DC Protection Relay Principles

Yarra Trams Power and Communications

DC protection relay settings

\\corp.tramco.com\fs\users\stuart.orr\My Documents\Word\DC prot relay calcs\DC protection relay calcs.doc Page 1 of 28 Created on 2/02/2012 8:58:00 AM ( tables from ABB Switch Gear Manual 10th rev edition)

Introduction. DC over current protection.




Calculation is continued on the next page.




DC Traction rectifier short circuit modeling The block diagram for a traction system is given below.

AC Side impedances. -Short circuit power Sm (ex1 or Lo1 for the AC system reactance.) -Short circuit power of the transformer (ext or Lot for transformer reactance, ert or Rot for transformer resistance.) -Cable impedance (exc or Loc for cable reactance, erc or Roc for cable resistance.) DC side impedances. - Rectifier. L rect & R rect. Generally L rect = 10E-6H, R rect =1 E-3 ohm, -Cable impedance (Lc for cable reactance, Rc for cable resistance.) The European Standard EN50327 provides a graphical method for approximating the Short circuit current response on a traction system. An extract is provided on the next page of the method used for the calculation. The full method is provided on J drive in, J:\E-Gate\Power\Staff Working Folders\Stuart creations\Sub SLD's & PROT DOT\DOT Prot settings & Calcs. A calculation has been performed on Sy3 feeder. See, J:\E-Gate\Power\Staff Working Folders\Stuart creations\South Yarra fdr settings.




The curve above varies depending on the configuration of the sub station. For accurate modeling the following quantities need to be provided in order to get the model correct. Several companies are available to do this modeling. Secheron (Imtram), Mot Macdonald (Melbourne), URS(Melbourne)




1. 12 pulse sub or 6 pulse sub. Single or Parallel rectifiers. Inter phase

transformers. Usually the Single Line Diagram will cover this. 2. Rectifier transformer full load copper loss 3. Rectifier transformer no load core loss 4. Coupling factor (K). Derived from Transformer Short circuit reactances and

losses. 5. Tappings applied on the transformer when the sub is put on load.

The transformer test certificate from the manufacture should state many of these factors. If newly designed, it must be specified in the build of the transformer. We can continue with the graphical solution demonstrated in EN50237/A1 or allow the consultants to model the system with MATLAB or there own software.




DC over load and full load rating factors. Cabling design help for DC systems. Type 76 DC, SEPCOS I, SEPCOS NG, MITRE and DDL protection relays are used extensively through out the tram network. A need has arisen for checking that the protection profile used in these relay matches the thermal heating characteristics of the 400mm2 screened V90 PVC cable & 185mm2 trolley wire used to feed the over head system. Many feeders exit the sub stations buried in conduit for a certain distance then run up to aerial connection points. Some subs such as Northcote go direct to O/H lines from the building. Tables will be presented here that give de rating factors for single core DC cables which are either

direct buried as a single core or a group run over head in air as a single core or as a group.

For under ground cabling de rating factors will be referenced back to AS3008 to ensure calculations are done to locally specified conditions. See clause 3.4.5 below of AS 3008.

These values must be used in selecting de rating factors for cables from tables used further on in this document.




Consideration may need to be given to conditions in clause 3.4.6 of AS 3008 also.

Permissible conductor temperatures for normal use (not under short circuit condition) and maximum permissible use are given in Table 1 of AS3008. This gives the maximum operating temperature of the cable conductor. (Not the outer sheath, screen etc)

Notes from the table are attached on the next page.




Given on the next page is an example of calculating how long (time) a conductor can run for with a current that exceeds the nominal rating of the cable. The cable time constant is required along with a rating factor a, that normalizes the over load current rating to the cable nominal rating. See on next page. This may be handy for calculations.




Direct buried cabling. (Current Rating) You may only use conductor ratings from column 2 and 7 in Table 13-44 below. These are applicable to DC systems with remote return conductors. The top of the table provides ratings for Copper. The bottom section for, Aluminum.

Conversion factors are given, f1 and f2. f1 is for ground temperature de rating. Table 13-54. (Attached) f2 is a de rating for cables grouped in multiples. Table 13-59. (Attached) Final rating = Table 13-44 rating x f1 x f2




Direct buried cabling. (De Rating under ground, f1) From AS 3000 clause 3.4.5 above the ambient conditions are stated.

Soil ambient specified at 25 Deg C. Depth of laying 0.5m. See definition given above. Thermal resistivity of soil taken as 1.2 Deg C.m/W. Load factor. Definition given below.

AS 3008 table 27(2), table 28 (1), table 28(2) and table 29 give optional de rating factors if any of these parameters are changed. See Appendix A. Load Factor Load factor is defined as follows.

A load factor of 0.7 will be used here.




From Table 13-54 conversion factor f1 is derived. By selecting the cable permissible operating temp (V90 cable), soil temp (25 Deg C) and using the 1 K.m/W value for thermal resistivity should meet AS 3008 requirements. K is for Kelvin in this table. AS 3008 uses Deg C which is identical.




Direct buried cabling. (De Rating for buried cables grouped in multiples, f2) Here the de rating factor takes into account the number of cables buried, load factor and soil thermal resistivity under which the cables operate. From Table 13-59 conversion factor f2 is derived. By selecting the number of cables used, (soil temp 25 Deg C and using the 1 K.m/W) one should easily meet AS 3008 requirements.




Layed or supported over head cabling. (Current Rating) You may only use conductor ratings from column 2 and 7 in Table 13-45 below. These are applicable to DC systems with remote return conductors. The top of the table provides ratings for Copper. The bottom section for, Aluminum.

Conversion factors are given, f1 and f2. f1 is for air temperature de rating. Table 13-51. (Attached) f2 is a de rating for cables grouped in multiples. Table 13-53. (Attached) Final rating = Table 13-45 rating x f1 x f2




From Table 13-51 conversion factor f1 is derived. By selecting the cable permissible operating temp (V90 cable) and air temp (40 Deg C), which should meet AS 3008 requirements. K is for Kelvin in this table. AS 3008 uses Deg C which are identical




Layed or supported over head cabling. (De Rating for cables grouped in air, f2) Here the de rating factor takes into account the number of cables grouped in air. From Table 13-53 conversion factor f2 is derived. By selecting the number of cables used, and their arrangement one should easily meet AS 3008 requirements.




Feeder cable time constant A guide to cable time constants is given here. Due to the variables involved it is best to test the relevant conductors in a test rig to determine cable time constants. Design guides are given below.




Test currents have been applied with a 3000A test rig (1000A applied) to 400mm2 cable buried in conduit enclosed in gravel. Heating results are shown here. A cable temperature rise of 60 deg C was attained over 4 hours. The time constant worked out to 56min.

Test currents have been applied with a 3000A test rig (600A applied) to 185mm2 trolley wire. Heating results measured show the trolley wire reaching 115 Deg C before interrupting the test. Lower test currents must be applied. See next page. The next test was set to 300A applied load with the Schwartz relay set to begin the trip timed sequence at 500A (14mV). Cable temperature rise matched the profile of the relay to within a few degrees. Once the cable temperature had stabilized at the 300A mark, 500 plus Amps were applied to see if the relay and cable would track once the trip timed sequence had begun. The relays temp rise trip sequence was observed to be double the rise of the cable. Tripping occurred within 5 min.




Appendix A. AS 3008 table 27(2), table 28 (1), table 28(2) and table 29 give optional de rating factors if any of the given design parameters from Clause 3.4.5 are changed.




The ABB switch gear manual 10th edition gives formulas for calculating the DC resistance of a cable from the Cross Sectional Area of the cable.