This document analyzes grounding considerations for large kVA pad-mount transformers. It summarizes the assumptions made in analyzing different transformer voltages and kVA sizes up to 5,000 kVA. Calculations of ground potential rise, touch potential and step potential are performed and compared to safety limits. Results show the standard two ground rod system may not provide adequate protection for transformers over 750 kVA or higher secondary voltages. Larger or engineered grounding systems are recommended for safety.

1. Grounding Considerations for Large kVA
Pad-Mount Transformers
Ruwan Weeransundara P.E.
Member, IEEE
ESC Engineering Senior Planning Engineer
3540 JFK Parkway, Fort Collins, Colorado 80525, USA
Phone 970-212-1525
ruwanw@thinkesc.com
Abstract - Utilities in the United States and Canada have done
extensive analysis of the ground grid design for substations in order to
limit step and touch potentials to safe values; however, few have
analyzed the potential hazard and designed grounding systems for
pad-mount transformers. Since the kVA size and voltage ratings of
pad-mounts have increased and the exterior of these facilities are
accessible not only to qualified electric utility workers but also the
general public, there is a need to analyze and properly design and
construct grounding systems which will render these facilities safe for
switching and fault conditions. This paper analyzes and develops the
design for several different pad-mount transformer voltages and kVA
sizes through 5,000kVA.
I. INTRODUCTION
The primary purpose of grounding is to limit voltages across
insulated or non-insulated portions of equipment within a person’s
reaching capabilities and the potential differences between different
points on the ground within one step. These voltages are known as
“touch” and “step potentials,” respectively. The evaluation of these
voltages is required not only during normal operating conditions, but
also during abnormal situations such as lightning strikes, faults and
switching surges.
A large Ground Potential Rise (GPR), with respect to remote earth
potential, can occur during abnormal conditions if the fault current (I)
and ground (Rg) are high (GPR = I x Rg). Voltage potential rises
during fault conditions can be significant even when established
grounding practices are used.
Pad-mount transformers are being used in ever increasing kVA
sizes and with higher primary and secondary voltages. Pad-mount
transformers are now available in sizes up through 5,000kVA.
Pad-mount transformers are frequently used in lieu of smaller kVA
size traditional substations when the primary voltage is 34.5kV and
lower. Secondary voltages up to 24.9/14.4kV are available. These
trends necessitate a review of established grounding practices for
pad-mount transformer installations to ensure that the grounding
system will limit step voltage and touch potential to safe levels.
Limiting the maximum step potential is especially important for these
transformer installations because they are often placed in locations
where the general public can be exposed to danger.
II. PAD-MOUNT TRANSFORMER CONNECTIONS
The large majority of three-phase pad-mount transformers are
connected grounded-wye on the secondary. Secondary voltages
include 208/120 volts, 480/277 volts, and common distribution
primary voltages (4.16/2.4, 12.47/7.2, 13.2/7.6, 24.9/14.4 kV). Delta
connected primary connections are not common due to the
susceptibility of this connection to ferro-resonance over voltages.
Although not common for utility application, grounded-wye primary
with impedance grounded secondary are used for industrial
applications to limit phase-to-ground fault currents. The primary Ho
and secondary Xo are separated when the secondary Xo bushing is
grounded through impedance.
III. ASSUMPTIONS
The following assumptions were used to determine the adequacy of
various grounding systems for different kVA size and voltage rated
pad-mount transformers. These assumptions are conservative for
most conditions but should be checked for particular installations to
insure they are applicable and provide safe results.
Pad-mount transformers rated 750kVA and larger were
assumed to have 5.75 percent impedance.
Top soil resistivity of 2,000 m was used. This is
typical for sand, gravel and dry soil.
Lower soil resistivity of 1,000 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 seconds
The maximum secondary line-to-ground fault currents for different
kVA size and voltage transformers were assumed to have nearly an
infinite primary source:
5,000kVA with 12.47/7.2kV secondary, available fault current
4,000 amps.
2,500kVA with 12.47/7.2kV secondary, available fault current
2,000 amps.
5,000kVA with 4.16/2.4kV secondary, available fault current
12,000 amps.
2,500kVA with 4.16/2.4kV secondary, available fault current
6,000 amps.
The current distribution factor equals 1.0 (i.e., all fault current goes
into the grid and none returns through the source distribution circuit).
The touch and step potential values calculated from the equations
specified in IEEE Guide for Safety in AC Substations Grounding
Std. 80-2000 are the maximum values which should be allowed for
pad-mount transformer installations.
IEEE Std 80 provides equations for the allowable touch and step
potentials for both 50 and 70 kilograms persons. The weight of 50 kg
was used because the potential limits for the lighter person are more
restrictive.
Two types of ground grid were initially investigated: the two
ground rod schemes required by Rural Utilities Services (RUS) for all
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2. three-phase underground pad-mounted equipment applications in their
unit UM 48-2 and the Canadian Standards Association Rule 36-302 in
the Electrical Safety Authority, which requires a ground rod on each
of the four corners around the pad-mount transformer. These
grounding schemes are shown in Figures 2 and 3, respectively.
In addition, a suggested sample design (Figure 4) for a 5000kVA,
34.5/4.16kV pad-mount transformer is included. The potential
contour for the grounding arrangement is shown in Graph 4.
IV. CALCULATIONS
The ground potential rise (GPR), maximum allowable touch
( and step ( potential calculations were performed using
the equations specified in IEEE Std 80-2000. The grounding analysis
was performed for different kVA sizes and voltage ratings of pad-
mount transformers with both the RUS and Canadian Standards. The
EDSA Advanced Ground Mat Program V4.70.00 was used to perform
sample calculations on pad-mount transformers, ranging from
500kVA to 5000kVA. These calculations were compared with the
maximum allowable values calculated from the equations specified in
IEEE Standard 80-2000.
Step and Touch Voltage Criteria
The safety of a person depends on preventing the critical amount of
shock energy from being absorbed before the fault is cleared and the
system is de-energized. The maximum driving voltage of any
accidental circuit should not exceed the limits defined as follows.
The limit for Step Voltage is: 1
(1)
For body weight of 50kg:
1 (2)
Similarly, the touch voltage limit is:
(3)
For body weight of 50kg:
(4)
(5)
Where:
Estep-The step voltage in volts
Etouch-The touch voltage in volts
Cs -The surface layer de-rating factor
S- The surface material resistivity in m
- The resistivity of the earth beneath the surface material in m
ts -The duration of shock current in seconds
hs -The thickness of the surface material
The following values were used for grounding analysis:
Cs=1.0 (No earth beneath surface material), = S = 2000 m
ts = 0.5 sec, hs=0-.5 ft.
From (2) & (4)
= 2312.6V (6)
= 656.2V (7)
1
IEEE – Guide For Safety in AC Substation Grounding, IEEE – Std. 80-2000, pp- 23-37
The actual step and touch voltages, metal to touch and step to step
voltages, should be less than the respective maximum allowable
voltage limits to ensure safety.
It should be noted that the above calculations assume 0.5 seconds
fault duration. With a square root of ts in the denominator of equations
(6) and (7) above, the allowable step and touch voltages will both be
decreased with an increase in fault clearing time. Increasing the fault
duration time from 0.5 seconds to 2.0 seconds will reduce the above
values by one-half.
Although not common in the electric utility industry, some
industrial electrical users separate the primary H0 and secondary X0
connection points to obtain an impedance grounded secondary
system. When this is done, the maximum voltages during fault
conditions will not be limited by the secondary windings and the
touch potentials as well as the step potentials will exceed those which
would occur for a grounded wye to grounded wye connection with the
H0 and X0 points bonded.
Figure 1 Human Tolerances Model
Figure 2 RUS Std UM48-2, 3-Phase Pad-Mounted Grounding
Figure 3 Canadian Standard for Pad-Mounted Grounding
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3. Graph 1 Potential 3D graph for 5000kVA, 34.5/4.16kV RUS Std
Graph 2 Absolute potential along an axis – 34.5/4.16kV RUS Std
Graph 3 Potential 3D Graph for 5000kVA, 34.5/4.16kV Canadian
Standard
Figure 4 Suggested grounding arrangement for 5000kVA 34.5/4.16kV
pad-mount transformer
Graph 4 Potential 3D graph for 5000kVA, 34.5/4.16kV with suggested
grounding arrangement
V. RESULTS
Table 1 shows the comparison of maximum surface voltage on both
RUS Std and Canadian Standard for different sizes and voltages of pad mount
transformers. The maximum allowable touch and step voltages are 656.2V and
2312.6V respectively.
Table 1
Transformer
Secondary
Voltage (kV)
Size
(KVA)
Maximum
Line-Ground
Fault
Current (A)
Maximum
Surface
Potential
(V)
Meets the
Safety
Requirement
Maximum
Surface
Potential
(V)
Meets the
Safety
Requirement
12.47 5000 4000 1655 707
12.47 2500 2000 875 338
12.47 1500 1250 601 205
4.16 5000 12000 4551 1491
4.16 2500 6000 1848 911
4.16 1500 3500 1454 515
4.16 1000 2250 933 404
4.16 750 2000 875 338
4.16 500 1000
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4. VI. CONCLUSIONS AND RECOMMENDATIONS
With 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 size pad-mount transformers with 4.16/2.4kV
and higher secondary voltages. The Canadian Electrical Safety Code
which requires four grounding rods, one on each corner, provides
better results but still fails for some larger kVA sizes and higher
secondary voltages. A detailed engineered grounding design should
be prepared by a qualified person for pad-mount transformer
installations exceeding 750kVA when the secondary voltage is
4.16/2.4kV or higher or if the fault clearing time exceeds 0.5 seconds.
Since the allowable step and touch potentials decrease with an
increase in the shock current duration time, it is important to consider
the maximum fault clearing time when designing ground systems for
pad-mount transformers. A detailed grounding design requires
knowledge or measurement of the soil resistance at the pad-mount
transformer location plus information regarding any possible
deviation from the assumptions used in this paper. In addition, a
detailed grounding design should be done for all grounded wye to
impedance grounded secondary when the H0 and X0 are not connected
to provide an impedance grounded secondary.
VII. REFERENCES
[1] IEEE for Safety in AC Substation Grounding IEEE
Standard 80-2000.
[2] Canadian Electrical Safety Code, 2009, by Electrical Safety
Authority.
[3] RUS Standard 1728F-806, June 2000, Specifications and
Drawings for Underground Electric Distribution.
[4] IEEE Recommended Practice for Grounding of Industrial &
Commercial Power Systems, ANSI/IEEE Std 142-1982
VIII. BIOGRAPHY
Ruwan Weeransundara received a BSEE from the University of
Peradeniya Sri Lanka in 1987 and a MSEE from the University of
Windsor, Ontario Canada in 2004. From 1987 to 2000, he worked as a
Power System Engineer for Ceylon Electricity Board in Sri Lanka
with an emphasis on Power Distribution, Protection of Transmission
and Generating Stations and Substation Design. In 2000, he joined
Siemens Canada as a Field Service Engineer. In 2004, he joined
Schneider Electric Canada as Senior Power System Engineer and in
2009, he joined ESC engineering, Inc. as a Senior Planning Engineer.
He is a member of IEEE and a Registered Professional Engineer in
the Province of Ontario Canada.
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