CTSizer — CT/VT Dimensioning
CTSizer is a free online tool for dimensioning current transformers (CTs) and voltage transformers (VTs) for protection and metering to IEC 61869. It checks the knee-point voltage, the accuracy limit factor (ALF), burden and transient dimensioning factors, and shows every formula and calculation step.
Supported applications
- Overcurrent protection
- Distance protection, including auto-reclosure (C-O-C-O) analysis
- Line differential protection
- Transformer differential protection
- Busbar protection
- Metering cores
- Voltage transformers: burden, voltage drop and thermal checks
Standards and CT classes
IEC 61869-2 (current transformers), IEC 61869-3 (voltage transformers) and IEC TR 61869-100 (transient dimensioning factor Ktd, C-O and C-O-C-O duty cycles); cable resistance to IEC 60228 and IEC 60287. CT classes 5P, 5PR, 10P, 10PR, PX, TPX, TPY and TPZ; metering cores with instrument security factor (FS).
Results can be exported as a documentation report (RTF) for Word or LibreOffice. Available in English and Danish.
Frequently asked questions
Is CTSizer free?
Yes. CTSizer is free and open source (GNU AGPL-3.0). It needs no account and runs in the browser; projects are stored only in the browser.
Which standards does CTSizer use?
IEC 61869-2 for current transformers, IEC 61869-3 for voltage transformers and IEC TR 61869-100 for transient dimensioning (Ktd, C-O and C-O-C-O duty cycles). Cable resistance follows IEC 60228 and IEC 60287, and the fault current factors IEC 60909. IEC 60044 and IEEE C57.13 (C-class) are not supported.
Which protection and metering applications are covered?
Overcurrent, distance (with auto-reclosure), line differential, transformer differential and busbar protection, metering cores and voltage transformers. CT classes 5P, 5PR, 10P, 10PR, PX, TPX, TPY and TPZ.
What does CTSizer not do?
It is not an electromagnetic transient (EMT) simulator: it uses the analytical formulas of the standards, not a non-linear magnetising (B-H) curve or a step-by-step core model. It does not calculate short-circuit currents from a network model; the fault current and time constant are entered by the user. Results must always be checked independently.
Can I export a report?
Yes. CTSizer exports a documentation report as an RTF file for Word or LibreOffice, with the input data, every formula and calculation step, the charts and the verdict per core.
Dimensionering af strømtransformere og spændingstransformere
CTSizer er et gratis onlineværktøj til dimensionering af strømtransformere (CT) og spændingstransformere (VT) til beskyttelse og måling efter IEC 61869-2, IEC 61869-3 og IEC TR 61869-100: knæpunktsspænding (Vk), nøjagtighedsgrænsefaktor (ALF), byrde, transient dimensioneringsfaktor (Ktd), genindkobling (C-O-C-O) og sløjfemodstand. Overstrøms-, distance-, linjedifferential-, transformerdifferential- og samleskinnebeskyttelse, målekerner og spændingstransformere, med kontrol efter Elmåling (TR 353). Alle formler og mellemregninger vises, og rapporten eksporteres som RTF. Beregningerne kører lokalt i browseren, uden konto.
CTSizer is a free, non-commercial open-source project (GNU AGPL-3.0): github.com/maar0005/CTSizer. No account, no cookies, no tracking.