US1534697A

US1534697A - Protective system

Info

Publication number: US1534697A
Authority: US
Publication date: 1925-04-21
Anticipated expiration: 1942-04-21

Description

A ril 21, 1925. 1,534,697

A. S. FITZGERALL'D.

PROTECTIVE SYSTEM Filedlug. 13, 1924 k .r f n r@ w o t t t mt A V Ins H m Patented Apr. 21, 1925.

UNITED STATES 1,534,692 PATENT OFFICE.

ALAN STEWART FITZGERALD, OF LONDON, ENGLAND, ASSIGNOR TO GENERAL ELEC- TR-IC COMPANY, A CORPORATION OF NEW YORK.

Application filed August 13, 1924.

To all whom it away concern:

it known that I, ALAN STEWART Frrzunnsrn, a subject of the British Empire, residing at London, England, have invented certain new and useful Improvements in ifi ing improved operating characteristics.

Current transformers for use in connection with high tension circuits, especially the kind known as bushing transformers, are inefiicient in comparison with low tension cur- 2o rent transformers, owing to the greater space occupied by the primary insulation of the former. The current'output from a high tension current transformer, moreover, is small in comparison with that from a low tension current transformer owing to the fewer primary ampere turns of the former (assuming a single primary turn in each). Owing to these facts, difficulty has been experienced in accurately balancing, over a wide range of primary load, the secondary currents of current transformers in circulating current protective systems for protect ing circuits of different potentials or the current transformers of which have different characteristics. An object of my invention is to provide means for overcoming this difficulty. It is, therefore, especially applicable to systems and apparatus arranged for the protection of a transformer, or of a pair of parallel feeders at different potentials.

According to my invention, the combined effect of the series connected secondary windings of two current transformers upon an additional secondary winding of one of the current transformers is neutralized by the primary winding thereof, when the primary currents are of normal values, that is, when there is no fault in. the circuit that is being protected. The additional secondary wind ing is connected to a tripping device or protective relay and serves to operate the same in the event of the normal relationship between the currents of the primary conductors Jeing disturbed, such as may be caused by a fault.

PROTECTIVE SYSTEM.

Serial No. 731,867.

My invention will be better understood from the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims. I

In the accompanying drawing, Fig. 1. represents a simple arrangement of circulating current protection according to prior practice, and Fig. 2 an arrangement embodying my invention.

Like reference numerals are used when ever possible to denote similar parts and in 1 the cores 10, 11 surround the high tension primary 12 and the lower tension primary 13 respectively. The secondary windings. latand 15 respectively, are connected for circulating current, a protective relay 16 being arranged across equi-potential points to operate in a manner well understood.

In Fig. 2 two secondary windings 1%, 14' are provided upon the core of the current transformer which has the poorer characteristic, that is upon the core 10 of the current transformer associated with the higher potential primary 12. There is usually am ple space on a bushing transformer for this purpose. The secondary winding 14 of the double-wound current transformer is con nected with the secondary winding 15 of the other current transformer in such sense and is so arranged that current induced in the latter winding and flowing through the former will produce in the core 10 of the double-wound current transformer a magnetic flux equal and opposite to that produced therein by the primary winding 12 thereof, when conditions in the circuit that is being protected are normal. In these circumstances the resultant flux in the core 10 of the double-wound current transformer is substantially zero since the magnetomotive force in the core 10 due to the series connected secondary windings 14c and 15 is neutralized by the magnetomotive force of the primary winding 1:2. The other or additional secondary winding 14 is connected to a tripping device or protective relay 16 and will be energized only when the normal relation in the protected circuit is disturbed. lVhen it is so energized, it may operate to open the main protected circuit, to give a warning signal, or in any other desired manner. Owing to the relatively large current output from the current transformer associated with the circuit of the lower potential, here is no difficulty in maintaining the desired ratio of ampere turns between the series connected secondary windings l4 and 15.

In some cases it may be necessary to introduce a restraining bias in a manner that is well known, The impedance representing the restraining winding may the-n be excited from a winding on the core of the current transformer which does not carry the additional winding, that is, on the core 11 of the current transformer associated with primary 13. Preferably it is connected in the circuit of the sericsconnected wind ings 14, 15.

If'at a given instant the currents in the

primary windings

12 and 13 are flowing in the directions indicated by the arrows 17, then the directions of currents in thesecondary winoings 14; and 15 are indicated by the arrows 18 in normal conditions. A con sideration of the windings 12 and let in Fig.

2 will show that they produce fluxes in the core 10 in opposite senses. Consequently, under normal conditions, that is, the desired current balance in the

conductors

12 and 13, substantially no current flows in the auXil-. iary secondary winding 14 and the relay 16 is not operated. Upon an unbalance in the currents in the conductors 12 and13, however, the resultant fiux in the core 10 will not be zero and current will flow in the secondary winding 14:. This current, depending upon the degree of unbalance for which the relay 16 is set to operate, will actuate the relay.

What I claim as new and desire to secure by Letters Patent of the United States, is:

l. A protective arrangement for an alternating electric current circuit wherein two current transformers have their primary windings connected in said circuit and their secondary windings connected in series, characterized by the fact that one of said transformers is provided with an additional secondary winding arranged so that the combined effect thereon of said series connected secondary windings is neutralized by the primary winding of said one transformer when circuit conditions are normal.

2. A. protective arrangement for an alternating electric current circuit comprising two current transformers haying primary windings connected in said circuit and secondary windings connected in. series, an auxiliary secondary winding on one of said transformers and a relay connected in circuit with said auxiliary winding, the connection and arrangement of said secondary windings being such that under normal conditions on said circuit the combined effect of said series connected secondary windings is neutralized by the primary winding of said one transformer] V 3. A protective arrangement for a pair of conductors carrying alternating electric currents at different potentials comprising a magnetic core around each conductor, a winding on each of said cores, said windings being connected and arranged to produce under normal conditions on said conductors a magnetomotiye force in the core around the conductorrat the higher potential neutralized by the magnetomoti e force due to the current in the higher potential conductor whereby the resultant flux in the core is normally substantially zero, an auxiliary winding on the core of the higher potential conductor, and a'relay connected in circuit with said auxiliary winding.

In witness whereof, I have hereunto set my hand this thirtieth day of July 192%.

ALAN STEWART FITZGERALDv