How is CT size calculated?

How is CT size calculated?

The CT ratio is the inverse of the voltage ratio. In this example, the voltage ratio is 1:5, so the CT ratio is 5:1. This means the current level is stepped down 5 times where, if the primary current is 200 amps, the CT output is 40 amps.

How do you choose CT ratio for protection?

For a metering CT, choose a burden VA equal to, or greater than, 125 \% of the total combined VA of instruments connected to the secondary side. For a protective CT, please contact the manufacturer for assistance. Accuracy • For a revenue CT, the accuracy should be at least IEC 0.2 or 0.5 CL or IEEE 0.3 or 0.6 CL.

How do you determine CT ratio?

When analog ammeters are installed, we can easily determine the CT ratio by observing the meter full scale value and then divide that value by 5.

How do you find the CT ratio test?

A CT ratio test can be performed by injecting a primary current and measuring the current output, or by injecting a secondary voltage and measuring the induced primary voltage.

What is MVA on a transformer?

The transformer is the heart of the substation. For example, a typical substation transformer would be rated 69-13 kV and 20 MVA; meaning the primary or high voltage is 69 kV, the secondary or low voltage is 13 kV and the transformer has a power rating of 20 MVA or 20,000 KVA. …

How is VA calculated in CT?

Burden for Metering type CT = Total Burden of Meters (Digital/Analog Ammeter, Voltmeter, Power Meter, MFM, Transducer) which are in series with CT + The total VA of the Cables connecting CT and the Meters. Cable VA = (2L x R x I^2). Cable VA= 5^2 x 0.02 x 2 x 2=2VA .

What does 30 VA mean on a transformer?

Volt-ampere (VA) is a measurement of power in a direct current ( DC ) electrical circuit. For example, a supply might be rated at 600 VA. This does not mean it can deliver 600 watts, unless the equipment is reactance-free. In real life, the true wattage rating of a power supply is 1/2 to 2/3 of the VA rating.

How to compensate CT ratio mismatch for transformer differential protection?

Compensating CT ratio mismatch for transformer differential protection using numerical relays is quite straightforward. Relay measured currents are expressed to their primary values by multiplying CTR 1 and CTR 2 to I 1 and I 2, respectively.

How do you find the CT ratio of a transformer?

V (p) / V (s) = I (s) / I (P) ———— 2. Look at the formula 1 and 2. The CT ratio is the inverse of the voltage ratio. If your transformer has a voltage ratio is 1:25 means, the CT ratio would be 25:1. It means if the CT has a primary current of 25 Amps, you get 1 Amp as output. The current will be reduced considerably.

How does the transformer differential protection respond to through fault conditions?

Let us examine the response of our transformer differential protection on through fault conditions. Consider a single line-to-ground through fault as shown on the figure below. The currents seen by the relay are affected by the CT ratio and connection hence the values of the secondary currents before compensation.

What is the tap setting on a 33mva transformer?

The transformer MVA rating is 33MVA while the voltage rating is 23kV. Using a CT ratio of 240, the TAP setting value is 3.45Amps secondary for the wye side.