Sizing and Selection of Grounding TransformersDecision Criteria Gg dufu
lecticity Company of Ghaa System Planing ivision P.O. Box 5278, Acca-oth, Ghaa geogeedu
[email protected]
Gdd Mh
lecticity Company of Ghaa System Planing ivision P.O. Box 5278, Acca-oth, Gha
[email protected]g
bc- Within a period oHwo years, the Electricity ompany duty tsfome of equal kVA ating. Fo this eason, of Gana (CG) ost a tota of sx grounng transformers n a partcuar substaton. Te stuaton create a ot of nstabty an resute n uge prouctvty osses to bot te company an ts customers. Te faures were beeve to be reate to wrong seecton of grounng transformer ratng. owever, usng te concept of capactve cargng current of a system, t was foun tat te sort tme ratng of te grounng transformers were rgty seecte. Anayss of te penomenon strongy nke te amages to protecton ecency. Ts paper scusses anayss of te probem an proposes ecson crtera for seectng a grounng transformer.
gounding tasfomes ae oen not sized by "kVA" but by thei continuous and shot time cent atings. They ae usually oil immesed and may be installed outdoo. Gonding tasfome is used fo diect gonding o though a cuent limiting esisto. Zeo sequence impedance of gonding tansfome is quite low, but it can be inceased if the puose is to limit cuent though the tansfoe duing eah fault. The easons fo limiting cuent may be:
Kw m, v , q , m
a. To educe tansient ove voltage incusion om phase-to-eah fault. b. To educe mechaical stesses in cicuits cicuits ad appaatus caing fault cuents.
I. NTRODUCTION A poposal fo a chage in specication of gounding tansfoe was pesented in esponse to pesistent failue of gounding tsfome in a paticula substation of the lecticity Company of Gha. Among othes, the poposal suggested a eduction in ow of ea fault cuent om 3180 A to 1245 A ad an incease in shot time ating om 10 seconds to 10 minutes.
As a ule of thumb, gounding tsfomes ae designed with a continuous cuent ating equal to appoximately 10% of its shot-time ating. Fo exaple, a gonding tsfoe ated 1000A fo 10 seconds may cr100A (10% of 1000A) continuously. In pactice, the size of a gonding tsfome is based on capacitive chaging cuent of a system. This is because the chaging capacitive cuent is the lowest level of eath fault cuent at which system tansient ovevoltage ca be effectively educed.
e ole of gounding tansfome in powe systems is so citical that issues elating to its quality ad eliability ae teated with the utmost seiousness. As a holistic appoach to solving the poblem, the epot st looks at the basic concept of gonding tansfome in powe systems to put the subject in pespective. Theeae, the poposal is examined in a boade context. Based on technical analysis, it was poposed that the existing gounding tansfome specication be maintained. This pape pesents epot of the analysis ad poposes decision citeia fo selecting a gounding tasfome.
As discussed above, gounding tansfomes ca safely ca about 10% of it shot time ated load. Tempeates ding its continuous ating should not damage the windings. Heating of gonding tsfomes ae caused by adom shot duation cuents. Tempeatues that cause excessive gas development in the oil should be avoided. The tempeatue fo the windings in diect contact with the oil should not exceed 140°C. Fo this eason, Bucholz elay ad tempeatue potection ae povided. eutal C.T is also installed at neutal point of gounding tasfomes to ensue that in an event of sevee eath-fault, it signals the appopiate eath-fault elay to initiate tipping to potect the tansfome
II.
OF
BASIC ONCEPT ROUNDING SFORR N OWER YSTEMS
Gounding tansfome is used to povide a gound path to a ungonded delta connected system. As a shot-time ating device, its size ad cost ae less compaed with a continuous
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III.
ISCUSSION
OF ROPOSAL
With the bief oveview of the geneal concept of gounding tansfomes in powe system, we now exaine the poposal 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 mimum of 3180A to 1245A. ................it is being proposed that the existing zero sequence impedance of 19.2Q be changed to 50.
Although the poposal did not give detail on the technical consideation that inuenced the choice of e 1245A, it is a geneal knowledge that zeo sequence impedance detemines the value of eath-fault cuent. The desied value of the zeo sequence impedance is dependent on the system chaging capacitive cent. Theefoe, it is necessa to deteine the chaging cuent befoe the zeo impedace value ca be selected. The geneally accepted citeion fo deteining the size of zeo sequence impedance (Z) is that
At this condition, the destctive voltage build up on the chaging capacitance of the un-faulted phases cannot occu [1, 2, 3, and 4]. Whee, X is the line-to-eath capacitive eactace of the system. Stated in aothe way, the cuent in the zeo sequence impedance IN duing a line-to-eth fault must be equal to o geate tha tee times the line-to-eath system chaging cuent, 3Ico.
Accoding to [4, 5], ding line-to-eath system, chaging cuent (3 Ico) is given as
c
31
=
x 1x
f x C o
1
X
Am p er es
(1)
The zeo sequence capacitance of tansfome is negligible. Howeve, fo ove headlines, zeo sequence capacitance can be high if consideable lengths e involved. As a geneal ule, the following appoximate capacitance values ae used: Tansfoe C = O.OFtransformer Ove headline C = 0.00625 Fkm As indicated above, value of 3I is citical fo sizing and selecting gonding tansfoes. Fo good appoximation of 3I value, we consideed all cables and the ovehead lines length in the system. Also consideed ae tansfomes ad a capacito bak of 1.8MVa at the station. Based on equation (2), the zeo sequence capacitace of the cables e calculated, see the table-. SIC value used fo the capacitance calculation is 3.5.e capacitance values of the tasfome and the ovehead lines ae based on the appoximate values as indicated above. Fom Table 1, total zeo sequence capacitance of the 33kV system is 104.3F. is
Accodingly, using equation (2), 3Ico fo the 33kv system
r
3
Theefoe,
=
50 104.3
10
3I = 1872.308 Amps
33000
1 624
=-
.
.Q
IV. GRD TRRR SELE CRER Whee, LL is the system line-to-line voltage in kilovolts, SIC Criterion Based on the geneal le that Z ' it is dielectic constt, D is the diaete of cables ove the insulation shield, d is the diamete of the conducto, system ca be said that gounding tansfomes with values of Z up to 30.5 is appopiate fo selection. equency d C is zeo sequence capacitance of the system In elation to the above citeia, the 50 zeo sequence Based on equation (1), the system chaging cuents fo e impedace value suggested by the poposal does not match system (33kV netwok) c be calculated d hence, detemine the appopiate zeo sequence impedance. The the popety of the system. Hence, the poposed 50 zeo chaging cuent is calculated by summing the zeo-sequence sequence impedance is not appopiate. capacitance of all the cable and equipment conected to the Criterion : It appeas that the existing specication of system. 19.4 at shot time cuent ating 3180A also satises the The zeo sequence capacitace of any type of cable ca be geneal citeia. Howeve, to take an infoed decision, it is necess to compute the values of tasient ove voltage calculated using the following foula: nde the existing specication ad the calculated one (30.5 at ating of 1872A). Fo compaative analysis, tansient = 7 j (2) ovevoltage fo Z=50 is also computed.
g-
l
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V amete over nsulatio D amete over codco d Nue
b m f h b Ov x5 ( x6) x) 8 8 6 6 8 8MY 85 6 56 856 5 Cabe
Cabe
A
Legh(J') Capacance n
J
I
8.702845932
Cae
NA
26.89527355
Fom 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 Whee, X/X\ is the Thevenin's atio of zeo sequence eactance to positive sequence eactace of the location of the gounding tasfome. The tansient ove voltage is then calculated om the following elation [6]:
uing a line-to-eath fault on one phase, the tansient voltages on the healthy phases in elation to the Z values ae given in P.U and kV as: Zo Values in
P.U
Tansient Ovevoltage value
19.4 30 50
1.47 1.48 1.49
48.56 48.99 49.31
As ca be seen, the tsient voltage values pesented by the espective zeo sequence impedaces to e healthy phases unde line-to-eath condition ae lowe fo Z= 19.4 ad fo Z=30.5 as compaed to Z=50. is conms that Z=50 does not satis the geneal condition of Z X. Howeve, elative to the closeness of the tansient ovevoltage values fo Z=19.4 ad Z=30, decision egding the selection of the gounding tsfome ca still not be made at this stage.
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NA
A
NA
52
NA
A
NA
A
NA
3.3776735
I-ak
043060236
A
NA
A
NA
08
5466854076
Criterion The thid citeion is to conside sensitivity of
the elaying system ad the theal stess that will be imposed on the system in an event of eath-fault. At this stage, system enginees e nomally guided by potection philosophies. The geneal philosophy is that in a event of fault, enough cuent should be allowed to ow such that potective devices ca detect eath-fault cuent and tip off line but not so much cent to cause majo daage. emal stess ating of powe system equipment depends on t. Using Z = 19.4 will esult in the following: a. Highe ea fault cuent ad faste opeating times fo the existing IMT potection schemes at the station. b. ffects of high eath cuent will affect; a. Gounding tasfome 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 •
cuent, =elay opeation time. b. Cable ad Ovehead lines if the daage cuve of these equipment ae lowe th the IMT cuve of potection scheme potecting these equipment. Fom IMT potection schemes at the station, the potection cuves ae all fa lowe tha the damage cuve of the cables ad feedes, see the Fig.1. V. CE SDY: TERL SRE ALY O TE EE GRD TRRER FLRE A TE S This case uses typical eath-fault data, obtained om the potection elays d technical data as specied on the most ecent failed gounding tsfome, to exaine impact of themal stess, if any, on the system duing the ecent faile at the station.
HX 2 3 4 5 7 2 7 l 7 C 7 OM m _ �="_ 6"k'Jt!I9B o 1�
:e 9
.
, ,
,
�
,
,
10 : 1 : CUR9(A
(a)3X240 C XLPE
(a)X630 AI XLPE
X500 C XLP Fige 1. Potection cuves fo cables at the station
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8
=
=
Calculated design stess:
I � x t
4800 X 10 20,400,000 S
on
tsfome
IN>140A, td= .3seconds, cve: ong Time Invese Cuve ( TI) I7.7kA (aticipated tip time=0.34secs, assuming no CT satuation) Calculated themal stess on e gounding tansfome due to the fault:
177S02 034 = 107,11,S0A2 s As shown om the calculation, the design stess of the gounding tansfome is about 200% geate than the stess imposed on the system duing the fault condition. Ideally, the fault stess should not daage the tasfome. The high level of the fault cuent could be attibuted to a shot in the tasfome winding due to insulation bedown. Insulation beadown might be due to the following: 1. Inability of tempeatue potection system to detect oveheating of gounding tasfome possibly om the ow of cuent exceeding the continuous ating of the gounding tansfomes. 2. Poo CT sensitivity to the ow of gond fault cuent. Ou aalysis was also extended to the pevious failues at the station. It was conmed that the themal stess om the phase-to-ea faults wee all fa lowe compaed to the equipment designed stesses. Based on the above analysis, it obvious that the existing specication (Zo = 19.4 at 3180A) has no conection with the equent damages. The existing specication even povides oom fo tue gowth of the substation compaed with the poposed ating of at 1245A
VI.
ONCLUSION
Fequent damage of gonding tasfome at the station is attibuted to inability of tempeatue potection system to detect oveheating of the gounding tsfomes possibly esulting om the ow of cuent exceeding the continuous ating of gounding tansfome and poo sensitivity of neutal CT to gound fault cuent. Sizing of zeo sequence impedance depends entiely on the total capacitive chaging cuent of the system. To avoid tansient ove-voltages, gonding tansfomes must be sized so that the amont of the eath-fault cuent allowed to ow exceeds the electical system's chaging cuent. Gounding tasfomes should be selected to limit phase to-gound fault cuent such that the theal stess imposed on the system will be less tha the equipment design stess. Gounding tasfome should be selected such that in event of fault, enough cuent will ow to allow potective device to detect gound 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 Understding, vol 38, pp 1633-1640, NovDec 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 Trnsactions on Indust pplications vol IA-15, pp 136- 140, Mar/Apr 1979. [5] . ridger, Jr., "High-Resistance Grounding, IEEE Trnsactions on Indust pplications vol IA-19, pp 15- 21, JanFeb 1983. [6] Electricity Compy of Gha Distribution Planning Manual, Revised Edition 2011.
ropos
Prolong the le span of the grounding transformers by increasing the short time duration rating om the current 10 seconds to 1 0 minutes.
ine-to-eah is undesiable condition ad must not be allowed to pesist fo long time. Shot-time ating is necess to limit daage in a event that the system eath fault escalates into a double line-to-eath fault o e impedace of the tansfome becomes shoted. The standad ating allowed fo gounding tansfome anges om 10 to 60seconds. Howeve, whee gounding tasfomes ae used to establish a neual point to enable connection of phase-to neutal loads, continuous neutal cuent ating of the device is allowed because of the attendat load imbalance.
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