1. PRACTICAL METHOD FOR ANALYSIS OF GROUNDING SYSTEM OF HV/MV SUBSTATIONS LOCATED IN URBAN AND SUBURBAN AREAS

2. BASIC ELEMENTS OF GROUNDING SYSTEMS SPONTANEOUSLY FORMED IN AN URBAN AREA
Substation grounding electrode (substation grounding grid and/or substation building foundation)
Outgoing MV cable lines acting as long grounding conductors and /or as conductive connections with grounding systems of supplied MV/LV substations
Different underground metal installations conductively connected to the HV/MV substation grounding electrode, mainly water pipelines
Reinforced foundations of the surrounding buildings conductively connected to the HV/MV substation grounding electrode through the metal sheaths of outgoing MV cables and LV network neutrals
Ground wire(s) of HV feeding line, when it is an overhead line

3. CHARACTERISTIC DISPOSITION OF HV/MV SUBSTATIONS AND OUTGOING CABLE LINES IN AN URBAN AREA

4. CHARACTERISTIC DISPOSITION OF THE CABLE LINES BELONGING TO ONE OF PRIMARY DIRECTIOS

5. SIMPLE CASE: GROUNDING GRID AND ONLY ONE CABLE LINE

6. ANALYTICAL EXPRESSIONS FOR: RESISTANCE OF GROUNDING GRID

7. EQUIVALENT CIRCUIT OF FINITE CABLE LINE AS GROUNDING CONDUCTOR OBTAINED BY DISCRETISATION OF ITS, IN REALITY DISTRIBUTED PARAMETERS

8. GENERAL EQUATIONS OF THE METAL SHEATH REPRESENTED BY DISCRETE PARAMETERS

9. CHARACTERISTIC PARAMETERS OF CABLE METAL SHEATH AS A SPECIFIC ELECTRICAL LINE

10. GROUNDING RESISTANCE OF THE CABLE LINE SECTION OF ONE METER
ρ – equivalent soil resistivity along the cable line (Ωm)
L’ - total cable line length (m)
d - outer cable diameter (m)
h - depth in which the cable line is buried (m)

11. SELF- IMPEDANCE OF METAL SHEATH OF THE CABLE LINE SECTION OF ONE METER

12. EARTH PENETRATION DEPTH OF FAULT CURRENT

13. GROUNDING SYSTEM COMPOSED OF A SUBSTATION GROUNDING ELECTRODE AND OF ONLY ONE OUTGOING CABLE LINE
Grounding system impedance: ?

14. PROXIMITY EFFECT

15. CABLE LINES BELONGING TO ONE OF THE PRIMARY DIRECTIONS AND SIMPLIFIED ELECTRICAL CIRCUIT

16. ANALYTICAL EXPRESSION FOR THE GROUNDING IMPEDANCE OF INFINITE CABLE LINE INCLUDING PROXIMITY EFFECT

17. ACTIVE CABLE LINE LENGTH AND THE RADIOUS OF AREA COVERED BY THE SPONTENIOUSLY FORMED GROUNDING SYSTEM

18. ACTIVE CABLE LINE LENGTH AS A FUNCTION OF EQUIVALENT SOIL RESISTIVITY ALONG THE CABLE LINE

19. EQUIVALENT CIRCUIT FOR DETERMINATION OF THE GROUNDING CONTRIBUTION OF CABLE LINE LAID IN ONE OF THE SECONDARY DIRECTIONS

20. ANALYTICAL EXPRESSIONS FOR THE GROUNDING CONTRIBUTION OF THE CABLE LINE LAID IN A SECONDARY DIRECTION

21. AXIAL (TRANSFER) IMPEDANCE OF THE METAL SHEATH SEEN FROM THE BEGINNING TOWARD THE END OF THE LINE

22. ONE OF THE SUPPLIED MV/LV SUBSTATIONS IS CLOSED TO THE POINT OF DIVERGENCY

23. ANALYTICAL EXPRESSIONS FOR TAKING INTO ACCOUNT GROUNDING CONTRIBUTION OF THE GROUNDING SYSTEM OF SUPPLIED MV/LV SUBSTATION

24. POTENTIAL TRANSFERRED TO THE NEAREST MV/LV SUBSTATION

25. ANALYTICAL EXPRESSIONS FOR THE ESTIMATION OF TRANSFERRED POTENTIAL

26. ANALYTICAL EXPRESSION FOR THE GROUNDING SYSTEM IMPEDANCE

27. TRANSFER POTENTIAL COEFFICIENT AS A FUNCTION OF THE LINE LENGTH TO THE SUPPLIED MV/LV SUBSTATION

28. SAFETY CRITERION THAT SHOULD BE SATISFIED

29. PRACTICAL EXAMPLE

30. RELEVANT NUMERICAL DATA
· The area covered by the substation is: 26 m x 52 m
· The total number of outgoing cable lines is 11;
· The places of divergences of the cable lines from the denoted primary directions are at the following distances seen from the substation building: for primary direction I: 210 m and 320 m; for primary direction II: 300 m and 390 m; for primary direction III: 220 m and 290 m; and for primary direction IV: 400 m;
· Cables with uncoated metal sheath have an outer diameter of 44 mm and are buried at a depth of 0.7 m;
· The self-impedance of the metal sheath of the cables obtained by measurements is: ( j0.002) Ω/m;
· The whole grounding system is located in an area with an equivalent soil resistivity estimated at about 30 Ωm; ;
· The influence of the conductive connections with the grounding system of the nearest supplied MV/LV substation is considered negligible.

31. MAIN CALCULATION RESULTS