Douglass, Philip James

You, Shi

English

Fine-grained under frequency load shedding called “demand as a frequency controlled reserve“ (DFCR) has been shown to be a promising method of providingfrequency regulation service from distributed loads [1]. Micro-grids with a large portion of intermittent renewable generation will benefit greatly from this technology because their low inertia. The paper proposes a operating procedure for utilizing DFCR loads for energy balancing, expanding DFCR’s well known role as a power balancing resource. The system operator can use DFCR for energy balancing by adjusting the frequency controller of generators to schedule off-nominal system frequency values. The feasibility of the proposed system is evaluated on an existing small island power system

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Direct Load Control by AC Frequency ModulationPhilip J. Douglass and Shi YouCentre for Electrical Technology, Technical University of DenmarkElektrovej-Building 325, 2800 Kgs. Lyngby, Denmark{pjdo,sy}@elektro.dtu.dkAbstractFine-grained under frequency load shedding called “demand as a frequencycontrolled reserve“ (DFCR) has been shown to be a promising method of providingfrequency regulation service from distributed loads [1]. Micro-grids with a large portionof intermittent renewable generation will beneﬁt greatly from this technology becausetheir low inertia.The paper proposes a operating procedure for utilizing DFCR loads for energybalancing, expanding DFCR’s well known role as a power balancing resource. Thesystem operator can use DFCR for energy balancing by adjusting the frequencycontroller of generators to schedule off-nominal system frequency values.The feasibility of the proposed system is evaluated on an existing small islandpower system.I IntroductionAs photovoltaic (PV) module prices continue to fall and diesel prices rise, thepenetration of PV generation is expected to increase in diesel fueled isolated powersystems. The amount of PV generation in an isolated system is limited by the minimumforecast daytime load in the absence of battery storage systems or curtailment. To providereliable electric power during periods of low renewable energy sources (RES) production,a dispatchable generator must have sufﬁcient capacity to serve the peak load. If thesystem contained dispatchable loads, they could be activated during periods of high RESproduction adding to the minimum load, and they could be deactivated during periods oflow RES production, lowering the capacity requirements of the dispatchable generator.When frequency responsive DFCR loads were ﬁeld tested in a small island powersystem, problems arose because the devices assumed the system’s mean value forfrequency was always the nominal frequency, but on the island, the system frequency wasobserved at off-nominal values for extended periods of time. It was clear from the sampledfrequency data that the system was operated in two distinct frequency regimes and thisobservation lead us to investigate whether these frequency regime changes could bedetected and utilized.Loads suitable for DFCR operation will typically have constraints on the energydemanded over a given time period, but some loads may have additional time constraintson their operation. These time constraints have precedence over the frequency response,and have the potential to lead to undesirable system behavior.Modulating AC system frequency has been used to curtail production in microgridsystems [2], and frequency responsive loads have been described in [1,3,4], but theauthors are unaware of previous work describing the modulation of frequency forcontrolling loads with time constraints.This paper is organized as follows. Section II describes the behavior of DFCR inmicro-grids. The proposed system operation concept is described in detail in Section III,followed in Section IV by a case study of the island of Christiansø in the Baltic Sea. Finally,Section V is a discussion and Section VI concludes the paper.II DFCR loads in Micro-gridsA micro-grid is modeled with 4 lumped entities: dispatchable generators with droopfrequency controllers, stochastic RES generators (PV modules), DFCR load and ﬁnallyresidual (high priority) load, see Figure 1.Typical automatically controlled loads (such as thermostat controlled loads) operatein two states, OFF and ON, with the duty cycle controlled to keep the energy level withindesired bounds. A DFCR controller will alter the phase of the duty cycle to defer energyconsumption away from time periods when the grid is overloaded. The only way to createLow Priority CircuitHigh Priority CircuitLow Priority Power MeterHigh Priority Power MeterDFCROn / Off f schedulerDroop controllerff++f offset=PV  modulesFigure 1: Oneline diagram showing relation between generator governor and DFCR. Solidlines represent electric conductors, dotted lines represent data transfer paths.a smooth and reliable frequency response with OFF/ON loads is to have a population ofdevices with a diversifying parameter such as differing frequency thresholds. A smallsynchronous system will have a small population of DFCR loads, where each one may bea signiﬁcant portion of the total load. Therefore, the diversity of a large population can notbe counted on.Energy constraints for loads are represented by the maximum time the device isallowed to be disconnected from the electric energy supply (tmax_off), and the minimumtime it must remain reconnected after being disconnected (tmin_on). Processes with startupcosts and shutdown costs, such as an induction motor’s inrush current, will have additionalconstraints besides a minimum demand for electric energy over a given time period. Thestartup and shutdown costs are represented by the minimum ON time (tmin_on) and theminimum OFF time (tmin_off).DFCR with time constraints is suitable to act as disturbance reserve, reducing loadduring large and infrequent drops in frequency, but is unsuitable to act as a continuousregulation resource within the normal frequency operating range. The problem with DFCRas a continuous regulation resources is acute both when RES production is high and theload low, and when RES production is low and the load is high.When the RES production is more than the residual load, the droop controlleddispatchable generator cannot further reduce production, and the DFCR load is critical toabsorbing the RES energy over-production. If the DFCR load is constrained by tmin_off,frequency will rise uncontrollably in the absence of alternative frequency regulationresources (i.e. dump load).When RES production is low, the tmin_on constraint of DFCR can compromise thesecurity of the system. In a scenario where the dispatchable generators do not have thecapacity to supply the DFCR and the residual load simultaneously, DFCR load constrainedby tmin_on can cause the system frequency to decline uncontrollably (until axillaryfrequency regulation resources such as shedding of high priority loads are activated), seeFigure 2.III Dispatch of DFCR loadsThe control of distributed energy resources is typically classiﬁed as either direct orindirect [5]. Direct control corresponds to the architecture of existing utility SCADAsystems where commands are issued from a centralized controller and remote terminalsrespond to the commands in a deterministic manner. Frequency controlled reserves arenot usually considered as directly controlled because the control signal (system frequency)is not actively generated, but rather results from the intrinsic behavior of the power system.Dispatchable generators participating in frequency regulation are able to alter the systemfrequency by altering the parameters of their frequency controller, see Figure 4. Whenfrequency measurements are averaged over long time periods to ﬁlter out the intrinsicvolatility of the system power balance, changes in the generators’ frequency regulationparameters can be detected by DFCR loads.A droop controller will produce power proportional to the system frequency, withlower frequencies resulting in higher power output. Thus, at steady-state conditions, thefrequency will indicate the amount of load on the droop controlled generator. In thepresence of limited generation resources and a signiﬁcant amount of frequency dependentdemand, lowering the system frequency can be used as a proactive measure to balanceload and generation by selective load shedding. Changing the system frequency target onsome frequency regulation resources, but not others reallocates the energy productionbetween the two groups of resources (ie. supply-side and demand-side).The DFCR algorithm for relay-based devices described in [1] has a frequencythreshold (foff) below which the load is disconnected. Loads are reconnected when0 20 40 60 80 100 1204949.249.449.649.85050.250.450.650.851t [s]P [pu]0 20 40 60 80 100 12000.20.40.60.81t [s]P [pu]Figure 2: DFCR with tmin_on constraints dur-ing a transient spike in RES output. Top graphshows frequency with the black horizontal linefrestore. The bottom graphs show loads (mi-nus RES production). The solid green linerepresents the residual load, the blue dashedline shows the addition of DFCR loads.0 50 100 150 200 250 300 350 40048.648.84949.249.449.649.85050.250.450.6t [s]f [Hz]0 50 100 150 200 250 300 350 40000.20.40.60.81t [s]P [pu]Figure 3: The top graph shows the systemfrequency (dark blue) and the frequency sub-jected to a low pass ﬁlter (light blue). frestorefor the ﬁltered frequency is the solid blackline. The system frequency does not stayabove the threshold long enough for the ﬁl-tered value to cross the reconnect value, sothe DFCR remains off.system frequency is above a second, higher threshold frequency (frestore), creating asimple deadband. A wide enough deadband would avoid spurious changes to DFCRstate, but this has the disadvantage that when the system resides inside the deadband theDFCR state is uncertain because it depends on historical frequency deviations rather thanthe current frequency.To narrow the deadband while avoid spurious switching DFCR state, the DFCR canpass the frequency samples through a low pass ﬁlter before comparing the value to presetthresholds. A moving average or exponentially-weighted moving average ﬁlter could beutilized.A similar algorithm was described in [6], where different cutoff frequencies existed forshort-term frequency deviations (i.e. 1s) and longer-term deviations (i.e. 5 min.). Thisalgorithm is well suited for the proposed scheme because a small but consistent deviationfrom nominal frequency on a long time scale will force devices into either the ON or OFFstate. Changes to the long-term average frequency are feasible to provoke, even when thefrequency varies widely in the short-term.Figure 3 shows how DFCR with a low pass ﬁlter will not change state when subjectto steep ramps of RES generation.f [Hz]P [pu]Load_ref1DFCRGenerator Droop Controllerf_restore f_offDFCR Deadbandf_DFCR_onf [Hz]P [pu]1f_restore f_off f_DFCR_offFigure 4: Left: DFCR devices dispatched in ON state by allocating droop controlled re-serves to well above foff. Right: DFCR devices dispatched in OFF state by operatingdroop controlled reserves well below frestore.IV Case Study: ChristiansøChristiansø is a decommissioned naval base which is a popular tourist destination.Around 100 people live permanently on the 0.22km2 island. Their electric power system iscomposed of 4 diesel powered generators, two with a rating of 180kW, one 130kW andone 60kW. At the moment, there is no renewable electricity generation, though the windand solar resources are available.Two bottle cooling refrigerators with DFCR functionality have been installed as partof an ongoing experiment [7]. Frequency measurements were collected when the DFCRfunctionality was disabled to characterize the system.Two modes of operation were observed, and within each mode, the frequencymeasurements ﬁt well into a normal distribution, with the average value of 50.00Hz forgroup one and 50.11Hz for group two. The standard deviation for group one was 43.4mHz,for group two 137.3mHz. The moving average of high resolution frequency measurementsfrom group two was found and the standard deviation of these ﬁltered values was 48mHz.The normal distribution of frequency samples implies that 99.9999% of the time thevalue will be within ±5σ of the mean. Conversely, the system frequency will be farther than±5σ from the mean approximately 50 milliseconds per day. If the generators were set toproduce at fDFCR_on = frestore + σ = foff + 5σ, then the ﬁltered frequency would beabove frestore 84% of the time, and below foff 25ms per day.With a deadband 4σ = 200mHz, then the average frequency to dispatch DFCR inthe ON state with high conﬁdence would be fDFCR_on = 50 150Hz. To be highly certainthat the DFCR devices are OFF, the average frequency would have to befDFCR_off = 49 850Hz. This range of frequency offsets is slightly larger that that alreadyobserved during normal operation on Christiansø, but considered feasible withoutsigniﬁcantly effecting the functioning of conventional loads. Alternatively, if fDFCR_on = fo,the fDFCR_off = fo − 6σ = 49 700Hz.The EN 50160 standard [8] used in Europe requires that island power systemmaintain their frequency at 50Hz±%2 (49Hz to 51Hz) for 95% of a week and 50Hz±15%(42 5Hz to 57 5Hz) for 100% of the time. A mean value of 49.700Hz gives a margin to thestatutory minimum frequency of 700mHz, which is acceptable during unfaulted operation.Figure 5: Relative placement of DFCR fre-quency thresholds and generator setpoints.Parameter Relative Value Valuefo 50Hzσ 50mHzfoff fo − 2σ 49.9Hzfrestore fo + 2σ 50.1HzfDFCR_on frestore + σ 50.150HzfDFCR_off foff − σ 49.850HzFigure 6: Suggested parameter values forDFCR operation on Christiansø with thresh-olds symmetric to foV DiscussionThe difference between target frequency values would have to be larger than thetypical variation of frequency (i.e. > 5σ) but if disturbances pushed the system frequencyinto a neighboring frequency band, the autonomous action of the DFCR would act torestore the frequency to the targeted value. The utility’s load control performance woulddepend on how closely the autonomous action of the DFCR conformed to pre-establishedschedules. By designing the DFCR threshold levels and target frequencies, arbitrary levelsof load control performance can be achieved. Metrics for measuring frequency qualitywould be redeﬁned as deviations from the target frequency, not deviations from nominal.VI ConclusionThe paper has proposed a new operating concept which utilizes DFCR, a highlydistributed under frequency load shedding method, as part of direct load control scheme.Frequency sensitive loads are dispatched by adjusting the generators’ frequency controllerto target off-nominal frequencies. The feasibility of this concept was demonstrated bysimulations, and by analyzing data collected from an operating small island power system.The analysis shows that DFCR loads can be dispatched with high reliability withoutendangering the system’s ability to remain within the range of acceptable frequencies.References[1] Zhao Xu, J. Østergaard, and M. Togeby. Demand as frequency controlled reserve.Power Systems, IEEE Transactions on, 26(3):1062 –1071, aug. 2011.[2] Steca Elektronik GmbH. Steca RCC-02 Operating Instructions. Onlinehttp://www.stecasolar.com/index.php?Steca_Xtender_Zubehoer_en. AccessedFebruary 9, 2012.[3] J.A. Short, D.G. Inﬁeld, and L.L. Freris. Stabilization of grid frequency through dynamicdemand control. Power Systems, IEEE Transactions on, 22(3):1284 –1293, aug. 2007.[4] David P. Chassin, Matthew K. Donnelly, and Jeffery E. Dagle. Electrical powerdistribution control methods, electrical energy demand monitoring methods, and powermanagement devices, 2011. Patent US 8073573.[5] D.S. Callaway and I.A. Hiskens. Achieving controllability of electric loads. Proceedingsof the IEEE, 99(1):184 –199, jan. 2011.[6] A. Molina-Garcianda, F. Bouffard, and D.S. Kirschen. Decentralized demand-sidecontribution to primary frequency control. Power Systems, IEEE Transactions on,26(1):411 –419, feb. 2011.[7] P. Douglass, R. Garcia-Valle, P. Nyeng, J. Østergaard, and M. Togeby. Demand asFrequency Controlled Reserve: Implementation and practical demonstration.Innovative Smart Grid Technologies Europe, 2011.[8] Standard. EN 50160: Voltage characteristics of electricity supplied by public electricitynetworks, 2010.

Direct Load Control by AC Frequency Modulation

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Direct Load Control by AC Frequency Modulation

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