VSC-HVDC Control and Application in Meshed
AC Networks

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Abstract

VSC-HVDC is a fully controllable transmission
device, allowing rapid and independent control of active power
transmitted over the dc page link and reactive power at each end of
the dc link. Embedding VSC-HVDC in meshed ac networks
opens up new possibilities to improve power grid reliability and
efficiency.

INTRODUCTION

SC-HVDC is a transmission technology based on voltage
source converters (VSC) and insulated gate bipolar
transistors (IGBT). The converter technology operates with
high frequency pulse width modulation (PWM) and thus has
the capability to rapidly control both active and reactive
power, independently of each other, to keep the voltage and
frequency stable. Fig. 1 shows the main circuit diagram of
HVDC Light © system, the ABB product name of VSCHVDC
transmission. In this paper, VSC-HVDC and HVDC
Light are thus used interchangeably.

AC GRID WITH EMBEDDED VSC-HVDC

It becomes a challenge to increase power delivery with AC
expansion options in meshed, heavily loaded ac networks. A
key constraint in adding transmission capacity to existing ac
grids is the requirement to neutralize environmental impact -
often making overhead grid extensions impossible. AC
expansion options, both overhead and underground, are often
limited by voltage or transient instability problems, risk of
increased short circuit levels.

CONTROL ARCHITECTURE OF VSC-HVDC

In principle, each VSC-HVDC converter is able to control
active and reactive power independently by simultaneously
regulating the amplitude and phase angle of the fundamental
component of the converter output voltage. The general
control scheme of one VSC-HVDC converter station is shown
in the Fig. 4 below [1, 8].
The control functions of VSC-HVDC system can be
classified by three control layers: system control layer,
application control layer, and converter control layer as shown
in Table I. The system layer controller establishes the
functions for achieving bulk electric grid objectives such as
power flow control, congestion management and voltage
support. These objectives determine the function of the
application control layer which will be discussed in the next
section. The following gives a brief discussion of converter
control functions.

CONCLUSIONS

The operational benefits of embedded VSC-HVDC systems
in meshed ac networks are discussed. A range of advanced
control functions can be implemented in VSC-HVDC systems
for enhancement of ac network steady-state and dynamic
performance. It can be envisioned that VSC-HVDC is an
enabling technology for realizing smart transmission grids
with improved reliability and economics.