In this situation, the capacitance
to earth draws a current through the line, which
may be capacitive. When a capacitive current
flows through the line inductance there will
be a voltage rise along the line.
To stabilize the line voltage the line inductance
can be compensated by means of series capacitors
and the line capacitance to earth by shunt reactors.
Series capacitors are placed at different places
along the line while shunt reactors are often
installed in the stations at the ends of line.
In this way, the voltage difference between the
ends of the line is reduced both in amplitude
and in phase angle.
Shunt reactors may also be connected to the
power system at junctures where several lines
meet or to tertiary windings of transformers.
Transmission cables have much higher capacitance
to earth than overhead lines. Long submarine
cables for system voltages of 100 KV and more
need shunt reactors. The same goes for large
urban networks to prevent excessive voltage rise
when a high load suddenly falls out due to a
failure.
Shunt reactors contain the same components as
power transformers, like windings, core, tank,
bushings and insulating oil and are suitable
for manufacturing in transformer factories. The
main difference is the reactor core limbs, which
have non-magnetic gaps inserted between packets
of core steel.
Figure shows a design of a single-phase shunt
reactor. The half to the right is a picture of
the magnetic field. The winding encloses the
mid-limb with the non-magnetic gaps. A frame
of core steel encloses the winding and provides
the return path for the magnetic field.
3-phase reactors can also be made. These may
have 3- or -5-limbed cores. In a 3-limbed core
there is strong magnetic coupling between the
three phases, while in a 5-limbed core the phases
are magnetically independent due to the enclosing
magnetic frame formed by the two yokes and the
two unwound side-limbs.
The neutral of shunt reactor may be directly earthed,
earthed through an Earthing-reactor or unearthed. |
