## 3 Proposed Three-Phase Energy Mutual-Aid Strategy of a Grid-Connected Inverter  
### Three-Phase Power Exchanging Control Strategy of a Grid-Connected Inverter  
The three-phase energy mutual-aid strategy of a grid-connected inverter is proposed. By adding a negative sequence current control, the grid current and output power of each phase can be controlled separately to realize mutual aid.  
As compared with the conventional control strategy, the proposed control strategy can also be divided into four steps, i.e., (a), (b), (c) and (d), as shown in Figure 3.  
Figure 1: Typical topology of a grid-connected inverter.  
Figure 2: Conventional control strategy of a grid-connected inverter.  
1. Conduct positive and negative sequence separation on the sampled three-phase line voltage \(V_{\text{line}}\) to obtain the positive and negative sequence of the line voltage \(V_{\text{line-P}}\) and \(V_{\text{line-N}}\), respectively. Only take \(V_{\text{line-P}}\) to do the following computation. First, convert \(V_{\text{line-P}}\) into the phase voltage \(V_{\text{phase}}\); and then lock the phase of \(V_{\text{phase}}\) to obtain \(\text{sin}\varphi/\text{cos}\varphi\) as the phase-lock result.
2. Conduct positive and negative sequence separation on the sampled three-phase grid current \(I\) to obtain the positive and negative sequence currents \(I_{\text{P}}\) and \(I_{\text{N}}\), and then take them to perform dq decomposition, respectively. For the dq decomposition of \(I_{\text{P}}\), the phase-lock result of positive sequence line voltage \(\text{sin}\varphi/\text{cos}\varphi\) is adopted. However, for the dq decomposition of \(I_{\text{N}}\), the phase-locking result of negative sequence voltage cannot be used, because there is a random initial phase difference between the positive and negative sequence line voltage. The phase-lock result of negative sequence \(\text{sin}(-\varphi)/\text{cos}(-\varphi)\) should be constructed according to the positive sequence voltage phase-lock result \(\text{sin}\varphi/\text{cos}\varphi\), so as to ensure that there is no initial difference between the positive and negative sequence. Therefore, the constructed phase-lock result \(-\text{sin}\varphi/\text{cos}\varphi\) is adopted for the dq decomposition of the negative sequence current \(I_{\text{N}}\).
3. For both the positive and the negative sequence currents, take the errors between \(I_{\text{d}}/I_{\text{q}}\) and target current \(I_{\text{dref}}/I_{\text{dref}}\); enter the errors into the PI controllers, respectively; and then perform dq/abc transformation, adopting the phase-lock result \(\text{sin}\varphi/\text{cos}\varphi\) or the constructed phase-lock result \(-\text{sin}\varphi/\text{cos}\varphi\) to obtain the three modulated waves, respectively, on both the positive and the negative sequence currents.
4. According to the circuit superposition theorem, the positive and negative sequence modulated waves are directly superimposed on each corresponding phase to create the final modulated waves, and then the final modulated waves are compared with the carrier wave to generate the six-channel PWM switching signals.  
Some modules in Figure 3 are marked in red, which will be discussed in detail in the following sections.