The right choice of current transformers
Ratio
The rated ratio is the ratio of the primary rated current to the secondary rated current and is specified as an unabridged fraction on the rating plate.
X / 5 A transformers are used most frequently, and most measurement devices have the higher accuracy class at 5 A. For technical, but above all for economic reasons, x / 1 A transformers are recommended for long measuring cable lengths. The line losses for 1 A transformers are only 4% compared to 5 A transformers. However, the measurement devices here often have a lower measuring accuracy.
Nominal current
The rated or nominal current (former designation) is the value of the primary and secondary current (primary rated current, secondary rated current) specified on the rating plate for which the current transformer is rated. Standardized rated currents (except in classes 0.2 S and 0.5 S) are 10 – 12.5 – 15 – 20 – 25 – 30 – 40 – 50 – 60 – 75 A, as well as their decimal multiples and parts thereof. Standardized secondary currents are 1 and 5 A, preferably 5 A.
Standardized rated currents for classes 0.2 S and 0.5 S are 25 – 50 – 100 A and their decimal multiples as well as secondary (only) 5 A.
The correct selection of the primary nominal current is important for measuring accuracy. A ratio directly above the measured / defined current (In) is recommended.
Example: In = 1,154 A; selected CT ratio = 1,250/5.
The nominal current can also be defined on the basis of the following considerations:
- Depending on the transformer nominal current times approx. 1.1 (next transformer size)
- Fuse protection (nominal fuse current = nominal transformer current) of the measured part of the system (LVMDB, UV)
- Actual nominal current times 1.2 (if the actual current is significantly lower than the transformer or fuse nominal current, this approach should be selected)
Overdimensioning of the current transformer must be avoided, as otherwise the measuring accuracy at relatively low currents (in relation to the primary rated current) can be considerably reduced.
Rated power
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 through a load within the fault limits.
The following parameters must be taken into account when selecting the right power: Measurement device power consumption (for series connection ...), cable length, cable cross section. The longer the cable length and the smaller the cable cross section, the higher the losses through the supply line, i.e. the nominal power of the transformer must be selected accordingly.
The consumer power should be close to the rated transformer power. A very low consumer power (underloading) increases the overcurrent factor, and measurement devices may not be adequately protected in the event of a short circuit. Excessive consumer power (overloading) has a negative effect on accuracy.
Current transformers are often already present in an installation and can be used when retrofitting a measurement device. The nominal power of the transformer must be taken into account here: Is this sufficient to drive the additional measurement devices?
Accuracy classes
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. The class symbol corresponds to an error curve with regard to current and angle error.
The accuracy classes of current transformers are related to the measured value. If current transformers are operated with a low current in relation to the nominal current, the measuring accuracy drops significantly. The following table shows the fault limit values taking into account the nominal current values: We always recommend current transformers with the same accuracy class for UMG measurement devices. Current transformers with a lower accuracy class lead to a lower measuring accuracy in the overall system – current transformer + measurement device – which in this case is defined by the accuracy class of the current transformer. However, it is technically possible to use current transformers with a lower measuring accuracy than the measurement device.
Current transformer error curve
Measurement transformers vs. protection transformers
While measurement transformers should saturate as quickly as possible above their operating current range (expressed by the overcurrent factor FS) in order to prevent the secondary current from increasing in the event of a fault (e.g. short circuit) and thus protect the connected devices, protection transformers are required to saturate as far outside this range as possible.
Protection transformers are used for system protection in connection with the corresponding switching devices. Standard accuracy classes for protection transformers are 5P and 10P. "P" here stands for "protection". The rated overcurrent factor (in %) is placed after the protection class designation. For example, 10P5 means that at five times the nominal current, the negative secondary-side deviation from the expected value corresponding to the transmission ratio (linear) is a maximum of 10%.
The use of transformers is strongly recommended when operating UMG measurement devices.
Transformer standard rail sizes
