1322 THE COLLIERY GUARDIAN. December 24, 1914. system for the maximum time, perhaps for several seconds. If leakage tripping devices are substituted, however, fixed time-limit delaying attachments can be used with great advantage, as they ensure positive selective action, and there is but little objection to allowing the limited fault current to flow in the system for the necessary interval. The Earth Connection. All three-phase systems have their mid-point earthed through the star-connected condensers formed by the capacity of the cable, and on a fault to earth a corre- sponding capacity current will flow which may be quite sufficient to operate core-balancing apparatus. If an earthing resistance is used, as is generally the case, it must allow a current to pass substantially larger than the minimum required to operate the automatic The relay connections are omibCed Switch Feeder* Fig. Alternator Transformer devices at the remote end of each feeder. When a fault occurs near the remote end of one feeder, the current may be shared nearly equally between them. The current flowing back into the fault from the common point at the remote end will actuate the reverse relay on the faulty feeder, and thus relieve the sound feeder. The time-limit overload devices prevent the disconnec- tion of the feeders until the reverse current device has separated them at the remote end, thus concentrating the fault on the faulty feeder. In addition to the disadvantages attached to the use of overload devices this combination possesses several inherent in the use of simple reverse current relays that depend for their discriminating action on the use of potential windings. Several attempts have been made to get over these difficulties, and although not one of them presents a general solution of the problem, they are worthy of further consideration. The more important devices will now be described. Generating station bus-bar Sub-station Sub-station Feeder bus-bar Feeder bus-bar ■*— Restraining Operating coil coil Contacts Fig. 11. Fig. 8. A, A, and B, B, from a generating station, the feeders being protected by overload devices at the points marked X. A further device is required to discriminate between the two feeders B, B, and this may be obtained by arranging that the more heavily loaded feeder shall be cut out, for it will be seen in the example illustrated that the faulty feeder will carry most of the fault current. This is shown in fig. 9, in which the fault currents have been added, assuming a fault to earth of 200 amperes. Fig. 10 shows diagrammatically a relay having the required characteristics. The operating coils are excited respectively from the two feeders, and the balance arm is biassed to the middle position. So long as the currents are equal, the balance arm will be unaffected, but with an increase of current on one side the arm will be drawn down on that side, and will close the corresponding pair of contacts to trip the switch in the feeder on that side. On the faulty feeder being JLWLW Fig. 9. Fig. 10. joo amperes zs amperes [Contacts Contacts! Reverse relay Fixed point Spring Solenoid & plunger 200 amps. Support for pivot loo amps, 2 75ampg. Solenoid & plunger Fig. 7. Fig. 6.—Source of supply protected by core-balancing. Fig. 7.—Protection of generators and feeders by core- balancing. Each switch is controlled by the adjacent protective transformer. Fig. 8.—Sub-stations fed through parallel feeders. Fig. 9.—Distribution of fault current in parallel feeders. Fig. 10.—Balanced overload relay. Fig. 11.—Erased protective relay. Fig. 12.—Protection of parallel feeders by interlocked reverse relays. Relays for one phase are shown, and with potential connections omitted ; similar relays are required in each phase releases of the switches so as to ensure the quick development of the fault and proper action in case the fault is of relatively high resistance. To keep down static disturbances, the resistance should be an effective shunt to the star capacity of the system, but there are insufficient data on metallic arcs in air and oil for us to lay down rules on this point. A high resistance absorbs less energy, and is likely to be cheaper. The section .of metal should, however, possess ample mechanical strength. It is recommended that on e.h.t. systems the resistance should not pass less than 50 ampdres or 100 amperes on high-tension and medium- tension systems.. In practice the current required is generally higher than this on account of the higher settings of the automatic releases in large generator switches. We are considering the protection of independent feeders, but it is convenient to digress at this point to draw attention to the current required to be carried by the earthing resistance in the case of parallel feeders. If these are feeders in parallel protected at both ends by overload or leakage devices, it should be noted that if a fault occurs on one of them near that end remote from the source of supply the fault current may be shared nearly equally between them, and as each will carry only its share of the current, this share must be sufficient to operate the protective apparatus. It has been shown that the use of leakage devices on feeder circuits has many advantages, and it remains to say that when core-balancing apparatus is applicable it will always give better results than overload devices. Leakage Protection for Sources of Supply. Hitherto we have been considering independent feeders conveying current to individual distributing or consuming points. The core-balancing principle has, however, been applied recently to independent feeders conveying current from sources of supply. Fig. 3 shows the device applied to an outgoing circuit, and it will be seen that if the insulation of the generator breaks down to earth the balance of current in the lines is unaffected and the switch does not operate. If, how- ever, the conductor from the generator’s neutral point to earth is included in the core-balancing transformer as in fig. 6, it will be found that the transformer is now unaffected by a feeder fault, but is affected by a fault in the machine or on the machine side of the i-ans- former. Core-balancing apparatus will cut out an indi- vidual faulty load circuit or source. Fig. 7 shows three machines and circuits so protected. This arrangement can be applied to any source having one point earthed, for example, a transformer, or a three-phase machine with one pole earthed. It gives a very simple solution in the case of Scott-con- nected transformers feeding a three-phase four-wire system, but it is not a general solution of the problem of completely protecting sources of supply, as it is opera- tive only on faults to earth. It gives complete pro- tection only if the design of the apparatus protected is such that a breakdown must involve an earth fault, and cannot occur simultaneously on all the phases. Protection of Parallel Feeders. The familiar and primitive solution of this problem lies in the use of time-limit overload devices at the end nearest the generating station, and reverse current Fig. 13. Feeder Fig. 14. Switch Cable Winding' “jo w Connections are shown for one phase only Fig. 15. Switch Trip colls Current transformers Overload fuses Alternator I Current transformers Fig. 16. Primary Secondary Trip/ COllSt 4 HT coils Switch Cable Transformer Alternator Fig. 18. Fig. 17. Fig. 13.—Protection of interconnector by balanced voltage system. Connections shown for one phase only. Fig. 14.—Machine winding and cable protected by circulating current system. Fig. 15.—Alternator protected by circulating-current system. Fi . 16. — A A transformer prot> cted by circulating current system. Fig. 17. - A Y transformer protected by circulating current system. Fig. 18.—Large alternator with step-up transformer and cable protected together by the circulating- current protective system. Inter-Locked Belays.—Perhaps the most important device is that which has been described as inter-locking, and is based on the principle that parallel feeders of the, same length, make, and cross section will normally share the current equally. If the relays are arranged so that they are inoperative except when this balance is upset, the arrangement should be proof against all conditions not accompanied by breakdown of one of the feeders in question. Overloads and surges, whether in the forward or reverse direction, will not operate the switches, whereas a leak to earth or between phases will immediately destroy the balance. The essential feature of the inter-lock is the establishment of a rela- tionship between the apparatus and the circuits such that the apparatus cannot operate to cut out the circuits unless the balance of current between the circuits is destroyed. Inter-Locked Overload Belays.—Fig. 8 shows two sub- stations fed in series through pairs of parallel feeders disconnected, the other becomes the more heavily loaded, and some device is required to prevent its being disconnected. The only sound method is to employ auxiliary switches on the oil switch mechanism to bring the coils into action automatically. Satisfactory service has been given by such a device which automatically converts the protection into time-limit overload protec- tion on one feeder on the failure of, or the deliberate disconnection of, the other. Sensitiveness of Inter-Locked Belays.—In practice a perfect balance of current between the feeders is not obtainable, and there will be an appreciable excess current on one side or the other on a severe overload. If now the maximum short circuit current is 100 times the normal load in each feeder, and the feeders balance within 1 per cent., there will be on a dead short circuit a difference in current equal to the normal load current, and the relays must not operate under these conditions. In practice the short circuit current may exceed this