CT (CURRENT TRANSFORMERS) IN ELECTRICAL DISTRIBUTION SYSTEM

What is CT or Current Transformer in an Electrical Distribution System

The CT an abbreviation of Current Transformer is a type of instrument transformer that is used to measure the current in AC or Alternating Current systems, taking the reduced amount from its secondary winding which is proportional to the primary winding that is being measured.

In the low voltage system where the current flow limit is within 100A, it is possible to measure directly or the whole current metering system. For low voltage system current more than 100A and all high voltage systems, current is measured by using CT. Actually, CT takes a reduced portion of current which is proportional to the flowing total current in the load circuit.

The instrumental transformer CT is required for low voltage high amount or high voltage system current measuring because conventional measuring instruments are not practical to design to measure in high voltage or high current directly.

So, the Current Transformer’s working principle is not different from ordinary conventional transformers. Current Transformers just reduce the high voltage current or low voltage high current to a much lower value that is possibly measured by a conventional ammeter.

Current Transformer and Traditional Transformer

Traditional transformers are basically formed with a fixed core and primary & secondary winding, both primary and secondary winding have multiple numbers of turns.

In the Current Transformer secondary winding contains many numbers of turns, but the primary has only one turn of winding, which means that on the primary side main current-carrying conductor works as a single-turn primary winding. Sometimes CT’s cores are split type which allows them to open and put into the main conductor without interrupting the circuit.

Difference Between Conventional and Current Transformers

How to Calculate CT Ratio

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Generally, we know the CT or current transformer is used to step down the current level to monitor the flow of current. We also know that standard current measuring instruments have a limitation to measure up to a certain level of current. Consequently, CT is used to step down the circuit's current level and that is the way we need to calculate the CT ratio.

CT ratio is actually expressed in terms of rated primary and secondary currents. To calculate the CT ratio we need to know the primary and secondary turns number of the CT. Note that often the primary turn is only one, practically the main current carrying conductor uses as CT primary turn.

Let us consider the following example to calculate the CT ratio to measure the actual current from the measured current.

1. we know the primary turn-off CT is one, so for our example, we can assume the primary turn is 1;
2. We need to find the total turns in the secondary of the CT. We assume the secondary turn is 100;
3. Now we can find the voltage ratio of the CT. The voltage ratio is the proportional of the ratio between the primary and secondary turns. So, the voltage ratio is 1:100
4. We know the CT ratio is inverse to the voltage ratio. So, our calculated CT ratio is 100:1, which means the current steps are down 100 times, another way if the primary current is 100 ampere then the CT secondary current is 1 ampere.

Using the equation in the above example we can show the secondary current of the transformer: I_s = I_p (N_p/Ns) = I_p (1/100)

if primary current is 100 ampere, then I_s = 100(1/100)= 1 ampere.

General Properties of CT Ratio

Generally, CT primary current I_p and secondary current I_s are standardized as per IEC 60044 and ANSI.  According to IEC 60044-1, the primary current series 1, 1.25, 1.5, 2, 2.5, 3, 4, 5,  6, 7.5; similarly according to ANSI the primary currents for single Ratio CT's Ip = 10, 15, 25, 40, 50, 75, 100, 200, 300, 400, 600, 800, 1200, 1500, 2000, 3000, 4000, 5000, 6000, 8000, 12000A.

According to IEC, the secondary current can be 0.5, 1, 2, or 5A and according to ANSI, the secondary current is always 5A.

Protection CT and Measurement CT in Electrical System

We already know that CTs are generally used to measure high currents, so the parameter CT ratio is important to understand a CT that gives the ratio of primary and secondary currents.

But also, CT can also be used to protect an electrical installation from overcurrent or short circuit current by using a protection relay.

As per function of protection or measurement CT or Current Transformer may have two major groups:

1. Protection CT or Protection Current Transformer
2. Measurement CT or Measurement Current Transformer

Protection CT

means protect electrical circuits if an overcurrent or short circuit occurs. CT operates to protection relay to protect the circuit before reaching the short circuit current over the rated current. As per IEC standard protection CT is defined as class P, class PX, class TP.

Measurement CT

means to measure the current of the circuit. Measurement CT is mainly used for metering systems for electricity billing purposes. According to IEC, the standard accuracy classes are class 0.2, 0.5, 1, 3, and 5.

Split Core and Solid Core CT in Electrical Distribution System

As per core construction CT may defined as split-core or core opening type and solid-core or fixed-core type.

Split-core provides the facility to connect the CT to the line without interrupting, splitting the core opening and placing over the current carrying conductor. This type of CT is suitable for temporary CT connections or re-assembling systems.

There are also flexible type core CT which is known as Coil CT, which is more simplified in use whereas other solid and split cores are difficult to use.

In the case of solid-core CT installation, the power line must be disconnected, and then the main current-carrying conductor placed through the CT window. This type of CT is suitable for new installation and permanent connection.

CT Connection Procedure

How do you connect CT safely? Typically the CT primary has only one turn whereas the secondary may take many turns depending on the amount of current carrying through it. Primary never has more than a few turns. So, the conductor or a bus goes through the CT window that makes the primary connection.

CT secondary connection is very important and must ensure safety. It’s notable that,  when the CT secondary is closed and the current present on the primary side then the secondary coil generates a back EMF to the primary magnetizing force; but if the secondary side is open in the presence of the primary current, the EMF is removed and extremely high voltage may produce in secondary due to primary magnetizing forces. This high voltage may cause danger for humans or CT. That is why CT secondary always shorted by ampere meter is recommended.

So, before using CT (Current Transformer) in the electrical distribution system should confirm the ratio, specification, and connection procedure.

current transformer calculation.