Protective devices' time-current curves made easy

Protective devices have different tripping units for different types of electrical equipment. If several protective devices are to interact effectively in a switchgear system, it is necessary to compare their tripping characteristics in order to evaluate their selectivity for the demands of enhanced system availability. The device-specific setting features of different protective devices have to be presented and assigned to the various types of electrical equipment. This is practically impossible with printed characteristic representations in catalogues.

ElectricalDesign Curves software tool

ElectricalDesign Curves is a free tool with which you can easily visualize and evaluate the tripping curves for a large number of protective devices. The tool is also integrated in TiSoft ElectricalDesign.

In the following example, we will go step-by-step through the process of displaying tripping curves and giving the discrimination limit of a typical electrical installation using ElectricalDesign Curves.

Project data

Project data
Click to enlarge

As shown in the singleline diagram on the left, the electrical installation is composed by a typical 20 kV private substation with a 1000 kVA power transformer.
The medium-voltage side of the transformer is protected by a 63A HRC fuse-link and the low-voltage side of the transformer is protected by an MCCB, In=2000A, with electronic protection trip unit with LSI (Long-Time, Short-Time and Instantaneous) functions.

  • HA1 = 20 kV switchboard
  • T1 = 1000 kVA power transformer
  • SA1 = 400 V, low voltage main switchboard
  • DB1 = low voltage distribution board

Create a new project

Enter project data and information
Click to enlarge

Run ElectricalDesign Curves and click on the New project button to create a new project.

Step 1, Add the medium voltage fuse-link to the chart

Medium voltage fuse-link library
Click to enlarge

Click on the Curves button and select the medium voltage fuses from the navigation tree.

The medium voltage fuse-link is added to the chart.
Click to enlarge

The tripping curve of the fuse-link is drawn on the chart. Keep in mind that the X-axis of the chart represents the low-voltage side current.
So, the medium-voltage protecive devices are drawn on the chart by multiplying their current by the ratio:

Ratio = Phase-to-phase Medium voltage / Phase-to-phase Low voltage = 20000/400 = 50

Step 2a, Add the main MCCB (In = 2000A) to the chart

The green tripping curve of the MCCB intersects with the red tripping curve of the fuse-link.
Click to enlarge

In the same way, select an MCCB with electronic unit, In=2000A, and change the protection functions of MCCB from LI (Basic UL protection) to LSI (Selective protection).

Step 2b, Push the green curve below the red curve

By adjusting the settings of the MCCB protection unit settings, tripping curves overlay properly and the red one is below the green one.
Click to enlarge

The green tripping curve of the MCCB intersects with the red tripping curve of the fuse-link. The area of intersection indicates conditions under which both protection devices may trip, needlessly interrupting power to some portions of the electrical distribution system. On the other hand, if the tripping curves of the protection devices are distinctly seperate and do not intersect, the protection devices coordinate. To properly overlay trip curves, adjust the settings of the MCCB protection unit settings.

  • Function S, I2=5760A
  • Function I, I3=1000A

Step 3, Add the feeder MCCB (In=1000A) to the chart

The feeder MCCB (blue curve) is added to the chart.
Click to enlarge

In the same way, the feeder MCCB (blue curve) is added. Adjust the settings of the protection unit so that the blue curve is below the green curve.

Step 4, Add the feeder MCCB of distribution board DB1 (In =160A) to the chart

The feeder MCCB of distribution board is added to the chart.
Click to enlarge

Finally, select an MCCB with thermal and magnetic protection unit, In=160A. To distinguish it, you may change its tripping curve color to magenta.





Step 5, Set the drawing properties of the tripping curves (optional)


Drawing properties of the tripping curves
Click to enlarge





Step 6, Preview or save the chart


Tripping curves chart
Click to enlarge





Step 7, Produce and view the project report


Tripping curves report
Click to enlarge