The magnitude of zero-phase sequencecharging
current is determined by the line-toground
capacitance associated with system
components. The value of the current must be
known to properly coordinate the Post Glover
High-Resistance Grounding System.
In an industrial power system where the design
and components are known, the charging
current can be estimated with reasonable
accuracy. With a complex array of machines and
cables this may be tedious and yield less-thanaccurate
results. The discussion that follows
outlines a suitable test for determining the value
of current in a system.
A transformer’s capacitance-to-ground is usually
negligible because of the large spacing between
the transformer’s core and winding, and the
shielding effect of the winding layer adjacent to
the core. Shielding prevents the other winding
layers from significantly increasing the windingto-
ground capacitance. Cable and overhead line
capacitance, on the other hand, can be
significant if the circuit is large.
Major contributions to the overall system
capacitance-to-ground are made by rotating
machines. Attributes such as the type of
insulation and the number and depth of slots
can produce wide variations. A typical value for
a 200-hp, 2300-volt, 1800-rpm induction motor
might be 0.002 microfarads phase-to-ground
capacitance.
The most accurate way to determine the
maximum value of the charging current is by
test, since extreme variations can exist. The
charging current per phase is represented by Ic
while 3Ic corresponds to the total line-to-ground
charging current. To obtain the zero-phase
sequence charging current, a one-phase
conductor is intentionally grounded (Figure 7).
First, an estimate of the total charging current is
required for sizing of the test resistor (Table A).
This estimate does not include the contribution
from surge-protective capacitors.
The test resistor value is found by the following:
R, = 2*(Line-to-Neutral Voltage)
Estimated 3Ic
For example, if 3Ic is assumed to be 3A and
VL-n is 1390V, then the value of the test resistor
Rt will be about 926 ohms.
This example yields an Rt of about 900 ohms.
The resistor should be variable and connected
from one of the phases to ground. The
resistance is used to bring one conductor to
ground potential by progressively decreasing
the resistance to zero. The resistor is sized to
have a short-time current rating of about one
minute and not less than the estimated total
system-charging current 3Ic.
By connecting a portable ammeter in the path
between the grounded conductor and earth
ground, Ic can be read. Conducting the test
under varying load conditions throughout the
day allows more accurate measurement of the
maximum value.
