©2012 QuEST Global Services Pte Ltd. The information in this document is the property of QuEST Global Services Pte Ltd. and may not be copied or communicated to a third party or used for any purpose other than that for which it is supplied without the written consent of QuEST Global Services Pte Ltd. CT & PT SIZING CALCULATIONS P.A.HARISH KUMAR (QI_4792) By

©2012 QuEST Global Services Pte Ltd. Confidential 2 PRESENTATION ON CT & PT SIZING CALCULATIONS

©2012 QuEST Global Services Pte Ltd. Confidential 3 POINTS OF DISCUSSION  Introduction  Current Transformers  Current Transformers Safety Precautions  Current Transformers Accuracy  Current Transformers Types  Voltage Transformers  Voltage Transformers Safety Precautions  Voltage Transformers Accuracy  Voltage Transformers Types  CT &PT Sizing Calculation Overview  Over view of Typical System SLD Of Power Plant.  Questions  Discussion

©2012 QuEST Global Services Pte Ltd. Confidential 4 INTODUCTION  This course describes the how to Design the Instrument Transformers  Basic about the CT & PT.  This information will prepare you for additional safety Precautions to be taken for CT & PT.  Overall view of the different types of CT & PT in the Indian Power Systems.  Single Line Diagram Of Power Monitoring & Controlling of an Power Plant.

©2012 QuEST Global Services Pte Ltd. Confidential 5 CURRENT TRANSFORMERS  Current transformers are used extensively for measuring current and monitoring the operation of the power grid. (Protection)power grid  The CT is typically described by its current ratio from primary to secondary

©2012 QuEST Global Services Pte Ltd. Confidential 6 CURRENT TRANSFORMERS SAFETY PRECAUTIONS  Care must be taken that the secondary of a current transformer is not disconnected from its load while current is flowing in the primary,  As the transformer secondary will attempt to continue driving current across the effectively infinite impedance.impedance  This will produce a high voltage across the open secondary (into the range of several kilovolts in some cases), which may cause arcing.arcing

©2012 QuEST Global Services Pte Ltd. Confidential 7 CURRENT TRANSFORMERS ACCURACY  The Accuracy of a CT is directly related to a number of factors including:  Burden  Knee-point voltage  Class  Load  External electromagnetic fieldselectromagnetic fields  Temperature and Temperature  Physical configuration.  The selected tap, for multi-ratio CTs

©2012 QuEST Global Services Pte Ltd. Confidential 8 CURRENT TRANSFORMERS TYPES  CURRENT TRANSFORMER for indoor application(Ref. STD IS/IEC/BS) up to 33 KV :  Donut or Window-Type CT  Bar -Type CT  Ring type CT  Outdoor CT

©2012 QuEST Global Services Pte Ltd. Confidential 9 CURRENT TRANSFORMERS TYPES

©2012 QuEST Global Services Pte Ltd. Confidential 10 CURRENT TRANSFORMERS TYPES

©2012 QuEST Global Services Pte Ltd. Confidential 11 VOLTAGE TRANSFORMERS  Potential Transformer or Voltage Transformer are used in electrical power system for stepping down the system voltage to a safe value which can be fed to low ratings meters and relays.electrical power system  The CT is typically described by its current ratio from primary to secondary

©2012 QuEST Global Services Pte Ltd. Confidential 12 VOLTAGE TRANSFORMERS SAFETY PRECAUTIONS  Manufacturer instructions,  Never short-circuit the secondary's of a voltage transformer when fed on primary, for it would be destroyed in a matter of seconds.  Location  Ventilation(1/2 Inch rod)  Clearance  Earthing  Winding resistance measurements,  Polarity verification,  Insulation power factor, and  e.tc

©2012 QuEST Global Services Pte Ltd. Confidential 13 VOLTAGE TRANSFORMERS ACCURACY  The accuracy of a PT is directly related to a number of factors including:  Burden  Class  Load  External electromagnetic fieldselectromagnetic fields  Temperature and Temperature  Physical configuration.  The selected tap.

©2012 QuEST Global Services Pte Ltd. Confidential 14 VOLTAGE TRANSFORMERS TYPES  CURRENT TRANSFORMER for indoor application(Ref. STD IS/IEC/BS) up to 33 KV :  Pole Insulated P.T.  Core & Coil Assy  Outdoor PT

©2012 QuEST Global Services Pte Ltd. Confidential 15 VOLTAGE TRANSFORMERS TYPES

©2012 QuEST Global Services Pte Ltd. Confidential 16 CT & PT SIZING  CT & PT SIZING :  SYSTEM DETAILS  FULL LOAD CURRENT  CLASS (ACCURACY)  PERCENTAGE OF OVER CURRENT  BURDEN FOR COMPONENT.  QUANTITY OF THE COMPONENT USED.  SUBTRANSIENT REACTANCE(XD”).  CT WINDING RESISTENCE  CABLE RESITENCE  RELAY RESITENCE  KNEE POINT VOLTAGE

©2012 QuEST Global Services Pte Ltd. Confidential 17 CT & PT SIZING  CT & PT SIZING :  SYSTEM DETAILS 1.MW 2.Generator Voltage 3.Power Factor  FULL LOAD CURRENT  PERCENTAGE OF OVER CURRENT 1. 10% 25%,125 %

©2012 QuEST Global Services Pte Ltd. Confidential 18 CT & PT SIZING  BURDEN  General Instrument Burdens For Reference

©2012 QuEST Global Services Pte Ltd. Confidential 19 CT & PT SIZING  BURDEN  Burden Value can be get from the manual for each component.

©2012 QuEST Global Services Pte Ltd. Confidential 20 CT & PT SIZING  BURDEN

©2012 QuEST Global Services Pte Ltd. Confidential 21 CT & PT SIZING  CLASS (ACCURACY)  Defining the accuracy class as per the application Standards (Metering) = Class 0.1 or 0.2 Tariff metering= Class S S Industrial division metering = Class 1 Measure display = Class 1 or 3 Protection = Class 3P, 5P or 10P & PS Remember that instrument errors add up to the errors of the transformers feeding them. This also applies when several transformers are mounted in tandem. In the case of protection class 5P or 10P, selects the accuracy limit factor, generally 5, 10, 15, 20, or 30.

©2012 QuEST Global Services Pte Ltd. Confidential 22 CT & PT SIZING  Accuracy of Current Transformers

©2012 QuEST Global Services Pte Ltd. Confidential 23 CT & PT SIZING  Accuracy of Voltage Transformers

©2012 QuEST Global Services Pte Ltd. Confidential 24 CT & PT SIZING  SELECTED CT & PT CHART

©2012 QuEST Global Services Pte Ltd. Confidential 25 KNEE POINT VOLTAGE  KNEE POINT VOLTAGE The knee-point voltage of a current transformer is the magnitude of the secondary voltage after which the output current ceases to follow linearly the input current.

©2012 QuEST Global Services Pte Ltd. Confidential 26 Knee Point Voltage Calculation Example.

©2012 QuEST Global Services Pte Ltd. Confidential 27 TYPICAL SYSTEM SLD OF A POWER PLANT. CT PT

©2012 QuEST Global Services Pte Ltd. Confidential 28

©2012 QuEST Global Services Pte Ltd. Confidential 29

©2012 QuEST Global Services Pte Ltd. The information in this document is the property of QuEST Global Services Pte Ltd. and may not be copied or communicated to a third party or used for any purpose other than that for which it is supplied without the written consent of QuEST Global Services Pte Ltd. CT & PT SIZING CALCULATIONS P.A.HARISH KUMAR QI_4792, EXTN : 1416