Section 7:
GROUNDING OF DISTRIBUTION SYSTEMS
7.1
General Requirements. Distribution systems using equipment with
low impulse strength (for example, underground cables, dry-type transformers,
and rotating equipment) shall be effectively grounded to avoid surge
voltages. Each individual section of a circuit shall be grounded only once,
at the source side. This action helps to control ground fault currents and
to facilitate ground fault relaying. Detailed discussions of distribution
system grounding are covered in ANSI/IEEE Standard 142-1982: IEEE
Recommended Practice for Grounding Industrial and Commercial Power Systems,
and ANSI/IEEE Standard 141-1986: IEEE Recommended Practice for Electrical
Power Distribution for Industrial Plants.
7.2
Methods. The type and characteristics of a grounding device shall
limit the maximum line-to-ground fault current to the value of the three-
phase fault. Select a ground connection from the types given in paras. 7.2.1
through 7.2.3.
7.2.1
Solid Grounding. The solid grounding system shall be used for
automatic clearing of ground faults. Use only on secondary systems or where
impedance of transformers is included in the zero sequence current path.
This connection shall be avoided for grounding of generators where the zero
sequence or ground fault current at the generator terminals will exceed the
three-phase fault current for which the generator is braced. For additional
guidance on recommended practices and restrictions in applying this system
grounding connection, refer to ANSI/IEEE Standard 141-1986.
7.2.2
Resistance Grounding.
The resistance grounding system is
applicable for:
a)
automatic clearance of ground faults,
b)
limiting ground fault currents to safe values, and
c)
reducing mechanical stresses caused by ground fault currents.
This type of connection may be used on wye-connected generators and
transformers. For additional guidance on recommended practices and
restrictions in applying this system grounding connection, refer to ANSI/IEEE
Standard 141-1986.
7.2.3
Reactance Grounding.
The reactance grounding system is applicable
to:
a)
grounding generator systems and
b)
automatic clearing of ground faults.
When considering this type of grounding, remember that the available
ground-fault current should have a value of at least 25 percent of the
three-phase fault current in order to limit the transient overvoltages during
the clearing of ground faults. For additional guidance on recommended
practices and restrictions in applying this system grounding connection,
refer to ANSI/IEEE Standard 141-1986.
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