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http://electrical-engineering-portal.com/an-overview-of-grounding-system-grounded

An Overview Of Groun rounding ding System System (Groun (Grounded ded))  Asiff Eqba  Asi Eqball

 An O ve rvi e w Of O f G ro und e d G ro und in g Sys te m ( o n p ho to : G ro und e d s o la r p ane l b y J .E.M . So la lar; r; je ms o la r.co m)

Continued from technical article: An article: An Overview Of Grounding System (Ungrounded) (Ungroun ded)

Topics covered

Solidly Soli dly groun grounde ded d syst em

 

 

Let us assume that R phase (Phase-3 in figure-3) is shorted to ground than: If  = Current Current thro ugh shorted path (Fault current ) In  = Current Current t hrough neutral to earth connection Icy  = Capacitive Capacitive current returning via the network Phase-2 Phase- 2 (Y   phase )-earth capacitances IcB = Capacitive Capacitive current returning via the network Phase-1 Phase- 1 (B  phase )-earth capacitances We can write:

Figure 3 - Solid ly grounded system

If = In + IcY IcY + IcB IcB + IrIr // EquationEquatio n-08 08 Where Ir  =  = Current ret urning via via netwo rk insulation resist ance which which is always negligibl negligiblee In case o f LV, system sys tem voltage available available between phase phas e and earth eart h is 415/1.732 = 240V . Resi Resist st anc ancee of earth plate, pla te, grounding grounding connections connections etc… is of the order of 1.5 Ohms so the earth eart h current is lim limited to approximately 240/1.5 =160 Amperes Amperes . This is not very high magnitude magnitude hence any intentional intentio nal impedance impedance is not requ require iredd in neutral to earth connection. connection.  As per pe r equation -08  f f or IfIf one can can see that that if IcY  and  and IcB is negligible than If = In which is the t he case in LV  system. At At 415V  level  level capaci capacitive tive ground currents current s are not signif icant hence we can can write: If  = In  f o r so lid lidly ly earthed earthed LV LV system // Equ Equatio ation-09 n-09

Go back to Index ↑ Top Resistance grounded system

In case case of o f MV system (3.3kV onwards to 33kV ) voltage between phase and earth is high. Al Also so capacitive charging curr current  ent  is  is not lar large ge enough to com compensate pensate the same, same, so earth f aul aultt curr current ent is  is likely lik ely to be excessive. Hence resist ance is Hence is connected co nnected between neutral to ground connection. Current through neutral is limited lim ited to 100-400 Amperes . Figure 4 - Resistance grounded system

Rest ri rict ct in ing g t he e art arth h f au ault lt current current / current current through neutral

 Altho ugh all a ll the th e compon co mponent ent o f po power wer syst s yst em at MV level are ar e rat ed at full MV system fault level , f or  instance: Windi Wi nding ng of transf ormer , Cables, Buss duc Bu ducts ts,,

 

Rotating machine winding, etc. Than what is getting protected p rotected by restricting restricting the earth fault curr current/curre ent/current nt through throug h neutral? 

The neutral of transformer or generator are grounded through impedance, the principal element of which is resistance. This method is used when the earth fault current would be too large if not restricted (e.g.) MV Generators. Generato rs. Here, Here, a resisto r is connected intentionally intent ionally between the neutral and earth. T his is to lim limitit the earth f aul aultt current. Go back to Index ↑ T he reasons t o limi limitt t he e art arth h f au ault lt current current The reasons to lim limit it the earth fault f ault current are: are:

1. In rotating electrical machines like motors and generators, if the earth f aul aultt current is high, as as in the case o f so lid earthing, the core damage would would be high. To limit limit t he damage damage to the core, co re, machi machine ne manuf manuf acturers allow only a lim limited ground gro und f ault current. This is given in the form f orm of a core damage damage curve.

2. A typical value would be 25A-100A for 1 second . T his value is used as a guide in selecting NGR and and sett ing st ato atorr earth f aul aultt relays relays in generator generator prot ection. 3. Winding damage in rotating electri electrical cal machines is not of serious concern (T (T hough windi windings ngs are rated rated f or f ull fault lev level). el). T he repairs to win winding ding damages damages can be done by the local lo cal re-winder. re-winder. But, in case case o f core damage, dam age, repairs repairs cannot be carried carried out at site. s ite. T he machin machinee has to be sent back to the manufact urer’s works f or repairs repairs thus resultingin resultingin prolonged peri periods ods o f loss o f production. Since Si nce rot ating electrical electrical machin machines es are no t pres ent in voltage levels f rom 22kV onwards, these systems are usually solidly grounded . 4. X0/X1 ratio ratio of the t he system syst em also decides type of neutral earthing. If the corresponding X0/X1 ratio falls under that predef ined range. It is a choice between to weather to deal with with higher voltage or o r higher current current while whi le under sho rt circuit. circuit. Ef Ef f ectivel ectivelyy earthed lowers the o ver voltage limit limit of o f the healthy phases while another phase is short ci circu rcuited ited to earth. But t he ground f aul aultt cur current rent is very high. high. That means system will need a high capacity breaker but insulation in sulation system has to be moderate BIL rating.

But as the neutral to earth impedance increases ground fault current reduces but doing so the over voltage f acto actorr will will rise even up to 1. 1.73 73 times! So requires a breaker with low current capacity but a HI HIGH GH BIL BIL f o r all insulation insul ation system s ystem.. Let us assume that R phase (Phase-1 in figure-4) is shorted to ground than: If  = Current Current thro ugh shorted path (Fault current ) In  = Current Current t hrough neutral to earth connection connection Icy  = Capacitive Capacitive current returning via the network Phase-2 Phase- 2 (Y phase)-earth capacitances IcB = Capacitive Capacitive current returning via the network Phase-3 Phase- 3 (B phase)-earth capacitances Repeating equation-8 equatio n-8 we can write:

If = In + IcY + IcB + Ir  Neglecting Ir and substituting the following:

 

In = -V1/Rn ( Negative sign indicates that capacitive charging & discharging current are in phase opposition to current through neutral ) IcY + IcB = Total capacitive charging and discharging current of healthy phase = j3CwV1 f rom equation-07  Phasor diagram representation will be:

So f ina inallllyy af ter substitution s ubstitution of In and IcY IcY + IcB IcB expression expression f or ground f ault current in MV MV system would be: If = - V1/ V1/Rn Rn + j3CwV1 j3CwV1 //// Equatio Equationn -10 - 10 Magnitude of ground fault current will be:

|I|Iff | = |V1|√(I/Rn)2+ |V1|√(I/Rn)2+ 9C2w2 9 C2w2 Go back to Index ↑ Top Syste m earthin earthing at EH EHV V level

Figure 5 - Phasor Diagram

In case case of o f HV system (above 33kV ) Capacitive Capacitive ground gro und current is large enough to neutraliz neu tralize e the earth fault curr currents ents  hence no res istance is required in in neutral to earth connection. Solid grounding is univer un iversally sally adopted for fo r following reasons:

1. As we already already understo o d that it is a choice cho ice between between weather to deal with with higher voltage or  o r  higher current current while under under sho rt circuit. At EHV EHV level level ifif we opt f or higher volt volt age than due to higher co co st o f  insulation at EH EHV V selectio selectionn of o f higher volt volt age will will not be a viable idea. idea. It is better to opt for higher current by selecting solid grounding.

2. Rotating machines are not present at EHV system so there is no use of limiting the ground fault current as we do in MV MV system. Ev Even en if rot ating machine machiness are present pres ent because o f higher volt volt age capacitive ground current is also large enough to neutralize the earth fault current. Go back to Index ↑

References:

1. Indus ndustr trial ial electrical network netwo rk design guide By By Schneider Schneider electric 2. Swi Switchge tchgear ar prot ectio n & po power wer syst em By Sunil S Rao Rao,, Khanna publicatio publicatio ns 3. EA EARTHI RTHING: NG: Your quest ions answered ans wered By By Geof Geof f Crons Cronshaw haw 4. IEE EEE E Recomm Recommended ended Practice f o r Electric Electric Po Po wer Dist Dist ribut ribution ion f o r Indus Industr trial ial Plants Asiff Eqbal - Bac Asi Bachelor helor of Eng Engineeri ineering ng in Electrical Electrical & El Electro ectronics nics engineering, engineering, f rom Manipal Manipal

University, (Karnataka), University, (Karnataka), India India in 2006. Presently involved in the des ign of EH EHV V out doo r  subst ation and coal fired thermal thermal power power plants plants f or more than seven years. years. Mot to of joini joining ng EEP EE P as a cont ributo ributorr is t o share my litt litt le engineering engineering experienc experiencee and help the budding engineers in bridging the t he conspicuous co nspicuous gap bet ween academics academics and Indust ndustrial rial practice. “I “If  f  you have knowledge, knowledge, let let o thers ligh lightt t heir candles candles with with it, so that people who are genuinely genuinely

 

helpingg one another ano ther develop new capacities capacities f o r action; it is about abo ut creating timeless timeless learni learning ng interest ed in helpin processes".

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