Nguyên lý hoạt động của biến dòng

How-current-transformer-works

Current transformers are measuring devices that accurately represent high-level current in low voltage so that it can be measured easily and safely using a standard ammeter. Current transformers thus serve to isolate power measuring instruments from high voltage circuits and supply sufficient current to protective relays. Newtek Electricals is the first current transformer manufacturer to introduce current transformers with nylon casing in India. The flame retardant nylon 66 material makes the current transformers more resistant to impact from fall and offers better protection from dust and fire hazard.

The basics of current transformers

Current transformer has two sets of windings. It receives the high-voltage direct current from utility line in its primary winding and generates alternate current in its secondary winding. The primary and secondary windings are wound upon an iron core. While the primary winding is connected in series with the load, the secondary winding is connected to measuring devices. The amount of current flowing through the high-powered lines into primary winding determines the number of turns the secondary winding will have. the more the current flow, the more the number of turns.

How current transformer works

Typically, current transformers work to reduce the high voltage current to a ratio of 5 Amp or 1 Amp under normal operational circumstances. This is known as the current transformation ratio of the CT. Most CTs have a high current transformation ratio. The primary current that CTs can step down ranges from 10 Amp to 3000 Amp, and suitable devices are chosen according to the needs of various applications. Since the load impedance within the CT is very small, it works under short circuit conditions. Secondly, the current flow in CTs is not affected by the load impedance but instead depends on the current flowing in the primary winding.

Classifications and types of CTs

Current transformers can be classified according to the use cases such as indoor or outdoor, and according to the purpose such as metering and protection type CTs. Metering and protection type CTs are designed specifically for their stated purpose and should not be used interchangeably.

Protection type CTs

Protection type CT are designed to monitor current during a fault in the electrical system. They will then give the input to the relay which will trip the circuit breaker to keep vital loads isolated. These CTs can function when the current flow is 10-20 times the rated current, and are not saturated during electrical faults. As the amount of current flowing through the protection type transformers is high, connecting it to the meter which is not designed for such high levels of current will damage the meter.

Metering type CTs

Accuracy and safety are the two key features of a metering CT. Metering CTs are connected to the meters that measure power consumption. Because of this, they come with a designated Instrument Safety Factor (ISF). The core of the CT saturates if the current is higher than rated current, and this protects the meters from being damaged. The same will happen if a metering CT is deployed for protection. As the core of the CT saturates, the secondary current will be distorted and might prevent the relay from operating.

CT types according to construction

Based on their construction, current transformers can be classified into the following types

Wound type current transformers

Wound type transformer has the basic make of primary and secondary windings. These windings cannot be disconnected This current transformer is noted for its high accuracy.

Bar type current transformers

In bar type current transformers, a bus bar of the main circuit is used as a primary winding. They are usually bolted to the device that carries the current and are isolated from high operating voltage in the system. Magnetization of the core can reduce their accuracy.

Window type transformer

The window type current transformers lack any kind of primary winding, and their secondary winding is situated around the conductor through which the current is flowing. Current is induced in the secondary winding by the magnetic electric field which is created by the current flowing through the conductor.

Bushing current transformer

The bushing current transformer is very similar to the bar type transformer. In this CT, the core and secondary are located in the place of the primary conductor. The secondary winding can be twisted into a circular or annular shaped core. This in turn connects to high voltage bushing inside the circuit breakers. The bushing works as primary winding when the conductor flows through it. An insulating bush can be arranged to work as a core. These types of current transformers tend to be cheap and are reused for relaying purposes in high-voltage circuits.

CT manufacturer and supplier

When current transformers are damaged, their replacements takes up considerable time. In that light, providing more durable, and high-performing CTs is a key concern which Newtek Electricals addresses efficiently. We have introduced current transformers with nylon casing which can deliver reliable performance in harsh conditions. Newtek has proven experience and expertise in providing quality CTs at large scale in sectors such as steel, cement, infrastructure, petroleum and power-in short lead times.

To avail of durable, reliable and high-performing current transformers, reach out to us at info@mtecviet.com or call on +84 – 908 188 361, 888 202 248