Services and Performance

from Future IBRs

Deepak Ramasubramanian
Electric Power Research Institute
dramasubramanian@epri.com
Learning Objectives
• Evaluate the added benefits to system reliability and stability upon increased
utilization of technical capability specified in IEEE 2800 and IEEE 1547.

• Recognize that performance based requirements for future inverter based resources
(IBRs) are more appropriate than control system element based requirements.

• A simulation model is a means to an end and no simulation model is perfectly

correct/accurate
INTRODUCTION

3
Transforming Power System

Central synchronous generators (SGs) replaced by transmission and distribution connected

inverter-based resources (IBR) can lead to operational and planning challenges
 Motivation to improve utilization of services from IBR

▪ Increased frequency of disturbances with high participation of IBRs

▪ Exponential increase of IBRs in generator interconnection queues
BENEFITS OF INCREASED
UTILIZATION OF CAPABILITY

6
Control Hierarchy of Traditional IBR Plant

▪ At the inverter/turbine level,

active/reactive power control
– Typically fast control

▪ At the plant controller level, voltage

and active power control
– Typically slow control

▪ These control modes have priority

at steady state and for disturbances
that do not activate fault ride
through modes
Focus of the
next few slides
 With traditional hierarchy of controls – fast reactive power

▪ Reduction in SCR below 2.0 results in

instability
▪ Initial reaction is to assume that presence of
PLL is the cause for instability
 What would happen if we shift to inverter level voltage control?

▪ Keeping the PLL and current controller gains

the same, switch to inverter level voltage
control.
▪ From a small signal sense, the control is now
stable even for SCR lower than 1.5!
 Would merge of both types of control be impactful?

▪ Marginal increase in stability with SCR of 1.5

▪ So what is happening here?
– Let us take a look at participation factors
Participation factors reveal influence of reactive power controller

SCR = 1.0 SCR = 0.5

• At both values of SCR, in addition to PLL states, reactive

power and active power controllers plays a role.
Interim takeaways
▪ Traditional hierarchy of inverter level reactive power control is a contributing
factor to instability in low short circuit conditions

▪ Conversion to inverter level voltage control provides improved stability and

reliability benefits

▪ To understand this from a power flow perspective:

– Traditional inverter level reactive power control can be related to a PQ bus in powerflow
– Switching to inverter level voltage control can be related to a PV bus in powerflow
– Increased number of PV buses is beneficial from a power flow solution
▪ A similar benefit is obtained in dynamic stability
Working example in a distribution feeder
 Different inverter controls that have been compared

Conventional inverter control Advanced inverter control Conventional inverter control

with coordinated Q/V with inverter level PV droop with inverter level voltage
control control
 Comparison of performance

▪ DVS and droop-based control can both stabilize the inverters following the fault
ride-through
▪ By using DVS or droop-based control for two PV plants, all the six PV plants in the
system are stabilized
Utilization of inverter level frequency response capability
▪ Traditional hierarchy of control has frequency response only at
plant level controller
– Slower closed loop response

▪ Inverter manufacturers also have capability to offer inverter level

frequency response
– Faster closed loop response

▪ Can utilization of this capability help with system performance?

– Could sometimes also be known as fast frequency response
Test system studied to explore this capability
▪ Real island network with:
– PV – 8.25 MVA
– BESS – 8 MVA
– DER – 3.25 MVA
– Load – 2.9 MW
– Sync condenser – 2.75 MVA
▪ System contains ac coupled PV-BESS hybrid plants, and
standalone PV and BESS plants.
▪ Total base case IBR MVA is 19.50 MVA
Scenarios considered
▪ Variety of scenarios based
on ability of PV and BESS
to provide fast frequency
response at inverter level
▪ In addition, the size of a
new advanced IBR* is to
be determined

*More on this in the next section of the presentation

Scenarios considered

▪ When fast frequency

response is not
utilized, new IBR of
around 25% is
needed for system to
be reliable
Scenarios considered

▪ When fast frequency

response of both PV
and BESS is used,
new IBR of only
around 7% is needed
for system to be
reliable
Scenarios considered

▪ When fast frequency

response of only
BESS is used, new
IBR of around 11% is
needed for system to
be reliable
Let’s Review
▪ If all inverters are assumed to operate as ‘PQ buses’,
– higher percentages of new IBR control methods (colloquially known as Grid
Forming IBR) will be needed

▪ Increased utilization of fast inverter level voltage and frequency

response
– Potentially reduces the percentage of new IBR control methods needed

▪ Newer IBR control methods are still important and needed

– But important to understand their behavior from a performance perspective
rather than a control structure layout
PERFORMANCE BASED
REQUIREMENTS

23
Why are they important?
• Newer IBR control methods are being Standardization of
developed to help improve reliability expected performance
and stability is important, without
• However, one cannot be too prescriptive being prescriptive
in how IBRs provide a service about control system
components
• Could stifle necessary innovation
• If a new IBR control method can provide
the necessary performance, then the
control elements used to provide that
behavior should not be important
Kirchhoff’s Laws apply to both current source and voltage source

Values of injected current to be

controlled in a timely manner for
network to be stable
Stability characterization across different new IBR control methods

• Using different control elements

(including PLL) it is possible to attain
stable operation of 100% IBR networks.
• Manufacturers should be allowed to
used these control elements, if they
wish
Contact presenter for many more of such examples
Towards development of UNIFI-ed performance specifications for IBR
• UNIFI Consortium brings
together diverse set of
stakeholders involved in the
transition of the power
network.
• These specifications provide
uniform technical
requirements for the
interconnection, integration,
and interoperability of GFM
IBR units and plants
• Feedback and suggestions
are most welcome.
SIMULATION MODEL
PERFORMANCE

28
Preliminary time domain tests for new inverter performance

• Tests can
check the
capability of
new IBR
device models
Preliminary time domain tests for new inverter performance

• Without needing to
know the exact control
details of an IBR,
performance against
specifications can be
verified
• However, only looking
at time domain
behavior is not
sufficient.
Frequency domain characterization is increasingly important

• Certain frequency domain characterization can

be used to identify characteristics and behavior
of different inverter based resources.
 Generic model structures
Entire IBR model
Vref/Vreg
or Qref/Qgen
• Generic models to represent inverter-based resources (IBR) in positive
At plant level Plant Level
Control
Modules within model sequence simulation environments are usually modular in nature
fref/freq and
Plant_pref/Pgen
Qgen Vt

•
Iqcmd
Qref
Electrical Ipcmd Generator/
Iq
Each module can have different generations of versions.
Pgen
Torque Pref Controls Converter Ip
Prefo
Control Pord • Typically named as _A, _B, _C, _D,…
w w
ref

Pgen
Drive-Train

• Newer generic modules continue to be developed and improved

Pitch
Aero
Control

• The characteristics of one named module within entire IBR model does
not necessarily characterize the behavior of the entire IBR model
Source: Draft report of IEEE TF on Simulation methods, models, and analysis techniques to
represent the behavior of bulk power system connected inverter-based resources

IBR control architectures are a continuum from ideal GFL to ideal GFM.

Operation of a system at 100% IBR can be achieved with different IBR simulation models
(combination of modules) and parameterization sets.
 Performance of generic model structures

Theoretical GFL inverter

• The performance of an IBR at the plant level can be more enhanced as compared to the
Legacy IBR plant just the performance at the inverter level
Modern IBR plant
• Many concepts related to GFM only consider a comparison of inverter level performance
Theoretical GFM inverter
• Given that an IBR plant model is comprised of multiple modules, it is important to
consider the performance of the entire plant’s model when evaluating its performance.
• Behavior of individual modules do not completely reflect entire plant’s performance
Summary
• Increased utilization of fast inverter level voltage and frequency response
• Potentially reduces the percentage of new IBR control methods needed

• Newer IBR control methods are still important and needed

• But important to understand their behavior from a performance perspective rather than a control
structure layout

• Performance specifications, such as the UNIFI Specifications, can help retain focus on expected
performance

• Avoid mapping the name of a simulation model/module to define an entire spectrum of

performance of a resource
• Instead utilize tests designed to verify performance specifications
QUESTIONS?

35
References
• Deepak Ramasubramanian, Wes Baker, Julia Matevosyan, Siddharth Pant, and Sebastian Achilles, “Asking for Fast Terminal Voltage Control in Grid Following Plants Could Provide Benefits of Grid Forming Behavior,” IET
Generation, Transmission & Distribution, vol. 17, pp. 411– 426 (2023)

• D. Ramasubramanian, “Differentiating between plant level and inverter level voltage control to bring about operation of 100% inverter based resource grids,” Electric Power Systems Research, vol. 205, no. 107739, Apr 2022

• D. Ramasubramanian, E. Farantatos, O. Ajala, S. Dhople, and B. Johnson, “A Universal Grid-forming Inverter Model and Simulation-based Characterization Across Timescales,” 2023 56th Hawaii International Conference on
System Sciences (HICSS), Maui, HI, USA, 2023, pp. 1-10

• Deepak Ramasubramanian, “Generic Positive Sequence Representation of Grid Forming Behavior and its Application in an Island System Case Study,” 2022 IEEE Power & Energy Society General Meeting (PES), Denver, CO,
USA, 2022, pp. 1-5

• B. Johnson, T. Roberts, O. Ajala, A. D. Dominguez-Garcia, S. Dhople, D. Ramasubramanian, A. Tuohy, D. Divan, and B. Kroposki, “A Generic Primary-control Model for Grid-forming Inverters: Towards Interoperable Operation &
Control,” 2022 55th Hawaii International Conference on System Sciences (HICSS), Maui, HI, USA, 2022, pp. 1-10

• Deepak Ramasubramanian, Wenzong Wang, Evangelos Farantatos, and Mohammad Huque, “Toward Performance-Based Requirements and Generic Models for Grid-Forming Inverters,” in Grid-Forming Power Inverters:
Control and Applications (1st ed.), N. Mohammed, H.H. Alhelou, & B. Bahrani (Eds.) Boca Raton: CRC Press, 2023

• UNIFI Consortium (https://sites.google.com/view/unifi-consortium/home)

• B. Kroposki et. al, “UNIFI Specifications for Grid-forming Inverter-based Resources—Version 1,” UNIFI-2022-2-1, December 2022

• Model Specification of Droop-Controlled, Grid-Forming Inverter [link]

• Proposal for Suite of Generic Grid Forming (GFM) Positive Sequence Models [link]

• Performance Requirements for Grid Forming Inverter Based Power Plant in Microgrid Applications: Second Edition. EPRI, Palo Alto, CA: 2022. 3002024431

• Zhangxin Zhou, Wenzong Wang, Deepak Ramasubramanian, Evangelos Farantatos, Garng M. Huang, “Small Signal Stability of Phase Locked Loop based Current-Controlled Inverter in 100% Inverter-based System,” IEEE
Transactions on Sustainable Energy, [accepted]

• Wenzong Wang, Deepak Ramasubramanian, Aminul Huque, Arun Kannan, and Diana Strauss-Mincu, “Benefit of Fast Reactive Power Response from Inverters in Weak Distribution Systems,” 2022 IEEE Rural Electric Power
Conference, Savannah, GA, USA, 2022

• Grid Forming Inverters: EPRI Tutorial (2022), EPRI, Palo Alto, CA: 2022. 3002025483
Presenter Bio

Deepak Ramasubramanian
Technical Leader, EPRI
Deepak joined EPRI in 2017 where his work is in the
area of modeling, control and stability analysis of the
bulk power system with recent focus on the
associated impacts of large-scale integration of
converter interfaced generation. He received his
Ph.D. degree in Electrical Engineering from the
Arizona State University, Tempe, USA in 2017 and his
M.Tech. degree in Power Systems from the Indian
Institute of Technology Delhi, New Delhi, India in
2013. He is a Senior Member of IEEE
Email: dramasubramanian@epri.com