Converter Transformer

Topics: Transformer, Alternating current, Direct current Pages: 9 (1793 words) Published: April 28, 2013
Converter Transformer

HVDC Systems for…
• Energy Transmission over long distances • Asynchronous coupling between AC Regional networks

Power Electronic circuits are used to convert AC to DC (Rectifier circuits) or convert DC to AC (Inverter Circuits). Both of these circuits are also called converter circuits.

A transformer that has one of its windings connected to one of these circuits, as a dedicated transformer, is a Converter Transformer

Converter Transformer in HVDC system….
•Supply of AC voltages into two separate circuits feeding the rectifier bridges with a phase shift of 30 electrical degrees for reduction of low order harmonics esp. 5th & 7th harmonics. •As a galvanic barrier between AC and DC systems to prevent DC potential entering into the AC system •Reactive Impedance in the AC supply to reduce short circuit currents and to control the rate of rise in valve current during commutation.

Converter Transformers for 12 pulse rectification….

Primary: Star 400 kV AC Secondary: Two windings connected to converter (Thyristors) connected in series to build up required level of DC voltage

Choice of transformer design is mainly ruled by….
– transport restrictions (dimensions and weight) – number of necessary spare transformers – technical possible solutions for core and windings

Converter Transformers for 12 pulse rectification
Type of Connections 3 Phase Star-Delta & StarStar Single phase 2 winding Singe phase 3 winding Extended deltaconnection 3 Phase 3 winding No. of design X No. of units 2X2 2X6 2x3 2X2 2X2 Spares required 2 2 1 1 1

Converter Transformer....
Design active part
Valve Winding (Y) Tap Winding Line Winding Valve Winding (D)



Simplified connection of line windings and tap windings Winding and core arrangement

Converter Xmer & Normal AC Xmer…..

• Polarity Reversal • Voltage Distribution in Oil Barrier System • Impedance variation influence • On load Tap Changer • Harmonic Currents • Losses • DC Magnetisation • Short Circuit Forces • DC Bushings

Under Polarity Reversal…..

Beginning voltage stress distributions capacitive – Oil is stressed more than PB Successively change over to Resistive distribution – PB is stressed more (almost all stress across solid insulation) PD stresses under DC – Sporadic pulses at random intervals -Discharges in oil gaps under rapid changes in voltage -Discharges in cellulose insulation due to imperfections in insulation -Discharges at the oil-cellulose interface

……. To meet above, special oil-barrier insulation system is required

Voltage Distribution in Oil-Barrier System...

Voltage Distribution in Oil-Barrier System...
• Main duct in HVDC transformers require more PB barriers than normal AC transformers as DC voltage is taken mainly by PB. Voltage distribution is by resistivity in steady state. - Transient DC voltages: • Start up of converter when full DC potential from bridges is developed almost instantaneously

- DC Voltage Polarity Reversal:
• • • • When direction of power flow is changed in HVDC system, current direction remains the same while polarity of the voltage will be reversed. This is done within a few number of power cycles. A sudden change in DC voltage is capacitive. Time constant for the transition from capacitive to resistive distribution is about an hour.

Voltage Distribution in Oil-Barrier System...



Insulation Design
• Valve windings to withstand AC voltages, superimposed DC voltages on AC voltages, DC Voltage Polarity Reversal Composite insulation of Pressboard or Paper (Solid) and oil (liquid) Voltage distribution between paper & oil under AC conditions depends on inverse ratio of dielectric constants of PB/ Oil (2:1) Voltage distribution between oil & paper under steady state DC voltage depends on direct ratio of resistivities (1:10 ~ 500), depends on oil quality, moisture content and temperature

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