Zaragoza Factory Railways Application En

8/12/2019 Zaragoza Factory Railways Application En

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Dry type transformers ZaragozaTraction application


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The product

Traction transformers

Special transformers

OverloadsEffects of harmonics on the transformer

Special design

Index


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The product

From 250 kVA up to 40 MVA.

High voltage: up to 72.5 kV.

Classes: E2, C2, F1.

Partial discharges:


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The product

Vacuum cast coil dry

type transformer


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Transformers used in variable speed drives that will feed traction

systems, such as systems in:

Railway application.

Undergrounds.

Tramways.

Traction transformers I


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Applications

AC

Transformers which feed systems in alternative current.

DC

Transformers which feed systems in direct current.

Traction transformers II


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Special transformers

Transformers for traction applications are non - standard

transformers due to:

1.Overloads from duty demand.

2.Harmonics from rectifiers.


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Overloads

According to EN 50329.

Each duty class correlates with an overload cycle.


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Overload cycles I


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Duty class according to UNE EN 50329

Example for a dutty class VI (rated power in kVA) and windingtemperature rise admissible.

Rated power SN: 1000 kVA.

(1) Each power at each overloading condition, is referred to the fundamentalcomponent of the rated power. Heating temperature test must be performed at the ratedpower (including the current harmonics).

(2) According to the IEC 60905 winding temperature rise during the overload,must not exceed 120 K.

Power Heatingp.u of Ib p.u of In Duration kVA (K)

1.215 1 2400 100a 1 0.823 Cont 1975.2 80b 1.5 1.234 2h 2961.6 120c 3 2.468 60s 5923.2 120


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Effects of harmonics on transformers I

Harmonics are distortions of the mains supply occurring at multiples of

the supply frequency. Any equipment which uses electronics to change

one voltage and / or frequency to another will generate harmonic

currents and consequently voltage distortion.


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Switching the line current with line frequency or

its multiple by means of electronic switches.

No-linear impedance.

Current dependant resistances: Arc

furnaces, welding machines, fluorescent

lamps

Voltage dependant inductance:

Transformers and core reactors.

Switching on saturable inductance as

induction motors or transformers.

Effects of harmonics on transformers IISource of current harmonics

Non linear load generate

harmonics


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Effects of harmonics on transformers III

Sources of current harmonics.

Rectifiers bridges feed.

Sources of voltage harmonics.

Voltage drops in circuit impedance due to current harmonics.

Voltage shape not fully sinusoidal.

Effects of voltage harmonics.

Increase of no load losses.

Increase of noise level.

Effects of current harmonics.

Increase of the load losses.

Local overheating due to uneven distribution of the eddy losses.

Eventually resonance over voltages.


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Effects of harmonics of transformers IV

Evaluation of harmonic content

Total harmonic distortion factor:

The ratio of the r.m.s. value of the sum of all the harmonic components up to

a specific order and the r.m.s. value of the fundamental component:

The limitation of THD is aimed to prevent the simultaneous presence of

several harmonics components with high amplitude.

Hh

h Q

QhTHD2

2

1


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Effects of harmonics of transformers V

Compatibility levels for voltage harmonics.

According to IEC 61000-2-4 for class 3 environments:

THD 10%

There are also limitationsfor individual value:

Odd order excluding multiple of three (3,9,15).

Even order.

Odd order multiple of three.

Inter harmonics.


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Special design I

Due to the flow of harmonic currentsin both low voltage and highvoltage windings, there are extra lossesand extra heating, thus thetransformer must be over rated according to a higher equivalentpower.

Due to the flow of harmonic currentsthrough the network and thetransformer impedance, there is a voltage distortion(voltageharmonics) on the transformer magnetic core, which could saturate it.In order to avoid core saturation, the magnetic core must be oversized.


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Special design II

To avoid capacitate coupling between high voltage and low voltage

and protect the power electronics devices on low voltage side from

over voltages in high voltage side, it is recommended to place an

electrostatic shield between high voltage and low voltage windings.

In some cases due to floating systems or high du/dt, higher insulation

levels on low voltage are needed.


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The number of turnsof the two low voltage windings must be modifiedin order to reach the voltage ratio between these two low voltagewindings.

The impedancebetween the two low voltage windings must be

matched by calculating and manufacturing carefully the windingdimensions.

Special design III


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The space factoris more critical because of the insulation gap

between windings, and the larger size of the transformer.

In order to guarantee the correct losses and good operation, the HV

winding is split in two or more parallel circuits with two o more tapchangers instead of one circuit.

Special design IV


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Zaragoza Factory Railways Application En