A New Protection Scheme for HVDC Converters Against DC-Side
Faults With Current Suppression Capability
Abstract:
In this project, a protection scheme for HVDC converters (classical VSCs as well
as MMCs) against dc-side faults is contemplated. The contemplated scheme provides complete
segregation between the ac side and the HVDC converters during dc faults which allow the dclink current to freely decay to zero (the grid current contribution into the dc fault is eliminated).
This posses combining and connecting the double thyristor switches a cross the ac output
terminals of the HVDC converter. The contemplated protection scheme provides advantages,
such as lower dv/dt stresses and lower voltage rating of thyristor switches in accumulation to
providing full segregation between the converter semiconductor devices and ac grid during dcside faults. A simulation case study has been carried out to reveals the effectiveness of the
contemplated scheme.
Existing Method:
Two-level voltage-source converters and half-bridge modular multilevel converters are
among the most popular types of HVDC converters. One of their serious drawbacks is their
vulnerable nature to dc-side faults, since the freewheeling diodes act as a rectifier bridge and feed
the dc faults. The main disadvantages of this method are: high dv/dt stresses across thyristors
during normal conditions, and the absence of by passing for the freewheeling diodes during dc
faults since they are sharing the fault current with thyristors.
Proposed Method:
In the single-thyristor switch scheme (STSS), a single thyristor switch is connected in
each submodule of the MMC. This can be realized by turning the thyristors on when a dc-side
fault is detected. The thyristor has higher capability for withstanding the surge current compared
to the freewheeling diode. As a result, most of the fault current flows through the thyristor and
not through the diode.
Further Details Contact: A Vinay 9030333433, 08772261612
Email: takeoffstudentprojects@gmail.com | www.takeoffprojects.com
Block Diagram:
Fig. Block Diagram and Description of the simulated case study.
A simulation case study has been carried out to demonstrate the effectiveness of the
contemplated scheme.
Advantages and Applications:
1. Depending on ac circuit breakers (ACCBs) to protect HVDC converters against dc-side
faults is a risk since the full ac fault current is passing through the freewheeling diodes
until tripping the ACCBs is achieved. Hence, the need for complex dc breakers has
emerged as the alternative.
2. A comparison between the contemplated scheme and other existing schemes (STSS, and
DTSS) is presented. With the same number of thyristors, the contemplated scheme is able
to accomplish the task of the DTSS, but with back-to-back thyristor switches exposed to
lower dv/dt stresses, and possessing lower voltage (33% compared to other schemes), but
with higher current rating (200% compared to other schemes).
3. Implementation of the contemplated scheme is less complex since it is connected across
the ac terminals of the converter and not across semiconductor devices as in the single
and double-thyristor switch schemes.
Further Details Contact: A Vinay 9030333433, 08772261612
Email: takeoffstudentprojects@gmail.com | www.takeoffprojects.com