Current Transformers (CTs) are defined by their primary conductor: wound-type for low currents, bar-type for high currents, and toroidal for bus-bar mounting. Core construction uses grain-oriented silicon steel or nanocrystalline alloys for precision.
Inductive Voltage Transformers (IVTs)
employ single-stage or cascaded designs to manage insulation stress and ensure accurate metering and protection.
Construction Types of Current Transformers (CTs)
Straton Electricals Pvt. Ltd. designs and manufactures instrument transformers for voltage classes ranging from 3.3 kV to 245 kV. CT construction is selected based on system voltage, current rating, environment, and accuracy requirements.
Window-Type Current Transformers use a toroidal core allowing the primary conductor to pass through the window. These are suitable for high-current applications up to 10,000 A and use cast-resin insulation for uniform dielectric strength.
Wound-Type Current Transformers
use a primary winding over the magnetic core and are commonly used in medium-voltage systems, converting currents up to 2,000 A into standardized 1 A or 5 A outputs.
Bar-Type Current Transformers incorporate a fixed primary conductor and provide excellent ratio accuracy for energy metering and revenue measurement.
Modern cast-resin CTs manufactured as per DIN 42600 standards are widely used in indoor substations due to their compact design, safety, and maintenance-free operation.
Construction Types of Inductive Voltage Transformers (IVTs)
Inductive Voltage Transformers (IVTs) use a core-and-coil assembly housed within suitable insulation to step down high system voltages to standardized secondary values for metering, protection, and control.
Dry-Type (Cast Resin) IVTs encapsulate windings in epoxy resin and are commonly used in indoor medium-voltage applications due to their safety and moisture resistance.
Oil-Immersed IVTs are preferred for high-voltage outdoor substations, offering higher dielectric strength and superior cooling performance.
In GIS and space-constrained environments, SF₆ Gas-Insulated IVTs are used for compactness and high dielectric reliability.
Comparison of CT and IVT Construction
Current Transformers (CTs)
and Inductive Voltage Transformers (IVTs) differ significantly in construction due to the electrical quantities they measure.
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Core Design: CTs use ring, bar, or wound cores, while IVTs use laminated electromagnetic cores or capacitive dividers.
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Insulation: CT insulation focuses on core-to-secondary separation, while IVTs require complex inter-winding insulation.
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Impedance: CTs operate with low-impedance secondary circuits, whereas IVTs maintain voltage accuracy over a defined burden.
Applications Based on Construction
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Protection-Grade CTs: Used in relaying schemes for fault detection with high thermal and dynamic withstand capability.
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Metering CTs: Used in revenue metering with accuracy classes up to 0.2S as per IEC 61869 standards.
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IVTs: Used for voltage measurement, protection, and monitoring in substations and switchgear.
Maintenance Considerations for CTs and IVTs
Routine inspection, insulation testing, burden verification, and thermal imaging are essential to ensure long-term accuracy and safety. CT secondary circuits must never be left open during operation due to dangerous voltage buildup.
FAQs
What is the primary purpose of a Current Transformer (CT)?
A Current Transformer safely steps down high current to measurable levels for metering and protection in power systems.
What are common CT construction types?
- Bar-Type CT
- Wound-Type CT
- Window-Type CT
What is the difference between CVT and IVT?
A CVT uses capacitive voltage division, while an IVT operates on magnetic induction for voltage measurement.
What are CTs and VTs?
CTs measure current, and VTs measure voltage. Both are instrument transformers essential for metering and protection.
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