Characteristics of Partial Discharge depending on SF6 Pressure in High Voltage Direct Current 1
Sun-Jae Kim1*, Hyang-Eun Jo1, Gi-Woo Jeong1, Gyung-Suk Kil1+ and Jae Ryong Jung2 Div. of Electrical and Electronics Engineering, Korea Maritime and Ocean University, Busan 2 Hyosung Corporation, Changwon Republic of KOREA + E-mail:
[email protected] sine wave in an alternating current is being applied. Direct current, however, doesn't have a phase that application to PRPD is unfeasible, so new research about it is required[3]. This paper contains simulating defects for the purpose of development on power facility diagnosis technology for HVDC, and analyzes a partial discharge pulse on the SF6 pressure into time and frequency domain.
Abstract— This paper describes the characteristics of partial discharge(PD) depending on SF6 pressure in high voltage direct current(HVDC). Electrode systems, such as a protrusion on conductor(POC), a protrusion on enclosure(POE), a crack on epoxy plate(Crack) and a free particle(FP) were fabricated to simulate insulation defects. Electrode systems were designed as a circular cylinder with 125mm in diameter and 86mm in height. The radius of curvature of the needle electrode is 10um. The plane electrode was made of a round-edged tungsten-copper alloy disc to avoid electric field concentration. Analysis system was designed for Time-Frequency(T-F) map algorithm programed based on LabVIEW. This can analyze the measured signal of frequency and time domain. A HVDC power supply consists of a transformer(220V/50kV) and a diode(100kV) and a capacitor (50kV, 0.5μF). The gap between the electrodes is 3mm, and the gas pressure of SF6 was set at 3 and 5bar. PD pulses were detected at 50Ω resistor and their characteristics were analyzed by a digital storage oscilloscope(DSO) with the resolution of 2.5GS/s and a data acquisition(DAQ) system with the resolution of 30MS/s. PD pulses distributed 20ns to 40ns at below 0.5MHz for POC, and 10ns to 125ns at below 1.7MHz for POE. The T-F map of Crack and FP were distributed 135ns to 250ns at below 0.1MHz, and 260ns at below 1.6MHz, respectively. From the results, it was confirmed that rise time of discharge pulses had the short period as SF6 pressure increased from 3 to 5bar.
II.
EXPERIMENTS
A. Electrode systems The insulation defects in the power facility are generated by a inflow of extraneous substance, electrical mechanical stresses and other environmental factors, and partial discharge's generated patterns are differently analyzed by the types of defects[4]. To simulate PD, which can be generated in the power facility, POC, POE, Crack, and FP electrode systems were fabricated as shown in Fig. 1.
Keywords – HVDC, PD, T-F map, Electrode systems, SF6 gas
I.
INTRODUCTION
Development on extra high voltage, high capacity in a power system is making progress for the purpose of improvement in reliability of power supply, power transmission cost reduction and rapid increase of the electrical demand. Recently, as an interest in renewable energies and smart grid like solar and wind energy are increasing, the study about HVDC transmission system is actively in progress. HVDC in the long distance transmission has a low power loss, easy insulation compared to alternating current, and its frequency is possible for grid-connection to other countries, so its demands are gradually increasing[1,2]. Internationally, the studies about diagnosis technology of power facility for HVDC as well as transmission are actively in progress as well, whereas the national interest in them has just started. Method of partial discharge in the diagnosis of power facility is the mainstream, and phase resolved partial discharge(PRPD) analysis that uses correlation with phase of
(a) POC
(b) POE
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standard deviations from signal in the range of frequency and time. T
T
(t t
0
)2 s~(t )2 dt
(2)
0
F
f
2
s~(f ) 2df
(3)
0
s~(f ) is a Fourier transform of s~(t ) , t 0 is the center of
(c) Crack
gravity of normalized signal, and marked as (4). T
t0 ~ t s(t)2 dt
(4)
0
With the use of formula above, the analysis by T-F map preserves the characteristic on the partial discharged pulse time and frequency. The system consists of a high-speed data acquisition with a onboard real time operating system(RTOS) and a T-F map analysis program designed by LabVIEW given in Fig. 2 (a) is a front panel, (b) shows a program formation, analyzing the signal in 6 domains. (1) sets the input port, and (2) sets the system for the signal analysis. (3) saves data in the inner memory of a data collecting device until the analysis is completed because sequential data processing in realtime uses much memory, and applies Fetch number of record to reduce the measuring time. (4) removes a low frequency element and has only the partial discharged signal processed and uses digital high pass filter with 1kHz low cut-off frequency. After (5) rearranges data saved in the memory in accordance with time domain, it measures the size of partial discharged pulse and its generated counts. (6) analyzes the partial discharged signal by T-F map.
(d) FP Fig. 1 Structure of electrode systems.
Electrode systems are closed structures of circular cylinder with 125mm in diameter, 86mm in height, and can charge 6 bar of SF6 the most. 30mm sphere gap in the upper of HVDCapplied connecting part is installed. As (a) and (b) are simulation for an internal conductor and enclosure projection defects in the power facility, needle electrode used in the inner part of electrode systems is 10μm in radius of curvature, and plane electrode is round-edged with 80mm in diameter and 10mm in thickness so that it prevents electric field concentration. As (c) is simulation for Crack which comes from insulator, plane electrode is identical to which is used in POC, POE, and solid insulator used epoxy of 70mm in diameter, 5mm in thickness. As (d) is simulation for defects caused by free particle, sphere-shaped aluminum with 1mm diameter is used for the particle, and designed for particle's easy free movement by concaving a bottom electrode. B. Analysis method Unlike alternating current, direct current doesn't have the phase that PRPD analysis is inapplicable. But if it uses T-F map, characteristics of defects would be analyzed by distributing a PD pulse into time and frequency domain[5,6]. As for signal processing of the partial discharge, normalization shown (1) is implemented. T ~ s (t) s(t)/( s(t)2 dt ) 0
s(t) [0, T]
(a) Front panel
(1)
The measured signal is marked as quadratic function using a standard deviation in (2), (3). In addition, T and F are the
(b) Block diagram Fig. 2 Analysis program of PD pulses.
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C. Experimental system The experimental system composes HVDC power using a high voltage transformer(220V/50kV), diode, and capacitor. It installs electrode system inside the shielding enclosure [1,020mm(L) × 720mm(W) × 760mm(H)] to minimize the influence from the external noise, and fills up to 3bar and 5bar of SF6. It measures the applied power using a potential divider at a ratio of 10,000:1, and detects the partial discharged pulse generated from the electrode with 50Ω non-inductive resistance. (c) Crack (13kV)
Fig. 3 Configuration of the experimental set-up. (d) FP (21.5kV)
III.
RESULT AND ANALYSIS
Fig. 4 T-F map at 3bar.
It shows result of the T-F map at 3bar, as shown in Fig. 4. In the number of PD pulses were 504 at POC with frequency below 0.4MHz and 25ns to 40ns pulse width. In the POE, PD pulses occurred 445 at 17kV with 0.5MHz to 1.6MHz frequency and 160ns to 250ns pulse width.
The PD pulses for Crack are 2,062 at 13kV with frequency below 0.1MHz and 160ns to 250ns pulse width. The reason why the frequency element is low might be caused by vibration reduction of the partial discharged pulse on account of electrostatic capacity of insulator. The PD pulses for FP were 182 at 21.5kV with frequency below 1.5MHz, pulse width below 75ns, 135ns to 260ns.
(a) POC (17kV)
(a) POC (30kV)
(b) POE (30kV)
(b) POE (17kV)
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It shows the analyzed result by defects on the pressure, as shown in Fig. 6. As the SF6 pressure increases, POC cluster decreases 20%, and pulse width shows up in 5ns preceding range. The range from 40ns to 60ns generated at 3bar for POE disappeared at 5bar. Crack reduces 15% of cluster, generating 30ns low in pulse width. FP distributes in the range excepting 75ns to 125ns of 0ns to 260ns at 3bar, but it does in all 0ns to 250ns at 5bar. In addition, FP includes the whole PD ranges generated from the third-class electrode, and its discharge inception voltage doesn't increase in any degree as the SF6 pressure increases.
(c) Crack (26kV)
IV.
CONCLUSIONS
The paper has fabricated 4 types of electrode systems to simulate the insulation defects, and designed T-F map algorism to analyze the partial discharged pulse. After charging 3 and 5bar of SF6 into electrode system, it generated the partial discharge with applying HVDC, and analyzed the partial discharged pulse on each of electrode system into time domain and frequency domain. If SF6 pressure increases from 3bar to 5bar, the total discharged pulse showed up generating with a short rising time. The characteristic of partial discharge from HVDC found it possible to distinguish the defects by T-F map. FP, however, has difficulty on distinguishing for the inclusion of other generating of defects ranges. The additional research on the pulse width for FP should be done for the future.
(d) FP (22kV) Fig. 5 T-F map at 5bar.
It shows result of the T-F map at 5bar, as shown in Fig. 5. It occurred 2,410 PD pulses for POC at 30kV with frequency below 0.5MHz and 20ns to 30ns pulse width. In the POE, PD pulses occurred 400 at 30kV with below 0.6MHz, 0.7MHz to 1.7MHz frequency and 10ns to 40ns, 60ns to 125ns pulse width. The PD pulses for Crack are 196 at 26kV with frequency below 0.1MHz and 135ns to 215ns pulse width. The PD pulses for FP occurred 199 at 22kV with frequency below 1.6MHz, pulse width below 250ns.
ACKNOWLEDGMENT This research was supported by the National Research Foundation of Korea(NRF) grant funded by the Korean Government. REFERENCES [1]
[2]
[3]
(a) 3bar
[4]
[5]
[6]
(b) 5bar Fig. 6 T-F map.
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