Analysis of power system transient induced radiation for substation plant condition monitoring

E.J.Bartlett and P.J.Moore

Abstract: Electromagnetic radiation in the form of atmospheric radio waves (or sferics) originate from power system apparatus when transient currents are present. This phenomenon can occur during switching events in substations where an arc forms part of the current path, causing nonlinearities. An experimental investigation over a two-year period has collected data from srerics induccd by power system substation switching transients. The paper introduces analysis techniques to extract information from the received switching transient induced sferics and to allow event identification. Digital signal processing (DSP) is used to investigate the time- and frequency-domain charactcristics of thc cxpcrimcntally recorded signals. Thc possible futurc application of the developed techniques for condition monitoring of substation switchgear is discussed.

1 Introduction

The monitoring of power system plant is a complex prob- lem with many different strategies and engineering solu- tions. By monitoring the condition of plant, such as substation switchgear, significant events such as a fault or network reconfigurations can be identified and the relevant power system engineer informed through instrumentation displays or via a PC interface.

Traditional systems for monitoring power system substa- tion plant are based on analogue transducers measuring current, voltage and switch state values and involve:

large numbers or difrerent analogue transducers for differ- ent plant

kilometres of cable to allow aiialogue information transfer to central station

central analogue to digital (AD) conversion and digital processing * individual solutions to each substdtiodplant system - a sipificant investment of material and labour to install, commission and maintain the equipment [I]. In thc modern business environment power systems opera- tioiial engineers are embracing methods that apply cutting edge teclinology to traditional engineering problems. New plant-condition monitoring techniques therefore take advantage of developments in digital signal processing, communications systems and computing to improve system availability and security [2]. For substation plant nionitor- ing these modem solutions include:

modular systems allowing for ease of installation, mainte- nance and expandability

standard transducer types

0 IEE, 2001 IEE Prweeclings online no. 20010190 D o t 10. IWY/ipgtd:200 I0 190 Papzr firs1 received 2211d March aid h revised foim 17th October 2ooO Thc authon an: with the Departmait or Electronic & Electrical Engineering, LJnivenily of Balh, Clavzrton Down, Bath. X A 2 7AY, UK

digital information transfer along information buses using optical fibresiothcr widcband technology (less cables)

distributed processing and automation reducing the need for communications

functionality dctermined by software not by hardware [l]. Monitoring of switching events using digital fault recorder (DFR) technology is a good cxamplc of the benefits of such modern systems. The application or digital transduc- ers and a PC-based monitoring system in a substation allows faults to be discriminated from nomial system oper- ation, the system thcn dispatches a short fault report with the i n k r " n n necessary for the operational engineer [3]. One drawback of these systems is that they still rely on directly connected transducers to measure the current and voltage waveform needed [or analysis purposes.

Thc dcsign of monitoring systems based on the reception of power system arc-induced transicnt radiation has been considered in recent research [46]. In essence such systems involve the recording and detailed analysis of switching generated radio frequency interference that has hitherto been regarded as unwanted noise. These systcms do not require a direct connection Lo the power system plant and arc not specific to a single circuit, instead covering a geo- graphic area. A further bcnefit is that these systems can be easily retrofitted without network outages.

This paper introduces techniques that can be used to identify differences between sn bstation switching operations from their induccd radiation signatures. The experimental evidence collected is briefly summariscd and the results processed using Fourier analysis; this allows event featurcs to be extracted for categorisation purposes. This analysis of the captured data will foim the basis for investigations into the potential application of the transient induccd radiation measurement technique to condition monitoring of substa- tion plant.

2 Previous developments

2.1 Arcing-transient-induced radiation A power system arc can occur during substation switching events; arc ignition occurs twice a cycle, after cach currcnt

215 IEE Pruc . -Genu Trmstn. Dislrib., Vu/. 14X No 3, MliI, 2001

zero crossing causes the arc current to cease momentarily. After the critical breakdown voltage is reached, the arc is reformed by an avalanche ionisation that causes nonlinear currents to circulate.

The circulation of the transient arcing current leads to the transmission of radio waves; this occurs when the construction of the powcr system plant near the switching arc includes antcnna like objects that are connected to or form part of the current path. In a power system substation structures that could act as antennas in this situation include busbars, bushings and any other supporting metal- work that is energised. The eftkiency of transmission at a given frequency will depend on the physical dimensions of the antenna like objects. For exaniple, at VHF (3C300MHz) antenna structures have a higher transmis- sion efficiency when elements of lengths between 0.25 and 10m exist (assuming a quarter wavelength multiple of possible VHF frequencies).

The radiated waves are termed power-system sferics since lightning induced sferics are produced in a similar way [7]. Transient radiation from substation switching events can be transmitted in the very low frequency (VLF) up to the ultra high frequency (UHF) regions of the electromagnetic spec- trum [8-101.

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2.2 Experimental investigation into reception and propagation of sferics Four sets of experiments took place at Midland Electrici- ty’s Hereford and Willenhall substations between July 1998 and October 1999. Sfenc data was captured using the

Table 1: Details of switching operations monitored

following equipment arrangement (Fig. I): monopole vertically polarised antennas used for reception

between 1 MHz and I GHz (MF - UHF) superheterodyne radio receivers using double sideband

@SB) narrowband amplitude demodulation gave an audio frequency output of the demodulatcd electric field

Texas Instruments TMS320C31 DSP board hosted by an EMC hardened rack mount PC, sampling the receiver out- put signal at 4 0 k k Table 1 shows the details of all the switching induced sfcric records captured in t h s investigation. Figs. 2 and 3 show thc type of configuration for the circuits where these switches are located. The preliminary results from the experimental investigations have concluded that transient induced radiation occurs as a direct consequence of the transient currents that flow during a substation switching event. The reception of the switching induced sferics is pos- sible with this equipment in the medium frequency (MF) to low ultra high frequency (UHF) bands.

3 Analysis of captured data

Analysis of the data records has been carried out to investi- gate techniques to allow identification of individual switch- ing events from the switching-transient induced sferic signals. The following differences between switching events have been analysed:

operation performed (openiclose) duration of switching event type of switch (oil circuit breakeriair break isolator etc) substation system (voltage levelhetwork configuration)

Both time- and frequency-domain attributes of the switch- ing transient induced radiation have been analysed to allow switching evcnt classification.

3. I Comparison of operations and sferic general signature description From Table 1 it can be seen that the 132kV air break isolator (ABI) switch (Sl) was the commonest event recorded and so this will be used for the opening and clos- ing comparison. 14 sfcric events (7 opening, 7 closing), were rccorded during one switching session with the same recep- tion antenna, superheterodyne receiver and reception frequency.

Ref. Switch type Operation Switching Operations Voltage, kV current, Nphase

Location

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S4 oil CB connection of incoming grid Hereford 66 2 300 transformer 4 feed from 66 kV bus

S5 oil CB connection of incoming grid Hereford 66 3 350 transformer 5 feed from 66kV bus

S6 oil CB connection of outgoing Bodenham feed Hereford 66 2 30 from 66 kV bus

57 oil CB connection of outgoing unloaded Hereford 66 2 < 5 * Hereford South feed from 66kV bus

S8 air break isolator isolation of oil CB 57 from 66kV bus Hereford 66 1 E 1 0 * S9 vacuumCB connection of outgoing feed from Willenhall 33 3 120

grid transformer 3

* Figures estimated by on-site MEB engineers.

216 IEE P,-oc.-Genm Transin. Dislrib.. Vol. 148. No. 3. MUJ> 2001

Fig.2

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Fig. 4 shows a time-domain plot of the induced sferic radiation recorded during an opening operation of S1; this result is typical of the S1 opening signature, with other S1 opening signatures appearing very similar. By compari- son, Fig. 5 shows the radiation signature when S1 closes; again, this result closely resembles other results recorded for the samc operation. Although for a specific switching operation the signatures recordcd at a given reception fre- quency were very similar, it can be seen from Figs. 4 and 5 that a number of differences are apparent between the sig- natures recorded for the opening and closing operations. The closing cvcnt is shorter, and the signatures of both

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IEE Pt-oc.-Genet-. 7i.nn.srn. l)i.sfrih., Vol. 148, No. 3. Miiy 2001

The donlcrin plot Of witch 132kV AB1 (SI) o p i n g (c(0norluluted

transients have a period of high frequency noise and a period of lower frequency spikes. The most striking differ- ence is that the signature of the closing event iii the time doindin looks like a reversal of the opening event.

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This variation in signature is caused by the number of arc ignitions, which is dependent on the physical length of the arc, which varies throughout the switching operation as the switch contacts move [XI. In the case of the opening operation the arc clongates as the switch contacts separate. At the beginning of the operation, when the contacts are

217

close together, there are many arc ignitions since the effec- tive insulation withstand voltage is far below the system voltage level. This effect causes the high-frequency modula- tion in the time-domain representation of the records.

As thc contacts move apart the arc elongates and the arc ignitions occur less frequently until they only occur after cach current zero, this causes the lower kequeiicy si&nature of the second part of the transient. Finally, after the arc elongation has caused the effective insulation withstand voltage to increase above the effective critical flashover voltage (CFO), the x c current ceases after a current zero for each phase. This process is reversed for the closing operation, where the arc is progressively shortened.

The short time Fourier transform (STFT), which takes successive windowed fast Fourier transforms (FFTS), is used to represent thc timc-changing frequency characteris- tics of the signal [ll]. STFT plots of the same opening and closing events shown in Figs. 4 and 5 are presented in Figs. 6 and 7. The symmetry between the opening and closing signatures with the high- and low-frequency stages of thc radiation transient is very apparent from this repre- sentation. The ability to extract this type of information providcs a basis for developing algorithms to distinguish between opening and closing operations.

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3.2 Analysis of switch operation times The duration of SI opening and closing events can be measured from the signatures; results are shown in Fig. 8. Both the opening and closing events have a consistent duration with avcragcs of 0.127s and 0.084s, rcspcctivcly. This shows that opening events for this switch are approx- imately 0.043s longer in duration than closing events.

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Opening a switch is more onerous than closing since the arc is extinguished by lengthening and increased energy dissipation, and not by the contacts meeting as with closing [ 121. This explains the longer duration of the current tran- sient (and therefore radiation transients) for opening opera- tions. The ability to extract the switching duration from the signatures allows a measure of the current interrupting ability of the switch to be monitored. This result can also be related to the mechanically assisted mechanism used to operate this switch [8].

3.3 Comparison of different switchgear types A number of different types or switchgear have been oper- ated and the corresponding switching-induced radiation analysed to see whether the switch design affects the sferic signal characteristics. The induced VHF radiation signa- tures after receiver demodulation induccd from these switching events are shown in Figs. 4, 5 and 9-13 (with reference to Table 1 for the switch details).

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The effect of the switch type can be directly compared using the results presented. The critical part of the switch

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design that affects the current transient, and therefore the induced radiation, is the insulation medium of' the switch. Three types of switchgear with different insulation medi- ums of air. oil and vacuum insulation have been investi- gated.

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IEE Prw-Gener . Trimsni. Uislrib., V d 148, ,Vo, 3, Mny 2001

fotming. Addihxially, Lhe metal clad vacuum CB was enclosed in a substation switching building and was fed by shielded 33 kV underground cables, which would have prevented radiation transmission. This ineans that the only valid comparison between switchgear types can be made between the air and oil insulated switches. Both switch types induced radiation for all switching events, Figs. 4, 5 and 13 show air break isolator (ABI) operations whereas Figs. 9-12 show oil circuit breaker (OCB) operations.

The shorter duration of the OCB events is an indication of the higher efficiency of the oil circuil breaker at conlrol- ling and extinguishing the arc, compared with the air break isolator, even when the OCB is operated at higher currents. This is to be expectcd, as oil is a bctter insulation medium than air and the OCB circuit breaker is designed to regu- larly break higher currents with different switchng mecha- nisms that separate the switch contacts in less time.

To be able to distinguish between different types of switch being operated, the modulation of the induced radi- ation signatures needs to be analysed in the time-frequeiicy domain. This is possible using moving-window spectral estimation with embedded demodulation using the Hilbert transform, auto-correlation and DC trend removal. This embedded deniodulation technique is based on a digital sig- nal processing (DSP) technique dcvclopcd for the dctcction of single-phase fault induced radiation [13, 141.

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219

As previously discussed, whcn the switching contacts are farthest apart (towards the end of an opening operation), the arc ignitions will occur after each current zero crossing. Each arc ignition takes the form of an avalanche ionisa- tion and generates a broadband noise spike; this leads to the signal being modulated by the frequency of the current zero crossings. For a three-phase switching event, there will be six zero crossings per power system cycle, since thc switching events recorded took place on the UK system with a frequency of 50Hz there should be a 300% modula- tion of the broadband sfenc signal present in the caplured data.

The results of this demodulation analysis are given in Fig. 14 for an AB1 opening operation and in Fig. 15 for an OCB opening operation, these plots give a time-frequency domain representation of the demodulated induced sferic radiation. There are significant components in the 300Hz region that are clearly visible: as expected, the peak of the 300Hz component occurs at the end of the switching- induced radiation period. From these two signatures, it is evident that there are a number of different frequency com- ponents in the modulation other than the 300Hz related to the system frequency. This is a further indication of differ- ences between sferics induced by different switchgear plant that could be used for switch identification purposes.

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3.4 Effect of system voltage and network configuration At different system voltage levels there will be changes in the application and design of switchgear for the same insulation medium. This leads to differences in the makingi breaking characteristics of the switchgear and the current/ radiation transients produced during switching. Studies have shown that the system voltage level does affect the induced electric field levels, an effect that is not evident in these results because of the automatic gain control (AGC) of the superheterodyne receiver [8].

As can be seen from Table 1 the currents that these switches were making/breaking varied considerably. This will affect the transient current levels, and therefore the induced sferic radiation, but is to be expected as each switch will have a different duty to perform, such as isola- tion, load current interruption or fault current interruption. The configuration of the network will also have an impact: if part of the network is in the process of being isolated for maintenance purposes, then the currents flowing will be dif-

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ferent from those in normal operation. and this will also affcct the induccd radiation from any switching transients.

3.5 Limitations of captured record data and analysis techniques Since the findings reported here represent the start of our investigations into substation transicnt induced radiation the results gathered have certain limitations. For the analy- sis of the captured signals, the main limitations are:

low signal bandwidth (40kHzj receiver automatic gain control (AGCj causes signal mag-

nitude distortion different superheterodyne receivers used have different

characteristics. Future investigations should seek to overcome these prob- lems to increase the amount of bandwidth and improve signal quality.

4

The knowledge of transient sferic radiation obtained from the experimental and analytical investigations can be used to propose systems to monitor the condition of power sys- tem substation plant. These systems are based on the prin- ciple of monitoring radio frequencies in a specific geographic area to detect power system transient-induced radiation. Such a developed system would have similar advantages to modern digital condition-monitoring systems already in use, with the added benefits of:

no need for direct transducer connection to power system low number of transducers needed for data acquisition distributed processing reduces need for communication to

minimum standard modular design of monitoring station.

The information that this switchgear monitoring system could usefully provide and possible monitoring techniques are:

identification of the switch that has operated (via the loca- tion of the switching arc induced radiation source and intel- ligent signal identificatiodclassification using artificial neural networks)

identification of the operation performed (by analysis of the timeifrequency domain of the induced switching sferic radiation and classification using ANNs)

extraction o f switching event duration (by analysis of the duration of the induced switching sferic radiation)

extraction of current levels (through measurement and analysis of the induced magnetic field)

detection of possible wearing of contacts (with high sampling rate for digitisation with the application of signal feature extraction techniques such as higher order statistics (HOS) wavelets)

detection of mechanism malfunction (using statistical analysis to show whether the switch duration is abnormal, or with classification analysis of calculated current levels or induced radiation). Experimental investigations have already shown that arc- induced sferic radiation occurs at frequencies up to and including the UHF region [8]. To capitalise on the band- width of the sferic, wideband analogue or digital techniques using direction finding (DF) or time of arrival (TOA) strat- egies could be implemented to locate the switch being oper- ated. Digitally sampling the signal at increased sampling

Application to switchgear condition monitoring

IEE Euoc.-Gcner.. Trurisni. Disfr.iD., Vol. 148. hlo. 3, .Way M O 1

frequencies will also give benefits, allowing more complex and in-depth induced sferic analysis.

A switchgear monitoring system can be implemented using a similar equipment configuration to that shown in Fig. 1. The positioning of the reception and monitoring equipment within the substation is not critical and can be dictated by the substation design.

5 Conclusions

The framework for developing substation switchgear condi- tion monitoring systems based on the reception of switch- ing transient induced radiation known as sferics has been presented. This paper has focussed on the analysis of exper- imental data recorded at substations in the UK during switching events. Techniques that categorise the different switching sferics have been investigated.

The results of the analysis have highlighted the differ- ences between opening and closing events and the symme- try between them. The duration of the induced sferic radiation signatures specific to a single switch is consistent. A comparative analysis has been performed on transient- induced radiation signals captured from operations of different types of switchgear: this has shown the vaiying individual characteristics of different switches. Analytical techniques, including the STFT and moving window spec- tral estimation with embedded demodulation, have been used to successfully extract the arc restrike 300Hz compo- nent from the captured data. The effects of network config- uration, insulatioiliswitch type and voltage level have also been discussed.

The application of the knowledge gained here to the development of a power system substation plant condition- monitoring system has been presented. Such systems do not require a physical connection to the power system and can be used to cover an entire substation location. The development of a system with a wide bandwidth for signal data acquisition has been introduced. Such a system intro- duces many opportunities for the location of the radiation source and the improved analysis of the signal.

6 Furtherwork

The Power and Energy Systems group has recently under- taken a further research project based on the work reported here to develop and install a substation switching-induced- radiation monitoring system. The system has been installed at a local EHV substation. Initially these captured events will be correlated with the substation switching records to identify the switches being operated to allow further mean- ingful analysis.

The analysis of the captured data will concentrate on the variations between the arcing-induced-radiation signatures

for different switch types and at different network voltages. The characteristics of the received transient sferic radiation signatures will be investigated using the techniques devel- oped here and further feature extraction analysis tech- niques. Work will also look into possible systems for the location of the arcing induced radiation s o w , and it is hoped to report on these developments in due course.

7 Acknowledgments

This work would not have been possible without the help and support from a wide range of people and institutions. The authors would particularly like to thank Mike Edwards of the University of Bath, the Engineering & Physical Sciences Research Council, Paul Daker and Mike Lovegrove of Midlands Electricity and the project tram.

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References

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