23-07-2012, 01:30 PM
HVDC Transmission Overview
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Abstract
High Voltage Direct Current (HVDC) technology
has characteristics which make it especially attractive in certain
transmission applications. The number of HVDC projects
committed or under consideration globally has increased in
recent years reflecting a renewed interest in this field proven
technology. New HVDC converter designs and improvements in
conventional HVDC design have contributed to this trend. This
paper provides an overview of the rationale for selection of
HVDC technology and describes some of the latest technical
developments.
INTRODUCTION
High Voltage Direct Current (HVDC) technology has
characteristics which makes it especially attractive for certain
transmission applications. HVDC transmission is widely
recognized as being advantageous for long-distance, bulkpower
delivery, asynchronous interconnections and long
submarine cable crossings. The number of HVDC projects
committed or under consideration globally has increased in
recent years reflecting a renewed interest in this mature
technology. New converter designs have broadened the
potential range of HVDC transmission to include applications
for underground, offshore, economic replacement of
reliability-must-run generation, and voltage stabilization.
Developments include higher transmission voltages up to ±
800 kV, capacitor-commutated converters (CCC) for weak
system applications and voltage-sourced converters (VSC)
with dynamic reactive power control.
SYSTEM CONFIGURATIONS
Fig. 3 shows the different common system configurations
and operating modes used for HVDC transmission. The most
common configuration for modern overhead HVDC
transmission lines is bipolar with a single 12-pulse converter
for each pole at each terminal. This gives two independent dc
circuits each capable of half capacity. For normal balanced
operation there is no earth current. Monopolar operation,
often with overload capacity, can be used during outages of
the opposite pole. Emergency earth return operation can be
minimized during monopolar outages by using the opposite
pole line for metallic return via pole/ converter bypass
switches at each end. This not only is effective during
converter outages but also during line insulation failures
where the remaining insulation strength is adequate to
withstand the low resistive voltage drop in the metallic return
path. For very high power HVDC transmission especially at
dc voltages above ± 500 kV, i.e., ± 600 kV or ± 800 kV,
series-connected converters can be used to reduce the energy
unavailability for individual converter outages or partial line
insulation failure.
HVDC CONTROL & OPERATING PRINCIPLES
Conventional HVDC
For conventional HVDC transmission one terminal sets the
dc voltage level while the other terminal(s) regulates the (its)
dc current by controlling its output voltage relative to that
maintained by the voltage setting terminal. Since the dc line
resistance is low, large changes in current and hence power
can be made with relatively small changes in firing angle,
alpha. Two independent methods exist for controlling the
converter dc output voltage. These are 1) by changing the
ratio between the direct voltage and the ac voltage by varying
the delay angle or 2) by changing the converter ac voltage via
load tap changers (LTC) on the converter transformer.
Whereas the former method is rapid the latter method is slow
due to the limited speed of response of the LTC. Use of high
delay angles to achieve a larger dynamic range, however,
increases the converter reactive power consumption. To
minimize the reactive power demand while still providing
adequate dynamic control range and commutation margin, the
LTC is used at the rectifier terminal to keep the delay angle
within its desired steady state range, e.g., 13-18 degrees, and
at the inverter to keep the extinction angle within its desired
range, e.g. 17-20 degrees, if the angle is used for dc voltage
control or to maintain rated dc voltage if operating in
minimum commutation margin control mode.