The phenomenon of sympathetic interaction between transformers, which is very likely to occur when a transformer is energised onto a system to which there are other transformers already connected, changes significantly the duration and the magnitude of the transient magnetising currents in the transformers involved. This phenomenon is discussed in the paper, with the harmonic content of the transient currents and its associated overvoltages being analysed. It is shown that the importance of taking into consideration the saturation characteristics of the transformers already in operation when studying transformer transient inrush.

Harmonic analysis of transient currents during sympathetic interaction

The paper describes a procedure for the representation of hysteresis in the laminations of power transformers in the simulation of electromagnetic transient phenomena. The model is based on the recognition that in today's iron cores the hysteresis loops are narrow and therefore the modeling details are only important in relation to the incurred losses and the associated attenuation effects. The resultant model produces losses proportional to the square of the flux density, as expected from measurement data. It is formulated as a simple, linear relationship between the variation B-B/sub re/spl nu, of the magnetic flux density B after a reversal point B/sub re/spl nu and the resulting additional field intensity H/sub hyst/. This idea can be easily implemented in existing transformer models with or without frequency dependent modeling of eddy currents in the laminations. It has been found that in simulation tests the representation of hysteresis is not necessary those situations have been described where the modeling of hysteresis appears to be more meaningful. >

A simple representation of dynamic hysteresis losses in power transformers

An adequate mathematical model of a three-phase power transformer is one of the important elements in the programs for the computer analysis of power system transients. Featured in this paper is the simulation model of a three-phase, three-limb core-type power transformer. Nonlinear effects of saturation, hysteresis and eddy currents are considered. Two ways of creating major and minor hysteresis loops are presented. The transformer model, described by a system of time dependent differential equations, is solved by an efficient numerical algorithm. The behaviour of the transformer model during switching-in and fault transients, as well as other types of transients, has been tested. The computed transient waveforms are compared with the measured ones and there exists very close agreement between them. >

Dynamic model of a three-phase power transformer

In power systems with appreciable resistance, the transformers already connected to the supply system can experience unexpected saturation during the inrush transient of an incoming transformer. This saturation, which is established by the asymmetrical voltage drop across the system resistance caused by the inrush current, demands offset magnetising currents of high magnitude in the already connected transformers. This generates a transient interaction between the transformers, a 'sympathetic interaction', that affects the magnitude and duration of the inrush current. As a consequence, problems in the operation of the system, such as false operation of transformer differential relays and prolonged temporary harmonic overvoltages, can occur. The sympathetic interaction between the transformers is analysed in the paper. System configurations with transformers in parallel and in series are investigated. The transformers and system models have been validated using small transformers.

Phenomenon of sympathetic interaction between transformers caused by inrush transients

In this paper, a model is proposed for numerical simulation of transient state in three phase transformers. The model derived from circuit theory is based on association of linear and nonlinear reluctances. A large number of existing models are dealt with and discussed in detail. New approaches for leakage and core fluxes as well as core losses modeling are highlighted. The transient behavior of the transformer model during switching-in has been tested. There exists a good agreement between the computed and measured transient waveforms.

A contribution to the modeling of three phase transformers using reluctances

This paper describes an electromaanetic circuit model of three ohase transformers capable of includina ferromacanetic non-linear effects includinga hvsteresis, suitable for anv number of windinas with a variety of interphase connections and applicable to any magnetic circuit confiauration. In particular, three of the common types of magnetic circuits are considered i. e. (1) bank of sinqle phase cores (2) three limbb core and (3) five limmb core. The model is applied to tne solution of the inrush currents in a variety of situations and the results are shown to compare favourably with experimental results.

H. L. Nakra

Papers

Three Phase Transformer Transients