Grid-Forming Inverters in Microgrids

Transcription

1 Axel Seibel, Peter Unruh Department: Converters and Drive Technology Fraunhofer IWES Slide 1

2 Contents Introduction Improving the control of grid-forming inverters SelfSync Improving SelfSync Robust control Practical tests Conclusion Fraunhofer IWES Slide 2

3 Introduction Grid-forming means that an operating device participates actively on forming the grid voltage. Grid-forming inverters act as voltage sources. f U P ff = ff 0 kk pp (PP PP 0 ) UU = UU 0 kk qq (QQ QQ 0 ) Q Fraunhofer IWES Slide 3

4 Introduction 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 4

5 Basic Idea for grid forming inverters Transfer droops of power plant behaviour with synchronous generators to inverters I Z L = jωl L f U U E 0 P Q E 0 δ U δ P U Q Fraunhofer IWES Slide 5

6 Grid-Forming Inverters in Microgrids Control SelfSyncTM SelfSync is a technique that based on conventionel droops (f(p)- respectively U(Q)-characteristic) f0 P Q [V] 0 Load Current Fraunhofer IWES Slide 6 f st1 - Tm TWR Q st2 ϕ u u SelfSync 250 Load Voltage P U0 500 Current Inv1 TWR st1 An additional angle feedforward improves stability and the dynamic behavior Current Inv2 Tm Disconnection [A] Load Voltage Load Current Current Inv1 Current Inv2

7 Control SelfSync TM Transfer droops of power plants with synchronous generators to inverters P T m T WR P f st 1 f 0 - ϕ Represent the Best of behaviour of a synchronous generator in the control structure Q T m T WR Q st 1 st 2 u U 0 u + working well and stable in well planned microgrids (industrial application) SelfSync - Line resistance is ignored: Quality criteria (performance) of control deteriorates Fraunhofer IWES Slide 7

8 Control 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 8

9 What happen at low voltage level? High voltage (HV) cable has X L /R L ratio of appr. 7 (mainly inductive ) Low voltage (LV) cable has a X L /R L ratio of appr. 2 HV I Z L = jωl L LV I Z L = R L + jωl L U E 0 U E 0 δ P δ P U Q U Q Fraunhofer IWES Slide 9

10 Control challenge I Z L = R L + jωl L PP = LL kk ss + RR kk LL kk ss + RR kk 2 + ωωll kk 2 UU2 UUEE 0 cos δδ ωωll kk LL kk ss + RR kk 2 + ωωll kk 2 UUEE 0 sin δδ U E 0 QQ = ωωωω kk LL kk ss + RR kk 2 + ωωll kk 2 UU2 UUEE 0 cos δδ + LL kk ss + RR kk LL kk ss + RR kk 2 + ωωll kk 2 UUEE 0 sin δδ δ U P Q Difficulty: distribution grid angle/frequency deviation results in reactive power flow. Arbitrary spatial distribution Fraunhofer IWES Slide 10

11 Control Improving SelfSync U N f N δ N Feedback matrix F U soll f soll δ f F G GG 11 GG 12 GG 13 GG 21 GG 22 GG 23 1 TT ff ss + 1 P Q k p, k q reflect the droops angle feedforward: improve stability and transient behavior (k p, k q ) U 1 TT ff ss + 1 k p = Δf/P max k q = ΔU/Q max Fraunhofer IWES Slide 11

12 Improving SelfSync - choice of the parameters k p, k q With a higher resistive grid impedance the parameter k q gets more relevance (here L k = 1 mh, R k = 0.4 Ω). Fraunhofer IWES Slide 12

13 Comparing grid parameters to choose k p, k q Different L k = xx mh (R k = 0.4 Ω) 10-3 L k = 1 mh L k = 3 mh Fraunhofer IWES Slide 13

14 Comparing grid parameters to choose k p, k q Different L k = xx mh (R k = 0.4 Ω) L k = 1 mh L k = 3 mh Fraunhofer IWES Slide 14

15 Comparing grid parameters to choose k p, k q Different L k = xx mh (R k = 0.4 Ω) L k = 1 mh L k = 3 mh Fraunhofer IWES Slide 15

16 Experimental results voltage step 20 Double angle feedforward I out [A] Single angle feedforward Voltage step from 230 V eff to 245 V eff and back Grid impedance (0.345 Ω, filter) Smooth settling with double angle feedforward I out [A] I out [A] Without angle feedforward Time [s] Fraunhofer IWES Slide 16

17 Experimental results frequency step Frequency step from 50Hz to 49.5Hz and back Smooth settling Instantaneous reaction P [kw] Double angle feedforward Frequency [Hz] Single angle feedforward 51 P [kw] Frequency [Hz] Time [s] Fraunhofer IWES Slide 17

18 Conclusion 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 inverters This will be the next story! Fraunhofer IWES Slide 18

19 Contact Axel Seibel Königstor 59 D Kassel Fraunhofer IWES Slide 19