INSTRUMENT TRANSFORMERSABB DESIGNS OPTIMIZEDINSTRUMENT TRANSFORMERSAND SENSORS WITH
MULTIPHYSICS SIMULATION

To protect and maintain power grids from system failure, ABB uses
multiphysics simulation to design instrument transformers that canwithstand very fast transient overvoltages and a split-core sensor forsubmersible underground vault applications.
By BRIANNE CHRISTOPHERA POWER OUTAGE CAN LAST a few hours,days, weeks — even months. Nomatter how long an outage lasts, it is afrustrating and disruptive experience.
A blackout can even be dangerous
when it occurs during extreme heator cold. To ensure power grid stability,
protection, and financial viability,
ABB uses multiphysics simulation
and applications to develop optimized
electrical designs, particularly
instrument transformers and sensors.

»ENSURING POWER GRIDPROTECTION WITH INSTRUMENT
TRANSFORMERSINSTRUMENT TRANSFORMERS (ITS)

are specialized, highly accuratetransformers that isolate, transform,or reduce high voltages and currents
in order to maximize safety andusability. Uses of ITs include metering,monitoring, protection, and control
of power systems. An IT is madeup of a primary winding, which isconnected to a high-voltage or highcurrent circuit, and a meter or relay,
which connects to a low-voltage or
low-current secondary circuit.
At ABB, current sensor designs(Figure 1) are based on the Rogowski

6

COMSOL MULTIPHYSICSIntelligent electronic devices (IED)
use more advanced technology andhave only been around for the past
20 years. Instead of ferromagnetic
materials, these transformers are
made up of solid-state components.
Since they are unable to transfer
power from primary to secondary
coils, they have a low energy
output. This makes them usefulfor many indoor and outdoor
applications, such as air- and gasinsulated environments, line posts,
and line-mounted transformers.
“IEDs are safer, more versatile,and have linear response for a wide
range of input signals,” says Nirmal
Paudel, consulting R&D engineer
at ABB, adding that they are“compatible with today’s electronic
devices and our level of usage.”»

MULTIPLE CONSIDERATIONS FOR
IT&S SIMULATION AND DESIGN
WHEN DESIGNING AN IT, multiphysics
must be taken into account. In fact,
Paudel calls this capability “critical.”A successful design should, of course,
capture resistive and inductive
heating, inductive and capacitive
coupling, magnetic saturation,and magnetostriction. However,phenomena such as fluid flow,
convective cooling, thermal expansion,
external loads and circuits, noise andvibrations, and the skin effect alsocoil principle. A Rogowski coil consistsof a uniformly wound coil with anonmagnetic core, and the output
voltage is proportional to the derivativeof the primary current. Sensors
that measure voltage in mediumvoltage scenarios use resistive voltage
dividers, while capacitive dividers, on
the other hand, are used for voltage
measurement and indication.
The IT and sensor (IT&S) industry
has made significant developments inmore recent years. Traditional ITs are
based on standard
technology andhave been aroundfor more than
100 years, mostly
used in metersand relays. They
are made up ofa ferromagnetic
circuit and areable to transfer
power fromthe primarycoil to thesecondary coil,with an outputof 1–5 amps or
120–240 volts.
Figure 1. An example of a sensor that uses the Rogowski coil principle.

DECEMBER 2019

look into the thermal curing process
and mold flow of the epoxies.
A third example is structuralanalysis. The ABB team computes
the stress level of the IT to optimize
the geometry (Figure 3). They also
look at the stress displacement levelsof the devices and components inorder to optimize them before 3D
printing of prototypes for testing— or, more serious yet, mass
production. Modeling is not only
helpful for seeing the performanceof the device in advance but also for
determining its structural integrity.

»VERY FAST RESULTS FOR VERYFAST TRANSIENT PHENOMENAVERY FAST TRANSIENT (VFT) PHENOMENA

Figure 2. Multiple physics affect an instrument transformer design.

need to be considered (Figure 2).
To account for a wide rangeof physical effects, ABB uses theCOMSOL Multiphysics® software.
One example is the simulation of
the IT’s electric fields both caused
by voids in the epoxy casting andduring the basic impulse level (BIL).
These results enable researchers
to see how effective the insulation
layers and dielectric materials
are at protecting the device.

The software is also used to performthermal analyses. In a line-line voltage
transformer, simulation is used tocalculate the core and resistive losses
in both the primary and secondary coilwindings. Another instance in which
thermal modeling is used is to findthe heat flux on the outer boundariesof the IT and the fixed temperatureboundary on the baseplate. These
results show the temperature rise andloss in the design, as well as provide a

is an important factor to consider
in power grid devices that involve
switching, such as vacuum circuit
breakers. When switching causes VFT,
it can stress the insulation systemand cause internal resonances in theprimary winding of a transformer.
The distribution of the transient
overvoltage, when it becomes
highly nonlinear, leads to internalfailure. There is a higher incidence
for VFT overvoltage (VFTO) nearrenewable energy sources, such as
wind power, because of the new grid
generation, loads, line characteristics,
and increase in switching. VFTO
steepness (i.e., how quickly theFigure 3. Structural simulations are used to optimize the geometry of an IT.

DECEMBER 2019

COMSOL MULTIPHYSICS7INSTRUMENT TRANSFORMERS

overvoltage is distributed) can beup to three MV/microsecs, which ismuch steeper than lightning! (Notethat steepness is just as damaging toinsulation systems as magnitude.)
Typical design approaches to IT&S
do not yield results that are sufficient
at withstanding VFTO. This is because
these designs require extensivemodeling of the high-frequency
voltage distribution in the winding,for which no software models existed— until now. ABB, in collaboration
with Hochschule für Technik(University of Applied Sciences) inRapperswil, created a tool to model
this behavior and understand voltage
distribution in an IT turn by turn. The
results? New design methods and anew dry type of insulation that can
resist the negative effects of VFTO.

»»DESIGNING A SPLIT-CORE
CURRENT TRANSFORMER FOR
UNDERGROUND SWITCHINGSPLIT-CORE DESIGN IS AN IMPORTANT

feature for transformers because it
enables power grid maintenancewithout any disruptions. ABB set
out to design a split-core current
transformer (sensor) that allows
high-accuracy current measurement
while the switching is done by otherdevices and the need for switchingis evaluated by IED based on
signals from the sensor. The sensor
is waterproof and submersible sothat it can be used underground.
(Underground power lines are
becoming the industry standard
because they are less likely to beaffected by high winds or severe
weather, especially in cities.) This
split-core sensor comes with its own
set of design challenges, including
its shape, size, and weight as well as

When a device suchas an IT or split-corecurrent sensor can survive in allconditions, everyone benefits.
—NIRMAL PAUDEL , ABB8COMSOL MULTIPHYSICS

Figure 4. Schematic of a split-core current transformer model.

the winding turns, core shape, andcore size (Figure 4). Besides that,
there is the risk of current crosstalk,depending on the configuration of
the device. Lastly, the sensor needs
to conform to industry standards
before it can even be tested for
production and use. “All kinds ofIEEE and IEC standards must be
met before the test lab,” says Paudel.
The ABB team turned to thesoftware yet again in order to optimize
the split-core current sensor design
before prototyping. Paudel has beenusing it for a long time and enjoys
its “ease of use, and the fact that it
has the same interface for multiple
physics and is easy to couple withother physics.” The COMSOL
software includes built-in settings
for implementing Maxwell–Ampère’s
law and an interface to solve for themagnetic fields in the frequency
domain (Figure 5). By using geometricsymmetry, the ABB team only has tomodel one-fourth of the coil, saving
time, effort, and computational
resources. A special coil modelingfeature enables the team to set up theprimary coil as a solid conductor andthe secondary coil as a homogenized

DECEMBER 2019multiturn coil. A boundary condition
describes the area where the tangential
component of the magnetic fieldand surface current density is zeroas a perfect magnetic conductor,
and the external boundaries are set
to be magnetic insulators. Solver
features enable the team to easilyadjust the settings between solidand homogenized conductors, andsolid conductors versus wires.

»»SIMULATION APPLICATIONSOFFER QUICK CALCULATIONSONE TIME-CONSUMING ASPECT of IT&S
design is the conversion betweena nonlinear magnetic B-H curve(DC magnetization) and equivalent
AC effective H-B curve. ABB used
an example application from theApplication Library to performthese calculations. After finding
the effective H-B curve with theapplication, they used the value
to model the magnetic core of thesplit-core current sensor. They foundthat the magnetic permeability isalmost linear throughout the entire
core because of the decrease inmagnetic flux density. Based on theseresults, the team concluded thatMATLAB is a registered trademark of The MathWorks, Inc.

homogenous anisotropic conductivity
and permeability should be used.
Looking at the magnetic fluxand current density results of thesimulation, the ABB team found thatthe flux level for their design is verysmall, which is ideal for their mediumvoltage use case. In addition, thegroup noticed something interesting:
Usually, when the number of turns
on the secondary coil is increased, theopen-circuit voltage also increases (asit did for one of their studies, from
130 to 196 V). However, when theload is connected across the coil, thevoltage does not always increase, andsometimes even decreases instead.
One of the final analyses that
ABB completed for this project was
looking at the three-phase crosstalkfor different configurations for thesplit-core current transformer design.
They found that the crosstalk differeddepending on whether the secondary
coils were placed closer to or farther
away from the transformer’s air gaps.

»»FINAL PRODUCT: OPTIMIZEDDESIGNS AND ENHANCEDDEVELOPMENT PROCESSESABB’S FINAL DESIGN ITERATION, theSubmersible Split-Core Sensor, met
the standards set out by IEEE andIEC (Figure 6). When asked about
their future plans, Paudel mentions
that his team is working on developing
a tool to advance the analysis ofVFTO and transformers, decreasing
the timeline of the analysis processfrom weeks to days. The tool will rely
primarily on MATLAB® but may offer
an integration with the COMSOL®
software via the LiveLink™ forMATLAB® interfacing product. The
plans for this new tool show that ABB
is just as dedicated to optimizing theirworkflow and processes as they areto the end results. They work hard tooptimize devices that increase power
grid accessibility. As Paudel says, whena device such as an IT or split-corecurrent sensor can survive in allconditions, “everyone benefits.”

Figure 5. The magnetic flux and current density of the split-core transformer.

[I enjoyCOMSOL’s]ease of use, andthe fact that it
has the sameinterface formultiple physics
and is easy tocouple withother physics.”—NIRMAL
PAUDEL , ABB

Figure 6. The submersible split-core sensor.

The ABB team, from left to right: Vivek Siddharth, Steve Shaw, David Raschka, and Nirmal Paudel.

DECEMBER 2019

COMSOL MULTIPHYSICS

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