Voltage-source converters (VSCs) and half bridge Modular
Multilevel Converters (MMCs) 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 severity of DC side faults can be limited by connecting double
thyristor switches across the semiconductor devices. By turning them on, the AC
current contribution into the DC side is eliminated and the DC-link current
will freely decay to zero. The main disadvantages of this method are: high dv/dt
stresses across thyrsitors during normal conditions, and absence of bypassing for
the freewheeling diodes during DC faults as they are sharing the fault current
with thyristors. This paper proposes a new protection scheme for HVDC
converters (VSCs as well as MMCs). In this scheme, the double thyristor
switches are combined and connected across the AC output terminals of the HVDC
converter. The proposed scheme provides advantages such as lower dv/dt
stresses and lower voltage rating of thyristor switches, in addition to
providing full separation between the converter semiconductor devices and AC
grid during DC side faults. A simulation case study has been carried out to demonstrate
the effectiveness of the proposed scheme.
KEYWORDS:
1.
DC side faults
2.
Double
Thyristor Switch
3.
Fault current
suppression
4.
Protection of
VSC-HVDC
5.
Protection of MMC-HVDC
SOFTWARE: MATLAB/SIMULINK
SCHEMATIC DIAGRAM:
Fig. 1. Description of simulated
case study
Fig. 2. Simulation results for VSC
case: (a) converter line voltage , (b) per-phase grid current, (c) DC-link
current, (d) thyristors currents for different protection schemes, (e)
freewheeling diode current for different protection scheme, and (f) dv/dt
stresses across each thyristor for different protection schemes.
Fig. 3. Simulation results for
three-level MMC (n=2): (a) converter line voltage , (b) per-phase grid
current, (c) DC-link current, (d) thyristors currents for different protection
schemes, (e) freewheeling diode current for different protection scheme, and
(f) dv/dt stresses across each thyristor for different protection
schemes.
CONCLUSION:
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. In this paper, a protection scheme for both VSC-HVDC and
MMCHVDC converters against DC side faults is proposed. The proposed scheme
provides complete separation between the AC side and the HVDC converters during
DC faults which allows the DC-link current to freely decay to zero (the grid current
contribution into DC fault is eliminated). A comparison between the proposed
scheme and other existing schemes (STSS, and DTSS) is presented. With the same number
of thyristors, the proposed 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
higher current rating (200% compared to other schemes). Implementation of the
proposed scheme is less complex since it is connected across the AC terminals
of the converter not across semiconductor devices as in the single and double thyristor
switch schemes.
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