The University of New South Wales School of Electrical Engineering and Telecommunications High Voltage Systems ELEC9712 Appendix Partial Discharge Content Introduction Quantities measured Test circuits and measuring systems Calibration Practical considerations
1 Introduction What is Partial Discharge? Localized electrical discharge that only partially bridges insulation between conductors Breakdown is due to presence of small defects in insulation, which creates local electric field enhancement
Different types of PDs Internal discharges Occurs in cavities within solid or liquid dielectric Cavities surrounded by dielectric or Bounded on one side by a conductor Surface discharges Occurs at interface between 2 different dielectrics Corona discharges Occurs in presence of strong inhomogeneous fields built up around sharp points or edges of HV electrodes Sharp protrusions from conductor can also cause corona discharge Damaging effects of PD Cause gradual damages to insulation Shorten lifespan of equipment Needs to measure PD When designing HV equipment, strict manufacturing process needed to ensure quality of insulation Non-destructive test techniques required to detect and identify defects present in insulation
AS 60270 2001 Standard High-voltage test techniques Partial discharge measurements Scope Quantities measured Test and measuring circuits Analog and digital measuring methods Calibration Test procedures Interference abc model C b C a C c Apparent charge: detectable PD quantity at terminals of test object. It is a representative measure of the PD, but it is NOT equal to the true amount of charge involved at the source of the PD.
Example: Breakdown strength of air 3kV/mm (uniform field), say 2kV/mm (field enhancement). Cylindrical void (0.5mm depth, 1mm diameter) Cc=0.0139pF Say C b =C c /100=1.39x10-4 pf (much larger depth) Terminal voltage (100kV) gives 1kV across void and cause breakdown True charge= 1kV x 0.0139pF = 13.9pC Say test object is 100pF so C a =100pF Voltage dip at terminal = 1.4 mv Apparent charge = 1.4mV x 100pF = 0.14pC Paschen s curve for air at 20 o C
can only measure apparent charge at terminals usually much smaller than actual charge at discharge site more charge larger current more damage to insulation 2 Measured quantities
Measured Quantities Basic quantities Derived (integrated) quantities Others Measured quantities Basic quantities related to each individual PD pulse apparent charge q (coulombs) and polarity phase angle (degrees) wrt 50Hz applied voltage
Measured Quantities Measured quantities Quantities derived over a time interval T (many AC cycles) to characterise overall trend of PD activity. 1. Discharge current I (A or C/s) 2. Discharge power P (W) 3. Quadratic rate D (C 2 /s) 4. Repetition rate n (pps)
Measured quantities Voltage Applied test voltage U PD inception voltage U i PD extinction voltage U e 3 Test circuits and measuring systems
Z C k C a Z mi CD CC MI Circuit for PD measurement C a : test object capacitance C k : blocking capacitor Z mi : input impedance of measuring system CD: coupling device CC: connecting cable MI: measuring instrument PD measuring system = CD + CC + MI Measurement system characteristics Transfer impedance Lower and upper limit frequencies Mid-band frequency and bandwidth Superposition error Pulse resolution time Integration error
Magnitude Frequency spectrum Calibration pulse -6 db PD pulse Measurement system f 1 f 2 Frequency Lower limit frequency Pass band Upper limit frequency Measuring system Wide-band Narrow-band
Measuring system Wide-band PD instruments Frequency band (recommended) 30 khz f1 100 khz f2 500 khz 100 khz Δf 400 khz Response to PD current pulse is a well-damped oscillation From this response, can determine apparent charge and polarity of PD current pulse Pulse resolution time 5μs to 20μs e.g 1V/div 10μs/div Measuring system Narrow-band PD instruments Frequency band (recommended) 50 khz f m 1 MHz 9 khz Δf 30 khz Response to PD current pulse is a transient oscillation +ve/-ve peak values of envelope proportional to apparent charge, independent of polarity of PD current pulse Large pulse resolution time, typically > 80μs
Measuring system Digital PD instruments Minimum requirement Display value of largest repeatedly occurring PD magnitude. Update display at least every second Additional requirements Measure q i, u i, ϕ i of each individual PD pulse. Derived quantities I, P, D, n Radio disturbance voltage U RDV Measuring system Largest repeatedly occurring PD magnitude apparent charge displayed by a peak reading meter with a charge/discharge time constant < 0.44s Equivalent pulse train response:
Coupling capacitor Coupling device (quadrupole) Acquisition unit+battery Optical USB fibre controller Computer A digital PD measuring instrument (Omicron MPD540) http://mtronix.de/
A digital PD measuring instrument (Lemke LDS-6) Ref.: http://www.ldic.de
Source: http://www.pd-systems.com/
4 Calibration Calibration To check that specified PD magnitude measured correctly Determine correct scale factor k for measurement of apparent charge q Calibration is required with each new test object
Calibration: procedure Use a calibrator, test object de-energised Inject current pulses of a known charge magnitude q o across terminals of test object In the range 50% to 200% of specified PD magnitude Calibrator capacitor C o < 0.1 C a C o C k U o C a CD CC MI Calibrator C a : test object capacitance C k : blocking capacitor Z mi : input impedance of measuring system CD: coupling device CC: connecting cable MI: measuring instrument
Indirect calibration Direct calibration
Typical capacitance of HV components / equipment: Insulators (post, suspension) some 10 pf Bushings (simple, graded) ~ 100 1000 pf Instrument transformers ~ 200 500 pf Power transformers < 1 MVA ~ 1000 pf > 1 MVA ~ 1000 10000 pf HV power cables Oil-paper impregnated ~ 250 300 pf/m Gaseous insulated ~ 60 pf/m 5 Practical issues
Permissible PD in equipment Permissible level varies with type of equipment. Refer to relevant standards, e.g. AS 60044.1-2007 Instrument transformers - Current transformers Need to specify if U m not less than 7.2 kv Permissible PD in equipment AS 60076.11-2006 Power transformers - Dry-type transformers Need to specify if windings have U m not less than 3.6 kv Voltage application for routine PD test Basic PD measuring circuit for a 3-phase transformer
Disturbances Sources of disturbances Detecting disturbances Disturbance levels Reduction of disturbances Screening and filtering Balanced circuits Electronic processing and recovering of signals Time-window method (phase position gating, noise sensor gating) Polarity discrimination methods Pulse averaging Frequency selection Digital signal processing Balanced circuit arrangement Polarity discrimination circuit arrangement Source: AS60270-2001
Red Yellow Blue 50Hz Y-phase 50Hz PD signal coupling between phases. interference PD pulse gating pulse active at 3.2kV, only noise, ~2500pC gating pulse not active at 6.4kV, detect PD of ~400pC Hoxton Park (14/9/01) A2 cable
Australian RF communications AM radio: CB radio: FM radio: Marine radio: VHF TV channels 0-11: UHF TV channels 28-43: 526-1605 khz 26-28 MHz 88-108 MHz 156-162 MHz 46-216 MHz 527-632 MHz 6 PD sensors
Capacitive: pick up PD signal through electric field Inductive: pick up PD signal through magnetic field Galvanic: sense PD conductive current Radiative: antenna to sense electromagnetic waves Capacitive sensors Pre-moulded cable accessories CIGRE WG21-16
Example of utilising coupling circuit inherent in the test object to measure PD. http://www.pd-systems.com/
LINE External Noise Permanent Permanent Coupler C1 Coupler C2 Ring Bus PD Coaxial Signal Cable Split 1 Split 2 NEUTRAL Coaxial Signal Cable + - PDA PDA capacitive HV coupler using a 28kV XLPE cable with coaxial connection to an external terminal box for on-line PD monitoring. Inductive sensors HF-CT clamp around conductor that connects the cable screen to Earth. can be installed while cable is energised.
Metallic ring clamped around the plastic outer sheath of a cable with helically wound wires of the earth screen. 132kV Endurance CT Sydney West 330kV/132kV substation. High-frequency CT wrapped around DDF link.
Rogowski coil HF-CTs may or may not use magnetic cores, e.g. ferrite. Rogowski coils have lower transfer impedances (hence poor sensitivity), and limited bandwith (not suitable for high frequency application). Galvanic sensors Sensor (made by a resistor or inductor) placed across metallic screen interruption of joints that is sensitive to the current. CIGRE WG21-16
6 Non-standard PD measurements (electrical) PD measurements - electrical Conventional (IEC 60270) Up to 1 MHz Non-conventional (beyond 1MHz) Part of MF band (1-3 MHz) HF band (3-30 MHz) VHF band (30-300 MHz) UHF band (300-3000 MHz)
UHF PD measurements Electromagnetic waves (TEM, TE, TM modes) Spec Frequency range: 300 MHz to 3GHz Narrow-band bandwidth: ~5 MHz or wide-band: 300MHz to 1.5 GHz Sensors (antennas to detect EM wave radiation) disc sensor, cone sensor micro-strip UHF sensors for GIS (ABB) http://www.pd-systems.com/
Conclusions (If you can only remember 2 things from this lecture ) AS 60270 Standard the single most important quantity to measure is the largest repeatedly occurring magnitude of the apparent charge.