Amorphous core material offers lower losses for distribution transformers

July 10th, 2015, Published in Articles: Energize

 

Traditionally transformer cores have been constructed of grain orientated steel alloys, but a new material which has an amorphous structure is being used increasingly in distribution transformers. The material offers advantages over traditional materials and transformers using the core material are being used in a wide range of applications.

Distribution transformers constitute the largest group of transformers in any electrical network, and therefore losses in these transformers can constitute the highest losses in the network. Distribution transformers carry a load which varies with time during the day and are sized to cater for the maximum load during the day, but often the average load is far below this and the minimum load can be far below the maximum.

Losses in a distribution transformer consist of no-load losses, which are independent of the load, and load dependant losses. A low load factor, or usage profile, means that no-load losses can form a high percentage of the total losses in the transformer, and the development of distribution transformers is focused strongly on reducing these no-load losses without compromising the performance of the transformer. The most efficient distribution transformers, which are in service continuously except for maintenance and failure breaks, record a loss of approximately 2 to 4% of the electricity they conduct, and electric utilities and industries are constantly searching for methods and technologies to reduce operating costs and energy losses [2]. Amorphous metal distribution transformers (AMDT) offer the possibility of achieving this goal.

Fig. 1: Hysteresis curve for AM compared to CRGO [2].

Fig. 1: Hysteresis curve for AM compared to CRGO [2].

Typical properties of AM cores are given in Table 1.

Cost of ownership

Cost of ownership (CoO) is an important factor in networks with many transformers, such as distribution networks. CoO is defined as the total cost of purchasing and operating the unit over its lifetime. CoO comprises both capital and operating costs. Operating costs consist of both maintenance costs and the cost of energy lost in the transformer. The cost of energy losses over the lifetime increases the cost of ownership. Losses consist of no load and load losses so the total cost of ownership may be simply stated as:

CoO = PP + NLL+ LL    (1)

Where

PP = Purchase price
NLL = Cost of no-load losses over transformer lifetime
LL = Cost of load losses over transformer lifetime

There are a number of standard methods for calculating NLL and LL for a particular transformer.

AMDTs may have a higher initial purchase price (PP) than traditional cold rolled grain orientated steel (CRGO) transformers, but the reduction in no-load losses can more than compensate for this.

Table 1: Typical properties of alloy magnetic (AM) cores.
Operating flux density (T) Saturation flux density (T) No load core loss (W/kg) Service temperature (ºC)

Core space factor

1,3 to 1,4 1,56 ≤0,18 150

0,86

No-load losses in transformers

No-load losses consist of hysteresis and eddy current losses.

Hysteresis loss

Hysteresis is the retention of magnetic domain orientation after the magnetisation force has been removed. This is illustrated by the magnetisation curve (Fig. 1). Hysteresis loss is the energy required to reverse the magnetic domain orientation in core materials when the magnetisation is reversed during an alternating current cycle. Core material with a high hysteresis has high losses. The width of the hysteresis loop is related to permeability of the material, and materials with a high permeability exhibit a narrow hysteresis loop.

Eddy current loss

Eddy current is a current circulating in the magnetic core of the transformer due to an electromotive force (EMF) induced in the core by stray magnetic fields. The value of the eddy current and the resulting losses are dependent on the resistivity of the core material and are also proportional to the square of the thickness of the laminations. Eddy current losses can be reduced by reducing the thickness of the core material layers or laminations.

Losses in amorphous core material

Amorphous core material (AM) offers both reduced hysteresis loss and eddy current loss because this material has a random grain and magnetic domain structure which results in high permeability, which ensures a narrow hysteresis curve compared to conventional CRGO material (Fig. 1).

Eddy current losses are reduced by the high resistivity of the amorphous material, and the thickness of the film. The laminations comprise thin ribbons and the thickness of the sheet is about 1/10th that of the CRGO, i.e. approximately 0,025 to 0,030 mm. Amorphous core transformers offer a 70 to 80% reduction in no-load losses compared to transformers using CRGO core material. Typical comparative figures are given in Table 2.

Table 2: No-load losses for CRGO and AM  transformers (Hitachi).
Transformer size (kVA) NLL Amorphous (W) NLL CRGO (W)
100 100 288
500 270 888
1000 460 1640

Disadvantages

AM cores have a lower stacking factor than CRGO. As a result of its hardness and thickness, the manufacturing surface of amorphous alloy is uneven, so the associated stacking factor is only 0,85 while the stacking factor silicon steel is 0,95 [3].

AM cores have a lower saturation point too: amorphous metal cores saturate at a lower flux density than CRGO, which requires larger coils for the same capacity, typical figures are in the region of T, compared to x t for CRGO.

The other significant difference between amorphous core transformers and CRGO transformers is the cross-sectional structure of the core. Because of the difficulty of producing amorphous strips, there are limited production sizes available (typically 213 mm, 170 mm, and
140 mm). Although conventional electrical steel transformers can be oval or round in cross-section, amorphous cores may be square or rectangular in shape. This is a disadvantage in terms of cost for amorphous core transformer.

History

The commercial production process of an amorphous material consisting of an alloy of iron, nickel, phosphorus and boron was developed in 1976, and was commercialised in the 1980s.

Production of the first transformers using amorphous cores started in the 1990s and the number of units currently in service runs into hundreds of thousands [5].

Production

The core material is generally formed from an alloy of iron, silicon and boron. The amorphous alloy is a non-crystalline substance created by rapidly cooling the alloy from a high temperature liquid to a solid form. Because there is no regular atomic arrangement, the energy loss (through hysteresis) is small when the flux of magnetic induction passes through the iron core. In addition, eddy current loss is decreased because the thickness is approximately 0,025 mm.

Amorphous metal casting process

Molten metal is fed through a small nozzle slot onto a rapidly moving, water cooled substrate, which causes rapid solidification at a cooling rate of approximately 106°C/s. The line operates at a speed of 100 km/h. The process is a continuous operation with a multi-hub, indexing winder. Careful raw materials and melt chemistry control is necessary. The production process is illustrated in Fig. 2.

Fig. 2: Production of amorphous material ribbons [3].

Fig. 2: Production of amorphous material ribbons [3].

Manufacture and assembly

Amorphous core material is supplied in the form of ribbons on reels in a five-ply format with a thickness of 0,115 to0,125 mm. Ribbons come in different widths, e.g. 142 mm, 170 mm and 213 mm. The first stage consists of forming the five-ply ribbon into layers of ribbon. The ribbons are fed to a cutting machine which combines three spools into a lamination of 15 layers of ribbon, with a thickness of approximately 0,375 mm. The stacking factor of 0,85 results in laminations that are slightly thicker.

The formed layers are then automatically cut into lengths depending on the shape and the size of the core required. Layers are cut at right angles to the length of the ribbon to give a joint angle of 0° and joints are arranged to overlap. The cut layers are stacked in the order of assembly.

The core is assembled by lacing the layers together over a special mandrel. The stacked laminations are laced over a rectangular mandrel which forms the inner core dimensions. The core is then slowly built (“laced”) layer by layer to form a rectangular shape which is almost similar to wound core making with conventional steel. This is done by hand. An inner U-shaped bracket or mandrel of CRGO helps to retain the shape of the core. The exterior sheet of the core is protected by a CRGO strip of the exact width of the amorphous core ribbon. This is primarily done to support the flexible amorphous ribbon core structure.

After forming the core goes through an annealing process of several hours at an elevated temperature and under a strong DC magnetic field. The purpose of the annealing is to reduce stress in the assembled core and induce magnetic anisotropy along the direction of the ribbon. Annealing improves the permeability of the core. After annealing the core receives a coat of resin on three of the arms, the arm containing the joints being left open.

Core shapes and structure

The production in ribbon format of fixed width makes the amorphous material suitable for the distributed gap wound core type of structure. The main difference being that in the AM core structure the gaps are placed in the short arm of the core. The word “gap” is used to describe the distributed butt joints, in the joint region where the core may be opened up to allow fitting of the coils. Each repeating sequence of gaps constitutes what is often referred to as a single book, pack or chapter. The number of chapters per core will vary depending on a number of factors, such as core radial build, gaps per chapters, gap spacing and sheets per step.

Single phase cores

A typical single phase transformer will make use of two wound cores, as shown in Fig. 3.

Fig. 3: Single phase AM core construction.

Fig. 3: Single phase AM core construction.

Three phase cores

Three phase transformers may make use of several configurations, the most common being the five limb shell structure shown in Fig. 4.

07-tt-mr-Amorphous-04

A variant is the Evans configuration, which consists of an outer core and two inner cores as shown in Fig. 5. Fitting the coils requires unlacing of both the inner and outer cores.

Fig.  5: Evans three phase core structure.

Fig. 5: Evans three phase core structure.

Coil mounting

Coil mounting requires the unlacing of the core joints, a process which is performed manually. Even when combined into 15-layer laminations, the material is very flexible and requires very little effort to unlace (Fig. 6). The laminations are organised in chapters and fall naturally back into the laced format (Fig. 7). Relacing is also performed manually.

Fig. 6: Unlacing requires little effort.

Fig. 6: Unlacing requires little effort.

Fig. 7: The chapters of the laminations fall back into place naturally,  making relacing relatively simple.

Fig. 7: The chapters of the laminations fall back into place naturally,
making relacing relatively simple.

Applications

AMDT are ideally suited for renewable energy applications such as wind farm and solar power, as they typically operate with 20 to 40% low load conditions. On transformers running at variable load factors such as distribution networks and particularly renewable energy plant, the no-load loss (NLL) can form a significant portion of the total losses. The NLL of AMDT transformers makes them very attractive for this type of application, particularly for wind which delivers energy to the grid continuously but has a capacity factor of less than 30%. Solar power units shut down outside of operating hours and are not so affected, but also deliver a variable load, with low output during early morning and late afternoon.

References

[1]    Hitachi: “Hitachi amorphous metal core”, www.hitachi-America.us/ice/transformer/products/amorphous_core/#sthash.SpMfCjkk.8KP015Br.pdf
[2]    R Hasegawa: “Energy Efficiency of Amorphous Metal Based Transformers”, www.leonardo-energy.org/sites/leonardo-energy/files/root-files/PL-7.pdf
[3]    BRG Energy: “Manufacturing of amorphous metals”, www.brgenergy.com/process.html
[4]    C Hsu: “Systematic Study of Low Loss Amorphous Core Transformers: Design and Testing”, Eighth WSEAS International Conference on Instrumentation, Measurement, Circuits and Systems,  www.wseas.us/e-library/conferences/2009/hangzhou/IMCAS/IMCAS39.pdf
[5]    Katsutoshi Inagaki: “Amorphous transformer contributing to global environmental protection”, Hitachi Review, www.hitachi.com/rev/pdf/2011/r2011_05_113.pdf

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