IEEE Electrification Magazine - December 2015 - 39

State Estimation
During GMD Events

transmission assets. Currently, the
records are composed of the following major real-time sources: GIC
monitors installed at critical locations
in the system, transformer harmonic
monitors, transformer DGA monitors, and transformer temperature
monitors. GIC monitors provide the
dc current magnitude at transformer
neutral grounds, while transformer
harmonics, DGA, and temperature
monitors provide supplementary
information that helps to indicate the
saturation status, potential overheating conditions, and transformer loss estimation that assists
in online health assessment of the transformers. All information is available on DVP's internal online portal and via
EMS data queries.
DVP has developed its own GMD visualization tool to
provide real-time information of GIC flows at key locations in the system (Figure 4). With this tool, SOs are able
to correlate weather forecast information with the realtime GIC measurements when making decisions on an
appropriate response. All historical event data are
promptly archived and used for postevent analysis and
methodical studies (Figure 5).

The saturation
condition causes
an increase in var
consumption by the
transformer and the
generation of even
and odd harmonics.

During GMD events, the reactive
power consumed by power transformers increases significantly. Consequently, the system may exhibit
abnormal voltages, and the risk of a
voltage collapse increases. Such
factors can affect the performance
of the online state-estimator module in the energy management
system (EMS) that assists with
power-system control. Impacts
may include equality constraints
at zero-injection buses, low voltages at regulated buses
where voltage and schedule are used as a pseudomeasurement, and statistical outlier detection methods.
At DVP, the performance of the state estimator is
assessed using the prevailing conditions during GMD
events. An analysis has been made on the computational
results of voltage magnitudes and var consumptions of
transformers under various storm severities. The findings are used to fine-tune the SOCs' EMS state-estimator
module and also to train SOs. Therefore, SOs gain the
knowledge necessary to be aware of any potential problems (for instance, low-voltage problems and stateestimator biasing).

equipment enhancement
Real-Time GIC Monitoring
DVP has long recognized the importance of establishing a
comprehensive observation library of GIC records for its

Solar-induced geomagnetic storms may have serious
impacts on transmission and distribution equipment, as
well as on protective relay systems. Among all substation
equipment, transformers and capacitor banks are likely to
experience the greatest impact and, hence, may cause the
most severe consequences to the power grid.

Transformers

Transformers with grounded-wye connections provide conducting paths for GIC to flow into transformer windings,

15
10
5
0

ac + dc

Saturation

dc
Offset

-5
-10
21

Mid

Time (h)

Time
ac + dc

Substation1_Solar.TX8_GIC.amps
Substation2_Solar.TX9_GIC.amps
Substation3_Solar.TX3A_GIC.amps
Substation4_Solar.TX7_GIC.amps

Figure 5. DVP's GIC monitor readings during a K7 storm that occurred
on 2 October 2013. The increase of neutral dc was captured by four
monitors at different locations in the DVP system. (Source: DVP.)

Only ac
Time

Figure 6. Half-cycle saturation of power transformers due to GIC.
Effects of GIC on power transformers and power systems. (Source:
Girgis et al., 2013.)

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

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