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Application of the network equivalent concept for external system representation in electromagnetic transient studies is well known. However, the challenge in application of an equivalent model, approximated by rational functions, is to guarantee passivity of the corresponding model. Therefore, there is a need to enforce passivity of the equivalent model. In this paper, the passivity enforcement problem is first formulated as a quadratic programming (QP) problem, and then solved based on an efficient implementation, using an infeasible-interior-point primal-dual method. An application of this method for passivity enforcement of a six-port admittance model (with large passivity violations) demonstrates its effectiveness and computational efficiency. Index Terms--Convex quadratic optimization problem, infeasible-interior-point primal-dual method, network equivalent, passivity enforcement.

The fast and very fast transient overvoltages can endanger the insulation of transformer windings. Therefore, it is necessary to develop models, to study these transients. This paper compares the two well-known multiconductor transmission line (MTL) and lumped parameter detailed (RLC ladder network) models. The high-voltage (HV) winding of a 220/35-kV, 50-MVA transformer has been modeled using these models. The simulation results of the MTL and the RLC ladder network models have been compared with the measurements results in the frequency domain, by application of the Pearson's correlation coefficients. It is shown that the RLC Ladder Network model should be used for fast transient overvoltage (FTO) studies, i.e. frequency range of 10 kHz < f < 1 MHz and the MTL model with frequency dependent loss factor, demonstrates a better agreement with the measurement results for very fast transient overvoltage (VFTO) studies, i.e., in the frequency range of 1 MHz [less than or equal to] f [less than or equal to] 5 MHz. Index Terms--Loss factor, modeling, multiconductor transmission line, power transformer, very fast transient.

Understanding the potential and electric field distribution in the insulation system of a transformer during transients is vital to its construction. Therefore, we have developed a method for electric field analysis inside a power transformer. The method consists of the following steps. 1) A lumped parameter equivalent model is constructed by dividing transformer windings into several blocks. 2) The electric circuit parameters of this model are calculated. 3) Employing the results of the transformer transient model analysis as boundary conditions, a 2-D asymmetrical electric field finite-element analysis is performed to determine electric fields through the windings. The method has been examined employing power frequency and impulse voltages. We have also demonstrated successful application of the method to a transformer and have verified the computed results by comparison with measured results. Index Terms--Electric field, finite-element analysis, insulation design, power transformer.

Online monitoring of high voltage (HV) cross linked polyethylene (XLPE) cables is a major requirement and interest of utilities for reliable operation of underground cables. Although XLPE cables have a relatively reliable insulation system, however since its insulating material is less resistant to partial discharges (PD), the failure risk increases significantly after occurrence of any partial discharge. Therefore sensitive sensors and a reliable detection system are required for an effective cable life management. Due to attenuation effects of semi-conducting layers of XLPE cables on high frequency components of partial discharge signal, the detection process is easily distorted with application of conventional sensors such as high frequency current transformers (CTs). In this paper a new sensitive internal sensor of partial discharge signal, the semiconducting layer, is introduced which is available normally in the H.V. XLPE cables and access can be easily provided at the terminals and also at some specific distances from the terminals, i.e. every one hundred meters, if the manufacturer was asked for. Two 11 kV, XLPE cable setups are employed for study of high frequency signals and partial discharge signals propagation in the XLPE cable and its semiconducting layers. The J Marti's frequency dependent cable model is employed for simulation of partial discharge signal propagation in the core conductor and semiconducting layers of single phase XLPE cables. Index Terms--Attenuation, cable model, signal detection, partial discharges, semiconducting layers, simulation, internal sensor.

An EHV transformer's insulation structure must be designed to withstand the internal stresses generated during transients. Computer models are employed for predetermination of these stresses. This paper develops a detailed transformer model and solution method which represents the nonlinear, saturable characteristic of the magnetic core during transients. The resulting set of stiff nonlinear differential algebraic equations (DAE's) are solved by application of Gear's method. Part 1 describes the transformer model and the necessary set of equations based on linearization of the iron core's saturable magnetic characteristic at each time interval. Part 2 presents data verifying the methodology.

An EHV transformer's insulation structure must be designed to withstand the internal electrical stresses generated during system transients. Computer models are employed for predetermination of these stresses. Part 1 of this paper developed a detailed transformer model and solution method representing the nonlinear, saturable characteristic of the core during transients. The resulting set of stiff nonlinear differential algebraic equations (DAE's) are solved by application of Gear's method. Part 2 presents the verification of this methodology. This is accomplished by comparing the computed and measured response of a 765/345/34.5kV 500MVA autotransformer during energization and transient excitation.

An EHV transformer's insulation system must be designed to withstand stresses generated during transient events. Computer models are employed for pre-determination of these stresses. The lack of accurate and consistent computer models for use by both utilities and manufacturers, is felt to contribute to the relatively high failure rate of EHV transformers. This paper develops a detailed nonlinear transformer model and a solution method that improves the accuracy with which the effect of the nonlinear characteristic of the magnetic core during transients can be predicted. This model is based on linearization of the core's saturable characteristic during solution time intervals. This paper describes a nonlinear, single phase, detailed transformer model and its verification against measured data.