Indian Standard for Distribution Transformers- Suggestions for Revision

 P Ramachandran, 06 may 2022

1. Introduction

Indian Standard on Distribution Transformers was first issued in 1958, covering distribution transformers up to 100 kVA. The first part of the fourth revision to this standard came out in 2014 with 4 amendments till 2021, covering transformers up to 2500 kVA 33 kV. This is titled “Outdoor/Indoor Type Liquid Immersed Distribution Transformers up to and including 2500 kVA 33kV -Specification- Part 1: Mineral Oil Immersed. Part 3 of the standard, “Natural /Synthetic Organic Ester liquid Immersed “was published in 2021. It is understood Part 2 of the standard, covering Self-Protected Mineral Oil Immersed Transformers is planned for the future.

National Standards (IEC/IEEE/EN) that cover distribution transformers can be listed below:

IEC Standards:

• 60076-13-2006 Self Protected liquid-filled Transformers

IEEE Standards:

• 12.20 -2017 – Overhead Type Distribution Transformers up to 500 kVA 33 kV
• 12.23- 2108 – Submersible Single-Phase Transformers up to 250 kVA 34.5 kV
• 12.24-2016 –Submersible Three Phase Distribution Transformers up to 3.75 MVA, 34.5 kV
• C57,12.34-2014- Pad-mounted Distribution Transformers – up to 5 MVA
• 12.36-2017 -Standard Requirements for Liquid immersed Distribution Substation Transformers
• C57,12.38-2014 – Single-Phase Pad-Mounted Distribution Transformers up to 167 kVA, 33 kV
• 12.40-2011- Network three-phase Transformers (Subway &Vault Type)
• 12.44-2014-Standard Requirements for Secondary Network Protectors

EN Standard

• EN 50464-4 Three-phase oil-immersed distribution transformers 50 Hz, from 50 kVA to 2 500 kVA with highest voltage for equipment not exceeding 36 kV — Part 4: Requirements and tests concerning pressurized corrugated tanks
• EN 50588-1-2015 -Medium Power Transformers up to 36 kV-General Requirements.
• EN 50588-2-2018 -General Requirements of Cable Boxes
• EN 50588-3-2018 -Medium Power Transformer with Type 1 Cable Box
• EN 50588-4-2018-Medium Power Transformer with Cable Box.

An effort is made to critically review the current 1180 Standards and some suggestions are put forward for consideration during the next revision of these standards.

2. Reducing the variety of Power (kVA) Ratings.

The rated power of distribution transformers is selected from the R10 number series given in ISO 3:1973 Preferred Numbers-Series of preferred numbers. It means ratings between 1-10 are in ten steps each incremental rating being multiplied by 1.26 (the tenth root of 10) ie 1-1.25 -1.6 -2 – 2.5 -3.15- 4-5 – 6.3- 8- 10 kVA or MVA and their multiples by 10 &100. This results in a large variety of transformer sizes.  If we use the R5 series for selection (5 steps in multiples of the fifth root of 10=1.58),the sizes will be 1-1.6 – 2.5 – 4 – 6.3- 10 kVA or MVA, and multiples of 10 &100 for higher ratings. This way the kVA ratings of distribution transformers can be halved. The loss of economy by using a larger power rating will be offset by economies of scale achieved by the variety reduction resulting from such standardization. When transformer rating goes up, from say, 100 kVA to 200 kVA, the weights, losses, and cost of the transformer will not be doubled but increase by only 1.7 times (2^0.75) Hence for a fixed load, it is always economical to select a slightly higher transformer power rating, considering the future power demands too.

3. Permissible Working Flux Density

In the IS 1180-1, under clauses 6.9 & 7.9 the following stipulation is given:

Clause 7.9.1 The maximum flux density in any part of the core and yoke at rated voltage and frequency shall be such that the flux density with + 12.5 percent combined voltage and frequency variation from rated voltage and frequency does not exceed 1.9 Tesla. NOTE — The design calculations in support of flux density shall be furnished by the manufacturer.

The above stipulation is applicable only for CRGO, but there is no mention of the flux density as far as the Amorphous core is concerned. Hence the following addition may be made.

“ ….. not exceed 1.9 T in case of CRGO core,1.48 T with conventional grade Amorphous core and 1.56 T with high permeability Amorphous core “

4. Energy Efficiency Levels.

IEC standard 60076-20: 2017” Power Transformers Energy Efficiency” proposes three methods for evaluating the energy performance of a transformer.  These are based on the practices followed in different countries of the world.

• the Peak Efficiency Index (PEI) should be used in conjunction with either a total cost of ownership (TCO) approach or any other means of specifying the load factor.
• the no-load and load losses at rated power for rationalization of transformer cores and coils for transformers generally produced in large volumes;
• the efficiency at a defined power factor and particular load factor (typically at 50 %).

From the above, it is clear that IEC is recommending Method (b) for Distribution Transformers from getting the full benefit from standardization (.…produced in large volumes…) ie specify the maximum no-load and load losses.

 

Country	Energy Performance Index	Standard
Australia/New Zealand	Efficiency at 50% load	AS 2374.1.2-2003
Brazil	Max no-load and load losses at 100% load	ABNT NBR 5356; 5440
Canada	Efficiency at 50% load	CSA C802.1
China	Max no-load and load losses at 100% load	JB/T 10317-02 GB 20052-2013
EU	Max no-load and load losses at 100% load,PEI > 3,150kva	EN50588-1:2014
India	Max total losses at 50% and at 100% load	IS 1180:2014 & Goal Gazette 2968
Israel	Max total losses at 100% load	IS 5484
Japan	Max total losses at 40% or 50% load	IS 5484
Maxico	Efficiency at 50% load	NOM-002- SEDE-1997
Korea	Efficiency at 50% load	KS C4306,C4316 and C4317
USA	Efficiency at 50% load	10 CFR 431
Vietnam	Efficiency at 50% load	TCVN 8525:2015

Table1 Energy Performance Indices of various countries for oil-filled Distribution Transformers (Ref: 2022-Bhaba P. Das)

 

But IS 1180-1 has a specified maximum of ( No-Load  + Load Losses) at 50 % and 100 % load factors.  It can be seen from Table 1, that no-other country or standard is specifying transformer losses in this way. IS:1180 is specifying 5 levels of losses for each transformer power rating.

315 kVA 11 kV Distribution Transformer Maximum Losses

1. EN 50588 -1: 2014 (European Standard)

From July 1, 2015 -Maximum no-load/load losses – 360/3,900 W (4 %) Impedance

From May1,2021 – 324 /2,800 W

2. IEC 60076-20: 2017 Power Transformer Energy Efficiency

Level 1 – 360/3900 W Level 2- 324/2800 W (Maximum no-load/load losses)

3. IS 1180-1: 2014 & Amendment 2021 Oil Filled Distribution Transformers

Maximum total losses at 50 % / 100 % load factor

Level 1- 1025/ 3100 W (IEC/EN- total loss at 100% load factor- 4260 W from 2015 & 3124 W from 2021)

Level 2- 955/2750 W

Level 3- 890/2440 W

Level 4- 829/2164 W

Level 5- 772/1920 W

The following can be concluded from the above figures:

• While Europe/IEC is specifying one or two levels of losses, Indian Standard is stipulating five levels of losses for each rating. This defeats the whole purpose of standardization and rationalization. It means five separate core-coil assemblies are required with each manufacturer and all thetype and special tests are to be done for each core-coil assembly.
• The losses stipulated by IS are far below the IEC/EN losses. Even Level 1 of Indian Standard is less than IEC/EN standard losses, requiring large quantities of copper and core material to achieve these losses. We should have a fresh look at maximum losses and capitalization rates because the price of active materials is going up steadily during the past decade while the cost of energy is going down after considering recent inflation rates.
• It is suggested that Indian Standard may stipulate only two-loss levels -one with low no-load losses and high copper losses for load factors below 50 % and high no-load losses and low copper losses for load factors above 50 %.
• Instead of total losses at 50 % and 100 % load, only maximum no-load and load losses may be specified.
• To arrive at the standard loss levels, three or four leading manufacturers may prepare competitive designs with a maximum current density of 3 A/mm2 and flux density of 1.65T (1.3 &1.35 T for Amorphous). Optimization (manufacturing cost + loss capitalization) may be checked using realistic loss capitalization rates.Based on this study new minimum energy performance (MEPS) standards with a maximum of two levels of losses may be formulated.
• The standardized losses may be frozen for at least ten years so that repeated type tests and special tests like short circuit tests can be avoided.

 

4. Separate Standard for Ester filled Transformers

Part 3 of IS 1180 was issued in 2021, titled “Outdoor/ Indoor Type Liquid Immersed distribution transformers up to and including 2500 kVA 33 kV -Specifications -Part 3 Natural/ Synthetic Organic Ester liquid Immersed “.Transformer performance parameters are not affected by the insulating fluid used, whether esters or silicon oil and IEC and other National Standards on Power Transformers do not have a separate standard for ester-filled transformers. The standards are titled Liquid immersed transformers to cover any type of insulating fluids. Hence this Part 3 is redundant and may be withdrawn. Part 1 may be revised from Mineral Oil-immersed to Liquid Immersed Transformers.

 

5. Outdoor/ Indoor Type

IEC /EN National Standards for Transformers are applicable for both indoor and outdoor use. To mention the same in the title seems superfluous.

6. Conclusions:

IS: 1180 was issued in 2014 and there were extensive revisions issued through four amendments. It is time for reviewing the standard for the next revision as it will take at least two years to finalize the Rev 5 of IS 1180. A few suggestions are put up for consideration while revising the standard for a new edition.

 

7. References:

2022- BhabaP.Das, Rob Milledge, Review of distribution transformer energy-efficient metrics: in the Australian and New Zealand Context, Transformer Magazine Vol 9, Issue 2,2022