Download A LARGE-POWER VOLTAGE SOURCE CONVERTER FOR FACTS

A LARGE-POWER VOLTAGE SOURCE CONVERTER FOR
FACTS APPLICATIONS COMBINING THREE-LEVEL
NEUTRAL-POINT-CLAMPED POWER ELECTRONIC
BUILDING BLOCKS
ABSTRACT
This paper presents a novel large-power voltage source converter (VSC) arrangement suitable for
flexible alternating current transmission system applications, achieved by combining four threelevel three-phase neutral-point-clamped inverters by means of intermediate magnetic elements
with lower VA rating and size than those of similar existing installations.
The 12 inverter poles forming the VSC share a common dc bus and are implemented
using standard power electronic building blocks of well-proven reliability. The output voltage is
doubled by using an H-bridge configuration, and only a single-secondary grid coupling
transformer is required. In addition, the intermediate magnetic elements ensure an equal power
sharing between the four inverters. Since they also allow for the harmonic cancellation between
the inverter poles and eliminate any third harmonic component created by the modulation, the
output voltage waveform has a high quality, often eliminating the need for additional filtering.
The two-level (2L) and three-level (3L) neutral-point clamped (NPC) inverters are widely
used in industry and also in FACTS devices. Most existing installations make use of these
inverters and combine them in series and/or parallel to increase the voltage and current ratings,
respectively, achieving output powers of tens of MVARs. This is done at the expenses of using
highly complex and bulky intermediate magnetic elements.
This paper has presented a large-power VSC suitable for FACTS applications, achieved
by combining four 3L-NPC inverters that share a common dc bus, by means of intermediate
magnetic elements with a low VA rating. The converter requires a single-line coupling
transformer and overall converter size, and footprint is much lower than that of other similar
existing installations without impairing the overall efficiency. Moreover, the output voltage has a
faster dynamic response and the simulation results are evaluated by MATLAB/SIMULINK.
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