With the rapid development of HVDC technology and the issue of smart grid, monitoring and diagnosing the operation of connected power facilities is a new challenge. The thesis dealt with the statistical characteristics of the partial discharge (PD) pulse in SF6 gas at HVDC in terms of discharge initiation voltage (DIV) and discharge extinction voltage (DEV), discharge magnitude and number of pulses, as well as statistical characteristics obtained from discharge distributions and density functions. DIV and DEV in Void were almost similar to the increase in SF6.
Therefore, it is possible to identify the type of defects in gas-insulated equipment operated under HVDC by analyzing statistical characteristics extracted from the discharge distribution and the density function.
Introduction
Partial discharge (PD), which is considered the initial stage of deterioration, has been widely investigated under AC voltage to evaluate the insulation of power devices. Until now, PD under DC voltage has not been investigated in detail, and experience with AC PD measurements cannot be applied directly to DC. Therefore, it is necessary to study the behavior of PD under DC voltage to monitor the condition of HVDC equipment[3].
The discharge onset and extinction voltage as a function of the gas pressure, the discharge magnitude and the number of pulses as a function of the applied voltage, as well as the statistical characteristics of the discharge distribution and the density function were analyzed.
Theory
Partial discharge
Thus, it is essential to detect PD for condition monitoring and diagnosis of the insulation system[5],[6]. Capacitance Cc represents the capacity of the cavity where the discharge takes place, the capacity of the dielectric in series with the cavity is represented by capacitance Cb and the sound part of the dielectric is represented by capacitance Ca. Due to the opposite field induced by space charge, voltage Vc drops to V+ at which the discharge extinguishes, this voltage is called partial discharge extinction voltage (DEV).
Unlike high voltage AC, due to the absence of change in both amplitude and polarity of the DC voltage, once PD occurs in the cavity, the opposing field induced by space charge, goes out.
Detection and analysis methods
- Detection methods
- Analysis methods
According to this method, PD analysis methods such as phase-resolved partial discharge (PRPD), time-frequency (TF) map, and time-resolved partial discharge (TRPD) can be established for the defect identification and condition assessment of power facilities. When PD occurs, the ultrasonic wave is generated in frequency ranges from 20 kHZ to 500 kHz, which can be detected by the AE sensor mounted on the outside of transformer tank or GIS room. The localization of PD can be performed by the whole-acoustic method and electro-acoustic method.
The AE method has the advantages of ease of use, non-invasive and immune to the electromagnetic noise, while it has the disadvantage of signal attenuation, which can be compensated by using the preamplifier[14],[15]. The UHF (300 MHz ~ 3 GHz) method shows obvious advantages such as high sensitivity, robustness against external noise as well as the possibility of on-line monitoring and is widely used for PD detection, identification and localization. In general, the UHF sensors can be classified into internal sensors such as valve sensor for power transformer, internal coupler for GIS and external sensors, for example barrier sensor and window sensor for GIS.
When electrical or abnormal thermal faults occur in oil or gas insulated equipment, gases such as H2, CH4, C2H2, C2H4, C2H6, etc. Defect types such as PD, low energy discharge, high energy discharge, and thermal defects can be distinguished by analyzing the three basic gas ratios. Three other ratios can be used to supplement the basic ratios, including the CO2/CO ratio, the O2/N2 ratio, and the C2H2/H2 ratio[18].
From the basic quantities and derived quantity, the discharge distribution and density function can be determined: PD size as a function of time q(t) as. Three parameters such as skewness, kurtosis and peak were used to describe the statistical characteristics of discharge distribution and density function[20]~[26].
Experiment and Analysis
Experiment
- Insulation defects
- Measurement system
- Experimental setup
The measurement system is developed based on real-time operating system (RTOS) by LabVIEW, which can run the applications with very precise timing and high degree of reliability. The virtual instrument (VI) in LabVIEW Project Explorer are categorized into two parts by the hardware platform on which they will run: the HOST VI that runs on the general-purpose computer and the RT VI that runs on the NI PXI. All the VI were developed on a PC and then connected to the NI PXI via Ethernet to download and run the RT VI.
A standard state machine design pattern has been built into HOST VI to allow the various states to operate in a software-defined dynamic sequence. The RT VI included four parts as shown in Figure 1) reading the waveform from the digitizer, (2) software bandpass filter, (3) peak detection, and (4) array arrangement. Multiple acquisition of more than available memory and a producer-consumer design pattern based on a queue operation were used in RT VI.
One was used to send commands such as digitizing setting, peak detection threshold and filter configuration from the PC to the PXI, while another was used to send data from the PXI to the PC. By using this proposed system, PD pulse can be measured and analyzed in real time within a specified period. The PD signal produced from the electrode systems was detected through a 50Ω non-inductive resistor and analyzed by a digital storage oscilloscope (DSO) with a sampling rate of 5 GS/s and a DAQ system based on the LabVIEW program.
In this thesis, DIV was defined as the voltage at which discharge of magnitude greater than 5 mV occurred at least once per minute, and DEV was defined as the voltage at which no discharge of magnitude greater than 5 mV was observed when the applied voltage gradually decreased from the initial PD voltage[5],[24]. DIV and DEV as a function of gas pressure, maximum discharge size (Qmax), average discharge size (Qmean) and pulse count in 5 seconds as a function of applied voltage, as well as statistical characteristics of each electrode system analyzed.
Results and Analysis
- DIV and DEV
- Discharge magnitude and pulse count
- Statistical characteristics
The maximum discharge size Qmax, the average discharge size Qmean and the pulse number in 5 seconds as a function of the applied voltage of each electrode system are shown in Fig. For each electrode system, Qmax and number of pulses increased with increasing applied voltage. When the voltage applied to the FP reached about 23 kV, the aluminum particle was thrown into the electrode system.
The discharge distribution and density function such as TRPD, relationship between discharge size and time interval to the previous discharge q(△tpre), relationship between discharge size and time interval to its successive discharge q(△tsuc), density function of discharge size H (q) and the density function of the time interval H(△t) of each electrode system are illustrated in figure. In POC, the peak value of q(△tpre) and q(△tsuc) was the lowest compared with that of other four electrode systems. In FP, the statistical characteristics of the discharge distribution and density function are distributed over a small range.
Both of them also had large value of skewness and kurtosis of H(△t) as the H(△t) peaked with a longer right tail. Therefore, it has been verified that defects have different patterns and the defects identification algorithm for HVDC power facilities can be developed by analyzing the statistical features.
Conclusions
1] Gi-Woo Jeong, Sun-Jae Kim, Guoming Wang, Gyung-Suk Kil, “Design and Fabrication of Online Lightning Arrester Analyzer for Traction”, Proceedings of the 2014 Spring Conference of the Korean Railway Association, pp. 2] Sun-Jae Kim, Gi-Woo Jeong, Guoming Wang, Gyung-Suk Kil, "Seasonal Variation of Soil Impedance for a Copper Rod and a Carbon Block", Proceedings of the 2014 Spring Conference of the Korean Railway Association, pp Design and Fabrication of a LED Working Light for Vessels”, Proceedings of the 38th the Korean Society of Marine Engineering Fall Conference, p.121, 2014.
6] Guoming Wang, Sun-Jae Kim, Hyang-Eun, Jo, Min-Young Yun, Gyung-Suk Kil, “Partial discharge characteristics in SF6 gas under HVDC”, Proceedings of the Korean Institute of Electrical and Electronic Materials Engineers Annual Fall Conference , p.91, 2014. 7] Hyang-Eun, Jo, Sun-Jae Kim, Guoming Wang, Gyung-Suk Kil, “Analysis of Partial Discharge Characteristics under AC and DC in SF6 Gas”, Proceedings of the 2015 Spring Conference of the Korean Society for Railway, KSR2015S306, 2015. Development of the LED search light for vessels”, Proceedings of the International Conference on Electrical Engineering, pp Korea, 2014.
Comparison of Partial Discharge Pulse Waveforms Between AC and DC Voltages in SF6 Gas”, Proceedings of the International Conference on Electrical Engineering, Hong Kong, 2015. 3] Jin-Wook Kim, Guoming Wang, Min-Young Yun, Gyung-Suk Kil, “A Condensing Technology for a LED Searchlight”, Proceedings of the International Conference on Electrical Engineering, Hong Kong, 2015. 4] Guoming Wang, Sun-Jae Kim, Hyang-Eun Jo, Gyung-Suk Kil, Sung-Wook Kim, “ Partial Discharge Localization in insulating oil by the electro-acoustic method", Proceedings of the 19th International Symposium on High Voltage Engineering, Czech Republic, 2015.
Algorithm for defect identification by analysis of PD pulses in SF6 gas”, Proceedings of the 19th International Symposium on High Voltage Engineering, Czech Republic, 2015. 6] Hyang-Eun Jo, Sun-Jae Kim, Guoming Wang, Sung-Wook Kim, "Analysis of DIV and DEV in SF6-N2 Mixture Gas", Proceedings of the 19th International Symposium on High Voltage Engineering, Czech Republic, 2015.
