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

.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

.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

.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

.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

.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

.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.

.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.

.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

.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)

.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.

. 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

Typical tower structures and rights-of-way for alternative transmission systems of 2,000 MW capacity.

Source: Arrillaga (1998)

.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.

.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.

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)

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.

$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

.HVDC Converter costs .HVDC Line costs & Transmission Corridor costs

Million Euros

.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 – as in the US.

.HVDC links can stabilize AC system frequencies and voltages, and help with unplanned outages. – A DC link is asynchronous, and the conversion stations include frequency control functions. – Changing DC power flow rapidly and independently of AC flows can help control reactive power. – HVDC links designed to carry a maximum load cannot be overloaded by outage of parallel AC lines.

Schematic of AC system

DC link Rectifier Inverter Sending Receiving End End Transformer RT

V1 V2

F Idc F

Harmonic Filter i i Idc i (Reactive Power)

1. Converters 2. Smoothing reactors 3. Harmonic filters 4. Reactive power supplies 5. Electrodes 6. DC lines 7. AC circuit breakers

Components of HVDC

Converters . They perform AC/DC and DC/AC conversion . They consist of valve bridges and transformers . Valve bridge consists of high voltage valves connected in a 6-pulse or 12-pulse arrangement . The transformers are ungrounded such that the DC system will be able to establish its own reference to ground Smoothing reactors . They are high reactors with inductance as high as 1 H in series with each pole . They serve the following: – They decrease harmonics in voltages and currents in DC lines – They prevent commutation failures in inverters – Prevent current from being discontinuous for light loads Harmonic filters . Converters generate harmonics in voltages and currents. These harmonics may cause overheating of capacitors and nearby generators and interference with telecommunication systems . Harmonic filters are used to mitigate these harmonics

Reactive power supplies . Under steady state condition, the reactive power consumed by the converter is about 50% of the active power transferred . Under transient conditions it could be much higher . Reactive power is, therefore, provided near the converters . For a strong AC power system, this reactive power is provided by a shunt capacitor Electrodes . Electrodes are conductors that provide connection to the earth for neutral. They have large surface to minimize current densities and surface voltage gradients DC lines . They may be overhead lines or cables . DC lines are very similar to AC lines AC circuit breakers . They used to clear faults in the transformer and for taking the DC link out of service . They are not used for clearing DC faults . DC faults are cleared by converter control more rapidly

• Back-to-Back DC 1 Station − frequency changing AC AC − asynchronous connection

• Point-to-Point Overhead DC Line Station 1 Station 2 AC AC − bulk transmission − overland

• Point-to-Point Submarine DC Cable Station 1 Station 2 AC AC − bulk transmission − underwater or underground Submarine Cables

Decrease voltage at station B or increase voltage at station A. power flows from A B Normal direction 33 © 2017 Electric Power Research Institute, Inc. All rights reserved. 34 © 2017 Electric Power Research Institute, Inc. All rights reserved. Power reversal is obtained by reversal of polarity of direct voltages at both ends. 35 © 2017 Electric Power Research Institute, Inc. All rights reserved. HVDC System Configurations

HVDC links can be broadly classified into:

• Monopolar links • Bipolar links • Homopolar links • Symmetrical Monopolar links • Multiterminal links • DC Grids

.It uses one conductor . .The return path is provided by ground or water. .Use of this system is mainly due to cost considerations. .A metallic return may be used where earth resistivity is too high. .This configuration type is the first step towards a bipolar link.

. Each terminal has two converters of equal rated voltage, connected in series on the DC side. . The junctions between the converters is grounded. . If one pole is isolated due to fault, the other pole can operate with ground and carry half the rated load (or more using overload capabilities of its converter line).

. It has two or more conductors all having the same polarity, usually negative. . Since the corona effect in DC transmission lines is less for negative polarity, homopolar link is usually operated with negative polarity. . The return path for such a system is through ground.

.An alternative is to use two high-voltage conductors, operating at ± half of the DC voltage, with only a single converter at each end. In this arrangement, known as the symmetrical monopole, the converters are earthed only via a high impedance and there is no earth current. The symmetrical monopole arrangement is uncommon with line- commutated converters (the NorNed interconnection being a rare example) but is very common with Voltage Sourced Converters when cables are used.

Line Commutated Converter Voltage Source Converter (or Current Source Converter ) • IGBT Based • Thyristor based (Insulated Gate Bipolar Transistor) • Switches on-off one time per cycle • Switches on-off many times per cycle

41 © 2017 Electric Power Research Institute, Inc. All rights reserved. .LCC Switch turn on by gate pulse but external circuit needed to turn off – VSC has turn on and turn off capability without external circuit due to self commutation

.LCC suffers commutation failures as a result of a sudden drop in the amplitude or phase shift in the AC voltage, which result in dc temporal over-current – Ability to turn on and off switches means VSC does not suffer from commutation failures

.Existing HVDC largely point to point .Multi – terminal being talked about more and more – few installations exist – Multi terminal LCC problematic due to difficulty changing polarity – VSC more suitable for multi terminal operation

.Large filters required due to low order harmonics generated

43 © 2017 Electric Power Research Institute, Inc. All rights reserved. Voltage Source Converters 2 Level • Most simple VSC design • Requires high harmonic filters • High switching frequency required • 1st generation VSC • Uses PWM

3 Level • Slightly more refined than 2 level • Still requires filtering but lower harmonics • Used in some installations but surpassed by MMC

• Modular Multilevel Converter ~ ~ ~ • In this case the converter arms = = = are constructed from identical sub-modules that are

1 individually controlled to obtain 2 the desired ac voltage.

n SM electronics

IGBT1 D1 Half-Chain Links shown here. Full-Chain Links can be used to 1 1 IGBT2 D2 reduce fault currents on DC side

2 n

• Much more complex control • Almost no requirement for AC filters • Most expensive and complex topology • Lower losses due to lower switching frequency per switch • Inherent redundancy • Modular design

• First introduced in 1997 with the 3MW, +/-10 kV dc technology demonstrator at Hellsjön, VSC Sweden

• In 2007 Cross Sound cable having a rating of 330 MW and ±150 kV dc

• Awarded projects not in operation yet – • France to Spain 320 kV, two bipoles (2x1000 MW), using underground extruded cable of 64 km (40 miles) • Skagerak 4 (one pole) at 500 kV, 700 MW by 2014 using DC submarine cable (140 km)+land cable(104 km) between Norway & Denmark .

.Currently all the HVDC VSC systems are designed with solid extruded cables XLPE cables, with the exception of the Caprivi HVDC inter-connector in Namibia, where the technology is applied to an overhead line. The project is rated at 300 MW at 350 kV.

.The use of VSC is being expanded to overhead lines and dc voltage can be increased to higher levels (above 320 kV because there is no limit of dc cable voltage)

.One of the important applications of HVDC VSC converters is integration of off-shore wind farms.

48 © 2017 Electric Power Research Institute, Inc. All rights reserved. HVDC Converter Technology: LCC vs. VSC Function LCC VSC Semi-Conductor Thyristors currently 6 IGBTs with anti-parallel free wheeling diode, with Device inch, 8.5 kV and 5000 controlled turn-off capability. Current rating 4.5 to 6 Amps. No controlled kV and turn off current of 1200 Amps. turn off capability DC transmission Up to +/- 800 kV Up to +/- 320 kV to 400 kV currently limited by voltage bipolar operation. HVDC cable if extruded XLPE cable is used. 1000 kV under Up to +/- 350 kV with Overhead line, can go higher consideration in China

DC power Currently in the range Currently in the range of 600 to 1000 MW per pole of 6000 MW per bipolar system Reactive Power Consumes reactive Does not consume any reactive power and each requirements power up to 60% of terminal can independently control its reactive power. its rating Filtering Requires large filter Requires moderate size filter banks or no filters at all. banks Black start Limited application Capable of black start and feeding passive loads 49 © 2017 Electric Power Research Institute, Inc. All rights reserved. HVDC Converter Technology: LCC vs. VSC

Function LCC VSC

Commutation failure Fails commutation for ac Does not fail commutation performance disturbances Over load capability Available if designed for up to Does not have any overload any required design value capability

Application with overhead lines Can be applied and dc line faults Can be applied but dc line faults can be cleared by converter are cleared by trip of ac breaker, control or the use of a dc circuit breaker. Currently one application of overhead line. It has mostly been applied with cables

Small taps Not economic and affects the Economic and seems not affect performance the performance Load rejection over voltage Large and has to be mitigated not large because of small size of because of the large reactive filters if required. power support

Function LCC VSC Foot print Can be large Small for the comparable rating to an LCC Off shore wind farms Can be applied with Straight forward some dynamic application voltage control Power losses Typically 0.8% per Typically 0.8 to 1.0% converter station at per terminal with rated power multilevel converters

52 © 2017 Electric Power Research Institute, Inc. All rights reserved. HVDC IN NORTH AMERICA Square Butte Quebec – (500 MW) Nelson River Coal Creek New England McNeill (1620 MW) (1000 MW) (2000 MW) (150 MW) Nelson River II (1800 MW) Chateauguay Miles City (1000 MW) (200 MW) Eel River (320 MW) Rapid City DC (200 MW) Madawaska LCC HVDC (350 MW) VSC HVDC Highgate (200 MW) PDCI (3100 MW) TBC (400 MW) Cross Sound Cable (300 MW) IPP (2400 MW) Neptune Lamar (210 MW) (600 MW) Blackwater (200 MW) Oklaunion Stegall Welsh (200 MW) (110 MW) (600 MW) Sidney Artesia (200 MW) (200 MW) Eagle Pass Sharyland 53 © 2017(36 Electric MW) Power Research Institute,(150 Inc. AllMW) rights reserved. Examples of HVDC Projects Around the World

Nelson River 2 Highgate Hällsjön Fenno-Skan CU-project Chateauguay Hagfors Gotland 1 Vancouver Island Quebec-New Skagerrak 1&2 Gotland 2 Skagerrak 3 Gotland 3 Pole 1 England Konti-Skan 1 Gotland Konti-Skan 2 Kontek Baltic Cable Tjæreborg SwePol Moselstahlwerke English Channel Three Gorges - Dürnrohr Changzhou Sardinia-Italy Sakuma Pacific Intertie Italy-Greece Cross Sound Cable Gezhouba- Pacific Intertie Eagle Pass Shanghai Upgrading Leyte-Luzon Pacific Intertie Chandrapur- Broken Hill Expansion Itaipu Padghe New Zealand 1 Intermountain Inga-Shaba Rihand-Delhi New Zealand 2 Blackwater Cahora Bassa Vindhyachal Brazil-Argentina Murraylink Interconnection I Directlink Brazil-Argentina HVDC Classic Converters Interconnection II CCC Converters Source: ABB HVDC Light (VSC) Converters 54 © 2017 Electric Power Research Institute, Inc. All rights reserved. DC Grids – The Future of DC Transmission

– DC Grids for Offshore Wind – Considered more in Europe than in other countries – Need to resolve many issues . Power & Voltage control DC Node . DC circuit breakers (a) AC Node . Standard DC voltages DC Line

. Communication needs

– CIGRE/IEEE WGs Two Topologies (b)

• Interconnection of remote renewable energy sources • Overcoming “bottlenecks” in the existing AC grids • Low loss (HVDC) transmission systems • Controllable power flows over a wide area 3000k • Avoidance of synchronisation over m a wide area • Less environmental impact than AC reinforcement

1Introduction 2HVDC grids – concepts and lessons learned from history 3Available Converter Technologies, VSC and LCC Comparison 4Motivation of an HVDC grid 5HVDC grid Configurations 6Fault Performance 7Protection Requirements 8New components in HVDC grid – Including Questionnaires to manufacturers 9Power Flow Control in DC Grids 10The Requirements on an HVDC grid – Security and Reliability 11Needed Standardization 12New working groups within the HVDC grid area

• Cigrè have started five further DC grid working groups;

– B4-56: Guidelines for the preparation of “connection agreements” or “Grid Codes” for HVDC grids

– B4-57: Guide for the development of models for HVDC converters in a HVDC grid

– B4-58: Devices for load flow control and methodologies for direct voltage control in a meshed HVDC Grid

– B4-59: Protection of Multi-terminal HVDC Grids

– B4-60: Designing HVDC Grids for Optimal Reliability and Availability performance

G. Asplund, B. Jacobson, B. Berggren, K. Lindén ”Continental Overlay HVDC-Grid”, Cigré conference, B4-109, Paris, 2010

63 http://atlanticwindconnection.com/download/AtlanticWindConnection_Brochure.pdf© 2017 Electric Power Research Institute, Inc. All rights reserved. Atlantic Wind Connection Project (see: www.atlanticwindconnection.com/ferc/2010-12-filing/Petition_for_Declaratory_Order.pdf)

What . A sub-sea HVDC backbone transmission system Where . Extending from northern New Jersey to southern Virginia. Who . Google . Marubeni . Good Earth . Elia Why . Serve as an efficient collector of ac power from offshore wind farms . Relieve transmission congestion on the eastern ac grid

AC PARAMETER DC PARAMETER Frequency Target DC Voltage  Vdc Voltage Change Voltage Change (Vsin()) V Impedance of Connection Resistance of Connection (X) R

Real Power Real Power VVsin VV X R

When closed the DC breaker must have very low losses • optimum solution mechanical switch

Unlike an AC breaker the DC AC current never experiences a current zero. Hence, to interrupt the DC current the DC DC breaker must drive the load current to zero. Main switch Modular hybrid solution to drive current to zero • critical component is the mechanical switch as it has to operate VERY fast to minimise the peak current to be interrupted by the auxiliary branch

I – Many ideas are explored G IS – Fast growing area IV

– Numerous R&D projects V – Minimize size, cost, & interruption time

New Hybrid Circuit Breaker

ABB develops world’s first circuit breaker for HVDC November 7, 2012 By PennEnergy Editorial Staff ABBSource:ABB(NYSE: ABB), the leading power and automation technology group, has announced a breakthrough in the ability to interrupt direct current, solving a 100-year-old electrical engineering puzzle and paving the way for a more efficient and reliable electricity supply system. After years of research, ABB has developed the world’s first circuit breaker for high voltage direct current (HVDC). It combines very fast mechanics with power electronics, and will be capable of ‘interrupting’ power flows equivalent to the output of a large power station within 5milliseconds- that is thirty times faster than the blink of a human eye. The breakthrough removes a 100-year-old barrier to the development of DC transmission grids, which will enable the efficient integration and exchange of renewable energy. DC grids will also improve grid reliability and enhance the capability of existing AC (alternating current) networks. ABB is in discussions with power utilities to identify pilot projects for the new development. ABB has written a new chapter in the history of electrical engineering,” said Joe Hogan, CEO of ABB. “This historical breakthrough will make it possible to build the grid of the future. Overlay DC grids will be able to interconnect countries and continents, balance loads and reinforce the existing AC transmission networks. “ The Hybrid HVDC breaker development has been a flagship research project for ABB, which invests over $1 billion annually in R&D activities. The breadth of ABB’s portfolio and unique combination of in-house

68manufacturing capability for power semiconductors, converters and high voltage cables (key components of HVDC systems) were© 2017 Electric distinct Power Research advantages Institute, Inc. All rightsin the reserved. new development. Current State of HVDC versus HVAC

.Many Existing HVDC systems are old (30 - 50 years old) – Life extension is taking place .Highest DC Voltage is UHVDC at +/- 800 kV in China & India – South Africa & Brazil are also considering – For long distances over 3000 km – For Bulk Power Transfer ( 3000 to 6000 MW) .UHVDC of +/- 1000 to 1100 kV is planned in Asia for up to 8000 MW - China .VSC HVDC is increasing (+/- 320 kV up to 1000 MW)

.Max AC Voltage in North America is 765 kV (EHVAC) .UHVAC (1000 kV to 1200 kV) is considered in China (highest in the world)

.For transfers of above 6,000 MW over 4,000 km, the optimum voltage rises to 1,000–1,200 kV. – Technological developments in LCC converter stations seem to be ready to handle these voltages.

.HVDC and HVAC overlays for regional interconnections

.Segmenting AC grids with DC back-to-backs for improved reliability

.Growth of VSC DC applications – more dc cable projects

.DC Grids for renewable integration