# US 2016/0252564 A1 · Method and apparatus for monitoring capacitor bushings for a three-phase ac system

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- Number: US 2016/0252564 A1
- Document ID: US20160252564A1
- Status: Published application
- Kind: A1
- Application number: 15/027,772
- Filed: 2014-11-11
- Published: 2016-09-01
- Earliest claimed: 2013-11-15
- Assignees: Maschinenfaberik Reinhausen GmbH
- Applicants: Maschinenfaberik Reinhausen GmbH
- Inventors: Junliang WU; Karsten VIERECK; Ulrich SUNDERMANN
- Web page: https://patentgrep.com/patent/US20160252564A1

## CPC

- G01R 31/1263: {of solid or fluid materials, e.g. insulation films, bulk material; of semiconductors or LV electronic components or parts; of cable, line or wire insulation}
- G01R 15/06: having reactive components, e.g. capacitive transformer
- G01R: MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES (indicating correct tuning of resonant circuits H03J3/12)

## Abstract

A method for monitoring capacitor bushings (2a, 2b, 2c) for a three-phase AC system, wherein each capacitor bushing (2a, 2b, 2c) has a conductor (4), which is connected to one of the system lines (5a, 5b, 5c) of the AC system, and an electrically conductive lining (3) that surrounds the conductor (4), comprises the steps that—an upper capacitance (C0a, C0b, C0c) and a lower capacitance (C1a, C1b, C1c) are ascertained for each capacitor bushing (2a, 2b, 2c); —a measured voltage (U1a, U1b, U1c) is detected at each capacitor bushing (2a, 2b, 2c), which measured voltage is present between the respective lining (3) and an earth potential (13); —an actual capacitance (C0a′, C0b′, C0c′) is computed for each capacitor bushing (2a, 2b, 2c), which actual capacitance is dependent on the respective measured voltage (U1a, U1b, U1c), the respective lower capacitance (C1a, C1b, C1c) and on the measured voltage (U1b, U1c, U1a), the lower capacitance (C1b, C1c, C1a) and the upper capacitance (C0b, C0c, C0a) of one of the other capacitor bushings (2a, 2b, 2c); the respective upper capacitance (C0a, C0b, C0c) is compared with the respective actual capacitance (C0a′, C0b′, C0c′) for each capacitor bushing (2a, 2b, 2c); a monitoring signal is produced that is dependent on the results of the capacitance comparisons.

## Claims

### Claim 1

A method of monitoring capacitor bushings for a three-phase alternating current mains, where each capacitor bushing comprises a conductor connected with one of the mains lines of the alternating current mains, and an electrically conductive lining surrounding the conductor, the method comprising the steps of:
determining an upper capacitance and a lower capacitance for each capacitor bushing;
detecting a measured voltage present between the respective lining and a ground potential at each capacitor bushing;
calculating an actual capacitance for each capacitor bushing, the actual capacitance depending on the respective measured voltage and the respective lower capacitance as well as on the measured voltage, the lower capacitance and the upper capacitance of one of the other capacitor bushings;
for each capacitor bushing comparing the respective upper capacitance with the respective actual capacitance; and
generating a monitoring signal dependent on the results of the capacitance comparison.

### Claim 2 (depends on 1)

The method according to claim 1, further comprising the steps of:
detecting the mains voltage for each phase;
comparing the detected mains voltages with one another; and
if the voltage comparison has the result that the mains voltages differ from one another by not more than a predetermined amount carrying out the calculation of the actual capacitances, the comparison of the capacitances and the generation of the monitoring signal.

### Claim 3 (depends on 2)

The method according to claim 2, wherein effective values and/or peak values and/or amplitudes of the mains voltages are used in the voltage comparison.

### Claim 4 (depends on 3)

The method according to claim 3, further comprising the steps of:
determining tolerance values UAB>0, UBC>0, UCA>0 for the voltage comparison; and
carrying out the voltage comparison to check whether |Uae−Ube|≦UAB and |Ube−Uce|≦UBC and |Uce−Uae|≦UCA.

### Claim 5 (depends on 1)

The method according to claim 1, wherein
the actual capacitance of the first capacitor bushing is calculated according to the following formula:
C
0

a
′
=
C
1

a
×
U
1

a
×
K
a
U
1

b
×
C
1

b
C
0

b
+
U
1

b
-
U
1

a
×
K
a
where Ka is a correction value for which Ka=1 or Ka=Ub/Ua; and/or
the actual capacitance of the second capacitor bushing is calculated according to the following formula:
C
0

b
′
=
C
1

b
×
U
1

b
×
K
b
U
1

c
×
C
1

c
C
0

c
+
U
1

c
-
U
1

b
×
K
b
where Kb is a correction value for which Kb=1 or Kb=Uc/Ub; and/or
the actual capacitance of the third capacitor bushing is calculated according to the following formula:
C
0

c
′
=
C
1

c
×
U
1

c
×
Kc
U
1

a
×
C
1

a
C
0

a
+
U
1

a
-
U
1

c
×
Kc
where Kc is a correction value for which Kc=1 or Kc=Ua/Uc.

### Claim 6 (depends on 1)

The method according to claim 1, wherein
tolerance values CA>0, CB>0, CC>0 for the capacitance comparisons are determined; and
if the capacitance comparisons have the result that |C0a′−C0a|≦CA and |C0b′−C0b|≦CB and |C0c′−C0c|≦CC
a monitoring signal is generated that indicates that the capacitor bushings are in an orderly state.

### Claim 7 (depends on 6)

The method according to claim 6, wherein otherwise a monitoring signal is generated that indicates that at least one capacitor bushing is not in an orderly state.

### Claim 8 (depends on 1)

The method according to claim 1, further comprising the steps of:
determining tolerance values CA>0, CB>0, CC>0 for the capacitance comparisons;
if the capacitance comparisons have the result that C0a′−C0a<−CA and C0b′−C0b CB and |C0c′−C0c|≦CC
generating a monitoring signal that indicates that at least the second capacitor bushing is not in an orderly state;
if the capacitance comparisons have the result that C0b′−C0b<−CB and C0c′−C0c CC and |C0a′−C0a|≦CA
generating a monitoring signal that indicates that at least the third capacitor bushing is not in an orderly state; and
if the capacitance comparisons have the result that C0c′−C0c<−CC and C0a′−C0a CA and |C0b′−C0b|≦CB
generating a monitoring signal that indicates that at least the first capacitor bushing is not in an orderly state.

### Claim 9 (depends on 8)

The method according to claim 8, wherein
otherwise a monitoring signal is generated that indicates that at least two capacitor bushings are not in an orderly state.

### Claim 10 (depends on 1)

The method according to claim 1, further comprising the steps of:
determining tolerance values CA>0, CB>0, CC>0 for the capacitance comparisons;
if the capacitance comparisons have the result that C0a′−C0a CA and C0b′−C0b<−CB and |C0c′−C0c|≦CC
generating a monitoring signal that indicates that the first and third capacitor bushings are not in an orderly state and have an identical fault;
if the capacitance comparisons have the result that C0b′−C0b CB and C0c′−C0c<−CC and |C0a′−C0a|≦CA
generating a monitoring signal that indicates that the second and first capacitor bushings are not in an orderly state and have an identical fault; and
if the capacitance comparisons have the result that C0c′−C0c CC and C0a′−C0a<−CA and |C0b′−C0b|≦CB
generating a monitoring signal that indicates that the third and second capacitor bushings are not in an orderly state and have an identical fault.

### Claim 11

An apparatus for monitoring capacitor bushings for a three-phase alternating current mains, wherein each capacitor bushing comprises a conductor, which is connected with the one of the mains lines of the alternating current mains, and an electrically conductive lining surrounding the conductor where the apparatus is constructed for carrying out a method according to and comprises:
a measuring device;
a measuring adapter for each phase which adaptor can be connected with the lining of the capacitor bushing belonging to the respective phase and is connected with the measuring device so as to detect a first electrical measurement variable; and
an evaluating device that is connected with the measuring device so as to communicate the first measurement variable to the evaluating device.

### Claim 12 (depends on 11)

The apparatus according to claim 11, additionally comprising:
a voltage converter for each phase, which converter can be connected with the mains line belonging to the respective phase so as to detect a second electrical measurement variable and is connected with the evaluating device so as to communicate the second measurement variable to the evaluating device.

### Claim 13 (depends on 12)

The apparatus according to claim 12, wherein
each voltage converter is constructed as a capacitive voltage converter or inductive voltage converter or resistive voltage converter.

### Claim 14 (depends on 13)

The apparatus according to claim 13, wherein
the measuring device comprises at least one measuring capacitor.

### Claim 15 (depends on 14)

The apparatus according to claim 14, wherein
the measuring device comprises three measuring capacitors;
in each phase a measuring adapter is connected with a measuring capacitor associated therewith; and
the capacitances of these three measuring capacitors are different.

### Claim 16 (depends on 15)

The apparatus according to claim 15, wherein
the three capacitances are in the ratio 1:2:3, 1:2:4, 1:2:5, 1:3:5, 1:3:7, 1:3:9, 1:4:7 or 1:4:9.

### Claim 17 (depends on 11)

The apparatus according claim 11, wherein:
the first electrical measurement variables are voltages respectively present at a lower voltage capacitor of the corresponding phase.

### Claim 18 (depends on 11)

The apparatus according to any one claim 11, wherein
the second electrical measurement variables are voltages respectively present between the corresponding mains line and ground potential.

### Claim 19 (depends on 11)

The apparatus according to any one of claim 11, wherein
the evaluating device is so constructed that it can calculate an actual capacitance for each capacitor bushing the actual capacitance depending on the respective measured voltage and a lower capacitance of the respective capacitor bushing as well as on the measurement voltage, a lower capacitance and an upper capacitance of one of the other capacitor bushings.

### Claim 20 (depends on 11)

The apparatus according to any one of claim 11, wherein the evaluating device is so constructed that it can compare the respective upper capacitance with the respective actual capacitance for each capacitor bushing.

### Claim 21

(canceled)

### Claim 22 (depends on 1)

An apparatus for monitoring capacitor bushings for a three-phase alternating current mains, wherein each capacitor bushing comprises a conductor, which is connected with the one of the mains lines of the alternating current mains, and an electrically conductive lining surrounding the conductor wherein the apparatus is constructed as an apparatus for carrying out a method according to claim 1 and comprises
means constructed to detect at each capacitor bushing a measurement voltage present between the respective lining and a ground potential;
means constructed to calculate for each capacitor bushing an actual capacitance that depends on the respective measurement voltage and a lower capacitance of the respective capacitor bushing as well as on the measurement voltage, a lower capacitance and an upper capacitance of one of the other capacitor bushings;
means constructed to compare the respective upper capacitance with the respective actual capacitance for each capacitor bushing; and
means constructed to produce a monitoring signal dependent on the results of the capacitance comparisons.

### Claim 23 (depends on 22)

The apparatus according to claim 22, additionally comprising
means constructed to detect the mains voltages of each phase;
means constructed to compare the mains voltages with one another; and
means constructed to cause or carry out calculation of the actual capacitances, comparison of the capacitances and generation of the monitoring signal if the voltage comparison has the result that the mains voltages differ from one another by not more than a predetermined amount.

### Claim at position 24

24-25. (canceled)