FUNDAMENTALS OF HARMONICS

Presented by: Syed Khundmir T Department of Electrical Engineering University at Buffalo khundmir@buffalo.edu

The BEST Group THE BUFFALO ENERGY SCIENCE AND TECHNOLOGY GROUP

-Winter Lecture Series

Prime  reference:  Electrical  Power  Systems  Quality  By  Roger  C.  Dugan,  Mark  F.  McGranaghan,  Surya  Santoso,  H.  Wayne  Beaty  

Harmonic Distortion

•  It is caused by non-linear devices.

•  In such devices, the current is not proportional to applied voltage.

• Most of the times, only odd harmonics are present in the system.

• Vast majority of nonlinearities in the system are found in shunt elements ie., loads.

Voltage vs Current Distortion

• Non-linear loads appear to be the source of harmonic current in shunt and injects harmonic currents into the power system. • Voltage distortion is the result of distorted currents passing through the

linear, series impedance of the power delivery system. • Harmonic currents passing through the impedance cause a voltage drop for

each harmonic. This results in voltage harmonic appearing at the load bus. • While the load current harmonics result in voltage distortion, it should be

noted that load has no control over voltage distortion. •  The above mentioned phenomenon is shown in the figure present in the next

slide:

Contd…

• Control over harmonic currents takes place at the end-user application. • Control over the voltage distortion, assuming the harmonic current

injection is within reasonable limit, is excercised by the entity having control over the system impedance, which is often the utility.

Harmonics  vs  Transients

•  Transients  exhibit  high  frequency  only  briefly  a:er  an  abrupt  change  in  power  system.  •  The  frequencies  of  transients  have  no  rela?on  with  the  fundamental  frequency.  • Waveform  distor?on  that  produces  harmonics  is  present  con?nually,  or  at  least  for  several  seconds.  •  Transients  are  usually  dissipated  within  a  few  cycles.  

Effect  on  Power

• No  real  work  will  result  from  the  por?on  where  the  current  is  not  in  phase  with  the  voltage.  •  In  the  non-­‐sinusoidal  case,  the  computa?on  of  the  ac?ve  power  must  include  contribu?ons  from  all  harmonics  components;  thus  it  is  the  sum  of  ac?ve  power  at  each  harmonic.  • Apparent  power  S  is  a  measure  of  the  poten?al  impact  of  the  load  on  the  thermal  capability  of  the  system.  •  The  reac?ve  power  component  at  fundamental  frequency,  may  be  used  to  size  shunt  capacitors.  

Power  factor:  Displacement  and  True

•  Power  factor  is  the  ra?o  of  useful  power  to  perform  real  work  to  the  power  supplied  by  the  u?lity.  •  In  other  words,  it  is  the  power  expended  for  its  intended  use.  •  In  the  sinusoidal  case,  the  power  factor  can  be  computed  as  the  cosine  of  the  phase  angle  and  is  commonly  referred  as  the  displacement  power  factor.  •  In  the  non-­‐sinusoidal  case,  the  power  factor  which  takes  into  account  the  contribu?on  from  all  ac?ve  power,  including  both  fundamental  and  harmonic  frequencies,  is  known  as  true  power  factor.  •  Considering  only  the  displacement  power  factor  will  give  a  false  sense  of  security.  

Harmonic  Phase  Sequences

•  The  method  of  symmetrical  components  allows  the  three  phase  system  to  be  transformed  to  three  single  phase  system  that  are  much  simpler  to  analyze.  •  It  allows  any  unbalanced  set  of  phase  currents  or  voltages  to  be  transformed  into  three  balanced  sets.  •  The  posi?ve  sequence  set  contains  three  sinusoids  displaced  120  degrees  from  each  other  with  normal  ABC  rota?on.  •  The  nega?ve  sequence  will  have  opposite  rota?on.  •  The  sinusoids  of  the  zero  sequence  are  in  phase  with  each  other.  

Triple  Harmonics

•  Triple  harmonics  are  odd  mul?ples  of  the  third  harmonics.  •  System  response  is  o:en  considerably  different  for  triplens  than  for  the  rest  of  the  harmonics.  •  These  have  become  an  important  issue  for  grounded-­‐wye  systems  with  current  flowing  on  the  neutral.  •  Two  typical  problems  are  overloading  the  neutral  and  telephone  interference.  •  Transformer  winding  connec?ons  have  a  significant  impact  on  the  flow  of  triplen  harmonic  currents  from  single-­‐phase  non-­‐linear  loads.  

Contd…

•  Two  cases:  1.  In  the  wye-­‐delta  transformer,  the  triplen  harmonic  currents  are  shown  

entering  the  wye  side.  Since  they  are  in  phase,  they  add  in  the  neutral.  They  can  flow  but  they  remain  trapped  in  the  delta  and  do  not  show  up  in  the  line  currents  on  the  delta  side.  

2.  Using  grounded-­‐wye  windings  on  both  sides  of  the  transformers  allows  balanced  triplens  to  flow  from  the  low-­‐voltage  system  to  the  high-­‐voltage  system  unimpeded.  

•  Transformers  are  suscep?ble  to  overhea?ng  when  serving  single  phase  loads  having  high  third-­‐harmonic  content.  •  These  rules  about  triplen  harmonics  apply  only  to  balanced  loading  condi?ons.