Current Transformer: What Is CT Burden?

Replies

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  er.h3m4l

  The load, or burden, in a CT metering Electrical Network is the (largely Electrical Resistance) Electrical Impedance presented to its secondary winding. Typical burden ratings for IEC CTs are 1.5 VA, 3 VA, 5 VA, 10 VA, 15 VA, 20 VA, 30 VA, 45 VA & 60 VA. As for ANSI/IEEE burden ratings are B-0.1, B-0.2, B-0.5, B-1.0, B-2.0 and B-4.0. This means a CT with a burden rating of B-0.2 can tolerate up to 0.2 Ohm of impedance in the metering circuit before its output current is no longer a fixed ratio to the primary current. Items that contribute to the burden of a current measurement circuit are switch-blocks, meters and intermediate conductors. The most common source of excess burden in a current measurement circuit is the conductor between the meter and the CT. Often, substation meters are located significant distances from the meter cabinets and the excessive length of small gauge conductor creates a large resistance. This problem can be solved by using CT with 1 ampere secondaries which will produce less voltage drop between a CT and its metering devices.
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  manoj_2all

  Current Transformers – Burden and Classification.
  
  Current Transformers occupy a vital part in the measurement and protection scheme of any electric installation.
  

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  Hence, it is imperative that the choice of CT is made with full knowledge of the application and the number of relays and meters which are to be connected to it. A current transformer with a wrong burden rating or a wrong accuracy class will seriously compromise the effectiveness of the measuring or protection system.
  
  Calculating the Burden of a Current Transformer.
  
  The burden of a current transformer is indicated in its nameplate. The burden is rated in VA. such as 15VA or 25 VA. The rated VA indicates the load the transformer can take.
  
  The current transformer is connected to a measuring instrument or a protective relay by means of wires. The burden on the current transformer is imposed by the connected device and the impedance of the connecting wires which connect it. The VA load of the device can be obtained from the datasheet provided by the manufacturer. The total burden is the sum of the burden of the connected devices and the resistance of the wires. The inductive component of the wire impedance is usually neglected as it is minimal.
  
  The burden of a current transformer can increase over time as the resistance of the connecting wires may slightly increase due to age, change in temperature and loosening of connections. Hence, the current transformer should never be loaded to 100% of its capacity.
  
  Classification of Current Transformers
  
  Depending on their application, current transformers can be classified into measuring and protection current transformers.
  
  Measuring transformers have high accuracy. However, they have a low saturation point. They are deliberately designed this way, so that the measuring instruments are not damaged by the high currents during a fault. During a fault, the measuring transformers get saturated and the output stays within the range of the measuring instruments.
  
  Measuring transformers are classified into 0.1,0.3, 0.5, 1. The values indicate the percentage error at the rated primary current. Thus a 100/5 transformer with 0.3 accuracy will have a maximum error of 0.3 when a current of 100 A passes through the primary.
  
  The current transformers used for protection have lesser accuracy as compared to measuring current transformers. They have a very high saturation limit. This is necessary as they need to continue sensing the current even at high fault values.
  
  Protection Transformers are classified as 5P10,10P10, etc. The first letter in the notation indicates the maximum percentage error. The last number indicates the number of times the rated current. Thus a 5P10 transformer would indicate a maximum error of 5 % at 10 times the rated current.