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Benefits of power flow control

Date post: 27-Jan-2015
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Describes a future in which the transmission grid can be controlled to optimize the use of cost-effective, clean generation resources while providing high-quality, reliable power. Summarizes the research that the Advanced Research Projects Agency - Energy (ARPA-E) is undertaking into hardware and software technologies that could significantly change the ability to control the flow of electricity in the power grid. This analysis is intended to describe potential benefits of a flexible transmission system achieved through power flow control technologies. Specifically, it categorizes the benefits of power flow control technologies and defines the impact of technologies used for power flow control. It also makes recommendations for further studies and analyses on power flow control. Written by ARPA-E interns Lotte Schlegel and Chris Babcock under the guidance of Josh Gould.
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Advanced Research Projects Agency for Energy, U.S. Department of Energy Benefits of Power Flow Control Hardware and Software Technologies Lotte Schlegel, Chris Babcock and Josh Gould 9/27/2013
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Page 1: Benefits of power flow control

 

 

 

   Advanced  Research  Projects  Agency  for  Energy,  U.S.  Department  of  Energy  

Benefits  of  Power  Flow  Control  Hardware  and  Software  Technologies  

Lotte  Schlegel,  Chris  Babcock  and  Josh  Gould  9/27/2013    

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Contents  Purpose  and  Scope  ......................................................................................................................................  3  

Characteristics,  Capabilities  and  Technologies  of  a  Flexible  Grid  ................................................................  3  

Power  Flow  Control  Technology  Defined  ................................................................................................  4  

Hardware  .............................................................................................................................................  5  

   High  Voltage  Direct  Current  ...............................................................................................................  5  

   HVAC  Power  Transmission  Controllers  (PTC)  .....................................................................................  7  

   Software  .............................................................................................................................................  9  

   Topology  Control  Algorithms  (TCAs)  ..................................................................................................  9  

Value  Analysis  of  Power  Flow  Control  .......................................................................................................  11  

Identification  of  Value  Propositions  ......................................................................................................  12  

Asset  Management  ............................................................................................................................  12  

Reliability  and  Security  .......................................................................................................................  13  

Congestion  Relief  ...............................................................................................................................  14  

   Integration  of  renewable  energy  .....................................................................................................  14  

   Economic  Efficiency  .........................................................................................................................  15  

Summary  of  Power  Flow  Control  Technology  Value  ..............................................................................  18  

Stakeholders  in  the  Transmission  Grid  Influence  Technology  Investment  Decisions  ............................  18  

Conclusion/Next  steps  ...............................................................................................................................  21  

References  .................................................................................................................................................  24  

   

 

 

 

 

 

 

 

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Purpose  and  Scope    Electricity  is  dynamic  –  supply  must  meet  demand  that  changes  by  the  second  in  the  electric  grid.  While  electricity  markets  have  evolved  to  price  supply  dynamically  and  demand  response  systems  have  developed  to  manage  demand  on  a  dynamic  basis,  the  transmission  grid  is  inflexible.  When  the  flow  of  electrons  is  disrupted  by  a  storm,  an  accident,  or  congestion  choking  the  lines  like  cars  on  an  interstate,  it  affects  the  wallets  of  people  and  businesses.    The  electric  transmission  grid  costs  consumers  billions  in  congestion  costs,  is  difficult  and  expensive  to  upgrade,  and  does  not  respond  quickly  to  contingency  events  –  costing  $79  billion  annually  in  power  interruptions  (Hamachi  LaCommare,  2004).  Transmission  infrastructure  in  the  U.S.  is  aging  -­‐  as  of  2008,  70%  of  transmission  lines  and  transformers  are  25  years  or  older  and  60%  of  circuit  breakers  are  30  years  or  older  (DOE,  2008).    

The  electric  grid  of  the  future  will  need  to  be  sufficiently  flexible,  responsive,  and  reliable  to  support  variable  generation  resources,  reduce  areas  of  transmission  congestion,  and  respond  quickly  to  system  disruptions  due  to  severe  weather  events.  The  impending  upgrades  to  infrastructure  present  an  opportunity  to  include  technologies  to  improve  resiliency  of  the  grid.  Increasing  the  flexibility  of  the  electric  transmission  grid  can  be  the  cornerstone  to  addressing  all  of  these  challenges.    

ARPA-­‐E’s  Green  Electricity  Network  Integration  “GENI”  program  envisions  a  future  in  which  the  transmission  grid  can  be  controlled  to  optimize  the  use  of  cost-­‐effective,  clean  generation  resources  while  providing  high-­‐quality,  reliable  power1.  To  that  end,  ARPA-­‐E  is  funding  research  into  transformative  hardware  and  software  technologies  that  could  significantly  change  the  ability  to  control  the  flow  of  electricity  in  the  power  grid.    

This  analysis  is  intended  to  add  to  the  conversation  about  the  benefits  of  a  flexible  transmission  system  achieved  through  power  flow  control  technologies.  Specifically,  it  will  describe  and  categorize  the  benefits  of  power  flow  control  technologies  and  define  the  impact  of  technologies  used  for  power  flow  control.  It  will  also  make  recommendations  for  further  studies  and  analyses  on  power  flow  control.      

Characteristics,  Capabilities  and  Technologies  of  a  Flexible  Grid      Historically,  the  electric  grid  was  designed  to  be  a  passive,  one-­‐directional  system.  To  improve  the  grid’s  reliability  and  turn  intermittent  power  sources  into  major  contributors  in  the  U.S.  energy  mix,  the  grid  needs  to  be  designed  and  operated  to  be  smarter  and  more  flexible.  Power  flow  control  is  one  way  to  increase  the  flexibility  and  resiliency  of  the  electric  grid.  Power  flow  is  determined  by  the  impedance  of  a  transmission  line  and  the  difference  in  voltage  at  each  end  (M.I.T.,  2011)2.  Power  flow  control  is  the  

                                                                                                                         1  More  at  http://arpa-­‐e.energy.gov/?q=arpa-­‐e-­‐programs/geni    2  From  MIT’s  Future  of  the  Electric  Grid.  “Two  factors  determine  power  flow:  the  impedance  of  a  line  and  the  difference  in  the  instantaneous  voltages  at  its  two  ends.  Impedance  is  the  combination  of  resistance  and  reactance.  Resistance  accounts  for  energy  that  is  lost  as  heat  in  the  line.  It  is  analogous  to  the  physical  resistance  exerted  by  water  on  a  swimmer  or  wind  on  a  cyclist.  Energy  lost  in  this  way  can  never  be  recovered.  Reactance  accounts  for  energy  associated  with  the  electric  and  magnetic  fields  around  the  line.  This  energy  is  analogous  to  

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ability  to  change  the  way  that  power  flows  through  the  transmission  grid  using  hardware  and  software  to  maximize  system  value.  These  technologies  can  change  the  effective  impedance  of  the  network  or  the  sending  and  receiving  voltages  to  influence  the  path  of  electrons  flowing  through  the  transmission  grid.  This  enables  the  ability  to  hold  power  on  a  transmission  line  at  a  certain  level  or  direction.  Electrons  follow  the  path  of  least  resistance  (or  lowest  impedance),  and  the  result  of  changing  the  pathways  of  the  grid  is  to  change  the  way  that  power  flows  through  the  transmission  system.  Specifically,  power  flow  control  can  be  used  to  remove  congestion,  respond  to  contingency  events  (e.g.  loss  of  a  generator  or  transmission  line),  and  mitigate  power  quality  issues.  

Power  flow  control  includes  the  faculties  to  control  the  voltage  or  impedance  on  given  major  transmission  lines,  switch  lines  on  and  off,  direct  power  from  one  line  to  another  to  increase  the  capacity  of  a  transmission  route,  provide  voltage  support,  transport  power  efficiently  over  long  distances,  and  quickly  reverse  the  direction  of  power  flow  from  one  area  to  another  in  response  to  contingencies.  A  system  planner  can  optimize  power  flow  on  a  system  by  choosing  among  technologies  to  enable  each  of  these  capabilities  as  appropriate.  In  order  to  fully  integrate  power  flow  control  at  the  system  level,  information  systems,  hardware  technology,  and  human  operators  at  ISOs/RTOs,  generators,  and  transmission  and  distribution  companies  coordinate  to  match  system  supply  and  demand  at  every  moment.  For  instance,  information  (such  as  forecasting  of  weather,  supply  and  demand),  sensors,  communication  devices,  and  control  technology  work  together  to  enable  physical  changes  to  the  transmission  grid.  As  power  flow  control  hardware  technologies  are  added  to  the  system,  coordinated  control  of  the  transmission  grid  will  maximize  the  efficacy  of  power  flow  control  and  ensure  reliability  across  the  system.  Changes  are  likely  required  to  optimize  the  coordination  of  the  grid  with  the  addition  of  power  flow  control  technologies  -­‐  for  instance,  as  variables  and  options  are  added  to  the  system,  either  a  central  operator  with  sufficient  computational  power  to  respond  to  dynamic  grid  conditions  or  coordinated  distributed  control  will  be  necessary  to  ensure  system  optimization.    

Power  flow  control  can  increase  reliability  and  resiliency,  optimize  transmission  asset  efficiency  and  help  prioritize  new  transmission  construction  by  increasing  the  capacity  of  the  transmission  grid,  reduce  cost  to  electric  consumers,  facilitate  grid-­‐interconnection  of  generation,  storage,  demand  response,  and  detect  and  minimize  the  impact  of  unforeseen  disruption  events  such  as  extreme  weather.  The  following  sections  will  describe  the  technologies  that  enable  power  flow  control  and  the  value  that  power  flow  control  capabilities  afford  to  different  stakeholders  in  the  electric  grid.    

Power  Flow  Control  Technology  Defined    Both  hardware  and  software  technologies  have  power  flow  control  applications.  This  analysis  will  focus  on  two  types  of  hardware  technologies  –  High  Voltage  Direct  Current  (HVDC)  transmission  cables  and  

                                                                                                                                                                                                                                                                                                                                                                                                       the  potential  energy  stored  when  riding  a  bicycle  up  a  hill.  It  is  recovered  (in  the  ideal  case)  when  going  down  the  other  side.  In  an  AC  line  in  the  U.S.,  this  energy  is  stored  and  recovered  120  times  per  second,  and  thus  is  quite  different  from  the  behavior  of  energy  stored  in  devices  such  as  batteries.  The  resistance  of  a  line  is  determined  by  the  material  properties,  length,  and  cross-­‐section  of  the  conductor,  while  reactance  is  determined  by  geometric  properties  (the  position  of  conductors  relative  to  each  other  and  ground).  In  practical  transmission  lines,  resistance  is  small  compared  to  reactance,  and  thus  reactance  has  more  influence  on  power  flow  than  resistance.”  

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substation  equipment;  and  High  Voltage  Alternating  Current  (HVAC)  power  transmission  controllers  that  use  power  electronics  to  augment  the  existing  AC  grid.  In  addition,  the  capabilities  enabled  by  software  control  algorithms  such  as  topology  control  are  discussed.    

Hardware  

Hardware  can  efficiently  direct  the  flow  of  power  on  the  grid,  help  stem  energy  losses,  and  enable  the  grid  to  be  more  responsive  and  resilient.  Advances  in  materials  and  engineering  are  decreasing  the  costs  of  power  flow  hardware;  many  of  the  concepts  of  which  have  been  around  for  a  long  time.  The  descriptions  below  include  power  flow  control  technologies  that  already  exist  and  are  in  wide  spread  use  in  the  grid  today,  as  well  as  emerging  technologies  not  yet  in  use  that  show  tremendous  promise  for  power  flow  control  applications.    

High  Voltage  Direct  Current  transmission  systems  are  composed  of  one  or  more  DC  transmission  lines  or  cables  between  a  converter  (combined  rectifier  and  inverter),  which  converts  AC  to  DC  or  vice  versa.  The  DC  lines/cables  in  concert  with  the  most  recent  voltage  source  converter  (VSC)  technology  enable  rapid  control  of  the  direction  of  power  flow.  Both  voltage  and  current  source  converters  can  invert  DC  to  a  matching  AC  frequency  of  an  interconnected  AC  grid,  which  affords  HVDC  the  ability  to  connect  two  asynchronous  AC  systems.  DC  poses  fewer  technical  challenges  compared  to  AC  because  it  is  not  necessary  to  match  frequency,  phase  or  voltage.  DC  can  be  configured  as  a  monopolar  (one  cable)  or  bipolar  (two  cable)  system  which  offers  cost  savings  over  tripolar  AC  designs  which  require  one  cable  for  each  of  the  three  phases.  Because  of  this  and  the  lower  line  losses  (30-­‐50%  lower  as  compared  to  AC),  HVDC  transmission  lines  are  the  least  expensive  option  for  transmitting  power  over  long  distances  (Reed,  2012).  HVDC  transformers  have  been  more  expensive  relative  to  HVAC.  The  distance  at  which  a  given  HVDC  line  becomes  more  cost  effective  than  HVAC  at  a  given  voltage  is  the  difference  between  line  and  terminal  costs  including  the  difference  between  losses  (see  Figure  1).  Also,  when  connected  with  an  AC  grid,  HVDC  can  mitigate  power  factor  issues  (current  lagging/leading  voltage)  by  providing  reactive  power  support,  and  can  provide  black  start  capabilities3  with  VSCs.    

HVDC  transmission  systems  are  used  to  transport  power  over  long  distances  and  sub-­‐sea.  HVDC  lines  with  VSCs  allow  for  bi-­‐directional  control  of  power  flow  and  can  be  directly  scheduled  and  dispatched.  Bi-­‐directionality  allows  for  the  export  of  energy  from  control  area  A  to  control  area  B  under  certain  conditions,  and  re-­‐dispatch  for  import  of  energy  from  area  B  to  area  A  in  other  scenarios.  One  example  of  bi-­‐directional  flow  is  the  HVDC  cross  channel  2,000  MW  link  that  imports  electricity  to  Britain  from  France  during  much  of  the  year,  but  exports  power  to  France  during  the  summer  when  demand  is  high  or  to  meet  load  during  scheduled  outages.  The  Cross  Sound  Cable  between  Connecticut  and  Long  Island  is  also  bi-­‐directional,  although  power  flows  from  Connecticut  to  Long  Island  for  most  hours  of  the  year.  

                                                                                                                         3  Black  start  is  the  process  of  restoring  power  to  a  power  plant,  normally  without  relying  on  the  power  of  the  transmission  grid.  Typically  in  the  case  of  a  wider  grid  outage,  black  start  is  provided  in  a  sequence:  a  portable  generator  is  used  to  start  one  power  plant,  the  proximal  transmission  lines  are  energized  and  the  power  used  to  start  the  next  base  load  generator,  and  so  on.  Voltage  Source  Converters  can  be  used  for  black  start  as  they  can  synthesize  a  balanced  set  of  three  phase  voltages.      

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HVDC  technology  has  gained  popularity  since  the  mid-­‐20th  century.  Historically,  the  limitations  to  its  practical  use  have  been  the  high  cost  of  the  power  electronics  required  for  the  converter.  Recent  technical  breakthroughs  have  reduced  the  cost  of  power  electronics  and  increased  their  application.  HVDC  is  now  being  deployed  globally,  with  dozens  of  projects  in  the  global  pipeline,  and  is  of  particular  importance  to  integrating  distant,  renewable  energy  generators  such  as  offshore  wind  farms.  When  considering  new  transmission  corridors,  HVDC  is  more  favorable  to  HVAC  because  of  the  smaller  footprint  of  the  transmission  towers.  HVDC  proponents  envision  a  future  in  which  DC  cables  are  embedded  within  the  existing  AC  grid  and  multi-­‐terminal  HVDC  allows  for  a  superimposed  HVDC  network  that  will  help  to  integrate  remote  resources,  improve  system  stability  and  reliability  via  AC-­‐DC  interties,  and  increase  control  of  power  flows  through  the  system.  HVDC  technologies  are  being  developed  by  numerous  vendors,  including  General  Electric  with  funding  from  the  ARPA-­‐E  GENI  program.  

 

 

 

 

 

 

 

 

 

GE  Global  Research  is  developing  two  ARPA-­‐E  funded  projects  to  improve  HVDC  technology  –  multi-­‐terminal  HVDC  and  improved  cable  insulation.    

The  multi-­‐terminal  HVDC  Networks  with  High-­‐Voltage  High-­‐Frequency  Electronics  project  is  developing  multi-­‐terminal  HVDC-­‐compatible  converters  to  improve  the  ability  to  network  HVDC  and  integrate  renewable  energy  into  the  grid.      Nanoclay  Reinforced  Ethylene-­‐Propylene-­‐Rubber  for  Low-­‐Cost  HVDC  Cabling  project  is  developing  low-­‐cost  insulation  for  HVDC  transmission  cables.  Cables  will  be  less  expensive  and  suppress  excess  charge  accumulation,  which  will  protect  the  insulation.    

 

 

 

Fig.  1.  Breakeven  distance  for  HVDC  transmission  lines  HVDC  becomes  cost  competitive  with  HVAC  over  a  distance  at  which  line  losses  at  a  given  voltage  are  lower  than  a  comparable  HVAC  line.    

Source:  Pike  Research,  2012  

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HVAC  Power  Transmission  Controllers  (PTC)  can  control  impedance,  voltage  and  phase  and  hold  power  at  a  desired  level  and  direction  of  flow.  PTC  devices  use  a  combination  of  solid  state  power  electronics  and  other  static  equipment  to  modulate  the  state  of  a  given  AC  transmission  line  by  injecting  and  removing  voltage  and  impedance.  These  coordinated  actions  result  in  controllable  voltage/current  phase  shift  to  manage  real  and  reactive  power  flows,  controllable  line  impedance  to  increase  or  decrease  current,  and  the  ability  to  balance  the  current  phase  between  the  three  phases  of  an  AC  transmission  system.    

Historically,  these  capabilities  were  accomplished  by  Flexible  Alternating  Current  Transmission  Systems  (FACTS),  which  employed  similar  power  electronic  devices  in  substations  and  were  typically  large  and  capital  intensive.  Today,  advances  in  technology  are  decreasing  the  cost  and  footprint,  and  increasing  the  reliability  and  operability  of  these  devices,  making  HVAC  PTC  viable  solutions  for  power  flow  control  applications.  Such  devices  are  being  developed  by  several  ARPA-­‐E  GENI  teams,  including  Smart  Wire  Grid,  Varentec,  Oak  Ridge  National  Laboratory,  and  Michigan  State  University.      Phase  Shifting  Transformers  

Phase  shifting  transformers  change  the  voltage  phase  angle  between  primary  and  secondary  windings,  changing  the  input  and  output  voltages  of  a  line  and  thereby  controlling  the  active  power  that  can  flow  in  the  line.  Effectively,  they  inject  a  voltage  in  series  with  the  line.  This  enables  control  of  power  flow  between  two  power  systems,  balances  loading,  and  improves  system  stability.  

   

 

Distributed  Series  Reactor  The  Distributed  Series  Reactor  (DSR)  is  a  technology  being  developed  by  Smart  Wire  Grid,  a  startup  based  in  Oakland,  California.  DSRs  are  small,  single-­‐turn  transformers  that  inject  inductance  onto  a  transmission  line.  The  level  of  inductance  is  tunable  to  alter  the  overall  line  impedance  and  thus  the  flow  of  current.  DSRs  are  distributed  along  transmission  lines,  in  all  3  phases,  and  can  communicate  with  each  other  to  form  a  variable  impedance  system.  They  can  also  operate  autonomously  to  alter  flows  at  a  specific  point  on  the  line.  As  such,  the  technology  can  help  to  reduce  congestion  and  balance  power  flow  within  a  system.  

 

Magnetic  Amplifier  for  Power  Flow  Control  Oak  Ridge  National  Laboratory  is  developing  an  electromagnet-­‐based  amplifier-­‐like  device  that  will  allow  for  complete  control  over  the  flow  of  power.  The  prototype  device  is  a  low  cost  iron-­‐based  magnetic  amplifier.      

Dynamic  Power  Flow  Controller  Varentec  is  developing  low  cost  transmission  controllers  to  dynamically  control  voltage  and  power  flow  with  ARPA-­‐E  funding.  The  technology  would  enable  early  detection  and  fail-­‐safe  protection  of  the  transmission  grid  to  maintain  its  operating  state.    

 

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Shunt  Compensators    Shunt  devices  are  used  to  control  transmission  voltage,  reduce  reactive  losses,  dampen  power  oscillations  and  are  connected  in  shunt  to  a  transmission  line.  A  Static  Synchronous  Compensator  (STATCOM)  is  a  VSC  usually  connected  to  the  grid  through  a  shunt  transformer.  STATCOMs  do  not  require  the  bulk  capacitors  and  inductors  that  are  used  in  the  thyristor-­‐based  Static  Var  Compensators  (SVCs)  which  are  still  in  widespread  use  today.  Instead,  the  STATCOM  generates  reactive  power  entirely  electronically  and  can  act  as  either  a  source  or  sink  of  reactive  power.  The  STATCOM  can  also  exchange  real  power  between  the  grid  and  an  energy  storage  device  connected  at  its  DC  terminals.  VSCs  based  on  Insulated-­‐gate  bipolar  transistor  (IGBT)  technology4  have  much  faster  switching  times  than  other  compensator  technologies,  which  makes  them  particularly  useful  for  dynamic  voltage  support  and  power  factor  correction.      A  STATCOM  does  not  affect  power  flow  on  a  transmission  line  directly.  However,  by  using  local  shunt  reactive  power  injection  to  change  the  voltage  profile  of  a  transmission  line  (e.g.  support  voltage  at  the  midpoint  of  a  long  line),  it  can  enable  a  line  to  be  loaded  more  heavily  (e.g.  to  thermal  limits)  without  exceeding  steady  state  stability  margins  or  voltage  drop  limits.  In  contracts,  a  power  flow  controller  is  connected  in  series  with  a  transmission  line  and  has  the  ability  to  force  a  change  in  power  flow  on  the  line,  essentially  by  introducing  a  controllable  voltage  in  series  with  the  line.  

Series  Compensators  A  Static  Series  Synchronous  Compensator  (SSSC)  is  a  VSC  connected  in  series  with  a  transmission  line.  It  has  the  ability  to  raise,  lower,  or  even  reverse  the  power  flow  on  the  line  by  injecting  a  relatively  small  voltage  in  series.  For  a  wide  range  of  power  flow  control,  only  reactive  power  output  from  the  VSC  is  needed.  However,  additional  control  capabilities  such  as  independent  control  of  real  and  reactive  power  flow,  can  be  obtained  if  a  source/sink  of  real  power  is  connected  to  the  DC  terminals  of  the  VSC.  Currently,  there  are  no  examples  of  SSSC  installations  in  transmission  grids  except  those  installed  as  part  of  the  three  Unified  Power  Flow  Controller  demonstration  projects.      A  stand-­‐alone  SSSC  is  a  more  versatile  (and  potentially  lower-­‐cost)  power  flow  controller  than  a  Thyristor-­‐Controlled  Phase  Angle  Regulating  Transformer  with  a  similar  MVA  rating,  which  is  the  closest  comparable  device.  At  present,  back-­‐to-­‐back  HVDC  is  being  considered  in  some  places  to  solve  loop  flows  and  other  transmission  problems,  but  requires  two  converters  rated  for  full  transmitted  power.  In  most  cases  the  problem  could  be  solved  with  a  single  fractionally  rated  SSSC.  

                                                                                                                         4  IGBT  technology  is  a  power  semiconductor  device  that  forms  an  electronic  switch.  They  are  high  efficiency,  fast  switching  and  can  handle  high  voltages  and  current  when  many  devices  are  stacked  in  parallel.    

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Thyristor  Controlled  Series  Capacitors  (TCSC)  are  a  family  of  equipment  that  provides  a  controllable  capacitance  (or  in  some  cases,  an  inductance)  connected  in  series  with  a  transmission  line  to  reduce  (or  increase)  the  total  reactance  of  the  line.    

Unified  Power  Flow  Controllers  (UPFCs)  UPFCs  provide  the  functionality  of  both  shunt  and  series  compensators.  They  control  real  and  reactive  power  flow  and  provide  voltage  support  for  the  connecting  bus5.  Historically,  UPFCs  have  taken  up  significant  space,  been  very  expensive,  and  required  the  construction  of  large  transformers.  There  are  only  three  operational  UPFCs  in  the  world,  each  of  which  was  tailored  to  meet  a  particular  utility’s  problem.  However,  grid  operators  are  largely  uncomfortable  with  the  series  compensation  capabilities  of  UPFCs,  and  as  a  result  these  operating  modes  are  rarely  used,  leaving  the  UPFCs  to  operate  largely  as  a  STATCOM  (for  more,  see  Marcy  UPFC  case  study  in  this  document).  Moreover,  the  company  that  built  the  UPFCs  –  Westinghouse  –  was  acquired  by  Siemens,  which  no  longer  sells  or  supports  the  devices.  An  ARPA-­‐E  team  from  Michigan  State  University  is  building  a  transformer-­‐less  UPFC  which  addresses  these  issues  and  can  control  power  flows  from  intermittent  resources  including  wind  and  solar  resources.                

Software  

Advancements  in  computing  and  data  communications  can  optimize  grid  operations,  match  power  delivery  to  real-­‐time  demand,  and  find  effective  ways  to  manage  sporadically  available  renewable  power  sources  and  grid-­‐level  power  storage.    

Topology  Control  Algorithms  (TCAs)  are  a  network  solution  to  optimally  activate  (close)  and  deactivate  (open)  transmission  lines  to  decrease  the  cost  of  the  transmission  system.  This  is  based  on  the  counter-­‐intuitive,  but  demonstrated,  phenomenon  that  closing  a  congested  pathway  improves  the  overall  system  flow6.  TCAs  are  integrated  into  software  that  controls  the  grid’s  hardware  infrastructure,  

                                                                                                                         5  Real  power  is  power  delivered  to  the  end  user  to  do  work  (measured  in  watts).  Reactive  power  is  current  energizing  the  system  components  (measured  in  volt-­‐amperes  reactive-­‐  VAR).    6  Closing  a  congested  pathway  can  open  the  electric  flow  at  the  system  level.  This  has  been  demonstrated  by  ISOs  and  researchers,  including  the  Brattle  Group,  Argonne  National  Labs,  and  a  team  from  Texas  A&M.  To  illustrate  this  concept  a  team  from  Texas  A&M  showed  that  when  a  50MW  line  was  dropped  in  a  3-­‐line,  3-­‐generator  system,  the  feasible  cost  to  serve  load  dropped.  This  concept  is  demonstrated  in  the  diagrams  below:  

Transformer-­‐Less  Unified  Power  Flow  Controller  Michigan  State  University  is  developing  a  power  flow  controller  to  improve  the  routing  of  electricity  from  renewable  sources  through  existing  power  lines.  The  UPFC  will  eliminate  the  need  for  a  transformer  and  construction  of  new  transmission  lines.  It  will  optimize  energy  transmission  and  help  reduce  transmission  congestion.    

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and  change  the  shape  of  the  grid  by  actuating  line  switching  hardware  or  by  controlling  the  HVAC  PTC  devices  listed  above.  The  net  effect  of  changing  the  shape  of  the  grid  is  to  change  the  way  that  power  flows  through  the  transmission  system.    

TCAs  are  not  a  new  concept;  they  have  been  employed  by  operators  of  wireless  ad-­‐hoc  networks  for  radios  (since  1970’s)  and  computers  (since  1990’s)  by  optimizing  the  transmission  power  of  each  node  to  improve  signal  flow  in  the  network.  For  the  electric  power  industry,  recent  advances  such  as  phasor  measurement  units  (PMUs),  low-­‐latency  communication  systems,  and  the  reduced  cost  and  improved  speed  of  computer  processors  allow  for  TCAs  to  be  an  effective  solution  for  power  flow  in  the  transmission  grid.  TCAs  are  being  developed  through  the  ARPA-­‐E  GENI  program  by  Texas  A&M  and  Boston  University.  

 

 

 

 

 

 

 

 

 

 

                                                                                                                                                                                                                                                                                                                                                                                                       

   

Automated  Grid  Disruption  Response  System  Texas  A&M  is  developing  a  Robust  Adaptive  Topology  Control  (RATC)  system  designed  to  detect,  classify,  and  respond  to  grid  disturbances  by  reconfiguring  the  grid  to  maintain  economically  efficient,  reliable  operations.  The  system  would  help  to  prevent  outages  and  minimize  the  time  it  takes  for  the  grid  to  respond  to  interruptions,  and  make  it  easier  to  integrate  renewable  resources  into  the  grid.  

 

Transmission  Topology  Control  for  Infrastructure  Resilience  to  the  Integration  of  Renewable  Generation  Boston  University  is  developing  a  technology  that  helps  grid  operators  manage  power  flows  and  integrates  renewable  resources  by  optimizing  the  transmission  system.  The  system  would  have  the  capability  of  turning  power  lines  on  and  off  to  manage  transmission  congestion,  increase  use  of  renewable  resources,  and  improve  system  reliability.  The  fast  optimization  algorithms  would  enable  near  real-­‐time  change  in  the  grid.    

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Value  Analysis  of  Power  Flow  Control    Power  flow  control  benefits  the  entire  transmission  system  as  well  as  transmission  owners,  generators,  operators,  planners,  regulators,  and  consumers.  Transmission  benefits  can  be  numerous  and  diverse,  including:  

• Reduce  energy  transmission  losses  • Mitigate  transmission  outages  • Defer  and  prioritize  transmission  investments  • Increase  transfer  capability  from  one  part  of  the  system  to  another  • Reduce  cycling  of  base  load  generators  to  increase  asset  efficiency  • Increase  wheeling  of  power  in  and  out    • Reduce  loop  flows  • Meet  public  policy  goals  

Any  one  of  the  technologies  described  above  can  help  to  achieve  these  benefits.  However,  to  maximize  the  benefits  of  power  flow  control  and  to  maintain  system  reliability,  some  system  coordination  is  required  in  order  to  understand  the  system-­‐level  effect  of  the  installation  of  power  flow  control  technologies,  to  plan  for  future  asset  mix,  and  to  optimize  operations  of  the  physical  grid  and  electricity  markets.  Power  flow  control  is  achieved  when  software  technologies  in  concert  with  well-­‐placed  hardware  work  together  to  optimize  the  transmission  system.  Ultimately,  planners,  operators  and  regulators  may  need  to  consider  several  additional  factors  to  realize  the  full  potential  and  system  benefits  of  power  flow  control  technologies,  including:  

• Market/regulatory  structure  for  wide  area  control  –  to  make  sure  that  market  structure  and  technical  capabilities  are  aligned  to  properly  value  the  benefits  of  power  flow  control  technologies  

• Software  –  synchronous  access  to  cloud  resources  for  optimized  coordinated  control  • Sensors  –  accurate,  real-­‐time,  dispersed  estimation  sensors  to  measure  and  communicate  the  

conditions  of  the  electric  grid  in  real  time  and  ensure  

This  analysis  does  not  consider  the  many  complementary  technologies  that  would  help  to  maximize  flexibility  and  control  including  PMUs,  advanced  metering  infrastructure  or  distribution-­‐level  technologies,  or  incentives  and  market  structures  that  could  enable  power  flow  control.  The  analysis  is  solely  focused  on  the  high-­‐voltage  transmission  technologies  and  software  applications  described  above.  

One  can  think  of  the  value  of  power  flow  control  technologies  in  terms  of  the  total  costs  and  benefits  of  a  transmission  grid  with  power  flow  control  capabilities  as  compared  to  the  total  cost  and  benefits  of  the  system  without  these  capabilities.  However,  one  of  the  difficulties  in  quantifying  the  value  of  power  flow  control  capabilities  is  that  system  optimization  requires  that  there  be  short-­‐term  beneficiaries  of  a  change  in  power  flow,  and  corresponding  entities  that  might  see  a  drop  in  revenue  in  the  short-­‐term,  as  any  change  to  the  physical  constraints  of  the  electric  grid  can  affect  the  price  that  generators  or  transmission  owners  are  paid  for  electricity.  This  analysis  explores  five  distinct  value  streams  of  power  

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flow  control,  defines  the  associated  benefits  and  costs,  and  identifies  the  stakeholders  and  how  they  might  be  affected  at  a  system  level.  

Identification  of  Value  Propositions  

Asset  Management    Transmission  infrastructure  in  the  United  States  is  built  to  meet  peak  demand,  which  leads  to  sub-­‐optimal  utilization  outcomes  at  a  system  level  during  non-­‐peak  periods.  Reliability  standards  and  favorable  FERC-­‐established  rates  of  return  provide  incentives  for  transmission  investment.  At  the  same  time,  much  of  the  existing  transmission  infrastructure  is  reaching  the  end  of  its  useful  life,  and  new  transmission  is  difficult,  expensive,  time-­‐consuming,  and  highly  litigious  to  build.  Transmission  owners  are  also  faced  with  competing  calls  for  capital  to  meet  reliability  and  environmental  priorities.  Research  from  the  Edison  Electric  Institute  shows  that  its  shareholder-­‐owned  utility  members  increased  their  investment  in  transmission  infrastructure,  investing  $11.1  billion  in  2011  and  planning  to  spend  $54.6  billion  through  2015  (Edison  Electric  Institute).  Several  power  flow  control  technologies  could  increase  the  capacity  of  existing  transmission  lines  and  defer  new  investment  in  construction  or  help  prioritize  construction  of  new  lines  to  optimize  the  use  of  the  transmission  grid.  While  increasing  the  capacity  of  transmission  lines  would  produce  system-­‐level  benefits,  ultimately  some  transmission  owners  and  electricity  generators  would  see  lower  revenues  in  cases  where  they  currently  benefit  from  congestion.    

HVDC  

In  some  scenarios,  power  flow  control  technologies  could  decrease  transmission  losses  and  increase  transmission  utilization.  Most  notably,  HVDC  lines  have  lower  losses  in  transporting  power  over  long  distances,  and  technological  advances  in  insulation  could  increase  this  benefit  further.  For  instance,  GE  Global  Research  is  developing  a  nanoclay  reinforced  ethylene-­‐propylene-­‐rubber  for  low-­‐cost  HVDC  cabling  that  could  bring  down  the  cost  of  HVDC  cable  by  as  much  as  80%.  Such  a  decrease  in  the  cost  of  HVDC  would  lower  the  distance  at  which  HVDC  is  cost  competitive  with  HVAC,  and  increase  its  affordability  as  an  option  for  integration  into  the  AC  grid.    

HVDC  requires  smaller  transmission  right  of  ways,  so  new  construction  or  reconductoring  of  transmission  lines  can  be  easier  to  achieve.    This  is  particularly  important  in  heavily  populated  areas,  which  often  suffer  from  transmission  congestion.  In  these  cases,  transmission  planners  may  consider  using  existing  transmission  right  of  ways  to  install  buried  HVDC  cable  to  increase  transmission  capacity  without  permitting  a  completely  new  transmission  pathway.    

Power  Transmission  Controllers  and  Topology  Control  Algorithms  

HVAC  PTCs  such  as  DSRs  and  STATCOMs  can  increase  the  capacity  of  AC  transmission  infrastructure  and  reduce  the  need  for  a  new  transmission  line,  to  optimize  the  existing  AC  transmission.  Because  repowering  existing  assets  could  be  less  costly,  a  transmission  owner  could  prioritize  capital  expenditures  and  deploy  resources  for  new  transmission  lines  in  parts  of  the  system  where  it  would  

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make  the  most  difference.  In  addition,  they  can  increase  the  flexibility  and  adaptability  for  grid  operators  to  use  existing  AC  lines.    

Topology  control  allows  for  line  switching  to  optimize  economic  efficiency  and  minimize  congestion.  In  some  cases,  employing  topology  control  alone  would  increase  the  utilization  of  transmission  lines  and  defer  the  need  for  new  transmission  construction.  One  common  concern  about  topology  control  is  that  it  might  increase  circuit  breaker  operations  and  maintenance  expenses.  Under  a  scenario  with  topology  control,  circuits  will  be  switched  more  frequently,  but  in  non-­‐fault  conditions  with  much  less  current.  Circuit  breakers  have  a  robust  design  to  deal  with  fault  conditions  are  expected  to  operate  will  in  a  topology  control  case.  However,  equipment  manufacturers  will  need  to  validate  and  support  this  new  use  case.  Circuit  breakers  that  are  old  and  past  warranty  may  be  of  greater  concern  in  than  newer  devices.  While  it  is  thus  possible  that  line  switching  could  increase  the  need  for  maintenance  on  breakers  that  are  used  more  frequently  in  switching  than  static  scenarios,  the  system-­‐level  benefits  should  outweigh  the  costs.    

Reliability  and  Security    Where  power  systems  are  designed  to  meet  one  or  two  contingency  extreme  events,  power  flow  control  capabilities  could  help  to  mitigate  the  impact  of  one  or  two  outages  by  providing  alternate  power  flow  paths  to  continue  to  serve  load.  The  economic  impact  of  the  infamous  northeastern  August  2003  blackout  was  estimated  to  be  $4  to  $10  billion  in  the  United  States,  highlighting  the  importance  of  the  electric  grid  in  today’s  economy  (U.S.-­‐Canada  Task  Force,  2004).  Reliability  is  top  of  mind  for  system  operators,  regulators,  policy  makers,  and  businesses  in  the  U.S.  today,  as  reflected  in  the  regional  implementation  of  North  American  Electric  Reliability  Corporation  (NERC)  standards.  Power  flow  control  technology  could  increase  the  flexibility  and  responsiveness  of  the  grid.      

HVDC  

HVDC  technology  provides  several  reliability  benefits.  Specifically,  a  DC  circuit  breaker  with  instantaneous  response  time  will  allow  for  quick  fault  detection  and  response,  which,  in  conjunction  with  other  power  flow  control  technologies,  can  prevent  a  system-­‐level  problem  and  re-­‐route  power  to  enable  continual,  uninterrupted  service.  Similarly,  directional  switching  of  power  flow  enables  routing  options  post-­‐contingency.  The  ability  to  reverse  power  flow  in  response  to  a  contingency  can  decrease  generation  capacity  requirements  for  ancillary  services.    

In  the  case  of  an  HVDC  intertie  between  two  asynchronous  grids,  VSCs  can  provide  black  start  service  from  one  grid  to  another,  significantly  decreasing  response  time  without  the  need  for  reserve  installations  that  would  otherwise  be  idle  much  of  the  year.    

 Power  Transmission  Controllers  and  Topology  Control  Algorithms  

DSRs,  STATCOMs,  and  TCAs  each  provide  reliability  benefits.  DSRs  can  control  potential  transmission  overload  and  bypass  congested  lines,  increasing  transmission  utilization,  decreasing  congestion,  and  thereby  increasing  dispatch  options.  The  built  in  device-­‐to-­‐device  communication  system  in  DSRs  

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enables  dynamic,  autonomous  response  and  eliminates  risks  associated  with  other  central-­‐control  communications  devices.  The  AC  regulation  function  of  STATCOMs  can  automatically  control  transmission  contingency  conditions  and  prevent  problems  or  decrease  recovery  time.  TCAs  will  optimize  transmission  line  switching  under  normal  and  contingency  conditions  –  bypassing  congested  lines  and  finding  the  optimal  path  to  serve  load.    

In  order  to  quantify  the  specific  benefits  of  power  flow  control  technologies  on  a  particular  system,  it  would  be  necessary  to  model  the  grid  response  under  contingency  conditions  using  reliability  software,  and  then  again  with  power  flow  control  technologies  built  in  and  estimating  the  economic  value  of  the  reduction  in  load  loss  (Budhraja,  Mobasheri,  Ballance,  Dyer,  Silverstein,  &  Eto,  2009).    

 Congestion  Relief    Transmission  congestion  happens  whenever  preferable  or  low  cost  generation  is  unable  to  serve  electric  load  due  to  a  physical  limit  on  the  transmission  system.  Market  efficiency  is  based  on  optimal  economic  operation  of  the  grid  by  dispatching  the  lowest-­‐cost  generation.  Congestion  disrupts  this  process  and  leads  to  dispatch  of  higher  cost  generation  to  meet  demand  in  the  importing  location,  and  exerts  downward  pressure  on  prices  in  exporting  areas.  Reducing  congestion  on  the  transmission  grid  will  reduce  congestion  pricing  for  energy  and  ancillary  services  and  allow  for  economic  dispatch  of  generation  while  balancing  transmission  lines.  At  a  system  level,  the  cost  of  constructing  new  transmission  lines  or  adding  power  flow  control  technologies  must  be  weighed  against  the  benefits  of  doing  so.  Congestion  is  often  a  problem  in  or  around  densely  populated  areas,  where  permitting  new  transmission  lines  can  be  particularly  difficult.  In  these  cases,  there  may  be  a  clear  system-­‐level  benefit  to  power  flow  control  technologies.  Congestion  relief  brings  multiple  benefits  in  terms  of  integration  of  renewable  energy  and  economic  efficiency  of  energy  markets.  

Integration  of  renewable  energy  Multiple  renewable  integration  studies  have  validated  the  substantial  system  level  and  societal  benefits  of  increased  renewable  energy  penetration.  Wind  and  solar  energy  generators  reduce  the  system  operating  costs  by  displacing  fuel  expenses  and  deferring  upgrades  to  existing  conventional  generators;  in  addition  to  lowering  generation  fleet  carbon  emissions.  In  the  Western  Wind  and  Solar  Interconnection  Study  (WWSIS),  it  was  found  that  by  tapping  the  large  solar  and  wind  resource  in  the  Western  Connection,  up  to  35%  of  the  required  energy  could  be  delivered  by  renewables  (GE  Energy,  2010).  This  results  in  a  40%  reduction  in  the  annual  system  OPEX.  In  the  Eastern  Wind  Integration  and  Transmission  Study  (EWITS),  a  10%  reduction  in  annual  system  OPEX  was  achieved  by  incorporating  30%  of  the  energy  requirement  from  wind  in  the  Eastern  Connection  (EnerNex  Corporation,  2011).  EWITS  also  calculated  an  18%  reduction  in  CO2  emissions.  

The  challenge  to  incorporating  variable,  uncertain  renewable  energy  is  that  the  current  system  infrastructure  and  operational  practices  were  designed  for  dispatch-­‐able  and  predictable  generation  supplies.  However,  renewable  energy  generators,  such  as  wind  and  solar,  are  variable  and  uncertain  (non-­‐perfectly  predictable)  due  to  the  nature  of  wind  and  cloud  coverage.  This  variability  and  

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uncertainty  has  the  potential  to  exacerbate  transmission  congestion  as  the  penetration  of  renewable  generation  increases.  Conversely,  there  might  be  an  under  supply  of  energy  or  system  frequency  disruption  if  the  renewable  generators  slow  or  stop  production  (due  to  ramping).    

To  mitigate  these  challenges,  system  operators  can  require  additional  reserve  capacity  to  supplement  renewables  and  come  online  quickly  to  stabilize  system  frequency  in  the  event  of  ramping  of  the  energy  resource.  Other  generators  must  perform  load  following  to  match  their  output  to  any  changes  in  the  energy  supply-­‐demand  balance.  Furthermore,  local  generators  are  called  upon  in  instances  when  congestion  prevents  renewable  energy  from  serving  the  load.  In  this  case,  current  practice  empowers  grid  operators  to  curtail  renewable  generators  if  their  supply  cannot  be  reliably  transmitted  due  to  congestion  elsewhere  in  the  system.  In  all  these  cases,  the  operation  of  reserve  generators  is  generally  higher  cost  than  the  renewable  generators.  Some  system  operators  have  begun  to  utilize  forecasts  of  renewable  energy  regions  to  aid  in  more  economic  reserve  scheduling  and  transmission  system  operation.  However,  the  accuracy  of  these  forecasts  at  present  is  marginally  better  than  assuming  persistence.  Poor  information  leads  to  inefficient  dispatching  and  un-­‐necessary  cycling  of  conventional  generators  which  is  a  less  efficient  operational  method  that  outputs  greater  emissions  and  more  wear  and  tear  on  the  asset.    

These  additional  operational  requirements  of  renewables  are  manageable,  but  lessen  the  total  achievable  system  benefits  due  to  the  increased  demand  for  real-­‐time  reserves  and  inefficiencies  in  the  near-­‐term  asset  scheduling  and  curtailment  practices.  For  example,  the  integration  of  wind  energy  in  ERCOT  is  estimated  to  cost  an  additional  $0.66/MWh  due  to  deployment/operation  of  reserves,  the  cost  of  base  load  cycling,  and  transmission  congestion  (Ahlstrom,  2013).  In  terms  of  capital  outlay  for  reserve  capacity,  it  is  estimated  that  PJM  spends  $3  per  each  additional  MW  of  wind  power  capacity  (The  Brattle  Group,  2013).  

The  renewable  integration  studies  have  found  that  these  practices  and  associated  costs  can  be  largely  avoided  if  the  grid  were  flexible  to  compensate  for  the  variable,  uncertain  supply.  Power  flow  control  technologies  can  achieve  sufficient  transmission  system  flexibility  to  lower  renewable  integration  costs,  reduce  congestion,  and  allow  for  even  further  economic  utilization  of  renewable  energy  by  minimizing  curtailment.  In  addition,  a  more  interconnected  and  controllable  transmission  system  will  facilitate  the  network  benefits  of  geographic  averaging  of  renewable  resources  and  more  accurate  wind  and  solar  forecasts.  

Economic  Efficiency  Power  flow  control  technologies  can  increase  the  economic  efficiency  of  the  electric  grid  through  lower  losses  and  by  enabling  economic  dispatch  of  transmission  and  generation  assets.  HVDC  devices,  DSRs  and  TCAs  can  be  installed  on  the  existing  transmission  grid  to  allow  for  the  necessary  flexibility  to  lower  integration  costs  through  the  mitigation  of  curtailment-­‐causing  system  bottlenecks  and  congestion.    

HVDC  

Long-­‐distance  HVDC  installations  improve  market  access  to  remote  resources.  When  congestion  is  appropriately  managed,  HVDC  facilitates  lower  energy  prices.  Lower  line  losses  of  HVDC  can  further  

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reduce  the  overall  cost  to  serve  remote  load  by  30-­‐50%.  The  most  economic  generation,  including  renewable  generation  resources,  are  often  not  located  in  close  proximity  to  major  load  centers.  To  tap  these  resources,  a  transmission  system  must  be  developed.  For  long  distance  connection,  HVDC  conductors  offer  the  most  value  because  of  5-­‐10%  less  line  loss  than  similar  capacity  AC  conductors  (Bahrman,  2009).  Along  with  the  advantages  of  smaller  transmission  towers  and  no  need  for  intermediate  substations,  lower  line  loss  equates  to  lower  overall  system  cost.  For  a  1000  mile  system  rated  for  6000MW  an  800  kV  HVDC  system  is  $670/MW-­‐mi  less  expensive  than  a  765  kV  AC  system  (Bahrman,  2009).    

HVDC  can  be  used  to  route  power  around  a  congested  area  of  the  AC  grid,  bringing  less  expensive  power  or  renewable  generation  situated  at  a  distance  to  market  in  an  area  of  higher  demand.  For  example,  the  Trans  Bay  Cable  delivers  power  from  Pittsburg,  California  to  San  Francisco,  providing  an  alternate  route  for  generation  to  serve  40%  of  the  city’s  peak  energy  needs.  Similarly,  the  Neptune  HVDC  cable  running  from  New  Jersey  to  Long  Island  enables  power  flow  directly  to  Long  Island,  skirting  areas  of  transmission  congestion  in  New  Jersey  and  New  York  and  serving  30%  of  electric  needs  of  Long  Island.    

The  bi-­‐directional  flow  capabilities  of  many  HVDC  installations  could  allow  for  the  change  of  flow  to  address  particular  points  of  congestion  where  congestion  stress  points  shift  with  changing  supply  and  load  patterns.  For  example,  the  Cross  Sound  Cable,  a  merchant  transmission  line  between  CT  and  Long  Island,  largely  sends  power  from  CT  to  Long  Island  but  on  occasion  sends  power  the  other  way  in  response  to  changing  conditions.  

Back-­‐to-­‐back  HVDC  –AC  intertie  capabilities  enable  ties  between  asynchronous  grids  and  can  thereby  increase  transfer  capacity,  allowing  for  access  to  supply  from  a  contiguous  grid  system  and  decreasing  the  cost  of  reliability  services.  HVDC  that  is  multi-­‐terminal  or  bi-­‐polar  with  bi-­‐driectional  capabilities  will  increase  the  interconnection  further  and  allow  for  economic  dispatch  in  multiple  directions.  For  example,  the  Cross  Sound  Cable  can  send  power  from  Connecticut  to  Long  Island  or  from  Long  Island  to  New  York  depending  on  system  conditions.    

With  greater  HVDC  connectivity  of  disparate  renewable  generators  and  loads,  the  negative  system  effects  of  renewable  intermittency  are  largely  displaced.  Using  multi-­‐terminal  HVDC  transmission  systems  with  VSCs  that  allow  bi-­‐directional  power  flow,  system  operators  can  take  advantage  of  varying  geographical  resource  profiles.  For  example,  the  proposed  Clean  Line  Energy  transmission  projects  leverage  periods  of  excess  wind  energy  in  the  SPP  to  deliver  power  to  MISO  or  PJM  (Galli,  2012).  When  SPP  is  not  producing  wind  energy,  MISO  might  be,  or  likewise  PJM  might  be  producing  solar  energy.  By  connecting  large  geographical  areas,  the  average  amount  of  energy  available  to  serve  loads  is  higher  and  more  predictable  than  an  individual  resource  area  alone;  and  HVDC  systems  are  the  most  cost-­‐efficient  manner  to  create  the  connection.  The  geographical  averaging  effect  improves  energy  forecasts  (as  forecast  error  is  smaller  for  larger  geographies),  reduces  the  system  impact  of  ramp  events,  and  thus  reduces  base  load  cycling  and  the  use  of/need  for  reserve  capacity.  Additionally,  a  more  interconnected  transmission  system  allows  for  reserve  capacity  sharing  between  balancing  areas,  which  reduces  the  total  reserves  required  below  that  which  any  single  balancing  area  would  need  to  carry  to  meet  load  and  frequency  regulation  requirements.  

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HVDC  collection  systems  enable  a  new  design  paradigm  for  renewable  energy  generation  stations.  With  AC  collection  systems,  solar  PV  electricity  is  collected  as  DC  at  each  panel  and  then  converted  to  synchronous  AC  electricity.  For  wind,  generators  produce  asynchronous  AC  electricity,  which  is  converted  to  DC  and  then  to  synchronous  AC  electricity.  If  renewable  generators  were  designed  to  connect  to  an  HVDC  collection  system,  PV  panels  would  not  need  an  inverter  and  wind  turbine-­‐side  converters  would  be  reduced  in  complexity  to  output  DC.  This  not  only  reduces  the  costs  of  developing  renewable  generator  stations  -­‐  by  7%  for  solar  (Goodrich,  2012)  -­‐  it  also  lowers  the  collection  losses  when  there  are  long  feeder  lines  connecting  the  generators  to  the  transmission  system.  Vestas  estimates  a  30%  improvement  in  reducing  energy  losses  for  wind  farms  developed  for  HVDC  collection  instead  of  AC  (Manjrekar).  

 Power  Transmission  Controllers  and  Topology  Control  Algorithms  

DSRs  and  topology  control  algorithms  could  increase  the  flexibility  of  the  transmission  grid  and  thereby  increase  the  economic  efficiency  of  generation  dispatch.  DSRs  allow  operators  to  bypass  congested  lines  by  increasing  capacity  and  distributing  power  flow  among  portions  of  the  AC  grid,  thereby  increasing  transmission  utilization,  decreasing  congestion,  and  allowing  for  economic  dispatch  of  generation.  For  instance,  variable  impedance  devices  such  as  Smart  Wire  Grid’s  DSRs  can  increase  AC  transmission  system  utilization.  A  Smart  Wire  Grid  simulation  of  3,000  modules  on  six  transmission  lines  in  an  eastern  RTO  reduced  the  average  bus  marginal  cost  by  over  6%  in  a  summer  peak  scenario  (Smart  Wire  Grid,  2013).  DSRs  balance  the  load  being  transmitted  across  each  phase  and  allow  for  the  increase  in  transmission  capacity.    

Power  flow  control  technologies  designed  to  alleviate  congestion  can  have  a  great  advantage  to  easing  the  integration  of  renewables.  Smart  Wire  Grid’s  DSRs  have  been  demonstrated  to  create  a  variable  impedance  transmission  network  that  allows  power  flow  to  bypass  congested  lines.  A  simulated  study  in  the  Pacific  North  West  found  that  with  an  investment  of  $58  million  (~3000  devices),  the  variable  impedance  system  created  was  able  to  unlock  and  additional  2.8GW  of  wind  energy  by  reducing  congestion  (Smart  Wire  Grid,  2013).  This  benefit  would  be  achieved  without  adding  any  additional  transmission  lines,  and  thus  deferred  significant  investment  for  the  transmission  owners.      

Likewise,  this  same  effect  can  be  accomplished  by  optimally  switching  transmission  lines  to  change  the  impedance  characteristics  of  the  transmission  system.  TCAs  can  be  deployed  by  system  operators  to  optimize  their  switching  decisions  based  on  real-­‐time  events  on  the  grid.  TCA  simulations  in  power  flow  modeling  software  has  shown  a  reduction  in  wind  curtailment  instances  from  33%  to  14%  by  switching  lines  (Qiu,  2013).  A  simulation  of  the  impact  of  TCA  using  historical  PJM  data  demonstrated  over  $100M  in  annual  savings  from  congestion  relief  (The  Brattle  Group,  2013).  Again,  these  benefits  were  gained  with  very  little  capital  investment  which  allows  transmission  owners  to  invest  elsewhere  in  their  system.    

Other  HVAC  PTC  devices  that  can  provide  voltage  and  frequency  support,  such  as  STATCOMs  and  phase-­‐shifting  transformers,  have  been  used  to  improve  the  integration  of  wind  and  solar  generators.  These  devices,  which  also  allow  for  power  flow  control,  provide  dynamic  response  to  fluctuations  in  the  power  quality  of  renewable  generators.  

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Summary  of  Power  Flow  Control  Technology  Value    One  power  flow  control  technology  can  have  multiple  benefits  depending  on  its  application  in  the  grid.  To  understand  the  possibility  of  various  power  flow  control  technologies  at  a  glance,  see  Table  1.  Technical  capabilities  alone  are  not  sufficient  to  achieve  economic  efficiency  of  the  system  with  the  deployment  of  a  power  flow  control  technology.  Market  and  regulatory  barriers  can  prevent  use  of  the  technical  capabilities  even  when  it  would  be  economic,  highlighting  the  need  for  clear  understanding  among  transmission  owners,  system  operators,  and  regulators  of  both  technical  capabilities  and  benefits  of  technology  at  the  system  level.    

Table  1.  Power  Flow  Control  Technology  Value  Categories.  Power  flow  control  technologies  can  have  different  or  multiple  benefits  depending  on  their  position  and  application  in  the  electric  grid  –  asset  management,  renewable  integration,  congestion  relief,  economic  efficiency,  and  reliability  and  security.  Classes  of  technologies  that  are  represented  by  one  or  more  of  ARPA-­‐e’s  GENI  technology  teams  are  represented  in  bold  font.    

Technology Value Categories

22

Power Flow ControlTechnology

Asset Management

Renewable Integration

Congestion Relief

Economic Efficiency

Reliability&

Security

Value

Impr

ove

U

tiliz

atio

n

Prio

ritiz

eor

def

er

new

inve

stm

ent

Impr

ove

inte

r –co

nnec

tion

Red

uce

curta

ilmen

t

Dis

patc

h &

pl

anni

ng

Rea

l Tim

e

Ene

rgy

Anc

illar

y

Impr

ove

Con

tinge

ncy

Bla

ck s

tart

HVDC VSC X X X X X X X

HVDC LCC X X X X

TCSC X

UPFC X X X X X X X X X

Shunt -STATCOM X X X X X X X X

Series - SSSC X X X X X

Phase-Shifting Transformer

X X

DSR X X X X X X X

TCA X X X X X

GENINon-GENI

 

Stakeholders  in  the  Transmission  Grid  Influence  Technology  Investment  Decisions    As  previously  discussed,  quantifying  the  benefits  of  power  flow  control  capabilities  is  particularly  difficult  due  to  the  dynamic  nature  of  the  electric  transmission  grid  and  the  differences  in  benefits  to  individual  stakeholders  as  compared  to  the  overall  system  benefits.  At  the  same  time,  multiple  

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stakeholders  are  often  involved  in  technology  investment  decisions,  and  a  level  of  agreement  among  them  is  necessary  in  order  to  optimize  system  efficiency.    

Differences  in  regulatory  structure  among  federal  power  authorities,  investor  owned  utilities,  merchant  transmission  owners,  municipal  utilities,  and  rural  electric  co-­‐ops  lead  to  substantial  differences  in  the  way  certain  groups  assess  power  flow  control  technologies,  even  within  similar  stakeholder  categories.  As  technology  vendors  consider  the  best  value  proposition  and  business  model  for  their  power  flow  control  technologies,  they  should  bear  the  regulatory  environment  and  degree  of  restructuring  of  the  electric  market  in  mind.  For  an  overview  of  influencers  in  the  electric  grid,  see  Figure  2.    

Numerous influences on Transmission Owner’s investment and siting decisions

16

Transmission Owner FERC

PUC

ISO/RTO

NERC/ coordinating

councils

Regulatory industry groups

$ Transmission CAPEX, OPEX, rate recovery

Tech

Decreasing influence

on investment

decisions Other regulators,

NGOs

 

Figure  2.  Overview  of  influencers  in  the  transmission  grid.  A  utility  or  transmission  owner  investing  in  technology  must  be  mindful  and  responsive  to  the  interests  of  multiple  stakeholders  in  the  grid:  the  ISO/RTO  that  dispatches  assets  and  determines  set  points  for  power  flow  control  technologies,  the  regulators  overseeing  investment  and  siting  decisions,  the  bodies  responsible  for  overall  reliability  of  the  electric  grid,  and  other  interested  parties  who  may  intervene  in  a  transmission  case.    

 

The  benefits  of  power  flow  control  technology  to  each  stakeholder  will  vary  by  their  business  model  and  geographic  and  regulatory  situation.  To  better  understand  the  business  models  and  motivations  of  various  stakeholders  in  the  electric  transmission  grid,  see  Table  2.    

For  the  most  part,  power  flow  control  will  have  positive  economic  outcomes,  with  the  exception  of  those  stakeholders  who  currently  benefit  from  transmission  congestion  such  as  reserve  generators  and  to  a  slightly  lesser  extent  base  load  generators,  renewable  energy  generators,  and  transmission  owners.  

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The  beneficiaries  of  a  change  in  power  flow  control  will  often  be  temporary  and  largely  situation-­‐dependent,  as  market  conditions  will  remain  dynamic  in  a  world  with  power  flow  control.  An  overview  of  how  each  stakeholder’s  situation  might  change  as  compared  to  current  conditions  is  presented  in  Table  3.  

Table  2.  Motivations  of  stakeholders  in  the  electric  transmission  grid.  This  table  demonstrates  the  motivations  of  each  stakeholder  involved  in  the  electric  transmission  grid,  including  their  motivations,  inherent  conflicts  and  considerations,  and  a  brief  description  of  their  revenue  model.  While  every  effort  was  made  to  provide  a  comprehensive  overview,  the  differences  in  regulatory  structure  among  federal  power  authorities,  investor  owned  utilities,  merchant  transmission  owners,  municipal  utilities,  and  rural  electric  coops  should  be  considered  when  assessing  the  position  of  each  stakeholder.

Conflicts of interest related to PFR investment

19ARPA-E Template

Stakeholder How do they make or recover $

Motivation Conflicts & Considerations

Transmission Owner

§ Rate of return (~13%) fortransmission investment

§ FERC technologyincentive rate

§ ~11.5% distributioninvestment

§ Projects that will be approved or financed –leadsto incremental build out of system (relativelyshort time horizon for utilities dependent onregulated rate of return)

§ Invest in what they know (wires) rather than newtechnology

§ Profit

§ Incentive towards construction to meetpeak - of new transmission lines ratherthan investment in technology toremove congestion etc.

§ Regulated: certainty of public benefitcase (to rate-base)

§ Merchant transmission need 20 year,low-risk opportunity

ISO/RTO Fees charged to:§ Generators§ Transmission owners (allocated to states and recovered in rate base)

§ Reliability (& compliance with standards)§ Reduced congestion§ Reserve margin§ Economic efficiency§ Known solutions

Split in priority/focus :§ Reliability§ Economic dispatch§ Capacity margins

Renewable Generator

§ Contracts (PPA, tariff) § Bankability§ Off-take certainty§ Reduced curtailment

Base Load Generator

§ Dispatch§ Regulated return (whereapplicable)

§ Bankability § Increase utilization § Compliance with regulations

§ Risk change schedule/dispatch§ No compensation for cycling & wear &

tear for slow ramping

Reserve /peakGenerator

§ Dispatch§ Ancillary services§ Regulated return (whereapplicable)

§ Increase utilization§ Ability to access ancillary services revenuestreams (where applicable)

§ Compliance with regulations

Risk lowering utilization by removingancillary service functions

FERC § Congressional approval§ Recovered from

regulated industries

§ Economic efficiency§ Reliability§ Policy implementation

TO needs to approach FERC with new technology to receive favorable return for new tech solution. Theoretically could change incentive for transmission technology investment over new wires.

PUC § Budget set at state level § Recovered from rate payers.

§ Customer rates § Economic efficiency§ Reliability§ Policy implementation

§Transmission investment on economicbenefits accruing to their state ratebase vs everyone else in market area

§ Public perception§ Re-election (where applicable)  

 

 

 

 

 

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Table  3.  Overview  of  beneficiaries  as  a  result  of  power  flow  control  improvements  in  the  electric  grid.  

Power flow control technology beneficiariesStakeholders Asset

ManagementRenewable Integration

Congestion Relief

Economic Efficiency

Reliability & Security

Value

Impr

ove

U

tiliz

atio

n

Prio

ritiz

eor

de

fer n

ew

inve

stm

ent

Impr

ove

inte

r –c

onne

ctio

n

Red

uce

curta

ilmen

t

Dis

patc

h &

pl

anni

ng

Rea

l Tim

e

Ene

rgy

Anc

illar

y

Impr

ove

Con

tinge

ncy

Bla

ck s

tart

Transmission Owner

ISO/RTO

RenewableGenerator

Base Load Generator

Reserve Generator

FERC

PUC

Consumer

Likely benefit (financial or operational)

Benefit is dependent on situation

Revenue losses likely in current system

PFC generally produces beneficiaries…except in cases where stakeholders currently profit off of system inefficiencies

 

Conclusion/Next  steps      This  document  defined  power  flow  control  and  identified  and  described  technologies  that  enable  power  flow  control.  It  identified  the  benefits  of  power  flow  control  and  how  these  benefits  accrue  to  various  stakeholders  involved  in  the  electric  grid.  It  did  not  perform  a  detailed  analysis  of  system-­‐level  benefits  or  provide  case  studies  quantifying  the  impact  of  power  flow  control  technologies.    As  power  flow  control  technologies  become  more  common  on  the  electric  grid,  further  analysis  will  be  required  to  optimize  their  use  at  a  system  level.  This  should  include:      

Technology  case  studies  and  models  

• Power  flow  control  technology  case  studies  and  data  sharing  to  document  lessons  learned    For  the  existing  cases  where  power  flow  control  technologies  are  installed  and  operated,  in-­‐depth  analyses  will  advance  the  understanding  of  the  technical  capabilities,  costs,  and  benefits  of  the  technology.  Where  possible,  case  studies  should  include  quantitative  analysis  of  the  

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effects  of  the  technology.  Data  sharing  at  a  high-­‐level  will  enable  deeper  understanding  of  the  applications  of  power  flow  control  technologies.  Possible  case  studies  include  

o HVDC:     Trans  Bay  Cable  and  its  use  and  effects  on  transmission  congestion   Bi-­‐polar  HVDC  applications  such  as  Cross  Sound  Cable  between  Connecticut  and  

Long  Island,  Cross  Chanel  Cable  between  the  UK  and  France  o UPFC:  

Marcy  station  UPFC  in  New  York.  What  was  the  economic  (market)  response  to  its  operating  mode  set  points,  before  and  after  installation  

o DSR:   Case  study  on  the  Tennessee  Valley  Authority  pilot  installation  

• Further  describe  and  quantify  the  benefits  of  power  flow  control  technology  to  a  particular  stakeholder  Interested  parties  will  seek  more  information  on  how  the  benefits  of  power  flow  control  technologies  change  the  economics  of  the  system,  particularly  for  cases  where  the  benefits  of  a  power  flow  control  vary  (e.g.,  those  situation  identified  as  “yellow”  in  Table  3–  in  what  situations  are  these  green  and  red?)  

• Develop  or  identify  a  uniform  model  for  analyzing  transmission  technologies                                      Numerous  stakeholders  expressed  interest  in  a  uniform  grid  model  of  sufficient  size  to  model  system-­‐level  effects  of  combinations  of  technological  installations.  

• Add  power  flow  control  technology  specifications  to  existing  grid  modeling  software              Recent  modeling  exercises  may  have  been  limited  by  the  technical  specifications  available  to  modelers.  To  the  extent  that  these  set  points  can  be  added  rather  than  programmed  for  each  specific  hypothetical  or  actual  installation,  decision  makers  would  have  more  accurate  models  and  understanding  of  the  effects  of  technological  installations.    

System  level  technical  and  market  analyses  

• Technological  analysis  of  what  is  required  to  enable  power  flow  control  at  a  system  operator  level  Analysis  may  include  modeling  of  optimal  physical  positioning  of  devices  in  the  grid,  reliability  modeling,  and  economic  modeling  of  the  impact  of  increased  transmission  capabilities  and  the  increased  fluidity  of  changing  grid  topologies.  

• Define  level  of  coordination  and  control  required  within  an  RTO  and  among  regions.                                In  order  to  increase  the  flexibility  of  the  transmission  and  distribution  grid  and  meet  goals  around  reliability,  integration  of  renewable  electricity  at  the  utility  and  distributed  scale,  energy  efficiency  and  demand  response  capabilities,  we  will  need  some  centralized  control  and  centrally  coordinated  distributed  control.  This  will  provide  quick,  responsive  voltage  support  and  meet  the  changing  needs  of  the  electric  grid.  

• Consideration  of  market  design  for  a  flexible  transmission  grid                                                                                                                                                          Changing  grid  topologies  can  change  the  economics  of  generator  and  transmission  positioning  

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and  dispatch  more  fluidly  than  in  the  past.  The  community  should  ensure  that  market  design  is  aligned  to  ensure  flexibility  and  resiliency  of  the  grid  under  a  scenario  with  power  flow  control.  

This  work  could  be  done  by  industry  groups,  academic  organizations,  or  via  coordinated  public  effort.  Groups  to  engage  for  the  purpose  of  research  include  

• Electric  Power  Research  Institute  (EPRI)  • Edison  Electric  Institute  (EEI)  • Electricity  advisory  committee,  DOE    • Power  Systems  Engineering  Research  Center  (PSERC)  • CMU  • MIT  • Western  Interconnection  modeling  stakeholders  • Eastern  Interconnection  Planning  Collaborative  • ISOs/RTOs  • Regulators  

Groups  to  contribute  experts  for  educational  purposes  include  

• CIGRE  Grid  of  the  Future  • IEEE  • Georgia  Tech  • Washington  State  • Penn  State  • Texas  A&M  • Iowa  State  • Regulatory  trade  associations  –  NARUC  etc  • Institute  of  Public  Utilities  at  Michigan  State  University  

 

 

 

 

 

 

 

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References  Ahlstrom,  M.  (2013).  Questioning  the  Problem.  EUCI  Fast  and  Flexi-­‐Ramp  Resources  Conference.  Chicago.  

Bahrman,  M.  (2009).  HVDC  Transmission:  An  Economical  Complement  to  AC  Transmission.  WECC  Transmission  Planning  Seminar.    

Budhraja,  V.,  Mobasheri,  F.,  Ballance,  J.,  Dyer,  J.,  Silverstein,  A.,  &  Eto,  J.  H.  (2009).  Improving  Electricity  Resource-­‐Planning  Processes  by  Considering  the  Strategic  Benefits  of  Transmission.  The  Electricity  Journal  .  

DOE,  D.  o.  (2008,  January  9).  Retrieved  July  31,  2013,  from  National  Energy  Technology  Laboratory:  http://www.netl.doe.gov/smartgrid/referenceshelf/presentations/PSC%20Missouri_MGS_Utility%20Meeting_010908_Final_NETL%20Review_AP.pdf  

Edison  Electric  Institute.  (n.d.).  Actual  and  Planned  Transmission  Investment  by  Shareholder  Owned  Utilties,  2006-­‐2015.  Retrieved  August  20,  2013,  from  http://www.eei.org/issuesandpolicy/transmission/Documents/bar_Transmission_Investment.pdf  

EnerNex  Corporation.  (2011).  Eastern  Wind  Integration  Study.  Golden:  National  Renewable  Energy  Lab.  

Force,  U.-­‐C.  P.  (2004).  Final  Report  on  the  August  14,  2003  Blackout  in  the  United  States  and  Canada.  Retrieved  from  FERC.gov  website  .  

Galli,  W.  (2012).  The  Role  of  HVDC  for  Wind  Integration  in  the  Grid  of  the  Future.  CIGRE.  Paris.  

GE  Energy.  (2010).  Western  Wind  and  Solar  Integration  Study.  Golden:  National  Renewable  Energy  Lab.  

Goodrich,  A.  (2012).  Residential,  Commercial,  and  Utility-­‐Scale  Photovoltaic  (PV)  System  Prices  in  the  United  States:  Current  Drivers  and  Cost-­‐Reduction  Opportunities.  Golden:  National  Renewable  Energy  Lab.  

Hamachi  LaCommare,  e.  a.  (2004).  Understanding  the  Cost  of  Power  Interruptions  to  U.S.  Electricity  Consumers.  Berkeley,  CA:  Ernest  Orlando  Lawrence  Berkeley  National  Laboratory,  2004.  

M.I.T.  (2011).  Future  of  the  Electric  Grid.  Cambridge:  Massachusetts  Institute  of  Technology.  

Manjrekar,  D.  M.  (n.d.).  Wind:  Challenges,  Opportunities  and  PCS.  

Qiu,  F.  (2013).  A  Study  on  Transmission  Switching  for  Improving  Wind  Utilization.  FERC  Technical  Conference  on  Increasing  Market  Efficiency  through  Improved  Software.  Washington,  DC.  

Reed,  e.  a.  (2012).  Medium  Voltage  DC  Technology  Developments,  Applications  and  Trends.  CIGRE  U.S.  National  Committee  2012  Grid  of  the  Future  Symposium.  University  of  Pittsburgh.  

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Smart  Wire  Grid.  (2013).  Power  Flow  Control  for  the  Grid.  FERC  Technical  Conference:  Increasing  Real  Time  and  Day  Ahead  Market  Efficiency  Through  Improved  Software  (p.  14).  Washington,  DC:  Smart  Wire  Grid.  

The  Brattle  Group.  (2013).  Advances  in  Topology  Control  Algorithms  (TCA).  FERC  Technical  Conference  on  Increasing  Market  Efficiency  through  Improved  Software.  Washington,  DC.  

 


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