L
o c a t i o n of substations fo r converting high-tension a.c.- J supply into d irect-cu rren t fo r feeders is a m ajo r problem in laying out any coal-m ine pow er-distribution system , because, if satisfacto ry volt
age conditions a re to be m aintained, m eans m ust be found to overcom e the handicap of a receding load center.
T h is problem is intensified in m echan
ized m ining, because u n d er th a t sys
tem load centers sh ift m ore rapidly th a n u n d er hand m ining. T o m eet th is situation the portable substation has been developed.
T h e m a jo rity of m otors driving underground equipm ent a re 275 o r 550 volt d.c. A d.c. m o to r will keep on ru n n in g long a fte r the c o rre sponding a.c. equipm ent has stalled because of low voltage, but it is op
eratin g a t a disadvantage u n d e r such conditions and th e o u tp u t suffers p ro portionally. T h e speed of the average d.c. m otor drops approxim ately 19 per cent fo r a 20 p er cent voltage drop and 40 to 50 per cent fo r a 50 p er cent voltage drop. A voltage d rop of 50 p er cent sounds ridiculous
— but such conditions do exist in re m ote sections o f m ines th a t are spread over a considerable area.
I t is alw ays desirable to locate the substations as n ear to the load center as possible, in o rd e r to m inim ize length o f th e feeder cables, because, in the first place, the heavy cables re quired to c a rry th e direct c u rre n t are expensive, and then, no m atter how heavy the cable used, th e re is bound to be a considerable voltage fluctua
tion due to th e high c u rre n t sw ings as th e load changes. W h en a well planned and correctly located substa
tion is first installed the d.c. voltage is good, but inevitably th e w ork in g face and the load center recedes fa rth e r
and fa rth e r from th e substation as the coal is taken out. G radually the feeders are lengthened until even
tually a condition is reached re q u ir
ing th eir reinforcing, in o rd e r th at they m ay carry the pow er fo r the g reater distance, o r it m ay m ean relo
cation o f the substation n earer the new load center, o r a new substation supplem enting the original installa
tion if a reasonably good voltage is m aintained a t the m otor term inals.
E x te n sio n and rein fo rcin g of the cables is expensive. M oreover, th ere is a considerable pow er loss in the cable and re tu rn circuit, and this loss increases as th e length o f th e feeder is increased. T h e kw. loss in the circuit equals am peres x am peres x resistance divided by 1,000.
F o r exam ple, let us consider a 300- kw. substation supplied w ith 2,300- volt, 3-phase, 60-cycle a.c. and
deliv-By F. P. B R IG H T M A N
Industrial Engineering D epartm ent, General Electric Co
Schenectady, N . Y.
ering 275 volts d.c. to a load center approxim ately 4,000 ft. from th e sub
station. T h e a.c. incom ing line con
sists o f a 3-conductor N o. 2 A W G cable, and the d.c. feeder consists of a 1,250,000-c.m. copper cable w ith rail
retu rn .
T heoretically, the rail re tu rn should have less resistance th a n th e copper cables, but, in a g rea t m any installa
tions, the resistance of th e re tu rn c ir
cuit is equal to o r g re a te r th a n the resistance of th e m ain feeder cable, ow ing to poor bonding o f th e rails.
B ut we will asum e th a t the re tu rn c ir
cuit has the sam e resistance as the outgoing circuit. T h e load facto r is 50 p er cent, so th a t the average load fo r a 10-hour period is 150 kw. (545 am p .). T h e total resistance o f feeder
Fig. 1— Portable M otor-G cncrator S e t fo r M ine Service
A pril, 1930 — C O A L A G E 229
and rail circuit is ap p ro x im ately .069 to fit th e m ine track , an d is designed ings a re n o t intended to stand. T h e ohm. U n d e r these conditions, th e to obtain m inim um height above th e base is stiff enough to p rev en t bend-pow er loss is 205 kw .-hr. w hich, a t rails, in o rd e r th a t it can be tak en ing, b u t it cannot p rev en t tip p in g if l^ c. p e r kw .-hr., costs $3.07. I f th e th ro u g h entries w ith a low roof, th e tra c k is uneven,
load fa c to r is 75 p er cent, and av er- P o rta b le featu res can be applied to M o to r and g e n e ra to r term inals are age load 240 kw. (820 a m p .), th e eith er synchronous con v erters o r m o- b ro u g h t o u t a t th e side fo r con- loss am ounts to 450 kw .-hr., o r $6.75. to r-g e n e ra to r sets, to g eth er w ith m an- venience in m aking connections. E x - Coupled w ith th is pow er loss is a ual, p a rt autom atic, o r full autom atic tra flexible dynam o cable w as used voltage drop o f 38 a t 50 p er cent load control equipm ent. fo r th e 2,300-volt m o to r leads be-and 57 volts a t 75 p e r cent load. I t is A n excellent exam ple o f p ortable tw een th e sta rtin g -ru n n in g contactor no t a t all uncom m on fo r loads o f 150 substation equipm ent a re th e tw o tru c k and the m achine. In operation p e r cent to be on fo r considerable 300-kw . synchronous m o to r-g en era- these leads a re perm an en tly attached periods and th en th e voltage drops to to r sets w ith full autom atic control a t the m otor end an d equipped w ith 152. I f the re tu rn circuit happens to w hich G eneral E lectric recently b u ilt disconnecting ty p e potheads a t . the have h igher resistance th a n w e have fo r use a t th e W ildw ood ( P a .) m ine o th e r end, so th a t it is necessary only assum ed it to be. the voltage d ro o of the B u tle r C onsolidated Coal Co. to pull th e plug a p a rt w hen it is de
sired to disconnect th e m otor.
A ll relays an d contactors f o r con
tro llin g the a.c. and d.c. ends o f the set a re m o unted on th e rig h t-h an d u nit, show n in F ig . 2, to g eth er w ith the oil circuit b reak er and the d.c. re closing contactor. T h e control equip
m ent provides full autom atic o p era
tion, the sam e as th e conventional type o f substation. S tub-m ultiple d.c.
reclosing feed er equipm ent is built into the g e n e ra to r panel. E ach of the tw o cables connecting th e u n its a t the bottom o f th e tru c k s is a 9-con- d u cto r control cable fo r th e low -volt- age control circuits, equipped w ith stan d ard railw ay -car ty p e coupling device. T h is la tte r device can be in serted into th e receptacle in only one position, w hich assu res speedy con- and pow er loss will be correspond- T h e D re sse r m ine o f W a lte r B ledsoe nection o f the control circuits and in ingly g reater. O bviously, m otors can- & Co., T e r r e H a u te , In d ., also has th e rig h t o rd er,
n o t be expected to operate satis fac- recently added a p o rtab le substation
to rily u n d er such conditions. to its equipm ent. T X 7"H E N it is desired to install the I f th e substation could be m oved E ach of th e sets in use a t th e W ild - V V substation in a given location, it to the new load center, so th a t th e w ood m ine is of sta n d a rd 3 -bearing is m erely necessary to provide a suit- long ru n w as m ade by th e 2,300-volt construction and consists of a 435- able room and a tra c k w hich has been cable, instead o f by th e d.c. cable, kva., 0.8-pf., 1,200-r.p.m ., 2,300-volt, carefu lly leveled up. T h e u n its can the pow er loss in th e cable a t 75 p e r 3-phase, 60-cycle synchronous m o to r th en be hauled in an d spotted in the cent load on a 300 kw . system feed- w ith field coils w ound to tak e ex cita- desired location. T h e p rim a ry and ing a load cen ter 4,000 ft. fro m th e tion c u rre n t fro m th e gen erato r, di- secondary cables and th e incom ing source o f pow er w ould be ap p ro x - rect-connected to a 300-kw . g e n e ra to r line circuit are connected m erely by im ately 15c. fo r a 10-hour day. T h e rated 2 5 0 /2 7 5 volts, com pound in sertin g th e plugs. T h e d.c. connec- voltage d ro p w ould be approxim ately w ound, good fo r 50 p er cent overload tions are th e only ones w hich have to 1 p e r cent, o r 23 volts. I f th e con- fo r 2 h o u rs and fo r 200 p er cent fo r be bolted and, as th e re a re only tw o version a p p aratu s is a synchronous 1 m inute. A ll o f th e equipm ent is cables betw een th e g en erato r and converter, this 1 p e r cent will show m ounted on 42-in. gage tru c k s w ith panel to tak e care o f outside o f the up in the d.c. voltage as a d ro p o f a w heelbase of 48-in. T h e over-all outgoing feeder, the tim e req u ired fo r 2.75 volts, b u t if a m o to r-g en e rato r height above th e rail is 59^-in., p e r- g ettin g the station in operation should set is used the d.c. voltage will n o t m ittin g o f fre e m ovem ent th ro u g h be v ery sh o rt. W h e n it is desired to be affected a t all. any of th e entries. m ove th e equipm ent to a new
loca-U n fo rtu n a te ly th e jo b o f m oving T h e set (F ig . 1) is m o unted on an tion, it is necessary only to uncouple the conventional type o f substation e x tra heavy electrically w elded stru c - the connections, hitch on th e loco- entails considerable expense and a tu ra l steel base m ade up o f 11-in. m otive, and haul the individual units prolonged shutdow n, w ith a tte n d a n t I-beam s. I t is sufficiently rig id to to th e new location. T h e re a re no loss o f production, unless th e re is re- p erm it o f ru n n in g th e m achine stan d - m iscellaneous p arts to be g ath ered up serve capacity on th e system . T h e in g on the w heels, provided th e tra c k and b ro u g h t along,
portable equipm ent, how ever, has all is level. T h e tra c k w here th e set is A room w ith adequate provision the advantages o f th e conventional to stan d m u st be carefu lly leveled; fo r p ro tectin g th e equipm ent ag ain st type o f substation plus th e added ad- otherw ise, the ro ta tin g p a rts are likely d rip p in g w a te r and fo r k eeping out vantages o f m obility. T h e equipm ent to press against th e bearings, resu lt- unau th o rized persons, should be pro - is m ounted on tru c k s of suitable gage ing in a th ru s t effect w hich th e b ear- vided fo r th e substation. T h e re also
Fig. 2— Control E quipm ent S e t Up fo r Factory T e st; W h en in Operation, . Equipm ent W ill B e Connected in Sam e M anner as S e t S h o w n at R eader’s
Left
230 C O A L A G E — V o l.3 5 ,N 0.4
should be adequate ventilation if sa t
isfac to ry operation and long life o f th e set are to be obtained. A n excel
lent m eans o f obtaining th e desired ventilation is to locate the substation in a crosscut betw een tw o entries c a r
rying a ir in opposite directions (F ig . 3 ) , so th a t the fresh incom ing air can be d raw n into the substation an d dis
charged into the re tu rn a ir cu rren t.
In this w ay the ventilating a ir is cleaner and cooler th a n if it had c ir
culated aro u n d the m ine b efo re it got to th e set.
A substation located lengthw ise in the crosscut as indicated w ith a ir flow from end to end of th e room gets m uch b etter ventilation th an a sub
station located parallel to th e entry w ith air flow across the room . T he la tte r arran g em en t inevitably causes dead-air pockets. T h e crosscut a r rangem ent also requires less excava
tion.
T h e over-all height of th e W ild wood equipm ent is determ ined by the size of th e m o to r-g en erato r set itself.
W h ile it m ight be possible to reduce this slightly, the added expense would be so g re a t th a t it could h ard ly be justified. T h e 150- and 200-kw. sizes can be designed fo r a low er head room . A ten tativ e layout on a 200- kw. set indicates th a t it will be pos
sible to build a set fo r 52-in. above th e rails. T h e sw itching equipm ent can be designed to m eet an y req u ire
m ents th a t the set can m eet. In d u c tion m o to r-g e n e rato r sets can be used as well as synchronous sets, in w hich case the sw itching equipm ent is simplified considerably, especially if the m otor is designed fo r full-volt- age startin g .
O n e of the leading coal m ining com panies is considering the applica
tion of the portable featu res to small units to be used to provide pow er to drive an individual group of m a
chines consisting of drill, cutting m a
chine, and loader. T h e proposed equipm ent consists o f a 50-kw. induc
tion m o to r-g en erato r set designed fo r full-voltage sta rtin g and provided w ith p a rt autom atic control. T his type o f control provides fo r m anual sta rtin g of th e set w ith d.c. reclosing service on the g e n erato r end to cut dow n the loss o f tim e due to pow er outages caused by overloads and short-circuits.
A num ber o f these equipm ents, of course, would be required in the m ine and th e total installed g enerating ca
pacity w'ould be considerably in e x cess o f th a t required in a centrally
located substation, w here advantage could be taken o f the fact th a t all the m achines would n o t be carry in g full load at th e sam e tim e. W h en th e in dividual units are furnished, they m ust be large enough to handle the m axim um dem and of th e m achines they are driving. Such an a rra n g e m ent has the distinct advantage th a t the g en erato r can be located very close to the load and good voltage can be m aintained a t the m o to r term inals.
T he heat generated by the set, o f course, is very m uch less than in the big m achine and, consequently, less a ir is required fo r ventilation. A s a consequence, it is very probable th a t it will be possible to locate such an equipm ent on a sp u r tra c k adjacen t to the w orking face, w ith some p ro tection provided to prev en t w ater from d ripping on the equipm ent.
O ne o f th e v ery im p o rtan t factors entering into the choice o f new types o f ap p aratu s is th e question o f price not only on the basis o f the bare equipm ent itself, but also on the basis o f a com pletely installed substation ready fo r operation. T h e portable substation equipm ent here described costs approxim ately 22 per cent m ore th a n the conventional type of equip
m ent o rdinarily furnished by the m an u factu rer. T h is portable equip
m ent, how ever, comes to th e m ine completely assem bled and w ired, ready to be placed on th e track and
m oved into location. It is only neces
sary to b rin g in th e incom ing cable, and provide the outgoing d.c. feeder cable and negative gro u n d connection.
O n the o th er hand, it is estim ated th a t labor and m aterial fo r installing th e conventional type substation, exclu
sive o f foundations and room con
stru ctio n , is ap p ro x im ately 27 per cent o f the. cost o f th e set, sw itching equipm ent, and th e accessories. W h en the cost fo r the labor and m aterial is added to th e cost o f th e original equipm ent, the conventional type o f substation installed costs a p p ro x im ately 4 p er cent m ore th a n the portable equipm ent, less tra c k and room.
T h e conventional type of sub
station is a m ore o r less p erm anent affair and usually is installed in a re
inforced concrete room . T h e portable equipm ent is intended only fo r tem p o rary occupation o f a given location and fo r th a t reason can be installed in a knock-dow n type o f shelter m ade o f steel fram ew ork and corru g ated m etal or asbestos sheeting, unless the ro o f stru c tu re is so w eak th a t rein forcing is necessary. In either case, substantial foundation floor m u st be provided.
W h e re conditions a re suitable fo r inside-of-m ine location o f the sub
station, portable equipm ent provides the o perating engineer w ith a m eans o f fu rn ish in g d.c. pow er to th e m o
to r term inals at good voltage w ith a m inim um o f pow er loss in the cables.
Good voltage m eans longer life of the m otors, and higher speed and horse
pow er, w ith a proportionally increased o utput of coal. A ll o f these ad v a n tages a re obtainable fo r a p p ro x i
m ately the sam e price as fo r th e con
ventional ty p e o f substation.
April, 1930 — C O A L A G E 231