Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – For fault location
Reexamination Certificate
1998-02-02
2002-07-16
Sherry, Michael J. (Department: 2829)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
For fault location
C324S501000, C324S547000
Reexamination Certificate
active
06420879
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to high power electrical apparatus, and particularly relates to sensing systems for detecting electrical partial discharges in high power electrical apparatus.
Partial discharges are pulse events with a sudden localized redistribution of charge in or on high voltage insulating materials at high electric stress. The detection of partial discharges is significant because partial discharge events are frequently an indicator of failure processes that are active within or on the insulation.
The pulse discharge event itself is typically of a very short duration. That is, the redistribution of charge, and hence pulse currents, associated with partial discharge events typically occur in the sub-microsecond time scale. Time duration values of 10 nanoseconds (10
−8
) and less can also occur.
Certain conventional partial discharge methods employ detection schemes based on a low-voltage external resonant circuit, typically of the R-L-C type, that is connected either in series or in parallel with the high voltage insulation of the power transformers. In both serial and parallel detection schemes, the coupling capacitor and the resonant detection circuit are in series to yield a closed-loop path for the current in the short duration partial discharge pulse event. Typically, an inductive impedance isolates the partial discharge pulse loop from the external source of high voltage. The R-L-C type resonant circuit is conventionally set to a resonant frequency in the range of 30 kHz to 300 kHz and set to be under-damped so as to yield a ringing waveform impulse response.
With this arrangement, the partial discharge event is typically of a duration much shorter than a period of the detection circuit resonant frequency and hence would act like an impulse and stimulate the resonant circuit natural response to yield a waveform referred to as a “ringing waveform”. This ringing waveform is at a defined frequency and could be readily detected with standard amplifier and display electronics. The ringing waveform has a beginning peak amplitude and an oscillation period as defined by the resonant circuit. The amplitude (or size) of the ringing response is also dependant on the size of the partial discharge event. Because the partial discharge events are much shorter in duration than the period of the resonant circuit, the response of conventional partial discharge systems is always the same basic ringing impulse response. This distinct response to all partial discharge events permits detection electronics to be quite simple, sometimes only utilizing the display of an oscilloscope.
The defined ringing response also enables the use of modern digital pulse height counting techniques to quantify the size of each event according to the peak size of the response. This digital acquisition and storage of pulse heights is a common method of partial discharge measurements since many events can be accumulated to yield a histogram of pulse height sizes versus the number of occurrences. Additionally, the time of occurrence (phase) relative to a 60Hz power frequency of an applied AC high voltage can be recorded and used to present the pulse height information according to phase position on the 60 Hz voltage waveform.
Each of these conventional partial discharge measurements begin from a condensation- of the partial discharge event into a single size or height quantity. Subsequently, additional information such as the number of occurrences within a specified time, is used to yield added information about many events. Also, information regarding the event moment relative to an applied AC voltage may be recorded to yield further information regarding many events.
This conventional method has been successful for the detection of partial discharge events in situations such as laboratories and factories where extraneous unknown pulse signals are eliminated. Because the resonant circuit will yield the same ringing response to any pulse drive signal that is short when compared to the ring frequency, the method cannot distinguish extraneous noise signals from actual partial discharge events. Attempts to apply the conventional partial discharge measurement to in-service applications, have not been fully satisfactory, in part, because external noise pulses cannot be distinguished from, and are often equal or larger in size than, actual partial discharge signals.
Noise detection and elimination have been attempted using various different frequencies and/or frequency spectral analysis, using digital methods such as neural networks, and also using background subtraction. Practical partial discharge measurement of in-service exposed apparatus, such as power transformers, is however, severely limited by external noise, even with the various additions to conventional partial discharge measurement. The resonant circuit detection requires that all pulses yield the same response. For this reason, the pulse origin is not identifiable.
Other more recently developed techniques for partial discharge measurement do not use a resonant circuit detection scheme. Instead, each partial discharge signal is recorded as a fast-pulse time waveform using a broadband recording device so that details of each individual event are preserved. Results of this type of measurement have shown that distinctive waveforms are recorded with time details in the sub-microsecond range and frequency content extending well beyond 20 MHz, even for the case of power transformers.
Moreover, detection of individual partial discharge signals at more than one location has shown that recorded signals at different locations are not the same. While certain frequencies may exhibit common responses at multiple terminals, when broader bandwidth signals are used clearly distinctive responses are detected at different locations. A cause for this difference in signal waveform at different detection sites is the different paths of propagation from the original site of the partial discharge signal to the locations of detection.
Other tests involving simulated partial discharge pulse signals with fast nanosecond timeframe transitions applied to power transformers confirm that the recorded waveforms are always different when detected at different locations. Both time delay and waveshape changes were detected at different detection locations.
These findings substantiate the view that partial discharge signals, being very localized and very rapid, therefore, release a pulse of energy that propagates out from its site of origin. The propagation occurs according to the structure surrounding the partial discharge event. An internally generated partial discharge pulse will thus propagate and appear at the end regions, such as a high or low voltage bushing, as specific pulse waves in accordance with the structure, the location of the origin of the signal, and the signal characteristic itself. Not only is there a received pulse height or size, but also a full wave shape including propagation time delays. Thus sufficiently broadband measurements reveal a distinctive pulse wave response for each partial discharge event, and not simply a size or magnitude component. This distinctive response may be compared with other signals to perform waveform recognition.
One example of the use of pulse waveform recognition is in the application of time domain reflectometry (TDR). In TDR an injected signal of known characteristics is injected and then recorded after propagation so as to evaluate the characteristics of the propagation path.
Another technique to better distinguish pulses associated with internal partial discharge signals from external noise, is to consider the nature of the apparatus being measured. In particular, for many power systems such as power transformers or power cables, the high voltage insulation is surrounded by a metallic tank or enclosure. A fully enclosed tank is often used to contain insulating oil and to protect against the elements of the external environment such as moisture. Such an enclosure also provides shielding, by the
Hollington Jermele M.
Massachusetts Institute of Technology
Samuels , Gauthier & Stevens, LLP
Sherry Michael J.
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