SELECTION OF CURRENT TRANSFORMERS

Transformer operated energy meters need instrument transformers to reduce high voltage- and current values to lower quantities for measurement purpose.
In context with power distribution we have different voltage levels (IEC 60038):
Low voltage (LV): voltages up to 1 kV
Medium voltage (MV): voltages between 1 kV and 35 kV
High voltage (HV): voltages between 35 kV and 230 kV

This article is dealing with the selection of low voltage current transformers.

A current transformer (CT) is producing a secondary current that is proportional to the primary current. It consists of a single primary winding window, through which an external bus-bar or cable runs.
Current transformers are used for metering- or protection purposes.

The primary current rating of a CT must be larger than the expected maximum operating current. Metering CT’s primary current rating should not exceed 1.5 times the maximum operating current.
Standard values for the primary current I_pr are: 10, 12.5, 15, 20, 25, 30, 40, 50, 60, 75 A, and decimal multiples of these values (see IEC 60044-1).

The secondary current rating of a CT is either 1 A or 5 A. CTs with a 5 A secondary rating are becoming less common as more CT driven equipment becomes digital. For long secondary cables, CTs with 1 A secondary can minimize the transformer and secondary cable size. In some countries still instrument transformer with a rated secondary current I_sr of 10 A or 20 A can be found.

Main criteria for current transformer selection

a) The primary voltage
Transformer operated CT/VT meters are measuring on the primary side of distribution transformers. These CTs need to be isolated versus the medium voltage on the primary side.
CT operated meters are connected to the secondary side of the distribution transformer. These meters get the secondary voltage (e.g. 230 V) and the transformed current (e.g. 5 A).

b) Indoor/outdoor use and connection
Current transformers for outdoor use should have the same ingress protection (IP) rating like the distribution transformer.
The CT window must match with your cable diameters or bus-bar size.

c) The primary current rating per phase from distribution transformer

d) The secondary current rating
usually 1 A or 5 A

e) The rated real output power
This output power is also called burden. The burden needs to be calculated for each measurement setup. Utilities usually have a set of standard installation plans.
If the CT output power is too low, the meter gets either no value or it is lower than it should be for an accurate measurement.

Calculation

R: resistance in Ω
l: total length in m
(to the meter plus return to the CT)
κ : conductivity in m/( Ω*mm²)
copper has a conductivity of 57 m/(Ω*mm²)
A: cross-section of the conductor in mm²
I: secondary current or the CT in A
P: output power in VA

Once we have the burden, we need to add 1 VA for the consumption of electronic energy meters. For electromechanical meters we need to add 3 VA.
You can use the calculator below to see the impact of the different parameters on the CT burden. The calculation is based on copper wires and reference temperature 20 °C.

Standard values for CTs are: 1, 2.5, 5, 10, 15 VA. You will choose the next higher burden rating after your calculation.

f) Accuracy class
The accuracy class indicates the accuracy of the CT secondary current from 5 % to 125 % of rated primary current. Above this level, the CT starts to saturate and the secondary current is clipped to protect the inputs of a connected metering instrument.

For metering and billing purpose the classes Cl. 1.0, Cl. 0.5 and Cl. 0.2 are typically in use.
The CT is part of the measurement circuit, means when we have a CT meter class 0.5 S and a CT class 1.0 the maximum error of this measurement location will be ± 1.5 %.
The eventual loss can be minimized by choosing CTs with a better accuracy class. This reduces the technical loss. It’s finally a matter of utility budget and objectives.

g) Frequency
This rating must match the system’s operating frequency. Standard frequencies are 50 Hz and 60 Hz.
A 50 Hz CT can be used in a 60 Hz system, but a 60 Hz CT cannot be used on a 50 Hz system.

Installation

Never leave the secondary side of a current transformer open. It can cause a high voltage of several kV. You can find a good description for the reason here.
We recommend the use of “test disconnect terminals”.
Always maintain the general safety rules.

After installation, you should check the proper measurement function of the whole system by doing a meter error measurement with primary current.
If something is suspicious you need to check the transformer ratio, the burden and the meter error individually.
Typical mistakes are:
– wrong energy meter ratio
– inverted CT wiring
– CT over-burdened

Increasing the burden after installation is a very classical mistake. It happens when later-on a backup-meter is installed or someone likes to make a consumption comparison with an electromechanical meter.
The comparison will show no fault, but the total current transformer power is too low to run two (or more) energy meters in series.

For the verification of installations with current transformers we recommend a portable meter test equipment, like our RS350. These installations should be verified regularly.

Thank you for reading. If you have some other selection criteria, please share your thoughts.