Industrial and Commercial Power SystemsTopic 4

DISTRIBUTION TRANSFORMERS

The University of New South Wales

School of Electrical Engineeringand Telecommunications

Pad-mountedsubstation

315kVA 22kVpole-mountedtransformer(Wilson Transformer)

140MVA 132kVgenerator transformer

A transformer is a static device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils.

Cut-away view of three-phase oil-cooled transformer.

The oil reservoir is visible at the top.

Radiative fins aid the dissipation of heat.

Range of EA standard substation transformers

Standard specifications

AS60076.1-2005: Power transformers – General

AS2374.7-1997: Power transformers - Loading guide for oil-immersed power transformers

AS2374.8-2000: Power transformers - Application guide

AS60076.11-2006: Power transformers - Dry-type transformers

AS3953-1996: Loading guide for dry-type power transformers

AS60044.2-2003: Instrument transformers - Voltage transformers

1

LIQUID INSULATION

Oil insulated distribution transformerONAN cooled type (Oil Natural Air Natural)

Liquid-insulated transformers

Kraft paper wound around winding conductors

paper impregnated with liquid dielectric to exclude air bubbles and to provide good thermal circulation for heat dissipation.

Use mineral oil in most applications

Such oil is highly flammable. Transformers generally limited to Class A materials temperature rise limits of about 60-70oC. Outdoor use only.

Moisture is a major problem.

Mineral oil

Refined from petroleum

Hydrocarbon compounds: paraffinic, napthenic, aromatic.

Additives sometimes added: Inhibitors to retard oxidation of oil Passivators to retard formation of copper sludge

Examples: Shell Diala Oil B: straight mineral napthenic solvent refined oil Shell Diala Oil BX: Diala B with added oxidation inhibitor

Askarel

essentially a Poly-Chlorinated Biphenyl (PCB)

artificial insulating oil which is almost non-flammable.

toxic effects, particularly if heated or burnt

now banned from use in most countries

some PCB insulated transformers still in service

Polychlorinated Biphenyls (PCBs)

Mixtures of synthetic organic chemicals with same basic

chemical structure and similar physical properties ranging

from oily liquids to waxy solids.

Non-flammable,

chemically stable,

high boiling point and electrical insulating properties.

Polychlorinated Biphenyls (PCBs) Used in many industrial and commercial applications

including: electrical, heat transfer, and hydraulic equipment; as plasticizers

in paints, plastics and rubber products; in pigments, dyes and carbonless copy paper and many other applications

Health effects: carcinogenic (cancer) and non-carcinogenic (effects on immune system, nervous system)

Silicone oil

tetrachloro-benzyl toluene with ~40% trichlorobenzene

essentially non-flammable and no toxicity problems

most favoured synthetic transformer insulating oil

convective heat dissipation coefficient not so good

electrical properties very similar to mineral oil

more expensive

Ester

Organic compounds that result from interaction of acids and alcohol

Natural or synthetic

Bio-degradable, no adverse toxic environmental or aquatic side-effects

Higher flash and fire points than mineral oil

More hygroscopic than mineral oil, less affected by moisture

Hygroscopic: Readily absorbing moisture, as from the atmosphere

Becoming more popular as alternative fluids for transformers

Ref: ABB Biotemp fluidH.T.H. : High Temperature Hydrocarbons

BIOTEMP® is currently in use in small power and distribution transformers across the United States.

BIOTEMP® is non-toxic to bird, animal and human life and is 97% biodegradable within 21 days when exposed to microbes.

The fluid does not contain any petroleum, halogens, silicones or other materials that might adversely effect the environment.

BIOTEMP Characteristics

2

DRY TYPE INSULATION

Open winding type

windings in open air paper-insulated or nomex-insulated windings or insulating varnish coating applied to windings

Problem with moisture ingress increase dielectric losses in insulation (dielectric

dissipation factor) reduce insulation strength

Dry-type transformer, open winding

Cast resin type

cast solid epoxy resin structure

much less susceptible to moisture ingress and absorption

differential expansion or contraction can cause cracking

more costly than open structure dry-type transformer and often more expensive than silicone oil transformers.

Dry-type transformer, cast resin

SF6 gas insulated transformer

non-flammable

used increasingly in buildings and in high-density areas

very expensive but very reliable

SF6 is a non-toxic gas with very good electrical insulation properties and thermal transfer properties

typically operate at ~2Atm where dielectric strength is similar to oil

greenhouse problems

SF6-N2 mixture as alternative

SF6 gas insulated transformer

Cost

Source: Midel web site - Transformer Expectation

Losses

Losses

Comparison of losses of different transformers types[for 1000kVA, 11kV/415V]

Source: Electrical Line Magazine Jan/Feb 2005

Reduction of insulation life loading effect on operating temperature

increased temperature causes increased chemical reactions in insulation and lead to deterioration by changing insulation composition

rule of thumb: increase of continuous operating temperature by 10oC causes reduction of insulation life by ~50%.

details in loading guides, e.g. AS 2374.7-1997 (oil-filled transformers) and AS 3953-1996 (dry-type transformers).

Transformer impedance and S/C

effect of transformer impedance on prospective fault current is significant

typically around 5% (4-6% for Dist. Trans.)

usually, leakage inductance component is major contribution to impedance

Transformer impedance and S/C

Transformer impedance and S/C

Tappings on windings

for adjusting voltage level

tapping points normally on HV windings (Why?)

allow ~10% voltage variation, in ~1% steps

manually changed (must de-energise and isolate transformer)

Tap changer can be installed on neutral point, mid or end points of the winding.

on-load tap changer (OLTC) not available

tapping points normally on HV windings (Why?)

HV winding has more turns which makes voltage control easier

HV winding is the outer winding and it is easier to install tap changer on that

HV winding has less current compared to LV winding. Thus, easier to manufacture a low current tap changer

Transformer with on-load tap changing [rating about 5000kVA]

Connections

many possible variations of winding connections

affect magnitude of voltages and phase shift between primary and secondary.

common winding connections (vector group; IEC): Dyn11 (most common, HV delta, LV star, neutral

brought out, LV leads HV by 30o) Dyn1 Dyn5 Dyn7

star-connected on LV side to eliminate circulating triplen harmonics.

HV side almost always delta connected

Vector group In the IEC vector group code, each letter stands for one set of windings.

The HV winding is designated with a capital letter, followed by medium or low voltage windings designated with a lowercase letter.

The digits following the letter codes indicate the difference in phase angle between the windings, with HV winding is taken as a reference.

The number is in units of 30 degrees.

For example, a transformer with a vector group of Dyn1 has a delta-connected HV winding and a wye-connected LV winding. The phase angle of the LV winding lags the HV by 30 degrees.

HV winding phase ‘A’ phasor points at 12 o’clock

LV winding phase ‘a’ phasor is oriented according to induced voltage relation which results from the connection.

Sense of rotation is counter-clockwise, giving the sequence ABC.

Ref: Annex D, AS60076.1-2005

Cable terminations

by means of a cable box

LV cable box usually air insulated

HV cable box is compound-filled with petroleum grease or similar viscous insulant

sealing of box to keep moisture out

moulded heat shrink terminations for XLPE cables: easy to construct

paper insulated termination required considerable expertise to make

Example of MV (11kV) cable terminations

Parrallel Operation

If transformers used in parallel, ensure they have:same voltage ratios

If not, will cause circulating current, overheatingsame tapping points in use (i.e. same voltage)same vector diagram (same phase shift)

If not, line and phase voltages will be intermixed, Overstress insulation.

same impedance angle (preferable) If not, will cause unequal loading

DISTRIBUTION TRANSFORMERS

PART 2

OPERATIONAL CHARACTERISTICSAND EFFICIENCY

1

CONSTRUCTION

Core and winding structure of a single phase transformer: (a) Core type, (b) Shell type

Core construction of 3-phase transformer(a) Core type (b) Shell type or 5 limb core

Core Form is the most prevalent type in use.

Types of transformer winding(a) Concentric, (b) Sandwich – made up of disc sections.

2

EQUIVALENT CIRCUITS

Ideal transformer with perfect flux coupling between primary and secondary windings.

Only winding resistance needed in equivalent circuit

Leakage flux

Equivalent circuit with leakage inductance

3

EXCITATION REQUIREMENT

(a) Magnetising current Im:

(b) Core loss and Ic:

Full equivalent circuit

(c) Total exciting current Io:

Equivalent circuit referred to the primary

Equivalent circuit referred to the secondary

Lumped equivalent circuit

Phasor diagram

Example:

Determine:

Solution:

4

TRANSFORMER LOSSES

Two components of transformer losses:

load (copper) loss in resistance of windingsload-dependent scales as square of load current temperature-dependent

core (iron) loss in the core materialcomprise of hysteresis and eddy current lossesconstant whenever transformer is energized

and independent of load

Copper loss:

Core loss:

5

TRANSFORMER EFFICIENCY

Efficiency

Power efficiency

Efficiency

very efficient, typically 95 – 99% varies with load maximum efficiency when core loss = load loss

Can you explain ?

Efficiency load will vary (usually) in a cyclic manner hence consider energy efficiency (instead of power

efficiency)

MEPS

AS2374.1.2-2003 Power Transformer Part 1.2: Minimum Energy Performance Standard (MEPS)requirement for distribution transformers

Power efficiency determined at 50% of rated load at unity power factor.

Apply to dry-type and oil-immersed type, 1φ and 3φ power transformers, 10kVA to 2.5MVA, used in 11kV and 22kV networks.

Standard also defines minimum efficiency levels for "High Power Efficiency Transformers".

Power efficiency for oil-immersed transformersAS2374.1.2-2003 Tables 1&3

Power efficiency for dry-type transformersAS2374.1.2-2003 Tables 2&4

6

TRANSFORMER TESTS

Open-circuit test

for core loss determination

require normal operating flux in core, hence need rated voltage applied.

no load connected so no load loss contribution in measured power, only (constant) core loss Po.

provides P0, I0 , Rc and Xm for equivalent circuit

Short-circuit test

for load loss determination

I2 (and I1) is rated current, but applied voltage V1 is impedance voltage level, only ~5%.

thus core flux density is ~5% and core loss is negligible.

but full rated currents flow in windings so measured power Psc is copper loss in winding resistances only.

test requires measurement of Psc, V1, I1 and I2.

test results give copper loss, total winding resistance Reqand leakage reactance Xeq

EFFECT OF HARMONICS

Harmonic problem fromnon-sinusoidal supply voltagenon-sinusoidal current (non-linear loads)

losses in transformer are frequency dependent core loss scaled with square of frequency copper loss increase with frequency due to skin

effect on the eddy current.

effect on core loss of load current harmonics is not generally significant and thus is neglected

only load loss increase is considered when de-rating calculations are performed.

thus assume pure sinusoidal supply voltage 2 methods to calculate de-rating factor:

CBMEA Crest Factor method IEEE K-factor method

CBEMA Crest Factor:

CBEMA crest factor method is not widely used.The more accurate K-factor method is preferred.

Find CF for triangular wave?

K-Factor method

Total harmonic distortionof a current waveform:

K-factor:

Example:100kW of PCs supplied from transformer rated at 150kVA and with a typical value of PEC(R) =10%.

harmonic current levels caused by the PCs

K-factor transformers

specifically designed for use with loads that produce harmonic distortion (without need to de-rate)

typically K-factor value of ~15

compare to standard type, K-factor transformers are: more expensive (twice) heavier (20% more) larger lower impedance

Fire of a pole-mounted distribution transformer(in Moscow)

Source: Intelligent Energy Europe Programme

Amorphous metal. Very low loss.Eg. HB1 from Metglass

Source: Intelligent Energy Europe Programme

1.6MVA amorphous iron core transformer (Ireland, 1998)No-load (core) loss = 384WLoad (copper) loss = 18.2kW

Thank you

Ester