• 한국공작기계학회:학술대회논문집
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• 한국공작기계학회 2004년도 추계학술대회 논문집
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• pp.304-308
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• 2004

One of the obstacles for a magnetic bearing to be used in the wide range of industrial applications is the failure modes associated with magnetic bearings. These failure modes include power amplifier faults, position sensor faults, and the malfunction of controllers. Fault-tolerant magnetic bearing systems have been proposed so that the system can operate in spite of some faults. In this paper, we designed and tested a fault-tolerant magnetic bearing system. The system can cope with the actuator faults as well as the faults in position sensors, which are the two major fault modes in a magnetic bearing system.

• Journal of Electrical Engineering and Technology
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• 제6권6호
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• pp.742-749
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• 2011

Power transmission lines are one of the most important components of electric power system. Failures in the operation of power transmission lines can result in serious power system problems. Hence, fault diagnosis (transient or permanent) in power transmission lines is very important to ensure the reliable operation of the power system. A hidden Markov model (HMM), a powerful pattern recognizer, classifies events in a probabilistic manner based on fault signal waveform and characteristics. This paper presents application of HMM to classify faults in overhead power transmission lines. The algorithm uses voltage samples of one-fourth cycle from the inception of the fault. The simulation performed in EMTPWorks and MATLAB environments validates the fast response of the classifier, which provides fast and accurate protection scheme for power transmission lines.

• Journal of Power Electronics
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• 제9권6호
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• pp.949-959
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• 2009

This paper presents a systematic investigation of the fault-mode behaviors of matrix converter systems. Knowledge about converter behaviors after fault occurrence is important from the standpoint of reliable system design, protection and fault-tolerant control. Converter behaviors have been, in detail, examined with both qualitative and quantitative approaches for key fault types, such as switch open-circuited faults and switch short-circuited faults. Investigating the fault-mode behaviors of matrix converters reveals that converter operation with switch short-circuited faults leads to overvoltage stresses as well as overcurrent stresses on other healthy switching components. On the other hand, switch open-circuited faults only result in overvoltage to other switching components. This study can be used to predict fault-mode converter behaviors and determine additional stresses on remaining power circuit components under fault-mode operations.

• Journal of Power Electronics
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• 제16권4호
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• pp.1415-1425
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• 2016

This paper presents a new approach for fault diagnosis in three-level neutral point clamped inverters. The proposed method is based on the average values of the positive and negative parts of normalized output currents. This method is capable of detecting and locating multiple open-circuit faults in the controlled power switches of converters in half of a fundamental period of those currents. The implementation of this diagnostic approach only requires two output currents of the inverter. Therefore, no additional sensors are needed other than the ones already used by the control system of a drive based on this type of converter. Moreover, through the normalization of currents, the diagnosis is independent of the load level of the converter. The performance and effectiveness of the proposed diagnostic technique are validated by experimental results obtained under steady-state and transient conditions.

• Journal of Electrical Engineering and Technology
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• 제9권6호
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• pp.2058-2064
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• 2014

In transformer fault diagnosis, dissolved gas analysis (DGA) is been widely employed for a long period and numerous methods have been innovated to interpret its results. Still in some cases it fails to identify the corresponding faults. Due to the limitation of training data and non-linearity, the estimation of key-gas ratio in the transformer oil becomes more complicated. This paper presents Intuitionistic Fuzzy expert System (IFS) to diagnose several faults in a transformer. This revised approach is well suitable to diagnosis the transformer faults and the corresponding action to be taken. The proposed method is applied to an independent data of different power transformers and various case studies of historic trends of transformer units. It has been proved to be a very advantageous tool for transformer diagnosis and upkeep planning. This method has been successfully used to identify the type of fault developing within a transformer even if there is conflict in the results of AI technique applied to DGA data.

• 대한전기학회논문지:전력기술부문A
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• 제48권1호
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• pp.16-21
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• 1999

The analysis of distribution line faults is essential to the proper protections of the power system. A high impedance fault test, which was carried in Korean electric power systems, it was found that a arcing phenomenon occurred during the high level portion of conductor voltage in each cycle. In this paper, we propose a new method for detection of high impedance faults, which uses the arcing fault current difference during high voltage and low voltage portion of conductor voltage waveform. To extract this difference, we diveded one cycle fault current into equal spanned four data windows according to the magnitude of voltage waveform and applied fast fourier transform(FFT) to each data window. The frequency spectrum of current wavefrom in each portion are used as the inputs of neural network and is trained to detect high impedance faults. The proposed method shows improved accuracy when applied to staged fault data and fault-like load.

• Journal of Electrical Engineering and Technology
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• 제12권4호
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• pp.1376-1385
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• 2017

This paper proposes a method for estimating the expected frequency of voltage swells caused by asymmetrical faults in a power system. Although voltage swell is less common than voltage sag, repeated swells can have severe destructive impact on sensitive equipment. It is essential to understand system performance related to voltage swells for finding optimal countermeasures. An expected swell frequency at a sensitive load terminal can be estimated based on the concept of an area of vulnerability (AOV) and long-term system fault data. This paper describes an effective method for calculating an AOV to voltage swells. Interval estimation for an expected swell frequency is also presented for effective understanding of system performance. The proposed method provides long-term performance evaluation of the frequency and degree of voltage swell occurrences.

• Journal of Electrical Engineering and Technology
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• 제1권1호
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• pp.8-15
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• 2006

Since the power swing depends on the operating time of the relay, the swing's magnitude can be reduced by an autoreclosure relaying system with an optimal reclosing algorithm. This paper proposes a method for stability improvement using optimal reclosure relaying. An optimal reclosure algorithm is applied to identify both temporary and permanent faults, and to predict system stability by applying WAM and high speed communication technology. It provides optimal control by predicting and determining the degree of stability, considering the real time transient stability using EEEAC. For temporary faults, the algorithm determines the system's stability and either recloses optimally for stable systems, or inserts series capacitance before optimal reclosure for unstable systems. It also applies an optimal reclosure algorithm to minimize shock and damage to the power system when reclosure fails due to permanent faults.

• Journal of Power Electronics
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• 제19권6호
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• pp.1467-1476
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• 2019

Grid unbalanced faults can cause core saturation of power transformer and produce lower-order harmonics. These issues increase the electrical stress of power electronic devices and can cause a tripping of an entire HVDC system. In this paper, based on the positive-sequence and negative-sequence impedance model of a VSC-HVDC system as seen from the point of common connection (PCC), the resonance problem is analyzed and the factors determining the resonant frequency are obtained. Furthermore, to suppress over-voltage and over-current during resonance, a novel method using a virtual harmonic resistor is proposed. The virtual harmonic resistor emulates the role of a resistor connected in series with the commutating inductor without influencing the active and reactive power control. Simulation results in PSCAD/EMTDC show that the proposed control strategy can suppress resonant over-voltage and over-current. In addition, it can be seen that the proposed strategy improves the safety of the VSC-HVDC system under unbalanced faults.

• Journal of Electrical Engineering and Technology
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• 제5권1호
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• pp.54-60
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• 2010

This paper presents a new algorithm for determining generator operation point for maintaining stability considering generator faults in Jeju-Haenam HVDC system. As the HVDC system consumes reactive power for the transmission of active power substantially, compensation of reactive power is essential. And the HVDC system is operated on frequency control mode. That is to say, the HVDC system almost manages system frequency. Therefore, we recognized that the Jeju system could be unstable if the reactive power consumed by the HVDC is insufficient when out-of-step occurs with large generators. When the solution of power flow analysis does not converge due to the unstable system phenomenon, we have difficulty in establishing countermeasures as the post-fault information is not available. In this paper, for the purpose of overcoming this difficulty in establishing countermeasures, we introduce the CPF(Continuation Power Flow) algorithm. This paper suggests an algorithm for calculating the output limitation of the generator to maintain the stability in case of generator fault in the Jeju system.