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TRANSCRIPT
© Fraunhofer IWES Slide 1
Grid-Forming Inverters in Microgrids
Axel Seibel, Peter Unruh
Department: Converters and Drive Technology
© Fraunhofer IWES Slide 2
Contents
Introduction
Improving the control of grid-forming inverters
SelfSync
Improving SelfSync
Robust control
Practical tests
Conclusion
© Fraunhofer IWES Slide 3
Grid-Forming Inverters in MicrogridsIntroduction
„Grid-forming“ means that an operating device participates actively on forming the grid voltage.
Grid-forming inverters act as voltage sources.
f U
P Q
𝑓𝑓 = 𝑓𝑓0 − 𝑘𝑘𝑝𝑝 ∙ (𝑃𝑃 − 𝑃𝑃0) 𝑈𝑈 = 𝑈𝑈0 − 𝑘𝑘𝑞𝑞 ∙ (𝑄𝑄 − 𝑄𝑄0)
© Fraunhofer IWES Slide 4
Grid-Forming Inverters in MicrogridsIntroduction
A high penetration of grid-forming inverters is inherently system stabilizing.
This approach can cover:
Virtual inertia
Uninterruptable power supply
Black start capability…
© Fraunhofer IWES Slide 5
Grid-Forming Inverters in MicrogridsBasic Idea for grid forming inverters
Transfer droops of power plant behaviour with synchronous generators to inverters
f U
P Q
∆δ
∆U Q
P
E0U
I ZL = jωLL
UE0
δ
© Fraunhofer IWES Slide 6
Grid-Forming Inverters in MicrogridsControl – SelfSyncTM
SelfSync™ is a technique that based on conventionel droops (f(P)- respectively U(Q)-characteristic)
An additional angle feedforward improves stability and the dynamic behavior
Q‘
TWRTm
f0
st1
st1‘
P‘ ∆f
TWRTm
st2
Q
P
∆u
U0
u
- ϕ
SelfSync™
-20
-10
0
10
20
-500
-250
0
250
500
[A][V]
Load Voltage
Load Current
Current Inv1
Current Inv2
Disconnection
Load Current
Current Inv1Current Inv2
Load Voltage
© Fraunhofer IWES Slide 7
Grid-Forming Inverters in MicrogridsControl – SelfSyncTM
Transfer droops of power plants with synchronous generators to inverters
Represent the “Best of” behaviour of a synchronous generator in the control structure
+ working well and stable in well planned microgrids (industrial application)
- Line resistance is ignored:Quality criteria (performance) of control deteriorates
Q‘
TWRTm
f0
st1
st1‘
P‘ ∆f
TWRTm
st2
Q
P
∆u
U0
u
- ϕ
SelfSync™
© Fraunhofer IWES Slide 8
Grid-Forming Inverters in MicrogridsControl – challenge 20xx
Wishes for the future of energy production (20xx):
100% renewables in all voltage levels
Arbitrary spatial distribution of grid-forming inverters
Stable control in all voltage levels
……
Handicaps for the future of energy production (20xx):
Storage of energy
Stability of the electrical energy system
Market (investments, politics, industrial interests, ……..)
© Fraunhofer IWES Slide 9
Grid-Forming Inverters in MicrogridsWhat happen at low voltage level?
High voltage (HV) cable has XL/RL ratio of appr. 7 (mainly inductive )
Low voltage (LV) cable has a XL/RL ratio of appr. 2
E0U
I ZL = jωLL
∆δ
∆U Q
P ∆δ
∆U Q
P
E0U
I ZL = RL+ jωLLHV LV
© Fraunhofer IWES Slide 10
Grid-Forming Inverters in MicrogridsControl – challenge
𝑃𝑃 =𝐿𝐿𝑘𝑘𝑠𝑠 + 𝑅𝑅𝑘𝑘
𝐿𝐿𝑘𝑘𝑠𝑠 + 𝑅𝑅𝑘𝑘 2 + 𝜔𝜔𝐿𝐿𝑘𝑘 2 𝑈𝑈2 − 𝑈𝑈𝐸𝐸0 cos𝛿𝛿 −𝜔𝜔𝐿𝐿𝑘𝑘
𝐿𝐿𝑘𝑘𝑠𝑠 + 𝑅𝑅𝑘𝑘 2 + 𝜔𝜔𝐿𝐿𝑘𝑘 2 𝑈𝑈𝐸𝐸0 sin 𝛿𝛿
𝑄𝑄 =𝜔𝜔𝐿𝐿𝑘𝑘
𝐿𝐿𝑘𝑘𝑠𝑠 + 𝑅𝑅𝑘𝑘 2 + 𝜔𝜔𝐿𝐿𝑘𝑘 2 𝑈𝑈2 − 𝑈𝑈𝐸𝐸0 cos𝛿𝛿 +𝐿𝐿𝑘𝑘𝑠𝑠 + 𝑅𝑅𝑘𝑘
𝐿𝐿𝑘𝑘𝑠𝑠 + 𝑅𝑅𝑘𝑘 2 + 𝜔𝜔𝐿𝐿𝑘𝑘 2 𝑈𝑈𝐸𝐸0 sin 𝛿𝛿
Difficulty: distribution grid
angle/frequency deviation results in reactive power flow.
Arbitrary spatial distribution
∆δ
∆U Q
P
E0U
I ZL = RL+ jωLL
© Fraunhofer IWES Slide 11
Grid-Forming Inverters in MicrogridsControl – Improving SelfSync
Feedback matrix F
kp, kq reflect the droops
angle feedforward: improve stability and transient behavior (kp‘, kq‘)
kp = Δf/Pmax kq = ΔU/Qmax
δ f
U
Q
PUsoll
fsoll
UN fN δN
𝐺𝐺11 𝐺𝐺12 𝐺𝐺13𝐺𝐺21 𝐺𝐺22 𝐺𝐺23
G
F1
𝑇𝑇𝑓𝑓𝑠𝑠 + 1
1𝑇𝑇𝑓𝑓𝑠𝑠 + 1
--
-
© Fraunhofer IWES Slide 12
Grid-Forming Inverters in MicrogridsImproving SelfSync - choice of the parameters kp’, kq’
With a higher resistive grid impedance the parameter kq‘ gets more relevance (here Lk = 1 mH, Rk = 0.4 Ω).
© Fraunhofer IWES Slide 13
-10-2
-10-4
-10-6
10-6
10-5
10-4
10-3
Grid-Forming Inverters in MicrogridsComparing grid parameters to choose kp’, kq’
Lk = 1 mH Lk = 3 mH
Different Lk = xx mH (Rk = 0.4 Ω)
© Fraunhofer IWES Slide 14
Grid-Forming Inverters in MicrogridsComparing grid parameters to choose kp’, kq’
Lk = 1 mH Lk = 3 mH
Different Lk = xx mH (Rk = 0.4 Ω)
© Fraunhofer IWES Slide 15
Grid-Forming Inverters in MicrogridsComparing grid parameters to choose kp’, kq’
Lk = 1 mH Lk = 3 mH
Different Lk = xx mH (Rk = 0.4 Ω)
© Fraunhofer IWES Slide 16
Grid-Forming Inverters in MicrogridsExperimental results – voltage step
Voltage step from 230 Veff to 245 Veffand back
Grid impedance (0.345 Ω, filter)
Smooth settling with double angle feedforward
19 20 21 22 23 24 25-20
-10
0
10
20Double angle feedforward
I out [A
]
24.5 25.5 26.5 27.5 28.5 29.5 30.5 31.5-20
-10
0
10
20Single angle feedforward
I out [A
]
36.3 37.3 38.3 39.3 40.3 41.3 42.3 43.3 44.3-20
-100
10
20
Without angle feedforward
I out [A
]
Time [s]
© Fraunhofer IWES Slide 17
Grid-Forming Inverters in MicrogridsExperimental results – frequency step
Frequency step from 50Hz to 49.5Hz and back
Smooth settling
Instantaneous reaction
36 37 38 39 40 41 42 43 44 450
2.5
5
7.5
10
P [k
W]
Double angle feedforward
36 37 38 39 40 41 42 43 44 4549
49.5
50
50.5
51
Freq
uenc
y [H
z]
10 11 12 13 14 15 16 17 18 190
2.5
5
7.5
10
P [k
W]
Single angle feedforward
Time [s]10 11 12 13 14 15 16 17 18 19
49
49.5
50
50.5
51
Freq
uenc
y [H
z]
© Fraunhofer IWES Slide 18
Grid-Forming Inverters in MicrogridsConclusion
Grid-forming inverters are inherently system stabilizing with regard to the power grid control
Improved control behavior due to the angle feedforward
For an optimal controller design an impedance estimation tool was applied
Outlook:Actual we are designing a robust control for grid forming invertersThis will be the next story!
