TESTING OF CURRENT TRANSFORMER
Amala Joseph
Electrical Protection, Testing and Commissioning Engineer at Aura Engineering LLC | PDO Approved Testing Engineer | MIC-North PDO
CURRENT TRANSFORMER
A current transformer (CT) is a type of transformer that is used to measure AC current. It produces an alternating current (AC) in its secondary which is proportional to the AC current in its primary.
Current transformers are designed to provide a scaled-down replica of the current in the HV line and isolate the measuring instruments, meters, relays, etc., from the high voltage power circuit.
A current transformer is a device that is used for the transformation of current from a higher value into a proportionate current to a lower value.
1.1????TESTING OF CURRENT TRANSFORMER
1.1.1????GENERAL CHECKS
The following general checks are performed before proceeding with the CT test.
1.1.2????INSULATION RESISTANCE
Checking the insulation resistance value by using insulation resistance tester.
Why IR test is always first?
Before confirm the characteristics of CT, Insulation healthiness to be ensured for transportation damage.
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1.1.3????POLARITY
The relative of CT primary and secondary terminals are identified either by painted polarity marks or symbols.
Why polarity test is always before winding resistance test?
To measure the winding resistance, CT will be saturated by injecting dc current. Once CT is saturated secondary will not reflect or follow primary. Hence, it is not possible to do the polarity test after winding resistance test.
1.1.4????WINDING RESISTANCE
Check S1&S2 by using multimeter or micro ohm meter, the value will be in mΩ or Ω.
Why winding resistance test is always before excitation characteristic?
CT will be saturated during winding resistance test. The saturated CT will be normalized or Desaturated during excitation characteristic test.
1.1.5????MAGNETIZING CHARACTERISTICS
To obtain the data the primary circuit is left open circuited and an AC voltage is applied across the secondary winding by using the auto transformer (variac).By raising the voltage one step at a times of reading of the corresponding current are taken with the ammeter. The point at which a CT begins to saturate is known as the knee point on the curve of its.
Magnetizing characteristics and is by definition the point where the exciting current increases by 50% for a 10% increase in secondary voltage.
How will you De-saturate the CT?
Prior to taking the reading for the characteristics and after they are obtained, the core of the CT should be demagnetized by slowly raising the voltage not more than the knee point voltage?and then slowly reducing it to zero two or three times.
1.1.6????RATIO
The rated transformation ratio is the ratio between the primary rated current and the secondary rated current. The ratio of the CT is checked by dividing the value of the primary current by that of the secondary current, and comparing with the rated valve.
Why ratio test is always last?
???????????????Saturated CT during winding resistance test will only be De-saturated / normalized during excitation characteristic test. After that only CT secondary will follow the primary to carry out the ratio test.
DISCUSSION:
The above mentioned are old methods, however today all the tests can be performed by a single testing kits. Popularly, IR test was performed by Megger MIT515 kit, remaining tests will be conducted using Omicron CPC100 or Omicron CT analyzer kit.
If you want to know how to perform the CT tests using above mentioned kits kindly comment.
Questions & Answers:
1.???Types of CT?
Current Transformers can be constructed in a number of different ways:
A.?????WOUND TYPE: with a fixed primary with a number of turns around the core the winding is fully insulated and form an integral part of the assembly
?B.?????BAR TYPE: this has a fixed insulated primary conductor passing through the core and essentially the primary conductor is equal to 1 turn
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?C.?????BUSHING TYPE: this has no primary winding it is for use with a fully insulated conductor acting as the primary winding.
?D.????RING or WINDOW TYPE: this has no primary winding but it normally has insulation in the window so that conductors can be passed through
?2.???Types of core?
Current Transformers are divided into 2 major categories:
A.???MEASUREMENT: Intended for use with indicating or integrating meters
Application: Ammeter, energy meter, Tariff meter, recording instruments
B.???PROTECTION: Intended for use with protection devices
5P20:
5?????????- Total error in % at accuracy limit and rated burden
P?????????- Protection class
20(ALF) - Minimum multiple of rated secondary current at which the accuracy limit is reached with rated burden
Application: Backup protection relays, OC&EF relays
CLASS X:
Application: Main protection relays, REF relays
3.???What is the different between protection and other core?
Protection core has high saturation point. And accuracy is very high. ?
4.???Which core is using for Backup protection? What is ALF in CT
For Backup protection 5P15 or 5P20 CT core is to be used.ALF Means Accuracy Limit Factor of CT.ie 5P15 means 5times fault on primary current of CT cause 15% error secondary side.
5.???How you decide the requirement of CT?
1. Expected Max loading of that feeder.
2. Burden on metering scheme
3. Type of protection.
4. Expected fault current.
6.???Give the parameter of CT using for different function.
Metering: Class 0.5 or 0.1 or 0.2 or 1.0, Burden: 50 or 20 or 10VA Rct
Backup Protn.: 5P15 or 5P20, Rct: 2ohm Burden: 25VA or 50VA
Main Protection: Class: X. Rct: 8ohm at max ratio.?Rct 5 ohm, Vk: 300V; Ie= 30mA at Vk/2
?7.???Different core application? what is metering core, protection core,’x’ spl core?
?***Share your answer in comment***?
8.???Diff between 5P10 and 5P20? Accuracy calculation? What ‘10’ and ‘20’ representing?
***Share your answer in comment***?
9.???What are the protection used in ‘x’ core?
?***Share your answer in comment***?
10.?What are things to consider when selecting ‘x’ class?
??***Share your answer in comment***?
11.?Testing of CT?
??***Already explained above***?
12.?How to test knee point voltage? Circuit drawing?
??***Share your answer in comment***?
13.?What will happen when CT gets saturate?
??***Share your answer in comment***?
14.?Knee point curve explanation,
??***Share your answer in comment***?
15.?IR value test? Winding resistant test? Polarity test? Ratio test?
?***Already explained above***
?16.?Can you use the backup protection CT for metering?
In case of metering core is not available or metering core is saturated condition and also the situation (power purchase agreement) temporary basis backup protn.CT is to be used for metering via low saturation ICT which having specified ISF.