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B - Connection to the MV publicdistribution network1 Supply of power at mediumvoltageB10© Schneider Electric - all rights reservedThe strategy in this case, is to reduce the resistance of the substation earthelectrode, such that the standard value of 5-second withstand-voltage-to-earth forLV equipment and appliances will not be exceeded.Practical values adopted by one national electrical power-supply authority, on its20 kV distribution systems, are as follows:b Maximum earth-fault current in the neutral connection on overhead line distributionsystems, or mixed (O/H line and U/G cable) systems, is 300 Ab Maximum earth-fault current in the neutral connection on underground systems is1,000 AThe formula required to determine the maximum value of earthing resistance Rs atthethesubstation,substation,totoensureensurethatthatthetheLVLVwithstandwithstandvoltagevoltage willwillnotnotbebeexceeded,exceeded,is:is:Uw UoRs = in ohms (see cases C and D in Figure B10). C10).ImWhereWhereUw = the lowest standard value (in volts) of short-term (5 s) withstand voltage for theconsumer’s installation and appliances = Uo + 1200 V (IEC 60364-4-44)Uo = phase to neutral voltage (in volts) at the consumer’s LV service positionIm = maximum earth-fault current on the MV system (in amps). This maximum earthfault current Im is the vectorial sum of maximum earth-fault current in the neutralconnection and total unbalanced capacitive current of the network.A third form of system earthing referred to as the “IT” system in IEC 60364 iscommonly used where continuity of supply is essential, e.g. in hospitals, continuousprocessmanufacturing, etc. The principle depends on taking a supply from anunearthed source, usually a transformer, the secondary winding of which isunearthed, or earthed through a medium impedance (u1,000 ohms). In these cases,an insulation failure to earth in the low-voltage circuits supplied from the secondarywindings will result in zero or negligible fault-current flow, which can be allowed topersist until it is convenient to shut-down the affected circuit to carry out repair work.Diagrams B, D and F (Figure B10)They show IT systems in which resistors (of approximately 1,000 ohms) are includedin the neutral earthing lead.If however, these resistors were removed, so that the system is unearthed, thefollowing notes apply.Diagram B (Figure B10)All phase wires and the neutral conductor are “floating” with respect to earth, to whichthey are “connected” via the (normally very medium) insulation resistances and (verysmall) capacitances between the live conductors and earthed metal (conduits, etc.).Assuming perfect insulation, all LV phase and neutral conductors will be raised byelectrostatic induction to a potential approaching that of the equipotential conductors.In practice, it is more likely, because of the numerous earth-leakage paths of all liveconductors in a number of installations acting in parallel, that the system will behavesimilarly to the case where a neutral earthing resistor is present, i.e. all conductorswill be raised to the potential of the substation earth.In these cases, the overvoltage stresses on the LV insulation are small or nonexistent.Diagrams D and F (Figure B10)In these cases, the medium potential of the substation (S/S) earthing system acts onthe isolated LV phase and neutral conductors:b Through the capacitance between the LV windings of the transformer and thetransformer tankb Through capacitance between the equipotential conductors in the S/S and thecores of LV distribution cables leaving the S/Sb Through current leakage paths in the insulation, in each case.At positions outside the area of influence of the S/S earthing, system capacitancesexist between the conductors and earth at zero potential (capacitances betweencores are irrelevant - all cores being raised to the same potential).The result is essentially a capacitive voltage divider, where each “capacitor” isshunted by (leakage path) resistances.In general, LV cable and installation wiring capacitances to earth are muchlarger, and the insulation resistances to earth are much smaller than those of thecorresponding parameters at the S/S, so that most of the voltage stresses appear atthe substation between the transformer tank and the LV winding.The rise in potential at consumers’ installations is not likely therefore to be a problemwhere the MV earth-fault current level is restricted as previously mentioned.Schneider Electric - Electrical installation guide 2009
B - Connection to the MV publicdistribution network1 Supply of power at mediumvoltageOverhead lineAll IT-earthed transformers, whether the neutral point is isolated or earthed througha medium impedance, are routinely provided with an overvoltage limiting devicewhich will automatically connect the neutral point directly to earth if an overvoltagecondition approaches the insulation-withstand level of the LV system.In addition to the possibilities mentioned above, several other ways in which theseovervoltages can occur are described in Clause 3.1.This kind of earth-fault is very rare, and when does occur is quickly detected andcleared by the automatic tripping of a circuit-breaker in a properly designed andconstructed installation.Safety in situations of elevated potentials depends entirely on the provision ofproperly arranged equipotential areas, the basis of which is generally in the form of awidemeshed grid of interconnected bare copper conductors connected to verticallydrivencopper-clad (1) steel rods.The equipotential criterion to be respected is that which is mentioned in Chapter Fdealing with protection against electric shock by indirect contact, namely: that thepotential between any two exposed metal parts which can be touched simultaneouslyby any parts the body must never, under any circumstances, exceed 50 V in dryconditions, or 25 V in wet conditions.Special care should be taken at the boundaries of equipotential areas to avoid steeppotential gradients on the surface of the ground which give rise to dangerous “steppotentials”.This question is closely related to the safe earthing of boundary fences and is furtherdiscussed in Sub-clause 3.1.B111.2 Different MV service connectionsAccording to the type of medium-voltage network, the following supply arrangementsare commonly adopted.Fig. B11 : Single-line serviceSingle-line serviceThe substation is supplied by a single circuit tee-off from a MV distributor (cable orline).In general, the MV service is connected into a panel containing a load-break/isolating switch-fuse combination and earthing switches, as shown in Figure B11.In some countries a pole-mounted transformer with no MV switchgear or fuses(at the pole) constitutes the “substation”. This type of MV service is very common inrural areas.Protection and switching devices are remote from the transformer, and generallycontrol a main overhead line, from which a number of these elementary service linesare tapped.Underground cablering mainFig. B12 : Ring-main service(1) Copper is cathodic to most other metals and thereforeresists corrosion.(2) A ring main is a continuous distributor in the form of aclosed loop, which originates and terminates on one set ofbusbars. Each end of the loop is controlled by its own circuitbreaker.In order to improve operational flexibility the busbarsare often divided into two sections by a normally closed bussectioncircuit-breaker, and each end of the ring is connectedto a different section.An interconnector is a continuous untapped feeder connectingthe busbars of two substations. Each end of the interconnectoris usually controlled by a circuit beaker.An interconnector-distributor is an interconnector whichsupplies one or more distribution substations along its length.Ring-main serviceRing-main units (RMU) are normally connected to form a MV ring main (2) orinterconnector-distributor (2) , such that the RMU busbars carry the full ring-main orinterconnector current (see Fig. B12).The RMU consists of three units, integrated to form a single assembly, viz:b 2 incoming units, each containing a load break/isolating switch and a circuitearthing switchb 1 outgoing and general protection unit, containing a load-break switch andMV fuses, or a combined load-break/fuse switch, or a circuit-breaker and isolatingswitch, together with a circuit-earthing switch in each case.All load-break switches and earthing switches are fully rated for short-circuit currentmakingduty.This arrangement provides the user with a two-source supply, thereby reducingconsiderably any interruption of service due to system faults or operations by thesupply authority, etc.The main application for RMUs is in utility supply MV underground-cable networks inurban areas.© Schneider Electric - all rights reservedSchneider Electric - Electrical installation guide 2009
Page 1 and 2: Technical collectionElectricalinsta
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Make the most of your energyISBN 97
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