Comparative study in clearing of transient faults in medium voltage networks by means of neutral or single-phase tripping

Abstract

Within power utility networks, transient and permanent faults cause power interruptions to customers. Research has indicated that most faults that result in breakers tripping within MV networks are temporary in nature. Other sources have also indicated that approximately 30% of permanent faults started as a transient fault. All types of faults in an electrical network do put strain on electrical equipment to a certain degree. A need therefore arises to explore alternative ways of clearing transient faults in order to increase network reliability. If transient faults are cleared more effectively, it will influence power quality positively by reducing the length of voltage dips and limiting voltage dip propagation. This dissertation contains research performed on the effectiveness of methods that aims to clear transient faults on MV networks (11 kV and 22 kV) without causing momentary supply loss to customers due to breaker ARC operations. Transient fault clearing is achieved by interrupting fault currents very quickly. For transient earth faults, this can be achieved by momentarily disconnecting the neutral earth connection of the NECR. For transient phase-to-phase faults, this can be achieved by singlephase tripping - which will open only one of the two affected phase breakers within 30 ms. The fast operation greatly reduces the amount of ionised air and damage that is formed during a fault condition. This, in turn, improves the overall success rate of both schemes. MV line - and transformer models were created, verified and validated by means of calculations, simulations and field testing. The validated line and transformer models were used to develop integrated models for each of the two fault-clearing schemes. The integrated models were used to simulate the expected network response under different fault conditions. After implementing both fault-clearing schemes on actual MV networks, the simulated results of the integrated models were validated with measurements. Neutral tripping is effective in the clearing of high and low impedance earth faults. It was found that by implementing neutral tripping, as many as 83.4% of earth faults were successfully cleared. The fault-clearing effectiveness of neutral tripping is primarily determined by the capacitive coupling of the MV network. Single-phase tripping is effective in the clearing of earth faults and phase-to-phase faults. It was found that by implementing single-phase tripping, 58% of earth faults and 94% of phase-to-phase faults were successfully cleared. The fault-clearing effectiveness of single-phase tripping is primarily determined by the feedback current magnitudes of Δ/Y transformers as well as the capacitive coupling on the MV line. Implementing these methods of transient fault clearing results in less stress being placed on breakers, conductors and transformers within the MV network. The efficiency with which transient faults are cleared positively influences network reliability as transient faults will not result in permanent faults. The speed with which transient faults are cleared improves the power quality of the MV network with regards to voltage dips, sustained and momentary interruptions. Lastly, the neutral breaker and single-phase breaker schemes will result in up to 50 times fewer burn wounds on people or animals in the unfortunate case where inadvertent contact is made with the MV network due to the fast tripping capabilities of both schemes. Tydelike en permanente elektriese foute veroorsaak ongewenste kragonderbrekings in elektriese verspreidingsnetwerke. Navorsing toon dat die meerderheid foute wat veroorsaak dat stroombrekers in 11kV- en 22kV-distribusienetwerke klink, tydelik van aard is. Ongeveer 30% van permanente foute het begin as gevolg van ‘n tydelike fout. Alle elektriese foute veroorsaak egter stremming op verskeie elektriese komponente. Daar is dus ʼn behoefte om alternatiewe oplossings te ondersoek wat tydelike foute effektief kan blus en sodoende netwerkbetroubaarheid verbeter. Indien tydelike foute effektief geblus kan word, sal die toevoerkwaliteit verbeter omdat spanningsknikke verkort word. Hierdie verhandeling omvat navorsing oor die effektiwiteit van verskeie tegnieke om tydelike foute in 11kV- en 22kV-kragnetwerke te blus sonder om ʼn drie-fase toevoeronderbreking te veroorsaak. Tydelike foute kan geblus word deur die foutstroomvloei so gou moontlik te onderbreek. Tydelike aardfoute kan geblus word deur die neutrale aardverbindingspunt van die neutraleaardingskompensator (NECR) tydelik te onderbreek. Fase-na-fase foute kan geblus word deur enkel-fase klinking, waarna slegs een van die twee geaffekteerde fases binne 30 ms sal klink. As gevolg van die hoë spoed waarteen die breker klink, sal die hoeveelheid geïoniseerde lug en skade aan die elektriese netwerk wat tydens ʼn fouttoestand veroorsaak word, verminder. Die 11kV- en 22kV-kraglyn en transformatormodelle is ontwerp en gestaaf deur middel van berekeninge, simulasies en praktiese toetse. Die gestaafde lyn- en transformatormodelle is gebruik om geïntegreerde modelle vir elk van die twee foutklinkingtegnieke te ontwerp. Die geïntegreerde modelle is gebruik om die kragnetwerk se reaksie onder verskeie fouttoestande te bepaal. Al twee foutklinkingtegnieke is op bestaande 11kV- en 22kV-kragnetwerke geïmplementeer en die gemete resultate is vergelyk met die simulasie resultate van die geïntegreerde modelle. Neutrale klinking is effektief om hoë en lae impedansie aardfoute te blus. Gedurende die navorsing is bevind dat 83.4% van al die aardfoute suksesvol geblus het ná die implementering van die neutrale klinkingtegniek. Die effektiwiteit van neutrale klinking is hoofsaaklik afhanklik van die kapasitiewe-koppeling van die kragnetwerk. Enkel-fase klinking is effektief om aardfoute sowel as fase-na-fase foute te blus. Gedurende die navorsing is bevind dat 58% van alle aardfoute en 94% van alle fase-na-fase foute suksesvol geblus het ná die implementering van die enkel-fase klinkingtegniek. Die effektiwiteit van enkel-fase klinking is hoofsaaklik afhanklik van die Δ/Y-transformators se terugvoerstroom, sowel as die kapasitiewe-koppeling van die kraglyn. Die implementering van die twee bogenoemde foutklinkingtegnieke het gelei tot minder stremming op stroombrekers, geleiers en transformators in die elektriese kragnetwerk. Die effektiwiteit waarmee foute blus, dra by tot die betroubaarheid van die elektriese kragnetwerk omdat tydelike foute nie ontwikkel in permanente foute nie. Die spoed waarteen foute geblus word, dra ook by tot ʼn verbetering in toevoerkwaliteit ten opsigte van spanningsknikke, en tydelike sowel as permanente kragtoevoeronderbrekings. As gevolg van die hoë klinkingspoed van al twee foutklinkingtegnieke sal tot 50 maal minder brandwonde veroorsaak word in ongewenste gevalle waar ʼn mens of dier kontak met ʼn elektriese kragnetwerk maak.