WO2021063473A1

WO2021063473A1 - Instrument transformer with bellow and method for pressure compensation during fluid volume changes

Info

Publication number: WO2021063473A1
Application number: PCT/EP2019/076392
Authority: WO
Inventors: Emanuele GASTALDO
Original assignee: Siemens Energy Global GmbH & Co. KG
Priority date: 2019-09-30
Filing date: 2019-09-30
Publication date: 2021-04-08

Abstract

The present invention relates to an instrument transformer (1) for high current and/or high voltage conversion and to a method for pressure compensation during fluid volume changes within the instrument transformer (1), comprising a housing with a high voltage side (14) and a low voltage side (15), and at least an active part, which is electrically insulated by a fluid (10) within the housing, and with a bellow (6) for pressure compensation during fluid volume changes. The bellow (6) is arranged at the low voltage side (15) of the instrument transformer (1).

Description

Instrument transformer with bellow and method for pressure compensation during fluid volume changes

The present invention relates to an instrument transformer for high current and/or high voltage conversion and to a method used by the instrument transformer, comprising a housing with a high voltage side and a low voltage side, and at least an active part, which is electrically insulated by a fluid within the housing, and with a bellow for pressure compensation during fluid volume changes.

Oil insulated instrument respectively measurement transformers are for example known from US 5391 835 A. The instrument transformers are used to measure high currents and/or voltages, particularly in the range up to some hundred Ampere and/or up to 1200 kV. An instrument transformer comprises a housing and at least an active part, which includes a measuring assembly. The measuring assembly comprises for example windings arranged around an electric conductor, which can be used to measure a current in the conductor by magnetic induction in the windings. The active part is electrically insulated by an isolation material from the housing of the instrument transformer. The whole measuring assembly or parts of the measuring assembly are wrapped by kraft paper sheets and the housing is filled by oil, to electrically insulate active parts.

The housing comprises for example a head housing at a high voltage side with a measuring assembly, arranged on a columnar insulator, standing straight upwards on a base at a low voltage side and/or ground. The housing is filled with oil, covering the measuring assembly and electrically isolating it from the housing. On top of the housing a bellow, comprising a burst disc for dissipation of critical excess pressure within the housing and a bellow cover is arranged, particularly with an oil level indicator for manually metering the oil level within the housing.

During high current and/or high voltage conversion, and by environmental respectively weather changes, the fluid like oil within the housing can change its volume, particularly expand or contract. The volume change is induced for example by waste heat from current-carrying conductors within the instrument transformer, particularly at high currents, and/or by temperature changes in the environment of the instrument transformer. Higher temperatures increase the volume of the fluid and low temperatures decrease the volume. To enable for example expansion and contraction of fluids like oil within the housing, space upwards of oil is filled by gas like air, and/or the bellow compensates respectively allows expansions or contraction of fluid volume. During expansion the bellow expands, increasing the volume within the housing of the instrument transformer, and during contraction the bellow contracts, decreasing the volume within the housing of the instrument transformer. Bellow deformation is analogous to an accordion deformation.

At very high temperature increases, excess pressure can lead to an explosion of the instrument transformer. To prevent the instrument transformer from explosion, to prevent injuries of personal near the instrument transformer and to enable dissipation of excess pressure in a predefined direction, a burst disc is arranged on top of the instrument transformer. At a critical pressure respectively when pressure exceeds a critical value, for example induced by temperature increase within the instrument transformer and/or expansion of insulation oil, the burst disc breaks respectively bursts, and fluids like oil and gas can dissipate from the housing of the instrument transformer, reducing the pressure to a value below a critical value. An explosion of other parts than the burst disc is prevented, particularly of parts like isolator, base and/or head housing, preventing staff to be hurt and enabling an easy reconstruction of the instrument transformer for example by changing the burst disc.

A further problem of fluid filled instrument transformers, for example filled with oil and/or esters, is leakage. Fluids can be environmental harmful, and a leakage can lead to destruction of the instrument transformer. If parts at high voltage like the measuring assembly within the housing are not particularly fully covered by oil, a measuring can be inaccurate, and parts of the instrument transformer can be damaged or staff can be harmed by high voltage. A detection of leakage manually using an optical oil level indicator, for instance a window with level lines to be seen outside of the housing, requires regular service inspections and is cost and labor-intensive. Changes in volume of fluid within the instrument transformer, detected manually by an oil level indicator, are only detected during inspections with local service personal.

For continuous monitoring of the instrument transformer conditions a manual metering is not sufficient. Sensors can be used to monitor continuously properties like for example oil level and/or pressure. Most sensors measure locally values and produce data from measured values. For data procession, storage and/or display a transmission for example by electrical wire to a central unit is necessary. In instrument transformers critical values like oil level and pressure particularly at the burst disc and/or at the bellow, require a measurement at the high voltage side. Data processing, storage and display units are located at low voltage side, for example to prevent damages and interferences by high voltage and to enable save readings from a display. A data transfer from high to low voltage side is necessary.

Data transfer from high to low voltage side by wire is difficult, since arcs or voltage drops can occur due to the wire and current and/or voltage flashovers via the wire can harm local staff. Alternatively, transmitter/receiver units enable a wireless data and/or energy transfer. Electronic devices like transmitter/receiver units are complex and expensive. A maintenance and replacement of defect electronic units like sensors, transmitters and/or receiver units at high voltage side is dangerous, requiring a shutting off of high voltage lines, including costs and maintenance time.

High voltage is able to affect correct measurements of sensors and to disturb data transfer by transmitter/receiver units, leading to measurement errors and failures.

An object of the present invention is to overcome the problems described above. Especially an object of the present invention is to reduce costs, reduce manual inspections and increase reliability and lifetime of the instrument transformer and particularly of sensors. A further object is to enable a particularly fully automatic measuring and monitoring of fluid levels within the housing of the instrument transformer, without wires from high to low voltage side and/or without costly transmitter/receiver units, safe and reliable, with an easy maintenance and exchange of sensors.

The above objects are achieved by an instrument transformer for high current and/or high voltage conversion according to claim 1 and/or by a method for an instrument transformer, particularly for an instrument transformer described above, according to claim 12.

An instrument transformer for high current and/or high voltage conversion according to the present invention comprises a housing with a high voltage side and a low voltage side, and at least an active part, which is electrically insulated by a fluid within the housing, and with a bellow for pressure compensation during fluid volume changes. The bellow is arranged at the low voltage side of the instrument transformer. A bellow at low voltage side allows sensors, particularly measuring volume and/or volume changes, to be electrically connected direct for example via electrical and/or optical wire to devices on low voltage side, particularly devices to control, measure, process, store, monitor/display and/or send values and/or data, with devices for example arranged in a terminal box. Since sensors and devices to control, measure, process, store, monitor and/or send values and/or data, are arranged on low voltage side, no arcs and/or faults caused by high voltages via wires occur, and sensors and devices are not affected or distorted by high voltage. Reliability and lifetime of the instrument transformer, particularly sensors and/or devices to control, measure, process, store and/or send values and/or data, is increased. Even without sensors, a reading of oil level indicators manually is easier on low voltage side than on high voltage side.

A use of sensors without faults, without the need to use units like transmitter/receiver units, reduces costs, reduces the need of manual inspections and increase reliability and lifetime of the instrument transformer and particularly of sensors. A fully automatic measuring and monitoring of fluid levels within the housing of the instrument transformer is enabled, without wires from high to low voltage side and without costly transmitter/receiver units, safe and reliable, with an easy maintenance and exchange of sensors.

The bellow can comprise as at least one sensor for example a fluid-level sensor, a pressure sensor, a temperature sensor and/or a bellow expansion sensor. These sensors can provide information about the status of the instrument transformer, particularly critical situations like high temperature, critical pressure and/or critical expansions of fluids within the housing, with for example risk of explosions and/or damages of the instrument transformer, particularly explosion of the burst disc. The at least one sensor can be comprised for automatic fluid- level measurements and an oil level indicator can be comprised for manual fluid-level measurements and/or control, particularly at or in the bellow. A manual fluid-level measurement can be used for a check of correct working of the at least one sensor during service at regular maintenance intervals. The use of at least one sensor can reduce needed frequency of maintenance and enables a continuous monitoring of for example fluid-levels. At critical values a fast reaction is possible, for example switching off high power lines producing for example heat, warning and ordering service staff into safe locations, and informing service staff to inspect the instrument transformer.

The sensor can comprise a hydro-static pressure sensor, a laser-level transmitter, a mechanical sensor, particularly a mechanical sensor generating a well-defined tone during fluid level changes or by reaching a pre-defined fluid level, and/or an electrical sensor, particularly an electrical sensor generating a signal during fluid level changes or by reaching a pre-defined fluid level, or a resistor particularly for continuous monitoring of the fluid level.

The at least one sensor can be or can comprise a hydro-static pressure sensor for example to detect and/or monitor fluid leakages, which particularly involves a reduction of fluid- level. The at least one sensor can be or can comprises a laser-level transmitter. A laser-level transmitter comprises for example a laser and an optical detector, detecting reflected or inflected laser light at the interface between liquid fluids, for example oil and/or esters, and a gas, for example air. The fluid-level of the liquid can be deduced from an intensity and/or angle value of a reflected or inflected laser beam. This permits the detection and/or monitoring of fluid-levels, detection of fluid leakage, and/or expansion or compression of insulating fluids, with the danger of explosions or the danger of fluid-levels below a critical value, not particularly complete covering parts of the instrument transformer in the housing at high voltage and/or measuring devices like coils, introducing errors in the measuring.

The at least one sensor can be or can comprises a mechanical sensor, particularly a mechanical sensor generating a well- defined tone respectively sound during fluid level changes or by reaching a pre-defined fluid level. The advantages are as described before for laser-level transmitters. A receiver can be a microphone, what gives a cost effective, easy to handle arrangement. Mechanical sensors can be or can comprise sensors with floaters additional or alternative to sound generating sensors, with advantages as described before.

The at least one sensor can be or can comprise an electrical sensor, particularly an electrical sensor generating a signal during fluid level changes or by reaching a pre-defined fluid level. An example for an electrical sensor is an electrical resistant bar or stripe, reaching from an area of low fluid level to an area of high fluid level, changing the resistance between two electrical contacts depending on the fluid level. Combinations of sensors, particularly of electrical and mechanical sensors are possible, increasing the reliability of fluid level measurements and flexibility to adjust to electrical and dimensional properties within the housing of the instrument transformer. Electrical sensors are reliable, at low price available, and with a need of little space arrangeable within an instrument transformer, and signals are easy to detect and easy to process particularly electronically.

The bellow can be fluidically connected with the housing by a pressure regulator. This enables a bellow on a spatial position below a fluid-level in the housing of the instrument transformer. Measurements of fluid-level changes in the bellow are proportional to the fluid level in the housing. At high pressures in the housing weak sensors can be protected in the bellow. The housing can comprise a base, an isolator, and/or a head, particularly with a head housing, high voltage terminals, particularly arranged at the head housing, and/or with a measuring assembly, particularly secondary windings and/or arranged within the head housing. This arrangement allows a high voltage side well separated spatial from the low voltage side, with high voltage side in spatial high position, far away from staff on ground side, well isolated and without the risk of arcs, damaging the instrument transformer and/or hurting staff.

The bellow can be arranged on the base, particularly opposite a terminal box. This allows easy access of staff if service is needed, for example manual reading of values and/or exchange of defect sensors. On the base the bellow can be mechanically mounted safe, stable and reliable. An arrangement of bellow opposite the terminal box enables short wires between sensors in the bellow and devices in the terminal box as for example devices to monitor, control, measure, process, store and/or send values and/or data. This particularly saves costs and increases reliability and lifetime.

The fluid can be or can comprise oil, particularly transformer oil, mineral oil and/or a synthetic oil, and/or the fluid can be or can comprise ester, particularly vegetable esters. The fluid within the housing can be filling the head housing and/or the isolator and/or the base, particularly filling space between the housing and active parts, particularly the measuring assembly and the housing. Oil and ester or combinations of oil and esters provide a fluid with high respectively good electrical insulation properties, filling particularly completely free space between parts of the instrument transformer, particularly parts like current conducting wires at high voltage and/or the measuring assembly and parts like isolator, base and/or head housing. A good electrical isolation between parts of the instrument transformer at high voltage respectively high electrical potential and parts at lower electrical potential is possible, by filling in fluids like oil and/or esters in- between, within the housing.

The bellow can comprise a bellow cover, particularly a cylindrical wrap, and/or at least one sensor as well as an oil level indicator can be arranged at the bellow cover. The bellow cover can mechanically protect the bellow and sensors, and protect the bellow and sensors against weather, particularly rain and wind. An arrangement of the oil level indicator at the bellow cover allows an easy and fast reading of values without the need to remove the bellow cover, saving time and effort.

The at least one sensor of the bellow can be optically and/or electrically connected with at least one device, particularly a device to control, measure, process, store, display and/or send values and/or data, particularly arranged in a terminal box. Optical and/or electrical connections are easy and cost effective particularly by wire, and a connection of the at least one sensor of the bellow to at least one device, particularly a device to control, measure, process, store, display and/or send values and/or data, allows a continuous monitoring of values as for example fluid-levels, particularly remote from a central service point, with low cost, easy, secure and reliable, with advantages as described before. An arrangement of devices to control, measure, process, store, monitor/display and/or send values and/or data in a terminal box allows an easy and fast service, check and if needed exchange of devices. The devices are saved against weather influences or sabotage.

A method according to the present invention for an instrument transformer, particularly for an instrument transformer as described before, comprise that volume changes of a fluid in a housing of the instrument transformer, with at least an active part of the instrument transformer electrically insulated by the fluid, are compensated by a bellow arranged at a low voltage side of the instrument transformer.

At least one sensor, particularly arranged at or in the bellow and/or on the low voltage side of the instrument transformer, can be measuring the fluid-level of the electrically insulating fluid, particularly liquid, particularly oil and/or ester, electrically, optically and/or mechanically.

The at least one sensor can emit an optical, an acoustic and/or an electrical signal particularly when a predefined fluid-level is reached and/or the fluid is falling below a predefined fluid-level, and/or continuously and/or periodically, particularly to a device on the low voltage side of the instrument transformer, particularly arranged in a terminal box, to control, measure, process, store, display and/or send values and/or data correlated to the fluid-level.

The advantages in connection with the described method for an instrument transformer according to the present invention are similar to the previously, in connection with the instrument transformer for high current and/or high voltage conversion described advantages.

The state of the art and the present invention are further described hereinafter with reference to illustrated embodiments shown in the accompanying drawings, in which:

FIG. 1 illustrates an instrument transformer 1 according to the state of the art for high current and/or high voltage conversion in section view, comprising a housing and an active part, with isolation fluid 10 in the housing and a bellow 6 on top of the housing on high voltage side 14 for pressure compensation during fluid volume changes, and FIG. 2 illustrates an instrument transformer 1 according to the present invention for high current and/or high voltage conversion in section view, comprising a housing and an active part, with isolation fluid 10 in the housing and a bellow 6 at a base 4 of the housing on low voltage side 15 for pressure compensation during fluid volume changes.

An instrument transformer 1 for high current and/or high voltage conversion is shown in section view in FIG. 1. The instrument transformer 1 comprises a housing and at least an active part, which is electrically insulated by an isolation fluid 10. In the embodiment of FIG. 1 according to the state of the art, an active part of the instrument transformer 1 includes a measuring assembly 11 with for example windings arranged around an electric conductor, which are wrapped for example by paper. The windings are used to measure a current in the conductor by magnetic induction in the windings. Further active parts are for example control electrodes and/or a discharge pipe.

The instrument transformer 1 comprises a head 2, an isolator 3 and a base 4. The head 2 comprises a head housing 12 with high voltage terminals 8, and a bellow cover 6, for example with burst disc. The active part, particularly the measuring assembly 11, is located within the housing of the instrument transformer 1. An oil level indicator 7 for manual metering the oil level is arranged at the head 2, particularly at the bellow cover 6. The isolator 3 is particularly composed of a hollow cylindrical body with fins outside. The isolator 3 is for example a ceramic, silicon and/or composite hollow body with plate fins at the outer sheath to increase leakage current length. The base 4 is for example in form of a cast- iron pedestal, with a terminal box 5 for particularly control units, data processing, monitoring, and transmission devices.

The isolator 3 is for example columnar with two ends of the column, arranged with the base 4 on one end and the head 2 on the other end. The head 2 is on top of the upstanding columnar isolator 3, with the high voltage terminals 8 to electrically connect the instrument transformer 1 with high voltage lines, electrical generators and/or electrical consumers, to measure current/voltage of electrical high voltage lines and/or devices. A measuring assembly 11 as active part within the housing of the instrument transformer 1 measures current and/or voltage in between the high voltage terminals 8. Transferred via active parts as for example a discharge pipe and/or VT primary, secondary windings and VT core, measuring results are recorded and/or read from meters within the terminal box 5 particularly arranged at the base 4.

The active part is electrically insulated by an isolation material from the housing of the instrument transformer. The whole active part or parts of the active part, for example coils, are wrapped by kraft paper and the housing is filled by a fluid like oil, to electrically insulate active parts. Oil impregnates the kraft paper, improves isolation properties and fills free space within the housing. An oil level is above the active part, for example filling the head housing nearly complete, with air on top of the oil filling. An optical oil level indicator 7 arranged on the upper end of the head housing 12 enables staff to manually detect the filling amount with fluids like oil and to inspect the condition of the instrument transformer 1. The oil is for example transformer oil 10, comprising mineral and/or synthetic oil, and/or esters. Alternatively, esters can be used as insulation fluid.

During high current and/or high voltage conversion, and by environmental respectively weather changes, the fluid within the housing can change its volume, particularly expand or contract. The volume change is induced for example by waste heat from current-carrying conductors within the instrument transformer for example at high currents, and/or by temperature changes in the environment of the instrument transformer 1. At very high temperature increases, excess pressure can lead to an explosion of the instrument transformer 1. To prevent the whole instrument transformer 1 from explosion, to prevent injuries of personal respectively staff near the instrument transformer 1 and to enable dissipation of excess pressure in a predefined direction, a burst disc is arranged on top of the instrument transformer. At a critical pressure respectively when pressure exceeds a critical value, for example induced by temperature increase within the instrument transformer and/or expansion of insulation oil, the burst disc breaks respectively bursts, and fluid can dissipate from the housing of the instrument transformer, reducing the pressure to a value below a critical value. An explosion of other parts than the burst disc is prevented, particularly of parts like isolator 3, base 4 and/or head housing 12, preventing staff to be hurt and enabling an easy reconstruction of the instrument transformer for example by changing the burst disc.

Further changes of fluid level in instrument transformers 1, for example filled with oil and/or esters, can occur by leakage. A leakage can lead to the destruction of the instrument transformer 1 and a problem is further that fluids can be environmental harmful. If parts at high voltage like the measuring assembly within the housing are not particularly fully covered by oil, a measuring can be inaccurate, and parts of the instrument transformer 1 can be damaged, or staff can be harmed by high voltage. A detection of leakage manually using an optical oil level indicator 7, for instance a window with level lines to be seen outside of the housing, requires regular service inspections and is cost- and labor-intensive. Changes in volume of fluid within the instrument transformer, detected manually by an oil level indicator, are only detected during inspections with local service personal. A reading of oil level indicator 7 by staff is difficult and can lead to reading errors, due to the arrangement of oil level indicator 7 on top of the instrument transformer 1, at high voltage side 14. In FIG. 2 a solution to the before described problems is shown, with an oil level indicator 7 in a bellow 6, arranged at the base 4 of an instrument transformer 1 according to the present invention. The instrument transformer 1 of FIG. 2 is analogous to the instrument transformer 1 of FIG. 1, but bellow 6 with cover and oil level indicator 7 is arranged on low voltage side 15 mounted at the base 4, enabling an easy reading by staff at a level staff normally is located and without the danger of high voltage injuries. A burst disc, not shown in the FIG., can be arranged on top of the instrument transformer 1, or can be arranged below the bellow cover.

For continuous monitoring of the instrument transformer 1, for example to early detect leakage or excess temperature/pressure and to avoid damages by leakage and explosions, a manual metering is not sufficient. Sensors 13 can be used to monitor continuously properties like for example oil level and/or pressure. Most sensors measure locally values and produce data from measured values. For data procession, storage and/or display a transmission for example by electrical wire to a central unit is necessary. In instrument transformers 1 according to the state of the art, critical values like oil level and pressure particularly at the burst disc and/or at the bellow, require a measurement at the high voltage side 14. Data processing, storage and display units are located at low voltage side 15, for example to prevent damages and interferences by high voltage and to enable save readings from a display. A data transfer from high 14 to low 15 voltage side is necessary.

In the instrument transformer 1 according to the present invention, shown in FIG. 2, bellow 6 is located at low voltage side 15, enabling an arrangement with sensors 13 at low voltage side 15, and avoiding problems described above. Sensor 13 is located in the bellow 6 and/or at the bellow cover, and is able respectively designed to measure a fluid- level, particularly oil-level, automatically. Sensor 13 is for example located at or near to the oil level indicator 7, enabling a calibration and control of proper work of sensor 13 by readings of the oil level indicator 7. Sensor 13 is for example a hydro-static pressure sensor, a laser-level transmitter, a mechanical sensor and/or an electrical sensor, like a resistor. In normal operation, the fluid level of the instrument transformer 1 is nearly constant. If there is a leakage, fluid like oil spills out of the housing of the instrument transformer 1, and fluid-level of oil drops. A drop of fluid-level is measurable with sensor 13.

Temperature increases, for example by high heat generation during operation, particularly by a short circuit-current, change a fluid-level due to expansion of fluids like oil, or increase the pressure within the housing. Bellow 6 will expand, and the expansion can be measured by sensor 13. In the same way compressions of bellow 6 can be measured, indicating a pressure reduction and/or temperature reduction or leakage within the housing of the instrument transformer 1. For monitoring proper conditions and work of the instrument transformer 1 without changes of fluid-level, sensor 13 can be designed to measure a constant fluid-level. At changes, a signal can alarm staff to inspect the state of the instrument transformer 1 or trigger automatic reactions, like switching off currents/voltages. Particularly at reaching critical values, sensor 13 is able to measure critical fluid-levels and release a signal. For example, a reaction is possible before an explosion, bursting of the burst disc, and/or dropping of fluid out of the instrument transformer 1 occurs.

An acoustic, for example mechanical sensor 13, particularly above a normal fluid-level or at a predefined expansion of bellow 6, is able to give an acoustic signal if fluid is reaching the sensor 13 or bellow 6 expands above a critical value. A sensor 13 below a normal fluid-level or a predefined compression level of bellow 6, is able to give an acoustic signal at dropping of fluid-level below the sensor 13 location or by compression of bellow 6 below a critical value. The same applies to electrical sensors 13, whereby for example resistive sensors 13 are able to measure fluid-level changes continuously over a wide range of fluid levels, and not only at certain positions. Optical sensors 13, for example laser-level transmitters, are able to detect an interface between liquid and gas, and/or the high of the bellow 6, and to determine the fluid-level from the position of the interface respectively the high of the bellow 6 or position of the bellow cover. The laser-level transmitters are for example located above and/or below the interface fluid/gas or above or below the bellow 6 or bellow cover. Hydro-static pressure sensors, particularly in the bellow 6, are able to detect pressure changes during changes of the fluid-level. Further examples of sensors to be used in the instrument transformer 1 according to the present invention are mechanical sensors 13, for example in form of a floater.

A bellow 6 at low voltage side 15, particularly mounted and/or arranged at the base 4 or the isolator 3, enables an easy and short wiring of sensors 13 with devices, particularly devices for monitoring, processing, storing, displaying and/or transmission of data, in a terminal box 5 particularly at the base 4, without the risk of arcs via wires and with easy maintenance and/or exchange of sensors 6. A pressure regulator in-between the bellow 6 and the fluid 10 in the housing of the instrument transformer 1 can be used, enabling a reduction of pressure in the bellow 6 to avoid destruction of sensors 13 and to enable a location of bellow 6 below the level of head housing 12. The Bellow 6 with base 4 and/or isolator 3 is fluidically connected for example by a pipe, to enable a fluid flow between the housing of the instrument transformer 1 and the bellow 6 and vice versa.

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, the instrument transformer 1 can be a current transformer, an inductive voltage transformer, a capacitive voltage transformer, a combined current and voltage transformer, a power voltage transformer, and/or an optical current transformer. Active parts can be located in a head housing 2, in an isolator 3 and/or in a base 4. A measuring assembly 11 is for example in the head housing 2 arranged. Alternative instrument transformer designs comprise an isolator 3 and a base 4 without a head housing, for example with measuring assembly 11 arranged in the base 4. Sensor 13 is able to measure for example a fluid-level, and/or pressure, and/or temperature, and/or a location/state of the bellow 6, and to transfer the information via wire or wireless, for example by light signal, sound and/or radiocommunication. Measured data are processed for example by sensor 13 itself, or transferred, stored, processed and/or displayed by data-transfer-, storage-, processing-, and/or displaying-devices. Energy for operation of sensor 13 can be provided by wire or wireless, or sensor 13 is an energy-self-harvesting-sensor. Data of sensor 13 can be send for example by handy network, telecommunication wire and/or internet to a central service station, analyzing data, triggering alerts and/or sending staff to inspect and service the instrument transformer 1 if needed. List of Reference Characters

1 instrument transformer

2 head 3 isolator

4 base

5 terminal box

6 bellow with cover 7 oil level indicator 8 high voltage terminals

9 high voltage insulation

10 transformer oil 11 measuring assembly, particularly secondary core/windings 12 head housing 13 sensor, particularly for fluid-level and/or pressure

14 high voltage side

15 low voltage side

Claims

1. Instrument transformer (1) for high current and/or high voltage conversion, comprising a housing with a high voltage side (14) and a low voltage side (15), and at least an active part, which is electrically insulated by a fluid (10) within the housing, and with a bellow (6) for pressure compensation during fluid volume changes, characterized in that the bellow (6) is arranged at the low voltage side (15) of the instrument transformer (1).

2. Instrument transformer (1) according to claim 1, characterized in that bellow (6) comprises at least one sensor (13), particularly a fluid-level sensor, a pressure sensor and/or a bellow expansion sensor.

3. Instrument transformer (1) according to claim 2, characterized in that the at least one sensor (13) is comprised for automatic fluid-level measurements and an oil level indicator is comprised for manual fluid-level measurements and/or control, particularly at or in the bellow (6).

4. Instrument transformer (1) according to any one of the claims 2 or 3, characterized in that the sensor (13) comprises a hydro-static pressure sensor, a laser-level transmitter, a mechanical sensor, particularly a mechanical sensor generating a well-defined tone during fluid level changes or by reaching a pre-defined fluid level, and/or an electrical sensor, particularly an electrical sensor generating a signal during fluid level changes or by reaching a pre-defined fluid level.

5. Instrument transformer (1) according to any one of the claims 1 to 4, characterized in that bellow (6) is fluidically connected with the housing by a pressure regulator.

6. Instrument transformer (1) according to any one of the claims 1 to 5, characterized in that the housing comprises a base (4), an isolator (3), and/or a head (2), particularly with a head housing (12), high voltage terminals (8), particularly arranged at the head housing (12), and/or with a measuring assembly (11), particularly secondary windings and/or arranged within the head housing (12).

7. Instrument transformer (1) according to claim 6, characterized in that bellow (6) is arranged on the base (4), particularly opposite a terminal box (5).

8. Instrument transformer (1) according to any one of the claims 1 to 7, characterized in that the fluid (10) is or comprises oil, particularly transformer oil, mineral oil and/or a synthetic oil, and/or the fluid is or comprises ester, particularly vegetable esters.

9. Instrument transformer (1) according to any one of the claims 1 to 8, characterized in that the fluid (10) within the housing is filling the head housing (12) and/or the isolator (3) and/or the base (4), particularly filling space between the housing and active parts, particularly the measuring assembly (11) and the housing.

10. Instrument transformer (1) according to any one of the claims 1 to 9, characterized in that bellow (6) comprises a bellow cover, particularly a cylindrical wrap, and/or at least one sensor (13) and/or oil level indicator are arranged at the bellow cover.

11. Instrument transformer (1) according to any one of the claims 2 to 10, characterized in that the at least one sensor (13) of bellow (6) is optically and/or electrically connected with at least one device, particularly a device to control, measure, process, store, monitor and/or send values and/or data, particularly arranged in a terminal box (5).

12. Method for an instrument transformer (1), particularly for an instrument transformer (1) according to any one of the preceding claims, characterized in that volume changes of a fluid (10) in a housing of the instrument transformer (1), with at least an active part of the instrument transformer

(1) electrically insulated by the fluid (10), are compensated by a bellow (6) arranged at a low voltage side (15) of the instrument transformer (1).

13. Method according to claim 12, characterized in that at least one sensor (13), particularly arranged at or in bellow (6) and/or on the low voltage side (15) of the instrument transformer (1), is measuring the fluid-level of the electrically insulating fluid (10), particularly liquid, particularly oil and/or ester, electrically, optically and/or mechanically.

14. Method according to any one of the claims 12 or 13, characterized in that the at least one sensor (13) emits an optical, an acoustic and/or an electrical signal particularly when a predefined fluid-level is reached and/or the fluid is falling below a predefined fluid-level and/or continuously and/or periodically, particularly to a device on the low voltage side (15) of the instrument transformer 1, particularly arranged in a terminal box (5), to control, measure, process, store, monitor and/or send values and/or data correlated to the fluid-level.