THE COLLIERY GUARDIAN AND JOURNAL OF THE COAL AND IRON TRADES. Vol. CVIII. THURSDAY, DECEMBER 24, 1914. No. 2817. Automatic Protective Switchgear for Alternating Current Systems.* By E. B. WEDMORE. In recent years great advance has been made in the art of constructing automatic devices for disconnecting defective feeders and apparatus from an electric distri- bution system with a minimum of disturbance to the supply. , The author proposes to consider the general conditions governing the choice and design of protective devices of this kind, and to describe some of the more generally useful devices now available. On plants rated at more than 20,000 kw. the systematic employment of selective devices throughout is commercially almost a necessity. On much smaller plants it is not sufficiently recognised that the use of selective protective devices often enables economies in the expenditure on cables to be effected, which alone will more than cover the expenditure on the apparatus. The expenditure on cables is likely to represent one- third to one-half of the total capital expenditure. If the selective apparatus employed is of a type independent of the arrangement of the cables, the distribution net- Fig. 1. F< Ring' main H station $ Q/ O o o o Fig. 1.—Elements of distribution system. Fig. 2.—Three-phase feeder with leak to earth. Fig. 3.—Fee er protected by core-balancing transformer. Fur 4.—Core-balancing with three transformers. Fig. 5.—Core-balancing combined with overload protection. work can be laid out on the most economical principles, unhampered by the limitations of reverse current and similar apparatus. Classification of Protective Devices and Feeder Systems. Protective devices may be roughly classified under three heads, as follow — (1) Overload devices, or devices which operate when the current exceeds a certain predetermined limit. (2) Reverse current devices, or devices which operate when the flow of current at any moment is in the reverse direction to what is normal at that moment. (3) Leakage devices, or devices which operate when current flows through other than the proper channels. Fig. 1 shows the elements of which distribution systems are constructed, and will serve to define the terms employed. The supply to the point A is through an independent feeder. The supply to B is duplicated by the use of parallel feeders, whilst that to C and D is duplicated by the use of an inter-connector. The ring main, feeding points E, F, G, and H, is a com- * From a paper read before the Institution of Electrical Engineers. bination of feeders and inter-connectors, the connection between any two of the distribution points being an inter-connector. All distribution systems are built up of combinations of the above. Independent feeders may be branched as at J and K, but are still considered as independent feeders or open-ended. In independent feeders a fault is characterised by an increase of the current in the normal direction in the individual feeder, and may be removed from the system by disconnecting the feeder at the end nearest to the source of supply. In closed systems, on the other hand, a fault on any feeder will cause an increase of current through the duplicate source of supply, varying in amount with the position of the fault. Protection of Independent Feeders. In the case of independent feeders it is impossible to remove the fault without causing a temporary loss of supply to that part fed by the faulty feeder; but trouble should be confined to the part in question, although Alternator Feeder Load Fig. 2. Alternator earthed Oil switch Transformer core and Feeder secondary Fig. 3. Battery £ Rel Transformers Feeder Instruments Bus-bars switch Trip coils i & 5 shunted by time-^iit fuses this is not always done. The following devices are employed for the purpose :— Simple Overload Devices.-^Fuses are satisfactory for small circuits, but are replaced by automatic circuit- breakers on large circuits in order to obtain better rupturing characteristics, and to save delay and expense in restoring the circuit. If instantaneous in operation, circuit breakers must be set high enough not to be dis- turbed by momentary heavy overloads incidental to the service. Such an arrangement necessitates all faults being retained on the circuit until they become heavy faults, and it may allow of serious injury to the plant on a sustained overload. Time-Limit Overload Devices. — This difficulty is partly met by the use of time-limit overload devices. The destructive heating will require a time roughly inversely proportional to the square of the current. A circuit breaker having an inverse time characteristic suitably adjusted would permit the plant to deal with any condition that it can safely meet, and it would dis- connect the circuit at any overload on the danger point being approached. In practice one relies on the attendant to prevent continuous overloading. ■ Leakage Devices.—A leak may occur between poles or to earth. A leak between poles generally develops into a short circuit. At the source of supply it is indistinguishable from an overload, and the case is met by the use of overload devices. A leak to earth may not cause overloading, and thus may continue for a long time on a circuit protected only by overload devices. This introduces a risk of fire or shock, serious in mining work and undesirable in all cases. In case the leak is through combustible insu- lating materials, as in machines or cables, it will probably develop into a short circuit between phases. Fig. 2 represents an independent three-phase feeder having a leak to earth at the point A. It will be observed that prior to the leak all current going out through one of the conductors must return through the others, and that the sum of these currents must be zero. When a leak occurs, however, some of the current returns through the earth, and the balance of the cur- rents in the three conductors is no longer zero. This principle has been known for many years, and it is remarkable that it has only recently been applied in the design of automatic switchgear. The principle finds its embodiment'in protective switchgear of the core- balancing type, so called because the currents in the several cores in one cable are balanced. In one form a current transformer is furnished with a core surrounding the feeder conductors, as in fig. 3, which conductors form the primary winding. The secondary is connected directly through a relay to the circuit breaker. Such devices are obtainable adjustable to operate on a leak of a few amperes. In another form separate transformers are furnished, one in each conductor, and the tripping device is con- nected in a common return circuit, as in fig. 4. With this arrangement direct-acting trip coils can commonly be employed, operating at about one-third normal load, and the same transformers may be employed to excite instruments and to operate the usual time-limit over- load devices which are still necessary to protect the system against faults between phases and heavy over- loading. Fig. 5 shows a particular combination for this purpose, in which the time-limit overload protec- tion is obtained by the use of the well-known time-limit fuse. The introduction of core-balancing apparatus enables selective action to be obtained to an extent quite impos- sible with overload devices. With leakage protective gear, used in combination with a suitable limiting resistance, a fault to earth may be supplied by the generating plant for several seconds or more without danger, whilst at the consumer's end the device may be set to operate instantaneously at a fraction of normal load without any danger of its causing a disconnection during momentary overloads. The settings of the auto- matic devices in series between the generators and the local supply circuits can be graded in large steps of current and time without sacrificing the requirements at either end. Use of Fixed Time-Limit Relays. Fixed time-limit relays have been employed success- fully to get selective action between switches near to and others remote from the sources of supply. There is one essential condition of success, namely, that the difference in time setting between two consecutive switches must be sufficient to admit of one switch com- pletely disconnecting the circuit before the other commences to operate. There are several stages in the operation, as follows :—(1) Action of the tripping device; (2) separation of the switch contacts after the catch is released; (3) breaking the circuit after the con- tacts are separated; (4) cessation of movement of the relay after the circuit is opened. The total will generally lie between one-quarter second and one second, according to the design. To this must be added a margin for safety. The several relays in series must be adjusted to give this time interval between the action of each and the next. The heaviest faults will be those near the generating station, and yet these must remain jn the