Power topic #6014 Technical information from Cummins Power GenerationGenerator set andUPS compatibility White paper By Gary Olson, Director, Power Systems DevelopmentThis document discusses problems that canbe encountered in operating UPS equipmenton generator set equipment, explains reasonsbehind some problems, and identifies stepsthat can be taken to minimize compatibilityproblems between generator sets and staticUPS equipment.Conventional wisdom,and associated problemsThere is a “conventional wisdom” that a generator setprovided for use with a UPS should be sized so that thegenerator set is always two to five times the capacity ofthe UPS.One problem with that guideline is that there is no firmconsensus in the industry of what exact number to useand little technical basis for a recommendation basedon sizing alone. There have been observed cases whereSCR loads as small as 3 kW have disrupted operationof a 100kW generator set. Would anyone suggest thatgenerator sets be derated to cover that situation?Moreover, even if one follows that “rule”, there is noguarantee that the generator set will successfully powerthe load. The UPS supplier generally can’t offer aniron-clad guarantee that a specific generator set willwork in an application, and the generator set supplierwon’t guarantee that a UPS will operate successfullyon a specific generator set.With as little as a “two times” derating factor, evenif the load is successfully operated, the over-sizingof the generator set can result in unnecessarily highinstallation and equipment costs, excess facility spacerequirements and design costs, and potential operatingand service problems with the generator set fromunder-loading the engine on the generator set.Over-sizing alone will not guarantee successful operational performance, and will result in unnecessarilyhigh installation and operation costs. So, what can bedone to improve the cost, performance and reliability ofpower systems that include UPS and generator equipment? The answers lie in understanding the problemsthat can occur and what suppliers can do to preventthem from happening.Generator set compatibility problems are mostpronounced on small (under 100 kW) generator sets,and particularly on natural gas fueled generator sets.Problems most commonly occur when the UPS is theonly load on the generator set, or is the single largestload on the system. Simply designing around situationsthat fit these situations could avoid many problems.Generator set and UPS incompatibility is particularlymystifying when identical equipment at two separatesites may perform differently; and especially when theUPS and generator set will both perform normally untilan attempt is made to run them together.When a system failure occurs, it is a natural reactionto assume that something must be wrong in theperformance of either the generator set or the UPS.That assumption is often not true. So, it is important tofirst understand that there are several different failuremodes in the UPS/generator set compatibility problem,and then apply the appropriate solution to the specificproblem encountered.The first action is to be sure that the generator set andUPS equipment are both operating correctly. This mayrequire testing of the genset under various loading

Power topic #6014 Page 2conditions to validate load carrying capability andtransient stability. From that point we can start to evaluate various possible compatibility issues by their failuremode symptoms. By categorizing these failure modes,some common areas of problems with generator setscan be identified:SymptomPotential ProblemFail to “lock on”to generator power improper generator frequencyor voltage poor generator frequency stability unrealistic performance requirementsInstability of generator voltage regulator sensitivitycontrol loop compatibilityfilter/control interactiongovernor or AVR problemFail to sync bypass frequency or voltage out of rangepoor frequency stabilityunrealistic performance requirementschanges to total load on the systemgenerator output voltage distortion poor frequency stability due to misadjustmentsor component malfunction voltage regulator sensitivity — output voltagedistortion poor frequency or voltage stability due tocontrol loop compatibility issues Inappropriate filter selection or operation on a UPSInstability at specific load levels control loop compatibilityIn addition, non-linear load heating effects should alsobe considered, in the interest of designing safe systemswith acceptable life spans for the equipment involved.Heating effects will not have immediate effects but maycause premature alternator failure.Instability at load changes control loop compatibilityMetering errors generator output voltage distortionLoss of voltage control excess capacitance in filters vs. loadMisadjustmentsGenerator sets that serve steady state linear loads,by nature of the power quality requirements of theloads, are normally not required to function at theirpeak steady state stability and transient responseperformance levels. They are often tuned at the factoryfastest response to load changes. Consequently, themany adjustments that can be made in governor andvoltage regulator circuits may not be optimized butoverall system performance may still be acceptable. Itis even possible that a generator set could be crippledby a partially functioning component (such as a stickingor binding actuator rod) and still function well enoughto serve many loads.Non-linear and other electronic loads often demand ahigher level of stability (that is, a lower rate of changeof frequency) than linear loads from a generator set.Consequently, it is possible that a generator set willstart, power up a load bank, and carry some loadswith no problems and still not carry specific UPS loadsdue to misadjustment or subtle component or systemdeficiencies. Or, a simple damping of especially thevoltage regulation system response will allow stableoperation.These problems will be extremely difficult to detectat a job site, because there typically is not monitoringequipment available at the site that would allow atechnician to detect these conditions. If no sample ofproper performance is available, it is difficult to evaluateon-site performance to know whether it is acceptable, 2007 Cummins Power GenerationTABLE 1 – Typical symptoms and potential problems associatedwith generator sets operating with UPS loads.or deficient in some way. Finally, it may be necessary totest at varying load levels to find the point of instabilitythat is causing an issue.Generator sets used in UPS applications should betested at the factory at rated load and power factor.Tests should include transient load testing at variousload levels with verification of voltage dip and recoverytimes, and observation of the a damped response ofthe system on recovery after loading.These results should be compared to prototype testdata, to verify that the unit is performing in the properfashion. Note that the absolute values are not necessarily as important as verifying the stability of the system,and the absence of malfunctioning components. Notealso that it is reasonable to require rated power factortesting for the factory test, since steady state voltageregulation is affected by the power factor of the load.When a stability problem is found at a job site andnormal testing and diagnosis fails to uncover a problem, a full load test with a load bank at various loadscan be helpful in verifying the ability of the system toperform properly. When this testing is done on the jobsite it is expensive and time consuming, but it is theonly way to verify that the generator set is performingcorrectly. It is reasonable to consider designing provisions for easy load bank connection into the system forany equipment that serves non-linear loads, for boththe generator set, and the UPS.

Power topic #6014 Page 3Voltage regulator sensitivityFIGURE 2AA generator set uses an automatic voltage regulator(AVR) to monitor the output voltage of the generator setand control the field strength of the machine to maintain a constant voltage on the output of the generatorset under varying steady state load conditions. FIGURE1 illustrates a typical generator set design.The voltage regulator senses output voltage level of thegenerator set directly from the output power connections and, based on a set reference point, changesoutput power to the generator exciter to maintainvoltage. Note that the power to operate the voltageregulator is derived from the output of the generatorset. This is termed a “shunt” type excitation system.80 KVA UPS/60KW Load Running on utility power 150A0A-150A75.0A/div vertical3.3ms/div horizontalFIGURE 2B 500V0ASHUNT-EXCITED ALTERNATOR-500VSensing and Power250V/div verticalElectrical Power Output3.3ms/div horizontalFIGURE 2C80 KVA UPS/60KW Load Running on 150KW Cummins Power Generation gensetOutputAutomacticVoltageRegulator 150ARotatingRectifiersMain StatorExiterRotorand StatorMain RotorRotatingMechanicalPowerInput0A-150AFIGURE 1 – In this schematic drawing of a shunt-excited alternator,note that the voltage regulator senses voltage level and draws excitation power from the output of the main field of the alternator.Since the AVR directly senses the output of thealternator, it must be designed to operate successfullywhen the sensed voltage waveform is distorted by thepowering of non-linear loads. The load current drawnby a UPS in normal operation does not follow a sinusoidal pattern. Consequently, the voltage waveform of thesource supplying power to the UPS is also distorted.The waveform distortion effects, which can be asignificant detriment in the operation of utility powereddistribution systems due to heating effects, will haveeven more pronounced effects on generator sets.This voltage waveform distortion can cause misoperation of generator sets with some voltage regulatortypes, especially AVRs that utilize SCRs to switchexcitation power on and off. These AVRs provide goodperformance when powering linear loads, but can failto operate in a situation where the voltage waveformis disrupted by non-linear loads. Under normal (linearload) conditions, the AVR will sense the voltage level onthe output of the alternator and based on that voltagelevel will time the firing of the SCR so that a 2007 Cummins Power Generation75.0A/div vertical3.3ms/div horizontal250V/div vertical3.3ms/div horizontal 500V0A-500VFIGURE 2 (a,b,c) – Comparison of current and voltage distortionseen at the input of an 80kVA UPS running with 60kW load whileoperating on the utility and on a 150kW genset with filters operating, and not operating. Total harmonic distortion of voltage is about11.4% when running on the genset without filters.amount of energy (the area under the excitation curve)will reach the exciter. (FIGURE 3a) Note that the SCRis “switched off’ by self-commutation as the voltagewaveform approaches its zero-crossing point.Consequently, voltage level on the output drops; theAVR tries to increase voltage level by switching on theSCRs sooner, and overcompensates for the problem.The net result is that voltage level on the output ofthe alternator begins to oscillate. Since the actualkilowatt load on the engine is a direct function of the

Power topic #6014 Page 4SCR-CONTROLLED EXCITATION SYSTEMInput AC WaveformTriggersExcitationFIGURE 3a – Note that a single trigger input “turns on” the excitation power, and the AC voltage crossing turns it off. When that regulator is applied with non-linear loads, the waveform notching causesthe SCR in the AVR to switch off at the incorrect time, so the exciterdoes not get the proper level of energy to maintain generator outputvoltage level (FIGURE 3b).voltage, the AVR voltage variation results in real powerpulsations to the engine of the generator set. Thesepower pulsations cause pulsating governor action,and oscillating frequency, which makes the problemeven worse, since most generator sets incorporate avoltage roll off with a change in frequency. The quicklychanging frequency can also result in misoperation ofUPS equipment. See “Control Loop Compatibility” inthis paper.Many governors on many generator sets were changedbefore the core problem was diagnosed. With theproper diagnosis in hand, the first attempts to correctthe problem revolved around isolating the AVR from thewaveform distortion by: applying filters to the AVR sensing input toprevent voltage distortion from reaching the AVR;These “solutions”, however, were not always withoutside effects.The same input filter that prevented waveformdistortion from disrupting the SCR-based AVR alsoprevented the regulator from quickly responding to realKVA demands, in large motor starting applications, forexample. So, even though the filter helped the systemremain stable, it could prevent proper operation ofother loads, unless the generator set was over-sized.Another problem with filtering was that it tended to besuccessful only when waveform distortion was not toosevere-such as when total SCR load on the generatorwas only 30-50% of the total generator capacity.The isolation transformers applied would not alwaysbe successful in removing enough of the waveformdistortion to allow stable operation since specializedtransformer arrangements are needed to successfullyremove significant waveform distortions (other than thetriplen harmonics).Because of the limitations of filtering AVR inputs, manymanufacturers designed high-speed voltage regulatorsthat provided pulse width modulated output to theexciter of the alternator. These AVRs accurately sensetrue RMS voltage value and provide excitation power inshort “bursts”, rather than depending on commutationof the AC voltage waveform to switch off the excitationpower. (FIGURE 3c)Consequently, the amount of excitation power deliveredto the exciter of the alternator is not affected by waveform notching, and