Effect of variations in load profile on power transformer cyclic ratings

Atkinson, Andrew Peter (2014) Effect of variations in load profile on power transformer cyclic ratings. [USQ Project]


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Cyclic rating and demand side management (DSM) schemes are employed by electrical supply authorities to manage peak demand as an alternative to costly network upgrades. This project investigated the currently unknown impact of DSM altered load profiles on the cyclic ratings of power transformers.

In response to electricity price increases, supply authorities are employing cyclic rating and DSM techniques to defer capital expenditure. A cyclic rating exploits transformer thermal inertia to permit loading above nameplate for parts of a 24 hour cycle, compensated for by loading below nameplate at other times such that insulation thermal limits are not exceeded. DSM aims to flatten load profiles commonly by transferring load from peak to off-peak times. Both techniques ensure that existing transformers can supply load peaks. Cyclic ratings achieve this by boosting capacity above demand while DSM reduces demand below capacity. In this way, the two techniques permit deferral of network upgrades. Since DSM alters the load profiles on which cyclic ratings are based, a relationship must exist between the two. The plant rating engineer must understand all such dependencies, hence the need for this project.

Initially, thermal transformer models proposed by various researchers and AS 60076.7 for the purpose of predicting transformer oil and paper temperatures were studied. The AS 60076.7 models and one selected from the literature were then implemented in Matlab and compared to assess their suitability for use in the project and by plant rating engineers. The selected thermal model then became the basis of a cyclic rating calculator employed to automate the computation of cyclic ratings based on the AS 60076.7 specification. DSM techniques were researched to reveal the load shifting version as the method favoured by supply authorities and a simulator was constructed in Matlab to modify load profiles accordingly. 12 diverse Ergon Energy transformers were then selected along with a set of DSM modified load profiles as inputs to the cyclic rating calculator which computed 600 cyclic ratings for analysis. The final phase of the project then involved analysis of the ratings to determine and quantify the effect of DSM caused load profile changes on cyclic ratings.

It was determined that the cyclic ratings of power transformers are negatively affected by load shifting DSM. That is, attening of load profiles causes reduction in cyclic ratings. The amount by which cyclic ratings change given a change in load profile varies according to several factors including: location, size and cooling mode of the
transformer. Regression models for simple but approximate prediction of cyclic rating changes were developed. Generalised expressions for predicting the change in cyclic
rating with change in peak load or load factor are:

ΔCRFd = 0:45 x ΔPLd
ΔCRFd = 0:37 x ΔLFi

where: ΔCRFd is the percentage decrease in CRF;
ΔPLd is the percentage decrease in peak load; &
ΔLFi is the percentage increase in load factor.

In addition to achievement of the main project objective - determination of the impact on cyclic ratings of load profile changes - a range of other outcomes from the project are useful. These include: a thorough investigation of the techniques and theory involved in transformer thermal modelling, insulation ageing, cyclic rating calculation and DSM; a program which automatically calculates cyclic ratings for power transformers; and a load shifting DSM simulation program, useful for generating altered load profiles for use with the cyclic rating calculator. This collection of knowledge and programs will
be of particular use to Ergon Energy, the project sponsor.

The impact of load shifting DSM on cyclic ratings, as identified in this dissertation, has the potential to negatively influence factors such as: peak capacity; transformer lifespan and maintenance; planning and budgeting for network augmentation; reduction of capital expenditure; and the valuation of DSM programs. The results and outcomes of this project have the potential to assist plant rating engineers in their understanding and application of cyclic ratings in the context of changing load profiles such that they may anticipate and therefore prevent many of the negative side-effects identified.

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Item Type: USQ Project
Item Status: Live Archive
Additional Information: Bachelor of Electrical and Electronic Engineering project.
Faculty/School / Institute/Centre: Historic - Faculty of Health, Engineering and Sciences - School of Mechanical and Electrical Engineering (1 Jul 2013 - 31 Dec 2021)
Supervisors: Ahfock, Tony
Date Deposited: 09 Sep 2015 05:20
Last Modified: 09 Mar 2016 02:52
Uncontrolled Keywords: power transformers; demand site management electrical supply; transformer thermal modelling; automated cyclic ratings; load profiles
Fields of Research (2008): 09 Engineering > 0906 Electrical and Electronic Engineering > 090607 Power and Energy Systems Engineering (excl. Renewable Power)
Fields of Research (2020): 40 ENGINEERING > 4008 Electrical engineering > 400805 Electrical energy transmission, networks and systems
URI: https://sear.unisq.edu.au/id/eprint/27292

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