This section provides step-by-step guidance to plan the EMI Monitoring installation.
Identify a suitable installation location for the EMI Monitoring System. Location suitability should be based on the following factors:
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Proximity to similar monitoring equipment – do not install the EMI Monitoring System in a location where each of the HFCT circuits exceed 200 ft in wire length. Cables exceeding this length can negatively affect signal quality. Note that the HFCT cable lengths for each run do not need to be equidistant relative to each other.
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Minimizing the distance is preferred to assure limited attenuation from the CT to the monitoring system. It is critical to understand that the monitoring system is measuring signals at the microvolt scale, and attenuation plays a significant role.
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The EMI Monitoring System has a minimum/maximum operating temperature range of –40 degrees C (–40 degrees F) to 70 degrees C (158 degrees F). Avoid mounting the enclosure in areas that approach or depart from this temperature range on a regular basis. If possible, avoid mounting the monitoring system in a location that experiences direct sunlight for extended periods of time throughout the day.
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Install the monitoring system in such a way that it does not complicate generator disassembly during outages.
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Ensure the monitoring system is accessible to personnel for servicing as needed. However, system surveillance and performance feedback is not viewable at the monitoring system panel.
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Ensure there is proximity to selected HFCT locations, 120VAC power source and data hub.
This section provides guidance for planning the location of the CT components.
If the installation has a stand-alone NGT cabinet and the lead coming into the NGT cabinet is in a conduit of a size that the EMMC-108 (95 mm) CT can fit around:
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Open the EMMC-108.
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Place the CT around the outside of the NGT lead conduit.
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Latch the CT closed.
This preferred location gives the best EMI signal strength.
If the installation does not have a stand-alone NGT cabinet or the lead coming into the cabinet is too large for the EMMC-108 CT, then place the CT around the cable going into the high side of the neutral ground transformer. This option is not preferred because:
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It requires a plant outage to perform
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The neutral ground lead must be insulated to the full generator output voltage
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CT must be restrained from moving or touching the cable
Warning
If the CT comes into contact with the cable, it may cause damage to the cable insulation and cause the NGT to be bypassed in a phase to ground fault condition. This condition could cause very high currents to be generated during the fault condition and cause major equipment damage.
On the GSU transformer, the high-voltage side (grid side) usually has a WYE configuration with a neutral ground resistor to limit phase to ground fault currents. The center tap of the transformer WYE ties through this resistor to a ground connection. The connection to the tank ground and then grid ground is sometimes a bus bar connection and sometimes just a cable. For Auxiliary transformers (Unit Auxiliary, Reserve Auxiliary, Start-up, etc.), the high-voltage side is a delta configuration, and the low-voltage side (plant load) is a WYE connection to the ground through a neutral ground resistor to limit phase to ground fault currents.
The GSU HFCT shall be installed around the tank ground that is NOT connected to the high voltage WYE connection.
The GSU HFCT shall not be installed on the neutral side of the WYE connection or below the WYE connection for a few reasons:
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It is susceptible to high levels of grid noise that can make EMI analysis for the transformer defects difficult to detect and cause false position indications.
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It commonly has higher current flow which can potentially damage the signal acquisition equipment.
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It is not a preferred location unless you are also looking for switchyard anomalies, although switchyard anomalies are very difficult to quantify and very little has been attempted for switchyard phenomena using this technique.
While monitoring of the Isolated Phase Bus (IPB) can be done with a CT monitoring generator, some clients may wish to have additional monitoring of the IPB area in an attempt to have better localization. Almost any ground attached to the outer PT enclosures can be used for this location. Multiple grounds ported to the earth through one ground lead can be used without much concern for external noise except the ground lead antenna affect: the longer the ground lead the more external noise it may pick up.
For locations where the NGT is inaccessible, you can use a generator frame ground for the CT location.
Portable monitoring with an interference analyzer should be performed during unit on line conditions to determine which frame ground location has the best signal strength.
Note
The best signal is not always the one with the with the highest value but the one with the most internal noise values.
After the location has been determined, the installation will most likely utilize the EMMC-107 (46 mm) CT. Some installations only use the EMMC-108 CT for all monitoring purposes, but the EMMC-107 CT has similar response characteristics and is more cost effective.
For many Combustion Turbine Generators, the Exciter end frame ground is the lead of choice. Take care that the CT around the frame ground is coming directly from the frame casing and is not a combination of multiple ground leads. Having multiple ground leads introduces external noise and hampers the analysis process.