Patent Description:
Initially, the system of an electrical switchgear can be filled with an insulating gas forming the gas filling of the electrical switchgear. For example, the insulating gas may consist of or comprise Sulfur hexafluoride (SF6). Over time, the gas filling may leak from the system so that the insulating function may decrease. To ensure a minimum of the insulating function, e.g. for switching devices arranged within the gas filled system, a minimum pressure value of the gas filling can be defined. Once the pressure of the gas filling drops below this minimum pressure value, operation of the electrical switchgear can get dangerous. That is why it is advisable to stop the operation of the electrical switchgear in such a situation. Unfortunately, such a situation may occur unexpectedly and lead to an unwanted downtime of the electrical switchgear.

<CIT> discloses an arithmetic unit <NUM> that receives pressure information from a pressure sensor <NUM>, temperature information from a temperature detection means and time information from a time measuring means <NUM>, and operates a gas pressure expressed in terms of a reference temperature, based on these pressure information, temperature information, reference gas temperature information, and reference gas pressure information, and then operates a gas pressure drop rate based on this gas pressure expressed in terms of the reference temperature and the time information, thus operating a residual time to an allowable lower limit gas pressure based on the gas pressure drop rate and the gas pressure expressed in terms of the reference temperature.

Accordingly, an object of the invention is the provision of a method of predicting a lifetime of a gas filling of an electrical switchgear. In particular, an unexpected downtime of an electrical switchgear shall be avoided.

The object of the invention is solved by a method of predicting a lifetime of a gas filling of an electrical switchgear, which comprises the steps of.

In this way, unexpected downtimes of the electrical switchgear can be avoided, and maintenance of the electrical switchgear can be planned better.

To avoid that fast temperature changes of the gas filling, which may be caused by a switching operation of a switching device arranged within the gas filled system, it is proposed to take pressure values at the same predefined temperature Tp or at least at temperatures within a predetermined temperature range. In particular, switching arcs during opening of the switching contacts of such a switching device can heat up the gas filling quickly and cause spikes in graphs of the pressure p and the temperature T of the gas filling. Tests and investigations revealed that there is a time difference between the spikes of the pressure p and the temperature T. Hence, using the gas equation <MAT> wherein R is the gas constant of the gas filling and Vgas is the volume of the gas filling, to take the influence of the temperature T on the pressure p into consideration, may lead to invalid lifetime predictions under unfavorable circumstances, in detail when measuring the pressure p and the temperature T is done during such transition phases like switch off of the switching device. However, that does not necessarily mean that pressure values are only taken at a predefined temperature Tp or in a temperature range, but pressure p can be measured continuously, and suitable values can be picked out of the data stream for the lifetime prediction calculation.

In particular the total lifetime LTtotal of the gas filling can be calculated by use of the formulas <MAT> <MAT> <MAT> wherein p<NUM> is the first pressure value, p<NUM> is the second pressure value, Δp is the pressure difference between the second pressure value p<NUM> and the first pressure value p<NUM>, t<NUM> is the first point in time and t<NUM> is the second point in time, Δt is the time difference between the second point in time t<NUM> and the first point in time t<NUM>, Δpptu is the nominal pressure drop per time unit (for example per year), LTnominal is the nominal lifetime of the gas filling, plow is the minimum pressure value of the gas filling and phigh is the maximum pressure value of the gas filling.

The minimum pressure value plow of the gas filling ensures a minimum level of insulation. Below that value, operation of the electrical switchgear may get dangerous. The maximum pressure value phigh of the gas filling normally is the initial pressure when the system is filled. The nominal lifetime LTnominal of the gas filling is the expected lifetime of the gas filling, e.g. <NUM> years. The nominal pressure drop per time unit Δpptu corresponds to this nominal lifetime LTnominal and the difference between the maximum pressure value phigh and the minimum pressure value plow.

A remaining lifetime LTremain of the gas filling can be calculated by use of the formula <MAT>.

In another embodiment, the total lifetime LTtotal of the gas filling can be calculated by use of the formula <MAT> wherein p<NUM> is the first pressure value, p<NUM> is the second pressure value, plow is the minimum pressure value of the gas filling, phigh is the maximum pressure value of the gas filling, Δp is the pressure difference between the second pressure value p<NUM> and the first pressure value p<NUM>, t<NUM> is the first point in time, t<NUM> is the second point in time and Δt is the time difference between the second point in time t<NUM> and the first point in time t<NUM>.

Further on, the remaining lifetime LTremain of the gas filling can be calculated by use of the formula <MAT> wherein p<NUM> is the first pressure value, p<NUM> is the second pressure value, plow is the minimum pressure value of the gas filling, Δp is the pressure difference between the second pressure value p<NUM> and the first pressure value p<NUM>, t<NUM> is the first point in time and t<NUM> is the second point in time and Δt is the time difference between the second point in time t<NUM> and the first point in time t<NUM>.

In an advantageous embodiment, the first pressure value p<NUM> can be replaced by p1corr and referenced to a reference temperature Tref and/or the second pressure value p<NUM> can be replaced by p2corr and referenced to a reference temperature Tref in the aforementioned formulas by use of the following formulas <MAT> <MAT> wherein T<NUM> is the first temperature, T<NUM> is the second temperature, Vgas is the volume of the gas filling, R is the gas constant of the gas filling, p1corr is the first corrected pressure value and p2corr is the second corrected pressure value. By these measures, the pressures p<NUM> and p<NUM> may be determined more accurately when a temperature range is defined for taking pressure measurements. Preferably, the reference temperature Tref is within said temperature range.

Preferably, the first pressure value p<NUM> and/or the second pressure value p<NUM> is calculated based on a volume change of a capacitor having an elastomer as a dielectric material between two electrodes of the capacitor.

In detail, a deformation of the elastomer or dielectric material based on a pressure change Δp may cause a proportional volume change ΔV in view of an initial volume V<NUM> at an initial pressure. The proportionality factor usually is denoted B, and the formula for the pressure change Δp reads as <MAT>.

A volume change may be calculated based on a change of the capacitance of the capacitive pressure sensor. Provided that the capacitor is ball shaped, the capacitance C can be calculated by use of the formula <MAT>.

Wherein ro is the radius of the outer flexible electrode, ri is the radius of the inner (rigid) electrode, ε<NUM> is the absolute permittivity or absolute dielectric constant and εr is the relative permittivity or relative dielectric constant. Based on the known radius ri of the inner (rigid) electrode and the radius ro of the outer flexible electrode at an initial pressure and an actual pressure, the volume change ΔV and thus the pressure change Δp can easily be calculated. Of course, other shapes of capacitive pressure sensors work as well, for example cylindrical shapes.

Preferably, an alarm is output if the first pressure value p<NUM> or the second pressure value p<NUM> drops below a minimum pressure value plow of the gas filling. For example, this alarm can be output via a signal lamp and/or a wireless interface so as to inform responsible personnel. In this way, exceptional pressure drops can be recognized.

Advantageously, the predefined temperature Tp or the predefined temperature range is chosen according to the IEC standard IEC <NUM>-<NUM>/<NUM> or a nominal temperature operating range of the electrical switchgear. In this way, suitable predefined temperatures Tp or temperature ranges may be chosen.

The invention now is described in more detail hereinafter with reference to particular embodiments, which the invention however is not limited to.

Generally, same parts or similar parts are denoted with the same/similar names and reference signs. The features disclosed in the description apply to parts with the same/similar names respectively reference signs. Indicating the orientation and relative position is related to the associated figure, and indication of the orientation and/or relative position has to be amended in different figures accordingly as the case may be.

<FIG> shows a schematic view of an electrical switchgear <NUM>, which comprises a gas filling <NUM> in a gas tank <NUM> within a housing <NUM> of the electrical switchgear <NUM>. The gas filling <NUM> has the pressure p, the volume Vgas and the temperature T.

In the gas tank <NUM>, the electrical switchgear <NUM> comprises a capacitive pressure sensor <NUM> with an outer electrode <NUM>, an inner electrode <NUM> and a dielectric material <NUM> in-between as well as a base <NUM>. The dielectric material <NUM> may consist of or comprise an elastomer. The pressure p of the gas filling <NUM> can be calculated based on a volume change of the capacitor formed by the electrodes <NUM> and <NUM> and the dielectric material <NUM>.

Moreover, a temperature sensor <NUM> is arranged on the base <NUM>. The capacitive pressure sensor <NUM> and the temperature sensor <NUM> are connected to an evaluation unit 11a, which comprises a processor <NUM>, a memory <NUM> and a wireless interface <NUM>. Moreover, a signal lamp <NUM> is connected to the evaluation unit 11a. Finally, the electrical switchgear <NUM> comprises an exemplary switching device <NUM> arranged in the gas filling <NUM>.

It should be noted that the gas tank <NUM> may be part of a larger system filled with the gas filling <NUM>. For example, the system may comprise additional chambers, tubes and so on. Accordingly, the pressure sensor <NUM> and/or the temperature sensor <NUM> are not necessarily arranged in the gas tank <NUM> but may be arranged at another position within the system filled with the insulating gas.

The function of the electrical switchgear <NUM> is now as follows:
Initially, the system and thus the gas tank <NUM> are filled with an insulating gas forming the gas filling <NUM> of the electrical switchgear <NUM>. For example, the insulating gas may consist of or comprise Sulfur hexafluoride (SF6). Over time, the gas filling <NUM> may leak from the system, in particular from the gas tank <NUM>, so that the insulating function may decrease. To ensure a minimum of the insulating function, e.g. for the switching device <NUM> and other devices as the case may be, a minimum pressure value plow of the gas filling <NUM> can be defined. Once the pressure p of the gas filling <NUM> drops below this minimum pressure value plow, operation of the electrical switchgear <NUM> can get dangerous. That is why it is advisable to stop the operation of the electrical switchgear <NUM> in such a situation.

To avoid unexpected downtimes of the electrical switchgear <NUM>, the lifetime of the gas filling <NUM> is predicted according to the following method, which comprises the steps of:.

In particular, the total lifetime LTtotal of the gas filling can be calculated by use of the formulas <MAT> <MAT> <MAT> wherein p<NUM> is the first pressure value, p<NUM> is the second pressure value, Δp is the pressure difference between the second pressure value p<NUM> and the first pressure value p<NUM>, t<NUM> is the first point in time and t<NUM> is the second point in time, Δt is the time difference between the second point in time t<NUM> and the first point in time t<NUM>, Δpptu is the nominal pressure drop per time unit (for example per year), LTnominal is the nominal lifetime of the gas filling <NUM>, plow is the minimum pressure value of the gas filling <NUM> and phigh is the maximum pressure value of the gas filling <NUM>.

As already mentioned, plow is the minimum pressure value of the gas filling <NUM> above which safe operation of the electrical switchgear <NUM> is ensured. The maximum pressure value phigh of the gas filling <NUM> normally is the initial pressure when the system or gas tank <NUM> is filled. The nominal lifetime LTnominal of the gas filling <NUM> is the expected lifetime of the gas filling <NUM>, e.g. <NUM> years. The nominal pressure drop per time unit Δpptu corresponds to the nominal lifetime LTnominal of the gas filling <NUM> and the difference between the maximum pressure value phigh and the minimum pressure value plow of the gas filling <NUM>.

A remaining lifetime LTremain of the gas filling <NUM> can be calculated by use of the formula <MAT>.

In detail, the pressure p of the gas filling <NUM> is measured by means of the pressure sensor <NUM>, in this embodiment by measuring the capacitance formed by the electrodes <NUM> and <NUM>. To avoid that fast temperature changes of the gas filling <NUM>, which may be caused by a switching operation of the switching device <NUM>, it is proposed to take pressure values at the same predefined temperature Tp or at least at temperatures T within a predetermined temperature range. In particular switching arcs during opening of the switching contacts of the switching device <NUM> can heat up the gas filling <NUM> quickly and cause spikes in graphs of the pressure p and the temperature T of the gas filling <NUM>. Tests and investigations revealed that there is a time difference between the spikes of the pressure p and the temperature T. Using the gas equation <MAT> wherein R is the gas constant of the gas filling to consider the influence of the temperature T on the pressure p, may lead to invalid lifetime predictions under unfavorable circumstances, in detail when measuring the pressure p and the temperature T is done during such transition phases like switch off of the switching device <NUM>. However, that does not necessarily mean that pressure values are only taken at a predetermined temperature Tp or within a temperature range, but pressure p can be measured continuously, and suitable values can be picked out of the data stream for the lifetime prediction calculation.

So, in detail the processor <NUM> of the evaluation unit 11a may continuously (i.e. in predetermined time intervals) take measurements of the pressure p by use of the pressure sensor <NUM> and measurements of the temperature T by use of the temperature sensor <NUM> and store the same in the memory <NUM>. For calculation of the lifetime of the gas filling <NUM>, measured values may be read from the memory <NUM> and used for the above calculations. Alternatively, the processor <NUM> may monitor the temperature T by use of the temperature sensor <NUM> and measure the pressure p at suitable temperatures.

The predefined temperature Tp or the predefined temperature range may be chosen according to the IEC standard IEC <NUM>-<NUM>/<NUM> or a nominal temperature operating range of the electrical switchgear <NUM>. In this way, suitable predefined temperatures Tp or temperature ranges may be chosen.

In a preferred embodiment, an alarm can be output if the first pressure value p<NUM> or the second pressure value p<NUM> drops below the minimum pressure value plow of the gas filling <NUM>. For example, this alarm can be output via the signal lamp <NUM> and/or the wireless interface <NUM> so as to inform responsible personnel. In this way, exceptional pressure drops can be recognized.

In another embodiment, the total lifetime LTtotal of the gas filling <NUM> can be calculated by use of the formula <MAT> and the remaining lifetime LTremain of the gas filling <NUM> can be calculated by use of the formula <MAT>.

In all embodiments, beneficially the first pressure value p<NUM> can be replaced by p1corr and referenced to a reference temperature Tref and/or the second pressure value p<NUM> can be replaced by p2corr and referenced to a reference temperature Tref in the aforementioned formulas by use of the formulas <MAT> <MAT> wherein p1corr is the first corrected pressure value and p2corr is the second corrected pressure value. By these measures, the pressure p may be determined more accurately when a temperature range is defined for taking pressure measurements. Preferably, the reference temperature Tref is within said temperature range.

Generally, the pressure p of the gas filling <NUM> can be calculated based on a volume change of the capacitor formed by the electrodes <NUM> and <NUM> and the dielectric material <NUM> as mentioned hereinbefore. In detail, a deformation of the elastomer or dielectric material <NUM> based on a pressure change Δp causes a proportional volume change ΔV in view of an initial volume V<NUM> at an initial pressure. The proportionality factor usually is denoted B, and the formula for the pressure change Δp reads as <MAT>.

A volume change ΔV may easily be calculated based on a change of the capacitance of the capacitive pressure sensor <NUM>. Provided that the capacitor is ball shaped, the capacitance C can be calculated by use of the formula <MAT> wherein ro is the radius of the outer flexible electrode <NUM>, ri is the radius of the inner (rigid) electrode <NUM>, ε<NUM> is the absolute permittivity or absolute dielectric constant and εr is the relative permittivity or relative dielectric constant. Based on the known the radius ri of the inner (rigid) electrode <NUM>, the radius ro of the outer flexible electrode <NUM> at an initial pressure and an actual pressure, the volume change ΔV and thus the pressure change Δp can easily be calculated.

<FIG> finally shows a more detailed embodiment of an evaluation unit 11b, which in addition to the parts of the evaluation unit 11a of <FIG> comprises a signal injector <NUM>, a signal conditioner <NUM>, a signal amplifier <NUM> and a capacitor <NUM>. The signal injector <NUM> is provided to apply a signal to the serial connection of the capacitor <NUM> and the capacitive pressure sensor <NUM>. The signal conditioner <NUM> is provided to generate a pressure signal based on the voltage across the capacitor <NUM> and based on the signal of the temperature sensor <NUM>. The signal amplifier <NUM> finally amplifies the signal from the signal conditioner <NUM> and outputs its signal to the processor <NUM>.

It is noted that the invention is not limited to the embodiments disclosed hereinbefore, but combinations of the different variants are possible. In reality, the electrical switchgear <NUM> and the evaluation unit 11a, 11b may have more or less parts than shown in the figures. Moreover, the description may comprise subject matter of further independent inventions.

Claim 1:
Method of predicting a lifetime of a gas filling (<NUM>) of an electrical switchgear (<NUM>), comprising the steps of
a) measuring a first pressure value p<NUM> in a system of the electrical switchgear (<NUM>) containing the gas filling (<NUM>) at a predefined temperature Tp at a first point in time t<NUM> and measuring a second pressure value p<NUM> in the system of the electrical switchgear (<NUM>) containing the gas filling (<NUM>) at the same predefined temperature Tp at a second point in time t<NUM> or
b) measuring a first pressure value p<NUM> in a system of the electrical switchgear (<NUM>) containing the gas filling (<NUM>) at a first temperature T<NUM> within a predefined temperature range at a first point in time t<NUM> and measuring a second pressure value p<NUM> in the system of the electrical switchgear (<NUM>) containing the gas filling (<NUM>) at a second temperature T<NUM> within the same predefined temperature range at a second point in time t<NUM>,
- calculating a pressure difference Δp between the first pressure value p<NUM> and the second pressure value p<NUM> and
characterized in that the method further comprises the steps of:
- calculating a total lifetime of the gas filling (<NUM>) based on said pressure difference Δp, wherein the first pressure value p<NUM> and/or the second pressure value p<NUM> is calculated based on a volume change of a capacitor having an elastomer as a dielectric material (<NUM>) between two electrodes (<NUM>, <NUM>) of the capacitor, and
- wherein the total lifetime LTtotal of the gas filling is calculated using the formula: <MAT>
wherein plow is a predefined minimum pressure value of the gas filling (<NUM>), phigh is the initial pressure value of the gas filling (<NUM>), and Δt is the time difference between the second point in time t<NUM> and the first point in time t<NUM>.