IEEE Electrification Magazine - December 2015 - 31

When the blocking device is added
to an autotransformer, this primarily
only reduces the GIC flow in the common winding and has limited impact
on the series winding GIC flows; in
fact, the GIC flow in the series winding might actually increase. Other
potential risks associated with the
blocking device, such as negative
impact on the reliability of protection
systems, maintenance, and compliance risks of blocking devices operated as special protection systems, must be studied as well.

The GIC capability curves confirm
that transformers of this design can be
subjected to GIC levels of as much as
155 A/phase for a duration of 30 min
without the need for reducing their
load while limiting the rate of loss of
life of insulation to less than 1% and
also reducing the risk of forming gas
bubbles in the oil. This type of comprehensive evaluation is recommended
for individual designs of transformers
determined to be critical to the transmission system and
located in areas of high susceptibility to high levels of GIC.

Transformer Thermal Assessment

Gmd mitigation

In addition to the GMD system vulnerability assessment to
ensure continued reliable system operation, AEP Transmission has been working with transformer vendors to perform
transformer thermal assessment to quantify the impact of
different levels of GIC on selected large EHV power transformer designs. The evaluation included the following:
xx
the impact of GIC on additional var demand and magnitudes of current harmonics injected into the transmission system as a result of part-cycle core semisaturation-these evaluations provide needed information
to perform further system simulations and voltage stability analyses, and the results of these studies help
develop/refine GMD operating procedures with predetermined triggers for initiating mitigation actions
xx
the impact of GIC on the performance parameters of
the transformer and the resulting hot-spot temperatures of the windings and structural parts
xx
determining the GIC capability of the transformer
design.
The GIC capability of a transformer design is defined as
the percentage of rated transformer MVA that would be
allowed when the transformer is subjected to different levels
of GIC current. The temperature limits recommended by the
industry standards for long-term overloading of transformers are 140 °C for cellulose insulation and 160 °C for metallic
parts not in contact with cellulose insulation. Figure 11(a)
and (b) presents the GIC capability of this 765-kV singlephase transformer design based on the IEEE standard limits
on temperatures of the windings and tie-plate hot spot for
longer-duration GIC. The figure shows that, for this transformer design, no reduction of load would be needed for a
GIC level of up to 275-A/phase for 30 min continuously when
considering winding hot-spot limits and 155-A/phase considering the structural parts' hot-spot temperatures. Because
of the low levels of temperature increases in the windings of
this design, the load capability of the transformer would not
be limited by overheating of the windings. Figure 11(a) shows
that, beyond a certain level of GIC, the losses in the windings
are the dominant component, and, hence, the load applied
to the transformer does not significantly affect the temperature of the windings.

Based on the historical GMD impact observations, the GIC
system vulnerability assessments, the transformer thermal assessments, and the newly implemented GMD monitoring system, AEP has further developed/refined its GMD
mitigation measures with
xx
GMD operating procedure development/refinement
xx
GMD monitoring system expansion
xx
equipment hardening
xx
spare transformer program enhancement.

The GMD monitoring
system is a
substation-based
corporate-wide area
monitoring network.

GMD Operating Procedure
Two major risks from GMD events identified in the NERC
2012 Report "Effects of Geomagnetic Disturbance on the
Bulk Power System" are 1) the loss of reactive power support, which could lead to voltage instability and power system collapse, and 2) damage to bulk-power-system assets
typically associated with transformers. To address these
risks, NERC Standard EOP-010-1 requires that each transmission operator shall develop, maintain, and implement a
GMD operating procedure or operating process to mitigate
the effects of GMD events on the reliable operation of its
respective system. AEP has developed the GMD operating

With Blocking

120

Mvar Losses (%)

100
80
60
40

20% Blocking
33% Blocking
66% Blocking
100% Blocking

20
0

0

50

100 150 200 250
Storm Direction (°)

250

300

Figure 10. The percentage of Mvar losses after blocking against
storm direction.

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

31



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

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