Safety and EMC aspects of the earth potential rise (EPR) of station and transfer potential

Thesis of the PhD work

József Ladányi

Consultant: Dr. György Varjú

Budapest, 2010. Safety and EMC aspects of the earth 2 potential rise (EPR) of transformer Thesis of the PhD work station and transfer potential

1. Introduction, aims

Electric power has become essential for people of developed countries for the XXI. century. It is necessary to everyday’s in numerous field of life with the opportunity of many especial, sudden dangers. Electrical safety and electromagnetic compatibility (EMC) requirements must be assured during the interaction of human beings and electric devices and equipments for the undisturbed co-existence. The actual problems of the earth potential rise (EPR) of earthing systems and the transfer potential have been investigated in the point of view of electrical safety and EMC requirements in this dissertation. These problems especially arise in the densely populated area because of consumers are electrically (transfer potential appearing at consumers) and physically (step and touch voltages near by the station) close to the transformer stations. Identifying, understanding and solving the problems appearing in practical cases are the main aims of this dissertation with the use of site measurements, existing simulation softwares and technical literatures. The new scientific results contained in the theses of the PhD work are contributing to the solution of the studied questions thus supporting the good practice of design and operation. This theme is linked to the CIGRE/CIRED JWG C4.202 and after it the JWG C4.207 (EMC) working groups’ Guide (under development) which deals with the transfer potential problems and the ITU-T SG 5 Recommendation (under development) of protection of low voltage networks and telecommunication lines against transfer potential.

2. Methodology of the research

On behalf of the earthing grid designers and operators a demand for comparison of steel and copper earthing grid has appeared from the point of view of electrical characteristics. It is consequence resulting from establishment of more and more copper earthing grids in Hungary. As a first step of my research work, I have examined many technical literatures and standards which deal with earthing grid design and provided proposals for the grid materials. I have examined those standards and technical literatures which have laid special emphases on withstand to corrosion in the selection of earth grid material for long lifetime (20 – 30 years). Nevertheless, the effect of the different materials on the electrical safety (step and touch voltages, earth potential rise) and electromagnetic compatibility requirements (earth potential rise, potential differences inside the station affecting secondary wiring) are not considered in these literatures. Simulation calculations have been done with the use of internationally well-known and accepted CDEGS (Current Distribution, Electromagnetic Fields, Grounding and Soil Structure Analysis) [1] software to compare electrical characteristics of earthing grids made of copper or steel material. The EPR, touch and step voltages and potential differences inside the station affecting the secondary wiring have investigated in case of same arrangements and boundary conditions. In the simulation calculations the nonlinearity of steel caused by the nature of ferromagnetism have be considered on the bases of the laboratory measurement. Safety and EMC aspects of the earth 3 potential rise (EPR) of transformer Thesis of the PhD work station and transfer potential

First of all I have examined the technical literatures in the beginning of the investigation of double earth fault potential and transfer potential. The recent practical experiences have shown that, the transient power frequency overvoltages appearing in the low voltage (LV) supply networks are causing increasing number of damages of electrical equipments. The internationally well-known and accepted MULTC [2] software has been used for the study of double earth fault potential and potential transferred to the low voltage network. When considering the public water pipe system, the nonlinearity of the steel pipe has been taken into account in the simulation by the use of laboratory measurement values. I have had the opportunity to prove the simulation results by site measurements.

3. New scientific results

1. Thesis

Standards and technical literatures lay special emphases on the corrosion withstand in the selection of earthing grid material to ensure long lifetime (20 – 30 years). Thermal and mechanical withstand ability, reliability (e.g. point of view of bonding) and good conductance have of special importance. The specific resistivity of steel is 8 – 10 times higher than the resistivity of copper. Both materials are recommended by standards and technical literatures. Unfortunately, no satisfactory attention is paid to the effect of the nonlinearity on the frequency and current dependence of the internal surface impedance of steel rod. Therefore, I have investigated the role and effect of this nonlinearity on the selection of earthing grid material. Laboratory measurement of the internal impedance of the 20 mm diameter steel rod electrode have done to determine nonlinearity characteristics that is the basis of accurate consideration. Simulation calculations based on measured values have done to compare electrical characteristics of earthing grids made of linear (copper) and nonlinear (steel) material. From the results the following statement can be drawn as a thesis: From the comparative analysis of earthing grid made of ferromagnetic and non- ferromagnetic material is stated that the material of earthing grid does not affect the earthing grid resistance, however, affects the potential differences inside the substation, the internal potential distribution and the step voltages, which are significantly influenced by current and frequency dependency of internal impedance of ferromagnetic conductor earthing grid elements [S1]-[S5]. I have calculated with the same arrangements and boundary conditions the earthing grid resistance, touch and step voltages inside and near the substation together with potential differences along the actual secondary cable lines. The study was made by CDEGS [1] software package, which is based on internationally well-known and accepted methods. It is able to calculate the current and potential distribution inside the grid in 3D and the evolved electromagnetic field at arbitrary grid arrangement and soil structure. Simulation calculations have done for the earthing grid of 400/120 kV substation called Szombathely with real operationally and faulty conditions. The thesis gives guidance for the earth grid designers in the planning period what effects must be considered in case of steel and copper earth grids.

Safety and EMC aspects of the earth 4 potential rise (EPR) of transformer Thesis of the PhD work station and transfer potential

2. Thesis

I have introduced and defined the equivalent earth resistance and the equivalent earth current towards accurate calculation of earth potential rise (EPR) of earthing system of substation. For derivation of the equivalent characteristics I have made distinctions between the earthed conductors connected to the earth grid. Thus, the earthed conductors are classified into passive conductors or return-conductors. The following statement can be drawn as a thesis: Earth conductors connected to the earthing grid of substation can be classified into two types. Thus, passive conductors and return-conductors are distinguished. The equivalent earth resistance of the substation is the resultant of the passive earthed conductors and the earth grid resistance itself. The equivalent earth current is the difference of the current injected to the earth grid (total fault current decreased with the transformer neutral current) and the current of return-conductor(s). The earth potential rise of substation is given as the product of the equivalent earth current and the equivalent earth resistance [S6]-[S11]. Passive conductor is that earth conductor which is connected to the earth grid and is only affected by conductive effects. These conductors behave like long horizontal earthing conductors and thus decreasing the earthing resistance of the grid. This decreasing effect is manifested itself in and can be taken into account with the parallel resultant of the input impedance of the passive conductor-to-earth loop and the resistance of the earth grid itself. This resultant value is considered as the equivalent resistance of the earthing system. Return-conductor is that earth conductor which is connected to the earthing grid and part of the lines feeding the fault. Therefore it is affected by both inductive and conductive couplings. In these conductors after the end-effect zone (correspondingly to length, the length constant) the stationer induced current is flowing, which is given by the current factor, i.e. the ratio of the mutual impedance and the self impedance of the return conductor-to-earth loop. The current in the return conductor is equal to the sum induced current with end-effect and the end-effect value of the conductive current. Determination of the current flowing in the return conductor(s) is the precondition of the accurate calculation of the earth potential rise. It is especially true in case of cable sheaths where the current factor is ranging between 0,90 – 0,95. The above mentioned basic terms are essential instruments of the accurate EPR calculation. The role of the above defined terms is not only the question of theoretical clarification but closely linked with appropriate specification and value of the substation earthing system and its measurement practice. The thesis gives guidance to avoid false determinations of the EPR related terms.

Safety and EMC aspects of the earth 5 potential rise (EPR) of transformer Thesis of the PhD work station and transfer potential

3. Thesis

There are no standard values or calculation method for the resultant earthing impedance of the neutral conductor of low voltage network in the technical literature or standards. On the bases of 4th chapter of the dissertation it can be concluded that the neutral lines connecting to a MV/LV transformer station are affecting both the equivalent earthing impedance thus the earth potential rise (EPR) of the transformer station and the potential transferred to the LV customers. Therefore, the reference values for equivalent earthing resistance of different substations are necessary to determine the EPR of MV/LV transformer station and voltage transferred to the LV network. The neutral line system together with the coupled pipe line, if present, has been simulated to determine the relative importance of different parameters. The nonlinear behaviour of pipe line has been taken into account based on laboratory measurement. The correctness of the simulation results has been checked an approved by site measurements. In addition guide-values for the equivalent earth resistance have been for the different types of MV/LV transformer stations. During site measurements two earthing resistance measurement techniques have been compared. From the results it can be concluded that the application of the technique available for the measurement of the resistance of tower footing, i.e. measurement with increasing frequency [3] is limited to separate the resistance of the substation earth grid from the equivalent earthing resistance. Accordingly to the simulation calculations and site measurements the following peculiarities are determining the resultant earth resistance of LV network:  in the case of cable network in urban area: o proportion of conventional cables with continuous leakage (lead sheathed armoured cables) and new plastic insulated cables; o external surface impedance of cable sheaths; o number of cable-branches and their lengths; o proportion of metal public pipe-lines connected to the EPH system at consumers; o the continuity of the neutral conductor at the feeding boundary.  in the case of rural overhead line networks: o the earth resistance provided by the earthing applied individually to the neutral conductor of the network; o number of lines and their lengths; o proportion of metal public pipe-lines connected to the EPH system at consumers; o the continuity of the neutral conductor at the feeding boundary. For the parameters and conditions affecting mostly the input impedance as the neutral- to-earth loop the following statements can be made as thesis:  The input impedance of the neutral conductor-to-earth loop is determined dominantly by the resistance value and the frequency of the individual earthing applied. The value of the input impedance can range between 0.24 to 74 Ω accordingly to the most favourable (below 50 Ω·m) and extremely wrong (above 1000 Ω·m) specific resistivity of the soil. Safety and EMC aspects of the earth 6 potential rise (EPR) of transformer Thesis of the PhD work station and transfer potential

 The input impedance of the neutral conductor-to-earth loop is decreased by the cross-connections between the neutral and pipe(s). Or accordingly to another approach the increasing use of plastic pipe instead of the metallic ones leads to higher EPR.  The input impedance of the neutral conductor-to-earth loop is decreased by the continuity of the neutral conductor at the feeding boundary, especially in environment where the specific resistivity of the soil is high [S6]-[S10].

4. Thesis

It could be identified that in many cases the short term power frequency overvoltages are originated from the earth potential rise (EPR) transferred to the LV network. The EPR can be caused by phase-to-earth fault (FN) in a high voltage (HV) substation of a HV network with directly earthed neutral or by double earth fault (2Ff) in a MV network with not directly earthed neutral. Simulation study has been done to investigate the two sources and to analyse the sensitivity to the key parameters relevant to the actual operational conditions in the point of view of potential transferred to LV network. The simulation calculations have been performed by the use of the internationally well- known and accepted MULTC software which is applicable to multi-conductor lines with multipoint boundary conditions. As a result of the simulation study the following thesis can be drawn: Double earth faults in MV system cause higher transfer potential in LV system both in number and amplitude than the phase-to-earth faults in HV/MV transformer station. Therefore, the short term power frequency overvoltages and damages caused them can mainly be traced back in double earth faults [S6]-[S10].

5. Thesis

The followings have of essential roles in the determining the magnitude of the potential due to 2Ff faults occurring in MV/LV transformer stations:

 The magnitude of the 2Ff fault current (its 3I0 zero sequence component). The magnitude of the 3I0 zero sequence current is affected by the relevant parameters in the following ways: o the 2Ff current increases with the short circuit power at MV side of the HV/MV substation which is essentially determined by the self short circuit power of the HV/MV transformer, o decreases with the increase of the faulty-point resistance(s) o decreases with the increase of the (electric) distance between the faulty- points.  In the case of MV cable-line the resistance of the sheath/screen and the possible continuous leakage of the sheath play an important role. These are affecting the sheath current factor of the sheath and thus the equivalent earth current and finally the EPR in the following ways: Safety and EMC aspects of the earth 7 potential rise (EPR) of transformer Thesis of the PhD work station and transfer potential

o with the decrease of the sheath resistance the return current factor increases, consequently a equivalent earth current and the EPR decrease, o due to the continuous sheath-to-earth leakage and the involved end-effect there are further increase in the current factor and decrease in the EPR, o with the distance between the faulty points (typically MV/LV stations) the EPR increases. This is valid only up to a given distance, which depends on the sheath resistance.  The equivalent resistance of the MV/LV transformer station has also an important role. This is determined by the followings: o the earth resistance of the station earthing itself, o the resultant input impedance of neutral conductors of the connecting LV lines. For the two parameters mostly affecting the EPR due to 2Ff fault the following can be stated as a thesis:

 The increase of the equivalent resistance of the MV/LV stations results in the increase of the EPR. Accordingly to the recent technologies the public pipe networks (water, gas, etc.) are changing from metallic to plastic or even in case of metallic pipe networks the inlet sections are plastic tubes. This involves the increase of the neutral system resistance and finally the increase of the EPR.  The MV lines are installed more and more as cable lines rather than overhead or in cases of overhead lines at least the terminal sections are cables. In these cases due to the improvement of the current factor, the equivalent earth current causing the EPR decreases accordingly. This, in contrast to the effect explained in the previous paragraph, tends to act in the opposite direction, i.e. tends to the decrease the EPR. To realize this EPR reduction the cross-section area of the screen must be high enough (at least 25 mm2 Cu per phase) resulting in low enough screen resistance [S6]-[S10].

4. Publications in theme

[S1] Ladányi József, Átviteli hálózati transzformátorállomások földelőháló anyag hatásának vizsgálata, Diplomaterv, BME, 2006. május. [S2] Bodnár Imre, Ladányi József, Réz vagy acél? Acél és réz földelőháló potenciál- eloszlás vizsgálata In: Bitay Enikő (szerk.) Fiatal Műszakiak Tudományos Ülésszaka XI. Kolozsvár, Románia, 2006.03.24-25. Kolozsvár: Erdélyi Múzeum- Egyesület, pp. 57-60. [S3] Bodnár Imre, Ladányi József, Szekunder kábelek EMC biztosítása különböző anyagú földelőhálók esetén In: MEE 53. Vándorgyűlés, Konferencia és Kiállítás. Budapest, Magyarország, 2006.08.23-25. pp. 1-4. [S4] Ladányi József, Villamos biztonsági és EMC szempontok a földelőhálók anyagának kiválasztásánál, Elektrotechnika 7-8. pp. 16-18. 2009. [S5] József Ladányi, Safety and EMC aspects of the selection of earthing grid material, Electrotehnica Electronica Automatica 58:(2) pp. 37-40. 2010 Safety and EMC aspects of the earth 8 potential rise (EPR) of transformer Thesis of the PhD work station and transfer potential

[S6] József Ladányi, György Varjú, Transient power frequency overvoltages transferred to low voltage networks from the EPR of HV and MV transformer stations, In: 7-th International Symposium On Electromagnetic Compatibility and Electromagnetic Ecology: The Proceedings. Saint Petersburg, Oroszország, 2007.06.26-29. pp. 32- 35. The proceedings of the EMC'2007 Symposium [S7] Ladányi József, Dr. Varjú György, Belvárosi, kis alapterületű alállomások földelési kérdései, Innotech Egyesület, K+F jelentés, 2009. november [S8] CIGRE JWG C4.207, GUIDE FOR ASSESSMENT OF TRANSFERRED EPR ON TELECOMMUNICATION SYSTEMS DUE TO FAULTS IN A.C. POWER SYSTEMS. Propagation of EPR (Earth Potential Rise) by metallic conduction, characteristics of used system configuration and precautions for telecommunication equipment.(Kidolgozás alatt) [S9] Ladányi József, Dr. Varjú György, KÖF/KIF transzformátorállomások kettős földzárlati potenciáljának és a kisfeszültségű hálózatokba kivitt (transzfer) potenciál vizsgálata, Innotech Egyesület, K+F jelentés, 2009. november [S10] Jozsef Ladanyi, Investigation of the earthing impedance and transfer potential of MV/LV transformer station, REVUE ROUMAINE DES SCIENCES TECHNIQUES-SERIE ELECTROTECHNIQUE ET ENERGETIQUE EB228:(10) pp. 1-12. (2010) [S11] J Ladányi, Gy Varjú, Analyses of the grid resistance measurements of an operating transformer station, PERIODICA POLYTECHNICA- 2009: pp. 9-15. (2009) [S12] Ladányi, Gy Varjú, Circumstances affecting the protection against electrode potential rise (EPR), PERIODICA POLYTECHNICA-ELECTRICAL ENGINEERING 2009: pp. 1-8. (2009) [S13] József Ladányi, György Varjú, Circumstances affecting the protection against electrode potential rise (EPR), In: EA4EPQ, M P Donsión, M Manana (szerk.) International Conference On Renewable Energies And Power Quality. Santander, Spanyolország, 2008.03.12-2008.03.14. Santander: pp. 1-5. Paper 432. [S14] József Ladányi, Analyses of the earthing resistance of HV/MV transformer stations with different earth electrode arrangements and soil structures, In: 7-th International Symposium On Electromagnetic Compatibility and Electromagnetic Ecology. Saint-Petersburg, Oroszország, 2007.06.26-2007.06.29. Saint-Petersburg: pp. 36- 39.

Safety and EMC aspects of the earth 9 potential rise (EPR) of transformer Thesis of the PhD work station and transfer potential

5. Bibliography

[1] IEEE-SA Standards Board: IEEE Standard 367-1996, IEEE Recommended practice for determining the electric power station potential rise and induced voltage from a power fault., 1996. [2] IEEE-SA Standards Board: IEEE Standard 80-2000, Guide for Safety in AC Substation Grounding, 2000. [3] International Electrotechnical Comission: IEC 61936-1:2002, Power installation exceeding 1 kV a.c. - Part 1: Common rules, 2002. [4] G. F. Tagg: Earth resistances. London: George Newness Limited, 1964. [5] Safe Engineering & Technologies Ltd.: Current Distribution, Electromagnetic Fields, Grounding and Soil Structure Analysis (CDEGS) software package manuals. Montreal, Quebec, Kanada, 2009. [6] J. Ma, F. P. Dawalibi Y. Li: Power Grounding Safety: Copper grounding systems vs. steel grounding systems, POWERCON2006 kiadványában, Chongqing, China, 2006. október 22-26. [7] R. D. Southey F. P. Dawalibi: Grounding & Electromagnetic Interference Analysis and Design - A Complete and Integrated Engineering Solution, , Southern Electric Exchange Engineering and Operations Conference, 1988. June, Bal Harbour, Florida. [8] R.D. Southey F.P. Dawalibi: New Computational Tool for Analysis of Transmission Line/Pipeline Interference Problems, , American Gas Association Distribution/Transmission Conference, 1988. May, Toronto, Canada. [9] J. Ma, R.D. Southey F.P. Dawalibi: Behaviour of Grounding Systems in Multilayer Soils: a Parametric Analysis, IEEE Transactions on Power Delivery, 9. kötet, 1. példány, 334-342. oldal, 1994. January. [10] F.P. Dawalibi I.B.K. Simpson: Computer Analysis of Impedance Effects in Large Grounding Systems, IEEE Transaction on IAS, IA-23. kötet, 3. példány, 490-497. oldal, 1987. May/June. [11] ETV-ERŐTERV Rt.: Szombathely 400/120 kV-os transzformátorállomás Földelési terv, rajzszám: P224210/3092/A. Budapest, 2006. [12] Geszti P. Ottó: Villamosművek I., 244-250. oldal. Budapest: Tankönyvkiadó, 1977. [13] ITU: The protection of telecommunication lines and equipments against lightning discharges, 1974. [14] ETV-ERŐTERV Rt.: Szombathely 400/120 kV-os transzformátorállomás zárlati áram- és feszültségeloszlása. Budapest, 2006. [15] PLINE 2.0 PC program for Calculation of Power Line Parameters. [16] International Telecommunication Union: ITU-T Recommendation K.68: Management of electromagnetic interference on telecommunication systems due to power systems and operators' responsibilities. Genf, 2008. [17] Dr. Varjú György: Földelőháló potenciál, lépésfeszültség és szekunder (JM) kábelezés EMC megfelelősségének vizsgálata az ELMŰ Kőbányai alállomás 120 kV-os részének rekonstrukciójához, F.1. függelék. Budapest, 2003.

Safety and EMC aspects of the earth 10 potential rise (EPR) of transformer Thesis of the PhD work station and transfer potential

[18] Dr. Varjú György: A Vérmező 120/10 kV-os transzformátor állomás földelő rendszerének tervezéséhez szükséges vizsgálatok, 2004. [19] Dr. Varjú György, Ladányi József: A Kecskemét Daimler 120/20 kV-os transzformátorállomás földelő rendszerének potenciál-emelkedés vizsgálata, 2010. [20] Varjú György F. J. Sollerkvist: A general model and numerical method for multiconductor systems in frequency domain, IEEE/KTH Stocholm Power Tech Konferencia kiadványában, Stockholm, Svédország, 1995., 227-232. oldal. [21] Ladányi József, Béla Viktor Dénes: Földelési ellenállás mérése nagyfeszültségű távvezetéki oszlopokon - Felülvizsgálat és hibafelderítés, Elektrotechnika, 9-11. oldal, 2009/12. [22] CIGRE/CIRED C4.207: Guide for assessment of transferred EPR on telecommunication systems due to faults in a.c. power systems., (Kidolgozás alatt). [23] Rózsa Lajosné, Varjú György Luspay Ödön: Erősáramú kábelvonalak. Budapest: Műszaki Könyvkiadó, 1985. [24] R. Calvas B. Lacroix: Earthing systems in LV Cashier technique no. 172. Párizs, 2002. [25] Varjú György: Laboratory measurement of the equivalent internal impedance of UIC type railway, BME, Budapest, Jelentés 2001. [26] CIGRE Working Group 36.04: Guide on EMC in Power Plants and Substations., 1997. [27] Magyar Szabványügyi Testület: MSZ 2364-442:1998 A kisfeszültségűvillamos berendezések védelme a nagyfeszültségű rendszerek földzárlata esetén., 1998. [28] International Electrotechnical Comission: IEC/TS 60479-1 Effects of current on human beings an livestock - Part 1: General aspects, 4th szerk., 2005. [29] Magyar Szabványügyi Testület: MSZE 19410: Villamosenergia-rendszerek távközlési létesítményekre gyakorolt elektromágneses zavarásának menedzselése. Budapest, 2007. [30] ITU-T SG 5: Directives concerning the protection of telecommunication lines against harmful effects from electric power and electified railways lines, Volume VI. "Danger, damage and disturbance". Geneva, 2008. [31] IEEE-Std Standards Board: IEEE Std. 487-2000, IEEE recommended practice for the protection of wire-line communication facilities serving electric supply locations., 2000. [32] ITU-T: Earthing and bonding Handbook. Geneva: Study Group 5, 2003.