Basics of HVDC: AC compared DC - University of Minnesota · 2019-07-25 · Dr. Ram Adapa Technical...

© 2017 Electric Power Research Institute, Inc. All rights reserved.

Dr. Ram Adapa

Technical Executive, EPRI

[email protected]

HVDC Lines and Cables Course

June 12, 2017

Basics of HVDC:

AC compared to DC

mailto:[email protected]

2© 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


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


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


5


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


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


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.


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.


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


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)


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.


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)


Typical Tower Structures

Typical tower structures and

rights-of-way for alternative

transmission systems of 2,000

MW capacity.

Source: Arrillaga

(1998)


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.


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.


17

• The cost of a DC link depends on:

the cost of the substations

the cost of the line or cable

• HVDC is more economical than

AC when the transmission distance :

is >300 miles for Overhead lines

Is>30 miles for underground cables

DC

SubstationBreak Even

Distance

Cost

DC

AC

Substation

AC

Transmission distance

Note: Assume right-of-way costs same for AC or DC

AC versus DC: Break Even Distances


AC versus DC: Typical Breakeven distances

Source: Arrillaga (1998)

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


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.


$0

$1,000

$2,000

$3,000

$4,000

$5,000

$6,000

$7,000

$8,000

0 50 100 150 200 250 300

Line Length - Miles

Tra

nsm

issio

n C

ost

- $/M

W-M

ile

Series1

Series2

Series3

Series4

Series11

138 kV

230 kV

345 kV

500 kV

765 kV

A Broader look: Example AC Transmission Costs in

North America


HVDC Transmission System Costs

HVDC Converter costs

HVDC Line costs & Transmission Corridor costs


HVDC Converter Cost Structure


AC versus DC Transmission Costs – Consider

cost of losses - Reduces break even distance

AC

AC

Million Euros


Source: ABB


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.

Special Applications of HVDC


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.

Special Applications (continued)


Principle of AC Transmission

Schematic of AC system


Basic HVDC Transmission

DC link

Transformer

F F

Harmonic Filter

(Reactive Power)

Receiving

End

Sending

End

Idc

RT

t

Idci

t

i

Iac

t

i

Iac

InverterRectifier

V1 V2

2T1DC RRR

2V1VI


Components of HVDC Transmission Systems

1. Converters

2. Smoothing reactors

3. Harmonic filters

4. Reactive power supplies

5. Electrodes

6. DC lines

7. AC circuit breakers

Components of HVDC


Components of HVDC Transmission Systems….

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

Components of HVDC Transmission Systems….


HVDC Scheme Types

AC ACDC 1 Station

• Back-to-Back

− frequency changing

− asynchronous connection

AC

DCStation 2

AC

Station 1• Point-to-Point Overhead

Line

− bulk transmission

− overland

AC

DC Station 2

AC

Station 1

Submarine Cables

• Point-to-Point Submarine Cable

− bulk transmission

− underwater or underground


Decrease voltage at station B or increase voltage at station A. power flows from A B Normal

direction

Decrease voltage at station B or increase voltage at station A. power flows from A B Normal

direction


Power reversal is obtained by reversal of polarity of direct voltages at both ends.


• Monopolar links

• Bipolar links

• Homopolar links

• Symmetrical Monopolar links

• Multiterminal links

• DC Grids

HVDC links can be broadly classified into:

HVDC System Configurations


Monopolar Links

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.


Bipolar Links

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).


Homopolar Links

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.


Symmetrical Monopolar Link

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.

https://en.wikipedia.org/wiki/NorNed


HVDC Converter Technology: LCC Versus VSC

Line Commutated Converter

(or Current Source Converter )

• Thyristor based

• Switches on-off one time per cycle

Voltage Source Converter

• IGBT Based

(Insulated Gate Bipolar Transistor)

• Switches on-off many times per cycle


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


Line Commutated Converters

Large filters required due to low order harmonics generated


Voltage Source Converters2 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


VSC : Recent new Topology (MMC)

~

=

~

=

~

=

1

2

n

1

2

n

SM electronics

1

2

IGBT2 D2

D1IGBT1

• Modular Multilevel Converter

• In this case the converter arms

are constructed from identical

sub-modules that are

individually controlled to obtain

the desired ac voltage.

Half-Chain Links shown here.

Full-Chain Links can be used to

reduce fault currents on DC side


Modular Multi Level Converters

• 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


VSC Short History

• First introduced in 1997 with the 3MW, +/-10 kV

dc technology demonstrator at Hellsjön,

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 .

VSC


VSC

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.


Function LCC VSCSemi-Conductor

Device

Thyristors currently 6

inch, 8.5 kV and 5000 Amps. No controlled

turn off capability

IGBTs with anti-parallel free wheeling diode, with

controlled turn-off capability. Current rating 4.5 to 6

kV and turn off current of 1200 Amps.

DC transmission

voltage

Up to +/- 800 kV

bipolar operation. 1000 kV under

consideration in China

Up to +/- 320 kV to 400 kV currently limited by

HVDC cable if extruded XLPE cable is used.

Up to +/- 350 kV with Overhead line, can go higher

DC power Currently in the range

of 6000 MW per

bipolar system

Currently in the range of 600 to 1000 MW per pole

Reactive Power

requirements

Consumes reactive

power up to 60% of

its rating

Does not consume any reactive power and each

terminal can independently control its reactive power.

Filtering Requires large filter

banksRequires moderate size filter banks or no filters at all.

Black start Limited application Capable of black start and feeding passive loads

HVDC Converter Technology: LCC vs. VSC


Function LCC VSC

Commutation failure

performance

Fails commutation for ac

disturbances

Does not fail commutation

Over load capability Available if designed for up to

any required design value

Does not have any overload

capability

Application with overhead lines Can be applied and dc line faults

can be cleared by converter

control

Can be applied but dc line faults

are cleared by trip of ac breaker,

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

performance

Economic and seems not affect

the performance

Load rejection over voltage Large and has to be mitigated

because of the large reactive

power support

not large because of small size of

filters if required.



Function LCC VSC

Foot print Can be large Small for the

comparable rating to

an LCC

Off shore wind farms Can be applied with

some dynamic

voltage control

Straight forward

application

Power losses Typically 0.8% per

converter station at

rated power

Typically 0.8 to 1.0%

per terminal with

multilevel converters



Trans Bay VSC DC Cable


HVDC IN NORTH AMERICA

Blackwater (200 MW)

Artesia (200 MW)Sidney

(200 MW)

Stegall

(110 MW)

Rapid City DC

(200 MW)

Miles City

(200 MW)

IPP (2400 MW)

PDCI (3100 MW)

TBC (400 MW)

McNeill

(150 MW)

Eel River

(320 MW)

Square Butte

(500 MW)Nelson River

(1620 MW)

Nelson River II

(1800 MW)

Coal Creek

(1000 MW)

Oklaunion

(200 MW)

Madawaska

(350 MW)

Highgate

(200 MW)

Quebec –

New England

(2000 MW)

Welsh

(600 MW)

Eagle Pass

(36 MW)

Cross Sound

Cable (300 MW)

Lamar (210 MW)Neptune

(600 MW)

Sharyland

(150 MW)

Chateauguay

(1000 MW)

LCC HVDC

VSC HVDC


Examples of HVDC Projects Around the World

Nelson River 2

CU-project

Vancouver Island

Pole 1

Pacific Intertie

Pacific Intertie

Upgrading

Pacific Intertie

Expansion

Intermountain

Blackwater

Itaipu

Inga-Shaba

Cahora Bassa

Brazil-ArgentinaInterconnection I

English

ChannelDürnrohrSardinia-ItalyItaly-Greece

Highgate

Chateauguay

Quebec-New

England

Skagerrak 1&2

Skagerrak 3

Konti-Skan 1

Konti-Skan 2

Baltic Cable

Fenno-Skan

Gotland 1

Gotland 2

Gotland 3

Kontek

SwePol

Chandrapur-

Padghe

Rihand-Delhi

Vindhyachal

Sakuma

Gezhouba-

Shanghai

Leyte-Luzon

Broken Hill

New Zealand 1

New Zealand 2

Three Gorges -

Changzhou

Brazil-ArgentinaInterconnection II

Gotland

Murraylink

Directlink

Moselstahlwerke

Cross Sound Cable

Eagle Pass

Tjæreborg

Hällsjön

Hagfors

HVDC Classic Converters

CCC Converters

HVDC Light (VSC) Converters

Source: ABB


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 circuit breakers Standard DC voltages Communication needs

– CIGRE/IEEE WGs Two Topologies

DC Node

AC Node

DC Line

(a)

(b)


DC Grid Configurations: Offshore Development – Point to Point System

Source: ALSTOM


DC Grid Configurations: Offshore Grid System

Source: ALSTOM


Macro Grid

HVDC Network Concept


59

Overlay DC Grid Gives Access to Renewable

Sources within Europe

• 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

• Avoidance of synchronisation over

a wide area

• Less environmental impact than

AC reinforcement

3000k

m


Cigrè B4-52: HVDC Grid Feasibility Study

1 Introduction

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


DC Grid Standardisation Activities

• 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


A Sample of European Proposals

G. Asplund, B. Jacobson, B. Berggren, K.

Lindén ”Continental Overlay HVDC-Grid”, Cigré

conference, B4-109, Paris, 2010


Atlantic Wind Connection

http://atlanticwindconnection.com/download/AtlanticWindConnection_Brochure.pdf


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

Improve regional system reliability.

2010 P

S36A

65© 2017 Electric Power Research Institute, Inc. All rights reserved.CENELEC meeting 29.06.11 P 65

Comparison of AC and DC parameters

AC PARAMETER DC PARAMETER

Frequency

Target DC Voltage

Vdc

Voltage Change

))sin(V(

Voltage Change

V

Impedance of Connection

)X(

Resistance of Connection

R

Real Power

XsinVV

Real Power

RVV


When closed the DC breaker must have very low losses

• optimum solution mechanical switch

DC Breakers

Main switch

Unlike an AC breaker the DC

current never experiences a

current zero. Hence, to

interrupt the DC current the DC

breaker must drive the load

current to zero.

AC

DC

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


HVDC Circuit Breaker Developments

– Many ideas are explored– Fast growing area– Numerous R&D projects– Minimize size, cost, & interruption time

Solid State Circuit

Breaker

V

VV

IG IS

IV

New Hybrid Circuit Breaker


New HVDC Circuit Breaker Developments –

Hot of the Press (as of November 7, 2012)

ABB develops world’s first circuit breaker for HVDC

November 7, 2012

By PennEnergy Editorial Staff

Source:ABBABB (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

manufacturing capability for power semiconductors, converters and high voltage cables (key components of

HVDC systems) were distinct advantages in the new development.

http://www.pennenergy.com/content/ppg/en/authors/pennenergy-editorial-staff.html

http://www.pennenergy.com/_search?q=ABB&x=0&y=0

http://markets.financialcontent.com/pennwell.ogj/quote?Symbol=321:1220985

http://www.pennenergy.com/power.html

http://www.pennenergy.com/_search?q=HVDC&x=23&y=15


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

Future Trends in HVDC


Together…Shaping the Future of Electricity

Basics of HVDC: AC compared DC - University of Minnesota · 2019-07-25 · Dr. Ram Adapa Technical...

Documents

Transcript of Basics of HVDC: AC compared DC - University of Minnesota · 2019-07-25 · Dr. Ram Adapa Technical...