Current Transformers Coursework

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Observation Sheet

Secondary resistance measurement
Protection CT secondary resistance= 0.3 Ω
Measuring CT secondary resistance= 0.4 Ω
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)|
4.89| 30|
6.93| 36|
8.93| 42|
11.08| 46|
12.82| 52|
14.9| 60|
17.11| 70|
19.05| 94|

Primary Current: 50 A
Protection CT:
Secondary voltage (V): 0.39
Measuring CT:
Secondary voltage (V): 0.371

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

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...
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