High Efficiency Distribution Transformers
Due to use of better grade materials and optimum design, sudden failures are reduced along with lower cost of maintenance, leading to increased life expectancy. These benefits add up and balance against the inevitable increase in purchase cost – as additional copper in the windings and better materials in the core will be used in the manufacture of high efficiency transformers...
- S V Varadarajan
Transformers convert electrical power from one circuit to another in the same frequency. In this process, they can raise or lower the voltage in one of the circuits with corresponding decrease or increase in current. Transformers achieve the above thorough mutual induction between the two circuits that are linked by a common magnetic flux in its core.
The invention of transformer
Transformers range in size from radio frequency transformers less than a few grams in weight to industrial transformers interconnecting the power grid, weighing hundreds of tons. A wide range of transformer designs is encountered in electronic and electric power applications. Since their invention in 1886, transformers have become the back bone of AC transmission, distribution, and utilisation of electrical energy.
Importance of transformers
Transformers play an indispensable role in the power distribution network. After transmission lines, transformers are the second large loss making equipment in electricity networks. Failure of a transformer causes sudden outage in the power supply, leading to loss in industrial production. High efficiency transformers create economic benefits in terms of lower operating costs besides reduced greenhouse gas emissions, improved reliability and potentially longer service life. In view of these important benefits, many countries including India have taken policy initiatives – to establish mandatory and voluntary programmes to conserve energy and help domestic markets be competitive by adopting high efficiency transformers. Moreover, it is relatively easy to replace the inefficient transformers with the efficient ones – when compared with laborious / time consuming efforts needed for change in lines or cables.
High efficiency vs life cycle cost
Life cycle cost of a transformer is calculated by adding the purchase cost (investment cost including bank interest), the cost of energy losses, cost of failure / repairs, cost of maintenance and de-commissioning cost after providing for resale price (residual value) of the transformer at the time of its replacement. The cost of energy losses (iron, copper and stray) can be reduced by improving the efficiency of the transformer. This in turn reduces the life cycle cost.
Also, due to use of better grade materials and optimum design, sudden failures are reduced along with lower cost of maintenance, leading to increased life expectancy. These benefits add up and balance against the inevitable increase in purchase cost as additional copper in the windings and better materials in the core will be used in the manufacture of high efficiency transformers.
Urgent need to increase efficiency
The demand for distribution transformers has been increasing at a rapid pace due to rising population and migration of people from rural areas to urban cities. This has further led to increased demand for reliable power supply systems within the country.
Reduction in transmission and distribution losses and providing reliable uninterrupted power supply have gained top most attention in our government's thinking. The emission of green house gas is reduced with decrease in energy loss. Also, every unit of energy saved is equivalent to about two units of energy generated. It is well known that the electrical energy tariffs are subsidised in certain segments in our country. The distribution transformers are special and critical as they are the final equipment – through which each unit of electricity consumed by the end user has to be delivered. Hence, it is essential that every unit of electricity reaches the consumer in a reliable and efficient way for ensuring a viable distribution.
The role of BIS
Bureau of Indian Standards (BIS) has brought out the revised standard IS 1180:2014, 'Outdoor Type Oil Immersed Distribution Transformers up to and including 2500 kVA, 33kV - Specification Part 1 Mineral Oil Immersed.' This standard extends the scope of coverage beyond 200 kVA and up to and including 2500 kVA and 33 kilovolts. This extension of the scope would bring India’s norms on efficiency more at par with other major countries such as USA, China and Australia. The above BIS standard (for the first time) stipulates standard total loss (no-load + load losses) levels against specific rating of transformers both at 50% loading and at 100% loading.
BEE's star rated transformers
Bureau of Energy Efficiency (BEE), Government of India, has brought out 'star rating plan,' through which the distribution transformers (for the first time) are classified into '1star' to '5star' classifications. The transformers under '5Star' grade are the most efficient. Total loss figures are stipulated both at 50% loading and at 100% loading for each star classification.
Power is generated at generating stations at voltage level ranging from 10 to 30kV. This power is converted to typically 230kV* to 400kV* by step-up transformers for transmission to the consumers' distribution networks, which are located at urban areas far away. At the distribution substations, the transformers step down the power to more usable levels of 110kV* for industries. For shorter distances, power flows at 110kV* level and at urban substations, voltage is again reduced to 11kV. Further along the streets/roads distribution is carried out at 11kV and distribution transformers are used to step down from 11kV to user voltage levels of 415/240V at the street level, very close to the consumers. Thus on an average, electrical power is transformed approximately 4 times from the generating station down to the consumer. [* Note: The voltage levels indicated in this paragraph are typical values. The exact levels are determined by the quantum of power to be transmitted and the distance between the two sub-stations (based on economics)]
Technical losses are present in all electrical equipments as all equipments offer some resistance to the flow of current causing, I2R losses. Integrated over a period of time, 't', this constitutes energy loss, namely, I2Rt. Technical losses are categorized and discussed in paragraphs below.
Line loss comprises energy loss in conductors and cables (due to selection of lower size), unbalanced loading (more than designed value of current flowing in one of the phase conductors), neutral conductor loading due to pre-dominant single phase loads, loosening of strands in ACSR conductors etc
Losses at joints and terminations including mid-span joints caused due to improper choice of materials and fasteners.
Losses in transformers (apply more to distribution transformers) such as:
- Loose connection at the bushings
- Bend in jumpers at the connectors where the strands are not tightly held
- High no-load losses due to the type of core used and/or improperly tightened cores in the case of repaired units
- High copper losses due to sub-optimal loading.
Losses in service cables and connections are caused due to under sizing of service cables, losses in joints in the poles and in junction boxes due to use of inappropriate fasteners, missing spring washers and non-use of torque spanners.
Loss due to high impedance faults occur due to the overhead wires touching trees, growth of creepers over the pole / the wires and bird nesting, broken insulators and tracking. Losses in rewired fuses and joints due to poor connections and inadequate sizing of the fuse wires leading to development of hot spot. As explained before, losses contributed by the transformers can be controlled through better design and installation practices besides proper evaluation of the load demand and choosing the right kVA rating to suit the demand requirement.
Industrial vs public distribution networks
Even though a large number of distribution transformers are used both in industrial and distribution networks, the quality of design, maintenance and application are considered superior in an industrial environment. The population of transformers in public networks is very high compared to the industrial networks.
At the same time, their working atmosphere is harsh both in the physical sense and in the electrical circuit point of view. Some of these factors may be seen as per the representative list given below:
- Industrial distribution transformers have a higher capacity range of 630 to 4000kVA. The public distribution transformers are rated from 15kVA to about 1000kVA
- The average load on an industrial transformer is higher when compared to the average load on an public distribution transformers, due to constant load monitoring practice adapted in the industry
- There is a huge tendency to go in for dry type transformers in the industrial network, which reduces the maintenance time and servicing cost when compared with oil cooled transformers till now almost exclusively used in public networks
- Even though the loads have a high harmonic content (caused by non-linear loads) in an industrial transformer, suitable harmonic reduction circuits are employed to bring down the harmonic content. The public distribution networks also suffer due to loads like UPS, computer power supplies and various other electronic gadgets used in households. However, harmonic reduction circuits are almost absent in public distribution networks, even though schemes are on the anvil to monitor and penalise polluting loads
- Load fluctuations are lower in an industrial network
- Public network transformers suffer from large unchecked unbalanced loads
- In industrial networks, the transformers are serviced or maintained better
- Accessibility for servicing / monitoring the transformers is poor in public networks
- Smaller rating transformers are predominantly used in the public distribution networks. When compared with higher ratings, these small transformers have a higher no-load and load losses
- Thus, we find that transformers in public distribution network needs more attention.
Losses in distribution transformers
It includes both hysteresis loss and eddy current loss. The core flux in a transformer is practically constant for all loads - about 1 to 3% variation from no-load to full load conditions. Due to this the core loss is assumed practically constant for a given transformer.
Also called as copper loss or short circuit loss, this loss is due to resistive losses in the windings/leads and stray losses that are due to flow of eddy currents in the structural steel work and windings. This loss is proportional to the square of current.
Cooling fan loss
This is caused by the power consumed by the fans that cool the radiator bank / body of the transformer. Higher the transformer losses, larger will be size of the fan. This will result in increased losses in the cooling units.
The losses are to be reduced in order to improve the efficiency. The major areas are (a) Core and (b) Windings. However, it will not be easy to work exclusively on core and windings ignoring the other considerations. Transformer design is complex – and let us look into some of the important requirements besides the losses, which are also to be optimised by the design engineer.
- Leakage field
- Short circuit impedance
- In rush current
- Stresses and dynamic behaviour under short circuits
- Transformer DC Bias
- Monitoring and diagnostics
- Incorporation of latest software (numerical methods) for design optimisation
- Weight and cost minimisation
Reduction in no-load losses
Better selection of the core material will minimise the core losses. The use modern Cold Rolled Grain Oriented (CRGO) Silicon Steel laminations have reduced core loss. More recently the emergence of amorphous metal has demonstrated significant reduction in no-load losses and capacity to tackle non-linear (harmonic) loads.
Apart from the type of core material, the method employed in the design, cutting, fabrication and assembly of the core materials also play an important role.
Increasing the size of the core will reduce the flux density, which will reduce the losses, as hysteresis loss is proportional to Bmax1.6 max and eddy current loss is proportional to Bmax2. But this method has to be optimised with respect to the attendant increase the weight of the transformer and the increase in transport and installation costs.
Reduction in load losses
Load losses are proportional to the square of the load current. Hence, the load current has to be monitored continuously and a fairly accurate estimation of the loads are needed to select the proper size of the transformer – as a fully loaded transformer achieves higher efficiency.
The size of the conductor can be increased thereby reducing the current density – and hence the losses, with attendant higher manufacturing cost. Increased quality control on the making of the windings through automated machines and their insertion into the cores also favour reduction in load losses. Superconducting windings with nitrogen cooling have been developed for special applications as their cost is high for normal applications or installations.
Points of concern
The high efficiency transformer apart from being an expensive purchase, also comes with a few other issues listed below, which are to be taken care by the designer or end user for extracting the advantages of the reduces losses. The higher initial cost of the transformer as such can be recovered over a period of time – as the lower losses will result in lower operating costs. Increase in size and weight occurs due to enlargement of the core as well as increase in conductor size. For amorphous iron transformers, the core size increases by about 50% over the conventional design. Hot spots developed due to non-linear loads (such as variable speed drives, computer power supplies, uninterruptible power supplies etc) can bring down the life expectancy of all the transformers.
As the financial burden is more in the case of high efficiency distribution transformers, the problem of hot spot is to be tackled vigorously. Non-linear loads cause harmonics, which increase the losses many fold, leading to the requirement of a higher sized transformer. Analysis of the loads prior to selection of the transformer is a prudent way to avoid failures. Mere installation of high efficiency distribution transformers alone is not sufficient. The following check list suggests some of the efforts to be taken at the distribution level to improve the voltage profile, reduce the single phase loads, balancing of loads, reduce the line losses etc., for ensuring the correct and optimum functioning of the high efficiency transformers.
- Construction of more high voltage distribution lines, which will improve the HT-LT ratio as well as reduce the distribution loss
- Strengthening of the sub-transmission network by upgrading voltage levels of distribution feeders from 11 to 33kV
- Replacement of conductors in the old LT lines
- Use of higher sized conductors at the substation end of
- 11kV feeders
- Power factor compensation through installation of shunt and/or series capacitors
- Introduction of auto voltage boosters in areas of recurrent low voltage
- Re-arrangement of LT feeders to avoid overloading as well as under loading of distribution transformers
- Reducing length of LT lines by suitable relocation of the distribution transformers
- Balancing of loads on distribution feeders through regular monitoring
- Frequency energy audits at the transformer level.
High efficiency distribution transformers are the need of the day, as we have seen in this article. However, the transformers alone cannot reduce the losses. They must be provided with a sound environment including a network devoid of overloads, under voltage, single phasing, unbalance, harmonics, etc., as discussed. Supervision with monitoring the transformers' health on a continuous basis including periodic checks on the loads and load pattern will enhance their life and ensure continuous performance near to designed parameters.
S V Varadarajan is Former Technical Director, M.N.Dastur & Company (P) Ltd, Chennai.
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