WO2020035494A1

WO2020035494A1 - Power transformer comprising a turret

Info

Publication number: WO2020035494A1
Application number: PCT/EP2019/071725
Authority: WO
Inventors: Janusz Szczechowski; Hamid JAHANGIR; Asghar AKBARI; Peter Werle
Original assignee: Abb Schweiz Ag; Gottfried Wilhelm Leibniz Universität Hannover
Priority date: 2018-08-15
Filing date: 2019-08-13
Publication date: 2020-02-20

Abstract

A power transformer (1), comprising a device (2) to monitor partial discharges, characterized in that the device (2) comprises a turret (3) of the power transformer (1) and an electrode (4), which is arranged inside of the turret (4), achieves the object to monitor a partial discharge in a power transformer as reliable as possible.

Description

Power transformer comprising a turret

Description

The invention is related to a power transformer, comprising a device to monitor partial discharges.

Partial discharge (PD) is a localized dielectric breakdown in an insulation system of a high voltage equipment, e.g. a power transformer, which does not bridge the space between two conductors. Detection and quantification of signals of partial discharges as the best indicator of an insulation defect could provide early warning on a failure of the equipment.

The electrical measurement of partial discharge according to IEC 60270 is the most frequently used method, which is based on an appearance of a pulse of partial dis- charge on the terminal of the test object. Unfeasibility of the localization of a partial discharge and an online application and a higher amount of external noises in onsite tests are the most important disadvantages of the conventional IEC method.

An UHF (ultra high frequency) PD measurement method is based on the detection of electromagnetic (EM) waves radiated by the source of partial discharge. UHF anten- nas as the heart of the measurement system, are used to capture electromagnetic waves. Due to a lack of connection between high voltage and the measurement sys- tem, the UHF method can be applied online.

Moreover, the location of the partial discharge can be determined by comparing the differences in arrival times of signals captured by four UHF antennas. Furthermore, because the main frequency components of onsite noise lay between few kHz to some tens of MHz and due to the shielding effect of a transformer tank, the UHF method has a greater robustness against external noises over the IEC method. Because of above advantages, UHF PD measurement is applied on GIS (gas insu- lated switchgear) in 1988 and later in 1997 for power transformers. However, there are serious problems and obstacles when using the UHF techniques on power trans- formers. Most of these obstacles are associated with UHF probes as the only UHF antennas which are used in power transformers through the oil drain valve.

For most power transformers there is no way to install an UHF antenna except through the oil drain valves. In recent years opening dielectric windows on a trans- former tank has been proposed for new power transformers. But this is not applied widely because this may create a weak point in terms of mechanical resilience and/ or moisture ingress. In some power transformers oil drain valves are completely shielded against electromagnetic waves due to an internal cover e.g. by a tube.

Thus UHF probes receive no UHF signals in these cases. Moreover due to safety consideration insertion depths of UHF probes are limited which leads to lower sensi- tivity in detection of partial discharge behavior. Beside of the problem, which is caused by the installation of UHF probes, the number of them is also limited due to a confined number of oil drain valves in power transformers to one or eventually two.

Thus, the localization of the source of partial discharge, which is the main reason of using of an UHF method would not be possible. Moreover there is no method to cali- brate an UHF partial discharge measurement system. Thus, the severity of partial discharge detected by an UHF system could not be determined. The sensitivity of an UHF detection system may be different for positive and negative voltage half cycles.

Furthermore for one type of partial discharge in one half cycle a large scattering in relationship between IEC and UHF may be observed. This may lead to UHF PRPD (phase resolved partial discharge) patterns being distinct from IEC well known pat- terns. Moreover, it is known that UHF PRPD may not be detected for some types of partial discharge at an inception stage when the mean charge density for negative polaritiy pulses is smaller than positive counterparts. According to above, even if an UHF probe could be installed without safety for shielding problems, UHF results with lack of information concerning the location of the partial discharge and its severity and with an unfamiliar PRPD are not productive. The object of the invention therefore is to monitor a partial discharge in a power transformer as reliable as possible.

The object of the invention is achieved by means of the features of claim 1.

According to this claim the device comprises a turret of the power transformer and an electrode, which is arranged inside of the turret.

According to the invention it has been found that opening a dielectric window on new power transformer walls is not usually performed by a transformer manufacturer be- cause this is considered as a weak point in terms of mechanical resilience and/ or moisture ingress. Therefore an installation of UHF probes through oil drain valves is the only way to capture UHF electromagnetic waves in power transformers.

Due to a limited number of UHF probes, e.g. one or two, and their single operation mode a localization of a source of partial discharge and its calibration are not possi- ble. Furthermore the installation of an UHF probe causes safety problems or may not be practical because of shielding of an oil drain valve. These critical obstacles make the UHF method not practical as much as an IEC method in power transformers.

According to the invention it has been found that to overcome the above mentioned obstacles as much as possible, a new UHF antenna can be used, whereas this UHF antenna is a kind of an open waveguide antenna. To overcome difficulties of UHF PD measurement on power transformers, an open waveguide UHF antenna, namely a turret-electrode-antenna, is proposed by this invention.

Due to this invention a turret-electrode antenna based on open waveguide theory is proposed to mitigate the above mentioned obstacles. The device insofar acts as an antenna which can monitor partial discharges in a power transformer.

Advantageously the device works as an antenna in a frequency range of 265 - 3000 MHz. The antenna, namely the turret-electrode-antenna, operates effectively in a fre- quency band of 265-3000 MHz, while the corresponding band for already commercial UHF probes is much more limited even with high and probably non-safety insertion depths. Through this a wide frequency range of detection is given.

The VSWR (voltage standing wave ratio) of the antenna shows a wide operation fre- quency. According to the radiation pattern of the antenna an overall coverage on the transformer active parts is expected. Due to a lower VSWR and a more effective ra- diation pattern the sensitivity of the antenna is higher for different types of partial dis- charges compared with UHF probes.

Advantageously the turret is a circular waveguide. The said antenna is built up by a preferably completely earthed loop electrode as a feed element and a turret of a power transformer as circular waveguide. An inventive study of the antenna parame- ter has shown that the diameters of turrets in power transformers with different volt- age levels are large enough for this application. Furthermore the dimensions of an optimized feed electrode are small enough to easily install it inside of the turret of the transformer.

Further advantageously the electrode is accessible through a manhole of the turret. Most of the turrets have a manhole which makes the installation easy. Dimensions of an optimized feed electrode of the antenna are small enough to easily install it inside of the high voltage or even low voltage turret of power transformers.

Advantageously the electrode is arranged in a safe region within the turret. The feed electrode is installed inside of the turret of the transformer in a safe region, thus it causes no safety problem.

Further advantageously the electrode is looped and/ or earthed. The feed electrode is not only earthed but also installed inside of the turret of the transformer in a safe region. Through this a safe operation is possible.

Advantageously the device acting as an antenna has no dielectric covered part. The antenna has no dielectric covered part, therefore it can be installed permanently in a power transformer while no inspection or maintenance is required. Through this the antenna is self-sufficient. Further advantageously the power transformer comprises at least three devices act- ing as antennas. For each power transformer six or at least three antennas can be installed. Therefore the problem of the limited number of antennas is resolved. The antenna is designed to be installed in the turret of high voltage or even low voltage windings. Thus for each power transformer six or at least three antennas are provid- ed. Therefore by resolving the problem of a limited number of UFIF probes, the locali- zation of a source of partial discharge can be done as before using a time difference between received signals.

Advantageously the device is able to work in two modes, namely conduction mode and radiation mode. Despite of UFIF probes the antenna works in two operation modes, conduction and radiation mode. The conduction mode shows a close correla- tion with IEC apparent charge and provides a feasibility of both calibration. The radia- tion mode which occurs at higher frequencies is used for localization of the partial discharge source. A detection of low radiant sources of partial discharges is possible. One of unique features of the antenna is the detection of sources of partial discharge with low radiation power. The operation of the antenna in two modes, namely radia- tion mode and conduction mode, provides the feasibility of detection of this type of sources of partial discharge compared with UFIF probes.

Further it is possible to distinguish external disturbances by using the antenna. While all disturbances can be detected in conduction mode of the antenna, including fre- quency range up to 200 MFIz, external disturbances can be distinguished due to a lack of data component in radiation mode, which concerns 200 MFIz and higher.

A great advantage of using the antenna proposed here is that the faulty phase can be easily determined according to comparison of the magnitude of received UFIF sig nals to each antenna. This is the case because for each phase there is an antenna, namely a turret-electrode-antenna, and each antenna has a greater sensitivity for the front winding rather than for the side winding, due to the radiation pattern and the closer distance.

Moreover the UFIF signals received by the antenna proposed here show a high ratio du/dt due to a wide range of operation frequency. Thus an estimation of the starting point of UHF signals and in consequence, localization of the source of partial dis- charge can be done more accurately compared with common UHF probes.

Data of the antenna in the conduction mode show a close correlation with an IEC apparent charge. Through this a calibration is possible. A calibration error due to type of partial discharge is 47% while the corresponding value of UHF probes is 700%.

A failure of a transformer while in service usually leads to significant revenue loss to the utility, potential environment damage, explosion and fire hazards and expensive repairing or replacement costs. Hence, it is desirable that the transformer should be utilized to the maximum extent consistent with adequate service life.

Using the proposed antenna, all power transformers can be equipped to an online PD monitoring, which is known as the best indicator of insulation deterioration long before total breakdown and leads to take remedial actions before catastrophic fail ures occur.

Advantageously, a turret to be installed in a power transformer, comprising an elec- trode. which is arranged inside of the turret, is used as an antenna as described be- fore. The turret may be delivered as a separate part. The turret and the electrode may have all the technical features mentioned in this description.

In the drawings:

Fig. 1 schematically shows the geometry of a turret electrode antenna,

Fig. 1 a a table describing the dimensions of the antenna according to Fig. 1 ,

Fig. 2 the VSWR of the antenna, and

Fig. 3 radiation patterns of the antenna at 0,5, 1 , 1 ,5 and 5 GHZ. Fig. 1 schematically shows a power transformer 1 , comprising a device 2 to monitor partial discharges. The device 2 comprises a turret 3 of the power transformer 1 and an electrode 4, which is arranged inside of the turret 3.

The device 2 is an antenna. The device 2 works in a frequency range of 265 - 3000 MHz. The turret 3 is a circular waveguide. The electrode 4 is accessible through a manhole 9 of the turret 3. The electrode 4 is arranged in a safe region 8 within the turret 3. The electrode 4 is looped and completely earthed.

The device 2 acting as an antenna has no dielectric covered part. The power trans- former 1 comprises at least three devices 2 acting as antennas. The device 2 is able to work in two modes, namely conduction mode and radiation mode.

To overcome difficulties of UHF PD measurement on power transformers addressed in previous sections, an open waveguide UHF antenna called turret electrode anten- na is proposed here. The turret 3 of power transformers 1 provides a suitable space to install a loop electrode 4 in its inside and produces an open waveguide antenna, e.g. the turret 3 and loop electrode 4 act as the circular waveguide and its feed ele- ment respectively.

Preferably the proposed antenna is just composed of a small copper piece as feed element. This reduces the costs for the antenna. The simple structure of the feed el- ement makes the construction of the antenna easy, fast and with no complex ma- chining.

A schematic view of the proposed turret electrode antenna including transformer bushing 5 and turret 3 and feed electrode 4 is shown in Fig 1. Most of transformer bushings 5 have an earthed 300 mm or more sub-section extension for installation of current transformer (CT), which is called CT space 6.

Thus, the region in front of CT space 6 is safe for installation of measurement equip- ment, e.g. bushing CT. The region below CT space 6 and above a line inclined with an angle of 30° with the turret 3, region 7 in Fig. 1 , may be used for equipment with uninsulated edges with a radius greater than 5 mm. The area below region 7 is known as the hazard region and no measurement equip- ment is allowed to be installed in there.

As it is shown in Fig. 1 , the electrode 4 of the proposed turret electrode antenna is earthed and placed in a completely safe region 8, thus it will not disturb a high volt- age electric field.

An antenna in accordance with the results obtained from a huge optimization work on antenna parameters has been constructed. Fig. 1 refers in principle to such a proto- type antenna with parameters listed in Table 1 of Fig. 1 a. The prototype antenna has been installed on a real 123 kV bushing 5 with 60 cm bushing CT space 6. The bush- ing 5 has been installed on the top of a transformer tank model with dimension of 1596 x 856 x 1236 mm. The prototype turret 3 could not endure the weight of a real bushing 5, thus the antenna has been installed inside of a tank. Flowever, the proto- type antenna has a same condition as in a real power transformer from the stand- point of electrical and EM terms.

Instead of using parameters with best expected performance, the minimum values of the proposed range for each parameter in previous range is chosen for the prototype antenna. Therefore, achieving to promising results with the prototype antenna en- sures applicability of proposed antenna in all cases.

VSWR of the antenna with parameters given in Table 1 obtained in CST Studio soft- ware is shown in Fig. 3. In regards to this figure, the prototype antenna works effec- tively in a wide frequency range of 265 - 3000 MFIz.

Flowever some notch frequencies are observed in 310, 700 and 900 MFIz. The proto- type turret-electrode antenna shows lower VSWR in most frequencies than commer- cial UFIF probes even with high and probably non-safety insertion depths.

Most of the UFIF antennas including commercial UFIF probes work in single operation mode. A unique feature of the turret electrode antenna is the operation in another mode in addition to a conventional mode. Similar to the conventional UHF antennas including UHF probes, the turret electrode antenna is capable of receiving the EM waves radiated from the PD site by the fre- quency band obtained above for the prototype antenna.

Because the electrode is placed in the turret 3 as a circular waveguide, received EM waves will be in TE and TM modes. This Operation mode can be used for PD locali- zation and other high frequency analysis in UFIF method.

Moreover when a PD occurs, induced current in winding passes through the bushing 5 and produces the TEM mode in the turret 3. The turret electrode 4 is oriented in a way to capture the TEM magnetic field. Because this operation mode is mostly inde- pendent of calibration parameters including the type of PD, radiation pattern and po- larization of the antenna, thus is very helpful to use in calibration of UFIF method.

The radiation pattern of the antenna describes the ability of the antenna to receive EM waves from different regions of space. Fig. 3 shows 3D power radiation patterns of the prototype antenna in four different frequencies, namely a) 0,5 GFIz, b) 1 GFIz, c) 1 ,5 GFIz and d) 2 GFIz. The intensity of each pattern is determined by a bar given for each pattern. The situation of patterns in respect to the antenna is determined according to the coordination system given in Fig. 1 and 3.

According to Fig. 3, a radiation pattern of the antenna is mostly directed by z-axis with low backlobes. Thus, regarding the position of turrets 3 in power transformers 1 , all radiation lobes effectively cover the region inside of the power transformer 1.

In lower frequencies, e.g. 0.5 GFIz, the antenna has a greater sensitivity for PD in front of the antenna rather than on its side. Therefore, in these frequencies, each tur- ret antenna has greater sensitivity for corresponding winding of that turret 3.

In higher frequencies, e.g. 1 GFIz and more, the antenna receives electromagnetic waves from both front and side windings. Thus considering the whole performance frequency range of the antenna, an overall coverage on the transformer active parts is expected. Reference numbers

Claims

1. Power transformer (1 ), comprising a device (2) to monitor partial discharges, characterized in that the device (2) comprises a turret (3) of the power trans- former (1 ) and an electrode (4), which is arranged inside of the turret (3).

2. Power transformer according to claim 1 , characterized in that the device (2) works as an antenna in a frequency range of 265 - 3000 MHz.

3. Power transformer according to claim 1 or 2, characterized in that the turret (3) is a circular waveguide.

4. Power transformer according to one of the preceding claims, characterized in that the electrode (4) is accessible through a manhole (9) of the turret (3).

5. Power transformer according to one of the preceding claims, characterized in that the electrode (4) is arranged in a safe region (8) within the turret (3).

6. Power transformer according to one of the preceding claims, characterized in that the electrode (4) is looped and/ or earthed.

7. Power transformer according to one of the preceding claims, characterized in that the device (2) acting as an antenna has no dielectric covered part.

8. Power transformer according to one of the preceding claims, characterized in that the power transformer (1 ) comprises at least three devices (2) acting as an- tennas.

9. Power transformer according to one of the preceding claims, characterized in that the device (2) is able to work in two modes, namely conduction mode and radiation mode.

10. Turret (3) to be installed in a power transformer (1 ), comprising an electrode (4), which is arranged inside of the turret (3).