6. Power flow can be modulated during disturbances on one of the ac systems,
resulting in increased system stability.
7. The two ac systems that are connected by the dc line do not need to be in
synchronization. Furthermore, the two ac systems do not need to be of the
same frequency. A 50-Hz system can be connected to a 60-Hz system via a
dc link.
The disadvantage of dc power transmission is that a costly ac-dc converter,
filters, and control system are required at each end of the line to interface with the
ac system.
Figure 4-24a shows a simplified scheme for dc power transmission using
six-pulse converters at each terminal. The two ac systems each have their own
generators, and the purpose of the dc line is to enable power to be interchanged
between the ac systems. The directions of the SCRs are such that current io will
be positive as shown in the line.
In this scheme, one converter operates as a rectifier (power flow from ac to dc),
and the other terminal operates as an inverter (power flow from dc to ac). Either
terminal can operate as a rectifier or inverter, with the delay angle determining
the mode of operation. By adjusting the delay angle at each terminal, power flow
is controlled between the two ac systems via the dc link.
The inductance in the dc line is the line inductance plus an extra series
inductor to filter harmonic currents. The resistance is that of the dc line conductors.
For analysis purposes, the current in the dc line may be considered to be a
ripple-free dc current.