Siemens 7SR45 Argus User Manual

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Contents

Chapter 7 - 7SR45 Applications Guide©2018 Siemens Protection Devices Limited Chapter 7 Page 3 of 10Section 1: Current Transformer Requirements7SR45 Self Powered/Dual Powered Non-Directional Overcurrent and Earth Fault Relay uses the current inputs toenergize the relay.Higher ohmic burdens in the current transformer (CT) secondary circuit results in greater saturationof the core and introduces larger errors in the secondary current waveform. For a given secondary current, itrequires more voltage from the CT for a higher burden.The total burden on the CT is the vector sum of the CT winding resistance, the connecting lead resistance, theimpedance of any auxiliary CTs, and the impedance of the connected relays and meters.After the ohmic burden is determined, the next step in predicting CT performance is to determine the required CTexcitation voltage or secondary limiting voltage by multiplying the calculated total ohmic burden by the accuracylimit factor (ALF) and rated secondary current.The secondary limiting excitation voltage (EALF) is given by the following expression:EALF = ALF * I sr * (Rct + Rbn )Where,ALF – Accuracy Limiting FactorI sr – Rated Secondary current of CT (A)Rct – CT Winding resistance (Ω)Rbn – Rated Burden of CT (Ω)If the secondary limiting excitation voltage (EALF) calculated exceeds the knee point voltage of the connected CT,the CT may get saturated.To select a proper CT, the accuracy limit factor (ALF) at a given burden needs to be considered. The ALF needsto be checked for entire range of currents the relay is expected to operate. For proper operation of the CT, thefollowing condition needs to be fulfilled.ALF * Isr * (Rct + Rbn) ≥ Is * (Rct + Rb )So,ALF = Is *(R ct + R b )Isr *(Rct + Rbn)I s – Actual Secondary current of CT (A)Rb – Actual secondary burden of CT (Ω) (Includes Burden of the relay (Rrel) and resistance (Rl ) of the cablebetween CT and Relay.The burden of 7SR45 relay depends on the input current. The impedance of the relay is inversely proportional tothe input current. Based on the field conditions, it is recommended to select the primary CT turns ratio should beat least 10 %xIn to reduce the dependency of the internal battery.The following graphs show the impedance of the relay for different currents when the relay is powered through asingle phase current input.NOTE:When all the three phases are available, the burden on each CT reduces.