IEEE Electrification Magazine - December 2015 - 41

alleviated concerns about adjacent structural members
becoming overheated during GIC events, which may cause
thermal degradation of adjacent insulation pieces.
In addition, new transformer designs have been
modeled and analyzed to confirm their effectiveness in
handling the specified GIC signature (Figure 7). The study was originally conducted using uniformly
interpolated GIC values and was
later amended with the maximum
GIC level under a 100-year GIC scenario derived by Dr. Emanuel Bernabeu from DVP. The analysis mainly focused on temperature rise and
loss density in metallic structures
and coils, generation of harmonics,
and var absorption.
The analysis revealed some patterns of how magnetic fields interact with quasi-dc GIC internally in
the transformer, much of which proves the methodical
description of a GMD event. In DVP, stability and reliability
are always the priority of system operations. Although all
analyses have verified the effectiveness of the GIC hardening in present transformer designs, DVP recognizes the
seriousness of a GMD event that could contribute to grid
instability and makes adequate preparations to withstand a potential 100-year scenario. Other reliability factors, such as the intensive growth of load consumption,
the trend of higher renewable-generation penetration,
and flexible ac transmission system deployments, are
also considered in new designs.
To deeply understand GIC phenomenon and test
how the new designs enable transformers to withstand
GMD, DVP has participated in a series of factory tests
with two major manufacturers (one with shell-form
and one with core-form technology). Though dc injection was limited to the capability of the factory's test
equipment, many enlightening findings were obtained.
One key finding is that the extra temperature rise of the
core due to dc current is caused entirely by the extra
no-load loss because of the higher induction and partial
saturation of the core at the middle of the wound legs
(for a single-phase transformer) during half a period.
Another key finding is that the increase in sound level
of a transformer during a GIC event is significant. Even
with low dc currents (less than 1 A), the sound increases
tremendously (up to 25 dB).
Besides the factory tests, DVP is tentatively planning to
perform a GIC test in the field on some 500-kV-rated transmission transformers (Figure 8). Hopefully, this test will
allow DVP to inject up to 30 A dc. This level of dc will be significantly lower than the predicted maximum tolerable levels but should serve to validate the manufacturer's models
and assess the electromagnetic and thermal performance
of transformers with respect to GIC.

Capacitor Banks
The study of GMDs has indicated a significant increase of
odd and even orders of harmonics during a GMD event. In
the circuit, capacitor banks are low-impedance paths for
harmonics. During the March 1989 solar storm, 13 capacitor banks within the DVP service
territory tripped because of a defective protection scheme. The disturbance did not have a severe consequence in terms of grid stability
since the event occurred in the latenight hours and the protective
relays cut off influence from adjacent utility tie lines. The event has
drawn close attention to DVP as it
pointed out the vulnerability of the
system. This vulnerability could
increase when the system is under
higher stress. During extreme operating conditions, there is an increasing risk for the entire system to collapse, similar to what
happened at Hydro Quebec.
The original capacitor-bank protection scheme used in
DVP was a voltage-unbalance scheme that used current
differentials at the neutral ground point to determine the
failure of capacitor units. Though equipped with a parallel
capacitor to filter third harmonics, the electromechanical
relay was unable to distinguish the excessive harmonics
of other orders from the fundamental sequence flowing

The performance of
the state estimator
is assessed using
the prevailing
conditions during
GMD events.

TX1

+
UHV1
-
X
+
ULVI
-

UTV1

AC
1~ 500 kV/√3 kV 60Hz

H

TX2
IAC1

Igrid

IAC2

H
+
UHV2

+
X
+

UAC

-

ULV2
-
Y1a

Y1b
+

Y1b
+

-
L > 2 mH
H0X0
-
+
UL

-
H0X0

dc
+

-

UTV2

Idc

Figure 8. A circuit diagram of DVP's 500-kV single-phase transformer
GIC field test. (Source: DVP.)

	

IEEE Elec trific ation Magazine / d ec em be r 2 0 1 5

41



Table of Contents for the Digital Edition of IEEE Electrification Magazine - December 2015

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