How three phase transformers work, selection criteria

Ortea Transformer

Since 1969, the year Ortea Next was founded, transformers have been our strong point. The design and production knowledge and experience gained over the years allow us to offer high-quality, optimized, and reliable products.

The flexibility and adaptability of the Ortea Next organization, ensure the development in short time frames of solutions designed beginning from the customer’s technical specifications, ensuring high levels of quality that distinguish our standard production.

To ensure both quality control and the full synergy between resources involved, all the design and production activities are carried out at our headquarters in Cavenago di Brianza.

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Construction features

The production of transformers is carried out with copper or aluminium windings, depending on the rated power, the economic optimisation of the project, or the customer’s specific request. The magnetic core is produced with thin sheet metal to reduce losses.

The material used for the main insulation towards the core and for the inner insulation of windings is selected according to the required insulation class. The solidly built winding, locked by plates and rods, is then impregnated with environment-friendly polyester resin. The impregnation and subsequent oven polymerisation processes consolidate the winding and achieve the required insulation class.

The specific request aspects needed for design include:

the temperature class (B, F, H up to class 220°C)
the presence of multiple windings
any adjustment sockets
the connection vector unit
the performance features: losses, short-circuit voltage, inrush current, etc.

The transformers can be in an open configuration (IP00) or housed in a metal enclosure with a standard protection rating of IP21. Higher protection ratings can be achieved on request. The transformers are typically designed for indoor installation, but housings suitable for outdoor installation can be provided upon request.

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Selection criteria

A three-phase transformer is designed to transform an input voltage into the required single-phase or three-phase output voltage.

To select the correct type of transformer you need to determine:

the type of load to be supplied: if the load consists of both single-phase and three-phase equipment, the single-phase equipment is connected to only one phase of the transformer.
primary transformer voltage: line input voltage (typically 400V)
secondary transformer voltage: voltage required by the load to be supplied
load frequency: it must be the same as the transformer’s (typically 50 Hz)
total load power: this is determined by the result of the voltage supplied to the load multiplied by its current.

The total load is often a combination of individual single-phase and three-phase loads (e.g. lights, heaters, motors, etc.). You need to know the active power, power factor, unbalance, etc. of each load. The total load must be evaluated taking into account the appropriate usage and contemporaneity factors. The transformer chosen must have a kVA value equal to or greater than the required load, also taking into account any unbalance.

ISOLATION TRANSFORMERS: APPLICATION FIELDS

Isolation transformers are specifically designed and manufactured to guarantee the load protection. Isolation transformers usually provide a unit transformation ratio (e.g. 400/400V) and are used to uncouple the load from the power supply grid, as the primary and secondary windings are galvanically isolated.

The typical sectors of application are telecommunication systems, data centre power supply systems, trafoless UPS power supply, by-pass line protection, public building systems.

Standard isolation transformers

Isolation transformers are always equipped with an electrostatic screen between the primary and secondary windings.

The windings are made in a delta/star or delta/zig-zag configuration (with star center available) to eliminate the third and multiple-ofthree harmonics.

Isolation transformers perform various functions and, in particular, in data center power supply systems, they can be used for the following purposes:

Within a UPS, to act as an integral part of power conversion circuits.
To create a locally grounded stable neutral point.
Inside distribution units to reduce harmonic currents.
If equipped with sockets, to allow voltage adjustment to comply with load requirements.
To eliminate ground loops with multiple generators or network sources.

Ortea Next manufactures a range of standard isolating transformers having K factor 4, 13 and 20, but we can meet any specific need of the Customer.

K-rated isolation transformers for non-linear loads

These isolation transformers are specifically designed to cope with the high level of harmonic distortion generated by non-linear loads. Typical non-linear loads are computers, variable speed drives, inverters, welding machines. The harmonics generated result in a higher effective value of current and therefore in an increase in Joule losses.

In order to ensure compliance with the temperature limits defined by the thermal class, the design of the transformer must take account of these harmonics. The degree of influence of harmonic distortion is indicated by the K factor: the higher the K value, the higher the harmonic content that the transformer must withstand without damage.

Isolation transformers for UPS

Uninterruptible power supply (UPS) transformers are used for AC/DC conversion and DC/AC rectification (inverters).

These transformers are highly customised and it is not possible to define standard types as their design is tied to that of the machine in which they are to be inserted. Both the electrical parameters and the mechanical characteristics respond to precise specifications drawn up by the UPS manufacturer.

Close collaboration is therefore an essential requirement for the development of the most suitable solution.

Thanks to decades of experience, Ortea is able to design and manufacture transformers for UPSs of all types.

The partnership with the world’s leading manufacturers provides evidence of the high degree of reliability achieved.

Isolation transformers for photovoltaic systems

These transformers are used to ensure adequate separation and protection between the photovoltaic system and the electricity grid. Their main functions:

Isolation and safety: they create an electrical barrier between the photovoltaic system and the grid, reducing the risk of damage or problems to the grid due to faults or disturbances in the photovoltaic system.
Voltage adaptation: they can adapt the voltage generated by the solar panels to make it compatible with the local grid voltage, allowing the safe injection of energy into the grid.
Ground fault protection: they help prevent the passage of ground fault currents from the grid to the DC part of the photovoltaic system, improving operator safety and reducing the risk of fire.
Efficiency improvement: they can improve the efficiency of the photovoltaic system, ensuring that the energy generated is transmitted to the grid with minimal losses, resulting in improved system economics.
Regulatory compliance: they are often needed to meet regulatory and safety requirements imposed by local and national authorities for the safe integration of photovoltaic systems into the grid.

ISOLATION TRANSFORMERS FOR RENEWABLE STORAGE SYSTEMS (BESS)

BESS (Battery Energy Storage System) systems include inverters, batteries and distribution cabinets. These systems allow energy from renewables (wind and photovoltaics) to be stored and made available when needed.

Between these energy storage systems and the main grid, galvanic isolation of the two circuits is appropriate in order to protect the inverter and batteries from any overvoltages and/or surges generated in the grid.

It is also often necessary to transform the voltage supplied by the renewables (400V) into the operating voltage of the BESS system (typically 690V-480V).

The solution to these needs is to introduce an isolation transformer into the electrical design that performs both functions.

Considering also that the transformer must supply the batteries during the charging phase and supply the load during the battery use phase, this type of transformer is designed to operate bidirectionally.

ISOLATION TRANSFORMERS FOR THE MEDICAL SECTOR

Due to the importance and peculiarities of electrical systems used in the medical sector, it is crucial to have highly reliable transformers, both from an operational and a safety point of view (IEC 61558-2-15 standard).

The medical isolation transformer is an isolated power supply used in medical electrical IT systems (isolated from earth) that allows the isolation to be monitored and minimises the risk of failure by maintaining operational continuity of the power supply.

The use of the isolation transformer in these medical contexts helps prevent short circuits, electrical discharges, interference and other issues that could jeopardise patient safety and equipment integrity.

Below are some examples of medical equipment that benefit from the use of isolation transformers:

Imaging equipment
Surgical equipment
Monitoring equipment
Therapy equipment
Diagnostic equipment.

DATA CENTER ISOLATION TRANSFORMER

Isolation transformers perform several functions. Generally, in Data Center power supply systems, they are used for the following purposes:

To act as an integral part of the power conversion circuits within a UPS.
To create a locally stable neutral point to earth.
To reduce harmonic currents within distribution units.
When equipped with sockets, to adapt the voltage to the needs of the load.
To eliminate ground loops with multiple generators or mains sources.

ISOLATION TRANSFORMER FOR MARINE

Until the recent past, the voltage in distribution boards was 440V. However, due to the ever-increasing size of ships and, consequently, the greater power required on board, there was an increasing difficulty in handling ever higher rated and short-circuit current values. This has led to the transition to a 690V and, in some rare and specific cases, 1000V power supply.

From the Low Voltage (LV) switchboards on board ship, a complex distribution network takes shape, supplying the various types of low-voltage consumers on board such as the rudder, the winches or the essential auxiliaries of the engine apparatus, as well as the lighting, entertainment, comfort and show facilities, and the hotel structure with kitchens and laundries. Considering that the operating voltage of such equipment on board is 400V, the installation of an appropriate LV/LV transformer is necessary.

The installation of an isolation transformer also eliminates, when the ship is connected to the grid (cold ironing), the electrical continuity between the grid and the vessel and is therefore essential for safety and to avoid the phenomenon of galvanic corrosion.

ISOLATION TRANSFORMER FOR RAILWAY

Single-phase or three-phase transformers are used for supplying the auxiliary signalling and control circuits in the relay rooms of railways or subways.

They comply with IEC EN 61558-2-4 with the integration of particular specifications required by the IS365 railway standard.

The main specifications of these transformers are:

Constant power operation with ±15% voltage variation.
Operation at constant power with frequency variation of ±5%.
All materials of construction must be of insulation class H.
Insulation between primary and secondary must be of double or reinforced type.
Allowable overtemperature for single-phase transformers ≤ 10kVA and three-phase transformers ≤ 60kVA shall not exceed 50°C.
The allowable overtemperature for transformers with sizes larger than those mentioned above must not exceed 80°C (thermal class B).
Efficiencies, no-load currents, in-rush current, losses in iron, and Joule losses at 75°C must comply with the tabulated values and the trends of the curves shown.

TRANSFORMERS FOR RECTIFIERS

In special industrial applications such as galvanic plants (chromium-plating, galvanising, etc.) rectifiers are used to convert the voltage from AC to DC with very high currents in the order of thousands of amperes.

In order to obtain these currents, a dedicated step-down transformer is inserted, which can have different configurations depending on the type of rectifier (three-phase bridge, polyphase, with or without interphase coil, etc.). Therefore, the transformer is developed and sized specifically for the rectifier for which it is intended.

The most critical components of the transformer for this type of application are therefore the output bars of the secondary.

However, thanks to the expertise and experience gained by its designers over the years, Ortea is able to design and develop the configuration of these components to the best of its ability.

GENEAL-PURPOSE TRANSFORMERS

Utilities often only need to adapt the mains voltage to that required by the plant or machinery to be powered.

In these cases, a transformer is inserted into the electrical system to perform this specific function.

Starting from the analysis of the data provided and the customer’s needs, Ortea, thanks to the experience and professionalism of its designers, is able to find the optimal solution in terms of efficiency, performance and economy.

REACTORS

Thanks to many years of experience and close collaboration with Customers, Ortea has developed a specific knowledge that allows to manufacture reactors in the light of the various problems that may arise from the wide variety of uses in which these magnetic parts are employed.

Production processes being very versatile, we can cover a wide range of uses.

In particular, we produce for both indoor and outdoor use:

Three-phase and single-phase reactors.
Reactors with iron or air core.
Detuning reactors.
Smoothing reactors.
Blocking reactance.
Inrush current limiting reactors.

Our reactors are manufactured in accordance with the relevant technical standard (IEC 60076).

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