Monitoring acoustic noise which is neither corona nor Aeolian, a novel sensing method for stability in the power grid
Acoustic noises are observed near 400KV circuits which are not related to corona or Aeolian tones. The sensing method adapted is to record the sound by using a laptop computer with various microphones and an adapted FFT program with output in a frequency -amplitude- time waterfall mode. Comparisons are made with magnetic spectra and using different types of microphone. Mainly sub- harmonic frequencies 8.33, 16.7,25, 33.3 and 37.5 Hz are recorded in the acoustic spectra corresponding with only the usual higher harmonics in the magnetic spectra. On a couple of occasions an inter-harmonic of 30 Hz corresponding with 180 Hz in the magnetic spectrum is observed and parametric conversion is postulated. The acoustic noise inter-harmonics generally become more accentuated with an increased wind generation contribution to the grid capacity. Possible theories of noise generation are advanced including electro-seismic effect with and without seismic feedback mechanisms. An alternative theory involving infra-structure micro-cracks is briefly discussed. Possible space weather implications are speculated upon.
in this respect the present author a keen hill walker recently noticed a very
peculiar low pitched quasi -periodic humming sound whilst walking early on
morning in hills near Llanllechid Gwynedd. The pitch was estimated to
be much lower than 50 Hz and although the whole hillside seemed to
resonate with the noise its centre of activity seemed to lie in between two
high voltage power circuits on 400 KV National Grid single circuit and one 132
KV Manweb single circuit. The noise was definitely not corona as it
was not 100 Hz and there was no 'crackling' nor was it Aeolian tones as it was
not windy and the noise simply didn't have the right characteristic. The
author remembered a similar noise being reported to the
Synchronous machines fed from harmonic sources can certainly generate unwanted and additional acoustic noise13,14. In this respect this section of 400 KV circuit connects Deeside to the Dinorwig Pumped storage generation scheme via the Pentir Inter-connector. There have been instabilities at Dinorwig in the past15,16 . The author wonders if increased power system harmonics and inter-harmonics are once again provoking a problem at Dinorwig. In such a respect ground borne vibrations may also be involved and they can travel huge distances17,18
what possibilities are there for the overhead lines? It is well
known that high voltage circuits can emit corona noise. But this type of noise
has a frequency component which again is predominantly at 100 Hz and
sounds 'crackly'. On the other hand Aeolian tones sound just that, rather like
the whaling or howling wind or like blowing over a bottle of water and
again did not fit the bill for the noise being experienced.
The hypothesis is perhaps that the dirtier the content injected into the grid,
the more this new acoustic phenomenon will manifest. A proposed way of
testing is to try and correlate the appearance of the phenomenon with total or
else a significantly relevant portion of
Another early morning visit to the same location where two single circuit sections of the grid run paralleled one 400 KV owned by National Grid and one at 132 KV owned by Scottish Power Manweb held the solution. The whole air, indeed the whole hillside for several hundred yards was resonating with the noise. This time however, a battery of equipment was available. Then and over the coming days a series of experiments was performed. Some measurements were also made under the 400 KV double circuit at Llanfair PG which links the Pentir Inter-connector to Wylfa Nuclear Power Station.
Experimental ; Apparatus and methods
Low frequency sound and infra sound is notoriously difficult to record using traditional methods due at least in part to filtered frequency responses of equipment. The approach here was to employ a laptop computer running FFT spectral analysis software such that near real-time frequency- amplitude -time plots could be obtained in the waterfall display mode. For audio recording the computer electret microphone could be used or a modified loudspeaker as a moving coil microphone and a small potted transformer was available as an inductor for obtaining magnetic spectra. Electric field spectra could be recoded simply by connecting an unscreened lead to the computer microphone input. The acoustic recordings are not closely calibrated. Traditional sound meters with dB(A) weightings under estimate low frequencies but most of the readings are in the range 65- 90dB(A) based on a separate acoustic spectrum and sound meter reading of moving car infra-sound. The scales on each waterfall plot are merely relative as the colour spectra were being adjusted to tease out the best signal to signal differentiations.
Series of acoustic waterfall mode recordings were made under the 400 KV single circuit at Llanllechid on three dates namely 22nd,24th and 31st August 20011. These were supplemented by electric and magnetic recordings. The mean magnetic field was 29 mG. A similar series of recordings was made under the 400 KV double circuit at LlanfairPG on the same dates allowing time to travel between the sites.
On 22nd and 24th August the noise described above could clearly be heard, on 31st August 2011 it could not. On 22nd August the weather locally was still and slightly damp. This does not of course imply that there was no wind offshore or anywhere else in the country for that matter. On 22nd August, corona noise could be heard at the Llanllechid site as a distinctly separate sound. . On 24th August the weather was cool, dry and breezy no corona noise could be heard. On 31st August the weather was still and dry, no corona noise could be heard at either site.
Results and Discussion
The results are presented as simple printouts of the computer waterfall displays.
Figure 1 Llanllechid 400 KV single circuit 0722
acoustic spectrum shows signals at 16.7, 30 and 50 Hz. The
signal at 90 Hz is an
Clearly the acoustic spectrum is showing up something which the magnetic spectrum is not and certainly whereas the signals at 16.7 and 30 Hz appear to be quite broad and continuous the signal at 50 Hz appears to a have a pulsating amplitude. It is thought that this could well account for the phenomenon heard by the author on the hillside. The observation of 30 Hz is fascinating. It seems to coincide with an observation of 180 Hz in the magnetic spectrum and one can speculate on a parametric conversion mechanism, 180EM -3x50EM = 30 AU. Where EM = electro magnetic and AU = audio. In order to investigate the persistence and extent of this phenomenon further measurements were made on subsequent days and at additionally at the LlanfairPG site. Parametric behaviour of power systems has been discussed elsewhere19 Non- integer harmonics including 25 Hz and 180 Hz are referred to by Jacobs et al (2002)20 and there is a specific reference to 180 Hz by Bollen.21
Figure 2 Llanllechid 400KV
Magnetic signals present 0-50 Hz noise, 50 Hz, 100Hz, 180 Hz.
signals 16.7 Hz , 25 Hz, 30 Hz, 33.3 Hz, 38 Hz, 50 Hz, 90 Hz
On this occasion the acoustic spectrum is slightly more complex than on the 22nd August and the signals at 30 Hz and 33.3 Hz are intermittent. All the signals appear to be pulsating more in amplitude. However the 180 Hz magnetic signal is now continuous. The signal at 38 Hz could conceivable be the .75 sub harmonic of 50 Hz. The other signals are as described previously except now a signal at 33.3 Hz is also prevalent being the inter-harmonic related to 1/3 of 100 Hz which has showed up in the magnetic spectrum.
Figure 3 Llanllechid 400KV
Magnetic signals present 50Hz, 150Hz, 250 Hz. Acoustic signals present 16.7,25,33.3 and 50 Hz.
Surprisingly some four minutes later the instability which gave rise to the 180Hz magnetic signal and 30 Hz audio signal has disappeared. The magnetic signal at 100 Hz has been replaced by the 3rd harmonic at 150 Hz but the other acoustic signals remain stable and the 33.3. Hz signal is somewhat less intermittent. Allowing for travelling time, measurements were next made at Llanfair PG in an attempt to assess how widespread these effects are.
Figure 4, 24th August LlanfairPG 400 KV Double circuit Top magnetic, bottom acoustic
At Llanfair PG Magnetic signals present 50Hz, 150 Hz and some low frequency noise mainly 0-70Hz.
Acoustic signals present approx 8.5 Hz, 16.7 Hz, 25 Hz, 33.3 Hz, 50Hz, and some noise in range 50-312Hz. The recordings here are strikingly similar to those in figure 3 except for the absence of the 5th harmonic in the magnetic spectrum and the presence of a signal centred on about 8.5 Hz in the acoustic (infrasonic) spectrum which could potentially be the sixth sub -harmonic at 8.33 Hz. The close similarity between the frequencies present now completely rules out Aeolian tones as the mechanism of sound production because the Llanfair PG circuit is a double circuit and would thus have completely different wind loading and aerodynamic characteristics.
It is noted that the acoustic sub-harmonics appear to be amplitude or pulse modulated in some way.
This could just be to load variations in the power system but it is interesting also to note that power quality output form wind turbines has been shown to contain amplitude modulation at the blade passing frequency typically 1.5 Hz for a three blade turbine and causes flicker in power syatems22
On 31st August, the grid is obviously behaving very differently and in a far more stable manner than on the previous two dates. The only acoustic signals of relevance are a very weak 25 Hz and a 50Hz signal. The acousto-magnetic transducer recorded signals at 50 and 250 Hz and very weak signals at 100 and 150 Hz. These signals were quite difficult to record on account of a JCB working some 400 metres away. This did emphasise however what a highly efficient acoustic radiator the hillside in question was.
The only relevant frequencies are 25 and 50 Hz; there is considerable background noise from other sources. There was no evidence of higher magnetic harmonics or lower frequency acoustic inter-harmonics.
general the inter-harmonics are worst on the 22nd and 24th August
and virtually nil on the 31st August. Irrespective of
the mechanism for the generation of these acoustic inter harmonics which will
be discussed briefly later, what has been discovered is a very powerful and
sensitive way of monitoring the stability of power grids or sections thereof
and in that respect the author has already filed for patent protection. The
hypothesis advanced earlier is that wind generation and many other embedded low
carbon forms of generation contain inverter systems which produce
inherently dirty electricity. Ideally to test the hypothesis one
should seek out sources attached to to the nearest nodes in the grid.
Unfortunately small sources producing less than 90MW are not registered with
Exelon as separate generating units. For larger units, one can find out how
much wind power is being generated from 'bm reports' part of the Exelon
electricity trading website23. On the assumption that wind
input from more minor sources would have a similar geographical distribution
and in any event is not all accessible through data at Exelon, it was decided
to use the sum total of injected power from three major wind farms
geographically well spread across the
One of the largest inshore wind
AUDIBLE NOISE Y/N
Y -large range
Y -large range
very weak 25 Hz only
to the above then, the hypothesis is very strongly supported by the present results.
It would seem there is a very low threshold in the section of grid examined for
disturbance by wind generation. As these farms are geographically well spread
it is fair to assume that a similar proportion of
The mechanism of sound production itself could involve parametric conversion. At least two separate and very different mechanisms are possible therefore further investigation will be necessary to confirm.
Firstly, it is known that geologists use electro-seismic prospecting and indeed the use of seismic prospecting with power lines as a source has also been described stating that harmonic and sub-harmonic frequencies of the power line fundamental may be generated as seismic signals.26
Electro-seismic conversion was first theorised on by Frenkle27 in 1944 and later by Biot28 in 1962 and more recently by Pride29 2003 and in one sense can take place at electrical double layers at soil or porous rock water interfaces and is therefore a commutative process.
It is interesting to note that one unstable mode of the power grid involving 30 Hz acoustic signals and 180 Hz magnetic signals was only noted adjacent to the Llanllechid section of the single 400 KV circuit where the MANWEB 132KV circuit runs nearby. It is possible that the instability involves electro-seismic and seismo-electric electric coupling between the two.
variation on this theme might be that higher harmonics excite increased
non-linear behaviour of the synchronous motor-generator sets at Dinorwig
causing stronger earthbound seismic coupling of bearing vibration
which has been quoted by
Alternative hypotheses could involve the cables, pylons and insulators themselves. Acoustic waves at 50 Hz are observed in all the above cases. It is also know that the electromagnetic component contains higher harmonics. Vibrations of these structures can take place by Lorentz force, magnetostriction and electrostriction. If micro-cracks and flaws are present at metallic contacts sub-harmonic generation can take place31,32. Presumably as higher harmonics are present there is more opportunity for sub and inter-harmonic generation by theses mechanisms. This mechanism would be expected to give rise to predominantly airborne vibrations. The hypothesis ought to be testable by examining an unstable power grid at night when low frequency atmospheric sound ought to propagate much further. Generation of the frequencies observed by insulators or their partial breakdown can probably be ruled out as such processes tend to give a flat frequency response from DC to about 50 KHz and with emissions still measurable in the radio frequency bands. Thermophone like noise generation by the cables cannot be entirely rules out but would be expected to be strongest on a working day when the load was greatest. This was not observed. There can also be sound patterns similar to corona with loose connection arcing33 but these don't fit the bill of what was observed either.
Although it has been established here that the instability in the sections of power grid monitored appear to be due to wind generation, the technique ought not to be limited to that. In one sense the 30 Hz phenomenon has been proven to be more of a transient phenomenon than the rest. On one occasion, it came and went over a four minute period. Kappenman, describes how space weather can cause havoc on power grids. Solar storms can cause black outs on earth due to powerful ground induced currents34.
However, although a considerable body of literature exists by those working in the field; it is generally far less well known that power grids can cause space weather! So called PLHRS or power line harmonic radiation may affect the earth's magnetosphere35-38
the author believes there is almost the possibility for the
The observation of low frequency acoustic and infrasonic radiation from 400 KV sections of the power grid not attributable to previously known causes and revealed in this paper yields numerous potential opportunities. In whatever case, it would seem an incredibly sensitive way of visualising sub- harmonics and inter-harmonics in power systems. Sub and inter-harmonics with period doubling have been observed which are known routes to chaos in a power system42 , for a stable grid they ought then to be minimised at all costs. There can be no such minimisation without monitoring and perhaps ultimately some kind of feedback control. In order to expand this method to cover more of the grid, there may be the opportunity to develop long fibre optic based acoustic sensors for in-situ monitoring in the future. Alternatively, several mobile phone companies are now making use of pylons for mounting of their antennas and back haul dishes. There could be the opportunity for co-operation in the development of wireless based monitoring technologies along these lines.
· A new and sensitive method for monitoring potential instabilities on a power grid is described.
· It is believed to be the first time that significant wide area acoustic noise is reported and recorded from overhead 400KV circuits which is neither arc breakdown nor corona nor Aeolian in origin.
· Sub-harmonic and inter-harmonic acoustic tones have been recorded which seem to be involved in a parametric conversion process with frequencies in the magnetic spectrum.
· Acoustic inter-harmonics have been shown to become more severe when there is a greater proportion of wind generation on the grid.
· Some possible hypotheses of the acoustic generation mechanism have been advanced.
1. S.M. Wilkinson1 and N.A.Henderson, USE OF NETWORK ANALYSIS TO ASSESS THE VULNERABILITY OF AN ELECTRICAL SUPPLY NETWORK The 14th World Conference on Earthquake Engineering October 12-17, 2008, Beijing, China http://www.iitk.ac.in/nicee/wcee/article/14_06-0082.PDF
Electricity Transmission Network a Vision for 2020’ URN 09/752
3. EWIS European Wind Integration Study TREN/07/FP6EN/SO7. 70123/038509 http://www.wind-integration.eu/downloads/library/EWIS-Grid-Reinforcement-Projects.pdf
4. Burak Ozpineci, Leon M. Tolbert Department of Electrical and Computer Engineering University of Tennessee-Knoxville Knoxville, TN 37996-2100 http://www.uv.es/emaset/iep00/IEP%2009-10/cycloconvertertutorial.pdf
Gunther, E.W.; Electrotek Concepts Inc.,
6. Wind Turbine Harmonics Caused by Unbalanced Grid Currents By Hans De Keulenaer / Published on Tue, 2008-05-06 http://www.leonardo-energy.org/wind-turbine-harmonics-caused-unbalanced-grid-currents
An Investigation of the Harmonic Emissions of Wind
Turbines, Sokratis T. Tentzerakis;
Issue Date: March 2007 Volume: 22 Issue:1 On page(s): 150 - 158 ISSN: 0885-8969 http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=4106019
8. Harmonic and interharmonic distortion analysis in the grid-connected wind electric generator, Kumar and Kannan, Intl.J.Energy Tecnol+Pol. 5(2) 2007 pp187-203
Modeling of Solar Inverters and Their Interaction with the Distribution Grid Bosman,
A.J.A.; Cobben, J.F.G.; Myrzik, J.M.A.;
Kling, W.L.; Dept. of Electr.
11. Harmonic current emission of wind farms exceeding their limiting values, http://www.dewi.de/dewi/fileadmin/pdf/publications/Publikations/S14_5.pdf
13. Analysis of the Audible
Noise of Three-Phase Squirrel-Cage Induction Motors Supplied by Inverters
Belmans, Ronnie J. M.; D'Hondt, Lieven; Vandenput,
Andre J.; Geysen, Willy; Department of Electrical
Engineering, Katholieke Universiteit Leuven, Laboratory for Electrical Machines
and Drives, Kard. Mercierlaan 94, B3030
and vibration from induction machines fed from harmonic sources Yacamini, R.; Chang, S.C.; Energy
Conversion, IEEE Transactions on Issue Date: Jun 1995 Volume: 10 Issue:2
On page(s): 286 - 292
ISSN: 0885-8969 Dept. of Eng.,
15. Dewi Jones and Sa'ad Mansoor, http://www.gwefr.co.uk/prevexp.htm
16. Reproducing oscillatory behaviour of a hydroelectric power station by computer simulation S.P Mansoor , D.I Jones, D.A Bradley, F.C Aris, G.R Jones Control Engineering PracticeVolume 8, Issue 11, November 2000, Pages 1261-1272 http://www.sciencedirect.com/science/article/pii/S096706610000068X
Components of the Seismic Noise in the NORSAR Area. E.Hjortenberg1, T.
Risbo2Geophysical Journal of the
18. Nature of narrow-band signals at 2.083 Hz Götz H. R. Bokelmann* and Stefan BaischBulletin of the Seismological Society of America; February 1999; v. 89; no. 1; p. 156-164© 1999 Seismological Society of America http://bssaonline.org/cgi/content/abstract/89/1/156
hybrid power filter control in power systems with interharmonics Jacobs,
J.; Schroder, S.; De Doncker, R.W.; Inst.
for Power Electron. & Electr. Drives,
Print ISBN: 0-7803-7640-4
21. Bollen, Electric Power Systems Research 66(1), 2003
for predicting flicker induced by large wind turbines Saad-Saoud,
Z.; Jenkins, N.; Centre for Electr. Energy, Univ. of
Manchester Inst. of Sci. & Technol. 790945 abstract Energy
Conversion, IEEE Transactions on Issue Date: Sep 1999 Volume: 14 Issue:3
On page(s): 743 - 748
the Effects of Subsynchronous Interharmonic Voltages on Power Transformers:
Single Phase Units Langella, R. Testa, A. Emanuel, A.E. Dipt.
di Ing. dell'Inf., Seconda Univ. di Napoli, Aversa Aversa : Power
Delivery, IEEE Transactions on
Issue Date: Oct. 2008 Volume: 23 Issue: 4
On page(s): 2480 - 2487 ISSN: 0885-8977 INSPEC Accession Number: 10235586
25. Improvement in the efficiency of
distributed power systems, G.Benysek, Bulletin of the
26. US patent 7227362
27. Frenkel (1944) studied 'seismoelectric potentials' associated with the propagation of longitudinal waves in moist soil. He used electrokinetic theory associated with quasistatic Poiseuille flowin charged, cylindrical capillaries to model the induced potentials.
28. Generalized theory of acoustic propagation in porous dissipative media, MA Biot - J. Acoust. Soc. Am, 1962 - pmi.ou.edu
electroseismic wave theory of FrenkelAuthor:Pride,
Steven R. Publication Date:11-10-2003Publication
investigation into the development of a condition monitoring/fault diagnostic
system for large reversible Francis type pump-turbinesS
32. John G. Kappenman, Storm
sudden commencement events and the associated geo-magnetically induced current
risks to ground-based systems at low-latitude and mid-latitude locations,
33. A theory of PLHR emissions to explain the weekly variation of ELF data observed by a low-altitude satellite OA Molchanov, M Parrot, MM Mogilevsky… - Annales …, 1991 - cat.inist.fr http://cat.inist.fr/?aModele=afficheN&cpsidt=4986567
34. Daily variations of ELF data observed by a low‐altitude satellite, M Parrot, OA Molchanov, MM Mogilevski… - Geophysical Research …, 1991 - agu.org http://www.agu.org/journals/ABS/1991/91GL01352.shtml
35. Identifying power line harmonic radiation from an electrical network 1, C. J. Rodger, N. R. Thomson http://hal-insu.archives-ouvertes.fr/hal-00317854/
man-made modification effects in the magnetosphereK Bullough,
T.R Kaiser, H.J Strangeways Journal of
Atmospheric and Terrestrial Physics
Volume 47, Issue 12, December 1985, Pages 1211-1223 http://www.sciencedirect.com/science/article/pii/0021916985900893
Effects of Power Line RadiationC.
G. Park and R.
38. Transmitter simulation of power line radiation effects in the magnetosphere, C.G.Park and DCD Chang, Geophysical Research Letters 5 (10), 1978 http://star.stanford.edu/~vlf/publications/214.pdf
39. DEMETER observations of ELF waves injected with the HAARP HF transmitter, M.Platino, U.S. Inan, T.F. Bell, M.Parrot, and E.J. Kennedy, Geophysical Research Letters, 33 (L16101), 2006.
of (almost forgotten?) infrasound from below on the upper atmosphere and
ionosphere, Lastovicka, J.;
Drobzheva, Ya. V.;
Krasnov, V. M.
36th COSPAR Scientific Assembly. Held 16 - 23 July 2006, in
41. Influence of infrasonic waves on the ionosphere. Krasnov , Ya.V. Drobzheva, Physics and Chemistry of the Earth, Part C: Solar, Terrestrial & Planetary Science, Volume 26, Issue 6, 2001, Pages 433-437 http://www.sciencedirect.com/science/article/pii/S1464191701000265