RU2368906C2 - High-voltage digital device for current measurement - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: invention is related to measurement equipment, in particular to instruments for measurement of AC and DC with application of digital measurement equipment, mostly at voltages from 10kV to 1500kV. Digital device comprises current detector, analog-digital converter and modulator. Besides device is placed inside current conductor with measured current, is under potential of high voltage in zone of magnetic and electric fields absence. Current conductor with measured current is used as antenna-feeder circuit for transfer of modulated digital signal.

EFFECT: simplified design of instrument, reduction of weight and dimensional parametres, increased accuracy of measurements and reliability of operation in part of high-voltage insulation.

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Description

Technical field

The invention relates to measuring equipment, in particular to digital devices for measuring AC and DC current, mainly at voltages from 10kV to 1500kV.

The technical result consists in simplifying the design of the device, reducing overall dimensions, increasing the accuracy of measurements and the reliability of operation in terms of high-voltage insulation. The digital device contains a current sensor and a modulator for transmitting encoded information about the magnitude of the measured current through the current lead with the measured current.

State of the art

High voltage current is traditionally measured by induction current transformers. These transformers are used as sensors that scale the current. Usually they have a primary winding and several secondary windings for relay protection, measurement and commercial metering. The windings are galvanically isolated. The primary winding is at a high potential of the measured current, and one of the terminals of the secondary windings is at the ground potential. The magnitude of the current is judged by the magnitude of the voltage (usually up to 100 volts at a current of up to 5A) at the terminals of the secondary windings. Measurements on the secondary winding are carried out using equipment installed at a distance from the current path in the substation building. Secondary windings are isolated from the primary by the amount of voltage that can occur during overvoltages as a result of switching or lightning. For example, for a voltage class of 110 kV - this voltage is 460 kV, and for equipment of 220 kV - more than 1 million volts. Thus, the insulation distances between the windings of the measuring induction current transformers with increasing voltage to ensure insulation increase significantly, the mass of current transformers increases in this case.

Considering that the main increase in the mass of insulators is connected not with the measured current, but with the voltage level in the measured circuit, it is currently considered promising to use electro-optical current measuring devices, as well as devices based on the Faraday effect: RU 2208798, RU 2223512, RU 2120128, RU 2262709 , SU 1337782 and others.

When using electro-optical current measuring devices, the problem of high-voltage isolation of the sensor and the measurement object from the ground potential and the place of use of the measurement results is quite simple, since optical radiation that carries information about the measured current does not conduct electricity. But at the same time, a large number of transformations of the measured quantities arise, leading to a deterioration in the accuracy of the measurement. In addition, the method is very sensitive to changes in temperature, vibration, a possible change in the optical transparency of the fiber channel, etc. The indisputable advantage of the method is a high level of electrical insulation, while the dielectric distance between the sensor, which is at high potential, and the recording unit can be arbitrarily large. Increasing, if necessary, the level of electrical insulation (dielectric distance) does not require an increase in costs and structural changes.

The main advantage of these devices associated with high-voltage isolation follows from the method of transmitting information about the magnitude of the current from the high-voltage potential to the earth potential using a light beam. This is implemented, in particular, in devices a.s. No. 354353, SU 1437786, SU 1597745, RU 2171996. In these devices, the measured current in the wire excites a magnetic field around, which is perceived by the secondary winding of the current transformer. All of these devices use an electromagnetic method of recording the magnitude of the passing current. The voltage of the secondary winding in analog form is supplied to the light emitting unit (usually an LED), which transmits the radiation to the photoelectronic sensor, while the radiation power is proportional to the voltage on the secondary winding. By measuring the voltage at the photoelectric sensor, we obtain information on the strength of the measured current in the primary winding (bus). The main disadvantage of all devices of this kind also remains a large number of transformations and, as a result, a high resulting measurement error. At the same time, other disadvantages related to the sensitivity of the sensors to vibration, temperature, to the influence of surrounding magnetic fields, etc. are not eliminated. Additionally, an error appears due to a change in the throughput of the optical system as a result of exposure to time, weather conditions and radiation itself.

To compensate for errors associated with temperature changes in the device SU 1310735, a divider with parallel branches of the circuit: measuring and shunt is used. Given that the shunt is inside the current path of the measuring circuit, it has the same conductivity drift with the measuring circuit and, accordingly, compensates for the possible measurement error due to wide temperature variations. However, such a device cannot be used for high voltage measurements, since the operating voltage is limited by the electrical strength of the insulation of the magnetomodulating transducer. And also the issue of outputting from the cavity of the current lead a signal about the measured current value is not resolved. When a signal is output to the ground potential via wires, the insulation level will be determined by the insulation of these wires.

In other devices, to eliminate the error that occurs in the transmission path of information along the light beam, in particular, to eliminate the temperature dependences of the radiation power, change the bandwidth of the optical channel, etc., the information signal must be transmitted in digitally encoded form. This is implemented in the device RU 2166218, which is the closest prototype of the claimed device. The device according to RU 2166218 contains several electromagnetic current transformers, an analog-to-digital converter and an LED for transmitting information to the earth potential in digital form. Using an analog-to-digital converter (ADC) allows you to completely eliminate errors when transmitting information to earth potential. The device has primary windings, through which the measured current flows, exciting a magnetic field proportional to the magnitude of the current. The magnetic field, in turn, generates in the secondary windings an EMF proportional to the magnitude of the magnetic field. After passing through the compensation schemes for a possible error, the ADC converts the information about the magnitude of the EMF into a digital form in which it is transmitted to the ground potential via an LED.

The main disadvantages of the prototype are as follows:

- low reliability associated with the life of the LED;

- the need to ensure direct visibility of the LED receiver;

- measurement error caused by the influence of magnetic and electric fields from conductors of neighboring phases, which are usually located at a small distance from each other and can induce currents in the conductors of the device;

- the sensitivity of electronic devices and components to electric and magnetic fields from the current path with a measured current. The LED may light up under the influence of induced electric potentials;

- the possibility of failure of the radio equipment when exposed to large internal electrical potentials induced in the radio by short-circuit currents or switching overvoltages.

If you place the electronic measuring device under high voltage, the equipment will inevitably fail. This is due to high voltages induced on parts, wires, equipment connectors when located near high-voltage conductors and buses. Moreover, in the case of an alternating current of industrial frequency of an ideal shape, the device can work. In fact, the overall potential of the device will “float” at a frequency of 50 Hz. But in real electrical networks there are not only harmonic components. When switching high-voltage devices, a sharp increase in voltage and frequency occurs. For example, when a vacuum circuit breaker is disconnected on a 35 kV line, the switching overvoltage can reach 190 kV, and the frequency can reach several megahertz. If the device contains sections of conductors longer than the wavelength of the megahertz range, an internal breakdown between the components or between the electrically conductive sections of the device is possible. In the frequency range of about 50 Hz, the wavelength is much larger than the dimensions of the conductors and the device itself. The dimensions of the device can be neglected, considering it as a point slowly "swinging" on an electromagnetic wave. With switching overvoltages, the wavelength becomes smaller than the size of the device, and the amplitude is larger, different parts of the device are under different potentials. Partial discharges and complete overlap by discharge of insulating gaps may occur inside the device. If the device is placed in a shielded case, there remains the possibility of electrical breakdown through the connected power wires, measurements, outputs. Up to 90% of all equipment failures in the electric power industry occur due to processes occurring during switching or lightning overvoltages. This is due to the lack of radio equipment in direct contact with high-voltage lines at substations. As current sensors, current transformers with insulation, designed for the effects of switching and lightning surges, are used. In particular, numerous studies of the failure of a large number of current transformers at a voltage of 500 kV have revealed the main reason - the effect of high-frequency switching overvoltages. Thus, current measurement can best be done by placing the sensor and the ADC under the high voltage potential, and transmit information about the current strength to the ground via an optical channel. But electronic digital equipment cannot be used due to strong electromagnetic pulses inherent in the operation of high-voltage equipment and possible damage to optoelectronic channels.

This contradiction is resolved in the proposed device.

OBJECTS OF THE INVENTION

The problem to which the invention is directed, is a simple reliable device for measuring the magnitude of alternating and direct current of high voltage, measuring the current strength at high voltage and transmitting a signal for use on existing channels of radio frequency communication.

Description

The goal is achieved by the fact that the device for measuring the current strength at high potential contains a shunt connected in parallel and having direct contact with the conductor on which the measurement is performed, an analog-to-digital converter and a modulator for transmitting digital information about the current strength using high electromagnetic waves frequency devices that are under the potential of low voltage (ground). The modulated signal is transmitted using the current path itself, on which the current is measured. After converting the signal about the voltage level at the ends of the shunt into a digital form, modulation is performed, the received high-frequency signal is sent back to the current lead. The modulated signal propagates in the current lead as in an antenna-feeder device with minimal losses. To obtain information on the low-voltage side, high-frequency communication equipment installed at each power plant is used. Via coupling capacitors, the high-frequency signal from the current measuring device is separated from the industrial frequency, demodulated and used for automation, metering and relay protection.

Since galvanic isolation between the high-voltage potential of the measured current lead and devices using information about the current strength in it is carried out by transmitting information on electromagnetic waves, there is no need for additional galvanic isolation through magnetic induction used in proximity sensors (measuring induction current transformers).

The entire device is in working condition under high voltage potential. With direct current, this potential is constant in time, with alternating current, the potential changes (floats) with an amplitude equal to the phase voltage in the current lead, and can reach 1500 kV. All elements of the device are low voltage.

To exclude the influence on the electronic equipment of electric and magnetic fields, the entire measuring device is completely placed inside the current lead with the measured current. Inside a completely enclosed electrically conductive region of space, the sum of the electric and magnetic fields is zero. Thus, digital equipment placed inside the current lead will not be affected by electromagnetic fields, however large they may be. Also, the conductor will protect the equipment from possible switching or lightning surges. The temperature gradient between the shunt and measuring part of the circuit will also be minimal. Due to this, the same change in the conductivity of the material of the shunt and measured current path circuit will be observed. In this case, in the divider, the temperature changes in the environment will be compensated. The measuring device is powered by a voltage divider, also located inside the current lead. The design of the measuring device allows you to place it completely inside the current lead. The lack of need for external devices allows the cavity of the current lead with a measuring device to be sealed. Since the electric current propagates along the surface of the conductor and with increasing current frequency the thickness of the near-surface layer along which the current propagates decreases, the device is not threatened by lightning and switching overvoltages of high frequency. A high-frequency signal with information about the magnitude of the current is transmitted through the current lead. Thus, a device is obtained which is hermetically mounted in the current lead, powered by the current lead, measuring the strength of the current flowing through the current lead, and transmitting a modulated signal about the measured current strength through the same current lead to the RF communication equipment.

The device operates as follows.

With the passage of current through the current path at the ends of the shunt, a voltage potential difference appears corresponding to the strength of the current flowing through the shunt. The conductor and the shunt are a voltage divider. The total current strength is scaled to correspond to the current passing through the shunt. Thus, a change in the current strength in the current lead leads to a change in the voltage drop at the ends of the shunt. The voltage from the terminals of the shunt is encoded by the ADC and controls the modulation of the high-frequency reference signal. Modulation of a digital signal can be amplitude, frequency, angular, phase, etc. Next, a high-frequency signal is sent to the current lead and propagates through it as in an antenna-feeder device. The signal from the measuring device on the low-voltage side is received using high-frequency communication equipment installed at the substation. The high-frequency signal with the help of coupling capacitors is separated from the industrial frequency, demodulated and used by automation, telemetry and relay protection.

The implementation of the invention

At the applicant enterprise, a model of the proposed device for measuring current was made. The device was mounted on a 220 kV high voltage rod insulator. The device was designed to measure currents from 10 to 100 amperes. The design accuracy class was in accordance with class 0.2S (IEC Class 0.2) of traditional current transformers.

The signal from the shunt was digitized by a 12-channel ADC manufactured by Rudnev and Shilyaev CJSC and subjected to phase modulation at a frequency of 50 kHz. The signal was received on the equipment of the Odessa plant "Neptune", the RF signal was filtered from the industrial frequency by coupling capacitors manufactured by the Serpukhov plant "Kvar".

Analysis of the test results showed high repeatability of measurements, high accuracy and reproducibility of the results, the absence of distortion at short circuit currents, the absence of distortion due to vibration, the absence of distortion when the magnetic field changes at the measurement site, the absence of distortion of the results when electric discharges pass through the air (shock simulation lightning).

The design of the device is illustrated in the drawing, which shows the electrical circuit of a digital device for measuring current.

The drawing shows the following notation:

1 - current lead of the measured circuit;

2 - a shunt connected in parallel to the measured circuit;

3 - ADC;

4 - modulator;

5 - an antenna forming an antenna-feeder path with a current lead.

Claims (1)

A high-voltage digital device for measuring current, containing a current sensor, an analog-to-digital converter and a modulator, characterized in that it is placed inside a current path with a measured current, is under high voltage potential in the zone of absence of magnetic and electric fields, and a current path with a measured current is used in as an antenna-feeder path for transmitting a modulated digital signal.