Voltage Dependent Current Order Limiter (VDCOL)

Figure 11 - 8 shows the operation of the voltage-dependent current order limiter. The object of this limiter is to ensure good recovery of the DC power transit following faults on the AC side.
The risk of commutation failure during recovery is also minimized.

Voltage-Dependent Current Order Limiter (VDCOL)

The limiter includes two functions:

• the dynamic filtering of the DC voltage
• the calculation of the reference based on the filtered voltage

Dynamic filtering of the voltage: The DC voltage Vd goes through a first order filter whose time constant T varies:

• T=T_d=0 when Vd decreases
• T=T_m=80ms when Vd increases

In this way, the current reference falls immediately when the voltage drops but increases more slowly when the voltage rises. This allows for quick but controlled recovery of the direct power transit.

The typical values of the following adjustable parameters are: