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Determination of dc-link voltage switching ripple in three-phase PWM voltage source inverters (VSI) is important for the selection and design of the dc-link capacitor. In this paper the complete analysis of the dc-link voltage ripple is given for three-phase PWM VSIs, in case of dc source impedance and balanced load. The dc voltage ripple amplitude is analytically determined as a function of amplitude...
Investigated in this paper is the use of state variable feedback control combined with a PI controller for a single-phase grid-connected inverter with an LC filter operating under weak grid conditions. This condition is characterized by a grid impedance much larger than the inductance in the LC filter. Under weak grid conditions, variations in the grid impedance can degrade the performance of the...
Single-phase inverters are connected to the grid at the distribution system level. Because the grid impedance can be large at this level, it is important to consider the effect of the grid impedance on the control performance of the inverter. A PI controller is often adopted for current control in a single-phase inverter. With an increase in the grid impedance, the open loop cut-off frequency decreases,...
Inverters in voltage control mode including droop control and virtual synchronous generator (VSG) are promising for interconnecting the renewable energy to the grid, which are both emulating the frequency and voltage regulating functions of synchronous generators through droop method. However, improper transient impact current and reactive power output are observed for inverters in voltage control...
This work will present a novel photovoltaic (PV) inverter with integrated short-term storage. The topology combines advantages of microinverter topologies, such as module-specific maximum-power-point tracking, failure-tolerance, safety due to limited voltages, modularity, use of low-cost low-voltage components such as silicon field-effect transistors (FET), and simple expandability, with those of...
The nonlinear loads in microgrid not only distort the voltage but also degrade the harmonic power distribution in a conventional droop-controlled microgrid. This paper starts with analyzing the trade-off between voltage total harmonic distortion (VTHD) and harmonic sharing effectiveness. A multi-framed droop controller is proposed to improve the harmonic power sharing without deteriorating VTHD by...
Power of generation systems based on renewable sources steadily grows up. New single-phase PV power generation systems (PGS) are widely built. PGS based on PV modules can operate in two modes: autonomous mode and grid-tied mode. The important part of PGS based on PV in the grid-tied mode is transformer, which provides galvanic isolation. It is possible to remove the transformer from the PGS in order...
The aim of this paper is to explain a novel technique to improve voltage and frequency profiles on microgrids controlled using the droop control scheme. The method consists on retuning the controller according to the present load scenario by adjusting the droop coefficients in a way that is proportional to the load changes. The proposed technique is tested through numerical simulations and the results...
When a severe grid voltage sag has happened, the transient rush current caused by grid voltage sag can not be effectively suppressed by the voltage controlled grid tied inverter based on the conventional virtual synchronous generator(VSG) control. In order to improve the capacity of voltage controlled grid tied inverter to withstand power grid voltage sag, the paper proposes an enhanced robust strategy...
The well-known active power-frequency and reactive power-voltage amplitude droop control is widely used in the coordinated control of parallel inverters in microgrids. However, the dynamics of the conventional droop method is slow due to the low-pass filters used for power calculation purposes. Stability concerns also exist in the conventional droop method. This paper proposes a novel droop control...
The microgrid can operate in grid-connected mode and islanded mode, and the parallel operation of inverters will be involved in both modes. However, due to the differences of output characteristic among various inverters, the load power can't be shared equally. As a result, some methods without communication lines such as droop control and VSG (virtual synchronous generator) control have been proposed...
In this paper the hosting capacity considering voltage unbalance is estimated for single-phase photovoltaic inverters (PVIs) in low-voltage distribution networks in Sweden. A stochastic approach is used to calculate the negative-sequence voltage unbalance for each of the possible locations in the network. The method has been applied to 6 and 28-customer networks for the connection of 6-kW single-phase...
This paper addresses harmonic magnification due to resonance circuits resulting from interaction between uncertain grid impedance and converter. The source of harmonic may be either the grid or inverter. It is demonstrated that unknown and unpredictable grid impedance may result in variable resonance frequency, which challenges robust design of LCL filter of inverter.
This paper presents a harmonic study of different parallel photovoltaic (PV) systems connected to a low voltage distribution network to identify potential resonance between PV inverters and system impedance. LCL filters are commonly employed in PV inverters and recommended by IEEE 929–2000. These filters, however, by interacting with each other as well as with the system, may generate multiple harmonic...
For the islanded microgrid which is connected with nonlinear loads, an effective harmonic sharing method should be designed to guarantee the system stable operation. In this paper, a novel method of virtual impedance and PCC voltage compensation are proposed to achieve the harmonic sharing and voltage quality improvement at the point of common coupling (PCC). The fundamental and each harmonic currents...
Multistage systems are highly prone to issues of instability due to the interaction between converters. In this paper, the operation principles of a novel triple-phase-shift based dual active bridge converter are elaborated. Additionally, the impedance model of the cascaded system based on small signal analysis is proposed and applied for analyzing the stability of the cascaded system. The theoretical...
This paper proposes a novel control method to realize unbalanced load power sharing in an islanded microgrid. The proposed method consists of typical real-power-frequency (P-f) and reactive-power-voltage (Q-V) droop control, virtual impedance control which only functions at fundamental negative sequence, and unbalanced-power-small-signal-frequency (SU-ωss) droop control. The typical droop control...
This paper presents an eigenvalue-based impedance stability analytical method of multiple paralleled grid-connected inverter system. Different from the conventional impedance-based stability criterion, this work first built the state-space model of paralleled grid-connected inverters. On the basis of this, a bridge between the state-space-based modelling and impedance-based stability criterion is...
This paper presents an ultra-low power chopper-stabilized amplifier intended for portable or implantable ECG applications. A new technique to fundamentally improve the noise efficiency of such amplifiers is introduced that is based on Gm-boosting by AC-coupling up-modulated signals onto stacked inverters. Designed and simulated in a 65 nm CMOS process, the amplifier consumes 17.7 nW from a 1 V supply...
The increasing awareness of the need to reduce pollution and global warming, and the increasing price of conventional energy resources, encouraged many countries to adapt new energy policies by including renewable energy resources, such as solar energy, wind energy, tidal energy. These new energy sources are usually connected to the gird via inverters, which presents fast dynamics due to their small...
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