Voltage and current transformers – what are they and what are they used for?
Voltage and current transformers
Instrument transformers are essential components in power engineering, enabling accurate and safe electrical measurements. They are widely used for monitoring and maintaining power quality across the grid. One of their key functions is integration with protective automation systems that safeguard electrical equipment from adverse phenomena such as short circuits, overloads, and other network disturbances. What types of instrument transformers exist, and what are their applications?
The two fundamental types of instrument transformers are current transformers (CTs) and voltage transformers (VTs), also known in technical literature as measurement transformers. With the continuous development of the power sector, the importance of these devices has grown, as they play a critical role in the operation of numerous metering and protection systems.
How do current and voltage transformers work?
Although current and voltage transformers are often grouped together within measurement systems, they are distinct devices with different functions and operating principles. Consequently, their applications also differ based on type.
Current transformers (CTs)
A current transformer is a single-phase transformer used to measure electric current in circuits where the primary current is too high to be measured directly due to safety concerns. It consists of a primary and a secondary winding and provides galvanic isolation between the high-voltage circuit and the measurement equipment to ensure operational safety.
The current flowing through the primary winding generates a magnetic field, which induces a proportional current in the secondary winding. Using the principle that the secondary current is proportional to the primary, you can connect a measuring device to the secondary winding to measure electric current. The ratio of the currents in the primary and secondary windings is equal to the ratio of the number of turns of these windings, which allows for precise current measurement. After making calculations, in an indirect way, you can find out the values for the current flowing through the primary winding. Current transformers operate under conditions close to a short circuit state.
Voltage transformers (VTs)
A voltage transformer is a single-phase or three-phase transformer with low rated power and low short-circuit voltage. It is designed to provide information on voltage levels in a monitored electrical system. It converts high primary voltage into proportionally lower secondary voltage suitable for powering measurement and protection devices.
Key characteristics of a VT include low leakage reactance and minimal no-load current. Voltage transformers are used in medium-, high-, and extra-high-voltage networks and operate under conditions close to no-load state.
Differences between current and voltage transformers
The main differences between CTs and VTs lie in the type of electrical quantity they measure and their method of operation. A current transformer provides information about current in the primary circuit, while a voltage transformer supplies data on system voltage. CTs are connected in series with the circuit, whereas VTs are connected in parallel.
Additionally, the secondary winding of a VT must never be short-circuited, and the secondary winding of a CT must never be left open. The grounding terminal of the primary winding must be securely earthed in the terminal box and must not be disconnected during operation.
The operation of substations and power lines would be impossible without instrument transformers. They enable the acquisition of precise data on electrical parameters such as current, voltage, power, and frequency, all of which are essential for system control and regulation. These transformers play a vital role in ensuring reliable and safe grid operation by protecting power equipment.
Current and voltage transformers are deployed in substations, switchgear, and distribution stations for various purposes, including:
- measurements with low-range instruments
- remote measurements from equipment located far from the monitored circuit
- galvanic isolation between instruments and high-voltage systems
The primary purpose of using instrument transformers is to provide engineers and technicians with real-time insights into the grid’s operating conditions. This enhances power quality control and enables early detection of potential anomalies or faults. In turn, this shortens outage response times and may even prevent failures through timely mitigation of threats to grid stability.Thus, instrument transformers are crucial for maintaining the reliability and performance of modern power systems.