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“Ratron” Loading Transformers Beyond Nameplate Rating
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Loading Power Transformers Beyond Nameplate Ratings(1)

Apr 03, 2018

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Page 1: Loading Power Transformers Beyond Nameplate Ratings(1)

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“Ratron” 

Loading Transformers Beyond

Nameplate Rating

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Outline

• Background

• IEEE C57.91-1995• Calculations

• Benefits and Applications

• Conclusions

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• Background

 – Most utilities limit their transformers to

nameplate ratings

 – Emergency ratings do not exist

 – Usually a rating philosophy is not in place

 – Settings of overload relays is inaccurate

 – Dispatchers reroute loads or load shed

costumers

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 – IEEE Guide for Loading Mineral-Oil-Immerse

transformers (Std C57.91-1995)

• Transformers can be loaded beyond nameplate

ratings during critical conditions and peak hours• Offers a method to calculate continuous, long term

emergency and short term emergency ratings

• Offers a method to calculate the loss of life of the

transformer 

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• Using the IEEE C57.91 standard to

calculate:

 – Normal loading

 – Long term emergency

 – Short term emergency

 – Loss of life

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• Normal Life expectancy loading

 – This results when the transformer is operated

continuously at a hottest spot conductor 

temperature of 110 deg C

 – The standard allows a maximum temperature

of 120 deg C only if the transformer has

operated below 110 deg C for longer periodsof time

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• Planned Loading Beyond Nameplate

Rating

 – Transformers that do not work at a continuous

temperature of 110 deg C, can be loaded to a

maximum temperature of 120-130 deg

 – Loss of insulation life should be considered on

the length of time at this temperature

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• Long Term Emergency Loading

 – During contingency conditions, transformers

hottest spot conductor temperature can reach

120-140 deg C

 – Time will depend on loss of life

 – It is expected that such occurrences happen

only 2-3 times during the life of thetransformer 

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• Short Term Emergency Loadings

 – During second or third contingency problems,

the transformer can be allowed to reach a

maximum hot-spot conductor temperature of 

180 deg C

 – Maximum time of 2-3 hours and an

occurrence of once or twice during the life of the transformer 

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• Calculations with Ratron:

 – Using the standard to calculate transformer 

ratings can be time consuming and difficult

 – Ratron makes it easier for the user to input

data he is familiar with and data that is

available to him

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• Data Required

 – Top-Oil temperature rise above ambient (test

report)

 – Winding average temperature above ambient

(nameplate 65 or 55deg C) (nameplate)

 – Tank radiator oil capacity (nameplate)

 – Tank and fittings weight (nameplate)

 – Cooling mode (FOA,OA, FA) (nameplate)

 – Load and no load losses (test report)

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Input Tab

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Results

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The Grid Tab

Here you can edit

load profile and

ambient temperature

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Graphical tab

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• Benefits and Applications

 – Transmission Planning•  Apply transformer emergency ratings for power 

flow and contingency analysis – Real time operations

• Prepare dispatching centers with emergencyratings until they mitigate power failures

 – Relay settings• Set overload relay settings based on the calculated

emergency ratings of the transformer 

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• Benefits and Applications

 – When two transformers are in parallel, emergency

ratings can help you know how much the other 

transformer can be overloaded.• Rating Philosophies

 – Ratron offers an easy way to create transformer rating

philosophies

• NERC compliance – Ratron allows you to back up your methodology and

gets you ready for NERC auditing

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