Audible noise spectral characteristics of high-voltage ac bundled conductors at high altitude

At high altitudes, the corona discharge around a conductor surface is severe, and the induced audible noise (AN) is irritating; this is due to the low air density at high altitudes. Therefore, AN has become a crucial limiting factor in the design of ac power lines of 500 kV or higher at high altitudes. An investigation of the spectral characteristics of AN should help provide a greater understanding of corona noise; however, only a few studies have investigated the spectral characteristics of AN in practical bundled conductors at high altitude. Therefore, it is difﬁcult for power utility companies to select suitable conductors. In this study, the AN spectral characteristics of 6 × LGJ400, 6 × LGJ720, and 8 × LGJ500 bundled conductors were investigated using an ultra-high-voltage corona cage (8 × 8 × 35 m) in Xining, Qinghai Province (altitude: 2261 m). The AN equivalent A-weighted level and the 1/3-octave frequency characteristics of these three conductors were obtained, and the inﬂuence of the electric ﬁeld (E-ﬁeld) on these characteristics was analysed. Subse-quently, the relationship between the AN A-weighted level and the 8-kHz level was examined. We found that, with the increase of the E-ﬁeld, the low-frequency components of AN level did not exhibit an obvious trend, but in the high-frequency band (1.6–20 kHz), a clear positive correlation was observed between the spectrum level and E-ﬁeld strength. Among these three conductors, the 8 × LGJ500 conductor was the optimal conductor for reducing the AN levels at high altitude. The results obtained in this study can provide a data reference for the construction of high-altitude ac power lines.


INTRODUCTION
The electric field (E-field) strength of a bundled conductor surface increases with the operating voltage of ac power lines. This increase can result in strong corona activity and thus induce some negative effects, such as corona loss, radio interference, and audible noise (AN) [1][2][3][4]. The spectral characteristics of AN generated by corona discharge differ from the spectral characteristics of environmental noise; AN is particularly irritating and adversely affects the acoustic environment around power lines. In China, the AN threshold of ac transmission lines (110-1000 kV) is clearly limited [5,6]. The AN of ac transmission lines consists of two main components: (1) A broadband crackling and hissing noise, and (2)   is mainly caused by an irregular corona pulse induced by corona discharge. The humming noise (main components: 100 or 120 Hz) is caused mainly by a periodic pressure wave that is produced from the drift of ions left behind by discharges [7,8].
The frequency of the pure tone is low, and pure tone noise can travel a longer distance than broadband noise, which occasionally renders the pure tone more irritating [7]. In Japan, the number of complaints due to pure tone noise is even higher than the number of complaints due to high-frequency noise [9,10].
Recently, in order to satisfy the energy needs for economic and social development, extra-high voltage (EHV) or ultra-high voltage (UHV) transmission project has developed strongly in some countries such as China and Brazil for fossil energy and clean energy transmission [11][12][13][14]. However, owing to the uneven energy distribution and terrain condition, some transmission lines must be constructed at high altitudes [15,16]. For example, some power lines must be constructed on the Qinghai-Tibetan Plateau in China for clear energy transmission where the average elevation is over 2000 m.
In high-altitude areas, the air density is low and the mean free path of charged particles is long, which can result in strong corona discharge and a serious AN level [17][18][19]. Without effective control, noise pollution will become a threat to surrounding areas. However, if the control is excessive, the project cost will increase sharply because of expensive methods, such as increasing the conductor cross-section and lifting the lines higher.
Therefore, considering the severe AN level at high altitudes, different spectral characteristics in the human response and the economy of power project construction, it is vital to investigate the AN spectral characteristics of different bundled conductors at high altitudes, which would help to provide data and theoretical support for power utility companies in the construction of power lines at high altitudes.
In the past decades, Kolcio et al. from the American Electric Power Service Corporation conducted AN measurements for 750-kV power lines over a four-year period; they studied the AN spectral characteristics under both rainy and fair weather conditions in 1979 [20]. In 1981, researchers from the Bonneville Power Administration (BPA) analysed the octave band spectra for a single 63.6-mm conductor on 500-kV ac lines; they found the pure tone level to be higher than high-frequency noise under rainy conditions [4,21]. Molino et al. studied the response of different humans to noise frequency and analysed the frequency spectral characteristics of five EHV ac lines [22]. In recent years, with the development of UHV transmission lines in China, some researchers have measured the AN frequency spectra of 1000-kV power lines. Lu and Zhang et al. from the China Electric Power Research Institute have analysed the AN spectra of a 1000-kV UHV single-circuit ac line and a 1000-kV UHV double-circuit ac line [23][24][25]. They have discussed the relationship between the A-weighted level and the 8-kHz level and also observed that for the 1000-kV UHV double-circuit power line, the 100-Hz pure tone level reached 61.4 dB outside 10 m from the outer phase under heavy rain conditions.
In recent years, some scholars have conducted in-depth research on the mechanism involved in the generation of ac corona noise and pure tone noise; these scholars have also discussed the relationship between time-domain noise performance and corona current pulses and investigated the impact of raindrop shape on noise under rainy conditions [7,[26][27][28]. Furthermore, other scholars have studied the frequency spectral characteristics of the AN level of conductors under dc voltage by using a corona cage [29][30][31][32][33]; they have analysed the impact of rain, ice, pollution, and other external conditions on the AN level and its frequency spectrum.
To investigate the AN characteristics of ac transmission lines in high-altitude areas, different institutes, such as the BPA from the United States, Eskom from South Africa, and the China Electric Power Research Institute, have studied the influence of altitude on the AN A-weighted level and proposed different cor-rection coefficients [19,34,35]. Because the human ear has different responses to different sound frequencies, the A-weighted network is proposed to make an approximation of ear response to different frequency levels of sound. It can generally describe the human annoyance to environmental noise. Therefore, the A-weighted level is usually used to evaluate and limit the noise level [36].
These studies have focused mainly on the A-weighted level of bundled conductors at high altitudes, and the in-depth research on the spectral characteristics of AN has not been considered and conducted. Moreover, practical transmission lines adopt mainly bundled aluminium conductor steel-reinforced (ACSR) wires with a large spatial scale and complex surface status, and the existing physics model, which is based mainly on thin wires, has not adjusted to predict the spectral characteristics of AN in ac bundled conductors. Therefore, experimental research on the AN frequency spectral characteristics of bundled conductors in high-altitude areas is necessary.
In this study, a UHV corona cage from the State Grid Corporation of China was deployed at a high altitude in Qinghai Province. The AN frequency spectral characteristics of three types of ACSR bundled conductors-6 × LGJ400, 6 × LGJ720, and 8 × LGJ500-were investigated under the condition of heavy rain. The six-bundle conductors are widely used in 750-kV power transmission projects in the high-altitude areas of Northwest China, and the 8 × LGJ500 conductor is used in 1000-kV UHV single-circuit transmission projects in China. The effect of the E-field strength around conductor surfaces on the low-frequency tone noise and broadband noise was discussed, and the relationship between the A-weighted level and the 8-and 16-kHz levels was analysed.

UHV corona cage at high altitude
To perform investigations on bundled conductors with six or more subconductors, a large corona cage test system is required to reproduce a conductor state that is similar to the conductor state used in operational power lines. The State Grid Corporation of China has established the UHV ac corona cage in Xining, Qinghai Province (at an altitude of 2261 m), where the cage is 35 m long and has a cross-section of 8 m × 8 m (Figure 1). Power is supplied by a single-phase ac test transformer with a rated capacity of 400 kVA and a rated voltage of 800 kV to reproduce the corona discharge of practical ac power lines up to 1500 kV. A rainfall system is used to simulate different rainfall conditions; the system can generate rain falling at speeds of 1.5 to 50 mm/h.

Test conductor types
The AN characteristics of the three types of ACSR bundled conductors-6 × LGJ400, 6 × LGJ720, and 8 × LGJ500-  were investigated in this study; the conductor configuration and ACSR structure are presented in Figure 2. The maximum voltage and the corresponding E-field around the surfaces of different conductors in the corona cage are listed in Table 1.

Measurement conditions and equipment
The corona test in this study was conducted at 2261 m; the temperature in this area was in the range 16-25 • C, and the wind speed was less than 3 m/s. Because of the randomness of corona discharge around the conductor surface under fair weather conditions, it would be difficult to obtain effective and stable data to study the AN performances of different bundled conductors. Moreover, the corona discharge and AN level under rainy conditions are more severe than the corona discharge and AN level under fair weather conditions for high-voltage ac (HVAC) power lines. Therefore, the AN test in the corona cage is usually conducted under the conditions of heavy rain by different institutes; the AN levels under different weather conditions can be inferred according to heavy rain data and empirical correction [1,4]. During the experiment in this study, the average rainfall speed was at least 18 mm/h. The values of meteorological parameters (such as precipitation, air pressure, temperature, wind direction, and wind speed) during the tests were obtained from the HOBO weather station (Onset, USA). A photograph of the experimental platform is depicted in Figure 3.
The Brüel & Kjaer (B&K, Naerum, Denmark) noise measurement system consisting of a microphone, data acquisition unit, and computer was employed in this study ( Figure 4). The microphone was a 1/2-in free-field probe of type 4189 (B&K), with a frequency range of 6.3 Hz to 20 kHz, a sensitivity of 50 mV/Pa and a measuring range of 14.6 to146 dB. The probe can meet the class I level of the IEC 61672 standard. The data acquisition module was a type 3050 (also by B&K), offering 4-6 high-precision input channels with frequency ranging from dc to 51.2 kHz [37]. After the signal was processed by the acquisition module, the Pulse Labshop software package was used   Figure 4, where the microphone is situated in the centre of the longitudinal direction of the corona cage and the distance between the microphone and the centre point of bundle conductor section was 4.8 metres. The relationship of microphone location, sound pressure level and acoustic power density level can be found in [35].
In this study, the 1/3-octave frequency was used for noise spectral analysis, and the process involved in the 1/3-octave frequency analysis for the AN measurement is shown in Figure 5 [38].

AN spectral characteristics
The E-field strength around a conductor surface is generally used as the primary reference in investigations of the AN levels in a corona cage. Owing to the internal shielding effect in conductor bundles, the E-field at the surface of a single subconductor is not uniform [18]. Therefore, the maximum E-field strength at each subconductor surface was determined and aver- aged as the primary variable to study the AN level; this variable is known as the average maximum E-field strength around conductor bundles [1]. The ambient, rain, and power transformer noises were measured first in the experiment. Subsequently, the background noise was eliminated according to [30] to obtain the actual corona noise. If the difference between the background noise and the measured noise is less than 3 dB, a valid sound test may not be possible.
In this study, a CoroCAM 504 ultraviolet (UV) imager (South Africa) was used to take the graph of corona discharge under different E-field strengths in heavy rain. The UV images obtained when the 8 × LGJ500 conductor was used are depicted in Figure 6 as an example. Clearly, when the energised voltage was 184 kV (corresponding to an E-field strength of 8 kV/cm, sporadic discharge occurred on the conductor surface. When the applied voltage was increased to 322 kV (14 kV/cm), the corona discharge became severe and occurred throughout the conductor. When the applied voltage was 414 kV (18 kV/cm), the corona discharge strengthened further. The 1/3-octave noise frequency spectrum histogram and the equivalent A-weighted value of the 8 × LGJ500 conductor for typical E-field strength values is presented in Figure 7. As presented in Figure 7(a), the high-frequency level of background noise was low (approximately 40 dB). The pure tone components at 100 and 200 Hz were 48.8 and 43.5 dB, respectively. The equivalent A-weighted value noise was 52.6 dB(A), which was caused mainly by the noise of rain and the noise of raindrops colliding with the corona cage and conductors. As displayed in Figures 7(b) and (c), when the corona discharge was strong, the pure tone level (100 Hz), high-frequency level (above 1 kHz), and A-weighted level increased. When the E-field strength increased from 0 to 16 and 18 kV/cm, the 100-Hz component increased from 48.8 to 66.7 and 68.2 dB, respectively; the 200-Hz component increased from 43.5 to 55.2 and 56.9 dB, respectively; and the equivalent A-weighted level increased from 52.6 to 64.2 and 67.8 dB(A), respectively. The increase was substantial.
In addition, the 1/3-octave noise frequency spectrum histograms and the equivalent A-weighted values of 6 × LGJ400 and 6 × LGJ720 conductors are also given in Figures 8 and 9. The trends of different frequency levels were similar.

AN spectral analysis of three bundled conductors
The 1/3-octave band spectral characteristics of the three bundled conductors with E-field strengths in the range of 14-22 kV/cm were compared and analysed as illustrated in Figure 10. As displayed in Figure 10, the curves of spectral characteristics obtained in this study followed a typical ac corona noise shape, similar to the trends of the measured AN spectra of 500and 750-kV practical transmission lines at lower altitudes [4,22].
In the low-frequency band (0-400 Hz), an increasing trend was observed in the 100-and 200-Hz components; however, no clear trend was observed in the AN spectrum in this band, mainly because ac corona discharge noise is composed of 2f sound levels and high-frequency noise and the low-frequency band was dominated by background noise. Therefore, even when the field strength was 22 kV/cm, the low-frequency noise did not increase significantly. In the medium-frequency range (400 Hz-1 kHz), the noise spectral level caused by corona discharge began to increase, and with the increase in the E-field strength, the spectral amplitude gradually increased, showing some regularity. In the highfrequency range , the noise at all E-field strengths was significantly higher than the background noise. A clear positive correlation was observed between high-frequency noise and the E-field strength around the conductor surface. Consider, for example, the 8-kHz level at 14 kV/cm, the 8-kHz components of the 6 × LGJ400, 6 × LGJ720, and 8 × LGJ500 conductors were higher than the background noise by 6.5, 13.3, and 10.8 dB, respectively.

FIGURE 9
Spectral characteristics of 6 × LGJ720 conductor in corona cage (a) 1/3-octave spectrum of background noise for 6 × LGJ720, (b) 1/3octave spectrum of 6 × LGJ720 conductor at 16 kV/cm, (c) 1/3-octave spectrum of 6 × LGJ720 conductor at 18 kV/cm The high-frequency noise level showed an increasing trend with E-field strength. When the E-field strength was low, the increase in AN in the high-frequency section was larger with an increase in E-field strength; this increase in AN was smaller when the E-field strength was high, which was more obvious for the 6 × LGJ720 conductor. The 8-kHz level increased by 4.2 dB when the E-field strength was increased from 14 to 16 kV/cm, but the 8-kHz level increased by only 0.8 dB when the E-field strength increased from 20 to 22 kV/cm. This is mainly because at low E-field strengths, when the E-field increased, the corona discharge around the conductor surface increased considerably, and the discharge intensity also increased causing the coronagenerated AN level to also increase. When the E-field strength exceeded 20 kV/cm the corona discharge on the conductor surface was in a relatively saturated state; thus, there was a reduction in the increase in the AN level.

Changes in AN characteristic spectrum
The common types of environmental noise, such as wind, vehicle, or aircraft noise, are mainly in the low-frequency band (less than 500 Hz). By contrast, the main component of corona noise is higher than 1 kHz. Therefore, when the noise measurement is affected by ambient noise during the investigation of AN in practical power lines, the corona noise can be detected by comparing the spectral characteristics of the environmental noise and corona noise. The 8-kHz noise spectrum is often used by researchers as a reference spectrum of corona noise to distinguish it from background noise [4,23,32]. Pure tone noise attenuation is slow and more irritating. Therefore, the 100-Hz, 200-Hz, and 8-kHz components of the three bundled conductors were further examined and compared with the equivalent A-weighted level. As presented in Figure 11, the three reference spectral levels and the A-weighted level all increased with the E-field strength. The increase in the 8-kHz level and A-weighted level was larger than the increase in the 2f pure tone level. For the 6 × LGJ400, 6 × LGJ720, and 8 × LGJ500 conductors, when the E-field strength increased from 12 to 22 kV/cm, the 100-Hz sound pressure level increased by 7.5, 8.2, and 10.5 dB, respectively; the 8-kHz level increased by 17.6, 18.8, and 18.0 dB, respectively; and the equivalent A-weighted level increased by 12.5, 17.4, and 16.0 dB, respectively. For the 6 × LGJ720 large cross-section conductor, when the E-field strength was 18 kV/cm, the corona discharge appeared to be saturated, and the increase in the AN level was attenuated.
When the corona discharge occurred, the 100-Hz pure tone amplitude was higher, and in some cases, it was even higher than the equivalent A-weighted level. These results are similar to the practical long-term test results reported in previous studies [22,39]. The different types of weighted sound pressure levels are illustrated in Figure 12 [38]. Low-frequency noise contributes little to the equivalent A-weighted level of AN. Early researchers investigating AN in bundled conductors focused mainly on the equivalent A-weighted level; they did not focus much on pure tone noise, partly causing the complaints of pure tone noise even more than high-frequency noise as reported in the literature [9]. Therefore, when designing transmission lines at highaltitude locations in the future, the effects of pure noise should also be considered.

AN comparison of three bundled conductors
The pure tone noise and A-weighted level of three conductors are compared in Tables 2 and 3, and the ratio of average maximum electric field strengths are shown in Table 4, where the voltage applied to the conductors in corona cage was 1 kV. The 100 Hz pure tone level was found to have no clear changing trend for the three bundled conductors. However, when the E-field strength was constant, the A-weighted level displayed obvious differences. The level of the 6 × LGJ400 conductor  was the least, the 6 × LGJ720 level was the highest, and the level of the 8 × LGJ500 conductor was in between. Considering the electric strength of the three conductor, the 8 × LGJ500 was the least and approximately 78.8% of the 6 × LGJ400 value. Combined with AN performance and E-field strength distribution, the 8 × LGJ500 conductor was the optimal conductor among these three conductors in reducing the AN levels at high altitude.
In addition, as shown in Figure 11, a correlation was observed between the 8-kHz level and the equivalent A-weighted level of AN under different E-field strengths. The relationship between these two components is discussed in the next section.

Discussion of 8-kHz level and the A-weighted level
Tables 5-7 presented the difference between the AN Aweighted level and the 8-kHz spectral component of the three   conductors with different E-field strengths. For the 6 × LGJ400 conductor, the difference fluctuated to some extent. As the E-field strength increased, the difference between the AN Aweighted level and the 8-kHz spectral component decreased. The median difference was 10 dB, and the error was within 1.5 dB. For the 6 × LGJ720 and 8 × LGJ500 conductors, the differences between the A-weighted level and 8-kHz spectral component varied very slightly. If the 9 dB value was set as a correction factor, the maximum error of the 8 × LGJ500 conductor was 0.6 dB and that of the 6 × LGJ720 conductor was just 0.3 dB.
The differences between the A-weighted level and the 16-kHz spectral component were also indicated in the tables. Overall, the fluctuation in the differences between L(A) and L(8 kHz) was less than the fluctuation in the differences between L(A) and L(16 kHz). Therefore, the 8-kHz component was determined to be more suitable as a reference spectrum for distinguishing corona noise from background noise.
Notably, the relationship between the A-weighted level and the 8-kHz component was discussed in this study mainly based on the sound pressure level measured at the corona cage side. When the relationship between these two levels is investigated for a practical transmission line, the type of microphone, the distance between the microphone and conductors, and the layout of the power lines should also be considered and corrected [4]. Because air has a high absorption coefficient for highfrequency noise, the difference between the A-weighted level and the 8-kHz component would increase slightly with distance in practical transmission lines. As reported in [23], the difference between these two levels was approximately 9-11 dB at 20 m outside the phase conductor for a practical 1000-kV transmission line with the 8 × LGJ500 conductor.

CONCLUSIONS
In this study, the AN spectral characteristics of 6 × LGJ400, 6 × LGJ720, 8 × LGJ500 bundled conductors were investigated using the UHV corona cage at a high altitude (2261 m). The equivalent A-weighted levels and 1/3-octave frequency characteristics of the three bundled conductors were obtained under the condition of heavy rain with different E-field strengths around the conductor surfaces. Based on the study findings, the following conclusions can be drawn.
1. When different energised voltages were applied in the corona cage, the low-frequency band (0-400 Hz) components of the three conductors did not exhibit any trend, mainly because of the background noise. In the highfrequency band (1.6-20 kHz), a clear positive correlation was observed between the spectrum level and E-field strength.
2. An analysis of the 100-Hz pure tone component of the three conductor types revealed that the 100-Hz level increased when corona discharge occurred. Considering that pure tone noise attenuates slowly and is more irritating, the pure tone level must be evaluated and restricted in the design of highaltitude ac transmission lines. 3. Combined with AN performance and electric field strength distribution, the 8 × LGJ500 conductor was considered as the optimal conductor among these three conductors in reducing the AN levels at high altitude. 4. The relationship between the A-weighted level and the 8-kHz level of the three conductor types was investigated. The difference between these two levels for the 6 × LGJ720 and 8 × LGJ500 conductors was approximately 9 dB, and the median difference between the two levels for the 6 × LGJ400 was 10 dB with some fluctuation.
This study presented data references for the estimation of AN levels for different conductors and can be useful in the construction of high-altitude ac power lines. In the future, it will be necessary to investigate the AN performances of more large cross-section conductors (630, 720, 900 mm 2 ) and more bundle number conductors (10,12) at higher altitude (3000 or 4000 m) to meet the need of ultra-high voltage transmission project construction.