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The role of inertia emulation and voltage control is fundamental to support the integration of distributed generation in weak power grids and Virtual Synchronous Machines (VSMs) represent an attractive solution to assume both of these roles. However, VSMs act as voltage sources and this makes it difficult to address power quality issues. Virtual impedances are commonly applied in the literature to...
In an islanded microgrid, harmonic current is controlled (or shared) by modifying the effective impedance in the path of harmonic current. This effective impedance includes line impedance and the internal impedance of the generation unit. Reported works consider the aforementioned impedances as separate entities and their combined effect in harmonic current flow has not been discussed so far. This...
This paper presents a proportional harmonic power sharing (PHPS) control scheme to improve the voltage quality at point of common coupling (PCC) and share harmonic power simultaneously for an islanded microgrid. The proposed control strategy employs P+ multiple resonant controller, and P+ plus Q+-based improved droop control scheme to improve the voltage quality at the load bus. Additionally, the...
Harmonic power can be shared by controlling the equivalent harmonic impedance of distributed generation (DG) units in islanded microgrids. This paper firstly reviews several methods for controlling the equivalent harmonic impedance of DGs and adopts the method based on resistive active power filter (R-APF). Then, a droop relationship between the equivalent harmonic impedance of DGs and their remaining...
Current-controlled LCL-filtered inverters are usually paralleled together for interconnecting the renewable energy resources into the grid. When the inverter output impedance is not well designed, the harmonic circulating currents out of the control bandwidth may be easily aroused. To reveal this resonant phenomenon and to enhance the stability of the parallel system, this paper builds the parallel...
In DC microgrids, constant power loads (CPLs) would lead to a destabilizing effect that may cause the main bus voltage oscillation. Hence, most researcher are exploring the stabilization methods for DC/DC buck converters feeding CPLs. In practical applications, buck converters are always cascaded with LC input filters to prevent the switching harmonics from returning into the source. With the addition...
Voltage-sensorless control has been investigated for Voltage Source Inverters (VSIs) for many years due to the reduced system cost and potentially improved system reliability. The VSI based Resistive Active Power Filters (R-APFs) are now widely used to prevent the harmonic resonance in power distribution network, for which the voltage sensors are needed in order to obtain the current reference. In...
The grid inductance is a sensitive parameter to be considered in order not to degrade the control system performance and to guarantee the system stability. In this paper, the Grid Inductance estimation based on Generalized Integrators (GIGI) method is proposed. It carries out an online grid inductance detection, which disturbs the grid by injecting a time-limited and small-amplitude harmonic voltage...
Capacitor-current-feedback active damping method is commonly used in LCL-filtered grid-connected inverters, which attenuates the resonance peak by emulating a virtual resistor in parallel with the filter capacitor. Taking the digital control delay into account, the emulation component becomes frequency-dependent, and it will turn negative when the filter resonance frequency becomes higher than one-sixth...
Frequency domain passivity theory is used to evaluate harmonic resonance instabilities in a system with multiple grid-tied voltage source converters (VSCs), and subsequently, a stabilizing controller using predictive current control (PCC) method is proposed to prevent such instabilities. The input admittance of VSCs using PCC can achieve passivity almost up to the Nyquist frequency. This research...
Harmonic resonances around the intersection frequency is triggered with the stability margin decreasing to zero and the system will be unstable with the stability margin reducing to be negative. For suppressing harmonics, the phase difference between the inverter admittance and grid admittance should be lower than 120° at their intersection. Based on this principle, a phase-reshaping strategy aiming...
The grid-connected inverter with a LCL filter has the ability of attenuating the high-frequency current harmonics. The current distortion caused by harmonics in the grid voltage is difficult to be eliminated. Applying proportion feedforward of the grid voltage can suppress the effect of grid voltage harmonics. However, weak grid is commonly seemed as a voltage source which contains a grid impedance...
In this paper, an eigenvalue-based harmonic stability analysis method for offshore wind farm is proposed. Considering the internal cable connection layout, a component connection method (CCM) is adopted to divide the system into individual blocks as current controller of converters, LCL filters, collection system cable impedance, transmission cable impedance and grid impedance. By using this method,...
This paper presents an approach for upgrading droop control aimed to improve its performance when used for two parallel inverters. The proposed control ensures equal voltage for the two inverters when different power flowing through them which guarantee zero current circulation. To fulfill the voltage regulation, the second inverter (slave) regulates its voltage to follow the master inverter reference...
Presented in this paper is grid current observer-based compensation control for a single-phase grid-connected inverter that is connected to a weak grid. Traditional single-loop PI current control performance suffers in the presence of the grid voltage distortion and grid impedance uncertainty. To compensate for disturbances from the grid side, two different compensation control structures are proposed...
Grid-connected converters have been widely used in distributed generations. However, the resonance brought by converters in weak grid has been a great threat to system stability. Impedance-based method provides a powerful tool for stability analysis. Active damping is also widely applied to improve system stability. This paper derives the impedance model of grid-connected converter system and analyzes...
This paper proposes an impedance-based active-filter additional control method for grid connected PWM-converters, by which a PWM-converter can help improve the voltage quality at a sensitive load bus (SLB) of a complex network with many distributed unbalanced loads and nonlinear loads. To realize the method, the information of the voltage to be compensated needs to be acquired and transferred to the...
A photovoltaic inverter control strategy based on the virtual impedance method is proposed, which makes the inverter compensate the harmonic of power grid to achieve the purpose of harmonic suppression while the inverter transmit the active power to the grid. An algorithm of finding the best virtual impedance is designed. The search range of the virtual impedance is determined by estimating the equivalent...
The paper presents a novel filtering concept for multi-cell switch-mode power amplifiers intended to be used for high quality power sources up to the kW-region. Multi-cell switch-mode amplifiers usually are operated in interleaved PWM resulting in effective output voltage harmonics being much higher than the cell's switching frequency. This major advantage, however, is only given for ideally balanced...
Capacitor-switching operation can be used to measure network harmonic impedance. Via the transient components of voltage and current after capacitor switching, the harmonic impedance of PCC (Point of Common Coupling) can be calculated. To improve measurement accuracy, the TLS-ESPRIT algorithm is introduced in this paper to filter out noise and fundamental components in voltage and current signals...
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