Research On Novel Principles Of Line Protection Adaptive To Renewable Energy Integration And Extra-long Distance Power Transmission

Nowadays,a series of large-scale wind power bases have been successively established in those regions with rich wind resources in China,leading to much more complexity of massive wind power integration.Furthermore,in view of the distribution reversal between energy/resource and load demand in China,the development of electricity transmission technology in terms of extra-long-distance and super-large-capacity not only has become the best solution to promote the reasonable utilization of mass energy and remote consumption of large-scale renewable energy,but exactly matches the requirement of structure adjustment of energy strategy in China.However,in recent years,the frequent occurrences of wind farm cascading off-grid incidents have drawn the attention of researchers on how to enhance the protection coordination between the crucial components within the wind farms,and therefore improve the fault ride-through ability of wind farms.On the other sides,the extra long-distance transmission of mass power requires to develop advanced UHV transmission technology,among which,the half-wavelength AC transmission technology is one of the especially promising solutions.However,the existing various protection principles that have been successfully applied to EHV/UHV transmission lines are basically unsuitable for this extralong transmission line.It is urgent to develop the new types of main protection principles and criteria for half-wavelength AC transmission line.In addition,the increasing complexity of the secondary system for protection increases the risk of lose-of-synchronization between the data acquired from two sides of the half-wavelength line due to the extra-long distance for data exchange.How to improve the ability of pilot protection against synchronization error is also the remaining problem of half-wavelength line protection.Therefore,taking the line protection in the scenarios of mass renewable energy integration and extra-long distance power transmission as the research object,this thesis focuses on how to improve the adaptive ability to different system operation modes and extensive complicated operation condition of secondary system.The main work are summarized as following:The influence of the voltage drop and Crowbar resistance value on the steady-state fault current AC component coming from the double-fed asynchronous wind turbine is analyzed,and the mechanism of lose-of-selectivity and operating speed becoming slower of the collecting line protection is disclosed.On this basis,an inverse time protection method for the collecting line protection based on hyperbolic tangent function is proposed to ensure the absolute selectivity between of collecting line protection and fuses,while improving the operating speed of the collecting line protection in responding internal faults;The influence law of DG infeed current on the operation characteristic of the inverse-time protection of the collecting lines is deduced,and the risk of lose-of-selectivity of the inverse-time protections between this line and downstream line is disclosed.On this basis,a voltage correction based inverse-time overcurrent(VCITO)protection scheme is proposed.Voltage correction factor and voltage correction gradient exponent,which are constructed depending upon the voltage natural distribution under fault condition and the changing law of measuring voltage with respect to the changing of fault location,is used to correct the inverse-time characteristic curve.By this means,the operating time of VCITO protection can be shorten appropriately on the premise that the protection coordination between the neighboring collecting lines and the sensitivity can be guaranteed.By means of establishing a fully distributed parameter model of half-wavelength AC transmission line,the applicability of existing pilot directional protection,pilot distance protection,and current differential protection on half-wavelength line are studied quantitively,and the mechanism of mal-operation and fail-to-trip of these protections is revealed.On this basis,novel pilot current differential criterion and pilot voltage differential criterion suitable for half-wavelength AC transmission lines,together with corresponding comprehensive main protection scheme,are proposed.On the premise of ensuring the security of criterion,this scheme can be used to identify various complicated internal faults under various operation conditions and fault scenarios.Meanwhile,it has the ability against Wire-Breakage of PT and strong ability of withstanding CT saturation.On this basis,an unequal-weighted pencentage restraint characteristics differential protection criterion for half-wavelength line with strong ability against synchronization error is proposed.By optimizing the shape and size of the restraint region,the ability against data synchronization error the sensitivity on responding internal faults of the differential criterion can be achieved in the meanwhile.