Inverter Design Shines in Photovoltaic Systems | Renewable Energy Applications

Jul 1, 2008 12:00 PM
By Eric Zhang, Senior Technical Marketing Engineer, Fairchild Semiconductor, Shanghai, China

The electric utility grid-connected photovoltaic (PV) system is an important technology for future renewable energy applications. This requires the design of a high-efficiency grid-connected inverter that delivers power to the grid with low total harmonic distortion (THD) and high power factor (PF).

There are two basic types of grid-connected inverters: voltage-source inverters (VSI) and current-source inverters (CSI). A VSI grid-connected system requires the system's output voltage to be boosted and regulated, which greatly increases its complexity and cost.

Compared with a VSI system, the output current of a CSI system is not influenced by grid voltage (U_GRID), so its grid current (I_GRID) has low THD and high PF. Also, when the input voltage to a CSI system is lower than the peak value of U_GRID, it can successfully interface with the grid. Consequently, the input voltage to a grid-connected inverter is not restricted by U_GRID. Therefore, the current-source grid-connected inverter is ideal for a PV generation system.

The immittance converter theory, which is a variation of the impedance-admittance converter, has been analyzed in detail in several papers. A novel topology is being proposed for a current-source grid-connected inverter based on the immittance converter theory. Compared with the traditional current-source inverter that employs power-frequency inductors and transformers, the proposed topology uses high-frequency inductors and transformers, resulting in a small-volume, low-cost system with low THD and high PF.

The new topology employs a disturbance observer derived by monitoring the PV cell output voltage and cycle-by-cycle current to determine the output power. By analyzing the disturbance, the injection direction can easily be obtained. By estimating the output power, the disturbance injection direction can be determined, which can achieve the maximum power point tracking (MPPT). This method is the traditional MPPT solution, which provides a quick response. However, its disadvantages are more components and higher costs.

A concept that will be explored here is the injection of a disturbance (δΠ that causes the system's duty cycle (D_CYCLE) to vary. The MPPT can be determined by tracking and programming the D_CYCLE variation caused by injection of the input δ. The direction of the D_CYCLE variation needs to be known, as it will affect the inverter's next switching cycle. This disturbance observer uses a new concept for dc MPPTs, obtained by monitoring the inverter output current as an input parameter. This simplifies the control algorithm and cuts down the voltage sense in the disturbance observer, providing significant cost savings.

System Topology

Fig. 1 is the circuit diagram for the current-source grid-connected inverter. The proposed system consists of a high-frequency full-bridge inverter, immittance converter, center-tapped transformer, high-frequency bridge rectifier, power frequency inverter and low-pass filter. For the purposes of this discussion, certain nodes in the circuit are highlighted as test points (TP) and given letter designations. For example, test point A is TP_A (the test point letter designations are circled in Fig. 1 for easy reference).

The immittance converter has two inductors, L1 and L2, and a capacitor, C2, which provides the voltage-source to current-source conversion. Inductances L1 = L2 = L, and the transfer function is:

where ω is the resonant frequency of the immittance converter. When the carrier-frequency of the high-frequency inverter is equal to the resonant frequency, that is ω=1/√LC, Eq. 1 becomes:

where Z₀ = √LC is the characteristic impedance of the immittance converter. From Eq. 2, the input voltage (u₁) of the immittance converter is proportional to the output current (i₂) of the immittance converter. Therefore, the immittance converter effectively converts a voltage source into a current source.

A sine-sine pulse-width modulator (SPWM) controls this high-frequency inverter. The immittance converter produces a high-frequency current with a sinusoidal envelope. The center-tapped transformer, high-frequency rectifier bridge, power-frequency inverter and low-pass filter deliver the sinusoidal current to the grid.

From the aforementioned analysis, the carrier frequency of the high-frequency inverter is equal to the resonant frequency of the immittance converter. Furthermore, to avoid core saturation, the positive-drive pulse width must be equal to the negative-drive pulse width during every resonant period.

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