0319 - 7018 CT106PM - 05 Current Transformer 1. Dimensions Diagram  Dimension Unit: (Millimeter mm)
Dimension Tolerance: ±0.5mm (Unless Otherwise Specified) Pin Tolerance: ±0.3mm
2. Electrical Performance L S1-S2 (mH MIN) | 470 （Inductance: testedat 10KHz & 10 mV） | Frequency | 200KHz Max | Turn Ratio | 1：500 | Dielectric Strength voltage | 4000V AC 2mA 1 SEC | DC Resistance | 16.0 OHMS Max(25℃) |
3. Test Circuit 
Current Transformer (CT) A Current Transformer (CT) is a specialized transformer that proportionally converts high current into low current. Its core functions are to provide standardized current signals (typically 5A or 1A) for measuring instruments and relay protection devices while ensuring electrical isolation between the high-voltage primary circuit and the low-voltage secondary control circuit. It is widely used in power systems for metering, monitoring, and protection applications.
I. Core Working PrincipleThe current transformer operates based on the law of electromagnetic induction. The primary winding is connected in series to the high-current circuit being measured, while the secondary winding is connected to measuring or protection equipment. The primary current I1and secondary current I2satisfy the inverse turns ratio relationship: I2I1=N1N2 During normal operation, the secondary side operates in a near-short-circuit state, with low magnetic flux density in the core. If the secondary side opens, it can generate extremely high induced voltage, endangering equipment and personnel safety.
II. Key Performance Parameters and AnalysisParameter Category | Core Metrics | Performance Analysis | Transformation Ratio Parameters | Rated Ratio Kn(e.g., 100/5, 500/1) | Transformation ratio error reflects the deviation between actual conversion and rated values. Metering CTs require ≤0.2%/0.5% error, while protection CTs allow ≤5%/10% error. | Ratio Error Δf |
| Accuracy Class | Metering Class: 0.2, 0.5, 0.2S, 0.5S | - Class 0.2S/0.5S are suitable for metering scenarios with significant load fluctuations, maintaining controlled errors even at low currents. | Protection Class: 5P, 10P | - Class 5P/10P withstand short-circuit current impacts, ensuring reliability of relay protection operations. | Rated Capacity | Rated Secondary Burden Sn(Unit: VA, e.g., 10VA, 20VA) | The secondary load (instruments + wiring) must be ≤ rated capacity; otherwise, errors increase. Overloading can cause core saturation and loss of linearity in the transformation ratio. | Insulation Performance | Rated Insulation Level (e.g., 10kV/35kV) | Withstands primary-side high voltage to prevent insulation breakdown and ensure safety of secondary-side low-voltage circuits. Must comply with standards such as GB/T 1208. | Power Frequency Withstand Voltage, Lightning Impulse Withstand Voltage |
| Dynamic and Thermal Stability | Dynamic Withstand Current Idyn | - Dynamic stability: withstands electromagnetic force impacts from short-circuit currents without mechanical deformation. | Thermal Withstand Current Ith(and duration) | - Thermal stability: withstands thermal effects of short-circuit currents without exceeding allowable winding temperature limits. | Saturation Characteristics | Saturation Voltage Us(Key metric for protection CTs) | Higher saturation voltage ensures CT is less prone to saturation during short circuits, enabling accurate transmission of fault currents and ensuring correct operation of protection devices. |
III. Core Performance Advantages
Electrical Isolation, Safety, and Reliability Complete isolation between the high-voltage primary circuit and the low-voltage secondary control circuit prevents high voltage from entering the secondary side, protecting instruments, relays, and operating personnel.
High-Accuracy Transformation for Precise Metering Metering-class CTs (e.g., Class 0.2/0.2S) exhibit minimal errors, accurately converting primary current to meet the precision requirements of energy metering.
Strong Short-Circuit Withstand Capability for Protection Protection-class CTs (e.g., Class 5P/10P) use core materials with high saturation flux density, reducing saturation during short circuits and accurately reflecting fault currents to trigger fast relay protection actions.
Compact Structure and Flexible Installation Available in various installation types such as bushing-type, support-type, and sleeve-type, CTs are adaptable to different scenarios like switchgear, distribution cabinets, and cable lines, occupying minimal space.
IV. Performance Limitations and Precautions
Secondary Side Must Never Be Open-Circuited During normal operation, the secondary side is short-circuited, maintaining low magnetic flux density. An open circuit can cause a sharp increase in core flux, inducing kilovolt-level voltages that may burn windings or endanger personnel.
High Demand for Load Matching The total secondary load (instrument resistance + wiring resistance) must be ≤ rated capacity; otherwise, errors exceed limits. For long-distance wiring, larger conductor cross-sections are required to reduce resistance.
Core Saturation Affects Accuracy When the primary current significantly exceeds the rated value, core saturation occurs, disrupting the transformation ratio and drastically increasing metering errors. Protection-class CTs mitigate this issue by optimizing core materials.
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 Characteristics1. 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 Characteristics1. 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 Characteristics1. 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 Characteristics1. 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.
Current Transformers: The "Sensory Nerves" of Power Systems Current Transformers (CTs) are ubiquitous in power systems. Their core value lies in providing safety, standardization, and electrical isolation. They convert high-voltage, high-current signals from the primary side into standardized, low-current signals that can be safely processed on the secondary side for measurement, protection, and control purposes. I. Core Application Areas1. Measurement and Metering (Metering CTs) This is the most fundamental and widespread application.
Current Measurement: Provide signals for analog or digital ammeters installed on control panels and instrument boards, enabling real-time display of operating currents in lines, transformers, generators, etc.
Energy Metering: Provide precise current input signals to electricity meters (kWh meters). Combined with voltage signals, they calculate active and reactive energy for billing and internal performance evaluation. This application demands the highest CT accuracy (commonly Class 0.2S, 0.5S).
Power and Power Factor Measurement: In conjunction with Potential Transformers (PTs), provide current signals to wattmeters and power factor meters.
2. Relay Protection (Protection CTs) This is the critical application ensuring the safe and stable operation of power systems. During system faults (short circuits, ground faults, etc.), CTs provide current signals reflecting the fault condition.
Line Protection: Overcurrent protection, instantaneous overcurrent protection, differential protection, distance protection.
Transformer Protection: Differential protection, overcurrent protection (as backup to Buchholz protection, etc.).
Generator and Motor Protection: Differential protection, negative-sequence current protection, ground fault protection.
Busbar Protection: Provide current from all incoming and outgoing feeders for busbar differential protection.
Characteristics: Protection CTs prioritize linearity and anti-saturation during faults over high accuracy at low currents. They must remain essentially linear and avoid saturation under fault currents several times or even tens of times the rated current to ensure reliable operation of protective relays (commonly Class 5P, 10P; TP class is used for EHV/UHV transient protection).
3. System Monitoring and Automation
Data Acquisition (SCADA): Provide real-time current data to grid dispatch centers and energy management systems for monitoring overall network status.
Fault Recording: Record fault current waveforms during disturbances for post-event analysis.
Synchronized Phasor Measurement (PMU): In smart grids, provide current phasor data with precise time stamps.
4. Electrical Safety and Control
Ground Fault / Earth Leakage Protection: Residual Current Transformers (RCTs) monitor the vector sum of three-phase currents (or the difference between line and neutral currents in single-phase systems). Any leakage causes an imbalance, triggering protection (e.g., in RCDs/GFCI breakers). This is vital for personnel and equipment safety.
Motor Control: Serve as current feedback signals for motor soft-starting, variable frequency speed control, and overload protection.
Load Control: Monitor circuit loads for automatic capacitor bank switching (reactive power compensation) or implementing load-shedding schemes.
II. Applications by Power System SegmentSystem Segment | Specific Application Scenario | Primary Role of CT | Power Plant | Generator terminals, auxiliary transformers, high-voltage auxiliary equipment | Metering, protection, monitoring | Substation | Incoming/outgoing lines at various voltage levels, busbars, transformer windings, capacitor/reactor banks | Core protection and measurement, serving as the station's "information source" | Transmission Lines | Line terminals, tee-off points | Line protection, metering | Distribution Network | Distribution transformers (pole-mounted, pad-mounted), switchgear, ring main units, feeder pillars | Measurement, protection (overcurrent, earth fault), customer metering | Consumer Side | Main incomer for factories/buildings/communities, critical branches, large equipment | Energy management, internal metering, protection |
III. Specialized Applications of Special CT Types
Split-Core / Clamp-On CTs
Application: System retrofits, temporary measurements, online monitoring, energy audits, adding metering to existing circuits. Their main advantage is installation without power interruption.
DC Current Transformers
Application: HVDC transmission systems, photovoltaic power stations, battery banks, electrolysis/electroplating industries, for measuring large DC currents.
Optical Current Transformers (OCTs)
Application: EHV/UHV substations, digital substations. Utilize the Faraday magneto-optic effect, offering strong immunity to EMI and simplified insulation. Represent a future development direction.
Rogowski Coils
Application: Measuring high-frequency currents, transient surge currents (e.g., lightning), harmonic analysis, power electronics testing. Offer no magnetic saturation and extremely high bandwidth.
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