3　Electrical Systems

3.1　Main Transformer

3.1.1　The number and capacity of main transformers shall be determined based on overall consideration of local power supply conditions，load characteristics，power demand and operation modes.

3.1.2　Two main transformers shall be installed in a substation serving Class I and Class Ⅱ loads.If technically and economically viable，more main transformers may be installed.Where sufficient power can be supplied from the medium voltage grid and low voltage grid，one main transformer may suffice.

3.1.3　For substations equipped with two or more main transformers，when one main transformer is out of service，the capacity（including overload capability）of the remaining main transformers shall be able to meet the power demands of all Class Ⅰ and Ⅱ loads.

3.1.4　For substations with three voltage levels，if the power of each winding is higher than 15% of the rated capacity of the transformer，a three-winding transformer should be used.

3.1.5　Main transformers should be of low loss and low noise type.

3.1.6　For substations with significant power flow variation and voltage deviation，on-load tap changer transformers shall be used if ordinary transformers cannot meet the voltage quality requirements of power systems and users.