Method and device to compensate for an asymmetrical dc bias current in a power transformer connected to a high voltage converter

A method to compensate for an asymmetrical DC bias current in a three-phase transformer, wherein the transformer is connected between an AC power system and an AC/DC or DC/AC high voltage converter, wherein the transformer is star connected on a side to the AC power system, wherein for each phase of the AC side of the transformer a current quantity is determined, wherein the current quantity reflects a time dependent behaviour of a magnetizing current in the phase, the method comprising:

• determining a first electrical phase angle range in the current quantity during which the current quantity reaches a positive maximum and a second electrical phase angle range is determined during which the current quantity reaches a negative maximum;
• demultiplexing the current quantity according to the first and second electrical phase angle ranges to create two current components;
• determining the amplitude of the positive maximum and the amplitude of the negative maximum for the two current components;
• determining a DC magnetizing quantity from a difference between the amplitude of the positive maximum and the amplitude of the negative maximum;
• determining an asymmetrical quantity by superimposing the DC magnetizing quantity of the phase with the DC magnetizing quantities of the other phases;
• and
• generating a control signal from the asymmetrical quantity and providing the generated control signal to a control device of the converter in order to adjust the generation of the AC or DC voltage in the particular phase of the converter which corresponds to the phase of the AC side of the transformer.

A device to compensate for asymmetrical DC bias current in a three-phase transformer, wherein the transformer is connected between an AC power system and an AC/DC or DC/AC high voltage converter, wherein the transformer is star connected on the side to the AC power system, the device comprising for each phase of the AC side of the transformer

• means to determine a current quantity, wherein the current quantity reflects the time dependent behaviour of the magnetizing current in that phase;
• means to determine a first electrical phase angle range in the current quantity during which the current quantity reaches a positive maximum, and to determine a second electrical phase angle range during which the current quantity reaches a negative maximum;
• a demultiplexer to create two current components by demultiplexing the current quantity according to the first and second electrical phase angle ranges;
• two peak detectors to determine for the two current components the amplitude of the positive maximum and the amplitude of the negative maximum, respectively;
• means to determine a DC magnetizing quantity from a difference between the amplitude of the positive maximum and the amplitude of the negative maximum;
• means to determine an asymmetrical quantity by superimposing the DC magnetizing quantity of the phase with the DC magnetizing quantities of the other phases;
• and
• means to generate a control signal from the asymmetrical quantity and to provide the control signal to a control device of the converter in order to adjust the generation of the AC or DC voltage in the particular phase of the converter which corresponds to the phase of the AC side of the transformer.