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The grid-feeding voltage controlled inverter (GF-VCI) based on droop control is vulnerable to the harmonic voltages of the utility grid. Because of the equivalent impedance of system is small, the slight distorted grid voltage will result in the harmonic component increasing of GF-VCI output currents. Therefore, the reason of generation of distorted grid-feeding current of GF-VCI under the distorted...
The use of the virtual admittance concept for harmonic and/or unbalance compensation in microgrids when multiple inverters operate in parallel is proposed in this paper. The virtual impedance concept has been traditionally used for these purposes. However, one drawback of the virtual impedance is that it can only be applied to distributed generation (DG) units operating in voltage control mode (VSI-VCM),...
With increasing penetration of grid-connected photovoltaic (PV) power generation in distribution network, the equivalent impedance of the distribution network and the distribution capacitance of the interconnection cables are changed. The changed impedance network may be excited by the harmonic current of inverters or the harmonics of the grid voltage, which will affect the power quality and stable...
This paper discusses the dynamic interaction of multi-paralleled inverters within a weak grid. Interactive current and common current models are proposed to explain the interaction among these inverters, which are studied with both open loop and closed loop analysis. An aggregated model is proposed to describe the totality of multi-inverters. Additionally, system stability is explicitly studied and...
Distributed generation (DG) has gained popularity in recent years due to the increasing requirement for renewable power sources. A problem that exists with DG systems is the islanding of DG units that creates safety issues for personnel as well as the potential for damage to utility infrastructure. Therefore, islanding detection methods are utilized to mitigate the risk of islanded operation of DG...
This paper analyzes the harmonics emission, propagation, and mitigation issues in an MVDC distribution system. Specifically, the DC current harmonics from a high power NPC DTC motor drive is analyzed. It is found that DC network resonance is a major cause of DC current harmonics amplification and propagation in the MVDC system. As a result, adding a DC-side input filter inductor is proposed to attenuate...
To address the generation uncertainty problems introduced by renewable energy power plants, STATCOMs are a typical solution to smoothen the point of common coupling (PCC) voltage in order to facilitate the integration of the renewables. A recently proposed control method — virtual synchronous machine (VSM), which controls grid-interfaced inverters to behave like synchronous machines so as to be more...
In this paper a unified Impedance-based Stability Criterion (UIBSC) has been proposed for paralleled grid-tied inverters. Compared to the traditional Impedance-based Stability Criterion (IBSC) which needs to evaluate all minor loop gains (MLGs) of each individual inverter, the UIBSC only checks the derived global minor loop gain (GMLG) once to determine system stability. As a result, the computation...
As the number of power electronics based systems are increasing, studies about overall stability and harmonic problems are rising. In order to analyze harmonics and stability, most research is using an analysis method, which is based on the Linear Time Invariant (LTI) approach. However, this can be difficult in terms of complex multi-parallel connected systems, especially in the case of renewable...
The increase of grid-tied distributed generators (DGs) and nonlinear loads may generate harmful low order harmonics, polluting the grid and deteriorating the operation of other power electronics based loads and DGs. In order to improve the output current of the voltage source inverter (VSI) in grid-tied DG system, two methods are proposed in this paper to compensate the output harmonic under distorted...
Characterized by grid impedance variation and background grid voltage harmonics, weak grid may give rise to injected grid current distortion and cause stability issues in a grid-connected AC/DC converter. To lessen the effect, a composite compensation method is proposed in this paper based on output impedance model, which can increase control bandwidth and improve system's robustness against grid...
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