This paper presents a performance of a bipole HVDC link
based on line commutated converter (LCC) with a Voltage
Source Converter (VSC). Connecting a VSC based link in a
bipole fashion with an LCC based link has never been practically
implemented yet.
In implementing such a system, as the commutation process
is different for the two. Therefore, study of issues such as
the control of the links, commutation failures and harmonics
are important for the proper operation of the system.
The control strategy for both the LCC and VSC links has
been presented. The VSC control is based on PWM techniques
which enables it to have full control of the active and
reactive power exchanged with the ac grid. However the
LCC is controlled by adjusting the firing angle of the converter
and can only control the active power exchanged with
the ac grid.
The LCC link has a higher probability to commutation failures
than the VSC. Hence, the performance of the bipole
link has been studied during normal operation and faulty
conditions. The faults which have been simulated deal with
both ac and dc side faults. For ac side faults, single phase
to ground, reduction in the amplitude and distortion of the
voltages and three phase to ground have been simulated. For
dc side, line to ground and sudden rapid increase in the dc
current have been simulated.
The results obtained by using the simulation program,
PSCAD are presented at the end. The results show that the
link operates satisfactorily during normal operation. During
faulty conditions, depending on the type of fault, the results
show that the link either operates through one line or is com-
IVpletely down in worst fault case, three phase to ground.