Patent ID: 12249455

DETAILED DESCRIPTION OF INVENTION

InFIG.1a high voltage instrument transformer1according to the present invention is shown. The high voltage instrument transformer1comprises a housing2with isolator5and vessel6, particularly a high-pressure vessel. The isolator5is hollow, columnar, with rips on the outer surface to increase leakage current lengths. The columnar isolator5is arranged upstanding on the vessel6, with excess pressure means8, particularly a bursting disc, and high voltage terminals12on top. High voltage in the range of kilovolt up to 1200 kV can be applied to the high voltage instrument transformer1via terminals12, to measure for example voltages in the grid and/or at electrical lines, and/or at electrical units like bushings, high voltage switches, transformers, and/or surge arresters.

The isolator5is made of materials and/or comprises materials as for example ceramic, silicone and/or composite materials. The vessel6is made of materials and/or comprises materials as for example metals like aluminum, steel, and/or cast iron. The vessel6is arranged on a base with earth connection7, for example made of steel on a concrete basement. Outside the vessel6a terminal box13is arranged with for example electric terminals and/or electronics, for example to measure voltages, process signals and transfer signals/information to monitoring devices. For example, computer, sense boxes, data receiver and/or transmitter for particularly online monitoring like mobile units and/or data cable units/online units for internet, are arranged in terminal box13.

A measuring assembly to measure high voltages is arranged for example in the vessel6, comprising particularly a primary winding3and a voltage transformer core4. High voltage terminals12of the high voltage instrument transformer1on top of the isolator5are electrically connected via a bushing11, particularly comprising a metal bar made of aluminum, steel and/or copper, to the measuring assembly in the vessel6. The metal bar is surrounded by control electrodes embedded for example in isolator paper, not shown inFIG.1for reasons of simplicity, and at the passage from isolator5to vessel6a ground screen10, for example made of a metal like copper, aluminum and/or steel, is arranged. Isolator5and vessel6are particularly filled by a fluid for isolation15, for example an isolation gas like SF6, Clean Air and/or an isolation liquid like oil.

According to the present invention, a partial discharge sensor9, particularly a Transient Earth Voltage sensor, is arranged at, particularly directly on the ground screen10and/or electrical connected to the ground screen10. Partial discharges preferably propagate along capacitive paths. When a partial discharge incepts in or at the isolator5, the partial discharge propagates through the bushing capacitance, for example represented by aluminum multi-shields, to the shielded ground, creating a closed circuit. A signal, after flowing through the bushing, passes through the screen to “close” the electrical circuit, enabling a reliable partial discharge measurement.

Alternatively, and/or additionally, not shown inFIG.1for reasons of simplicity, an outer shield of the bushing of the high voltage instrument transformer can be electrically connected to ground screen. Alternatively, and/or additionally, not shown inFIG.1for reasons of simplicity, further sensors can be arranged, particularly directly arranged on the outside surface of the isolator5, and/or inside of the isolator5, particularly directly on the surface of the isolator.

The partial discharge sensor9, particularly Transient Earth Voltage sensor, is electrically and/or optically connected for example by a connection wire14to terminal box13. Signals measured with sensor9are directly or processed transmitted to terminal box13, for example to be processed further and transmitted to a central control room, to mobile devices and/or to be locally displayed for example at an alarm light. A monitoring, continuously or in particularly predefined periods of time, of the condition of the high voltage instrument transformer1is possible with sensor9, for example via mobile and/or cable/internet connection to a central control and/or devices in the cloud.

The partial discharge is a localized dielectric breakdown of the electrical isolation at high voltages between electrical poles, introducing alternating voltages and/or currents in or at the isolator5. Partial discharge signals are for example measured in form of voltage with time. The partial discharge signal is small compared to the high voltage at terminals12, with high voltage for example in the range of a kilovolt up to 1200 kV, compared to some picocoulomb signals of partial discharge, i.e. small voltage and current signals. The partial discharge at or in the isolator5starts for example within voids, cracks, contaminants or inclusions, particularly at conductor-dielectric interfaces, and in liquids partial discharge starts particularly in bubbles, contaminants and/or inclusions. The partial discharge is only limited to a portion of isolation. The discharge is only partially bridging the distance between the poles respectively between electrical conductors.

As the partial discharge is initiated, high frequency transient current pulses appear and persist for nano- to microseconds. Then the current pulses disappear and reappear repeatedly as for example a sinewave passes through the zero crossing. The partial discharge signals are short in duration and exhibit rise times of currents in the range of nanoseconds. High levels of electrical background noise, for example due to corona, crosstalk and other effects, make a correct measurement and simple recognition and determination of partial discharge within a measured signal of high voltage/currents difficult.

Due to the short duration and due to rise times in the range of nanoseconds of partial discharge currents in high voltage applications, a visualization and distinguishing between noise and partial discharge in voltage/current against time plots is difficult. To identify partial discharge and/or to determine a value of partial discharge, according to the present invention the information content I of a signal S is investigated. Measured signals with time S (t) are transformed from time to frequency domain for example by Fourier Transform and/or Discrete Cousin Transform. Frequencies f above a predefined frequency limit flimare cut off and truncated signals are obtained, by removing the signal components at frequencies higher than flim. The value of frequency limit flimis for example defined every time at the beginning of a monitoring activity respectively measurement. In a step after removing frequencies f above a predefined frequency limit flim, signals are transformed back respectively retransformed from frequency to time domain.

Partial discharge signals contain an information, while noise signals do not contain information. Noise signals are completely uncorrelated among themselves. By switching from the time domain to the frequency domain, each component has a part of the global information Ii. By cutting off parts of the signal above frequency limit flim, information is lost, if the measured signal comprises partial discharge and does not consist only of noise. According to the present invention the method respectively algorithm is to extract the information from the sampled signals, for example by calculating the Shannon entropy, and compare the information content of the sampled signal with the information content of the truncated signal, derived by cutting off frequencies of the signal above frequency limit flim.

The information content of the measured respectively sampled signals is calculated, particularly by a computer locally and/or in the cloud, as described above for example by calculating the Shannon entropy, and the information content of the truncated signal respectively signal after cutting off frequencies is calculated, particularly by a computer locally and/or in the cloud, likewise as described above for example by calculating the Shannon entropy. Both calculations are for example performed in time with measurements or separately one after another, or in a predefined order. After removing frequency components, a time domain signal for the truncated signal is obtained for example by the inverse integral transform, particularly by Fourier Transform and/or Discrete Cousin Transform. The determination of information content of measured and truncated signals is analogous, for example in both cases by calculating the Shannon entropy.

A comparison of information content of the measured and truncated signals results in a recognition and/or determination of partial discharge. A comparison is performed for example manually and/or automatically, for example by a computer and/or in the cloud. Methods to compare signals include for example division and/or subtraction of the information content of the measured and truncated signals. For division a result is smaller than one, if a measured signal comprises partial discharge. Without partial discharge is a division result, which is exactly or mainly one, since a signal consists only of noise, and no information is lost by cutting off frequencies.

A recognition and/or determination of partial discharge has advantages as described above, for example a warning can be provoked and/or transmitted and further actions triggered, for example switching off voltage and/or disconnecting high voltage units from the grid, to avoid damage and/or destruction/failure of the units. Partial discharge measurements are also providing quality assessment and diagnostic results. The partial discharge is for example an indicator for the status of the high voltage instrument transformer1under test. It is performed in factory quality tests, to show proper function of produced units. In operation in the grid, with a lot of background noise, an online monitoring on high voltage units, for example high voltage switches, bushings, and/or transformers, is possible.

The above described embodiments of the present invention can be used also in combination and combined with embodiments known from the state of the art. For example, partial discharge can be detected by once performing the steps, particularly all steps as described before. A higher reliability is reached by performing the steps, particularly all steps as described before, repeated. An iteration method to determine partial discharge can include determining a value of frequency limit flim. An iteration method can comprise repeating steps according to the present invention, and changing cut off frequency limit flim, till a change of Shannon entropy occurs and/or no change of Shannon entropy occurs with changed frequency limit flim.

Additional sensors, particularly Transient Earth Voltage sensors, optical, acoustic, and/or electrical sensors, can be comprised by the high voltage instrument transformer1and/or combined used with external sensors, to improve sensitivity and/or reliability, and to add additional information to measuring signals, enabling a high sensitivity, continuous measuring with time, with simple equipment at low cost and complexity, to reliably detect and determine partial discharge, with advantages as described above, for example prevention of damage and/or destruction of electrical units and/or the grid.

LIST OF REFERENCE CHARACTERS

1high voltage instrument transformer2housing3primary winding of measuring assembly4voltage transformer core of measuring assembly5isolator6vessel7base with earth connection8excess pressure means, particularly bursting disc9partial discharge sensor10ground screen11bushing12high voltage terminals13terminal box14electrical connection of the terminal box with the partial discharge sensor15fluid for electrical isolation, particularly isolation gas and/or oil