Figure 26. Response on motor energization.
The field current for the DFIG went beyond its limit of 2.2pu because it was energized at a slightly higher excitation level. With proper setting of the DFIG reactive power reference, it is possible to energize the motor at lower excitation thus keeping the energization transient to below 2.2pu. The voltage dip associated with motor starting last for about a second and this is permissible as per most low voltage ride through criterion.
4. Conclusion
Analysis on SCIG and DFIG based hybrid windfarm in emergency restorative conditions with focus on load pickup and component re-energization is provided. The range of operation of both the windfarms was first analyzed from both an active and reactive power response perspective. An additional pitch control based on dump load power was implemented to prevent the excess production of power at a time when the dump load power absorption power has been exceeded. The ability to pick up inductive load for the SCIG based system is governed by the SC capability and availability of capacitor banks while for the DFIG system, it is dependent on the both the SC and DFIG reference settings which can be adjusted to lower the excitation levels and pick up more inductive load.
The 2 most common power quality issues related to windfarm operation, harmonics and flicker, have been used to determine the hosting capacity of the respective hybrid systems. Due to reduced network sources in restorative conditions, harmonic and flicker effect will be more magnified. As expected, flicker has a significant effect on the SCIG based system while harmonics have a significant effect on the DFIG based system. For maximum wind speed variation (worst case scenario) the voltage and frequency of the SCIG based hybrid system could not stay within its technical limit thus flicker mitigation techniques are required. This was not an issue for the DFIG based system. The harmonic levels were well below the limit for the DFIG based hybrid system.
The capability of the hybrid systems to perform re-energization functions, similar to conventional blackstart units was investigated. This has been successfully proven for the case of transformer, HV underground cable, overhead lines and motor energization. Cranking up of other WECSs in the same windfarm was also examined using previous work (by the same author) proposed windfarm starting model. Regulation of the excitation levels is important for proper re-energization. Careful consideration is required for DFIG based hybrid systems on energization of compensated lines due to the increased risk of missing zero phenomenon. Apart from that, the DFIG based hybrid system was superior in performance and flexibility.
Acknowledgments
The authors acknowledge funding support to carry out this research from Ministry of Business, Innovation and Employment, New Zealand’s National Science Challenge- Resilience to Nature’s Challenge and in particular the project on Electricity Distribution Resilience Framework informed by West Coast Alpine Fault Scenario; University of Auckland under Faculty Research Development Fund (FRDF) Grant 3709532 and QuakeCoRE-New Zealand Centre for Earthquake Resilience.
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