An Investigation into Substation Grounding and Its Implementation on Gaza Substation

Transcription

1 Enery and Power Enineerin,, 3, doi:.436/epe Published Online November ( An Investiation into Substation Groundin and Its Implementation on Gaza Substation Ahmed Hammuda, Hassan Nouri, Mohammed Saleh Al-Ayoubi Department o Enineerin Desin and Mathematics, University o the West o Enland, Bris, United Kindom Faculty o Mechanical & Electrical Enineerin, University o Damascus, Damascus, Syria ahmedhammuda@hotmail.com, Hassan.Nouri@uwe.ac.uk, ayoubi.msaleh@mail.com Recieved October, ; revised November, ; accepted November 8, Abstract An investiation into the optimal desin o a substation roundin system or the transmission substation in Gaza City, Palestine has been carried out. A research into the most inluential parameters on the eectiveness o the substation rid system has been perormed and its results have been incorporated into the Gaza case study. Throuh modellin and simulatin the power station in Gaza while considerin some ield data, an optimal substation roundin rid has been desined and has shown complete conormance to saety. It is thus considered that such a desin will protect personnel in any area o the substation in addition to the installed machinery i the larest possible ault current was to traverse the earth. Keywords: Gaza, Groundin Grid, Ground Resistance, Substation, Step Voltae, Touch Voltae. Introduction Groundin is by ar one o the most imperative aspects o electrical systems desin the siniicance o which has attained modest mention. The desin o the substation is complex and constitutes a copious number o interlinked actors that all need takin into account. A substation roundin system is an underround, reular mesh conductor network that serves the purpose o providin the path o least resistance to the traversin current so that in the case o a ault it is distributed in all directions o the underlyin earth. I eicient, the resultin round potential due to a ault and the ensuin touch and step voltaes will be low enouh to uarantee the saety o personnel workin on the substation in addition to saety o the installed machinery. This paper research investiates the eects o alterin certain parameters on the eectiveness o the roundin system, ocusin on the most relevant parameters beore applyin the indins o the investiation on the substation o the Gaza Power Generatin Company in Gaza City, Palestine. The selection o this case study is due to the intrinsic characteristics o the substation earth bein sandy and in close proximity to the sea, and or this reason, at least the accessible top layers will be o considerably hih resistivity. Additionally, the power station bein the only one locally eneratin power exhibits a substantial ault current in such a case. The substantial necessity to protect the personnel and the dependable machinery stipulates the desin o a completely trustworthy and eective roundin system exhibitin touch and step voltaes within erable marins. Seein as the round resistance R is a major determinant o the system saety, it becomes o interest to study parameters that aid in reducin this quantity. It should be noted that a low round resistance does not necessitate an even distribution o surace potentials across the rid thus it becomes necessary to study some parameters that help to reulate surace voltaes. For this parametric analysis and or the correspondin desin pertainin to Gaza substation, the roundin rid analysis module in ETAP is utilised. While to determine the ault current that can potentially be available at Gaza power station, PSCAD is used.. Ground Potential Rise, Touch Voltae and Step Voltae.. Ground Potential Rise The round potential rise (GPR) is the product o the round resistance R which is a unction o the number o rid conductors, its area, its depth and the resistivity o the surroundin soil multiplied by the current I G enterin the rid durin a ault []. Copyriht SciRes.

2 594 A. HAMMUDA ET AL... Touch Voltae and Step Voltae At the instant o a ault, the potentials that occur at the surace o the earth are such that voltae spikes appear above the rid conductors while depressions occur above the mesh areas. At typical operational requencies, this potential distribution is relatively equal reardless o the point o current injection []. The touch voltae results rom a person makin contact with a rounded piece o equipment which resembles the GPR while standin on any point on the substations surace. Thus the touch voltae becomes V GPR V () T e where Ve is the voltae at the point where the person is standin. Clearly, where V e is lowest the touch voltae is reatest [3]. The step voltae is then simply the dierence o potential occurrin between two points on the surace o the earth, m apart. I p and q are the locations where a mans eet touch the earth surace, the step voltae becomes V V V () S Both phenomena can be diarammatically as in Fiure. In the eneral case, the human can erate a reater step voltae than the touch voltae seein as in the ormer case, a iven erable current level I b will traverse rom one oot to the other each with resistance R, encounterin the body resistance, all in series (Fiure ) while in the touch voltae case the current will traverse a body resistance in series with two parallel oot resistances (Fiure 3) [4]. I V s V t p q Earth surace Grid section Earth surace voltae Fiure. Diarammatic representation o the step voltae and the touch voltae appearin on the substation earth durin a ault. Fiure. Touch voltae and body resistance, adapted rom [4]. Fiure 3. Step voltae and body resistance, adapted rom [4]. 3. Comparin Simulation with Calculation For the purpose o addin validity to the results o the parametric analysis in the orthcomin section, where possible, the curves obtained throuh ETAP simulation will be shown alonside those calculated utilisin the accepted expression or R derived by Sverak [5], R ( h) (3) LT A h A where is the soil resistivity in Ωm, LT is the total lenth o all o the conductors combined, h is the depth o the rid and A is its area. Simulations not involvin round rods will be carried out usin the Finite Element (FEM) unctionality in ETAP as oppose to the IEEE method. Copyriht SciRes.

3 A. HAMMUDA ET AL Groundin Grid Perormance Results and Analysis 4.. The Ground Resistance aainst the Area Bounded by the Grid The irst consideration ater conductin any thorouh ield study is the area o the substation in which the roundin system is to be installed. It can be seen in Fiure 4 that increasin rid size is one o the most undamental and eective actors in reducin the round resistance. Since most substations are well above m, it is clear that the entire area o the substation should be covered by the rid to ensure the lowest possible resistance ( Ω or less). The results were obtained or a rid at a depth o.5 m in soil o resistivity Ωm with a constant mesh size o 5 m. The other advantae o such desin is to ensure that the substation work area is not built over the rid perimeter where the step voltae and touch voltaes are reatest due to the abrupt chane in surace potential. 4.. The Ground Resistance aainst the Conductor Lenth The typical relationship between the two variables, L c and R is most appropriately shown on a loarithmic scale to demonstrate the saturation eect that occurs when the lenth o the conductor is minimal. This eect is the result o interaction between the neihbourin rid conductors, such that as the conductors tend towards one another, the mutual interaction beins to limit the amount o current that can be ejected thus increasin saturation (Fiure 5). The results were obtained or a rid o size 36 m at a depth o.5 m in soil o resistivity Ωm. It can be said that the reduction in the conductor lenth or the increase in the number o meshes alon one Ground resistance (Ohms).8 Calculated.6 Simulated Conductor lenth (m) Fiure 5. The variation o the round resistance with respect to the rid conductor lenth (mesh size). side o the rid has limited eect in reducin the round resistance beyond a certain number o meshes. For this reason, this particular enhancement can be optimized The Ground Resistance aainst the Soil Resistivity Quite understandably, the relationship between the soil resistivity and the round resistance is linear or the simulated curve usin the FEM method and likewise throuh calculation as is the external multiplyin term in Sverak s round resistance calculation ormula as can be seen rom Fiure 6. It can be seen rom Fiure that or low resistivities, an increase rom Ωm to Ωm will result in a two old increase in R. The results pertain to a 5 m rid with meshes o 5 m at a depth o.5 m modelled in sinle layered soil The Ground Resistance aainst the Inclusion o Rods An enhancement that shows remarkable decrease in Ground resistance (Ohms) Calculated Simulated Ground resistance (Ohms) (ohms) Calculated Simulated 5 5 Grid size (metres squared) Soil resistivity (ohm-m) (Ohm-m) Fiure 4. The variation o the round resistance with respect to the area occupied the rid. Fiure 6. The variation o the round resistance with respect to the soil resistivity surroundin the rid. Copyriht SciRes.

4 596 A. HAMMUDA ET AL. roundin resistance when a two soil model is used, yet small decrease in R when a homoenous soil model is used is the inclusion o round rods bound to the rid. The two layer soil model is a more accurate model that more closely resembles the practical situation o the earth beneath the substation. This assumes that the soil is split into two layers, one above the other, each with its own resistivity value. This model is characterized by the relection actor K deined by K (4) where is the upper soil resistivity and is the lower soil resistivity [6]. The more the relective actor tends towards or the reater the respective dierence in resistivity between the two layers. A relective actor o denotes that the soil is uniorm as was the assumption or the previous analysis. I the lower soil resistivity is much lower than the upper soil resistivity the value o K will tend towards and vice versa. Fiure 7 shows the eect o addin 8 m rods to a rid o size 5 m when the relection actor is ( Ωm homoeneous soil) and.8 where the Ωm upper soil extends to a depth o 5 m beore the presence o the lower soil o resistivity Ωm. The mesh size in either case is 5 m and the rod diameter is cm. It can be seen rom Fiure 7 that rods are only reatly eective i they penetrate the lower resistivity soil layers, reducin R by a actor o.5 ollowin the inclusion o rods in the above situation. Thereore the easibility o their addition is determined by the studied soil model in concern and is constrained by their lenth Touch and Step Potentials aainst the Lenth o the Grid Conductor Increasin the number o meshes, or equally reducin the lenth o the rid conductors will siniicantly draw nearer the dierence o surace potential between any two points on the rid (Fiure 8). This has the eect o reducin the dierence between two points on the earth s surace, thus the step potential and will draw nearer the value o the surace potential to the GPR thus reduce the touch voltae as shown raphically in Fiure 8. The rid occupies an area o 36 m and resides in the soil o resistivity Ωm at a depth o.5 m. The meshes are square thus 6 m is the extreme mesh scenario showin the reatest touch and step voltaes as anticipated. Seeminly the eect o mesh size reduction on the touch voltae is reater. This is conirmed by the numerical results where it is ound that the drop in the touch voltae between the reatest mesh size and the Ground resistance (Ohms) (ohms) Number o rods in rid area K = K = -.8 Fiure 7. A raph showin the relationship between the roundin resistance aainst the number o round rods in two layer soil. Voltae (v (V) ) Conductor lenth (m) Touch Step Fiure 8. The touch and step voltaes aainst the conductor lenth (number o meshes). smallest is 8.5% while that o the step voltae is 57.% Surace Layer Incorporation aainst the Tolerable Touch (T-) and Step Voltaes (S-) The siniicant eect o addin a surace layer or the purpose o increasin the series resistance o the personnel hence raisin erable voltae levels [7] is not emphasised enouh. Fiure 9 shows the results obtained throuh incorporatin a Ωm and a 5 Ωm (typical or pea ravel) surace layer. The rid is 5 m with 5 m meshes and the soil is o Ωm resistivity. It can be seen that the incorporation o a Ω surace layer as thick as just 5 cm can improve the erable touch voltae siniicantly, precisely by 3.5% while increasin the erable step voltae by 374.9%. It is Copyriht SciRes.

5 A. HAMMUDA ET AL. 597 Tolerable voltae (V) Ohm ohm TTouch T- Ohm ohm TStep S- Ohm ohm TTouch T- Ohm ohm TStep S Resistive layer thickness (m) Fiure 9. The resistive layer thickness aainst erable voltaes. noted that surace layers o much hiher resistivity are widely available. This cost eective approach can considerably increase the saety o the system while avoidin excessive structural improvements. This technique was employed or improvin the Gaza roundin system ater reachin the saturation o structural improvements as will be shown in the succeedin section. 5. Modelin the Gaza Study 5.. Substation Field Study Followin consultation with the manaement o the Gaza Power Generatin Company (GPGC) [8] it was ound that the substation occupies an area o 467 m (4 m 9 m). The substation is approximately 7.5 km rom the Mediterranean shoreline and built above a reion o dry sandstone [9] almost 7 m above sea level []. Dry sandstone can resemble a resistivity as hih as Ωm. It is suested that the earth at depths below 7 m will be both rich in moisture and will possess a hih salt content due to sea water intrusion. Accordin to the curves shown in [6] this can reduce the resistivity up to old due to the electrolytic nature o water and salt thus the resistiveity or the second sandstone layer below a 7 m depth is modelled at Ωm. ratin o MVA. They develop a voltae o kv and are connected to separate transormers. The transormers step up the eneratin voltae at the substation to 66 kv and are both rated at 3 MVA. The reactance o enerators and are arbitrarily assumed to be.7 Ω. The enerators 3 and 4 can potentially produce a power output o 59.3 MW, are rated at MVA and develop a voltae o kv. Generators 3 and 4 are arbitrarily assumed to have a reactance o.5 Ω. They are connected to a bus coupled with two parallel connected transormers rated at 6.5 MVA each steppin up the voltae to 66 kv. It is assumed that a ault occurs at the transmission bus coupled with the aorementioned transormers and that the ault is o three phase to round nature as shown in the PSCAD model o Fiure Simulatin the Three Phase Fault The phases are aulted ater.5 s o normal operation assuminly lastin a period o.5 s. The resultant waveorm is shown in Fiure, attainin a peak o 5.46 ka beore settlin at a manitude o 6.5 ka. Closer evaluation o the resultant waveorm yielded a DC component attenuation time constant o s. This time constant T a and the ault period t can be placed into the ormulation or the decrement actor D [7] and multiplied by the symmetrical current I to obtain the symmetrical ault current equivalent over the.5 s period where D t T a Ta (5) t yieldin a decrement actor o.6. The symmetrical ault current I F, over the initial period o.5 s, has a value o 5.. Generatin Station Model or Fault Determination Further consultation with the manaement o the (GPGC) [8], it was ound that the power station constitutes 4 major enerators. Generators and can potentially produce a power output o 3.47 MW and have a MVA ABC > G Fiure. Fiure 8 PSCAD model o Gaza power station with a connected 3 phase to round ault loic component. Copyriht SciRes.

6 598 A. HAMMUDA ET AL. Fiure. Three phase ault current waveorm produced by Gaza power station. 6.5 ka.6 = 9.68 ka. Finally, and assumin the current that traverses the rid is 8% o the ault, uided by the curves deduced by Garett et al. [] and allowin or the worst case scenario, the split actor, S.8 and the rid current I 6.5 ka.6.8. G 5.4. Initial Desin and Tolerable Voltaes The conductor lenth is commonly in the reion o 5 - m dependin on the area o the substation. I the conductor lenth is 8.5 m or a square mesh, 4 and 4 meshes can be placed in the x and y direction respectively. The rid contains 7 copper conductors and resides in a soil o resistivity Ωm. The erable touch V and erable step T@ VS@ voltaes or this particular situation are 554. V and 555. V respectively or an averae 7 k individual. The objective o the system is to develop voltaes below these limits Initial Simulation Followin a 7.7 ka current injection, the resultant touch and step voltaes reatly exceed the permitted limits at V and 39.8 V correspondinly. ETAP valued R at 3.6 Ω. The imperative task is thus to reduce the touch voltae, and on reulatin this, it is expected that the step voltae will also adhere to saety Improvement o Desin by Rod Incorporation As aorementioned, a two layer soil structure with a neative K actor can be harnessed to the advantae o the enineer by incorporatin resistance reducin rods. Fiure shows this eect and demonstrates how the inclusion o 8 m penetratin rods has caused the round resistance to all to a value well below.5 Ω a very satisactory decrease o 83.7%. Addin more than 5 rods is uneasible in terms o reducin the roundin resistance. The curve reaches almost a horizontal radient and any urther addition o Ground resistance (Ohms) Number o rods in rid area Fiure. Addin resistance reducin rods to the initial desin. rods will only aect the potential radient rather than the roundin resistance and the overall GPR o the system. The new touch voltae ater re-simulation has allen 35.% to 79 V while the step voltae has dropped 8.4% to 893. V. More enhancements are required to meet the erable criteria Improvement o Desin by Mesh Size Reduction Revisitin the previous section, it is seen than a reduction in mesh size promotes a respectable reduction in the touch voltae. Decreasin the mesh size to 5. m 5. m resultin in 39 and 3 meshes in the x and y direction correspondinly, the system can be re-simulated to yield a urther drop in the touch voltae o 3.% (5 V). It is interestin to note that the new step voltae is.% reater at 5. V. The increase in the step voltae is an exceptional case and has occurred due to the act that the rid potential has been raised with respect to the area at the immediate vicinity o the rid. It is suested that this exceptional case occurs when the roundin resistance reaches a saturated low value that no loner alls siniicantly when physical structural enhancements are made to the rid. The result o this is that the GPR essentially remains at its previous value. The decrease in touch voltae then occurs due to raised surace potentials Incorporation o a Hih Resistivity Surace Layer It is orecasted that urther physical improvement to the metallic roundin structure is uneconomical, thus the system s solid structure is ready to incorporate a layer o hih resistive surace material to raise erable voltaes. Addin a surace layer o pea ravel o depth.5 m and resistivity 5 Ωm accordin to [7] and as valued by ETAP produces new results showin ull conormance to saety or both the touch and step voltaes. This adjustment shows a remarkable increase in the Copyriht SciRes.

7 A. HAMMUDA ET AL. 599 touch and step erable voltaes, 84.6% and 63.7% correspondinly. The results are summarised in Table. 6. Conclusions A roundin system or the transmission substation in Gaza City, Palestine has been desined and simulated and is believed to saely dissipate the larest possible ault current at the plant. The inal desin shows that the resultant touch and step voltaes are within erable reions and no more enhancements are necessary. It was ound that in the study, the incorporation o round rods lon enouh to penetrate the moist soil layers deemed reachable at depths beyond 7 m decreases the overall roundin resistance by above 8%. The touch and step voltaes are reduced by 35% and 8% correspondinly. Further structural improvements or the purpose o reducin the touch voltae included the reduction o the mesh size beore the incorporation o a pea ravel surace layer o depth.5 m and o resistivity 5 Ωm. This adjustment remarkably increased the touch and step erable voltaes to 84.6% and 63.7% respectively and ormed the adequate roundin system or the Gaza study. It also establishes the importance o includin a hih resistivity surace layer i it is ound that the touch and step voltaes are beyond acceptable while urther structural improvements in the rid are uneasible. In this particular case its inclusion is indispensable. The perormed parametric analysis and research establishes that the most eectual rid improvement actor is its area bein inversely proportional to the roundin resistance. This is ollowed by the soil resistivity bein directly proportional. Other desin modiications are useul in obtainin speciic results. Rods are only eective in two layer soils o neative K coeicient when they are lon enouh to penetrate the lower soil. Reducin the mesh size is an admirable touch voltae reducin actor and when accompanied by a reduction in the overall round resistance assists in reducin the step voltae. 7. Acknowledements Dr Raiq Maliha, the plant manaer at the main power station in Gaza, Palestine is acknowleded or supplyin useul inormation and or his correspondence in the ormation o the research. 8. Reerences [] IEEE, IEEE Recommended Practice or Determinin the Electric Power Station Ground Potential Rise and Induced Voltae From a Power Fault, IEEE Std , 996, pp. -5. [] J. Ma and F. P. Dawalibi, Modern Computational Methods or the Desin and Analysis o Power System Groundin, Proceedins o International Conerence on Power System Technoloy, Beijin, Vol., 8- Auust 998, pp. -6. [3] S. Ghoneim, H. Hirsch, A. Elmorshey and R. Amer, Surace Potential Calculation or Groundin Grids, IEEE Power and Enery Conerence, Jaya, 8-9 November 6, pp doi:.9/pecon [4] IEEE, IEEE Recommended Practice or Industrial and Commercial Power System Analysis, IEEE Std , 997, pp [5] J. G. Sverak, Simpliied Analysis o Electrical Gradients above a Ground Grid, Part I: How Good Is the Present IEEE Method? IEEE Power Enineerin Review, Vol. PER-4, No., 984, pp doi:.9/mper [6] F. Dawalibi and D. Mukhedkar, Parametric Analysis o Groundin Grids, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-98, No. 5, 979, pp doi:.9/tpas [7] IEEE, IEEE Guide or Saety in AC Substation Groundin, IEEE Std 8-,, pp. -9. [8] R. Maliha, Gaza Power Station Manaement, (rmaliha@pc.ps). Part o Gaza power station roundin project at UWE,. (ahmedhammuda@hotmail.com) [9] H. Baalousha, Analysis o Nitrate Occurrence and Distribution in Groundwater in the Gaza Strip Usin Major Ion Chemistry, Global NEST Journal, Vol., No. 3, 8, pp Table. A summary o the results produced in the iterative rid enhancement process. [] Goole Earth, Gaza Aerial View,. Desin R (Ω) V (V) V (V) V (V) V (V) T T@ S S@ Stae [] D. L. Garrett, J. G. Myers and S. G. Patel, Determination o Maximum Substation Groundin System Fault Current Usin Graphical Analysis, IEEE Power Enineerin Review, Vol. PER-7, No. 7, 987, pp doi:.9/mper Copyriht SciRes.