Download Transmission of Three Phase Power

Transmission of Three Phase Power
Transmission of Three Phase Power
The three windings discussed
above can be made to supply three
individual circuits, when all six ends
must be used. It is possible,
however, to link the three circuits
electrically with the result that the
number of conductors necessary for
The three windings discussed
above can be made to supply three
individual circuits, when all six ends
must be used. It is possible,
however, to link the three circuits
electrically with the result that the
number of conductors necessary for
the transmission of the power is reduced.
In the first instance it is possible to affect an economy by using a common return, this
being permissible since it does not disturb the electrical arrangement. This implies that the
three rear ends of the turns, A, B', and C must all be joined, together with the three rear
ends of the three resistances used as loads. This arrangement is illustrated in Fig. which
shows the three generator windings connected together at one end, the other ends being
connected to three conductors for the purpose of transmitting the power. The return
conductor carries the vector sum of these three currents back to the common junction of
the three generator windings.
If the three e. m.f.'s are all equal and the three load resistances are also all equal, the
three currents will also be all equal and will have a phase difference of 120° from one
another. In these circumstances the system is said to be balanced.
Three-Wire Transmission. — In a balanced three-phase system the three currents are
equal and can be represented by the graphs shown in Fig. 6, substituting current for e. m. f.
The resultant current in the fourth (return) conductor is, at any instant, the algebraic sum of
the three line currents and, on examination of the graphs, it is found that this algebraic
sum is zero at every instant. The fourth (return) conductor thus carries no current and it
can be omitted. The connections now take the form shown in Fig., three conductors only
being employed. Each conductor now acts in turn as the return for the other two. This can
be checked from Fig., where it is seen that the reverse current in one phase is always equal
to the forward current in the other two. (At certain instants, two conductors act as the
return for the forward current in the remaining conductor). It is also general practice to
earth the system at one point, this being done conveniently at the generator common
junction as shown in Fig.
Three-Phase Four-Wire System. — This is a system of connections which permits
the employment of a three-phase load and three single-phase loads simultaneously, as
shown in Fig. 9. It re-introduces the fourth conductor to act as
a common return for the three single phase loads, this
conductor being called the neutral. The other three conductors
are called the line conductors. The system is earthed as before,
by connecting the neutral conductor to an earthed plate or
other earthing connection.
Color Scheme. — For purposes of standardization it is
now the general practice to mark each particular phase by a
distinctive color. The three phases are colored red, white (or yellow), and blue,
respectively, while the neutral conductor is colored green.
the transmission of the power is reduced.
In the first instance it is possible to affect an economy by using a common return, this
being permissible since it does not disturb the electrical arrangement. This implies that the
three rear ends of the turns, A, B', and C must all be joined, together with the three rear
ends of the three resistances used as loads. This arrangement is illustrated in Fig. which
shows the three generator windings connected together at one end, the other ends being
connected to three conductors for the purpose of transmitting the power. The return
conductor carries the vector sum of these three currents back to the common junction of
the three generator windings.
If the three e. m.f.'s are all equal and the three load resistances are also all equal, the
three currents will also be all equal and will have a phase difference of 120° from one
another. In these circumstances the system is said to be balanced.
Three-Wire Transmission. — In a balanced three-phase system the three currents are
equal and can be represented by the graphs shown in Fig. 6, substituting current for e. m. f.
The resultant current in the fourth (return) conductor is, at any instant, the algebraic sum of
the three line currents and, on examination of the graphs, it is found that this algebraic
sum is zero at every instant. The fourth (return) conductor thus carries no current and it
can be omitted. The connections now take the form shown in Fig., three conductors only
being employed. Each conductor now acts in turn as the return for the other two. This can
be checked from Fig., where it is seen that the reverse current in one phase is always equal
to the forward current in the other two. (At certain instants, two conductors act as the
return for the forward current in the remaining conductor). It is also general practice to
earth the system at one point, this being done conveniently at the generator common
junction as shown in Fig.
Three-Phase Four-Wire System. — This is a system of connections which permits
the employment of a three-phase load and three single-phase loads simultaneously, as
shown in Fig. 9. It re-introduces the fourth conductor to act as
a common return for the three single phase loads, this
conductor being called the neutral. The other three conductors
are called the line conductors. The system is earthed as before,
by connecting the neutral conductor to an earthed plate or
other earthing connection.
Color Scheme. — For purposes of standardization it is
now the general practice to mark each particular phase by a
distinctive color. The three phases are colored red, white (or yellow), and blue,
respectively, while the neutral conductor is colored green.
М.А. Беляева и др. «Сборник технических текстов на англ. языке»
М.А. Беляева и др. «Сборник технических текстов на англ. языке»