1/25/23, 2:23 PM How to Size Current Transformers HOME (HTTPS://MYELECTRICAL.COM/) Login (https://myelectrical.com/login?returnurl=%2fnotes%2fentryid%2f99%2fhow-to-size-current-transf NOTES (HTTPS://MYELECTRICAL.COM/NOTES) TOOLS (HTTPS://MYELECTRICAL.COM/TOOLS) (https://myelectrical.com/) How to Size Current Transformers By Steven McFadyen (/user-profile/userid/3)on November 7th, 2011 The correct sizing of current transformers is required to ensure satisfactory operation of measuring instruments and protection relays. Several methods exist to size current transformers. This note will look at several methods, with particular attention being paid to protection class CTs sized in IEC 60044, which is adopted internationally. 110 or 230 Volts (https://myelectrical.com/notes/entryid/22/110-or-230volts) I've been considering a blog on the 110 or 230 Volt issue for a while. While browsing the Internet I came across a great summary by Borat over at engineering... (http://myelectrical.com/Portals/0/SunBlogNuke/2/Windows-Live-Writer/How-to-Size-CurrentTransformers_E50F/ABBCT_2.jpg) Introduction to Current Transformers ABB Current (https://myelectrical.com/notes/entryid/209/introduction- Transformer to-current-transformers) Current transformers (CTs) are used to convert high level Example of a CT specification: - a very common specification for a protection class CT would be an accuracy currents to a smaller more reasonable level for use as class 5P (1%), with rated accuracy limiting factors of 10 or 20. Typical burdens would be 5, 10, 15 or 20 VA. A inputs to protection relays and metering... typical specification would be 5P10 15 VA. How Electrical Circuits Work (https://myelectrical.com/notes/entryid/139/howContents [hide] electrical-circuits-work) 1. The IEC 60044 Method If you have no idea how electrical circuits work, or what 1. What the Manufacturer Wants people mean then they talk about volts and amps, hopefully 2. Connection Leads I can shed a bit light. I’m intending... 2. Other CT Sizing Methods and Requirements 1. BS 3938 and BS 7626 Electrical Engineering 2. ANSI/IEEE C57.13 (https://myelectrical.com/notes/entryid/4/electricalengineering) Electrical engineering is a field that covers a wide variety of sub-fields, including electricity and electronics. It is a field The IEC 60044 Method that goes back to the... IEC 60044 specifies the requirements for protection CTs (in addition to measuring CT's, VTs and electronic sensors). The key to CT dimensioning under the standard, is the symmetrical short circuit current and transient dimensioning factors: Kssc - rated symmetrical short-circuit current factor K’ssc - effective symmetrical short-circuit current factor Ktd - transient dimensioning factor The factor Kssc is relatively easy to understand and relates to the liner portion of a CT characteristic. The voltage and current across a CT are linear only up till a certain value (normally specified as a multiple of the nominal rating), after which the CT will saturate and the curve will level off. A CT rated at say 5P20 will stay linear to approximately 20 times its nominal current. This linear limit is the Kssc (i.e. Kssc = 20). As a reminder, the 5 [in the 5P20] Slightly more complicated is the effective factor, (https://myelectrical.com/notes/entryid/219/introductionto-cathodic-protection) If two dissimilar metals are touching and an external conducting path exists, corrosion of one the metals can take place. Moisture or other materials... Resistors Example IEC 60044 Calculation (https://myelectrical.com/notes/entryid/195/resistors) Resistors are electronic components that oppose the flow of Consider a CT with the following current. Manufactured in various types and ranges they specification and protection have a wide application to electronics... requirements: would be the CT accuracy class and the ‘P’ signifies a protection class CT. K’scc. Introduction to Cathodic Protection This is a calculated value which takes into account the burden (resistance) of the relay, resistance of the CT windings and resistance of the leads: https://myelectrical.com/notes/entryid/99/how-to-size-current-transformers How to Calculate Motor Starting Time CT: 600/1 5P20 15 VA, Rct = 4 (https://myelectrical.com/notes/entryid/107/how-to- Ω calculate-motor-starting-time) CT Leads: 6 mm2, 50 m long - use R=2 ρ l /a to calculate = 1/6 1/25/23, 2:23 PM How to Size Current Transformers 0.0179 Ω/m HOME (HTTPS://MYELECTRICAL.COM/) NOTES (HTTPS://MYELECTRICAL.COM/NOTES) Request to look at induction motor starting time have come TOOLS (HTTPS://MYELECTRICAL.COM/TOOLS) Relay: Siemens 7SJ45, Ktd = 1 up a few times on the site. Hopefully in this post, I give you Short circuit current, Iscc max = guys some idea on how to calculate... 30 kA IEC Reference Designations (https://myelectrical.com/notes/entryid/24/iec-reference- To find the lead resistance Rleads (two designations) leads – supply, return) we can use the The IEC publishes a series of documents and rules standard formulae for resistivity: governing the preparation of documents, drawings and the referencing of equipment. Depending on country... Rleads = 2 ρ l /a = 2 x 0.0175 x 50 / 6 = 0.3 Ω Surface Treatment – Ladders, Trays and Baskets (https://myelectrical.com/notes/entryid/186/surface- Numerical relays have low burdens, treatment-ladders-trays-and-baskets) typically 0.1 Ω (where possible the relay Steel ladders, trays and baskets form the backbone of cable manual should be consulted). containment systems. Often these items need some form of surface treatment to prevent corrosion... Plugging everything into the equations: Microsoft OneNote Rb = 15 VA / 1 A2 = 15 Ω (https://myelectrical.com/notes/entryid/32/microsoftonenote) R’b = Rleads + Rrelay = 0.3 + 0.1 = 0.4 Ω A couple of months ago I came Microsoft's OneNote and downloaded the 60 day free trail. Since then I have been K’scc = Kscc (Rct + Rb)/(Rct + R’b) using it regularly and now have a full license... (http://myelectrical.com/Portals/0/SunBlogNuke/2/WindowsLiveWriter/HowtoSizeCurrentTransformers_FC74/7345944727c6cb5e8f2423756b28f0bb_2.png) = 20 (4 + 15 )/ (4 + 0.4) = 86.4 Rct - secondary winding d.c. resistance at specified temperature Required K’scc > 1 x 30000/600 = 50 Rb - rated resistive burden of the relay R’b - Rleads + Rrelay; this is thel connected burden In this case the effective K’scc of 86.4 is CTs need to be able to supply the required current to drive the relays during transient faultthan conditions. The ability theand CT greater the required K’scc ofof50 and relay to operate under these conditions is a function of K’scc and the transient performance thestability relay, Kcriteria. the CT meetsofthe td. The factor, Ktd is supplied by the relay manufacturer. Correct functioning is achieved by ensuring the following is valid: (http://myelectrical.com/Portals/0/SunBlogNuke/2/WindowsLiveWriter/HowtoSizeCurrentTransformers_FC74/c176f53a1ee1cd44331dd80e552366ff_2.png) Issc max - maximum symmetrical short-circuit current Ipn - CT rated primary current That it. Once you have confirmed the above is ok, you know your CT is ok. What the Manufacturer Wants There is a slight complication in the manufacturers know their relays better than we (or the IEC) do. As general advice, you should always refer to the manufacturers information: firstly it is the only way to get the factor Ktd secondly manufacturers sometimes have additional requirements; for example Siemens’ overcurrent, motor protection, line differential (non-pilot)and transformer differential are good to go with the above, while their line differential (pilot wire) and distance relays require the above and have additional limitations on K’scc Connection Leads In the sizing of protection transformers, the resistance (burden) of the connection leads can have a considerable effect. In calculations, the resistance of the connection leads can be estimated from: where: l is the connection lead length in m ρ is the resistivity in Ω mm2 m-1 (=0.0179 for copper) A is the cross sectional area in mm2 https://myelectrical.com/notes/entryid/99/how-to-size-current-transformers 2/6 1/25/23, 2:23 PM How to Size Current Transformers Other CT Sizing Methods and Requirements NOTES (HTTPS://MYELECTRICAL.COM/NOTES) HOME (HTTPS://MYELECTRICAL.COM/) TOOLS (HTTPS://MYELECTRICAL.COM/TOOLS) BS 3938 and BS 7626 BS 3938 and BS 7626 are older British Standards which deal the the specification and sizing of current transformers. Both of these have been withdrawn and are superseded by the IEC 6044 standard. The standards adopted the concept of knee voltage and it is still common to find knee voltage voltage being used as a CT sizing parameter. Knee voltage is defined as the point at which a 10% increase in voltage across the terminals, causes a 50% increase in excitation current Utilising the British Standards, CTs were defined by the knee point voltage UKN and the internal secondary resistance Ri. To convert an IEC design the following can be used: where: I2N is the nominal secondary current ANSI/IEEE C57.13 The IEEE standard C57.13 covers the requirements for CT sizing in the North American markets. Class C of the standard defines CTs by their secondary terminal voltage at 20 times nominal current (for which the ratio error shall not exceed 10%). Standard classes are C100, C200, C400 and C800 for 5 A nominal secondary current. This terminal voltage can be calculated from the IEC data as follows: with and If anyone has any questions, comments or suggestions on how to improve the post, please add them below. Current Transformers (https://myelectrical.com/notes?tag=current+transformers), Equipment Ratings (https://myelectrical.com/notes?tag=equipment+ratings), Protection (https://myelectrical.com/notes/tag/protection) More interesting Notes: Cable Insulation Properties (https://myelectrical.com/notes/entryid/178/cable-insulation-properties) Motor Efficiency Classification (https://myelectrical.com/notes/entryid/155/european-motor-efficiency-classification) Induction Motor Equivalent Circuit (https://myelectrical.com/notes/entryid/251/induction-motor-equivalent-circuit) Famous Scientists (https://myelectrical.com/notes/entryid/143/famous-scientists) Star-Delta Motor Starting - Performance (https://myelectrical.com/notes/entryid/253/star-delta-motor-staringperformance) Steven McFadyen (/user-profile/userid/3) Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author(/user-profile/userid/3) https://myelectrical.com/notes/entryid/99/how-to-size-current-transformers 3/6
