To confirm the physical condition and electrical characteristics of the current transformer installed in the installation. Ensure the CT is connected to the system properly in all respects (primary and secondary).
TEST EQUIPMENT REQUIRED:
Insulation tester
Polarity tester
Digital low ohmmeter
Current source, multimeter
Variac, step-up transformer (0-2kv)
Primary current injection set
INSULATION RESISTANCE TEST:
The voltage shall be applied between
- Primary to secondary plus ground (covered during switchgear test).
- Secondary to primary plus ground.
- Secondary core to core.
.
POLARITY TEST:
Polarity test is to confirm the polarity marking on the CT primary and secondary and verify it is matching with drawing. More ever it is giving an idea, how to connect the secondaries to make the protection (like directional, differential)
and metering function properly.
Isolate CT secondary from the load and make a circuit connection as shown in Close and open the battery switch connected on the primary. Observe the pointer is moving +ve direction while closing and –ve direction while opening for correct polarity.
BURDEN TEST: (OPTIONAL TEST)
The burden test is to ensure the connected burden to CT is within the rated burden, identified on the nameplate. Injected the rated secondary current of the CT, from CT terminals towards load side by isolating the CT secondary with all connected load and observe the voltage drop across the injection points. The burden VA can be calculated as, Burden VA = Voltage drop x rated CT sec. Current. Limits:
The calculated burden should be less than rate CT burden.
Note: The ammeter selector switch should be at the respective phase during the test. High-impedance relays shall be shorted during the test.
MAGNETIZATION CURVE TEST: (OPTIONAL TEST)
Mag. Curve test is to confirm the magnetization characteristics of CT with nameplate specification.
This test shall be conducted before the ratio test and after the secondary resistance and polarity test, since residual magnetism is left in the core due to DC test (polarity, resistance), which leads to additional error in the ratio test. The meters used for this test shall have true RMS measurements.
The circuit connection shall be made as shown The primary should be open during the test.
Demagnetisation:
Before starting the test demagnetize the core by Injecting voltage on secondary terminals and increasing up to where considerable increment in current with a small voltage increment. Now start decreasing the voltage to zero, the rate at which increases.
Magnetization test:
Now increase the voltage and monitor the excitation current up to the CT reaching near saturation point. Record the reading of voltage and current
at several points. Plot the curve and evaluate the Vk and Img from the graph.
TURNS RATIO TEST: (OPTIONAL)
This test is to ensure the turn ratio of CT at all taps. The circuit connection shall be made as shown Fig 6.4. The primary current of minimum of 25%rated primary current to be injected on primary side of CT with secondaries shorted and the secondary current can be measured & recorded for all cores. Limits: The obtained turn's ratio should match with the rated nameplate ratio.
PRIMARY INJECTION TEST:
This test is to ensure the CT circuits are properly connected with respected cores and there is no mix-up in the circuit (phase identification).The circuit
connections shall be made as shown in Fig 6.4. Single point grounding shall be verified for CT circuits, before starting this test.
Inject 25% of rated primary current between one phase and the earth with all connected burdens. Measure secondary current at all points of CT circuits. It shall be done for other phases.