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Resources 20
Line Amperage (3-Phase System): 694.487A / 3 = 231.496A
Now that we know are maximum expected current flow is 232A, we can choose an
appropriate CT ratio.
For metering, we may choose a 300:5. The 232A falls into the mid-range of the 300A
rating, however we could also safely choose a 200:5 so long as the CT has an
appropriate “rating factor” which will typically allow a CT to operate at 1.5 or 2.0 of its
nameplate while still maintaining proper accuracy. To make this determination, the CTs
rating curves should be consulted to ensure the CT rating is appropriate for your
application.
For relaying however, we may choose a higher ratio CT, possibly a 400:5. This higher
ratio will provide better measurements of fault current conditions that are typically much
larger than the system’s normal operating conditions. A 400:5 CT with a rating factor of
2.0 will provide measurements of up to 800A before saturating.
Regardless of what PT or CT ratio is ultimately selected, the meter and protection relay
devices are configured to understand what instruments are connected via the PTR
(Potential Transformer Ratio, 60: 1 = 60) and CTR (Current Transformer Ratio, 300:5 =
60) settings variables.
The selection of instrument transformers is specific to the application, the
interconnection and facility electrical characteristics, and also an Engineer’s preference.
For the myPV ClearSky™ PLUS, we have assembled the following table of standard
part numbers for PTs and CTs to use, unless otherwise specified buy the Customer:
The Interconnection Voltage: 12.47 kV * 1000 = 12,470V L-L
The Line Voltage: 12,470V L-L / sqrt(3) = 7,199.558V L-N
Facility Wattage: (5,000kW * 1000) = 5,000,000W
Facility Total Amperage: 5,000,000W / 7,199.558V L-N = 694.487A
The Math (Expected Current Flow for a 12.47kV Interconnected, 5,000kW Facility):