IEEE-Grounding-Transformers

Report
Grounding Transformers
Medford, Oregon
Medford,
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
Arcing Ground Faults
Intermittent or Re-strike
•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 overvoltages.
– i.e. re-striking due to ac voltage waveform or loose wire caused by vibration
4 8 0 V D e lta S o u rc e
3Ø Load
R fe
V
V
Cb
Cb
S fa
THE HIGH RESISTANCE GROUNDED
POWER SYSTEM
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 unfaulted lines will rise in in voltage by the square root of 3,
possibly overstressing the transformer insulation, and
other components, by 173%
UNGROUNDED SYSTEM NORMAL
CONDITIONS
UNGROUNDED SYSTEM
GROUND FAULT ON PHASE A
• 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
Grounding
Transformer
HRG
Wye-Delta
Grounding
Transformers
HRG
HRG
Broken Delta
Grounding
Transformers
• 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
B
C
A
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
to a winding from another which
is out of phase
C
c
B
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 5legged 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
• Phase to Phase continuous
primary current (or Rated
kVA)
• Continuous Neutral current
• Available Fault Current and
Duration
• Impedance as a % or as an
ohms/phase value
• Primary Winding
connection
• Secondary connection
• Basic overall construction
• 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 overvoltage 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
mount or unit
substation design
• Indoor or outdoor
• Fluid type (Mineral Oil,
Silicone, or Envirotemp
FR3)
• Site Elevation or
Environmental
conditions
• Connectivity
• Dead Front or Live Front
• Spade terminals
• Cover-mounted or
sidewall
• Exposed or enclosed
• Temperature Rise
• 65 degrees C
• 55 degrees C
• 55/65 deg. C
• 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

similar documents