At a glanceCTDim is a software program for cur-rent and voltage transformer (CT, VT)dimensioning. Both protection andmeasuring cores can be considered.

The benefits of working with CTDiminclude:

· dynamic check and optimization ofCT requirements

· simulation of CT transient behavior

· automated report generation andproject oriented documentation

· CT dimensioning and settings calcu-lation for high impedance differen-tial protection

The challengeThe development of the digital protec-tion technology and the introductionof gas insulated switchgear (GIS) havelead to a paradigm change in CT andVT dimensioning. As a result, the accu-racy limiting factor of the CT plays animportant role, rather than the nomi-nal burden of the CT. Unfortunately,sometimes specifications regarding CTshave not been changed and are now incontradiction with the limited spacereserved for CTs in GIS substations.Therefore, the optimization of CTs re-garding both technical requirements ofmodern digital relays and economicaspects is becoming more and moreimportant.

Our solutionCTDim makes CT and VT dimensioningmore efficient. It saves engineeringand production costs by optimizing theCT data.CTDim is comprised of the followingfeatures:

· easy dimensioning of CT electricaldata

· database of specific CT require-ments for protection devices (Sie-mens and other manufacturers)

· input of CT data according to thefollowing standards: IEC 61869 P,PR, PX, PXR, TPX, TPY, TPZ andANSI class C

· conversion between standards

· automatically customized documen-tation (the user can modify the re-port with additional comments)

· transient simulation of CT behaviorduring short circuit

· export functions, e.g. transfer of thesimulated curve in COMTRADE fileformat

· dimensioning of measuring CT

· dimensioning of VTs (accuracy bur-den check)

Figure 1: CT dimensioning report

Figure 2: Transient simulation of CT behavior

Figure 3: COMTRADE export (Microsoft® Win-dows®)

Current and voltagetransformer dimensioningCTDim

siemens.com/power-technologies

Figure 4: Data input for measuring CT

Application examplesIn new substations, the process of CTdimensioning takes place at the begin-ning of the project planning for theswitchgear. Since the CTs act as an in-terface between primary and second-ary system, a lot of clarification workhas to be done at this stage of the pro-ject planning. The function of CT,which is specified according to thesecondary equipment it will be con-nected to, has a straightforward influ-ence on its size and in consequence onthe sizing of the whole switchgear.

Therefore, it is very important that atthat time, the protection philosophyand the characteristic data of theequipment are known. In practice,however, it is often impossible to col-lect all of the necessary data at thisearly stage. On the other hand, theprimary equipment design should befixed in order to start production of theswitchgear. Moreover, not only speci-fied secondary functions, but custom-

ers’ habits also play an important rolein the selection and sizing of currenttransformers.

Thus, the CT dimensioning process is amulti-level optimization task, involvingknowledge not only about the primaryand secondary equipment, but alsoabout the surrounding network inwhich the switchgear will be put intooperation. Direct cooperation betweenthe primary/secondary engineeringand the manufacturer of the CTs is of-ten indispensable. In this relation, a lotof knowledge and personal experienceis necessary to manage this process.

CTDim can help to handle this process,since many scenarios can be quicklycalculated and the data influencing theCT size can be optimized. The user cancarry out the simulation of CT transientbehavior using the transient simulationmodule of CTDim. Thereby not only CTparameters, but also fault current val-ues, primary system time constant andfault inception angles can be defined.

As a short example, the figure belowshows the transient simulation of a CTclass 5PR used for bus bar protection(Siemens 7SS52). The performance of3000 A / 1 A CT, 5PR30, 5 VA, RCT<10 Ω is simulated for 40 kA short cir-cuit current with 100 ms primary timeconstant and fault inception angle of0°.

Figure 5: Transient simulation window

The resulting primary current (bluecurve) and secondary current (redcurve) are shown.

Figure 6: Transient simulation of a 3000 A /1 A, 5PR30, 5 VA CT with RCT<10 W

In Figure 6, the time to saturation ofthis CT is approximately 10 ms, whichis much more than the required timeby 7SS52 relay (3 ms).

Published bySiemens AG 2017

Energy Management DivisionFreyeslebenstrasse 191058 Erlangen, Germany

For more information, please contact:power-technologies.energy@siemens.com

AL=N, ECCN=EAR99Subject to changes and errors. The infor-mation given in this document only con-tains general descriptions and/or perfor-mance features which may not alwaysspecifically reflect those described, orwhich may undergo modification in thecourse of further development of theproducts. The requested performance fea-tures are binding only when they are ex-pressly agreed upon in the concluded con-tract.