Studies On Grid Integration Remote Test For Offshore Wind Turbines

Energy transition is the historical necessity of continuous progress of human society,vigorously developing renewable energy has become an important measure to promote the transition of the energy structure to the green and low-carbon mode.China has rich wind resources,superior conditions and huge potentials for the development of offshore wind power(OWP),which has come to a significant support for promoting energy transition.Due to the harsh operating environment,the high investment and cost of operation and maintenance as well as high risks,integration of large scale OWP poses a great challenge to the operation of China's eastern ultra-high voltage AC/DC hybrid receiving-end grid.Therefore,the quality and reliability of wind power equipment are the foundation for the healthy,orderly and sustainable development for the whole industry and are also essential to the safe and stable operation of the grid.Test is the most effective means to improve the quality and reliability of wind turbines(WTs).The European leading countries in OWP fields have experienced a complete design cycle,thus they have relatively complete developing and manufacturing standards,large-scale testing platforms and certification system for offshore WTs,which can be directly connected to the grid if passing certification,which has effectively supported the fast development of OWP in recent years.China's OWP started late,the WT development technology is immature,the independent innovation capability,the accumulation of offshore wind resources data and WT operation experience is not are insufficient.There are also lacking of complete technical standards and certification systems for OWP products,developing,public testing and certification platform,the reliability of WTs is defective.All these issues restrict the development of OWP in China.To ensure the safe and reliable operation of grid-connected offshore WTs,China clearly requires the grid-integration test for WTs,the grid-integration testing capability is seriously lagging behind the testing demand,it is thus urgent to conduct the research of key technologies and development of testing equipment for offshore WTs grid-integration test.In view of the lack of testing technology,insufficient capacity of existing testing device and difficulty in local test for offshore WTs,this paper analyzes the differences in testing conditions between offshore and onshore WTs on the basis of summarizing the requirements for WTs grid integration testing.Then the impacts of offshore wind power electrical systems,especially the impacts of long-distance submarine cables on test are researched,and the requirements for offshore WTs grid integration testing are clearly clarified.This dissertation concentrates on the research of testing technology and the development of testing device,which includes the following three aspects:1)Selective harmonic damping and dynamic deviation compensation strategy based harmonic adaptability(HA)remote testing method;2)Stepwise voltage generation strategy based fault ride through(FRT)remote testing method;3)Testing capability analysis of the proposed remote testing methods and development of testing devices.The major contributions and innovations are summarized as follows.(1)After analyzing the testing requirements and the testing characteristics of offshore WTs,the impacts of submarine cables and other elements on harmonic distortion and voltage sag and swell depth are researched respectively.The influence mechanism of cables on HA and FRT test is revealed.Based on the HA test characteristics,the equivalent circuit for resonance analysis is established after modelling the testing system elements in the harmonic domain.The effects of submarine cables on HA test are analyzed with the frequency scanning method.According to the FRT testing characteristics,the impacts of submarine cables,currentlimiting and short-circuit reactors,grid equivalent impedance on FRT test overvoltage and voltage sag/swell depth are analyzed.The simulation analysis method is used to study the basic law of the influence of submarine cables on harmonic and fault ridethrough testing under different test conditions.Theoretical research and simulation analysis show that the introduction of submarine cables will incur resonance and thus the distortion of testing harmonics will exceed the required values.The introduction of submarine cables will also stimulate the participation of WTs including capacity and operating states in resonance,which cause the variation of resonance frequency under different testing conditions.Severe overvoltage resulting in the FRT testing failure is caused by the interaction of submarine cables and current-limiting reactor.The voltage sag/swell depth will deviate from the required values and the testing result is not convincing.(2)A selective harmonic damping(SHD)and dynamic deviation compensation(DDC)strategies based HA remote testing method is proposed for offshore WTs.By using the strategy of selective harmonic resonance suppression,the shunt active filter is used as a controlled reactance to change the GAP's impedance characteristics,which can dynamically suppress the resonance that may occur under different test conditions.The designed resonance controller and control strategy have a good control performance when resonance frequency shifts are in a certain range.In order to eliminate the distortion deviation of the testing integer harmonics caused by the change of WT operating states,a dynamic adjustment strategy based on deviation compensation is proposed,The deviation is detected in real time with the proposed instantaneous reactive power theory based arbitrary integer harmonic detection method and compensated with the series APF to maintain the required distortion.The designed control strategy has sufficient bandwidth to ensure that the testing harmonic can be adjusted accurately.The simulation results of a 5MW DFIG WT show that the proposed method can effectively suppress the possible resonance under various testing conditions and can maintain the harmonic distortion complying with the testing requirements.(3)A stepwise fault voltage generation strategy based low/high voltage ride through(LVRT/HVRT)remote testing method is proposed for offshore WTs.For the LVRT test,the overvoltage can be suppressed by the stepwise fault voltage generation method.The theoretical analysis of the stepwise fault voltage generation strategy is given,and the relationship between the current-limiting reactance and short-circuit reactance invested in each stage is deduced.As for the HVRT test,the voltage at the high-voltage side of WT transformer is firstly reduced to the rated voltage with the stepwise voltage generation method and then the capacitor branch is adjusted to produce required voltage swells.The impedance configuration method of the FRT testing device,which takes into consideration of safety constraints,is given to achieve the FRT under various testing conditions by eliminating the effects of submarine cables on voltage sag/swell depths deviations.The simulation results of a 5MW PMSG WT show that the proposed FRT testing method can effectively suppress the overvoltage and eliminate the voltage saps/swells deviation caused by the introduction of submarine cables and generate various fault voltage in accordance with the testing standards and thus can be used for offshore WTs FRT test reliably.(4)The capability of the proposed HA and FRT remote testing methods are carried out and a large-capacity grid adaptability testing device and an integrated LVRT/HVRT testing device are developed and verified by onsite testing.The capability of the HA remote testing method is determined by the resonance suppression capability of shunt APF and active transmission capacity of submarine cables and the capability of the FRT remote testing method is determined by the maximum feasible current-limiting reactance value allowed by the overvoltage constraint and also the active transmission capacity of submarine cables.The relationship between the capacity of the shunt active filter and the resonance suppression capacity and the transmission capacity of the submarine cable are deduced.The calculation method of the transmission capacity considering the constraints of the current-carrying capacity and terminal voltage rise is given.As for the grid adaptability testing device,a cascaded multi-power module topology based on the superposition of harmonics and fundamental waves is designed.Multi-carrier phase-shifting control is applied to improve the equivalent switching frequency and thus reduce the output harmonics.In the meantime,a feed-forward power compensation based DC bus voltage control strategy is applied to eliminate the effects of dynamic loads.Theoretical analysis shows that the designed technical scheme and control methods improve the equivalent switching frequency and output performance of the device,and can adapt to changing test loads.For the FRT testing device,the integrated FRT topology and the inductance/capacitance branch are designed,and the impedance configuration method is also given to achieve the integration of low/high voltage ride through testing functions.The performance of the developed testing devices were verified through on-site testing.The testing results show that the developed devices comply with the design indexes and can generate all the required grid disturbances,which can be successfully used for grid-integration test of WTs.This dissertation absorbs the practice results and useful experience of testing for onshore WTs that has been widely carried out in China in recent years and conducts research on the grid integration testing requirements,testing methods and development of testing devices for offshore WTs.The research in this paper will facilitate the formulation and improvement of the testing standards and certification systems,the design and testing capability of WTs.This work has important practical theoretical significance and application value to promote the establishment of offshore wind power testing center and to speed up the development and grid integration of offshore WTs by ensuring the safe and stable operation of WP equipment.