## 3 Rotor-Side External Retrofit Techniques
### DC-Link Energy Storage System  
The ESS acts as a buffer in DFIG-WTs with the function of keeping the DC-link power flow through charging/discharging [32]. Typical DC-link ESS configurations include indirect DC-link ESSs and direct DC-link ESSs. Figure 9 shows the topology of an indirect DC-link ESS, which comprises a battery and a buck-boost converter acting as a DC/DC converter. The battery is connected in paralleled with the DC-link, and the energy storage system is able to share the DC-link with the GSC when it is  
Figure 8: Coordinated control of a series rotor-side crowbar (SRSC) and DC-link capacitor crowbar (DCCC).  
Figure 7: Series dynamic resistance (SDR) protection circuit.  
activated [33]. By applying an indirect ESS to the DC-link, DFIG-WTs have better capability to control DC-link voltage and balance the power flow in the system [34]. Due to the specific structure, the ESS can smooth the steady-state active power output. During transient periods, the ESS can endow the RSC with the ability to provide enough reactive power support via additional demagnetizing current injection. Thus, the terminal voltage dynamics can be improved. To reduce costs and increase efficiency by avoiding converter losses, a direct DC-link ESS configuration was proposed in Reference [35]. In this topology, the energy storage is connected in parallel with the DC-link without any extra converter. Moreover, the direct DC-link ESS also shows the desirable capability to smooth output power during voltage dips. However, the demagnetizing current which is injected from converters into the rotor circuit may result in rotor overcurrent.  
An advanced auxiliary ESS control system was proposed in Reference [36], and the auxiliary control configuration is shown in Figure 10. The operation of the ESS is controlled by power electronic switches. During normal operation, power electronic switches SW1 and SW2 are off and the control system is disabled. When disturbances occur in the output power, the primary control system can participate in smoothing the output power by switching SW1 off and SW2 on. During the fault condition, the auxiliary control system is activated by switching SW1 on and SW2 off to restrict DC link overvoltage. This auxiliary ESS control system provides different functions based on different conditions. Therefore, both steady-state and transient stability can be enhanced by applying this control system.  
Figure 10: Auxiliary control system of ESS.  
Figure 9: Indirect DC-link energy storage system (ESS).  
In the DC-link ESS, super-capacitors can replace traditional energy storages due to the function of matching the intermittent wind energy. Super-capacitors do not have an electrochemical reaction but have electric charge absorption and desorption during charging and discharging periods [37]. Super-capacitors can be connected with the DC-link capacitor directly or via an interface. By implementing a super-capacitor to the DC-link, the oscillation of stator current, stator voltage, and DC-link voltage can be significantly restrained [38]. Although the super-capacitor imposes a higher cost, it can enhance the transient stability of the DFIG, and the operation of the machine does not need to be modified [39].