Early 110 kV substations typically employed the "internal bus connection" method on the power side, where the power source side mostly utilized the "internal bridge connection" method (often seen in a particular 220 kV substation where 110 kV buses from different transformers were used in the "same direction dual power" supply configuration). This involved installing two transformers, with the 10 kV side employing a single bus segmented connection method. The advantages included simplified wiring, convenient operation, simple auto-switching, requiring only three switches for the power side of two transformers. Additionally, the bus on the power side didn't require separate protection (within the scope of transformer differential protection), and it required less investment. However, limitations included each bus could only accommodate one transformer, restricting the development of the 10 kV load it could support. Moreover, when one transformer was operational, half of the station needed to be shut down, leading to the risk of a total station blackout if the other half experienced equipment failure.
To increase station capacity and enhance supply reliability, a mid-term approach for 110 kV substations employed the "expanded internal bus connection" method, with the power side mostly adopting the "expanded bridge connection" method. This involved installing three transformers, where the power was supplied to the station via two "side buses" from different 110 kV buses of the same direction dual power supply from a particular 220 kV substation and one "middle bus" from a different direction single power supply from another 220 kV substation. The 10 kV side still employed a single bus segmented connection method, with the middle transformer's 10 kV side ideally segmented into A and B sections. This approach increased the number of outgoing lines for the 10 kV load and allowed load distribution from the middle transformer to the other two in case of its shutdown. However, it came with complexities in operation, auto-switching, and required higher investment.
As urban areas expanded, land became scarce, and electricity demand surged, there was an urgent need to further increase station capacity and enhance supply reliability. The current approach for 110 kV substations primarily utilizes single bus segmented connection on the power side, with four transformers each connected to different buses (where the two middle transformers are cross-connected to the upper power source). On the 10 kV side, A and B section connections are employed for eight-bus segmented lines forming a "ring connection" powered by four transformers. This design increases the number of outgoing lines for the 10 kV load and enhances supply reliability by cross-connecting the two middle transformers on the 110 kV side to the upper power source, ensuring uninterrupted power supply to the eight-segment bus on the 10 kV side even if one 110 kV bus is shut down. However, drawbacks include the need for protection equipment on the 110 kV bus, high initial investment, and operational complexities.