Sizing and Selection of Grounding TransformersDecision Criteria

Sizing and Selection of Grounding TransformersDecision Criteria Gg dufu

lecticity Company of Ghaa System Planing ivision P.O. Box 5278, Acca-oth, Ghaa geogeedu[email protected]

Gdd Mh

lecticity Company of Ghaa System Planing ivision P.O. Box 5278, Acca-oth, Gha [email protected]g

bc- Within a period oHwo years, the Electricity ompany duty tsfome of equal kVA ating. Fo this eason, of Gana (CG) ost a tota of sx grounng transformers n a partcuar substaton. Te stuaton create a ot of nstabty an resute n uge prouctvty osses to bot te company an ts customers. Te faures were beeve to be reate to wrong seecton of grounng transformer ratng. owever, usng te concept of capactve cargng current of a system, t was foun tat te sort tme ratng of te grounng transformers were rgty seecte. Anayss of te penomenon strongy nke te amages to protecton ecency. Ts paper scusses anayss of te probem an proposes ecson crtera for seectng a grounng transformer.

gounding tasfomes ae oen not sized by "kVA" but by thei continuous and shot time cent atings. They ae usually oil immesed and may be installed outdoo. Gonding tasfome is used fo diect gonding o though a cuent limiting esisto. Zeo sequence impedance of gonding tansfome is quite low, but it can be inceased if the puose is to limit cuent though the tansfoe duing eah fault. The easons fo limiting cuent may be:

Kw  m, v  ,  q ,  m  

a. To educe tansient ove voltage incusion om phase-to-eah fault. b. To educe mechaical stesses in cicuits cicuits ad appaatus caing fault cuents.

I. NTRODUCTION A poposal fo a chage in specication of gounding tansfoe was pesented in esponse to pesistent failue of gounding tsfome in a paticula substation of the lecticity Company of Gha. Among othes, the poposal suggested a eduction in ow of ea fault cuent om 3180 A to 1245 A ad an incease in shot time ating om 10 seconds to 10 minutes.

As a ule of thumb, gounding tsfomes ae designed with a continuous cuent ating equal to appoximately 10% of its shot-time ating. Fo exaple, a gonding tsfoe ated 1000A fo 10 seconds may cr100A (10% of 1000A) continuously. In pactice, the size of a gonding tsfome is based on capacitive chaging cuent of a system. This is because the chaging capacitive cuent is the lowest level of eath fault cuent at which system tansient ovevoltage ca be effectively educed.

e ole of gounding tansfome in powe systems is so citical that issues elating to its quality ad eliability ae teated with the utmost seiousness. As a holistic appoach to solving the poblem, the epot st looks at the basic concept of gonding tansfome in powe systems to put the subject in pespective. Theeae, the poposal is examined in a boade context. Based on technical analysis, it was poposed that the existing gounding tansfome specication be maintained. This pape pesents epot of the analysis ad poposes decision citeia fo selecting a gounding tasfome.

As discussed above, gounding tansfomes ca safely ca about 10% of it shot time ated load. Tempeates ding its continuous ating should not damage the windings. Heating of gonding tsfomes ae caused by adom shot duation cuents. Tempeatues that cause excessive gas development in the oil should be avoided. The tempeatue fo the windings in diect contact with the oil should not exceed 140°C. Fo this eason, Bucholz elay ad tempeatue potection ae povided. eutal C.T is also installed at neutal point of gounding tasfomes to ensue that in an event of sevee eath-fault, it signals the appopiate eath-fault elay to initiate tipping to potect the tansfome

II.

OF

BASIC ONCEPT ROUNDING SFORR N OWER YSTEMS

Gounding tansfome is used to povide a gound path to a ungonded delta connected system. As a shot-time ating device, its size ad cost ae less compaed with a continuous

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III.

ISCUSSION

OF ROPOSAL

With the bief oveview of the geneal concept of gounding tansfomes in powe system, we now exaine the poposal in detail.



ropos



Reduce the thermal stress on network components, and hence failure rates, by reducing currents that ow during earth faults om the current mimum of 3180A to 1245A. ................it is being proposed that the existing zero sequence impedance of 19.2Q be changed to 50.

Although the poposal did not give detail on the technical consideation that inuenced the choice of e 1245A, it is a geneal knowledge that zeo sequence impedance detemines the value of eath-fault cuent. The desied value of the zeo sequence impedance is dependent on the system chaging capacitive cent. Theefoe, it is necessa to deteine the chaging cuent befoe the zeo impedace value ca be selected. The geneally accepted citeion fo deteining the size of zeo sequence impedance (Z) is that

At this condition, the destctive voltage build up on the chaging capacitance of the un-faulted phases cannot occu [1, 2, 3, and 4]. Whee, X is the line-to-eath capacitive eactace of the system. Stated in aothe way, the cuent in the zeo sequence impedance IN duing a line-to-eth fault must be equal to o geate tha tee times the line-to-eath system chaging cuent, 3Ico.

Accoding to [4, 5], ding line-to-eath system, chaging cuent (3 Ico) is given as

c

31

=

x 1x

f x  C o

1

X

Am  p er  es

(1)

The zeo sequence capacitance of tansfome is negligible. Howeve, fo ove headlines, zeo sequence capacitance can be high if consideable lengths e involved. As a geneal ule, the following appoximate capacitance values ae used: Tansfoe  C = O.OFtransformer Ove headline  C = 0.00625 Fkm As indicated above, value of 3I is citical fo sizing and selecting gonding tansfoes. Fo good appoximation of 3I value, we consideed all cables and the ovehead lines length in the system. Also consideed ae tansfomes ad a capacito bak of 1.8MVa at the station. Based on equation (2), the zeo sequence capacitace of the cables e calculated, see the table-. SIC value used fo the capacitance calculation is 3.5.e capacitance values of the tasfome and the ovehead lines ae based on the appoximate values as indicated above. Fom Table 1, total zeo sequence capacitance of the 33kV system is 104.3F. is

Accodingly, using equation (2), 3Ico fo the 33kv system

r

3 

Theefoe,

=

   50 104.3 

10

3I = 1872.308 Amps

33000

1 624

=-

.

.Q

IV. GRD TRRR SELE CRER Whee, LL is the system line-to-line voltage in kilovolts, SIC Criterion  Based on the geneal le that Z  ' it is dielectic constt, D is the diaete of cables ove the insulation shield, d is the diamete of the conducto, system ca be said that gounding tansfomes with values of Z up to 30.5 is appopiate fo selection. equency d  C is zeo sequence capacitance of the system In elation to the above citeia, the 50 zeo sequence Based on equation (1), the system chaging cuents fo e impedace value suggested by the poposal does not match system (33kV netwok) c be calculated d hence, detemine the appopiate zeo sequence impedance. The the popety of the system. Hence, the poposed 50 zeo chaging cuent is calculated by summing the zeo-sequence sequence impedance is not appopiate. capacitance of all the cable and equipment conected to the Criterion : It appeas that the existing specication of system. 19.4 at shot time cuent ating 3180A also satises the The zeo sequence capacitace of any type of cable ca be geneal citeia. Howeve, to take an infoed decision, it is necess to compute the values of tasient ove voltage calculated using the following foula: nde the existing specication ad the calculated one (30.5 at ating of 1872A). Fo compaative analysis, tansient  = 7      j (2) ovevoltage fo Z=50 is also computed.

 

g-

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Cabe

A

Legh(J') Capacance n

J

 I  

8.702845932

Cae

NA

26.89527355

Fom ASP One-line modeling of the substation fo Z=19.4, X/X\=33.6177 fo Z=30, X/X\=62.5156 fo Z=50, X/X1=168.732 Whee, X/X\ is the Thevenin's atio of zeo sequence eactance to positive sequence eactace of the location of the gounding tasfome. The tansient ove voltage is then calculated om the following elation [6]:

uing a line-to-eath fault on one phase, the tansient voltages on the healthy phases in elation to the Z values ae given in P.U and kV as: Zo Values in 

P.U

Tansient Ovevoltage value

19.4 30 50

1.47 1.48 1.49

48.56 48.99 49.31

As ca be seen, the tsient voltage values pesented by the espective zeo sequence impedaces to e healthy phases unde line-to-eath condition ae lowe fo Z= 19.4 ad fo Z=30.5 as compaed to Z=50. is conms that Z=50 does not satis the geneal condition of Z X. Howeve, elative to the closeness of the tansient ovevoltage values fo Z=19.4 ad Z=30, decision egding the selection of the gounding tsfome ca still not be made at this stage.

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A

NA

52

NA

A

NA

A

NA

3.3776735

I-ak

043060236

A

NA

A

NA

08

5466854076

Criterion  The thid citeion is to conside sensitivity of

the elaying system ad the theal stess that will be imposed on the system in an event of eath-fault. At this stage, system enginees e nomally guided by potection philosophies. The geneal philosophy is that in a event of fault, enough cuent should be allowed to ow such that potective devices ca detect eath-fault cuent and tip off line but not so much cent to cause majo daage. emal stess ating of powe system equipment depends on t. Using Z = 19.4 will esult in the following: a. Highe ea fault cuent ad faste opeating times fo the existing IMT potection schemes at the station. b. ffects of high eath cuent will affect; a. Gounding tasfome  2 . xt d . ifl  J 2 t. I  p >I d e  e Sl g n Whe r e IFf au Sl g n   lt •

cuent, =elay opeation time. b. Cable ad Ovehead lines if the daage cuve of these equipment ae lowe th the IMT cuve of potection scheme potecting these equipment. Fom IMT potection schemes at the station, the potection cuves ae all fa lowe tha the damage cuve of the cables ad feedes, see the Fig.1. V. CE SDY: TERL SRE ALY O TE EE GRD TRRER FLRE A TE S This case uses typical eath-fault data, obtained om the potection elays d technical data as specied on the most ecent failed gounding tsfome, to exaine impact of themal stess, if any, on the system duing the ecent faile at the station.



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(a)X630 AI XLPE

 X500 C XLP Fige 1. Potection cuves fo cables at the station

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=

=

Calculated design stess:

I � x t

4800 X 10 20,400,000 S

on

tsfome

IN>140A, td= .3seconds, cve: ong Time Invese Cuve ( TI) I7.7kA (aticipated tip time=0.34secs, assuming no CT satuation) Calculated themal stess on e gounding tansfome due to the fault:

177S02  034 = 107,11,S0A2 s As shown om the calculation, the design stess of the gounding tansfome is about 200% geate than the stess imposed on the system duing the fault condition. Ideally, the fault stess should not daage the tasfome. The high level of the fault cuent could be attibuted to a shot in the tasfome winding due to insulation bedown. Insulation beadown might be due to the following: 1. Inability of tempeatue potection system to detect oveheating of gounding tasfome possibly om the ow of cuent exceeding the continuous ating of the gounding tansfomes. 2. Poo CT sensitivity to the ow of gond fault cuent. Ou aalysis was also extended to the pevious failues at the station. It was conmed that the themal stess om the phase-to-ea faults wee all fa lowe compaed to the equipment designed stesses. Based on the above analysis, it obvious that the existing specication (Zo = 19.4 at 3180A) has no conection with the equent damages. The existing specication even povides oom fo tue gowth of the substation compaed with the poposed ating of   at 1245A

VI.

ONCLUSION

Fequent damage of gonding tasfome at the station is attibuted to inability of tempeatue potection system to detect oveheating of the gounding tsfomes possibly esulting om the ow of cuent exceeding the continuous ating of gounding tansfome and poo sensitivity of neutal CT to gound fault cuent. Sizing of zeo sequence impedance depends entiely on the total capacitive chaging cuent of the system. To avoid tansient ove-voltages, gonding tansfomes must be sized so that the amont of the eath-fault cuent allowed to ow exceeds the electical system's chaging cuent. Gounding tasfomes should be selected to limit phase to-gound fault cuent such that the theal stess imposed on the system will be less tha the equipment design stess. Gounding tasfome should be selected such that in  event of fault, enough cuent will ow to allow potective device to detect gound fault. EFERENCES

IEEE

[1] J.R. Dunki-Jacobs, "The Reality of High-Resistance Grounding, vol IA-13, pp 469-475, Sept/Oct 1977.

Transactions on Indust pplications

[2] J.P. Nelson, "System Grounding and Ground Fault Protection in the Petrochemical Industry: A Need for a etter Understding, vol 38, pp 1633-1640, NovDec 2002.

IEEE Transactions on Indust pplications [3] W.C. loomquist, KJ. Owen and R.L. Gooch, "High-Resistance Grounded Power Systems  Why Not? IEEE Transactions on Indust pplications vol IA-2, pp 574-580, Nov/Dec 1976. [4] D.S. aker, "Charging Current Data for Guesswork-Free Design of High Resistance Grounded Systems, IEEE Trnsactions on Indust pplications vol IA-15, pp 136- 140, Mar/Apr 1979. [5] . ridger, Jr., "High-Resistance Grounding, IEEE Trnsactions on Indust pplications vol IA-19, pp 15- 21, JanFeb 1983. [6] Electricity Compy of Gha Distribution Planning Manual, Revised Edition 2011.

ropos 

Prolong the le span of the grounding transformers by increasing the short time duration rating om the current 10 seconds to 1 0 minutes.

ine-to-eah is undesiable condition ad must not be allowed to pesist fo long time. Shot-time ating is necess to limit daage in a event that the system eath fault escalates into a double line-to-eath fault o e impedace of the tansfome becomes shoted. The standad ating allowed fo gounding tansfome anges om 10 to 60seconds. Howeve, whee gounding tasfomes ae used to establish a neual point to enable connection of phase-to neutal loads, continuous neutal cuent ating of the device is allowed because of the attendat load imbalance.

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