FR2790881A1 - PROTECTION AND CONTROL DEVICE OF ELECTRIC POWER SYSTEM - Google Patents

Abstract

A power electrical system protection and control apparatus has an input transformer (2) and a current-to-voltage converter (5). The input transformer (2) collects a current signal from an electrical power system (21). The current-to-voltage converter (5) has almost zero input impedance, and is connected to a secondary winding of the input transformer (2) to convert a current signal from the input transformer into a voltage signal. This enables the secondary voltage of the input transformer (2) to be brought back to substantially zero.

Description

The present invention relates to an apparatus for pro-

  tection and control of an electrical power system to protect and control, when an abnormal condition occurs, an electrical power system by collecting current and voltage information from the electrical system

power stick.

  Fig. 4 is a block diagram showing a system current and voltage input section

  of a conventional system protection and control device

  electric power unit described, for example, in Japanese patent publication No. 3-79932 / 1991. In fig. 4, the reference symbols Ia to Id each denote a current signal (system current) sent from a system

  electric 21 power; the reference number 22 is

  signs an input transformer incorporating current-voltage conversion resistors 27 connected to its windings

  secondary; 23 designates a reference signal generator

  this to produce a reference signal Eref; 24 designates

  filters to remove parasitic components of fre-

  quence contained in one of the system current signals passing through the input transformer 22; 25 designates sampling and holding circuits for maintaining one of the output signals for a fixed period of time

  filters 24 at predetermined intervals; and 26 dice-

  a multiplexer for sequentially selecting one of the voltage signals maintained by the sampling and holding circuits 25 and for sending it to an analog-digital converter 12. The reference number 9 designates a digital processing section for processing digital data transmitted by the analog-digital converter 12 and to produce a signal 31

  alarm when it detects a fault.

  Then the operation of the conventional apparatus

  protection and control of electrical system of

sance will be described.

  The input transformer 22 transforms, according to a transformation ratio, the current signals, Ia to Id,

  sent to its primary windings, sends them to resistances

  tances 27 of current-voltage conversion connected to the windings-

  secondary elements to transform them into voltage signals determined by the product of the current values by the resistance values, and sends them to the filters 24. The voltage signals emitted by the filters 24 are maintained by the sampling circuits 25 and holding, sequentially converted to digital signals at

  by means of the multiplexer 26 and the analog converter

  digital 12, and processed by processing section 9

digital.

  With the previous configuration, it is necessary that the conventional apparatus for protecting and controlling the electrical power system is designed so that the secondary voltages do not saturate the input transformer 22. For this purpose, the input transformer 22 must

  have a core section and a number of turns of its

  sufficiently large bearings. Consequently, the

  input transformer 22 becomes rather bulky and cost-

  teux, which leads to an increase in size and cost

of its case.

  In addition, since the secondary voltages of the input transformer 22 induce primary voltages

  determined by the transformation ratio of the transforma-

  22 input voltage, the primary voltages constitute a

  load on a current transformer which sends the

  current from the power electrical system

  this to the input transformer 22. This presents another

  problem of increase in size and cost for trans-

current trainer.

  The present invention is implemented for solving

  dre the previous problems. It is therefore a goal of the pre-

  It is an invention to provide an electrical power protection and control apparatus which is capable of reducing both the size and the cost of the transformer

  input and current transformer.

  According to a characteristic of the present invention,

  a protection and system control device is created

  electrical power unit comprising: an input transformer for collecting a current signal from a

  electric power system; a current converter

  voltage connected to a secondary winding of the input transformer to convert a current signal emitted by the input transformer into a voltage signal, the input impedance of the current-voltage converter being practically zero; and an analog-to-digital converter for converting a transmitted analog signal into a digital signal

  by the current-voltage converter.

  Here, the current-voltage converter whose impe-

  input dance is practically zero can include: an operational amplifier whose inverting input terminal is supplied by the output current of the transformer

  input, and whose non-inverting input terminal is

  lied by a mass potential; and a resistor

  troaction connected between an output terminal of the operational amplifier and the inverting input terminal. The electrical power protection and control apparatus may further comprise diodes connected head to tail between the inverting input terminal and

the non-inverting input terminal.

  The electrical power protection and control apparatus may further comprise a filter for eliminating parasitic frequency components contained in the output signal of the current-voltage converter. The filter may include a capacitor connected in

  parallel with the feedback resistor.

  Fig. 1 is a block diagram showing a first embodiment of a protection and control apparatus for an electric power system according to the present invention; fig. 2 is a circuit diagram representing a current-voltage converter, main portion of a second embodiment of the protection device and

  electric power system control according to the pre-

  invention; fig. 3 is a circuit diagram representing a current-voltage converter, main portion of a third embodiment of the protection device and

  electric power system control according to the pre-

  invention; and fig. 4 is a block diagram representing

  a classic system protection and control device

electric power me.

  The invention will now be described with reference to

rant to the accompanying drawings.

EMBODIMENT # 1

  Fig. 1 is a block diagram showing a first embodiment of a protection and control apparatus for an electric power system according to the present invention. In fig. 1, the reference number 1 designates a current transformer (CT) to transform a current signal from an electrical system 21 of power 2 designates an input transformer to transform the current signal sent by the current transformer 1

  at a current level suitable for treatment, and 5 dis-

  a current-voltage converter to convert the

  general current sent by the input transformer 2 into a voltage signal. The current-voltage converter 5 has a practically zero input impedance. Reference numeral 6 designates a filter for removing parasitic frequency components contained in the current signal

  of system converted into the voltage signal by the conver-

  current-voltage weaver 5; and 7 designates a processing section comprising an analog-digital converter for converting the analog signal emitted by the filter 6 into a digital signal, and a processor for processing the signal

  digital in order to implement the protection functions

tion and control.

  The current-voltage converter 5 comprises an am-

  operational planner 3 and a feedback resistor 4

  tion. The operational amplifier 3 has an inverting input terminal to which the output current of the

  input transformer 2, and a non-inverting input terminal

  threshold which is placed at ground potential. The feedback resistor 4 is connected between the output terminal and the inverting input terminal of the operational amplifier 3,

  thus constituting an inverting amplifier.

  The operation of the present first embodiment

sation will now be described.

  The system current is transformed into a secondary current I1 of the current transformer 1, determined by the transformation ratio of the current transformer 1,

  and is injected into the input transformer 2. The trans-

  input trainer 2 turns it into a secondary current

  12 determined by the transformation ratio of the transforma-

  input 2 and sends it to the inverting input terminal

  of the operational amplifier 3 constituting the convert-

  current-voltage generator 5. In this case, the secondary voltage E2 of the input transformer 2 is created as being the

  secondary current product 12 of transformer 2 of

  entered by the input resistance of the current converter

tension 5.

  The internal operation of the current-voltage converter 5 will be described here by treating the following two points. (1) The secondary voltage E2 of transformer 2

  input is zero regardless of the value of the current se-

  of the current transformer 1, in other words, of

  input to input transformer 2.

  (2) The current-voltage converter 5 emits a voltage

  sion E3 which is directly proportional to the primary current

  mayor I1 and to the secondary current 12 of the input transformer 2, which thus correctly performs the function of

current-voltage conversion.

  (1) SECONDARY VOLTAGE E2 OF INPUT TRANSFORMER 2

IS THEORICALLY NULL.

  As is well known, the operation amplifier

  nel 3 basically works so the difference

  potential between the inverting input and the non-input

  jug is kept at zero. This behavior is called "imaginary short circuit". Since the inverting input terminal has an imaginal short circuit relationship with the non-inverting input terminal, the potential of the inverting input terminal of the operational amplifier 3 is

  maintained at zero whatever the value of the secondary current

  12 of transformer 2 input. This indicates that the

  secondary voltage E2 of input transformer 2 is theo-

  virtually zero. In the real input transformer 2, however, a very small secondary voltage E2 appears as the product of the secondary current 12 by a resistor

  secondary winding which is inevitable in practice.

  (2) THE OUTPUT E3 VOLTAGE IS DIRECTLY PROPORTIONAL

  AT PRIMARY CURRENT I1 AND SECONDARY CURRENT 12 OF TRANS-

ENTRY TRAINER 2

  The imaginal short circuit described above is based on the principle that the total sum of the circulating current

  in the inverting input terminal of the operating amplifier

  tional 3 is zero, which is obtained by canceling the cost

  secondary rant 12 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, via the feedback resistor 4. In other words, 12 = -IF. Thus, the output voltage E3 of the operational amplifier 3 is expressed by the following equation (1): E3 = IF x R2 (1) Since 12 = -IF, equation (1) can be replaced by the following equation (2): E3 = -I2 x R2 (2) Equation (2) shows that the output voltage E3 is directly proportional to the secondary current 12 of the

output transformer 2.

  As described above, the present embodiment

  tion n 1 can reduce the secondary voltage E2 of the transformer

  input mateur 2 at a very small value due to the resistance of the secondary winding, which in turn can reduce the core section and the number of turns of the windings to a value such that the very small secondary voltage E2 does not saturate the nucleus. In addition, this allows the input transformer to fulfill its original function of creating the voltage proportional to its current.

secondary.

EMBODIMENT # 2

  Although the previous embodiment constitutes a current-voltage converter 5 using a circuit

  electronics including input impedance or resistance

  is almost zero (practically zero), this requires that the secondary current I2 of the input transformer 2 be canceled by the feedback current IF supplied by the

  output terminal to the inverting input terminal of the amplifier

  operational cator 3. However, when the secondary current

  re 12 is too large, operational amplifier 3 cannot provide sufficient IF feedback current to

  maintain the imaginal short circuit, because the amplification

  operational 3 cannot produce higher voltage

  lower than the supply voltage.

  In such a case, the operational amplifier 3 can

  to be destroyed. In view of this, the present embodiment

  sation n 2 connects two diodes 9a and 9b head to tail between the

  inverting input terminal and the non-inverting input terminal

  se from operational amplifier 3, as shown in

fig. 2.

  We will then describe the operation of the diodes 9a and 9b mounted head to tail.

  The diodes 9a and 9b which are mounted head to tail can-

  wind maintain the potential difference between the two input terminals at a value substantially zero. It's the

  case even if the operational amplifier 3 cannot

  hold the imaginal short circuit between the input terminals

inverting and not inverting.

  Thus, embodiment n 2 can maintain the potential difference between the two input terminals of the operational amplifier 3 at a substantially zero value by using head-to-tail diodes, which thus prevents the operational amplifier 3 d be damaged by

entry level too high.

EMBODIMENT # 3

  Although the output of the current-voltage converter

  5 is sent to the analog filter 6 to eliminate the com

  frequency interference in the embodiments

  tion n 1 and n 2, the present embodiment n 3 constitutes the filter to eliminate the high frequency components by connecting a capacitor 10 in parallel with the feedback resistor 4, as shown in the

fig. 3.

  We will then describe the functioning of the present

embodiment n 3.

  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 by the impedance of the feedback circuit, the output voltage E3 is reduced towards the high frequencies, which constitutes an elimination filter.

high frequencies.

  As described above, the present embodiment

  tion n 3 can suppress the high frequency components without increasing the output voltage E3 of the input transformer 2. This allows the use of a small input transformer, which is not saturated by the very low output voltage E2 while retaining the sound section.

  core and the number of turns of its windings at va-

their weak.

Claims (5)

  1. An electrical power system protection and control apparatus comprising: an input transformer (2) for collecting a current signal from an electrical power system (21);   a current-voltage converter (5) connected to an   secondary bearing of said input transformer (2) for converting a current signal emitted by said input transformer into a voltage signal, the input impedance of said current-voltage converter (5) being practically zero; and an analog-to-digital converter (7) for converting a transmitted analog signal into a digital signal   by said current-voltage converter (5).   2. System protection and control device   electric power according to claim 1, in the-   which said current-voltage converter (5) having an impedance   practically zero input dance comprises: an operational amplifier (3) having an inverting input terminal supplied by the input current of said input transformer (2), and having a non-inverting input terminal connected to the potential of the mass; and a feedback resistor (4) connected between a   output terminal of said operational amplifier (3) and the so-called inverting input terminal.   3. Apparatus for protecting and controlling an electrical power system according to claim 2, comprising   further diodes (9a, 9b) connected head to tail between the-   said inverting input terminal and said non-inverting input terminal. 4. Apparatus for protecting and controlling an electrical power system according to claim 2, comprising   additionally a filter (6; 4, 10)) to remove components   your frequency noise contained in the output signal   tie of said current-voltage converter (5).   1 1 5. System protection and control device   electric power according to claim 4, in the-   which said filter comprises a capacitor (10) connected in   parallel with said feedback resistor (4).