WO2012022377A1

WO2012022377A1 - Measurement input circuit of a measuring or protective device

Info

Publication number: WO2012022377A1
Application number: PCT/EP2010/062023
Authority: WO
Inventors: Carsten Braun; Harald Strohmaier
Original assignee: Siemens Aktiengesellschaft
Priority date: 2010-08-18
Filing date: 2010-08-18
Publication date: 2012-02-23

Abstract

The invention relates to a measurement input circuit (20, 30) of a measuring or protective device, comprising a current transformer (21), to which a primary current to be measured can be supplied on the input side, wherein said primary current causes a proportional secondary current at the output of the current transformer (21), and a first load resistor (22), which is connected to the output of the current transformer (21) and at which a first load voltage can be tapped when a secondary current is flowing across the first load resistor. In order to further develop such a measurement input circuit in such a way that the measurement input circuit can cover a greater measurement range without significant additional costs, an operational amplifier (23) is connected to the output of the current transformer (21), said operational amplifier being provided with a first feedback branch (24) led back to the inverting input of the operational amplifier (23), said first feedback branch being wired in such a way that the operational amplifier (23) causes an amplification of the first load voltage present on the first load resistor (22) of approximately -1; and said operational amplifier being provided with a second feedback branch (25) led back to the inverting input of the operational amplifier (23), said second feedback branch being wired to a first feedback load resistor (28); and a first switching device (29) is provided, which is designed to switch on either the first (24) or the second feedback branch (25) and to block the other feedback branch, respectively.

Description

description

The invention relates to a measuring input circuit of a measuring or protective device with a current transformer, which can be acted on the input side with a primary current to be measured, which causes a proportional secondary current at the output of the current transformer, and a related to the output of the current transformer in connection first load resistor to which in the case of a secondary current flowing over a first burden ^¬ voltage can be tapped.

When measuring value of high alternating currents, as they must be measured, for example, in the field of automation of electrical power grids of measuring devices or protective devices, the currents to be measured are usually reduced by means provided in the respective devices measured value detection means to the electronic further processing suitable levels. Such a measuring ^¬ value detection device in this case has a measurement input ^¬ circuit, with a potential separation and a signal adjustment to be made. The adjusted by the measuring circuit input signal levels of the currents to be measured are then converted by an analog / digital converter in ent ^¬ speaking digital samples, which are then fed to the actual further processing and evaluation. For example, the currents detected by a protective device are examined in the course of further processing using so-called protection algorithms to determine whether they indicate a permitted or an unauthorized operating state of the monitored energy supply network. In case of an unauthorized operating state (eg, a short circuit on a line) are accepted by the apparatus automatic countermeasures, such as opening of a circuit breaker, eingelei ^¬ tet. The detection of different operating conditions with teilwei ^¬ se very large, but sometimes very low currents, places high demands on a measurement input circuit, which must be suitable for measuring high and low currents accordingly. Usually this will be suitable for this purpose

Current transformer (usually inductive current transformer) ^¬ is introduced, which serve the primary side to detect a to be measured Pri ^¬ märstromes, is caused by the on the secondary side of the power converter ^¬ a reduced level with respect to its secondary Ström. Thus, the current transformer on the one hand the potential separation and on the other hand, the reduction of the signal level. The amount of the secondary current is ei ^¬ nerseits of the ratio of the numbers of turns of the windings on the primary side and the secondary side of the current transformer and on the other hand, depending on a load resistor, whose output is connected to the current transformer and through which the secondary current flows. In the simplest case, the load resistance as so-called "passive load" directly Zvi ^¬ rule is connected in the outputs of the current transformer.

Specifically, in the case of a measurement input circuit in the form of a passive load as shown in FIG. 1, a primary current Ii flowing on the primary side IIa of a current transformer 10 is transformed into a lower secondary current I ₂ emitted on the secondary side IIb of the current transformer 10, which in turn is transformed by the output signal at the output side Current transformer 10 is provided ^¬ nen load resistor 12 is converted into a proportional to the secondary current I ₂ ^¬ burden voltage U ₂ . In ni and n ₂ , the number of turns of the windings on the primary side IIa and on the secondary side IIb are designated in FIG.

In the circuit shown in FIG. 1, the following relationship applies to the load voltage U ₂ :

where R _{2 is} the value of the burden resistance. The area covered by the embodiment shown in Figure 1 measuring input circuit measurement range and the error limits of this circuit on the one hand by the design, the size, the Kernma ^¬ TERIAL and the turns ratio n i / n ₂ of the used

Current transformer 10 and on the other hand by the value R ₂ of Bür ^¬ denwiderstands 12 determined. In this case, care must ^¬ to the value R ₂ of the load resistance 12 is not too large, otherwise the error limits of this measuring input circuit to increase in an unacceptable manner due to saturation effects of the current wall ^¬ toddlers 12th In addition, at small (and thus cost) current transformers is that in particular ^¬ sondere with little core material, it is often not possible to obtain sufficient for the downstream Messwertverarbei ^¬ tung voltage level of the load voltage U ₂ via a passive load. This is the case in particular for the measurement of very small currents, for which, according to equation (1) given above, in principle a burden resistor 12 with a very large value R ₂ would have to be provided. The invention has for its object to provide a measuring input ^¬ circuit of the type mentioned above, which can cover a larger measuring range in particular when using cost-effective current transformers with little core material. This object is achieved in that is connected to the output of the current transformer, an operational amplifier, which is provided with a first fed back to the inverting input of the operational amplifier feedback ^¬ branch, which is connected in such a way that the operational amplifier tion amplification of the first burden resistor applied to the first load voltage of about -1 (minus 1), and which is provided with a second feedback path fed back to the inverting input of the operational amplifier, which is connected to a first feedback load resistor. For this purpose a ^¬ ers th switching device is provided which is adapted to either the first or the second feedback branch mono- switch on and lock the respective other feedback branch.

An advantage of the measurement input circuit according to the invention is given by the fact that it has two modes of operation by the switchable burden resistors. While in one ers ^¬ th mode large currents can be measured in a comparatively large range with the first load resistance, is of suitable in the second feedback branch of the Opera- tion amplifier provided feedback load resistor in a second mode for measuring lower currents in a small measuring range, since the virtual load resistance caused by the operational amplifier is nearly 0 ohms in this case. Moreover, the use of a switchable binding of the current transformer makes it unnecessary to use an operational amplifier for high powers, so that no unnecessarily high costs are associated with the measuring input circuit according to the invention. An advantageous embodiment of the measurement input circuit according to the invention provides that the first load resistor is smaller than the first feedback load resistor.

In this way, the measuring ranges of the switchable operating modes can be adapted such that high primary currents can be measured when the first feedback branch is switched on (use of the first load resistor), while low primary currents are measured when the second feedback branch is switched on (use of the first feedback load resistor) can be.

A further advantageous embodiment of the invention ^shown SEN measurement input circuit further provides that the operational amplifier is wired while the first feedback branch as an inverter, and when the second feedback branch as an inverting current to voltage converter. These special - but known - wiring, that species of an operational amplifier, the secondary streams ^¬ advantageous in the output side to the Operationsver ^¬ more voltages applied reacted, whereby a good measurability of the streams is allowed used in all operating modes.

According to a further advantageous embodiment of the measuring input circuit according to the invention, it is provided that the position of the first switching device is determined by a parameterization provided in the measuring or protective device.

In this way, the user of such a measurement or protection device can determine the operating mode of the gearshift through a Messein ^¬ rätes within the parameterization of the overall fixed setting. A particular advantage of such a parameter-controlled definition of the operating mode is the fact that a change of the operating mode in a simple manner, namely by a corresponding changed setting of the corresponding parameter is possible without having to make changes to the egg ^¬ tual input circuit.

An alternative advantageous embodiment of the inventive measuring input circuit, however, provides that the

Position of the first switching device is set dynamically depending on the primary current.

In this embodiment, the setting of the operating mode of the measuring input circuit is quasi-tracked to the current level of the primary current, so that the respective covered by the measuring input circuit measuring range is adjusted in each case the height of the primary current. The tracking can be realized by simple measuring devices on the primary or secondary side of the current transformer, the need to make any exact Bestim ^¬ mung of the current, but only need to specify the region (for example, by multistage

Threshold value transmitter), in which the primary or secondary Ström is currently located, so that a corresponding control of the first switching device can be made.

Specifically, it may be provided in this connection that the first switching device is set up in such a way that it switches on the first feedback branch with a high primary current and switches on the second feedback branch with a low primary current. A further advantageous embodiment of the invention ^shown SEN measurement input circuit is provided in that a further load resistance at the output of the current transformer in addition to the first load resistance provided at least, and a second switching means is provided for the switchover between the first and the at least one further Burden resistance is set up.

This allows the operating modes - and thus the covered measuring range in each case - to be adapted even more precisely to the current to be measured. The activation of the second

Switching device can also be done either parameter-controlled or depending on the primary current to be measured.

In this context, it can be concretely provided that the first and the at least one further burden resistor are arranged as a voltage divider.

According to a further advantageous embodiment of the measurement input circuit according to the invention it is further provided that in addition to the first and the second feedback branch at least one further fed back to the inverting input of the operational amplifier feedback path is provided, which is connected in each case with a further feedback burden resistor, and the first Schalteinrich - Direction is adapted to switch ^¬ either the first Rückkopp ^¬ ment branch or one of the other feedback branches ^¬ . In this way, the respective operating mode-and thus the respective covered measuring range-can be adapted even more precisely to the current to be measured. The appropriate control of the first switching means may again either parameter-controlled or a function of the measured Pri ^¬ märstromes.

The invention will be explained in more detail below with reference to Ausführungsbei ^¬ games. Show this

Figure 1 is an exemplary diagram of a measuring ^¬ input circuit according to the prior art;

Figure 2 shows a first embodiment of a

Measuring input circuit with two switchable operating modes; and

Figure 3 shows a second embodiment of a

Measuring input circuit with five switchable operating modes.

Figure 2 shows a first embodiment of a Messein ^¬ gears 20. The measurement input circuit 20 comprises a current transformer 21 on the primary side of a to be measured primary current Ii flows which induces on the secondary side of the current wall ^¬ coupler 21 a secondary current I2. On the output side, a first load resistor 22 is connected to the current transformer 21 with the value R2. Furthermore, an operational amplifier 23 is provided at the output of the current transformer 21, with its inverting terminal (-) via a resistor 26 with the value R with a terminal of the current transformer 21 in contact and with its non-inverting terminal (+) with the other terminal of the current transformer 21 is connected. At the output of the operational amplifier two feedback paths are provided, which each produce a feedback of the output of the operational amplifier to its invertie ^¬ in power input, namely a first feedback branch 24, which has a resistor 27, and a second feedback branch 25, which comprises a feedback burden resistor 28 with the value R3. In addition, a first

Switching device 29 is provided which is set up such that it either the first feedback branch 24 turns on ^¬ and simultaneously blocks the second feedback branch 25 or the first feedback branch 24 blocks and simultaneously turns on the second feedback branch 25. In addition, the measurement ^¬ voltage U3 can be tapped at the output of the operational amplifier, which is a measure of the primary current Ii.

By the first means of the switching device 29 umschaltba- ren feedback branches 24 and 25 are for the Messein ^¬ shifter 20 two different modes of operation reali ^¬ Siert whose measurement areas by the value of the resistors 22, 26, set 27 and 28th In the first mode the first Schalteinrich ^¬ tung 29 switches the first feedback branch 24 and blocks the second feedback branch 25. Thereby, the first Bür ^¬ the resistor 22 determining the value R ₂ of the value of the output side can be tapped measurement voltage U3. Concretely, it is converted in this first mode, the secondary current I ₂ on the first load resistor 22 into a proportional first clamping load ^¬ voltage U _2, - is applied via the resistor 26 at the Operati ^¬ onsverstärker 23rd Since the first switching device 29 has turned on the first feedback branch 24 and the values R of the resistors 26 and 27 coincide, the operational amplifier 23 is operated in this case as an inverting operational amplifier with a gain of -1. The following relationship applies to the value of the measuring voltage U3 in the first operating mode:

U ₃ = - (R ₂ - (ni / n ₂ ) · Ii) (2) In the second mode of operation of the measurement input circuit 20, the first switching device 29 disables the first feedback branch 24 and turns on the second feedback branch 25. As a result, the value R _{3 of} the feedback

Burden resistance 28 determining the value of the measuring voltage u ₃ .

Specifically, in this second mode of operation, the operational amplifier is connected as a current-to-voltage converter and converts the secondary current I ₂ into the measurement voltage U ₃ proportional thereto. The measurement voltage U ₃ drives the output of the Opera ^¬ tion amplifier 23 via the second feedback branch 25 a compensating current through the feedback load resistance 28 corresponding to the negative value of the Sekundärstro ^¬ I mes. ₂ Due to the feedback through the second feedback branch 25, a virtual zero point arises at the point NP, which acts as a short circuit to ground for the secondary current I _{2 of} the current transformer 21. Thus, the current transformer 21 in the second mode sees a burden of nearly 0 ohms.

The following relationship applies to the value of the measuring voltage U ₃ in the second operating mode:

U ₃ = - (R ₃ - (ni / n ₂ ) -I). (3)

It should be noted that the relationships between the primary current Ii and the measuring voltage U ₃ given by equations (2) and (3) must be taken into account in the further processing of the measured values in a measuring or protective device comprising the measuring input circuit 20.

If the value R _{2 of} the first load resistor 22 is selected to be smaller than the value R _{3 of} the feedback load resistor 28, a measurement of high secondary currents I ₂ in a large measurement range can be achieved in the first operating mode (switched on feedback branch 24). while at the second mode (feedback path 25 is turned on) ei ^¬ ne measurement of low secondary currents I2 is possible in a small measuring range. The position of the first switching device 29 can be determined, for example, by parameterizing the measuring or protective device comprising the measuring input circuit. Here ^¬ at a setting value (parameter) may be set to a entspre ^¬ sponding value specifying the position of the switching device 29th

Alternatively, the position of the first switching device can also be dynamically adapted to the height of the measured primary current Ii that, in dependence of the primary current Ii (or proportional thereto, the secondary current I ₂₎ with simp ^¬ chen threshold value switches on the range limits of the measuring ranges of the first and second operating mode are set, a drive signal for the switching device 29 is generated. In this case, it is expressly intended that in a high primary current Ii (or secondary current I _2), a drive signal is generated which causes the first switching means for a ^¬ switching the first feedback path 24, and (at a low primary current Ii or secondary current I ₂ ), a drive signal is generated, which causes the first switching device for switching on the second feedback branch 24.

The introduced measuring input circuit 20 explains the use of smaller, lighter and therefore cheaper current transformers with less core material and thus also less space requirement with switchable small and large measuring ranges for low or high currents. Figure 3 shows a second embodiment of a Messein ^¬ gears 30. The basic function of the measured input circuit 30 agrees with that of the measurement input circuit 20 (see Figure 2), so that on the one hand corresponding components are denoted by the same reference numerals, and on the other hand covers only different features in the De ^¬ tail with respect to the description of FIG. 3

In the measuring input circuit 30, five operating modes are realized, each with different measuring ranges.

For this purpose, on the one hand in addition to the first load resistor 22, a further load resistor 31 with the value R4 vorgese ^¬ hen. The two load resistors 22 and 31 are arranged in the form ei ^¬ nes voltage divider. A second Schalteinrich ^¬ device 32 allows switching between the complete value of the voltage divider or a corresponding partial amount of its resistance. Thereby, as it were, the first Be ^¬ triebsart, wherein the first feedback path 24 is inserted ^¬ on, are divided into two sub-modes, where either the combined resistance of the load resistors 22 and 31 or a corresponding component of a burden used.

In addition, two additional feedback branches 33 and 34 are provided to extend the operating mode scope to ^¬ addition to the first and the second feedback branch 24 and 25, each having their own feedback load resistors 35 (with the value R5) and 36 (with the value Re). The first switching device 29 is correspondingly extended to the added feedback branches 33 and 34 and ^{ausgestal ¬} tet that it either the first feedback branch 24 (to realize the two sub-modes of the first mode) or one of the other three feedback branches 25, 33 or 34 turns on, whereby - depending on the switched feedback path 25, 33 or 34 - and the second operating ^{¬ type is} divided into three sub-modes quasi.

These additional sub-modes R 35 and 36, the measuring ranges of the measuring equipment can be given an appropriate choice of the values of R _2, R3, _4, R5 and R6 are the reflection ^¬ stands 22, 28, 31, very carefully set the starting circuit 30. As explained with reference to FIG. 2, this can also be done either by a corresponding parameterization of the measuring or protective device or by a dynamic adaptation to the current value of the primary current (or of the secondary current proportional thereto).

In a corresponding manner, both more and less than five operating modes (or sub-modes) of the measuring input circuit can be realized.

Claims

claims

1. measuring input circuit (20, 30) of a measuring or Schutzge ^¬ advises with

- A current transformer (21), the input side is acted upon by a primary current to be measured, which causes a proportional secondary current at the output of the current transformer (21), and

a first load resistor (22) which is connected to the output of the current transformer (21) and to which a first load voltage can be tapped in the case of a secondary current flowing thereabove,

d a d u r c h e c e n c i n e s that

- With the output of the current transformer (21) an operational amplifier (23) is connected, the

- Provided with a first to the inverting input of the operational amplifier (23) fed back ^¬ feedback branch (24), which is connected such that the operational amplifier (23) an amplification of the first load resistor (22) adjacent first

Burden voltage of about -1 causes; and the

- Provided with a second to the inverting input of the operational amplifier (23) fed back ^¬ feedback branch (25), which is connected to a first feedback load resistor (28); and

- A first switching device (29) is provided, which is adapted to turn on either the first (24) or the second feedback branch (25) and to lock the respective other feedback branch.

Second measurement input circuit (20, 30) according to claim 1,

d a d u r c h e c e n c i n e s that

- The first load resistor (22) has a smaller value ^¬ than the first feedback load resistor (28).

3. measurement input circuit (20, 30) according to claim 1 or 2, characterized in that - The operational amplifier (23) when switched on the first feedback branch (24) as an inverter and switched ^¬ tem second feedback branch (25) is connected as an inverting current voltage converter.

4. measurement input circuit (20, 30) according to any one of the preceding claims,

d a d u r c h e c e n c i n e s that

- The position of the first switching device (29) is determined by a provided in the measuring or protective device parameterization.

5. measurement input circuit (20, 30) according to one of claims 1 to 3,

d a d u r c h e c e n c i n e s that

- The position of the first switching device (29) is set dynamically in response to the primary current.

6. measurement input circuit (20, 30) according to claim 5,

d a d u r c h e c e n c i n e s that

- The first switching device (29) is arranged such that it turns on the first feedback branch (24) at a high primary current and the second feedback path (25) turns on at a low primary ^¬ .

7. measurement input circuit (30) according to one of the preceding claims,

- At the output of the current transformer (21) in addition to the first load resistor (22) at least one further load resistor (31) is provided, and

- A second switching device (32) is provided which is adapted for switching between the first (22) and the at least one further burden resistor (31).

8. measurement input circuit (30) according to claim 7,

d a d u r c h e c e n c i n e s that

- The first (22) and the at least one further burden resistance ^¬ stand (31) are arranged as a voltage divider.

9. measurement input circuit (30) according to one of the preceding claims,

d a d u r c h e c e n c i n e s that

- addition to the first (24) and the second feedback branch (25) at least one further to the inverting input of the operational amplifier (23) recirculated back ^¬ feedback branch (33, 34) is provided, which in each case (with a further feedback load resistor 35, 36) is connected, and

- The first switching means (29) is adapted to turn on either the first feedback branch (24) or one of the other feedback branches (25, 33, 34).