09 DGModelingGuidelines Revised

8/6/2019 09 DGModelingGuidelines Revised

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Section 9Distributed GenerationModeling Guidelines


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Modeling Versus Screening

l For distributed generation to be widely successful, acomplete modeling study of each application is notpractical.

l

A simpler screening approach is desirable that wouldrequire:

Basic parameters of the interconnection point (voltage,system impedance, load connected in the nearestpossible island, protection scheme, and information onother DR on the circuit) and,

The DR unit (type of unit, rated kVA, power factor,and interconnection transformer configuration.l Detail modeling should be done after an initial screening

shows possibility for potential interactions with distributionsystem.


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IEEE 1547 Screening Tools

l Stiffness Ratio Calculation

l Penetration Ratio Calculation


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Penetration Factor

lPenetration factor

Aggregate DR rating on an EPS feeder in kVA

Peak EPS feeder load in kVA

Low penetration is described by a penetration factor of less than orequal to 0.30. High penetration is described by a penetration factor of

greater than 0.30


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Sample Calculation Penetration Ratio

Assume a feeder with 6 MVA of peak demand load. A customer

wants to install a synchronous generator with a rating of 1 MVA.

There is currently a generator on the feeder with a rating of 1.5

MVA.

The Penetration Factor is:

1 + 1.5

6

= .42

Therefore the installation of the new generator would result in a

high penetration ratio.


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Stiffness Ratio

l Stf = (SC kVA (area EPS) + SC kVA (DR) ) SC kVA (DR)

Where: Stf = Stiffness ratio SC kVA (Area EPS) = the short circuit

contribution in kVA of the area EPS, and SC kVA (DR) = the short circuit contribution

in kVA of the DRNote: The stiffness ratio is calculated at the PCC, except when there is atransformer(s) dedicated to one customer in which case the stiffness ratio iscalculated on the high-voltage side of the dedicated transformer(s).


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Sample Calculation Stiffness Ratio

Assume a 12.47 kV feeder with available fault power of 43200 kVA

at the PCC. A 1 MVA synchronous generator is proposed with a

3000 kVA fault availability.

Stf= (43200 + 3000) / 3000 = 15.4

We will discuss what this means in the following slides.


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General Guidelines for Stiffness Ratio

l An important part of the screening and classificationprocess is to determine the relative stiffness of the utilitysystem at the point of DR interconnection.

l The term stiffness applied here means the ratio of the

utility systems available fault current at the DR location tothe maximum rated output current of the DR unit(Stiffness Ratio = Ifault/I DR-output).

l Experience shows that if this ratio is large (>200), it ishighly unlikely that any problems caused by the DR

related to voltage flicker, steady-state voltage regulation,or harmonics will occur on the utility system


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General Guidelines for Stiffness Ratio

l On the other hand, if the stiffness ratio is small(


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Application of Stiffness Ratio forScreening as per IEEE 1547

l Disconnection for Faults

DR applications with a Stiffness Ratio of10 or less shall beequipped with current-based protective devices and ground faultprotection suitable for the detection of Area EPS faults.

l

Loss of Synchronism Synchronous Generator DRs in applications with a stiffness ratioof 20 or less shall be equipped with protective functions suitablefor detecting loss of synchronism (out-of-step, pole slipping) andinstantaneously (without any intentional time delay) isolating theDR from the area EPS.

With a Stiffness Ratio at the PCC of less than 20, then, at thetime of connection, the frequency difference shall be less than 0.2Hz, the voltage difference shall be less than 10%, and the phaseangle difference shall be less than 10 degrees


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General Analytical Models for DR

l Advantage

Does not require detail modeling parametersfor DR

In essence DR is modeled the same way as ageneration bus with constraints (voltage,power etc.)

Suitable for steady state analysis such asregulation, unbalance, voltage drop, etc.

l Disadvantage Cannot be used for transient or stability

modeling


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Generic Power System Tools for DRModeling

Product Company Contact

CH-Analysis forElectric

Cook Hulbert, Inc. www.cook-hurlbert.com

CYMEDist CYME www.cyme.com

DESS Dromey Design www.dromeydesign.com

DEWorkstation EPRI www.epri.com

DistriView Aspen, Inc. www.aspeninc.com

EDSA EDSA www.edsa.com

FeederAll ABB www.abb.com

WindMill Milsoft, Inc. www.milsoft.com

PSS/Adept Power Technologies, Inc. www.pti-us.com

SynerGEE Electric Stoner Associates www.stoner.com

V-Flow Cooper Power Systems www.cooperpowersystems.com


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Analytical Models for DR

l Circuit models of DRs to be considered are

Voltage Sources

Voltage-Dependent Current Sources.


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Voltage Source Model

l A 3-phase voltage source model may beconsidered to model a generic DR.

l

Here the DR may be viewed as a stiff voltagesource, and thus by varying the DR voltagemagnitude and/or phase angle the power flows inthe circuit may be controlled.

l It is believed that this model should only be used

to represent DRs for very small variations involtage magnitude and phase angle.


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Application of Voltage Source Model

l For the 3-phase voltage source model, a circuit withbalanced loads but unbalanced impedances can be usedto study the per phase power supplied by the DR.

l This model may be applied to any DR that has the ability

to affect its terminal voltage.l The circuit will be designed so that when the DR voltage

is equal to the substation voltage, on a per phase basis,power flow conditions in the circuit are essentiallybalanced.

l Then, as the voltage magnitude or angle of the DR are

varied slightly away from the substation levels, theresulting power flows and unbalances in the circuit will beobserved.


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Voltage Dependant Current Source Model

l A voltage-dependent, current source model applies tomulti-phase (I.e, either single-phase or 3-phase)synchronous generators.

l In this model the current supplied by each phase of thegenerator is a function of the phase terminal voltage, andthe terminal voltages of the machine are not necessarilybalanced (due to unbalances in the distribution circuit).

l With unbalanced terminal voltages the currents suppliedby the machine will be unbalanced.

l For the voltage-dependent current source model, voltage

magnitude control, with a power factor override, will beused to control the output of the machine. Via this controlthe machine may affect the terminal voltages.


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Modeling DR for Steady State Analysis

l The simplest model of DR would be as anegative load and this can be used for manytypes of DR as a first approximation.

l Negative loads may not be allowed in somecomputer programs.

l The normal load models include constant power,constant impedance, and constant current.

l

The constant-power load is most appropriate formost types of DR.


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Load-Flow Analysis

l The load-flow analysis is important for analyzingthe voltage effects of DR. The analysis will beused primarily for:

Voltage drop (or rise) caused by the DR unit. Line losses caused (or reduced).

Line and equipment loading.


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Short Circuit Modeling

l Short-circuit analysis can be done using astandard short-circuit program.

l Although the addition of distributed resources will

increase the time varying nature of the faultcurrent, it is not enough to warrant using astability or EMTP-type model for fault analysis.

l Industrial systems with large local generatorscan still be analyzed properly with standard short-circuit analysis programs.


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DG Modeling for Protection Coordination

l When considering the effects on protectioncoordination with DG units, standard protectionprograms can be used.

l With fault contributions from only one source,curves of two overcurrent devices can be plottedon a time-current curve that will demonstratevisually the coordination over a range of currents.

l With contributions from different sources, it ismore difficult to plot curves in a time-currentgraph. It is more useful to do a table of operatingtimes of different devices for faults at differentlocations.


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Short Circuit and Protection Studies

l It is important that the DG transformer connection beproperly modeled. If a grounding transformerinterconnection is used, its effects on line-to-ground shortcircuits will be significant. Therefore, it must be included

in the modeling.l The short circuit contribution of DGs is also important.

Synchronous generators will supply the most current. Thesub-transient reactances are usually between 0.1 and 0.3per unit on the machine rating (so the fault current

contribution would be between 3.3 and 10 times rated full-load current).

l The fault-current contribution decays within a few cyclesto 2 to 5 times rated full-load current


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Short Circuit Contribution

l Induction generators can also feed fault current. Thelocked rotor impedance determines the short-circuitcontribution (typically 0.2 per unit )

l Short-circuit contributions of inverter-based distributed

generators vary by inverter design. In many cases, theinverter controls will act within milliseconds of an overloadto protect the inverter electronics.

l In some cases, an inverter can output two to four timesfull load output for several cycles. The duration of fault

current output also depends on the amount ofcapacitance on the inverter DC bus.


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Modeling Synchronous Generators

l Most distributed synchronous generators use a voltagefollowing control mode.

l This is different from large generators on the electricsystem with controls to regulate voltage (within the limits

of the VAR capability).l Distributed generators normally follow the utility voltageand inject a constant amount of real and reactive power(so the power factor stays constant).

l Given this, the most accurate model is the constant powermodel.

l For induction generators, the reactive power will changewith the voltage, but the changes are small enough thatthey can be modeled as constant power devices.


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Synchronous Generator in VoltageRegulating Mode

l If a synchronous generator is operated in avoltage-regulating mode, the best load-flowmodel is as a voltage-regulating, constant real

power device (the PV bus in transmissionplanning load flow programs). The reactive powerlimits would also need to be specified.


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Synchronous Machine Parameters

l Typical constants for a synchronous generator rangewidely depending on construction.

l The subtransient reactance that is primarily used for faultcalculation (the positive-sequence reactance, X1=Xd"), is

between 10 and 30 percent on the machine rating with 20percent being typical.

l Values as low as 8 percent have been found on smallermachines with oversized windings.

l The X2 is equal to the average of Xd" and Xq" and istypically 1.2 to 1.4 times the value of X1.

l However, it is common practice to assume X2 is equal toX1 if actual data is not available. The zero-sequencereactance, X0, is usually 3 to 7 percent on the machinerating with 5 percent being typical.


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Synchronous Machine Parameters

l The voltage behind sub-transient reactance, E",is normally in the range of 1.0 to 1.3 per unit atunity power factor to 0.8 power factor lagging

with a typical value of 1.1.l The actual value can be calculated from the

generator phasor diagram, but using 1.1 per unitis a practical method for a small generator

without a full-scale test of the generator.


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LV Synchronous Machine Data EPRI TR1000418


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LV Synchronous Machine Data EPRI TR1000418


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MV Synchronous Machine Data EPRI TR1000418


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Induction Machine Data EPRI TR 1000419Vrate

d

HP kVA Xs Rs Xr Rr Xm H RPM SLIP

460V 50 50 0.185 0.022 0.185 0.022 2.65 0.085 1800 .0222

460V 76 73 0.155 0.025 0.164 0.014 2.50 0.108 1800 .0133

460V 125 115 0.147 0.022 0.147 0.014 3.33 0.134 1800 .0144

460V 200 180 0.131 0.021 0.131 0.013 3.37 0.179 1800 .0133

208V 75 65 0.133 0.007 0.133 0.027 4.30 1800 .0278

480V 123 101 0.141 0.098 0.138 0.015 3.56 0.140 1800 .0133

460V 400 335 0.147 0.007 0.147 0.011 3.56 0.205 1200 .0167

480V 202 201 0.056 0.010 0.061 0.018 1.83 0.168 900 .0144

480V 670 624 0.060 0.010 0.066 0.017 2.23 0.162 900 .0156

2400

V

938 873 0.111 0.010 0.130 0.013 2.76 0.198 900 .0122

2400

V

1340 1255 0.098 0.010 0.120 0.012 2.48 0.176 900 .0111


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Inverter Model for Steady StateAnalysis

l Forced-commutated inverters (for microturbines,fuel cells, photovoltaics, etc.), such as thoseusing pulse-width modulation, can also bemodeled using a constant power model.

l The inverters have control loops that control bothreal and reactive power. Line-commutatedinverters can also be modeled as a constantpower.

l They are limited in that they can only absorbreactive power and cannot supply VARs to thesystem.


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Harmonic Modeling for DG

l There are several analytical techniques used forexamining harmonics on power systems :

Frequency Scans:

At a point on the network where a harmonic source would be

located, a 1.0 per unit current is injected over a range offrequencies.

The voltages and currents at different points in the networkcan then be plotted as a function of frequency.

The most useful is the voltage at the point of the injection;this is the driving point impedance (Z=V/I=V/1.0=V).

This is useful for identifying system resonances that appear aspeaks in the impedance scan.


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Current Source Model

l Current Source Model:

To model the effect of one or more harmonicsources, the most common modelingtechnique is to model all harmonic loads asconstant-current sources.

The network voltages and currents are solvedat each frequency that has harmonic sources(by solving I=YV where Y is the system

admittance matrix and I contains the currentinjections at each node--note that Y will alsochange with frequency).


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Harmonic Power Flow Model

l Harmonic Power Flow: One of the disadvantages of the current source model is that a

constant current source does not accurately model the behaviorof nonlinear loads under all conditions.

One case where the error is significant is where converters areoperating under heavy voltage distortion.

Another case that is significant is where multiple harmonicsources are modeled on a distributed system (the phase angles ofthe current sources will vary depending on the voltage angle ateach harmonic source).

To achieve a more accurate solution, more accurate nonlinearload models are used that are a function of the voltagemagnitude and angle (and possibly other parameters as well).

The system with the models is iteratively solved using load-flowsolution techniques. The Harmflo program is the most widelyused example of this. The main disadvantage of these methods isthat the load modeling is more complicated.


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Time Domain Analysis for HarmonicModeling of DG

l Time-Domain Analysis:

Another technique to achieve high accuracy of theperformance of nonlinear loads is to use a time-domain analysis most commonly done using anelectromagnetic transients program (EMTP).

A transient simulation of the system is run until theinitial simulation starting transients have died out andthe system reaches steady state.

The harmonics of the voltage and current waves canbe examined with standard FFT tools.

The advantage of this method is the capability tomodel virtually any scenario. The main difficulty is that it takes much more time to

model the system and the harmonic loads, and theanalysis takes more time to perform.


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Comparing Harmonic ModelingMethods

l Of the different methods, the frequency scans and currentsource model are the most commonly used.

l The current source model is thought to be accurateenough for most applications. Note that some of the

available programs only model balanced three-phaseharmonics.

l For distribution applications, and especially for analyzingmultiple single-phase DR applications, capability ofmodeling all three phases independently is important.

l

For distribution modeling, it is important to include systemcapacitor banks as they will greatly affect resonancepoints.


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Modeling of Generators for HarmonicStudies

l Another consideration of DR is how to model thegenerators since some DR will also be a sink ofharmonics.

l Synchronous generators are normally modeled as theirnegative-sequence reactance, X2, which can be

approximated by (Xd+Xq)/2.l Induction generators can be modeled as the locked rotor

inductance that is generally about 20% on the machinebase.

l Inverter modeling is difficult since it is an active source

that can correct the voltage waveform.l Data from manufacturers should be requested for

calculating voltage distortion on inverter output due toharmonic current.


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Transient Analysis

Origin Frequency Range (Typical)

Transformer energization and ferroresonance (DC) 0.1 Hz - 1 kHz

Load rejection 0.1 Hz - 3 kHz

Capacitor switching 50/60 Hz - 3 kHz

Fault clearing 50/60 Hz - 3 kHz

Fault initiation 50/60 Hz - 20 kHz

Line energization 50/60 Hz - 20 kHz

Line reclosing (DC) 50/60 Hz - 20 kHz

Transient recovery voltage 50/60 Hz - 100 kHz

Breaker restrikes 10 kHz - 1 MHz

Lightning surges 10 kHz - 3 MHz

The study of electrical transient phenomena involves a frequency range from DC to

about 60 MHz. Generally, the frequency of the resulting disturbance will fall

somewhere in the above range


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Reasons for Transient Modeling

l Analyzing the effect of DR overvoltages, such asferroresonance or neutral shift, and the effect of theseovervoltages on equipment, such as arresters.

l Evaluating the effectiveness of anti-islanding protectionthrough detailed islanding scenarios.

l Evaluating the surge protection against lightning surges.

l Converters waveform notching or high frequency noise.

l Evaluation of voltages during out-of-phase reclosing with

DR (e.g., for a failure investigation).l Converter harmonics and non-integer harmonics.


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Time Step Selection for TransientModeling

l Transients are generally modeled in a time-domainelectromagnetic transients program (generically referredto as EMTP). Selection of the time step is one of the mostimportant decisions an EMTP user must make.

l Improper time step selection is analogous to selecting amonitoring device that samples a waveform at a slowenough rate that the transient of interest is partially orcompletely missed.

l There needs to be a balance between computational

effort and accuracy when the time step is selected. Theselection, much like the modeling question, is more of afeel then a hard and fast rule.


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Guidelines for Time Step Selection

Obtain shortest travel time for lines and cables:

l=

10

t

Determine period of oscillation for each LC loop:

LCf

T 21==

10

Tt

Find RC and L/R time constants for lumped elements:

RCt


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Maximum Simulation Time forTransient Analysis

l At least one cycle of pre-transient power frequencyvoltage for slow to medium frequency events. (Severalcycles of post transient may also be desirable for viewingthe event.)

l At least 10 to 20 cycles of dominate transient frequency(to observe damping rates and to ensure that resonancesdo not occur).

l One to five seconds for machine dynamics.

l Sufficient time for one transient event to decay beforebeginning another.

For example, in reality, a pre-insertion contact maytake 10 cycles to close. However, if the transientwaveform has decayed in three cycles, there would beno reason to delay the second event the full 10 cycles.


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Modeling Tools for Detail Analysis

Product Organization Contact

ATP ATP North American Users Group www.ee.mtu.edu/atp

EMTP96 EPRI www.epri.com

PSCAD/EMTDC Manitoba HVDC ResearchCentre,Inc.

www.ee.umanitoba.ca/~hvdc

Harmflo EPRI www.epri.com

PSS/E Power Technologies, Inc. www.pti-us.com

ETMSP EPRI www.epri.com

PSLF General Electric www.ps.ge.com


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Suggested Procedure for DGModeling Studies

l Develop component models.

Each component model (transmission line,transformer, breaker, etc.) will depend on thefrequency range of interest and the specific harmonicevent being evaluated.

l Run a steady-state solution case. This case will verify system connectivity and provides a

sanity check on many of the system components. Thisis a very important step that must not be skipped orgross errors could result.

l

Estimate the expected results. This can be done from previous studies, from theliterature, or from hand calculations. It is important toknow what to expect from the simulation so that majorproblems can be identified quickly.


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Suggested Procedure for DGModeling Studies

l Use a sensitivity analysis for unknown orimportant quantities.

Important variables from the simulation shouldbe evaluated to determine their impact onresults. These could include capacitor banksize, transformer size, line length, sourcestrength, etc.

l Develop solutions.

Possible solutions (i.e., filters) are evaluatedand design specifications are developed.

09 DGModelingGuidelines Revised

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