Introduction
Continuous detection and monitoring of partial discharges in high voltage (HV) electrical equipment is essential to avoid reaching the breakdown state of the insulation so that life time of the equipment can be enhanced. Gas insulated switchgear (GIS) is gaining considerable importance in recent times due to its ability to transmit high power in limited space [1]. Partial discharges (PDs) occur in high voltage GIS equipment due to sharp protrusions, free metallic particles, and voids in spacers which are created during production, transportation and installation [2]. PDs can be detected using acoustic, optical, chemical and ultrahigh frequency (UHF) sensing methods [3]. Among all methods, UHF sensing technique is popular due to its ability to detect all types of PDs, ease of installation, and suitability for online condition monitoring in GIS [4]–[6]. Several types of UHF sensors were used such as Hilbert antenna [7], slot antenna [8], spiral antenna [9], [10], disc sensor [11], and monopole antenna [12] were reported for PD detection.
Among the UHF PD sensors, spirals have the advantages of low profile, wide bandwidth (BW), good gain and circular polarization [10]. Among the spiral antennas, Archimedean spiral antenna (ASA) is widely used as it exhibits circular polarization over wider frequency band than the log spiral, but it has relatively large aperture area. The UHF spiral antennas reported for PD detection have large aperture (
There are several ways to miniaturize the ASA [19]–[21]. One such way is to meander its arms, which increases the electrical length of spiral arms. As meandering decreases the outer radius of the spiral, lowering of operating frequency was reported with periodic Z plane meandering [19]. Spiral arms with rectangular wave meandering at the end, full rectangular wave meandering and saw tooth wave meandering were also proposed for broadband operation and miniaturization [20]. Recently, numerical simulations of cosine wave meandered ASA operating over 0.92 – 3 GHz was reported for PD detection [22] with an wideband balun feed for symmetric bi-directional radiation from two electrically conductive spiral arms. This simulation study did not present PD detection capability or free space measurements of the meandered ASA.
Knowing these limitations of UHF PD sensors, we present a miniaturized planar ultrawideband circularly polarized UHF spiral antenna for PD detection with unidirectional radiation pattern. The proposed UHF PD sensor is cosine slot Archimedean spiral antenna (CSASA) optimized with a protective radome. Instead of metal spiral arms, we present a pair of planar slot spiral on the ground plane of 1.6 mm thick dielectric substrate. The planar slot spiral arms are center fed on the back plane of the dielectric substrate using aperture coupling. This type of feeding has advantages of broad bandwidth and low cross polarization. Furthermore, it eliminates the need for a wideband unbalanced to balanced transition typically used to feed the dual arms of the planar slot antenna. Thus, a simplified feed design is proposed unlike the large and delicate ultrawideband balun. The cavity backing of the UHF CSASA loaded with EM absorber is optimized for circular polarization over ultrawideband frequency of 0.5-5 GHz. The optimized CSASA design is fabricated and experimentally verified for its radiation characteristics in free space. The PD detection capability and sensitivity of the fabricated UHF CSASA sensor were assessed using GIS test cells with Corona, particle movement and surface discharge type of PD defects. The PD measurements were compared with conventional UHF PD sensor and its free space radiation characteristics were compared with existing UHF spiral antenna-based PD sensors.
The organization of the paper is as follows: The proposed UHF sensor design and optimization are presented in Section II. The optimized sensor fabrication and free space characteristics are presented in Section III. PD detection capability of the fabricated sensor and its comparison with reference UHF PD sensor are presented in Section IV followed by conclusion Section V.
Uhf Sensor Design and Optimization
A. Sensor Design
Figure 1 shows the cavity backed CSASA with radome investigated in this work. The outer diameter of the antenna determines the lower operating frequency, while the inner diameter determines the upper operating frequency. The slotted arms of spiral were meandered as cosine wave to reduce the antenna aperture size. The slot was defined as, \begin{equation*}r=r_{0}+a\phi +m\left ({\frac {\phi -\phi _{st}}{\phi _{end}-\phi _{st}} }\right)\cos \left ({n\phi }\right).\tag{1}\end{equation*}
Illustration of the ultrawideband cavity backed UHF cosine slot spiral Archimedean antenna (CSASA) sensor for PD detection. (a) Slotted metal plane in which the black region is metal and white space is air, (b) cross sectional view.
B. Numerical Modelling
The antenna was modelled in electromagnetic (EM) simulation software, Ansys HFSS® (Ansys Corp., USA). The substrate and superstrate were assigned the material properties of FR4 laminate. The substrate thickness was 1.6 mm and the copper cladding was set as 17
C. Design Optimization
CSASA design optimization was initially carried out only for the antenna design parameters namely, inner radius (
Firstly, the outer radius
Power reflection coefficient of CSASA for varying (a) spiral outer radius
Next, the amplitude of the cosine wave
Power reflection coefficient of CSASA for varying (a) amplitude of cosine wave meandering
The optimized CSASA was covered with ABS plastic radome for electrical isolation of the UHF sensor from HV environment. The radome thickness
Sensor Fabrication and Characterization
A. Uhf Sensor Fabrication
The UHF CSASA was fabricated on 1.6 mm thick FR4 laminate for the optimized antenna dimensions arrived in Section II using printed circuit board manufacturing process. The top and bottom sides of the fabricated CSASA are shown in Figures 5(a) and 5(b), respectively. The CSASA sensor with the radome has bidirectional radiation pattern as the radome is transparent to EM waves. The radiation pattern was converted to unidirectional radiation pattern using cavity backing in order to minimize EM interference pick up from the environment. An air filled metal cavity of height
Fabricated CSASA sensor showing (a) antenna top view, (b) bottom view and (c) complete assembly of antenna, cavity and radome.
Free space radiation measurements of the cavity backed UHF CSASA sensor in an anechoic chamber gathered using vector network analyzer (VNA) (E5071C, Keysight Technologies, USA) for varying
B. Free Space Measurements
Figure 6 shows the
Simulated and measured power reflection coefficient of CSASA sensor and its comparison with the reference UHF disc sensor.
Figure 9 shows the radiation pattern of the CSASA sensor measured in an anechoic chamber for line-of-sight illumination in XY (
Radiation pattern of the UHF CSASA sensor in (a) E-plane (XY) and (b) H-plane (XZ).
PD Measurements
A. Experimental Setup
Figure 10(a) shows the laboratory setup used to determine the PD detection capability of the fabricated UHF CSASA sensor. The sensor performance was evaluated for its ability to measure raw PD signals as well as phase resolved PD (PRPD) pattern in an unshielded laboratory environment. The setup consists of function generator, Trek amplifier (Model 20/20C) connected to the SF6 gas filled GIS test cells with three different electrode configurations for simulating PD activities (Figures 10(b)-(d)), reference UHF disc sensor, CSASA sensor, and digital storage oscilloscope (DSO) for raw PD signal acquisition and analysis (Figure 10(e)).
(a) Experimental setup for PD detection showing CSASA sensor, reference disc sensor and test cell; electrode configurations inside test cell for generating PDs of type (b) Corona, (c) particle movement, (d) surfaced discharge; Illustration of experimental setup for detecting (e) raw PD signals and (f) PRPD signals.
The DSO was replaced with signal analyser for PRPD analysis as illustrated in Figure 10(f). Three types of defects commonly occurring in GIS and other HV equipment namely, Corona (Cor), particle movement (PM), and surface discharge (SD) were simulated using the electrode configurations shown in Figures 10(b)-(d). Corona type of PD was generated using needle shaped HV electrode and flat ground electrode (needle-plane configuration) with 5 mm separation distance as shown in Figure 10(b). PM type of PD was generated using spherical upper electrode and a concave shaped bottom electrode with 5 mm separation distance. A 1 mm diameter aluminium particle was placed on the ground electrode to initiate PM type of discharges as shown in Figure 10(c). SD type of PD was generated by sandwiching 1.5 mm thick epoxy insulating disc between IEC(b) electrode and flat ground electrode as shown in Figure 10(d). The test cells were filled with SF6 gas maintained at 3 bar. A continuous power frequency (50 Hz) sinusoidal voltage from function generator was delivered to Trek amplifier and its magnitude was increased gradually until PD phenomenon was initiated in the test cell. The minimum voltage at which the PD signals generated is called inception voltage. The inception voltage was increased by 10% for the respective minimum value for all PD defect types so that the PD signals can be detected with good strength by both UHF sensors.
The radiated PD signals from the test cells were captured using CSASA sensor as well as the reference UHF disc sensor placed equidistance (200 mm) from the test cell. The sensors were connected to the two channels of the DSO using two identical low loss coaxial cables. Table 2 shows the minimum voltage required for initiating PDs when exciting the test cells individually. The inception voltage tabulated for each PD defect in Table 2 is an average of 10 measurements at 3 bar SF6 gas pressure. It can be observed that the standard deviation is less than 1 kV for all three defect types. As PD events are random in nature, 200 PD signals were collected per defect for both sensors.
For measuring PRPD pattern for each PD defect type, the output of UHF sensors was connected to the signal analyser one at a time as shown in Figure 10(f). The signal analyser was operated in zero span mode with center frequency 1 GHz, as identified from the spectral content of the UHF signal. Sinusoidal excitation with 25 cycles at 50 Hz and amplitude equal to the inception voltage was applied from function generator. Total 2000 sinusoid cycles were excited for each type of PD defect. The function generator and signal analyser were synchronized, and sweep time of 500
B. Raw PD Signal Analysis
Figure 11 shows the sequence of raw PD signals captured by the CSASA UHF sensor for Cor, PM, and SD type of activities for the experimental setup in Figure 10(e). PD signals captured by reference disc sensor simultaneously for the three PD defects are also shown in Figure 11. It is observed that the both reference disc sensor and CSASA sensor detected all PD events, but the amplitude of PD signals detected by CSASA sensor is always higher than reference disc sensor for all PD signals. This is due to the fact that the CSASA sensor has positive gain throughout its operating frequency band. It is further noticed from Figures 11(b) and 11(e) that CSASA sensor even detected small PD signals that were not detected by reference disc sensor due to its better sensitivity. This confirms the ability of the CSASA sensor to detect all the PD events detected by the reference UHF PD sensor with a relatively higher signal strength compared to the reference sensor.
Measurement of PD signals. (a)-(c) Reference disc sensor and (d)-(f) CSASA sensor PD measurements for Corona, particle movement and surface discharge type of defects.
Figure 12 shows the time domain PD signals and their corresponding spectrum gathered using CSASA sensor and reference disc sensor for the three PD defects in the simulated GIS test cells. Figure 12 clearly shows higher signal strength and broader spectral content for the proposed UHF sensor in time and frequency domains, respectively when compared to the reference sensor. The raw PD signals clearly demonstrate that the UHF CSASA sensor has better PD detection sensitivity than the reference UHF PD sensor. Furthermore, the cost of the fabricated sensor is significantly lower than the commercial disc sensor used in this study for PD measurements.
PD signal analysis. (a)-(c) Time domain signals and their (d)-(f) spectral magnitude for Corona, particle movement and surface discharge type of defects.
C. Phase Resolved Partial Discharge Analysis
PRPD signals were collected for all the partial discharges presented above. PRPD analysis presents the magnitude of the PD and the phase at which the PD event occurred during the HV excitation in the power apparatus. This information is essential to determine the sensitivity of the UHF sensor to the PD event and validate the source of the PD event. Figure 13 shows the PRPD analysis of the UHF EM emissions due to Cor, PM and SD type of PDs in the GIS test cells gathered by the UHF CSASA and reference disc sensors. Each data point in Figure 13 measured by the UHF sensor using the signal analyzer represents the magnitude of PD event and phase at which it occurred. It can be observed that the PRPD plots has varying patterns for the three PD defects which demonstrate the sensitivity and PD detection capability of the proposed UHF sensor. The PRPD patterns for the three PD defects in Figure 13 clearly indicate larger magnitude for the PRPD signals detected by UHF CSASA sensor than reference UHF disc sensor. Figure 13 (d) shows that Corona discharges occurred from 48 to 126° centered about 90° in the positive half cycle, and over 228 to 300° centered about 270° in the negative half cycle for the CSASA sensor. Secondary electron emission around the cathode is responsible for negative Corona discharges. Due to nonuniform electric field and space charge accumulation around the tip of protrusion, negative Corona discharges form around
PRPD signal analysis. (a)-(c) Reference disc sensor and (d)-(f) CSASA sensor PD measurements for Corona, particle movement and surface discharge type of defects.
PRPD pattern for PM type of PD activity occurred over the entire AC cycle in Figures 13(b) and 13(e). This behaviour is expected when a particle is dancing in the HV GIS test cell. Thus there is no obvious phase dependency in Figures 13(b) and 13(e) [25]. The frequency of PM type of PD discharges is about 407 in the positive and negative half cycles for both UHF sensors. The PD magnitude variation was relatively high for PM than the other two PD defects. Meijer et al. studied the free moving particles influence on the insulation strength in 380 kV GIS and observed similar PRPD pattern when the particle is dancing [27]. Minh-Tuan Nguyen et al. classified four types of PDs including particle movement by extracting features from their respective PRPD patterns, and reported similar observation for the PRPD pattern of PM defect [28].
PRPD patterns of SD shown in Figures 13(c) and 13(f) indicate that SD type of discharges occurred mostly in the rising edge of the AC voltage with phase windows of 0 to
D. Comparison with Other Uhf Spiral Sensors
Table 3 presents the comparison of the fabricated UHF CSASA sensor with existing UHF spiral sensors reported for PD detection. It can be observed that most of the PD sensors do not have electrical isolation which is essential for PD detection in HV environment. As the presence of the radome deteriorates the antenna performance, it was included in the numerical model during sensor design optimization. It is well known that the spectrum of PD signals varies in the UHF range depending on the dimensions and type of the PD defects, defect orientation and location, and properties of the medium [6]. Thus, UHF sensors used for PD detection should have wide fractional bandwidth. Table 3 indicates that the spiral UHF antennas reported for PD detection cover only a fraction of the desired spectrum and their size is also large compared to their operating bandwidth unlike the proposed UHF sensor. It should be noted that the smaller aperture size of the UHF spiral antenna reported in [16] is due to the higher operating bandwidth. The electrical size of the UHF CSASA sensor with the lower operating frequency of 0.5 GHz is the smallest (
Conclusion
A CSASA sensor was designed for continuous detection of partial discharges in HV equipment. The sensor was designed to operate from 0.5–5 GHz with unidirectional radiation pattern and reduced aperture size (158 mm diameter). The sensor characteristics such as return loss, gain and radiation pattern measurements were in good agreement with the simulation results. The proposed CSASA UHF sensor is electrically smaller and has polarization purity over the ultrawideband bandwidth when compared to the existing UHF spiral antennas for PD detection. PD measurements indicate that the proposed CSASA sensor has the ability to detect the commonly occurring electrical discharges due to Corona, particle movement and surface discharge with 120% higher signal amplitude than the reference UHF disc sensor widely used for PD detection in GIS and power transformers. PRPD analysis confirms the PD detection capability with higher sensitivity for the CSASA sensor compared to the disc sensor. It is concluded that the proposed UHF sensor could be used for remote detection and monitoring of PDs occurring in HV electrical equipment such as GIS.