ABSTRACT:
Multi-Terminal HVDC based on
three-level neutral-clamped voltage source converters (VSC) is an ideal approach
for the integration of DFIG wind farms to the power grid. However, dc-link
faults and ac faults are major concerns for the safety and consistency of
VSC-HVDC system. This paper demonstrates methods employing both full bridge and
half bridge DC-DC converters for the fast clearance and protection of dc and ac
ground faults respectively. In addition, control strategies incorporating
decoupling control and feed-forward compensation on both grid side and wind
farm side VSCs are also presented. Normal operations are observed to examine
the performance of the MT-HVDC system, and also dc-link fault and three-phase ground
fault at inverter side are simulated to verify the effectiveness of the
approach employing DC-DC converters to suppress dc current overshoot in case of
dc-link fault and mitigate dc voltage overshoot during three-phase ac ground
fault. This proposed MT-HVDC transmission system and the fault-ride through
capabilities provided by the dc choppers is validated by the simulation studies
using detailed Matlab/Simulink model for normal operation, dc and ac ground
faults.
KEYWORDS:
2. DFIG
3. DC chopper
4. Faults
SOFTWARE: MATLAB/SIMULINK
BLOCK DIAGRAM:
Fig. 1 Topology of the proposed multi-terminal VSC-HVDC system.
EXPECTED SIMULATION RESULTS:
Fig. 2 Simulation results of MT-HVDC during normal
operation: (a) active power of wind farm, (b) dc voltage, and (c) ac rms
current.
Fig. 3 Simulation results of 6 DFIG units during
normal operation: (a) active power, (b) reactive power, (c) ac rms voltage, and
(d) back-to-back dc-link voltage of DFIG unit.
Fig. 4 Simulation results of MT-HVDC during dc
pole-to-pole fault with and without full bridge dc chopper protection: (a) dc
voltage, and (b) dc current.
Fig. 5 Simulation results of MT-HVDC during
three-phase ac ground fault at inverter side with and without half bridge dc
chopper protection: (a) ac rms voltage at inverter side, (b) dc voltage
overshoot without protection measures, and (c) dc voltage with protection
measures.
CONCLUSION:
This paper investigates a multi-terminal VSC-HVDC system,
which integrates two DFIG wind farms to the ac grid. The control strategies of
both WFVSC and GSVSC stations are discussed in detail, and two approaches
employing both full bridge and half bridge dc choppers are extended and
displayed. Simulation studies are carried out in normal, dc pole-to-pole and ac
ground fault operations, and the result verifies the effectiveness of the
proposed MT-HVDC system in both the performance of wind power delivery and the
protection measures for various fault conditions. Specifically, the dc voltage
drop and dc current overshoot are eliminated during dc fault with full bridge
dc choppers, while only a 8% voltage overshoot is observed with the implementation of half
bridge dc choppers in case of three-phase ac ground fault.
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