EATON ELECTRICAL
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Predictive Diagnostics
pulling. Therefore, a varying range of PD is expected. If insulation can vibrate against
support or core rotating machines, load current is a good parameter to correlate. Also
zero load currents could be used to suspend monitoring during equipment outage.
2.1.6 Networking and Communicating with InsulGard
InsulGard provides several interfaces for alarms, SCADA, and data retransmission
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Always wire alarm relays to an annunciation system. Use two relays for Red and
Yellow alarm and the third relay for device status. If necessary, all three relays can
be wired in series to the same enunciator.
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20mAmps output is for SCADA systems. A SCADA system can trend the most
important parameters.
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RS-485/232 with database software allows for remote device setting, initiating
measurements, and downloading of data. If an expert opinion will occasionally be
needed, designate a permanent or temporary phone line and specify a modem.
Cellular modem subscriptions are available. If a Cellular modem is used, extra
conduits for RS-485 networking can be excluded.
2.1.7 Application Specific for Motors and Small Generators
PD sensors set for small generators and medium voltage motors typically include three
sensors located at the line terminal and six to twelve RTDs. A necessity of having too
many sensors comes from very strong high frequency signal attenuation in motor
winding. Therefore, sensors located at the line terminals will not produce valuable
information on a winding depth; likewise, a sensor embedded into a winding will not
sense discharges at the line terminal.
Two types of sensors can be used at line terminals.
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Radio Frequency Current Transformers (RFCT) placed on incoming feeders’ shield
grounding conductor (one per phase) or on single-phase surge capacitor grounding
conductor (one per phase). Using a single three-phase surge capacitor is not
recommended. In such a case, phase-to-phase PD signals are shorted inside the
capacitor and will not be detected by any of RFCT. An advantage of RFCT is that
they are non-invasive (no connection to a live part is required). A disadvantage of
using RFCT on a cable shield ground is, in some cases, a ground noise. This means
that RFCT measures all radio frequency signals coming through the cable shield.
Using RFCT on a single-phase surge capacitor ground is more noise immune.
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Coupling Capacitors (IPDS) commonly have an 80pF capacitance. They have
sufficient insulation and connect directly to motor line terminals. These sensors
exhibit the most noise-immune characteristics due to their low capacitance and,
therefore, can suppress most low frequency noise by themselves. Using IPDS is a
highly recommended option for line terminals.
Note: Installation of line terminal
sensors always requires an outage.
As stated above, additional sensors embedded into winding sensors are required. RTD
outputs are commonly used for this purpose. RTD supplies DC temperature signal to a
