Transformer winding movement detection by high frequency...

Details Transformer winding movement detection by high frequency... Transformer winding movement detection by high frequency...

2001-05-29

2002-10-15

Le, N. (Department: 2858)

Electricity: measuring and testing

Fault detecting in electric circuits and of electric components

Of individual circuit component or element

C324S527000, C361S038000

Reexamination Certificate

active

06466034

ABSTRACT:

TECHNICAL FIELD
This invention pertains to a highly sensitive method of detecting small deformation or movement of the windings of a high voltage transformer over a broader frequency range (1 kHz to 20 MHz) than prior art techniques (which are typically restricted a frequency range of about 1 kHz to 3 MHz), without requiring external leads or removal of the transformer from service.
BACKGROUND
High voltage power transformers (i.e. 50 or 60 Hz oil-filled transformers with primary voltages ranging from 69 kV to 750 kV and ratings from 5 kVA to over 500 MVA) are the most expensive pieces of equipment in a typical interconnecting power system. Keeping such transformers in service is critical to the operation of the power system. Transformers normally operate quite reliably over their typical 30 year design life spans. However, transformer failures do sometimes occur, with consequential severe impacts including loss of service and resultant loss of revenue; equipment damage which can be very expensive to repair or replace; and, potentially serious explosion, fire or other safety hazards to utility operations personnel.
It is well known that movement, looseness, deformation or distortion of a transformer's windings can lead to catastrophic electrical or mechanical failure of the transformer. A transformer's windings may move, etc. if the transformer is subjected to an electrical short circuit, which is not uncommon. Furthermore, as a transformer ages, the paper insulation material applied to the transformer's windings tends to shrink, thereby reducing the overall winding clamping pressure and allowing the windings to move, etc. It is very difficult to reliably detect small movement, etc. of a transformer's windings, yet early and reliable detection of such conditions is highly desirable in order to avoid the aforementioned catastrophic consequences.
The conventional prior art method of detecting movement, etc. of a transformer's windings is to remove the transformer from service, open the transformer, and visually inspect the windings. The transformers in question are normally oil-filled, so it is necessary to drain the oil to facilitate inspection, and replace the oil after inspection. This is very time consuming, requires a long transformer outage interval and is quite expensive.
Another prior art method of detecting movement, etc. of a transformer's windings is the so-called “transformer short circuit impedance test”. However, this technique requires a relatively large AC power source which is usually unavailable at the site of the transformer to be tested. Moreover, measurements of a transformer's short circuit impedance at the frequencies of interest (i.e. the transformer's normal 50 or 60 Hz operating frequency) are insufficiently sensitive to detect small winding deformation or movement, etc. and even less sensitive to detection of winding looseness.
The prior art has evolved an alternative “frequency response analysis” or “low voltage impulse” testing technique, which is considerably more sensitive to the detection of transformer winding deformation, etc. than the short circuit impedance test. This alternative test can be performed without opening the transformer and without a large AC power source. As shown in
FIG. 1
, transformer
10
is removed from service and a signal source
12
such as a recurrent surge generator is electrically connected to one of the transformer's input windings
14
. A current shunt
16
is electrically connected to the transformer's input winding (as shown) or another winding (not shown). A recording device
18
such as a digital oscilloscope is electrically connected to the input winding and the output of the current shunt. Actuation of signal source
12
applies a test signal to input winding
14
, producing a current in the input winding to which current shunt
16
is connected or, if current shunt
16
is connected in the output winding, a capacitively coupled current signal. Alternatively the current shunt is disconnected and the voltage coupled into the output winding is measured. Both the applied signal and the resultant capacitively coupled signal are recorded by recording device
18
, and the data so obtained is then used to calculate a transfer function for the transformer, in conventional fashion. The data and/or transfer function are retained for future comparison with additional data and/or transfer function(s) obtained during subsequent testing of the same transformer under identical test conditions. The comparison can also be done with another transformer of identical design; although such comparisons are not as accurate as comparisons of test results obtained for the same transformer over time. The objective is to detect differences between transfer functions obtained at different testing times, with such differences possibly being indicative of transformer winding movement, etc.
Either one of two different test methods can be used to perform the frequency response analysis test to obtain the desired transfer function; namely, the swept frequency test method or the pulse test method. A swept frequency test is performed by applying a variable frequency voltage or a white noise voltage input signal to the transformer's high voltage winding terminal (normally the input winding terminal) and recording the output response signal produced in another winding terminal (normally the output winding terminal) of the transformer. The output signal divided by the input signal for each test frequency yields the transformer's transfer function as a function of frequency. A pulse test is performed by applying an input pulse signal containing energy at all frequencies of interest to the input winding, and recording the output response signal produced in another winding (normally the output winding). The recorded data (applied voltage input and capacitively coupled current output signal) are then each subjected to a Fourier transform. The Fourier transform of the capacitively coupled current output signal is divided by the Fourier transform of the applied voltage input signal to obtain the transformer's transfer function as a function of frequency.
The input and output signals can be measured on any combination of input and output terminals of the transformer. All combinations will have some sensitivity to transformer winding deformation, etc. Normally, the most sensitive measurement is obtained using the high voltage winding for the input signal and the low voltage winding for the output signal. Other combinations are sometimes used e.g. input to high voltage winding and output on neutral to obtain more information, or due to transformer design limitations.
A transformer's transfer function is independent of the applied signal source, but dependent upon the transformer's internal structure. More particularly, a combination of factors including winding inductances and winding capacitances (inter-turn capacitance, interwinding capacitance, winding-to-tank capacitance, etc.) determine the transfer function. Any movement, looseness, deformation or distortion of a transformer's windings can change the transformer's capacitance characteristics, thereby changing the transformer's transfer function. By carefully comparing a transfer function obtained via testing while a transformer is in a known satisfactory operating condition with a transfer function obtained via later testing of the same transformer, one may detect changes indicative of transformer winding movement, etc. The higher the maximum frequency of the transfer function, the more sensitive the test is to winding movement.
FIG. 2
provides further details of the prior art frequency response analysis test using the pulse test method. A signal source such as a recurrent surge generator
12
A is used to apply a pulse of about 300 volts to each one of transformer
10
A's input winding terminals in turn. The transformer's neutral terminal H
0
/X
0
and the corresponding tertiary winding terminals Y
1

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