The terms 'primary', 'secondary', and 'tertiary' winding has absolutely nothing whatsoever to do with voltage levels. The primary winding is simply that winding connected to the supply, while the secondary winding is that winding connected to the load. The voltages of these windings depend on whether you are dealing with a step-up or step-down transformer.
The function of an isolation transformer is to electrically-isolate the secondary circuit from the primary circuit, without changing the voltage levels. So, its secondary voltage will be the same as the primary voltage.
To electrically isolate the secondary circuit from the primary circuit, without changing the voltage.
A voltage transformer takes a primary voltage and steps it down to a smaller secondary voltage. This type of transformer will attempt to keep the secondary voltage at a specific ratio of the primary voltage. If you short it, massive current flow in the secondary is required to do this. For a similar reason a CT should never be open circuited - because it attempts to push a specific ratio of primary current through the secondary. If you open circuit the secondary, it takes a massive voltage on the secondary to accomplish this.
A voltage is never applied to the secondary It can be used as a source but hen it becomes the primary by definition
Voltage on primary/Primary turns = Voltage on secondary/Secondary turns
A step- down transformer is one whose secondary voltage is less than its primary voltage, it is used to reduce 'step down' the voltage applied to it. The number of coils in the primary circuit is greater that the secondary circuit. For instance, to step down 480 Voltage to 240 voltage, you need a step down transformer whose ratio of primary coils to secondary coils is 2:1.
Voltage doesn't 'pass through' anything! Voltage is another word for 'potential difference', and is measured between two points in a circuit. For a transformer to work, it's necessary to apply an a.c. voltage across the transformer's primary terminals.
The current in the secondary when the voltage is twice the primary will be one half of the primary. The current in the primary when the voltage is twice the secondary will be twice the secondary.
The primary coil is the one with voltage applied, or the 'input'. The secondary coil is the one in which a voltage is induced by electromagnetism, or the 'output'. In a step up transformer, the secondary coil voltage is higher than the primary. In a step down transformer, the secondary coil voltage is lower than the primary. In an isolation transformer, the secondary coil voltage is the same as the primary. Here, the point of the transformer isn't to raise or lower voltage, but to keep a particular circuit electrically disconnected from another circuit, while still allowing the circuits to function together (through electromagnetism).
Many substation transformers have three windings - the primary, secondary, and tertiary. The tertiary bus is the bus connected to the tertiary of the transformer. this is often used as station power for the substation, since the tertiary is very often a distribution level voltage (2 - 15kV).
A third winding - primary, secondary, and tertiary. The third winding is typically sized much smaller than the primary and secondary, and is very often a lower voltage; it can be used for stabilization, removal of 3rd harmonics, power factor correction injection (lower voltage inductors are cheaper), and station service.
I assume the primary has 12 volts applied. The voltage ratio from primary / secondary is equivalent to the turns ratio = 10/20, so the primary voltage is 1/2 of the secondary voltage. The secondary voltage is 24.