Study On Novel Algorithms For The Fault Processing In Advanced Grid

A strong grid with the ultra-high voltage grid (UHV) as the mainstay grid and grids of different voltages developing harmoniously will be built in China. On this basis, a new electric network which is highly integrated of modern advanced sensing measurement technology, communication technology, information technology, computer technology, control technology and physical power grid will be built. While with continual development of modern grid, and enlargement of power grid interconnection scale in China, the difficulties on the safe operation and management of grid is increasing greatly. The new gird will take full of customers' demand for electricity and optimize allocation of resources to guarantee the safety, reliability, economy and quality of power supply. For transmission grid, relay protection is the first line of defense of a safe and stable grid; for distribution grid, fault location is the basis of achieving automatic fault isolation and self-healing of smart grid, and also necessary for achieving high reliability of power supply. To improve the safety and reliability of power supply in the new era, this paper delves into some key issues of relay protection of UHV transmission line and fault location of distribution network, covering mainly the following aspects:Firstly, the author analyzes the impacts of variation of transmission line parameters on the longitudinal differential protection using capacitive current compensation method. Because the condensance of UHV line is much smaller than that of extra-high voltage (EHV), capacity current will reach a high level thus making the traditional split-phase current differential protection no longer applicable for UHV line. The Bergeron method and the time-domain compensation method based on Bergeron line model and ?-type equivalent model can compensate capacity current theoretically and are free of the impacts of transient process, but the compensation effects in practice rely heavily on the accuracy of the circuit parameters. And as the parameters are susceptible to operation mode and external conditions, this kind of methods are rather limited. One the basis of single-phase instantaneous power theory, the author proposes an extracting method for single-phase active current through decoupling second harmonics. The method can separate the active component from the reactive component through a coordinate system rotating synchronously with fundamental voltage phasor, getting rid of relying on the accurate transmission line parameters.Secondly, based on the analysis of instantaneous fundamental active differential currents on the both sides of protected area in case of internal and external faults, the author proposes a split-phase differential current protection theory and presents the setting principle. The protection theory is simple and clear and can avoid the impacts of distributed capacity current and compensating current caused by shunt reactors. Compared to capacity current compensating differential protection algorithm, it has an obvious advantage of requiring no impedance parameter and heavy computing, better meeting the requirements of UHV line for the selectivity, quickness and sensitivity of differential protection.Thirdly, the author put forwards a practical longitudinal phase-splitting active current differential protection, which uses the three-phase Decoupled Double Synchronous Reference Frame Phase-locked Loop (DDSRF-PLL) instead of single PLL. When serious grounding or phase to phase faults occur at voltage measurement point, and lead to voltage flicker, voltage sag or discontinuity of phasor and frequency, the three-phase PLL can still provide accurate system fundamental positive-sequence voltage phasor information. This thesis also puts forward a protection theory combining the new protection method with the traditional phase-splitting current differential protection method, improving the reliability of the protection criterion further.Fourthly, aimed at the single-phase grounding (SPG) fault location in neutral ineffectively earthed loop distribution network, this thesis proposes a hierarchical location method based on loop closing. The method locates the fault section according to the current variation before and after closing the switch, then uses db4 wavelet transform to process the transient signal of zero-sequence to extract eigenvalues, which are then input into trained SVM to predict the specific fault location, thus accurate fault location can be achieved. Then in simulation verification, Artificial Neural Network (ANN), BP Neural Network (BPNN) and Genetic Algorithm (GA) are tested respectively using the same trained and testing data. The results show that compared to these three methods, the proposed method is not only more accurate, but also less subject to the size of training sample. And the special advantage lies in that the precise fault location is on the basis of fault section location, thus even is the precise fault location is not achieved, we can still locate the fault section.Lastly, aimed at the multi-phase fault location in the loop distribution network containing multiple distributed power supplies, the author conducts in-depth analysis into the traditional fault location matrix algorithm, pointing out that the traditional algorithm needs to set positive direction for all the power supplies and requires heavy computing. On this basis, this thesis puts forwards a matrix fault location theory applicable for loop distribution network containing multiple distributed power supplies. The theory uses the fault current measured at each FTU point instead of the anticipation value used in the traditional algorithm, thus saving the process of setting positive direction for every power supplies. The method is characterized by simple and clear criterion and small calculating amount. Simulations conducted verify the effectiveness of the method in locating single point and multiple points of failure.