In the modern power system network, every stage-from generation to transmission to consumption-requires precise and safe measurement of current. This ensures stable, efficient, and secure system operation. Currents in high-voltage systems often reach several thousand amperes, making direct measurement with instruments impossible. Therefore, current transformers are used to safely and accurately convert these currents. Current transformers act as the “sensors” and “guardians” of the power system. They convert high currents into standard, safe low currents, providing accurate data for metering, monitoring, and protection devices. At the same time, they isolate high voltage, protecting both personnel and equipment. Their performance and proper use have a direct impact on the reliability and economic efficiency of the power system. This article will introduce the structure, principle, function, application, selection, use and maintenance methods of current transformers.
Current Transformer Structure and Operating Principle
Structure： A current transformer primarily consists of an iron core, a primary winding, a secondary winding, insulating supports, terminals, and (optional) auxiliary windings.
Operating Principle： A current transformer works on electromagnetic induction. It has a closed iron core and windings. The primary winding has few turns. Some current transformers do not have a separate primary winding. The primary circuit conductor, like a busbar, can pass through the iron core and act as the primary winding (one turn). The primary conductor has a large cross-section and connects in series with the current to be measured. It carries the full current of the circuit. The secondary winding has many turns and a thinner conductor. It connects to a measuring instrument or protection circuit. The secondary circuit stays closed during operation. The coils connected to the measuring instrument and protection circuit have very low impedance. The current transformer works almost like a short circuit. It converts the high primary current into a proportionally lower secondary current.
Operating Principle： A current transformer works on electromagnetic induction. It has a closed iron core and windings. The primary winding has few turns. Some current transformers do not have a separate primary winding. The primary circuit conductor, like a busbar, can pass through the iron core and act as the primary winding (one turn). The primary conductor has a large cross-section and connects in series with the current to be measured. It carries the full current of the circuit. The secondary winding has many turns and a thinner conductor. It connects to a measuring instrument or protection circuit. The secondary circuit stays closed during operation. The coils connected to the measuring instrument and protection circuit have very low impedance. The current transformer works almost like a short circuit. It converts the high primary current into a proportionally lower secondary current.
Functions and Applications of Current Transformers (CTs)
Main Functions
1. Measurement: Work with measuring instruments to measure line current and other parameters. Provide accurate data for electricity metering and system monitoring.
2. Protection: Work with relay protection devices to protect power systems and equipment from overload and overcurrent. Keep the system safe and stable.
3. Isolation: Keep measuring instruments and protection devices separate from high-voltage power. Protect people and equipment.
Industry Applications
Current transformers are used in power systems, industry, new energy systems, transportation, commercial and public facilities, and automation. They measure current, monitor equipment, manage load, and provide protection.
Current transformers in transformers and systems are used for:
1. Main Transformer Winding Monitoring: Measure high- and low-voltage winding currents. Provide real-time load information and prevent overcurrent damage.
2. Relay Protection: Provide current signals to protective devices. Devices act quickly when faults happen. Faulty lines are disconnected to keep the system safe.
3. Metering and Billing: Convert high currents to standard currents for meters. Ensure accurate measurement and fair billing.
4. Overload and Fault Monitoring: Detect abnormal currents. Find overloads, short circuits, and other problems quickly. Prevent damage to transformers and the system.
5. Differential Protection: Detect internal faults. Compare currents on high- and low-voltage sides. Isolate faults fast.
6. Ground Fault Monitoring: Measure zero-sequence or residual current. Detect insulation damage or grounding problems. Protect people and equipment.
7. Busbar/Bushing CT (Internal): Install on transformer bushings or busbars. Save space and provide precise measurements. Make system monitoring easier.
8. Transformer Testing and Diagnostics: Provide accurate current data for testing, maintenance, and fault analysis. Help evaluate performance and do preventive maintenance.
Classification of current transformers
Current transformers (CTs) used in power transformers are auxiliary devices designed to convert high primary currents into proportional low secondary currents for measurement, protection, and control. They ensure safe monitoring of transformer operation, accurate metering, and reliable relay protection. Depending on their purpose, structure, and insulation method, these CTs can be classified as follows:
| Classification Basis | Type | Description |
| By Function | Measuring CT | Provides accurate current signals for meters and instruments; high accuracy class (e.g., 0.2, 0.5). |
| Protection CT | Designed to operate correctly during fault conditions without magnetic saturation; used for relay protection (e.g., 5P, 10P). | |
| Combined CT | Combines measuring and protection functions within one unit. | |
| By Structure | Wound Type | Primary winding consists of several turns wound on the core; it is suitable for small current applications. |
| Bar Type (Bushing Type) | The transformer’s bushing conductor serves as the primary winding, widely used in power transformers. | |
| Ring Type | Fully enclosed core; secondary winding surrounds the primary conductor, providing good accuracy and insulation. | |
| By Insulation Medium | Dry Type | Uses air or epoxy resin insulation; mainly for indoor transformer applications. |
| Oil-Immersed Type | Immersed in transformer oil for cooling and insulation, common in oil-filled power transformers. | |
| SF₆ Gas-Insulated Type | Used in GIS or sealed transformer systems with high voltage levels. | |
| By Installation Location | Built-in CT | Mounted inside the transformer tank and immersed in oil; typically installed on bushing leads. |
| External CT | Installed outside the transformer tank on the bushing or cable connection; convenient for maintenance. |
How to Choose a Current Transformer
1. Voltage and Current Selection
– The rated voltage of a current transformer should not be lower than the rated voltage of the circuit at the installation point.
– The rated current of the primary winding varies depending on the voltage level (e.g., 5A, 10A, 15A, 20A, … 1500A). When selecting a current transformer, the primary rated current should not be less than the calculated current of the circuit (a certain margin is recommended). – The rated current of the secondary winding is typically 5A, with a few being 1A, depending on the current load of the secondary equipment.
2. Selecting the Accuracy Grade
– A current transformer must meet the accuracy grade requirements. That is, its secondary load S₂ must not exceed the rated secondary load S₂ₙ corresponding to that accuracy grade:
S₂ₙ > S₂
How to Use a Current Transformer Correctly
1. The Secondary Side Must Not Be Open-Circuited
If the secondary circuit is disconnected during operation, the induced voltage on the secondary side will rise sharply, causing a rapid increase in magnetic flux. This can saturate the core and generate high-voltage spikes, endangering the measuring instrument and personnel. If a secondary side open circuit is detected, the circuit must be closed immediately and the fault corrected.
2. Load Matching and Protection
When selecting a current transformer, consider the instrument input impedance, the protection relay interface, and other factors to ensure that the load impedance Z_b is within the rated range. For protection systems requiring high speed and reliability, a CT with a low error grade and good open-circuit protection characteristics is recommended.
3. Installation Position and Orientation
The orientation of the CT should align with the direction of the primary conductor to avoid phase reversal and signal errors. When installing ring or window-type CTs, ensure that the conductors pass through the core the correct number of times and in the correct direction, as specified in the design.
4. Temperature and Ventilation
High loads or high ambient temperatures can increase CT temperature rise, affecting copper and iron losses. Ensure good ventilation in the equipment room, proper wiring, and avoid installing the CT near strong heat sources to ensure long-term stable operation.
Current Transformer Maintenance
During operation, on-duty personnel should regularly inspect the current transformer to ensure safe operation. During inspection, pay attention to the following points:
1. The current transformer should have no abnormal noise or burning odor.
2. Check the current transformer connectors for overheating.
3. The porcelain insulation of the current transformer should be clean and intact, free of cracks and discharges.
4. Check the current transformer oil level for normal operation and no oil leakage.
5. Regularly test the insulation properties of the current transformer oil. For oil-filled current transformers, oil quality should be sampled regularly to test to prevent the degradation of insulation capacity, which may lead to heat expansion and cause an explosion or fire.
