Time-Current Curve (TCC)

Time-Current Curve usually referred as TCC's are primarily used as a tool to verify equipment under protective zone is properly protected and coordinated with the system.

As a part of Protection Study it is desirable to have the IEC/ANSI protection characteristic of the primary and secondary circuit breaker of the equipment which needs to be protected and coordinated with various criteria/operating conditions.

Usually a time current curve (TCC) is plotted at a specified voltage level and contains following information (as a minimum):

1. Interrupting time of an overcurrent device Vs Current.
2. Continuous rating of various equipment (Rated Current).
3. Damage curves (also called as short-time rating) of various equipment (Abnormal Current Vs acceptable withstand duration of equipment).
4. Equipment transients (like transformer inrush, motor starting curves at different voltage levels, combined transformer inrush in a bus etc;).
5. Available maximum & minimum short-circuit currents.

   Electrical networks with different supply (generator & power utility; only generator etc.) the values of SCC may be very different and consequently, the selectivity, make different results.

6. Operating time at through-fault current seen by Relay.

As a good practice on the same sheet adjacent to TCC engineer should attach details of Network i.e; Key protection SLD of the equipment indicating following details as a minimum: Tag of Equipment, IED detail, CT ratio, ANSI codes enabled in IED, Main Equipment data, Interconnecting power cable, Bus rating (continuous and short-time) etc.

An example for Color code (for plotting cures of Relay's indicated on a graph for a switchboard) followed when TCC's are plotted

• Black = Branch
• Pink = Largest
• Red = Main
• Blue = Primary Feeder
• Brown = Upstream

Special Comments which are worth mentioning in a TCC:

1. Fully selective or partially selective
2. Any suggestion to reduce clearing time if any
3. Recommendation to change settings from the interface team

Quality Checks in TCC:

1. Nomenclatures for each breaker / Relay Tag on the Final/original report should match with information mentioned in final As-built drawing of switch-gear (i.e; ensure capturing right manufacturer details).
2. Settings as captured on Co-ordinations graphs should be within range properly rounded-off to the nearest value (select-able in relay-configuration).
3. When mentioning settings in report ensure the ANSI codes are Enabled & programmed in relay to avoid surprises while commissioning at site.

Revised & Updated on 08/02/2015 based on comments received :)

Shailesh Kumar Chetty