• Journal of Power Electronics
• /
• v.18 no.1
• /
• pp.195-203
• /
• 2018

In the variable speed Wind Turbine based on ElectroMagnetic Coupler (WT-EMC), a synchronous generator is coupled directly to the grid. Therefore, like conventional power plants, WT-EMC is able to inherently support grid frequency. However, due to the reduced inertia of the synchronous generator, WT-EMC is expected to be controlled to increase its output power in response to a grid frequency drop to support grid frequency. Similar to the grid frequency support control of Type 3 or Type 4 wind turbine, inertial control and droop control can be used to calculate the WT-EMC additional output power reference according to the synchronous generator speed. In this paper, an experimental platform is built to study the grid frequency support from WT-EMC with inertial control and droop control. Two synchronous generators, driven by two induction motors controlled by two converters, are used to emulate the synchronous generators in conventional power plants and in WT-EMCs respectively. The effectiveness of the grid frequency support from WT-EMC with inertial control and droop control responding to a grid frequency drop is validated by experimental results. The selection of the grid frequency support controller and its gain for WT-EMC is analyzed briefly.

• International Journal of Fluid Machinery and Systems
• /
• v.8 no.2
• /
• pp.102-112
• /
• 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.

• Journal of Power Electronics
• /
• v.15 no.2
• /
• pp.518-529
• /
• 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 radiological science and technology
• /
• v.41 no.5
• /
• pp.463-469
• /
• 2018

Morphological information such as shape and margin of micro lesion is important information for diagnosis of disease in clinical imaging. In this study, we investigated the morphological changes of the micro lesions by comparing the contrast and area in grid suppressed DR images according to grid frequency. In the profile analysis of the image, the mass showed an average intensity variation of 8.6 ~ 72.4 after suppression, The higher the grid frequency, the more the contrast was increased. However, in the images obtained using 103 lp / inch, which is a grid frequency less than the sampling frequency, the contrast of the mass in the vertical direction decreased after suppression. In the binary image, the area change of the mass was also large. As a result, the shape, size, and margin of the mass changed. In the case of very small calcification, the higher the grid frequency is the larger the change in contrast, so that a clear image can be obtained in the post-suppression image. However, we could confirm that the margin of the lesion was blurred and the lesion was lost in some of the images using the 103 lp / inch grid. The higher the frequency of the grid, The change of the contrast of fiber occurred largely and clear boundary was confirmed. The decrease of the number of pixels was small and morphological change was small. In conclusion, when using a grid frequency that is not suitable for the sample frequency, morphological changes or lesion loss of micro lesions in the post- suppression image may give the possibility of misdiagnosis in diagnosis and differentiation of the image.

• Proceedings of the KIPE Conference
• /
• 2013.11a
• /
• pp.125-126
• /
• 2013

ESS(Energy Storage System) for Improve the quality of the power grid, supply reliability, system stability and the efficient operation method of power is drawing attention. According to changes in the load of the power system frequency will be adjustable in real time in response to changes in the frequency of the grid, so thermal power output is mainly controlled in order to keep the grid frequency stable. ESS for adjusting the frequency of the grid when the frequency rises to grid and charge the energy storage device. If the frequency drops to discharge the battery power to the grid and the future is expected to replace the thermal power plant. This paper describes 4MW ESS for the frequency regulation and find out about the characteristics through the PSCAD/EMTDC.

• Journal of Power Electronics
• /
• v.11 no.4
• /
• pp.599-605
• /
• 2011

This paper proposes a gain scheduling method that improves the stability of grid-connected systems employing an LCL-filter. The method adjusts the current controller gain through an estimation of the grid impedance in order to reduce the resonance that occurs when using an LCL-filter to reduce switching harmonics. An LCL-filter typically has a frequency spectrum with a resonance peak. A change of the grid-impedance results in a change to the resonant frequency. Therefore an LCL-filter needs a damping method that is applicable when changing the grid impedance for stable system control. The proposed method instantaneously estimates the grid impedance and observes the resonant frequency at the same time. Consequently, the proposed method adjusts the current controller gain using a gain scheduling method in order to guarantee current controller stability when a change in the resonant frequency occurs. The effectiveness of the proposed method has been verified by simulations and experimental results.

• Journal of IKEEE
• /
• v.22 no.3
• /
• pp.539-547
• /
• 2018

Grid-connected inverter is required to cut off the power supplied to the grid at the islanding condition, immediately. For this reason, an islanding detection is an indispensable function for grid-connected distributed generation system. Slip-Mode frequency Shift (SMS) islanding detection method is very popular method to determine the grid state. SMS method supplies the reactive power to the load according to the grid frequency. In the islanding condition of grid, this injected reactive power pulls out the grid frequency from the allowable range, then the inverter system can detect the islanding condition of the grid. The SMS method can detect the islanding state well and does not generate any harmonics of the grid current. However, the reactive power would be generated and the power quality is reduced even though the grid is not islanding condition, but normal condition. In this paper, a modified SMS method is proposed to remove the reactive power in the normal condition. The performance of the proposed method is evaluated by 600W single phase inverter experimental results.

• Journal of Power Electronics
• /
• v.18 no.3
• /
• pp.853-862
• /
• 2018

Frequency coupling in the phase domain is a recently reported phenomenon for phase locked loop (PLL) based three-phase grid-inverter systems. This paper investigates the mechanism and stabilization method for the frequency coupling to the stability of grid-inverter systems. Self and accompanying admittance models are employed to represent the frequency coupling characteristics of the inverter, and a small signal equivalent circuit of a grid-inverter system is set up to reveal the mechanism of the frequency coupling to the system stability. The analysis reveals that the equivalent inverter admittance is changed due to the frequency coupling of the inverter, and the system stability is affected. In the end, retuning the bandwidth of the phase locked loop is presented to stabilize the three-phase grid-inverter system. Experimental results are given to verify the analysis and the stabilization scheme.

• Journal of Power Electronics
• /
• v.16 no.1
• /
• pp.18-26
• /
• 2016

This paper proposes a compensation algorithm for reducing a specific ripple component on synchronous reference frame phase locked loop (SRF-PLL) in grid-tied single-phase inverters. In general, SRF-PLL, which is based on all-pass filter to generate virtual voltage, is widely used to estimate the grid phase angle in a single-phase system. In reality, the estimated grid phase angle might be distorted because the phase difference between actual and virtual voltages is not 90 degrees. That is, the phase error is caused by the difference between cut-off frequency of all-pass filter and grid frequency under grid frequency variation. Therefore, the effects on phase angle and output current attributed to the phase error are mathematically analyzed in this paper. In addition, the proportional resonant (PR) controller is adapted to reduce the effects of phase error. The validity of the proposed algorithm is verified through several simulations and experiments.

• Journal of Electrical Engineering and Technology
• /
• v.10 no.3
• /
• pp.1335-1341
• /
• 2015

The utilization of electric vehicles has been suggested to support the frequency regulation of power system. Assuming that an intermediate aggregator exists, this study suggests a decentralized vehicle-to-grid operation scheme in which each vehicle-to-grid aggregator can behave independently of the power system operator. To implement this type of decentralized operation, this study adopts a price-based operation that has been proposed by many researches as an alternative operation scheme for the power system. In this environment, each vehicle-to-grid aggregator can determine its participation in vehicle-to-grid service in consideration of its residual energy of aggregated system and real-time market price. Consequently, the main purpose of this study is to verify whether or not the vehicle-to-grid power can effectively support the current frequency regulation function within the price-based operation scheme. Specifically, a frequency regulation method is proposed based on the real-time price signal, and a feedback controller for battery management is designed for decentralized vehicle-to-grid operation.