An apparatus for detecting partial discharge in electrical equipment

Abstract

First conductor 104 comprises two conductive layers 104A, 104B separated by a dielectric. Connecting voltage source 108 provides an electromagnetic field which interacts with a field of microwave sens

Description

FIELD This disclosure relates to an apparatus for detecting partial discharge in electrical equipment and in particular to an apparatus for detecting partial discharge in underground power cables, transmission lines and other electrical equipment where partial discharge is likely to occur. BACKGROUND Partial discharge is caused by the localised electrical breakdown of electric insulation materials under high voltage stress. Detecting and measuring partial discharge events is a useful way of identifying underlying electrical breakdown and other incipient faults in electrical apparatus, to enable preventative maintenance interventions. Partial discharge can occur in cable insulation, cable joints and accessories, and cable terminations. For the detection of partial discharge, several methods exist. These methods can be classified into two types: online and offline. Online detection is carried out without disconnecting the component under observation from the power system, while offline detection requires putting the component out of service. For online detection, partial discharge occurs under operational system parameters such as voltage level. However, offline methods rely on a dedicated source to send pulses through the observed component. Online methods, although less accurate than offline methods, take precedence as they are non-invasive. Existing methods of detecting partial discharge events involve the use of sensors attached to the apparatus being monitored to detect electrical pulses caused by partial discharge events. These sensors include High-Frequency Current Transformers, Transient Earth Voltage sensors, ultrasonic sensors, and Ultra-High Frequency sensors to name but a few examples. Different combinations of these sensors may be utilized for monitoring partial discharge in different power system components. The most common method for the detection and localization of partial discharge in underground cables is the use of High-Frequency Current Transformers. These sensors can detect the high-frequency electrical current generated due to partial discharge, which travels along the cable. However, the length of cable monitored by such a system is limited to a few kilometres. Due to the low energy content of the electrical signal, this type of sensing can often be inaccurate if system noise is not effectively filtered. For underground power cables, partial discharge sensing may primarily be done in two ways: estimation from high frequency partial discharge current sensed at cable terminals using High-Frequency Current Transformers and optic fibre sensing. The first method is prone to error due to noise and attenuation during signal transmission, while second method requires extensive sensing fibre layout along the length of the power cables. The accuracy of determined location of partial discharge activity by High-Frequency Current Transformers is dependent on several factors, including length of monitored cable and the transmission voltage level. Precise partial discharge sensing for underground cables may also be performed by sensing microwaves generated during discharge. However, microwave partial discharge sensing for underground cables may be impractical for at least the following reasons. Firstly, the microwaves produced from partial discharge can be of any frequency ranging from a few kilohertz up to several hundred megahertz and sensors capable of detecting such a broad range of frequencies are expensive and complex. Secondly, attenuation of partial discharge microwaves before reaching the surface makes them difficult to detect from above ground. There is accordingly scope for improvement. The preceding discussion of the background is intended only to facilitate an understanding of the present disclosure. It should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was part of the common general knowledge in the art as at the priority date of the application. SUMMARY In accordance with an aspect of the disclosure there is provided an apparatus for detecting partial discharge in electrical equipment, the apparatus comprising: a microwave sensor operable at a given frequency; a first conductor disposed adjacent the sensor to define a distance between the first conductor and sensor, wherein the first conductor includes two conductive layers separated by a dielectric, the layers configured to be connectable to a voltage source for providing an electromagnetic field between them which interacts with a field of the sensor to cause a shift in a frequency response of the sensor; and a control module configured to control the shift in frequency response by controlling a voltage applied to the first conductor, thereby tuning the sensor to a different frequency for sensing a partial discharge. The apparatus may include a plurality of additional conductors each disposed adjacent one another and each connectable to a voltage source and the control module may be configured to control a shift in frequency response by successively controlling a voltage applied to each of the first conductor and additional conductors thereby tuning the sensor to a multitude of frequencies for sensing a partial discharge. The distance between the sensor and first conductor may be less than a quarter-wavelength Further, the distance between the sensor and first conductor and the distance between each subsequent additional conductor may be less than a quarter-wavelength The sensor may be configured to be connectable to a current for reducing reflective losses to improve directivity of the sensor at a given frequency and the control module may be configured to control the directivity by controlling a current applied to the sensor. Applying a positive current may increases directivity and applying a negative current may decreases directivity of the sensor. The apparatus may be operable to detect surface partial discharge by frequency tuning and may be operable to detect underground partial discharge by both frequency tuning and directivity tuning. In accordance with an aspect of the disclosure there is provided a method for detecting partial discharge in electrical equipment, the method comprising: providing a microwave sensor operable at a given frequency; disposing a first conductor adjacent the sensor to define a distance between the first conductor and sensor, wherein the first conductor includes two conductive layers separated by a dielectric, connecting the layers to a voltage source for providing an electromagnetic field between them which interacts with a field of the sensor to cause a shift in a frequency response of the sensor; and controlling the shift in frequency response by controlling a voltage applied to the first conductor, thereby tuning the sensor to a different frequency for sensing a partial discharge. The method may include disposing the first conductor a distance of less than a quarterwavelength from the sensor; scanning for a partial discharge event at the given frequency of the microwave sensor; if no partial discharge event is detected, controlling a voltage applied to the first conductor to cause a shift in the frequency response of the sensor; and scanning for a partial discharge event at the new frequency. The method may include disposing a plurality of additional conductors adjacent one another; and connecting each conductor to a voltage source. The first conductor and each subsequent additional conductor may be disposed a distance of less than a quarter-wavelength from one another and the method may include scanning for a partial discharge event at the given frequency of the microwave sensor; if no partial discharge event is detected, successively controlling a voltage applied to each of the first conductor and additional conductors thereby tuning the sensor to a multitude of frequencies; and scanning for a partial discharge event at each new frequency. The method may include connecting a current to the sensor for reducing reflective losses to improve directivity of the sensor at a given frequency and controlling the directivity by controlling a current applied to the sensor. Applying a positive current may increase the directivity of the sensor and applying a negative current may reduce the directivity of the sensor. The method may include scanning for a partial discharge event at the given frequency of the microwave sensor; controlling the current applied to the sensor to increases its directivity; and scanning for a partial discharge event. The method may include, if a partial discharge event is detected, collecting and storing the sensor data for further analysis including analysing a partial discharge event to determine one or more of a partial discharge magnitude, partial discharge location, or a cavity size within an insulation for internal partial discharge. Embodiments of the technology will now be described, by way of example only, with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Figure 1 is a schematic diagram which illustrates an exemplary apparatus for detecting partial discharge in electrical equipment according to aspects of the present disclosure; Figure 2 is a schematic diagram which illustrates an exemplary apparatus for detecting partial discharge in electrical equipment according to an embodiment of the present disclosure; Figure 3 is a plot which illustrates an exemplary shift in frequency response of a microwave sensor upon controlling a voltage applied to a conductor of the apparatus of Figure 2; Figure 4 is a plot which illustrates an exemplary improvement of reflective losses of a microwave sensor upon controlling a current applied to the sensor; Figure 5 is a schematic diagram which illustrates an exemplary improvement in directivity of a microwave sensor upon controlling a current applied to the sensor; Figure 6 is a schematic diagram which illustrates an exemplary apparatus for detecting partial discharge in electrical equipment according to another embodiment of the present disclosure; Figure 7 is a plot which illustrates exemplary shifts in frequency response of a microwave sensor upon successively controlling a voltage applied to each conductor of a plurality of conductors of the embodiment of Figure 6; Figure 8 illustrates an exemplary method for detecting partial discharge in electrical equipment according to an embodiment of the present disclosure; and Figure 9 illustrates an exemplary method for detecting partial discharge in electrical equipment according to another embodiment of the present disclosure. DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS Aspects of the present disclosure provide an apparatus for detecting partial discharge in electrical equipment and in particular for detecting partial discharge in underground power cables, transmission lines and other electrical equipment where partial discharge is likely to occur. The apparatus may include a microwave sensor operable at a given frequency. The microwave may be used to scan for partial discharge at the given frequency. However, the microwaves produced from partial discharge can be of any frequency ranging from a few kilohertz up to several hundred megahertz and the microwave sensor may not be capable of detecting such a broad range of frequencies. The apparatus may include a conductor disposed adjacent the sensor to define a distance between the conductor and sensor. The conductor may include two conductive layers separated by a dielectric. The layers may be configured to be connectable to a voltage source for providing an electromagnetic field between them which interacts with a field of the sensor to cause a shift in a frequency response of the sensor. The apparatus may further include a control module configured to control the shift in frequency response by controlling a voltage applied to the conductor. Therefore, the sensor may be tuned to different frequency for sensing a partial discharge at the new frequency. In some embodiments, the apparatus may include a plurality of conductors each disposed adjacent one another and to the microwave sensor. Each conductor may be connectable to a voltage source and the control module may be configured to control a shift in frequency response of the sensor by successively controlling a voltage applied to each of the conductors thereby tuning the sensor to a multitude of frequencies for sensing a partial discharge. In some embodiments, the sensor may be configured to be connectable to a current for reducing reflective losses to improve directivity of the sensor at a given frequency. The control module is configured to control the directivity of the sensor by controlling a current applied to the sensor. A positive current may increase directivity of the sensor, and a negative current may reduce its directivity. Microwaves generated because of partial discharge span a wide band of frequencies and require an array of sensors of different design frequencies to ensure detection. A single sensor designed to operate at a certain frequency is not suited to monitor all types of partial discharge events. Dedicated sensors for partial discharge monitoring are more complex and expensive to manufacture. A tuneable sensor wherein the sensor can be configured to operate at the required frequency may be preferred for monitoring partial discharge events. Tuning of the sensor can be achieved electronically by capacitive tuning. Two microwave sensors placed at a distance less than a quarter-wavelength disturb the resonant frequency due to the interaction of electric. Such phenomenon can also be used to tune the sensor at any desired frequency. If a voltage source is connected to two conductive layers separated by a dielectric, the apparent permittivity of the dielectric material can be changed. This leads to a change in the effective capacitance between the conductive layers. It is possible to achieve a similar affect by placing a current carrying conductor at a distance less than a quarter-wavelength The electric fields of the two components interact, which shifts the frequency response of the microwave sensor. If multiple conductors are positioned adjacent to the sensor at similar spacing such that they can influence the electric field of the sensor, it is possible to tune the sensor to a multitude of frequencies. To improve the directivity of a microwave sensor, an input current could be applied in a way that reduces its reflection losses, thus improving the directivity, which could be utilised for distant sensing. Controlling the input current by some viable means will allow enhancement of directivity, which can be used to detect the partial discharge events in the underground cables from the surface level. This non-invasive method of partial discharge sensing can save time and cost associated with traditional partial discharge localizing methods. The apparatus as will be further described herein below, may be operable to detect surface partial discharge by frequency tuning and to detect underground partial discharge by both frequency tuning and directivity tuning. Specific embodiments of the invention are described with reference to the Figures. Figure 1 illustrates an exemplary apparatus (10) for detecting partial discharge in an underground power cable (12). An underground power cable (12) may have a cavity (14) in its insulation, which may be the site for the partial discharge event. The partial discharge may produce microwave emissions (16). The frequency of the microwave emissions (16) produced may depend on several factors, including the type of partial discharge, cavity size within the insulation when internal partial discharge occurs, or operating voltage for the electrical equipment. Due to these factors, the frequency of the generated waveforms can lie anywhere from a few kilohertz (kHz) up to several hundred-megahertz (MHz) range. Further, theses microwave emissions (16) are often weak and may be nullified after a short distance due to attenuation within the ground. The apparatus (10) may include a microwave sensor which may generate strong electromagnetic waves (18). These strong electromagnetic waves (18) interact with these weak electromagnetic waves (16) to detect the partial discharge as will be further described herein below. Figure 2 illustrates an exemplary apparatus (100) for detecting partial discharge in electrical equipment. The apparatus (100) includes a microwave sensor (102) operable at a given frequency and a conductor (104) disposed adjacent the sensor to define a distance (106) less than a quarter-wavelength between the conductor (104) and sensor (102). The conductor (104) includes two conductive layers (104A, 104B) separated by a dielectric. The layers (104A, 104B) are configured to be connectable to a voltage source (108) for providing an electromagnetic field between them. In the absence of a potential gradient, the dielectric has a certain permittivity value. When a voltage source (108) is connected to the conductive layers (104A, 104B), an electric field is established between the two conductive layers (104A, 104B). This electric field changes the permittivity of the dielectric, and it attains an apparent permittivity, which is a function of the frequency of applied voltage. A strong electric field is produced between the conductive layers (104A, 104B) by the application of voltage, which changes the apparent permittivity of dielectric material between them. The conductor (104) is placed at a distance of less than a quarter-wavelength ^from the sensor (102) to enable the electromagnetic field produced by the conductor (104) to interact with the near field of the microwave sensor (102). This causes a shift in the frequency response of the sensor (102) as a function of applied voltage. Figure 3 is a plot which illustrates an exemplary shift in frequency response of the microwave sensor (102) upon controlling a voltage (108) applied to the conductor (104). The frequency response is presented with normalized frequency as the independent parameter while the transmittance S11 as the dependent parameter. The frequency response (122), centred at frequency f’, depicts the response of the microwave sensor (102) in the absence of a voltage source (108). When the conductor (104) is excited by the voltage source (108), it produces a shift Af’ (124) in the original frequency response of the microwave sensor (102). Controlling the voltage applied to the conductor (104) may accordingly be used to control the shift in frequency response. The apparatus (100) includes a control module (120) configured to control the shift in frequency response by controlling the voltage applied to the conductor (104), thereby tuning the sensor (102) to a different frequency for sensing a partial discharge. The microwave sensor (102) may accordingly be tuned to a desired frequency without altering its physical parameters. This may be termed frequency tuning of microwave sensor and may be used to detect partial discharge events for aboveground cables. The microwave sensor (102) may further be configured to be connectable to a current (110). The current could be applied in a way that reduces its reflection losses, thus improving the directivity of the sensor (102) at a given frequency. Figure 4 is a plot which illustrates an exemplary improvement of reflective losses and Figure 5 is a schematic diagram which illustrates an exemplary improvement in directivity upon controlling a current (110) applied to the sensor (102). The frequency response (132) depicts the reflection characteristics of the microwave sensor (104) without any input current (110). When current (110) is applied to the sensor (102), the reflection response of the sensor (102) improves, as depicted by (134). The directivity of the sensor (102) without any input current (110) is shown in (142). To enhance directivity, a current of a specific magnitude must be applied in a particular direction. For example, when a positive current is applied to the sensor (102), the directivity of the sensor (102) increases as depicted in (144) and when a negative current is applied to the sensor (102), the directivity of the sensor (102) decreases as depicted in (146). Controlling the input current by some viable means will allow enhancement of directivity. The control module (120) may be further configured to control the directivity by controlling the current (110) applied to the sensor (102). This may be termed directivity tuning of microwave sensor and may be used to detect partial discharge events in underground cables. The apparatus (100) may thus be operable to detect surface or aboveground partial discharge by frequency tuning and to detect underground partial discharge by both frequency tuning and directivity tuning. Figure 6 illustrates an exemplary apparatus (200) for detecting partial discharge in electrical equipment according to another embodiment. The apparatus (200) is substantially similar to the apparatus (100) and includes a microwave sensor (202) operable at a given frequency, and a control module (220) configured to control the directivity by controlling the current (210) applied to the sensor (202). In this embodiment, the apparatus (200) includes a plurality of conductors (221, 222, 223, 230) each connectable to a voltage source and each disposed adjacent one another and to the microwave sensor (202). In this this embodiment, the apparatus (200) includes four conductors (221,222, 223, 230), however it is appreciated that the apparatus may include a series of “n” number of conductors each connectable to a voltage source and each disposed adjacent one another. A first conductor (221) is disposed adjacent the sensor (202) to define a distance (206) less than a quarter-wavelength between the first conductor (221) and sensor (202) and the distance between each subsequent additional conductor (222, 223, 230) is also less than a quarterwavelength Q). This ensures that interaction of the electromagnetic fields when the conductors are connected to their respective voltage sources. Figure 7 is a plot which illustrates exemplary shifts in frequency response of the microwave sensor (202) upon successively controlling a voltage applied to each conductor (221,222, 223, 230). The frequency response (122), centred at frequency f’, depicts the response of the microwave sensor (102) in the absence of a voltage source. When the first conductor (221) is excited by a voltage source, it produces a shift in the original frequency response of the microwave sensor (202). The microwave sensor (202) then attains a new frequency fi’. When a second conductor (222) is subsequently excited by a voltage source, microwave sensor (202) attains a new frequency f2’. When a third conductor (223) is subsequently excited by a voltage source, microwave sensor (202) attains a new frequency ‘fa’ and when a nth conductor (230) is subsequently excited by a voltage source, microwave sensor (202) attains a new frequency ‘fn’. Controlling the voltage applied to each conductor of the plurality of conductors may accordingly be used to control the shift in frequency response. The control module (220) may thus be configured to control a shift in frequency response of the sensor (202) by successively controlling a voltage applied to each of the conductors (221,222, 223, 230) thereby tuning the sensor (202) to a multitude of frequencies for sensing a partial discharge. It is appreciated that the apparatus (200) according to this embodiment provides a wider range of frequencies compared to the apparatus (100) and the microwave sensor (202) may be tuned to a desired frequency without altering its physical parameters. Figure 8 illustrates an exemplary method (300) for detecting partial discharge in electrical equipment. The method (300) may include providing (302) a microwave sensor operable at a given frequency and scanning (304) for a partial discharge using the microwave sensor at that frequency. If a partial discharge event is detected, the method may include collecting and storing (306) the sensor data for further analysis including analysing the data to determine one or more of a partial discharge magnitude, partial discharge location, or a cavity size within an insulation for internal partial discharge. If no partial discharge event is detected, the method (300) may include disposing (308) a first conductor adjacent the sensor. The first conductor may be disposed a distance of less than a quarter-wavelength from sensor. The conductor may include two conductive layers separated by a dielectric. The method (300) may include connecting (310) the layers of the first conductor to a voltage source for providing an electromagnetic field between them. The electromagnetic field interacts with the field of the sensor to cause a shift in the frequency response of the sensor. The method (300) may include controlling (312) a voltage applied to the first conductor to cause a shift in the frequency response of the sensor and scanning (314) for a partial discharge at the new frequency. If no partial discharge event is detected, the method (300) may include disposing (316) disposing a plurality of additional conductors adjacent one another and connecting (318) each additional conductor to a voltage source. The first conductor, and each subsequent additional conductor may be disposed a distance of less than a quarter-wavelength from sensor to ensure interaction of the electromagnetic fields when the conductors are connected to their respective voltage sources. The method (300) may include successively controlling (320) a voltage applied to each of the first conductor and additional conductors thereby tuning the sensor to a multitude of frequencies, and scanning (322) for a partial discharge event at each new frequency. For example, the method (300) may include controlling a voltage applied to a second conductor to cause a shift in the frequency response of the sensor and scanning () for a partial discharge at the new frequency. If no partial discharge event is detected, the method may include controlling a voltage applied to a third conductor to further cause a shift in the frequency response of the sensor and scanning for a partial discharge at the new frequency and so on. The method (300) may continue scanning for microwaves over a band of frequencies to monitor any possible partial discharge events in the observed electrical component. Figure 9 illustrates an exemplary method (400) for detecting partial discharge in electrical equipment according to another embodiment. The method (400) may include providing (402) a microwave sensor operable at a given frequency, disposing (404) disposing a plurality of conductors adjacent one another and to the sensor, connecting (406) each conductor to a voltage source, and connecting (408) a current to the sensor. When current is applied to the sensor, the reflection response of the sensor improves. Applying a current of a specific magnitude in a particular direction may further enhance directivity of the sensor. For example, when a positive current is applied to the sensor, the directivity of the sensor increases and when a negative current is applied, the directivity of the sensor decreases. The method (400) may include controlling (410) a voltage applied to the first conductor, and controlling (412) the current applied to the sensor to increases its directivity at a specific frequency. The method (400) may further include scanning (414) for a partial discharge and if no partial discharge is detected, controlling (416) a voltage applied to a subsequent conductor and repeating steps (412) and (414). If a partial discharge event is detected, the method (400) may include collecting and storing (418) the sensor data for further analysis including analysing the data to determine one or more of a partial discharge magnitude, partial discharge location, or a cavity size within an insulation for internal partial discharge. The disclosure accordingly provides an apparatus and method for detecting partial discharge in electrical equipment over a wide band of frequencies. It will be appreciated that numerous variations and modifications may be made to the resonator apparatus as described. The foregoing description has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the technology to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above disclosure. The language used in the specification has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. It is therefore intended that the scope of the present disclosure be limited not by this detailed description, but rather by any claims that issue on an application based 5 hereon. Accordingly, the present disclosure is intended to be illustrative, but not limiting, of the scope of any accompanying claims. Finally, throughout the specification and any accompanying claims, unless the context requires otherwise, the word ‘comprise’ or variations such as ‘comprises’ or ‘comprising’ will be understood 10 to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Claims

1. An apparatus for detecting partial discharge in electrical equipment, the apparatus comprising:a microwave sensor operable at a given frequency;a first conductor disposed adjacent the sensor to define a distance between the first conductor and sensor, wherein the first conductor includes two conductive layers separated by a dielectric, the layers configured to be connectable to a voltage source for providing an electromagnetic field between them which interacts with a field of the sensor to cause a shift in a frequency response of the sensor; anda control module configured to control the shift in frequency response by controlling a voltage applied to the first conductor, thereby tuning the sensor to a different frequency for sensing a partial discharge.

2. The apparatus as claimed in claim 1, wherein the distance between the sensor and first conductor is less than a quarter-wavelength3. The apparatus as claimed in claim 1 or claim 2, including a plurality of additional conductors each disposed adjacent one another and each connectable to a voltage source.

4. The apparatus as claimed in claim 3, wherein the control module is configured to control a shift in frequency response by successively controlling a voltage applied to each of the first conductor and additional conductors thereby tuning the sensor to a multitude of frequencies for sensing a partial discharge.

5. The apparatus as claimed in claim 3 or claim 4, wherein the distance between the sensor and first conductor and the distance between each subsequent additional conductor is less than a quarter-wavelength6. The apparatus as claimed in any one of the preceding claims, wherein the apparatus is operable to detect surface partial discharge by frequency tuning.

7. The apparatus as claimed in any one of the preceding claims, wherein the sensor is configured to be connectable to a current for reducing reflective losses to improvedirectivity of the sensor at a given frequency and wherein the control module is configured to control the directivity by controlling a current applied to the sensor.

8. The apparatus as claimed in claim 7, wherein applying a positive current increases directivity and applying a negative current decreases directivity of the sensor.

9. The apparatus as claimed in claim 7 or claim 8, wherein the apparatus is operable to detect underground partial discharge by both frequency tuning and directivity tuning.

10. A method for detecting partial discharge in electrical equipment, the method comprising: providing a microwave sensor operable at a given frequency;disposing a first conductor adjacent the sensor to define a distance between the first conductor and sensor, wherein the first conductor includes two conductive layers separated by a dielectric,connecting the layers to a voltage source for providing an electromagnetic field between them which interacts with a field of the sensor to cause a shift in a frequency response of the sensor; andcontrolling the shift in frequency response by controlling a voltage applied to the first conductor, thereby tuning the sensor to a different frequency for sensing a partial discharge.

11. The method as claimed in claim 10, including disposing the first conductor a distance of less than a quarter-wavelength from the sensor.

12. The method as claimed in claim 10 or claim 11, including scanning fora partial discharge event at the given frequency of the microwave sensor; if no partial discharge event is detected, controlling a voltage applied to the first conductor to cause a shift in the frequency response of the sensor; and scanning for a partial discharge event at the new frequency.

13. The method as claimed in any one of claims 10 to 12, including disposing a plurality of additional conductors adjacent one another; and connecting each conductor to a voltagesource.

14. The method as claimed in claim 13, including disposing the first conductor and each subsequent additional conductor a distance of less than a quarter-wavelength from one another.

15. The method as claimed in claim 13 or claim 14, including scanning for a partial discharge event at the given frequency of the microwave sensor; if no partial discharge event is detected, successively controlling a voltage applied to each of the first conductor and additional conductors thereby tuning the sensor to a multitude of frequencies; and scanning for a partial discharge event at each new frequency.

16. The method as claimed in any one of claims 10 to 15, including connecting a current to the sensor for reducing reflective losses to improve directivity of the sensor at a given frequency and controlling the directivity by controlling a current applied to the sensor.

17. The method as claimed in claim 16, including applying a positive current to increase the directivity of the sensor; or applying a negative current to reduce the directivity of the sensor.

18. The method as claimed in claim 16 or claim 17, including scanning for a partial discharge event at the given frequency of the microwave sensor; controlling the current applied to the sensor to increases its directivity; and scanning for a partial discharge event.

19. The method as claimed in any one of claims 10 to 18, including, if a partial discharge event is detected, collecting and storing the sensor data for further analysis including analysing a partial discharge event to determine one or more of a partial discharge magnitude, partial discharge location, or a cavity size within an insulation for internal partial discharge.

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