17-09-2012, 12:01 PM
VOLTAGE SOURCE CONVERTER TRANSMISSION TECHNOLOGIES - THE RIGHT FIT FOR THE APPLICATION
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Abstract
Voltage Source Converter (VSC)
technology has been selected as the basis for several
recent projects due to its controllability, compact
modular design, ease of system interface and low
environmental impact. This paper describes the
rationale for selection of VSC technology and the latest
technical developments utilized in several recent
projects.
INTRODUCTION
Traditional HVDC and FACTS installations have often
provided economic solutions for special transmission
applications. HVDC is well-suited for long-distance, bulkpower
transmission, long submarine cable crossings, and
asynchronous interconnections. Static var compensators
(SVC) provide a reserve source of dynamic reactive power
thereby raising power transfer limits. HVDC and FACTS
technologies permit transmitting more power over fewer
transmission lines.
Deregulated generation markets, open access to
transmission, formation of RTO’s, regional differences in
generation costs and increased difficulty in siting new
transmission lines, however, have led to a renewed interest
in FACTS and HVDC transmission often in non-traditional
applications. This is especially true with the lag in
transmission investment and the separation in ownership of
generation and transmission assets. HVDC and FACTS
transmission technologies available today offer the planner
increased flexibility in meeting transmission challenges.
DC Transmission System – Current Source
Converters vs. VSC
Conventional HVDC transmission employs linecommutated,
current-source converters requiring a
synchronous voltage source in order to operate. The
conversion process demands reactive power from filters,
shunt banks, or series capacitors which are part of the
converter station. Any surplus or deficit in reactive power
must be accommodated by the ac system. This difference in
reactive power needs to be kept within a given band to keep
the ac voltage within the desired tolerance. The weaker the
system or the further away from generation, the tighter the
reactive power exchange must be to stay within the desired
voltage tolerance.
Cross Sound Cable – HVDC Light
The Cross Sound Cable provides a directly controllable
merchant interconnection between the New England and
Long Island systems bypassing the congested New York
City transmission network. The interconnection is rated 330
MW, ± 150 kV. The 40 km submarine cable is buried on the
sea bottom. The Cross Sound Cable increases regional
reliability by increasing the ability of the New England and
New York networks to share generating capacity. It can also
reduce the overall cost of power to consumers as well as
reduce overall CO2 emissions by allowing the shared use of
more efficient generation units.
Holly STATCOM – SVC Light
The VSC used in the Holly STATCOM located in Austin
Texas is based on the Polarit design described in Section 3.4.
It consists of a single ± 95 MVAR, 32 kV VSC and 15
MVAR of high frequency filters for the PWM switching
harmonics. This combination gives a continuous dynamic
range from –80 MVAR to +100 MVAR plus a short time
capacitive overload of 10 percent. The VSC has built in
redundancy. Three 31.2 MVAR, 138 kV mechanicallyswitched
capacitor banks are included and controlled to shift
the VSC dynamic range depending on system operating
conditions.
CONCLUSION
Recent projects have demonstrated that VSC-based
transmission technology has come of age both for HVDC
transmission schemes and for enhancing performance of ac
transmission through application of new FACTS devices.
Although VSC-based schemes may not always be the most
economical solution for the higher rated transmission
applications, their special attributes and ease of application
provide special benefits which merit serious consideration.
In some applications, VSC transmission may be the only
solution. As the technology matures and ratings increase,
more applications can be expected.