using grid-forming inverters to improve transient

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Using Grid-Forming Inverters to Improve Transient Stability of Autonomous Microgrids and Dynamic

Distribution Systems

Wei Du

April 29, 2019, Seattle

2

OUTLINE

Grid-Forming Inverter & Control

Modeling Work at PNNL & Validation against CERTS Microgrid Field Test Results

Transient Stability Study of Large-Scale Distribution Systems with High Penetration of PVs

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Grid-Following vs. Grid-Forming

EXL

ω

E δ

P,Q

I θ+φ V θ

P,Q

grid

I (θ+φ)

+ Current Control (PLL+ Current loop)

+ Control P & Q

- Do not control voltage & frequency

- ‘Negative Load’ to Power System

- Reduce System Inertia

+ Voltage & Frequency Control

+ Can Work in islanded mode

+ Autonomous Load Tracking

- No Direct Control of Current

- Overload Issues

Grid-Following (Current Source) Grid-Forming (Voltage Source)

Most grid-connected sources (PV, fuel

cells) use grid-following control

Grid-forming source is crucial for

islanded microgrid operation

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Overload Mitigation Controller

• Droop control has already been widely used in microgrids

• Grid-Forming inverters have overload issues: currents are not controlled

• Address the two overload issues by actively controlling sources’ frequency

• Part of the sources are overloaded: Overload Transfer

• All the sources are overloaded: Under Frequency Load Shedding

Overload Mitigation Controller

-+

mp

Pset

P ++

ω0

+

kppmax+kipmax/s

+

+

-

-+

+

Pmin

0

0

Pmax

P

ω

kppmax+kipmax/s

Δω +

5

0

20

40

60

80

A1 Power kW

A2 Power kW

-500

0

500

Load Voltage Phase a

Load Voltage Phase b

Load Voltage Phase c

0 0.2 0.4 0.6 0.8 1

-100

0

100

Time Seconds

A1 3 Phase Currents

A2 3 Phase Currents

Overload

Overload Transfer

Without Pmax Controller

• When one source is dispatched near its

maximum generation, a load step can result in

overload

• Collapse the dc bus of inverters, stall the

synchronous generators, etc.

A Two-Source System

Z1

A1

XLA1

Z2

K

A2

XLA2

(EA1,δA1) (EA2,δA2)

Load 1 Load 2

0 10 20 30 40 50 60 70 8059

59.2

59.4

59.6

59.8

60

60.2

60.4

60.6

Power[kW]

Fre

quency[H

z]

P

max=60kW

Pset-A2

=5kW

Pset-A1

=55kW

PA2

=20kW PA1

=70kW


6

0

20

40

60

80

A1 Power kW

A2 Power kW

-500

0

500




0 0.2 0.4 0.6 0.8 1

-100

0

100

Time Seconds

A1 3 Phase Currents

A2 3 Phase Currents

Overload

Overload Transfer


With Pmax Controller

• Transfer the extra load to non-overloaded

sources by reducing the frequency rapidly

• The change of phase angle redistributes

power flow between sources

0

20

40

60

80

59.6

59.8

60

0 0.2 0.4 0.6 0.8 1

2

3

4

Time Seconds

A1 Power kW

A2 Power kW

A1 Frequency Hz

A2 Frequency Hz

Phase Angle Degree

-500

0

500

-100

0

100




A1 3 Phase Currents

A2 3 Phase Currents

Overload

Change of Phase Angle

A Two-Source System

Z1

A1

XLA1

Z2

K

A2

XLA2

(EA1,δA1) (EA2,δA2)

Load 1 Load 2

0 10 20 30 40 50 60 70 8059

59.2

59.4

59.6

59.8

60

60.2

60.4

60.6

Power[kW]

Fre

quency[H

z]

P

max=60kW

Pset-A2

=5kW

Pset-A1

=55kW

PA2

=20kW PA1

=70kW

With Pmax ControllerWithout Pmax Controller

0 10 20 30 40 50 60 70 8059

59.2

59.4

59.6

59.8

60

60.2

60.4

60.6

Power[kW]

Fre

quency[H

z]

P

max=60kW

PA2

=30kW

Pset-A1

=55kW

Pset-A2

=5kW

PA1

=60kW

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How do thousands of grid-forming/grid-following

inverters impact the transient stability of large-scale

distribution systems?

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• Challenges in simulation: Neither the electromagnetic simulation nor the positive

sequence-based transient stability simulation works for large-scale, three-phase,

unbalanced distribution systems simulation

• Solutions: PNNL developed an open-source software—GridLAB-D, which can

conduct three-phased transient stability simulations for distribution systems.

Dynamic models include:

• Synchronous generators

• Motors

• Grid-Forming/Grid-Following inverters

• …

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XLE∠δa

E ω

XL

E ω

XLE∠δc

E ω

E∠δb Distribution

System

Network

Solution

Grid-Forming Controller

Pi,Qi,Vgi

E,ω

Inverter model and the interface to the

distribution system

Modeling of Grid-Forming Inverters in GridLAB-D

• Grid-Forming Inverter: Three-phase balanced voltage source behind XL &

Controller

• Network Solution: Extension of Current Injection Method, which allows three-

phase power flow calculation

P-f Droop and Overload Mitigation

Q-V Droop

Inverter model and the interface to the

distribution system

Phase-Lock Loop

Current Loop

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Modeling of Grid-Following Inverters in GridLAB-D

• Grid-Following inverters are usually modeled as controllable PQ node in

traditional transient stability analysis software, PLL and current loops are ignored

• In GridLAB-D, they are modeled as controllable voltage sources, and detailed

PLL and current loop are modeled

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-50

0

50

100

150

A1 Power kW

B1 Power kW

ESS Power kW

LB4 Power kW

-200

0

200

A1 Current phase a A

A1 Current phase b A

A1 Current phase c A

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

-200

0

200

Time Seconds

B1 Current phase a A

B1 Current phase b A

B1 Current phase c A

AEP Test Data 3/18/2016 10:53:09.5829

-50

0

50

100

150

A1 Power kW

B1 Power kW

ESS Power kW

LB4 Power kW

-200

0

200




0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

-200

0

200

Time Seconds

B1 Current phase a A

B1 Current phase b A

B1 Current phase c A

Simulation

Field Test Results

PSCAD Simulation, Time

Step = 50 μs

Under-Frequency Load Shedding: Generator vs. Inverter

Feeder A

Feeder B

Inverter-based Source Inverter-based Source

Energy Storage Synchronous Generator

Load Bank 3 Load Bank 4

Load Bank 5

Feeder C

Load Bank 6Grid

Static Switch

A1

ESS B1

A2

• In most microgrids, load shedding is achieved through fast

communication. The CERTS microgrid can do under-frequency

load shedding.

• By ignoring electromagnetic transients, GridLAB-D is able to

conduct transient analysis of large-scale, three-phase,

unbalanced distribution systems

CERTS/AEP Microgrid

Time Step = 5 ms

Validation against CERTS/AEP Microgrid Test Results

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An Islanded IEEE 123-Node Test Feeder with High Penetration of PVs

• Synchronous Generators: 3*600 kW

• 6 Distributed PV Inverters: 1400 kW

• Peak Load: 2150 kW + 490 kvar

• PV Penetration: 65% (compared to

peak load)

• Substation voltage is lost due to

extreme weather events, three

microgrids work as an islanded

networked microgrid

• Contingency: Loss of Generator 3

Simulation on a Modified IEEE 123-Node Test Feeder

Gen PV

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PV: Grid-Following controlled at MPP

• PV operates at Maximum Power Point

• Contingency: Loss of Generator 3

• Frequency drops fast due to the low inertia, 12%

Loads are tripped

GridLAB-D Simulation


Load shedding

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• As the penetration of PVs continues increasing,

dynamic reserve of PVs has the potential to

improve the transient stability

• How do we use this reserve?

33% Reserve


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33% Reserve

• PV: Grid-Following with Frequency-Watt

• Not only reduces the system inertia, but also results in

oscillations between generators and inverters

• Loads are still tripped

Load shedding

Grid-Following with Frequency-Watt

0.05s 4% droop 0.25s

Frequency-Watt Control

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Load shedding


Improve frequency control


• Grid-Forming PV with dynamic reserve can improve

frequency stability, load shedding is avoided

• Grid-forming inverters response to load changes

instantaneously

Grid-Forming

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Load shedding


Mitigate voltage unbalance

Grid-Forming


• Grid-Forming inverters can improve voltage control,

voltage unbalance is mitigated

• All PV inverters become three-phase, balanced voltage

source behind XL, system is stiffer

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• The voltage profile of the entire distribution feeder is

significantly improved

• The three-phase unbalanced characteristic cannot be

easily reflected by positive-sequence-based

simulation

Grid-FormingGrid-Following

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Conclusion

• A three-phase, electromechanical modelling approach is proposed to model Grid-Forming/Grid-Following inverters for large-scale distribution system study

• The developed Grid-Forming inverter model in GridLAB-D is validated against CERTS Microgrid test results

• PV Grid-Forming inverters with dynamic reserve can improve Frequency & Voltage stability of distribution systems

• More work needs to be done in the Dynamic Distribution Systems research area

Thank you

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21

-50

0

50

100

150

A1 Power kW

A2 Power kW

ESS Power kW

LB4 Power kW

-200

0

200




0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

-200

0

200

Time Seconds




AEP Test Data 5/12/2016 09:18:51.8316

-50

0

50

100

150

A1 Power kW

A2 Power kW

ESS Power kW

LB4 Power kW

-200

0

200




0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

-200

0

200

Time Seconds




Simulation

Under-Frequency Load Shedding: Inverter vs. Inverter

Feeder A

Feeder B

Inverter-based Source Inverter-based Source

Energy Storage Synchronous Generator

Load Bank 3 Load Bank 4

Load Bank 5

Feeder C

Load Bank 6Grid

Static Switch

A1

ESS B1

A2

Validation against CERTS/AEP Microgrid Test Results

CERTS/AEP Microgrid

Time Step = 5 ms

By ignoring electromagnetic transients, GridLAB-D is

able to conduct transient analysis of large-scale,

three-phase, unbalanced distribution systemsPSCAD Simulation, Time

Step = 50 μs

Field Test Results

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Tecogen’s response to fault

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Current Loop PI gains: kp=0.05, ki=5, but with feedforward term F=0.3

Coordinate

Transformation

ugqi

δPLLi

kpPLL

∆ωi

1/sδPLLi Ugi∠δgi

ω0

fPLLi

2π

+

+

kiPLL/s

++

+

-

+-

kpc+

-

+

++

+

igdi_ref

igdi

igqi_ref

igqi

ωL

ωL

ugdi

ugqi

edi

eqi

kic/s+

+

kpc

kic/s+

+

Pref changes from 0.5 pu to 1 pu at 2.1 s

Grid-Following Inverters

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Grid-Forming Inverter & Droop Control

• Grid-Forming inverter: voltage source behind a coupling reactance XL

A well designed XL is important for controller design: P∝δ, Q∝E

• Droop Control: make multiple inverters work together in a microgrid

P vs. f droop ensure sources are synchronized

Q vs. V droop avoids large circulating reactive power between sources

P vs. f droop Q vs. V droop

using grid-forming inverters to improve transient

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