US2978065A - Regulating electric precipitators - Google Patents

Description

April 4, 1961 B. BERG REGULATING ELECTRIC PRECIPITATORS Filed June 27, 1958 CLEANING DEVICE VOLTAGE f FIG. I.

l l l l l 1 I I F SHOV d FEE]. r

1" cu n RENT APPLIED FILTER VOLTAGE VARIATLT FLASHOVER LIMIT i i j 4 4 4 4 4 +-5TEP UP BY TIME RELAY AT REGULAR TIME INTERVALS Q-STEP DOWN RESULTING FROM FLASHOVER TIME INVENTORI BENGT BERG ATTYS,

REGULATIN G ELECTRIC PRECIPITATORS Bengt Berg, Bandhagen, Sweden, assignmto A.B. Svenska Fliiktfabriken, Stockholm, Sweden The present invention relates to a system for governing regulating means for electrical precipitators and consists of electrical control elements placed in the primary circuit of a high tension transformer. The control elements produce impulses, either directly or through an amplifier, resulting in the regulation of the voltage or current and con trol of the cleaning unit in the filter etc., in the event of flash-overs in the filter.

The efficiency with which particles are filtered in an electrostatic precipitator is determined by a large number of factors, inter alia, current and voltage. The amount of voltage required to produce corona current sufficient to cause a good particle charge is determined by the geometric configuration of the electrodes, the composition of the gas, its temperature and pressure, and the nature of the particles. in the case of certain simple gases and gas compositions the voltage and current can be estimated theoretically if the filter is completely clean. Similarly, under such conditions, the flash-over voltage can be calculated. It is, however, quite obvious that these conditions can never be satisfied in practice and the only solution is to use values based on experience.

As regards the flash-over voltage, it is found that the actual value is considerably below that calculated theoretically for a clean filter. Causes of the flash-over occurring at lower voltages may be changes in the distance between the electrodes, e.g., in that some of the very large number of emission wires accidentally swing as a result of heat stresses caused by rapid variations in temperature or the particle load. The particles may, owing to their electrical properties, cause the corona discharge to vary rapidly and unevenly, thus giving rise to an increased tendency towards isolated flashovers in the precipitator. The fall of particles caused when the filter is shaken for cleaning purposes may also cause hash-overs etc. it is evident from the foregoing that flash-overs may occur in the precipitator at any voltage, even though the tendency to flash-overs is, of course, increased at higher voltages. It is therefore preferred to define the flash-over limit as that voltage at which the flash-over frequency starts rapidly to increase. This limit is raised or lowered depending on the variations in filter operation. From the point of view of precipitation, the best results are obtained if the filter voltage can be kept in the region. of the flash-over limits as thus defined.

A primary object of the present invention, therefore, is to provide regulating means for an electric precipitator which automatically maintains the voltage applied to the precipitator at the highest possible value without causing excessive fiashovers.

More specifically the present invention provides in the primary circuit of a preciptator, a transducer which serves 69 the triple function of quenching flashovers, providing an impulse to a regulating circuit upon occurrence of flashoveis, and regulating the voltage applied to the precipitator to maintain the highest possible voltage without excessive flashovers.

With the foregoing in mind, the invention is more fully atent O period of time when the precipitator is controlled in accordance with the present invention.

The current and voltage characteristics of an electrostatic precipitator under varying operational conditions are, in principle, as shown in Fig. l. Curve a is the mean curve considered as applicable to the filter. Curves b, c and d indicate variations in operational conditions. I indicates the current which, from the point of View of particle charging, is calculated as desirable for the filter. The filter or precipitator-electrcdes have been so dimensioned that when the current 1 is passed through the precipitator, the voltage U normally obtained is at a value, which lies at a suitable safety-factor distance from the flash-over limit. To illustrate the matter further, let us consider the following conditions. Suppose that the flash-over limit and the mean points of the filter characteristics are as shown by U and a respectively. This results in the desirable condition whereby U is very near the flash-over limit. If the operational conditions are now altered so that curve b is'applicable, while U remains unchanged, the filter voltage will exceed U if the current i is maintained. It will then be necessary to reduce the current to the value 1. True, the filter voltage will still be in the region of U but the filter current will be too low to charge the particles effectively. if, instead, the filter charctcristic is altered to position c, the voltage will be lower than U if 1 is maintained. In order to restore the voltage to the region of U the filter current must be greatly increased to the value I, and this often means an impossible overloading of the rectifier if this latter unit is economically designed for normal operation.

It will be seen from the foregoing that it is impossible alwaysor even oftento keep the filter voltage near the flash-over limit. For the best possible function under all conditions, the filter should be so calculated that the lowest value for the flash-over limit is in the region of the highest value for the filter voltage (at current 1 curve b). The electrostatic precipitator is fed from a. rectifier which, for reasons of economy, ought not to be designed for currents notably higher than 1 It has been mentioned earlier that, under normal circumstances, it should not be reckoned that the filter voltage can always be kept in the region of the flash-over limit. However, it is possible under certain circumstances and, in such cases, the rectifier must be regulated. If the operational conditions are subject to rapid and frequent change, the supervisory staff must male constant adjustments to the rectifier. As a rule, the current wiil be reduced and maintained at such a low level that the supervisory staff will not be troubled by a high frequency of flash-overs. As a result, the capacity of the pro-- cipitator will not be fully utilized. In this case, it is easy to augment the rectifier with an automatic regulater.

The present invention refers to an improved form of such automatic control, characterized in that the impulse generator for the automatic unit consists of a transducer designed to function simultaneously as a voltage regulator, an "impulse generator, and as a device for markedly quenchirv the fiashovers. The transducer of the present system is a direct-current magnetized alter- 3 r mating-current choke coil, and thus includes such devices as saturable reactors and magnetic amplifiers. The amplification of the emitted impulses is achieved by the use of a suitably designed amplifier consisting of a thyratrou relay. One form of the device especially intended for regulation of the current or voltage to the maximum value as regards lash-over includes an amplifier unit consisting of a thyratron relay, combined with a time relay, both controlling a stage regulator in such a way that the current or. voltage are stepped-up at regular intervals, While in the case of flash-over, the current or the voltage are stepped-down in stages. The device according to the invention can also be used for starting the cleaner in the precipitator when the current in the precipitator has been stepped-down below a predetermined minimum value, this device being characterized in that the stage regulator is fitted with a limit switch for connecting or energizing the cleaner.

in the illustrated embodiment of the control system, the main line connection 1 has a main switch 2 for the entire installation. A control relay 3 can beswitched on and off by means of the manually operated contacts 3a and 312. A transducer 4 of special design is coupled into the primary circuit of the electrostatic precipitator 7. The electrostatic precipitator is provided with a high tension transformer 5, and a high tension rectifier 6. A rectifier 8 for the directecurrent pre-magnetization of the control windings of the transducer is connected to an intermediate transformer 9 for this rectifier. A regulating transformer 16 at one side is connected to the supply line 1'7 for the operating current to the automatic device and at the other side to the primary circuit of the transformer 9. The transformer 10 is fitted with a reversible regulating motor 11, which is caused to rotate in different directions by the alternative connection of the relays 12 and 13. This switch-over can either be effected manually by contacts 12a and 13:; or automatically by switch contacts 12b and 13b. The switch 12b is operated by a time relay 15, while switch 137) is operated by a thyratron relay 14 consisting of a thyratron 14a, a capacitive time regulation circuit 14b and a rectifier 14a. The latter rectifier is connected to transducer 4 by an intermediate transformer 16. A time relay 18 with an exceedingly delayed action (about 30 seconds) is connected in the operating circuit to the main control relay 3, this time relay breaking the supply of current to the electrostatic precipitator '7 in the event of a state of short circuit in the precipitator. A limit switch 20 responsive to the regulating arm of the transformer 10 is actuated by the regulating motor 11 when the current in the filter 7 is stepped down below a predetermined minimum. The limit switch, when closed, switches in the cleaning devices built in to the precipitato-r. These cleaning devices (not shown in detail on the drawing) are connected to the automatic control system at the points marked 19.

The control system according to the invention functions in the following manner: Assuming the motoroperating regulating transformer 1% is applying a low voltage to the precipitator 7, the voltage may be increased without danger of'excessive fiashovers. To this end, the timer operates at regular intervals to close the switch contacts 1% for a timed period. Upon closure of the contacts 1212, the relay 13 is energized which both maintains the relay 12 de-energized and energizes the motor 11 of the transformer 10 to cause rotation of the motor in a direction to step-up the voltage applied to the precipitator 7. The timed period causes the voltage to rise a predeterminedamount during each closure of the contacts 12b. The voltage increases periodically the predetermined amount until excessive fiashovers occur in the precipitator 7. 'When a flash-over takes place in the precipitator, the transducer 4 momentarily accepts the entire line voltage. According to the invention the transducer 4 is designed to have a marked flash-over quenching function, so that the transducer in this case serves both as a regulator for the rectifier and as a flash-over sensor. The rise in voltage in the transducer 4 generates an impulse which is transmitted through the intermediate transformer 16, the capacitive time regulation circuit 14b, and the rectifier 14a to the 'thyratron 140 which, Whenever the frequency of flashovers is sufiicient to accumulate enough energy in the circuit 14b to operate the thyratron 14a, closes the switch 13b, energizing. the relay 12 which both maintains the relay 13 de-energized and energizes the motor 11 to cause rotation thereof in the direction to step-down the voltage applied to theprecipitator 7.

In this way, the current to the electrostatic pre'cipitator 7 is reduced, as is also the voltage. The lengh of time the thyratron relay remains closed to cause such rotation of the motor, and thereby the magnitude of the drop in voltage for every flash-over, can be regulated by means of the variable resistances in the capacitive time-regulation circuit 14-11. 'As soon as the intended step-down has ceased, the regulating motor 11 is reversed by an impulse from the timer 15, which energizes the relay 13. As a result, the voltage is steppedup again periodically and this regulation continues until a new step-down process is introduced. A typical sequence of operation is illustrated in Fig.- 3. Inthis way it is possibleto maintain the filter voltage in the region of the above-mentioned flash-over limit, i.e., the voltage above which the flash-over frequency rapidly starts to rise. Should a state of short-circuit arise in the filter, the supply of current to the electrostatic p-recipitator 7 is disconnected via the time relay 18, which is so designed as to function with an exceedingly delayed action. When the flashovers become so frequent that the voltage is stepped-down to the point where the arm of the regulating transformer 16 closes the limit switch 20, the cleaning devices of the precipitator are energized through the operated regulating transformer connected to the control windings of said transducer and operable selectively to raise and lower the voltage applied to said precipitator, a timing device connected to .said control circuit to opcrate at regular intervals the transformer in one direc tion for a timed period and thereby periodically raise said applied voltage a predetermined amount, and a sensingcircuit responsive to said flashover-induced voltage in said transducer including a capacitive time-regulation circuit operable upon occurrence of repeated flashovers and resulting accumulation of energy in said capacitive circuit to disconnect said timing device from said control circuit and to operate the transformer inthe opposite direction for a timed interval and thereby lower the applied voltage a predetermined amount, whereby said transducer serves as both a flashover sensor and a regulator to apply the highest possible voltage to the precipitator at all times without excessive flashovers.

References Cited in the file of this patent UNITED STATES PATENTS Schaelchlin et a1; July 9, 1957

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