Solar PV Modeling and Impacts on
Distribution Systems
Greg Shirek
Milsoft Utility Solutions
Outline
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Generator Modeling
Inverter Ratings and Operating Modes
Step Voltage Regulators
–  Settings
–  Modes
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Developing Solar Irradiance Generation Profiles
–  Plane of Array Levels
–  Tracking Systems
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Generation and Feeder Load Coincidence Levels
Solar Irradiance Variability
Cases in WindMil
–  Large 1 MW PV Plant with Voltage Regulator Interactions and Inverter Modes
–  Feeder Distributed Roof-Top PV
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Summary
WindMil Generator Models
•  Rotating Type
–  Synchronous Generator (adjust kVAR by field winding).
–  Induction (needs external source of excitation).
–  Wind Turbines Types 1-4 machine based but coupled
with grid via converters (Types 3-4).
•  Inverter based PV
WindMil Generator Model Algorithms
•  Generators (Load Flow)
–  Negative load is a current source with real and reactive
current injections.
–  Swing kVAR – Iterate until the level of reactive current
(positive or negative) is found that is required to hold the
specified voltage given the specified kW.
•  Generators (Fault Flow)
–  Simply a voltage source behind and impedance (Zsm).
WindMil Generator Models
Solar PV Plant Components
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Panels – DC Ratings in Watts.
Panels create collectors.
Collectors mounted on tracking system.
Inverters.
Interconnect step-up transformer.
Inverter Operating Modes
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Converts DC to AC via solid state switches (IGBT’s or SCR’s)
Fixed Power Factor (Unity, Q=0)
Fixed Power Factor (Q≠0)
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Voltage Droop
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Maintain a Fixed PF at the POI, e.g. maintain between 0.98 lag and unity.
Increase/Decrease Q linearly based on Voltage at POI within limits. Inverters rating based on voltage, so
voltage decrease means can increase Q, and vice versa. Many times used in micro-grids with controllers
with autonomous control.
Voltage Control
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Regulate a set point voltage at the POI by allowing inductive or capacitive Q within the limits of the
inverter.
q  Traditional deployment is to size inverters for PV Array de-rated AC kW due to IEEE 1547 standard,
and operate in Unity PF mode (Quadrant 1).
Increasing Inverter Potential
PPV = Max Total Power
Output of Solar PV Plant
PQinv = kVA Rating of Inverter
Increase kVA rating by just
10% provides (100 kVA →
110 kVA):
+/- 45 kVAR or
(90% PF lead/lag PF)
Step Voltage Regulator Operating Modes
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SVR’s include up to 7 control settings to accommodate varying real and reactive power flow
directions:
1.
2.
3.
4.
5.
6.
7.
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Locked forward
Locked reverse
Neutral idle
Reverse idle
Bi-directional
Co-generation
Reactive bi-directional
Modes 1-3: One Direction Tap Changing Control regardless of power flow direction.
Modes 5-7: Voltage Control on either primary or secondary side.
Mode 7: Forward/Reverse reactive current direction determines if Forward/Reverse mode settings will be
used, whereas Modes 1-6 use the real component of current.
MODES 6 and 7 MOST COMMONLY USED FOR HIGH PENETRATION DG APPLICATIONS WITH DG
GENERATION EXCEEDING MINIMUM CIRCUIT LOAD.
Voltage Regulator Operating Modes
Bi-Directional -V1 or V2 controlled based on real
current component direction
Co-Generation –
V2 controlled during reverse
active current flow
Reactive Bi-Directional -V1 or V2 controlled based on
reactive current direction
WindMil Regulator Models
•  Defaulted to use co-generation mode during reverse power (power
flowing opposite of parent-child connectivity) since the algorithm
assumes the downline (child) side is the voltage control point.
•  Regulators with LDC during reverse power:
–  Still uses a co-generation mode.
–  With positive LDC values, will “buck” (lower voltage on child side).
–  With negative LDC values, will “boost” (raise voltage on child side).
•  Don’t use SET REGULATION analysis with DG or meshed circuits.
•  Overall, pay attention to regulator voltages during looped analysis
(feeder/source ties, and/or DG) and reverse power.
WindMil Regulator Mode Limitations
•  Limitations
–  WindMil regulators will not operate in a bi-directional
mode with generators downline during reverse power.
–  Needed enhancements on the list
•  Bi-directional mode so that power flow direction will control the
forward or reverse settings.
•  Reactive Bi-Directional modes.
•  Co-generation mode so that either parent or child side of
regulator will be the control point.
•  User Defined Tap Number, similar to Locked Tap but
independent of prior voltage drop run.
Regulators – Forward with LDC
Regulators – Reverse w/ (+) LDC & Gens
Regulators – Reverse w/ (-) LDC & Gens
PV Plant Array Types
Fixed Tilt
Fixed Tilt, one axis rotation
Clear Sky Insolation Profiles at Site
Source: National Renewable Energy Laboratory (NREL) METSTAT (Meteorological/Statistical) solar radiation
model. Developed to support production of the NSRDB (National Solar Radiation Database. Estimates hourly
values of direct normal, diffuse horizontal, and global horizontal radiation.
Sample System
1 MW PV Plant
Substation Regulator
Line Regulator
Contingency Substation
Sample System Load Profiles
A
B
C
D
E
A.  Peak = 3.6 MW, Used to
Find LDC Settings
B.  Max kW
2.5 MW
C.  Net Max
1.5 MW
D.  Min kW
0.8 MW
E.  Net Min:
-0.2 MW
Case A – 6 p.m. peak, No PV (3.6 MW)
Used to find LDC
settings with load center
at 120V at end of circuit
Substation SVR
124V
No LDC
Line SVR
120V
R=9, X=12
Case B – Max 12 p.m. , no PV (2.5 MW)
Substation SVR
124V
No LDC
Line SVR
120V
R=9, X=12
Line SVR at Tap 8
Case C – Max 12 p.m. ,with PV (1.5 MW)
Substation SVR
124V, No LDC
Line SVR
120V
R=9 , X=12
Less regulator
LDC boost due to
less current
Meets ANSI
Case D – Min 12 p.m., no PV (800 kW)
Substation SVR
124V
No LDC
Line SVR
120V
R=9, X=12
Notice Line
Regulator
actually bucks
voltage
Case E – Min 12 p.m., with PV (net -200 kW)
•  Need to establish cogeneration settings
•  For case study use 120 V
with no LDC
•  Remember, WindMil gen
model assumes co-gen
mode
•  Notice Voltage Rise due to
reverse power
•  Voltages meet ANSI
Reverse Power Voltage Rise
V = IZ
I = DG Current
Ø = PF Angle
V = I * (R+jX)
V ≈ Ir*R + Ix*X Vrise ≈ IRCosØ + IXSinØ
Time-Frame Considerations
•  Previous cases were steady-state by allowing regulators to
operate assuming necessary time constants were met to allow
regulator step changes.
•  Overall, for all load cases both with and without PV, voltages
appear to meet ANSI Standards.
•  However, with DER intermittent generation, small time-frame
cases need to be considered due to cloud shading as well as the
DER disconnecting during a temporary feeder fault to meet IEEE
1547 Standards.
•  Make use of WindMil regulators LOCKED setting.
Ramping Rates from Solar PV
Source: EPRI Distributed PV Monitoring Project. High
Penetration PV Workshop, April 19, 2012.
Ramping Rates from Solar PV
Source: EPRI Distributed PV Monitoring Project. High
Penetration PV Workshop, April 19, 2012.
Voltage Changes due to Intermittency
•  Regulators have too long of a time delay to operate during
intermittency.
•  From EPRI studies, the 1 MW PV plant will ramp up or ramp down
completely within 30 seconds.
•  Need to check multiple cases at noon to see impacts and find worst
case.
1.
2.
3.
4.
Minimum feeder load with no PV transitioning to peak PV
Maximum feeder load with no PV transitioning to peak PV
Minimum feeder load with PV transitioning to no PV
Maximum feeder load with PV transitioning to no PV
•  Worst case voltage change for case 2 (Max kW with No PV → w/
PV).
WindMil Procedure for Ramping and/or DG
Disconnect for Faults
•  No PV → PV
1.  Run Voltage Drop with Regulators ON and NO generation.
2.  Run Voltage Drop with Regulators LOCKED and turn
generators ON.
•  PV → No PV
1.  Voltage Drop with Regulators ON with Generation ON.
2.  Run Voltage Drop with Regulators LOCKED with
Generators OFF.
DER Shall Cease to Energize Area EPS
1.
2.
3.
4.
5.
6.
7.
8.
9.
A temporary fault occurs.
A fault-detection device in the Area EPS detects the fault (i.e. substation
recloser).
The device opens isolating the Area EPS.
The DR interconnection system detects the island, open-phase, or
undervoltage condition.
The DR ceases to energize the Area EPS.
Fault was temporary, so substation device closes restoring power to the EPS.
DR stays off-line due to industry standards.
Regulators maintain tap settings before outage due to time delay (i.e. 30-45
seconds).
Possible low or high voltages ensue on the Area EPS.
Maximum feeder load with no PV → peak PV
2.5 MW, No PV
2.5 MW, with PV (1.5 MW Net)
Note the Δ Voltage at end of feeder is ~ 4 V (124.5 – 120.5)
Remediation Options
•  After ramping, voltage exceeds 127 volts.
•  Need to lower voltage downline of regulator to meet ANSI 126 volt
standard. Leverage Swing kVAR Generation Mode.
•  Real-world case studies demonstrate making use of PV inverter operating
modes for kVAR import or export may be very beneficial.
•  PV Inverters can use their reactive power kVAR capabilities
“instantaneously.”
•  Further analyses are warranted to determine forward and co-generation
mode settings.
•  WindMil Procedure: Lock Regulator Taps and Set Generator to Swing
kVAR with 1 MW output at 1.016 p.u. voltage (122 V).
Inverter Reactive Power Capability
Generator Imports 800 kVAR to
help increase the voltage “drop.”
Regulator remains at tap 8.
This 800 kVAR with 1000 kW at
max irradiance means the inverter
needs to have +/- 80% PF
capability.
More options need to be explored
on how the regulator forward and
co-gen modes should be set along
with allowing the inverter to operate
in reactive power mode.
Rooftop Residential PV
•  Two ways to model
–  Method 1 – Consumer and Generator Elements.
Consumer with actual demand along with separate
generator with generated kW.
–  Method 2 – Just Consumer element. Calculated load tab
value to be customer kW demand minus coincidental PV
kW generation. Therefore may be negative kW at high
irradiance periods.
Rooftop Residential Modeling
Method 1– With Generators
Rooftop Residential Modeling – Method 1
Rooftop Residential Modeling – Method 2
Summary
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Demonstrated WindMil generator model modes – simple negative load versus swing
kVAR.
Demonstrated voltage regulator settings affects on line voltages during forward/
reverse power flow with and without Line Drop Compensation and based on parentchild relationship.
When analyzing large PV integration, need to know the inverter capabilities and
modes so they can be modeled correctly in WindMil.
Investigated Solar Irradiance profiles and coincidence with feeder loads for a case
study.
Reviewed how to investigate ramping/intermittency with Solar PV in WindMil.
For simple rooftop PV, customers preference to model generator element along with
consumer, or simply use consumer with net load.
Thanks for your attention!
Questions/Comments?
Greg Shirek, PE
Lead Support Engineer
greg.shirek@milsoft.com
Milsoft Utility Solutions