5　Design Conditions

5.1　Environmental Conditions

For system design and equipment design，detailed ambient data for converter station shall be obtained as far as possible，mainly including:

5.1.1　Climatic data

1　Ambient temperatures

（1）Extreme maximum temperature.

（2）Extreme minimum temperature.

（3）Annual average temperature.

（4）Average temperature in hottest month.

（5）Average temperature on hottest day.

2　Atmospheric pressure

Average atmospheric pressure over years.

3　Relative humidity

（1）Average relative humidity.

（2）Minimum relative humidity.

4　Wind direction and velocity

（1）Average wind velocity over years.

（2）Maximum wind velocity（the average maximum velocity over 10 minutes measured 10m above ground，with a 50-year or 100-year return period）.

（3）Prevailing wind direction.

5　Rainfall

（1）Annual rainfall.

（2）Maximum monthly rainfall.

（3）Maximum 24h rainfall.

6　Others

（1）Maximum snow thickness over years.

（2）Solar radiation.

（3）Average thunderstorm days.

（4）Maximum thunderstorm days.

（5）Design ice coating.

5.1.2　Pollution level

The natural pollution level of converter station shall be indicated by equivalent salt deposit density（ESDD，mg/cm2）.For equipment subject to DC voltage，the pollution level shall be indicated by natural pollution level of its external insulation under DC voltage.In areas where such pollution level cannot be obtained directly，the value can be derived by measuring the natural pollution level of the equipment's external insulation under AC voltage，for which a proper conversion coefficient is required.

5.1.3　Earthquake

The earthquake intensity or peak ground motion acceleration for the converter station site shall be provided.

5.1.4　Altitude

The altitude of converter station site shall be provided.

5.1.5　Hydrogeological data

The ground conductivity at converter station site，electrode site，along HVDC transmission lines and electrode line routes，ground water depth and soil resistivity at converter station site shall be provided.