cigre e-session 2020

CIGRE e-Session 2020

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JWG C2/B4.38

Capabilities and requirements definition for Power Electronics based technology for secure and efficient system operation and control

Summary Presentation to NERC IRPWGPresenters: Renuka Chatterjee, Neil Kirby

07-22-2021 1


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Content

Introduction Renuka Chatterjee Identified SO Challenges Renuka Chatterjee Power Electronics Capabilities Neil Kirby Practical Experiences Neil Kirby Enabling more PEID Neil Kirby Conclusions and Recommendations Renuka Chatterjee

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Introduction

Dr. Renuka Chatterjee is the executive director for system operation at MISO inIndianapolis, USA. Renuka has a long history in the industry and was awarded the2020 CIGRE Technical Council award. Neil Kirby works as a Business Development Manager for FACTS & HVDC in the

Renewables division of General Electric in Philadelphia, USA. Neil has a long-standing experience in the industry and was awarded the 2020 DistinguishedMember Award.

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Presenters/ panellists


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Introduction

We need to prepare for the ongoing energy transition and the operation of thefuture, low inertia power system Technology and system operation (SO) experts need to better understand each

others reality: Create insight in system operational challenges and capabilities of PE devices Optimal use of the available PE capabilities, now and in the future Open the door for new concepts to enable more efficient operation of the system To facilitate the advanced use of power electronics in system operations and control

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Why bridging the gap between system operators and vendors?


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Introduction

Step 1 consisted of two work streams: Stream A: identification of system operational challenges through literature review, surveys

and bilateral discussions with TSOs Stream B: description of capabilities of several power electronics interfaced devices (PEID)

Step 2: Mapping PEID capabilities to SO challengesStep 3: Identfication of best practicesStep 4: Guidelines for enabling proliferation of PEID in the power system

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Way of working


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Identified SO Challenges

Introduction Identified SO Challenges Power Electronics Capabilities Practical Experiences Enabling more PEID Conclusions and Recommendations

Panel Session

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SO Challenges from the Energy Transition

7

Overview of Generation DevelopmentIreland

1990 2004 2017

Year

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40

60

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100

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al E

lect

rical

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gy G

ener

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n (%

)

UK

1990 2004 2017

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Denmark

1990 2004 2017

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Australia

1990 2004 2017

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40

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lect

rical

Ener

gy G

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n (%

)

Coal

Wind + Solar PV

LEGEND

Annual electrical energy production coming from coal and combined wind and solar PV for Ireland,UK, Denmark and Australia. Values are normalized using the sum of these three sources. Other fossilfuel and renewable energy sources are not considered in the figure.

Demand, wind and net import of All Island of Ireland on 7 December 2019


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With increasing power electronicsinterfaced devices, the systembehaviour and response are bound tochange. It identifies issues on newpower system behaviour (e.g. lowerresonance frequencies due toincreasing HVAC underground cables).

Identifies areas where how we operate the powersystem needs to change. It includes the people,processes and tools in system operation thatobserve the bulk electric system and take necessaryactions to maintain operational reliability. The newoperation of the power system will require toincrease the level of automatic control actions tocope with the expected faster and more frequentdynamic power system behavior. Some phenomenaare expected to be too fast for a manualoperational response.

System stability will remain crucial. Thiscategory deals with issues that result fromlack of support for a stable voltage andfrequency (e.g. increasing RoCoF resultingfrom decreasing synchronous generation).

SO Challenges

8

Presenter

Presentation Notes

Three categories are identified to classify the challenges Survey of System Operators and literature review provided multiple challenges that were grouped into these three categories


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SO Challenges - Overview

9

Presenter

Presentation Notes

Three categories are identified to classify the challenges Survey of System Operators and literature review provided multiple challenges that were grouped into these three categories


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SO Challenges - Overview

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Detailed description of SO Challenges

Reference:V. N. Sewdien, R. Chatterjee, M. Val Escudero, and J. van Putten,“System Operational Challenges from the Energy Transition,” CIGRE Sci. Eng., vol. 17, pp. 5–20, 2020.


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Category 1: Lack of Voltage and Frequency Support

Increasing RoCoF Estimation of operating reserves Ramps management

The voltage and frequency support of the grid is altered by the large-scaleintegration of PEIDs

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Category 2: New Operation of the Power System

Observability of RES Increased TSO – DSO coordination Operators need to be equipped with more skills

The large-scale integration of PEIDs alters the power system requiring usto review the people, process and tools in system operation that monitorthe bulk electric system and provide decision support to operate reliably

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Category 3: New Behavior of the Power System

Resonance instability Decreased damping of existing power oscillations Power electronics capability to withstand asymmetry

The behavior of a power system can be described by its steady state and dynamic characteristics.The increased cabling, reduction of conventional synchronous generation and the increase ofPEIDs alter the power system behavior

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Power Electronics Capabilities


Panel Session

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Introduction to Technologies HVDC Transmission FACTS Devices Other Inverter-based Interfaces

Comparison of Network Support Capabilities Mapping capabilities of power electronics interfaced devices in

mitigating system operational challenges

15

Power Electronics Capabilities

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HVDC Transmission

Scheme Types Long-Distance Overhead Line Submarine / Underground Cable Back to Back

Technologies Line-Commutated Converter (Thyristor) Voltage Source Converter (Transistor)

Configurations Monopole Bipole Multi-terminal DC Grid

High Voltage Direct Current

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FACTS Devices

Device Types Shunt Systems (SVC, STATCOM) Series Systems (FSC, TCSC, SSSC) Combination Systems (IPFC, UPFC)

Technologies Line-Commutated Converter Voltage Source Converter

Flexible AC Transmission Systems

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Other inverter-based interfaces

Applications PV Solar inverters Wind turbine generator converters Pumped hydro storage systems Energy storage systems Static frequency converters

Technologies Mostly Voltage Source Converter

Power Electronic Interface Devices

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Capabilities of PEIDs

Physical Characteristics Fault current contribution Harmonic emissions Overload capability

Grid forming capabilities Synthetic inertia Frequency control / RoCoF control Voltage control Black start capability

The Technical Brochure elaborates on different capability areas of HVDC and FACTSelements as well as of inverter-based generation and a few non-PEID equipment

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Capabilities of PEIDs

Damping Capabilities Power oscillation damping SSTI damping Resonance damping Active filtering

Other capabilities Active power control Reactive power control Phase unbalance compensation Steady state angle difference control

The Technical Brochure elaborates on different capability areas of HVDCand FACTS elements

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Capabilities of PEID in Mitigating SO ChallengesCategory 1: Lack of voltage and frequency support

21

Power Electronics Interfaced Devices HVDC PE based FACTS PE based generation

Operational Challenges VSC HVDC

LCC HVDC STATCOM SVC TCSC SSSC UPFC IPFC PV WTG

Type III WTG Type IV BESS

Lack

of v

olta

ge &

fre

quen

cy su

ppor

t Increasing RoCoF ✓ ✓ ✓ (✓) (✓) ✓ Decreasing frequency nadir ✓ ✓ ✓ (✓) (✓) ✓ Excessive frequency deviations ✓ ✓ ✓ ✓ ✓ ✓ Static reactive power balance ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ Dynamic reactive power balance ✓ (✓) ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ Ramps management ✓ ✓ ✓ ✓ ✓ ✓ Estimation of operating reserves

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Not related to PEIDs


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Capabilities of PEID in Mitigating SO ChallengesCategory 2: New operation of the power system

22





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New

ope

ratio

n of

the

pow

er sy

stem

Increased congestion ✓ ✓ Observability of RES Controllability of RES Vertical coordination Horizontal coordination Operation of hybrid systems Control of bi-directional flows ✓ ✓ ✓ ✓ ✓ ✓ New operator skills Power system restoration ✓ ✓ ✓ ✓

Not related to PEIDs


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Capabilities of PEID in Mitigating SO ChallengesCategory 3: New behavior of the power system

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New

beh

avio

r of

the

pow

er sy

stem

Resonances due to cables and PE ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ Reduction of transient stability ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ Resonance instability ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ Sub-synchronous controller interaction ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ New low frequency oscillations ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ Decreased damping ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ Voltage dip induced frequency dip ✓ ✓ (✓) (✓) ✓ Larger voltage dips ✓ ✓ ✓ (✓) (✓) (✓) (✓) PLL instability ✓ ✓ ✓ ✓ ✓ LCC commutation failure ✓ ✓ ✓ ✓ ✓ ✓ ✓ Asymmetry withstand capability of PE ✓ (✓) ✓ ✓ ✓ ✓ ✓ ✓ ✓ (✓) ✓ ✓ Supply of single-phase traction load SFC


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Practical Experiences


Panel Session

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Avoid increasing RoCoF In Canada, wind farms are required to

deliver under frequency inertial responsecontrol: momentary overproduction of ≥ 6%during ≥ 9s. limits Δf after a major loss ofgeneration on the system

Keep frequency nadir under control Contingency in Zambian – Namibian HVDC

interconnector (Caprivi Link): suddenislanding in Namibia, restoring stable gridoperation through the HVDC link

Challenges from Category 1

26

Zambian side Namibian side

IslandingRestoration of Namibiangrid, while keepingfrequency nadir undercontrolY.-J. Häfner, M. Manchen, "Stability Enhancement and Blackout

Prevention by VSC Based HVDC”, CIGRE Symposium, Bologna, B4-232, 2011


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Resonance instability PEIDs have the potential to excite existing or

unknown resonances However, PEIDs can provide active damping

and improve the stability Example: STATCOM connected to a wind farm

could actively damp an existing resonance


27

T. Bagnall et al., "PCS6000 STATCOM ancillary functions:Wind park resonance damping", European Wind EnergyConference EWEC, Marseille, 16-19 March 2009


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Power electronics capability to withstand asymmetry Effect of asymmetry in power grids:

• induction of double fundamental frequency current in generators• Induction of double fundamental frequency voltage in converter DC

links• This is unwanted because the equipment may be overloaded

PEIDs can be designed to withstand defined asymmetry Applications where PEIDs are designed to withstand heavy

asymmetry• Static frequency converters supplying single-phase railway grids:

withstand 100% asymmetry• STATCOMs compensating heavily fluctuating power supply for

electric arc furnaces:withstand up to 100% asymmetry and a virtually random harmonic load


29

For more details refer to:C. Zhao, C. Banceanu, T. Schaad, P. Maibach, S. Aubert, "Static frequency converters - a flexible and cost efficient method to supply single phase railway grids in Australia", AusRAIL 2015, Melbourne, 2015

R. Grünbaum, T. Gustafsson, J.-P. Hasler, T. Larsson, B. Aigner, D.C. Park, "STATCOM for Grid Code Compliance of a Steel Plant Connection", 19th International Conference on Electricity Distribution, paper 0519, CIRED, Vienna, 2007

Arc furnace currents: upper curves, and currents to the grid with STATCOM in operation: bottom curves.

R. Grünbaum, T. Gustafsson, J.-P. Hasler, T. Larsson, M. Lathinen, "STATCOM for safeguarding of power quality in feeding grid in conjunction with steel plant expansion”", B4-306, CIGRE Session 2004


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Enabling more PEID


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Enabling more PEID

Ever increasing utilization of PEID is a fact and should not a priori be seen as athreat but could rather be considered a chance to improve the behavior of thegrids. Many practical examples are encouraging this What steps should be taken to turn this chance into real benefits?

Embrace the fact that PEIDs can bring substantial benefits Collaboration TSOs – vendors Validation Standardization Further development, PEID performance improvement

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Enabling more PEID

Close collaboration between Grid System Owner and Operators with vendors Multi-stage test strategy

Main steps for the integration of new PEID in power systems

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Enabling more PEID

Market based solutions Placement of requirements (e.g. in Grid Codes) Open requirements as ancillary service (e.g. the UK «Stability Pathfinder» project)

Grid forming converter controls PEIDs mimicking properties of synchronous machines

Coordinated control of different PEIDs, especially in emergency situations Master controller

Different scopes of supply Standardized interfaces between subsystems of a PEID

New directions under investigation

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Conclusions and Recommendations

Introduction Identified SO Challenges Power electronics Capabilities Practical experiences Enabling more PEID Conclusions and recommendations

Panel Session

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Due to the changing power system we loose several physical characteristics ofconventional production units that help to maintain operational security This results in a number of challenges. An industry-validated list of operational

challenges was developed. The PEIDs that replace these production units can help in mitigating some of

these challenges. Practical cases were presented as examples.

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Conclusions


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The main recommendation is that System Operators and vendors should havetransparent discussions on the use and capabilities of PEID. Often, SystemOperators are not aware of the full range of PEID capabilities. This is a lostopportunity, as many of such capabilities can be used to enhance system operation.Furthermore, the following topics need further attention:

Grid forming controls Standardisation Protection Modelling and simulation tools

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Recommendations


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Panel session


Panel Session

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Panel session

Jan van Putten Renuka Chatterjee Philippe Maibach Neil Kirby Susana de Graaff

Vinay Sewdien is event moderator

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Presenters/ panellists


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Panel session

Raise your hand to ask a question (click symbol in control panel) You will be un-muted by the event moderator There will be discussion with the panel Once finished you will be muted again and we go to the next question

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Closing

Thank you for your interest

JWG C2/B4.38 [email protected]

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Are solar PVs capacitive? NERC IRPWG Monthly Meeting

July 22, 2021PI: Lingling Fan (USF)

Co-PIs: Zhixin Miao (USF), Shahil Shah (NREL), Przemyslaw Koralewicz(NREL), Vahan Gevorgian (NREL)

1

Outline• First, a direct answer to the question via inverter’s impedance

measurements obtained by the NREL team• V. Gevorgian, S. Shah, P. Koralewicz, “Modeling Challenges: Dynamic Interactions Between

Inverters, Grid Forming Inverters,” presentation for Challenges for Distribution Planning, Operational and Real-time Planning Analytics Workshop (DOE SETO), Washington, DC May 17, 2019

• Event analysis:• AC overcurrent• Sub-cycle overvoltage • Hydro One 80-Hz oscillations

2

L. Fan, Z. Miao, “Root Cause Analysis of AC Overcurrent in July 2020 San Fernando Disturbance,” IEEE TPWRS 2021.

L. Fan, Z. Miao, “Analysis of 80-Hz Oscillations in Solar PV Farms,” submitted, IEEE PES letters. L. Fan, Z. Miao, “Analysis of sub-cycle overvoltage”, submitted, IEEE PES letters.

4

Angle: 0 -180 degree: capacitive-90 -180 degree: negative resistance

5

Crossover frequency: when the two (inverter and grid) meet each other.

If the phase angle difference is close to or greater than 180 degree, poorly damped or undamped oscillations.

• Sub-cycle overvoltage (2017, 2018 solar PV tripping events)• AC overcurrent (2020 San Fernando disturbance)

• Hydro One 80-Hz oscillations

6

7

AC overcurrent: i = -Y vg

Znetwork Y

ZVSC

ZVSC-Y

total Z: VSC + network

Capacitive!

Series LC resonance at ~100 Hz!

L. Fan, Z. Miao, “Root Cause Analysis of AC Overcurrent in July 2020 San Fernando Disturbance,” IEEE TPWRS 2021.

A dip in |Z|

Hydro One data

C. Li, “Unstable operation of photovoltaic inverter from field experiences,” IEEE Transactions on Power Delivery, vol. 33, no. 2, pp. 1013– 1015, 2018.

(2015) Hydro One observed 20-Hz poorly damped oscillations in RMS voltage measurements at a 44-kV distribution feeder upon switching in a 30-Mvar capacitor in a substation. Three 10- MVA solar PV plants were connected to the utility substation through a 30-km feeder. Fault level at the 44-kV point of connection (PoC) is approximately 120 MVA.

Hydro One also observed that the instantaneous currents and voltage have a large 80-Hz oscillation component. This component reflects in the RMS voltage as 20-Hz oscillations. The left figure presents the recorded instantaneous current and the voltage measurement. The oscillation issue was resolved by closing a tie breaker to reduce grid impedance.

8

FFT results of the phase current

Source: Chester Li

Voltage RMS 20 Hz peak to peak: 2%. Current 60 Hz component: 11%Current 80 Hz component: 8.25%

Rated current amplitude: 181 A

At 80 Hz, impedance (voltage/current) is 0.25 -27 db (a dip in |Z| at 80 Hz)

Does it look like series LC resonance at 80 Hz? (Grid: L, PV: C )

9

10

Sub-cycle overvoltage: Canyon 2 event

NERC report page 18

Key Finding: During fault events, there appears to be an interrelationship between momentary cessation, in-plant shunt compensation, and transient overvoltage conditions that result in inverter tripping. While this has been observed at multiple locations for multiple events, the causes and effects are not well understood and require detailed EMT simulations for further investigation.

Recommendation: EMT studies should be performed by the affected GOPs, in coordination with their TO(s), to better understand the cause of transient overvoltagesresulting in inverter tripping. These studies should also identify why the observed inverter terminal voltages are much higher than the voltage at the point of measurement (POM), and any protection coordination needed to ride through these types of voltage conditions.

• In our previous analysis, the shunt cap is assumed to be connected at the inverter bus. We demonstrated the possibility of sub-cycle overvoltage due to grid LC resonance, but have not demonstrated why the inverter sees overvoltage while the PoM (the substation bus) does not see overvoltage.

X = 0.6; Shunt: 0.2 pu R/X = 0.1

t = 0.5: current 1 0.5t = 0.51: current 0.50

PCC voltage

For X = 0.6 pu (SCR about 2), shunt compensation 0.2 pu, the oscillation frequency is about 2.88*60 = 173 Hz.

11

L. Fan, Z. Miao, M. Zhang “Subcycle Overvoltage Dynamics in solar PVs,” IEEE TPWRD 2020.

Thinking hard:

If the inverter can be viewed as a shunt cap, then the inverter bus should have a severe overvoltage compared to the substation bus where a shunt cap is connected.

PV admittance is a cap vs. PV admittance is an inductance

PV bus has more severe overvoltage

PV bus has less severe overvoltage

12

All three cases indicate the following:

Are solar PVs capacitive at certain frequency range?

13

Sub-cycle overvoltage: mechanismFor example, KVL + control logic:

ZPV

PCCi

14

L. Fan, Z. Miao, “Analysis of sub-cycle overvoltage”, submitted, IEEE PES letters.

Z*PV

PV Impedance

Inductive

Capacitive

Negative resistance

15

Current control bandwidth: 225 Hz

Delay: 100 𝜇𝜇s

Sub-cycle overvoltage demonstration

16

0.30 0.15

30%

Sub-cycle overvoltage demonstration

At 0.18 s, grid voltage is subject to 10% dip.

At 0.2 s, current order ramps down to 0.

Overvoltage is more severe at the PV inverter than at the PoM.

PV bus: 1.53 p.u.POM: 1.18 p.u.

17

What if scenarios

• There is no feedforward unit in inverter current control

• No overvoltage

• There is no shunt compensation

• Much less severe overvoltage at the inverter bus

18

80-Hz oscillation demonstration

19

L. Fan, Z. Miao, “Analysis of 80-Hz Oscillations in Solar PV Farms,” submitted, IEEE PES letters.

ZPV Zgrid

Yes, series LC resonance at 80 Hz! This manifests as a dip in the impedance magnitude, indicating current oscillations.

-27 dB can be created for the total impedance.

-27 dB!

Reduced grid impedance, better for stability. Larger proportional gain, better for stability. 20


EMT simulation studies

• Case 1: The PLL bandwidth is 13 Hz and the current PR control parameters are (0.035, 2) with a delay of 150 μs.

• Interaction of current control and the circuit generates 80-Hz oscillations


• Interaction of PLL and current control generates 20-Hz oscillations in dq, 40/80 Hz in abc


• More aligned to Case 1, and similar to Hydro One’s observation.

21

SimScape circuit diagram: the shunt cap will be switched in at 2 seconds.

22


Case 1: If current control effect is dominant, we see 80-Hz oscillations.

23


Case 2: if PLL effect is dominant, we see both 40-Hz and 80-Hz components.

24


Case 3: Current control is dominant, PLL plays a less dominant role.

25


Concluding remarks

• Are solar PVs capacitive? • Current control employing proportional integral (PI) or proportional resonant

(PR) control may lead to capacitive impedance in the frequency range of 60-300 Hz range.

• This characteristics (+negative resistance) may lead to: • LC resonance (Hydro One 80-Hz oscillations)• sub-cycle overvoltage (2017 Canyon 2 event and 2018 events), and • ac overcurrent (2020 San Fernando disturbance).

26

© 2021 Electric Power Research Institute, Inc. All rights reserved.w w w . e p r i . c o m

Wes Baker, Deepak Ramasubramanian, Jens Boemer (EPRI)

Rich Bauer (NERC)

July 22, 2021

IBR field data recordingsIRPWG Meeting

This effort is, in part, supported by the U.S. Department of Energy, Solar

Energy Technologies Office under Award Number DE-EE0009019

Adaptive Protection and Validated MODels to Enable Deployment of High

Penetrations of Solar PV (PV-MOD).

This effort is, in part, supported by the North American Electric Reliability

Corporation (NERC) under EPRI contract 20011165 Inverter-Based

Resources Dynamic Response Characterization for Bulk Power System

Protection, Planning, and Power Quality.

http://www.epri.com/

https://www.linkedin.com/company/epri

https://www.facebook.com/EPRI/

https://twitter.com/EPRINews

© 2021 Electric Power Research Institute, Inc. All rights reserved.w w w . e p r i . c o m2

Objective

▪ Analyze data recordings from IBR facilities to:– Characterize IBR response to system faults

– Improve models

– Validate models

▪ Challenge: The availability of high-resolution data has been identified in recent NERC reports as a major issue hindering event analysis involving IBRs. – April and May 2018 Fault Induced Solar Photovoltaic Resource

Interruption Disturbances Report link

– July 2020 San Fernando Solar PV Reduction Disturbance link


https://www.nerc.com/pa/rrm/ea/April_May_2018_Fault_Induced_Solar_PV_Resource_Int/April_May_2018_Solar_PV_Disturbance_Report.pdf

https://www.nerc.com/pa/rrm/ea/Documents/San_Fernando_Disturbance_Report.pdf


Description

▪ BC fault on a 138kV line, 3-terminal line▪ Event occurred on June 2nd, 2018, at ~6:15 AM local time. Low plant output pre-disturbance

likely due to reduced solar irradiance.– pre-disturbance active power = ~0.32 MW

– pre-disturbance reactive power = ~0.82 MVAR

▪ Recordings are from the relay at the solar facility 138kV terminal of the 3-terminal line. – Event report: 4 samples / cycle – filtered data

▪ SEL 5601-2 SynchroWAVe used to analyze data

40 MWsolar facility

relay

BC fault in LZOP of relay(exact location unknown)

138kV



Overview of the event in terms of V and IA

mp

skV

138kV bkr opens (52A=0)

~46ms



Instantaneous active and reactive powerkW

/kV

Ar

kV𝑃𝛼𝛽 ≅ 0𝑀𝑊

𝑄𝛼𝛽 ≅ 10𝑀𝑉𝐴𝑅Legend:

𝑃𝛼𝛽 =3

2𝑣𝛼𝑖𝛼 + 𝑣𝛽𝑖𝛽

𝑄𝛼𝛽 =3

2(−𝑣𝛼𝑖𝛽 + 𝑣𝛽𝑖𝛼)

~46ms



Sequence components of the currentA

mp

skV

Legend:∠V1 − ∠𝐼1

∠V2 − ∠𝐼2deg

rees

40 MW facility→ ~ 167 amps at

full output

~46ms



Model Validation – German FGW example framework

▪ FGW TG4 details model validation requirements using laboratory testing measurements. While not specific to model validation from field data recordings, the framework provides insight into the model validation process. – Ultimately P, Q, ip, and iq (both positive and negative sequence) of the model needs to closely match the measured response of

the equipment.

– For each time period, statistical measures of error (integrated) between the equipment’s measured response and the model’s response are calculated.

– If all margins of error are within the defined allowable tolerance, the model is validated.

The FGW approach is highly quantitative; the U.S. approach relies more on engineering judgement

Example framework used for model validation in FGW Guidelines

Time period P Q ip iq

Pre-fault

Fault

Post-fault

Reference: FGW TG4: Demands on Modelling and Validating Simulation Models of the Electrical characteristics of Power Generating Units and Systems, Storage Systems as well as for their Components https://wind-fgw.de/shop/technical-guidelines/?lang=en


https://wind-fgw.de/shop/technical-guidelines/?lang=en


Active and reactive components of current in positive

and negative sequence

kVA

mp

s

LegendPositive sequence:𝐼𝑃1 = 𝐼1 cos(∠𝑉1 − ∠𝐼1)𝐼𝑅1 = 𝐼1 sin(∠𝑉1 − ∠𝐼1)

Negative sequence:𝐼𝑃2 = 𝐼2 cos(∠𝑉2 − ∠𝐼2)𝐼𝑅2 = 𝐼2 sin(∠𝑉2 − ∠𝐼2)

~46ms



Phasor diagram snapshot ~ 40ms after grid fault inception

At the line relay:I1 lags V1 ~ 77°I2 leads V2 ~ 117°

Based on Sbase = 40MVA:|V1| ~ 0.74pu|I1| ~ 0.25pu|V2| ~ 0.27pu|I2| ~ 0.27pu

→ Unknown how many inverters online.

→ P2800 requirements at the inverter terminal,

would be useful to have inverter level data



P2800/VDE parameterized EMT model vs data recording

0

10

20

30

40

50

60

70

80

90

100

0.06 0.08 0.1 0.12 0.14 0.16

Voltage Magnitude

V1 V2 V1_model V2_model

~46ms

0

10

20

30

40

50

60

70

80

90

100

0.06 0.08 0.1 0.12 0.14 0.16

Current Magnitude

I1 I2 I1_model I2_model

Plant bkr opens

Am

ps

kV



-10

0

10

20

30

40

50

0.06 0.08 0.1 0.12 0.14 0.16

+VE Current

IP1 IR1 IP1_model IR1_model

~46ms

P2800/VDE parameterized EMT model vs data recording

-70

-60

-50

-40

-30

-20

-10

0

10

0.06 0.08 0.1 0.12 0.14 0.16

-VE Current

IP2 IR2 IP2_model IR2_mode

Plant bkr opens Positive sequence:𝐼𝑃1 = 𝐼1 cos(∠𝑉1 − ∠𝐼1)𝐼𝑅1 = 𝐼1 sin(∠𝑉1 − ∠𝐼1)

Am

ps

Am

ps

Negative sequence:𝐼𝑃2 = 𝐼2 cos(∠𝑉2 − ∠𝐼2)𝐼𝑅2 = 𝐼2 sin(∠𝑉2 − ∠𝐼2)



Observations

▪ Data from 2018 → prior to IRPTF/WG guidelines

▪ Positive and negative sequence current injected for asymmetrical fault on system. At plant POM:→ Δ𝑉1~0.25𝑝𝑢 … Δ𝐼1𝑅~0.25𝑝𝑢

∗

→Δ𝑉2~0.25𝑝𝑢 … Δ𝐼2𝑅~0.25𝑝𝑢∗

*assuming all inverters online

▪ Details of the plant design and inverter level data needed

▪ Likely a wealth of data from uP relays that can be useful for IBR response characterization, model improvement, model validation.



Together…Shaping the Future of Electricity


© 2021 Electric Power Research Institute, Inc. All rights reserved.w w w . e p r i . c o m

Deepak Ramasubramanian ([email protected])

NERC IRPWG Virtual Meeting22nd July 2021

Blackstart and System

Restoration using Inverter

Based ResourcesSupply of critical load

This presentation is, in part, supported by the U.S. Department of

Energy, Solar Energy Technologies Office under Award Number DE-

EE0008776 Solar Critical Infrastructure Energization (SOLACE) System


https://www.linkedin.com/company/epri

https://www.facebook.com/EPRI/

https://twitter.com/EPRINews

mailto:[email protected]


Objective from transmission system analysis

▪ To supplement resources on distribution system to serve critical loads following a blackout

▪ To energize a meshed transmission or sub-transmission network to serve multiple substations

▪ To utilize transmission connected BESS and PV as black start resources



Proof of concept test network

▪ One black start capable IBR available at bus 102

▪ Objective is to energize a path to critical load at Bus 153

Identification of cranking path



Representation of cranking path for study

• A 25 MVA grid forming inverter control developed at EPRI conceptually based upon FERC Orders Nos 827 and 842. • Representative of a BESS and has an average model for the inverter• Transformer between B1 and B2 is 25 MVA• Transformer between B2 and B3 is 250 MVA• Transformer between B4 and B5 is 500 MVA

• All transformers have saturation represented:• Leakage reactance of 0.08pu on self MVA base• Magnetization current of 0.3%• Inrush decay time constant of 1.0s• Knee voltage of 1.15pu

• Rengr is a transformer energization variable resistance used for soft energization

• The three induction motors (IM) (7 MVA, 2.5 MVA, 3.0 MVA) have a soft starting mechanism



Sequence of operations

1. All breakers and loads open/disconnected initially

2. At t = 0s, 25 MVA INV (1.8 MW auxiliary load) starts up

3. At t = 1.5s, INV voltage and frequency controls get activated

4. At t = 6.0s, INV breaker B1 is closed energizing 0.575/21.6 kV Xfmr

5. At t = 7.0s, breaker B2 is closed energizing 21.6/500 kV Xfmr

6. At t = 12.0s, breaker B3 is closed energizing the T-lines

7. At t = 17.0s, breaker B4 is closed energizing the 500/230 kV Xfmr

8. At t = 25.0s, breaker B5 and B6 are closed resulting in 7 MW const. I load pickup

9. Three scenarios for motor starting1. Staggered soft start

2. Same time soft start

3. Same time direct start



Start up of inverter – at t = 0.0s

1. Inverter starts at t = 0.0s2. Constant current control

enabled at 1ms3. Outer voltage and

frequency control disabled until 1.5s

Successful inverter start up



Energizing 0.575/21.6kV 25 MVA transformer – at t = 6.0s by closing B1

1. Transformer is energized using resistance based soft energization2. Resistance is reduced gradually3. There are numerous other methods of transformer energization that

can be applied



Energizing 21.6/500.0kV 250 MVA transformer – at t = 7.0s by closing B2

1. Again, transformer is energized using a Resistance based soft energization2. Impedance of 25 MVA transformer also helps during energization



Energization of complete cranking path up to load bus

Further controller tuning required to damp oscillations



Picking up load on distribution network at t=25.0s

▪ A reduced distribution feeder with path to critical load

▪ Both single phase and three phase induction motors present



Load consumed by distribution feeder from transmission

network

▪ Load on the feeder is unbalanced and IBR control loops do not yet have negative sequence voltage control

▪ However, successful energization of critical load



Possibility of control interactions between large motor

soft-start scheme and single-phase induction motors

▪ Distribution feeder energized as cold load pick-up

▪ Control interactions when three phase motor tries soft start– Solved by carrying out staggered start of three phase induction

motors



Grid forming and grid following resource in black start

▪ A 100 MVA grid forming solar plant

▪ Modeling assumptions:

– Each transformer has its own energization resistor to reduce inrush currents.

– No soft starter is utilized for induction motor loads. Motors are all 3 phase.

– A 50 km transmission line connects the grid forming plant to the load center.



Instability if control loops are not tuned appropriately

▪ More chance of instability with increase in reactive power loading of network

▪ Aggressively tuned grid following converters can dominate over a grid forming converter

Dynamic characteristics of load play a role

Varying levels of static load with induction motor load introduce new unstable regions

Stable Stable but < 5% damping Unstable



Black start of IEEE 14 bus test system

▪ PV at bus 2 and 6 are grid forming

▪ PV at bus 1 is grid following

▪ First black start bottom portion of the network

▪ Then bring PV6 online

▪ Then restore rest of the network



Black start of bottom portion of the network

▪ t=0, PV2 transformer and line energization

▪ t=1, load on bus 2 energization

▪ t=2, line 1 – 2 energization

▪ t=2.5, PV1 transformer energization

▪ t=3.5, PV1 breaker closing (with aux load) and enable its PLL

▪ t=4.5, PV1 control de-blocking

▪ T=7 – 9s, lines 2 – 5 and 1 – 5 energization

Grid forming

Grid following



Black start of bottom portion of the network (cont’d)

▪ t=0, PV2 transformer and line energization

▪ t=1, load on bus 2 energization▪ t=2, line 1 – 2 energization▪ t=2.5, PV1 transformer

energization▪ t=3.5, PV1 breaker closing (with

aux load) and enable its PLL▪ t=4.5, PV1 control de-blocking▪ T=7 – 9s, lines 2 – 5 and 1 – 5

energization

Oscillations between grid forming and grid following devices during initial restoration time



But if controllers are tuned well…

▪ Second GFM synchronizes at 22s

▪ Large variety of induction motor load present

▪ Start up of induction motors have to be coordinated



Key takeaways

▪ Inverter based resources can provide black start services.

▪ Many methods to reduce energization current of transformers– Soft start energization resistor method used– Detailed analysis to be carried out to determine appropriate soft start method

▪ Induction motor start-up can significantly use up inverter capacity and to be considered while sizing black start resources

▪ Load model interactions between different motor types can also introduce challenges during black start

▪ Control interactions can occur between grid forming and grid following devices



Together…Shaping the Future of Energy™
