On-Line Continuous Partial Discharge measurements of medium voltage substations have not attracted the same attention in the electrical community as compared to similar PD measurements of rotating machines. The cost of an unexpected outage however may be considerably more significant of a main distribution bus than just for a single motor failure. This paper discusses pros and cons of PD measurements using multiple sensors for additional noise reduction and localizing PD activity for measurements made in higher frequency bands. The importance of continuous PD monitoring vs. periodic walk in PD measurements of switchgear is also discussed considering that PD activity in switchgear can have very unstable activity highly dependent on ambient conditions. Negative periodic PD tests in switchgear can result in a false sense of security. Case studies from field experience are presented to support the discussions.

On-Line Continuous Partial Discharge (PD) measurements of medium voltage substations are an excellent way to determine the overall health of the equipment. Partial discharge measurements can provide maintenance alerts to allow scheduling of equipment outages based on actual condition data as opposed to scheduling based on time intervals. PD measurements provide information about the insulation system that is impossible to extract with other methods. Remote monitoring adds additional value to continuous monitoring by relieving customers of the burden of data analysis, readily involving remote experts. This paper discusses the effectiveness of PD measurements using multiple sensors. Multiple sensors provide additional noise reduction and the ability to localize PD activity for measurements made in higher frequency bands. The importance of continuous PD monitoring vs. periodic walk in PD measurements for switchgear is also discussed. PD activity in switchgear can be unstable and dependent on ambient conditions. Negative periodic PD tests in switchgear can result in a false sense of security. Case studies from field experience are presented to support the discussions. INTRODUCTIONOn-line Partial Discharge measurements in industrial environments face three major problems: high noise in the low frequency range, signal attenuation in the high frequency range and significant variation in PD activity over time. A measurement system operating within the 1-20 MHz range with multiple PD sensors distributed throughout the equipment can limit the PD attenuation problem while providing the location of the PD activity. Microprocessor-based signal processing can significantly reduce the background noise. PD sensors include Coupling Capacitors (CC) and Radio Frequency Current Transformers (RFCT). These sensors are used in switchgear line-ups and dry type transformers. Continuous monitoring, compared to infrequent periodic testing, assures that variable PD activity is captured and its correlation to operating parameters such as temperature, humidity, and system voltage is established. Adding remote communication capability to the continuous monitoring system opens the possibility for Remote Monitoring (RM), featuring automatic alarms, remote data access/storage, and instant analysis by PD experts without bringing experts on site. NOISE AND ATTENUATION ISSUESIndustrial noise typically originates from radio communication systems, thyristors firing in exciters and rectifiers, digital metering systems and electronic transducers. Radio noise is typically found in 0.8-1 MHz range and can be avoided by signal acquisition in a higher frequency range. Thyristor firing commonly has pulse widths wider than PD pulses and can be rejected by microprocessor-based signal processing. Rejecting "digital noise" is the most difficult. This problem is resolved by designing "noise immune" sensors. Coupling capacitors directly connected to HV conductors are not susceptible to low frequency noise and "digital noise" commonly does not exist in HV circuits....

Authors share their experience with 15-channel continuous Partial Discharge (PD) monitors applied to medium voltage rotating machines, apparatus and substation equipment, using variety of PD sensors. Real life examples illustrate the condition-based paradigm "measured-inspectedconfirmed-repaired" that clearly purports this totally nonintrusive and effective on-line diagnostic technology. Use of stator RTDs as PD sensors offers an inexpensive solution for attenuation problems, especially in large rotating machines. Continuous monitoring assures that high variations of PD activity will not be overlooked in comparison with periodic tests. The discussed technology will help in making intelligent and accurate maintenance decisions, significantly improving effectiveness and quality of insulation repairs.

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