EP0030320A1 - Process for recording discharges in an electrofilter - Google Patents

Abstract

A method for detecting breakdowns in an electrostatic filter in which single measured values of equal phase of successive half waves of the filter voltage and crest values of successive half waves of the primary current are compared with one another and in which the differences of the measured values at which a breakdown signal is delivered are made dependent on the existing filter voltage or the primary current.

Description

The invention relates to a method for detecting breakdowns in an electrostatic filter, which is fed via a rectifier, a high-voltage transformer and an actuator from an AC voltage network and in which the breakdown criterion is the exceeding of a predetermined differential voltage value of individual measured values of the same phase position of successive half-waves of the filter voltage.

This generic term refers to a method as is known for example from DE-OS 25 40 084.

It is common to detect a breakdown by increasing the primary current. The sole detection of the over However, current peaks exceeding the nominal current are not sufficient, since breakdowns often occur during operation, the peaks of which do not exceed the nominal current. These weak current breakthroughs must also be detected by the control. The current detection must therefore be dimensioned such that current peaks are recognized after the percentage of the operating current just exceeded and control processes are then triggered.

In addition to this indirect breakdown detection provided on the primary side of the voltage system, there is also the direct breakdown detection to be used on the high voltage side. This is e.g. the voltage drop at the moment of the breakdown is exploited. It does not matter whether the breakdown current has exceeded the nominal current limit or not. Since the voltage drop also occurs with low-current breakdowns, this is determined within the current half-wave. In the case of indirect detection via the primary current, on the other hand, the detection usually only takes place via the charging current peak of the current half-wave following the breakdown. The primary current shows only a slight reaction at the moment of the breakthrough. The direct detection of the breakdown on the high-voltage side can thus be regarded as the cheapest.

A method for detecting the breakdown on the high-voltage side can be e.g. consist in that the voltage amplitudes of successive half-waves of the fluctuations in the voltage at the separator are continuously compared with one another and a predetermined deviation of assigned measured values serves as a criterion for a breakdown.

As experience has shown that the arcing depends on the voltage Maximum of the half-wave occur, it can suffice in many cases according to the solution of the aforementioned DE-OS 25 40 084 if the voltage profiles are compared with each other according to the amplitude maximum. For this purpose, for example, individual voltage values can be recorded and stored in the falling flank of the separator voltage at predetermined times and these voltage values can each be compared with the measured voltage values of a subsequent half-wave at the corresponding times shifted by the period. If the comparison shows a considerable deviation, this can serve as a criterion for the rollover.

Such detection of breakthroughs has proven itself; However, specifying a sufficient difference is somewhat problematic across the entire filter voltage range, as it may this value should be smaller for low voltage values than for relatively high voltage values.

The object of the present invention is to ensure immediate detection of a breakdown in the entire possible filter voltage range.

This object is achieved in that the permissible differential voltage value is specified as a percentage of the filter voltage measured in each case. In this way, you have a comparison standard at any time that is adapted to the respective operating conditions and can therefore immediately decide whether there is a breakdown or not.

To increase the security of the breakthrough detection, it is advantageous if the peak values of the half-waves in each period of the primary current are additionally ge stores and compared with the assigned peak values in the following network period. Here too, exceeding a peak current difference proportional to the peak current value serves as a breakdown criterion.

If only the current criterion occurs, this can be interpreted as an indication of a fault in the system, since the voltage-dependent signal should normally also be present.

To further increase the safety for the essential variable "breakdown", it is also advantageous if the drop below a predetermined minimum voltage in the filter also serves as a breakdown criterion. Here too, the sole occurrence of this criterion - without the voltage comparison criterion mentioned at the beginning - can serve as an indication of a fault in the system.

The calculations and storage of the measured values required for calculating the breakdown detection are advantageously carried out digitally, with the aid of a microcomputer system.

• Figur 1 das elektrische Schaltbild einer Elektrofilteranlage mit Durchschlagserfassung,
• Figur 2 den Verlauf der Filtergleichspannung in Abhängigkeit von der Zeit und
• Figur 3 den Verlauf des Primärstromes abhängig von der Zeit.

The invention will be explained in more detail with reference to a drawing;
show it:

• FIG. 1 shows the electrical circuit diagram of an electrostatic precipitator with breakdown detection,
• Figure 2 shows the course of the DC filter voltage as a function of time and
• Figure 3 shows the course of the primary current depending on the time.

An electrostatic filter 1 is connected to a high voltage rectifier 2 and a high-voltage transformer 3 fed from an AC network 5. For voltage regulation or current regulation, a thyristor actuator 4 ′, for example, is provided in the primary circuit and is controlled by a digital regulator 6. This digital controller forms the necessary control signals for the thyristor actuator 4 from current and voltage-dependent values in conjunction with the breakdowns.

As already mentioned in the introduction, three criteria are used for the breakdown detection, firstly the voltage comparison of successive half-waves of the filter voltage, then the comparison of peak values of the primary current and additionally a minimum voltage monitoring.

gebildet. Jede der so im Vergleicher 76 gebildeten Einzelspannungsdifferenzen wird mit einem Grenzwert ΔU_Gr verglichen, der wie folgt aus der Filterspannung berechnet wird:

wobei X zwischen 0,05 und 0,2 liegen kann.The half-waves of the filter voltage U _F (see FIG. 2) are sampled, for example, twenty times - sampling i = 0.1.2 ... - per half-wave, the half-wave beginning with, for example, the voltage U _FO (N) due to the external signal crossing zero Mains voltage is signaled with the period T. Twenty of these individual measured values U _Fi (N) of the half-wave N distributed over half the period T / 2 are stored in the memory 74. Subsequently, twenty individual measured values U _Fi (N + 1) of the subsequent half-wave N + 1 are sampled and stored in the memory 75, with the same phase position as that of the previous half-wave N. The individual measured values of the same phase position of successive half-waves are compared with one another and the voltage differences

educated. Each of the individual voltage differences thus formed in the comparator 76 is compared with a limit value ΔU _Gr , which is calculated from the filter voltage as follows:

where X can be between 0.05 and 0.2.

Breakthrough processing in controller 6 is initiated via gate 79 if

Für die Dauer der restlichen Halbwelle wird die Durchbruchserfassung ausgesetzt.

Breakthrough detection is suspended for the duration of the remaining half-wave.

In the schematically illustrated circuit diagram, the storage and the comparison of related measured values is indicated by switch 77, which are actuated by a synchronization control, not shown.

To monitor the current criterion (see FIG. 3), the primary current I _{p is} rectified with a rectifier 8 and the peak values I _p (N) and I _p (N + 1) of a half wave and the following half wave are stored and with the corresponding values Ip (N + 2), Ip (N + 3) of the following network period compared. This results in the determination of the following current differences at the comparison stage 73 connected to the memories 71 and 72.

Jeder der beiden Scheitelwertdifferenzen wird mit einem Grenzwert verglichen, der sich nach folgender Beziehung berechnet:

wobei X_i die zulässige Abweichung des Primärstromscheitelwertes ist. Die Durchbruchsverarbeitung wird angestoßen wenn

oder

Each of the two peak value differences is compared with a limit value, which is calculated according to the following relationship:

where X _{i is} the permissible deviation of the primary current peak value. Breakthrough processing is triggered when

or

During the first half wave after a breakdown and During a run-up phase after a breakthrough with several subsequent breakthroughs, this type of breakthrough detection is suspended. The signal of the comparator 73 also goes via the gate 79 to the digital controller 6. The storage and sampling and the comparison of the individual current measured values is also indicated by the switch 77 actuated by the control unit (not shown).

If the filter voltage U _F falls below a threshold value, which is detected, for example, by the threshold element 78, the breakdown processing is also initiated if the detection has not been achieved according to the above two criteria, for example: due to a fault in a converter module.

The sole triggering according to criterion 3, falling below the minimum voltage, just like the sole occurrence of criterion 2 - excessive primary current - represents an abnormal operating result and is additionally output via gate 10 as fault signal St.

The structure described above with function blocks was chosen only for the sake of simplicity. In the technical implementation, the memory and computing functions will be transferred to a microcomputer system today, as represented by the equal sign 9 between the areas outlined with dashed lines.

This microcomputer system essentially consists of a central unit 91, the actual arithmetic and control unit, the memory 92 and the input and output devices 93, all of which are connected to a common bus 94.

Claims (4)

1. A method for detecting breakdowns in an electrostatic filter, which is fed via a rectifier, a high-voltage transformer and an actuator from an AC voltage network and in which the breakdown criterion is to exceed a predetermined differential voltage value of individual measured values of the same phase position of successive half-waves of the filter DC voltage, characterized in that the differential voltage value (ΔU _Fi ) is specified as a percentage value of the filter voltage (U _F ) measured in each case. 2. The method according to claim 1, characterized in that additionally the peak values (Ip (N + 1), 1 (N + 2) of the half-waves in each period (T) of the primary current (Ip) are stored and with the associated peak values of the following network period are compared and that exceeding a peak current difference (Δ3) proportional to the peak current value also serves as a breakdown criterion. 3. The method according to claim 2, characterized in that the sole occurrence of the current criterion is evaluated as an error signal. 4. The method according to claim 1, characterized in that falling below a minimum voltage (U _F ) on the filter (1) is also evaluated as a breakdown criterion and the sole occurrence of this minimum voltage criterion is used as a fault signal (St).

Images