Partial discharge spectral response to variations in the supply voltage frequency Cuthbert Nyamupangedengu and Ian R Jandrell University of the Witwatersrand, Johannesburg School of Electrical and Information Engineering, Private Bag 3, Wits 2050, Johannesburg, South Africa ABSTRACT Partial discharge (PD) spectral response to variations in the supply voltage frequency was experimentally investigated through laboratory-based tests. The PD spectral content of each defect type responded uniquely to variations in the sinusoidal supply voltage frequency in the range 20 to 400 Hz. The findings are interpreted using the theory of space charge dynamics in PD mechanisms. Prospective diagnostic applications of the findings include PD recognition using supply voltage frequency sweeps. Knowledge on supply voltage frequency dependency of partial discharges also helps in comparing PD diagnostic test results obtained at different test voltage frequencies. Index Terms — Partial discharges, supply voltage frequency, frequency spectra. 1 INTRODUCTION IN partial discharge (PD) tests there is flexibility in the choice of the supply voltage frequency (SVF) depending on the type of equipment being tested. Questions arise concerning the extent to which the changes in test voltage frequency affect PD mechanisms. These questions have sustained efforts in research into understanding the relationship between PD characteristics and supply voltage frequency. Frequencies other than power frequency (50/60 Hz) in PD detection are used in order to reduce power ratings of the test equipment. A test voltage frequency of 50/60 Hz may not be technically and economically viable for testing largely capacitive equipment such as power cables. In such cases lower frequencies are desirable. Similarly, higher frequencies are more suitable for inductive equipment such as transformers and generators [1]. Consequently, technologies such as 0.1 Hz and dumped alternating voltage (DAC) have been developed and commercialised [2]. The ringing frequencies of DAC can range from a few hundred to thousands of Hz depending on the length of the cable under test. The flexibility in the choice of frequency of PD test voltage poses a question on the comparability or trending of PD results that are obtained using test voltages of different frequencies [2-4]. In laboratory based research, higher voltages and frequencies are usually used as accelerating agents in PD ageing tests [5-7]. This is done in order to increase the rate at which an insulation defect is exposed to PD induced degradation. The rate of insulation degradation in service under power frequency can then be inferred. The technique of accelerated ageing of PD defects through increasing the test Manuscript received on X Month 2010, in final form XX Month 2010. voltage frequency is based on the assumption that the change in the frequency only alters the rate of ageing mechanisms and not the nature of the mechanisms [7]. There is however no evidence in literature that supports this assumption. Discharges can occur in insulation of equipment that is exposed to power supplies polluted by harmonics. Whether and how the harmonics aggravate the PD induced insulation degradation is a question that is yet to be fully explored [3]. The effectiveness of PD diagnosis technologies in condition based maintenance practice depends on how well the PD tests results are interpreted. Correct interpretation of the results requires sound understanding of the nature of PD mechanisms. The relationships between test voltage characteristics and PD mechanisms have been the subject of research by many researchers as evident in the literature review presented in Section 2. Aspects that are not yet fully explored and understood are highlighted, and these motivated the research work presented in this paper where the influence of the SVF on PD frequency spectra was investigated. The rest of the paper is structured as follows; Section 3 presents the experimental investigation procedure. The results are presented, analysed and discussed in Section 4. The prospective application and the conclusion are Sections 5 and 6 respectively. 2 INFLUENCE OF SUPPLY VOLTAGE FREQUENCY ON PD CHARACTERISTICS: a literature review The frequency ranges considered by various researchers working on PD characteristics dependency on SVF varies significantly from case to case. A summary of some of the important aspects of the literature on PD dependency on SVF is presented in Table 1. Other literature related to the effect of SVF on insulation includes that by Mason who investigated the influence of SVF on PD inception voltage (PDIV) and short time breakdown voltage of insulation [8]. Mason concluded that in frequency ranges up to a few kHz, the effect on PDIV was too small to explain the observed reduction of short time breakdown voltage of insulation. Localised heating due to repetitive PDs was suggested as the main cause of the reduction in short time breakdown voltage of insulation. Gockenbach & Hauschild [9] also reported results on work involving investigation of SVF effect on the insulation withstand voltage where they concluded that in the frequency range 20 Hz to 300 Hz, the insulation experienced the same electric stress effects. It is notable that there has been a considerable mix of agreements and controversies in the conclusions on the various research results. An example is that generally most of the researchers except for Miller & Black [10] agree that the minimal PD magnitude is independent of the SVF while the maximal magnitude decreases with frequency. Forssen & Edin Table 1: presented a remarkable clarification on the question of PD magnitude where they concluded that the dependency of PD magnitude on supply voltage frequency was influenced by the cavity size [11]. In small cavities (diameters equal to or less than 1.5 mm) the PD magnitudes (both maximal and minimal) were independent of the SVF. In larger cavities however (diameters more than 1.5 mm) the PD magnitudes decreased with increase in the SVF. This was attributed to the possibilities of influence of multiple and simultaneous occurrences of PDs in larger cavities. Another common agreement among researchers is on how the PD inception voltage increased and repetition rate decreased with increase in SVF for cavity PDs. An example of an area of controversy or disagreement is on the extent of influence of SVF on PD phase-resolved patterns. As an example while conclusions from Bodega et al [4] point towards minimal influence, Cavallin et al [3] as well as Forssen & Edin [11] reported significant changes, confirming that work is still needed in this regard. A scrutiny of most PD parameters studied, as summarised in the fourth column of Table 1, shows that most of the workers Summary of literature highlights on the subject of PD dependency on supply voltage characteristics. Researchers Frequency range considered Type of defects PD characteristics studied Key findings Miller & Black [10] 0.1 to 50 Hz 2-8 mm diameter Cavities in epoxy as well as polyethylene insulation (also surface discharges in cable and stator samples) PD inception voltage (PDIV) and PD magnitude - PD characteristics in the frequency range were generally independent of the supply voltage frequency. - Experimentation with polyethylene was more difficult than with epoxy because of significant induced insulation condition changes in the polyethylene. - Surface PDs increased with increase in frequency of supply voltage. Radu et al [5] 1-30 kHz 0.5 mm gap between dielectric in Helium at atmospheric pressure Glow and Pseudo – glow pulse height, repetition rate, width and rise time - Pulse height remained quasi constant. - Repetition rate reduced with increase in supply voltage frequency. - Pulse width and rise time reduced with increase in the supply voltage frequency. Wester et al [2]; Bodega et al [4]; Bodega et al [12] 50 Hz; 0.1 Hz; DAC (260, 520 and 930 Hz) 2-3 mm cavities in cast polyester resin PD inception delay, PD magnitude and phase patterns - At frequencies below 50 Hz there were equal chances that the PD characteristics could either be similar or different to those at 50 Hz. - Above 200 Hz PD maximal magnitude decreased with increase in supply voltage frequency. - Overall shape of the phase resolved patterns (PRPDP) were generally independent of the supply voltage frequency variations. Cavallini & Montanari [3]; Hauschild [13] 0.1; 20; 50 & 300 Hz 2 mm diameter spherical cavities in polyethylene, surface and corona discharges PD maximal magnitude PDIV PD repetition rate PRPDP - Maximal magnitude decreased with frequency. - PDIV increased with increase in supply voltage frequency. - PRPD patterns changed. Forssen & Edin [11] 0.01 to 100 Hz 1.5 – 10 mm diameter disc shaped cavities in polycarbonate insulation PD maximum and minimum magnitude, PD repetition rate and PDIV - PDIV increased with voltage. - Average & maximal PD magnitude depended on cavity diameter such that no change occurred for 1.5 mm cavity but for bigger cavities decreased with increase in supply voltage frequency. - No change in minimal PD magnitude. - Maximum PD magnitude decreased with increase in supply voltage frequency. - PRPD patterns changed. have been focusing mainly on conventional PD characteristics such as PDIV, extinction voltage (PDEV), magnitude and phase-resolved PD patterns (PRPD).These parameters are conventionally used to characterise partial discharge signals as guided by popular standards such as the IEC60270 [14]. Of late, however, there has been growing interest in the unconventional characterisation of PDs such as in the high frequency (HF) and ultra high frequency (UHF) techniques [15]. In such cases interest is in the information contained in PD pulse shape (in the time domain) or spectral characteristics (in the frequency domain). The current knowledge on PD characteristics dependency on supply voltage type should therefore be extended to PD pulse shape and frequency content. It is in this context that the work in this paper focused on PD spectral characteristics. Furthermore from the third column in Table 1 it is evident that most of the studies on the influence of SVF on PD characteristics have been conducted using cavity defects. This trend could be attributed to an assumption that cavities are among the most common defects found in solid insulation as stated by Gutfleisch & Niemeyer [16] as well as Cavallini & Montanari [3]. It is however known that surface discharges and corona are also a concern in insulation systems although to a lesser extent. An extension of knowledge on the relationship between SVF and PDs to other types of discharges such as surface discharges and corona could reveal valuable knowledge that enriches the effectiveness of PD diagnosis technology. An experiment was therefore conceived in which equal attention was given to cavity, surface and corona discharges as explained in the following section. 3 THE EXPERIMENTAL SETUP The experimental setup used for investigating the PD spectral content dependency on supply voltage frequency is depicted in Figure 1. The test specimens were designed to give undistorted frequency response up to 1 GHz and this was verified through measurement using a network analyser. The PD signal detection was through a ring guard electrode and measuring electrode setup. The measuring electrode disc was connected to the grounded guard ring electrode using 3 x 150 Ω resistors in a star formation. This ensured impedance matching with the 50 Ω signal cable. The test cells showing the type and dimensions (not drawn to scale) of the artificial PD defects are schematically shown in Figure 2. The defect dimensions were chosen such that in all cases the PDIV was the same, approximately 6 kV at 50 Hz. All the test samples were preconditioned by continuous stressing at 7 kV for an hour. This was to avoid taking measurements in the first hour of voltage application where PD behavior, particularly in voids, could be more influenced by rapid physiochemical changes in the defect than the controlled variables [6]. Each test cell was then tested separately at 7 kV. The PD signals were captured as frequency spectra using a Rohde & Swarz model FS300, 9 kHz - 3 GHz sweep tuned spectrum analyser (SA). The SA was set in full span and maximum amplitude hold mode. The SA settings were kept at constant optimal values throughout all the tests. Each spectral record at every SVF was a resultant trace of maximum signal magnitude registered at each spectral frequency component over a period of 2 minutes of continuous repetitive sweeps. The SVF was varied from 20 to 400 Hz and incremented in steps of 20 Hz. Five independent measurements were taken at every step. The 2-minute test duration and number of measurements per every test step for each sample were optimally minimal in order to limit PD induced ageing effects that would otherwise adversely influence the test results. In order to check results repeatability, a number of independent but similar tests were conducted. The experimental tests were performed in a screened laboratory environment at ambient atmospheric conditions of 20 to 25oC and 50% humidity. Each set of measurements did not take more than 3 hours including the preconditioning time. Related tests, but in the time domain, were conducted on similar test samples. The same experimental setup and procedures were used but with a suitably fast digital oscilloscope as the detection instrument instead of the spectrum analyser. Details of this work are described elsewhere in [17]. 4 RESULTS Variations of the PD spectral bandwidth in response to changes in the SVF were plotted for each defect type. Best fitting trend lines were generated for each scatter plot. A qualitative comparison of the trends showed close similarity for each defect type in all three sets of independent but identical measurements. This section presents the measurement results, analysis and Figure 1. The experimental setup for investigating the influence of supply discussion for respectively: void, surface and corona voltage frequency on partial discharge characteristics. discharges. Figure 2. Test samples showing the various PD defect models and dimensions used in the experimental investigation of PD dependency on supply voltage frequency. The tip radius of the corona discharging copper needle was about 50 m. 800 4.1 VOID DISCHARGES and is given by equation 1. 600 500 400 300 200 100 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 Supply voltage frequency (Hz) (a) -30 -35 -40 -45 Magnitude (dBm) 4.1.2 CAVITY DISCHARGE MECHANISM ANALYSIS Figure 4 illustrates the sequence of events in a cavity discharge process used to interpret the experimental observations on cavity PD dependency on SVF. All equations and analytical expressions used in explaining the cavity discharge process are adopted from Niemeyer’s generalised PD model [18]. The process begins with establishment of the resultant field ( E i ) that is responsible for initiating a discharge in the cavity PD spectral bandwidth (MHz) 700 4.1.1 MEASUREMENTS AND OBSERVATIONS The spectral bandwidth of cavity PD was generally not responsive to changes in the SVF from 20 to 400 Hz, and this was irrespective of the cavity position between the electrodes. The cases investigated were: HV electrode bound, earth electrode bound and fully embedded cavity in insulation. The scatter plot of the spectral bandwidths as a function of the SVF for an HV bounded cavity is shown in Figure 3a. Examples of the spectra recorded at SVF of 20 Hz and 400 Hz are shown in Figure 3b. In Appendix A results of the embedded and earth electrode bound cavities are given. It is notable that the degrees of stochastic scatter, as indicated by the confidence range at each measuring point in the plots, were more pronounced than in surface discharges and corona. A closer look at the scatter plots also suggests some quasi-modulation in the trends. In the time-domain, except for negative PD pulse height that decreased with increase in SVF, all the other PD pulse parameters were not responsive to the changes in the SVF in range 20 to 400 Hz as published earlier by the authors [17]. At 400 Hz supply voltage frequency -50 -55 -60 -65 -70 -75 0 At 20 Hz supply voltage frequency 100 200 300 400 500 600 700 800 900 1000 Frequency (MHz) (b) Figure 3. (a) Cavity PDs spectral bandwidth as a function of the supply voltage frequency. (b) Typical spectra at lowest and highest frequency of the supply voltage. E i = fE 0 + E res (1) Where: E 0 = electric field in the insulation due to the externally applied voltage [V/m]. f = stress enhancement factor, which is a function of the cavity dimensions. In spherical cavities 3ε r and ε r is the dielectric constant f = 2ε r + 1 of the insulation [F/m]. Eres = stress created by the space charge deployed on the cavity walls after a discharge event [V/m]. The stress in the cavity increases until the stress conditions become conducive to PD inception, that is, the resultant stress ( E i ) becomes at least equal to the streamer initiation involves detrapping of electrons from residual charge that would have been deployed on the cavity surfaces by the previous discharge events. In small cavities not exposed to radioactive radiation and where the cavity surfaces have a high work function, the dominant seed electron source can be assumed to be surface emission process [3]. Some of the residual charge is lost through conduction across the cavity walls and this process is governed by a decay time constant ( λcond ) derived from the resistance and capacitance (RC) model of the cavity and is given by: λ cond = Where: threshold stress ( E str ). In an air filled spherical cavity the threshold stress is given by: E str = p ( Ei p ) cr [1 + B 2 rp ] Ei p ) cr = B = p = r = εr D cavity = the insulation dielectric constant [F/m]. = the cavity diameter [m]. σs = the cavity surface conductivity and this (3) parameter increases with ageing of the cavity under continuous exposure to PD [S]. (2) Where for air in the cavity: ( ε r D cavity 4σ s 25 [V/Pa.m]. 8.6 [Pa.m]. gas pressure in the cavity [Pa]. cavity radius [m]. Even if the stress is sufficiently high for streamer discharge initiation a discharge only occurs when an initiatory or seed electron becomes available in the cavity at strategic positions such as near the anode. The source of such an electron can be either volume or surface emission processes. Volume generation of the seed electron involves release of free electrons by gas molecules or negative ions due to ionisation by cosmic/radioactive energy. The surface emission process The remaining electrons, that would have survived loss through conduction, further suffer decay as some migrate deep into the insulation. This loss is accounted for through a decay term given by: (− e t λtr ) (4) Where: λtr t = the time constant of loss through the migration [s]. = the time elapsed since the last PD event [s] Figure 4. An illustration of the sequence of events in a cavity discharge process showing the role of the residual space charge The rate of detrapping electrons from the cavity walls avalanches that can be characterised through the discharge current pulse shape or spectral content. Assuming the residual charge decay parameters remain constant, λ cond (due to cavity . ( N e ) is given by: . N e = N e v 0 exp( − Φ− eE i ( 4 πε 0 ) kT ) (5) Where: (Φ − eE i ( 4 πε 0 ) is the Schottky term. v0 = fundamental phonon frequency for the insulation [s-1]. e = the elementary charge [C]. = the effective detrapping work function [eV]. = permittivity of vacuum [F/m]. Φ ε0 k T Ne = the Boltzmann constant [JK-1]. = the temperature [K]. = total number of electrons available for migration. E = the electric field on the emitting cavity surface temperature [V/m]. At instances when the cavity surfaces are negatively charged, the total number of electrons available for detrapping ( N e ) is scaled down by a factor ( ξ ) that accounts for the more difficult process of detrapping electrons from negatively charged insulation surfaces [16]. At any given instant the production of an initiatory electron from the cavity surface is therefore a stochastic process that can be expressed as a probability function and denoted by; P ( dt ) = 1 − e − ( N e dt ) (6) When initiated the discharge event causes the field in the cavity to change from E i to a residual value E res . The difference in the electric field is associated with a charge transfer given by: ∆ q ( t ) = ε 0 π r 2 ( E i ( t ) − E res ) (7) Where: r E i (t ) = the cavity radius [m]. = the resultant field in the cavity [V/m]. E res = the field remaining in the cavity after a discharge event [V/m]. The resultant field in the cavity therefore varies with time as a function of the sinusoidal external stress. Using equations 6 and 7, the pulse phase positions and corresponding charge magnitudes can be determined. 4.1.3 CAVITY DISCHARGE TEST RESULTS ANALYSIS AND DISCUSSION It is evident from the discharge model outlined above that the residual space charge in the cavity plays a critical role in determining the point at which discharge occurs along the test voltage cycle. This in turn affects the nature of the discharge surface conductivity) and λ tr (due to charge migration to deeper insulation traps), the amount of residual charge generated at the instant of a new PD depends on the length of the time ∆ t PD between consecutive pulses. With reference to Figure 4, ∆ t PD is proportional to the rate of increase of the resultant field E i (t ) which in turn is a function of the rate of change of the supply voltage. If the rate of change of E 0 ( t ) is much slower relative to λ cond and λ tr then the discharge characteristics are not influenced by the SVF. This is considered as a possible explanation for the observation in this experimental work where the positive PD pulse parameters were not influenced by the variations of the SVF in the range 20 to 400 Hz. An increase in the SVF causes reduction of the time slot ( ∆ t PD ) that is available for PD pulses to occur and therefore causes reduction in the number of PD pulses. This relationship was noted by some researchers while investigating the influence of SVF on PD characteristics [3,5]. In the zero crossing regions, particularly on polarity reversal from positive to negative, different conditions exist. The amount of residual charge remaining after time ∆ t 1 PD , (and assuming no significant changes in λ cond and λ tr ) is proportional to the number of PD pulses in the period preceding this point in time. Since the latter is a function of the SVF, the residual charge in the zero crossing region can therefore be assumed to be dependent on the SVF. It follows that the PDs associated with this residual charge are expected to respond to changes in the SVF. In the zero crossing region discharges occur at bigger gap overvoltages because E 0 ( t ) and E res are in phase. Furthermore where polarity changes from positive to negative, it is more difficult to extract seed electrons from negatively charged surfaces. These factors mean that maximal PD magnitudes occur in this region and the PDs are more sensitive to changes in the SVF. This was confirmed by the findings in the time-domain measurements where the negative PD pulse magnitude decreased with increase in SVF as presented in [17]. Other researchers have attributed the appearance of ‘rabbit ear’ like portion of PRPD patterns to these type of PDs. The ‘ears’ were observed to disappear with increase in SVF [3] thus further confirming the model. When observed in the frequency domain in this work, the cavity PD frequency spectral characteristics were generally not responsive to changes in the SVF as shown in Figure 3. This is explained as follows: Since the frequency spectra were recorded using a spectrum analyser in maximum hold mode, the frequency components of both positive and negative PDs were overlayed. Although the negative PDs reduced in magnitude with increase in SVF, the resultant overall spectral profile did not change as it was dominated by the positive PDs that did not change with changes in the SVF. 4.1.4 KEY FINDING ON CAVITY DISCHARGES In small cavities the cavity PD spectral content is immune to variations in the supply voltage frequency. The life span of residual space charge in the cavity (after each PD event) relative to the rate of change of the supply voltage, determines how the cavity discharge characteristics respond to the supply voltage frequency. PD spectral bandwidth (MHz) 300 250 200 150 4.2 SURFACE DISCHARGES The experimental observations and results discussion of surface discharge tests are presented in this section. 100 50 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 Supply voltage frequency (Hz) (a) -30 -35 -40 -45 Magnitude (dBm) At 400 Hz supply voltage frequency -50 -55 4.2.1 MEASUREMENTS AND OBSERVATIONS Surface discharge spectral bandwidth increased with increase in the supply voltage frequency from 20 to 400 Hz as shown in the scatter plot of Figure 5a. The same trend was observed for the other two cases of surface discharges, that is, on the earth electrode and those on the high voltage (HV) electrode. As the supply voltage frequency was increased from 20 Hz to 400 Hz the average spectral bandwidth increased by about 170% for HV electrode surface discharges and about 40% in the case of the earth electrode surface discharges. Other forms of energy such as optical and sound increased in intensity as the SVF increased. Related measurements conducted in the time domain showed that negative surface discharge PD pulse shapes changed in response to changes in the supply voltage frequency in the range 20 to 400 Hz. More details of this work are described elsewhere in [17]. -60 4.2.2 -65 -70 At 20 Hz supply voltage frequency -75 0 50 100 150 200 250 300 350 400 450 500 Frequency (MHz) (b) Figure 5. (a) Surface discharge spectral bandwidth as a function of the supply voltage frequency. (b) Typical spectra at lowest and highest frequency of the supply voltage. SURFACE DISCHARGE MECHANISM ANALYSIS AND RESULTS DISCUSSION The response of surface discharges to variations in the supply voltage frequency can be analytically explained in terms of space charge dynamics along the insulation interface with the metallic electrode. Murooka et al (as summarised in [19]) studied surface discharge mechanisms using dust and photographic figure methods, and this gives useful basis for interpreting surface PD mechanisms under varying SVF. Figure 6 gives an illustration of the surface discharge mechanism. Figure 6. An illustration of surface discharge mechanisms showing the influence of the slower positive ion cloud. The space charge dynamics’ response to increase in SVF can be deduced from the illustration of the mechanisms. In the positive half cycle at the tip of the positive ion cloud the stress due to this space charge superimposes on the background stress. The maximum electric field therefore occurs at the advancing plasma tip [19,20]. Assuming constant space charge decay, an increase in the supply voltage frequency gives less time for the positive ion space charge to disperse during the full cycle of the supply voltage. By increasing SVF, the amount of positive ion space charge available in each discharge event increases. An increase in the amount of the positive ion space charge results in more stress enhancement at the tip of the positive ion space cloud causing faster avalanches and further extension of the discharge streamer. This manifests as increased discharge frequency bandwidth as well as optical and audible emissions as observed in the experimental tests conducted for this work. In the negative half cycle the positive ion cloud near the cathode causes enhanced stress region between the ion cloud and the cathode. This region is known as the cathode fall [19]. More avalanches are consequently initiated producing more positive ions as electrons (being much lighter) are quickly swept away to the opposite electrode. Since increased SVF results in increased positive ion space charge, it follows therefore that the size of the cathode fall region increases with increase in the SVF. The shielding effect of the positive ion cloud increases resulting in further limitation of the negative discharge streamer growth. This is the reason why in the timedomain negative discharge pulse magnitudes were observed to decrease with increase in the SVF. In the frequency domain, where both positive and negative discharges were detected simultaneously using a spectrum analyser in a maximum hold mode, the frequency components of the positive discharges prevailed over those of the negative discharges. The overall surface discharge spectral bandwidth therefore increased as the SVF increased although that of the negative discharges would have decreased due to the increased cathode fall. Increasing pulse rise-time is expected to be associated with decreasing spectral bandwidth. The time-domain pulse parameter measurements taken as a function of SVF compared with the corresponding frequency domain spectral measurements, showed non-conformance to this principle. In the time-domain generally the positive pulse parameters remained unchanged as the SVF increased and yet in the frequency domain the bandwidth increased. A possible explanation for this could be the effect of pulse superpositions. Incidences of pulse superposition have a high probability of occurrence in surface discharges particularly with the type of samples used in this work, where the test sample was made up of a disc electrode pressed against a layer of dielectric insulation. The phenomenon of PD pulse parameters distortion due to superposition was also experimentally and analytically explored by Reid et al [21] as well as Brosche et al [22]. They reported that current pulses could occur in bursts of up to ten individual pulses in as short time as 1 ns. Such pulses are difficult to resolve using ordinary signal measuring instruments. 4.2.3 KEY FINDING ON SURFACE DISCHARGES Surface discharge spectral bandwidth increases with increase in supply voltage frequency. Positive ion clouds effectively determine how surface discharges respond to variations in the SVF through either influencing streamer mechanisms in the positive half cycle or the cathode fall magnitude in the negative half cycle. 4.3 CORONA DISCHARGES The point to plane corona experimental test results as well as observations are presented and discussed in this section. 4.3.1 MEASUREMENTS AND OBSERVATIONS The variation of spectral bandwidth of point to plane corona on HV electrode in air as a function of SVF (in the range 20 to 400 Hz) is shown in the scatter plot in Figure 7a. Examples of the corona spectral signatures recorded at the lowest (20 Hz) and at highest (400 Hz) SVF frequencies are shown in Figure 7b. Corona on the earth electrode behaved generally the same as that on the HV electrode as shown in the plots in Appendix A. The corona discharges spectral bandwidth decreased with increase in the SVF. The trends for both HV electrode and earth electrode corona had a step-wise profile. Equivalent measurements were conducted on corona in the time-domain. The corona pulse parameters (rise-time, width, fall-time and height) exhibited similar trends as those of frequency domain measurements as discussed in detail elsewhere [17]. As in the frequency domain the pulse rise-time and height decreased and in distinct steps as the SVF increased. The corresponding audio and optical emissions from the corona PDs also decreased with increase in SVF. 4.3.2 CORONA MECHANISM ANALYSIS AND TEST RESULTS DISCUSSION The behaviour of corona discharges under varying SVF is remarkable as it contrasts that of surface and cavity discharges. With reference to Figure 8, showing schematic illustrations of corona discharge mechanisms derived from literature [23-26], the tests results and observations are discussed. 350 PD spectral bandwidth (MHz) 300 250 200 150 100 50 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 Supply voltage frequency (Hz) (a) -10 -20 Figure 8. An illustration of positive corona mechanism [23]. Magnitude (dBm) -30 -40 At 20Hz supply voltage frequency -50 -60 -70 -80 0 At 400 Hz supply voltage frequency 100 200 300 400 500 600 700 800 900 1000 Frequency (MHz) (b) Figure 7. (a) Corona discharge spectral bandwidth as a function of the supply voltage frequency. (b) Typical spectra at lowest and highest frequency of the supply voltage Corona in air gives distinct phase-resolved patterns [26]. Discharges that occur while the needle tip is at negative potential are typically very small and uniform and this is negative corona. With the needle tip at positive potential, much bigger pulses occur on the voltage cycle crest (positive corona). When an oscilloscope was used to detect the corona pulses for pulse shape analysis as a function of frequency, the negative corona (also known as Trichel pulses) were below the detection level of the system. Only positive corona pulses could therefore be detected [17]. Similarly in the frequency domain when both negative and positive corona were detected simultaneously using a spectrum analyser in the maximum hold mode, the resultant spectrum was that of the positive corona pulses. Analysis and discussion of experimental findings in this work are therefore only about positive corona as they were the only corona discharges detected under the given experimental conditions. The corona discharge models in literature [23-26] are used to analyse and discuss the experimental results obtained in this work where the behaviour of corona under varying SVF was investigated. According to literature, distinctly different modes of corona discharges occur depending on the stress conditions at the anode. At an electric stress that is significantly above the discharge inception level the discharges are dominated by the onset corona streamer [23,24]. As illustrated in Figure 8, after initial avalanches, a space charge cloud enhances the field in the gap and this initiates secondary avalanches that extend radially into the lower field regions of the gap. Unlike in the negative corona where negative ions influence the process [25], in positive corona, electrons are always under the influence of strong electric field and therefore are quickly swept away by the anode without a chance to form negative ion molecules. An increased size of the positive ion space charge forms around the anode thereby effectively shielding the anode from the opposite electrode. The local electric field at the anode drops and the discharge extinguishes. The positive ion cloud then drifts away under the influence of the electric field thus clearing up the shielding effect [23,27]. The reestablishment of the high stress conditions at the anode initiates another discharge event, and the process then repeats. Under alternating voltage conditions the stress in the gap changes in magnitude and polarity as a function of time. After the peak, the stress decreases until it gets to zero and then increases again in the opposite direction. The effect on the positive ion cloud (still present in the gap after the last positive corona discharge) is a reduction in the rate of drift. On stress polarity reversal the space charge cloud drifts in the reverse direction. An increase in the rate of change in stress polarity results in increased retention of the residual positive ion cloud and therefore causes more effect on the nature of the subsequent discharges. This is the reason why the corona spectral bandwidth and magnitude decreased as the SVF increased from 20 to 400 Hz as shown in Figure 7. APPENDIX A 4.3.3 KEY FINDING ON CORONA DISCHARGES Point to plane corona spectral bandwidth in air decreases with increase in the supply voltage frequency. Positive corona discharge mechanisms depend on the dynamics of the positive ion cloud in the discharge gap. Factors such as increase in the SVF that cause prolonged presence of the space charge cloud in the discharge gap inherently alter the positive corona discharge characteristics as observed in the experiments. The influence of the supply voltage frequency on the PD spectral content was unique for each defect type. The response was however independent of the relative positions of the defects between the electrodes. Figure A1 and A2 show the embedded cavity and earth electrode cavity respectively. Figure A2 and A3 are scatter plots of the spectral bandwidth variations and examples of spectra recorded at highest and lowest supply voltage frequency for surface discharges on earth electrode/insulation interface and corona on earth electrode respectively. In the case of surface discharges and corona, the changes in the spectral content as a function of the supply voltage frequency also manifested as changes in the audio and optical energy emissions from the discharges. The way in which the PD frequency spectra responded to variations in the supply voltage frequency depended on the defect type. Void defect PDs were immune to changes in frequency of the supply voltage. Spectral bandwidth of surface discharges increased with increase in supply voltage frequency. The opposite was observed in corona discharges where frequency components diminished as the supply voltage frequency increased. The observed supply frequency dependent behaviour of PDs suggests the following possible diagnostic application-: • By sweeping the test voltage frequency through a range such as 10 Hz to 400 Hz, unknown PD source can be recognised from how the PD spectra change during the sweep. This technique can be used as a quick PD defect pre-classification or elimination of corona interference by elevating the frequency of the test voltage. ACKNOWLEDGMENT The authors would like to acknowledge with gratitude Eskom for their support of the High Voltage Engineering Research Group through TESP. They would also like to express gratitude to the Department of Trade and Industry (DTI) for THRIP funding and to thank the National Research Foundation (NRF) for direct funding of the research group. 250 PD spectral bandwidth (MHz) 5 PROSPECTIVE APPLICATION 6 CONCLUSION The supply voltage frequency (in the 20 to 400 Hz range) influences the frequency content of partial discharges in a manner that depends on defect type causing the discharges. Implications of this knowledge include the following: 200 150 100 50 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 Supply voltage frequency (Hz) (a) -35 ii) Where frequency spectra are used as characteristic signatures for each defect type in the frequency domain PD diagnosis, the influence of the SVF can pose a challenge in the reliability of the diagnostic results. As an example, spectral patterns of surface discharges in a power cable termination that is energised at 0.01 Hz test voltage can be remarkably different form that obtained at 50 Hz test voltage. It is imperative therefore that in classifying PD sources using spectral characteristics, the influence of SVF on the spectral features is appropriately taken into account. -40 -45 Magnitude (dBm) i) Comparative analyses of PDs obtained through test voltages of different frequencies should be done in cognisance of the different behaviour of PD types under different test voltage frequencies. -50 At 400 Hz supply voltage frequency -55 -60 -65 -70 -75 0 At 20 Hz supply voltage frequency 100 200 300 400 500 600 700 800 900 1000 Frequency (MHz) (b) Figure A1. (a) Imbedded cavity PDs spectral bandwidth dependency on supply voltage frequency. (b) Typical spectra at lowest and highest frequency of the supply voltage. 300 450 250 350 PD Spectral bandwidth (MHz) PD spectral bandwidth (MHz) 400 300 250 200 150 100 50 200 150 100 0 50 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 Supply voltage frequency (Hz) 0 (a) 20 40 60 80 100 120 140 180 200 220 240 260 280 300 320 340 360 380 400 (a) -40 -10 -45 -50 -20 At 400 Hz supply voltage frequency -55 -30 -60 Magnitude (dBm) Magnitude (dBm) 160 Supply voltage frequency (Hz) -35 -65 -70 -75 At 20 Hz supply voltage frequency -40 At 20 Hz supply voltage frequency -50 -60 -80 0 100 200 300 400 500 600 700 800 900 1000 Frequency (MHz) -70 (b) Figure A2. (a) Earth electrode bounded cavity PDs spectral bandwidth as a function of the supply voltage frequency. (b) Typical spectra at lowest and highest frequency of the supply voltage. 250 At 400 Hz supply voltage frequency -80 0 100 200 300 400 500 600 700 800 900 1000 Frequency (MHz) (b) Figure A4. (a) Earth electrode corona PDs spectral bandwidth as a function of the supply voltage frequency. (b) Typical spectra at lowest and highest frequency of the supply volt. PD spectral bandwidth (MHz) 200 REFERENCES 150 [1] 100 [2] 50 [3] 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 [4] Supply voltage frequency (Hz) (a) -30 [5] -35 -40 At 400 Hz supply voltage frequency Magnitude (dBm) -45 [6] -50 -55 [7] -60 -65 -70 -75 0 [8] At 20 Hz supply voltage frequency 50 100 150 200 250 300 350 400 450 500 Frequency (MHz) (b) Figure A3. (a) Earth electrode/insulation interface surface PDs spectral bandwidth as a function of the supply voltage frequency. (b) Typical spectra at lowest and highest frequency of the supply voltage. [9] IEEE C57.113, (1991) Guide for Partial Discharge Measurement in Liquid Filled Power Transformers and Shunt Reactors. F. J. Wester, E. Gulski and J. J. 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Fouracre, “Partial discharge current pulses in SF6 and the effect of superposition of their radiometric measurement”, Journal of Physics: Applied Physics, Vol. 39, pp. 4167 - 4177, 2006. [22] T. Brosche, W. Hiller and E. Fauser, “Novel characteristion of PD signals by real-time measurement of pulse parameters”, IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 6, No. 1, pp. 51-59, 1999. [23] S. P. Maruvada, “Corona and Gap Discharges”, In S. P. Maruvada, Corona Performance of High Voltage Transmission Lines. Research Studies Press Ltd, 2000. [24] M. G. Comber, D. W. Deno and L. E. Zaffanella, “Corona Phenomena on AC Transamission Lines”, In EPRI Transamission Refence Book, 345 kV and Above, pp. 169-186, EPRI, 1987. [25] W. L. Lama, and C. F. Gallo, “Systematic Study of the Electrical Characteristics of the "Trichel" Current Pulses from Negative Needleto-plane Coronas”, Journal of Applied Physics, Vol. 45, Issue no.1, pp. 103-113, 1974. [26] F. Kreuger, Partial discharge detection in high-voltage equipemnt. London: Butterworths, 1989. [27] H. Ryzko, “Drift Velocity of Electrons and Ions in Dry and Humid Air and in Water Vapour”, Proceedings of the Physics Sociaty (Proc. Phys. Soc.), Vol. 85, No. 6, pp. 1283-1295, 1965. Cuthbert Nyamupangedengu was born in Zimbabwe. He received a Bachelor of Technology honours degree in electrical engineering from the University of Zimbabwe in 1994. At the University of the Witwatersrand, Johannesburg, in 2004, he was awarded the M.Sc. Engineering (with distinction) and in 2011, the PhD. He is a lecturer at the University. Before moving to the University of the Witwatersrand, Cuthbert was an engineer in the Zimbabwe Electricity Supply Authority from 1995 to 2005 in various capacities mainly in power system planning and development. His passion is research on diagnosis of high voltage dielectric insulation. Ian R. Jandrell (M’85) was born in East London, South Africa. He received a B.Sc. degree in electrical engineering, GDE (Elect.) and Ph.D. degree from the University of the Witwatersrand, Johannesburg, RSA in the period 1981 to 1990. Ian Jandrell is the “CBI Electric Professor of Lightning” and holds the rank of “Personal Professor” in the School of Electrical and Information Engineering at the University of the Witwatersrand, Johannesburg. Since June 2001 he has been the Head of the School of Electrical and Information Engineering. He is a registered professional engineer in South Africa and is also a director of a number of companies.