The invention relates to a combination of a conductor, such as a busbar, and a device for measuring the AC voltage in the conductor, which device comprises:

- an insulation layer arranged on the conductor;

- a capacitor plate arranged on the insulation layer to position the capacitor plate at a fixed distance from the conductor to form a first capacitor;

- a second capacitor arranged electrically between the capacitor plate and ground to provide a capacitive voltage divider with the first capacitor; and

- means for measuring the voltage at the capacitor plate .

Such a combination is for example known from US 5017859. Capacitive voltage dividers are known to reduce the voltage of the conductor to a level, which is more suitable to be measured with conventional measurement equipment. The use of capacitors allows one to measure the voltage without any leakage of current .

A resistive voltage divider could also be used for measuring the voltage of a conductor, but with high voltages, the components become considerably large and current will leak via the resistors.

The use of a capacitive voltage divider allows one to use smaller components, as no current will flow through the capacitors. However, the capacitors have an impedance which is dependent on the frequency of the voltage to be measured. As long as the frequency is virtually constant, the voltage in the conductor can be measured accurately. However, if the frequency varies, the accuracy of the measurements will vary.

Furthermore, with the combination according to US 5017859, the distance of the capacitor plate to the conductor could vary due to manufacturing tolerances or due to heating of the combination, which causes the insulation layer to expand. As a result the capacity of the first capacitor will change having an adverse effect on the voltage measurement.

It is an object of the invention to reduce or even remove the above mentioned disadvantages.

This object is achieved according to the invention with a combination according to the preamble, which is characterized by:

- means for measuring the frequency of the voltage in the conductor; and

- means for calculating the AC voltage in the conductor based on the capacities of the first and second capacitors, the measured voltage and the measured frequency.

Because the frequency of the voltage in the conductor is also measured, one can take into account the changes in the capacitive voltage divider due to any frequency changes .

The means for calculating the AC voltage in the conductor will first determine, based on the known capacity of the first and second capacitors and the measured frequency, what the capacitive reactance of each capacitor is. Similar equations can be used for the capacitive reactance as for resistors. So, with the measured voltage at the capacitor plate and the ratio between both capacitive reactances of the two capacitors, one can accurately calculate the AC voltage in the conductor.

In a preferred embodiment of the combination

according to the invention the means for calculating the AC voltage in the conductor comprise a microcontroller with an analog to digital converter input connected via a low pass filter, precision rectifier to the capacitor plate.

Calculations can be quickly and accurately done by a microcontroller. However, the AC voltage needs to be

rectified, such that the voltage is only of a changing

positive value, which can then be fed to low pass filter and then to the analog to digital converter for further processing by the microcontroller.

Another preferred embodiment of the combination according to the invention, further comprises a temperature sensor for measuring the temperature of the insulation layer and wherein the means for calculating the AC voltage in the conductor take into account the measured temperature.

When the temperature of the insulation layer

changes, the insulation layer will typically expand or

compress, such that the distance of the capacitor plate to the conductor will change. Especially in high voltage

applications, in which the insulation layer will typically be an epoxy resin, temperatures can rise substantially and expansion of the epoxy resin will be considerable. This will cause the capacity of the first capacitor to reduce when the temperature rises.

In order to take the changes in the capacity of the first capacitor into account, the temperature is measured and based on an equation or based on calibration data, the changes in the capacity of the first capacitor will be taken into account based on the measured temperature.

Yet another embodiment of the combination according to the invention, further comprises storage means for storing calibration data and wherein the means for calculating the AC voltage in the conductor take into account the calibration data out of the storage means.

Although the manufacturing process could be very accurate, there will still be some small differences between similar combinations. In order to further increase the

accuracy of the calculation of the AC voltage in the

conductor, a combination can be tested with known voltages directly after manufacturing. The resulting data can be stored in the storing means, such that this calibration data can be taken into account during the calculation of the AC voltage in the conductor.

In a preferred embodiment of the combination

according to the invention, the combination is a high voltage bushing .

These and other features of the invention will be elucidated in conjunction with the accompanying drawing.

Figure 1 shows a schematic view of an embodiment of the combination according to the invention.

A bushing 1 is shown in figure 1 having a conductor

2 embedded in an epoxy resin 3. A capacitor plate 4 is

concentrically around the conductor 2 with the epoxy resin 3 serving as insulation layer in between. Furthermore, a

temperature sensor 5 is embedded near capacitor plate 4 in the epoxy resin 3 for measuring the temperature of the epoxy resin

3.

The conductor 2 and the capacitor plate 4 embody a first capacitor, which is connected to a second capacitor 6, which in turn is connected to ground 7. The first capacitor 2,

4 and the second capacitor 6 provide a capacitive voltage divider .

The voltage of the capacitor plate 4 is first fed to a buffer and low pass filter 8 to protect the electronics behind the buffer and low pass filter 8, as well to filter any high frequency noise.

The voltage of the capacitor plate 4 is then fed via a precision full wave rectifier 9 and low pass filter 10 to an analog to digital converter 11 of a microcontroller 12 for converting to a digital value of the measured voltage.

The voltage of the capacitor plate 4 is also fed to a zero crossing detector type voltage converter 13, which feeds to the microcontroller 12for frequency measurement.

Also the signal of the temperature sensor 5 is fed via a low pass filter 14 and an amplifier 15 to the

microcontroller 12 for measuring and taking into account the epoxy inner temperature.

Taking into account the epoxy temperature, the microcontroller 12 can calculate based on the digital value of the measured voltage on the capacitor plate 4, the measured frequency and the known capacities of the first capacitor 2, 4 and the second capacitor 6, what the real voltage in the conductor 2 is.

Using the temperature measurements of the temperature sensor 5, the microcontroller 12 can

calculates the change in the first capacitor 2, 4 capacity due to the expansion or compression of the epoxy resin for any change in the epoxy inner temperature using a software algorithm and the stored pre-test data in the

microcontroller. The microcontroller 12 adjusts the measured voltage as per the new calculated first capacitor 2,4 capacity using the software algorithm for accurate voltage measurement.