SE1400395A1 - Method for sensing a space charge characteristic - Google Patents

Description

15 20 25 30 35 l1850SE 2 13.08.2014 locations of these charges can help to determine the mechanisms for charge accumulation and facilitate mitigating dielectric breakdown. Techniques for measuring the spatial locations of these space charges include acoustic methods, wherein an acoustic pulse is excited and propagates through the sample under consideration. One acoustic method is the pulsed electro-acoustic (PEA) method, which may involve applying an extemal electric field impulse over the sample, which produces a mechanical pulse on the charges embedded in the dielectric. The external electric field induces a perturbation force on the charges, causing a slight and rapid oscillation of the charges whereby mechanical (acoustic) vibrations in the dielectric are excited. Thus, the perturbation forces on the charges generate acoustic pressure waves, which acoustic pressure waves are proportional to the charge distribution in the sample. The acoustic pressure waves can be detected by a (e. g., piezoelectric) transducer, or pressure transducer or pressure sensor, on one side of the dielectric. By measuring or detecting the anival time and magnitude of the acoustic pressure waves, the output signal from the transducer can be used to derive information on the magnitude and location of the charges in the dielectric. An example application of the PEA method is to measure the space charge distribution in an XLPE cable or another cable including a solid insulation material, e. g. polyethylene. As discussed above, a voltage pulse may be applied to the electrically insulated layer of the cable, and an acoustic response which depends on the charge distribution in the electrically insulated layer may be detected or measured. The acoustic response may for example be measured or sensed by means of a pressure sensor which may be based on a polyvinylidene fluoride, or polyvinylidene difluoride (PVDF), film or Lithium Niobate (LiNbOz).

SUMMARY PEA measurements are usually carried out within a so called measurement cell or compartment, in which the sample and possibly also the pressure sensor are located. PEA measurements may be perfonned at constant or substantially constant temperature conditions, for example at room temperature (about 20-25 °C), at about 50 °C, or at about 70 °C. When PEA measurements are perfonned with the sample being heated so as to attain an elevated temperature, heat may be provided to the measurement cell to ensure that the sample temperature at the site or location on the sample where the measurements are made accurately represents the temperature of the overall sample. The pressure sensor, which as mentioned above may be located within the measurement cell, may then also attain an elevated temperature. The output from the pressure sensor is in general temperature dependent. The speed of sound in the electrically insulated layer (e. g., including XLPE) is temperature dependent. 10 15 20 25 30 35 11850SE 3 13.08.2014 When performing PEA measurements, the response or output from the pressure sensor from a first, initial application of an external electric field impulse or voltage pulse over the sample may be used for calibration, or normalization, of subsequently obtained measurement data, or output sigr1al(s) from the pressure sensor, obtained from subsequent PEA measurements. It may be desired or required to perfonn subsequent PEA measurements during temperature cycling of the sample, e. g. during a period when the sample is repeatedly heated and subsequently allowed to cool (or even is actively cooled, using some appropriate means as known in the art, between periods of heating). In that case, the temperature of the sample and the temperature of the pressure sensor may differ to some extent, at least during a part or parts of the PEA measurement process, from the temperature which the pressure sensor had during the calibration process of the initial application of an extemal electric field impulse or voltage pulse over the sample and the obtaining of the corresponding response or output from the pressure sensor. Since, as mentioned above, the output from the pressure sensor is in general temperature dependent, that difference may introduce an error in infonnation on the magnitude and location of the charges in the sample derived from the output signal from the pressure sensor. In tum, this could result in introduction of error or inaccuracy in information derived from PEA measurements when carried out during non- constant temperature conditions of the power cable, for example during temperature cycling of the power cable, e. g. during a period when the power cable is repeatedly heated and subsequently allowed to cool (or even is actively cooled between periods of heating).

To that end, according to embodiments of the present invention, a correction factor may used when deriving infonnation on e. g. the magnitude and location of the charges in the sample from the output signal from the pressure sensor, which correction factor accounts for any difference between the temperature of the sample and the temperature of the pressure sensor during for example temperature cycling of the sample and the temperature which the pressure sensor had during the initial calibration process.

According to an aspect of the present invention, there is provided a method for sensing a space charge characteristic of an electrically insulating (or insulated), dielectric solid material, e. g., for sensing space charge distribution in the material. The method comprises: (i) applying an external electric field impulse to the material, whereby acoustic pressure waves are generated within the material; (ii) using a pressure transducer, sensing the acoustic pressure waves on one side of the material, wherein output from the pressure transducer depends on temperature of the pressure transducer; and (iii) based on the acoustic pressure waves, generating a signal indicative of at least one of magnitude and location of space charges in the material. 10 15 20 25 30 35 l1850SE 4 13.08.2014 The steps (i)-(iii) may be carried out repeatedly at varying temperatures of the material such that the generated signals correspond to respective temperatures. A temperature correction factor may be applied to at least one of the generated signals. The temperature correction factor may be a function of temperature, and may be based on temperature dependence of output from the pressure transducer.

By application of a temperature correction factor to the signal which is indicative of at least one of magnitude and location of space charges in the material, which temperature correction factor is a function of temperature and is based on temperature dependence of output from the pressure transducer, accuracy in for example the determined magnitude or location of space charges in the electrically insulated, dielectric solid material may be increased, for example for the case where the steps (i)-(iii) are carried out repeatedly at varying temperatures of the material. This case may occur for example during temperature cycling of the material, such as during a period when the material is repeatedly heated and subsequently allowed to cool (or even is actively cooled between periods of heating).

The temperature dependence of output from the pressure transducer, on which the temperature correction factor is at least in part based, can for example be determined by means of carrying out steps (i)-(iii) in which a well-defined external electric field impulse or voltage impulse is applied to the material for a number of different temperatures of the site or location on the material where the external electric field impulses or voltage impulses are applied and of the pressure transducer. For example, the method may hence further comprise determining temperature dependence of output fi°om the pressure transducer by carrying out steps (i)-(iii) repeatedly, wherein a predefined external electric field impulse is applied to one side of the material in step (i) for a number of different temperatures of the material where the external electric field impulse is applied, and of the pressure transducer, respectively.

The application of an extemal electric field impulse to the material may be carried out for example by any appropriate electrical field generating means as known in the art. Such means may perhaps most simply be implemented by a pair of electrodes coupled to a power (voltage) source, between which electrodes the material can be arranged.

The application of an extemal electric field impulse to the material may be carried out by means of generation of a voltage pulse over or across the material. The voltage pulse may be generated over or across the material at an instant during a period when there is a DC voltage applied over or across the material. The applied DC voltage may at least in part cause charges to accumulate at the outer surface of the material and possibly within the insulating material.

According to another example, a DC voltage is applied over or across the material. At some point in time, the material is connected to ground (earth), and a voltage pulse over or across the material is generated, so as to cause an external electric field impulse to be applied to the material. 10 15 20 25 11850SE 5 13.08.2014 According to examples, the applied DC voltage may be relatively high, e. g. about 100 kV or more, and may for example be applied during a relatively long period of time, e. g. an hour or more. The voltage pulse generated over or across the material may for example have a duration of one or a few nanoseconds, and may have a relatively low magnitude, e. g. about 1 kV.

The generation of signal indicative of at least one of magnitude and location of space charges in the material based on acoustic pressure waves sensed by the pressure transducer, and possibly the application of the temperature correction factor to the generated signal, may be carried out by means of any appropriate processing unit, such as a general central processing unit (CPU) or the like, which may be directly or indirectly coupled to the pressure transducer.

Inforrnation derived from PEA measurements may for example include a so called Field Enhancement Factor (FEF), which may be defined as: FEF = Ax / D, where D = (U / Ufef) Axfef.

Axis a sensed signal indicative of location of space charge in the material, and Axfef is the corresponding value obtained during an initial calibration process. U is the DC voltage applied to the material, and Ufef is the corresponding value obtained during the initial calibration process.

Claims (2)

10 15 20 ll850SE 6 13.08.2014 CLAIMS

1. l. A method for sensing a space charge characteristic of an electrically insulating, dielectric solid material, the method comprising: (i) applying an external electric field impulse to the material, whereby acoustic pressure waves are generated within the material; (ii) using a pressure transducer, sensing the acoustic pressure Waves on one side of the material, wherein output from the pressure transducer depends on temperature of the pressure transducer; and (iii) based on the acoustic pressure waves, generating a signal indicative of at least one of magnitude and location of space charges in the material; wherein the steps (i)-(iii) are carried out repeatedly at varying temperatures of the material such that the generated signals correspond to respective temperatures; the method further comprising: applying a temperature correction factor to at least one of the generated signals, the temperature correction factor being a function of temperature and being based on temperature dependence of output from the pressure transducer.

2. A method according to claim 1, further comprising determining temperature dependence of output from the pressure transducer by carrying out steps (i)-(iii) repeatedly wherein a predefined extemal electric field impulse is applied to one side of the material in step (i) for a number of different temperatures of the material where the external electric field impulse is applied, and of the pressure transducer, respectively.