Current Transformers perform two critical roles in the power supply eco-system. They are used as current measurement devices and as protection relays. However, there are different types of Current Transformers and several factors govern the appropriate choice of the Current Transformer.
The objective of this study is to understand the key considerations before choosing the right Current Transformer for a particular application.
If you are looking to read about the basic function of Current Transformers, compliance standards and applications, read our earlier blog here.
Current Transformers play a critical role in Generator protection and load management. The design of the generator excitation protection/control system needs to accommodate field forcing during faults to aid in maintaining transient stability. Starting a large motor load may tax the generator and exciter system. Modern exciters put out 3-5 times of the generator’s normal full load excitation current. Field forcing is used in case the generator is taking its excitation power from its own line side. A Current Transformer helps to regulate the excitation power. The power potential transformer (PPT) and Saturable Current Transformers (SCT) are the main source of power for the AC machine field. At no load, all the field power is obtained from the PPT. At full load, both are used with the SCT supplying most of the power.
KSC Series – Saturable Current Transformers from KSI
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These Current Transformers are used as source for relays and meters to maintain the field to generate the same load or under fault condition till the relay operates. The CT is responsible for current maintenance, but only during the short time until the fault is rectified. The relay’s operating characteristics have a large influence on how the CT should be selected for a given application.
Learn more: KSCM Series – SCM Current Transformers from KSI
These current sensing CTs are used for feedback and control in Electronics. The minimum current that can be sensed is limited by the noise level of the sensing circuit. They are widely used in utility meters and can measure from 10mA to 100A with 0.1% accuracy. These CTs come with proven performance features for accuracy, linearity, low-end resolution and temperature sensitivity.
KMA Series – Milli Ampere Sensing Current Transformers from KSI
Learn more: KMA Series – Milli Ampere Sensing Current Transformers
Core Balance Current Transformers (CBCT) are used for Earth Leakage Protection. Earth leakage currents must not be allowed for long periods of time as they can cause insulation failure and earth faults. To protect cable circuits and overhead transmission lines with cable terminations, these CTs are employed. The secondary of the CBCT is connected to an amplifier to get sufficient power to operate the relay. They are different from normal protective & metering current transformers due to their performance requirement.
KCBT Series – Core Balance Current Transformers from KSI
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Interposing Current Transformers (ICT) are used to balance, match or isolate the current supply for the equipment connected to it. ICTs are installed between the main CT and the equipment whose current supply has to be matched. They can be used on the primary side or secondary side of the power transformer being protected or both. These CTs also provide a convenient method of establishing a delta connection for the elimination of zero sequence currents where ever necessary.
KIC Series – Interposing Current Transformers from KSI
Learn more: KIC Series – Interposing Current Transformers
These CTs are used in precision instruments for the protection of power transformers. They are used in oil cooled transformers to monitor the transformer winding temperature to activate high temperature alarms, circuit breakers and coolers. These instruments operate on the principle of thermal imaging and it is not an actual measurement.
KWTI Series – WTI Current Transformers from KSI
Learn more: KWTI Series – WTI Current Transformers
Universal Auxiliary Current Transformers (UACT) are used as interposing CTs for protection to obtain various combinations from a single CT. An interposing CT is installed between the secondary winding of the main CT and the relay. They can be used on the primary side or secondary side of the power transformer being protected or both. These CTs are used for feeding protective or measuring devices which require operating current that is different from the main CT secondary current. The ICT’s internal burden is to be fed by the output of the main CT.
KUAT Series – Universal Auxiliary Current Transformers from KSI
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These Current Transformers are primarily used in motor control applications for various 3 phase circuits such as relaying, metering and overload protection. They can also be used in switchboards and control panels where space and installation requirements are a consideration.
KTP Series – Three Phase Current Transformers from KSI
Learn more: KTP Series – Three Phase Current Transformers
In several applications for power measurement, the device under test has higher levels of current than the prescribed limits that can be applied to the input terminals of the power analyzer. A Precision or Standard Current Transformer with High Accuracy Class (generally of 0.1) is used for to step down high AC and DC currents to a lower level that can be measured directly by the power analyzer. The CT can safely step down currents while preserving the highest accuracy. These CTs find wide application in Laboratories and Test Benches.
KPC Series – Precision Current Transformers from KSI
Learn more: KPC Series – Precision Current Transformers
These toroidal Current Transformers have a “split core” so that they can be opened, installed, and closed without disconnecting the circuit to which they are attached. They are used where the primary conductor is difficult to open for installation of regular ring type CTs. These high current CTs find common application in Furnaces, Electroplating Plants and Short Circuit Tests.
KHSC Series – High Current Split Core Current Transformers from KSI
Learn more: KHSC Series – High Current Split Core Current Transformers
KSINSTRUMENTS has a wide range of catalogue products to suit all needs. Despite a wide range of catalogue products, in several cases, a customer application may demand a custom solution. With the backing of a strong design team and an in house testing facility, KSI can offer custom solutions to various design challenges with ease. Get in touch with us to know more. Hope you found the information in this study useful. Please leave your comments below and let us know your thoughts and feedback.
Summation Measuring Current Transformers for Power Systems
Selection of Current Transformers for Protection Relays or Tripping Relays
Selection of Current Transformers for Metering and Measurement Applications