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J a nu nu ar ar y 21, 2013
Aut ore clos closing ing in t ransm ransmission ission and dist ribut ion syst sys t em ems s (on phot p hot o: A disc disconnec onnectt swit ch f or a powe r lin line e t hat 's maybe 200kV 200kV.. T he moveab le members are t he horizontal horizontal poles t hat are hinged hinged at op t he lef lef t set of in insul sulat at ors. - b y Xylographi Xylographile) le)
Introduction Two main functions of Utility Electric System Operations department are Control and and Protection. Control function deals with all operational aspects involved in efficient and optimal dispatching of available real power (with reactive power support) from the Generating units units (and reactive s ources) t o geographically distributed consumers consumers through through available system of T ransm ransmission ission and and dist ribu ribution tion (T (T &D) network network (s).
Control f unct unction ion thus thus perf orms all operat ions ions line connect connect ion and and disconnect disconnection ion of circuits circuits,, equipment, equipment, changin changing g levels of voltage volta ge (e.g. Power Transf ormer tap changes, changes, changing changing power f low direct direc t ions, load shedding, shedding, synchroniz synchronizing, ing, decreasing/increasing decr easing/increasing generation etc). Some of these functions are performed automatically and some
manually by syst em operators. Various monitoring and alarming systems help perf orm these control functions. Protection function deals with automatic det ection of f aults or abnormal/unsaf e syst em operat ion conditions and automatically disconnects involved circuits or equipment. Thus protective syst em helps in minimizing the damage to circuit components and equipment. Circuit components and equipment are t hus available f or reuse in most cases.
Automatic Reconnection Once a circuit component or equipment is automatically disconnected by prot ective f unctions, its reconnection is mostly a control f unction. Reconnection of a circuit component or equipment (after its trip due to protect ive device action) is perf ormed either manually or automatically. It is often essential to make sure that reconnecting it will not thermally degrade the life of major equipment or damage it beyond accept able level or results in major f inancial burden. Protection action for equipment like Generators, Transformers, Reactors, Capacitor banks, Filter banks, Power cables et c. are always results in ‘Locked out’ (meaning are blocked f rom closing bot h manually or automatically) . T his is because it is always a permanent f ault needing some kind of maintenance/repair. Well, only time I f eel requires no maintenance af ter a prot ection trip is when it is known f or sure that it is a ‘protect ion mal-operation’ or it is a sympathetic protect ion operation due to some other f ault/abnormal condition in the syst em. In any case, reco nnecting without any test is a SCARY business and involves a bit of personal risk!! !!
Every utility will have well decided operation policy in regard to reconnection of equipment or circuit component af ter a protection trip. Before reconnecting a major equipment considerable t esting and maintenance procedures are f ollowed (requires some equipment down time). Overhead (OH) Transmission and distribution circuits are t he only circuits which are of ten reconnected automatically without any testing or maintenance performed after its trip due to protection action. This is because most often the protection action is due to a f ault which is transient in nature and the f ault vanishes once it is de-energized. T riMod™ 600R Vacuum Reclose r w/ Single or T hree-Phase Operation
Depending on the magnitude of f ault current and st rength of electric potential (voltage level), some ionization occurs in the surrounding dielectric media (air) which neutralizes with time and regains required dielectric strength making it available f or successf ul reconnection. T hus Automat ic reconnection called Auto-reclosing, can be perf ormed f or OH Transmission and Distribution circuits.
From Auto-reclosing perspective, it becomes very important therefore to understand types of f aults which can occur in OH T&D circuits and amount of risk involved in autoreclosing and its advantages. Finally, a quick question to readers… Whether Auto-reclosing is a Control f unction or a Protection function? I guessed your answer… if you are a Protection Engineer, you will surely say it is a Control f unction and… he he, if you are a Control system Engineer (operation person) you will surely say it is a Protection function!!!! Because it is perf orming a closing operat ion not a t rip action, a Protection Engineer says it is a control f unction. Control Engineer says ot herwise because it is perf ormed by a protect ive relay or device f ollowing a def initive and sequential act ion decided (major part of sequence and conditions f or reclosing action) by Protec tion Engineers. Psst… I strongly believe most common real reason behind the views of Control and Protection Engineers is the risk involved with unsuccessf ul re- closing and f ailing t o initiate it in the first place…analyzing the events, making reports etc… LOL … guys accountability is a major issue in competitive power companies (excepting some utilities in corrupt systems)… anyway, we will go back to Auto-reclosing folks! Resource: Autoreclosing in Transmission and Distribution Systems by mr. S.R. Javed Ahmed
