Klaus Fröhlich

Elimination of transformer inrush currents by controlled switching. I. Theoretical considerations

Transformer inrush currents are high-magnitude, harmonic-rich currents generated when transformer cores are driven into saturation during energization. These currents have undesirable effects, including potential damage or loss-of-life to the transformer, protective relay misoperation, and reduced power quality on the system. Controlled transformer switching can potentially eliminate these transients if residual core and core flux transients are taken into account in the closing algorithm. This paper explores the theoretical considerations of core flux transients, Based on these studies algorithms were developed which allow controlled energization of most transformers without inrush current.

Elimination of transformer inrush currents by controlled switching. II. Application and performance considerations

For pt. I see ibid., vol.16, no.2, p.276-80 (2001). Transformer inrush currents are high-magnitude, harmonic-rich currents generated when transformer cores are driven into saturation during energization. These currents have undesirable effects, including potential damage or loss-of-life to the transformer, protective relay misoperation, and reduced power quality on the system. Controlled transformer switching can potentially eliminate these transients if residual core and core flux transients are taken into account in the closing algorithm. This paper explores the practical considerations of core flux transients, performance of control strategies, and the application of circuit breakers to control transformer inrush transients.

The Impact of Inrush Currents on the Mechanical Stress of High-Voltage Power Transformer Coils

From failure experience on power transformers very often it was suspected that inrush currents, occurring when energizing unloaded transformers, were the reason for damage. In this paper it was investigated how mechanical forces within the transformer coils build up under inrush compared to those occurring at short circuit. 2D and 3D computer modeling for a real 268 MVA, 525/17.75 kV three-legged step up transformer were employed. The results show that inrush current peaks of 70% of the rated short circuit current cause local forces in the same order of magnitude as those at short circuit. The resulting force summed up over the high voltage coil is even three times higher. Although inrush currents are normally smaller, the forces can have similar amplitudes as those at short circuit, with longer exposure time, however. Therefore, care has to be taken to avoid such high inrush currents. Today controlled switching offers an elegant and practical solution.

A new algorithm enabling controlled short circuit interruption

Controlled interruption of short circuit currents in medium- and high-voltage systems by means of point-on-wave control has not been feasible so far for technical reasons. The determination of the optimal tripping moment to achieve minimal contact wear should not take more time than the protection relay takes to indicate the fault, which makes it a difficult problem. As a solution, a so-called safe-point algorithm was developed, which overcomes the problem by an approximation of sufficient accuracy and with a time request of not much more than half a cycle of the power frequency. Computer simulations demonstrate that the mean I/sup 2/t can be reduced significantly. The method provides the foundation for implementing controlled switching of fault currents.

Calculation of stress-dependent life cycle costs of a substation Subsystem-demonstrated for controlled energization of unloaded power transformers

This paper presents an algorithm for calculating the life cycle costs of a power apparatus taking into account the deterioration process of the apparatus subjected to stress during its operational lifetime. The algorithm models the life-cycle of a power apparatus using a continuous-time Markov chain in which costs related to the deterioration stages of an apparatus during its lifetime are considered. The methodology is used to calculate the financial implications of introducing new equipment which reduce the stress for the adjacent apparatus in a substation. As an example, in this work the financial benefits of controlled energization of an unloaded substation power transformer are calculated. The analysis demonstrates the high financial benefits resulting from the introduction of controlled transformer energization

A novel hybrid current-limiting circuit breaker for medium voltage: principle and test results

Summary form only given. Although many attempts have been made to design a fault-current limiting (FCL) circuit breaker (FCLCB) for medium voltage electric power systems, no economically attractive solution has been achieved so far. In this paper, a novel concept for a FCLCB is introduced based on a hybrid arrangement of semiconductors, temperature-dependent resistors, and a newly developed fast-opening mechanical switch. The latter utilizes one part of an electrodynamic repulsion drive, which is concurrent with the moving double-contact system. Laboratory tests as well as computer simulations of the complete FCLCB verify as an example the feasibility of the goal ratings 12 kV and 2/20 kA (single-phase). A cost analysis shows the FCLCB to be more expensive than a conventional generator circuit breaker, but to be in the price range of the Is-limiter and below the cost of the superconducting FCL principles. It is concluded that the presented method provides the basis for further commercial product development.

Verification of a model-based diagnosis system for on-line detection of the moisture content of power transformer insulations using finite element calculations

It is a well known fact that the moisture content of a power transformer insulation system is a key parameter for the estimation of its aging condition and operation reliability. Therefore, detection of the moisture content is a very essential task within power transformer diagnostics. While conventional methods for estimation of the water content of the insulation system require a transformer shutdown of several hours, the proposed method calculates the profiles of water concentration in the cellulosic part of the insulation system, as well as the concentration of water in transformer oil from operational parameters. An one-dimensional model for the moisture diffusion in paper estimates the temperature dependent moisture distributions in the paper. It includes temperature dependent diffusion coefficients and acts as a non-linear control loop for the water concentration in transformer oil. Beside experimental work, two-dimensional finite element (FE) calculations of the temperature dependent water concentration distribution in the transformer insulation system have been performed for model verification. The calculations are based on an opensource Whitney-Element solver. The computed results show the validity of the diffusion model and give important guidelines for the required complexity of thermal models.