CURRENT TRANSFORMER
SCOPE:
This method statement covers the procedure for doing site tests to be carried out on current transformer.
OBJECTIVE OF THE TEST:
The objective of the test is to verify the readiness & healthiness of the current transformer.
SAFETY PRECAUTIONS
Secondary circuit of current Transformer should be open circuited during testing since the high voltage injected during insulation resistance may damage the insulation and other components in the circuit.
Standards rules for working on high voltage equipment will be followed.
Company’s safety codes will be followed.
STANDARD/SPECIFICATION REFERENCE OF THE TEST:
Current Transformer specification and approved drawings.
PROCEDURE OF TEST:
The tests to be carried out in site for current transformer are Insulation resistance, Polarity, Winding Resistance, Magnetization curve, Ratio Check, and Burden test.
INSULATION RESISTANCE:
To check and confirm the insulation of current transformers, “Insulation Test” is carried out on individual core of each current transformer. Insulation test is done between Primary – Secondary,Secondary to Earth and Secondary to Secondary in case of two or more secondary winding in one CT.
apply the voltage for 60 Seconds and record the values in the test sheet.
POLARITY
Each current transformer will be individually tested to verify that the primary and the secondary polarity markings are correct.
When primary current enters the P1 terminal, secondary current leaves the S1Terminal (or) when primary current enters the P2 terminal, secondary current leaves the S2 terminal.
The primary and secondary terminals of the CT to be connected to the CT Analyzer as shown in figure - 1 and the test to be started after testing CT Analyzer will give the report as polarity ok or not ok.
Care should be taken that from the CT Analyzer red colored terminals are connected to P1, S1 terminals of the CT and the black colored terminals are connected to P2, S2 of the CT. Refer figure-1.
WINDING RESISTANCE
The secondary winding of the current transformer has its own resistance, this winding resistance to be measured and noted for protection calculations.
This can be measured using milliohm meter or CT Analyzer.
The primary and secondary terminals of the CT to be connected to the CT Analyzer as shown in figure-1 and the test to be started after testing CT Analyzer will give winding resistance of the CT secondary. Refer figure-1.
MAGNETIZATION CURVE
Definition: AC voltage is applied across the secondary winding of the current transformer and the voltage is increased slowly from zero to saturation point in stepwise, the point where the exciting current increases by 50% for a 10% increase in secondary voltage is called the saturation point.
The primary and secondary terminals of the CT to be connected to the CT Analyzer as shown in figure and the test to be started after testing CT Analyzer will give magnetization curve of the CT.
Refer figure - 1.
RATIO TEST
The rated transformation ratio is the ratio between the primary rated current to the secondary rated current.
The ratios of the CT are checked by giving the current in the primary and monitor the induced
secondary current and compare the results with the name plate ratio.
The primary and secondary terminals of the CT to be connected to the CT Analyzer as shown in figure and the test to be started after testing CT Analyzer will give the CT ratio. Refer figure - 1.
BURDEN MEASUREMENT
Burden measurement is the load connected between the CT secondary terminals, like protection or metering or monitoring devices.
It can be measured using CT Analyzer or CPC 100.
Isolate the CT secondary circuit from CT secondary winding by opening the terminal block link as shown in the figure.
Connect the current and voltage leads from the kit to the CT secondary terminals as shown in the Figure - 3
Switch on the kit and press the start button, kit will give the circuit burden measurement.
ONE POINT EARTHING
Current transformer secondary star point (Neutral) to be earthed in only one point to make the metering and protection functions accurate.
Open the neutral earth link of the CT secondary under test and all other earth links to be closed.
Using 500V insulation tester do IR test between CT Secondary star point terminal to earth, the value of the insulation should be high (MΩ).
Close the neutral earth link of the CT secondary under test and then do the same IR test the insulation should be zero.
Thus the one point earthing is confirmed.
ACCEPTANCE CRITERIA:
Record all the values in the test report and compare the values with factory test repot and the name plate. All the readings should be within criteria limit.
