Open-core power voltage transformers: Concept, properties, and application

January 26th, 2015, Published in Articles: Energize


The power voltage transformer, which is a single-phase unit used for direct conversion of power from high to low voltages, is becoming more and more relevant for substation applications. This article aims to present the properties and advantages of the open-core design concept, which has been functioning very well for a number of years in inductive voltage and combined instrument transformers, enhancing the transformer reliability and operational safety.

A power voltage transformer is a single-phase unit used for direct transformation of power from a high, transmission level voltage (up to 550 kV) to a low voltage (under 1 kV). Typical power ratings of open-core power voltage transformers are in the range from 10 to 167,5 kVA. The basic concept of these transformers is that they are instrument transformers by design and power or distributive transformers in application. This means that these transformers are designed using the principles and according to standards relevant for instrument transformers, retaining the same insulation requirements, short-circuit withstand capabilities and the ability of permanent operation at increased voltages (usually 1,2 times the rated voltage) [1 – 3].

This approach eliminates the necessity for complex and expensive protection and monitoring systems and by default assumes a long-term, maintenance-free operation. On the other hand, the function of these transformers is inherited from power transformers as they are used to supply power for various applications, which stem precisely from the way these transformers are designed. In some cases, these transformers can serve a dual function, incorporating both power transmission and measurement or protection utilisation in one enclosure. The open-core design, available only with paper-oil as the main insulation, provides several extra benefits when viewed both from designer and user standpoints, such as: ferroresonance immunity, inherent explosion safety and decrease of inrush current.

Transformer concept and component overview

An example of an open-core power transformer can be seen in Fig. 1. The transformer is designed around an open core, which is basically a pole in the centre of the transformer. The main magnetic flux passes through the surrounding non-magnetic material.

1. Primary terminal 2. Bellows cover 3. Stainless steel bellows 4. Heat sink 5. Insulator 6. Mineral oil 7. Main insulation 8. Primary winding  9. Secondary winding 10. Open core 11 Termianl boxes 12. Transformer bases Fig. 1: Transformer cross-section, with an  outline of the main components.

1. Primary terminal; 2. Bellows cover; 3. Stainless steel bellows; 4. Heat sink; 5. Insulator; 6. Mineral oil; 7. Main insulation; 8. Primary winding; 9. Secondary winding; 10. Open-core; 11. Terminal boxes; 12. Transformer bases.
Fig. 1: Transformer cross-section, with an outline of the main components.

The design of all other components is dictated by the open core. This is most apparent with the design of the primary winding and the main insulation cylinder. The open core allows the primary winding to be divided into serially connected sections which are completely exposed to the natural oil flow, as well as the construction of the capacitively graded main insulation, which is essential in order for the transformers to comply with the requirements for higher voltage levels. Furthermore, the capacitive screens from the main insulation are connected to the sections of the primary winding, thus assuring a constant synergy between the two, leading to transformer insusceptibility towards overvoltages [4]. This also allows for the total control of parameters during the design process.

Additionally, as the main insulation is geometrically cylindrical, this also draws some technological benefits, such as the ability to produce the main insulation by machine, thus greatly speeding up the production process and reducing the amount of human intervention. This ensures uniformity and high precision during in-factory production processes [5].

The active part of the transformer is placed into a porcelain or composite insulator, essentially making the transformer its own bushing, contributing to the slender design of the transformer and consequently decreasing the overall weight and dimensions, while improving the weight distribution along the transformer. The tall, lean build also positively influences the leakage reactance and the impedance voltage of these units [6]. The aforementioned concept also eliminates practically all metallic constructive components in the path of the main or the leakage magnetic flux, which could potentially lead to increased additional stray losses [7]. The only metal components in the path of the flux are the insulator flanges, whose losses are controlled during the electrical design process [6]. The impedance voltage of these units is comparable to that of closed core transformers of the same ratings.

Open-core design specific features

The main advantages of the open-core concept are:

  • Ferroresonance immunity
  • Inherent explosion safety
  • Inrush current decrease
  • Insusceptibility to overvoltages

Ferroresonance immunity

Ferroresonance can be defined as a non-linear resonant phenomenon which manifests in oscillations between the non-linear inductance of the transformer and grid capacitance. This phenomenon can lead to increased voltages or currents appearing across the transformer, possibly resulting in damage to the transformer itself [8]. This has been found to be an issue with closed core inductive voltage transformers in service.

The open magnetic core can, for calculation purposes, be substituted with a closed core with an equivalent air gap. The width of the air gap can be determined using both analytical and numerical approaches [9]. For power voltage transformers this value typically ranges from 10 to 30 mm, depending on the core geometry. This air gap influences the magnetising curve of the transformer, making it more linear and smoothing the curve knee-point. The comparison of typical magnetising curves for open and closed core transformers can be seen in Fig. 2.

Fig. 2. Comparison of the open and cor network capacitance.

Fig. 2. Comparison of the open and cor network capacitance.

When the grid capacitance range is added to the same chart (Fig. 2), it is apparent that the probable range of intersection between the magnetising curve and capacitance, which is a prerequisite for ferroresonance, is vastly smaller (non-existent even) for open-core transformers than for closed-core transformers. Although this chart is based on instrument transformers, the same logic extends to power voltage transformers as both types share the same common denominator [6].

This fact has been recognised by the IEC, so the recent technical report by the IEC 61869 workgroup, which deals with topics connected to ferroresonance of inductive transformers, recommends the inclusion of an air gap into the core of any inductive transformer to decrease or eliminate ferroresonance [10].

Ferroresonance immunity of open-core transformers is verified with multiple criteria, including digital simulations of most common situations for ferroresonance inception in transmission networks, most importantly the situation when a transformer is energised through a capacitor for field grading optimisation of the open circuit breaker [8]. However, the most important criterion is field experience with open-core inductive voltage transformers, where no fault has ever been recorded that could be connected to ferroresonance.

Inherent explosion safety

Protection from transformer explosions is usually associated with SF6 insulated transformers, because of the rupture disc incorporated into the design of their enclosure [11]. This concept successfully mitigates the effect of the internal fault.

The open-core concept introduces explosion safety to paper-oil insulated power voltage transformers. The explosion safety principle eliminates the possibility of internal arcing under normal operating conditions. This feature is one of, if not the most important asset of open-core power voltage transformers, which ensures a better operational security and reliability.

The primary winding consists of multiple independent sections (whose number is dictated by the voltage level and/or power rating of the transformer). This fact causes the transformer to be insusceptible to internal arcing.

If a fault originates from inside the transformer, then it is most probably located in the primary winding, either between turns or layers of one section or between two adjacent sections. In either of these cases, due to the sectioned design, the fault remains localised to only one section and does not spread through the entire winding.

The grid voltage is then distributed across all other sections, thus limiting the fault current and preventing a direct short circuit connection between high voltage and ground. Even if the fault is located in the main insulation, due to the fact that screens from the insulation cylinder are galvanically connected to the corresponding primary winding sections, the fault again remains localised to one section of the winding, while the rest of the sections prevent a catastrophic breakdown of the transformer. This feature makes the power voltage transformers comparable to capacitive voltage transformers, in that their insulation also consists of many elements (capacitive) connected in series and prevents direct line-to-ground faults in the same way. Essentially, this means that an internal fault is reduced from a fast and disruptive occurrence to a slower, noticeable phenomenon.

These hypotheses were confirmed by testing open-core inductive voltage transformers, where a worst case scenario was simulated. One winding section was intentionally short circuited. The transformer was then subjected to its rated voltage while electrical parameters and pressures inside the transformer were monitored. While the transformer was connected to a high voltage, the primary current (mainly consisting of the magnetising current), was increased approximately two-fold and remained well in the value range of mA, meaning that it did not venture into ranges associated with network short-circuits (values in kA). Furthermore, the increased thermal stress of the faulty winding caused an increased release of gasses from the transformer oil, which led to an increase in volume and consequently to a pressure rise inside of the transformer. When the oil pressure exceeded the design value of the intentional weak spot of the transformer (bellows and bellows cover), a controlled release of pressure occurred in the dilatation bellows, without any oil spill, resulting only in detaching the bellows cover from the transformer.

This occurrence is illustrated in Figs. 3 and 4. Conveniently, the connection of the primary terminal is established by means of the bellows cover, meaning that its detachment also disconnects the transformer from the grid, thus providing an automatic disconnection of the transformer under fault conditions. Another advantage is that the information about the pressure rise inside the transformer can be obtained well before the bellows cover detaches either by checking the bellows position in the oil level indicator window or by monitoring the pressure rise itself. The latter is actually the basis for an efficient and reliable monitoring of power voltage transformers. The most common solution is an overpressure switch which can trip the monitoring or the protection system when the oil pressure exceeds a certain predefined value, indicating that the transformer needs to be inspected. This ensures an unparalleled reliability of open-core transformers and is more than sufficient for all power voltage transformer monitoring needs, with very little or no additional cost.

07-tt-otok-open-core-power-Fig.03Fig. 3: Inductive voltage transformer before the bellows. 07-tt-otok-open-core-power-Fig.04Fig. 4: Inductive voltage transformer after the bellows.

Inrush current decrease

Inrush current is present when any transformer is switched on. It can achieve an instantaneous value of ten or more times higher than the rated current of the transformer [12]. The peak value of the current is defined by equation 1.

07-tt-Otok-consulting-eqn-01                                          (1)



Al is the core cross section; A is the area enclosed by one turn of the winding; i0 is the mean length of the magnetic path; B, Bs and Br are the rated flux density, saturation flux density and remanent flux density respectively [13].

The magnitude of the inrush current is influenced by several factors, ranging from the moment of connection and the angle of the voltage waveform to the remanent magnetism displayed in equation 1 by the variable Br.

Remanent magnetism is a consequence of the hysteresis loop and the inability of the magnetic core to demagnetise instantly [14].

With closed-core transformers, the remanent flux density can range from 50 to 90% of the rated flux density value, depending on the core material and core assembly [13].  According to equation 1, the remanent flux density is superposed to the rated flux density, meaning that the core can be deeply saturated, thus resulting in a high inrush current value.

The introduction of the air gap into the magnetic circuit can drastically decrease the remanent magnetism and consequently the peak value of the inrush current [13,14]. Additionally, we can draw a parallel to the current transformer cores with transient response requirements such as TPY or TPZ classes, which according to IEC 61869-2 require the remanent magnetism (i.e. remanence factor Kr) to be less than 10 % of the rated flux. If the typical air gap values of such cores, which range from 4 – 12 mm are compared to typical equivalent air gap values of open-core power voltage transformers, it can be concluded that the open-core practically eliminates the remanence flux altogether.

Insusceptibility to overvoltage

Fig. 5 shows the equivalent diagram of the open-core transformer [16].

Fig. 5. Equivalent diagram of the open-core power voltage transformer active part.

Fig. 5. Equivalent diagram of the open-core power voltage transformer active part.

The main goal of a well-designed insulation system is an equalised distribution of dielectric stresses along all areas of the insulation, both internal (oil impregnated paper insulation, between layers and sections of the primary winding) and external (insulator – air boundary). As shown in Fig. 5, the transformer is actually a complex LC network (resistance values, although present, do not influence the dielectric stress distribution of the transformer), which means that the dielectric stress distribution is hugely dependent on the frequency of the overvoltage applied to it [16]. The number of nodes in this network corresponds to the number of primary winding sections. Each section is directly connected to a capacitive screen in the main insulation. The capacitance between two adjacent screens is represented by C1 to Cn capacitances in the equivalent diagram.

The self-capacitance of the section is small by comparison and can be disregarded [16]. Each screen has a capacitance to ground designated by Co1 to Con. Each section is represented by its self-inductance (L1 to Ln) and an array of mutual inductances towards all other sections. These two sets of parameters comprise the inductance matrix which is calculated using FEM analysis, while the capacitances can be calculated analytically, because of the cylindrical geometry of the insulation [16, 17]. Once all parameters have been determined, they can be imported into an EMTP solver, which can then calculate the voltage distribution across the primary winding for various operation scenarios, most notably a lightning impulse, a switching impulse and  sinusoidal overvoltages at different frequencies. Each of these parameters can be easily controlled by design: The section inductance can be fine-tuned by changing the number of turns of each section, and capacitances can be optimised by changing the geometry of capacitive screens in the main insulation. An example for the calculation of lightning impulse voltage distribution for a 362 kV, 100 kVA power voltage transformer can be seen in Fig. 6.

Fig. 6. Lightning impulse voltage distribution for 362 kV power voltage transformer.

Fig. 6. Lightning impulse voltage distribution for 362 kV power voltage transformer.

As can be seen from Fig. 6, the voltage distribution is practically linear during the entire duration of the voltage impulse.

This ability to withstand overvoltages of various types (waveform, amplitude, frequency) has been confirmed by a range of tests and positive service experience of the open-core inductive voltage transformers [16]. The feature that separates power voltage transformers from inductive voltage transformers is their behavior under sinusoidal voltages at different frequencies. This is because the power voltage transformers have a lower primary winding inductance (due to a smaller number of turns) than their instrument transformer counterparts, which results in a very stable voltage distribution over a typical range of lower frequencies. While the voltage distribution for instrument transformers is fine tuned for both the rated frequency and the test frequency of the power-frequency withstand voltage test (usually 100 to 150 Hz), experience has shown that for power voltage transformers it is enough to adjust the distribution only for the rated frequency, and a wide range of other frequencies will exhibit the same distribution.

This fact is illustrated in Figs. 7 and 8, which show calculated and measured voltage distributions for a 145 kV 50 kVA power voltage transformer unit. This behaviour is inherent to all transformers of this type.

Fig. 7: Voltage distribution for 145 kV. Power is expressed in p.u.

Fig. 7: Voltage distribution for 145 kV. Power is expressed in p.u.

Fig. 8: Percentage deviation from ideal voltage distribution.

Fig. 8: Percentage deviation from ideal voltage distribution.

Figs. 7 and 8 show that voltage distribution is highly linear in all cases. Furthermore, a strict control of dielectric stress in the entire active part also controls the stress distribution on the insulator surface. These two combined cause the transformers to be able to achieve the insulation requirements for the highest voltage levels.

Additionally, the transformers are partial-discharge (PD) free (PD less than 10%) even at power frequency withstand voltage. This fact greatly enhances the lifetime of the transformer and reduces breakdowns due to insulation ageing influenced by PD [18].


The most common application of these transformers is for the “in-house” supply of various substations, meaning the supply of power for auxiliary equipment, such as protection and monitoring systems.

In this configuration, the power voltage transformer substitutes a distributive transformer or diesel generator. This solution is very convenient in remote substations which have no access to the distribution system, which is common in countries which have large territories with low population density, such as the USA, Canada and Australia. An example of such transformers for this application is shown in Fig. 9.

Fig. 9. 362 kV transformers for auxiliary supply of a substation in Estonia

Fig. 9. 362 kV transformers for auxiliary supply of a substation in Estonia

The other conventional, albeit less common, application is for the power supply of remote communication towers or antennae, mines, pump stations, etc. In this case, by building a lightweight station it is possible to ensure an economical, robust and reliable power supply. Due to their modest weight and ease of transport, these transformers can be used for a temporary supply of local demand while the substation is being built, or as an emergency supply after faults or environmental disasters.

The idea is to use the power voltage transformer as a primary unit in a substation to provide electrical energy to remote communities and users. This way it is possible to reach numerous rural communities or mountainous areas where the construction of the distribution grid is economically unviable, or even technically impossible, thus providing stable electrical energy to those who do not have it.

A substation based on a power voltage transformer as the main transformer can be very compact and straightforward, resulting in savings in both the primary and secondary equipment, protection and monitoring alike. HV switchgear (which would most probably exceed the transformer itself in sheer cost) can be replaced by a HV fuse, no extensive monitoring is necessary and maintenance is basically limited to periodic visual checks and routine site measurements, just as with instrument transformers. Some confirmation of this has already been reported, as cost-benefit analyses of pilot projects have shown that this type of substation can cost approximately one-third of a regular substation [20].


The open-core power voltage transformer is an ideal solution for an auxiliary supply of substations or remote consumers, both communities and industry. The advantages these transformers offer support the “maintenance free” philosophy, but also, due to their explosion safety, ferroresonance immunity and other traits, can truly be labelled as “install-and-forget” units which are proven to operate reliably in all conditions, which is the premise of the entire concept. Furthermore, precisely because of these advantages, substation design can also be simplified and made more financially attractive. Therein lies the potential for a more widespread use of power voltage transformers as primary units for power supply and distribution to remote areas in the vicinity of overhead power lines.


[1]    IEC 61869-1:2009: “Instrument Transformers – Part 1: General Requirements”, 2009.
[2]    IEC 61869-3: “Instrument Transformers – Part 3: Additional requirements for voltage transformers”, 2011.
[3]    IEEE C57.13: “IEEE Standard requirements for instrument transformers”, IEEE engineering society, 2008.
[4]    M Poljak, B Bojanić, T Hafner, J Tomašević: “A new Concept of Combined Transformers”, European Transactions on Electrical Power, Vol. 6, pp. 2563-259, August 1996.
[5]    M Poljak, B Bojanić: “Dilemma – Inductive or Capacitor Instrument Transformers”, Cigré Croatia, 2003.
[6]    I Žiger, D Krajtner, Z Ubrekić, M Brkić: “Design of the open-core power voltage transformer”, International Colloquium Transformer Research and Asset Management, Dubrovnik, Croatia, 2012.
[7]    Ž Janić: “Improvement of Power Transformer Design in Order to Reduce Stray Losses”, Doctoral dissertation, Faculty of Electrical Engineering and Computing, Zagreb, 2008.
[8]    D Krajtner: “Influence of the Inductive Transformer Construction on Ferroresonance Appearance in High voltage Networks”, Cigré Croatia, 2005.
[9]    V Bego: “Instrument Transformers” (In Croatian), 1977.
[10]    IEC/TR 61869-102 Ed.1.0: “Instrument Transformers – Ferroresonance oscillations in substations associated with inductive voltage transformers” – Draft approved for publication, January, 2013.
[11]    TIP – SF6 Station Service Voltage Transformer: Cost-effective solution for insulated applications, ABB product catalogue, 2013.
[12]    S Kahrobaee, MC Algrain, S. Asgarpoor: “Investigation and Mitigation of Transformer Inrush Current During Black Start of an Independant Power Producer Plant”, Energy and power Engineering, Vol. 5, 2013.
[13]    SV Kulkarni, SA Khaparde: “Transformer engineering – Design and Practice”, Marcel Dekker, New York, 2004.
[14]    WT McLyman: “Transformer and Inductor Design Handbook”, Marcel Dekker, New York, 2004.
[15]    IEC 61869-2: “Instrument transformers – Part 2: Current transformers”, 2010.
[16]    M Poljak: “Insulation System of Combined Instrument Transformers”, Doctoral dissertation, Faculty of Electrical Engineering and Computing, Zagreb, 2006.
[17]    D Filipović-Grčić: “Optimisation of condenser type insulation system made of oil impregnated paper”, Doctoral dissertation, Faculty of Electrical Engineering and Computing, Zagreb, 2010.
[18]    M Poljak, B Bojanić: “How to prevent instrument transformer explosions”, Cigré Croatia, 2007.
[19]    F Birol: “Energy for all: financing access for the poor”, Energy for All Conference, Oslo, Norway, October 2011.
[20]    OR Calvo, RG Ibarra, A Solano, E Acosta: “Rural Electrification in Chihuahua, Mexico at one third of the cost vs a conventional substation”, World Energy Congress, Montreal, Canada, September 2010.

Contact Stanislav Kolenic,Otok Consulting, Tel 011 807-0993,

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