Module 6: Transformers - WECC
Course Outline1.2.3.4.5.6.7.8.9.Introduction to WECCFundamentals of ElectricityPower System OverviewPrinciples of GenerationSubstation OverviewTransformersPower TransmissionSystem ProtectionPrinciples of System Operation
Module Overview This module presents the following topics:Principle of OperationTypes of TransformersOperating Considerations and Limitations
Principle of Operation The purpose of a transformer is to change anelectric system quantity (e.g., voltage orcurrent) from one level to another. A transformer is made up of two or moreconductors wound around a single magneticcore, usually iron. The wound conductors,usually copper, are called windings.
Two-Winding TransformerThe primary winding is electrically connected to the power source.The secondary winding is electrically connected to the energy output or load side.There is no electrical connection between the primary and secondary windings.
Tertiary Winding Sometimes a third winding, the tertiary winding,is present. A tertiary winding provides power toan auxiliary circuit or a reactor. The core and the windings are mounted in a steeltank filled with mineral oil or some other liquidsuitable for insulating and cooling. Insulatedbushings, usually mounted at the top of the tank,connect the windings to other power systemequipment.
How do Transformers Work? Passing an alternating current through a coil causes analternating magnetic flux in the magnetic core. The magnetic flux circulates in the magnetic core,passing through another coil (the secondary winding),inducing an alternating voltage in this coil. The amount of induced voltage depends upon fourfactors:– 1) core composition and shape– 2) number of turns in primary coil or winding– 3) number of turns in the secondary coil or winding– 4) primary voltage
How do Transformers Work?In a transformer there are two or more coils linked together by a commoncore conducting the magnetic flux. Flux from one coil (the primary winding)passes through the other coil (the secondary winding), inducing a voltage inthe secondary winding. Mutual induction links the two windings.
Turns Ratio At the beginning of this section, we state that thetransformer's purpose is to change an electricsystem quantity from one level to another. Theamount a quantity changes is determined by theturns ratio, which is the ratio of the number ofturns in the two windings. The magnetic flux links the turns of the primaryand secondary windings. This induces a voltagein each winding. Since the same flux cuts bothwindings, the same voltage is induced in eachturn of both windings.
Turns Ratio The total voltage in each winding isproportional to the number of turnsin that winding:V1 V2 N1 N2 V1 and V2 are the voltages in the primary and secondarywindings, respectively. N1 and N2 are the number of turns in the primary andsecondary windings, respectively.
Turns Ratio We know from Module 2 that inductance is theelectrical circuit property that opposes the change ofcurrent. The following statements describe therelationship between flux and inductance:– Increasing the current flow increases the magnitudeof the flux.– Increasing the turns in the conductor increases theconcentration of flux.– Increasing the flux concentration increases induction.– Inductance causes the current to lag the voltage. Thecurrent may lag the voltage in a transformer by amaximum of 90 .
Turns Ratio V1 Primary VoltsV2 Secondary VoltsN1 Primary TurnsN2 Secondary Turns I1 Primary CurrentI2 Secondary CurrentP1 Primary Power InP2 Secondary Pwr OutThere is one additional relationship we must consider – the relationshipbetween P1 and P2. In an ideal transformer, the power into thetransformer is equal to the power out of the transformer. In other words,there are no losses.Therefore, the following relationship exists: P1 P2Using the relationships, we can determine the changes across atransformer.
Step-Down Transformer24,000 volts100 ANP50010 NS50150 turns10:12400 volts1000 A10EP 1ES10IS 1IPLoadSource500 turns
Step-Up Transformer500 turns2400 volts1000 ANP501 NS500101:101EP 10ES24,000 volts100 A1IS 10IPLoadSource50 turns
Determining Output Voltage Let's apply these relationships to an example: A transformer has 300 turns on its primary windingand 600 turns on its secondary winding. The inputvoltage is 120 volts. What is the output voltage? The given quantities are: V1 120 volts N1 300 turns N2 600 turns
Determining Output Current If we are given I1, we can determine the secondarycurrent, I2, by using the following equation:If I1 800 amps, by substituting the given valuesinto the equation we have:
Determining Power Remember, the power remains the same acrossthe transformer. So, let's check to make sure thepower into the transformer is the same as thepower coming out of the transformer. (Forpurposes of this example, assume a resistiveload; therefore, cos θ 1) P1 V1 x I1 x cos θ P1 120 V x 800 A x 1 P1 96,000 Watts P1 96 kW
Determining Power P2 should be the same. Let's see.P2 V2 x I2 x cos θP2 240 V x 400 A x 1P2 96,000 WattsP2 96 kWThese calculations show that the power on theprimary side of the transformer equals thepower on the secondary side of the transformer.
Step-Up/Step-Down Transformer In the example, the transformer changed theprimary-side voltage from 120 V to a secondaryvoltage of 240 V to decrease the current on thesecondary side. This is an example of a step-uptransformer; the voltage was stepped up from120 V to 240 V. Conversely, a transformer inwhich the energy transfer is from a high-voltagecircuit to a low-voltage circuit is a step-downtransformer.
Step-Up/Step-Down Transformer We can see from the example that whatever happensto the voltage through the transformer, the oppositehappens to the current.– If the voltage is stepped down, current is stepped up bythe same ratio.– Likewise, when voltage is stepped up, current is steppeddown by the same ratio. Some important concepts to remember abouttransformers include:– Transformers do not produce electricity. They onlytransform it from one level to another; i.e., step thevoltage or current up or down.
Step-Up/Step-Down Transformer Although transformers take certain levels ofvoltage and current and change them to otherlevels, the total amount of power does notchange from one side of the transformer to theother, if losses are ignored. The power on the primary side equals the poweron the secondary side, if the transformer iswithout losses. In reality, transformersexperience some losses. We discuss losses laterin this module.
Step-Up/Step-Down TransformerFor a given value of power, the higher the voltage, thelower the current for use by the transmission system.Using lower current decreases losses. This is why transmission systemsuse high voltage. In Module 3:Power System Overview, wediscussed the step-up transformersat generating stations (GSU) . Thesetransformers raise the generatoroutput voltage.
Step-Up/Step-Down Transformer While the transformer is operating, someelectrical energy is converted into heat. Butwe know the purpose of the transformer isnot to provide heat. The purpose is to transferelectrical energy from the primary to thesecondary winding. Therefore, any heat thetransformer produces is an energy loss andrepresents inefficiency.
Transformer Efficiency The efficiency of a transformer is the ratio ofthe output power to the input power.But we stated earlier that the power into a transformer isequal to the power out of the transformer, therefore theefficiency equals 100%. This is the ideal case. In reality, thetransformer consumes some of the power. Mosttransformers have an efficiency of between 97% and 99%.
Losses The power consumed is called power loss. It iscaused by the following:– hysteresis losses– eddy current losses– copper (I2R) losses Hysteresis and eddy current losses occur in thetransformer's core. Copper losses occur in the windings. All three loss types involve the conversion ofelectrical energy into heat energy.
Residual Magnetism Hysteresis loss is due to residual magnetism, which isthe magnetism that remains in a material after themagnetizing force is removed. The transformer corereverses magnetic polarity each time the primarycurrent reverses direction. Every time the magneticpolarity reverses, the residual magnetism of the previouspolarity has to be overcome. This produces heat.Hysteresis loss is the energy required to reduce theresidual magnetism to zero and occurs every half cyclejust before the core is re-magnetized in the oppositedirection.
Transformer LossesTransformer(Core Losses)LossesHeatHysteresisLosses
Eddy Currents Eddy current is the current that flows in thetransformer's core and results from the voltage that isinduced in the core by the primary winding. We knowthat the primary coil creates a flux that induces avoltage in the secondary coil. The flux also cuts thecore, and we know that when a varying flux passesthrough a conductor it induces voltage. The core isitself a conductor. So a voltage is induced in the coreas well as in the secondary winding. In the core, theenergy is converted to heat. Eddy current can bereduced by laminating the transformer's core with ahigher resistance material.
Transformer LossesHeatEddy CurrentLosses
Copper Loss (I2R Losses) Copper loss is the power dissipated in thetransformer windings. Using largerconductors for the transformer windingsreduces the copper loss, but the conductorsize is limited by the openings in the coreinto which the winding must fit. However,larger conductors may be required tosustain higher currents.
Transformer LossesHeatHeatCopperLossesCurrentCurrentI2 R Losses
Voltage Control Most high-voltage transformers contain taps on thewindings for changing the transformer's turns ratio. Atap is a connection at some point on a primary orsecondary winding which permits changing the turnsratio. Changing the turns ratio alters the secondaryvoltage and current. If the need for voltage adjustments is infrequent (e.g.,adjustments are made for load growth or seasonalvariations), utilities use no load de-energized tapchangers. As the name implies, the transformer is deenergized prior to changing taps.
Tap Settings
Load Tap Changer Where frequent voltage adjustments arenecessary, or in cases of a transformer thatcannot be de-energized without jeopardizingcustomer service, utilities use load-tap-changing(LTC) transformers. LTC transformers, sometimescalled tap-changing under load (TCUL)transformers, change transformer tapsautomatically, remote manually via SCADA, ormanually by local control, while the transformeris energized.
Load Tap Changer The tap changer is operated by a motor thatresponds to relay settings to hold the voltage at apre-determined level. Special circuits allow the tap to be changedwithout interrupting current. The load-tap changing equipment is usuallyhoused in a separate compartment on the side ofthe transformer. Load-tap changing equipment isused on power transformers, autotransformers,and distribution transformers.
Load Tap Changer
Load Tap Changer(LTC)
How Transformer Taps Adjust VoltageTap ChangerTapsvoltageSecondary WindingPrimaryWindingChanging taps adjusts the turnsratio between windings
Three Phase Transformer Up to this point we have been discussing single-phasetransformers. Three-phase transformers operate using the sameprinciples: passing an alternating current through aprimary winding causes an alternating magnetic flux in thecore, which induces an alternating voltage in thesecondary winding. In three-phase transformers there are three primarywindings and three secondary windings. Some three-phase transformers include windings for allthree phases in one tank. Other three-phase transformers have three single-phasetransformers connected together.
Three Phase Transformer
Three Phase Transformer
Transformer Bank The connection of two or more single-phase transformers as a unitis called a transformer bank. The most common methods forconnecting the windings are:– Wye or Y (sometimes called star) connection– Delta connection We discussed the Wye and Delta connections in Module 2:Fundamentals of Electricity. Some methods of connecting thewindings result in a voltage phase difference between the primaryand the secondary windings. This is called a phase shift. Theprimary and secondary windings need not have the sameconfiguration.
Transformer Bank
Transformer ConnectionsNote: In some transformers, the neutral point in the Y connection isgrounded.
Transformer Connections We must consider these phase shifts beforetying together circuits fed through differenttypes of transformers. For example,connecting a circuit fed by a Wye-Delta bankto a circuit fed by a Wye-Wye bank results inexcessive current flow because of the 30ºphase difference.
Types of Transformers Power TransformersAutotransformersPhase Shifting TransformersInstrument TransformersDistribution Transformers
Power Transformers Power transformer is aterm given to atransformer used totransfer power forvoltages higher than 69 kV.Most power transformersare three-phase. Powertransformers can step-upor step-down the voltage.Other capabilities can beadded to a step-up orstep-down transformer,such as tap changingequipment.
Autotransformers An autotransformer is asingle-winding transformerwith a terminal that dividesthe winding into twosections. Autotransformersare useful because they aresimply constructed andcost relatively littlecompared with multiwinding transformers. Autotransformers arevariously designed to raiseor lower the voltage at 5%, 7.5%, or 10 %ranges.
Autotransformers
Phase Shifting Transformers Phase shifting transformers,sometimes called phase angleregulators (PARS), control powerflow over parallel lines byadjusting the voltage phase angleat one end of the line.Phase shifting transformersincrease or decrease the phaseangle differences between buses.Inserting a phase shiftingtransformer on a transmission linechanges the power flow over theline by changing the phase anglebetween locations thusredistributing the power flow.
Phase Shifting Transformers
Phase Shifting Transformers The Phase A series winding's secondary is connected to Phase B'sexciting winding. Phase B's voltage lags Phase A's voltage by 120º (or 60º leading ifthe polarity is reversed). The Phase B exciting winding induces a voltage in the Phase Aseries secondary winding. This small out-of-phase voltage advancesthe supply voltage
Instrument Transformers In high-voltage systems, directmeasurement of voltage or current is notpractical. We must scale down the valuesfor use by meters and relays. Instrumenttransformers perform this function. Instrument transformers include currenttransformers (CTs) and potentialtransformers (PTs) (sometimes calledvoltage transformers [VTs]). Both of thesetransformers reduce system current andvoltage to lower values for use by therelays and control circuitry. We discussCTs and PTs in more detail in Module 8:System Protection.
Distribution Transformers A distribution transformer reduces voltage to a level that isusable by customers. Distribution transformers are mountedon poles, on concrete pads, or in underground vaults. Theiroperation is similar to a power transformer.
Transformer Cooling Systems Excessive heating in the transformer causes theinsulation to deteriorate; therefore, it isimportant to prevent overheating. Thetechnology for this is based on the idea that oilcools the core and windings. Transformermanufacturers equip transformers with coolingsystems that prevent the permissibletemperature rise of the insulating oil fromexceeding specifications.
Transformer Cooling Systems Cooling systems for large power transformerstypically include:– radiators in which outside air cools thetransformer oil that circulates by convectionthrough the radiators– pumps to increase the circulation rate whenadditional cooling is needed– fans that blow air on the radiators for addedcooling
Transformer Cooling Systems
Transformer Ratings Heat generated within the transformer tank causes thetransformer insulation to deteriorate gradually. Whilesome heating is unavoidable, excessive heating cancause rapid deterioration and breakdown of thetransformer insulating materials. The transformer rating is the maximum power thatthe transformer can safely carry without exceeding atemperature limit and is expressed in MVA.Transformers typically have more than one ratingdepending on the portion of the transformer coolingsystem that is operating.
Transformer Ratings The forced-oil and air (FOA) rating is themaximum rating that applies when oil pumpsand cooling fans are operating. The forced air (FA) rating applies when the fansare running but the oil pumps are not running(oil is flowing by natural circulation). This isapproximately 80% of the maximum rating. The oil to air (OA) rating applies when neitherthe fans nor the oil pumps are running. This isapproximately 60% of maximum rating.
Transformer Ratings It is important to detect faults in thetransformer windings before damage occurs.Major problems that cause extensive damagein transformers usually start out as smallshort-circuits between turns. These shortcircuits usually develop into an arc, whichproduces large volumes of gas by chemicallydecomposing the insulating oil.
Transformer Ratings Relays that detect rising internal gas pressurein the tank are able to detect such faults whilethey are still relatively minor. However, theserelays cannot be too sensitive, or theyoperate needlessly for pressure surges causedby sudden changes in current flow, such asthose caused by external faults.
Transformer Ratings It is important to be able to determine:– whether a transformer relay operated incorrectly, inwhich case the operator should restore thetransformer to service.– whether there is a minor internal fault that should berepaired prior to re-energizing the transformer toprevent more extensive damage. Following a transformer relay operation,substation personnel typically performinspections to determine whether an internalshort circuit is present.
Transformer RatingsThey may: Perform a resistance check to determine whethernormally energized parts have come in contact withnormally non-energized parts. Draw gas and oil samples from the tank and havethe samples analyzed to determine whetherexcessive decomposition due to arcing has occurred. Measure the turns ratio to determine whether ashort circuit has occurred between turns.
Transformer Ratings If test results indicate that no internal fault exists,the transformer can be re-energized. As a preventive measure, utilities periodicallyinspect transformers to identify possible problems.Most transformers include gauges for readingtransformer loading, oil levels and temperatures,and gas pressures and temperatures to assist inperforming these inspections.
Transformer RatingsSo what happens when you exceed thosetransformer ratings?
Salem and Hope Creek Nuclear Power Stations, Hancock’s Bridge NJ
Whoops! Wrong kind of transformers.
Questions?
QUIZ TIME
transformers. Three-phase transformers operate using the same principles: passing an alternating current through a primary winding causes an alternating magnetic flux in the core, which induces an alternating voltage in the secondary winding. In three-phase transformers there are three primary windings and three secondary windings.
applications including generator step-up (GSU) transformers, substation step-down transformers, auto transformers, HVDC converter transformers, rectifier transformers, arc furnace transformers, railway traction transformers, shunt reactors, phase shifting transformers and r
7.8 Distribution transformers 707 7.9 Scott and Le Blanc connected transformers 729 7.10 Rectifier transformers 736 7.11 AC arc furnace transformers 739 7.12 Traction transformers 745 7.13 Generator neutral earthing transformers 750 7.14 Transformers for electrostatic precipitators 756 7.15 Series reactors 758 8 Transformer enquiries and .
2.5 MVA and a voltage up to 36 kV are referred to as distribution transformers; all transformers of higher ratings are classified as power transformers. 0.05-2.5 2.5-3000 .10-20 36 36-1500 36 Rated power Max. operating voltage [MVA] [kV] Oil distribution transformers GEAFOL-cast-resin transformers Power transformers 5/13- 5 .
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Jun 17, 2015 · WECC Load Model Sensitivity Study “An Analysis of the Sensitivity of WECC Grid Planning Models to Assumptions Regarding the Composition of Loads ” Task 1 Progress Update June 17, 2015 Nick Tenza, Scott Ghiocel Mitsubishi Electric Power Products, Inc. (MEPPI) Power System
- IEC 61558 – Dry Power Transformers 1.3. Construction This dry type transformer is normally produced according to standards mentioned above. Upon request transformers can be manufactured according to other standards (e.g. standards on ship transformers, isolation transformers for medical use and protection transformers.
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