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Grid Control 2.0 – Control and Stability in Inverter Dominated Power Systems Philipp Strauss with contributions of Thomas Degner, Daniel Duckwitz and Maria Nuschke

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German Project 1.12.2017 – 30.11. 2021, Budget ca. 10 Mio. Euro

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Motivation – Actual Situation

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Grid control is mainly based on bulk power plants with synchronous machines.

Share of power electronics is increasing considerably (PV/Wind)

Conventional power plants are inevitable to support system stability

Even during very high renewable power infeed conventional power plants need to run

Under certain splitting scenarios new control technologies of inverters could be needed to support stability

Screen shot: www.energy-charts.de

Solar Wind

Conventional > 100 MW

Power Production Germany, May 2016

Export

Non-representative day with massive inverter participation In Germany

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Project Objectives

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Proving, that interconnected power systems can be operated stably with very high shares of inverters by applying suitable control technology

Investigate stability for separated system parts during system split

Preparation of the implementation -

concretely for the German part of the continental European power system

= ~

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Stakeholders in the project

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Grid Control 2.0 – Consortium

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Hypothesis

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„Momentary reserve and grid forming in the power system can be sufficiently provided by power units with grid forming inverters.“

Example: SelfSync*)- control with - f(P) and V(Q)-droops - phase feed-forward control - here: decoupled control of P and Q

Provision of momentary reserve by inverter coupled power units can be integrated seamlessly into the existing control structure

*) Selfsync: Engler,A.:„Regelung von Batteriestromrichtern in modularen und erweiterbaren Inselnetzen“, Dissertation, Fachbereich Elektrotechnik der Universität Kassel 2001

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Grid Control 2.0 - Workflow

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Detailed inverter models with grid forming and grid following controls

Simulation Frequency control

One-Machine/ One -Inverter-Modell

Studies: - f/V-stability - Angle stability - Grid protection - Islanding - Quality of supply

Inverter models

Models: - Grid - Electrical loads - Conv. generators WP 5:

Laboratory tests Field tests

WP 6: Transformation path, Preparation for implementation

WP1: Simulation-methodology

WP 2: Szenarios, critical grid states

WP 3: Studies for grid integration and system stability

WP 4: Development of robust inverter control methods

Power System

Requirements

for inverters

Inve

rter

ca

pabi

litie

s

© Fraunhofer

Project Transstabil-EE: Experimental Short-Circuit Test of Grid-forming Inverters

Experimental validation of LVRT-behavior using state of the art test equipment. Device under test: Grid forming inverter with virtual

impedance Control is able to comply with connection rules, Inherent, instantaneous (first cycle) reaction by applying

three-stage current limitation Virtual Impedance Fall-back current control PWM Blocking

Duckwitz, Knobloch et. al.: Experimental Short-Circuit Testing of Grid-Forming Inverters in Microgrid and Interconnected Mode, NEIS Conference, September 20, 2018, Hamburg

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FRT- Test Setup at Fraunhofer IEE

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Experimental Short-Circuit Testing of Grid-Forming Inverters NEIS 2018

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Test result: 50% voltage dip Grid forming inverter with 3-stage current limitation

Virtual impedance 𝑘𝑘 = 2 Δ𝑖𝑖𝐵𝐵 = 𝑘𝑘 ⋅ Δ𝑢𝑢 = 1

xΣ Δ𝑢𝑢

Inherent active current contribution.

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Experimental Short-Circuit Testing of Grid-Forming Inverters NEIS 2018

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Test result: 12% voltage dip - Grid forming inverter with virtual impedance and fallback current control

PWM blocking Current control

Virtual Impedance

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Test system and grid data Single-Line-Diagram

offen Maria Nuschke, 17.10.2018

Grid data and topology from German TSOs [1], Generator regulators assumed from DigSilent [2]

Implementation of the test model in DigSilent Powerfactory 2018

RMS simulations with system split

Scenarios:

Variation of export by load change

Variation of inverter ratio by change of installed capacity of the SG

VU … Full converter SG … Synchronous generator

Terminal VU1

Terminal VU2

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0 5 10 15 20

50

50.2

50.4

Freq

uenc

y [H

z]

FrequencyROCOF=0.73Hz/s

Test with different inverters SG + PV

offen

Grid-supporting PV generators according to WECC-Model [4] and [5], current controlled behaviour

PV generators with rated power 1,4 GVA at terminals VU1 and VU2 with power curtailment 20 p.u. Power/ p.u. Frequency at over-frequency, here without deadband

40% export

40% inverter ratio

Maria Nuschke, 17.10.2018 19

© Fraunhofer

0 5 10 15 20

50

50.2

50.4

Freq

uenc

y [H

z]

Frequency SG,VUsFrequency COIROCOF=0.43Hz/s

Test with different inverters SG + SelfSync

offen

Grid-forming inverters according to Self-Sync approach [6]

At terminals VU1 und VU2 SelfSync inverters with rated power of 1.4 GVA

40% export

40% inverter ratio

Determination of frequency stability indicators with center of inertia frequency (COI)

Maria Nuschke, 17.10.2018 21

© Fraunhofer

0 5 10 15 20

50

50.2

50.4

Freq

uenc

y [H

z]

Frequency SG,VUsFrequency COIROCOF=0.47Hz/s

Test with different inverters SG + VCI

Björn Oliver Winter, 17.10.2018

Grid-forming inverters according to VCI approach, [7]

At terminals VU1 and VU2 VCI inverters with rated power of 1.4 GVA

40% export

40% inverter ratio

Determination of frequency stability indicators with center of inertia frequency (COI)

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Result overview comparison to reference case Sensitivity export, inverter ratio 40%

offen

Grid-forming approaches (SelfSync and VCI) Same steady state frequency

deviation as in reference case „only SG“

Damped dynamic compared to „only SG“: Lower maximum frequency,

smaller RoCoF, higher 𝑇𝑇𝑁𝑁

Grid-supporting approach (PV) Deterioration in transient and

steady state frequency behavior Higher maximum frequency,

larger steady state frequency deviation, higher RoCoF and smaller 𝑇𝑇𝑁𝑁

Maria Nuschke, 17.10.2018 25

© Fraunhofer

Conclusions and next steps The project Grid Control 2.0 is assessing the application of grid

forming inverters also for the interconnected power system

FRT tests for grid forming inverters have been performed successfully within the Project Transstabil-EE

Splitting scenarios – in which system parts with very high shares of inverters are separating – are being investigated

In the frame of the project Grid Control 2.0 we will organize the a regular knowledge exchange within the international project cluster „Inverter Dominated Power Systems”

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© Fraunhofer

Project: Grid Control 2.0

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The project underlying this report was funded by the Federal Ministry for Economic Affairs and Energy under grant number 0350023A. The responsibility for the content of this publication lies with the authors and does not necessarily reflect the opinion of the consortium of the project Grid Control 2.0 (Netzregelung 2.0).

Contact: Dr. Philipp Strauss, Fraunhofer IEE Netzregelung-2.0@iee.fraunhofer.de