US2876861A - Electrical control circuits - Google Patents

Description

March 10, 1959 Filed Feb. 25, 1954 DC SOURCE i 2; RE/J REE/ PIPE C; /P/ TA T 0/? T. P. ROBINSON ET AL ELECTRICAL CONTROL CIRCUITS 2 Sheets-Sheet 1 Inventors T. R ROBINSON ELWGLOVER B WZZQ ZZB A ttorney 1959 I T.'.P.QROBINSON ETIAL 2,875,851 p ELECTRICAL CONTROL CIRCUITS I Filed Feb. 25, 1954 2 Sheets-Sheet 2 f AUXILIARY wm g 'Inveniors T. P. ROBINSON- B. -W.GLOVER ELECTRICAL coNTnor. cincturs Thomas Philip Robinson and Bertram Walter Glover, London, England, assignors to international Standard Electric Corporation, New York, N. Y.

Application February 25, E54, Serial No. 412,594

Claims priority, application Great Britain Februaryz'l, 1953 6 Claims. ct. issn This invention relates to controlling equipment for the supply of electrical power to a load circuit, with particular reference to protective arrangements therefor.

According to the invention, there is provided equipment for protecting an electrical circuit against sustained overloads without the necessity for breaking the circuit, which comprises a relay responsive to load voltage and a relay responsive to load current so arranged that when the load voltage is lower than a predetermined value and the load current is higher than a predetermined value, means are brought into use which reduce the voltage applied to the load.

The invention will be described with reference to the accompanying drawing which illustrates a preferred embodiment comprising a flash-over protection circuit for an electrical precipitator equipment. Such an equipment is fully described in British patent specification No. 684,024 (Air Preheater Corporation).

in the drawing in the interest of simplifying the circuit a convention is used which is now common in electrical circuits and more particularly, telephone switch-- ing system circuits. The relay windings are indicated by a reference and a sub-numeral which represents the number of contact sets controlled by the relay. The contacts controlled by the respective relays are not shown adjacent the relay windings but are placed in such a position on the drawing that numerous cross-over points in the circuit are avoided but the contacts are identified with their relays by the use of related references, for example, the reference RE3 T indicates th "he relay has four sets of contacts,

these contacts being given the references REEH, 1255.2, RE3.3, RES/i respectively. Similarly, the reference indicates that the relay REE has one contact set Which is given the reference REitl in the drawing. Make contacts are indicated by an open triangle and break contacts by a blacked-in triangle.

in a single figure cf the accompanying drawing, which is shown on two sheets representing a complete circuit diagram of the precipitator and its controls the main sup ply circuit to the precipitator consists of saturable reactors LSll-LSd in series in pairs in each phase of a 3-phase supply to a rectifier (MR1) feeding a load, in this instance an electrostatic precipitator, and further saturable reactors LPL-LPfi connected in permutation across the lines feeding the rectifier and following the series reactors in the circuit.

A step up transformer T1 is normally employed to provide a suitable high voltage to feed the rectifier, and. the first saturable reactors LSl-d may then be connected in series with the primary and the second saturable retates hatent actors LP1-6 in parallel with the primary of this transformer.

These series and shunt saturable reactors are normally employed to obtain control of the D. C. output from the rectifier by means of variations in the magnitude of saturating current applied to auxiliary windings of the reactors. The series reactors LSl-LS6 are adapted to he energized from an A. C. source via transformer T2 and rectifier MR2 whilst the parallel reactors LPl-LP6 are adapted to be energized from the same source via transformer T3 and rectifier MR3. This type of supply circuit is well known.

The present invention provides means for ensuring quick suppression of flash-overs occurring in a precipitator in the following manner.

When the precipitator is working normally and ionisation is taking place, the D. C. voltage and current stand at their normal values. When the rectifier output voltage is reduced, as, for instance, for cleaning of the precipitator, the 1). C. voltage is reduced to a value below ionisation level and the current is sensibly Zero. These two sets of conditions are normal to the operation of the precipitator and the protective device is not required to function. When a flash-over occurs in the precipitator, however, the impedance of the load circuit is reduced and the load current is mainly limited by the impedance of the series saturable reactors LS16, the transformer Tl (if present) and the rectifier MR1. The current in the precipitator rises to a high value and the voltage across the precipitator falls to a low value, but the are is maintained. These changes in magnitude of D. C. voltage and current are then made to operate the protective circuit by t c independent operation of a voltage sensitive relay REZ and a current sensitive relay RBI The current sensitive relay RBI is connected in series with, or across a resistor R1 in series with, the earthed pole i the rectifier MR1 D. C. output and will operate at any normal load current or higher value of current.

The voltage sensitive relay REZ is connected in the anode circuit of a cold cathode relay tube CC. This tube is supplied with anode voltage from the A. C. source via transformer T5 and rectifier MR5 and is arranged to be triggered by a voltage obtained from the A. C. source via transformer T4 and an auxil iary rectifier MR4 which is connected to the grid of the tube. in reverse series with this grid supply is the voltage developed across resistor R2 which is proportional to the voltage applied to the precipitator. This voltage may be derived from a resistor in series with the HV D. C. voltmeter V as shown in the fi ure or from a separate potential-divider resistor. When the voltage applied to the precipitator falls to a low value (as it does on to the cathode to fire the tube. Anode current then flows from rectifier MR through relay REZ contact RELZ being by then closed.

Relay contacts RELZ, RE4.3 and RE3.2 are connected into the anode circuit for the following reasons: RE1.2 prevents ignition of the tube CC when starting up, i. e. when the increasing D. C. voltage is approaching ionisation level but before there is any current flow in the precipitator; RE3.2 extinguishes the tube CC as soon as relay is closed, and permits relay REZ to release, and in particular opens contact M321, and R1343 prevents the completion of the anode circuit until the timing circuit relay 3 is de-energised.

When both voltage and current relays are closed, their contacts in series will permit relay and to become energised from an auxiliary D. C. source as shown in the figure.

The immediate operation of removes all D. C. saturation from the series saturable reactors in the main circuit by RE3.3 and applies saturation to the parallel saturable reactors in parallel with the transformer primary by RESA. The change of impedance in these series and parallel saturable reactors has the effect of transferring the major part of the voltage on the transformer primary to the series saturable reactors with the result that the output voltage from the main rectifier is reduced to a very low value and the arc in the precipitator is extinguished. Condenser C1 becomes charged via resistor R3 and after a predetermined time delay, relay 3 is operated.

By this time, both the current and voltage relays will be de-energised and contacts RELI and RE2.1 will be open but by-passed by RE3.1 (closed) and RE4.1 (not yet open). Contact RE4.1 new opening will de-energrse RES T Contact RE4.1 will also disconnect from the source of supply and as contact RE4.2 will quickly discharge condenser C1, relay RE4 T will quickly reset. The release of RE3 T will remove D. C. saturation from the parallel saturable reactors and restore it to the series saturable reactors. The precipitator will thus be restored to normal working conditions.

A time delay is necessary between reduction of saturation on the series reactors and increase of saturation on the parallel reactors, on the one hand, and the eventual restoration of normal saturation conditions, on the other, to allow the arc to be extinguished and to prevent hunting of the protective circuit which would otherwise occur owing to the time required for the saturable reactors to assume their new values of impedance and thus extinguish the arc.

A suitable choice of time delay in the operation of relay 3 will enable the flash-over to be suppressed and the precipitator returned to normal operation with a minimum loss of time and precipitation efiiciency.

It will be seen that are suppression is effected without a break in the normal supply circuit, and the interruption to normal working may be made as brief as desired by a satisfactory choice of delay times.

While the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

What we claim is:

1. Equipment for protecting an electrostatic precipitator against sustained flash-overs which comprises: series controlling saturable reactors and shunt controlling saturable reactors; a load-current-sensitive relay and a loadvoltage-sensitive relay; a third relay means responsive to the operation of both said relays on the occurrence of normal or greater than normal load current combined with less than normal load voltage to increase the impedances of the said series saturable reactors and to decrease the impedances of the said shunt saturable reactors, whereby the voltage applied to the said precipitator is materially reduced; and a fourth relay means also responsive, after predetermined delay, to the operation of both said relays to de-energise said third relay means and itself, thereby to restore said precipitator and said protection equipment to normal operation.

2. Device for protecting an electrical supply circuit against sustained overloads, comprising first relay means responsive to a load voltage lower than a predetermined value, second relay means responsive to a load current higher than a predetermined value and means controlled by said first and second relay means for reducing the voltage applied to the load on the occurrence of an overload.

3. Device for protecting an electrical supply circuit against sustained overloads, comprising a load-currentsensitive relay, a load-voltage-sensitive relay, a saturable reactor, a main winding for said reactor connected in said supply circuit, an auxiliary winding for said reactor, a source of current for energizing said auxiliary winding, switch means for controlling the connection of said source to said auxiliary winding, and means responsive to the operation of said load-voltage-sensitive and loadcurrent-sensitive relays for actuating said switch means.

4. Device for protecting an electrical supply circuit against sustained overloads according to claim 3, further comprising a time delay circuit and further relay means under the control of said time delay circuit for restoring said reactor to its normal current condition.

5. Device for protecting an electrical supply circuit against sustained overloads, comprising series controlling saturable reactors and shunt controlling saturable reactors, a load current-sensitive relay and a load-voltagesensitive relay, a third relay means responsive to the operation of both said relays on the occurrence of normal or greater than normal load current combined with less than normal load voltage to increase the impedances of the said series saturable reactors and to decrease the impedances of the said shunt saturable reactors, whereby the voltage applied to the said load is materially reduced, and fourth relay means responsive to the operation of both said relays for restoring the impedances of said reactors to their normal values after a predetermined time interval.

6. Device for protecting an electrical supply circuit against sustained overloads according to claim 5, in which said load-voltage-sensitive relay is connected in the anode circuit of a gas discharge tube arranged to be triggered when the load voltage drops below a predeter- 5 mined value.

References Cited in the file of this patent UNITED STATES PATENTS 2,365,611 White Dec. 19, 1944 10 2,672,947 Klemperer Mar. 23, 1954 2,675,092 Hall Apr. 13, 1954

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