Testing Current Transformers

CT or current transformer is an intermediate flow measurement, which limits the ability to read gauges. Suppose the system of high voltage lines, the current flowing is the 2000A while the gauges are only limited to 5A. CT is needed to transform a representation of the actual value of 2000A at 5A so that the field be read by the measuring instrument.

CT is generally only used as a medium readings are also used in power system protection system. Protection systems in electrical power systems are so complex that CT itself was made to the specifications and classes that varied according to the needs of the existing system.

Specifications on CT include:

1. CT ratio, CT ratio is the basic specifications that must be present on CT, where the representation of the current value on the ground in calculating the size of the CT ratio. Eg CT 2000/5A ratio, the measured value in the secondary CT is 2.5A, then the actual value of current flowing in the conductor is 1000A. Presentation or the magnitude of the error ratio error (% err.) Can affect the magnitude of error in measuring instrument readings, the error rate calculation, and fault protection system operation.
2. Burden or a maximum value of power (in VA) that can be borne by the CT. The value of this power must be greater than the measured value of the CT secondary terminal to the relay coil protection work. If smaller, then the relay protection will not work for mengetripkan CB / PMT when an interruption occurs.
3. Class, class of CT to determine whether the type of protection system for the CT core. Eg used for overcurrent protection 5P20 class, for class use tariff metering class 0.2 or 0.5, busbar protection system used for Class X or PX.
4. Kneepoint, is the point of saturation / saturation when CT did excitasi voltage. Busbar protection is generally used as the driving coil voltage. Voltage can be generated by CT when CT secondary impedance is given as it appears on Ohm's Law. Kneepoint only in CT with a Class X or PX. The magnitude of the voltage can reach 2000Volt kneepoint, and of course kneepoint magnitude depending on the value or the desired design.
5. Secondary Winding Resistance (RCT), or impedance in CT. Impedance in CT are generally very small, but in the Class X this value is determined and must not exceed the values ​​listed there. For example: <2.5Ohm, the impedance of the Class X CT should not be more than 2.5Ohm or CT were returned to the factory to do the replacement.

Based on the above criteria, then the CT testing can be done as follows:

These examples along with a description in this article I took from my experience doing SAT on CT and HV Equipments Project: Cikarang Listrindo 4x60MW Gas Power Plant Project, Inalum 275kV OHL Prot'n Panel Replacement Project, and Muara Karang 2x250MW Gas Power Plant Project. 

Ratio Test

Example: CT Ratio = 2000/5A
To perform the test that is true if the value of the CT secondary rated current of the primary line 2000a is 5A, the current injection device is needed here that can divert the current of 2000A. Of course this tool is very rare and very large.
Commonly used alternative way is to inject a smaller device, such as 500A. To get the 2000a then we can make as many rolls or loops 2000A/500A = 4 times roll.

Of course the value is not exactly as it appears on the calculator but at least that value can be achieved. Metering or instruments attached to show the value of approximately 2000A.

In the general case that occurred in the field, it kind of test tool that can generate large amounts of current are quite hard to find (because it is expensive so generally we rent from my friends)

Behind it was a lot of the CT measurement results are not linear / or not directly proportional to the ratio indicated. In other words, the percentage of error-reading of his varied and generally the smaller the applied current, the percentage of error, the greater his reading beyond the specifications listed on the nameplate CT. Though to some of the protection system such as Distance Relay using the current parameter readings at a low value.

Then IEC standardization issue that must be linear CT measurement values ​​of at least s / d 10% of current value or the nominal current rating indicated. Of course, this beneficial for me as the SAT and commissioning team. To test required enough current 2000A CT by 10% x 2000A = 200A only. Hmm .. consequently ujinya tool was not too heavy and not many places to eat. Hot smile "still loading luggage for souvenirs nyimpen ..".

Then how to test and calculation error presentation-reading it how?

 

Example for 2000A :

• CT 2.1 - Core # 3 
• Serial No. CT: 0805451CT primary terminal and secondary terminal 
• Tap terminal is used 3S1 ~ 3S3
• Class 0.5 Security Factor (FS) <20, a maximum of% err. is 0.5% 
• Ratio 2000/5 A 
• Injection current of 200A, the current measured on the primary side of CT were: 199.96 A, of course there are losses in the cables and connections on the primary side. 
• Rated current on CT secondary side is: 501.55 mA
• With the above formula, the value of primary current is: 2000A and skundernya current value is 5.0165 A 
• So that% err. = 0.33% [OK]

Because the count is less hobby, so I've made in the form of Excel formulas, and the result will be like this. Simply insert the actual value of primary current and secondary current actual value. Pretty simple is not it?

 Secondary Winding Resistance Testing (RCT)

CT Secondary Winding testing generally refers to the standard IEC 60076-1. Formula and test systems should be based on the setandar. Testing out the standard unauthorized and does not meet the criteria for standard CT testing.

Based on IEC 60076-1, the measurement of elements that must be taken when testing CT Secondary Winding is as follows:

• IDC: DC current is injected into the actual CT secondary terminal. I usually use the current value is of type 5A to 5A nominal CT secondary output. 
• VDC: measured voltage generated by the injection of DC current in the coil / winding CT. 
• Meas R: The value of winding resistance or resistance in the CT, which is obtained from the calculation VDC / IDC (Ohm's Law). 
• Time: Total time spent in testing. 
• Dev: deviation angle is expressed as% of maximum and minimum values ​​are measured and evaluated at least 10 seconds of measurement. Results declared stable if the Dev <0.1%. 
• Tmeas: ambient temperature or room temperature 
• TREF: operating temperature of CT, most commonly used value is 75 ° C. FAT Data should refer to the reference manual from the manufacturer or CT.

  Formulation so that the calculation of CT Secondary Winding Burden can be made as follows:

Secondary testing burden is quite significant, given that this test was also a CT to check on the circuit load such as relay panels, metering, buspro, logger, etc.. The series of CT must always be closed (short-circuit) in order to mengasilkan flow.

Circuit impedance should not be any large or even cut off, in case the current can not flow and CT into heat and overload. As a result the CT can be damaged, broken, or even explode. This testing while ensuring decent conditions rangkain CT operated or not.

CT or CT excitations testing Kneepoint

In the test CT saturation point or kneepoint there are three types of standards are set, all three have different values ​​kneepoint but all three are considered legitimate, depending on what standard to be used at least a CT Manufacturer and End-User use the same standard.

• IEC / BS - According to IEC 60044-1, the knee point is defined as the point on the curve where a voltage increment of 10% increases the current by 50%. 
• ANSI 45 ° - According to IEEE C57.13, the knee point is the point where, with a double logarithmic representation, the tangent line to the curve forms a 45 ° angle.Applies to current transformer cores without an water gap. 
• ANSI 30 ° - Like ANSI 45 ° but forming a 30 ° angle.Applies to current transformer cores with an the water gap.

In Indonesia generally refers to the IEC Standard, as a standard of high and medium voltage intalasi.

To conduct CT testing, it would require an AC voltage source is capable of being used to test the CT Class X, where its value could reach kneepoint 2000Volts. Excitation voltage is supplied to the secondary terminals of each Core-CT at her, then the voltage was increased slowly to reach the nominal current value of CT. Current measurements can be done by placing the Ampere-meter with a linked series of injections or the use of a clamp meter on the output cable voltage injection devices.

Model testing is generally I use is as below:

Any significant change in the current or any multiple of how many volts of voltage, measurement and recording can be done simultaneously in order to chart a smooth and precise. Examples of such graphs is as follows:

 


If you created a graph in Excel, then the graph it will be shaped like this:

 

Unfortunately, not all manufacturers or rarely mentioned CT Kneepoint value that is obtained when the done FAT (because not everyone is easy to determine and understand the value of the measurements obtained). Usually the only manufacturer to attach a data value with the value of excitation current in the can and attach the graph.

The key core excitation voltage on CT testing is only to determine the value of what the Volt, CT has reached saturation point and did not produce significant changes in flow.

Eg CT specification is Vk> 1.7 kV excitation voltage is 1.7 kV CT must exceed to produce 5A, 5A reached at least 2kV. Thus CT has specifications as stated.

Isolation or Megger Testing

Tests on a whole is to determine that the CT is feasible to operate the system design specifications and current measurement errors do not occur exactly where CT is an element metering and protection.

To determine whether CT voltage is feasible or not, it must be done or Megger Insulation testing. 5kV Megger is used for the primary side and secondary side to 1kV.

The point of testing can be done is:

• Primary to Ground terminal should not be any relationship 
• Primary to secondary terminals should not be any relationship 
• Secondary to the Ground terminal should not be any relationship

Physical checks

CT time comes and when installed must physically check diulakukan first as a quality control form. There should be no cracks, or even a transformer oil seepage.

"Hopefully, the article mentioned above were able to add insight and improve the quality control of products or development projects of electricity infrastructure in Indonesia. Electricity is better for the future, and let air-Save Energy."