Electrical Installation Guide 2009 - the global specialist in energy ...

B - Connection to the MV publicdistribution network1 Supply of power at mediumvoltageB13O21- Cycle 1SRI O1 fI o15 to 30 sI nSRO3fault0.3 s 0.4 sPermanent fault2 - Cycle 2SRa - Fault on main feederI O1fO2SR1O3SR2O4I nI o0.3 s 0.4 s15 to 30s15 to 30 sfault0.4 sPermanent fault0.45 sb - Fault on section supplied through Automatic Line SwitchI fO1O2SR1 O3I n15 to 30 s15 to 30 sSR2I oFault0.3 s 0.4 s0.4 sOpening of ALSFig. B14 : Automatic reclosing cycles of a circuit-breaker controlling a radial MV feederUnderground cable networksFaults on underground cable networks are sometimes the result of carelessworkmanship by cable jointers or by cable laying contractors, etc., but are morecommonly due to damage from tools such as pick-axes, pneumatic drills and trenchexcavating machines, and so on, used by other utilities.Insulation failures sometimes occur in cable terminating boxes due to overvoltage,particularly at points in a MV system where an overhead line is connected to anunderground cable. The overvoltage in such a case is generally of atmosphericorigin, and electromagnetic-wave reflection effects at the joint box (where the naturalimpedance of the circuit changes abruptly) can result in overstressing of the cableboxinsulation to the point of failure. Overvoltage protection devices, such as lightningarresters, are frequently installed at these locations.Faults occurring in cable networks are less frequent than those on overhead (O/H)line systems, but are almost invariably permanent faults, which require more time forlocalization and repair than those on O/H lines.Where a cable fault occurs on a ring, supply can be quickly restored to all consumerswhen the faulty section of cable has been determined.If, however, the fault occurs on a radial feeder, the delay in locating the fault andcarrying out repair work can amount to several hours, and will affect all consumersdownstream of the fault position. In any case, if supply continuity is essential on all,or part of, an installation, a standby source must be provided.Centralized remote control, based on SCADA(Supervisory Control And Data Acquisition)systems and recent developments in IT(Information Technology) techniques, isbecoming more and more common in countriesin which the complexity of highly interconnectedsystems justifies the expenditure.Remote control of MV networksRemote control on MV feeders is useful to reduce outage durations in case of cablefault by providing an efficient and fast mean for loop configuration. This is achievedby motor operated switches implemented in some of the substations along the loopassociated with relevant remote telecontrol units. Remote controled substation willalways be reenergized through telecontroled operation when the other ones couldhave to wait for further manual operation.© Schneider Electric - all rights reservedSchneider Electric - Electrical installation guide 2009