Medium Voltage Application Guide
Medium Voltage Application Guide
Medium Voltage Application Guide
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SWITCHGEAR<br />
Exercise 2: Protection CT1-2 for transformer incomer circuit:<br />
Step1: Select ratings common to both the metering and protection CTs<br />
Primary/secondary rated current<br />
Short-time withstand rating, I th<br />
Primary circuit voltage, U p<br />
Step 2: Select real output power<br />
= Use 300/1 A<br />
= Use 10 kA rating<br />
= Use 12 kV rating<br />
Real output power: typically > 1VA for digital type protection relay<br />
= Use 2.5 VA (this allows 1.5 VA for cable burden, etc)<br />
Step 3: Calculate protection class 5PX<br />
The instantaneous trip current level of protection relay OC1 is set to 15 x I n .<br />
I_ TRIP = 15 x 262<br />
= 3930 A (primary current)<br />
(Note: In most digital protection relays, the trip current levels are set with respect to the secondary current. In this<br />
case<br />
SEC<br />
300<br />
The instantaneous trip current level for the CT secondary is 13.1 A<br />
The trip current level should fall between 100 to 50% of the accuracy limit factor (ALF).<br />
Using an ALF of 10 (5P10), the trip current level of 3930 A falls outside the range 100% to 50% ALF, so a 5P10<br />
protection class CT is not suitable.<br />
)<br />
)<br />
Using an ALF of 15 (5P15), the trip current level of 3930 A falls within the range 100% to 50% ALF so a 5P15<br />
protection class CT is suitable.<br />
Use protection class 5P15<br />
Page 116 <strong>Medium</strong> <strong>Voltage</strong> <strong>Application</strong> <strong>Guide</strong> 710-12280-00A
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