WO2011138434A1

WO2011138434A1 - Current measuring method for switchgear having current paths connected in parallel

Info

Publication number: WO2011138434A1
Application number: PCT/EP2011/057297
Authority: WO
Inventors: Ralph Kriechel; Alexander Zumbeck
Original assignee: Eaton Industries Gmbh
Priority date: 2010-05-06
Filing date: 2011-05-06
Publication date: 2011-11-10
Also published as: CN103038848A; DE102010019533B4; DE102010019533A1; EP2567390A1

Abstract

The invention relates to a current measuring method for switchgear having current paths (L21; L22) connected in parallel. The digital measured values (Ι1'; Ι21'; Ι21'; I3') derived from the path currents (i1; i21; i22; i3) are converted into corrective values (I1; I2; I3) to compensate (Kn; Kp) for the non-linear transmission characteristics (N) associated with the transformer-based current transformation. The respective total value and difference values are determined from the digital measured values (I21', I22') associated with the current paths (L21, L22) connected in parallel, wherein at least one respective difference value is included in forming the associated corrective values (12) and the associated corrective values (12) represent the total of the associated path currents (i21, i22).

Description

Current measuring method for switching devices with parallel current paths

The invention relates to a current measuring method for switching devices, in particular circuit breaker or motor protection switch, for low voltage and alternating current with parallel-connected current paths according to the preamble of claim 1.

There are designs of switching devices, e.g. Circuit breakers, known in which

Current paths for increasing the current carrying capacity are connected in parallel. Also are

Versions of four-pole switching devices are known in which in each case only the two middle current paths are connected in parallel in order to increase the rated current. If those

Switching devices are designed with overcurrent releases, in particular with electronic overcurrent releases, the measurement signal of both the outer current paths as well as the two parallel-connected inner current paths must be detected and processed with sufficient accuracy.

Usually, the measurement of the overcurrents is performed by transformer transformers with iron cores. Alternatively, Rogowski coils are also used for the measurement. Furthermore, it is necessary to separately detect the measurement of the current signals in all switch poles of the device. In this case, in the two parallel current paths of the middle switch pole theoretically flows in each case half the current of the outer switch poles. For measurement, either the current transformers of the two parallel-connected current paths are usually designed with a different transformation ratio than the outer current paths and connected in series, or they are operated in parallel at the same transmission ratio. Another embodiment is a separate version of the analog

Preparation of the current signals by the measuring arrangement, consisting of a

Measuring resistor and possibly a measuring amplifier, the middle switch pole relative to the measuring arrangements of the outer poles are dimensioned differently.

The current transformers usually have a certain non-linear transmission characteristic as a function of the primary current, which compensates accordingly in the tripping electronics becomes. If the switching device is loaded symmetrically with current as described above, the rated current is divided approximately halfway across the two middle current paths and in each case into entire parts on the outer current paths. Here, the nonlinearity of

Current transformer in the transmitter sufficiently compensated. However, as is often the case in reality, an unbalanced load on the current paths or are the

Rail resistances states of the current paths differently, the distribution of the currents in the parallel current paths of the switching device is no longer the same. Due to the

different operating points on the transfer characteristic of the two current transformers, it depends on the primary flowing partial flow to increase the

Total tolerance of the converter pair. This has an incorrect detection of the currents and thus undesirably a greater trip tolerance of the overcurrent release, i. the risk of premature or late release.

From the document DE 1 950 319 A is a circuit breaker for low voltage and

AC with an even number in parallel and in the same sense durchflo Ssener current paths, each containing a switching point and busbars, known. To stabilize the current distribution in the parallel current paths are within the

Circuit breaker each two busbars on a portion of their length with opposite directions of flow arranged parallel to each other. These are parallel

Busbar sections are enclosed by an iron core. The switching points are arranged side by side, and the busbars are then guided symmetrically to the switching points cross and lie in the course of each other. The disadvantage of this arrangement is the volume and material consuming structure. In addition, the problem of increased tripping tolerance in unbalanced pole currents remains unresolved.

Reference DD 144 192 AI describes a current measuring device for electronic release for detecting over- and short-circuit currents in circuit breakers with two or more parallel-connected current paths per switch pole. Rogowski coils comprise geometrically each of a Schalterpol associated parallel-connected current paths and are electrically connected to integrators whose outputs act on the electronic release via an OR link. Even with this device, the problem of increased release tolerance in unbalanced pole currents is not solved. The invention is therefore based on the object to show a method for avoiding the larger release tolerance in asymmetric load of the switching device.

Starting from a current measuring method of the type mentioned, the object is achieved by the features of the independent claim, while the dependent claims advantageous developments of the invention can be found.

According to the invention, not only the sum value but also at least one difference value is formed in a method step from the determined digital measured values of the current paths respectively connected in parallel to a switching pole. With two current paths connected in parallel, there is only one difference value. At e.g. three parallel-connected current paths, these are three difference values and six difference values for four current paths connected in parallel. In the subsequent process step to be carried out correction of the sum value to compensate for the non-linearity associated with the current conversion is now at least one (in two parallel current paths so exactly one) difference value taken into account in terms of magnitude as another parameter. Thus, when compensating for the non-linear characteristic of the current transformers, there is a functional relationship associated with the asymmetrical distribution of the currents in the parallel-connected current paths. In general, the larger the amount, the more the difference value will influence the result of the correction. The current values determined and corrected by the proposed method are available for further evaluation, in particular for overcurrent tripping.

The proposed method limits the tripping tolerances to an admissible extent in the case of asymmetrical loading of the current paths. In practice, the

Correction functions for individual current paths as well as for the parallel connected current paths are recorded by the device manufacturer for each usable current transformer type and evaluated by software as a formula program or table of values in the

Electronic part of the shutter stored.

With more than two current paths connected in parallel, it is quite sufficient for not a few applications if only the largest is involved in the correction value formation

Difference amount value is taken into account as a parameter. If the current measurement method according to the invention is applied to a four-pole switching device, then it is advantageous to operate the two middle current paths in parallel and the two outer current paths individually. In this way, three switching poles with optimum rated current carrying capacity are available.

If current paths connected in parallel are distributed over a plurality of switching devices, the process-related digital processing and the further evaluation of the determined current values are carried out by the triggering electronics of only one of these switching devices.

Further details and advantages of the invention will become apparent from the following, explained with reference to figures embodiment. Show it

Figure 1: the schematic representation of a four-pole switching device with two parallel

switched current paths;

FIG. 2 shows a further schematic representation of the switching device according to FIG. 1 for explaining the method according to the invention;

FIG. 3 shows a functional diagram as a detail from FIG. 2.

Fig. 1 shows as a switching device, a four-pole circuit breaker with four current paths LI, L21, L22 and L3, which are each connected to a switching contact 1, 2, 3 and 4 respectively. In contrast to the two outer current paths LI and L3, the two inner current paths L21 and L22 are connected in parallel. Thus, the phase current of the middle current phase becomes two

Trajectory currents i21 and i22 split. The two outer phase currents are not split and are thus equal to the trajectory currents il and i3. With this measure, the

Rated current carrying capacity of the switching device increased overall. The switching device is equipped with a

Triggering device 5, which contains a triggering electronics 7 (see Fig. 2) equipped. The trajectory currents il, i21, i22 and i3 flowing in the case of closed switching contacts 1 to 4 are detected by the tripping device 5. In the event of a short circuit causes the triggering device 5 without appreciable delay via an actuating mechanism 6, the opening of the contacts 1 to 4 and thus the interruption of the current flow. Upon the occurrence of overcurrents, ie, the respective rated current of the switching device bordering trajectory il and / or i21oder / and i22 or / and i3 causes the triggering device 5 via the actuating mechanism 6, the opening of the contacts 1 to 4 with one of the height of the overcurrent or the overcurrents dependent delay time. The latter case is called overcurrent tripping. By different causes, in particular by unequal path resistances in the two parallel current paths L21 and L22 or by asymmetries between the

Phase currents, the average trajectory currents i21 and i22 are no longer distributed in equal parts. In order to achieve as precisely as possible triggering conditions even in the case of these cases of uneven distribution of the trajectory currents i21 and i22, which are not uncommon in reality

ensure, is on the switchgear assembly of FIG. 1, the inventive

Applied current measuring method, which will be described below with reference to FIG. 2.

In a first process step, in a known manner by the individual

Current paths LI, L21, L22 and L3 flowing trajectory il, i21, i22 and i3 by

transformer current transformers Wl, W21, W22 and W3 detected on the primary side and

secondary side as secondary currents il ', Ϊ2Γ, i22' and i3 'provided. In the following, the apostrophe at the end of the reference symbols is intended to indicate that the relevant physical values or values deviate from the actual values or values. This deviation is caused by the non-linear transfer characteristic N of the current transformers Wl, W21, W22 and W3, which is caused in particular by saturation of the magnetic material. The secondary currents il ', Ϊ2Γ, i22' and i3 'are supplied to the tripping electronics 7, and in the following method step, voltage-measuring measuring signals ul', u21 ', u22' and u3 'are generated therefrom by means of measuring arrangements 8. The measuring arrangements 8 usually each consist of a measuring amplifier and a measuring resistor.

The analog measuring signals ul ', u21', u22 'and u3' are fed to a microcontroller μC within the tripping electronics 7 and converted in the next method step into digital measured values ΙΓ, 12, 122 'and 13'. The analog-to-digital conversion 9 can be hardware or

be realized by software. The digital measured values ΙΓ, Ι2, 122 'and 13' do not correspond with sufficient accuracy to the trajectory currents il, i21, i22 and i3 in the current paths LI, L21, L22 and L3, respectively, since they originate from the nonlinear transfer characteristic N of the current transformers W1 , W21, W22 and W3.

The digital measured values ΙΓ and 13 'for the two outer switch poles, whose current paths LI and L2 are not connected in parallel with other current paths, are subsequently processed by means of a compensation Kn, which corresponds to that with the transformer

Current conversion associated nonlinearity N corrected numerically in first correction values II and 13 transformed. The compensation Kn is stored in the software as a table or formula program in the microcontroller μC. The first correction values II and 13 correspond with sufficient accuracy to the web flows il and i3 and are now available as determined current values of the triggering electronics 7 for further evaluation.

The method step described above for obtaining the first correction values II and 13, applied to the digital measured values 12 and 122 'assigned to the parallel-connected current paths L21 and L22, would lead to unacceptable tolerance deviations. Therefore, according to the invention from the two digital measurements 12 and 122 'in one

subsequent process step first the sum value Ι2 + Ι22 ¹ and the

Difference amount value 1 121'- 122 'I formed. The summation 10 and the

Difference amount formation 11 are also realized by software. In a final process step, the sum value Ι2 + Ι22 ¹ by means of processing by a

Compensation Kp, which said nonlinearity N taking into account the

Difference amount value 1 121'- 122 'I corrected numerically converted into a second correction value 12. The of the two input values sum value Ι2 + Ι22 ¹ as well

Differential value 1 121'-122 'I dependent compensation Kp is also stored as a table or formula program in the microcontroller .mu.C. The second correction value 12 now also corresponds to the sum of the partial currents i21 and i22 with sufficient accuracy for unequal splitting of the traction currents i21 and i22 and is now available as a determined current value of the tripping electronics 7 for further evaluation.

In the functional diagram of FIG. 3, the broken line is the nonlinear one

Transmission characteristic N of the transformer current transformer Wl, W21, W22 and W3 shown schematically. It can be seen how the secondary-side current (here with V

with rising values due to the saturation phenomena of the current transformers increasingly deviates from a linear relationship with the primary-side current (denoted here by i). In the functional diagram, the compensation Kp of the non-linear transfer characteristic N is also shown schematically by the solid lines. It can be seen that the formation of the second correction value 12, which represents the sum of the parallel mean trajectory currents i21 and i22, is associated with an increasing correction as the cumulative values Ι2 + Ι22 ¹ increase. In Fig. 3, the three waveforms for the compensation Kp are intended to differ from each other correction curves at different Difference amount values I Ι2Γ - 122 'I, ie, the functional relationship of the compensation Kp of the current split on the two average current paths L21 and L22 indicate.

The present invention is not limited to the embodiment described above, but also includes all the same in the sense of the invention

Embodiments. For example, the invention can also be applied to switching devices or switchgear assemblies having more than two current paths connected in parallel, which if necessary are distributed among a plurality of switching devices.

LIST OF REFERENCE NUMBERS

1 ... 4 switching contact

5 triggering device

6 operating mechanism

7 External electronics

8 measuring arrangement

9 analog-to-digital conversion

10 summation

11 difference formation il; i21; i22; i3 traction current

il '; Ϊ2Γ; i22 '; i3 'secondary current

II; 12; 13 correction value

ΙΓ; Ι2Γ; 122 '; 13 'digital reading Kn; Kp compensation

L1; L21; L22; L3 current path

N transfer characteristic ul '; u21 '; u22 '; u3 'measurement signal

WI; W21; W22; W3 current transformer

Claims

claims

1. Current measuring method for switching devices with at least two parallel connected

Current paths (L21; L22), the switching devices being equipped with triggering electronics (7) and each current path (LI; L21; L22; L3) being connected to a switching contact (1; 2; 3; 4) by

A) the trajectory currents (il; i21; i22; i3) flowing through the individual current paths (LI; L21; L22; L3) are detected on the primary side by transformer current transformers (W1; W21; W22; W3) and provided on the secondary side for further processing become,

B) each secondary current (ϋ '; Ϊ2Γ; i22'; i3 ') is converted by means of a measuring arrangement (8) into an analog measuring signal (ul'; u21 '; u22'; u3 '),

C) the analog measuring signals (ul '; u21'; u22 '; u3') are converted into digital measured values (ΙΓ; Ι2Γ; Ι2Γ; 13 '),

D) the digital measured values (ΙΓ; 13 '), which are to be assigned to current paths (LI; L3) not connected in parallel, with compensation (Kn) those with the transformer

Transformed nonlinear transfer characteristic (N) numerically into first correction values (II, 13) are formed, which are available as determined current values of the associated trajectory currents (il; i3) for further evaluation, characterized in that for each group of parallel current paths ( L21, L22)

E) from the associated digital measured values (12, 122) both the sum value

(Ι2Γ + Ι22 ¹ ) and at least one difference amount value (I Ι2Γ - 122 'I) is formed,

F) the sum value (Ι2 + Ι22 ¹ ) for the compensation (Kp) of the nonlinear transmission characteristic (N) associated with the transformer current conversion

Inclusion of at least one differential amount value (I Ι2Γ - 122 'I) is numerically converted into a second correction value (12), which is the determined current value of the Sum of the associated trajectory currents (i21, i22) is available for further evaluation.

2. current measuring method according to the preceding claim, characterized in that from three parallel current paths in step F only from the

Process step E determined highest difference amount value is taken into account.

3. Current measuring method according to claim 1, characterized in that when using a four-pole switching device, the method steps E and F are applied to the two average current paths (L21; L22) associated with digital measured values (Ι2Γ; 122 ').

4. Current measuring method according to one of the preceding claims, characterized in that when distributing parallel-connected current paths to a plurality of switching devices, only the triggering electronics (7) of one of the switching devices is active and performs the method steps C to F.