Global sensitivity analysis of impedance measurement algorithms implememented in intelligent electronic devices.

Abstract

A novel methodology for testing performance of impedance measurement algorithms used in transmission line protection schemes is developed. Nowadays, impedance measurement algorithms are software functions implemented in the multifunction Intelligence Electronic Devices (IEDs) responsible for overall monitoring, protection and control of transmission lines. Accurate impedance measurement during fault conditions is the key in successful performance of the line protection as well as fault location functions of an IED. This thesis investigates a typical practical situation where only short-term fault records of voltage and current measurements from one side of a transmission line are used as inputs in the impedance measurement algorithm. Current flowing into the fault from the remote terminal of transmission line as well as fault impedance can influence significantly accuracy of impedance measurement. Since these two quantities are not measured, we require a systematic tool which will assess sensitivity of impedance measurement to those factors. At present, these sensitivities are obtained in heuristic and ad hoc manner during application testing done by utilities before commissioning of new IEDs. Situation in practice can be increasingly complex and this kind of unsystematic testing approach can fail. The thesis addresses those practical complex cases in the systematic manner. In these cases we encounter the following configurations of transmission lines with new not measured factors: - parallel closely spaced lines, where the effect of electromagnetic mutual coupling can be significant; - series capacitive compensation of transmission line, where capacitance of the compensation device can be unknown; - three-terminal lines, where measurements on the tapped line are not available. The proposed systematic sensitivity testing tool comprises of a transmission line electromagnetic simulation module and a Global Sensitivity Analysis (GSA) module. The software packages commonly used by industry are employed to implement those modules: the DIgSILENT software for the line simulation module and the SIMLAB software for the GSA module. The simulation module is used to simulate large number of fault scenarios for all samples in the factor space, while the GSA module is responsible for creating a set of specific samples in the factor space as well as for sensitivity analysis. The commercial multifunctional IED SEL-421 from the Schweitezer Engineering Laboratories has been used to demonstrate the proposed sensitivity analysis tool. The IED functions has been modelled in DIgSILENT environment and integrated into the simulation module. Test automation program has been written using the DIgSILENT Programming Language (DPL) so fully automatic and integrated performance of the simulation and the GSA modules has been achieved. The GSA module relies on the Quasi-Monte Carlo (QMC) technique with the Sobol’s quasi random sampling and the Morris method is used in fast factor pre-screening in order to remove non-influential factors before applying the QMC GSA. The results of systematic tests of the impedance measurement algorithm implemented in the SEL-421 IED, for various line configuration cases, are presented in this thesis. The results verify the usefulness of the proposed testing methodology for practical applications.