SUBSTATION SURVEILLANCE USING RADIO FREQUENCY INTERFERENCE (RFI) MEASUREMENTS Eduardo Pabuna WESTCO ELECTRICAL Eduardo.pabuna@westco-phil.com
Agenda Introduction. Partial Discharge a precursor to failure. Radio Frequency Interference (RFI) detection - understanding what we are measuring. Complimentary EMI detection techniques using different sensors Case studies Conclusions
Key Drivers A successful substation asset management strategy depends heavily on predictive maintenance assessments conducted periodically: Surveillance based techniques to identify potential sites of insulation degradation Followed by more investigative and invasive quantitative techniques Adds value to the maintenance work, selectively identifying and replacing those items of plant that have deteriorated significantly. Identifies HV apparatus that presents a potentially safety hazard to staff and public. Cost effective and non expert use.
External discharges Surface discharges caused by pollution on the insulator surfaces. Leakage currents on insulator surfaces. Insulator damage. Corona. Loose connections Internal discharges Voids in the insulation. Poor conductor-insulation interfaces. Metal irregularities/contaminants
PDS100 PD Surveyor PD Detection by RFI/EMI
Partial Discharge (1) PD occurs when insulation defects exist which produce distorted and enhanced electric field stress. Causes of PD in insulation system: Voids in epoxy resins, polymers, paper Bubbles in liquids/oils Metal depositions/irregularities/contaminants Electrodes and insulation surfaces Poor terminations/loose joints Environmental stresses can act to accelerate the deterioration of the insulation system in HV apparatus.
Partial Discharge (2) Can also arise through: Poor design and manufacture Damage of equipment Poor installation processes/workmanship General ageing or deterioration of materials Lightening strikes, transients, overloading
Partial Discharge (3) PD is a symptom of degradation Once present it dominates as it s own inherent stress degradation mechanism Precursor to complete insulation failure and breakdown poor preparation erosion traces poor preparation poor preparation Cable Termination Area
Detection of partial discharges Detection is based on the energy exchanges that take place during the discharge Dielectric Heat Light losses Electromagnetic Radiation Chemical changes, gases Impulse current pulses Sound/ noise
RFI Emissions IEC60270 measurements Quantify apparent charge involved in PD event Integration of current pulse Requires physical connection to measure current pulse RFI measurements Radiated signals depends on the dynamics of charge motion Signals generated when charge is accelerated, e.g. Charge oscillation Charge movement PD current pulse comprises moving charges thus radiation will ensue
RFI Case in UK, 1984: Experience of a transformer fire that followed a bushing failure: During 6 months before this failure neighbors were complaining about Radio & TV interference, foretelling what was coming. Since then, RFI surveys are routine.
Understanding What We Are Measuring Partial discharge pulse RFI emission frequency spectrum PD current pulse comprises moving charges thus radiation will ensue It emits an impulsive electromagnetic wave that travels in the surrounding media.
Understanding What We Are Measuring Detecting and measuring a partial discharge event is analogous to listening for the strike of the hammer on the bell at distance what we hear are the resonant harmonics.
RFI propagation discharge source Amplitude of higher frequency emissions drop of quicker with distance Source of discharge may be located to an area or specific item of equipment Discharge pulse propagation Radio frequency, or RF signals, weaken as they travel through free space because of divergence or dispersal, much the same way light travels away from a light bulb.
How? RFI from PD source radiates intermittently PDS100 scans VHF/UHF in steps of 6 MHz for 1-1000 ms (set by user) Captures RF energy by amplitude
Scanning for RFI emissions
How? 1. Establish baseline outside substation or switch room 2. Walk from point to point inside substation in service 3. Look for changes in amplitude of the RF-activity 4. If indications of PD approach the apparatus and see if the activity increases
Typical RFI spectrum without discharge source (frequency sweep mode)
Compare Background signal with signal next to source
Frequency Range Contributions 1) Corona in air around sharp metal will be dominant up to 10 MHz but will go as high as ~250 MHz 1) Surface discharge on dirty porcelains will dominate from 10 to 250 MHz. Heavily influenced by time of year, rain washing etc. 3) PD will extend RFI up to 1GHz on the PDS100 (but actually can go much higher). The higher the frequency the greater the attenuation Localization of PD requires the using the attenuation to your advantage! The closer you get to the source of PD the more sensitive (elevated) the higher frequencies
Fault Characterization Measured impulse events from an arcing discharge source (time resolved mode)
Case Study 1 Survey of Switchyard revealed high RFI in vicinity of Isolator The time resolved plot shows a typical arcing type discharge Deviations from Background (Black) becoming larger when approaching the isolator
Case Study 1 Survey of Switchyard revealed high RFI in vicinity of Isolator It was recommended to pay particular attention to the isolator and to look for floating metal parts. Several Cu braids and springs were found to be burned off and just lying in the centre pole enclosure of the isolator. In addition the insulators on the load end (CSBP) were replaced due to glazing damage of the surface due to a previous flashover. A repeat test after repair of the isolator showed no significant deviation from background for all HV plant in this switchyard.
Case Study 2 11KV switchboard During a routine Survey some RFI activity was found in front of only one 11kV switchgear panel.
Case Study 4 400/275/22kV 800MVA Transformer RFI scans performed on places indicated by red dots L2 L11 L12 L13 Radiator 400 kv 275 kv L3 L4 L5 L1 Tapchanger L7 L6 L10 L9 L8
Case Study 4 400/275/22kV 800MVA Transformer The PD scans at location L3 and the baseline is shown. The PD measurements showed increases in amplitudes when compared with the baseline measurement indicating evidence of discharge activity.
Case Study 4 400/275/22kV 800MVA Transformer
Case Study 4 400/275/22kV 800MVA Transformer RFI scan at L8 (TDM at arrow 790MHz) Time domain mode at 780MHz Pos. L8
Case Study 4 400/275/22kV 800MVA Transformer Clear signs of severe PD The transformer failed 2 weeks after this survey
Case Study 4 132kV CT Strong RFI emissions were detected in the range of 750 to 900 MHz indicating significant PD activity in the oil dielectric. Further interrogation of RF signals were done in time domain mode at different PD frequencies.
Case Study 4 132kV CT Time Resolved Mode showing correlation with system voltage DGA results confirm internal PD C & TD Test data of CT s insulation measured Tan Delta at 3.0%.
Case 5 300 kv - Surge Arrester RFI measurement was carried out during routine substation survey. Very high amplitudes at the highest frequencies indicated serious PD. Other arresters in the same switchyard followed the baseline.
Other decoupling options TEV Joint Metal Cladding HV Busbar EM Wave PD EM Wave EM Wave TEV Probe Far-Field Propagation PDS 100 Photo showing the deployment of a TEV probe Graphic Showing Transient Earth Voltage Detection of Partial Discharge Activity.
Other Sensors: High Frequency CT s
UHF Transformer Sensors UHF Plate Sensors (Mounted in Hatch) UHF Drain Valve Sensors
Use of Complementary EMI Couplers Frequency plot of injected pulse Using a UHF coupler on a GIS Substation. Time resolved trace @ 810 MHz
Example of a Detected GIS Defect Frequency trace for a floating particle UHF signal @ 2.5 GSs -1 Time resolved trace @ 871 MHz; -repetition rate 1 s -1
Summary Impulsive broadband emissions are a characteristic of insulation defects in HV substations. The PDS100 instrument platform is specifically designed for substation surveillance and can greatly assist in early stage recognition and reporting of PD. RFI offers a routine non-invasive and costeffective surveillance technique.
Thank You! QUESTION?