• International Journal of Fluid Machinery and Systems
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• 제8권2호
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• pp.102-112
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• 2015

In the Nordic grid, a trend observed the recent years is the increase in grid frequency variations, which means the frequency is outside the normal range (49.9-50.1 Hz) more often. Variations in the grid frequency leads to changes in the speed of rotation of all the turbines connected to the grid, since the speed of rotation is closely related to the grid frequency for synchronous generators. When the speed of rotation changes, this implies that the net torque acting on the rotating masses are changed, and the material of the turbine runners must withstand these changes in torque. Frequency variations thus leads to torque oscillations in the turbine, which become dynamical loads that the runner must be able to withstand. Several new Francis runners have recently experienced cracks in the runner blades due to fatigue, obviously due to the runner design not taking into account the actual loads on the runner. In this paper, the torque oscillations and dynamic loads due to the variations in grid frequency are simulated in a 1D MATLAB program, and measured grid frequency is used as input to the simulation program. The maximum increase and decrease in the grid frequency over a 440 seconds interval have been investigated, in addition to an extreme event where the frequency decreased far below the normal range within a few seconds. The dynamic loading originating from grid frequency variations is qualitatively found by a constructed variable $T_{stress}$, and for the simulations presented here the variations in $T_{stress}$ are found to be around 3 % of the mean value, which is a relatively small dynamic load. The important thing to remember is that these dynamic loads come in addition to all other dynamic loads, like rotor-stator interaction and draft tube surges, and should be included in the design process, if not found to be negligible.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2018년도 추계학술대회
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• pp.104-106
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• 2018

The harmonics and the DC offset in the grid can cause serious synchronization problems for grid connected inverters (GCIs) which leads not able to satisfy the IEEE 519 and p1547 standards in terms of phase and frequency variations. In order to guarantee the smooth and reliable synchronization of GCIs with the grid, Phase Locked Loop (PLL) is the crucial element. Typically, the performance of the PLL is assessed to limit the grid disturbances e.g. grid harmonics, DC Offset and voltage sag etc. To ensure the quality of GCI, the PLL should be precise in estimating the grid amplitude, frequency and phase. Therefore, in this paper a novel Robust PLL technique called Digital Lock-in Amplifier (DLA) PLL is proposed. The proposed PLL estimate the frequency variations and phase errors accurately even in the highly distorted grid voltage conditions like grid voltage harmonics, DC offsets and grid voltage sag. To verify the performance of proposed method, it is compared with other six conventional used PLLs (CCF PLL, SOGI PLL, SOGI LPF PLL, APF PLL, dqDSC PLL, MAF PLL). The comparison is done by simulations on MATLAB Simulink. Finally, the experimental results are verified with Single Phase GCI Prototype.

• Journal of Power Electronics
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• 제15권2호
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• pp.518-529
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• 2015

Repetitive and proportional-resonant controllers can effectively reject grid harmonics in grid-connected inverters because of their high gains at the fundamental frequency and the corresponding harmonics. However, the performances of these controllers can seriously deteriorate if the grid frequency deviates from its nominal value. Non-ideal proportional-resonant controllers provide better immunity to variations in grid frequency by widening resonant peaks at the expense of reducing the gains of the peaks, which reduces the effectiveness of the controller. This paper proposes a repetitive control scheme for grid-connected inverters that can track changes in grid frequencies and keep resonant peaks lined up with grid frequency harmonics. The proposed controller is implemented using a digital signal processor. Simulation and practical results are presented to demonstrate the controller capabilities. Results show that the performance of the proposed controller is superior to that of a proportional-resonant controller.

• Journal of Power Electronics
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• 제15권3호
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• pp.786-795
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• 2015

In this paper, an active damping control scheme for LLCL filters based on the PR (proportional-resonant) regulator is proposed for grid-connected three-level T-type PWM converter systems. The PR controller gives an infinite gain at the resonance frequency. As a result, the oscillation can be suppressed at that frequency. In order to improve the stability of the system in the case of grid impedance variations, online grid impedance estimation is applied. Simulation and experimental results have verified the effectiveness of the proposed scheme for three-phase T-type AC/DC PWM converters.

• Journal of Power Electronics
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• 제18권6호
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• pp.1879-1888
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• 2018

With power level of grid-connected converters rising, the switching frequency of the switching devices is commonly greatly reduced to improve its power capacity. However, this results in serious couplings of the dq current components, which leads to degradation of the static and dynamic performances of grid-connected converters and fluctuations of the reactive power in dynamic processes. In this paper, complex vector models under low switching frequency are established for an L/LCL grid-connected converter, and the relationship between the switching frequency and the coupling degree is analyzed. In addition, a series decoupling current control strategy is put forward. It is shown that the proposed control strategy can eliminate the couplings, improve the performances and have good robustness to parameter variations through static and dynamic characteristics analyses and a sensitivity analysis. Experimental and simulation results also verify the correctness of the theoretical analyses and the superiority of the proposed control strategy.

• Journal of Electrical Engineering and Technology
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• 제12권3호
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• pp.1101-1107
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• 2017

In this paper, a robust and fast grid synchronization method of a three-phase power converter is proposed. The amplitude and phase information of grid voltages are essential for power converters to be properly connected into the utility. The phase-lock-loop in synchronous reference frame has been widely adopted for the three-phase converter system since it shows a satisfactory performance under balanced grid voltages. However, power converters often operate under abnormal grid conditions, i.e. unbalanced by grid faults and frequency variations, and thus a proper active and reactive power control cannot be guaranteed. The proposed method adopts a second order generalized integrator in synchronous reference frame to detect positive sequence components under unbalanced grid voltages. The proposed method has a fast and robust performance due to its higher gain and frequency adaptive capability. Simulation and experimental results show the verification of the proposed synchronization algorithm and the effectiveness to detect positive sequence voltage.

• 전기학회논문지
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• 제67권11호
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• pp.1447-1454
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• 2018

In this paper, the design of robust DSC-PLL(Delayed Signal Cancellation Phase Locked Loop) is proposed for coping with frequency variation. This method shows significant performance for detection of fundamental positive sequence component voltage when the grid voltage is polluted by grid unbalance and frequency variation. The feedback frequency estimation of DSC-PLL is tracking the drift in the phase by unbalance and frequency variation. The robust DSC PLL is to present the analysis on method and performance under frequency variations. These compensation algorithms can correct for discrepancies of changing the frequency within maximum 193[ms] and improve traditional DSC-PLL. Linear interpolation method is adopted to reduce the discretized errors in the digital implementation of the PLL. For verification of robust characteristic, PLL methods are implemented on FPGA with a discrete fixed point based. The proposed method is validated by both Matlab/Simulink and experimental results based on FPGA(XC7Z030).

• Nuclear Engineering and Technology
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• 제24권2호
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• pp.111-120
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• 1992

본 논문은 안전정지지진에 대한 spacer grid충격하중의 동적특성을 고찰한 것이다. 안전정지지 진 (SSE： Safe Shutodown Earthquake)을 가진력으로 사용하였고 모델로는 가장 긴 15 row의 핵연료집합체 모델을 사용하였다. 외부에서 제공되는 입력데이타의 불확실성을 보상하기 위하여 흔히 사용되는 10%증가된 입력값과, NRC 권고사항인 입력응답스펙트럼의 이동효과의 영향이 평가되었다. 평가대상으로는 spacer grid의 충격력이 사용되었다. 해석결과로서 입력응답 스펙트럼의 이동효과의 영향은 거의 무시할 수 있으나 증가된 입력값의 영향은 spacer grid의 충격력에 큰 영향을 줌을 알 수 있었다. 따라서 원자로노심의 동적응답을 위해서는 입력응답 스펙트럼을 이동시키면서 해석할 필요가 없음을 확인하였다.

• Journal of Electrical Engineering and Technology
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• 제13권2호
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• pp.790-799
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• 2018

This paper proposes a single-phase unified power quality conditioner (S-UPQC) for maintaining power quality issues in a microgrid. The S-UPQC can compensate the voltage and current harmonics, voltage sag, and swell as a dynamic voltage restorer (DVR), regardless of variations in the grid frequency. Odd harmonics are treated as even-order harmonics in a rotating frame to implement the harmonic compensators with only one repetitive controller (RC) without any harmonic extractor. The dynamic performance is improved and the delay time is reduced in the RC. The S-UPQC control scheme is designed to maintain accurate and stable operation under deviations of the grid frequency by using the Lagrange interpolation-based finite-impulse-response (LIFIR) filter approximation method. The proposed control schemes were validated through a simulation and experiment.

• 한국정밀공학회:학술대회논문집
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• 한국정밀공학회 2004년도 추계학술대회 논문집
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• pp.630-633
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• 2004

Generally, component and FR-4 board are connected by solder joint. Because material properties of components and FR-4 board are different, component and FR-4 board show different coefficients of thermal expansion (CTE) and thus strains in component and board are different when they are heated. That is, the differences in CTE of component and FR-4 board cause the dissimilarity in shear strain and BGA solder joint s failure. The first order Taylor series expansion of the limit state function incorporating with thermal fatigue models is used in order to estimate the failure probability of solder joints under heated condition. A model based on plastic-strain rate such as the Coffin-Manson Fatigue Model is utilized in this study. The effects of random variables such as frequency, maximum temperature, and temperature variations on the failure probability of the BGA solder joint are systematically investigated by using a failure probability model with the first order reliability method(FORM).