GB2347755A

GB2347755A - Protection system with smaller current detection transformer

Info

Publication number: GB2347755A
Application number: GB9917116A
Authority: GB
Inventors: Hiroshi Sato
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 1999-03-11
Filing date: 1999-07-21
Publication date: 2000-09-13
Anticipated expiration: 2019-07-21
Also published as: FR2790881B1; GB9917116D0; JP2000258469A; FR2790881A1; GB2347755B

Abstract

An electric power system protective and control apparatus comprises an input transformer 1 that captures a current signal I1, I2 from a power system 21. The circuitry may comprise a second input transformer 2. The apparatus further comprises current to voltage means 5 which may be connected to filter 6 and processing means 7 which can include analogue to digital conversion. The current to voltage means 5 has a substantially zero input impedance and may comprise an operational amplifier (op-amp) 3 in an inverting arrangement with a feed back resistor 4 between output and inverting input, with the non-inverting input to ground. Protection diodes (9a,b fig. 2) may be placed anti-parallel across the op-amp, and filter means, such as a capacitor (10 fig. 3), may be placed in the feedback loop.

Description

ELECTRIC POWER SYSTEM PROTECTIVE AND CONTROL APPARATS WITH DOWNSIZED INPUT TRANSFORMER The present invention relates to an electric power system protective and control apparatus for protecting and controlling, when an abnormal condition takes place, an electric power system by capturing current and voltage information from the electric power system.

Fig. 4 is a block diagram showing a system current and voltage input section of a conventional electric power system protective and control apparatus disclosed for example in Japanese patent publication No. 3-79932/1991,. In Fig. 4, reference symbols Ia-Id each designate a current signal (system current) fed from an electric power system 21; the reference numeral 22 designates an input transformer incorporating current-to-voltage converting resistors 27 connected to its secondary windings; 23 designates a reference signal generator for generating a reference signal Eref ; 24 designates filters for eliminating spurious frequency components contained in one of the system current signals passing through the input transformer 22; 25 designates sample-and-hold circuits for holding for a fixed time period one of the output signals of the filters 24 at predetermined intervals; and 26 designates a multiplexer for sequentially selecting one of the voltage signals held by the sampleand-hold circuits 25, and for supplying it to an A/D converter 12. The reference numeral 9 designates a digital processing section for processing digital data output from the A/D converter 12, and for generating an alarm output 31 when detecting a failure.

Next, the operation of the conventional electric power system protective and control apparatus will be described.

The input transformer 22 transforms, in accordance with a transformation ratio, the current signals Ia-Id fed to its primary windings, supplies them to the current-to-voltage converting resistors 27 connected to the secondary windings to transform them into voltage signals determined by products of the current values and resistance values, and supplies them to the filters 24. The voltage signals output from the filters 24 are held by the sample-and-hold circuits 25, sequentially converted into digital signals through the multiplexer 26 and A/D converter 12, and processed by the digital processing section 9.

With the foregoing configuration, it is necessary for the conventional electric power system protective and control apparatus to be adapted so that the secondary voltages do not saturate the input transformer 22. To achieve this, the input transformer 22 must have sufficient cross-sectional core area and numbers of turns in its windings. Accordingly, the input transformer 22 itself becomes rather bulky and expensive, leading to an increase in size and cost of its housing.

Besides, since the secondary voltages of the input transformer 22 induce primary voltages determined by the transformation ratio of the input transformer 22, the primary voltages constitute a load on a current transformer that feeds the current signals from the electric power system to the input transformer 22. This presents another problem of increased size and cost for the current transformer.

The present invention is implemented to solve the foregoing problems. It is therefore an object of the present invention to provide an electric power system protective and control apparatus capable of reducing the size and cost of both the input transformer and current transformer.

According to one aspect of the present invention, there is provided an electric power system protective and control apparatus comprising: an input transformer for capturing a current signal from an electric power system; a current-to-voltage converter connected to a secondary winding of the input transformer for converting a current signal output from the input transformer into a voltage signal, an input impedance of the current-to-voltage converter being substantially zero; and an A/D converter for converting an analog signal output from the current-to-voltage converter into a digital signal.

Here, the current-to-voltage converter whose input impedance is substantially zero may comprise: an operational amplifier whose inverting input terminal is supplied with the output current of the input transformer, and whose non-inverting input terminal is supplied with a ground potential; and a feedback resistor connected across an output terminal of the operational amplifier and the inverting input terminal.

The electric power system protective and control apparatus may further comprise diodes connected in antiparallel across the inverting input terminal and the non-inverting input terminal.

The electric power system protective and control apparatus may further comprise a filter for eliminating spurious frequency components contained in the output signal of the current-to-voltage converter.

The filter may comprise a capacitor connected in parallel with the feedback resistor.

Fig. 1 is a block diagram showing an embodiment 1 of an electric power system protective and control apparatus in accordance with the present invention; Fig. 2 is a circuit diagram showing a current-to-voltage converter, a major portion of an embodiment 2 of the electric power system protective and control apparatus in accordance with the present invention; Fig. 3 is a circuit diagram showing a current-to-voltage converter, a major portion of an embodiment 3 of the electric power system protective and control apparatus in accordance with the present invention; and Fig. 4 is a block diagram showing a conventional electric power system protective and control apparatus.

The invention will now be described with reference to the accompanying drawings.

EMBODIMENT 1 Fig. 1 is a block diagram showing an embodiment 1 of an electric power system protective and control apparatus in accordance with the present invention.

In Fig. 1, the reference numeral 1 designates a CT (current transformer) for transforming a current signal of an electric power system 21; 2 designates an input transformer for transforming the current signal fed from the CT 1 into a current level suitable for the processing; and 5 designates a current-to-voltage converter for converting the current signal fed from the input transformer 2 into a voltage signal.

The current-to-voltage converter 5 has an input impedance of substantially zero.

The reference numeral 6 designates a filter for removing spurious frequency components contained in the system current signal converted into the voltage by the current-to-voltage converter 5 ; and 7 designates a processing section including an A/D converter for converting the analog signal output from the filter 6 into a digital signal, and a processor for processing the digital signal to implement the protective and control functions.

The current-to-voltage converter 5 comprises an operational amplifier 3 and a feedback resistor 4. The operational amplifier 3 has an inverting input terminal to which the output current of the input transformer 2 is supplied, and a non-inverting input terminal which is placed at a ground potential. The feedback resistor 4 is connected across the output terminal and the inverting input terminal of the operational amplifier 3, thereby constituting an inverting amplifier.

Next, the operation of the present embodiment 1 will be described.

The system current is transformed into a secondary current li of the CT 1 determined by the transformation ratio of the CT 1, and is introduced into the input transformer 2. The input transformer 2 transforms it to a secondary current Iz determined by the transformation ratio of the input transformer 2, and supplies it to the inverting input terminal of the operational amplifier 3 constituting the current to-voltage converter 5. In this case, the secondary voltage E2 of the input transformer 2 is generated as the product of the secondary current la of the input transformer 2 and the input resistance of the current-to-voltage converter 5.

Here, the internal operation of the current-to-voltage converter 5 will be described in terms of the following two points.

(1) The secondary voltage E2 of the input transformer 2 is zero regardless of the magnitude of the secondary current of the CT 1, that is, the input to the input transformer 2.

(2) The current-to-voltage converter 5 outputs a voltage Es that is directly proportional to the primary current Ii and secondary current 12 of the input transformer 2, thereby achieving the current-to-voltage converting function correctly.

(1) THAT THE SECONDARY VOLTAGE E2 OF THE INPUT TRANSFORMER IS THEORETICALLY ZERO.

As is well known, the operational amplifier 3 basically operates such that the potential difference across the inverting input and non-inverting input is maintained at zero. This behavior is called"imaginal short". Since the inverting input terminal has an imaginal short relationship with the non-inverting terminal, the potential of the inverting input terminal of the 3 is maintained at zero regardless of the magnitude of the secondary current I2 of the input transformer 2. This indicates that the secondary voltage Ea of the input transformer 2 is theoretically zero. In the actual input transformer 2, however, a very small secondary voltage E2 takes place as a product of the secondary current I2 and a secondary winding resistance which is unavoidable in practice.

(2) THAT THE OUTPUT VOLAGE Es IS DIRECTLY PROPORTIONAL TO THE PRIMARY CURRENT Ii AND SECONDARY CURRENT 12 OF THE INPUT TRANSFORMER 2.

The imaginal short described above is based on a principle that the sum total of the current flowing into the inverting input terminal of the operational amplifier 3 is zero, which is implemented by canceling the secondary current I2 of the input transformer 2 by supplying the feedback current IF from the output terminal to the inverting input terminal of the operational amplifier 3 through the feedback resistor 4. In other words, I2 =-IF. Thus, the output voltage Es of the operational amplifier 3 is expressed by the following equation (1).

E3 = IF X R2 (1) Since I2 =-IF, equation (1) can be replaced by the following equation (2).

Es =-I2 x R2 (2) Equation (2) shows that the output voltage Es is directly proportional to the secondary current I2 of the input transformer 2.

As described above, the present embodiment 1 can reduce the secondary voltage Ea of the input transformer 2 to a very small value due to the secondary winding resistance, which in turn can reduce the cross-sectional area of the core and the numbers of times of windings of the input transformer to such a level that the very small secondary voltage Es does not saturate the core. In addition, this makes it possible for the input transformer to achieve its original function of generating the voltage proportional to its secondary current.

EMBODIMENT 2 Although the foregoing embodiment constitutes a current-to-voltage converter 5 using an electronic circuit with its input impedance or input resistance being nearly zero (substantially zero), this requires that the secondary current 12 of the input transformer 2 must be canceled by the feedback current IF supplied from the output terminal to the inverting input terminal of the operational amplifier 3.

However, when the secondary current 12 is too large, the operational amplifier 3 cannot supply enough feedback current IF to maintain the imaginal short because the operational amplifier 3 cannot output a voltage beyond a power supply voltage.

In such as case, the operational amplifier 3 can be destroyed. In view of this, the present embodiment 2 connects two diodes 9a and 9b connected in antiparallel across the inverting input terminal and the non-inverting terminal of the operational amplifier 3 as shown in Fig. 2.

Next, the operation of the antiparallel diodes 9a and 9b will be described.

The diodes 9a and 9b, which are connected in antiparallel, can maintain the potential difference between the two input terminals at substantially zero. This is the case even if the operational amplifier 3 cannot maintain the imaginal short across the inverting and non-inverting input terminals.

Thus, embodiment 2 can maintain the potential difference between the inverting and non-inverting input terminals of the operational amplifier 3 at substantially zero using the antiparallel diodes, thereby preventing the operational amplifier 3 from being damaged by an excessive input.

EMBODIMENT 3 Although the output of the current-to-voltage converter 5 is fed to the analog filter 6 to eliminate the spurious frequency components in the foregoing embodiments 1 and 2, the present embodiment 3 constitutes the filter for removing the high frequency components by connecting a capacitor 10 in parallel with the feedback resistor 4 as shown in Fig. 3.

Next, the operation of the present embodiment 3 will be described.

Since the capacitor 10 has a low impedance for high frequency components, the impedance of the feedback circuit consisting of the capacitor 10 and the feedback resistor 4 connected in parallel is reduced to a high frequency. Since the output voltage of the operational amplifier 3 is the product of the feedback current IF and the impedance of the feedback circuit, the output voltage E3 is reduced toward a high frequency, thereby resulting in a high frequency elimination filter.

As described above, the present embodiment 3 can remove the high frequency components without increasing the output voltage E2 of the input transformer 2.

This makes it possible to use a small size input transformer, which is not saturated by the very low output voltage E2, with maintaining the cross-sectional area of its core and numbers of turns of its windings at small values.

Claims

CLAIMS: 1. An electric power system protective and control apparatus comprising: an input transformer for capturing a current signal from an electric power system; a current-to-voltage converter connected to a secondary winding of said input transformer for converting a current signal output from said input transformer into a voltage signal, an input impedance of said current-to-voltage converter being substantially zero; and an A/D converter for converting an analog signal output from said current-tovoltage converter into a digital signal.
2. An electric power system protective and control apparatus according to claim 1, wherein said current-to-voltage converter having an input impedance of substantially zero comprises: an operational amplifier having an inverting input terminal supplied with the output current of said input transformer, and having an non-inverting input terminal supplied with a ground potential; and a feedback resistor connected across an output terminal of said operational amplifier and said inverting input terminal.
3. An electric power system protective and control apparatus according to claim 2, further comprising diodes connected in antiparallel across said inverting input terminal and said non-inverting input terminal.
4. An electric power system protective and control apparatus according to claim 2, further comprising a filter for eliminating spurious frequency components contained in the output signal of said current-to-voltage converter.
5. An electric power system protective and control apparatus according to claim 4, wherein said filter comprises a capacitor connected in parallel with said feedback resistor.
6. An electric power system substantially as hereinbefore described with reference to figure 1 ; or figure 2 ; or figure 3 of the accompanying drawings.