Load Modeling EEE 576
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This 57 page Class Notes was uploaded by Shammya Saha on Sunday April 3, 2016. The Class Notes belongs to EEE 576 at Arizona State University taught by Dr. Vittal in Fall 2015. Since its upload, it has received 48 views. For similar materials see Power System Dynamics in Electrical Engineering at Arizona State University.
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Date Created: 04/03/16
EEE 576 Spring 2016 Power System Dynamics Lecture 14 & 15 1 Load Representation for Dynamic Performance Simulation 1)Load Performance for Dynamic Performance Analysis, IEEE Committee Report, IEEE Trans. on Power Systems, V ol. 8, No. 2, pp. 472-482, May 1993. 2)Bibliography on Load Models for Power Flow and Dynamic Performance Simulation, V ol. 10, No. 1, pp. 523-538, Feb. 1995. 3)Standard Load Models for Power Flow and Dynamic Performance Simulation, IEEE Trans. on Power Systems, V ol. 10, No. 2, pp. 1302-1313,Aug. 1995. 2 The representation of loads has not received much attention. Load representation can have significant impact on analysis results. 3 Accurate load modeling is difficult because of the following factors: • Large number of diverse load components • Ownership and location of load devices not directly accessible to utility • Load composition changes with time of day and week, seasons, weather, and through time • No precise information on load composition • Load characteristics uncertain – particularly for large voltage or frequency variations 4 Basic Concepts Substation Transformer Given the description shown in the figure above, the load is a portion of the system that is not explicitly represented in the system mode, but rather is treated as if it were a single power-consuming device connected to a Bus in the system model 5 Therefore in this context “load” includes – Connected load devices together with some or all of the following: - Substation step-down transformers - Subtransmission feeders - Primary distribution feeders - Distributions transformers - Secondary distribution feeders - Shunt capacitors - Voltage regulators - Customer wiring, transformers and capacitors 6 If we want to represent the load accurately the elements listed above must be accounted for. To a large extent this depends on what is and is not represented in the system model. For bulk power system studies much of the subtransmission as well as the distribution system are omitted. 7 In describing the composition of the load, the following nomenclature is adopted. 8 LoadComposition-Fractionalcomposition of theload byload components-couldbefractionapplied to Busload or toloadclass. LoadClassMix -Fractionalcomposition of the Busload by loadclass. Loadcharacteristics-Aset of parameterssuch as pf ,P etc.that V describesbehaviorof a specifiedload.This couldbe applied toa specificloaddevice,a loadcomponent, a loadclassor the totalBusload. 9 Load Model-Represents the relationship between P&Q of theloadand Bus voltagemagnitudeand frequency-staticand dynamic Static Models- P&Q at t expressedasfunctionsof Bus voltagemagand freqat t.Used to represent staticloadslikeresistiveand lighting load,andas an approximation todynamicloadslikemotors. 10 DynamicLoadModel-Expresses P&Q at a given timet asa function of voltagemagnitudeand frequencyat past instantsin timeandatt.Usuallyrepresented by differenceor differentialequations. Typesof Static LoadModels 1)Constant Z- P&Q vary with thesquare voltage of magnitude 2)Constant I- P&Q varydirectly with voltagemagnitude 3)Constant Power or Constant MVA Load- P&Q do not vary with changein voltagemagnitude.Could alsobe a constant MVA model 11 Polynomial Load Model This is referred to as the ZIP Model Power consumed at rated voltage Rated voltage In manystabilitystudies when this modelis used to representloadat a Bus P ,Q ,a0dV a0e the va0ues at theinitialoperatingcondition. 12 13 For an aluminumreduction plant 2.5 2.7 V V P jQ P 0 V 0 jQ V0 Bus Frequency Frequency of the Bus voltage. This is not a variable inherent to any network analysis calculation. It is computed in simulation programs by taking the numerical derivative of the voltage phase angle. 14 Consumption of feedersand transformers minus shunt capacitance,includedin the"Busload"to give thespecifiedinitial reactiveflow Q at the Bus. 0 15 Static Model Used in ETMSP 16 Induction Motor Model Most stability programs have an induction motor model based on the following equation criteria s=Freq of Bus voltage – motor speed Some programs contain additional features: Rotor circuits, saturation, low voltage tripping, and variable rotor resistance. See Kundur’s book pg.279 for details 17 There is a definite need to improve load representation. If present load modeling is pessimistic – improved modeling will differ modifications and additions and operating limits will be increased. If present load modeling produces overly optimistic results – improved modeling will avoid system inadequacies and will prevent system emergencies. Poor modeling may also produce results that miss significant phenomena. 18 Load Modeling Considerations First Swing StabilityAnalysis • System voltages depressed in the first angular swing after the fault. • The power consumed by loads will affect the generator-load imbalance and hence angular excursion • Load response to voltage variation is important • There is some frequency excursion during transient • Frequency characteristics of loads close to accelerating and decelerating generator should be represented. 19 Small-Signal Damping Studies • Low frequency oscillations involving inter-area modes cause wide excursions in voltage and frequency. • Frequency variations are greatest at machines with high participation in the critical mode. • Voltage variations tends to be greatest at the electrical center between groups of machines swinging out-of-step 20 Generation-Load Imbalance • Frequency is the key variable of interest • Response of load components to frequency decay is important • For low frequency decay rates – loads follow long term voltage and frequency characteristics • For high frequency decay rates – load inertia and time constants of motors play a big role 21 Induction Motor Stability Important to know if motor will reaccelerate or stall after fault clearing. Modeling motor dynamics and parameters including contactor holding characteristicis important. Cold Load Pickup In these studies all customer load characteristics are important – thermostats, loads added when feeder is open. Motor starting characteristic, distribution LTC’s. 22 Voltage StabilityAnalysis This is a long term phenomenon. Static load model is not sufficient. Dynamic load models are important. LTC’s should be modeled. Characteristics of loads at very low voltage should be represented. Dynamic Overvoltages Usually occur following sudden loss of line or loading, blocking of HVDC converters, load rejection. Load response to voltage and saturation. Characteristicsof load devices should be modeled. 23 Load Characterization • Loads with fast dynamics – both mechanical and electrical – induction motors, adjustable speed drives. • Loads whose response to voltage excursions exhibit discontinuities - Discharge lighting - Adjustable speed drives that shut down at low voltages - Motor contactors that open during faults and voltage swings removing load - Motor overload protection that removes stalled motors after about 10s 1 • Loads whose response to voltage excursions does not exhibit discontinuities or time lags. - Very small motors - Incandescent lighting - Uncontrolled resistive loads • Load with slow dynamic characteristics - Loads controlled by thermostats - Manually controlled loads that are initially const resistance but change to const power over a 10-20min period after a change in voltage 2 Some Specific Load Characteristics to Note Motors • Inertia and rotor flux determine motor’s active and reactive power response to voltage and frequency swings. • Reaccelerating motors near generators improve stability • Reaccelerating motors in weak areas pull voltage down- reduces synchronizing power degrades stability • For 2300-4000V motors – contactors drop at 65-75% of rated voltage • For 460V or lower motors – contactors drop at 55- 65% of rated voltage 3 - Motors could drop during the fault or during voltage excursions following the fault – at sending end generators this will lower stability as lost loads will increase acceleration. At receiving end generators this will improve stability – 70% of system load Air Conditioners They fall into the same category as motors - They do not have contactors. Hence, they do not drop at low voltages. - Because of low inertia they slow significantly during faults 4 - As a result when the fault is cleared they impose large, low power factor “starting currents.” - For weak networks when air conditioner motors reaccelerate after fault simultaneously, voltages become depressed and motors speeds will decay. - At low voltages overload protection operates slowly. - During summers this could be up to 50% of system load 5 Discharge Lighting - Mercury vapor, sodium vapor, fluorescent lamps, etc. - About 20% of the load in commercial areas - Extinguishes at 80% of rated voltage – this causes load to drop following fault clearing. V oltage excursion is not as severe and improves stability near electrical center - At sending end it has the opposite effect 6 7 Incandescent Lighting -Assumed to haveconstant resistancecharacterisitcs - When voltage changes largetemperature swings occur asa result V P P 0V 0 Thermostat-Controlled Loads Space heaters, molding and packaging machines, soldering machines, water heaters – close to constant resistance characteristic in short term – This means the loads operate in a temperature range where changes in power input does not change temperature enough to 8 However, just a few seconds after a drop in voltage, the heat output from these devices drops – this is sensed by thermostats and the “on” cycle of the thermostat is extended. Thermostats in the “off” cycle when voltage changes will not respond until they enter the “on” cycle. 9 Short term – constant resistance Long term – constant resistance → constant power with a 10 Manually Controlled Loads Similar to thermostat-controlled loads. Cooking on an electric range.A soup pot will be left to simmer until the soup boils. If voltage is low, the soup will remain longer on the range. Electronic Power Supplies Draw constant dc output for voltage down to about 90% of normal rating – constant power. Below this voltage they stop functioning even though some will continue to draw power. 11 Adjustable Speed Drive Are like computer and electronic equipment. Shutdown occurs when voltage drops below 90% of normal voltage.Act like constant power loads. LTCs These should be modeled for long term voltage studies.August 1995 paper has a detailed model. 12 13 Voltage/freq dependent terms Nominal loadpowers P0and Q l0nearly reducedto zerostarting at a specifiedthreshold voltage(V ,a1);a2e powersarezerofor voltagebelowa second threshold voltage(V ,V ) b1 b2 Nominal loadpoweris ramped up for voltage recovery 14 15 16 17 18 19 20 21 22 We will now examine the various models suggested in the IEEE bibliography and see what specific aspects of the issues we had discussed earlier are addressed by each model. 23 24 25 26 27 28 29 30 31 32 33 34