Article

Optimal gang-operated switching for transformer inrush current reduction

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Abstract

Controlled switching technology with independent-pole-operated circuit breakers is an effective way to eliminate transient transformer inrush currents. This technique cannot usually be applied at lower voltage levels where three-pole-operated circuit breakers are more frequent. In this paper, the optimal instant for a simultaneous closing is obtained as a solution of a min-max problem. The proposed strategy has been evaluated in a test system using EMTP/ATP and has presented highly satisfactory results, even when actual characteristics of circuit breakers are taken into account.

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... The optimal closing instant is the instant at which residual flux and prospective flux match for the phase with the lowest absolute value of the residual flux, and the polarities of the residual flux and the prospective flux coincide in the other two phases. This strategy has been described in [12] and is illustrated in Fig. 5. ...
... Since the use of independent-pole-operated circuit-breakers represents an additional cost, three-pole-operated circuit-breakers are usually employed at voltage levels below about 145 kV [11]. In these cases, optimal gang-operated switching [12] (strategy S2) has proved to be the most highly recommended strategy, due to the inrush current mitigation obtained and its straightforward application. ...
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... For example, electrical protection is a Mini Circuit Breaker (MCB) that protects from overload and short circuit conditions which results in rising conductor temperatures and damage to electrical equipment. The working principle of the MCB is to cut off the electric current if the value of the current through it exceeds the nominal limit [18][19][20]. ...
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When a transformer is energized, a transient current, known as magnetizing inrush current, generally flows for a short period of time until normal flux conditions are established. Under most practical system conditions, this current transient is of little consequence. However, in very rare cases a combination of circumstances may be obtained which results in this inrush being of such consequence as to impair momentarily the proper operation of the system. Because of the numerous faetors bearing upon this general problem, an investigation has been made to determine the effects of transformer inrush currents under a wide variety of system conditions. It is the purpose of this paper to discuss the mechanism by which inrush currents are produced, the results of tests and calculations, and studies made with the miniature-system analyzer. Factors that determine the significance, of inrush current from the standpoint of system operation and methods for reducing the inrush current or mitigating its effects are also discussed.
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Controlled switching taking into account the residual flux can - in theory - completely eliminate transient transformer inrush currents. Real factors such as closing time scatter of the circuit breaker and uncertainty in the residual flux measurement significantly reduce its performance in the field. To quantify the acceptable tolerances of these factors if no inrush currents have to occur, a systematic inrush current study was carried out in the laboratory. Instantaneous switches and ideal residual flux measurement in combination with a controller unit are used to emulate the scatter respectively the uncertainty by presetting deterministic deviations. The analyses were carried out for ldquorapid closing strategyrdquo and ldquodelayed closing strategyrdquo and showed that the acceptable tolerances for both strategies can be evaluated independent of the controlled switching strategy with the inrush current studies of the first phase that is energized.
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The modeling of inrush currents that occur upon energization of a transformer is a challenge for Electromagnetic Transients Programs due to limitations in available transformer models and the ability to determine and specify initial flux. The estimation of transformer model parameters is also an issue. This paper presents a transformer model for low- and mid-frequency transient studies with a focus on the behavior in saturation and the estimation of residual fluxes. The comparison of the simulation results with analytical calculations and measurements proves the capability of the model to accurately represent energization and de-energization transients of a three-legged-core distribution transformer. A novel property is the ability of auto initialization after disconnection, made possible by the implementation of a hysteretic core model which properly simulates and remembers residual flux from the previous de-energization. Special attention is paid to parameter estimation. Detailed core and winding design data are not used as they are seldom available from the manufacturer. Sensitivity analysis is performed to verify the influence of each parameter on the inrush current envelope curve. It is observed that the parameter that most influences the current amplitude is the slope of the magnetization curve at extreme saturation.
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Transformers are essential elements, which facilitate the transmission of electric power at high voltages over long distances and transformer energization is a common occurrence. The energization can lead to excessive transient inrush current, especially when the transformer core has remnant flux that adds to the flux build-up after switching. Inrush current sags the system voltage, thereby affecting the power quality of the network in proximity of the transformer. The extent to which power quality is degraded depends on short circuit MVA at the source bus, and the magnitude and decay time constant of the transient current. Present day thyristor-controlled machinery used in mining, pulp and paper industries and semiconductor manufacturing require high quality power. Some of these industries even pay a premium price for high quality power. It is, therefore, necessary to first assess the impact of energizing a large transformer on power quality and then develop a technique to limit this impact. This paper presents results from field tests and simulations using Electromagnetic Transients Program a 138 kV 315 MVA transformer energization in the BC Hydro system. The transformer is situated close to a pulp and paper mill with voltage sensitive loads. Methods to assess and to limit the voltage sags during energization are discussed.
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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.
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From failure experience on power transformers, it was very often suspected that inrush currents, occurring when energizing unloaded transformers, were 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. Two-dimensional and three-dimensional computer modeling for a real 268 MVA, 525/17.75 kV three-legged step up transformer was 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 normally are smaller, the forces can have similar amplitudes as those at short circuit however with longer exposure time. Therefore, care has to be taken to avoid such high inrush currents. Today controlled switching offers an elegant and practical solution
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