Brief Review of Latest Technological Trends in Distribution Transformers (A Paper published in TRAFOTECH DT 2nd Edition on 06th May 2022)

 

A BRIEF REVIEW OF LATEST TECHNOLOGICAL TRENDS IN DISTRIBUTION TRANSFORMERS

Rajeev Shevgaonkar
Director,
North Star Electricals Pvt. Ltd.
Indore, India
rajeev.shevgaonkar@northstar4you.co.in
 
Dr Mrunal Deshpande
Associate Professor, EEE Department
Sri Sivasubramaniya Nadar College of Engineering,
Chennai, India
mrunald@ssn.edu.in

ABSTRACT:

The day by day increasing demand of power and the steps taken to meet them without blackouts indicate towards the capacity and progress of the Nation to handle the demands. The efforts being made at all the levels by government and private agencies in India indicates bright growth prospects and the results are encouraging. In this era of power crises when renewable energy sources are the most preferred topics of research, distribution transformers (DT) still play vital role in the power system. Manufacturing technologies for DT at global level have advanced a lot. Though efforts are being taken in India to improve the efficiency of these transformers, the manufacturing still follows the age-old inefficient technologies. So, the need of the day is to adopt and implement advanced technologies, material, and manufacturing processes to deliver very high-quality transformers in terms of performance and aesthetics. Encouragement from all the stakeholders to develop latest transformer technologies will help to improve economic as well uninterrupted delivery of power.

This paper presents a review of the various manufacturing technologies for DTs. Foil wound coils, amorphous cores, pad mounted units, K-class insulations, Smart transformers etc. are briefly reviewed and their suitability to Indian conditions is discussed. There are some challenges in adopting these to our benefit, however, if right efforts are made then the new technology can be used for obtaining more stability and improved performance. 

Key Words: Distribution Transformers, Energy Efficiency, Technology, operation and maintenance, field failures, manufacturing practices.   

1.     INTRODUCTION

As compared to other transformers, distribution transformers are placed closer to consumers’ end. These also outnumber the other transformers in the whole power network because of which the demand of new transformers is ever increasing. On the other hand, being close to the load centres distribution transformers face different performance challenges like loading conditions, maintenance challenges, mal practices like power theft, oil theft, sabotages etc. Besides energy wastages, sabotages and misuse of the transformers is the biggest challenge for utilities. Leaving few, most of the utilities till now have failed to nail down the true causes of misuse at their end. Conveniently the manufactures are blamed for all the failures of transformers and penalized.

A lot of efforts have been made at government level through Bureau of Energy Efficiency (BEE) and Bureau of Indian Standards (BIS) in regulating power losses especially no-load loss or core loss. But due to various limitations in implementing policies, the results of these efforts have been perceived of mixed nature.

In this paper, other possibilities for distribution transformers are explored besides energy efficiency, to make them more capable of withstanding Indian operating conditions without much affecting the cost. The paper intends to discuss new technologies and manufacturing practices which are either developed or being developed and which can make DTs a better proposition at overall level.

2.     BEYOND ENERGY EFFICIENCY

Probably, Transformer is the most efficient machine. Unlike other electrical equipments like motors, air conditioners, refrigerators, heating systems, lighting devices, transformers consume very little energy. However, with reference to power distribution system they are the highest energy consuming assets. To keep cost of power distribution low, transformers must be energy efficient.

The energy efficiency regulations by way of BIS 1180[1,2] are in place and are getting more stringent. When measures are taken to reduce the losses in transformer, it in turn also reduce its operating cost. Flip side to this is higher cost of acquisition of transformers, underutilised materials, and increased per KVA weights of costly materials like copper, CRGO etc. Consequently, higher raw material demand is also overburdening the natural resources and thereby adversely affecting the environment.

Its right time now that all stakeholders of distribution transformers must consider aspects other than energy efficiency like design, manufacture, and operation of transformers. Latest technological advancements and development of new materials in this field must be given due considerations so that better performance, and cost effectiveness can be aimed at.

3.     PERFORMANCE CHALLENGES

Distribution transformers in India have to work under very trying circumstances. Some of the operational challenges DTs have to face are:

1.      Maintenance and Upkeep: The transformers across the country are poorly maintained equipment. Smaller the transformer worse is the level of its maintenance. Oil leaks, poor termination, missing/bypassed fuses, damaged bushings, low oil levels, missing breathers, little or no preventive care, by passed protection (if any), dust and dirt accumulation, water ingress etc. are some of the issues associated with small distribution transformers.     
2.       Loading and operational issues: The DTs are often overloaded[3,4]. Long duration overloading of 20 to 30% is quite common. The effect of overloading multiplies if oil level is low, and climate is hot.
3.        Unbalanced load: DTs supplying single phase power to domestic households often deal with the load unbalance as number of connections on each phase are rarely equally distributed. The consumption pattern of each connection is also different, causing more unbalancing of load.
4.    Fires: As per recent media reports there are frequent fire incidences involving distribution transformers. Cumulative effect of low oil level and over loading is cited as one of the major causes of these accidents.
5.     Diagnosis and health assessment: It is near to impossible to carry out health assessment and diagnosis of DTs unlike power transformers due to high costs and practical difficulties.
6.     Sabotage, pilferage, and thefts: DTs in remote areas are often subjected to theft of oil, windings etc. In the effort, they are vandalized and sabotaged beyond recovery.
7.      Sub-standard quality: There is  tough competition and huge pressure on selling prices. To fight the competition, some manufacturers dilute the expected quality levels by using sub-standard raw materials and process short cuts. Such transformers even though qualify energy efficiency criterion, many times fail prematurely within warranty period.

All these issues have been haunting distribution transformer industry. The energy efficiency regulations have helped utilities up to some extent, but it has not given even a respite to manufacturers. It is required  to adopt new innovative trends in technology and manufacturing that can provide the desired outcomes.

4.     LATEST TECHNOLOGICAL TRENDS

Compared to power transformers, distribution transformers have been witnessing more technological advancements at global level. These developments are well grooved in most part of the world, however most of the south Asian countries continued to follow old traditional methods in making the transformers. The various options available as mentioned below can be evaluated in context of our nation.

a)     a)   Foil wound transformers
b)     Hybrid transformers
c)     Semi sealed dry type small DTs
d)     Pad mounted transformers
e)     3D transformers
f)      Amorphous core type
g)     K-class transformers
h)     Tap-less transformers
i)      Smart transformers 

a)     Foil wound transformers

The foil wound transformers[5] have low voltage windings (LV) made from thin sheets also called foil[6] of copper or aluminium sandwiched between interlayer insulation paper. The width of the sheet is equal to the axial height of the coil thus number of layers are same as number of turns. Now, high voltage coils can also be made with foil or sheet. Foil wound transformers offers many advantages over conventional strip wound types. 

 

Fig 1: Leakage flux and forces in windings[7]

Advantages:

1.     Superior short circuit strength[7]: Foil wound low voltage coils provide perfect ampere-turns balancing with high voltage coils. The axial forces because of this are practically nil. Service life of such transformers will be longer as they can effectively withstand short circuits.
2.       Better heat dissipation: Foil/sheet has large surface area available for heat dissipation.
3.       Lighter clamping structure: Core frames have to manage only radial forces; therefore, they can be made lighter, Axial clamping mechanisms like pressure rings and screws are not required.
4.     De-skilled process: Foil winding is easier to make. Complicated and high-risk operations like transposition, ride over or lead bending is not required to be done.

 

Fig 2: Automatic TIG welding for leads of foil winding [8]

 5.     High productivity: coils are made much faster compared to conventional windings.

Limitations:

1.  Performance wise there is no disadvantage of foil wound transformers reported.
2.  Sourcing: Due to limited demand, availability of foils in small quantities is a challenge. Copper foils beyond 400 mm widths are to be imported however aluminum foils have no such       limitations.
3.  Equipment: Foil windings are to be made on special machines. However, in India good quality and feature rich foil winding machines are manufactured and even exported. 

b)    Hybrid Transformers

This type of transformers brings more benefits. The LV coils are foil wound and HV coils are layer type strip wound like foil wound transformers. Material of LV and HV are kept different. If LV is of aluminum foil, then HV will be of copper and vice versa.

Advantages:

1. Aluminum strip wound HV coils are bigger in size and occupy more space compared to copper strip wound ones. LV of Aluminum foil and HV of copper strip is very compact. The footprints are also very small
2. Superior performance: Jointing of aluminum is difficult, if not done correctly, the HV windings can fail due to poor joints. This is not so with copper wound HV coils. Thus, LV aluminum foil with HV copper strip type hybrid transformer is as strong as transformers having both windings of copper.
3. Cost effective: Compared to copper wound transformers, hybrid transformers are cost effective. Saving in oil quantity is additional cost advantage.
4. Aluminum foil is easily available in India in all sizes and there are many sources.
5.  All other advantages of normal foil wound transformers

Limitations:

1. In India, copper is still considered a better material mostly because of its higher salvage value.
2. Special machines are required to make foil windings. However, good machines are available in India. 

c)     Semi sealed dry type DTs

Dry type transformers specifically cast resin transformers (CRT) in India are primarily limited to ratings between 200 KVA to 2500 KVA. Transformers below 200 KVA are mostly oil filled type.

 

Fig 3: A dry type outdoor transformer in service[9]

CRTs can be a better option especially for non-urban areas because of their numerous advantages over oil filled ones.

Advantages:

1. Maintenance: Being dry and semi sealed type they are almost maintenance free.
2. Failure safe: These transformers are free from problems arising due to low oil level and consequent faults. Damages due to moisture ingress are also comparatively less.
3.  Fire safety: These are comparatively more fire safe as there will be no chance of hot oil eruption. There are some incidences where pole mounted oil filled transformer exploded splashing hot oil on people. No such possibility is there with CRTs
4.  Ease of repair: On-site repair even replacement of damaged coils is easier. Coils can be replaced at site without need of oil filtration machines, drying out. Oil spillage and soil contamination also does not occur in case of CRTs.

Limitations:

1. Weather protection: Dry type transformers are air cooled type. Their enclosures are not water ingress proof. However, in some specific cases these transformers with specially designed enclosures (Fig 3) employed in outdoor applications are found to be  successfully working since many years.
2. Prone to failures due to partial discharges (PD). The design and manufacturing process should be capable of keeping the PD levels within safe limits.
3. Manufacturing set up: CRTs requires casting plants and PD test facility, which are costly. Especially, repairing companies would find it difficult to get good return on investments. 

d)    Pad mounted transformers

Pad mounted transformers are self-protected distribution transformers. These are more popular in Americas. They are installed on a concrete platform hence the name. They have HT and LT lockable panels integrated with  transformer body. The cables can be directly terminated. HT isolators, bayonet fuses, LT MCCBs etc are the inbuilt protection systems.

As the live terminals are housed in locked cabinets, they offer high level of safety. They can be installed in residential areas without any fence or guarding.

Fig 4: Panel side view of a pad mounted transformer [10]

Advantages:

1.  Most Compact: These transformers are very compact. They are smaller than compact or packaged substations.
2. Cost effective: Due to inbuilt protection and isolation systems they are cost effective compared to compact substations.
3. Safe: Live terminals are difficult to access. They are safer to the humans and tougher to be sabotaged.
4. Easy to manufacture: No special technology is required to make.

Limitations:

1. Protection devices are not available in India. A joint program for indigenization is required to make them cost effective.
2. Sealed type construction: Pad mounted are normally hermitically sealed fin wall tank construction. In India this technology is still not available to manufacturers, it is also not popular among utilities. 

e)     3D transformers

These transformers have different constructional details. The core is in the delta form, means 3 limbs of the core are at the vertices of an equilateral triangle. 3D wound cores are continuously wound 1-piece core ring assembled to form delta shaped construction. Negligible number of joints and air gaps results in low stray magnetic loss [11]

Advantages:

1. Magnetic circuit is shorter in length, and balanced (Fig 5), so better core optimization is possible
2. Core can be fabricated in-house.
3. Lesser level of CRGO inventory.
4. As core is continuously wound the process scrap is almost nil.

 

Fig 5: Phase wise exciting current in 3D and conventional stacked core transformers [11]

Limitations:

1. Cores are to be annealed. Special furnace and high energy consumption are the main drawbacks.
2. Special winding machines: Coils are wound over the core. Winding machines are to be specially designed. 

f)      Amorphous core type transformers

These are most familiar non-conventional transformers. Amorphous core transformers although have many advantages, are still not that widely popular as should have been. A transformer with amorphous core and foil winding is an excellent proposition.

Compared to conventional transformers, amorphous core transformers have- low core losses, low magnetizing current, less zero sequence current, less noise, higher inrush current, more harmonic problem, bigger size, higher initial cost, higher efficiency, and longer life. Advantages of amorphous core transformer outnumber its disadvantages [12].

Advantages:

1. Energy efficient: Typically, the iron loss of amorphous cores is 60 to 70% of CRGO core. This is the ideal core material for small transformers of energy efficiency level 3.
2. Their design with foil windings gives better results. Amorphous metal is very delicate and brittle. Under effect of transfer of forces from coils, it is susceptible for crushing/damage during short circuits. As axial forces are absent in foil type design, its combination with amorphous core can be very advantageous.

Limitations:

1. Special care in design and construction is required to prevent transfer of forces to the core packets. Most of the Indian manufacturers are not aware of it.
2. Amorphous core’s tiny flakes can be dangerous and may cause even failure of the transformers. They are to be contained within the core and have to be prevented from travelling along with circulating oil to coils/ live parts.
3. Reparability: If not designed rightly, the amorphous core packets get severely damaged during short circuits. These core packets have to be replaced with new ones. Secondly, core flakes can contaminate the coils and may cause failure post repair. 

g)     K-Class transformers

K-Class transformers as per IEC 60076 part 14 (2013) are liquid immersed transformers using “high temperature insulation materials” [14]. IS 1180 part 3 is specifically for distribution transformers filled with natural/ or synthetic ester fluids. Esters have the fire point higher than 300^OC degree centigrade and fall under K-Class insulation.

IEC 60076 part 14 has classified insulation in to four categories namely-

a)     Conventional insulation system

b)     Hybrid insulation system-

a.      Semi hybrid

b.     Mixed hybrid, and

c.      Fully hybrid

c)     High temperature insulation system

This categorization is based on type of insulation component – liquid insulation, conductor insulation, spacers/ strips, and solid barrier.

Out of above, IS 1180 – 3 has considered only Semi hybrid winding insulation in addition to conventional insulation. The thermal capabilities of ester fluids can be more effectively utilised with mixed or fully hybrid insulation system.

Advantages:

1.       Ester fluids offer many other benefits like biodegradability, fire safety, higher moisture absorption, slower aging [15], and delayed filtration.
2.       Easy to manufacture: No special equipment required. Standard set up of mineral oil transformers can be used with specific handling care.
3.       Overloading: These transformers can be subjected to continuous overloads without comprising on their useful life.

Limitations:

1.       Permissible temperature rise as per IS 1180-3 is 50^OC for top liquid and 55^OC for winding rise. At these levels the advantages of high temperature fluids are not fully utilised.
2.       Ester fluids properties are different than mineral oil like higher viscosity. The flow rate of mineral oil is about 28% higher than natural ester. Appropriate design considerations must be used in ester filled transformers[16].
3.       Natural ester fluid has higher affinity to oxygen so any direct contact with air must be prevented. Small distribution transformers have to be sealed type as it is not possible to have conservator with air-cell due to its small size. 

h)     Tap-less transformers

Off-circuit tap switch is helpful in tackling the voltage fluctuations on primary side. Except this it has many disadvantages. Moreover, it is not operated most of the times and because of this it has got no use.

IS 1180[1,2] has also suggested to do away with tap switch up to 100 KVA. And for higher ratings also it has left it for the user to decide.

A few private utilities in India have long opted for tap less transformers. However, if fine voltage control is desired by the users, On Load Tap Changer (OLTC) type transformers can be provided.

Advantages:

1.       Stronger HV coils: Ampere-turns balancing in tapped coils is challenging. Certain number of turns depending upon tap position are either removed or added to the circuit to get the desired output voltage on LV. This cause unbalance of  forces. In tap less coils all the turns are always remain in the circuit.
2.       Joint free coils: As there are no taps to be taken out the coils are without breaks and joints. Joints are the weak points in the coils, fewer the joints stronger is the coil.
3.       Reduced on-site failures: The tap switch is a mechanical device. Often while operating it, it’s moving and fixed contacts do not engage fully, causing arcing and erosion. This leads to failure of the transformers. After bushings, failure of transformers due to taps switches rank highest. Tap-less transformers will greatly reduce the failure of transformers.
4.       High productivity with less skill: Taking out taps in HV winding is a skill full operation and it also slows down the winding process. Tap-less coils can be made on high-speed machines by moderately skilled operators.
5.       Reduced oil volume: Tap switches are normally mounted on the top of the core. This space and thus the oil volume is saved if tap switch is eliminated.

Limitations:

1.                    No limitation as such is noticed with tap-less transformers. 

i)      Smart/ connected transformers

The DTs are in very remote and scattered areas. It is practically impossible to physically inspect, monitor and service them. This is the basic reason of high failure rates of DTs.

With modern advancement of technologies, remote access of any devices is possible. In case of transformers also this can be achieved easily. There are host of sensors in wide variety and range are available for almost any application. The distribution transformers can be made with in-built sensors. The sensors can be fixed on the joints, layers of coils, on bushings etc. to monitor their healthiness. The data so acquired can be transmitted to cloud and mobile networks and analysed suitably for transformer condition. Preventive measures can be taken well in time to prevent breakdowns and power outages.

Many companies are working on this concept and have already developed their products for power transformers. For DTs low-cost technologies can be developed.

Advantages:

1.       Catastrophic failures and site return of transformers will be reduced.
2.       Data based design improvements of DTs.
3.       Transparency in establishing causes of failure.
4.       Transformer over-loading can be monitored
5.       Assessment of residual life.
6.       Planned preventive care will be more efficient.
7.       Purchase of new DTs and phasing out old ones can be planned better.

Limitations:

1.       Data security: confidentiality of data its misuse by competitors.
2.       Depending upon the technology adopted, low to moderate price increase of DTs may be there.
3.       Disadvantage to small manufacturers, they lack required infrastructure for development.
4.       As the technology is at its initial stages, the costs are higher.   

CONCLUSIONS

In India manufacturing of distribution transformers even though having higher demand is losing its sheen. In a growing economy like ours, requirement of DTs will be large and growing. The DTs manufacturing must be strong and capable of delivering high-quality, long-lasting transformers to support the national growth objectives. Since few years manufacturers have been migrating from small distribution transformers which are primarily used by utilities to medium distribution and power transformers which are used by industries and private sector. Strict regulations like CRGO imports have helped utilities on energy saving, not much is done by them to ensure right use of transformers. Manufacturers are still searching for respite from transformer failures due to wrong use and poor operation and maintenance in the field.

This paper has attempted to touch upon recent technological trends in transformer manufacturing. These trends if adopted will be helpful in making more robust transformers. Such transformers can withstand tough working conditions; however, utilities must reform their operating mechanism to stop misuse and overuse of transformers.

The paper mentions key advantages and limitations of various non-conventional technologies. Many of these technologies are quite promising. The joint and concentrated efforts are required to be made to overcome the limitations. A national level research and development program may deliver solutions to these challenges.

REFERENCES

[1] BIS 1180 part 1 – Indian Standard for outdoor type oil immersed distribution transformers upto and including 2500 KVA, 33 KV (for mineral oil)

[2] BIS 1180 part 3- Indian Standard for outdoor type liquid immersed distribution transformers up to and including 2500 KVA, 33 KV (natural and synthetic ester filled).

[3] IEEE Loading guide for oil immersed transformers IEEE C57.91 1995

[4] IS 6600: Guide for loading oil immersed transformers

[5] Leonard Rabins, “Winding arrangement for foil wound transformers”, GE, Massasuchets, Patent no 3054974A, (expired 1979).

[6] CEA guidelines for specifications of energy efficient outdoor type three phase and single-phase distribution transformers, 2008.

[7] Nina Mesko, Branimir Crucic, Damir Zarko, “Short circuit stress calculations in oval windings”, 4th International Colloquium "Transformer Research and Asset Management”, 2017.

[8] Trishul Winding Solutions Pvt. Ltd., product catalogue “Programable foil winding machine”

[9] CESC Ltd, Kolkata

[10] WEG Instruction book #7-8102, 2020

[11] Thai Maxwell Electric, “TME introduction to 3D-1TC wound core technology (Eng) V1.1 180504”

[12] Nanda Hucharaddi, Mohsin Mulla, Swapnil Ghatage, “An overview of amorphous metal core transformer”, International journal of advanced computing and electronics technology, 2017.

[13] CEA guidelines for usage of amorphous and CRGO cores in distribution transformers. 2018

[14] IEC 60076 part 14 (2013), “Liquid immersed transformers using high temperature insulation materials”, 2013

[15] Myeong-Seop. Shim. “Comparative evaluation of aging of insulating material in natural ester and mineral oil”, Department of Electrical Engineering Inha University, Incheon, Republic of Korea, 2010.

[16] Ankita Garg, Thapar Institute of Engineering and Technology, Jeyabalan Velandy, CG Power and Industrial Solutions, “Thermal hydraulic network model for prediction of oil and temperature distribution in ester oil transformer under air natural and air forced cooling conditions”, IEEE 4th International Conference on condition assessment techniques in Electrical Systems (CATCON), 2019.