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How current transformers are used as Protection Transformers

June 13th, 2023

Introduction Current Transformers (CTs) are used in power systems to measure current levels and provide accurate readings for various purposes, including billing, monitoring, and control. In addition, CTs are also used as protection transformers in protective applications, where they help to protect electrical infrastructure against overloading and short-circuits. In this blog, we will discuss how current transformer is used for protection.

Sizing and Characteristics of Protection Current Transformers

In protective applications, CTs are sized differently than in normal measurement applications. This is primarily because the burden of the protective relay may be larger than that of the measuring instrument. As a result, CTs used in protective applications are generally larger in size to ensure that they can handle the higher burden. The core must also be large enough to avoid saturation when the CT experiences a high current on the primary. It is during this overcurrent that the CT must maintain its proportionality to ensure adequate protection.
 

Indoor Protection Current Transformer

 

11RC03

2.2kV to 36kV

11RC09

2.2kV to 36kV

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Outdoor Protection Current Transformer

 

25OC-B

2.2kV to 36kV

33OC-B

2.2kV to 36kV

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The key characteristics of a protection CT are as follows:

  1. Rated primary current: This is defined by standards, such as 10 – 12.5 – 15 – 20 – 25 – 30 – 40 – 50 – 60 – 75 A and their decimal multiples.
  2. Rated secondary current: The rated secondary current is either 1A or 5A.
  3. Rated accuracy power: This is the apparent power supplied to the secondary circuit for the given secondary current and accuracy load. It is measured in VA.
  4. Accuracy class: The accuracy class is the guaranteed error range on the CT ratio and on the phase shift in known power and current conditions.     The Protection current transformer accuracy class is defined by its composite error at a specific accuracy limit factor. Standard classes for protection CTs include 5P 10 and 10P 10, where “P” designates protection and the number before “P” represents the composite error percentage.
     
    The number after “P” indicates the factor of primary current up to which composite error will be achieved, such as 10x the rated primary current for 5P 10 and 10P 10. Class 5P and 10P protective current transformers are generally used in overcurrent and unrestricted earth leakage protection.    
     
    Limit of Error for Accuracy Classes 5P and 10P:

    Accuracy Class ± % Ration Error at Rated current ± Phace displacement error (In minute) at Rated Current % Composite error at rated Accuracy Limit Primary Current
    5P 1 60 5
    10P 3 – 10
    15P 5 – 15
  5. Accuracy limit factor (ALF): A protection CT must saturate sufficiently high so that an accurate measurement of the fault current by the protection is possible. Its operating threshold can be very high, so the ALF of the CT is also usually high. The Current Transformer is generally installed in conjunction with a protection relay. The associated relay is also able to withstand high over-currents.
  6. Knee Point Voltage: In protection class current transformers, the knee point voltage is defined as the voltage at which a 10% increase in CT secondary voltage causes a 50% increase in secondary current. This relationship also implies that a 50% increase in current results in only a 10% increase in voltage.
       
    The knee point voltage is particularly crucial for protection CTs, which are designed for protective purposes. Protection CTs typically have a higher burden than metering class CTs, leading to a high voltage drop across the burden. As a result, the knee point voltage of a protection class CT must exceed the voltage drop across the burden to maintain the CT core within its linear zone.

why is knee point voltage important?

It is particularly crucial for protection CTs, which are designed for protective purposes. Protection CTs typically have a higher burden than metering class CTs, leading to a high voltage drop across the burden. As a result, the knee point voltage of a protection class CT must exceed the voltage drop across the burden to maintain the CT core within its linear zone.

How CTs are used in Protective Applications

In protective applications, the secondary leads of the CTs are connected to sensitive measuring equipment, known as protective relays. These relays trip a protective device (breaker) when the circuit sees an over-current, caused by overloading the circuit or typically caused by a short circuit. The CT supplies current magnitudes to the relays to carry out protection functions during a fault condition while permitting other parts of the plant to continue in operation.

During a fault condition, the current in the primary circuit will increase, causing a proportional increase in the current in the secondary circuit of the CT. The protective relay will sense this increase in current and send a signal to the protective device to trip, isolating the faulty part of the circuit. The CTs used in protective applications must be accurate, reliable, and able to handle high currents and overloads. They must also have a high ALF to ensure that they can accurately sense the fault current.

Conclusion: Current Transformers are used in power systems for various purposes, including measuring current levels and protecting electrical infrastructure against overloading and short-circuits. CTs used in protective applications must be sized and designed differently than those used in normal measurement applications.

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