Analytical Study On HVDC Line Protection And Novel Line Protection Schemes

Dc (direct current) transmission line, as one of the core components of HVDC (HighVoltage Direct Current) systems, is always concerned as a result of its poor protectionperformance. Dc line protection is a kind of transient protection closely related to travellingwave dispersion and HVDC control. It cannot utilize the traditional analysis methods of ac(alternating current) power frequency protection. At present, dc line protection is mainlystudied based on digital simulation using trial and error type studies. The protection boundaryand the effect of main factors are easily swamped with massive simulation data, which goagainst the performance evaluation and optimization. Besides, the present dc line protectionhas some inherent drawbacks; that is to say, the main protection is effected by fault resistanceand the back-up protection has a long time delay. To solve the problems, this paper devotes tothe analytical calculation of dc line faults, the analytical analysis of dc line protections and theproposal of novel protection principles.The work is summarized as following:(1) The analytical model of HVDC system is derived for dc line faults. It is very difficult,if not impossible, to obtain the accurate time-domain solutions because the dc line parameteris frequency dependent and the pole control system has non-linear responses. Therefore, afirst-order system transfer function is introduced to represent the dispersion and attenuation oftravelling waves, and a proportional-integral system transfer function is introduced torepresent the effect of the pole control. Based on the equivalents, the time-domain expressionsare derived for the faults at the end of dc line and at the converter side of smoothing reactor.The analytical curves are compared with the simulation curves obtained in PSCAD/EMTDC,and the results verify the validity of the analytical method.(2) An analytical method is proposed, as a complement and alternative to digitalsimulation, for the analysis of dc line protection. The analytical expression of the discreterelaying quantity is calculated referring to the realization principle of the dc line protection.Under the condition of out-zone fault, the maximum value of the analytical function iscalculated as the variation boundary of the relaying quantity. Under the condition of in-zonefault, a given-width ruler is used to cut the analytical function curve by moving the ruler until the two terminals intersected the function curve. Then, a few pairs of intersection will beobtained, and the maximum value of the intersections is calculated as the variation boundaryof the relaying quantity. With the boundaries, reliability coefficient and sensitivity coefficientare further introduced to determine the thresholds of du/dt criterion, u criterion and icriterion. The analytical method is applied to calculating and analyzing the travelling waveprotection threshold in CSG (China Southern Power Grid), and the results show that themethod makes it possible to quantitatively evaluate and optimize the protection performance.(3) In allusion to the defect that the high-speed and high-sensitivity performances areincompatible for the current dc line protections, two novel protection schemes for dc lines isput forward. One protection scheme performs by using a startup and two auxiliary criteriawith high sampling rates to ensure the sensitivity of fault detection; the scheme is able toachieve an excellent high-speed performance because it acts only based on the first waveformfollowing a fault. Another protection scheme is based on the detection of the first peak time offault current. The scheme transforms the detection of electrical quantity to the detection oftime quantity and, as a result, eliminates the effect of fault resistance and guarantees theprotection sensitivity performance; the scheme is able to achieve an excellent high-speedperformance because it mainly uses the local fault information only when the fault occurs atthe end of the dc line.(4) The existed HVDC travelling wave fault location methods commonly have badaccuracy and can’t adapt to different working and fault conditions. To solve the problem, thispaper presents a novel fault location method based on broadband travelling wave information.Firstly, a deep research is made on the travelling wave speed variation characteristic and itsinherent relationship with the broadband information, pointing out how it impacts on faultlocating results. Then, to deal with the variant speed properly, a complete wave speedtreatment scheme is carried out based on the broadband information，significantly improvingthe accuracy of the excited fault location algorithms. Furthermore, applying the scheme intoone terminal fault location, a novel one terminal algorithm is proposed, which does not needdetect the second wavefront of the fault travelling wave and hence considerably reliable. Theproposed method is applied to detecting dc line faults of Tian-Guang HVDC system, and theresults verify the correctness and feasibility of the method. This paper is supported by the National High Technology Research and DevelopmentProgram (“863” Program) of China (2012AA050209) and the National Science Foundation ofChina (51077055). Some of the research results have been applied in CSG, and the economicbenefits and technical advantages verify the correctness and feasibility of the work.