Current Transformer

Current Transformer
Current Transformer

The current generated by the current transformers powers the vast majority of alternating current protective relays used in various protection systems. It is not easy to measure alternating currents of huge magnitudes with low range ammeters. Furthermore, high-current-rated relays are required for operation with these high alternating currents. This means that high currents can be reduced to a more manageable range by use of the current transformer. Many things must be considered for current transformers to be installed correctly. These include the kind of mechanical construction, the ratio of main to secondary currents, the insulating material (dry or oil), temperature conditions, precision, and service type.

CTs, or current transformers, are tools for directing current flow.
The primary current is directly proportional to the secondary current in this type of current transducer. These are used to convert the high currents produced by the power circuit into a range of currents that might be measured by control devices and sensors. In addition, they make sure that control devices, like ammeters, are separated from circuits that handle high voltage power. When other methods of measuring current, such as digital meters or moving coil vane meters, fail, this simple and inexpensive method remains the gold standard.

Current Transformers (CTs)

The primary winding of the current transformer, which has a large number of turns and a large cross-sectional area, is connected in series with the circuit that will measure the current flow. The conductor serves as the main winding in bar type CTs, which have a primary winding consisting of a single turn. A secondary winding made of fine wire with a small cross-sectional area and many turns is what you’d expect. This winding is connected to the instrument’s current coil or the operating coil of the relay, as shown in the picture. There is a common practice in CT design for the secondary terminals to provide 5A or 1A current at the rated or full primary current.

Current Transformers: How They Work

How a current transformer works is quite similar to how a standard power transformer operates. Central to CTs is the fact that they are step-up transformers for voltage and step-down transformers for current. This occurs because current is negatively correlated with voltage and positively correlated with low voltage. When the CT’s primary is turned on, it causes the primary side ampere to turn, creating a magnetic field inside the core. This magnetic flux’s connection to the secondary generates an electromagnetic field (EMF), which in turn drives the current in the CT secondary. The main ampere cycles are tried to be balanced by the secondary current. Therefore, the primary-secondary relationship is described as

  • It is true that I1N1 equals I2N2.
  • N2 divided by N1 is equal to I1 divided by I2.
  • (I divided by II) equals n
  • The current transformer’s transmutation ratio is the name for this.
  • The principal current is represented by I1 and the secondary current by I2.
  • The main and secondary turns are denoted by N1 and N2, respectively.
  • “n” stands for the secondary winding rotational ratio to that of the primary winding.

Current transformer with a switch

The nominal ratio of a 100-to-5A current transformer is 1:20, which means that there is one primary turn for every twenty secondary turns. This equation makes it easy to determine the current flowing through the main line-connected primary by using the current ratios and secondary ammeter current. A power transformer’s primary current can’t flow without the secondary current. The CT’s main winding, on the other hand, experiences almost no voltage loss due to its direct series connection with the power circuit. Because of this, the secondary current has no effect on the main current.

Importantly, while the primary is being energized, the secondary of the CT should not be left open. When the secondary is open, the current through it is zero, but in reality, the movement of the secondary ampere is counter to that of the primary ampere. Therefore, a large magnetic flux is generated inside the nucleus of an unopposed primary mmf when a countersecondary mmf is not present. The core temperature rises as a result of the increased core losses caused by this. Furthermore, this causes the primary and secondary sides to generate higher EMFs, which harms the insulation. For this reason, you must either short the secondary or connect it in series with the low resistance current coils on the instruments. Furthermore, the secondary side needs to be grounded to avoid impact dangers. In a practical sense, CTs’ secondary terminals are where short circuit switches are typically put.

The Evolution of Current Transformer Technology

Both wound and bar configurations are possible for the current transformer. A wound type CT is quite similar to a two-winding conventional transformer. At its heart, the principal winding is wound because it contains numerous turns, or more than one full turn. The construction of a low voltage wound type CT involves winding the secondary turns on a Bakelite former and the primary turns directly on top of the secondary winding, with the necessary insulation in between. These CTs can be round, rectangular, or window-shaped, depending on the core’s structure. When a single bar passes through the core’s center as the main winding, it forms a primary winding in bar type CT that has one turn.

CTs use flux densities that are far lower than those of power transformers. This means that core materials should be selected in a way that minimizes core loss, reluctance, and compatibility with low flux densities. The ring cores’ strength and the lack of joints give them minimal reluctance. The most common materials used to make cores are silicon steel, cold rolled grain oriented silicon steel, or nickel iron alloys. The central component of the CT is made of Mu meal, an extremely high-grade alloy steel, which guarantees accurate dosing. Tiny line voltages are insulated using varnish and tape. High line voltages, however, call either compound-filled or oil-filled CTs. Insulating the secondary windings and HV conductors with oil-impregnated paper is necessary for the use of CTs at higher transmission voltages. Another option for the construction of such CTs is the use of either living or dead tank shapes.

Different Types of Modern Transformers

Factors such as mounting method, circuit voltage, and intended application classify current transformers. Some instances of this type include

Indoor current transformers

Low voltage circuits frequently make use of these transformers, which can also be classified as winding, bar, or window varieties. Just like any other transformer, a wound type transformer has two windings: the primary and the secondary. In aggregating applications, for example, these are used for very low current ratios. These CTs are able to achieve an impressive level of accuracy since their principal ampere turns have high values. Primary bars that are essential to secondary cores make up the bar type CT.

The accuracy of bar-type CT is diminished since the core’s magnetization requires a substantial amount of total ampere cycles at low current ratings. Installing window type CTs—which do not have a primary—around the primary (or line) conductor is a common practice. These CTs, whether with a solid core or a divided core, are the most common. The main conductor must be unplugged before the solid window CT can be installed. Nevertheless, the main conductor need not be severed during the installation of split core CT.

Extant outdoor transformers

Due to their greater voltages, substations and switch yards use these. These CTs are insulated with SF6 gas or oil. SF6-insulated CTs are lighter than oil-filled ones. CTs connect the higher tank to the main conductor, making them live tank construction. Small bushings are utilized since the primary conductor and tank have the same potential. See how this container is attached to the insulator framework. The secondary terminals are in a terminal box at the bottom. An earthing connector is at the bottom.

Energy Converter for the Great Outdoors

Transformers with several ratios of current use a split-type principle winding. The main winding can be accommodated by the tank’s taps. These transformers allow for varying current ratios to be accomplished by valves on either the primary or secondary. When applied to the secondary, operating amp-turns change; when applied to the primary, most of the copper area is wasted save for the lowest range.

Transformers for Current Bushings

Core and secondary conductors encircle the primary conductor in a bushing type CT, same as in a bar type CT. High voltage bushings of switchgears, circuit breakers, power transformers, and generators house secondary windings that have a circular or annular core. The conductor passes through the bushing as the main winding, and the core is arranged to surround the insulating bush. The majority of high voltage circuit relays use bushing CTs because of how inexpensive they are.

Portable convex transformers

Power analyzers and ammeters that demand pinpoint accuracy often employ these high precession CTs. These portable CT scans are clamp-on models with flexible, split-core imaging. A common portable CT has a current measurement range of 1000 to 1500 A. Furthermore, these CTs prevent high voltage circuits from damaging the measuring devices.

Real-Time Transformer Problems

A perfect current transformer matches primary to secondary current and secondary to primary turn ratio. Anti-phase mmfs from the two windings’ currents match perfectly. Actually, the turning ratio and current ratio are distinct and at an angle to the opponent. We discuss phase angle and ratio faults when we hear these. When using CTs for accurate metering and measurement, minimizing errors is crucial.

Real-Time Transformer Problems
Real-Time Transformer Problems
CT2 was

Using a primary current, CT keeps the iron core excited. There are two parts to this current excitation current, as shown in the figure: one that magnetizes and another that is wattful. The secondary winding’s intrinsic reactance and resistance, along with the secondary current passing via the EMF-induced load, causes the secondary voltage to dropThe dotted line in the phasor demonstrates that I2 represents the principal current, indicating an angle β between the primary and secondary currents.

Error in Ratio

The component of the phasor that is responsible for excitation current is I1, also known as the main current. So, by applying the triangle OBC, we can determine the actual ratio error with the vector components of I2, Io (which depends on the magnetizing and wattful components), and I1. The reactance, resistance, and power factor of the windings all have an impact on secondary current. Having said that, the secondary-to-primary turn ratio and the nominal or rated current ratio are identical. Therefore, the difference between the transformed real ratio and the nominal ratio is the CT ratio error.

Ratio or Current Error = (real ratio – nominal ratio) / actual ratio

is equal to (Kn – R) divided by R multiplied by 100%.”

The Phase Error Angle

In an ideal current transformer, the secondary current would be at a precise 180-degree angle to the primary current. Or, put another way, there should be no phase angle at all between the main current and the reversed secondary current. The phasor diagram above has an inaccuracy in phase angle because the primary current is led by a specified angle by the reversed secondary current. The phase angle is positive when the reversed secondary current comes before the primary current, and negative when it follows.

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