Abstract
This paper illustrates a study on HVDC Light for itsenhancement of AC/DC hybrid transmission system and multiinfeedHVDC system. In the sense of the realizing independentcontrolling of active and reactive power, HVDC Light is moreeffective compared to the conventional HVDC system. An AC/DCparallel transmission system and a three-infeed HVDC system aremodeled using PSCAD/EMTDC. Simulations corresponding todifferent situations are performed. From the simulation results,HVDC Light is proven to be able to improve the rotor anglestability and dynamic performance of AC/DC interconnectedtransmission system, and support the AC voltage during thesystem fault to enlarge the margin of the voltage stability which isthe basic difference from conventional HVDC. HVDC Light canalso prevent commutation failure in multi-infeed HVDC systemand help the recovery of HVDC system too.Index Terms—HVDC Light; AC/DC transmission system;power stability; commutation failure
I. INTRODUCTION
High-Voltage direct current (HVDC) transmission is aneconomic way for long distance power delivery and/orinterconnection of asynchronous systems with differentfrequency [1]. With the development of modern power system,HVDC system plays much more important role in power gridsdue to their huge capacity and capability of long distancetransmission. Towards 2015, there will be 7 HVDCtransmission systems constructed in the China South PowerGrids (the CSGs) [2]. Conventional HVDC transmissionsystem is based on line-commutated thyristor rectifier [3].With the advent of high-voltage and high power gate-turn-offthyristor (GTO), insulated-gate controlled transistor (IGBT)and more recently, insulated-gate controlled thyristor (IGCT),high power solid-state switches have symmetrical turn-on andturn-off capabilities. They have given the birth to a newgeneration of HVDC stations, HVDC Light, which is alsocalled as voltage-source-converter (VSC) HVDC [4].Unlike conventional HVDC scheme that employs linecommutatedcurrent source converters with thyristors, HVDCLight is a new technology utilizing forced-commutatedvoltage source converters [5]. HVDC light can control bothactive and reactive power independently without commutationfailures in the inverter side. It doesn’t require reactive powercompensators resulting much smaller equipment size. HVDClight can be applied to the voltage support in the receiver systems [6], interconnection between asynchronous powersystems [7], grid connection of large wind farm [8] oroffshore wind farm [9] and subsea power transmission [10].So far, there are 12 HVDC Light projects for differentpurposes already in operations world wide.HVDC systems are responding and can be controlled withintens of milli-seconds. HVDC systems can also improve thestability, especially transient stability of power system [11].The appearance of the HVDC Light, which has moreadvantages than conventional HVDC systems, can bring anew approach for the AC/DC hybrid transmission systems andmulti-infeed HVDC transmission system. If the receiversystem is weak, voltage stability and commutation failure maybecome serious problems in conventional HVDC system [12].By pulse wide modulation (PWM) control, HVDC Lightrealizes independent control of active and reactive powercontrol, and dose not need reactive compensation in bothrectifier and inverter side. It can also be used with staticsynchronous series compensation (SSSC) characteristics todamp the power system oscillations [13]. HVDC Light appliesself-commutated solid-state device, so there are nocommutation failure issue.For AC/DC interconnected transmission systems, theintroduction of HVDC Light can enhance the voltage supportand improve the system stability. The following sectors willdiscuss the effect of HVDC Light for improvement of theAC/DC interconnected systems. Transient dynamicsimulations are performed with PSCAD/EMTDC. Twosimulation models have been set up with HVDC Light built in;one is an AC/DC parallel transmission lines system, and theother is a multi-infeed HVDC systems. The performance ofHVDC Light in enhancing both power angle stability andvoltage stability in these systems is taken into considerations.
II. PRINCIPLE OF HVDC LIGHTHVDC
Light is composed of transformer, filters, convertersand DC capacitors, as shown in figure 1. Transformer is usedto step down the AC voltage to satisfy the demand of selfcommutatedsolid-state devices, such as series and parallel ofGTOs, IGBTs or IGCTs. High frequency components causedby the switches of valves are isolated from power system byfilters. The key parts of HVDC Light are converters, whichcan realize the conversion from AC to DC bi-directly. DCcapacitors are used as the DC voltage source in HVDC Light,which need being charged and recharged.


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