Current Transformers: The Core Sensing Device in Power Systems
The characteristics of current transformers (CTs) can be derived from comparisons with voltage transformers and ordinary transformers, as well as from their design principles and functional requirements.
I. Core Working Principle Characteristics
1. Series Connection for Current Measurement
Unlike voltage transformers, which are connected in parallel, CTs are connected in series in the primary circuit. Their primary winding (or single-turn conductor) directly carries the measured current.
2. "Constant Current Source" Characteristic
Under normal operating conditions, the secondary circuit of a CT functions as a current source. Its output current Isis primarily determined by the primary current Ipand remains largely unaffected by changes in secondary load (within the rated load range).
3. Electromagnetic Induction Principle
Based on transformer principles, CTs utilize electromagnetic induction to proportionally convert large primary currents into small secondary currents.
II. Key Performance Characteristics
1. High Transformation Ratio and Standardized Output
Capable of proportionally reducing currents ranging up to tens of thousands of amperes. The secondary output current is standardized, typically 5A or 1A, ensuring compatibility with all standard meters and relays.
2. Accuracy and Error
CTs exhibit ratio error and phase angle error. Different accuracy classes are defined based on application (e.g., Class 0.2S for metering, Class 5P for protection), each with corresponding error limits.
3. "Instrument Security" and "Protection Security"
Metering CTs: Designed to saturate rapidly when the primary current exceeds a certain threshold, preventing hazardous high currents in the secondary circuit and protecting precision instruments. This is termed "instrument security."
Protection CTs: Must maintain a certain level of linearity even under extremely high fault currents to accurately relay fault current information to protection devices. This is defined by the "accuracy limit factor."
4. Mandatory Reliable Grounding
To prevent high voltage from the primary side entering the low-voltage secondary circuit—endangering personnel and equipment—the secondary side of a CT must be grounded at a single point only. This is a critical safety regulation.
5. Secondary Side Must Never Be Open-Circuited
The most critical and hazardous characteristic. If the secondary side of a CT is open-circuited:
The primary current fully becomes magnetizing current, causing severe core saturation and overheating, potentially burning insulation.
Dangerously high voltages (thousands of volts or higher) are induced at the open point, posing serious risks to personnel and insulation breakdown.
Remanent magnetism in the core degrades accuracy.
Therefore, fuses must never be installed in the CT secondary circuit, and secondary terminals must be shorted before disconnecting any instrument.
III. Structural Design Characteristics
1. Primary Winding: Few Turns, Large Cross-Section
The primary winding typically has only one or a few turns but must carry large currents, requiring conductors with large cross-sections, such as copper bars or bus ducts.
2. Secondary Winding: Many Turns, Small Cross-Section
The secondary winding has many turns to output standardized small currents, using finer conductors.
3. Diverse Core Materials
Metering/Measurement CTs: Use high-permeability silicon steel or permalloy cores, ensuring high accuracy at low currents but designed to saturate at high currents (achieving "instrument security").
Protection CTs: Use silicon steel with better linearity, maintaining reasonable accuracy under high fault currents (strong anti-saturation capability).
4. Wide Variety of Types
By Installation: Bushing-type, support-type, sleeve-type, busbar-type, etc.
By Insulation Medium: Dry-type, cast resin, oil-immersed, SF6 gas-insulated, suitable for different voltage levels.
By Function: Measurement, protection, metering, transient protection (TP class).
IV. Application Characteristics
1. Electrical Isolation
Safely isolates high-voltage primary circuits from low-voltage secondary control and measurement circuits, ensuring the safety of personnel and equipment.
2. Extends Instrument Measurement Range
Enables standardized, low-cost instruments (e.g., 5A ammeters) to measure currents of virtually any magnitude.
3. Standardizes Equipment Interfaces
Provides uniform input signals for standardized relays, energy meters, and data acquisition devices, ensuring equipment compatibility and interchangeability.
4. Core Protective Sensing Element
Serves as the "eyes" of power system protection, providing current signals that reflect fault conditions to relay protection devices, ensuring safe and stable system operation.
