How to enhance transformer efficiency

As the transformer acts as a mediator for the smooth passage of electricity, a small efficiency improvement in the transformer results in significant electricity savings by Supriya A Oundhakar, Associate Editor.

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How to enhance transformer efficiency

The power and distribution transformer sector has been witnessing an upstick due to renewed infusion of investments in transmission and distribution (T&D) sector as a positive impact of the government’s UDAY scheme for strengthening of T&D infrastructure. International Energy Agency (IEA) report predicts that the power demand in India will triple between 2018 and 2040. Moreover, the government’s schemes such as the Deendayal Upadhyay Gram Jyoti Yojana (DDUGJY), the Integrated Power Development Scheme (IPDS) and the recently launched Sahaj Bijli Har Ghar Yojana (Saubhagya) have generated a surge in the demand for transformers.

The Ministry of New and Renewable Energy under Green Energy Corridor (GEC) scheme has planned to invest Rs 430 billion for enhancing transmission network under which projects of about Rs 11,500 crore have been awarded so far both through bidding and regulatory tariff-based route, for the expected commissioning over the next two-year period. This paradigm shift towards alternative energy resources like nuclear and solar energy for power generation is expected to further boost the transformer deployments in the country in near future. Thus, demand for distribution transformers is projected to grow at a CAGR of over 10 per cent till 2020.

Need for efficiency
Transformers are considered as efficient machines that help in maintaining the safety and efficacy of a power system by controlling the rise and fall of voltage levels as and when needed. As the transformer acts as a mediator for the smooth passage of electricity, a small efficiency improvement in the transformer can result in significant electricity savings. With an increase in residential and industrial applications, transformer plays an important role in the distribution and regulation of power across long distances.

Efficiency is a key element in the performance of a transformer. In theory, a transformer is designed never to suffer from load losses. However, in reality, transformers used in real-world applications suffer from load as well as no load losses.

Enhancing efficiency
Transformers have undergone technology transformation from the first transformer built in 1885 based on Faraday’s Law of electromagnetic induction. The efficiency of transformer has improved significantly and total mass has reduced considerably. With the growth in demand for electricity, the unit capacity of transformer has increased along with transmission voltage which has now reached 1200 kV.

According to M Vijayakumar, Senior Transformer Expert, Primemeiden, the transformer efficiency is the ratio of output/input. The reduction in output is due to generation losses, which are categorised as no-load loss and load loss.

“The efficiency of a transformer solely depends upon the inherent loss of the transformer. An ideal transformer would have no losses, and would, therefore, be 100 per cent efficient. But in reality, transformer suffers from losses.

Losses can be classified into two types firstly load losses and secondly no-load losses. Load losses result from resistance in the copper or aluminium windings while no load losses result from resistance in the transformer’s laminated steel core. The relative values of these losses decide the load at which maximum efficiency should occur. However, one cannot do much about changing this ratio since these figures are mostly decided or rather specified by the purchaser of the utility,” states Atul Agrawal, Managing Director, Uttam (Bharat) Electricals Pvt Ltd.

He further informs that the magnitude of power loss determines the efficiency of a transformer. Large power transformers attain efficiency as high as 99.75 per cent whereas small transformer’s efficiency can be measured around 97.5 per cent. The ideal efficiency limit for an electrical transformer should be anywhere between 98 and 99.5 per cent. With the shift towards environment friendly designs in building, maximising transformer efficiencies is essential as the owners are striving for more high-performance building. Nowadays, consumers are also more concerned about rising electricity bills. So, they look for efficient transformer which can save their power consumption. The more efficient transformers require less cooling, which in turn saves more energy.

According to M Vijayakumaran, no load loss and load loss are analysed separately below. He also suggests solutions to reduce these losses, leading to enhanced efficiency of transformers.

No load loss
No load loss is also known as core loss. Power transformer core is made up of Cold Rolled Grain Oriented (CRGO) steel laminations. CRGO steel is available 0.18 to 0.5 mm thick and 1,000 mm wide. Modern CRGO has a silicon content of about 3 per cent that gives rather a high resistivity.

The main constituent of core loss is hysteresis loss and eddy current loss. Hysteresis loss depends on permeability and eddy current loss inversely depend on thickness. Therefore, to minimise core loss, we use high permeability and thinner CRGO. Specific loss of CRGO varies between 0.65w/kg ~1.5 w/kg. at 1.7 T. Therefore, for minimising core loss, it will be better to use high permeability thin core lamination. High permeability thinner core material will be costlier and availability can be difficult at times.

Building factor
The specific core loss guaranteed by the supplier at operating flux density multiplied by total core weight gives a value which is lower than value that measured in assembled core. This discrepancy is due to the fact that stacked core requires joints which change the induction direction and create a gap between different laminations. There are other factors such as cutting, burrs, type of joints, handling etc. affecting the losses. Building factors are generally in the range of 1.2~1.4 but will depend on CRGO grade and make, operating flux density, step lap, number of limbs etc.

Interlaminar losses
The core laminations are coated with very thin inorganic coating (few microns) to keep the space factor high. The coating is not a perfect insulator. Thus, Eddy current driven by the bulk flux in the core can flow across the stacked laminations, the coating must be good enough insulator to keep these low.

Load losses
The constituents of load losses are: I²R loss, stray losses, Eddy current losses in the coil, tie plate losses, tie plate and core loss due to unbalanced cross flux, tank and clamp losses, tank losses due to nearby bus bar, tank losses in bushing turret, and winding losses.

I²R loss
I2R loss is the major component of load loss. They are normally computed value of resistivity. This value varies depending on current density used. The reference temperature for loss measurement is 75C.

Stray loss
These are losses generated by leakage flux. The stray flux depends on the winding sizes and spacing, the tank size, the clamp position etc. The losses generated by this flux depend on whether shunt or shield is present. Also, the geometric and material parameters can have impact on loss reduction. In addition to the coil flux, there is flux produced by the leads.

Eddy current losses
In order to study the effect of stray fluxes in the coils, the individual strand which could be part of transposed cable, the strand is assumed to have rectangular cross section. The magnetic field of the strand segment will point in a certain direction relative to the strand’s orientation. Losses associated with each component of magnetic field are analysed separately and added for the results. It may be noted that continuously transposed cable (CTC which comprises several small strands) is used instead of bigger rectangular conductor to minimise eddy current losses.

Tie plate losses
The tie plate is located just outside the core in the space between the core and innermost coil. It is a structural plate which connect the upper and lower yoke clamps. Tension in the plate provides the clamping force necessary to hold the transformer together should a short circuit occur.

Tank and clamp losses
Tank and clamp losses are very difficult to calculate accurately. Here, we are referring to the tank and clamp losses produced by the leakage flux from the coils. The eddy current losses can be obtained from the finite element calculations. For reducing the losses in tank and clamps, shunts are used.

Winding losses
When two or more wires are in parallel and used to make a coil, it is necessary to interchange their positions at suitable points along the winding in order to cancel the induced voltages produced by the stray flux. Otherwise, any net induced voltage can drive currents around the loop established when parallel turns are joined at either end of the winding and these circulating currents can cause extra losses.

IEC/BIS standards
BIS has introduced STAR ratings for distribution transformers up to 2500 kVA, 33kV wherein losses are standardised. IEC has issued standard IEC 60076-20 for transformer energy Efficiency where efficiency is standardised for all ratings of transformers. IS will also adopt IEC in the near future so that the minimum efficiency required for all range of transformers will be standardised. Consequently, losses for all transformers will be standardised.

Indian government is working towards controlling the losses in the transformers. “With the introduction of star ratings by Bureau of Energy Efficiency is one such step towards improving energy efficiency. Due to this, the losses in the transformer have been reduced considerably which were never experienced or achieved in the past. The element of compulsion for the companies, acted a driving force for in the whole process. This has resulted in the increase or enhancement in the efficiency figures of the transformer,” informs Agrawal, from Uttam (Bharat) Electricals.