Detailed transformer representations are needed in the analysis of electromagnetic transients and in the analysis of unbalanced steady-state conditions. This paper describes the derivation of mdels for three- phase and single-phase N-winding transformers in the form of branch impedance or admittance matrices, which can be calculated from available test data of positive and zero sequence short-circuit and excitation tests. The models can be used for many types of studies as long as the frequencies are low enough so that capacitances in the transformer can be ignored. The inclusion of saturation effects is briefly discussed.

Matrix Representation of Three-Phase N-Winding Transformers for Steady-State and Transient Studies

A technique is described to evaluate harmonic interaction. It is based on linearizing the harmonic interaction between AC and DC sides of the converter, including the effect of control action. The technique provides an efficient tool for the evaluation of possible adverse amplification of low-order harmonics as well as the design of mitigating measures. It is especially valuable for complex HVDC systems where performance must be evaluated for many contingencies. >

Low-order harmonic interactions on AC/DC systems

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

Transformer saturation affects system over-voltages caused by transformer energization, fault application and clearing, and load rejection. Only some of these effects are desirable. The paper presents an analysis of such effects supported by results obtained on a typical long EHV transmission system by means of a power system simulator. An analysis of harmonic currents generated by a saturated transformer is also presented as a contribution to a better understanding of the phenomena considered.

Transformer Saturation Effects on EHV System Overvoltages

In systems with pronounced resonance conditions and a low degree of damping, temporary overvoltages may develop when transformers are switched in or at fault clearing. Overvoltages can occur e.g. in generator networks feeding HVDC stations and in high voltage cable systems.

Analysis of Overvoltages Caused by Transformer Magnetizing Inrush Current

The Cabora Bassa-Apollo HVDC Transmission is the first UHV dc long distance transmission system using thyristor valves. The overvoltage protection and insulation co-ordination for the terminal stations of this systemaredescribed. It is shown, how the specific requirements of the thyristor valves for strict and reliable overvoltage protection have been solved by direct surge arrester protection with reasonable insulation levels of the valves and the other HVDC station equipment. The selection of post and suspension insulators used in the d.c. section of the HVDC station, which is fully outdoor, is described as well.

D. Povh

Papers

Analysis of Overvoltages Caused by Transformer Magnetizing Inrush Current

The Cabora Bassa D.C. Transmission System: Overvoltage Protection and Insulation Coordination