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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 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 over- voltages. – i.e. re-striking due to ac voltage waveform or loose wire caused by vibration 480V Delta Source R fe 3Ø Load 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 un- faulted 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 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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