IEEE Power & Energy Magazine - July/August 2019 - 41

Because TRV is a determining parameter for successful
current interruption, breakers are normally type-tested
in a laboratory to withstand a standardized TRV.
any transformer stray, circuit breaker grading, or voltage
transformer capacitance will have a great influence on containing the TRV within the breaker's rated envelope.
Figure 4(a) illustrates the high-frequency voltage oscillations, depicted in the blue trace, resulting from the natural
frequency of the network between the breaker and fault location. The high natural frequency from low shunt capacitance
causes the recovery voltage to exceed the breaker envelope, depicted in the orange trace. The envelope represents
the maximum allowable TRV the circuit breaker is able to
withstand. The circuit-breaker-withstanding envelope is set
according to standardized testing and characterized by a
first period of higher rate of rise and a second period of lower
rate of rise. Adding shunt capacitance of 100 nF reduces the
frequency oscillations drastically, as shown in Figure 4(b),
where the recovery voltage is within the breaker envelope.

The design conclusions and recommendations that result
from a grounding system study include the following:
✔ thermal design
* determination of the required cross sections for different grounding components (conductors, electrodes,
rods, and foundation electrodes)
* general conductor material, considering temperature,
mechanical stresses, and corrosion
✔ ground grid layout
* direction for a type of meshed earth grid
* recommended foundation ground electrodes
* recommended grounding separation or interconnection.

Insulation Coordination Study
Insulation coordination studies determine whether equipment insulation will be able to withstand potential transient

Grounding Study

july/august 2019

0
Voltage (kV)

-100
-200
-300
-400

TRV Voltage in kV
Breaker Envelope in kV

-500
-600

(a)
0
-100
Voltage (kV)

-200
-300
-400
-500

00
94

00
92

0.
00
9

00
88

00
86

00
84

00
82

-600

0.
00
8

High-voltage installations require grounding systems to protect human life against excessive touch voltages and keep
transferred potential to a minimum. To minimize potentially
dangerous step and touch voltages, substation grounding
systems need to have resistances as low as possible. Step and
touch voltages are two key quantities in grounding studies.
Step voltage is the difference in surface potential that could
be experienced by a person striding 1 m while not contacting any grounded object. Touch voltage is the potential difference between a ground grid's ground potential rise and a
surface potential where a person is standing with a hand in
contact with a grounded structure.
Synchronous condensers, like any new fault current source,
can adversely impact a substation's grounding system. IEEE
Standard 80-2013 (IEEE Guide for Safety in AC Substation Grounding) is the definitive standard for evaluating all
aspects of grounding systems. Key parameters in performing grounding evaluations include maximum earth fault currents and fault durations at various voltage levels.
Another important factor for an extended grounding system is soil resistivity. For soil at greater depths, special measurements are necessary to identify its structure.
All parts of the fault current return within the grounding system (i.e., transformer neutrals, earth wire, and cable
sheaths) must be considered, including the return fault
current considered for the design of the grounding system
of the high-voltage station. Hence, determination of the
resulting current flowing into the earth electrodes is an
important task.

Time (s)
(b)

figure 4. The TRV simulation results with different network capacitances. (a) The high-frequency voltage oscillations, depicted in the blue trace, resulting from the natural
frequency of the network between the breaker and fault
location. (b) Adding shunt capacitance of 100 nF reduces
the frequency oscillations drastically.
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IEEE Power & Energy Magazine - July/August 2019

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