Future of HVDC Technology-OCT-2012新一代高压直流输电技术

Spotlight Figure 1: Economics of HVDCSource: Alstom Gridgate-commutated thyristors (IGCTs) are being used in VSCs,IGBT is the most promising as it requires little power for itscontrol. IGBTs also use relatively high frequency switchingand pulse width modulation (PWM) technology for voltageand current regulation, thus making them smaller and lighterin size. This is turn makes the IGBT-based power device muchsmaller in size, lighter in weight, more energy efficient andmore cost effective than the traditional LCC-based controlcircuits. Previously, VSC technology was only used forunderground/submarine cables, but now it is being deployedin both overhead and underground projects.The HVDC VSC transmission system is offered underdifferent names by the three leading manufacturers. ABB hasnamed this concept HVDC Light, Siemens calls it HVDC PLUS(Power Link Universal System) and Alstom Grid markets thisconcept as HVDC MaxSine.VSC-LCC hybrid: This emerging concept entails developing amulti-terminal HVDC system (MTDC) by combining VSC andLCC technologies to help eliminate the disadvantages of boththe HVDC technologies. Such an arrangement is presentlybeen explored by both engineers and power utilities. It isexpected to gain significant importance in the coming years,especially to connect offshore wind facilities. However, such asystem will require complex control schemes to ensure itfunctions efficiently under different operating conditions.As per Alstom Grid estimates, the installed HVDC capacityhas grown five-fold from 3 GW per year to 15 GW per year.Ultra HVDC (>600 kV) and VSC is expected to drive the futureexpansion of the industry. Alstom Grid also estimates that theHVDC market (convertor business) is expected to grow toEUR60 billion by 2020.Technical AdvantagesAC systems have certain inherent challenges such asexistence of relative elements (inductance and capacitance)in lines, which limits their transmission capacity; additionalcomponents such as series capacitors or shunt reactors needto be installed in an AC network to address issues related tocompensation; connecting several AC systems may lead tosystem instability. HVDC transmission offers manyadvantages over traditional AC transmission:•HVDC eliminates reactive capacitance associated with longAC underground/submarine cables;•power transfers and voltage in a DC system can becontrolled and modulated;•DC transmission has no fault currents and acts as a fault‘firewall’, thus preventing faults in the AC systemembedded with HVDC interconnections;•DC systems have a smaller footprint as they require lessright-of-way (RoW), less land and cause less disturbance;•DC reduces electromagnetic fields from transmission linesto insignificant levels;•DC has the ability to link asynchronous power systems(different voltages and frequencies);•DC system installtion involves lower capital costs (aftercentain distance) and results in lower losses as comparedto an AC system; and•DC technology based on VSC can help black start AC powergrids after grid failure or blackout.HVDC SchemesHVDC has a ‘more power per tower’ advantage as it requiresless space and infrastructure to transmit power as comparedto traditional HVAC technology. The same amount of electricitythat can be transferred by five 500 kV AC towers can betransferred by only two ±500 kV DC towers or just one ±800 kVDC tower. While HVDC is uneconomical for transferring powerover short distances as DC convertor stations requiresubstantial investment as compared to AC stations, at distancesover 700 km for overhead lines and over 50 km for submarinecables, the HVDC becomes a more economical option.Broadly, HVDC can be classified into three types ofschemes: back-to-back (BTB), point-to-point connections andoffshore HVDC grid connections. LCC HVDC technology canbe deployed for developing both point-to-point and BTB HVDCsystems while VSC HVDC technology is ideal for connectingoffshore wind farms as well as black starting the grid.A BTB HVDC interconnection is when two convertorstations are built under the same roof. Here, one convertorstation converts AC power (e.g. 380 kV, 60 Hz) to DC and thenthe second convertor station re-converts the DC to the desiredAC voltage and frequency (e.g. 400 kV, 50 Hz), which can betransmitted further to the connected grid. Here the length ofHVDC line or cable is zero.Global Transmission Report|October 201239。