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IEEE Rural Electric Power Conference 2011 Chattanooga, TennesseeGrounding Considerations for Large kVA Pad-Mount Transformers

Ruwan Weerasundara, P.EngMember IEEEESC Engineering, Senior Planning Engineer3540 JFK Parkway, Fort Collins, CO, 80525Ph: 970 212 1525ruwanw@thinkesc.com

Grounding Considerations for Large KVA Pad-Mount TransformersAbstract:Utilities in the United States and Canada have done extensive analysis of the ground grid design for substations in order to limit the safety parameters such as GPR, Step and Touch potentials to the acceptable safety limits.However, few have analyzed the potential hazard and designed grounding systems for Pad-Mount Transformers. This paper analyzes and develops the design for several different transformer voltages and kVA sizes through 5000kVA.Grounding TerminologyGround A conducting connection, whether intentional or accidental, by which an electric circuit or equipment is connected to the earth or to some conducting body of relatively large extent that serves in place of the earth.

Ground Potential Rise (GPR) The maximum electrical potential that a grounding grid may attain relative to a distant grounding point assumed to be at the potential of remote earth. This voltage, GPR, is equal to the maximum grid current times the grid resistance.

Grounding TerminologyGrounding Grid A system of horizontal ground electrodes that consists of a number of interconnected, bare conductors buried in the earth, providing a common ground for electrical devices or metallic structures, usually in one specific location.Touch VoltageThe potential difference between the ground potential rise (GPR) and the surface potential at the point where a person is standing while at the same time having a hand in contact with a grounded structure. [Also the potential difference between two different surfaces being touched with two hands.]

Grounding TerminologyStep Voltage The difference in surface potential experienced by a person bridging a distance of 1 m with the feet without contacting any grounded object.

Transferred Voltage A special case of the touch voltage where a voltage is transferred into or out of the substation from or to a remote point external to the substation site.

Grounding TerminologyRemote Earth A point on earth located at an effectively infinite distance from the location being analyzed. The remote earth potential is the reference voltage for ground potential rise and other voltages developed during fault conditions.

Reasons for GroundingPersonnel (human) safety by limitingTouch PotentialStep PotentialTransferred VoltageTolerable Current through the body

Improve Equipment Protection and Performance

Reduce Liability ExposureHuman TolerancesTolerable current through body 1 mA- threshold of sensation

6 mA- unpleasant cant let go current

25 mA- painful, hard to let-go of energized object, breathing difficult, death in minutes

100 mA- critical injury, ventricular fibrillation, heart stops, inability to breath, death in seconds

Human Tolerances

Human TolerancesTolerable current through body (calculation)

Dalziels equation- 99.5% of 50 kg (110 lbs) people can survive with a body current of:116 mA for 1 sec

164 mA for sec

367 mA for 0.10 sec

For a person weighing 110 lbs, the equation is IB = 0.116 / (TS)

Liability ConcernsSome concerns for liability for touch and step potential as contained in IEEE Std 80-2000 for substationsHowever, Pad-Mounts are commonly located where they have greater access by general publicPoints to consider1. Size(weight) of individuals2. Type of shoes if any3. Weather conditionsGrounding and Power QualityOver 50% of reported power quality problems are associated with improper grounding schemes.

Missing equipment grounds Missing or damaged connection between the ground conductor (neutral) and the grounding electrode at the service entrance. Multiple connections between the ground throughout the system. Improperly applied ground fault relays

Substation Grounding Design - IEEE 80In principle, a safe grounding design has the following two objectives:

To provide means to carry electric currents into the earth under normal and fault conditions without exceeding any operating and equipment limits or adversely affecting continuity of service.

To assure that a person in the vicinity of grounded facilities is not exposed to the danger of critical electric shock.

PAD MOUNT TRANSFORMERS(> 750KVA)Grounding of Large Pad Mount Transformers are equal or more important than Substation Grounding due to the greater potential exposure to the general public

Pad Mount Transformers are now available in sizes up through 5000kVA

The present standards on Pad-Mount grounding applications in both USA and Canada are not adequate in every situation especially for large kVA units the secondary voltage is 4.16kV/2.4kV or higherPAD MOUNTED GROUNDING STANDARDSRUS Std UM48-2, 3Phase Pad-Mounted Grounding

PAD MOUNTED GROUNDING STANDARDSCanadian Standard (Rule 36-302)for Pad Mounted Grounding

Sample CalculationsThe Ground Potential Rise (GPR), Maximum allowable touch (E touch) and Step (Estep) potential calculations were performed using IEEE Std 80 -2000.Potential contour and grounding analysis for Pad Mount transformers ranging from 500kVA to 5000kVA with 4.16/2.4kV and 12.47/7.2kVsecondaries were performed using EDSA Advanced Ground Mat ProgramThese Calculations were compared with the maximum allowable values calculated from the IEEE Std 80-2000 equations

Sample CalculationsAssumptions:Pad-mount transformers rated 750kVA and larger were assumed to have 5.75 percent impedance.Top soil resistivity of 2,000 Ohm-m was used. This is typical for sand, gravel and dry soil.Lower soil resistivity of 1,000 Ohm-m was used.The weight of the person is 50 kilograms (110 lbs.)Thickness of the surface material is 0.5 ft.Fault duration is 0.5 secondsCurrent Distribution Factor equals to 1Maximum Allowable Step and Touch VoltageThe maximum driving voltage of any accidental circuit should not exceed the limits defined as follows.

Estep = (RB+2Rf)Ib(1)Etouch = (RB+Rf/2)Ib(2)

Estep(50KG) = 2312.6VEtouch = 656.2V

RESULTS

Potential 3D graph for 5000kVA, 34.5/19.9kV to 4.16/2.4kV RUS StdResults

Potential 3D Graph for 5000kVA, 34.5/19.9kV to 4.16/2.4kV Canadian Standard

ResultsSuggested grounding arrangement for 5000kVA 34.5/4.16kV Pad Mount Grounding

Conclusions and recommendationsWith the RUS two ground rod standard, the maximum surface potential as well as the maximum 1.0 meter gradient potential can be exceeded for large kVA Pad Mount Transformers with voltages equal or greater than 4.16/2.4kVThe Canadian standard which requires four ground rods provides the better results but still fails for some large kVA transformersRecommendation:A detailed grounding design requires for large Pad Mounted Transformers on case by case basisReferencesIEEE Guide for safety in AC substation grounding (IEEE Std 80-2002)IEEE Recommended practice for grounding of industrial and commercial power systems (IEEE Std 142-1991)Transmission and distribution electrical engineering, Bayliss & HardyTransmission and distribution reference bookCanadian Standard (Rule 36-302)for Pad Mounted GroundingRUS Std UM48-2, 3Phase Pad-Mounted Grounding

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