Converter Transformers

Power Electronic circuits are used to convert AC to DC (Rectifier circuits) or 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.

Type-

The converter transformer can have different configurations:

(i) Three phase, two winding (Star-Delta & Star-Star)

(ii) Single phase, three winding; and

(iii) Single phase, two winding.

Converter Transformer are multi pulse, multi winding rectifier Transformers for Motor Drives applications. The number of secondary windings can be 12, 24 &36 in case of Oil cooled transformer. The valve-side windings are connected in star and delta with neutral point ungrounded. On the AC side, the transformers are connected in parallel with the neutral grounded. The leakage impedance of the transformer (typical value between 15% and 18%) is chosen to limit the short-circuit currents through any valve.

Application-

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 duringcommutation.

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

Our design-

The insulation of the transformer windings is specially designed to withstand a large DC potential to earth. While designing, various influencing factors are considered such as- high harmonic content loadsin the voltage and in the current loads. It is provided with an electrostatic screen/shieldlinked to the ground between the primary and secondary windings. It is very useful against the capacitive effect arriving from the secondary windings and reduces at the same time the transmission of interferencesbetween the primary and secondary coils.

Reference Standard-

IS 2026 : 2011

IEC 61378-1 (1.0) 1997-09 Converter Transformers ?Part I Transformers for Industrial Applications

IEC 61378-2 (1.0) 2001-02 Converter Transformers ?Part 2 Transformers for HVDC Applications

Range- Up to 5 MVA

Salient Features

1. Induction / Thyristor duty

2. Suitable for 3 Pulse / 6 Pulse / 12 Pulse / 18 Pulse drive configurations

3. Extra precaution for mitigating harmonics

4. Confirming to latest Indian Standard (IS 2026)

5. Up to 5000kVA

6. Tailor made for various input voltages