Page 43 - Energize August 2021

TECHNICAL

Main benefits of an open-core design References
The air gap in the open-core concept offers two main 1. I Žiger, D Krajtner, Z Ubrekić and M. Brkić: “Design of the Open-core Power
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benefits: ferroresonance immunity and a decrease in Voltage Transformers”, International Colloquium Transformer Research and
inrush current. Asset Management, Dubrovnik, Croatia, May 2012.
Other benefits result from the absence of 2. I Žiger, B Bojanić, D Krajtner: “Open-core Power Voltage Transformers:
the return core limbs which allows for a different Concept, Properties, Application “, IEEE Energycon 2014, Cavtat, Croatia,
insulation system concept including a sectioned May 2014.
primary winding. This high-quality, paper-oil main 3. M Poljak: “Insulation System of Combined Instrument Transformers “, Ph.D.
insulation design solution ensures a better distribution Dissertation, Faculty of Electrical Engineering and Computing, Zagreb,
of overvoltages at any frequency and a lower overall Croatia, 2006.
dielectric stress in the main insulation. This satisfies 4. I Žiger, B Bojanić, D Krajtner: “Internal fault performance of instrument
the insulation requirements for higher voltage levels transformers with sectioned active part”, 5th International Colloquium on
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(245 kV and above). This feature also effectively Transformer Research and Asset Management, 2019.
localises a fault on one segment of the active part, 5. I Žiger, D Krajtner: “Influence of HV inductive voltage transformers core design
thereby limiting the energy under internal fault to the ferroresonance occurrence probability”, International Conference on
conditions and making the PVTs explosion safe. All Power Systems Transients (IPST2015) in Cavtat, Croatia, June 2015.
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these benefits allow for a more fail-safe operation and 6. I Žiger, D Papić, M Čukman: “Accuracy Performance of Power Voltage
remove occurrences which could potentially cause Transformers”, Mediterranean Conference on Power Generation, Transmission,
transformer faults. Distribution and Energy Conversion, Dubrovnik, Croatia, November 2018.
The secondary power winding can be serial-
parallel reconnectable, offering one or two power Contact Stan Kolenic (PrEng), Otok Consulting Enterprise,
windings for individual use - serial, parallel or stanko@telkomsa.net
open delta connections. Additional metering or
relaying windings with various accuracy options
can be added.
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Off-load tapping can be provided on either the
secondary or the primary winding. In both cases,
the tap connections are available in the secondary
terminal box. The tap positions are defined by the
voltage level and are performed manually inside the
secondary box. Additionally, the secondary box can
be equipped with a low voltage fuse or circuit breaker,
and probes for continuous thermal measurement and
internal overpressure.
Every transformer is equipped with an oil
sampling valve, oil level indicator and a terminal for
dielectric dissipation factor (tgδ) measurement.
The single identifying characteristic of these
transformers is that they are built according to the
technological principles of instrument transformers
rather than power or distribution transformers.
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Thus, they are still designed, produced and tested
according to the requirements of IEC and IEEE
standards.

Conclusion
Open-core PVT are a practical solution for an auxiliary
supply for substations or remote consumers. They
offer advantages which support the maintenance free
philosophy, but also up the ante, meaning that, due to
their explosion safety, ferroresonance immunity and
other characteristics, can truly be labelled as “install-
and-forget” units which have been proven to operate
reliably under all conditions.
Furthermore, these advantages make substation
design simpler and more financially attractive.
Therein lies the potential for a more widespread
use of PVTs as primary units for power supply
and distribution to remote areas in the vicinity of
overhead power lines. n

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