Login (https://myelectrical.com/login?returnurl=%2fnotes%2fentryid%2f99%2fhow-to-size-current-transfo NOTES (HTTPS://MYELECTRICAL.COM/NOTES) TOOLS (HTTPS://MYELECTRICAL.COM/TOOLS) HOME (HTTPS://MYELECTRICAL.COM/) (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. Laplace Transform (https://myelectrical.com/notes/entryid/173/laplacetransform) Laplace transforms and their inverse are a mathematical technique which allows us to solve differential equations, by primarily using algebraic methods... (http://myelectrical.com/Portals/0/SunBlogNuke/2/Windows-Live-Writer/How-to-Size-CurrentTransformers_E50F/ABBCT_2.jpg) Robots - Interesting Videos ABB Current (https://myelectrical.com/notes/entryid/41/robot-folds- Transformer towels) Example of a CT specification: - a very common specification for a protection class CT would be an accuracy class 5P (1%), with rated accuracy limiting factors of 10 or 20. Typical burdens would be 5, 10, 15 or 20 VA. A The robot folding towels post below was interesting enough at the time to post a link. Recently I’ve come across a couple of other interesting videos... typical specification would be 5P10 15 VA. Bows and Arrows (https://myelectrical.com/notes/entryid/42/bows-andContents [hide] arrows) 1. The IEC 60044 Method It starts with me reading one of the Horrible History books 1. What the Manufacturer Wants with my son (Groovy Greeks). Arrows were mentioned which 2. Connection Leads lead to the discussion of the bodkin... 2. Other CT Sizing Methods and Requirements 1. BS 3938 and BS 7626 Understanding LV Circuit Breaker Fault Ratings 2. ANSI/IEEE C57.13 (https://myelectrical.com/notes/entryid/101/understandinglv-circuit-breaker-fault-ratings) I think this post is going to be helpful to several of our readers. While the IEC low voltage circuit breaker Standard The IEC 60044 Method [IEC 60947-2, Low voltage switchgear... 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: - 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] would be the CT accuracy class and the ‘P’ signifies a protection class CT. Slightly more complicated is the effective factor, K’scc. 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/181/operationalamplifier) The fundamental component of any analogue computer is Kssc - rated symmetrical short-circuit current factor K’ssc Operational Amplifier the operational amplifier, or op amp. An operational amplifier (often called an op-amp,) is a high... Lighting - Lamps Example IEC 60044 Calculation (https://myelectrical.com/notes/entryid/187/lightinglamps) Consider a CT with the following Lamps are the essential part of any luminaire. These are the specification and protection light generating components. Since the advent of electrical requirements: lighting in the middle of the... CT: 600/1 5P20 15 VA, Rct = 4 Frame Leakage Protection Ω (https://myelectrical.com/notes/entryid/218/frame- CT Leads: 6 mm2, 50 m long leakage-protection) - use R=2 ρ l /a to calculate = 0.0179 Ω/m HOME (HTTPS://MYELECTRICAL.COM/) NOTES (HTTPS://MYELECTRICAL.COM/NOTES) While not as popular as it once was, frame leakage TOOLSSiemens (HTTPS://MYELECTRICAL.COM/TOOLS) Relay: 7SJ45, Ktd = 1 protection does still have some use in some Short circuit current, Iscc max = circumstances. In essence frame leakage is an earth fault... 30 kA Differential protection, the good old days (https://myelectrical.com/notes/entryid/90/differential- To find the lead resistance Rleads (two protection-the-good-old-days) leads – supply, return) we can use the This morning I was explaining how differential protection standard formulae for resistivity: works to a junior engineer. To give him something to read I opened up the NPAG (Network Protection... Rleads = 2 ρ l /a = 2 x 0.0175 x 50 / 6 = 0.3 Ω Why a Sine Wave? (https://myelectrical.com/notes/entryid/36/why-a-sine- Numerical relays have low burdens, wave) typically 0.1 Ω (where possible the relay I received this question by email a few weeks. First thoughts manual should be consulted). was that it is a product of the mathematics of rotating a straight conductor in a magnetic... Plugging everything into the equations: How to Write an Electrical Note Rb = 15 VA / 1 A2 = 15 Ω (https://myelectrical.com/notes/entryid/149/how-to-writean-electrical-note) R’b = Rleads + Rrelay = 0.3 + 0.1 = 0.4 Ω Electrical notes are a collaborative collection of electrical engineering information and educational material. Any K’scc = Kscc (Rct + Rb)/(Rct + R’b) registered user can add content. ... (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 fault than 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 relay, Kcriteria. the CT meets of thethe stability 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 Other CT Sizing Methods and NOTES Requirements (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. Try it free for 14 days Keep translations in sync with your code, save time and costs, and scale globally fast. Lokalise Open Knee voltage is defined as the point at which a 10% increase in voltage across the terminals, causes a 50% increase in excitation current Try it free for 14 days Lokalise 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: HOME (HTTPS://MYELECTRICAL.COM/) NOTES (HTTPS://MYELECTRICAL.COM/NOTES) TOOLS (HTTPS://MYELECTRICAL.COM/TOOLS) 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) (http://myelectricalengineering.co.uk) ALSO ON MYELECTRICAL.COM Understanding Motor Duty Rating Induction Motor Equivalent Circuit Load Flow Study – how they work Photovolt - Perform 6 years ago • 1 comment 10 years ago • 3 comments 9 years ago • 3 comments 9 years ago Understanding Motor Duty Rating Induction Motor Equivalent Circuit Load Flow Study – how they work Photovolta Performan HOME (HTTPS://MYELECTRICAL.COM/) 9 Comments NOTES (HTTPS://MYELECTRICAL.COM/NOTES) 1 Login TOOLS (HTTPS://MYELECTRICAL.COM/TOOLS) Join the discussion… LOG IN WITH OR SIGN UP WITH DISQUS ? Name Best Share 1 T Newest Oldest − Thành Trần Thành 10 years ago ⚑ Hi Steven McFadyen Thanks for your useful post, but it seems that how to size a neutral current transformer (for function 51G) has not been included in your post, could you please add this? 34 Reply 1 O • Share › − origin789 4 years ago ⚑ Hi Mr. Steven, The formula, (rated secondary limiting emf), "Esl= Accuracy Limit Factor(CT rated burden in ohm+CT secondary resistance in ohm) x CT secondary current" is correct or not? and CT required secondary emf=transient dimension factor x ALF x (CT secondary resistance in ohm + CT real burden) x CT secondary current OK or not! would you please answer! 0 Reply 0 RE • Share › − Rouhollah Emami 5 years ago ⚑ Hi Steven can you share am link for a Siemens or GE-Schnider( micom series) protection relay which shows Ktd transient dimensioning factor ? 0 Reply 0 N • Share › − Nilesh 5 years ago ⚑ For example , if we consider 300/5 A CT as per calculation for measuring purpose. so that means we want to consider 100/5 A for each phase . Does it means any sense? 0 Reply 0 S • Share › Shivakumar Thayanithy 8 years ago − ⚑ − ⚑ − ⚑ is there a possibility to size ct's using calculation tools like ETAP? Reply • Share › 0 0 S sureshkumar > Shivakumar Thayanithy 8 years ago It is not possible in ETAP. We can do only manual calculation as per IEC and ANSI/IEEE. 0 0 Reply • Share › Steven McFadyen 8 years ago > sureshkumar A while ago, I also confirmed it is not possible. I thought about writing some external interface to the ETAP database to do the calculations, but did not take it further. There is other software available, CTDim from Siemens for example (although I have never used it). • 0 0 Reply Share › HOME (HTTPS://MYELECTRICAL.COM/) NOTES (HTTPS://MYELECTRICAL.COM/NOTES) J TOOLS (HTTPS://MYELECTRICAL.COM/TOOLS) − Jojo Alex 7 years ago ⚑ Can you provide the values for the above example with 5P10 CTs? Also i have checked the IEC 60044-1 IEC standard, however the above calculation formulaes used above are not mentioned in the IEC reference. Can you provide where is the calculation references.? 0 Reply 1 • Share › Steven McFadyen > Jojo Alex 5 years ago You can Google it quiet easily. 0 Subscribe 0 Reply • Share › Privacy Do Not Sell My Data − ⚑ HOME (HTTPS://MYELECTRICAL.COM/) NOTES (HTTPS://MYELECTRICAL.COM/NOTES) TOOLS (HTTPS://MYELECTRICAL.COM/TOOLS) View 7 Comments (old system) Popular Tags Automation (https://myelectrical.com/notes/tag/automation) Batteries (https://myelectrical.com/notes/tag/batteries) Buildings (https://myelectrical.com/notes/tag/buildings) Cable Sizing (https://myelectrical.com/notes?tag=cable+sizing) Cables (https://myelectrical.com/notes/tag/cables) Circuit Breakers (https://myelectrical.com/notes?tag=circuit+breakers) Commissioning (https://myelectrical.com/notes/tag/commissioning) Control Systems (https://myelectrical.com/notes?tag=control+systems) Earth Fault (https://myelectrical.com/notes?tag=earth+fault) Editing (https://myelectrical.com/notes/tag/editing) Education (https://myelectrical.com/notes/tag/education) Electrical Installations (https://myelectrical.com/notes?tag=electrical+installations) Electronics (https://myelectrical.com/notes/tag/electronics) Energy (https://myelectrical.com/notes/tag/energy) Equipment (https://myelectrical.com/notes/tag/equipment) Equipment Data (https://myelectrical.com/notes?tag=equipment+data) Equipment Ratings (https://myelectrical.com/notes?tag=equipment+ratings) Equipment Sizing (https://myelectrical.com/notes?tag=equipment+sizing) Fault Calculation (https://myelectrical.com/notes?tag=fault+calculation) Fundamentals (https://myelectrical.com/notes/tag/fundamentals) Generation (https://myelectrical.com/notes/tag/generation) High Voltage (https://myelectrical.com/notes?tag=high+voltage) History (https://myelectrical.com/notes/tag/history) IEC Standards (https://myelectrical.com/notes?tag=iec+standards) Innovation (https://myelectrical.com/notes/tag/innovation) Lighting (https://myelectrical.com/notes/tag/lighting) Materials (https://myelectrical.com/notes/tag/materials) Medium Voltage (https://myelectrical.com/notes?tag=medium+voltage) Motor Control (https://myelectrical.com/notes?tag=motor+control) Motors (https://myelectrical.com/notes/tag/motors) myElectrical (https://myelectrical.com/notes/tag/myelectrical) Off-Topic (https://myelectrical.com/notes/tag/off-topic) Power Quality (https://myelectrical.com/notes?tag=power+quality) Power Systems (https://myelectrical.com/notes?tag=power+systems) Protection (https://myelectrical.com/notes/tag/protection) Protective Devices (https://myelectrical.com/notes?tag=protective+devices) Relays (https://myelectrical.com/notes/tag/relays) Safety (https://myelectrical.com/notes/tag/safety) Scientists (https://myelectrical.com/notes/tag/scientists) Software (https://myelectrical.com/notes/tag/software) Standards (https://myelectrical.com/notes/tag/standards) Switchboards (https://myelectrical.com/notes/tag/switchboards) Testing (https://myelectrical.com/notes/tag/testing) Th (h // l i l / / /h ) Theory (https://myelectrical.com/notes/tag/theory) HOME (HTTPS://MYELECTRICAL.COM/) NOTES (HTTPS://MYELECTRICAL.COM/NOTES) TOOLS (HTTPS://MYELECTRICAL.COM/TOOLS) Transformers (https://myelectrical.com/notes/tag/transformers) Voltage (https://myelectrical.com/notes/tag/voltage) Wire Sizes (https://myelectrical.com/notes?tag=wire+sizes) Writing (https://myelectrical.com/notes/tag/writing) View All Tags (https://myelectrical.com/notes/viewtag/true) Click here to view the notes list in a table format Have some knowledge to share If you have some expert knowledge or experience, why not consider sharing this (/notes/notes-table-view) Click here to view the notes indexed by tag (/notes/notes-index-view) with our community. 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