Worth knowing – current transformers
Current transformers are special transformers for the proportional conversion of large currents to directly measurable values. A current transformer achieves galvanic separation between the primary circuit and the measuring circuit and, in the event of a malfunction, provides a protective effect for the downstream measuring devices. Current transformers are mainly used where currents cannot be measured directly
A current transformer converts a high primary current into an electrical signal that is easy to process. Such a current transformer emits a secondary current of milliamps up to a few amps (usually 1A or 5A) as an output signal. The secondary current is essentially proportional to the primary current.
There are different versions of current transformers for all voltage levels for use in the power grid. These have different characteristics depending on the application:
⦁ For measuring purposes, to generate a current that is reduced as proportionally as possible within the measuring range for energy meters, current measuring devices and universal measuring devices. Such converters protect themselves and the connected measuring devices from overcurrent by going into saturation.
⦁ for protection purposes, for the transmission of a reduced current to protection relays, control and regulating devices. Such converters deliver an output signal that is dependent on the primary current even with a high overcurrent.
Connect the current transformer correctly
The connections of the primary winding are marked with the capital letters “K” and “L” or “P1” and “P2”. The polarity has to be done in such a way that the “energy flow” flows from K to L or from P1 to P2. The connections of the secondary winding are with the lower case letters “k” and “l” or “s1” and “s2”. The polarity has to be done in such a way that the “energy flow direction” runs from K to L.
If the connections S1 and S2 are interchanged, this will lead to incorrect measurement results.
The correct determination of the power of a current transformer:
When determining the power of the current transformer, the type of connected measuring devices and the length and cross-section of the power must be taken into account. The power required by the current transformer results from the power requirements of the measuring devices and the cable.
A current transformer with a VA output that is too low, overloads, cannot maintain the class accuracy and the measurement error increases.
In the example given, you have a power requirement of approx. 3.8 VA. This is made up of the internal consumption of the lines, here eg 3VA and the internal consumption of the ammeter of 0.8VA. In order to maintain the class accuracy, the internal consumption of the measuring devices/lines should be in the range of 25 to 100% of the converter power. With the rated power of the selected current transformer, we are in this range with 5VA.
A brief overview of the power consumption of Cu power cables:
Avoid open operation of the current transformer
A current transformer operated openly on the secondary side induces very high voltage values on the secondary side. This voltage level can reach values of up to a few kilovolts and thus represents a great danger for people and the system.
Therefore, the secondary circuit should be opened under no circumstances as long as the current is flowing in the primary circuit. The open operation must be avoided and the converters must be short-circuited at the secondary terminals when connecting/exchanging measuring devices.
Current transformers are divided into classes according to their accuracy. Standard accuracy classes are 0.1; 0.2; 0.5; 1; 3; 5; 0.1 S; 0.2 S; 0.5 S, as well as the extensions ext150 and ext 200 in classes 0.1; 02; 0.5 and 1. The class symbol corresponds to an error curve with regard to current and angle errors.
Rated current (Ipn, Isn)
The rated current is the value of the primary and secondary current indicated on the rating plate (primary rated current, secondary rated current) for which the current transformer is rated. Standardized rated currents are (except in classes 0.2S and 0.5S) 10 – 12.5 – 15 – 20 – 25 – 30 – 40 – 50 – 60 – 75A, as well as their decimal multiples and parts thereof. Standardized secondary currents are 1 and 5A, preferably 5A.
Standardized rated currents for classes 0.2S and 0.5S are 25 – 50 – 100A and their decimal multiples as well as secondary (only) 5A.
Rated power Sn
The rated power of the current transformer is the product of the rated burden and the square of the secondary rated current and is specified in VA. Standardized values are 2.5 – 5 – 10 – 15 – 30 VA. Values above 30 VA may also be selected depending on the application. The rated power describes the capacity of a current transformer to “drive” the secondary current within the error limits through a load.
Types of Transformers
Straight-through or window-type current transformers
The conductor to be measured (busbar or cable) is passed through the opening of the current transformer and forms the primary circuit of the straight-through transformer. Straight-through converters are mainly used for mounting on busbars. A straight-through transformer or plug-in current transformer is the most common type of current transformer. These have the disadvantage that the primary conductor has to be interrupted during installation. This is why these current transformers are mainly used when setting up new systems.
In the case of small currents, the plug-in current transformer can be used as a thread-through current transformer for smaller currents for reasons of cost. The primary line is passed through the current transformer several times. The nominal primary current to be measured is reduced accordingly.
Folding transformers (divisible current transformers)
If current transformers have to be retrofitted, conversion current transformers are often used. With these converters, the converter cores can be opened during installation and thus mounted around the bus bars. This enables assembly without interrupting the primary conductor.
Winding current transformer
Winding current transformers or winding transformers have a primary winding for smaller currents of 1 to 40 A.
While measuring transformers should go into saturation as quickly as possible above their use current range (expressed by the overcurrent factor FS) in order to avoid an increase in the secondary current in the event of a fault (eg short circuit) and thereby protect the connected Devices, protection transformers are required to be as far outside as possible lying saturation.
Protection converters are used for system protection in connection with the corresponding switching devices. Standard accuracy classes for protection transformers are 5P and 10P. “P” stands for “Protection”. The nominal overcurrent factor (in%) is placed after the protection class designation. For example, 10P5 means that with five times the nominal current, the negative secondary-side deviation from the corresponding ratio (linear) to the expected value is a maximum of 10%.
Current transformer – measuring transducer
technology The advantages of the 4… 20 mA current signal are increasingly recognized and appreciated. With this signal, a base current of 4 mA is the zero value and the 20 mA signal is the 100% value. In addition to the general advantages of the current signal, a current of less than 4 mA can be used as an error signal.
The particular advantages of the 4 … 20 mA signal are always evident when the transmitter is a device that works on the 2-wire principle.
The 4 … 20 mA signal, which is also called the current loop, allows the power supply and signal evaluation to be carried out with only 2 wires. The transmitter is constructed using a special circuit technology in which the 4 mA base current is used to supply the converter.
With these transducers, the auxiliary power for the internal components of the device, such as the transducer amplifier, in particular, is drawn from the signal current.