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Basics of HVDC: AC Compared DC Basics of HVDC: AC compared to DC Dr. Ram Adapa Technical Executive, EPRI [email protected] HVDC Lines and Cables Course June 12, 2017 © 2017 Electric Power Research Institute, Inc. All rights reserved. Increased Benefits of Long Distance Transmission .Carrying energy from cheap generation sources which are far away from the load centers. .Long distance transmission increases competition in new wholesale electricity markets . Long distance electricity trade could include across nations or multiple areas within a nation and allows arbitrage of price differences .Long distance transmission allows interconnection of networks and thus reducing the reserve margins across all networks. .More stable long distance transmission is needed to meet contractual obligations 2 © 2017 Electric Power Research Institute, Inc. All rights reserved. Transmitting Fuel versus Transmitting Energy .Load centers can be served by: – Long distance transmission with remote generation – Transmitting fuel to the local generation facilities .Bottom line is Economics to see which option is better .Depends on many factors – Type of fuel – coal can be transported, hydro can’t – Cost of transporting fuel to local generators – Availability of generation facilities close to load centers – Allowable pollution levels at the local gen. facilities 3 © 2017 Electric Power Research Institute, Inc. All rights reserved. Long Distance Transmission – AC versus DC .AC versus DC debate goes back to beginnings of Electricity – DC was first (Thomas Edison) – AC came later (Tesla / Westinghouse) .AC became popular due to transformers and other AC equipment .Long Distance Transmission – AC versus DC - based on economics and technical requirements 4 © 2017 Electric Power Research Institute, Inc. All rights reserved. 5 5 © 2017 Electric Power Research Institute, Inc. All rights reserved. Long Distance AC Transmission .Allows step up and step down of voltages .Intermediate substations are possible to serve load .Reduces current & losses at high voltages .Limited maximum MW capability due to steady state stability limits (surge impedance loading limits) & transient stability limits .Series capacitor compensation can increase loading on the lines but sub synchronous resonance issues need to be addressed .Needs reactive power support (shunt capacitors, SVCs, STATCOMs) to keep acceptable voltages .Lines operating at ratings lot lower than the thermal capability of the lines 6 © 2017 Electric Power Research Institute, Inc. All rights reserved. Long Distance DC Transmission .Converts AC to DC, transmits dc power over long distances, and inverts DC to AC .Controls the power flow on the DC line to a desired value .Most economical for long distance transmission .Can operate the DC lines close to thermal limits .DC can provide direct control between regional AC grids .DC converter stations are more expensive than AC substations .Intermediate substations require multi-terminal DC which is not prevalent in use because of complexity 7 © 2017 Electric Power Research Institute, Inc. All rights reserved. HVDC Opportunities .The potential for long distance transmission for bulk power transfer .The potential for asynchronous interconnection. For example, it allows for connecting networks of 50 Hz and 60 Hz frequencies. .Higher system controllability with at least one HVDC link embedded in an AC grid. – In the deregulated environment, the controllability feature is particularly useful where control of energy trading is needed. .Lower overall investment cost. 8 © 2017 Electric Power Research Institute, Inc. All rights reserved. HVDC Advantages .Lower losses. Typically, because HVDC comprises active power flow only, it causes 20% lower losses than HVAC lines, which comprise active and reactive power flow. .Less expensive circuit breakers, simpler bus-bar arrangements in switchgear, and simpler safety arrangements because HVDC links do not increase the short circuit currents, as converters ensure that the current added never exceeds a preset value. .Increased stability and improvements in power quality. .Enhanced environmental solutions. 9 © 2017 Electric Power Research Institute, Inc. All rights reserved. HVDC Benefits .Management of congestion .Increasing transmission capacity .Frequency control following loss of generation .Voltage stability control, recovery following faults .Capability of providing emergency power and black start during grid restoration following major transmission contingencies 10 © 2017 Electric Power Research Institute, Inc. All rights reserved. HVDC Benefits .Power oscillation damping .Avoidance of cascading blackouts .Precise power transfer control between interconnected transmission areas during emergencies .Rating of HVDC systems as determined only by the real power demand of transmission capacity (versus HVAC system ratings as determined by both real and reactive powers) 11 © 2017 Electric Power Research Institute, Inc. All rights reserved. Relative Cost of AC versus DC .For equivalent transmission capacity, a DC line has lower construction costs than an AC line: – A double HVAC three-phase circuit with 6 conductors is needed to get the reliability of a two-pole DC link – DC requires less insulation – For the same conductor, DC losses are less, so other costs, and generally final losses too, can be reduced. – An optimized DC link has smaller towers than an optimized AC link of equal capacity. 12 © 2017 Electric Power Research Institute, Inc. All rights reserved. HVDC has lower losses than AC for the same power transfer (1200 MW Example) . HVDC line has lower losses than AC line for same power . Converter losses are extra (~ 0.6% of total power) . Total HVDC System losses are lower than AC system losses Source: ABB (2003) 13 © 2017 Electric Power Research Institute, Inc. All rights reserved. Typical Tower Structures Typical tower structures and rights-of-way for alternative transmission systems of 2,000 MW capacity. Source: Arrillaga (1998) 14 © 2017 Electric Power Research Institute, Inc. All rights reserved. AC versus DC (Continued) .Right-of-way for an AC Line designed to carry 2,000 MW is more than 70% wider than the right-of-way for a DC line of equivalent capacity. – This is particularly important where land is expensive or permitting is a problem. .HVDC cables can reduce land and environmental costs, but is more expensive per km than overhead line. 15 © 2017 Electric Power Research Institute, Inc. All rights reserved. AC versus DC (Continued) .The remaining costs also differ: – The need to convert to and from AC implies the terminal stations for a DC line cost more. – There are extra losses in DC/AC conversion relative to AC voltage transformation. – Operation and maintenance costs are lower for an optimized HVDC than for an equal capacity optimized AC system. .The cost advantage of HVDC increases with the length, but decreases with the capacity, of a link. .For both AC and DC, design characteristics trade-off fixed and variable costs, but losses are lower on the optimized DC link. 16 © 2017 Electric Power Research Institute, Inc. All rights reserved. AC versus DC: Break Even Distances Note: Assume right-of-way costs same for AC or DC Cost • The cost of a DC link depends on: DC the cost of the substations DC AC the cost of the line or cableSubstation Break Even AC Distance Substation • HVDC is more economical than Transmission distance AC when the transmission distance : is >300 miles for Overhead lines Is>30 miles for underground cables 17 17 © 2017 Electric Power Research Institute, Inc. All rights reserved. AC versus DC: Typical Breakeven distances This graph is based on late 1990s technologies – old numbers are 500 miles but present breakeven distances are estimated as 300 miles for 2000 MW power transfer Source: Arrillaga (1998) 18 © 2017 Electric Power Research Institute, Inc. All rights reserved. AC versus DC: Cost Comparison When comparing costs for AC and DC, the following need to be considered: .DC Converter / AC substation costs .Line costs .Corridor costs .Operation & Maintenance costs .Costs associated with losses (e.g. DC losses are lower than AC) Bottom line – Complete life cycle cost should be considered over an estimated life span (30 to 40 years) of the equipment. 19 © 2017 Electric Power Research Institute, Inc. All rights reserved. A Broader look: Example AC Transmission Costs in North America $8,000 $7,000 $6,000 Series1138 kV Series2230 kV $5,000 Series3345 kV Series4500 kV $4,000 Series11765 kV $3,000 Transmission Cost Transmission- $/MW-Mile Cost $2,000 $1,000 $0 0 50 100 150 200 250 300 Line Length - Miles 20 © 2017 Electric Power Research Institute, Inc. All rights reserved. HVDC Transmission System Costs .HVDC Converter costs .HVDC Line costs & Transmission Corridor costs 21 © 2017 Electric Power Research Institute, Inc. All rights reserved. HVDC Converter Cost Structure 22 © 2017 Electric Power Research Institute, Inc. All rights reserved. AC versus DC Transmission Costs – Consider cost of losses - Reduces break even distance Million Euros AC AC 23 © 2017 Electric Power Research Institute, Inc. All rights reserved. Source: ABB 24 © 2017 Electric Power Research Institute, Inc. All rights reserved. Special Applications of HVDC .HVDC is particularly suited to undersea transmission, where the losses from AC cables are large. – First commercial HVDC link (Gotland 1 Sweden, in 1954) was an undersea one. .Back-to-back converters are used to connect two AC systems with different frequencies – as in Japan – or two regions where AC is not synchronized
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