Atmospheric overvoltages in electric-power plant can be
developed in two ways: during the direct lightning hit in some parts of
a plant and the travelling wave from overground that enters into that plant.
PROTECTION FROM DIRECT HIT is made of lightning
rod sticks or protection ropes, which we described with some specific
qualities. Usualy, ends of lightning-rod sticks are grounded. We need
to have in mind that the lightning current creates overvoltage that appears
on casing of grounded devices in electic plants (transformators, generators,
etc.) That overvoltage should not be so high to indanger the insulation
of devices. The only way to influence that overvoltage is to reduce ground
resistance as low as we can.
Transformer is a device that needs to be most protected. Often the casing
of transformer is grounded about 15 meters from lightning rod groundning.
If that solution is not possible, then we protect the transformer with
extra overvoltage drains.
PROTECTION FROM TRAVELLING WAVES is managed
with overvoltage drains. Overvoltage drains are devices, which operate
On overvoltage drain could appear a voltage smaller than voltage which
goes through drain. In that case, it behaves like insulator and it will
not couduct current. When voltage in drain exceeds the voltage that goes
through drain, it starts conduct current. The current through drain is
variable, but the voltage in the overvoltage is same as overvoltage that
goes through drain.
Overvoltage drains are installed on the entrance into the electric plant.
One side is connected on phase conductor and the other side is grounded.
If the travelling wave passes through overground transmission line, it
will come across overvoltage drain on the entrance into plant, which starts
to work and loses only smaller overvoltages. That overvoltage is same
like voltage that goes through drain and it is not dangerous for devices
in plant. However, if lightning hits overground transmittion line near
by plant, than overvoltage drain can' t conduct away all overvoltages
and could be destroyed. To avoid that, the overground transmittion lines
near electric plants are higher protected and that eliminates possibility
of direct lightning hit near plant. It is called "protected approach".
Travelling wave originated far avay from plants is not dangerous because
of conductor resistance influence and phenomena called "corona".
Such wave through drain isn't dangerous for drain and plant any more.