Current Transformers Coursework Essay

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Observation Sheet Secondary resistance measurement Protection CT secondary resistance= 0. 3 ? Measuring CT secondary resistance= 0. 4 ? Polarity check Primary current flow is from P2 > P1 Secondary current flow when, Switch ON S2 > S1 Switch OFF S1 > S2 Ratio check Protection CT secondary current= 0. 6 A Measuring CT B secondary current= 0. 57 A Magnetizing Curve Measuring CT Secondary voltage(V)| Secondary current(A)| 6| 0. 1| 7. 04| 0. 23| 9. 06| 0. 98| 11. 08| 2. 25| 13. 09| 4. 05| Protection CT Secondary voltage(V)| Secondary current(mA)| . 89| 30| 6. 93| 36| 8. 93| 42| 11. 08| 46| 12. 82| 52| 14. 9| 60| 17. 11| 70| 19. 05| 94| Burden Primary Current: 50 A Protection CT: Secondary voltage (V): 0. 39 Measuring CT: Secondary voltage (V): 0. 371 Calculation Magnetizing curve for measuring CT Knee point The knee point of an excitation curve is defined as the point at which a further increase of 10% of secondary e. m. f. would require an increment of 50% of exciting current. Knee point voltage = 12. 72 V Magnetizing curve for protection CT Knee point voltage = 17. 7 V

DISCUSSION 1) Reason out as to why the secondary resistance of a protective transformer is low compared to that of a measuring transformer * Measuring transformers are used to measure the currents at normal operation normally flows small current, used in small range of currents. * Protective transformers are used to measure currents at fault conditions. At fault conditions the drown current is several times higher than the normal operation condition. * So protection transformer should be able to handle a high current than the measuring CT. Therefore the secondary resistance of the protection transformer needs to be a lower value compared to measuring transformer in order to handle high currents and to reduce power dissipation. 2) Explain the significance of knee point voltage of the magnetizing curve * That point on the magnetizing curve where an increase of 10% in the flux density (secondary voltage) causes an increase of 50% in the magnetizing force (exciting current). * Beyond this point specified ratio may not be * Beyond the knee point the CT is said to enter saturation. In this region almost all the primary current is utilized to maintain the core flux. It is a prime indicator of the characteristic of a particular current transformer (CT) because the voltage level reflects the structural integrity of the CT. * If a CT has a low knee point voltage, it mentions deterioration in the core material (delimitation, crack, etc. ) or a defect in the winding (inter-turn shorting). 3) Discuss about the selection of a current transformer for a given application To select best fit transformer we have to consider following factors. * CT ratio * The ratio of primary current to maximum secondary current under full load conditions. The most common CT secondary full-load current is 5A * It matches the standard 5A full-scale current rating of switchboard indicating devices, power metering equipment, protective relays. CT’s with a 1A full-load value and matching instruments with a 1A full-range value are also available. Many new protective relays are programmable for either value. * CT Burden * Maximum load that can be connected to the CT’s secondary side is called the burden. If you want to connect several relays to the same CT you should make sure that the CT can handle it.

Therefore the connected relay burden should not exceed the CT’s rated burden. * Accuracy Class * it says the accuracy level of the CT * For measuring CTs very high accuracy is need * A measuring CT will have a higher accuracy until 120% rated current and will saturate after that. * A protection CT will maintain its ratio for few times the rated current. These characteristics can be identified from the accuracy class of a CT. STUDY ON CURRENT TRANSFORMERS INSTRUCTED BY: Ms. pumi Name: R. M. A. Karunarathne Index Num: 080222E Group: G6 Date of Per: /02/2012 Date of Sub: /02/2012

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