Performance Assessment and Management of Large Wind Farms under Abnormal Grid Operating Conditions

Abstract

Increased penetration of wind generation into high voltage transmission systems necessitates the development of grid integration standards. Wind farms during normal system operation have been well assessed for impacts like forecasting, reliability, power quality, and transient stability following its large scale integration. Fault responses and protection schemes are currently not standardized and neither following consistent practices for existing wind farms which provides research opportunities. This is because different wind generation technologies exhibit diverse dynamic characteristics following grid disturbances, compared to synchronous generators. In this research, wind farms are illustrative of increasing influence of renewable on grids with several large scale deployments being integrated with transmission networks. The New Zealand (NZ) grid has been used in this thesis to develop methodology for ride-through criteria. During abnormal operations, ride-through requirement is prerequisite to handle large grid disturbance. Network characteristics and their interaction with available wind technologies also impact dynamic behaviour. One of the most important integration criteria for large wind farms are Fault Ride-through (FRT). Development of the FRT voltage envelope depends on transmission network characteristics along with considerations like contingency and existing protective relay settings. The first objective utilises the real New Zealand North Island network diagrams and data to test the proposed methodology, and summarise the results. Finally proposed actual voltage ride-through criteria for New Zealand grids (North and South Islands) are also presented with international benchmarking. To address the second objective, Type-1, Type-2 and Type-3 machines are modelled and connected to the same Point of Common Coupling (PCC). Large scale grid disturbance scenarios are created and identical faults are exposed to individual wind farm type. Fault current dynamics of wind farm type are then recorded from simulations and assessed for protection schemes like over-current, distance and differential. These responses are then compared from a protection relay operation viewpoint to assess the protection performance for the various Wind Turbine Generator (WTG) technologies. The third research objective explores an important aspect for fault level estimation that is sequence network equivalence. Realistic fault impedances can be used to estimate fault current levels without including the whole model of wind farms for the purpose of simplified analyses. It is known that dynamic fault impedance, or short-circuit impedance of convertor based wind farm cannot be directly evaluated through conventional methods without explicitly factoring internal machine control actions. The dynamic fault impedance of machine with positive sequence control is hard to standardize. A case study involving a DFIG (Doubly Fed Induction Generator) based wind farm having multiple units, is developed and utilise to record fault data in case of unsymmetrical faults. The data obtained is thereafter used to evaluate sequence impedance values for that wind farm during the fault period. Results obtained from analysis reveal that it is possible to estimate dynamic value of impedance of the WTG during fault factoring its internal control action The systematic development of FRT criteria proposed in this thesis provides a consistent pathway for any transmission utility to identify the important parameters and procedure required to assess and develop FRT. Fault current responses from WTGs, another outcome of this research, is useful for providing guidelines for relay manufacturers and transmission engineers to deal with protection performance issues. The proposed sequence network equivalent method proposed in this thesis helps to estimate dynamic fault impedance value without theoretical calculations during the fault. All three objectives delivered through this thesis revolve around understanding and exploring performance assessment for fault responses of large integrated WTGs during abnormal grid operation. In summary, this work is aimed towards the better understanding of wind farms during abnormal grid operating conditions to help establish improved protection and control strategies. Keywords: Demand Side Management, Doubly-Fed Induction Generator (DFIG), Energy Policy, Fault Current, Fault Model, Fault Ride-Through (FRT), Fixed Speed Induction Generator (FSIG), Full Scale Frequency Convertor (FSFC), Grid Codes, Large Scale Integration, Low Voltage Fault Ride-Through (LVRT), Power Generation, Power System Faults, Power System Protection, Protection Relaying, Renewable Integration , Sequence Network Equivalent, Short-circuit Analysis, Short-Circuit Model, Thevenin Equivalent, Wind Energy Generation, Wind Farm Modelling, Wind Farm Protection, Wind Farms, Wind Generation Technology, Wind Integration