Electricity + Control

By Peter Middleton, Crown Publications

Crown Publications editor, Peter Middleton, discusses Cast resin transformer technology with Mervyn Low, Managing Director of Greenergi.

Mervyn Low MD of GreenergiMervyn Low, Managing Director of Greenergi.

PM: What makes cast resin transformer technology different?

ML: A dry-type transformer has no oil in it, which has a number of benefits. There are several types of dry-type transformers. The first-ever transformer were open wound transformers where the coils were visible and these were air-cooled. One of the major drawbacks of this type of transformer is that the coils are not kept structurally rigid in fault conditions. Currents passing through a transformer coil produce forces – (Fleming’s left hand rule) – and if these currents are excessive, such as in short circuit conditions, then the transformers are subjected to very large radial and axial forces, which very likely will damage the coils. Mechanically, it is very important to keep a transformer’s coils as rigid as possible and prevent any movement of the windings, which is where cast resin type transformers come in.

PM: How is a typical cast resin transformer constructed?

ML: Instead of rolling transformer wire onto a cotton-reel-like core, we use flat foil windings like those on a roll of paper towel or an old-fashioned film reel. Usually the windings are made from aluminium foil but copper is also used. Separating the winding is a double layer of insulating film. For the HV coil for cast resin transformers we connect 10 or more of these pancake coils in series and stack them in columns to form the complete coil. Compared to conventionally wound transformers, this pancake/foil coil construction reduces inter-turn stresses with the benefit of increased resistance to high-voltage impulses. The HV coils and the LV coils are nested in the same column on a common core, with an air gap between them for cooling.

For transformers with higher power ratings, the LV coils are manufactured with gaps in the LV winding itself to promote airflow for better cooling. The cast resin HV coils, due to their construction, have the benefit of significantly reduced partial discharge – typically less than 10 pC (picocoulomb).

CTRs used instead of rolling transformer wireFor CRTs, instead of rolling transformer wire onto a cotton-reel-like core, flat foil windings are used.

PM: What makes these transformers reliable?

ML: For three-phase cast resin transformers, we use an EI-core with the I forming the yoke across the top to close the magnetic flux circuit. Mechanically, the construction is very simple and this enables us to make these transformers robust and reliable.

PM: What makes this technology electrically efficient and safe?

ML: It is the construction and materials used that make this technology electrically efficient and safe. The material used for the cores is Grain Oriented Silicon Steel (GOSS), which reduces the induced losses associated with the magnetic flux. Transformers are constantly running at 50 Hz. Depending on the grade of steel, the losses in the core can be minimised by reducing the materials ‘resistance’ to the magnetic flux. Reduced losses translate into less heat generated in the core which, over the life of the transformer, are significant.

From an efficiency perspective, distribution transformers are typically connected all the time. From an 11 kV three phase supply, these would typically be stepping the voltage down to 400 V phase to phase (or 230 V phase to neutral) on the LV winding. Even if no LV current is being drawn, the transformer is still idling, with switching 50 Hz flux heating the core – and this is going on 24/7/365 over the life of the transformer.

PM: What is the anticipated lifespan of the cast resin-type transformer?

ML: A resin type transformer was installed in 1983 at the BMW Rosslyn plant and this is still in operation today. If it had a more energy efficient core, just think how much energy could have been saved over those 30+ years and we can also now use an amorphous core material, which offers even better efficiencies as the composition of the core reduces the eddy current losses significantly.

As well as core losses, all transformers exhibit I2R or copper losses, which produce waste heat in the windings as the transformer is loaded. Transformers can be made more efficient and the losses reduced by using more/thicker winding material, which reduces the resistance and hence the losses.

Regarding the choice of coil winding material, aluminium foil/strip is used as the conductor material, for a number of reasons: It’s cheaper than copper; the expansion coefficient of Al is closer to that of the resin we use, which reduces the expansion stresses and the likelihood of expansion cracks; over and above this, aluminium is not as great a target for theft compared to copper.

PM: Describe the HV coil manufacturing process.

ML: A double layer of insulation is placed between the flat aluminium strip during the winding process. This creates a double layer of insulation between each loop of the pancake coil whereas some manufacturers use a single layer. The coils are then connected in series and stacked on top of one another – suitable spaced of course. Once the full stack of coils has been connected, the stack is reinforced, inside and out, with glass-fibre matting and placed into a mould. The moulds are placed inside a vacuum chamber to remove air. The resin must be pumped in under vacuum to prevent bubble formation, which would very likely become a source of partial discharge (PD) in the HV coil.

Once the correct vacuum level is reached, the heated epoxy resin mixture is pumped into the mould to encapsulate the entire coil.

The coils are then heated and cooled in an autoclave at closely controlled rates to maximise the strength of the cast HV coils. This vacuum casting and baking process is crucial and ensure that each HV coil is very solid and rigid and able to withstand mechanical stresses and exhibit extremely low levels of partial discharge.

In addition, the fibre-reinforcement gives the coil the lateral strength to resist cracking due to thermal expansion or shock loading forces. The result is an extremely strong coil that can safely operate at transformer temperatures between -25°C to 120°+.

PM: To what do you attribute the low fire risk associated with cast resin transformers?

ML: A fire retardant resin composition is responsible for the extremely low fire risk, while precise outside and inside resin thicknesses enable sufficient air-cooling. The enemy of coil-based machines such as transformers, motors or generators, is heat.

For CRTs, air gaps between the HV and LV coils as well as the LV coils and the core allow cool air to enter the bottom which rises due to convection and cools the transformer. The upright design enables cooling via natural convection in most cases, but if the transformer is placed inside an enclosure, then the enclosure needs to be designed to allow for adequate ventilation to enable the heat to dissipate into the atmosphere.

If needed, however, squirrel cage fans can be incorporated in the system, one placed on each side of each transformer coil. Then, when the LV winding temperatures reach 70°C or so, the fans kicks in to force-cool the transformer until the temperature subsides to a temperature of 60°C.

Pt100 temperature probes measure the temperature on each of the LV coils, and a temperature controller designed for cast resin transformers manages the fans and alarm and trip alerts are made available to prevent the transformer being damaged due to over-heating.

PM: Is the use of cast resin transformers growing?

ML: Many projects have and plan to implement dry-type cast resin transformers. This is mostly related to the much higher fire risk associated with oil-filled transformers. The capital costs are largely dependent on infrastructure as typically oil-cooled transformers are separated from the main building with a bund wall to contain the oil in the event of a leak and a fire suppression/ detection system.

Furthermore high current LV cable has to be run much longer distances to connect into the facility’s electrical systems.

By installing a resin cast transformer, which can be located in the centre of a building in a basement very close to the LV switchgear, cabling costs can be significantly lower – 120 m of LV-cabling for a 2,0 kVA transformer can cost close to R1 M – and no additional civil works are required for an external outdoor substation.

More importantly, the long-term operational costs come down dramatically, first because of lower energy losses (I2R) in the LV cables, but also because cast resin transformers require lower maintenance requirements. An oil-filled transformer should be constantly monitored and if possible an annual DGA (dissolved gas analysis) performed. This all adds to the TCO (total costs of ownership). Cast resin transformers simply need cleaning occasionally and the bolts re-torqued.

Conclusion

GreenErgi can offer 11, 22 and 33 kV cast resin transformers, with the largest supplied to date in South Africa being a 5,0 MVA unit for the Stortemelk Hydro plant near Clarens. Grid connected hydro, wind and PV plants are ideal applications for cast resin technology. The plants are often geographically remote, so ease-of-maintenance becomes more important as well as product reliability over the lifetime of the plant.

Total ownership costs are a big thing for owner operator plants on tariff-based procurement contracts, because ongoing costs directly impact long-term profitability. More efficient and maintenance friendly cast resin transformer technology is, therefore, often a preferred solution.

General industry is the biggest user of power, however, and here too, the long term savings can be significant. We have transformers in hospitals, hotels, office parks, exhibition centres, fuel refineries, water treatment plants, and factories.

 

 
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