Ang duha ka pangunahon nga klase sa instrument transformers sa usa ka power system.

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I. Voltage Transformer (VT)

Ang Voltage Transformer (Potential Transformer, abbreviated as PT; Voltage Transformer, abbreviated as VT) usa ka electrical device nga gamiton para mubag-o ang voltage levels sa power circuits.

1. Operating Principle

Ang voltage transformer nagoperasyon batas sa electromagnetic induction ug adunay structure sama sa conventional transformer, consist primarily of a primary winding, a secondary winding, ug a core. Ang primary winding giconnect in parallel sa high-voltage circuit nga gigamit ug adunay dako nga bilang sa turns.

Ang secondary winding, nga adunay mas gamay nga bilang sa turns, giconnect sa measuring instruments, protective relays, ug uban pang loads. Sa normal nga pag-operasyon, ang secondary side kasagaran naa sa open-circuit state. Sumala sa electromagnetic induction, ang ratio sa primary ug secondary voltage sama sa turns ratio (U₁/U₂ = N₁/N₂). Kini naghatag sa pag-step down sa high voltage proporsyonal sa standardized low voltage (typically 100V or 100/√3 V), nagpadali ug safe kini para sa measurement ug protection devices.

Ang electrical symbol niini mao kini:

2. Functions

Voltage Measurement: Steps down high system voltages to standardized low voltages (e.g., 100V or 100/√3 V) for use by voltmeters, energy meters, and other measuring instruments, enabling real-time monitoring of power system voltage.
Relay Protection: Provides reliable voltage signals to protective relays for overvoltage, undervoltage, and other protection functions. When abnormal voltage conditions occur, the protection system responds quickly, triggering a trip command to isolate the faulty circuit and ensure system and equipment safety.
Energy Metering and Billing: Works in conjunction with energy meters to accurately measure power consumption in high-voltage circuits. It serves as a critical basis for utility billing and energy settlement.

3. Characteristics

High Accuracy: Measurement-grade voltage transformers have high accuracy classes (e.g., 0.2, 0.5) to ensure precise voltage measurement and energy metering. Protection-grade VTs prioritize fast response and have relatively lower accuracy classes (e.g., 3P, 6P).
High Insulation Requirements: High-voltage VTs must withstand high operating voltages and typically use oil-immersed, SF₆ gas, or solid resin insulation for stable and reliable performance. Low-voltage VTs are mostly dry-type, with simple structure and easy maintenance.
Secondary Side Must Not Be Short-Circuited: A short circuit on the secondary side can generate extremely high currents, potentially overheating and destroying the windings. Therefore, the secondary circuit must be protected by fuses or miniature circuit breakers.

4. Application Scenarios

High-Voltage Applications: Suitable for transmission lines and substations with voltages of 1 kV and above (e.g., 10 kV, 35 kV, 110 kV systems). Used to monitor busbar or line voltages and provide input to protection systems, ensuring safe and stable grid operation.
Low-Voltage Applications: Applicable to distribution systems below 1 kV (e.g., 220V residential circuits, 380V industrial systems). Commonly installed in low-voltage switchgear for monitoring consumer-side voltage or interfacing with energy meters for power measurement.

II. Current Transformer (CT)

Ang current transformer (CT), usab gitawag og current transducer, usa ka instrument transformer nga, sa normal nga pag-operasyon, nag-produce og secondary current nga substantially proportional sa primary current, ngadto sa phase difference approaching zero when correctly connected.

1. Operating Principle

Ang current transformer nagoperasyon batas sa electromagnetic induction ug adunay structure sama sa conventional transformer, consist of a primary winding, a secondary winding, ug a magnetic core. Ang primary winding giconnect in series sa circuit nga gigamit ug adunay gamay nga bilang sa turns (sometimes only one turn), carrying the high primary current.

Ang secondary winding, nga adunay daghan pa kaayo nga turns, giconnect in series sa measuring instruments, protective relays, ug uban pang loads, forming a closed loop. Sa normal nga pag-operasyon, ang secondary side kasagaran naa sa short-circuit condition. Sumala sa electromagnetic induction, ang ratio sa primary ug secondary current inversely proportional sa turns ratio (I₁/I₂ = N₂/N₁). Kini naghatag sa pag-step down sa large currents proporsyonal sa standardized low-level currents (typically 5A or 1A), facilitating measurement, monitoring, ug protection.

Ang electrical symbol niini mao kini:

Ang ratio sa primary ug secondary rated current sa current transformer gitawag og current transformation ratio (Ke). Ang expression sa current transformation ratio mao kini:

Note:

W₁, W₂ ang number of turns sa primary ug secondary windings sa transformer, respectively;
I₁ₑ, I₂ₑ ang rated currents sa primary ug secondary windings, respectively;
I₁, I₂ ang actual currents sa primary ug secondary windings, respectively.

2. Functions

Current Measurement: Steps down high primary currents to standardized low secondary currents (e.g., 5A or 1A), enabling ammeters, energy meters, and other instruments to monitor load current in real time.
Relay Protection: Supplies current signals to protective relays for overcurrent, differential, and distance protection. When faults such as short circuits or overloads occur, the protection system triggers a trip signal to disconnect the power supply, preventing equipment damage and system instability.
Electrical Isolation: Provides galvanic isolation between the high-voltage/high-current primary circuit and the low-voltage secondary circuits used for measurement, control, and protection. This ensures the safety of personnel and secondary equipment.

3. Characteristics

High Reliability: Must withstand high mechanical and thermal stresses during short-circuit events. CTs are designed with excellent dynamic and thermal stability to remain intact under extreme fault conditions.
Multiple Winding Design: High-voltage CTs often have multiple secondary windings—one for metering (high accuracy, e.g., class 0.5) and another for protection (wide range and fast response, e.g., class 5P or 10P). Low-voltage CTs typically have single or dual windings to meet basic application needs.
Secondary Side Must Not Be Open-Circuited: An open circuit on the secondary side can induce extremely high voltages (up to several kV) across the winding, posing serious risks of insulation breakdown, equipment damage, and electric shock. Therefore, the secondary circuit must remain closed during operation—opening it is strictly prohibited.

4. Application Scenarios

High-Voltage Applications: Used in transmission lines and substations with voltages of 1 kV and above (e.g., 10 kV, 35 kV, 110 kV systems). Widely applied in current monitoring and protection of critical equipment such as transformers, circuit breakers, and busbars, playing a vital role in ensuring grid reliability and safety.
Low-Voltage Applications: Applied in distribution systems below 1 kV (e.g., industrial workshops, commercial buildings, residential complexes). Typically installed in low-voltage switchboards or distribution panels for branch circuit monitoring, energy metering, or integration with residual current devices (RCDs) and smart meters to enable safe and efficient power usage management.