1. Dry type transformers Zaragoza Traction application
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2. Index The product Traction transformers Special transformers Overloads
Effects of harmonics on the transformer
Special design
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3. The product From 250 kVA up to 40 MVA. High voltage: up to 72.5 kV.
Classes: E2, C2, F1.
Partial discharges: <10 pC.
Insulation class F or H.
Overloads from duty class.
6, 12 pulses, or pseudo 24 pulses.
Shift phase (+7,5º +15º …).
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4. The product Vacuum cast coil dry type transformer © ABB Group 2009
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5. Traction transformers I
Transformers used in variable speed drives that will feed traction systems, such as systems in:
Railway application.
Undergrounds.
Tramways.
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6. Traction transformers II
Applications AC
Transformers which feed systems in alternative current. DC
Transformers which feed systems in direct current.
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7. Special transformers 1. Overloads from duty demand.
Transformers for traction applications are non - standard transformers due to:
1. Overloads from duty demand.
2. Harmonics from rectifiers.
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8. Overloads According to EN 50329.
Each duty class correlates with an overload cycle.
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9. Overload cycles I
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10. Duty class according to UNE EN 50329
Example for a dutty class VI (rated power in kVA) and winding temperature rise admissible.
Rated power SN: 1000 kVA.
(1) Each power at each overloading condition, is referred to the fundamental component of the rated power. Heating temperature test must be performed at the rated power (including the current harmonics).
(2) According to the IEC winding temperature rise during the overload, must not exceed 120 K.
Power
Heating
p.u of Ib
p.u of In
Duration
kVA
(K)
1.215 1
2400
100 a
0.823
Cont
1975.2 80 b
1.5
1.234 2h
2961.6
120 c 3
2.468
60s
5923.2
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11. 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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12. Effects of harmonics on transformers II Source of current harmonics
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.
Non linear load generate harmonics
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13. 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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14. 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.
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15. Effects of harmonics of transformers V
Compatibility levels for voltage harmonics.
According to IEC for class 3 environments:
THD  10%
There are also limitations for individual value:
Odd order excluding multiple of three (3,9,15…).
Even order.
Odd order multiple of three.
Inter harmonics.
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16. Special design I
Due to the flow of harmonic currents in both low voltage and high voltage windings, there are extra losses and extra heating, thus the transformer must be over rated according to a higher equivalent power.
Due to the flow of harmonic currents through the network and the transformer impedance, there is a voltage distortion (voltage harmonics) on the transformer magnetic core, which could saturate it. In order to avoid core saturation, the magnetic core must be over sized.
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17. 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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18. Special design III
The number of turns of the two low voltage windings must be modified in order to reach the voltage ratio between these two low voltage windings.
The impedance between the two low voltage windings must be matched by calculating and manufacturing carefully the winding dimensions.
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19. Special design IV
The space factor is 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 tap changers instead of one circuit.
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20. © ABB Group 2009
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