IEEE Electrification Magazine - December 2015 - 42

Line

SC144M
1,200 A
800 AIC

MARK 5

Capacitor

3

200 A
0.75 mH

90-kV MO
(S)
73-kv MCOV

3
15-Unit Fuse

LV Capacitor
(167 kvar 825 V)
3

1,200/5

1

30/5

X1
59

Resistor
(100 Ω)
H2

H1

X2

H1
To
Neutral

H2
To
Neutral

X2
X1
Transformer
(800~250 V)

To Single-Point
Ground
(a)

(b)

Figure 9. (a) Old versus (b) new capacitor-bank protection schemes. (Source: DVP.)

and, therefore, malfunctioned. In the early 2000s, DVP
upgraded its capacitor-bank protection scheme (Figure 9).
The adapted version uses a voltage-differential scheme that
compares voltage differences between readings from two
taps of the bank, providing immunity
to harmonic interference. In addition,
the electromechanical relays were
replaced with digital relays to acquire
the ability to decompose harmonics
and dc offsets.
The other approach that has been
applied is to overrate the capacitor
banks. Harmonics present during
GMD conditions stress capacitor
banks and can create a full-load or
overload condition. This operating
condition greatly increases the
potential for capacitor unit degradation and thermal problems. A study
has been conducted on DVP's major capacitor banks. For
example, for Mt. Storm substation's generation step-up
unit, when running at 150-A root-mean-square phase current, a 100-A GIC injection will introduce more than 11%
total harmonic distortion into the system. To compensate

for this, DVP applies higher ratings for its major capacitor
banks with the guidance of IEEE 18 and 2036 Nominal Ratings, IEEE 1531 Guide for Specification and Application of Harmonic Filters, and IEEE C37.99 section 11.2 Capacitors Rated
for Higher Voltages . A 554.7-kV,
92.3-Mvar capacitor bank rated at
500 kV, 75 Mvar will increase the
overload current margin by 10.9%.
Transmission line series capacitors efficiently mitigate GIC impacts
on transformer neutrals since they
prohibit dc from flowing through
the line. DVP installed two series
capacitor banks at critical locations
on its backbone network to primarily increase system stability, but they
also help to increase the system's
capacity to handle GMDs.

The study of GMDs
has indicated a
significant increase
of odd and even
orders of harmonics
during a GMD event.

42

I E E E E l e c t r i f i cati o n M agaz ine / december 2015

Geoelectric Scenario Studies and modeling
The study of GMDs in power systems is a wide-open,
unprecedented task. Researchers are inspired to look into
complicated correlations, such as with solar activities and
the Earth's magnetic system, geospace electromagnetic



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