Current Transformer: 7 Answers for Beginners

Current Transformer: 7 Answers for Beginners

Current transformer, I can remember that day, When I was an electrical engineering student, current transformer was a confusing topic form me. Back then, it didn’t make any sense for me to transform the current. Now I’m answering important questions about CT for beginners. Let’s get started.

What is the current transformer burden?

The burden of the current transformer stands the terminating impedance of the measuring instrument. The instrument that is being used to measure can be an energy meter, either digital or analog, and a data logger or recorder.

Any instrument that uses an electric CT to gauge current in the line must terminate the CT by resistance (impedance in the same way that there is an inductance).

There are some essential points to understand the CT burden mentioned below:

  • The most common burden rating for CT: 2.5, 5, 10, 15, and 30VA.
  • The external load that is applied on the secondary of a current transformer is referred to as “burden”
  • The burden can be described as the highest amount (in VA) applied on CT secondary side.
  • It is possible to have the burden expressed in two different ways:
  1. The burden could be defined by the sum of impedance in ohms of the circuit or as the total voltage-amperes (VA)
  2. power factor at a certain amount of voltage or current and frequency.
  • It is common to describe the CT burden using the volt-amperes (VA) or power factor (PF), the volt-amperes consumed in the burden’s impedance at secondary rating current. So, a burden of 0.5 O impedance could be described by using “12.5 VA at 5 amperes” when we consider the standard 5-ampere secondary rating.

CT Burden calculation formula and example?

Formula to calculate the CT Burden is mentioned below:

The total burden of measuring CT =the total burden of meters in VA (Ammeter and Wattmeter), Transducer, etc.) that are connected to CT secondary circuit + the Secondary Circuit Burden of cable in VA.

  • Cable burden =I2 x R x 2 L. In this equation, I is the CT second current, and R is the resistance of the cable per length, 2L represents distance of the cable length L from CT to meters. If the correct length and size of wire are utilized, the cable burden is left out.
  • The CT Secondary circuit load must not exceed the CT VA ratings. When the CT burden is smaller than the CT load, the other meters attached to the CT should give the correct reading.
  • In a Measuring Current transformer, the burden is determined by the connected meters. Amount of meters that are connected to the secondary side of the cable, i.e., Ammeters, Kvar meters, KWh meters the Kwh Meters transducers as well as a load of connecting cables(I 2 x R x 2 L) to metering should be considered.
  • Note: Meter burdens values can be taken from the manufacturer’s catalog.
  • The selected CT burden should be greater than the burden calculated.

CT burden Calculation Example:

 If the resistance for the relay equals 0.2 Ohm, the resistance of the wires that are connected is 0.2 Ohm, the secondary winding resistance of the is CT 0.2 Ohm.

Total resistance in the Secondary side circuit = 0.2+0.2+0.2=0.6 Ohm.

The total burden on CT is 0.6 Ohm.

If the rated current of the secondary side of CT is 5 amps.

Secondary voltage = 0.6*5 = 3 Volts.

The main burden for an existing transformer within VA is;

CT (VA) = CT secondary current rated by CT * Secondary Voltage of CT

=5*3 = 15 VA.

CT ratio meaning and calculation, with example?

The ratio of the primary current input to the secondary current output at the entire load is called Current Transformer (CT) Ratio.

For an example:

An CT with a ratio of 300:5 is assessed to be 300 primary amps when fully loaded. This will result in 5 amps of the secondary voltage if 300 amps flow across the primary.

When the current in the main fluctuates or changes, the output of the secondary current changes according to that change.

For example, if 150 amps flow through a primary with a 300-amp capacity secondary current, the secondary current will be 2.5 amps.

What is the ALF and ISF of current transformer?

 ALF (Accuracy Limit Factor):

CT’s Accuracy limit value (ALF) means the ratio of the rated accuracy primary current limitation to the rated primary current.

ALF has been utilized in the protection class Current Transformers (CTs). The purpose of the Protection Class CT is to detect current during fault conditions and then feed the result to the protective relay/circuit breaker, which will then cut off power.

The class of protection for the CT is identified with 5P10 and 5P20. In this case, P represents the protected class. 20 represents the accuracy limit of the primary current, and 5 represents the Composite error for the CT when the accuracy limits current flows through the primary transformer.

ISF (Instrument Safety Factor):

Instrument Safety Factor (ISF) is the ratio between saturation current and highest-rated current at primary section.

Metering CTs is devised with ISF, having typically value less than 5 or less than 10. CT will be saturated if the amount of current generated is greater than 5 or 10 times the maximum output.

Instrument Safety Factor (ISF) = CT saturating Current/ Rated current

Why does a current transformer burn or break?

All electrical devices and equipment can get damaged or burnout. Each device has factors that cause damage. Current transformers are not an exception. Here under some common causes of the current transformer burning and breaking:

  • Secondary open circuits of the current transformer generate a high voltage; the secondary voltage instantly jumps into thousands of volts, which reduces the secondary insulation. It produces arcs internally, which generates lots of heat that burns out or breaks the transformer.
  • Long-term use of the transformer causes the insulation to overage due to poor insulation resulting in an overvoltage due to this local discharge, or break down occurs, and the current transformer burns out or breaks.
  • The contact surface of the aluminum of the primary connections of the current transformer is severely oxidized, resulting in an enormous resistance to contact. The contact resistance releases heat and burns out the current transformer.
  • The long-term overloading process causes the current transformer to emit heat and burn out.
  • High Voltage fluctuation from sub-station can be cause of burn or break current transformer.

Read also my article: Transformer moisture, causes and protection, and the other one, Transformer burnout, causes and solutions


What happens when current transformer secondary opens on live?

The prospect of open circuiting the CT secondary can be hazardous for the personnel working on the second side.

The secondary part of a transformer’s current should not be left in an open state because, if left in the open position, a high voltage is present on this secondary portion due to this action.

This high voltage triggers an intense magnetizing current to accumulate on the secondary end, which creates high flux and causes the intense flux could saturate the core and cause enormous residual magnetism within the core, increasing the magnetization current and creating an error in the conversion ratio.

Due to exceptionally high secondary voltage across of CT can cause flashovers and damage the insulation due to the rupture of the dielectric layer between two points of CT. So, Its secondary requires to be connected to a low-resistance device like an ammeter at all times.

Is current transformer a step up or a step down?

High currents exist on power lines and within high-power circuits. It is often difficult to measure these currents directly. Current transformers are used to reduce the line current to make it easier to determine.The current Transformer, also known as CT, is usually a step-down transformer. It will lower the current’s level to that level measured with an ammeter.

As we all know, ammeters can measure a range of current that can be approximately 500 to 750 A. However, the current levels of AC circuits are high enough that they can’t be measured with an ammeter. Thus, the CT is installed in series, with the primary winding connected to the circuit and the secondary winding attached to a current measurement instrument.

The power of the transformer is the same for both the primary side and the secondary side. The only way to reduce the current is to step-up the voltage.

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