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IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 40, NO. 2, MARCH/APRIL 2004

MV Generator Low-Resistance Grounding and Stator
Ground Fault Damage
Alex Wu, Senior Member, IEEE, Yousin Tang, Senior Member, IEEE, and Dale Finney, Member, IEEE

Abstract—Most of the in-plant medium-voltage generators
are grounded through resistors ranging from 200 to 400 A. In
some unusual situations,the resistors may even be as high as
1200 A. Low-resistance grounding is a preferred choice for a
medium-voltage power distribution system. However, extensive
generator stator ground fault damages had been reported due to
the prolonging generator neutral ground current, which would
continue to flow even after the main and field breaker opened.
This ground fault current could continue to flowfor as long as 5
. As
s depending on the generator open-circuit time constant
a result, the higher the generator neutral current, would lead to
the higher stator ground fault damages. An IEEE Working Group
has recently completed its study and made recommendations
for medium-voltage generator grounding in a multiple source
industrial environment. Based on the considerations of transientovervoltage, ground fault damages, and ground fault protection, the working group suggests a few variations of grounding
systems, which basically are low-resistance grounding systems
during normal operating conditions, and the generator would be
switched to a high-resistance grounded system from a normal
low-resistance grounded system when a ground fault occurs in the
generator stator. Thepurposes of this paper are: 1) to examine
the generator transient over-voltage and currents under the
low-resistance ground fault conditions, and also to evaluate their
corresponding stator ground fault damages, and 2) to establish an
acceptable maximum system ground fault level. For comparison
purposes, three versions of low-resistance grounding systems
have been considered and they are: alow-resistance grounding
system with a neutral breaker at the generator (hereinafter
called a “neutral breaker system”); a low-resistance grounding
system with the generator neutral low resistor being switched to
a high-resistor after a stator ground fault (hereinafter called a
“hybrid system”); and a low-resistance grounding system similar
to the current practice (hereinafter called a“traditional system”).
The simulation study is conducted with the aid of the Electro
Magnetic Transient Program. An experimental analog generator
model is also used to verify the simulation results.
Index Terms—Neutral breaker grounding system and hybrid
grounding system, stator ground fault damage, transient overvoltage.

I. INTRODUCTION

R

EFERENCE [2] presents seven generator-groundingmethods. There are: distribution-transformer grounded;
high-resistance grounded; low-resistance grounded; reactor

Paper PID 03–31, presented at the 2003 IEEE Pulp and Paper Industry Conference, Charleston, SC, June 15–20, and approved for publication in the IEEE
TRANSACTIONS ON INDUSTRY APPLICATIONS by the Pulp and Paper Industry
Committee of the IEEE Industry Applications Society. Manuscriptsubmitted
for review June 20, 2003 and released for publication December 8, 2003.
A. Wu and Y. Tang are with Global Engineering Company, Roswell, GA
30076 USA (e-mail: globaleng@earthlink.net).
D. Finney is with GE Power Management, Markham, ON L6E 1B3, Canada.
Digital Object Identifier 10.1109/TIA.2004.824499

Fig. 1.

System configuration for study Cases 1, 1A, and 3.

grounded;ground-fault neutralizer grounded; grounding-transformer grounded; and ungrounded. Due to the concern of
transient overvoltage under ground fault condition, ungrounded
system is generally not recommended for the medium-voltage
generators. The statistical data of the transient overvoltage for
medium-voltage grounding systems are scarce. However, in
1978, Brown, et al. [3], based on their research...