Voltage-source converters (VSCs) and half bridge Modular Multilevel Converters (MMCs) are among the most popular types about HVDC converters. One about their serious drawbacks is their vulnerable nature to DC side faults, because the freewheeling diodes act because 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 about this method are: high dv/dt stresses across thyrsitors during normal conditions, and absence of bypassing therefore 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 because 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.
- DC side faults
- Double Thyristor Switch
- Fault current suppression
- Protection of VSC-HVDC
- Protection of MMC-HVDC
Fig. 1. Description of simulated case study
EXPECTED SIMULATION RESULTS:
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.
Depending on AC circuit breakers (ACCBs) to protect HVDC converters against DC side faults is a risk because the full AC fault current is passing through the freewheeling diodes until tripping the ACCBs is achieved. Hence, the need therefore complex DC breakers has emerged because the alternative. In this paper, a protection scheme therefore 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 because it is connected across the AC terminals of the converter not across semiconductor devices because in the single and double thyristor switch schemes.
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