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Titlea new approach to assessment and utilisation of distribution power transformers
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Page 1

Chapter One: Introduction

A New Approach to Assessment and Utilisation of Distribution Power Transformers – S. Corhodzic PhD Thesis 1


1. INTRODUCTION



1.1. Distribution Transformer – Key Power System Component

Electrical power systems utilise several voltage levels. By the time electrical energy is

received at most consumers’ connection points (at 415 V for three phase supply and at 240

V for single phase supply), it has been usually transformed through up to five voltage

levels: initially being stepped up to 500 kV by the generator transformer, then down to 220

or 110 kV via a terminal substation transformer, then down to 66 - 33 kV at a bulk-supply

point, further down to 22 - 11 kV in a zone substation and finally at a local distribution

substation to a level acceptable by the Low Voltage (LV) networks - 415 V. The last of

these voltage transformations is being performed in one of the key components of the

electrical power system - a distribution power transformer.

Majority of the consumers of electrical energy are connected at this LV level (all residential

and bulk of smaller industrial and commercial customers). Some larger customers, such as

factories, mines, large office buildings or hospitals are connected to the electrical networks

at 11 - 33 kV (or even at higher sub-transmission and transmission voltages 66 - 500 kV).

These High Voltage (HV) customers can operate some of their specialised equipment at

higher voltages, however, they still have to employ their own (non-utility owned)

distribution transformers to provide supply for their local general LV loads. Distribution

transformer is also used, albeit to a much lesser extent, to enable connection of embedded

generators to the distribution networks.

Distribution transformers can be identified by voltage and rating (capacity). The voltage is

most commonly specified as a pair of input/output values (e.g. 11/0.415 kV). The rating

Page 2

Chapter One: Introduction

A New Approach to Assessment and Utilisation of Distribution Power Transformers – S. Corhodzic PhD Thesis 2


of a transformer indicates the amount of power it can transfer between its two sets of

terminals. For example, a transformer with a rated capacity of 500 kVA is designed to

continuously transfer its full load of 500 kVA under standard operating conditions (as

defined in the AS 2374.1: 1997).

By convention, the electrical power system comprises of the “transmission” and the

“distribution” networks. However, the voltage levels allocated to each type of network

slightly differ between Australian electrical utilities. Generally, transmission transformers

operate in the voltage range from 66 to 500 kV and within range of ratings from 3 MVA

to few hundreds MVA. These transformers are sometimes called “power transformers”,

although this term, according to AS 2374.1, encompasses “distribution transformers” as

well. The commonly used term “distribution transformer“ as defined in Australian

Standard AS 2374.1 (Power Transformers) describes group of power transformers, which

operate in the voltage range up to 33 kV and have ratings 10 – 2,500 kVA. It is estimated

that there are about 600,000 distribution transformers owned by Australian electrical

distribution and transmission utilities. The bulk of these transformers are owned by the

electrical distribution companies which operate LV networks (415 V) and Medium Voltage

(MV) networks (generally 11 - 33 kV, although some distribution companies own some

assets at higher voltages). Number of distribution transformers installed in electrical

distribution networks is estimated to grow at approximately 1.5% per annum, GWA

(2002).

Strong growth of the Australian economy in the last 10 years suggests that the current

stock of non-utility owned transformers (estimated at 115,000 in 2005) increases at a rate

of 2.5% per annum, ABARE (2004).

Page 46

Chapter Four: Distribution Transformer Engineering Analysis – Technology Assessment and Design Issues



A New Approach to Assessment and Utilisation of Distribution Power Transformers – S. Corhodzic PhD Thesis 46


The distribution transformer changes the alternating current from a primary voltage to a

secondary voltage. For the most common step-down distribution transformers in Australia

the primary voltage is usually 11, 22 or 33 kV (HV side) and the secondary voltage is 415 -

433 V (LV side). Distribution transformer transforms the voltage through an alternating

magnetic field in the core, which is created by the primary winding. The magnetic field

induces the secondary voltage in the secondary winding. The change in voltage is made

possible through the different number of turns in the primary and the secondary windings.

Distribution transformers are very efficient devices as their losses are generally very small, in

order of a few percent of the total power transferred through the transformer windings. The

transformer losses include two types of losses:

no-load losses (core or iron losses);

load losses (winding or copper losses).

No load losses are constant energy losses, which occur as soon as distribution transformer is

energised (even if the load is not connected). No load losses result in generation of heat in

the core. These losses consist of two major components:

hysteresis losses caused by the magnetic reluctance of the core;

eddy current losses due to currents induced in the core by the magnetic field.

The load losses occur in both the primary and secondary windings. They are consequences

of the electrical resistance in the windings. The load losses increase with the square of the

load connected to the transformer. In principle, increases in transformer efficiencies are

oriented towards design options, engineering practices and manufacturing techniques related

to reduction of transformer losses associated with these two assemblies: the core and the

windings.

Page 47

Chapter Four: Distribution Transformer Engineering Analysis – Technology Assessment and Design Issues



A New Approach to Assessment and Utilisation of Distribution Power Transformers – S. Corhodzic PhD Thesis 47


Generally, reduction of distribution transformer losses is a trade-off issue against higher

manufacturing costs, i.e. more economical design means higher losses and lower losses are

associated with a more expensive distribution transformer.



4.3. Technology Assessment

Technology assessment for distribution transformers under consideration is limited to

assessment of the active parts of typical Australian distribution transformers. For

comparison purposes, some consideration is also given to alternative design options

available in the USA and Europe.

Conductor materials presently used in windings for distribution transformer applications

include aluminium and copper. In a very limited number of cases, aluminium and copper

alloys have also been applied. Conductors for distribution transformers are utilised in form

of standard size wires and foils.

The following summary includes comparison of aluminium and copper used in an

identical distribution transformer application:

copper has a higher electrical conductivity and about 40% lower resistive losses;

aluminium has lower eddy current losses due to higher resistance;

aluminium has lower mechanical strength, but it is easier to form and work with;

aluminium is also less expensive than copper;

there are low load loss designs which utilise aluminium, however due to larger

conductor cross sectional area a required bigger distribution transformer core,

these designs have higher no load losses.

Page 91

Chapter Five: Capitalization of Distribution Transformer Losses



A New Approach to Assessment and Utilisation of Distribution Power Transformers – S. Corhodzic PhD Thesis 91


where IC is the initial installed cost, OCn is the operating cost in year n (including value of

losses and maintenance costs) and DR is the discount rate.

The Total Operating Cost (TOC) is calculated in Equation [18] and includes the capital

cost and cost of losses.

The LCC and TOC methods are very similar and in some cases TOC and LCC

methodologies do not produce significantly different answers.

These two methodologies, however, are often used in different contexts. The LCC analysis

used by the DOE evaluates costs and benefits before taxes and analyses economics in real

inflation-adjusted dollars. The TOC analysis, used by many US and Australian utilities,

considers after-tax revenues and costs as well as nominal prices and discount rates IEEE

(2001).

The LCC model developed by DOE annualises capacity costs by applying a capital

recovery factor, which is based on the real discount rate. The capital recovery factor

multiplied by the unit capacity cost gives the annualised unit cost of capacity. This

annualised capacity cost is then applied to the annual capacity requirement and included in

the cost stream that is evaluated for the LCC analysis DOE LCC (2002).

“The TOC analysis method uses a combination of levelised cost components to calculate

loss evaluation coefficients. The TOC uses a methodology common in utility rate-making

to calculate revenue requirements through the use of a Fixed Charge Rate (FCR). The FCR

includes mark-up for taxes and other expenses to assure that revenue streams set on the

basis of the FCR will maintain the value of the company. For a TOC calculation, the

capacity costs are multiplied by a fixed charge rate and then divided by the fixed charge

Page 92

Chapter Five: Capitalization of Distribution Transformer Losses



A New Approach to Assessment and Utilisation of Distribution Power Transformers – S. Corhodzic PhD Thesis 92


rate when calculating loss evaluation coefficients. The result is an answer that depends on

the capacity cost, but which is insensitive to the fixed charge rate.

The LCC calculates forecasted annual costs, aggregates them into the annual operating

costs, and calculates the present value of the annual cost stream. This method uses a

simple capital recovery factor and assumes that there is no net impact from taxes and

other utility expenses that are not explicitly accounted for in the analysis.

With recent changes brought about by the restructuring of the electricity industry, in which

utilities obtain electricity from wholesale markets at the margin, the generation capacity

costs are implicit in the correlations between peak prices and loads. The TOC

methodology currently does not have a mechanism for incorporating these economic

effects. The LCC methodology used by DOE with its hourly load profiles does capture the

impact of peak wholesale market prices on the operating costs of transformer losses by

including the impact of peak wholesale prices on the economic value of load losses DOE

LCC (2002).

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