• The Transactions of The Korean Institute of Electrical Engineers
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• v.67 no.1
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• pp.1-7
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• 2018

IEC 61000-3-7 provides guidance for limiting flicker and enabling the connection of fluctuating load installations, that is, producing flicker in MV, HV and EHV power systems. In Korea, the flicker have been restricted by Japanese standard of ${\Delta}V_{10}$ method. ${\Delta}V_{10}$ was developed only for arc furnaces in 1960's. And now it is revealed that it is not suitable for application to other fluctuating load installations through many researches. $P_{st}$ which is a flicker index used in IEC 61000-3-7, indicates visual inconvenience due to voltage fluctuation across large range of frequency and can be applied to fluctuating load installations as well as arc furnaces. In this paper, we introduce how to calculate and assess flicker emission level for the individual fluctuating load installations connected in EHV system and how to manage the emission levels in the power system according to IEC 61000-3-7.

• Wind and Structures
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• v.24 no.1
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• pp.79-93
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• 2017

Wind-induced fluctuating internal pressures in a building with a dominant opening can be described by a second-order non-linear differential equation. However, the accuracy and efficiency of the governing equation in predicting internal pressure fluctuations depend upon two ill-defined parameters: inertial coefficient $C_I$ and loss coefficient $C_L$, since $C_I$ determines the un-damped oscillation frequency of an air slug at the opening, while $C_L$ controls the decay ratio of the fluctuating internal pressure. This study particularly focused on the value of loss coefficient and its influence factors including: opening configuration and location, internal volumes, as well as wind speed and approaching flow turbulence. A simplified formula was presented to predict loss coefficient, therefore an approximate relationship between the standard deviation of internal and external pressures can be estimated using Vickery's approach. The study shows that the loss coefficient governs the peak response of the internal pressure spectrum which, in turn, will directly influence the standard deviation of the fluctuating internal pressure. The approaching flow characteristic and opening location have a remarkable effect on the parameter $C_L$.

• Special Issue of the Society of Naval Architects of Korea
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• 2005.06a
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• pp.44-50
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• 2005

Since a cavitation pattern in model scale can be different from that in full scale, it has been highly demanded to measure a fluctuating pressure induced by propeller in full scale. For the verification of the cavitation test for 105K lanker in the large cavitation tunnel in Samsung Ship Model Basin(SSMB), an effective pressure fluctuation measurement system was developed and a series of full scale measurements was carried out. These results were compared with those of cavitation tests in SSMB. The measured results in full scale gave good agreements to those in model tests. The fluctuating pressure at $2^{nd}$ blade frequency in full scale seems to be highly dependent upon tip loading.

• Wind and Structures
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• v.23 no.2
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• pp.143-155
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• 2016

The interaction between two different shaped structures is very important to be understood. Fluid-structure interactions and aerodynamics of a circular cylinder in the wake of a V-shaped cylinder are examined experimentally, including forces, shedding frequencies, lock-in process, etc., with the V-shaped cylinder width d varying from d/D = 0.6 to 2, where D is the circular cylinder diameter. While the streamwise separation between the circular cylinder and V-shaped cylinder was 10D fixed, the transverse distance T between them was varied from T/D = 0 to 1.5. While fluid force and shedding frequency of the circular cylinder were measured using a load cell installed in the circular cylinder, measurement of shedding frequency of the V-shaped cylinder was done by a hotwire. The major findings are: (i) a larger d begets a larger velocity deficit in the wake; (ii) with increase in d/D, the lock-in between the shedding from the two cylinders is centered at d/D = 1.1, occurring at $d/D{\approx}0.95-1.35$ depending on T/D; (iii) at a given T/D, when d/D is increased, the fluctuating lift grows and reaches a maximum before decaying; the d/D corresponding to the maximum fluctuating lift is dependent on T/D, and the relationship between them is linear, expressed as $d/D=1.2+{\frac{1}{e}}T/D$; that is, a larger d/D corresponds to a greater T/D for the maximum fluctuating lift.

• Proceedings of the KSME Conference
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• 2003.11a
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• pp.845-850
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• 2003

Unsteady nature of a tip leakage vortex in an axial flow fan operating at a design and off-design operating conditions has been investigated by measuring the velocity fluctuation in a blade passage with a rotating hotwire probe sensor. Two hot-wire probe sensors rotating with the fan rotor were also introduced to obtain the cross-correlation coefficient between the two sensors located in the vortical flow as well as the fluctuating velocity. The results show that the vortical flow structure near the rotor tip can be clearly observed at the quasi-orthogonal planes to a tip leakage vortex. The leakage vortex is enlarged as the flow rate is decreased, thus resulting in the high blockage to main flow. The spectral peaks due to the fluctuating velocity near the rotor tip are mainly observed in the reverse flow region at higher flow rates than the peak pressure operating condition. However, no peak frequency presents near the rotor tip for near stall condition.

• Wind and Structures
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• v.7 no.2
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• pp.71-88
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• 2004

In this paper the capacity of Covariance Proper Transformation (CPT) analyses to provide information about the wind loading mechanisms of bluff bodies is investigated through the application to square cylinders. CPT is applied to the fluctuating pressure distributions on a single cylinder, as well as on a pair of cylinders in the tandem and side by side arrangements, with different separations. Both smooth and turbulent flow conditions are considered. First, through the analysis of the contributions of each CPT mode to the total fluctuating aerodynamic forces, a correspondence between modes and aerodynamic components is sought, which is then verified through examination of the mode shapes. When a correspondence between modes and aerodynamic components is found, an attempt is made to separate the different frequency contributions to the aerodynamic forces, provided by each mode. From the analyses it emerges that (a) in most cases each mode is associated to one single force component, that (b) retaining a limited number of modes allows reproducing the aerodynamic forces with a rather good accuracy, and that (c) each mode is mainly associated with one frequency of excitation.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.67 no.6
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• pp.695-701
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• 2018

For a power grid that has a high wind penetration level, when wind speeds are continuously fluctuating, the maximum power point tracking (MPPT) operation of a variable-speed wind turbine (VSWT) causes the significant output power fluctuation of a VSWT, thereby significantly fluctuating the system frequency. In this paper, an improved power-smoothing scheme of a VSWT is presented that significantly mitigates the frequency fluctuation caused by varying wind speeds. The proposed scheme employs an additional control loop based on the frequency deviation that operates in combination with the MPPT control loop. To improve the power-smoothing capability of a VSWT in the over-frequency section (OFS), the control gain of the additional loop, which is set to be inversely proportional to the rotor speed, is proposed. In contrast, the control gain in the under-frequency section is set to be proportional to the rotor speed to improve the power-smoothing capability while avoiding over-deceleration of the rotor speed of a VSWT. The proposed scheme significantly improves the performance of the power-smoothing capability in the OFS, thereby smoothing the frequency fluctuation. The results clearly demonstrate that the proposed scheme significantly mitigates the frequency fluctuation by employing the different control gain for the OFS under various wind penetration scenarios.

• Journal of the Korean Society of Combustion
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• v.3 no.1
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• pp.71-78
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• 1998

The flame stabilization and the combustion characteristics of diffusion flame formed in the wake of a cylindrical bluff body with fuel injection are studied. With the turbulence generator, the flame stability limits and ion currents were measured and analyzed. The results from this experimental study are summarized as follows. The region with highest average value of ion currents in the middle of flame is moved to the upstream side by the turbulent components of main stream. The flame mass with partially active reaction is moved fast for uniform flow and turbulence generator G3, but the flame mass with relatively slow reaction is moved slowly for turbulence generator G1. If the turbulence generator with strong turbulent component is installed, the turbulent time scale is increased with movement from main stream side to recirculation zone as well as the flame stability limits is deteriorated. Though the special dominant frequency is not appeared in the eddy which exists in flame, high frequency characteristics are appeared in uniform flow and turbulence generator G3, and low frequency characteristics are appeared in uniform flow, turbulence generator G3 and G1.

• Journal of Power Electronics
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• v.16 no.5
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• pp.1851-1860
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• 2016

The traction motors in electric multiple unit (EMU) trains are powered by AC-DC-AC converters, and the DC link voltage is generated by single phase PWM converters, with a fluctuation component under twice the frequency of the input catenary AC grid, which causes fluctuations in the motor torque and current. Traditionally, heavy and low-efficiency hardware LC resonant filters parallel in the DC side are adopted to reduce the ripple effect. In this paper, an analytical model of the ripple phenomenon is derived and analyzed in the frequency domain, and a ripple control scheme compensating the slip frequency of rotor vector control systems without a hardware filter is applied to reduce the torque and current ripple amplitude. Then a relatively simple discretization method is chosen to discretize the algorithm with a high discrete accuracy. Simulation and experimental results validate the proposed ripple control strategy.

• Wind and Structures
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• v.24 no.2
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• pp.145-169
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• 2017

The auxiliary structures of a high-rise building, such as balconies, ribs, and grids, are usually much smaller than the whole building; therefore, it is difficult to simulate them on a scaled model during wind tunnel tests, and they are often ignored. However, they may have notable effects on the local or overall wind loads of the building. In the present study, a series of wind pressure wind tunnel tests and high-frequency force balance (HFFB) wind tunnel tests were conducted on rigid models of an actual super high-rise building with vertical ribs protruding from its facades. The effects of the depth and spacing of vertical ribs on the mean values, fluctuating values and the most unfavorable values of the local wind pressure coefficients were investigated by analyzing the distribution of wind pressure coefficients on the facades and the variations of the wind pressure coefficients at the cross section at 2/3 of the building height versus wind direction angle. In addition, the effects of the depth and spacing of vertical ribs on the mean values, fluctuating values and power spectra of the overall aerodynamic force coefficients were studied by analyzing the aerodynamic base moment coefficients. The results show that vertical ribs significantly decrease the most unfavorable suction coefficients in the corner recession regions and edge regions of facades and increase the mean and fluctuating along-wind overall aerodynamic forces.