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Partial Discharge Sensors
Coupling Capacitors
The Only Known Test Instrument that can be used to test bushing monitoring systems
- Capacitance Value of 80 pF
- Comes with Silicone Insulating Boot
- Capacitor is ground at point of installation for personnel safety
- Can over-potential test equipment with capacitor installed
- Used on motors, generators and switchgear
- Higher PD extinction voltage than most competitors
- Outdoor rated
- 7.2, 16 and 27 kV rating
Capacitors are used as a coupling device to pass high frequencies (low reactance at the high PD frequencies) from the point of detection to the input of the measuring equipment. A capacitor which is to be used in on-line monitoring applications should meet the following requirements:
- Withstand the impulse and 50/60 Hz overpotential test voltages of the machine winding over the range of possible operating temperatures.
- Have a partial discharge extinction voltage (PDEV) higher than the 1.73 times the phase-to-phase operating voltage of the machine over the range of possible operating temperatures.
- Show no signs of deterioration of the above properties following thermal cycling and mechanical vibration representative of operating conditions.
- Operating life in excess of 50 years.
- Have certified ratings to be used outdoors and in heavily contaminated environments.
- Grounded at the point of installation providing increased safety to personnel.
- No need to disconnect when performing insulation tests on connected equipment.
Radio Frequency Current Transformers (RFCTs)
- Used on ground Leads of Surge Capacitors in motor termination enclosure.
- Placed around ground sheath of cable terminations in switchgear and motors
- Wide Frequency Band
- Allow PD signal detection up to 2500 feet of cable.
- Solid Core, Split Core and Clamp-On sizes available.
A RF current transformer can be effective for monitoring motors, switchgear and cables that may be prone to PD activity. A RF current transformer that has a wide band pulse response can be used to measure high frequency PD pulses. It is constructed of a ferrite core. The CT is usually installed in a low voltage, ground or neutral leg of the monitored apparatus.
One common location or RFCTs are around the ground cable to the surge capacitor or on cable shield grounds on motor and switchgear terminations.
RTD Partial Discharge Module
- Used on Motors and Generators
- Just using sensors installed in termination enclosure can be precarious
- Detects PD they may be occurring deeper in the winding which sensors installed at the terminations can not detect
- Acts like an RF antenna deep in the winding
- Using both line sensors and RTDs sensors provides more coverage of the insulation system.
- Non-invasive
- 3 or 6 RTD Modules Available
PD events cause a high frequency signal to be generated which travels through the winding of a motor or generator. Due to the construction of a winding, these signal quickly attenuate and lose both their frequency characteristics and magnitude.
Thus, sensors installed at the line terminals, such as coupling capacitors, have limited "view" as to what is happening deep in a winding. Typically coupling capacitors will see 10 - 15% of the entire winding, which means the ring bus and maybe the first 2 or 3 coils. PD occurring deeper in the winding will not be detected.
By using the existing RTDs installed in machines one can obtain additional information as to what PD may be occurring deep in the winding.
It is strongly suggested both type of sensors be used since they compliment each other well.
