Xiaolong Yue

A Novel Adaptive Frequency Injection Method for Power Electronic System Impedance Measurement

Power electronic systems are prone to negative impedance instability due to the constant-power nature of the individual components. The impedance measurement, therefore, is very important during the system designing because of the close relationship between impedance and stability. However, traditional frequency sweep methods, by injecting a series of sinusoidal perturbation signals equidistantly, always select the frequency interval on experience accumulating without guiding theory, which result in low accuracy, low reliability, and low efficiency. In this paper, the relationship between measured results and the frequency interval is built first, then the frequency interval selection criteria of injection signals are derived accordingly. Afterward, based on the above analysis, a novel adaptive frequency injection method of impedance measurement is proposed to figure out the practical issues in power electronic system impedance measurement process. With this method, variable frequency interval can be selected according to characteristics of the measured network automatically. Compared to the traditional frequency sweep method, the proposed one can measure impedance more accurately and efficiently. The simulation and experimental results validate the effectiveness of the proposed method.

A Matrix-Based Multifrequency Output Impedance Model for Beat Frequency Oscillation Analysis in Distributed Power Systems

Power electronic converters are single input multiple output (SIMO) systems in frequency domain. In distributed power system (DPS), one converters switching frequency ripples are another converters perturbations, and beat frequency components are generated due to their SIMO characteristics. As the control loops of power converters always take high gains in low-frequency regions if the beat frequency components are relatively low, they may be magnified and presented as oscillations, which will introduce system instability. This paper proposes a matrix-based multifrequency output impedance model to describe the SIMO characteristics of power converters and to analyze the stability of DPS. The buck converter is specifically illustrated as demonstration. The proposed model indicates that the variation of switching frequency, as well as those traditionally discussed parameters of control loops and passive components, could also change output impedance characteristics and introduce system instability. In DPS, the beat frequency oscillation that traditional models fail to explain could be accurately predicted by the proposed model. In addition, based on the proposed model, the switching frequencies for power converters can be optimized to improve the stability of power electronic-based systems. Simulation and experimental results validate the accuracy and the effectiveness of the proposed method.

A modulation and sampling based modeling method for the nonlinearities of power converters and its application analysis

Power electronic converters are single input multiple output (SIMO) systems in frequency domain. In distributed power system (DPS), one converters switching frequency ripples are another converters perturbations. However, traditional modeling methods only take the frequency of input perturbation into consideration and the power converters are simplified as linear models, which are inappropriate to analyze the high frequency interactions. This paper proposes a modulation and sampling based modeling method to describe SIMO frequency characteristic of power converters and to analyze the oscillations introduced by switching frequency ripples in DPS.

Harmonic impedance measurement for an islanded microgrid using current injection

Harmonic impedance is the important parameter to analyze the stability of power system. In the microgrid, due to the existence of power converters with constant power characteristics which may result in negative impedance and thus decreases the system stability, harmonic impedance measurement is also of great significance. In this paper, the droop control strategy for parallel inverters in microgrid with virtual impedance to increase its stability is briefly introduced. Three most commonly used methods for harmonic impedance measurement, including the capacitor switching method, the thyristor-controlled sub-circuit switching method and the direct harmonic current injection method, are studied. By comparing advantages and disadvantages of different methods, the harmonic current injection method is chosen to measure the harmonic impedance of the microgrid system. The simulation results under PSCAD demonstrate that the harmonic current injection method is valid to measure the harmonic impedance of microgrid.

Modeling and analysis for output impedance of buck converters

In distributed power systems, the impedance model of power modules is very important because of the close relationship between impedance and stability. For buck converters with a sinusoidal current perturbation at the output terminal, the output voltage will contain multiple components in frequency domain. The output impedance characteristics of power converters are actually single input multiple output (SIMO). However, traditional models and modeling methods only take the perturbation frequency into account and ignore the other components. The obtained impedance models, with single input single output (SISO) forms, are actually simplifications of the real impedance. In this paper, the output impedance of buck converters with SIMO characteristics is modeled by a matrix based method. Detailed analysis shows that it is reasonable to only consider the perturbation frequency itself in low frequency regions, but in high frequency regions, especially those around the switching frequency and its multiples, the corresponding low frequency component is also very significant and should be taken into account. Simulation and experimental results validate the accuracy and effectiveness of the proposed modeling method and conclusions.

Quantified evaluation and criteria analysis for DMPPT PV system

Distributed Maximum Power Point Tracking (DMPPT) concept provides effective anti-mismatch feature, however, it fails in extreme mismatch situation. To get full profit from PV system, quantitatively static analyzes of major DMPPT structures are presented. The existence criteria and quantitative evaluation method for optimal power region of different DMPPT structures are first proposed. Simulation and experimental case study are provided to prove the effectiveness of the proposal.

Modeling for input impedance of buck converters and its application analysis

Impedance is very important for power electronic systems because of the close relationship between impedance and system stability. The power converters are typical nonlinear dynamic systems in frequency domain. With a sinusoidal input voltage perturbation excitation, the input current contains not only the input perturbation frequency, but also multiple other frequency components. Therefore, the input impedance has single input multiple output (SIMO) characteristic in frequency domain. However, traditional impedance modeling methods only take the component at input perturbation into consideration and simplify the model in a single input single output (SISO) form. They are actually simplifications of real input impedance in low frequency situation, which fail to explain some oscillations in distributed power system and micro-grid. This paper proposes a new input impedance model to describe its SIMO characteristic. The buck converter is introduced specifically as demonstration. The proposed model could accurately explain the oscillations in a system consist of two cascaded power electronic converters, whereas traditional models fail to do so. The simulation results validate the accuracy and effectiveness of the proposed model and obtained conclusions.

A new current injection method for impedance measurement using superposed modulated square pulse

Harmonic impedance measurement is very important in Micro-grid because of the close relationship between impedance and stability. This paper presents a new current injection method for impedance measurement. Sinusoidal amplitude modulation is a method to shift signals to different frequency bands. A signal with large frequency bandwidth and small amplitude attenuation can be obtained by superposing square pulse signals with sinusoidal amplitude modulation. When the superposed signal is injected to the system as a current source, impedance information in the frequency range of interest can be calculated with short time cost and high accuracy. The simulation results demonstrate the effectiveness of this method.

Calculation and measurement of harmonic impedance for a microgrid operating in Islanding mode

Stability analysis is very important in the design and in the operation of microgrid, because microgrid always contains many Distributed Generations(DGs), power electronic converters and constant power loads. Harmonic impedance is a useful parameters in the stability analysis. In this paper, the structure of microgrid is introduced detailed. The control strategies of inverter operating in different mode are discussed. The calculation of each component of microgrid is studied, and then a harmonic impedance theoretical calculated value of microgrid is given. An example microgrid model is developed in PSCAD. The harmonic impedance of microgrid is measured by injecting a current disturbance. Simulation results show that the theoretical calculated value is coincident with the measured value. The calculation of harmonic impedance for microgrid is valid.

A new impedance measurement method based on high frequency compensation

Impedance is very important in Micro-grid and other distributed power system based on power electronics because of the close relationship between impedance and stability. This paper presents a new current injection method for impedance measurement. Sinusoidal amplitude modulation is a method to shift signals to different frequency bands. A signal with large frequency bandwidth and small amplitude attenuation can be obtained by superposing square pulse signals after sinusoidal amplitude modulation. When the superposed signal is injected to the system as a current source, impedance information in the frequency range of interest can be calculated with short time cost and high accuracy. The simulation results demonstrate the effectiveness of this method.