Plan the Current Transformer (CT) Locations

Related Products:

EMMK-101, EMMK-102, EMMK-103, EMMC-107, EMMC-108

Related Documents:

EZDP-2056

Type:

Installation

Keywords/Labels:

transformer CT, WYE, Potential transformer (PT) casing ground

Summary:

Guidance for planning the locations of the CT components.

Option 1: Neutral Ground Transformer (NGT)

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:

  1. Open the EMMC-108.

  2. Place the CT around the outside of the NGT lead conduit.

  3. 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:

  • It requires a plant outage to perform

  • The neutral ground lead must be insulated to the full generator output voltage

  • 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.

Option 2: Generator Step Up (GSU) Transformer WYE Connection to Ground

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. Although you can use this configuration, 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.

  • Not 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.

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. If the CT is located around this lead to ground, then internal plant noise may interfere with the noise signals generated from faults within the transformer proper. You can use this ground lead but ensure the EMI analyst is aware of the location, so analysis of internal plant components become part of the analysis process.

Option 3: Potential Transformer Casing Ground

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.

Option 4: Generator Frame Ground

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 vendor will most likely use the EMMC-107 (46 mm) CT. Some vendors use only 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.

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