## 3 Smart Inverter Control Architecture
### Control Architecture of Three-Phase Photovoltaic Full-Bridge Inverter  
The circuit architecture of the three-phase photovoltaic full-bridge inverter adopted in this paper is shown in Figure 4. The dual-loop P-I controller [11; 12] was used, and the outer loop served as the DC-link voltage loop that generated a current command from the error between the DC-link voltage set and then measured. The inner loop served as the inverter output voltage and current control loop, where the error was used to generate the pulse width modulation (PWM) control signal [13; 14; 15] for controlling the output voltage and current. With the addition of \(L_{1}\)-\(C\)-\(L_{2}\), the three-level low-pass filter [16; 17] forms to attenuate the high-frequency harmonic wave of the inverter, so the output voltage becomes a low-frequency AC sine wave. Figure 4: Circuit architecture of three-phase photovoltaic full-bridge inverter. Compared with the LC filter, the LCL filter requires less inductance to remove low frequency harmonics of the converter. However, the small amount of inductance is smaller, the cost is lower, the system's overall efficiency is not affected, and the dynamic response is better. As the LCL filter is a third-order filter, it will cause system instability. Therefore, a damping resistor should be connected in series to the capacitor of the LCL filter to avoid oscillation of the system. The damping resistor is divided into a passive and an active damping resistor. For passive damping system, a damping resistor is connected in series to the capacitor to achieve the system's stability, and its control is simple and low-cost; The active damping system achieves the system's stability by controlling the current of the capacitor. Although its efficiency is high, the control is more complicated and expensive. According to the above comparison, this paper adopts the passive damping system with simple system architecture as shown in Figure 4. In addition, the adopted LCL filter with passive damping resistor is relatively easy to realize. The following will be explained by the design method of the LCL filter of one phase, and the three-phase circuit parameters are equivalent to \(L_{1}=L_{1a}=L_{1b}=L_{1c}\), \(L_{2}=L_{2a}=L_{2b}\) = \(L_{2c}\), \(R_{d}=r_{d1}=r_{d2}=r_{d3}\) and \(C=C_{a}=C_{b}=C_{c}\). Since the output power factor of the general grid-connected system is controlled as 1.0, if the converter adopts the bipolar switching control method, the total inductance of the LCL filter is \(L_{t}\), which can be obtained from Equation (4) [16; 17]. \[L_{t}=\frac{V_{dc}}{4I_{rms}f_{sw}\Delta I_{ripple}}(1-m_{a})m_{a} \tag{4}\]  
where \(V_{dc}\) is the input DC voltage of the inverter, \(I_{rms}\) is the effective (or root-mean-square, RMS) value of the inverter output current, \(\Delta I_{ripple}\) is the percentage of output current ripple; \(m_{a}\) is the amplitude modulation ratio, and \(f_{sw}\) is the switching frequency of the switch. \(L_{t}\) can be rewritten as shown in Equation (5) [16; 17]. \[L_{t}=\frac{L_{1}L_{2}}{L_{1}+L_{2}} \tag{5}\]  
Different degrees of harmonic attenuation can be obtained when \(L_{1}\) and \(L_{2}\) are distributed in different proportions. The relationship between the two is shown in Equation (6). The proportional parameters \(\alpha\) of the two can achieve a better harmonic attenuation effect within the range \(4\leq\alpha\leq 6\), according to the conclusions of the literature [18]. \[L_{1}=\alpha L_{2} \tag{6}\]  
The value \(\alpha\) used in this paper is 5, which is substituted into Equation (6) and then back into Equation (5) to obtain \(L_{1}\) and \(L_{2}\) values. The design of capacitors is limited by the reactive power \(Q\) generated. Generally speaking, it should not exceed 5% of the output-rated power \(P_{out}\), then the capacitance value can be calculated through Equation (7) [19].