Chapter B Connection to the MV utility distribution network B Contents  Supply of power at medium voltage B2 1.1 Power supply characteristics of medium voltage B2 utility distribution network 1.2 Different MV service connections B11 1.3 Some operational aspects of MV distribution networks B12 2 Procedure for the establishment of a new substation B4 2.1 Preliminary informations B14 2.2 Project studies B15 2.3 Implementation B15 2.4 Commissioning B15 3 Protection aspect B6 3.1 Protection against electric shocks B16 3.2 Protection of transformer and circuits B17 3.3 Interlocks and conditioned operations B19 4 The consumer substation with LV metering B22 4.1 General B22 4.2 Choice of MV switchgear B22 4.3 Choice of MV switchgear panel for a transformer circuit B25 4.4 Choice of MV/LV transformer B25 4.5 Instructions for use of MV equipment B29 5 The consumer substation with MV metering B32 5.1 General B32 5.2 Choice of panels B34 5.3 Parallel operation of transformers B35 6 Constitution of MV/LV distribution substations B37 6.1 Different types of substation B37 6.2 Indoor substation B37 6.3 Outdoor substation B39 d e v er es hts r g all ri ectric - El er d ei n h Sc © Schneider Electric - Electrical installation guide 2008 B - Connection to the MV public  Supply of power at medium distribution network voltage B2 The term "medium voltage" is commonly used for distribution systems with voltages above 1 kV and generally applied up to and including 52 kV (see IEC 601-01-28 Standard). In this chapter, distribution networks which operate at voltages of 1,000 V or less are referred to as Low-Voltage systems, while systems of power distribution which require one stage of stepdown voltage transformation, in order to feed into low voltage networks, will be referred to as Medium- Voltage systems. For economic and technical reasons the nominal voltage of medium-voltage distribution systems, as defined above, seldom exceeds 35 kV. . Power supply characteristics of medium voltage The main features which characterize a power- supply system include: utility distribution network b The nominal voltage and related insulation levels Nominal voltage and related insulation levels b The short-circuit current The nominal voltage of a system or of an equipment is defined in IEC 60038 Standard b The rated normal current of items of plant as “the voltage by which a system or equipment is designated and to which certain and equipment operating characteristics are referred”. Closely related to the nominal voltage is the b The earthing system “highest voltage for equipment” which concerns the level of insulation at normal working frequency, and to which other characteristics may be referred in relevant equipment recommendations. The “highest voltage for equipment” is defined in IEC 60038 Standard as: “the maximum value of voltage for which equipment may be used, that occurs under normal operating conditions at any time and at any point on the system. It excludes voltage transients, such as those due to system switching, and temporary voltage variations”. Notes: - The highest voltage for equipment is indicated for nominal system voltages higher than 1,000 V only. It is understood that, particularly for some categories of equipment, normal operation cannot be ensured up to this "highest voltage for equipment", having regard to voltage sensitive characteristics such as losses of capacitors, magnetizing current of transformers, etc. In such cases, IEC standards specify the limit to which the normal operation of this equipment can be ensured. 2- It is understood that the equipment to be used in systems having nominal voltage not exceeding 1,000 V should be specified with reference to the nominal system voltage only, both for operation and for insulation. 3- The definition for “highest voltage for equipment” given in IEC 60038 Standard is identical to the definition given in IEC 62271-1 Standard for “rated voltage”. IEC 62271-1 Standard concerns switchgear for voltages exceeding 1,000 V. The following values of Figure B, taken from IEC 60038 Standard, list the most-commonly used standard levels of medium-voltage distribution, and relate the nominal voltages to corresponding standard values of “Highest Voltage for Equipment”. These systems are generally three-wire systems unless otherwise indicated. The values shown are voltages between phases. The values indicated in parentheses should be considered as non-preferred values. It is recommended that these values should not be used for new systems to be constructed in future. It is recommended that in any one country the ratio between two adjacent nominal voltages should be not less than two. Series I (for 50 Hz and 60 Hz networks) Nominal system voltage Highest voltage for equipement (kV) (kV) 3.3 (1) 3 (1) 3.6 (1) d 6.6 (1) 6 (1) 7.2 (1) e v er 11 10 12 hts res 2- 2 2105 2147 .5 g all ri 33 (2) - 36 (2) ectric - -( 1) These values sh3o5u (l2d) not be us4e0d.5 f o(2r) public distribution systems. El (2) The unification of these values is under consideration. er eid Fig. B1 : Relation between nominal system voltages and highest voltages for the equipment n h Sc © Schneider Electric - Electrical installation guide 2008 B - Connection to the MV public  Supply of power at medium distribution network voltage B3 In order to ensure adequate protection of equipment against abnormally-medium short term power-frequency overvoltages, and transient overvoltages caused by lightning, switching, and system fault conditions, etc. all MV equipment must be specified to have appropriate rated insulation levels. A "rated insulation level" is a set of specified dielectric withstand values covering various operating conditions. For MV equipment, in addition to the "highest voltage for equipment", it includes lightning impulse withstand and short-duration power frequency withstand. Switchgear Figure B2 shown below, lists normal values of “withstand” voltage requirements from IEC 62271-1 Standard. The choice between List 1 and List 2 values of table B2 depends on the degree of exposure to lightning and switching overvoltages(1), the type of neutral earthing, and the type of overvoltage protection devices, etc. (for further guidance reference should be made to IEC 60071). Rated Rated lightning impulse withstand voltage Rated short-duration voltage (peak value) power-frequency U (r.m.s. withstand voltage value) List  List 2 (r.m.s. value) To earth, Across the To earth, Across the To earth, Across the between isolating between isolating between isolating poles distance poles distance poles distance and across and across and across open open open switching switching switching device device device (kV) (kV) (kV) (kV) (kV) (kV) (kV) 3.6 20 23 40 46 10 12 7.2 40 46 60 70 20 23 12 60 70 75 85 28 32 17.5 75 85 95 110 38 45 24 95 110 125 145 50 60 36 145 165 170 195 70 80 52 - - 250 290 95 110 72.5 - - 325 375 140 160 Note: The withstand voltage values “across the isolating distance” are valid only for the switching devices where the clearance between open contacts is designed to meet requirements specified for disconnectors (isolators). Fig. B2 : Switchgear rated insulation levels It should be noted that, at the voltage levels in question, no switching overvoltage ratings are mentioned. This is because overvoltages due to switching transients are less severe at these voltage levels than those due to lightning. Transformers Figure B3 shown below have been extracted from IEC 60076-3. The significance of list 1 and list 2 is the same as that for the switchgear table, i.e. the choice depends on the degree of exposure to lightning, etc. Highest voltage Rated short duration Rated lightning impulse for equipment power frequency withstand voltage (r.m.s.) withstand voltage (peak) (r.m.s.) List  List 2 (kV) (kV) (kV) (kV) y 1.1 3 - - 3.6 10 20 40 7.2 20 40 60 d e 12 28 60 75 erv 1274. 5 3580 7955 91525 hts res g 36 70 145 170 all ri 5722 . 5 9154 0 235205 ectric - El (1) This means basically that List 1 generally applies to Fig. B3 : Transformers rated insulation levels der ei switchgear to be used on underground-cable systems while n h List 2 is chosen for switchgear to be used on overhead-line Sc systems. © Schneider Electric - Electrical installation guide 2008 B - Connection to the MV public  Supply of power at medium distribution network voltage B4 Other components It is evident that the insulation performance of other MV components associated with these major items, e.g. porcelain or glass insulators, MV cables, instrument transformers, etc. must be compatible with that of the switchgear and transformers noted above. Test schedules for these items are given in appropriate IEC publications. The national standards of any particular country are normally rationalized to include one or two levels only of voltage, current, and fault-levels, etc. The national standards of any particular country General note: The IEC standards are intended for worldwide application and consequently are normally rationalized to include one or two embrace an extensive range of voltage and current levels. levels only of voltage, current, and fault-levels, These reflect the diverse practices adopted in countries of different meteorologic, etc. geographic and economic constraints. A circuit-breaker (or fuse switch, over a limited Short-circuit current voltage range) is the only form of switchgear Standard values of circuit-breaker short-circuit current-breaking capability are capable of safely breaking all kinds of fault normally given in kilo-amps. currents occurring on a power system. These values refer to a 3-phase short-circuit condition, and are expressed as the average of the r.m.s. values of the AC component of current in each of the three phases. For circuit-breakers in the rated voltage ranges being considered in this chapter, Figure B4 gives standard short-circuit current-breaking ratings. kV 3.6 7.2 2 7.5 24 36 52 kA 8 8 8 8 8 8 8 (rms) 10 12.5 12.5 12.5 12.5 12.5 12.5 16 16 16 16 16 16 20 25 25 25 25 25 25 40 40 40 40 40 40 50 Fig. B4 : Standard short-circuit current-breaking ratings Short-circuit current calculation The rules for calculating short-circuit currents in electrical installations are presented in IEC standard 60909. The calculation of short-circuit currents at various points in a power system can quickly turn into an arduous task when the installation is complicated. The use of specialized software accelerates calculations. This general standard, applicable for all radial and meshed power systems, 50 or 60 Hz and up to 550 kV, is extremely accurate and conservative. It may be used to handle the different types of solid short-circuit (symmetrical or dissymmetrical) that can occur in an electrical installation: b Three-phase short-circuit (all three phases), generally the type producing the highest currents b Two-phase short-circuit (between two phases), currents lower than three-phase faults Current (I) b Two-phase-to-earth short-circuit (between two phases and earth) b Phase-to-earth short-circuit (between a phase and earth), the most frequent type 22I''k (80% of all cases). 22Ib When a fault occurs, the transient short-circuit current is a function of time and I comprises two components (see Fig. B5). DC 22Ik b An AC component, decreasing to its steady-state value, caused by the various I rotating machines and a function of the combination of their time constants p ed b A DC component, decreasing to zero, caused by the initiation of the current and a v er function of the circuit impedances es hts r Time (t) Practically speaking, one must define the short-circuit values that are useful in Electric - all rig tmin stbbhe eIIl’be ’k:fic :rr trmsinmt sgps vo svalyealsu lutoeeep mo eof n fet shtq heaue tisp tiynmmmiteiinman l(t e msatyrniinmcdiamm tlh uecemtur rpi crdreaoenltl etac cyiunt)ritroeenrnr utspystetedm b:y the switching device when © Schneider Fpeigr .I EBC5 :6 G09ra0p9hic representation of short-circuit quantities as bbb IIIkpD::C rm:m DasCx iv mvaaululmuee oi nofs ft thtahene ts actneuearrodeuyn-sts vtaatleu es yomf tmhee tcriucrarle cnut raret nthte first peak Schneider Electric - Electrical installation guide 2008 B - Connection to the MV public  Supply of power at medium distribution network voltage B5 These currents are identified by subscripts 3, 2, 2E, 1, depending on the type of short-circuit, respectively three-phase, two-phase clear of earth, two-phase-to-earth, phase-to-earth. The method, based on the Thevenin superposition theorem and decomposition into symmetrical components, consists in applying to the short-circuit point an equivalent source of voltage in view of determining the current. The calculation takes place in three steps. b Define the equivalent source of voltage applied to the fault point. It represents the voltage existing just before the fault and is the rated voltage multiplied by a factor taking into account source variations, transformer on-load tap changers and the subtransient behavior of the machines. b Calculate the impedances, as seen from the fault point, of each branch arriving at this point. For positive and negative-sequence systems, the calculation does not take into account line capacitances and the admittances of parallel, non-rotating loads. b Once the voltage and impedance values are defined, calculate the characteristic minimum and maximum values of the short-circuit currents. The various current values at the fault point are calculated using: b The equations provided b A summing law for the currents flowing in the branches connected to the node: v I’’ (see Fig. B6 for I’’ calculation, where voltage factor c is defined by the k k standard; geometric or algebraic summing) v I = κ x 2 x I’’, where κ is less than 2, depending on the R/X ratio of the positive p k sequence impedance for the given branch; peak summing v I = μ x q x I’’, where μ and q are less than 1, depending on the generators and b k motors, and the minimum current interruption delay; algebraic summing v I = I’’, when the fault is far from the generator k k v I = λ x I, for a generator, where Ir is the rated generator current and λ is a factor k r depending on its saturation inductance; algebraic summing. Type of short-circuit I’’ k General situation Distant faults cUn cUn 3-phase 3Z1 3Z1 cUn c Un 2-phase Z1+Z2 2Z1 c Un 3 Z c Un 3 2 2-phase-to-earth Z1 Z2+Z2 Z0+Z1 Z0 Z1 +2Z0 c Un 3 c Un 3 Phase -to-e+arth 0+ 1 0 Z1+Z2 +Z0 2Z1+Z0 Fig. B6 : Short-circuit currents as per IEC 60909 Characterization There are 2 types of system equipment, based on whether or not they react when a fault occurs. Passive equipment This category comprises all equipment which, due to its function, must have the capacity to transport both normal current and short-circuit current. This equipment includes cables, lines, busbars, disconnecting switches, switches, transformers, series reactances and capacitors, instrument transformers. For this equipment, the capacity to withstand a short-circuit without damage is defined in terms of: d e v b Electrodynamic withstand (“peak withstand current”; value of the peak current er ebx Tphreesrmseadl wini tkhAst)a, ncdh a(r“ashctoerrti ztiimnge mweitchhsatannicda cl urerrseisntta”;n rcmes t ov aellueec teroxpdryensasmedic isnt rkeAs s ghts res for duration between 0,5 and 3 seconds, with a preferred value of 1 second), all ri characterizing maximum permissible heat dissipation. ectric - El er d ei n h Sc © Schneider Electric - Electrical installation guide 2008 B - Connection to the MV public  Supply of power at medium distribution network voltage B6 Active equipment This category comprises the equipment designed to clear short-circuit currents, i.e. circuit-breakers and fuses. This property is expressed by the breaking capacity and, if required, the making capacity when a fault occurs. b Breaking capacity (see Fig. B7) This basic characteristic of a fault interrupting device is the maximum current (rms value expressed in kA) it is capable of breaking under the specific conditions defined by the standards; in the IEC 62271-100 standard, it refers to the rms value of the AC component of the short-circuit current. In some other standards, the rms value of the sum of the 2 components (AC and DC) is specified, in which case, it is the “asymmetrical current”. The breaking capacity depends on other factors such as: v Voltage v R/X ratio of the interrupted circuit v Power system natural frequency v Number of breaking operations at maximum current, for example the cycle: O - C/O - C/O (O = opening, C = closing) The breaking capacity is a relatively complicated characteristic to define and it therefore comes as no surprise that the same device can be assigned different breaking capacities depending on the standard by which it is defined. b Short-circuit making capacity In general, this characteristic is implicitly defined by the breaking capacity because a device should be able to close for a current that it can break. Sometimes, the making capacity needs to be higher, for example for circuit-breakers protecting generators. The making capacity is defined in terms of peak value (expressed in kA) because the first asymmetric peak is the most demanding from an electrodynamic point of view. For example, according to standard IEC 62271-100, a circuit-breaker used in a 50 Hz power system must be able to handle a peak making current equal to 2.5 times the rms breaking current (2.6 times for 60 Hz systems). Making capacity is also required for switches, and sometimes for disconnectors, even if these devices are not able to clear the fault. b Prospective short-circuit breaking current Some devices have the capacity to limit the fault current to be interrupted. Their breaking capacity is defined as the maximum prospective breaking current that would develop during a solid short-circuit across the upstream terminals of the device. Specific device characteristics The functions provided by various interrupting devices and their main constraints are presented in Figure B8. Current (I) Device Isolation of Current switching Main constrains two active conditions networks Normal Fault Disconnector Yes No No Longitudinal input/output isolation I Switch No Yes No Making and breaking of normal AC load current Short-circuit making capacity Contactor No Yes No Rated making and breaking capacities Maximum making and breaking capacities Time (t) Duty and endurance characteristics Circuit-breaker No Yes Yes Short-circuit breaking capacity Short-circuit making capacity I d DC Fuse No No Yes Minimum short-circuit breaking hts reserve IIADCC:: PAepaekri oodf itch ec opmerpioodniecn ctomponent ccMaaappxaaimcciittuyym short-circuit breaking g all ri Fig. B7 : Rated breaking current of a circuit-breaker subjected c - to a short-circuit as per IEC 60056 Fig. B8 : Functions provided by interrupting devices ectri El er d ei n h Sc © Schneider Electric - Electrical installation guide 2008 B - Connection to the MV public  Supply of power at medium distribution network voltage B7 The most common normal current rating for Rated normal current general-purpose MV distribution switchgear is The rated normal current is defined as “the r.m.s. value of the current which can be carried continuously at rated frequency with a temperature rise not exceeding that 400 A. specified by the relevant product standard”. The rated normal current requirements for switchgear are decided at the substation design stage. The most common normal current rating for general-purpose MV distribution switchgear is 400 A. In industrial areas and medium-load-density urban districts, circuits rated at 630 A are sometimes required, while at bulk-supply substations which feed into MV networks, 800 A; 1,250 A; 1,600 A; 2,500 A and 4,000 A circuit-breakers are listed as standard ratings for incoming-transformer circuits, bus-section and bus-coupler CBs, etc. For MV/LV transformer with a normal primary current up to roughly 60 A, a MV switch-fuse combination can be used . For higher primary currents, switch-fuse combination usually does not have the required performances. There are no IEC-recommended rated current values for switch-fuse combinations. The actual rated current of a given combination, meaning a switchgear base and defined fuses, is provided by the manufacturer of the combination as a table "fuse reference / rated current". These values of the rated current are defined by considering parameters of the combination as: b Normal thermal current of the fuses b Necessary derating of the fuses, due to their usage within the enclosure. When combinations are used for protecting transformers, then further parameters are to be considered, as presented in Appendix A of the IEC 62271-105 and in the IEC 60787. They are mainly: b The normal MV current of the transformer b The possible need for overloading the transformer b The inrush magnetizing current b The MV short-circuit power b The tapping switch adjustment range. Manufacturers usually provide an application table "service voltage / transformer power / fuse reference" based on standard distribution network and transformer parameters, and such table should be used with care, if dealing with unusual installations. In such a scheme, the load-break switch should be suitably fitted with a tripping device e.g. with a relay to be able to trip at low fault-current levels which must cover (by an appropriate margin) the rated minimum breaking current of the MV fuses. In this way, medium values of fault current which are beyond the breaking capability of the load-break switch will be cleared by the fuses, while low fault-current values, that cannot be correctly cleared by the fuses, will be cleared by the tripped load-break switch. Influence of the ambient temperature and altitude on the rated current Normal-current ratings are assigned to all current-carrying electrical appliances, and upper limits are decided by the acceptable temperature rise caused by the I2R (watts) dissipated in the conductors, (where I = r.m.s. current in amperes and R = the resistance of the conductor in ohms), together with the heat produced by magnetic-hysteresis and eddy-current losses in motors, transformers, steel enclosures, etc. and dielectric losses in cables and capacitors, where appropriate. The temperature rise above the ambient temperature will depend mainly on the rate at which the heat is removed. For example, large currents can be passed through electric motor windings without causing them to overheat, simply because a cooling fan fixed to the shaft of the motor removes the heat at the same rate as it is produced, and so the temperature reaches a stable value below that which could damage the insulation and result in a burnt-out motor. The normal-current values recommended by IEC are based on ambient- air temperatures common to temperate climates at altitudes not exceeding 1,000 metres, so that items which depend on natural cooling by radiation and d air-convection will overheat if operated at rated normal current in a tropical climate ve and/ or at altitudes exceeding 1,000 metres. In such cases, the equipment has to be eser derated, i.e. be assigned a lower value of normal current rating. hts r The case of transformer is addressed in IEC 60076-2. all rig ectric - El er d ei n h Sc © Schneider Electric - Electrical installation guide 2008 B - Connection to the MV public  Supply of power at medium distribution network voltage B8 Earth faults on medium-voltage systems Earthing systems can produce dangerous voltage levels on Earthing and equipment-bonding earth connections require careful consideration, LV installations. LV consumers (and substation particularly regarding safety of the LV consumer during the occurrence of a short- operating personnel) can be safeguarded circuit to earth on the MV system. against this danger by: Earth electrodes b Restricting the magnitude of MV earth-fault In general, it is preferable, where physically possible, to separate the electrode currents provided for earthing exposed conductive parts of MV equipment from the electrode b Reducing the substation earthing resistance intended for earthing the LV neutral conductor. This is commonly practised in rural to the lowest possible value systems where the LV neutral-conductor earth electrode is installed at one or two spans of LV distribution line away from the substation. b Creating equipotential conditions at the substation and at the consumer’s installation In most cases, the limited space available in urban substations precludes this practice, i.e. there is no possibility of separating a MV electrode sufficiently from a LV electrode to avoid the transference of (possibly dangerous) voltages to the LV system. Earth-fault current Earth-fault current levels at medium voltage are generally (unless deliberately restricted) comparable to those of a 3-phase short-circuit. Such currents passing through an earth electrode will raise its voltage to a medium value with respect to “remote earth” (the earth surrounding the electrode will be raised to a medium potential; “remote earth” is at zero potential). For example, 10,000 A of earth-fault current passing through an electrode with an (unusually low) resistance of 0.5 ohms will raise its voltage to 5,000 V. Providing that all exposed metal in the substation is “bonded” (connected together) and then connected to the earth electrode, and the electrode is in the form of (or is connected to) a grid of conductors under the floor of the substation, then there is no danger to personnel, since this arrangement forms an equipotential “cage” in which all conductive material, including personnel, is raised to the same potential. Transferred potential A danger exists however from the problem known as Transferred Potential. It will be seen in Figure B9 that the neutral point of the LV winding of the MV/LV transformer is also connected to the common substation earth electrode, so that the neutral conductor, the LV phase windings and all phase conductors are also raised to the electrode potential. Low-voltage distribution cables leaving the substation will transfer this potential to consumers installations. It may be noted that there will be no LV insulation failure between phases or from phase to neutral since they are all at the same potential. It is probable, however, that the insulation between phase and earth of a cable or some part of an installation would fail. Solutions HV LV A first step in minimizing the obvious dangers of transferred potentials is to reduce the magnitude of MV earth-fault currents. This is commonly achieved by earthing the 1 MV system through resistors or reactors at the star points of selected transformers(1), 2 located at bulk-supply substations. A relatively medium transferred potential cannot be entirely avoided by this means, 3 however, and so the following strategy has been adopted in some countries. N The equipotential earthing installation at a consumer’s premises represents a remote Fault earth, i.e. at zero potential. However, if this earthing installation were to be connected by a low-impedance conductor to the earth electrode at the substation, then the I equipotential conditions existing in the substation would also exist at the consumer’s f Consumer installation. If V= IfRs Low-impedance interconnection This low-impedance interconnection is achieved simply by connecting the neutral conductor to the consumer’s equipotential installation, and the result is recognized as Rs the TN earthing system (IEC 60364) as shown in diagram A of Figure B0 next page. The TN system is generally associated with a Protective Multiple Earthing (PME) scheme, in which the neutral conductor is earthed at intervals along its length (every 3rd or 4th pole on a LV overhead-line distributor) and at each consumer’s service d ve position. It can be seen that the network of neutral conductors radiating from a er es Fig. B9 : Transferred potential substation, each of which is earthed at regular intervals, constitutes, together with hts r the substation earthing, a very effective low-resistance earth electrode. g all ri ectric - El er d ei n (1) The others being unearthed. A particular case of earth-fault h Sc current limitation is by means of a Petersen coil. © Schneider Electric - Electrical installation guide 2008 B - Connection to the MV public  Supply of power at medium distribution network voltage B9 Diagram Rs value A - TN-a B - IT-a Cases A and B MV LV MV LV 1 1 No particular resistance value for Rs is imposed in these cases 2 2 3 3 N N R R S S C - TT-a D - IT-b Cases C and D MV LV MV LV Uw - Uo 1 1 R s y Im 2 2 Where Uw = the rated normal-frequency withstand 3 3 voltage for low-voltage equipment at consumer installations N N Uo = phase to neutral voltage at consumer's installations Im = maximum value of MV earth-fault current RS RS E - TT-b F - IT-c Cases E and F MV LV MV LV Uws - U 1 1 R s y Im 2 2 Where Uws = the normal-frequency withstand voltage 3 3 for low-voltage equipments in the substation (since the exposed conductive N N parts of these equipments are earthed via Rs) U = phase to neutral voltage at the substation for the TT(s) system, but the phase-to- phase voltage for the IT(s) system RS RN RS RN Im = maximum value of MV earth-fault current In cases E and F the LV protective conductors (bonding exposed conductive parts) in the substation are earthed via the substation earth electrode, and it is therefore the substation LV equipment (only) that could be subjected to overvoltage. Notes: b For TN-a and IT-a, the MV and LV exposed conductive parts at the substation and those at the consumer’s installations, together with the LV neutral point of the transformer, are all earthed via the substation electrode system. b For TT-a and IT-b, the MV and LV exposed conductive parts at the substation, together with the LV neutral point of the transformer are earthed via the substation electrode system. b For TT-b and IT-c, the LV neutral point of the transformer is separately earthed outside of the area of influence of the substation earth electrode. Uw and Uws are commonly given the (IEC 60364-4-44) value Uo + 1200 V, where Uo is the nominal phase-to-neutral voltage of the LV system concerned. Fig. B10 : Maximum earthing resistance Rs at a MV/LV substation to ensure safety during a short-circuit to earth fault on the medium-voltage equipment for different earthing systems The combination of restricted earth-fault currents, equipotential installations and low resistance substation earthing, results in greatly reduced levels of overvoltage and limited stressing of phase-to-earth insulation during the type of MV earth-fault situation described above. d e v Limitation of the MV earth-fault current and earth resistance of the substation er es sAeneonth tehra wt iind ethlye- uTsTe ds yesatertmhi,n tgh es ycsotnesmu mis esrh’so ewanr tihni ndgia ignrsatmal laCt ioofn F (ibgeuirneg Bis1o0la. Itte wd iflrl obme ghts r that of the substation) constitutes a remote earth. all ri iTnhsiusl amtieoann osf tthhaet ,c aolnthsouumgehr ’tsh eeq turaipnmsfeenrrte, dth peo ptehnatsiael- twoi-lel naortth s itnressusla tthioen p ohfa aslel t-htore-peh ase ectric - El phases will be subjected to overvoltage. er d ei n h Sc © Schneider Electric - Electrical installation guide 2008 B - Connection to the MV public  Supply of power at medium distribution network voltage B0 The strategy in this case, is to reduce the resistance of the substation earth electrode, such that the standard value of 5-second withstand-voltage-to-earth for LV equipment and appliances will not be exceeded. Practical values adopted by one national electrical power-supply authority, on its 20 kV distribution systems, are as follows: b Maximum earth-fault current in the neutral connection on overhead line distribution systems, or mixed (O/H line and U/G cable) systems, is 300 A b Maximum earth-fault current in the neutral connection on underground systems is 1,000 A The formula required to determine the maximum value of earthing resistance Rs at the substation, to ensure that the LV withstand voltage will not be exceeded, is: the substation, to ensure that the LV withstand voltage will not be exceeded, is: Uw(cid:60)Uo Rs= iinn oohhmmss ((sseeee ccaasseess CC aanndd DD iinn FFiigguurree BC1100)).. Im Where Where Uw = the lowest standard value (in volts) of short-term (5 s) withstand voltage for the consumer’s installation and appliances = Uo + 1200 V (IEC 60364-4-44) Uo = phase to neutral voltage (in volts) at the consumer’s LV service position Im = maximum earth-fault current on the MV system (in amps). This maximum earth fault current Im is the vectorial sum of maximum earth-fault current in the neutral connection and total unbalanced capacitive current of the network. A third form of system earthing referred to as the “IT” system in IEC 60364 is commonly used where continuity of supply is essential, e.g. in hospitals, continuous- process manufacturing, etc. The principle depends on taking a supply from an unearthed source, usually a transformer, the secondary winding of which is unearthed, 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 secondary windings will result in zero or negligible fault-current flow, which can be allowed to persist 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 included in the neutral earthing lead. If however, these resistors were removed, so that the system is unearthed, the following notes apply. Diagram B (Figure B10) All phase wires and the neutral conductor are “floating” with respect to earth, to which they are “connected” via the (normally very medium) insulation resistances and (very small) capacitances between the live conductors and earthed metal (conduits, etc.). Assuming perfect insulation, all LV phase and neutral conductors will be raised by electrostatic 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 live conductors in a number of installations acting in parallel, that the system will behave similarly to the case where a neutral earthing resistor is present, i.e. all conductors will be raised to the potential of the substation earth. In these cases, the overvoltage stresses on the LV insulation are small or non- existent. Diagrams D and F (Figure B10) In these cases, the medium potential of the substation (S/S) earthing system acts on the isolated LV phase and neutral conductors: b Through the capacitance between the LV windings of the transformer and the transformer tank b Through capacitance between the equipotential conductors in the S/S and the cores of LV distribution cables leaving the S/S b Through current leakage paths in the insulation, in each case. At positions outside the area of influence of the S/S earthing, system capacitances exist between the conductors and earth at zero potential (capacitances between cores are irrelevant - all cores being raised to the same potential). The result is essentially a capacitive voltage divider, where each “capacitor” is d e shunted by (leakage path) resistances. v er es In general, LV cable and installation wiring capacitances to earth are much hts r larger, and the insulation resistances to earth are much smaller than those of the all rig cthoerr seusbpsotnadtiionng bpeatrwameeente trhse a ttr athnesf oSr/mS,e sr ota tnhka ta mndo stht eo fL tVh ew vinodltiangge. stresses appear at Electric - wThhee rreis teh ein M pVot eenatritahl -afat uclot ncusurrmenetr sle’ vinesl tiasl lraetsiotrnicst eisd n aost lpikreevlyio tuhselrye fmoreen ttioo nbeed a. problem er d ei n h Sc © Schneider Electric - Electrical installation guide 2008