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					Grounding Transformers




        Medford,
       Medford, Oregon
                 Oregon
        John S. Levine, P.E.
 Levine Lectronics and Lectric, Inc.

            March 2011
          February, 2007
                                       1
•   It is used to provide a ground path on either an
    ungrounded Wye or a Delta connected system
     • The relatively low impedance path to ground maintains the system
       neutral at ground potential
• On Ungrounded systems you can have
  overvoltages of 6 to 8 times normal with
  arcing faults
Arcing Ground Faults
Intermittent or Re-strike

•Plot of transient over-voltage for an arcing ground fault
•Intermittent ground fault: A re-striking ground fault can create a high frequency
oscillator (RLC circuit), independent of L and C values, causing high transient over-
voltages.
     – i.e. re-striking due to ac voltage waveform or loose wire caused by vibration
CONTROL OF TRANSIENT OVERVOLTAGE
• It supports the voltage on a faulted
  phase
  – If a single line-to-ground fault occurs on an ungrounded or
    isolated system, no return path exists and no current flows
  – The system will continue to operate but the other two un-
    faulted lines will rise in in voltage by the square root of 3,
    possibly overstressing the transformer insulation, and
    other components, by 173%
• Provides a metering point to measure faults
A typical example is a Wind Farm. They utilize
grounding transformers for fault protection on
ungrounded lines
        When a ground fault occurs on a collector cable causes the substation
circuit breaker to open, the wind turbine string becomes isolated

      Turbines do not always detect the fault and the generators continue to
energize the cable.

      Voltages between the un-faulted cable and the ground rise by the 173%

      The transformer, placed on the turbine string, provides the ground path
                                A typical example is a Wind Farm.




When the feeder breaker
Opens, the collector bus and
the step up transformer delta
connected MV windings
rely on the Grounding
Transformer for their ground
path and voltage support.
Two different constructions:


             ZIG ZAG (Zn)

          WYE CONNECTED
      •What if no neutral exists (i.e. delta systems)?
              – A grounding transformer is installed (either a zig-zag or a wye-delta)
                from all three phases to create an artificial neutral for grounding
                purposes only.
              AØ    BØ    CØ          AØ     BØ   CØ          AØ      BØ   CØ




Zig-Zag
                                                                                         HRG
Grounding
Transformer


                                             Wye-Delta               Broken Delta
                                             Grounding               Grounding
                                             Transformers            Transformers
              HRG              HRG
• The zigzag transformer contains six coils on
  three cores. The first coil on each core is
  connected contrariwise to the second coil on
  the next core.
• The second coils are then all tied together to
  form the neutral and the phases are
  connected to the primary coils.
• Each phase, therefore, couples with each
  other phase and the voltages cancel out
                                                        A
      Typical Wye connection with
      Neutral end of windings connected
      Together
    A          B          C               C


                                                            B

                                              Vector diagram of
                                              balanced system


A       B      C




    Symmetrical three phase source
                                       a


                                   A




                         c     C


The Zig-Zag connection has 2
windings on each leg.
                                       B

                                           b
                                                        a



                                                    A




                c             C

            A
                b
                                                           B
    a
                        Each leg of the Zig-Zag
                        Connection is connected                b
                         to a winding from another which
C                       is out of phase



        c           B
The resulting Zig-Zag connection
Is phase shifted with respect to the
incoming three phase source



                                 Az

   Cz




                     Bz
• Limits circulation of triplen harmonics
    ( 3 rd, 6th, 9th, etc)
• Can be used without a Delta connected or 5-
  legged core
• Elimination of secondary winding results in
  smaller footprint, lower cost (25-30%)
• Includes a Delta or Wye connected secondary
• Utilizes 4 or 5 legged core when Wye
  connected secondary is specified
• Multi-functional, provides benefit of auxiliary
  power
                Know the basic parameters

• Primary Voltage               • Impedance as a % or as an
• Phase to Phase continuous       ohms/phase value
  primary current (or Rated     • Primary Winding
  kVA)                            connection
• Continuous Neutral current    • Secondary connection
• Available Fault Current and   • Basic overall construction
  Duration
• This is the system voltage to which the
  grounded winding is to be connected.
• Don’t forget to specify the BIL also.
• In some cases the BIL will be dictated by
  equipment considerations, such as 150 kV BIL
  on 34.5 kV wind farms because of the
  limitation of dead front connectors
• The Transformer must be sized to carry the
  rated continuous, phase-to-phase current
  without exceeding its temperature limit
• The higher the current, the larger and more
  costly the transformer
• Typical values can be as low as 5 amps to as
  high as a few hundred
• Include any auxiliary loading requirements
• Is defined as 3X the Phase-to-Phase current
  (Zero Sequence Current)
• It is the value that is expected to flow in the
  neutral circuit without tripping protective
  circuits
• Used to design for thermal capacity of the
  transformer
• Used to determine the short time heating
  resulting from a fault on the system which
  returns through the transformer
• Typical ranges run from a few hundred to a
  few thousand amps
• Duration is expressed in seconds (i.e. 400
  amps for 10 seconds)
• Can be expressed as either a percentage or as an
  ohmic value
• Either should be chosen such that the un-faulted
  phase voltages are within the temporary over-
  voltage capability of:
      • The Transformer
      • Associated equipment (i.e. arresters, terminal connectors, etc.)
• Typical values can be as low as 8% and as much as
  100%
• Must be determined by the system designer
• Zig Zag or Grounded Wye
• Specify the secondary voltage and
  connection for primary Wye connected
  transformers
• Specify size of auxiliary loading to be
  connected
• If two winding with no secondary load,
  advise if the delta winding can be
  “buried” (not brought out) or if only one
  bushing is to be brought out for
  grounding to the tank or testing
• Compartmental pad          • Connectivity
  mount or unit                    • Dead Front or Live Front
  substation design                • Spade terminals
                                   • Cover-mounted or
• Indoor or outdoor                  sidewall
• Fluid type (Mineral Oil,         • Exposed or enclosed
  Silicone, or Envirotemp    • Temperature Rise
  FR3)                             • 65 degrees C
                                   • 55 degrees C
• Site Elevation or                • 55/65 deg. C
  Environmental
  conditions                 • Special Coating
                               requirements
      Round Coils




360 degree cooling ducts




              Radial forces are equalized during short circuits & overloads
Cruciform stacked core construction
   The following transformers are for
 reference only to let you be aware of
different type transformers for different
              applications.
• A Scott – T Transformer is a transformer that
  is designed for converting 2 phase current to 3
  phase current or vice-versa.
• An autotransformer is a transformer that only
  have one winding with taps. No isolation is
  provided between the primary and secondary.
• A Buck-Boost
  transformer is one
  that is designed to
  lower (buck) or raise
  (boost) the voltage
  in the range of 5%
  to 25%. This is a
  great (low cost) way
  to get between 208
  and 240 Volts, or
  between 480 and
  575 Volts. Used on
  1 and 3 Phase
  applications.
• A drive isolation
  transformer is one
  that is typically used
  on AC or DC Drive
  systems. It typically
  has the same primary
  and secondary
  voltages and is used
  for isolation. In
  addition you typically
  have a shield to
  attenuate line to
  ground noise.
• A constant voltage transformer is one in which
  the secondary stays constant with large
  swings on the primary voltage. This is
  accomplished by operating the primary in a
  saturated mode so even when the primary
  voltage dips you can still maintain a constant
  output. Typical input may be 80% to 110%
  with the output maintaining 1% voltage
  regulation.
A copy of this
 presentation
can be found
      at:

www.L-3.com
For more information and specification sheets




  www. PacificCrestTrans.com

				
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posted:7/27/2013
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