Dynamic voltage restorer

INTRODUCTION:

The  various  power  quality  problems  are  due  to  the increasing  use  of  non  linear  and  power  electronic  loads.Harmonics  and  voltage  distortion  occur  due  to  these  loads. The  power  quality  problems  can  cause  malfunctioning  of sensitive  equipments,  protection  and  relay  system.Distribution  system  is  mainly  affected  by  voltage  sag  and swell  power  quality  issue.  Short  circuits,  lightning  strokes, faults  and  inrush  currents  are  the  causes  of  voltage  sags. Start/stop  of  heavy  loads,  badly  dimensioned  power  sources,  badly regulated transformers, single line to ground fault on the  system lead to voltage swells.  Voltage sag is a decrease of the  normal voltage level between 10 and 90% of the nominal rms voltage at the power frequency, for durations of 0,5 cycle to 1 minute. Voltage swells are momentary increase of the voltage,  at  the  power  frequency,  outside  the  normal  tolerances,  with duration of more than one cycle and typically less than a few seconds .   The  use  of  custom  power  devices  is  one  of  the  most  efficient  method  to  mitigate  voltage  sag  and  swells.

There are many custom power devices. Each of which has its own  benefits  and  limitations.  Custom  power  device(CPD) is a powerful tool based on semiconductor switches concept to  protect  sensitive  loads  if  there  is  a  disturbance  from  power  line.  Among  the  several  novel  CPD,  the  Dynamic  Voltage  Restorer  (DVR)  are  now  becoming  more  established  in industry  to  mitigate  the  impact  of  voltage   disturbances  on sensitive loads.

DYNAMIC VOLTAGE RESTORER:

Dynamic  voltage  restorer  is  a  static  var  device  that  has  applications  in  a  variety  of  transmission  and  distribution  systems. It  is  a  series  compensation  device,  which  protects  sensitive  electric  load  from  power  quality  problems  such  as  voltage  sags, swells, unbalance and distortion through power  electronic  controllers  that  use  voltage  source  converters (VSC). The first DVR was installed in North America in 1996 -  a  12.47  kV  system  located  in  Anderson,  South  Carolina.

  Since then, DVRs have been applied to protect critical loads in  utilities,  semiconductor  and  food  processing.   Today,  the  dynamic  voltage  restorer  is  one  of  the  most  effective  PQ  devices in solving voltage sag problems.

PRINCIPLE OF DVR:

The  basic  principle  of  the  dynamic  voltage  restorer  is  to  inject a voltage of required magnitude and frequency, so that it  can restore the load side voltage to the desired amplitude and  waveform  even  when  the  source  voltage  is  unbalanced  or  distorted. Generally, it employs a gate turn off thyristor (GTO)  solid  state  power  electronic  switches  in  a  pulse  width  modulated (PWM) inverter  structure. The DVR can  generate  or  absorb  independently  controllable  real  and reactive  power  at the load side. In other words, the DVR is made of a solid  state DC to AC switching power converter that injects a set of  three phase AC output voltages in series and synchronism with  the distribution line voltages. Dynamic  voltage  restorer  is  a  series  connected  device  designed  to  maintain  a  constant  RMS  voltage  across  a sensitive load.

SYSTEMATIC DIAGRAM OF DVR:

COMPONENTS OF DYNAMIC VOLTAGE RESTRORER:

The DVR consists of:

A.   Voltage Source Converter (VSC)

B.   Injection Transformer

C.   Passive Filters

D.   Energy storage device/ Control system

The description of each component of DVR is given below

Voltage Source Converter (VSC):

Voltage  Source  Converter  converts  the  dc  voltage  from  the energy  storage  unit  to  a  controllable  three  phase  ac  voltage. The  inverter  switches  are  normally  fired  using  a  sinusoidal Pulse Width Modulation scheme.

Injection Transformer:

Injection transformers used in the DVR plays a crucial role in ensuring  the  maximum  reliability  and  effectiveness  of  the restoration  scheme.  It  is  connected  in  series  with  the distribution feeder.

Passive Filters:

Passive Filters are placed at the high voltage side of the DVR to  filter  the  harmonics.  These  filters  are  placed  at  the  high voltage  side  as  placing  the  filters  at  the  inverter  side introduces  phase  angle  shift  which  can  disrupt  the  control  algorithm.

   Energy storage device/ Control system:

Examples  of  energy  storage  devices  are  dc  capacitors, batteries,  super-capacitors,  superconducting  magnetic  energy Storage and flywheels. The capacity of energy storage device has a big impact on the compensation capability of the system. Compensation  of  real  power  is  essential  when  large  voltage sag occurs.

VOLTAGE INJECTION METHODS

Voltage  injection  or  compensation  methods  by  means  of  a DVR  depend  upon  the  limiting  factors  such  as;  DVR  power ratings,  various  conditions  of  load,  and  different  types  of voltage  sags.  Some  loads  are  sensitive  towards  phase  angel jump and some are sensitive towards change in magnitude and others  are  tolerant  to  these.  Therefore  the  control  strategies depend  upon  the  type  of  load  characteristics.  There  are  four different methods of DVR voltage injection which are

i. Pre-sag compensation method

ii. In-phase compensation method

iii. In-phase advanced compensation method

iv. Voltage tolerance method with minimum energy injection

A.   Pre-Sag/Dip Compensation :

The  pre-sag  method  tracks  the  supply  voltage  continuously  and  if  it  detects  any  disturbances  in  supply  voltage  it  will inject  the  difference  voltage  between  the  sag  or  voltage  at PCC  and  pre-fault  condition,  so  that  the  load  voltage  can  be restored  back  to  the  pre-fault  condition.  Compensation  of voltage  sags in the both phase angle and amplitude sensitive loads  would be achieved by pre-sag compensation method. In this method the injected active power cannot be controlled and it  is  determined  by  external  conditions  such  as  the  type  of faults and load conditions.

VDVR = Vprefault – Vsag

B.  In phase Compensation method :

This is the  most straight  forward  method. In this  method the injected  voltage  is  in  phase with  the  supply  side  voltage irrespective  of  the  load  current  and  pre-fault  voltage.  The  phase angles of the pre-sag and load voltage are different but  the  most  important  criteria  for  power  quality  that  is  the  constant  magnitude  of  load  voltage  are  satisfied.  One  of  the  advantages  of  this  method  is  that  the  amplitude  of  DVR  injection  voltage  is  minimum  for  a  certain  voltage  sag  in comparison with other strategies.

C.  In Phase advanced compensation :

In this method the real power spent by the DVR is decreased  by  minimizing  the  power  angle  between  the  sag  voltage  and  load  current.  In  case  of  pre-sag  and  in-phase compensation method  the  active  power  is  injected  into  the system during disturbances. The active power supply is limited  stored energy in the DC links and this part is one of the most  expensive parts of DVR.The minimization of  injected energy  is  achieved  by  making  the  active  power  component  zero by having  the  injection  voltage  phasor  perpendicular  to  the load current phasor.In this method the values of load current  and voltage are fixed in the system so we can change only the phase  of  the  sag  voltage.  IPAC  method  uses  only  reactive power  and unfortunately,  not  al1  the  sags  can  be  mitigated without  real  power,  as  a  consequence,  this  method  is  only suitable for a limited range of sags

D.  Voltage  tolerance  method  with  minimum  energy

injection:

A small drop in voltage and small jump in phase angle can be tolerated  by  the  load  itself.  If  the  voltage  magnitude  lies  between  90%-110%  of  nominal  voltage  and  5%-10%  of  nominal  state that will not disturb the operation characteristics of loads. Both magnitude and phase are the  control parameter for  this  method  which  can  be  achieved  by  small  energy injection

CONTROL TECHNIQUES

A.  Linear controllers :

The three main voltage controllers, which have been proposed  in  literature,  are  Feed forward  (open  loop),  Feedback  (closed loop) and Multi-loop controller .The feed-forward voltage controller  is  the  primary  choice  for  the  DVR,  because  of  its  simplicity  and  fastness.  The  supply  voltage  is  continuously  monitored  and  compared  with  a  reference  voltage;  if  the  difference  exceeds  a  certain  tolerance,  the  DVR  injects  the  required voltage. The drawback of the open loop controller is  the  high  steady  state  error.  In  the  feedback  control,  the  load  voltage is measured and  compared with the reference voltage,  the missing voltage is supplied by the DVR at the supply bus  in  a  feedback  loop.  This  controller  has  the  advantage  of  accurate response, but it is complex and time-delayed. Multiloop control is used with an outer voltage loop to control the  DVR  voltage  and  an  inner  loop  to  control  the  load  current.  This  method  has  the  strengths  of  feed-forward  and  feedback  control  strategies,  on  the  expense  of  complexity  and  time  delay.

B. Non-linear Controllers:

It  appears  that  the  nonlinear  controller  is  more  suitable  than  the linear type since the DVR is truly a non-linear system due  to  the  presence  of  power  semiconductor  switches  in  the  inverter  bridge.  The  most  non-linear  controllers  are  the  Artificial  Neural  Networks  (ANN),  Fuzzy  Logic  (FL)  and  Space Vector Pulse  Width Modulation (SVPWM). ANN  control  method  has  adaptive  and  self-organization  capacity.

The  ANN  has  inherent  learning  capability  that  can  give  improved  precision  by  interpolation.  FL  controllers  are  an  attractive choice  when precise mathematical formulations are  not possible. When a FL controller is used, the tracking error  and  transient  overshoots  of  PWM  can  be  considerably  reduced. SVPWM control strategy is to adopt a space vector  of  the  inverter  voltage  to  get  better  performance  of  the  exchange is gained in low switching frequency conditions.

V.  CONCLUSION

           By  the  use  of  different  control  techniques  it  is  viewed  that  DVR  is  suitable  for  voltage  sag  and  swell  mitigation.  The  basic structure of DVR, its operation, compensation methods  and  control  techniques  are  discussed  in  detail.  DVR  has  the  advantage of low cost,  require less computational efforts and  its  control  is  simple  as  compared  to  other  methods.   DVR  provides  simpler  implementation  for  voltage  profile  improvement.  Linear  controllers  provide  simpler  operation  and  less  computational  efforts  when  compared  to  other  methods.