Modeling Challenges: Dynamic Interactions Between ...
Transcript of Modeling Challenges: Dynamic Interactions Between ...
Modeling Challenges:
Dynamic Interactions Between Inverters, Grid Forming Inverters
V. Gevorgian, S. Shah, P. Koralewicz, NREL
Challenges for Distribution Planning, Operational and Real-time Planning Analytics Workshop
Washington, DC
May 17, 2019
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Services by Multi-Technology (Hybrid) Plants
– Dispatchable renewable plant operation
• Long-term and short-term production forecasts
• Capability to bid into day-ahead and real-time energy markets like conventional generation
– Ramp limiting, variability smoothing, cloud-impact mitigation
– Provision of spinning reserve
– AGC functionality
– Primary frequency response (programmable droop control)
– Fast frequency response (FFR)
– Inertial response:
• programmable synthetic inertia for a wide range of H constants emulated by BESS
• Selective inertial response strategies by wind turbines
– Reactive power/voltage control
– Advanced controls: power system oscillations damping, wide-area stability services
– Resiliency services: black-start, islanded operation
– Stacked services
– Plant electric loss reduction, AEP increase
– Selective plant configuration for BESS: ability to serve a whole wind power plant, or selected rows/turbines
– Battery SOC management
– Optimization model-predictive control strategies – work in progress
– Revenue optimization
NREL-First Solar Project
NREL-PG&E project
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Control Interactions
Governor
Controls
Field Control
(AVR)
Pitch Control
Q & V Control
DC Bus
Control
PLL
Current
Control
Sy
nch
ronou
s G
ener
ato
rs
Win
d T
urb
ines
, P
V &
Sto
rage
Inver
ters
,
HV
DC
AGC
Transmission Network
Dynamics
Filters
Torsional Modes
Grid Support Functions
PWMHVDC Controls
• Power electronics-based generation and T&D systems
have increased control interaction problems
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Impedance-Based Analysis
• Impedance responses of inverter/plant and grid are compared– Impedance intersection points give frequencies of resonance modes
– Phase difference at intersection points gives damping
Grid( )gZ s
Loop gain:
Zg(s)/Zp(s), Zg(s)/Zn(s)
( )
( )
p
n
Z s
Z s
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4 kHz300 Hz 2 kHz100 Hz 1 kHz150
100
50
0
50
100
4 kHz300 Hz 2 kHz100 Hz 1 kHz20
30
40
50
60
( )pZ s
Mag
nit
ud
e (d
B)
Ph
ase
(deg
.)
( )gZ s
( )gZ s
( )pZ s
SCR: 5
SCR: 2
172o
182o
195o
205o
SCRGrid
Inductance, Lg
Resonance
Frequency
Phase
Margin
5 4.6 mH 641 Hz +8o
4 5.7 mH 584 Hz -2o
3 7.6 mH 512 Hz -15o
2 11.5 mH 441 Hz -25o
• Unstable Resonance for Weak
Grids
– Unstable for SCR<5.0
– Resonance Frequency Decreases
with SCR and its “Severity”
Increases
-
-
-
Resonance: Frequency and Damping
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NREL Flatirons Campus
Siemens
2.3 MW
GE/Alstom
3 MW
GE 1.5 MW
Gamesa
2 MW
Research Turbines
2 x 600 kW
PV Array
1 MW
2.5 MW dynamometer
5 MW dynamometer,
7MVA CGI1MW / 1MWh BESS
FS PV array
• Total of 12+ MW variable renewable generation currently • 7 MVA Controllable Grid Interface (CGI) • Multi-MW energy storage test facility• 2.5MW and 5 MW dynamometers (industrial motor drives)• 13.2 kV medium voltage grid• 1.5 MW total PV capacity
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NWTC Controllable Grid Platform
NWTC Wind Turbines
SunEdison1 MW PV Array
Controllable Grid Interface (CGI)for Grid and Fault Simulation
Switchgear Building
XcelSubstation
1 MW / 1 MWh BESS
Controlled grid,CGI Bus
Regular grid,Xcel Bus
(7 MVA continuous / 40 MVA s.c.)
115 kV
13.2 kV tie-line
GE 1.5 MW
Siemens 2.3 MW
Alstom 3 MW
Gamesa 2 MW
DC
AC
DC
DC
DC
DC
AC
13.2 kV 13.2 kV
First Solar430 kW PV array
GE 1.25 MW / 1.25 MWh BESS
13.2 kV
Aerial view of the siteAC
AC
AC
AES
Grid forming
Flexible testbed for many black start schemes
Image source: NREL
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Impedance Measurement Using CGI
• 7-MVA, 13.2-kV grid simulator (CGI)
+
• Perturbed voltages
• Response currents
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Impedance of 1-MW/13.2-kV BESS System
• Inverter-interfacing 1-MW/1-MWh battery energy storage system
– Voltage perturbation magnitude
• Impedance response
• Different control
elements dominate at
different frequencies.
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Impedance of First Solar PV Plant
Shahil Shah, 04/19/2019
Mag
nit
ud
e (d
BW
)P
has
e (D
EG
.)
Vendor 1, 4x40 kW Inverters: 160 kW, 0 VAR
Vendor 2, 2x125kW Inverters: 250 kW, 0VAR • Phase response goes
below -90 degrees
between 60 Hz and
200 Hz: Can
potentially interact
with grid and create
undamped resonance
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Resonance of Sungrow Inverters for Weak Grid
Shahil Shah, 04/19/2019
Mag
nit
ud
e (d
BW
)P
has
e (D
EG
.)
2 x125kW Inverters: 250 kW, 0VAR
Grid Impedance: Lg = 1.2 H (SCR: 1.5) • 125 kW inverters will form undamped
resonance between 100 and 200 Hz if the
grid is inductive with SCR less than 1.5
on base of 250 KW, 13.2 kV
– Corresponding grid inductance is 1.2 H
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Power-Domain Impedance
• Transfer function from Active Power to Frequency at Point of Interconnection
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Frequency Response Characterization
Loss of generation
Applications:– Real-time Estimation of
Inertia, Primary Frequency Response (PFR), Nadir, etc.
– Frequency Support Design by Renewable Generation
20%
droop 10%
droop
5%
droop
5%
droop10%
droop
20%
droop
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Grid-Forming Inverter from Outside
• BESS Inverter
• Impedance
measurements can
quantify different
aspects of grid-
forming ability
Forming
Following
Forming
Following
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Black-start of Wind Power Plant (13.2 kV system)
Grid forming inverter – 7 MVA
Main problems:• Harmonics / resonances• Grounding / protection
• Inrush currents
Significant overvoltage
Solutions:• Additional filtering• Transformer neutral point grounding resistor• Grounding transformers• Oversized inverter (40 MVA SC capability)
Real event: Resonance observed between power converters during black start at NWTC
Image source: NREL
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Challenges and Future Research
• Adoption of impedance characterization by industry
– Root-cause finding, grid codes, control design, impedance specs
• Impedance measurement using grid simulators
– High-fidelity model validation
– Control design; Testing for grid codes
• New impedance-based tools
– Design of grid-support functions
– Testing of grid-forming ability of inverters
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CGI RTDS
DC
AC
AC
DC
DC
AC
DC
AC
DC
AC
7 MVA Controllable Grid Interface
Regular grid
RjX
Programmable line impedance
40 MVA for 2 sec
Real-time model of a power system
Distribution System Testbed for Islanded Testing
=~
NREL BESS 1MW/1MWh230kV
Z1Z2
70/230kV12.47/70kV
T1T2T3
0.4/12.47kV
CGI POI
12.47kV, Old River
CB1CB4
CB2
SEL-487
Z3
Z4
F1
650 kVAPF=0.6893
Z5Z6
F2Z7
PV
924 kWZ8
SW1 800 kVAR
Z9
R
Z10
SW2
800 kVAR
Z11
F3
560 kVAPF=0.6825
F4
Z12
PV
150 kW 150 kW 150 kW
Z16
CB3
T4
M1000HPPF=0.85
12.47kV/480V
CHIL
PHIL
SEL-487 SEL-487
CHIL
12.47/0.4kVT5
ISLAND
ISLAND FEEDER BREAKER
630631632
640
711712
642
652
643
662
644
645
646
647
691
692
671
672
682
651661
633
641
Grid Forming Control NREL – PG&E project
Image source: NREL
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Example: Bus 630 L-to-L Fault
Low Z fault Hi Z fault
Time (sec)
Time (sec)
Bu
s #
Bra
nch
#
Time (sec)
Time (sec)
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Islanded Distribution Circuit Restoration with BESS
BESS Active and Reactive Powerin Grid Forming Mode
Motor start eventPHIL testing results using 1 MW/1MWh BESS
Image source: NREL
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BESS Inverter Fault-ride Through in Grid Forming Mode While Operating an Islanded System
PHIL testing results using 1 MW/1MWh BESS
Image source: NREL
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Definition of Black Start
NERC definition: “A generating unit(s) and its associated set of equipment which has the ability to be started without support from the System or is designed to remain energized without connection to the remainder of the System, with the ability to energize a bus, meeting the Transmission Operator’s restoration plan needs for Real and Reactive Power capability, frequency and voltage control, and that has been included in the Transmission Operator’s restoration plan”
Image source: NREL
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Black Start Stages
The black start process can be divided into three stages:
• Preparation stage
• Network reconfiguring
• Load restoration
A typical restoration plan for bulk power system includes the following
essential steps:
System status identification: blackout boundaries and location in
respect to critical loads, status of circuit breakers, capacity of available
black start units, etc.
Starting at least one black start unit to supply critical loads such as
nuclear or large thermal power plants
Progressive restoration: step-by-step supply of other loads avoiding
over and under voltage conditions
The restoration strategies:
• Serial – simpler strategy, slower but more stable
• Parallel – quicker but more complex
Conventional top down approach
Conventional centralized BS units to start the network
Passive loads, waiting to be energized
Changing black start paradigm
Microgrids
Bottom-up approach
Image source: NREL
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Configurations of Integrated PV/BESS Plants for Black Start
Co-located starter for a black start resource
Remote starter for a black start resource
PV + storage as fully functional black start resource
Collective black start resource
Image source: NREL
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PV-BESS Black starting a Gas Turbine Generator
Main challenge:• Energizing transformers and feeders• Mid-size gas turbines employ starting motors• Black start inverters need to be sized to provide necessary inrush current
Possible solutions:• Oversized inverters for inrush current • Equip all plant motor loads with soft starters of VFDs • Partial solution – energize transformers with tap positions at highest number of turns
Image source: NREL
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Consecutive Start of Aux Plant Motors
4x250 kW
1 MW
• Inverter current limit is exceeded during motor starting
• Limits capacity of motors that can be started with inverters
• Solutions:• Oversized inverters • Soft starters or VFDs for all
motors
Energizing transformers and feeder
Motor 1 start
Motor 2 start
Motor 3 start
Motor 4 start
MVA limit of inverters
Motor 1 start Motor 2 startMotor 4 startMotor 3 start
Energize transformer 1
Energize transformer 1
Energizefeeder
Image source: NREL
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Inverters in Current limiting Mode
4x250 kW
1 MW
• Inverter current limit is not violated but lack of inrush current causes voltage collapse
• Motors fail to start
MVA limit of inverters
Image source: NREL
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Constant V/Hz Soft Start
4x250 kW
1 MW
Inverters operate as VFDs with constant V/Hz ratio
• The whole circuit is started as a single VFD with constant V/Hz ratio
• Smooth start of all motors• No overcurrent conditions
during energizing/start up• No need to oversize
inverters
MVA limit of inverters
Image source: NREL
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HPP / Supercapacitor Energy Storage for Improved Restoration Process
Super cap storage response
• INL-NREL project• Industry partners:
• Idaho Falls Power• Siemens• Maxwell
• Black start of the feeder circuit with a fully black-start capable hydro power plant (slow governor)
• Supercapacitor energy storage is used to assist in frequency stabilization during system restoration
Image source: NREL
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Challenges, Conclusions
• Today and future restoration strategies should align with the changing network paradigm
• Modern grid forming inverters can contribute into black start / restoration with more superior reactive power capabilities compared to conventional synchronous generators
• Inherent inverter current limit is one most important factor for black start applications
Q [MVAR]
P [MW]
Pmin Pmax
(prime mover limit)
Lagging power factor
Leading power factor
Field current limit
Armature heating constraints
Under excitation limit
Lagging Qmax
Leading Qmax
0
Synchronous Generator
Winding end region heating limit
Synchronous generator
PV Inverter and Type 4 wind turbine
BESS Inverter
Synchronous condenser
Type 3 wind turbine
Typ
e 3
WTG
STATCOM
Recommendations for future studies• Fault performance of grid forming inverters – needs to be robust and standardized• Robust seamless transition between grid forming and grid following• If grid forming is present, do we really need grid following anymore? If so, what shares of GM and GFL are optimal?• Impedance characterization of grid forming inverters• Grid stability impacts of grid forming• Validated grid forming inverter models are needed for various renewable and storage technologies for successful black start studies• At scale PHIL testing of black start-capable renewable resources is an important tool to discover potential issues, test mitigating solutions
and validate models
Image source: NREL
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Summary of CGI#2 SpecificationsPower rating• Continuous AC rating - 19.9 MVA at 13.2kV and 34.5 KV• Overcurrent capability (x5.7 for 3 sec, x7.3 for 0.5 sec)• 4-wire 13.2 kV or 35.4 kV taps• Continuous operational AC voltage range: 0 - 40 kVAC• Continuous DC rating – 10 MW at 5 kVDC
Possible test articles • Types 1, 2, 3 and 4 wind turbines• PV inverters, energy storage systems• Conventional generators• Combinations of technologies / hybrid systems• Responsive loads
Voltage control (no load THD <1%)• Balanced and unbalanced voltage fault conditions (ZVRT, LVRT and 140% HVRT) –
independent voltage control for each phase on 13.2 kV and 34.5 kV terminals• Response time – less than 1 millisecond (from full voltage to zero, or from zero
back to full voltage) • Programmable injection of positive, negative and zero sequence components• Long-term symmetrical voltage variations (+/- 10%) and voltage magnitude
modulations (0-10 Hz) – SSR conditions• Programmable impedance (strong and weak grids, wide SCR range corresponding
to a POI with up to 250 MVA of short circuit apparent power) • Injection of controlled voltage distortions • Wide-spectrum (0-2kHz) impedance characterization of inverter-coupled
generation and loads• All-quadrant reactive power capability characterization of any system
Frequency control• Fast output frequency control (3 Hz/sec) within 45-65 Hz range• 50/60 Hz operation• Can simulate frequency conditions for any type of power system • PHIL capable (can be coupled with RTDS, Opal-RT, Typhoon, etc.)• Coupled with PMU-based wide-area stability controls validation platform
New features • 5 kV MVDC grid simulator (PHIL capable)• Voltage or current source operation• Seamless transition between voltage and current source modes• Emulation of full set of resiliency services:
• Black start• Power system restoration schemes• Microgrids
• Flexible configurations are possible when combined with CGI#1:• Two independent experiments• Parallel operation• Back-to-back operation• Emulation of isolated, partially or fully grid-connected microgrids
20 MVA
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Variable Generation with Storage Ongoing PV, wind, hydro + storage projects (DOE and industry funded):• WETO – storage to enhance APC by wind power• GE – wind storage, hybrid wind/solar/storage• SETO - First Solar hybrid solar PV/storage systems• WPPTO – integrated storage with ROR hydro plants• PG&E EPIC project – synthetic inertia, grid forming, distributed
applications• AES – DC-coupled PV-storage peaker plant
Main current areas of research for storage integration: • Essential reliability services• Co-optimized controls development• Grid stability issues • Resiliency: Grid forming/black start/microgrids• Storage value streamsNew research enabled by Flatirons Campus:• Fundamental operational characteristics of other storage technologies:
• SMES, Supercaps, Flywheels, BESS, Flow batteries• Multi-POI PHIL experiments up to 20MW with CGI#1 and CGI#2 – 13.2kV, 34.5 kV• Virtually interconnected experiments for 100s of MWs – What are the limits of scalability?• PSH and CAES emulation using by-directional dynamometers• Fast EV charging• Thermal storage, hydrogen / CH4 storage• DC and MVDC microgrids
www.nrel.gov
Thank you
This work was authored in part by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding provided by U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Solar Energy Technologies Office and Wind Energy Technology Office. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.