1. Field of the Invention
This invention relates to locating RFI sources on power distribution and transmission systems.
2. Description of the Prior Art
Sparks on/in power system equipment are a significant source of radio frequency interference (RFI). When these RFI sources are of sufficient strength they can disrupt radio and TV reception and all types of radio communications. This leads to complaints to the operator of the power system and the source must be found and corrected. Often the RFI source is a faulty device or appliance in the home or office of the complaining party. However a significant percentage of RFI sources are caused by faulty devices or equipment on the power distribution/transmission system itself. Generally these sources are found on the overhead cables and associated equipment that distribute commercial electrical power.
After receiving a complaint the power company personnel typically locate the pole or structure from which the RFI is originating with special radio direction finding (RDF) equipment. Then they use very short range radio or ultrasonic detectors to pinpoint the source. For many years attempts have been made to design ultrasonic detectors with sufficient sensitivity, directivity and reliability to locate the spark RFI source from the ground, thus eliminating the need for a bucket truck and line crew to locate the fault. The limiting factor has been the achievable sensitivity and signal to noise ratio (S/N).
The present invention is a pinpointer, locator or detector of RFI event sources, with increased discrimination of power system RFI sources from random, ambient and other sources. This improved discrimination provides benefits in reducing the time and cost required to locate an RFI source on a power system. This and other improvements are achieved by taking into account the specific ultrasonic characteristics, or signatures, of power system line RFI sources so that they can be discriminated from background noise.
As in the prior art, some embodiments of the invention use a parabolic reflector to focus ultrasonic wave energy on a microphone mounted at the reflector focal point, although alternatively other types of ultrasonic sensors (such as direct contacting stethoscope probes) can be used. Unlike prior art, which typically treated the fundamental technical problem as one of detecting certain ultrasonic frequencies emitted by spark sources and mixing them down to provide audio indication to the operator, the concept of the invention evolved from understanding that the ultrasonic energy coming from the spark is essentially a series of shock waves in the atmosphere, rather than any particular ultrasonic frequency, and that these shock waves arrive at the microphone in predictable patterns directly related to the frequency of the AC power system and to the type of interference source from which they emanate.
The shock waves are converted to an alternating electrical voltage by a piezo ceramic transducer that resonates at an ultrasonic frequency when stimulated by the atmospheric shock wave. This signal is then amplified and filtered to remove out of band noise such as internal electronic noise and external radio noise picked up by the circuits. The signal then goes to a logarithmic amplifier/detector circuit, the output of which is a voltage proportional to the log base 10 of the AC input signal level. The time constants in the detector are adjusted so that it amplitude demodulates the ultrasonic frequency, thereby providing a signal with bandwidth from DC up to an upper cutoff frequency chosen for optimum system performance. The AC output of the amplifier/detector is sent to the AC amplifier and the bandpass filters, the outputs of which are converted to proportional DC voltages by active rectifiers. (The filter pass bands are designed for the frequency of the power system that the instrument will be used on, generally either 60 HZ or 50 HZ. If the instrument is to be used on more than one type of system the filter pass bands can be designed to be switchable.) The DC voltages from the bandpass-rectifier paths are then compared to each other and the DC output of the AC amplifier-rectifier path by a bank of comparator circuits. The resulting high/low comparator outputs are combined and screened logically so that when a predefined condition is met, based on prior analysis of signals collected from power line RFI sources, an indicator light is turned on to notify the operator. At the same time the unfiltered outputs of the bandpass-rectifier circuits, now providing higher harmonics of the power line frequency in the audible range, are fed to an audio amplifier and speaker. The sound amplitude and timbre from the speaker advise the operator, in a relatively gross way, of what is coming in through the microphone. At the same time the output of one or more of the bandpass-rectifiers (depending on mode of operation) is used to drive a signal strength meter which gives the operator a more precise indication of the strength of the power line component of the received signal.
The meter and light indicators are attached to the parabolic reflector in such a way that they are easily visible to the operator while sighting to point the highly directional parabolic microphone unit.
When practicing the invention the operator will usually have identified a particular pole or structure from which the RFI is emanating using RDF equipment. Standing at ground level, he/she will then point the ultrasonic parabolic microphone with one piece integrated electronics, meter, indicator and speaker, at various suspect devices on the pole until a spark source is identified by the speaker and meter and confirmed by the indicator light. The operator will then scan the pole from various angles to confirm which of various closely spaced items is the faulty component. Typically the operator will have to work with high levels of ambient ultrasonic noise, such as from traffic, wind, rustling leaves, etc. Since these noise sources will not have the. specific spectral characteristics of power line sources the indicator will not illuminate (or sound in the case of an audio indicator). Also, because of the response characteristics of the logarithmic amplifier/detector, the speaker and meter will both tend to quiet in the presence of loud out-of-band noise, further enhancing ease of operation.
It is anticipated that the invention will be used mostly for locating spark RFI sources, but the same fundamentals apply to corona RFI. In fact, since there are predictable differences in the signals emitted from sparks and corona, a variation of the invention can be used to distinguish between them. For example, a transmission tower may have a spark RFI source which is interfering with local UHF reception and at the same time, because of the high voltage, a corona RFI is present which is chronic but not a source of complaint. The operator has located the tower with RDF equipment which is not precise enough to locate the specific faulty device causing the spark. Using the invention he can select a logic mode to xe2x80x9cseexe2x80x9d only spark type sources. This was not possible with the prior art.
Additional features and advantages will be made apparent from the following detailed description of the illustrated embodiment which proceeds with reference to the accompanying drawings.