AGN 238 General Application And Operational For STAMFORD .

When looking at the international design standards common in rotating electrical machines forrating, IEC 60034-1:2010, there is an easy graphical representation of operational frequencyand voltage envelopes, as shown on the previous page, around the nominal Voltage andFrequency rating points (crossing points in the target for operation), with zones to identify thevariance of operation.The closer the machine is operated to the centre crossing point of Voltage and Frequency, theless impact may be seen on temperature that each wound component runs at for a continuousload condition. If the alternator is operated away from the centre, there will be some impact toeither 1 or both main wound assemblies, resulting in higher peak temperatures, and a potentialreduction in operational life.There are just two operating frequencies associated with power distribution in worldwide ACgrid networks, 50Hz and 60Hz.There are many countries around the world that operate at different voltages. It is important toensure that a generating set, which is designed to operate at a given voltage and frequency.Some designs of alternators can accommodate a wide range of system voltages, but someare very limited. Always check the alternator rating plate to confirm how the machine willoperate.It may be possible to reconfigure the stator winding to meet the power system requirements,or alternatively, if not, it may be possible to utilize a de-rate factor to allow operation atconditions less than optimal. Always ask and confirm the alternator voltage and frequencyrating capability to operate at a voltage level outside the designed bandwidth.To cope with the wide variety of combinations of voltage and frequency used in differentnational systems, every alternator in the STAMFORD and AvK ranges may have severaldifferent winding designs.Alternator RatingAlternator ratings are specified in accordance with national and international Standards, IEC60034-1 for alternators and (IEC 60034-22: AC generators for reciprocating internalcombustion (RIC) engine driven generating sets). There are two types of rating that arespecified, Basic Continuous Rating (BR) and Peak Continuous Rating (PR).Basic Continuous Rating (BR)This is a maximum continuous rating derived for continuous running duty. The alternator willhave a maximum continuous rating at which the machine may be operated for an unlimitednumber of hours per year, while meeting the thermal operational temperatures laid down inrequirements of the appropriate standards.This drives an assessment of thermal stress and therefore insulation system life (how long thealternator is designed to operate for at the given rated continuous load before the insulationfails).AGN 238 ISSUE A/8/20

For industrial use alternators, three Base Continuous Rating levels are offered, based ontemperature rise limits and a standards ambient temperature of 40 C:Class “H” (125 C/40 C)Class “F” (105 C/40 C)Class “B” (80 C/40 C)Peak Continuous Rating (PR)Peak continuous rating is also known as Standby Rating or Peak Standby Rating, andrepresents the absolute maximum load rating which the alternator can be run at, but with therecognition that the insulation life will be significantly reduced as the higher rating causeshigher operating temperatures in the windings, resulting in accelerated thermal degradation.The increased rating from base continuous rating will reduce the lifetime of the alternator by afactor of between 2 to 6 times.There are variations on the two main ratings, which involve definitions of continuous anddiscontinuous load factors, as generating set builders optimise cost effective solutions to meetmarket needs. Some will also identify limited number of operating hours at that identified kVArating, to limit thermal degradation, especially for well-defined standby applications.The following table summarises the rating definitions in ISO8528-3 and IEC 60034-1 for theAlternator and ISO 8528-1 for the Generating Set:Summary of various International Standards Ratings for Alternators and Generating SetsAGN 238 ISSUE A/9/20

It is always suggested that the specific alternator Nameplate is checked to ensure the ratingand duty are clearly understood.OverloadIf an alternator is operated above its designed base continuous rating, the operatingtemperatures will increase to a level that will degrade and so, shorten the life of, the woundcomponents of the insulation system.For a fixed percentage overload, the rate at which the operating temperatures increase is aproduct of the alternator’s Thermal Time Constant. This thermal time constant factor varies inproportion to:1. The physical mass of the wound components, which in turn affects the rate of thermalrise. For example; larger alternators up to 300kVA cope better with overloads thansmall alternators with outputs around 50kVA.2. The percentage level of overload above the alternator’s designed Class ‘H’ rating.3. The duration of the overload condition.4. The operating temperature of the wound components just before the overload conditionbegins.5. The ambient temperature during the overload condition.Ultimately, thermal overload should be avoided where at all possible. In all cases and as ageneral practical rule of thumb, continuous overload above 110% of the alternator Nameplate,applied for longer than 1 hour, will cause accelerated thermal degradation and bring aboutearlier end of life condition. Also, be aware that the relationship of overload and thermaldegradation is not linear.The rule of thumb for insulation design life is, the life of the insulation system is halved forevery 10 Degrees higher thermal operating condition.This importantly applies for every condition of increasing thermal stress. If for instance theambient temperature is 10 Degrees hotter than the ambient for rating, say 50 Degrees at theair inlet, and the alternator is run with restricted cooling air due to the generating set air filtersbeing partially clogged, and the alternator is running at 110% of rated load, there is everylikelihood that the thermal degradation can be accelerated, reducing the design life from say40,000 Hours to less than say 5,000 Hours.Example: with 3 thermal acceleration factors:40K design life:40K / 2 [10% overload] 20K20k / 2[10Deg increase in ambient temperature] 10KAGN 238 ISSUE A/10/20

10k / 2[ restricted air flow reduces effective cooling] 5KThe result is total operational life reducing to around 0.8 years, (based on average 6Kper year) of life from the nominal 8 years.The above illustration details only 3 of the many possible factors impacting thermalinsulation life of the alternatorPower FactorPower factor is a way of identifying the electrical relationship between the active real power,also known as working power (kWe), required to do the job, and the consumed apparent power(kVA). The difference being due to the electrical characteristics of the electrical load appliedto the alternator.In electrical engineering terms, the power factor of an a.c. power system is the ratio of the realpower (kWe) flowing to the load to the apparent power (kVA) in the electrical circuit.The nature of the load in an a.c. circuit will determine if the current drawn is in phase or out ofphase with the generated voltage. The load again determines if that current waveform ‘leads’the voltage waveform or ‘lags’ the voltage waveform. For normal industrial loads (e.g. motors)the current will lag the voltage by some time interval or phase angle. The optimum situation iswhere current and voltage are in phase. This makes the power factor unity (1.0) and hence thereal power (kW) the same as the product of voltage and current (kVA). Conventionally,alternator kVA ratings are based on a lagging power factor of 0.8. In this case the current willlag the voltage by an amount that causes the real power level supplied (kW) to fall below thekVA level by a factor of 0.8 times.Unity PFLagging PFLeading PFWhat Causes Power Factor?Power factor basically is a measurement of the timing - phase angle difference - of the currentwaveform relative to the voltage waveform. The idea being to identify how effectively theAGN 238 ISSUE A/11/20

supplied power (kVA) is working in relation to the real work being done (kWe). The power isbeing used more effectively when the power factor is closer to unity.It is possible for a load to demand a current that is almost totally out of phase with the generatedvoltage. Also, that current may be lagging the voltage (inductive or motor loads) or leading thevoltage (capacitive loads).When operating in parallel, care must be taken to manage power factor to ensure thegenerating set alternator is not overloaded.When choosing to operate the generating set at power factors away from the range of 1.0(Unity) to 0.8PF lagging, it is very easy to thermally overload the alternator by running a lowlagging PF, as even though only a small real power (kW) is demanded by the load, which iswell within the machines capability, damage can easily result if the load is a very low powerfactor load demanding a very high kVA level, and so result in very high alternator phasecurrent.Load SharingWhile the active power (kWe) sharing between generating sets during parallel operation isgoverned by the engine control systems, the reactive power (kVAr) sharing is controlled bythe alternator’s control systems.There are various recognised load sharing methods in the generator industry, such asisochronous load sharing, cross-current compensation and quadrature droop load sharing.Quadrature droop load sharing is the standard kVAr load sharing method utilised by CumminsGenerator Technologies. For AvK and STAMFORD alternators, quadrature droop load sharingrequires a Droop Current Transformer (CT) to be fitted on one of the alternator’s output phases.For AvK and STAMFORD alternators fitted with digital Automatic Voltage Regulator’s (AVR),the quadrature droop should be programmed on installation. For STAMFORD alternators fittedwith analogue AVRs, the CT secondary winding should be connected to the AVR droop S1 S2 inputs.The current signal from the Droop CT secondary winding is converted to a voltage signalacross a burden resistor within the AVR. This voltage signal is then added vectorially to theAGN 238 ISSUE A/12/20

AVR sensing voltage so that the AVR will be “tricked” to droop the voltage by adjusting theexcitation levels accordingly, in proportion to the levels of load current and power factor.Example of AVR and Quadrature Droop CT ConnectionIt is important to have the droop CT on the same phase and the same AVR droop setting forall paralleled alternators (usually 3% at 0.8pf) to ensure proportional kVAr sharing among theGenerating Sets.Example of vectoral addition of droop signal to the AVR sensing voltageBecause the percentage ‘droop’ of the output voltage is related mainly to the reactivecomponent of the load current, the droop is negligible when the load current is at unity powerfactor. For this reason, any attempt to check the operation of the ‘droop’ characteristic bytesting with a resistive load bank will not result in any observed ‘droop’ in the generating set’soutput voltage.When considering setting the generating set alternator quadrature droop circuit, please referto the Owner’s Manual. It is vital that you understand all controls and settings before anyadjustment is made. It is very easy to trip a generating set when operating in parallel with onlyvery small adjustments, depending on the installed generating set protection settings.AGN 238 ISSUE A/13/20

Cooling Air for an AlternatorBoth open air ventilated alternators and enclosed alternators with cooling sub-systems, musthave a cooling system that operates at a certain temperature and volume of air through thealternator, to cool the components to a satisfactory temperature so that alternator does notoverheat. The temperature that must be maintained is influenced by the air surrounding thealternator. This is commonly referred to as ambient temperature.An example of a Generating Set cooling system design.Ambient TemperatureAmbient temperature can be defined as the temperature of the surrounding air at a particularlocation. The internationally accepted standard value for industrial use alternators is 40 C.The ambient temperature measured should be, in the case of an air-cooled machine such asan AvK or STAMFORD alternator, the air inlet air temperature. This may be higher than thesurrounding air ambient temperature, due to the heat generated by the prime mover within theconfined space of an engine house.It is essential that the total actual temperature does not exceed the limits set by the Nameplaterating and ambient temperature identified. In some cases, especially marine machines, or forspecial rating cases, ambient temperatures higher than 40 C, may be specified and allowedfor.An industrial alternator operating in an ambient temperature greater than 40 C, must be derated to ensure that total actual temperature does not exceed the specified maximum.The converse of this is also true; that by reducing temperature a greater output can be obtainedfrom an alternator for the same actual temperature. This is permitted in most standards downto an ambient of 30 C.Airflow Through the AlternatorAs a standard, industrial alternators are designed for use in a maximum ambient temperatureof 40 C, and altitude no higher than 1000 meters above sea level. (Air density affects coolingcapability of the air – the higher the altitude, the less dense the air is, so less dense air hasless cooling capability.)AGN 238 ISSUE A/14/20

Where ambient temperatures in excess of 40 C and altitudes above 1000 meters exist, theseenvironmental operating conditions can be tolerated with reduced ratings. Refer to AGN 012for further information.The internal air flow is moved by a fan fixed on the machine shaft which is designed to createa pressure drop which in turn cools the alternator rotor and stator windings. The air inlet islocated at the non-drive-end of the alternator while the air outlet is located at the drive-end ofthe alternator as show in the following illustration. Cooling air flows into the alternator throughthe air inlet at 40oC, cools the windings as it passes through the air gap (air path) and exitthrough the air outlet at about 70oC.Example of the air-cooled alternatorAirflow requirementsIf the generating set is installed in a building or enclosure, efficient cooling will depend onmaintaining the condition of the cooling fan and air filters. There shall be no recirculating air inthe generating set room to maintain the ambient temperature entering the room. If theoperating environment differs from the values shown on the alternator nameplate, the ratedoutput must be adjusted.Alternator Air FiltersThere are two types of Air Inlet Filter used on AvK and STAMFORD alternators. The type offilter design will be selected for specific environmental conditions, based on the customersidentified operating conditions, sized to meet those declared conditions, and specified on theoriginal alternator order confirmation. The two types are: Dry Dust Filter Moisture FilterAGN 238 ISSUE A/15/20

Both types of filters are manufactured by companies specialising in air filtration systems andeach type of filter kit is a proprietary package from that company’s standard range ofappropriate products.Environmental ConditionsWhen considering a proposed operational site with a contaminated atmosphere, carefulconsideration must be given to the design and operational functionality of the generating sethousing, with special attention to air inlet and air outlet apertures. The next area of concernmust be the internal airflow management system in conjunction with the type of contaminationthat will inevitably enter the generating set housing. Finally, the type of supplementary filterrequired at point of entry to the alternator, the engine and its cooling system.The decision-making process regarding the need to consider an abnormal site and associatedcontaminated atmosphere must be a controlled function within the generating setmanufacturer’s commercial operation, where direct contact exists with the end user, theirneeds and required duty for any proposed generating set to identify the following points forsizing considerationRequirements, i.e.1.2.3.4.the type of contamination, particulate sizethe medium of contamination, (carrier: air, moisture, water spray, etc)The chemical composition (Acid/neutral/Alkoline)The expected level of contamination (concentration of contamination verses carriervolume / density)It is very important that the generating set supplier and installer are fully aware of the risk theyplace on the generator installation and equipment, to prevent operational problems occurringrelated to site conditions.Air inlet filters are constructed into a specially fitted filter framework at the factory. Each filterassembly is designed specifically for a particular alternator frame size and should be fitted atthe factory during alternator manufacture.It is strongly recommended that all alternators fitted with air inlet filters are specified to befactory fitted with stator winding temperature detection devices, or differential pressureswitches, thereby providing an automatic control system that can be employed to ensure thatblocked air filters are detected before a winding overheats, with the risk of burn-out occurring.Remember that maintenance of Air filters is vital to ensuring an adequate volume andtemperature of ambient cooling air is supplied to the alternator.Also remember that seasons change, and the regime of inspection and clean / replacementmay also change, depending on the prevailing operational conditions.As a rule of thumb, at commissioning, it is always wise to monitor air filters, so that it is possibleto track alternator changing operational conditions, over time, and then confirm filter change /cleaning, when a tip up in alternator RTD temperatures or greater than 5 to 7oC, or an increaseto 30 to 40% blocked condition is reached on the differential pressure switch.AGN 238 ISSUE A/16/20

IP CodesIEC 60034-5:2000 describes the system for classifying the degrees of protect

the STAMFORD alternator’s Owner’s Manual, where fundamental information regarding ac generator (alternator) care is provided. The site operating procedures must include the necessary guidance and instructions to ensure the prevailing site conditions are appropriately considered and the necessary maintenance regime is being applied.