EXPERIENCE WITH GROUNDED NEUTRAL IN A

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HIGH TENSION PLANT

C. W. RICKER

Electrical Superintendent, Power Stations, Interborough Rapid Transit Company

HE Interborough Rapid Transit Company of New York oper ates two steam power stations in parallel, containing seventeen engine-driven and one turbine-driven, 10 000 volt, 25 cycle generators of 90 500 kw aggregate rated capacity; and seventeen sub-stations, containing eighty-four 1 500 kw rotary converters, aggregating 120 000 kw. There is a separate lighting system in the subway supplied by three 1250 kw, 11 000 volt, 60 cycle, turbine driven generators.

The capacity rating is very conservative, all the generators and rotaries being able to carry double load for considerable periods. without injury. The sustained short-circuit current of the generators is about three times the full-load current, and that of the rotaries, running as alternating-current generators, in virtue of their stored energy of rotation, is, for a brief period, even greater. The momentary current to a ground or short-circuit may be much greater, depending upon the phase of the e.m.f. wave at the instant. The possible momentary flow of energy to any point in the high tension system, at which the insulation between conductors is broken down, is enormous, and although both generator and feeder oil switches are controlled by relays that are virtually instantaneous in operation on short-circuits, the switch mechanisms have a measurable time element which may be several tenths of a second, so that great damage may be done by a short-circuit.

The high tension distributing system consists of eighty-eight 25 cycle, 10 000 volt feeders, containing 308 miles of No. 000 and 13 miles of No. 0 000 cable, all operated in multiple, and three 60 cycle, 11000 volt, lighting feeders, containing 22.3 miles of No. 6 cable. All of the feeders are three-conductor, paper-insulated, lead-covered, underground cable, except one and one-half miles of three-conductor, No. 000, rubber-insulated, lead-covered, steel-armored, submarine cable. The longest feeders are about 8.6 miles long, and the average length is 3.8 miles.

With all of the high tension generating, distributing and transforming apparatus and conductors insulated from ground, a break

down of the insulation of a single conductor does not at once allow enough current to flow to operate the relays and trip out the switches through which the damaged conductor is fed, but a flow or discharge occurs at the fault, great enough to destroy very rapidly the insulation between conductors in the immediate neighborhood and cause a violent short-circuit. When this happens in a feeder cable, a portion of the cable is at once completely destroyed leaving an air gap of several inches or even several feet between the burned ends. If such a short-circuit occurs in a manhole where other cables are exposed to the arc, they may be seriously burned, even so badly as to short-circuit in themselves, and so spread the damage until a number of feeders are interrupted. All cables in manholes are protected by wrappings of asbestos and steel ribbon, but the amount of energy that can be concentrated at any point in the distributing system by a short-circuit is quite sufficient to destroy all the cable that can be put into a single manhole in an almost indefinitely short time, and leave no trace either of cable or protective wrapping. The damage which can be produced or rather provoked, by such an unconfined arc lasting a quarter of a second is not limited to its immediate vicinity. The arc, acting as a vibrator, gives rise to potential surges capable of weakening or breaking down the insulation in other and distant parts of the system, the effect of which may be apparent at once or only after several hours or days. Under such circumstances, one breakdown is usually accompanied or followed by others.

For instance: A three-conductor, No. 000, 10 000 volt, Subway division feeder cable, short-circuited at a joint where the insulation between conductors was insufficient. At the same instant a feeder oil switch in the Manhattan division power station (about five miles distant along the cables) was short-circuited and grounded on all three phases and badly damaged. The overload relays tripped the generator switches and shut down the power station. This particular switch had been in service for several years. It had been overhauled and cleaned within a few days, and was known to be in thoroughly good condition. At the same time the insulation of the potential transformer leads in one of the sub-stations was punctured. but the result was no more serious than the inconvenience of having to run the sub-station for some hours without any potential indicators or wattmeters and without synchroscopes in the primary side of the transformers which supply the rotary converters. This instance is cited because it is typical, although more serious break

downs have occurred. The accidental momentary grounding of a high tension conductor will produce effects similar to those described, although not necessarily of the same degree. Such a ground is liable to strain the insulation at some part of the system so that a break-down may be expected shortly afterward.

Another very troublesome difficulty is added, in such a completely insulated system, when a feeder cable breaks down anywhere in the conduit line outside the power stations and sub-stations. As just described, all three conductors and the lead sheath are almost invariably burned up or melted, leaving a gap in the cable and a considerable and altogether uncertain resistance across the carbonized insulation between the conductors and the sheath. No determination of the position of the break can be made from either end of the cable, and after many trials extending over a period of years, the most satisfactory method of locating a fault was found to consist in

sending from the power station, through the conductors and the lead sheath in series and across the insulation at the fault, a continuous constat current (from an arc dynamo). This current is reversed at short intervals, and can be traced by a galFIG I LISTENING COIL AND TELE- Vanometer or a listening coil carried PHONE RECEIVER USED IN LO- by an observer and placed near the exposed portions of the cable in the manholes. See Fig. 1. 1. By this

means the observer can distinguish the direction to the break, but in the case of a long cable, it may involve the opening of a great many manholes with hours of hard labor and weary searching, especially if the resistance at the fault is high and variable or if the fault is submerged in water, thus making the reversals indistinct. A short-circuit at a joint, when the sleeve is not wholly destroyed and still holds some compound which is melted by the testing current and heals up the break-down between the conductors and the sheath, is exasperating. The average feeder passes through about fifty manholes and the longest through more than one hundred.

All conducting parts of the high tension generating and distributing system are more or less enclosed in grounded envelopes, which, except in the case of the three-conductor feeder cables and the transformers, include only single conductors. Even in the feed

ers and transformers the insulation from ground is less than between conductors.

To facilitate the arc-dynamo method of locating faults, which must still be used in case a feeder short-circuits and has two or three

FIG. 2-SCHEME OF CONNECTIONS IN EACH GENERATING STATION.

PLIFIED DIAGRAM SHOWING THREE GENERATORS

SIM

conductors burned in two, the lead sheaths of all cables are grounded in the generating stations and are bonded together in all manholes by lead straps with wiped joints. Outside of the generating stations, the lead sheaths are kept as free from grounds as possible

and are practically insulated, except where they lie in very wet ducts. Bonding the cable sheaths together divides the return testing current so that it does not neutralize the effect upon the listening coil or galvanometer, of the current in the conductor within. This arrangement is also of material use in minimizing the effects of electrolytic action.

In order to prevent, as far as possible, the formation of shortcircuits, it was determined to ground the neutral points of all the generature armatures, which are star-connected. The scheme of connections in each generating station is shown in Fig. 2. By interposing a resistance in the lead to ground, the ground current at a fault in the insulation of any single conductor can be controlled, or at least damped, so as to allow a maximum flow great enough to open the automatic oil switch in the grounded conductor, but not enough to do serious damage. This arrangement constitutes a complete protection against damage proceeding from breakdowns. between single conductors and ground, though none at all against those occurring directly between conductors. As breakdowns to ground appear to be much more numerous, this partial protection is of great value and has been effective in every breakdown since its adoption, except one which was a short-circuit between conductors. In the generators, each phase of the winding is carried clear around the armature, so that the central point of the star is axially opposite the main leads on the opposite side of the armature. A lead is taken off at this point and conected, through a lever disconnect; ing switch and a current transformer, to an insulated bus-bar running along the station past all of the generators. Near the middle, this neutral bus-bar is grounded through a rheostat of six ohms resistance. See Fig. 3. The ground lead passes through a current transformer and is connected at several places to the discharge piping of the steam condenser.

The neutral ground rheostat is built up of cast iron grids, assembled in sections which are separated from each other by porcelain insulators. The whole rheostat is supported upon large porcelain petticoat line insulators. It is necessary to have the parts of this rheostat thoroughly insulated from each other, as the momentary current and fall of potential through it may be very large, depending upon the phase of the e.m.f. wave at the instant of breakdown and the resistance to ground, together with the inductance of the complete circuit including the armatures of the grounded generators and the rheostats, and the capacity of one-phase of the