Current transformer

A transducer is a transducer for potential-free measuring large AC currents. There are various embodiments of a conventional design is preferably implemented as special transformer for supplying power meters, power meters and relays. It is under normal operation conditions a secondary current, which is virtually proportional to the primary current, and with respect to it differs by a phase shift angle that is at a predetermined connection point close to zero. For use in the power grid, there are designs of current transformers for all voltage levels.

Construction and application

Inductive current transformers have only one or a few primary turns, which are traversed by the current to be measured, as well as a larger number of secondary turns. Often the primary winding consists of a guided through the annular core of the converter bus bar, which corresponds to a single primary winding.

The secondary current is reduced compared to the primary current to be measured - and inversely proportional to the ratio of the numbers of primary and secondary windings. The secondary winding before the switching of the primary current

• Unilaterally connected to ensure safety at higher voltage and
• To the intended external circuitry, such as a flow meter is connected.

The primary terminals are marked with P1 ( or K) and P2 ( or L), the secondary terminals S1 (or k ) and S2 ( or l).

Current transformers can be built up in the MHz range for frequencies of 16 Hz. The lower frequency limit is determined by the secondary inductance and the sum of winding resistance and external terminating resistor (load). The aim is therefore to use nuclear material with the highest possible permeability.

Two current transformers (left and right) for 1 kA in medium voltage networks

Current transformer for laboratory applications with multiple ranges

Current transformers for cabinet assembly

For the measurement of direct current, such current transformers are not suitable. However, there are DC-DC converter to be made available using field plates or Hall sensors and servo amplifiers, and DC - or mixed-flow detection.

In high voltage applications are for current measurement, magneto-optical current transformers, based on the Faraday effect, are used. In this case, an optical fiber is wound in a coil shape around the conductor, the current flow in the conductor results in rotation of the polarization plane of the light beam in the optical waveguide.

Indicator for the operation

Regular operation

Until the primary rated current Ipn and up to a predetermined load, a current transformer as part of its allowable measurement error behaves like an ideal current source or constant current source whose output current ( secondary current ) only from the input (primary current ) and does not depend on the load and pending the secondary Rated current Isn (preferably 5 A) according to the rated transformation ratio Kn = Ipn / Isn. This is given as an unabridged fraction, eg 200 A / 5 A, abbreviated to 200/5 A.

The relative current error ( translation measurement error) is defined by Ip = actual primary current and Is = actual secondary current when Ip flows under measurement conditions. The highest at the rated frequency permitted amount of current measurement error is determined by an accuracy class - different for current transformers for measuring purposes and current transformers for protection purposes.

The secondary current enabling impedance of the secondary circuit is referred to as a burden. It is usually expressed by the apparent power, which is received at a fixed power factor, and the rated secondary current. For the accuracy requirements is a special value of the burden of the tax burden will be taken. Standard values ​​of the corresponding rated power is 2.5 ... 30 VA.

Ingress limit

Current transformers for measuring purposes are to 120 % of the rated current specified in accuracy classes 0.1 ... 1 (also 0.2 S, 0.5 S, 3 and 5, not treated here). The statements of the class label to different primary currents on the example of Class 0.2 shows the first table.

Current transformers for protection purposes are specified up to a rated accuracy limit current, which is about the accuracy limit factor on the primary rated current. Standard values ​​for this factor are 5 ... 30 For this converter, the accuracy class with the letter P ( for protection - protection) marked; Standard values ​​are the classes 5P and 10P. The accuracy limit factor must be specified after the class with accuracy. The statements by the example of class 5P 20 shows the second table.

Limiting the output side

The details of the bias 100% of rated load, the data valid for current transformers for measuring purposes only secondary burdens in the range 25 ... with current transformers for protection purposes only rated burden. For purposes of testing performance factors are also prescribed.

A non-closed secondary circuit acts on the transducer as a burden with an almost infinitely high. The curve form of the secondary signal is greatly distorted due to the force acting on the core and the coil voltage-time area is then high; see also principle of operation of the transformer. The magnitude of the voltage occurring is dependent on the current, core cross-section and the secondary turns. Thus, with current transformers up to 500 /5 A and low rated performance of the peak value of the "open" tension usually remains below 200 V. For larger transformers, however, higher voltage peaks occur which constitute a hazard when touching the terminals are open for the people in the converter can lead to breakdowns and between terminals to rollovers. The secondary circuits of current transformers should never be operated openly, because especially at high currents and high-performance cores may occur in hazardous voltages at the secondary terminals. In other words, current transformer must be short-circuited at its terminals when operating without an adequate measuring instruments or other burdens in the secondary circuit.

Summation current transformer

A special form of the current transformer is the summation current transformer, such as is used in ground fault interrupters. Two or more conductors (usually return conductor or phase and neutral conductors in a single-phase AC power, or all three phase conductors and the neutral conductor for three- phase alternating current ) are guided by a current transformer. Changed then only the difference current from the conductors. For instance, fault currents can be detected.