48
Additional components on the DC converter
Additional components (catalog D23.1) are required for operation of built-in devices 6RA80.
The following section seeks to aid checking for correct rating of the additional components in
case of a fault. Catalog D23.1:
https://www.automation.siemens.com/mcms/infocenter/content/de/Seiten/order_form.aspx?n
odeKey=key_9181486
Protection with fuses:
Semi-conductor protective fuses are mandatory for protection semi-conductors. Types other
than those specified in the catalog/Operating Instructions may only be used if the rated
voltage is larger or equal and the I
2
t value is less than or equal to the prescribed type. The
fuse protects the semi-conductor if the current shows a particular rate of rise during a fault.
The fuse may not be able to protect the semi-conductor under the following conditions:
On a bolted short-circuit at the current converter output if a very slight commutation induction
is pending (current rise too steep: very high line short-circuit output and commutation reactor
too small), or if the current rises too slowly (very weak network or inductance in load circuit
too large). In both cases, a smaller fuse type could provide a solution, if the system allows it.
In any case, protection is provided by the additional CCP (see below).
Note:
Semi-conductor protective fuses of duty class "aR" cannot assume additional
protection of the line; in this case, extra precautions must be taken. Semi-conductor
protective fuses of duty class "gR" (e.g. 3NE1..) give additional line protection cover. The I
2
t
value of the 3NE1 type usually means that the prescribed fuses cannot be replaced directly
by this type.
Commutation reactor, Catalog D23.1: and
http://support.automation.siemens.com/WW/view/en/84138681
If line-commutated current converters are used, commutation induction is always required in
the supply circuit. In the simplest case, a commutation reactor is used.
To protect the current converter, a commutation inductance is required that produces a 4%
voltage drop at the rated current of the converter; Vimp = 4%. A separate commutation
reactor or separate transformer winding with 4 % Vimp must be used for every current
converter used on one power system. The permissible Vimp of the reactor/transformer
winding for the current converter is 4% to 10% (worst case 15%).
According to DIN EN 61800-3, voltage dips of up to 20% of peak line voltage are permissible
during commutation. The following applies: If the short-circuit power of the power system is
higher than the connected load of the current converter by at least a factor of 100,
commutation notches of maximum 20 % of the peak voltage of the network will occur if a 4%
commutation reactor is used.
If several current converters are installed on the same power system, it can be assumed that
these will not commutate at exactly the same time, so the reactor can be dimensioned for
each current converter individually as a function of its power, as described above. Normally
the commutation reactor is dimensioned as a function of the rated motor current; this saves
costs as the rated current of the device is usually higher. If high acceleration currents are
required on the motor, the reactor should be designed for a
peak current / 1.6 due to the saturation that occurs during high currents.
The alternating current of the reactor for the armature circuit infeed is: direct current
Id * 0.816
Help with calculation of the commutation notches:
U[%] = (x
N
+ x
T
)/ (x
N
+ x
T
+ x
D
) * 100%
;
where (x
N
+ x
T
) = U
N
2
/ S’
k2
; x
D
= L
D
* 2 * * f; x
D
: Reactor reactance, f = line frequency,
L
D
: Inductance of the reactor; S’
k2
: Short-circuit power at connecting point of current
converter; U
N
: Line-to-line nominal line voltage at connecting point of current converter; U
N
:
Line reactance; x
T
:transformer reactance related to the connecting point of the current
converter,
U: Commutation notch power system at current converter connecting point (line side
directly before the commutation reactor) as a percentage.