## 5 Internal Control Techniques
### RSC Control
#### 5.2.1 Linear RSC Control Methods  
Vector control is the most widely used method in DFIG to achieve the independent control of different system parameters. In vector control, the induction generator is controlled in a synchronous reference frame to decouple electromagnetic torque and rotor excitation current [64]. Moreover, the current vector is decomposed into components of stator active power and stator reactive power in the stator-flux-oriented reference frame. Typical RSC proportional-integral (PI) controllers are capable of regulating rotor speed, reactive power, and rotor current [65; 66]. Particle swarm optimization (PSO) was proposed in Reference [67] to tune the PI controller parameters, thereby improving the control performance of PI controllers.  
However, the closed-loop structure of conventional vector control may lead to complex calculation and low dynamic response in DFIGs. To solve these problems, a combined vector and direct power control (CVDPC) was proposed in Reference [66] to provide faster dynamic response, lower computation, and simpler implementation.  
Based on the mechanism that DC flux can be transformed into compensation current, current compensation (CC) control was proposed in Reference [68] to improve the LVRT capability of DFIG-WTs. As an effective RSC control method, this control technique makes use of compensation current to counteract the DC flux caused by faults; thus, transient current and rotor voltage can be limited. To coordinate with the current compensation control, a crowbar circuit is installed in the DC-link to act as a release channel for extra power. The optimal active current compensation controller can effectively improve the LVRT capability of DFIG-WTs. Due to the mechanism that reasonable active current injection can significantly improve the small signal stability of DFIG-WTs, a novel optimal active and reactive current proportion method was proposed in Reference [69]. The optimal active injection current varies with the active current reference, which can be calculated from the fault position.  
Figure 20: Rotor-side converter (RSC) controller.  
Simulation and experimental results showed that the proposed scheme can effectively reduce transient peak voltage and the voltage oscillations can be reduced during transient periods.  
A feedforward transient compensation (FFTC) controller was proposed in Reference [70]. Compensation terms for DFIG stator transient voltages are feedforward injected into the current control loop and power control loop to improve the control effect of DFIG-WTs in transient conditions. For the current loop, complete transient voltages are injected into the rotor-current control loop by feedforward terms to regulate the transient rotor current. For the power loop, 60- and 120-Hz components are feedforward injected into the rotor current; thus, desired tracking of the rotor current is achieved. In this case, an FFTC controller can improve the capability of transient rotor current control and minimizes the DFIG control lost during both balanced and unbalanced grid faults. Therefore, the LVRT capability of DFIG-WTs can be enhanced. There are other two feedforward control methods that improve the transient capability of DFIG-WTs by adding feedforward terms: (1) feedforward current reference control (FCRC) was proposed to achieve zero-error tracking for the rotor current references after grid faults. By introducing the additional rotor current feedforward to the current loop, the rotor current can be maintained in a balanced and sinusoidal manner [71]; (2) feed-forward transient current control (FFTCC) was proposed to reduce transient current stress on the RSC and ensure that the active and reactive power supply remains uninterrupted by adding the stator flux-related feedforward components to the current loop [72].