Inrush Current Reduction Research Articles

Reduction of inrush current by demagnetization of magnetic core

The peak value of three-phase transformer's inrush current depends, among other factors, on value of the residual flux in the iron core. Higher residual flux produces the inrush current of greater value. Diminishing of magnetism in the core can reduce value of the current. This article presents demagnetization of the transformer as a method of inrush current reduction. In the method proposed a condenser is the source of energy for demagnetization. Different condenser capacitances and voltages are implemented for calculation test to determine optimal values of these parameters. Later the residual flux established after demagnetization of the core is calculated for several initial values. Influence of the flux new value upon the inrush current is discussed.

Single-Phase Transformer Inrush Current Reduction Using Prefluxing

Power transformers can experience large inrush currents upon energization, the severity of which depends on the source strength, the leakage impedance and residual flux of the transformer, and the angle of the applied voltage at energization. A novel inrush current reduction strategy has been implemented which involves setting a single-phase transformer's residual flux to a known polarity after the transformer has been de-energized, a process called “prefluxing,” and controlling the instant of transformer energization based on the flux polarity, seeking not to eliminate inrush current but to substantially reduce it. Unlike a popular suggested solution, this strategy does not require prior knowledge of the transformer's flux. The device used for prefluxing is simple in construction and operates at substantially lower voltage levels when compared to the transformer's rated voltage. The presented strategy has been successfully implemented on an 18-kVA laboratory transformer with inrush current levels reduced below the rated current of the transformer even when accounting for typical breaker deviations. This paper describes the operation of the reduction strategy, including theory, device sizing, and implementation, and presents the successful laboratory results, all of which provide the basis for implementing inrush current reduction in three-phase transformers using a three-pole circuit breaker.

Hybrid winding configuration in high-current injection transformers based on EMC issues

In designing high-current injection transformers (CITs), many factors need to be considered for its compatibility with the internal and external propagation and generation of disturbances within electromagnetic environment, i.e. the magnetic leakage field must be reduced to a level that will make the operation of CIT system safe. On the other hand, inrush current (transient current) may inadvertently affect the operation of the protective relay system and necessitate strengthening of the mechanical structure of the CIT. The inrush current and leakage field do not favour each other, but they are considered as two aspects of EMC considerations. The prior art has only addressed asymmetric winding configuration (S-P-S-P) for inrush current reduction. For comparative purposes, the authors propose a novel hybrid approach that is a composition of auxiliary windings and S-P-S-P structures based on an active shielding concept for EMC improvement. To design an optimal choice of the distributive ratios of S-P-S-P of the main transformer windings, the formulae of the leakage inductance are calculated from the structural parameters of the transformer. Hence, as a case study, there comes the need for advanced numerical techniques for correctly calculating the leakage flux distributions and EMC considerations within a 25 kA CIT enclosure.

A Sequential Phase Energization Method for Transformer Inrush Current Reduction—Transient Performance and Practical Considerations

This paper presents an improved design method for a novel transformer inrush current reduction scheme. The scheme energizes each phase of a transformer in sequence and uses a neutral resistor to limit the inrush current. Although experimental and simulation results have demonstrated the effectiveness of the scheme, the problem of how to select the neutral resistor for optimal performance has not been fully solved. In this paper, an analytical method that is based on nonlinear circuit transient analysis is developed to solve this problem. The method models transformer nonlinearity using two linear circuits and derives a set of analytical equations for the waveform of the inrush current. In addition to establishing a set of formulas for optimal resistor determination, the results also reveal useful information regarding the inrush behavior of a transformer and the characteristics of the sequential energization scheme. This paper also proposed a method, the use of surge arrester, to solve the main limitation of the sequential phase energization scheme-the rise of neutral voltage. Performance of the improved scheme is presented

A Sequential Phase Energization Technique for Transformer Inrush Current Reduction—Part II: Theoretical Analysis and Design Guide

This paper presents a theory to explain the characteristics of a sequential phase energization based inrush current reduction scheme. The scheme connects a resistor at the transformer neutral point and energizes each phase of the transformer in sequence. It was found that the voltage across the breaker to be closed has a significant impact on the inrush current magnitude. By analyzing this voltage using steady-state circuit theory, the simulation and experimental results presented in a companion paper are explained. The results lead to the establishment of a guide to select the optimal value of the neutral resistor. The applicability of the proposed scheme to different transformer configurations has also been investigated in this paper. It is shown that the idea of sequential phase energization leads to a new class of techniques for limiting switching transients.

A Sequential Phase Energization Technique for Transformer Inrush Current Reduction— Part I: Simulation and Experimental Results

This paper presents a new, simple and low cost method to reduce inrush currents caused by transformer energization. The method uses a grounding resistor connected at a transformer neutral point. By energizing each phase of the transformer in sequence, the neutral resistor behaves as a series-inserted resistor and thereby significantly reduces the energization inrush currents. The proposed method has been tested by computer simulation and laboratory experiments. Both results show that the method has a performance similar to that of the resistor pre-insertion scheme. The proposed method is much less expensive, however, since there is only one resistor involved and the resistor carries only a small neutral current in steady-state.

Mitigation of Magnetizing Inrush Current using Sequential Phase Energization Technique

Energization of a transformer under no load or lightly loaded conditions may result in inrush current with high amplitude that can be comparable with the fault currents. These currents have undesirable effects, including malfunction of the differential relay, mechanical and thermal stresses on transformer and reduced power quality of the system. In this paper, a sequential phase energization technique has been used to mitigate magnetizing inrush current in a grounded system. Because in this technique, the size of the resistor, which is connected to the neutral point of the transformer, has an important role, analytical formulas have been derived to calculate inrush current amplitude with a neutral resistor. These analytical formulas have been implemented in the M-File of the MATLAB software for calculation of different inrush current reduction ratio. Then for the verification of the results, the obtained neutral resistor has beenapplied to the equivalent circuit of a typical three-phase transformer and inrush current reduction has been determined by MATLAB SIMULINK. Ill. 7, bibl. 13 (in English; abstracts in English and Lithuanian).http://dx.doi.org/10.5755/j01.eee.108.2.147