The sources of reactive power are:
1. Synchronous machines
These machines are used as generators (of active power) when they transform mechanical energy into electric power; otherwise, they are used as motors. These machines can produce reactive power by varying the excitation. In some cases, the machine does not supply any active power. It is then used as a synchronous condenser. Synchronous motors are rarely used as modulating devices.
2. Asynchronous machines
They are different from the machines described above in that they draw from the network the reactive power they need. This power can be considerable: 25–35% of the active power at full load, much more at partial load.
Asynchronous motors, being the most common, are the main consumers of reactive power produced by industrial networks.
3. Lines and cables
The L and C characteristics of overhead lines and cables are such that they consume reactive power at full load.
Overhead lines, depending on the load current, either absorb or supply reactive power. At loads below the natural (surge impedance) load, lines produce net reactive power; at loads above the natural load, lines absorb reactive power.
Underground cables, owing to their high capacitance, have high natural loads. They are always loaded below their natural loads, and hence generate reactive power under all operating conditions.
Transformers consume reactive power that corresponds to approximately 5–10% of the apparent power, which ﬂows through them. They always absorb reactive power regardless of their loading; at no load, the shunt magnetizing reactance effects predominate; and at full load, the series leakage inductance effects predominate.
Inductances consume primarily reactive power such as current limitation inductances and stabilization inductances (for arc furnaces, ﬂuorescent lamps).
By deﬁnition, capacitors generate reactive power at a very high rate. They are usually added to supply or absorb reactive power and thereby control the reactive power balance in the desired manner.