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Electrical Performance Characteristics of aNew Micro-Turbine Generator

M. Etezadi-Amoli, Senior Member, IEEE K. Choma, Member, IEEEElectrical EngineeringDepartmentk UNversity ofNevx Reno

Reno,Nevada 89557Abstract This paper presenta the nxxdts of an investigation

regarding the electrical performance characteristics of a new,three-phase, 480 volt (L-L), 30 kW, micro-turbine generator andits suitability as an electrical power source for applications at theelectrical power distribution voltage level. This unit is tiled bynaturrd gas and may be used as a dispersed generation source.The unit was purchased by the local utility company and given tothe University of Nevada, Reno for the investigation. TheElectical Machine and the Power System Laboratory was used toperform the study.Testing was performed at several load set-point levels. Foreach level, the micro-turbine speedj exit temperature, inlet

temperature and pressure, inverter currents, inverter phase toneutral voltages, inverter single phase and three-phase powerwere recorded. Also, results of noise, pollutio~ etliciency, andharmonic measurements for the micro-turbime are presented. Theinvestigation includes the turbine behavior under a weak systemthat consists of the micro-turbine, a variable three-phase load,and a three-phase 15 kVA synchronous machine in thelaboratory. The speed of this machine is controlled by a fbzzylogic algorithm. Conclusions are made regarding the suitabilityof the unit as a distributed generation source.Keywords: micro-turbine, voltage and cummt harmonics, lowvoltage power geuerationj distribution systems.

I. INTRODUCTIONThe ever increasing demand for energy is forcing

electric utility companies to generate more electricityevery year. From 1980 to 1996, the annual growth rate forelectricity generated in the U.S. was approximately 1.9percent [1]. In 1990, the total U.S. generating capacitywas 735 GW. In 2000, this figure is projected to reach 817GW [2]. Because of increasing costs and more stringentenvironmental regulation the construction of large powerplants to meet rising energy demands is economicallyunfeasible in many regions. Subsequently, there is anincentive for the utility companies as well as otherinvestors and small businesses to develop small powerproduction and co-generation facilities.A recent announcement regarding the development of a30 kW, compact micro-turbine generator has brought forthnew possibilities regarding the appl@tion of dispersedgeneration at the distribution voltage level. Many utilitycompanies, including the Sierra Pacific Power Company

(SPPCO) in Reno, Neva@ have become extremelyinterested in the potential contributions of these genemtors.

If the performance characteristics and the price of theseunits are acceptable, then electric utility companies willhave a powerful new tool to help them meet the increasingenergy demand. Utility companies will be able toctmatruct small distribution stations in regions that demandmore power by combining seveml of these micro-turbines.It is imperative that the installation of these new devices

will not affect the performance or safety of the powersystems already in place.II. DESCRIPTION OF TESTING PROCEDURE

The alternating current (at) output of the turbine isrectified to direct current (de) and then inverted back to acusing a puke-width modulation inverter [3] [6]. Theinverter control system limits the current output to preventdamage to the solid state switching devices. Themaximum continuous current output is 54 A with 200 Apossible for 10microseconds.A. Typical ConfigurationThe micro-turbine generator was installed in theElectrical Machinery Laborato~ at the University of

Nw* Reno. A separate, 4-wire, wye supply was usedto connect the turbine to the SPPCO system. Appropriatefuel supply, electrical connection and ventilation systemwere provided for the unit. This arrangement wouldrepresent the typical configuration for connecting themicro-turbine to the utility system.B. Load SetpointsFor the typical configuration, testing was performed at 1,5, 10, 15, 20, 25, and 30 kW load settings. However, due

to turbine tripping measurements at 1 and 5 kW werediflicult to obtain.For each level, the micro-turbine speed, exittemperatuna inlet temperature and pressure, invertercurrents, inverter phase to neutral voltages, inverter singlephase and three phase power were recorded. Given thealtitude of the test location (approximately 4,500 f=t), theunit was only able to supply a maximum of 22 kW, eventhough its nameplate rating is 30 kW.

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C. Noise MeasurementsNoise measurements were taken with the turbine

operating at 23 kW. These measurements were takendirectly in front of the turbine and at a distance of 10meters.Noise level in front of the turbine is approximately 85dB. Although tolerable, this somewhat high pitch turbinenoise would not provide a comfortable working

environment. Noise level 10 meters in front of the turbineis approximately 64 dB. Note tha~ because the turbine isdesigned for outside use, we do not see the noise as a bigproblem for its use. It is interesting to mention that perour conversation with the manufacturer, noise level forfuture turbines will be significantly lower.D, Pollution MeasurementsA gas analyzer was used for pollution measurements.

With the unit rmming at 22 kW, the sampling probe wasplaced at approximately 4 to 6 inches from the exhaustoutlet. The analyzer measured 20.3 Yo oxygen and 6 partsper million carbon monoxide.E. Efficiency MeasurementsTurbine efficiency was calculated using the followingprocedure:Svmbols used:

*Pw j.PWW*PP=TT.Hv

w=

RPp .

Q

m

Total (overall efficiency)Power in (Energy/time)Power out (Energy/time)Natural gas pressureStandard pressureNatuml gas temperatureStandard temperatureHeating value of the natural gas(energy/mass)Heating value of the natural gas at standardtempemture and pressure (enerj@volume)Gas constant of the natural gasNatural gas densityNatuml gas density at sta&iard temperatureand pressureNatural gas volume flow rateNatural gas mass flow rate

Calculations:~ = PW~*/PWi.PWin=HV*rn HVS/fi*?h

=W,/p. *Q*p=Ws *Q) *(P@ (1)Ideal gas lawP = @?T, where T and P are absolute temperatureand pressure, respectively.Then using,P = @T, Ps = pJ?T,and knowing R is constant one can obtaimp/p, = PT=/P=T (2)Substituting (2) into (1) gives:PWj=~~ *Q) *(P/P$ *(T./T)

Assumptions:For the calculations given below, the following areassumed:

The specified heating value for the natural gm (lIV, =1042Btu/ft?), is at standard pressure and temperature.Also,

T== 20 CP= = 14. 7psi

Input Data (Natural Gas = NG):HVs, heating value of NG (Btu/ft3) 1042Q, volume flow rate of NG (ft3/hr) 291T, temperature of NG ( K) 273 + OP, pressure of NG (psi) 14.7 + 5.4PWti, power out (kW) 19.986P,, standard pressure (psi) 14.7T,, standard temperature (K) 273 +20

Efllciency Calculation:p/p,, densitylstandard density 1.468PWrn,power in (Btu/hr) 444984.009PWrn,power in (kW=kJ/s) 130.414w, total (overall) efficiency 15.325(Yo)Our sample calculation for turbine-genemtor eficiency

is below the values given by the efficiency curves thatwere provided by the manufacturer. As demonstrated inthe efficiency computation calculation of the efficiency isdependent on many factors including the temperature ofthe natural gas. Better accuracy can be achieved by usingthe average of the pressures at the inlet and outlet of thevolume flow rate measuring device. Also, a mass flow

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measuring device can be used to achieve a more accurateresults.F. Harmonic MeasurementsWith the micro-turbine directly comected to the SPPCosystem, a recording poly phase power meter and threecurrent sensors were used to measure voltage and current

harmonics of each phase at various load levels. Voltage,current and power waveforms were also recorded.The summary of measurements for turbine loads of 5,

10, 15 and 22 kW are presented in Tables 1and 2.Table 1.

I Summarv of VoltwzeHarmonic Meaaurementaat the Micro-turbhe Bus IDuringhe Initial Cti@ationLoAD PHASE VOLTS THD 3m Sm 7 9m Ilm(kW) RMs 90 % % % % %5 A-N 276.9 2.3 0 0 1 1 05 B-N 276.7 2.6 0 0 1 0 15 C-N 276.6 2.7 0 1 2 1 110 A-N 277.4 2.3 0 0 1 0 110 B-N 277.6 2.4 0 1, 1 0 110 C-N 276.1 2.6 0 1 1 1 115 A-N 278.2 2.4 0 1 1 1 115 B-N 278.6 2.4 0 0 i 1 115 C-N 278.4 2.5 0 1 1 1 122 A-N 279.1 2.4 0 1 1 0 022 B-N 278.9 2.6 0 1 1 1 122 C-N 278.8 2.6 0 0 2 1 1

Table 2.

Summary of Cumnt Harmonic Measurements at the Micro-turbine BusDuring the Initial ConjurationLoAD I PHASE I AMPS I THD I 3d I 5m 71 9~ 1lm(kW) RMs 0/0 ?/0 0/ 0 % 90 0/ 0

5 A 7.48 46.2 7. 17 33 5 115 B 7.30 49.3 7 18 40 4 105 c 4.20 66.7 15 28 43 4 2010 A 12.60 24.7 1 6 20 3 110 B 12.45 22.3 1 6 17 4 310 c 12.70 26.2 3 7 22 1 415 A 19.01 16.4 2 1 14 2 215 B 18.98 17.9 2 3 15 1 215 c 18.85 18.9 5 5 16 2 222 A 27.17 11.3 3 1 9 1 222 B 26.89 10.5 3 1 8 1 122 c 26.69 12.1 3 2 10 1 2

As shown in Table 1, the maximum total harmonicdistortion (IHD) for the voltage is 2.7 % for thesemeasurements. This distortion limit is not dependent onthe load output setpoint. However, as shown in Table 2,the maximum THD for the current is 66.7/0for a turbineoutput setpoint of 5 kW. The THD is definitely dependenton the load output setpoint. As the load output setpoint forthe turbine is in- the THD for the current decreases.Still, at the maximum load output of 22 kW, the minimumTHD for the current is 10,5 %. Clearly, the rather highlevel of voltage and cument harmonics for this turbine maynot be acceptable to some customers [7]. Becauseconversion of the dc waveform to a 60 Hz ac waveform isaccomplished through the use of power electronics, webelieve that an improved conversion mechanism wouldprovide a more acceptable output.G. Alternate ConfigurationIn order to better evaluate the behavior of the

microturbine when it is not directly connected to the utilitysystem, two commercial grade, three phase tmnsformerswere used to connect the turbine to the 3 phase, 208 voltgrounded wye supply that exists in the electricalmachinery kborato~. The first transformer is rated at 480volt grounded wye to 480 volt delta. The secondtransformer converts 480 volt delta into 208 volt groundedwye. This configumtion provided the flexibility formeasuring the turbine output parameters at variouslocations. Additionally, this scheme is suitable forinvestigating the behavior of the turbine when excited by aweak power system. This weak system essentially consistsof a 3 phase supply provided by the 15 kVA synchronousmachine in the laboratory.With the micro-turbine connected to the utility throughthe two local transformers, harmonic measurements and

waveforms were recorded at multiple locations again forcomparison purposes. The results are very similar to thoseoutlined in Tables 1and 2.H. Turbine Behavior with a Weak SystemSince the unit cannot operate without a three-phasesupply system attempts were made to operate themicroturbine with a 15 kVA synchronous machine as thereference source. The speed of this machine is controlledby a fhzzy logic algorithm. This was to allowmeasurement of operating parameters without the

influence of the utility system. Multiple attempts weremade to achieve a small power system consisting of themicro-turbine, the laboratory 15 kVA machine and avariable load bank however, none were successful.During the initial trials, the micro-turbine was to startusing the 15 kVA machine as the source, as the micro-

turbine only uses 2 to 3 kW during startup. Using thisprocedure, the micro-turbine would not fully start as the

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voltage and /or the tlequency was not stable enough for itsprotective device settings. Even with the settableprotection as liberal as possible, a successtid start did notoccur because some of the protection settings, such as Af/At is not adjustable from the user keypad.The next sequence of tests had the 15 kVA machine

synchronized with the utility during the micro-turbinest@ to provide sufficient capacity and stability.Following the successful starL a load slightly greater thanthe output of the micro-turbine was applied to the powersystem. Following tha~ the utility connection was opened.In all attemp@ the micro-turbine would enter a shutdownmode.As the voltage would drop as soon as the utility breakerwas open~ a variable, three-phase capacitor back wasinstalled on the bus of this small power system, to providevoltage support during the transition from the utility.Aga@ the micro-turbine would not stay on line.

I. Load FollowingWhile the micro-turbine was connected in the alternate

configuration, the load following capability was testedusing a variable resistive load bank and a pulse outputpower meter. The output of a specific power meter, whenconnected to the micro-turbine, should contiol the loadsetpoint and power output of the unit.he meter was connected to the variable load bank.

During all attempts, the micro-turbine output powerclimbed to full output regardless of the load bank setting.Discussions with the manufacturer confirmed that this unitwas not equipped wi~ the load following option. Thisoption is apparently implemented in the units that weremanufactured at a later stage.J. Computer SimulationComputer simulations were pefiormed to investigateferroresonant and switching over-voltages of the SPPCO

Steamboat feeder #268 in the presence of dispersedgeneration systems (DGS), which will represent thesemicro-turbines. Two utility grade software packages wereused for this purpose.Regarding the ferroresonance study, several different

parameters were varied in order to find the DGScombination that resulted in the highest ferroresonanceover-voltage. The ferroresonance condition was initiatedby a single line to ground fault which results in the trippingof the SPPCO supply system. A small subsystemconsisting of the DGS units, their associated transfonnem,a capacitor bank, and some feeder load is then created. Asa result a resonant circuit will form between thetransformers magnetizing impedance and the capacitorbanks shunt capacitance. The highest over-voltage did

not correspond to syste& with 30 kVA micro-turbines.For the 30 kVA case, because the load is generally greaterthan the generation capacity, the voltage quickly drops tobelow the nominal voltage. Disconnecting the load in thiscase increases the over-voltage to approximately 1.5 perunit.

Regarding the switching over-voltage study, the micro-turbines will not have any significant impact on the voltagelevel for the distribution feeders due to their small size. Ifthere is a reduction in the fders load, the DGS systemwill not cause an over-voltage to the system if the mainsource is still connected. On the other hand over-voltageconditions may be exhibited if an island is formed with themicro-turbines. The system response depends on the DGSsize, removed loads, and the location of the loads removed.

III. CONCLUSIONSThe following conclusions are made regarding the 30kVA micro-turbine:

q

q

q

b

q

q

The final connection between the micro-turbineand the electrical distribution system is through asolid-state inverter using pulse width modulationto convert the power to the correct voltage andfrequency of alternating current.The micro-turbine, which we tested, cannot runwithout a reference source present. As such, itcannot operate as a stand-alone source for anisolated or emergency system.During start-ups, the three-phase micro-turbineacts as a motor load by drawing power from theutility system. Once the desired operatingconditions are reached, the device becomes agenemtor. As such synchronization with theutility system is automatically achieved.Since the generated power by the micro-turbine isdependent on the turbine - the machine ishappier at higher outputs.The micro-turbine is equipped with variousprotective devices that can be adjusted.We made every effort to make the turbine part ofan alternate configuration as described earlier.The objective of the alternate configuration wasto run the turbine under an island consisting of thethree-phase, 15 kV4 synchronous machine andthe variable three-phase load in the electricalmachinery hihoratory. Although we opened upthe baud for various protective devices as much aspossible, the hard wiring of the micro-turbine willtrip the machine under transient conditions. Inour opinioq this is a very desirable feature if the

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turbines are to be installed on distribution systemsby the utility companies.

. Since power electronics are used to genemte thefinal output power, the tested micro-turbineproduces a significant amount of voltage andcurrent harmonics. However, an improved dc toac conversion scheme could easily eliminate thisproblem.

q Ferroresonance and switching over-voltageswhich are generally produced for an islandedsystem are literally non-existent because thesensitive hard-wired setting of the micro-turbinestrips the unit immediately when the supply systemis lost.

Overall, we are delighted with the state-of-the-attechnology that this micro-turbine represents and believethat it delivers the fimctions that it is designed for. Themicro-turbine that we tested was one of the very earlymodels that was manufactured As we understand thenewer models exhibit much-improved characteristics.Given their small size and versatile fuel capabilities, theimproved versions of these micro-turbines can be installedin any area of a power distribution system. Also, the smallsize of the micro-turbine makes the new stand-alonemodels of the micro-turbines ideal for bachp units forfacilities such as hospitals, casinos, shopping centers,banks, and schools. In the event of a power outage,facilities could use a micro-turbine to maintain power fortheir security systems, computers, cash registers, elevators,life support systems, etc.

IV. ACKNOWLEDGMENTSWe thank Sierra Pacific Power Company for the supportof the project. V. REFERENCES

[1] Annual Energy Review 1996, Published by En~Information &trn in istra t ion , JuIY1996.[2] Glover, J. endM. Sarma, Power System Aoalysis andDesi~PWS PublishingCompany, 1994.[3] J. Holtq Tulsewidth Modulation A survey, IEEE Trans.Ind. Elect., vol. 39,no. 5,1990, pp. 410-420.[4] A. M. Trzynadow sk i , Int roduc t ion to Mo&m power

Electronics, JohnWiley & Sons, Inc., 1998.[5] J. Holtz, P. Larnrn@ end W. Lotzl@, High-speed drivesystem with ultrasonic MOSFET PWM in,verterand single-chip microprocessor control, IEEE Trans. Ind. Appl., vol. IA-23, no. 6,1987, pp. 1010-1015.

[7] IEEE Recommended Pmetiees and Requirements forHarmonic Control in Electrical Power Systems, IEEE Std.519-1992.VI. BIOGRAPHIES

Mehdi Etezadi-Amoli received the BSEE in 1970. MSEE in 1972 andPh.D. degree in 1974 from New Mexico State University. From 1975-1979, he worked as so assistant professor of Electrical Engineering atNewMexico State andthe University of New Mexico. From 1979-1983,he worked as a Senior protection Engineer at Arizona Public ServiceCompany in PhoeniL AZ. In 1983, hejoined the fmlty ofthe ElectricalEngineering Depmtment et the University of Neva4 Reno where he isresponsible for the power system program. Hk present interest is inpower system protection, large-scale systernsj neural networks and fuzzycontrol applications. Dr. Etezadi is a Registered Professional Engineer inthe states ofNevada endNew Mexico.Kent N. Choma wasborn in kketoq Canadaj in 1967. Heobtainedhis B.S. degree in Electrical Engineering from the University of Alberiain 1990. He has worked for utility and consulting organizations in tbepower industry. He is presently working for Ektro-Test in Reno,Nevada and is a graduate student in Electrical Engineering et theUniversity of Nev~ Reno where his main focus is in power systems.Areas of interest include power system modeling and power apparatusmaintenance. Kent is a Registered professional Engineer in the State of

Nevada andthe Province of Alberta.

[6] A. M. Trzynad lowsk i , .4n O verview of M od em PWMTect ii q ue s f or T h re e-P ha se , Vol ta ge -Con t ro ll e4 Vol ta ge -Source Invertcrs Proceedings of the IEEE InternationalSymposium on Indust ri alE le ti on i cs , June 1996 .

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