Safety Compliance of Substation Earthing Design A. Hellany, M.Nagrial, M. Nassereddine, J. Rizk

Transcription

1 World Academy o Science, Enineerin and Technoloy International Journal o Electrical, Computer, Eneretic, Electronic and Communication Enineerin ol:5, No:12, 2011 Saety Compliance o Substation Earthin Desin A. Hellany, M.Narial, M. Nassereddine, J. Rizk International Science Index, Electrical and Computer Enineerin ol:5, No:12, 2011 waset.or/publication/240 Abstract As new challenes emere in power electrical workplace saety, it is the responsibility o the systems desiner to seek out new approaches and solutions that address them. Desin decisions made today will impact cost, saety and serviceability o the installed systems or 40 or 50 years durin the useul lie or the owner. Studies have shown that this cost is an order o manitude o 7 to 10 times the installed cost o the power distribution equipment. This paper reviews some aspects o earthin system desin in power substation surrounded by residential houses. The electrical potential rise and split actors are discussed and a ew recommendations are provided to achieve a saety voltae in the area beyond the boundary o the substation. T Keywords EPR, Split Factor, Earthin Desin I. INTRODUCTION HE beneits o electricity are numerous but mishandlin it can cause damaes to properties and may inlict injuries and atalities. Electrical manaement is the key element in human saety, also not underestimatin the importance o protection equipment in the wide area o electrical inrastructure can be another added actor in raisin the risk actor. Earthin must be the primary concern durin the desin and operations o electrical inrastructure. People oten assume that any rounded object can be saely touched. A low substation round resistance is not, in itsel, a uarantee o saety. Step and Touch voltae need to be assessed in and around the substation boundaries. A serious hazard may result durin a round ault rom the transer o potential between the substation round rid area and outside locations. This transerred potential may be transmitted by communication circuits, conduits, pipes, metallic ences, lowvoltae neutral wires, etc. The daner is usually rom contact o the touch type. An investiation into possible transerred potential hazards is essential in the desin o a sae substation roundin network. This paper discusses the manaement o earthin system desin to meet the saety requirements as per the Australian and IEEE standards. In addition this paper investiates the area o concerns when dealin with the earthin system and what actors should be taken into when reviewin the desin Dr. Ali Hellany is with the University o Western Sydney, Locked Ba 1797 Penrith South DC 1797 NSW Australia ( a.hellany@uws.edu.au). M. Nassereddine, Research student at the University o Western Sydney, Australia ( m.nassereddine@uws.edu.au) Proessor M. Narial is with the University o Western Sydney, Locked Ba 1797 Penrith South DC NSW Australia ( m.narial@uws.edu.au) Dr Jamal Rizk is with the University o Western Sydney, Locked Ba 1797 Penrith South Dc 1797, NSW Australia ( j.rizk@uws.edu.au)). II. EARTHING DESIGN; SCOPE OF WORKS The hih demand on electricity and expansion o the power rid nowadays made it essential to have hih voltae (H) substations within the residential area and in some cases some zone substations and transmission substation are surrounded with residential houses. It is important to remember that the earthin desin scopes can be summarised in the ollowins but not limited to: Grid Resistance Current Split Study Earth Potential Rise (EPR) Touch oltae Step oltae oltae Transer The neihbourin between the H substations and the residential houses will orce the desiner and the reviewer to pay extra attention on the EPR contour o the substations under malunction and ault conditions. Also this neihbourin will hihliht the touch voltae on pipeline. Usually in an earthin desin, the main saety criteria will be determined usin the clearance time and the surace soil resistivity o the area to compute the touch and step voltae. It is a common practice to use IEEE 80:2000 or AS/NZS 60479:2002 standards and the recommended calculation method to determine the saety requirement or touch and step voltae. The close neihbourin will raise the requirement or the pipeline saety, as determined in the Australian and New Zeeland Standards AS/NZS 4853:2000, Table 5.3 in this standards state the saety requirements or dierent clearance time but it doesn t speciy or relate this to soil resistivity value [1-2]. Normally such substations are supplied either by underround cable (UG) or overheard conductor (OH). These eeders can have pole electrodes or joint bay earthin system. The neihborin between the substation and residential houses, orce the pole electrodes to be sometimes in the public ootpath. For the same reason the joint bay earthin system can be located in an area accesses by children, prenant women and bare oot passin by residents. These electrodes and the whole earthin system will acilitate and assist in transerrin the EPR rom the substations to the surroundin area requented by the community and closer to residents. This will require rom the desiner and the reviewer to carry more extensive study and analysis around these electrodes and to apply the coordination desin technique stated in the substation Earthin Guide which is the equivalent o IEEE80 in Australia that is accomplished throuh 23 steps. It is not recommended in some cases to use an electrode with International Scholarly and Scientiic Research & Innovation 5(12)

2 World Academy o Science, Enineerin and Technoloy International Journal o Electrical, Computer, Eneretic, Electronic and Communication Enineerin ol:5, No:12, 2011 International Science Index, Electrical and Computer Enineerin ol:5, No:12, 2011 waset.or/publication/240 less 10 Ω or 5 Ω or even less than 1 Ω to ensure the saety and the compliance o the earthin system. In some cases these 10 or 5 Ω do not meet the saety requirement, since the touch and step voltae may have a value that above the saety limit. There is a need to ollow step by step the coordination desin technique and to ensure that saety aspects are ully considered durin the desin and the commissionin o the system. This paper is addressin only the EPR aspect o the earthin desin and explorin how EPR can lead to unsae conditions III. EPR AND SAFETY CRITERIA In an urban substation-feeder systems, where there is no houses near by, applyin the IEEE and AS standards ormula can enerate a limit or saety criteria. Takin as example a zone substation o 132/11k with an EPR contour o 500 at around 150 meters radius outside the boundary ence. I we consider that the substation is in an area with a uniorm soil resistivity o 100 Ω m and a i we take the clearance time to be 500ms, ater applyin the IEEE and AS ormulas then table Soil Resistivity Ω/m TABLE I REPRESENTS THE SAFETY THRESHOLD FOR THE DESIGN 50 K Person 70 K Person step step Touc Touch h Based on table I, the desin needs to be limited to or the touch voltae and or the step voltae. Movin this substation to a residential area where there are water pipe line connected to houses where people resides with the possibility o havin children, elderly with bare eet livin inside these house. Usin AS/NZS 4853:2000 standard, the Australian standards which contain the allowable voltae under dierent clearance time, yields a maximum touch voltae on any pipe line or un-skilled people to be 100 [3-4]. Now the new maximum limit or the touch voltae outside the substation is 100 as per AS/NZS 4853:2000 requirement. The EPR contour o the 100 should be plotted and assessed to ensure that there is no pipe line within the boundary o this contour. I water pipe exists within the 100 contour, a touch simulation should be conducted to ensure that the touch voltae will be under the acceptable 100 limit. Due to the eedin arranement, not only the area within the 100 contour need to be studied, also the area alon the eedin route need to be assessed as each pole electrode and joint bay earthin system will create an EPR that can ive rise to touch voltae issue [5-8]. The ault current at the H inrastructure will be split into the round system and the auxiliary system. This split will create an EPR in the Auxiliary earthin system that needs to be studied to ensure the compliance with the correspondin standards and reulations. Split Study The ault current at the substation can be divided into subcurrents: Current low in the earth rid Current low in the auxiliary earthin system Auxiliary earthin system can be represented by Over Head Earth Wire (OHEW) knows as a round wire, and by the cable sheath o the cable. This auxiliary path acts as a path or partial o the ault current, and reduces the current that low into the round. The return current value can be determined ater calculatin the slip actor. The split actor determines the percentae o the current that low into the round the portion that low into the auxiliary path. A. Split Factor Split actor S, is vital to be determined when desinin an earthin system that have an auxiliary path or the ault current. Split actor is essential or determinin the actual EPR at the substation and ives an indication o the EPR around the auxiliary path. In addition, it ensures the compliance o any transer voltae or EPR alon the auxiliary path. The split actor can be determined usin equation 1: Z m S = 1 (1) Z w Where: Z w is the sel impedance o the OHEW in Ω/m. Z m is the mutual impedance per meter between OHEW and phase conductors in Ω/m. The Ground current is determined usin equation 2: I = S I (2) The auxiliary path current is determined usin equation 3: I e = I I (3) The auxiliary path current value will assist in assessin the saety o the auxiliary path under any ault at the nominated H inrastructure. The split actor is used to determine the inal impedance o the line as shown in equation 4 [3]: Z total B. EPR and Split Factor S R = (4) 1 S The EPR at the substation is determined usin the earth rid resistance and the ault current. Equation 5 shows the EPR calculation, This EPR is under the assumption that there is no auxiliary system which mean the split current is 1: International Scholarly and Scientiic Research & Innovation 5(12)

3 World Academy o Science, Enineerin and Technoloy International Journal o Electrical, Computer, Eneretic, Electronic and Communication Enineerin ol:5, No:12, 2011 International Science Index, Electrical and Computer Enineerin ol:5, No:12, 2011 waset.or/publication/240 EPR () EPR = I R (5) substation Recalculatin EPR Usin the Split Factor, shows how the split actor value can reduce the EPR value at the nominated H inrastructure. This EPR is or the substation EPR under the existence o auxiliary path or the ault current EPR = S I R (6) The maximum possible value or the split actor is 1, Fiure 1 shows a simulation value or the EPR under rid resistance o 1 ohm and ault current o 1000 amperes. EPR Split Factor Fi. 1 EPR s Split Factor Fiure 1 clearly shows how the split actor values impact on the EPR at the Substation, it is vital to determine this value as it will reduce the EPR at the nominated H inrastructure which will lead to a less expensive earth rid. The auxiliary current depend on the split actor and can create an unsae condition alon the auxiliary path,. or more inormation reer [9-10] I. SPLIT FACTOR AND AUXILIARY PATH The split actor depend on the type and coniuration o the auxiliary path, or example i the auxiliary path represented by an OHEW, then dierent type o OHEW will result in dierent value o split actor For more inormation reer to [4]. The EPR at the Auxiliary earthin system is determined usin equation 7, this EPR is or the auxiliary system. A part o the ault current uses the auxiliary system as a path to the source, the auxiliary system has resistance value, then EPR can be determined takin into account the auxiliary current and the auxiliary resistance ( S ) RAux EPR = I 1 The auxiliary earthin path is normally includin a number o electrodes runnin alon the transmission eed. A current distribution study and analysis needs to be conducted in order to determine the shunt and section current in each electrode alon the line. Current Distribution, Electromanetic Fields, Groundin and Soil Structure Analysis (CDEGS) sotware is used to determine the lowin current in each section o the EPR (7) auxiliary earthin path, Fiure 2 and 3 shows the ault current in the OHEW under a ault condition. Fiure 2 clearly shows that the ault current in the irst 10 electrodes is hih. In addition, it is possible to determine the value o the current lowin in each electrode. The calculated current is used to determine the EPR around each electrode. Fiure 2 shows the need tor the earthin system at the irst ew poles, due to hih current, to be assessed more than the poles between section 10 and section 60 [11-12]. Fiure 3 shows the section current manitude that is lowin in the OHEW and returnin to the ault source. Fiure4 shows the manitude o the current that lows in the electrode at pole No 3, the value is around 300A, this value will assist in controllin the EPR around this pole, the desiner can taret an earthin system or this electrode to meet the saety requirement. Equation 8 is used to determine the maximum rid resistance or this electrode to ensure the compliance with saety voltae sae. Fi. 2 Shunt Current Manitude R Electrode = I sae electrode For example i the saety voltae is 100 and the current is 300A, an electrode system o 0.33 Ω will insure the saety compliance o the system. In some cases and due to practical reasons this value cannot be met. Then it is recommended to conduct more analysis as required under the EG1 coordination desin technique [13]. (8) International Scholarly and Scientiic Research & Innovation 5(12)

4 World Academy o Science, Enineerin and Technoloy International Journal o Electrical, Computer, Eneretic, Electronic and Communication Enineerin ol:5, No:12, 2011 Terminal round system (man./anle) Total Earth Current Amps/ de. Earth Potential Rise olts/ de. Total Fault Current Amps Total Neutral Current Amps/ derees Total Earth Current Amps/ derees Ground Potential Rise (GPR) olts / derees Fiures 5 and 6 show that the 100 contour will exist around the irst 20 pole rom Touikia Zone Substation ZS, and in the last 15 poles to Harbata Transmission Substation TS. For a metallic pipeline runnin three meter rom the electrode, CDEGS simulation ives the ollowin results: International Science Index, Electrical and Computer Enineerin ol:5, No:12, 2011 waset.or/publication/240 Fi. 3 Section Current Manitude Fi. 4 Pole number 3 shunt current manitude. CASE STUDY Touikiya is a 132/11k zone substation that ed rom Harbata transmission substation situated 60 spans away; each span is 100 meters in lenth. Touikiya earthin system is 1 Ω and the Harbata earthin system is 0.5 Ω. The maximum sinle line to round ault is 5000 amperes with 0.5s clearance time. Each electrode at the bottom o each pole is a 5 Ω system. Usin CDEGS enineerin sotware to compute the split actors ive the ollowins: Earth Potential Computations Main Electrode Potential Rise (GPR) volts Buried Metallic Structure Potential Rise (GPR) volts Fi. 5 shunt Current Manitude The EPR at the metallic line will be , this value exceed the 100 value, then more analysis is required to ensure that this transer EPR does not create an unsae condition. This can be achieved studyin the current low in the round to determine the amount that is impactin on the pipeline; more details can be ound in EG1, IEEE80, publication [1-4] Fiures 7 and 8 show the EPR contour around the pole electrode and the touch contour. A close look at these iures shows that the 100 contour will include the pipe line. International Scholarly and Scientiic Research & Innovation 5(12)

5 World Academy o Science, Enineerin and Technoloy International Journal o Electrical, Computer, Eneretic, Electronic and Communication Enineerin ol:5, No:12, 2011 I. CONCLUSION This paper shows that hih voltae substation earthin desin in a residential area need to take into account the area beyond the boundary ence o the substations. Further study need to take place to ensure that the auxiliary path o the ault condition is not creatin unsae conditions. This paper recommends the ollowin steps to be included in the desin process o an earthin system o power substation: Spilt study even when the substation is compliance under any ault condition Examine the EPR around each auxiliary path Assess the EPR alon the auxiliary path International Science Index, Electrical and Computer Enineerin ol:5, No:12, 2011 waset.or/publication/240 Y AXIS (METERS) Fi. 6 Shunt Potential Manitude Fi. 7 Simulated contour around the pole electrode Touch oltaes (All - 2D Spot) [ID:Scenario1] LEGEND Maximum alue : Minimum alue : REFERENCES [1] IEEE uide to saety in AC substation roundin, 2000 (IEEE, New York, 2000). [2] AS/NZS 4853:2000 electrical hazards on metallic pipelines [3] Nassereddine M, Hellany A, Rizk J, 2009, How to desin an eective earthin system to ensure the saety o the people, 2009 International Conerence on Advances in Computational Tools or Enineerin Applications, pp [4] Nassereddine M, Hellany A, 2009, AC Intererence Study on Pipeline: the Impact o the OHEW under Full Load and Fault Current, Proceedin in the 2009 International Conerence on Computer and Electrical Enineerin, pp 497- [5] Nassereddine M, Hellany A, 2009, Desinin a Lihtnin Protection System Usin the Rollin Sphere Method, Proceedin in the 2009 International Conerence on Computer and Electrical Enineerin, pp [6] Hellany A, Nassereddine M, Narial M, 2010, Analysis o the impact o the OHEW under ull load and ault current, International Journal o Enery and Environment, vol 1, no. 4, pp [7] Nassereddine M, Hellany A, 2010, OHEW Earthin Desin Methodoloy o Traction Substation, World Academy o Science, Enineerin and Technoloy. Proceedins, vol 1, no. 66, pp ,Paris, France ISSN [8] Nassereddine M, Hellany A, 2010, Earthin Desin Improvement: correlation between Desin and Construction, World Academy o Science, Enineerin and Technoloy. Proceedins, no. 66, pp Paris, France ISSN [9] C.N. Chan, computation o current-division actors and assessment o earth-rid saety at 161/69k indoor-type and outdoor-type substations IEE proc.-gener. Transm. Distrib., ol. 152, No. 6, November [10] A. Campoccia, A method to evaluate voltaes to earth durin an earth ault in an H network in a system o interconnected earth electrodes o M/L substations IEEE transactions on power delivery,ol 23, No. 4, Oct 2008 [11] S. Manione, A simple method or evaluation round ault current transer at the transition station o a combined overhead-cable line, IEEE transaction on power delivery, ol, 23, No, 3, July 2008 [12] E. iel, Fault current distribution in H cable systems IEE proc- Gener. Transmission distribution, ol. 147, No. 4, July 2000 [13] F. Dawalibi, Measurements and computations o ault current distribution on overhead transmission lines IEEE on transactions on power apparatus and systems, ol. PAS-103, No. 3, March X AXIS (METERS) Touch oltae Man. (olts) [Near] Fi. 8 Touch oltae Contour International Scholarly and Scientiic Research & Innovation 5(12)