In the electrical field, CT and PT stand for Current Transformer (CT) and Potential Transformer (PT) respectively. Capacitor Voltage Transformer (CVT) is abbreviated as CVT.

Current Transformer (CT)

CT is a device used to measure large currents. It is able to convert high current values into lower standard current values (usually 5 amps or 1 amp) for easy measurement and protection of circuits. CT is widely used in power systems for functions such as current measurement, overload protection, and ground fault detection.

Potential Transformer (PT)

PT is used to measure high voltage and convert it into lower standard voltage values (usually 100 volts or 110 volts). It is mainly used in voltage measurement, energy metering, and protection relays. PT ensures safe and accurate monitoring of the status of high voltage circuits in low voltage environments.

Capacitor voltage transformer (CVT)

Capacitor voltage transformer (CVT) is a voltage transformer that is divided by series capacitors and then reduced and isolated by electromagnetic transformers. It is used as a meter, relay protection, etc. Capacitor voltage transformer can also couple the carrier frequency to the transmission line for long-distance communication, remote measurement, selective line high-frequency protection, remote control, teletypewriting, etc. Therefore, compared with conventional electromagnetic voltage transformers, capacitor voltage transformers can prevent ferromagnetic resonance caused by saturation of the voltage transformer core, and have many advantages in economy and safety.

Capacitor voltage transformers are mainly composed of capacitor voltage dividers and medium-voltage transformers. The capacitor voltage divider consists of a porcelain sleeve and several series capacitors installed in it. The porcelain sleeve is filled with insulating oil that maintains a positive pressure of 0.1MPa, and a steel bellows is used to balance different environments to maintain oil pressure. The capacitor voltage divider can be used as a coupling capacitor to connect the carrier device. The medium voltage transformer consists of a transformer, a compensating reactor, a lightning arrester and a damping device installed in a sealed oil tank. The space on the top of the oil tank is filled with nitrogen. The primary winding is divided into a main winding and a fine-tuning winding. A low-loss reactor is connected in series between the primary side and the primary winding. Since the nonlinear impedance and inherent capacitance of the capacitive voltage transformer sometimes cause ferromagnetic resonance in the capacitive voltage transformer, a damping device is used to suppress the resonance. The damping device consists of a resistor and a reactor, which is connected across the secondary winding. Under normal circumstances, the damping device has a high impedance. When the ferromagnetic resonance causes overvoltage, before the medium voltage transformer is affected, the reactor has been saturated and only the resistance load is left, so that the oscillation energy is quickly reduced.

Transformation ratio analysis

PT and CVT are voltage transformers, which are used to convert high voltage into low voltage. The secondary side is generally 100V, such as 10KVPT is 10KV/100V. The secondary side of PT cannot be short-circuited; CT is a current transformer, which converts large current into small current, generally 5A or 1A. The secondary side of CT cannot be open. Whether it is an electromagnetic or capacitive voltage transformer, it converts high voltage into low voltage for measurement, control, and protection systems. From the working principle, it is a small (small capacity) transformer, or it is better to call the equipment according to the standard name. Especially in the power operation department.

Working principle and composition details

The voltage transformer is actually a transformer with an iron core. It is mainly composed of primary and secondary coils, iron core and insulation. When a voltage U1 is applied to the primary winding, a magnetic flux φ is generated in the iron core. According to the law of electromagnetic induction, a secondary voltage U2 is generated in the secondary winding. Changing the number of turns of the primary or secondary winding can produce different ratios of primary voltage to secondary voltage, which can form voltage transformers with different ratios. The voltage transformer converts high voltage into low voltage in proportion, that is, 100V. The primary side of the voltage transformer is connected to the primary system, and the secondary side is connected to measuring instruments, relay protection, etc.; it is mainly electromagnetic (capacitive voltage transformer is widely used), and there are non-electromagnetic types, such as electronic and photoelectric types. CT-current transformer, spelled Current Transformer in English, is a conversion device that converts the large current on the primary side into a rated current of 5A or 1A suitable for use by instruments or relays. The current transformer consists of a closed iron core and a winding. Its function is to convert a large primary current into a smaller secondary current through a certain transformation ratio for protection, measurement, etc. For example, a current transformer with a transformation ratio of 400/5 can convert an actual current of 400A into a current of 5A.