This paper proposes a new parallel-LC-resonance-type fault current limiter (FCL) that uses a resistor in series with a capacitor. The proposed FCL is capable of limiting the fault current magnitude near to the prefault magnitude of distribution feeder current by placing the mentioned resistor in the structure of the FCL. In this way, the voltage of the point of common coupling does not experience considerable sag during the fault. In addition, the proposed FCL does not use a superconducting inductor which has high construction cost. Analytical analysis for this structure is presented in detail, and simulation results using power system computer-aided design/electromagnetic transients, including dc software are obtained to validate the effectiveness of this structure. Also, an experimental setup is provided to show the accuracy of the analytic analyses and simulation results.

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Parallel-Resonance-Type Fault Current Limiter

Survey results suggest that 92% of interruption at industrial facilities is voltage sag related. The voltage sag compensator, based on a transformer-coupled series-connected voltage-source inverter, is among the most cost-effective solution against voltage sags. When voltage sags happen, the transformers, which are often installed in front of critical loads for electrical isolation, are exposed to the disfigured voltages and a dc offset will occur in its flux linkage. When the compensator restores the load voltage, the flux linkage will be driven to the level of magnetic saturation and severe inrush current occurs. The compensator is likely to be interrupted because of its own overcurrent protection, and eventually, the compensation fails, and the critical loads are interrupted by the voltage sag. This paper proposes an inrush current mitigation technique together with a state-feedback controller for the voltage sag compensator. The operation principles of the proposed method are specifically presented, and experiments are provided to validate the proposed approach.

An Inrush Mitigation Technique of Load Transformers for the Series Voltage Sag Compensator

This paper proposes a technique for preventing saturation in series transformers from dynamic voltage restorer (DVR) systems. The method consists in correcting the voltages which are injected through the transformers into the power system to compensate voltage sags. It restricts the compensating voltages during the sag whenever it predicts that a maximum limit for the flux linkage is about to be exceeded. The prediction is carried out at the beginning of a stabilized voltage sag. Moreover, the technique allows a certain level of sag compensation even when the estimated flux is expected to exceed the saturation limit. The voltage sag level and phase are computed through an adaptive recursive least squares (RLS). The RLS estimation incorporates a transient period before it achieves a stable state whenever there is a sag event. The DVR is not supposed to operate in this period. Therefore, this paper also outlines a simple procedure for detecting the RLS estimation stable level. Simulation of different scenarios of voltage sags and the results from the experimental implementation of a DVR show the effectiveness of the method.

A Method for Averting Saturation From Series Transformers of Dynamic Voltage Restorers

Survey results suggest that 92% of interruption at industrial facilities is voltage sag related. The voltage sag compensator, based on a transformer-coupled series- connected voltage source inverter, is among the most cost- effective solution against voltage sags. Transformers are often installed in front of critical loads for electrical isolation purposes. When voltage sags happen, the transformers are exposed to the disfigured voltages and a DC offset will occur in its flux linkage. When the compensator restores the load voltage, the flux linkage will be driven to the level of magnetic saturation and severe inrush current occurs. The compensator is likely to be interrupted because of its own over-current protection, and eventually the compensation fails, and the critical loads are interrupted by the voltage sag. This paper proposes an inrush current mitigation technique together with a state feedback controller for the voltage sag compensator. The operation principles of the proposed method are specifically presented, and experiments are provided validate the proposed approach.

An inrush mitigation technique of load transformers for the series voltage sag compensator

This paper proposes a model that adequately describes the operation of the transformer at deep saturation, suitable for power-electronics applications, and a method for determining its parameters. Simulation and experimental results are presented to confirm the validity of the model and the method.

Transformer Operation at Deep Saturation: Model and Parameter Determination

Voltage sags are major power quality problems encountered by industries. Especially for high-tech companies inside industry parks, any voltage sag event in the distribution system could affect many manufacturers and inflict significant losses. The voltage sag compensator, based on a transformer-coupled series-connected voltage source inverter, is among the most cost- effective solution to protect sensitive loads. One important issue of this system is the transformer inrush at the start of sag compensation. The inrush current may trigger the over-current protection of the compensator inverter, and the compensation voltage is greatly reduced due the magnetic saturation of the coupling transformer. Oversizing the transformer is a common approach to avoid the inrush current. However, this would dramatically increase the size and weight of the sag compensation system. This paper proposes a new technique for mitigating the inrush of the coupling transformer and preserving the output voltage for effective sag compensation. Detailed explanations of the proposed inrush mitigation technique are presented, and the effectiveness of the proposed scheme is verified by laboratory test results. The inverter-assisted commutation sequence of the bypass thyristors for fast start of the compensator is also presented and verified by test results.

Chia-Long Ni

Papers

A Transformer Inrush Mitigation Method for Series Voltage Sag Compensators