6.8　DC Biasing Current

In system design，the total DC current flowing in the windings of converter transformer due to various causes shall be calculated to determine the corresponding characteristics of the main equipment and control，protection functions，so that the HVDC transmission system can operate continuously and unrestrictedly in any of the specified operation modes without affecting the service life of equipment or degrading the specified performance requirements.

For long distance HVDC transmission systems，due to unbalanced firing angle，power frequency current flowing in the DC line，positive sequence 2nd harmonic voltage occurring on the converter AC bus，and during monopolar ground return operation，the earth potential of the converter station rising in relation to remote earth due to injection of current into the electrode，all of these will produce DC current in the converter transformer windings.

To calculate the DC current produced in converter transformer windings，the following are considered:

1　Unbalanced firing angle，the maximum possible unbalance shall be taken according to the design and manufacturing capability.

2　Positive sequence 2nd harmonic voltage on the converter AC bus，which is usually taken as 1% of the power frequency voltage.

3　Power frequency current flowing in the DC line，which is usually determined through special studies，such as The Influence between AC and DC Transmission Lines Built in Same Corridor.

4　Rise of earth potential of the converter station due to current in the electrode，DC current produced in the neutral circuit of the converter transformer.

While for a back-to-back HVDC transmission system，only the first two factors need to be considered.

The DC biasing current component used for the design of converter transformer shall be taken as 1.2 times the sum of the calculated DC currents due to the above four causes.