CT Basics
Measurement of energy flow is extremely important to monitor and protect the power system. Current Transformer is widely used for this purpose.
It becomes practically impossible for any measuring instrument to handle the high current values and hence Current Transformer comes as a handy solution. Current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments. A current transformer isolates the measuring instruments from very high voltage in the monitored circuit.
Engineers often tend to think of a current transformer as an ideal device. Hence let’s have a look at ideal CT.
Ideal Current Transformer
An ideal current transformer would proportionally scale down the value of the power system current to a useable known value. Second, the scale-down output should faithfully reproduce the power system current waveform. An ideal current transformer should perform these two tasks over the range of a few amps up through ten thousands amps. The ideal current transformer should be able to meet these requirements. In reality, a current transformer has limitations.
Today’s current transformer hasn’t changed since it was developed. A current transformer consists of the following components:
A laminated steel core
A secondary winding around the core
Insulating material
When current travels through a current carrying device, such as a cable or bus duct it develops a magnetic field at right angles to the flow of current. The strength of the magnetic field varies as the current magnitude changes during all operating conditions. As learnt in transformer theory, when a magnetic field strikes a wire, it will cause a current to flow in the wire. By using the strength of the magnetic field and knowing the turns ratio, we can obtain a value of current that is useable for meters, relays and other current sensing devices.
In order to scale a value of high current flowing in a conductor, the engineer needs to introduce a specific number of uniformly distributed turns of wire around the core to scale down the system current. This will ensure that the output current is always proportional to the current flowing in the conductor. The current carrying conductor is referred to as a primary or P1 and the ends of the wire surrounding the core are referred to as a secondary or S1 and S2 for single winding current transformer.
In the past, there were two main values of secondary current typically used in measuring current. In India and most of Asian countries and also in the United States, engineers typically use a 5-amp output. Other countries have adopted a 1-amp output.
Application
RISHABH Current Transformers are manufactured to meet indoor or internal devices for Switch gear, Distribution Systems, Generator Sets and Control Panels. Rishabh CTs are intended for supplying measuring instruments and protection circuits of electrical power devices with a maximum operating voltage of 0.72KV and frequency of 50Hz or 60Hz. These Current Transformers are supplied in Ring/Rectangular/Split core type in a wide range of ratios and accuracies with Primary current ranging from 1A up to 7500A and with Secondary current of 5A and 1A.
CT Assembly
Accuracy of CT depends on the magnetic performance of the steel core. Hence we can call it as a heart of CT. Torridly wound cores with high permeability and low loss are used to optimize performance and physical size of the transformers. High grade insulation is used to insulate between the windings and the core and between winding layers.
Maximum mechanical and electrical performance is achieved by distributing all windings evenly around periphery of core.
The casing of the CTs is made with UL 94 V-0 approved 10% glass filled Polycarbonate material having self-extinguishing and non-drip feature that provides an excellent mechanical / fire protective body & look and long term dielectric performance.
CT Selection
Factors to be considered while selection of the current transformer.
Primary Current :- The nominal value of the primary current (Ipn) should be selected from the offered series range of available types to provide the closest match with the expected primary current of the system. All low voltage transformers manufactured by Rishabh have an extended rating of 120% which makes conversion possible within a range 20% higher than the rated value.
Secondary Current :- Secondary current (Isn) is to adapt to the system found on the secondary side of the transformer. 5A and 1A are the standardized secondary current values. Primary current/secondary current (Ipn/Isn) is the standard transformation ratio.
Power – the transformer load :- The total load (VA) that will be connected to the secondary side of the transformer should be considered, including the load from the connected device as well as losses on connection leads and terminals. According to standard IEC 60044-1 the current and phase errors of the transformer should not exceed values specified in below given table at any secondary load in the range from 25% to 100% of the rated load.
Dimensions:- Internal and external dimensions of the transformer should be specified to ensure the possibility to install the transformer on the current circuit and that it can be accommodated in the planned locations.
Accuracy class to obtain satisfactory measurement accuracy :- In case of metering CTs with accuracy class 0.2, 0.5, 1 or 3, CT has to be accurate from 5% to 120% of the rated primary current and CTs with accuracy class 0.2S and 0.5S, it has to be accurate from 1% to 120% of the rated primary current at 25% and 100% of the rated burden at specified power factor.
Factors Affecting CT Prices
Selection CT VA burden and accuracy class should be according to the actual requirement of the application wherein the CT is going to be installed. Selecting higher VA or Class than necessary usually results in a higher cost. The cost generally increases as the CT internal diameter increases. 1A secondary CTs are usually more expensive than 5A CTs.
Instrument Security Factor (FS)
In order to protect the instruments and meters from being damaged by high currents during fault conditions, a metering core must be saturated typically between 5 and 20 times the rated currents. The rated instrument security factor (FS) indicates the over current as a multiple of the rated current at which the metering core will saturate. It is thus limiting the secondary current to FS times the rated current. ISF for Rishabh CTs are designed to 5 or less than 5. The safety of the metering equipment is greatest when Rishabh CTs are used.
Test Set up Photo used in Rishabh with Test Certificate supplied with each CT.
CT Testing
Each and every CT is tested in accordance with IEC 60044-1 or IS-2705 for ratio errors and phase angle errors with microprocessor based automatic transformer test set (Omicron make CT Analyzer) with facilities for automatic printout of test results. Test comparisons are made with standard traceable NPL to validate ratio accuracy performance for all CTs. For protection class CT the performance is verified by excitation measurements.
Test Set up Photo used in Rishabh with Test Certificate supplied with each CT…
Metering Class CTs
In general the following applies :
Accuracy class requirements:
0.1 or 0.2 for precision measurements.
0.5 for High grade Killowatt-hour meters.
1.0 for commercial grade killowatt-hour meters.
1 or 3 for general industrial measurements.
3 or 5 for approximate measurements.
Burden requirements:
Ammeter : 1.0 VA.
Current coil of Watt/var meter : 1.5 VA.
Current coil of energy meter : 2.0 VA.
Current coil of PF indicator : 2.5 VA.
Current coil of Tri-vector meter : 3.0 VA.
Protection Class CTs
In addition to general specification required for CT design, Protection CTs require an Accuracy Limit Factor (ALF). This is the multiple of rated current up to which the CT will operate while complying with the accuracy class requirements. In general the following applies:
Instantaneous over current relays & trip coils : 2.5VA class 10P5.
Thermal Inverse time relays : 7.5VA class 10P10.
Low consumption relays : 2.5VA class 10P10.
Inverse definite min. time (IDMT) Over current relays : 15VA class 10P10/15.
IDMT earth fault relays with fault stability or accurate time grading required : 15VA class 5P10.
Special Type CTs
Class PS CTs
Class PS CTs are special CTs used mainly in balanced protection systems (including restricted earth fault) where the system is sensitively dependent on CT accuracy. Further to the general CT specifications, we now need to know:
Vkp – Voltage knee point
Io – Maximum magnetizing current at Vkp
Rs – Maximum resistance of the secondary winding.
Knee Point Voltage: That point on the magnetizing curve where an increase of 10% in the flux density (voltage) causes an increase of 50% in the magnetizing force (current).
Summation CTs
In electrical supply practice, it may become necessary to obtain sum of currents in a number of feeders. To achieve this, Summation CTs are used. These CTs are used with feeder CTs. Which may or may not have same ratios. Each feeder is provided with its own CT and the secondary windings of these are connected to the appropriate primary windings of the Summation CT. The summation CT has a single secondary winding which is connected to the burden. It is essential that Summation CTs are used on currents of same frequency and phase. Summation CTs are generally manufactured confirming to IS 6949.
Rish Xmer Sigma 75
Rish Xmer Sigma 150
Conclusion
As Rishabh is having the complete basket of low tension current transformers manufactured in a world class CT manufacturing setup consisting of an automatic testing facility for all CTs with a printed Test Certificate, we have a real technological excellence in current transformers.
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