Loading Power Transformers Beyond Nameplate Ratings(1)

7/28/2019 Loading Power Transformers Beyond Nameplate Ratings(1)

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“Ratron”

Loading Transformers Beyond

Nameplate Rating



Outline

• Background

• IEEE C57.91-1995• Calculations

• Benefits and Applications

• Conclusions



• 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



– 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



• Using the IEEE C57.91 standard to

calculate:

– Normal loading

– Long term emergency

– Short term emergency

– Loss of life



• 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



• 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



• 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



• 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



• 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



• 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)



Input Tab



Results



The Grid Tab

Here you can edit

load profile and

ambient temperature



Graphical tab




– 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




– 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



Please visit our website for more informationwww.synchrogrid.com

Loading Power Transformers Beyond Nameplate Ratings(1)

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