Field of the Invention
The invention relates to a method and a device for determining the location of a partial discharge (PD) in a transformer or a reactor, in which signals produced by a partial discharge are detected and supplied to an evaluation device.
The power transformer and reactor play important roles in power systems. They must operate safely; therefore the integrity of their insulation is very important. The criterion for the integrity of insulation is given by the level of partial discharge (PD). If partial discharges occur, then the engineers must find where they are and eliminate them.
Currently there is no method available to detect, locate and measure PD sources without taking the equipment out of service if the level of PD is low. The detection of low levels of PD can give an early indication of potential insulation problems in the equipment. Early detection can effectively reduce the probability of the occurrence of serious power plant breakdown. Locating the sources of PD is of great help in eliminating them, thereby reducing the maintenance costs of the substation equipment.
Keeping electrical equipment in the substation working continuously and without fault is the highest priority for the power network operators. Taking equipment out of service, under any circumstances, whether for periodical testing or fault repair, is highly costly. Online condition monitoring is strongly preferable for the electrical companies.
At present, periodic preventive tests have been widely adopted in the electricity industry to determine the insulation status of substation and other equipment subject to PD. These current tests can provide a reasonably accurate reflection of the insulation status in terms of the levels of partial discharge and other parameters. However, to carry out the test, the equipment must be taken out of service periodically (generally every six months), and during the intervening time between tests provides no information about the insulation condition. Although these tests are performed regularly, many serious faults do develop in the intervening time, and may lead to serious incidents.
An alternative and widely adopted method is based on using ultrasound to detect partial discharge sources. This method has been used for many years. It requires the manual operation of the detector by experienced engineers. This testing method can be carried out while the equipment is in service, however, it is only effective in finding large PD sources, and it is not always effective. In situations when the PD source occurs inside the winding of the apparatus, it is difficult to detect the PD signal using ultrasonic detectors.
A review of the prior art in this field reveals a number of other methods protected by patents
for detecting partial discharges and for monitoring insulation status. Of the existing prior art, the following are the only ones which appear to achieve a similar effect:
JP1270681 Detecting Method of Position of Partial Discharge of Transformer This method works by detecting the impression voltage of an auxiliary source.
JP7335445 Insulation Monitor of Transformer This method works by computing the sum of signal data within a specified voltage phase range.
DE19758087 Method to Identify Partial Discharge and Monitor Transformer This method works by comparing the direction of first half wave of pulse at different locations. Only discusses distinguishing partial discharges from interference.
JP3037580 Detection of Partial Discharge of Three-Phase Transformer Winding Voltage signals from two winding terminals are output to a differential circuit to determine differential voltage. The EM wave released with generation of the Partial Discharge is analyzed with a spectrum analyzer to determine if it is contained in a frequency component output from the differential circuit.
JP2056028 Monitor for Partial Discharge in Oil Filled Transformer This is a method based on ultrasound.
JP6317625 Partial Discharge Detecting Device under Operation of Transformer This method uses phase sensors on wires connecting coupling capacitors to the ground. Noise sensors connected to the coupling capacitors. Insulation deterioration is detected in a phase comparison between signals from the sensors.
JP9152462 Partial Discharge Detection Method for Transformer This method works with a main circuit transformer and a preliminary circuit transformer. Neutral points on HV side windings are connected. A through type current transformer detects unbalanced current flow, and a judging device judges whether or not Partial Discharge has occurred.
None of these nor any other prior art address all the operational problems mentioned above, and all methods work in a considerably different and less efficient manner to the present invention.
There is a need therefore for a new method that is more sensitive and reliable in detecting and locating PD sources, while the equipment is in service. The present invention provides a method for determining the location of partial discharges in a transformer (or a reactor), which can be carried out at low expense and with high precision, without taking the equipment offline.
The present invention extends the earlier work of the one of the inventors, patent number ZL99237957.1 Online Monitoring and Locating Device of Partial Discharge for High Voltage Apparatus. This earlier patent covers a rudimentary monitoring and location system, in which the foci are 1 ) the number of sensors, how the sensors are linked using coaxial cable, and the number of amplifier groups to compose a series of signal channels, and 2) the multiple digital switch used to connect the channels.
Summary of the Invention
The invention provides a method and a device that makes possible straightforward, rapid and accurate location of partial discharges in a transformer (or reactor). It can be used for PD location of a transformer at test or for on-line monitoring supervision.
A method, a device and a series of formulae are derived according to the invention, and are used for the data processing and positioning of the PD source.
Description and Details
Three cases are considered, corresponding to the three possible configurations of high voltage transformers found in industry. These are shown in Figure 1 , Figure 2 & Figure 3: a) The neutral of the transformer is grounded; b) The neutral of the transformer is not grounded; c) The transformer is an autotransformer
The following describes the details for detecting and locating PD sources in one winding in a transformer or reactor. The procedure is identical for each winding in the apparatus.
The sensors comprise: a core, coils and resistor wherein the core of the sensors comprises ferrite and the geometry of the sensors allow a wire to pass through the core. The coils are mounted on the core of the sensors and number at least 10. The components of the sensor are placed in an EM shielded protective container and filled with moisture retarding/absorbing materials to prevent moisture build-up. A hole is left in the container and the moisture retarding/absorbing materials to allow the wire to pass through the sensor.
To operate the system, the sensors are installed on the apparatus to be monitored. The sensors are placed at the terminals of the windings whereby: • the wire from the HV (high voltage) bushing tap (Figure 1.1 , Figure 2.1 & Figure 3.1 ) of the transformer (or autotransformer, or reactor) to the grounding goes through one sensor (Figure 1 .7, Figure 2.7 & Figure 3.7); • the wire from LV (low voltage) bushing tap (Figure 2.9) of the transformer (or autotransformer or reactor) or neutral (Figure 1.6, Figure 2.6 & Figure 3.6) to the grounding goes through a second sensor (Figure 1 .2, Figure 2.2 & Figure 3.2); and • where the device to be tested is an autotransformer, the wire from MV (middle voltage) bushing tap (Figure 3.8) of the autotransformer to the grounding goes
through a third sensor (Figure 3.3)
Figure 1 shows the sensor arrangement for the transformer (or reactor) in which the neutral is grounded. The winding of the transformer (or reactor) (Figure 1.10) has two terminals, the HV terminal (Figure 1.4) and the LV (or neutral) terminal (Figure 1.6). The wire from the bushing tap of the HV terminal (Figure 1.7) to the grounding passes through the first sensor (Figure 1.1 ). The wire from the neutral (Figure 1.6) to the grounding passes through a second sensor (Figure 1.2). The amplifier (Figure 1.11 ) and the PD Location System (Figure 1.12) are then used to amplify and process the data from the sensors.
Figure 2 shows the arrangement of the sensors for a transformer (or reactor) in which the neutral is not grounded. The first sensor is attached as the equivalent sensor described in 1 . above. The wire from the bushing tap (Figure 2.9) of the LV terminal of transformer (or reactor) (Figure 2.10) passes through the second sensor (Figure 2.2).
The arrangement of the sensors is more complicated in the case of an autotransformer; Figure 3 shows the sensor arrangement for an autotransformer. Two separate windings are used to represent an autotransformer. One is the HV (Figure 3.4) -MV (Figure 3.5) winding, the other is the MV (Figure 3.5) - neutral (Figure 3.6) winding. A third sensor (Figure 3.3) is attached to the wire from the bushing tap (Figure 3.8) of the MV terminal to the grounding. The data from both pairs of sensors (Figure 3.1 ), (Figure 3.3) and (Figure 3.3), (Figure 3.2) are fed to an amplifier (Figure 3.11 ) and then to the PD Location System (Figure 3.12) for data processing.
Once the system has been installed, a series of calibrations of the apparatus must be carried out whilst it is not in operation, and the details are fed to the PD location system. These calibrations need only be carried out once, unless the configuration of the apparatus is modified, for example by adding a further transformer. Once the calibrations have been carried out the system is ready to monitor the apparatus.
A series of calibrations are conducted to determine the passband of the digital filters according to the premise that the filtered data from sensor pairs (in Figure 1 - 1.1 & 1.2, in figure 2 - 2.1 and 2.2 and in Figure 3 - 3.1 and 3.2 or 3.2 and 3.3) in Figures 1 , 2, and 3 should have a similar geometry. These calibrations must be made in order to obtain the characteristic parameters of the transformer (or reactor).
The geometric similarity of the output of the two filters is defined as follows: • They contain same number of PD pulses. • For each PD pulse in one filtered output, there is a corresponding PD pulse in the other filtered output, such that the peaks of the PD pulses in these two outputs occur within a time range of +l.5μs .
If these conditions are satisfied we consider these two pulses valid and label them an "Available Pair".
The parameters (α, IL1 , IN1 , ILO, INO) obtained from the calibrations are input to the PD location system before energizing to test the transformer (or reactor).
(A) The a = J— of the winding is obtained by a calibration test, where c and k are the total shunt capacitance and series capacitance of the winding respectively. Figure 4 shows the calibration circuit for α parameter measurement.
A high frequency generator (Figure 4.21 ) is connected to the HV terminal (Figure 4.4) of the winding. An oscilloscope with at least two channels (Figure 4.22) is used to measure the voltages V1 between the HV terminal (Figure 4.4) and ground, and V2 between the neutral (Figure 4.6) and the ground respectively. The α value is calculated from V1 and V2 by« = arccos/z
For the autotransformer, as there are two windings to be considered, the HV (Figure 3.4) — MV (Figure 3.5) (middle voltage), and the MV (Figure 3.5) — neutral (Figure 3.6), these windings are considered individually, and each has its own α value.
(B) Calibration using PD pulse to determine the parameters, lu, INI , ILO, and lNo described below of a transformer (or reactor)
A calibrator (Figure 5.14, Figure 6.14 & Figure 7.14) and a calibrating capacitor (Figure 5.13, Figure 6.13 & Figure 7.13) are used to inject the PD pulse into the winding (Figure 5.4 - Figure 5.6)in Figure 5, (Figure 6.4 - Figure 6.6) in Figure 6 & (Figure 7.4 - Figure 7.6) in Figure 7. Three types of calibration circuit are used for different types of transformer, as described below.
(1 ) For a transformer (or reactor) in which the neutral is grounded, as shown Figure 5.
As illustrated in Figure 5, a rectangular voltage is supplied by a calibrator (Figure 5.14) to a calibrating capacitor (Figure 5.13) to form a pulse which then enters the HV terminal (Figure 5.4) of the transformer (Figure 5.10), passing the sensors (Figure 5.1 ), (Figure 5.2). The signals detected by the sensors are sent to the PD Location System (Figure 5.12) through the amplifiers (Figure 5.11 ). Following this the parameters lu , INI are obtained, where:
• lu is the first peak value of the pulse wave detected in sensor 1 (Figure 5.1 )
when the calibrating pulse is injected into the HV terminal (Figure 5.4) of the winding. • lN1 is the first peak value of the pulse wave detected in sensor 2 (Figure 5.2) when the calibrating pulse is injected into the HV terminal (Figure 5.4) of the winding. • In this case lN0 is set to zero, and lL0 is set to one.
(2) For a transformer (or reactor) in which the neutral is not grounded.
Four parameters (lu, INI , ILO, and lNo) must be obtained from the calibrations. The transformer must be calibrated twice in different ways, as shown in Figure 6 and Figure 7 respectively.
• First calibration
A pulse generated by a calibrator (Figure 6.14) and calibrating capacitor (Figure 6.13) is injected into HV terminal (Figure 6.4). IL1 and lNι are obtained in the same way as described (1 ) above. Figure 6 illustrates the calibration circuit used where the pulse is injected into the HV terminal (Figure 6.4) of the winding when the neutral is not grounded.
• The second calibration
A pulse generated by calibrator (Figure 7.14) and calibrating capacitor (Figure 7.13) is injected into the neutral (Figure 7.6). Figure 7 illustrates the calibration circuit where the pulse is injected into the neutral (Figure 7.6) when it is not grounded. lLo, and lNo are obtained after the calibration, where
• lLo is the first peak value of the pulse wave detected in sensor (Figure 7.1 ) when the calibrating pulse is injected into the neutral (Figure 7.6) of the winding in Figure 7. • lN0 is the first peak value of the pulse wave detected in sensor (Figure 7.2) when the calibrating pulse is injected into the neutral (Figure 7.6) of the winding in Figure 7.
(3) For an autotransformer
As described above, an autotransformer is represented by two separate windings, the HV (Figure 3.4) -MV (Figure 3.5) winding and the MV (Figure 3.5) - neutral (Figure 3.6) winding. The calibration methods displayed in Figure 6 and Figure 7 and described in (1 ) and (2) above have to be followed for calibrating the HV (Figure 3.4) - MV (Figure 3.5) winding, while the method described in (1 ) above and shown in Figure 5 has to be followed for the calibration of the MV (Figure 3.5) — neutral (Figure 3.6) winding.
Once the system has been calibrated, in the preferred embodiment, these parameters are saved and used in the processing system. The monitoring proceeds as follows:
4. Signals obtained by the sensors (Figure 8.15) are passed into the amplifier (Figure 8.16) which pre-filters the signals to leave only the signals between 10 kHz and 400 kHz and are then amplified by a factor of 20-40.
5. The amplified signals are then processed in a subsystem - PD Location System (PLS) to deduce the positions of the PD sources, using the parameters obtained from the calibrations described above. The processing procedure in the PLS is as follows:
First the amplified signals are sampled by an A/D converter (Figure 8.17a) with a sample rate of at least 20MHz.
Secondly, the digitized signals are fed into digital filters the parameters of which have been determined by means of the calibration above (Figure 8.17) to select the Available Pairs. The output of the digital filter gives a series of PD pulses. When the PD pulses from two sensor pairs (such as, (Figure 1 .1 , Figure 2.1 or Figure 3.1 ) and (Figure 1.2, Figure 2.2, Figure 3.2), or (Figure 3.2) and (Figure 3.3) in Figures 1 ,2 and 3 have the geometric similarity described above, we call them an "Available Pair". Those pairs will be used for PD location calculation.
Thirdly, these Available Pairs are sent to the data processing system (Figure 8.19) to locate any PD source (Figure 8.20). The first peaks of the Available Pair (lL, IN) are valid values and are passed into the system for location calculation. The processing system will then interpret the positions of PD sources from the data received and provide metrics including locations of the PD pulses, distribution of the PD pulses and apparent charge of the PD pulses.
The position, x, of a PD source is given by:
are the first peaks of available pair; α, lL
, are obtained from the calibration as described in section 3, and where
D = N
- i^ cos a-I^I^ si ύia
The PD source position x can be interpreted as the "electrical distance" between the neutral and the HV terminal of the winding. The neutral and the HV terminal correspond to 0 and 1 respectively.
The output from the PD Location System is monitored using custom software, which can provide analysis of the results. The system running the monitoring software may be configured to allow remote monitoring access, trigger alarms if certain levels of PD are detected, maintain historical records, make predictions of future problems et cetera.