Burden of the current transformer is expressed in VA. The total VA burden should be taken into account when CT is used for measuring or protection purpose.
The total resistance of the secondary circuit of CT known as a burden is the sum of resistances of CT secondary winding,connecting wires(lead resistance) and the resistance of the relay/meter.
Thus the total VA burden of the CT can be calculated by adding the VA burden of the followings.
- The VA burden of the measuring equipment like protection relay and measuring instruments
- The VA burden of the leads connected in between the current transformer and the relay/measuring meter
- The secondary resistance of the current transformer
VA Burden of meters and the protection relay:
The VA burden of the energy meters,voltmeter, ammeter,power factor meter and the protection relay is mentioned in the catalog of the instrument. The electronic meters and the numeric relays have less VA burden as compared to the analog meters and the electro-mechanical relay.
While designing the protection system or measurement system the VA burden of all the measuring instruments and protection relay must be taken into account for building a reliable measurement and protective system.
The VA burden of various electrical equipment is as given below.
VA Burden of the meter or the protection relay can be taken from the equipment manufacturer data sheet.
VA Burden of the connecting lead wires:
The VA burden of the lead wires can be calculated by following mathematical formula.
Lead Wire Burden in VA= I2 * 2 D/ (CS x 57)
Where: I = Secondary Current in Amps
D = Lead wire distance in meter.
CS =Cross section area of lead wire.
The thicker the wire the lesser will be the resistance, and as a result of larger cross sectional area of the conductor, the VA burden of the connecting wire will be less.The measuring instrument or the protection relay must be installed as near to the current transformer as possible so that the lead resistance can be minimized. The resistance of the conductor is temperature dependent and the resistance is measured at 75°C.
|Read More : Correction of Resistance at 75°C|
The resistance of the copper wire of different cross sectional area at 75°C is as given below.
The lead wires resistance can be calculated for the 6-wire and 4-wire configuration. The three sets of CT connection can be brought in the relay or metering panel by two methods.
- The 2 wires of each CT can be brought in the panel. The six wires of the three CTs can be connected to the meter/ protection relay.
- The common point can be made at the test terminal block and the four wires can be brought in the relay/metering panel.
The scheme of the wiring is as given below.
If the distance between CT and the relay or metering panel is 10 meters, the total distance is 10 x 2=20 meters for 6-wire connection. however the distance for the 4-wire connection when one wire is used as return wire is equal to 1.2 x 10 =12 meters. This rule is applicable for three phase connection.
|Read More : Accuracy Class of Current Transformer|
The resistance of the secondary of the CT:
The secondary of the current transformer has many turns of the copper wire and the internal resistance of the CT is taking into account while calculating the VA burden of the CT.
VA Burden of Current Transformer:
Thus, the burden of the current transformer depends on the internal resistance of the CT, leads resistance and the resistance of the relay.
Rct – Resistance of the secondary of the CT
RL – Lead Resistance
RR – Relay or meter resistance
R(sec)- Total CT burden in Ohm
CT burden Calculation:
Example: If the relay resistance is 0.2 Ω, connecting wires resistance is 0.2 Ω, and secondary winding resistance of CT is 0.2 Ω. The total resistance of the secondary circuit is 0.2+0.2+0.2=0.6 Ω.
Total burden of the CT is 0.6 Ω.
If the rated secondary current of the CT is 5 Amp. The secondary voltage is 0.6*5= 3 volts,
The burden of the current transformer in VA is ;
CT (VA) = CT rated secondary current x CT secondary Voltage
CT Burden Ratings According to IEC/ANSI Standard:
The burden of the CT is basically the resistive impedance of secondary circuit of the CT.The burden of the CT can be expressed in VA or in impedance. The IEC standard specifies the CT burden in VA and typically burden ratings are 1.5 VA, 3 VA, 5 VA, 10 VA, 15 VA, 20 VA, 30 VA, 45 VA and 60 VA. The ANSI standard specifies the CT burden in Ω and typically CT burden ratings are B-0.1, B-0.2, B-0.5, B-1.0, B-2.0 and B-4.0. The CT of B-0.1 will maintain the stated accuracy with up to 0.1 Ω on the secondary circuit.
Why CT burden data important for protection system?
The reliability of the protection system depends on the operation of the CT below the saturation level on the B-H curve. The knee point voltage gets changed with change in the burden of the CT. Therefore, the burden of the CT must be calculated during designing of the protection system.
|Read More – Calculation of Knee Point Voltage of CT|
How operating Voltage of CT gets affected with change in burden of CT?
The operating voltage of current transformer gets changed with change in the burden of CT. If protection CT goes defective, the new CT must be checked for its rated burden because the increased burden will cause CT to operate at voltage above the normal voltage of CT on magnetization curve of CT, and if the CT operates at knee point voltage of BH curve during fault with CT increased burden, the protection relay may not operate. The working of CT near knee point may cause CT saturation during fault.
If the burden of CT increases, the net magnetic flux flowing through the CT core will get increase because the secondary current gets decreased with increase of CT burden, and the core of the CT will get heated up due to core saturation which may cause permanent failure of the CT.
When the circuit burden gets too large, the CT secondary voltage becomes distorted. This is because the CT is beginning to have a flux density that is much greater than normal operation. The CT excitation voltage also increase above normal voltage with an increase in the CT burden and it may cause CT core saturation.
Read Next :
- Knee point voltage of CT
- Accuracy class of CT
- PS class CT
- CT secondary Grounding Practices
- Why CT secondary should not left open?