Calculation of the dangerous effects of magnetic influences
One of the main factors that determine the extent of the impact of electric lines on FOCL is the nature of their convergence. Under convergence relative position FOCL and high voltage lines (HVL) is understood (lines or electrified railroad contact networks ), where in OF may have dangerous voltage and currents. Convergence may be parallel, oblique and complicated.
Plot
convergence is parallel if the shortest distance between the lines
(width approximation) a
varies
in length convergence is not more than 10% from the average value. If
this condition is not fulfilled, the area of convergence is oblique.
This convergence is replaced by a stepped parallel, while choosing
the length of the parallel equivalent plots so that the ratio of
maximum to minimum width closer to the ends of the plot was not more
than three. Then the equivalent width of approximation аекв
is determined by formula
.
Dangerous magnetic interference can occur during normal operation of HVL or in its emergency mode at the precipice and grounding phase of EL wire or contact wire. A large amount of current short circuit creates an intensive magnetic field. As a result, in cable emf is induced, which may exceed the permissible value. This emf is called longitudinal because the induced electric field is directed along wire.
Longitudinal EMF - a potential difference at the beginning and at the end of the metal shell of OF at length of convergence.
The absolute value of longitudinal emf, which is present in a metal shell of OF (Figure 7.2) is calculated at 50 Hz using the formula:
, (7.7)
where Е – Longitudinal EMF, V;
n – number of areas of convergence;
– impacting
current, A;
– coefficient
of mutual induction between single-wire
circles
ofLVN
and "shell OF-land"
on the i-th field approximation, H / km;
– length
of the i-th region
of
convergence km.
To find the coefficient of mutual induction theoretically is difficult because it depends on the conductivity of the ground at the region of convergence and conductivity of the earth because of the heterogeneity of the structure of construction varies widely. In practice, the coefficient of mutual induction, depending on the width of the convergence and the conductivity of the earth is determined by the Mikhailov nomogram (Fig. 7.3) under the assumption that the structure of the ground in the area of convergence is uniform [12]. You can determine the coefficient of mutual induction by the approximate formula, which is valid in the range of tonal frequencies:
, (7.8)
where aekv - equivalent width of convergence, m;
f - frequency affecting current, Hz.
з - the conductivity of the earth, S / m
Figure 7.3 - Mikhailov Nomogram
Calculation
of longitudinal emf induced in the metal shell of OF is done in
2-mode HVL (emergency and normal). To do this, in the expression
(7.7) instead
substituted
value
(short-circuit current at the end of the i-th
region of convergence), and the second - value
(operating current HVL).
Determining
the coefficient of mutual induction
for
each region of longitudinal EMF is calculated, for example, in
emergency mode of HVL.
.
(7.9)
In normal operation mode of HVL, longitudinal EMF calculation is performed by the expression:
. (7.10)
Results
of calculation, such as
,
are
placed
to the table. 7.5.
When
calculating
or
on
remote power supply, it should be considered in the expressions
(7.7), (7.9) and (7.10) coefficient of protective action of metal
jacket of OF (
)
by multiplying the right-hand side of these expressions by this
factor.
If
the value
or
given
of remote power shell of OF is greater than
,
it is necessary to provide protection measures. As a protection
measures, you can consider the use of shielding cables ПС-70
according
the table. 7.3 and Table. 7.4. Value
=
42 V.
Values of dangerous voltages and currents in metallic elements of OF are established for the safety of staff that works at the station and linear structures, as well as to protect these structures from damage (breakdown of insulation, protective hose cable damage, damage of equipment, etc.).
Acceptable values of dangerous voltages and currents are taken in such way, for which special protection measures are not required, taking into account the time and the condition of their influence on people and structures of communication. Short period dangerous voltage and currents can occur in OF in areas closer to the ET lines and ECD in their short circuit to ground. Time of the voltage and current is (0.15 ... 1.2) s (while wearing disabling devices), so for this emergency mode allowed a relatively high voltage. In normal modes and forced working modes dangerous voltage and currents are lasting long, so the normes for these modes are significantly lower (42 V).
Short-term harmful effect on the HVL on the length of galvanically undivided section of cable lines maximum allowed values of longitudinal emf can be determined according to Table. 7.5.
The
value of the test voltage
depends on the type of cable and the voltage remote power line
regenerator
that
feeds the veins remote power of OF - the type of the transmission
system.
According
to TU on OF, wound on a drum, the norm f
or
protective hose is 10000 V [13], and the laid cables - 2000 [3].
Table 7.5 - Valid values of longitudinal EMF at short-time exposure
Figureof remote power supply (J) |
Possible EMF, V |
|
ЛЕП |
ЕЗД |
|
Without power supply |
|
0,6 |
"Wire-land" DC |
|
|
"Wire- Wire " DC |
|
|
