Design of Power Decoupling Strategy for Single-Phase Grid-Connected Inverter Under Non-Ideal Power G

Design of Power Decoupling Strategy for Single-Phase Grid-Connected Inverter under Non-Ideal Power Grid

Abstract: Because a single-phase inverter has a power coupling between the dc bus and the ac side, the dc bus always requires large electrolytic capacitors for the power decoupling. Although the active power decoupling circuit can restrain the ripple voltage with twice fundamental frequency on the dc bus and reduce the required capacitance, previous researches mainly focused on the power coupling of an ideal power grid, and just decoupled the ripple power with twice fundamental frequency.

This study analyzes the power coupling in a non-ideal power grid, and designs a novel decoupling method for the power with multi harmonic frequency on the dc bus. By modifying the reference values of dc series split capacitors, the system control structure can be simplified, and the multi-frequency-coupled power decoupling can be realized. Moreover, a notch filter is introduced into the dc voltage feedback path for further reducing the influence of the power coupling on the inverter output current quality. The proposed method can achieve the objective of the multi frequency- coupled power decoupling, even under a weak power-grid environment. Finally, numerical simulations and experimental results are provided to verify the effectiveness of the proposed method in comparison with the

traditional capacitor decoupling framework and the dual-voltage control decoupling scheme.

Existing system: Introduced a new decoupling method with a bidirectional buck-boost circuit paralleled with the dc-bus capacitor as the decoupling topology. The decoupling controller in utilized a proportional-integral (PI) controller for stabilizing the dcbus voltage while a multi-resonant controller was paralleled for suppressing the ripple voltages. In some particular situations, such as photovoltaic power generation systems, some researchers have proposed multi-objectives decoupling topologies. The topologies in can alleviate the leakage current, and reduce the total required capacitance by achieving the power decoupling on the dc bus. The method in can achieve the objectives of the leakage current suppression and the ripple power reduction .

Proposed system: proposed a new control method for the decoupling topology in and discussed the effects of the capacitance mismatch on the power decoupling. In , each dc-bus capacitor voltage was forced to track a different dc offset and a double-linefrequency, rather than the sinusoidal line-frequency . Although the capacitance mismatch in will not result in odd harmonic frequencies, it may reduce the decoupling ability. The topologies proposed in changed the full-bridge inverter in to a half-bridge inverter, i.e., two power switches were saved. In these topologies, one bridge arm is used to regulate the grid current and the dc-bus voltage, and another bridge arm is used to achieve the power decoupling.

Advantages: Although it is relatively simple, it is sensitive to the parameter variations of the energy storage component, and is difficult to achieve the accurate power decoupling. The closed-loop control method can realize the power decoupling automatically by controlling the ripple voltage on the dc bus.

Moreover, it is less sensitive to the parameter variations, and the accuracy of the decoupling control can be effectively increased. Parallel capacitors are usually introduced into the active decoupling circuit for absorbing the ripple power.

Depending on the location of the parallel capacitors, it can be further divided into the ac-side decoupling and the dc-side decoupling.

Disadvantages: Besides, the electrolytic capacitor has the disadvantages of the low reliability and short life expectancy. Thus, the active power decoupling method has attracted more and more attention in recent years.

In general, the goal of active power decoupling is achieved by adding a decoupling circuit for removing the ripple power from the dc bus to an additional energy storage device. Due to the inductor loss and its parameter fluctuation, capacitors are often used as the energy storage device.

Modules:

Active Power Decoupling Method : The active power decoupling method can reduce the second-order ripple power coupling between the dc bus and the ac side. The dual-voltage control decoupling circuit in is depicted, where is composed of a half-bridge inverter circuit and two series capacitors. The mid-point of the two capacitors is connected to the halfbridge inverter circuit through a filter inductor. Except that, this system did not require additional energy storage devices. By modulating the half-bridge inverter circuit, the average voltage on each of two series capacitors is Udc / 2. Meanwhile, there is a line-frequency sinusoidal component superimposed on them with the 180o phase shift.

Multi-Frequency-Coupled Power Decoupling Under Weak Power Grid : As the penetration of distributed generation systems increases, the power-grid impedance is correspondingly high, which makes the power grid to gradually perform a weak power-grid status. This high power-grid impedance will lead to the distortion on the grid voltage. Therefore, the voltage at the point of common coupling (PCC) contains more low frequency harmonics. By taking the 3rd harmonic component as an example, the expression of the grid voltage (ug) and the grid current (ig) can be represented.

multi-proportional-resonant : where s is the Laplace operator; krp and kri are the proportional gain and the resonance gain, respectively; ωhi and ωb are the resonance frequency and the bandwidth of the ith resonance controller, respectively. The multi-PR controller can generate higher gain and achieve favorable tracking without static error at the specified frequency. When the ripple voltage command is zero, the half-bridge inverter can be controlled to restrain the ripple voltage (urip). The output of the multi-PR controller is the current reference of the half-bridge inverter inductor, and it is compared with the corresponding feedback value of if. Then, the comparison will be sent to a proportional controller and generate a modulation.

Phase - locked loop : In this study, the discussion is mainly focused on the decoupling method. Owing to the relative independence between the control of the decoupling circuit and the inverter, the kind of the phase-locked loop (PLL) in this study is the commonlyused synchronous reference frame PLL (SRF-PLL) [35]. As for the situation of weak power grid, the stability should be analyzed including the consideration of the SRF-PLL. The analysis process of the influence of the SRF-PLL adopted in this study is the same as the one in [35], and the corresponding detailed derivations can be referred to [35]. In order to demonstrate the stability of the inverter under the weak grid, the bode plots of the additional admittance introduced by the SRFPLL and the impedance criterion (Zg/Zo), which is the ratio of the grid impedance (Zg) and the inverter impedance (Zo), is depicted .