity analysis is used to show how transmission line modeling and measurement schemes affect the performance of harmonic state estimation. The overall performance of the system is described in terms of confidence level versus error. These concepts are illustrated with simple systems. The overall harmonic measurement system is scheduled for installation and field evaluation on the NYPA/New York Power Pool transmission system by the end of 1993. >

Transmission level instrument transformers and transient event recorders characterization for harmonic measurements

A technique for laboratory characterization of instrument transformers designed for transmission level voltage and current measurements is presented. The technique is also extended to transient event recorders (TERs). The objective of the method is to determine the suitability of existing substation instrument transformers for harmonic measurements, particularly in the frequency range of 60 to 1500 Hz covering the first 25 harmonics. Specifically, the following characteristics are of interest: transfer function magnitude and phase, linearity, and sensitivity of the frequency response to burdens. The measurement technique is based on exciting the instrument transformer primary with an impulsive waveform. Both input and output waveforms are recorded using laboratory grade probes and digitizers. Digital signal processing techniques are used to compute the instrument transformer frequency response. Several voltage transformers and current transformers in the 230 kV-765 kV voltage range were tested. The results are described. >

Calculation of secondary cable losses and ampacity in the presence of harmonics

A simplified procedure for computing ohmic losses in secondary distribution cable systems by extending the Neher-McGrath model for 60 Hz losses is presented. Specifically, simplified formulae for evaluating ohmic losses due to harmonics are given. These results are subsequently used to compute the cable ampacity or the derating factor due to harmonics. The overall method is simple to follow and can be performed with a calculator. A typical example is given. >

An alternative method for transient analysis via wavelets

This paper presents an alternative method for the transient analysis of dynamical systems. The method consists of the traditional frequency domain analysis to capture the steady state operation of the system and a wavelet-based transient analysis which captures the disturbance. The total solution is obtained from the superposition of the steady state and disturbance solutions. This paper focuses on the second part of the solution method. The authors have named this method WBTA (wavelet-based transient analysis). It can be implemented using any set of orthogonal wavelets. In this paper, they present an implementation with Daubechies wavelets. The results obtained using this method are compared and verified with a numerical time-domain analysis method. A concise description of the method is presented followed by examples.

A PC based ground impedance measurement instrument

A PC-based instrument for ground impedance measurements is presented. The instrument injects a transient electric current between the ground under measurement and an auxiliary ground, and it measures ground potential differences (GPDs) around the ground under measurement. The GPD measurements are processed with software which rejects external noise by the use of correlation methods. Estimation methods are used to extract the ground potential rise and ground impedance. The final result is the ground impedance versus frequency in a user-selected frequency range. The instrument has been constructed and field tested by three electric utilities. >

Power distribution practices in USA and Europe: impact on power quality

Design practices for electric power distribution systems in USA and Europe are compared and the differences are identified. The differences have a profound impact on several aspects such as safety, protection and power quality. This paper concentrates on the impact of the European and USA practice on power quality. The paper reviews the design practices in the USA and in Europe in terms of: use of neutrals, grounding, load connectivity, fault protection, and overvoltage protection. The differences are discussed. Then the paper focuses on the implications of these differences on power quality issues. Commentary on the following issues is given: harmonic propagation, lightning disturbance propagation including associated voltage sags and swells, and safety.

Effects of modeling on the accuracy of harmonic analysis

Electronic switching in AC/DC converters generate harmonics. The level of the harmonics depends on the interaction of the interconnected AC and DC systems. In analysis methods for computing harmonic levels, the modeling of the AC power system and its effect on the computed harmonic spectrum, and the convergence of the solution method have not been adequately researched. A method which addresses these issues is proposed. The method is used to show that the choice of the AC system model is very critical since the harmonic distortion at the interface bus is the result of an interaction between the converter and the AC-system. The method consists of a comprehensive model of a converter substation, the AC power system, and the DC-system equivalent. A solution algorithm based on Newtons method is proposed with improved convergence characteristics. A parametric study is presented to illustrate the effects of the AC power system model and the converter control system on the harmonic generation. >

A new method and instrument for touch and step voltage measurements

This paper describes a new method and instrument for measuring touch and step voltages near a grounding system, for example in and around a substation. The hardware, software, and testing procedures are an extension of the smart ground multimeter, developed under EPRI sponsorship. The instrument injects a transient electric current between the ground under testing and an auxiliary ground, and it measures ground potential differences (GPDs) at up to six locations. The GPD measurements are processed with software which rejects external noise using correlation methods. Subsequently, statistical estimation methods are used to extract the touch or step voltages from the thousands of measurement points normalized with the system short circuit capability. Knowledge of the short circuit capability of the substation allows the quick measurement of the touch and step voltages. >

Modeling and analysis of URD cable systems

A method for modeling URD cables with explicit grounding representation is presented. The model accurately represents the skin effect in the cable conductor and shield, as well as in the soil and multiple grounds connected to the cable shield. The method is capable of representing insulated cables, semiconducting jackets, or bare concentric neutral cables. The accuracy of the model is validated by comparison with closed-form solutions of simple cable arrangements. The URD cable model has been integrated into EPRIs SMECC program, a program designed to analyze the ground potential rise of grounding structures. The overall model is also useful for analysis of effects due to loss of cable neutral. Typical results illustrate the effectiveness of URD cables in reducing ground potential rise and effects of loss of neutral. >

Comparison of SS-GIC and MHD-EMP-GIC effects on power systems

A comparative study of the effects of solar storm geomagnetically induced currents (SS-GIC) and nuclear detonation geomagnetically induced currents (magnetohydrodynamic electromagnetic pulse GIC or MHD-EMP-GIC) on the power system. The earth surface electric field of the MHD electromagnetic pulse is given to be of the order of 100 V/km, with a duration up to several minutes; and the electric field of the solar storms is of the order of 10 V/km, and lasts from several minutes to one hour. Both phenomena cause flow of almost direct current in the windings of power transformers through the grounding system. For long transmission lines, i.e. 300 miles or longer, this DC current offsets the 60 Hz AC and may saturate transformer cores, with secondary results such as high magnetization currents, increased harmonics, and concomitant effect on power system operation. The level of the transformer core saturation depends on the time constant of the saturation process, and on the duration and magnitude of the direct current through the transformer windings. Thus, although the solar storm electric field is much lower than MHD-EMP, the solar storm effects on the power system are greater due to their much longer duration. This paper presents a technique for the computation of the induced and/or transferred voltages and currents to an electric power system from geomagnetic disturbances. For this purpose, models of transmission lines which explicitly represent grounding, earth potential, and frequency dependent phenomena, and power transformers which explicitly represent nonlinear magnetization characteristics, are utilized. >