Information from Koncar Instrument Transformers, Croatia

Power voltage transformers (PVTs) are a special hybrid of instrument transformer product which incorporate features of a single-phase inductive voltage instrument transformer with that of a distributive power transformer to offer a compact, reliable power source [1]. Via that direct energy transfer, the distribution grid is bypassed entirely.

This eliminates the necessity for the intermediate transformation of power, consequently removing the related costs of primary and secondary equipment as well as the costs of the corresponding infrastructure. Therefore, the main application of such transformers is power supply of auxiliary systems and services within a substation or remote industrial consumers (communication towers and antennae, mines, pump stations).

Furthermore, PVTs can be one of the most economical solutions for rural electrification. They can act as reliable power sources to provide rural communities with electricity. Additionally, they are used as backups for temporary supply to local consumers during substation construction.

Končar’s PVT is a line-to-ground connected single-phase transformer which can be used as a single unit or in a three-phase bank to power loads. These single-phase units can be used for direct transformation of power from transmission level voltages of between 72,5 kV to 550 kV (1800 kV BIL), to low voltage according to customer requirements (typically between 120 to 1000 V).

With that in mind, the rated power of such units spanning from 10 to 333 kVA single phase (1 MVA three-phase). An example of such a unit, used for auxiliary supply in a substation is given in Figure 1.

Figure 1: PVT in a substation application

Končar PVTs are unique in design due the open core, whose main trait is that the main flux is partially closed through the non-magnetic material, meaning that an air gap is inherently present in the main magnetic circuit [2]. The main flux path is dictated by the flux-directing yoke, which steers the flux near to the main core limb [2].

PVT enclosures consists of the base assembly, insulator, heat sink, bellows, and bellows cover. The design allows PVTs to achieve minimal oil volume. The active part of the transformer is located inside the insulator. As per customer request, external insulation can be either porcelain or composite. Porcelain insulators are made of the highest quality C130 aluminous porcelain and the composite insulators are composed of a glass-fibre reinforced resin tube and silicone rubber sheds.

Two main benefits of the open-core concept, the ferroresonance immunity [5] and the inrush current decrease are a direct consequence of the aforementioned air gap [2]. Other benefits are caused by the fact that the absence of the return core limbs allows for a different insulation system concept including a sectioned primary winding.

This high-quality paper-oil main insulation design solution ensures a better distribution of over-voltages at any frequency and a lower overall dielectric stress in the main insulation, which makes satisfying the insulation requirements for higher voltage levels (245 kV and above) much easier [3].

This feature also effectively localises the fault on one segment of the active part which limits the energy under internal fault conditions making the PVTs explosion safe [4]. All these benefits allow for a more fail-safe operation of open-core PVTs and removes occurrences which can result in either a major or a minor transformer fault.

The secondary power winding can be serial-parallel reconnectable offering one or two power windings for individual use, serial, parallel or open delta connections. Additional metering or relaying windings with various accuracy options can be added [6].

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, plus a PT-100 probe for the continuous measurement of thermal performance and internal overpressure indicator.

Every transformer is equipped with an oil sampling valve, oil level indicator and a terminal for dielectric dissipation factor (tgδ) measurement.

The single identifying property of these transformers is that they are built under the technological principles of instrument transformers rather than their power or distributive transformer counterparts [1]. This means that they are designed, produced, and tested per the requirements of IEC and IEEE standards.

These PVTs can be considered “install-and-forget” units because of their explosion safety, ferroresonance immunity and other traits. They are proven to operate reliably in all conditions, which is the premise of the entire concept.

Furthermore, precisely because of these advantages, the substation design can also be simplified and made 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.

References

[1] I Žiger, D Krajtner, Z Ubrekić and M Brkić: “Design of the Open-core Power Voltage Transformers”, International Colloquium Transformer Research and Asset Management, Dubrovnik, Croatia, May 2012.

[2] I Žiger, B Bojanić, D Krajtner: “Open-core Power Voltage Transformers: Concept, Properties, Application “, IEEE Energycon 2014, Cavtat, Croatia, May 2014.

[3] M Poljak: “Insulation System of Combined Instrument Transformers “, Ph.D. Dissertation, Faculty of Electrical Engineering and Computing, Zagreb, Croatia, 2006.

[4] I Žiger, B Bojanić and D Krajtner: “Internal fault performance of instrument transformers with sectioned active part”, 5th International Colloquium on Transformer Research and Asset Management, 2019.

[5] I Žiger and D Krajtner: “Influence of HV inductive voltage transformers core design to the ferroresonance occurrence probability”, International Conference on Power Systems Transients (IPST2015), Cavtat, Croatia, June 2015.

[6] I Žiger, D Papić and M Čukman: “Accuracy Performance of Power Voltage Transformers”, Mediterranean Conference on Power Generation, Transmission, Distribution and Energy Conversion, Dubrovnik, Croatia, November 2018.

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