Application of HFCT and UHF Sensors in On-Line Partial Discharge Measurements for Insulation Diagnosis of High Voltage Equipment
2. PD Sensors and Frequency Converter for PD Measurements in UHF
2.1. HFCT Sensors
- The sensitivity is not so pulse shape dependent as in conventional PD measuring instruments.
- The signal to noise ratio (SNR) can be improved, analyzing the data in certain frequency bands.
- High sensitivity is obtained when the sensors are located closed to the PD source and also when they are far from it. In a power cable system, when a PD pulse propagates through the cable shield, although the high frequency content of the signal is filtered, the pulse can be measured at distances exceeding one kilometer holding their spectral content up to units of megahertz .
- If two or more HFCT sensors are placed in a HV installation, the measurement of the PD pulses with a common time reference allows the determination of the location of defects by the time-of-flight analysis.
- PD pulses waveform can be recorded for post processing purposes. The recorded signals can be classified by the characterization of the pulse shape with the aim of discriminate different PD or noise sources. A proper classification of the recorded pulses and a subsequent analysis of the associated phase resolved PD (PRPD) patterns, improves the sensibility in the detection of defects and facilitates more accurate diagnoses.
- For the frequency range specified, ferrite cores are commonly available and a high quality HFCT sensor manufacture results easy and inexpensive.
2.2. UHF Sensors
- High immunity to electric noise, interferences and corona discharges in air, since the frequency spectrum of these signals in the UHF range is very low and in some emplacements negligible.
- High sensitivity in PD detection inside shielded GIS compartments, metal enclosed switchgears and transformer tanks, due to the inner electrical resonance, very low inherent losses and low noise levels.
- Possibility of PD source location. In the case of GIS and transformers the location is performed using several sensors and analyzing the arrival times of the pulses (time-of-flight measurements).
- Accurate defect locations are achieved in cables and accessories due to the selectivity in the distance that this technique presents.
- Possibility to discriminate between internal and external defects to the HV equipment under consideration.
2.3. UHF-HF Converter
3. Measuring Instrument and Signal Processing Tools
3.1. Measuring Instrument
3.2. Signal Processing Tools
3.2.1. Noise Filtering Tool
3.2.2. Classification by Location Tool
3.2.3. PD clustering Tool
4. HF and UHF PD Measurement
4.1. Generation of PD Sources
4.2. Experimental Setup
- Substation. Formed by the GIS compartment described above and a 12/20 kV XLPE cable of 15 m length and a 240 mm2 aluminum conductor. This cable interconnects by a plug-in terminal (position B) and a cable junction (position C) the GIS compartment with the cable system.
- Cable system. A 12/20 kV XLPE cable system with a 240 mm2 aluminum conductor was configured with a total length of 867 m. The line was composed by two joined cable coils with lengths of 350 and 517 m. The connection between them (position D) was made with a splice.
- Distribution grid. To simulate the continuity of the distribution grid a 12/20 kV XLPE cable system with a total length of 585 m and a 240 mm2 aluminum conductor was connected to the cable system. The connection (position E) was made simulating a junction of the switchgear cabinets of a distribution transformer substation.
|1. Corona in a GIS compartment||A (inside the GIS chamber)|
|2. Internal defect type 1 in a cable termination||C (in the power cable)|
|3. Corona in air||C (in the power cable)|
|4. Internal defect type 2 in one of the cable terminations of a splice||D (in the power cable)|
4.3. Measuring Procedure
|Invasive UHF Sensors||Position|
|Non-invasive HFCT Sensors||Position|
|Non-invasive UHF Sensor||Position|
|Mobile UHF 3||In different emplacements along the HV installation to determine the exact PD source location|
|Measuring Unit 1||Channel 1||Invasive UHF 1 (position A)|
|Channel 2||Invasive UHF 2 (position B)|
|Channel 3||HFCT 1 (position C)|
|Channel 4||Mobile non-invasive UHF 3|
|Measuring Unit 2||Channel 1||HFCT 2 (position E)|
|Rest of channels||Free for more sensors if required|
4.4. Experimental Results and Analysis
- When the pulses are positioned by the location tool in the same site where a HFCT sensor is coupled the location of the defect can not be totally assured, i.e., the source can be in that position or in a previous one. This is because in both cases the delay in the arrival time of the pulses to the sensors is the same, see Figure 25a,b, so the pulses are always positioned in the emplacement where the HFCT sensor is placed.
- Only when the defect is in an intermediate point between the sensors it can be assured that the positioning of the focus is correct; a certain time delay corresponds only to one emplacement of the PD source, see Figure 25c.
- In position C two types of sources were identified: corona in air and an internal defect.
- The location of the corona defect in position C was ratified. However, the emplacement of the internal defect could not be confirmed.
- In position D an internal defect in a joint was detected and located.
- A corona type PD source related with a tip referenced to ground (cluster 6) was detected with the sensor HFCT 1. The location of this source could not be determined.
Conflicts of Interest
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Álvarez, F.; Garnacho, F.; Ortego, J.; Sánchez-Urán, M.Á. Application of HFCT and UHF Sensors in On-Line Partial Discharge Measurements for Insulation Diagnosis of High Voltage Equipment. Sensors 2015, 15, 7360-7387. https://doi.org/10.3390/s150407360
Álvarez F, Garnacho F, Ortego J, Sánchez-Urán MÁ. Application of HFCT and UHF Sensors in On-Line Partial Discharge Measurements for Insulation Diagnosis of High Voltage Equipment. Sensors. 2015; 15(4):7360-7387. https://doi.org/10.3390/s150407360Chicago/Turabian Style
Álvarez, Fernando, Fernando Garnacho, Javier Ortego, and Miguel Ángel Sánchez-Urán. 2015. "Application of HFCT and UHF Sensors in On-Line Partial Discharge Measurements for Insulation Diagnosis of High Voltage Equipment" Sensors 15, no. 4: 7360-7387. https://doi.org/10.3390/s150407360