Monitoring acoustic noise which is neither corona nor Aeolian, a novel sensing method for stability in the power grid

Dr Chris Barnes, Bangor Scientific Consultants.

 

Abstract

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.  

 Introduction

 Britain's power grid is presently quite robust, more so than the US grid1. Nevertheless, certain sections have problems. The Pentir – Deeside 400KV single circuit section of the UK power grid is known to have transient instability2 and lacks capacity for future developments in low carbon energy3. Also, linked to the Pentir inter-connector is the Pumped Storage Hydroelectric Plant at Dinorwig and the Wylfa nuclear plant linked via a 400kV double circuit across Anglesey. Power grids are known to be coming under more strain as they carry dirtier power with a higher prevalence of harmonics due to cyclo-converters4,5 and wind farms6,7,8   despite earlier claims to the contrary9 . There are also problems with smaller solar inverters10.   As more and more embedded generation takes place power grids will have the potential to become more unstable. New sensors and monitoring methods will be essential to keep Britain's' lights on. Such harmonics are already exceeding grid codes11.

 

Further, 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 University of Bangor some twenty years or so ago as a problem heard indoors by a Tregarth resident and this was investigated at the time by Dr Kevin Doughty who drew a blank (personal communication).  Unknown low frequency noises which cannot always be heard by all or necessarily traced to a single source are loosely referred to as the 'Hum' and the author has published extensively on this elsewhere12. Coincidentally, Tregarth also overlooks this same section of grid. The author suspects the noises heard then and now could possibly be related. 

 

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

 

But 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 UK wind generation.

 

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.     

 

Experimental Recordings

 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 GMT  22nd Aug 2011 top acoustic (electret) bottom magnetic

 

The acoustic spectrum    shows signals at 16.7, 30 and 50 Hz. The signal at 90 Hz is an HDD artefact.   The magnetic spectrum shows noise centred on about 25 Hz, a signal at 50 Hz and bursts of signal at 180Hz. There are also very weak  2nd  (100Hz) and 5th harmonics (250 Hz)  present.

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 24TH August  1342 GMT  Top magnetic bottom acoustic. 

 

Magnetic signals present 0-50 Hz noise, 50 Hz, 100Hz, 180 Hz.

Acoustic signals 16.7 Hz , 25 Hz, 30 Hz, 33.3 Hz, 38 Hz, 50 Hz, 90 Hz HDD artefact.  

 

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 24TH August  1346 GMT  Top acoustic bottom magnetic  

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     

 

 
Figure 5 31st August 0839 GMT Llanllechid 400KV no noise perceivable except JCB working 400 metres away.  Top acoustic (electret), bottom acousto-magnetic ( moving coil).  

 

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.   

 

 

 

Figure 6 31st August 0937 GMT LlanfairPG  400KV double circuit no noise perceivable except saw and grinder working 100 metres away.  Top acousto-magnetic (moving coil), bottom acoustic (electret).

 

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.

 

In 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 UK to test the hypothesis.   

 

One of the largest inshore wind farms in Scotland is the Crystal Rig II which  has a maximum capacity of  138 MW,  the Burbo Wind farm Liverpool Bay has a capacity of 90 MW maximum.  The Thanet offshore farm in South Eastern England has a capacity of 300 MW maximum.  The results are clearly summarised below. These three wind farms have a total maximum capacity of 528 MW.

 

 

 

DATE 

MW WIND 

AUDIBLE NOISE Y/N 

Inter-harmonics 

22ND Aug 

88

Y

Y -large range 

24th Aug 

119

Y

Y -large range 

31st Aug 

34

N 

very weak 25 Hz only

 

According 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 Britain's wind energy would be being generated by all systems main generation and embedded   on the days in question. The implication is even when Britain is only generating at some 20 % of maximum wind capacity, or about 0.5% of total UK generation, there is considerable sub-harmonic acoustic noise generation by the monitored sections of 400 KV circuit examined.  If this is repeated in other sections of the grid   Nationwide then the grid itself could possibly become  a  significant source of  low frequency noise pollution. Indeed since one possible method of noise generation involves potential seismic signals, large coverage of noise and unwanted geo-seismic interactions could even occur. Not wishing to exaggerate but with the UK aiming to boost production of wind energy by several thousand percent by 2030, there might be catastrophe waiting round the corner?  In this context it is interesting to note that sub-synchronous inter-harmonic voltages have been noted to have a significant effect in power transformers even with magnitudes of less than 1% of the rated voltage24 Inter-harmonics have been noted for causing general acoustic noise in distributed power systems by Benysek very recently indeed but the study does not state where25. It would seem this study certainly confirms the same. 

 

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.

 

Another 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 Roberts  as being 8.33 Hz for Dinorwig30 and indeed such a frequency was observed at LlanfairPG.  The author has recorded ground vibrations of 8.33 Hz in other parts of Gwynedd not in the direct neighbourhood of power lines. Certainly ground vibrations of the order of 2-2.5 Hz in connection with hydro power stations elsewhere  are known to propagate for huge distances17,18.  Thus there is the potential for seismo-electric coupling to feed back such signals into the grid itself, presumably either reinforcing or limiting instability due to precise phase relationships involved.   

 

 

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

Thus the author believes there is almost the possibility for the US and UK power grids to modulate each via space!   This could offer a possible alternative account for the spontaneous appearance of frequencies such as 30 and 180 Hz more related to the US power grid appearing on the UK grid.  There is certainly reference to interference in space at a power line harmonic frequency of 180 Hz in studies of ELF injection by the HAARP US Navy research programme39. As well as potential electromagnetic interaction, feedback to the ionosphere could even involve this sound and other infrasound itself40. The blade passing frequency of wind turbines is indeed infrasonic.  Infrasound from small sources can influence a wide area41.  Clearly, there is scope for a lot of further investigation. 

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.

 

Conclusions 

 

·    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.  

 

References

 

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