Byeong-Heon Kim

Anti-islanding detection method using Negative Sequence Voltage

This paper presents an active Anti-Islanding Detection (AID) method using Negative Sequence Voltage (NSV) injection to electrical grid through a three-phase photovoltaic (PV) inverters. Because islanding operation mode can cause a variety of problems, the islanding detection of grid-connected PV inverter is the mandatory feature. The islanding mode is detected by measuring the magnitude of Negative Sequence Impedance (NSI) calculated by the negative sequence voltage and current at the point of common coupling (PCC). Simulation and experiment are performed to verify the effectiveness of the proposed method which can detect the islanding mode in the specified time. The test has been done in accordance with the condition on IEEE Std 929–2000.

Image retrieval based on color coherence

The color histogram is a simple method for representing and comparing images, but this approach has a drawback that it does not capture spatial image attributes. Previously, color coherence vectors based on color homogeneous regions have been introduced for image retrieval since they include some spatial information. The color histogram constructed by connected components whose size is larger than a certain number of pixels. This definition takes only the spatial size into consideration. In this paper, we present new definitions of color coherence that can include spatial information, say, distance between regions. As a result, we have three types of color coherence measures. Pixels are grouped according to (1) region size only, (2) distance only and (3) both size and distance. We apply them to actual visual image retrieval applications. For the simulation, we proposed a fast segmentation and merging method. The experimental result showed that the distance information is more effective than the size information for the image retrieval.

Stability-Oriented Design of Frequency Drift Anti-Islanding and Phase-Locked Loop Under Weak Grid

The stability of a grid-tied, power conditioning system (PCS)—equipped with frequency drift anti-islanding—is analyzed in this paper. To reduce the nondetection zone (NDZ), the positive feedback gain (PFG) of the frequency drift method should be increased. However, it was reported that PFG is restricted due to stability issues of the system under a weak grid. Careful analysis revealed that such a restriction is also related to the parameters of the phase-locked loop (PLL). As the parameters of PLL are represented by the damping coefficient and natural frequency, which demonstrate the second-order filter characteristics, the investigation shows that the damping coefficient has a close relation to the maximum PFG for a stable operation of the system in the weak grid. To enhance the stability margin and increase PFG, an alternative method is proposed to estimate grid frequency. Since the proposed method reduces the negative resistance region in PCS, it can be stabilized more than the conventional estimation method. Furthermore, the PFG can be increased to reduce NDZ.

Shaping of PWM Converter Admittance for Stabilizing Local Electric Power Systems

This paper suggests the concept of shaping the pulsewidth-modulated (PWM) converter admittance. The analytic admittance matrix equation, which is derived from the small signal model of a PWM converter, is formulized. Based on the damping coefficient and the natural frequencies of phase-locked loop and dc voltage loop, the variations in admittance frequency responses are discussed. In addition, the frequency response can be adjusted through pole placement using the gain of the current controller. The controller gain for a stable operation is designed by detailed understanding of the frequency response shape. Furthermore, to stabilize the local electric power systems (EPSs) that invoke instability through the interaction between constant power load PWM converters, the admittance shaping loop is utilized for the distributed generator PWM converters. Through computer simulation and hardware experiments, the proposed shaping guidance can effectively stabilize local EPS.

Anti-islanding accelerated by grid unbalance component

In this paper, a method to detect islanding operation of distributed generator (DG) is proposed, which exploits frequency drift accelerated by grid unbalance component, namely negative sequence component of grid voltage. The negative sequence component could be added to the gain of frequency feedback loop in the conventional anti-islanding scheme. To keep the power factor regulation, the displacement power factor (DPF) is considered in the tuning of feedback gain. The PLL (Phase-Locked Loop) with low bandwidth is used to replace the nominal frequency in calculating frequency deviation, and this brings the reduction of unnecessary reactive power injection during normal grid connected operation. The experimental results are shown to verify the effectiveness of the proposed method, which show that both detection time and NDZ (Non-Detection Zone) are improved. Also, the performance of the proposed method with multiple DG systems is discussed by introducing the equivalent single virtual inverter. The simulation and experimental results show validity of discussions.

Analysis of Non Detection Zone for multiple distributed PCS based on Equivalent Single PCS using reactive power approach

In this paper, the methodology to analyze Non-Detection Zone (NDZ) of islanding detection algorithm for multiple Power Conditioning Systems (PCSs) connected to grid is proposed. The PCSs with renewable energy source or battery energy storage system are equipped with anti-islanding functionality and the method using frequency positive-feedback is one of the most popular methods to detect islanding condition. Because it is figured out that the positive feedback of grid frequency is only valid on reactive current, not the phase of current or active current, the reactive power according to the frequency drift algorithm is only considered for NDZ analysis in the proposed analysis. Additionally, the reactive power component by frequency drift anti-islanding can be linearized in normal operation range of grid frequency. By incorporated with this concept, Equivalent Single PCS (ESPCS), which equivalently supplies whole reactive current of multiple PCSs, is proposed and this could emulate the operation of multiple parallel PCSs without loss of the detection capability. The NDZ is analyzed by ESPCS and is compared to the NDZ of parallel PCSs. Through the computer simulation and hardware experiments, it is confirmed that NDZ based on ESPCS and operation dynamics is well matched to those of multiple PCSs.

Effect of current measurement error in angle estimation of permanent magnet AC motor sensorless control

This paper discusses the effect of current measurement error in angle estimation of permanent magnet AC motor for position sensorless control based on motor back EMF. The current measurement errors due to non-ideal elements in the system are classified into the scaling error, the offset error, and the discretization error. Those errors have been represented in the estimated synchronous reference frame. The current measurement errors affect the sensorless observer input and the estimated angle. With these results, the effects of the current measurement errors on the estimated angle for the sensorless control are analyzed. Through the analysis, it has been figured out that the estimated angle for the sensorless control is directly affected by the current measurement. The analysis is verified by computer simulation and experimental results.

Three-level back-to-back converters with ESS directly connected to the lower DC-link

With the conventional circuit configuration of DC microgrids, two power conversion stages are required to transfer the energy from the Energy Storage System to loads or sources: DC/DC and DC/AC conversions. This paper proposes a new circuit configuration which puts the battery directly to the lower DC-link of 3-level back-to-back converters. The proposed circuit reduces the two-step power conversion stage to one-step. The working principle of the proposed circuit is analyzed, and the controllers for the circuit are designed. The validity of the control algorithm is verified by computer simulation.

Anti-islanding Detection of Photovoltaic Inverter Based on Negative Sequence Voltage Injection to Grid

This paper presents an active anti-islanding detection method using negative sequence voltage injection to the grid through a three-phase photovoltaic inverters. Because islanding operation mode can cause a variety of problems, the islanding detection of grid-connected photovoltaic inverter is the mandatory feature. The islanding mode is detected by measuring the magnitude of negative sequence impedance calculated by the negative sequence voltage and current at the point of common coupling. Simulation and experimental test are performed to verify the effectiveness of the proposed method which can detect the islanding mode in the specified time. The test has been done in accordance with the condition on IEEE Std 929-2000.