What is a medium voltage current transformer?
Medium-voltage current transformers are one of the essential components used in electrical distribution systems to safely and accurately measure current. Since directly measuring high currents is both risky and costly, these currents must be converted to lower values before measurement. This is where medium-voltage current transformers come into play.
A current transformer reduces the high current in the primary circuit to levels that measuring instruments can operate at – typically 1 A or 5 A – and transmits it to the secondary circuit. It also isolates the measuring instruments from the high current in the primary. These transformers are connected in series with the circuit, and under normal conditions, there is no significant phase difference between the primary and secondary currents.
Working Principle of Current Transformers
Current transformers operate on the principle of a classic transformer. As alternating current passes through the primary winding, it creates a magnetic field. This magnetic flux is transmitted through the core to the secondary winding, induced by a voltage. This induced voltage creates a current in the secondary circuit, transmitting data to measuring devices.
Structurally, primary windings have fewer turns and thicker conductors because they carry high currents. Secondary windings, on the other hand, are made of thinner conductors with many turns. This structure allows for the correct conversion of current and ensures insulation.
Current Transformer Classes and Error Coefficients
Depending on their application, current transformers are classified as measurement and protection types. Measurement transformers maintain their accuracy only up to a certain current limit, while protection transformers are manufactured to transmit accurate data even at high currents that may occur in the system. This difference is determined by the saturation characteristic of the transformer’s magnetic core.
Current transformer class values determine the measurement accuracy. For measurement transformers, classes are generally 0.1, 0.2, 0.5, 1, 3, and 5. Protection current transformers are defined by classes such as 5P or 10P. The “P” (Protection) on the label indicates the protection class, and “Fs” (Security Factor) indicates the measurement class. Transformers in the measurement class have values such as Fs5 or Fs10.
The saturation factor (n) is the ratio of the primary current to the rated current. Measurement transformers generally operate with n<5 or n<10; Magnetic saturation starts late in protective transformers. This is critical for the correct operation of protective relays, especially in situations such as short circuits.
Considerations When Selecting a Medium Voltage Current Transformer
When selecting a medium voltage transformer, class information alone is insufficient. Thermal and dynamic withstand values must also be considered.
Ith (thermal withstand current) indicates the transformer’s capacity to withstand short-term high currents and should generally be at least 100 times the nominal current.
Idyn (dynamic withstand current) is typically 2.5 times the thermal withstand current.
Furthermore, allowing current to flow through the primary terminal while the secondary terminals are open can cause a high voltage to build up in the secondary. Therefore, the secondary terminals must be short-circuited before any work is performed on the transformer.
Types of Medium Voltage Current Transformers
Three types of current transformers are commonly used in medium-voltage systems:
Support Type Current Transformer
These transformers, insulated with epoxy resin, are mounted inside medium-voltage switchgear. They are common at medium-voltage levels such as 24 kV and 36 kV.
Toroidal Current Transformer
These ring-shaped transformers are generally used at low levels such as 0.72 kV. They provide an output current of 1 A or 5 A. Their compact structure provides a space advantage in medium-voltage switchgear.
LPCT (Low Power Current Transformer)
Similar to toroidal transformers, these produce voltage at the mV level and are directly connected to protection relays via RJ45 cable. They are generally preferred in switchgear with digital relays. They have an adjustable current range between 0–1250 A. The secondary output is voltage, not current, as in classic transformers.
Application Areas and Advantages of MV Current Transformers
Medium voltage current transformers (MV current transformers) are generally used in substations, power distribution panels, MV switchgear, and power generation facilities. They are integrated into the system for both measurement and protection purposes. In MV systems, toroidal transformers are preferred due to their space-saving properties; they are ideal, especially in scenarios where short-circuit protection is not critical.
Today, both domestic manufacturers and global companies offer a wide range of MV current transformers and MV voltage transformers. Selecting the correct transformer type and class according to the application directly affects the healthy and long-lasting operation of the system.