• Proceedings of the KIEE Conference
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• 2009.07a
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• pp.2229_2230
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• 2009

전력계통은 끊임없는 외란에 의한 손실이 발생하며 이를 막기 위한 많은 보호설비 및 기술들이 개발되어왔다. 이러한 외란 중 부하 불평형은 중성선 전류를 증가시킬 수 있으며 이는 전력품질을 저하 시키고 전력기기 등에 악영향을 미치게 된다. 따라서 부하 불평형이 배전계통에서 미치는 영향을 분석하고자 EMTP(Electro Magnetic Transient Program)를 사용하여 배전계통의 부하 용량 변화에 따른 불평형을 모의하고 영향을 분석하였다. 각 상별 부하 용량 변화에 따른 전압과 전류 파형을 분석하고 그에 따른 계통 영향 및 불평형율을 계산하였다.

• Proceedings of the KIEE Conference
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• 2006.07d
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• pp.1791-1792
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• 2006

이 논문에서는 저속 회전체의 불균일 질량측정을 위한 새로운 신호처리 방법을 제안하고 이를 이용한 자동차 타이어의 보정(balancing) 방법에 관해 설명한다. 타이어를 포함한 시스템의 동력학 모델을 기반으로 타이어의 불균일 질량에 의해 발생되는 진동과 이로 인해 로드셀에 작용하는 힘의 관계식을 유도하고, 센서의 측정계수(scale factor)를 포함한 매개변수를 실험 데이터로부터 계산한다. 또한, 기존 방법과 달리 불균일 질량의 크기와 위치를 정확하고 효율적으로 측정할 수 있는 방법을 소형 마이크로 프로세서를 이용하여 구현하고 실험을 통해 그 성능을 검증한다.

• The Transactions of the Korean Institute of Power Electronics
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• v.17 no.1
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• pp.8-13
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• 2012

DSTATCOM is active filter that reduces nonlinear and unbalanced currents. Researches about DSTATCOM are mainly divided two parts, one is the reference value calculation of compensation current depending on the calculation of the load-side average active power and dc-link capacitor average voltage, the other part is actual current control depending on the reference value of compensation current. This paper proposes a calculation of dc-link capacitor average voltage ripple voltage extraction instead of conventional method using LPF. The utility of the proposed algorithm is verified through the theoretical analysis and the experiment under unbalance loads and non-linear load.

• The Transactions of the Korean Institute of Electrical Engineers B
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• v.52 no.1
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• pp.1-8
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• 2003

Generally, thrust force of double-sided LPM is more powerful than that of single-sided LPM. Also, double-sided LMP can be effectively applied to the high-precision position system with its simple control scheme. However, An equality between two airgap lengths Is a necessary condition for its high performance. If a little difference between two airgaps is existed, unbalanced normal force and undesirable vibration will be generated. Additionally, even though two airgap lengths are absolutely same, undesirable vibration can be generated because the direction of instantaneous normal force on mover is changed according to excitation methods. An effect of inequality of two airgap lengths is presented in this paper. Distribution of airgap permeance vs. relative displacement is analyzed by two dimensional finite element method. Excitation From this results, current waveforms to reduce the undesirable vibration is proposed.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.29 no.9
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• pp.71-80
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• 2015

This paper proposes a convenient thermal modeling method for loss distribution control method of 3-level Active NPC(Neutral Point Clamped) inverter. In the drawback of conventional 3-level NPC, the generated losses can occur unbalance in each switching device, as a result, thermal utilization of designed system has been decreased. In order to compensate unbalanced losses, Active NPC inverter performed loss balancing control with thermal modeling during operation of each switching device. Therefore, this paper deals with a convenient thermal modeling method based on newton's law of cooling rather than conventional thermal modeling method. Both simulation and experimental results based on 10kW 3-level Active NPC inverter confirm the validity of the analysis performed in the study.

• Journal of Power Electronics
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• v.16 no.1
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• pp.351-361
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• 2016

Power quality is a critical issue in distribution systems, where a dynamic voltage restorer (DVR) is commonly used to mitigate the voltage disturbances for loads. This paper deals with a nonlinear control for the three-phase four-wire (3P-4W) DVR under a grid voltage unbalance and nonlinear loads in the distribution system, where a novel control scheme based on the feedback linearization technique is proposed. Through feedback linearization, a nonlinear model of a DVR with a PWM voltage-source inverter (VSI) and LC filters is linearized. Then, the controller design of the linearized model is performed by applying the linear control theory, where the load voltages are kept constant by controlling the d-q-0 axis components of the DVR output voltages. To keep the load voltage unchanged, an in-phase compensation strategy is employed, where the load voltages are recovered to be the same as the previous voltage without a change in the magnitude. With this strategy, the performance of the DVR becomes faster and more stable even under unbalanced source voltages and nonlinear loads. The validity of the proposed control strategy has been verified by simulation and experimental results.

• Polymer(Korea)
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• v.32 no.6
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• pp.501-508
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• 2008

Small injection molded articles such as lens and mobile product parts are usually molded in multi-cavity mold. The problem occurring in multi-cavity molding is flow unbalance among the cavities. The flow unbalance affects the dimensions and physical properties of molded articles. First of all, the origin of flow unbalance is geometrical unbalance of the delivery system. However, even the geometry of the delivery system is well balanced, cavity unbalance occurs. This comes from the temperature distributions in the cross-section of runner. Temperature distribution depends upon injection speed because heat generation near runner wall is high at high injection speed. Among the operational conditions, injection speed is the most significant process variable affecting the filling unbalances in multi-cavity injection molding. In this study, experimental study of flow unbalance has been conducted for various injection speeds and materials. Also, the filling unbalances were compared with CAE results. The dimensions and weights of multi-cavity molded parts were examined. The results showed that the filling unbalances vary according to the injection speeds and resins. Subsequently, the unbalanced filling and pressure distribution in the multi-cavity affect the dimensions and physical states of molded parts.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2007.11a
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• pp.840-846
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• 2007

This paper investigates the dynamic behavior of a HDD spindle system due to the imperfect roundness of a rotating shaft. The shaft of a spindle motor rotates with eccentricity by the unbalanced mass of the rotating part. The eccentricity generates the run-out of a spindle motor which results in the eccentric motion of a rotating part. Roundness of a shaft affects this motion which limits the memory capacity of a HDD. This research proposes a modified Reynolds equation for the coupled journal and thrust FDBs to include the variable film thickness due to the roundness. Finite element method is used to solve the Reynolds equation for the pressure distribution. Reaction forces and friction torque are obtained by integrating the pressure and shear stress, respectively. The dynamic behavior is determined by solving the equations of a motion of a HDD spindle system in six degrees of freedom with the Runge-Kutta method to characterize the motion of a rotating part. This research shows that the roundness of a rotating shaft causes the excitation frequency with integer multiple of a rotating frequency.

• Journal of Power Electronics
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• v.16 no.3
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• pp.1163-1175
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• 2016

In recent years, microgrids have been the focus of considerable attention in distributed energy distribution. Microgrids contain a large number of power electronic devices that can potentially cause negative impedance instability. Harmonic impedance is an important tool to analyze stability and power quality of microgrids. Harmonic impedance can also be used in harmonic source localization. Precise measurement of microgrid impedance and analysis of system stability with impedances are essential to increase stability. In this study, we introduce a new square wave current injection method for impedance measurement and stability analysis. First, three stability criteria based on impedance parameters are presented. Then, we present a new impedance measurement method for microgrids based on square wave current injection. By injecting an unbalanced line-to-line current between two lines of the AC system, the method determines all impedance information in the traditional synchronous reference frame d-q model. Finally, the microgrid impedances of each part and the overall microgrid are calculated to verify the measurement results. In the experiments, a simulation model of a three-phase AC microgrid is developed using PSCAD, and the AC system harmonic impedance measuring device is developed.

• The Journal of Asian Finance, Economics and Business
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• v.8 no.9
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• pp.189-199
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• 2021

This study attempts to examine the relationship between corporate social responsibility (CSR) disclosure and earnings management practices in the context of Saudi Arabia after mandatory IFRS adoption. It is carried out on an unbalanced panel of 277 observations over the period 2017-2019. For this purpose, CSR disclosure is measured by Bloomberg ESG scores, while the residuals from the modified Jones model are considered for earnings management. As control variables, we have retained the firm performance, market-to-book ratio, firm size, financial leverage, board independence, ownership concentration, managerial ownership, and lagged discretionary accruals. Using the system GMM estimator in the dynamic panel, the results show a positive association between CSR disclosure and earnings management practices, thus supporting the perspective of agency theory. Managers engage in socially responsible activities beforehand to conceal their wrongdoing and convince stakeholders that the organization is transparent. They probably use ethical codes as a tool to achieve their own goals rather than the firm's goals. Our contribution is the use of recent data (2017-2019) taking into account the mandatory adoption of IFRS in Saudi Arabia. Additionally, to our knowledge, this study is the first to address CSR disclosure and earnings management practices using GMM system estimates.