Transmission substation:  Where you connect two or more transmission lines you would use a transmission substation.

In a very basic situation you would have two transmission lines with the same voltage.  In such a situation the transmission substation contains high voltage switches that allow lines to be isolated or connected for maintenance or fault clearance.

A transmission substation may have transformers to convert between two or more transmission voltages.  It would also have voltage control devices such as capacitors, reactors and equipment such as phase shifting transformers to control power flow between two adjacent power systems.
These transmission substations can range from basic to intricate.  A basic “switching station” may be a bit more than an electrical bus (a common electrical connection between multiple electrical devices) plus a few circuit breakers.  The biggest transmission substations can cover an extensive area with multiple voltage levels, large amount of circuit breakers, protection and control equipment (voltage and current transformers, relays and SCADA (Supervisory Control And Data Acquisition) systems)

Step-Up Transmission Substation

Step-down transmission Substation

Transmission Line Substation (step-down)

A step-up transmission substation receives electric power from a nearby generating facility and uses a large power transformer to increase the voltage for transmission to distant locations. A transmission bus is used to distribute electric power to one or more transmission lines. There can also be a tap on the incoming power feed from the generation plant to provide electric power to operate equipment in the generation plant.

A substation can have circuit breakers that are used to switch generation and transmission circuits in and out of service as needed or for emergencies requiring shut-down of power to a circuit or redirection of power.

The specific voltages leaving a step-up transmission substation are determined by the customer needs of the utility supplying power and to the requirements of any connections to regional grids. Typical voltages in USA are:

High voltage (HV) ac:     69 kV, 115 kV, 138 kV, 161 kV, 230 kV
Extra-high voltage (EHV) ac:     345 kV, 500 kV, 765 kV
Ultra-high voltage (UHV) ac:     1100 kV, 1500 kV
Direct-current high voltage (dc HV):     ±250 kV, ±400 kV, ±500 kV

Direct current voltage is either positive or negative polarity. A DC line has two conductors, so one would be positive and the other negative.

Step-Down Transmission Substations

Step-Down Transmission Substation

Step Down Transformer

Step-Down transmission substations are located at switching points in an electrical grid. They connect different parts of a grid and are a source for subtransmission lines or distribution lines. The step-down substation can change the transmission voltage to a subtransmission voltage, usually 69 kV. The subtransmission voltage lines can then serve as a source to distribution substations. Sometimes, power is tapped from the subtransmission line for use in an industrial facility along the way. Otherwise, the power goes to a distribution substation.