Current transformer: What it is and which one to choose

Current transformer, or simply CT, is an electromagnetic device capable of transforming high currents, in the tens, hundreds, and thousands of amperes, into smaller currents, in the order of 1, 5, or 10 amperes.

They are used in alternating current circuits, and enable access to measuring, control, and protection devices in systems that do not allow such direct access, due to the low resistance of these instruments. (Equipment that would naturally be damaged (read: burned, toasted, exploded?) if connected directly).

How to specify a current transformer?

There are several details to be checked regarding the selection of the appropriate CT for each application. Therefore, from now on, we will perform a summary specification, in order to allow the reader to apply it algorithmically for energy measurement and power factor control .

For the following examples, we will focus on the option of CTs suitable for measurement in meters and controllers, so that the reader is aware of the precision involving consumption, power, and the possible concatenation of errors in systems with poorly dimensioned CTs or ranges. 

The applications here are restricted to those mentioned, therefore, for different applications, it is important for the reader to validate their design and choice, together with the current standards and considerations of each manufacturer.

CT destination, accuracy and saturation

The purpose is the first piece of information required to size a current transformer. We highlight here some of the best-known purposes, focusing on measurement.

CT for FP measurement and control

Here we want to measure current, and/or power or power factor. In this case, we need to pay more attention to the accuracy of the device, which can vary between 0.3%, 0.6%, 1%, 1.2%, and 3.0% (value not recommended due to the high phase shift error, which impairs the measurement, or control of power and power factor, respectively).

The accuracy class must nominally represent the expected error of the transformer.

current, taking into account the transformation ratio error (the value of the current in magnitude or amplitude), and the phase shift error (the insertion of a delay or advance in the signal) between the primary and secondary currents.

NBR 6856/92 guarantees accuracy by taking into account both errors, as well as major market players, respecting other international standards. However, be very careful with TCs that have little documentation, or little information, and are therefore cheaper financially.

For most of them, the application is restricted to analog ammeters, that is, only current, so the information regarding phase shift error is omitted, and in applications for measuring energy, consumption, or power factor (mainly), the accuracy can vary in absurd values ​​and totally inconsistent with reality.

For energy and power factor measurement applications, choose CTs with higher saturation (or higher nominal dynamic current). This is important because in different measurement situations we may have groups of different loads being measured by the same CTs (considering 1 per phase). Often, in this measured group, it is possible to have a number of motors, so checking options that support starting currents is necessary. See the application in MCCs below.

Below we list other possible uses for current transformers:

CT for Protection: For circuits that must be protected against overcurrents, using protection relays (It is recommended to opt for saturation at 20 times the nominal current or more).

CT for CCMs (Motor Control Centers): Since we are in circuits with motors, pumps, or machines, the CT must withstand current peaks at start-up, without saturating (saturation at 10 times the nominal current or more, depending on the motor), since this behavior is natural, to take the equipment out of inertia. A measurement is advisable, since the types of start-up, and their simultaneity, are factors in defining the CT.

Auxiliary CTs: more specific application due to non-standardized currents such as 10/0.1A or 0.05/1A. Possibly customized.

Types and usage

When referring to use, it is important to decide whether it is for indoor use (sheltered) or outdoor use (exposed to some weather conditions, “on the street”). In the overwhelming majority of cases, use is sheltered in a substation protected from the weather conditions, or even inside panels, but it is possible to see some in the utility distribution system, on the street, or substations.

We have numerous types that mainly influence mechanical and construction aspects. We highlight for our application (energy measurement and FP control, for example) the fixed and bipartite window models (preferred), in thermoplastic or epoxy material .

• Busbar CT: Primary winding consisting of a busbar fixed through the core;

• Wound CT: Primary winding consisting of one or more turns surrounding the core;

• Fixed window CT: It has a fixed primary in the transformer and consists of an opening through the core, through which the conductor that forms the primary circuit passes.

• Bushing type CT: Similar to the bar type CT, however it is installed in the bushing of the equipment (transformers, circuit breakers, etc.), which function as the primary winding.

• Split CT : Similar to window type CTs, except that the core can be separated to allow the conductor that functions as the primary winding to be wrapped around it.

• CT with multiple primary windings:  The primary windings are isolated and there is only one secondary winding. In this type, the primary coils can be connected in series or parallel, allowing various transformation ratios to be obtained.

• CT with multiple secondary cores: This consists of two or more secondary windings mounted separately, each of which has its own core, forming, together with the primary winding, a single set.

• Multiple secondary winding CT: This type consists of a single core surrounded by the primary winding and several secondary windings.

• Secondary shunt type CT: This consists of a single core surrounded by primary and secondary windings, which is provided with one or more shunt. However, the primary may consist of one or more windings.

CT with bipartite nucleus	CT with fixed window

CT ratio

CT ratio is the ratio of the primary current to the secondary current. A multiplication factor establishes the transformation. See: 

For a CT with a ratio of 500/5A (read as five hundred to five), the multiplication factor is 100, because if we multiply the secondary current, 5A, by 100, we have the primary current, 500A.

We usually stick to the ratio for application and selection, and not to the multiplication factor, that is, we choose according to the nominal current of our circuit, by the value of the CT primary. See the table below with some available ratios.

!! ->> BE VERY CAREFUL!!!! <<- !!

It is very common when choosing the ratio to opt for currents in the primary, checking circuit breakers or protection devices, to have an idea of ​​the current that will circulate in the primary, however, this does not always favor the measurement.

For safety reasons, there is a tendency to have systems with dimensions greater than the installed load, i.e., imagine a 400A circuit breaker, but in it, the loads or the total load never reach 50A. If we dimension a 400/5A CT for this application, it will always be working underutilized, with values ​​close to the lower range limit, introducing an error in the measurement. This error increases when we work close to the lower and upper limits of the measurement range (40A to 400A, respectively).

It is extremely important to study the load and its dynamics (load inputs and outputs and their departures) to know which CT ratio to use! This way, ensuring that most of the measurement remains within the range, between 10 and 100% of the primary value.

In this previous example, a 50/5 or 100/5 TC could be sufficient and much more accurate, if of course the window size (internal measurements) match the cable size! More on that later!

Below, see a real specification situation, based on a current measured with a power analyzer. Note that if there were no measurement, the CT would possibly be oversized.

For the graph above, a 1200/5 TC was adopted, and note that the current never exceeded 1040A. The circuit breaker was 2000A, which leads to the choice of a 2000/5 TC. 90% of the loads were connected simultaneously in this study, and the 10% installed were estimated.

The specification considered inrush currents and had sufficient time to ensure the best interpretation of the system's operating dynamics over time. If the circuit is changed, the CT must be re-evaluated!

Window and dimensions

Define where the CTs will be installed, and measure the distances between cables or busbars, and their cross-section. Remember, the CT must be well accommodated and fixed, and in terms of fixing, outline the appropriate strategy for the location.

Note that in addition to the interior window, the external measurements are important to know if the TC will fit in the space or between busbars, for example.

There are CTs with windows with geometry suitable for cables and bars! Always check the catalogs and datasheets!

CT power

When sizing, we must pay attention to the power at the secondary output. When we connect a load to the secondary, there is a power limit established by the transformer manufacturers. See below an example of a table taken from a catalog, where the manufacturer explicitly states the different powers for the same class or same ratio.

We have the transformation ratios in the first column, and the others are separated by power: 2.5VA; 5.0VA; 12.5VA; 25VA. Note that a model can have more than one 

We have to choose the CT model suitable for the installed load, taking into account the contribution of the cables that connect it to this load.

Longer cable runs require greater power, so consider the power dissipated by the cable impedance, adding it to the power dissipated by the load impedance, be it a meter, controller or other device.

Load (VA) = Device consumption (VA) + power consumed in the 2 wires for connection (VA).

Application care

Never leave the transformer secondary open when under load. Keep it connected to the load or shorted! This will prevent damage to the system and the CT.

The image on the side shows a system with CTs installed, and short-circuited, not connected to loads, or purposely deactivated for insertion of coils, in red.

In all installations, use calibration switches! They ensure safe operation, preventing disconnections or damage to a CT with an open secondary.

In case of doubt or incompatibility between technical characteristics…

This content aims to summarize a broad subject. However, it helps those looking for a solution for current sensing, for cases of energy measurement and power factor control.

By following the previous steps, you will probably get the sizing right, but there are cases where it is necessary to move on to other types of sensing. Let's look at a practical example:

In the photo on the side, we see a 500A circuit breaker, connected with 240mm² cables. In this system, an energy meter and 3 CTs compatible with the circuit breaker current are installed: 500/5A.

Over time, large differences were noted in the measured kWh compared to what was billed on the energy bill, and again an analyzer was used to audit this difference.

It was found that over the measured time of 30 days, the maximum current did not reach 50A, remaining below 10% of the CT primary current 100% of the time. A classic case of oversizing.

The structure is designed for a much greater load than the one installed, which is not difficult to find in countless cases for n reasons.

At first glance, a TC50/5 would solve the problem, however, the size of the cable does not allow it. The TC window is smaller than the cable cross-section. It does not fit. Therefore, we have to increase the TC ratio until we find a window that fits.

Basically, it is a matter of evaluating the table until the window allows the cable to pass through. Now, if you do this, is it correct in terms of measurement? The answer is NO! because we are again oversizing the ratio in relation to the nominal current due to a mechanical incompatibility.

So what to do in this case?

Use coils, not transformers! 

Flexible sensors are Rogowski coils . This type of sensor allows the equipment to be calibrated to measure any current, with a coil of any size. In other words, a coil of the appropriate size for the cable can be made, with the calibration taking into account the actual nominal current. In this case, the precision in both module and phase is much higher than using CTs.

Example for specification

Here is a summary specification that helps you correctly specify your TC:

Minimum aspects for application of measurement and control:

• Purpose (measurement and/or protection): Measurement or Control;

• Use (indoor or outdoor): Indoor (within the frame);

• Accuracy class: 1.2% or 0.6% or 0.3%;

• Transformation ratio; 200/5 or other according to the table;

• Dimensions: ? 42 mm or according to conductor and installation;

• Nominal load (Power): 2.5VA min or more depending on installation and load;

• Nominal frequency: 60HZ or 50HZ for other countries.

Important aspects not mentioned and more comprehensive to other applications:

• Voltage class: 600V or 1kV;

• Thermal factor: 1.2 x In or more;

• Insulation or impulse level: 3 kV or more;

• Type: Encapsulated in epoxy or other;

• Temperature class: A – 105ºC;

• Nominal dynamic current: 2.5 x In.

Do you need help sizing the current in your system, meter or controller?