IEEE Electrification Magazine - December 2015 - 49

out to provide a linkage between
observed and estimated GIC, reactive power margins, and operating
instructions. In other words, if a predetermined GIC is flowing through
this transformer and it is confirmed
by GIC measurements in key stations,
then the appropriate preplanned control actions will take place.
By 2010, the new real-time GIC
solver was completed, and the commissioning of the full eXtreme Space
Weather (XSW) tool started. A webbased graphical user interface was
developed to manage and display the information generated by the solver, and version 1.0 was put into service in
2012 (see Figure 2). By the time version 1.0 was launched,
version 2.0 was essentially complete, and it will be in
service in early 2016.

The geomagnetic
field input data
are obtained through
a direct link
to NRCan's
geomagnetic
laboratory in Ottawa.

Overview of the XSW Application
The network model consists of the Hydro One 500and 230-kV transmission network. GIC network
equivalent networks are used at the interconnection
points. At any point in time, the modeled network
typically contains
xx
530 wye-grounded transformers
xx
800 500- and 230-kV circuits
xx
30 230-kV shunt capacitor banks.
The SCADA data (acquired at 60 s intervals) consist of
xx
measured and estimated power flow
xx
measured and estimated bus voltages
xx
currents from the GIC monitoring network
xx
ambient temperatures
xx
estimated top oil and winding temperatures due to
loading calculated with a separate dynamic transformer rating (DTR) application; calculated incremental
transformer hot-spot temperatures due to GIC are
added to the DTR values.
The geomagnetic-field input data are obtained
through a direct link to NRCan's geomagnetic laboratory
in Ottawa. The sampling rate is 1 s, and it is acquired in
60-s packets. The geoelectric field, GIC flows in every circuit and transformer, transformer hot-spot temperatures, reactive power absorption, and total harmonic distortion are calculated every second, and the XSW display
is updated every 15 s. Transformer construction (singlephase, three-limb core, five-limb core, and shell) are
taken into account in all calculations. Bus voltages from
either measurements or the state estimator are also
taken into consideration in all calculations of reactive
power absorption, harmonics, and hot-spot temperature.
The typical solution time is around 80 ms, and the datatransfer time is around 500 ms. The computational-cycle
time is comfortably under the geomagnetic-field sampling rate of 1 s.

Preplanned mitigation
measures

Control instructions are preplanned
using a large number of offline studies.
The control actions typically involve
switching transmission circuits to
redirect GIC flow. In most cases, taking
a transformer out of service because of
excessive hot-spot temperatures is not
the preferred option because it would
have the effect of redirecting GIC to
other transformers in the bus or station. It is also interesting to note that
GIC redirection can be counterintuitive
at times. For example, one mitigation action to reduce GIC
in a transformer station is to bypass series compensation
capacitor banks.

Transformer Hot-Spot Temperature
A key part of the development is the capability to estimate
transformer hot-spot heating due to GIC in real time using
thermal-step and asymptotic responses. The thermal
responses of a limited number of Hydro One transformers
were measured at the manufacturer's facilities before final
assembly. These measurements were then complemented
by manufacturer calculations as well as measurements
carried out by other utilities to arrive at a series of conservative thermal responses that take into account transformer construction.
With the thermal response of winding and metallicpart hot spots, it becomes relatively straightforward to calculate real-time hot-spot temperatures with real-time
measured or estimated GIC values. This resulting hot-spot
temperature is added to top oil and winding temperatures
estimated from the DTR and then compared to emergency
overloading limits suggested in IEEE Standard C57.91-2011 to
trigger preplanned control actions. The emergency overloading limits were adjusted to take into account the specific
condition of each transformer. Figure 3 shows a simulation
of the hot-spot temperature of one of the transformers in the
system, using GIC(t) calculated from geomagnetic-field
recordings of the March 1989 GMD event.

Some modest Innovation-recursive
convolution Techniques
The calculation of the induced geoelectric field from the
geomagnetic-field measurements uses the well-established plane-wave method. The geoelectric-field calculations take into consideration that there are five Earth
models corresponding to distinct geological/physiographic
regions in the Ontario service territory (see Figure 4). Each
Earth model has different layers with different thicknesses and conductivities (all the way to the Earth's mantle). In
general, lower conductivity results in higher induced geoelectric fields. Different layer thickness results in a different frequency-dependent behavior of the Earth impedance
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Table of Contents for the Digital Edition of IEEE Electrification Magazine - December 2015