Presentation on theme: "Levine Lectronics and Lectric, Inc."— Presentation transcript:
1 Levine Lectronics and Lectric, Inc. Grounding TransformersMedford, OregonMedford, OregonJohn S. Levine, P.E.Levine Lectronics and Lectric, Inc.February, 2007March 2011
2 What is a Grounding Transformer? It is used to provide a ground path on either an ungrounded Wye or a Delta connected systemThe relatively low impedance path to ground maintains the system neutral at ground potential
3 Why do I need to have a Grounding Transformer? On Ungrounded systems you can have overvoltages of 6 to 8 times normal with arcing faults
4 Arcing Ground Faults Intermittent or Re-strike Plot of transient over-voltage for an arcing ground fault1
5 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 vibration1
6 THE HIGH RESISTANCE GROUNDED POWER SYSTEM CONTROL OF TRANSIENT OVERVOLTAGE1
7 What is a Grounding Transformer? It supports the voltage on a faulted phaseIf a single line-to-ground fault occurs on an ungrounded or isolated system, no return path exists and no current flowsThe 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%
10 What is a Grounding Transformer? Provides a metering point to measure faults
11 Where are Grounding Transformers used? A typical example is a Wind Farm. They utilize grounding transformers for fault protection on ungrounded linesWhen a ground fault occurs on a collector cable causes the substation circuit breaker to open, the wind turbine string becomes isolatedTurbines 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
12 Typical Wind Farm System Design A typical example is a Wind Farm.When the feeder breakerOpens, the collector bus andthe step up transformer deltaconnected MV windingsrely on the GroundingTransformer for their groundpath and voltage support.
13 Types of Grounding Transformers Two different constructions:ZIG ZAG (Zn)WYE CONNECTED
14 How do you create the Ground? 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.AØBCAØBØCØAØBØCØHRGZig-ZagGroundingTransformerHRGWye-DeltaBroken DeltaGroundingGroundingTransformersTransformersHRG1
15 ZIG ZAGThe 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
16 Vector diagram ofbalanced systemABCTypical Wye connection withNeutral end of windings connectedTogetherABCSymmetrical three phase sourceABC
17 aAcCThe Zig-Zag connection has 2 windings on each leg.Bb
18 Each leg of the Zig-ZagConnection is connectedto a winding from another whichis out of phaseBCAcbaCBAbca
19 The resulting Zig-Zag connection Is phase shifted with respect to theincoming three phase sourceAzCzBz
20 ZIG ZAG Limits circulation of triplen harmonics ( 3 rd, 6th, 9th, etc) Can be used without a Delta connected or 5-legged coreElimination of secondary winding results in smaller footprint, lower cost (25-30%)
21 Wye Connected Includes a Delta or Wye connected secondary Utilizes 4 or 5 legged core when Wye connected secondary is specifiedMulti-functional, provides benefit of auxiliary power
22 How to Specify a Grounding Transformer Know the basic parametersPrimary VoltagePhase to Phase continuous primary current (or Rated kVA)Continuous Neutral currentAvailable Fault Current and DurationImpedance as a % or as an ohms/phase valuePrimary Winding connectionSecondary connectionBasic overall construction
23 Primary VoltageThis 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
24 Primary CurrentThe Transformer must be sized to carry the rated continuous, phase-to-phase current without exceeding its temperature limitThe higher the current, the larger and more costly the transformerTypical values can be as low as 5 amps to as high as a few hundredInclude any auxiliary loading requirements
25 Continuous Neutral Current 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 circuitsUsed to design for thermal capacity of the transformer
26 Fault Current and Duration Used to determine the short time heating resulting from a fault on the system which returns through the transformerTypical ranges run from a few hundred to a few thousand ampsDuration is expressed in seconds (i.e. 400 amps for 10 seconds)
27 Impedance 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 TransformerAssociated 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
28 Primary Winding Connection Zig Zag or Grounded Wye
29 Secondary ConnectionSpecify the secondary voltage and connection for primary Wye connected transformersSpecify size of auxiliary loading to be connectedIf 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
30 Construction Features Compartmental pad mount or unit substation designIndoor or outdoorFluid type (Mineral Oil, Silicone, or Envirotemp FR3)Site Elevation or Environmental conditionsConnectivityDead Front or Live FrontSpade terminalsCover-mounted or sidewallExposed or enclosedTemperature Rise65 degrees C55 degrees C55/65 deg. CSpecial Coating requirements
39 What is a Scott – T Transformer? A Scott – T Transformer is a transformer that is designed for converting 2 phase current to 3 phase current or vice-versa.
40 What is an Autotransformer An autotransformer is a transformer that only have one winding with taps. No isolation is provided between the primary and secondary.
41 Buck-BoostA 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.
42 Drive Isolation Transformers 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.
43 Constant Voltage Transformer 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.
44 A copy of this presentation can be found at: www.L-3.com
45 www. PacificCrestTrans.com For more information and specification sheetswww. PacificCrestTrans.com