Niagara 2016 Symposium on Microgrids October 2021, 2016 Niagara, Canada Parallel Operation of Distributed Generators by Virtual Synchronous Generator Control in Microgrids Jia Liu* and Toshifumi Ise Osaka University, Japan October 21, 2016
2 Contents Introduction Parallel Inverters Synchronous Generator Inverter Conclusion
3 Contents Introduction Parallel Inverters Synchronous Generator Inverter Conclusion
Inertial feature of Synchronous Generators 4 Synchronous Generator Kinetic Energy of Rotating Mass Swing Equation Frequency fluctuation under active power transition limited by the inertia DC/AC Inverter No intrinsic relation Undesirable frequency dynamics
5 Concept of VSG Control Conventional Droop Control Load Sharing Smooth Transition between Islanding and Gridconnection Swing Equation Emulation Inertia Support Virtual Synchronous Generator (VSG) Control A New Concept of Inverter Control in AC Microgrid
6 Objectives Conventional droop control To apply VSG control to DCAC inverters in microgrids Two topics to be discussed Parallel Inverters Synchronous Generator Inverter
What is the benefit? 7 Without Frequency Restoration Slower frequency variation rate Frequency during Loading transition With Frequency Restoration Less maximum frequency deviation Frequency during Threephase ground fault cleared in 0.1 s Less maximum frequency deviation
8 Contents Introduction Parallel Inverters Synchronous Generator Inverter Conclusion
.. 9 Microgrid: Parallel Inverters DG1 S base1 =10kVA DG2 S base2 =5kVA L f1 :1mH 0.0942pu L f2 :1mH 0.0471pu 2 parallel DGs equipped with VSG control C f1 :10μF 66.31pu C f2 :10μF 33.16pu * P 0 _ i V out1 V out2 *, Q 0_ i I out1 I out2 MGCC Grid 100m 22mm 2 (0.14j0.015)Ω R * line1:0.035pu L * line1:0.00375pu 100m 22mm 2 (0.14j0.015)Ω R * line2:0.0175pu L * line2:0.00188pu BK3 Fluctuated loading BK1 BK2 V bus BUS Load1 Load2 Microgrid Central Controller (MGCC) Frequency and voltage restoration
... VSG Control Scheme (Basic Part) 10 ^ V bus E 0. (Constant) k q V Q Droop Controller k q :V Q droop coef. Q 0 :Set value of reactive power Q 0 Q 0 ^ Distributed Generator P in Swing Equation Function Energy Storage V Q Droop Q ref PI bus E 0 Governor Model P 0 V bus Estimator ω g Q ref. Z ls ω m LPF 1/s E Stator Impedance Adjuster θ m P out Q out V pwm θ pwm Frequency Detector Power Meter L f V out(abc) PWM C f V out(αβ) abc/αβ Z line I out(abc) I out(αβ) BUS. Swing Equation J:Virtual inertia D:Damping factor ω m :Virtual rotor frequency ω m ω 0 (Constant) k p P 0 P in ω P Droop Controller k p : ω P droop coef. P 0 :Set value of active power
... Z ls Q 0 ^ Distributed Generator P in Swing Equation Function Energy Storage V Q Droop Q ref PI bus E 0 Governor Model P 0 V bus Estimator ω g ω m LPF v out_β i out_α i out_β R line ω m L line R line v out_α v line_α v line_β 1/s P out Q out VSG Control Scheme E Stator Impedance Adjuster θ m v^ bus_α v^ bus_β V pwm θ pwm Frequency Detector αβ/ρθ Power Meter Estimate bus voltage for proper reactive power sharing (Enhanced Part) ^ V bus L f V out(abc) PWM C f V out(αβ) abc/αβ Z line I out(abc) I out(αβ) BUS. I out I thresh 0 E θ m i out_α i out_β ρθ/αβ R ls ω m Bus voltage estimator L ls R ls Stator Impedance Adjuster e α v pwm_α e β v pwm_β k Z v Zls_α v Zls_β ΔZ ls 1 1 ( X / R) 1 1 ( R / X ) 2 2 αβ/ρθ Stator Impedance Adjuster V pwm θ pwm L ls0 11 Virtual ΔR ls = R ls Stator Impedance Calculator ΔL L 1/ω ls ls 0 Constant part Transient power sharing Increased damping Transient part Overcurrent limiting
12 Control Parameters Parameter Value Parameter Value 10 kva 5 kva 200 V 0.0125 pu 376.99 rad/s 8 s 1 pu 17 pu 0.05 s 1 pu 6.39 mh 0 pu 13.81 mh 20 pu 1.79 pu 5 pu 5
Droop Control Simulation Results Loading Transition VSG Control 13 Reduced frequency deviation Islanding Loading Transition Islanding Loading Transition
Simulation Results ThreePhase Ground Fault Droop Control VSG Control 14 Reduced frequency deviation 3P Ground Fault 3P Ground Fault
.. Fault Current During ThreePhase Ground Fault 15 Without Transient Stator Impedance With Transient Stator Impedance I out I thresh 0 k Z ΔZ ls 1 1 ( X / R) 1 1 ( R / X ) 2 2 Virtual ΔR ls = R ls Stator Impedance Calculator ΔL L 1/ω ls ls 0 L ls0 E θ m ρθ/αβ e α v pwm_α e β v pwm_β αβ/ρθ V pwm θ pwm i out_α ω m i out_β R ls L ls R ls v Zls_α v Zls_β Stator Impedance Adjuster Fault Current is limited by Transient Stator Impedance
16 Effect of Constant Stator Reactance Without Constant Stator Reactance With Constant Stator Reactance Simulation Oscillation eliminated Oscillation damped Islanding Loading Transition P 0_2 Change Islanding Loading Transition P 0_2 Change Experiment Oscillation eliminated Oscillation damped Loading Transition P 0_2 Change Loading Transition P 0_2 Change
StateSpace Model of Islanded Microgrid 17 Disturbance: Loading Transition Change of Active Power Set Value Output: Frequency Active Power The damping ratio of oscillation in output parameters is determined by the eigenvalues of state matrix A
18 Relation between Output Reactance and Damping Ratio Total output reactance X increases Damping ratio ζ increases Oscillation can be damped by increasing output reactance
19 Transient Load Sharing When the disturbance is a loading transition If the total output reactance of each VSG is of the same per unit value, poles are cancelled by zeros and oscillation is eliminated
.. 20 Design of Constant Stator Reactance I out I thresh 0 k Z ΔZ ls 1 1 ( X / R) 1 1 ( R / X ) 2 2 Virtual ΔR ls = R ls Stator Impedance Calculator ΔL L 1/ω ls ls 0 L ls0 E θ m ρθ/αβ e α v pwm_α e β v pwm_β αβ/ρθ V pwm θ pwm i out_α R ls ω m L ls i out_β R ls v Zls_α v Zls_β Stator Impedance Adjuster
21 Effect of Constant Stator Reactance Without Constant Stator Reactance With Constant Stator Reactance Simulation Reactive power sharing improved Islanding Loading Transition P 0_2 Change Islanding Loading Transition P 0_2 Change Experiment Reactive power sharing improved Loading Transition P 0_2 Change Loading Transition P 0_2 Change
22 Cause of Poor Reactive Power Sharing If the input of droop controller is equal, reactive power should be shared properly ω P Droop V Q Droop
. Bus Voltage Estimator for Proper Reactive Power Sharing 23 ^ V bus E 0. (Constant) k q V Q Droop Controller k q :V Q droop coef. Q 0 :Set value of reactive power Q 0 Q ref. Z ls Q 0 ^ Distributed Generator P in Swing Equation Function Energy Storage V Q Droop Q ref PI bus E 0 Governor Model P 0 ω g ω m 1/s E Stator Impedance Adjuster θ m V pwm θ pwm Frequency Detector L f V out(abc) PWM C f V out(αβ) abc/αβ Z line I out(abc) I out(αβ) BUS V bus Estimator LPF P out Q out Power Meter. v out_β i out_α ω m L line i out_β R line R line v out_α v line_α v line_β v^ bus_α v^ bus_β αβ/ρθ Estimate bus voltage for proper reactive power sharing ^ V bus Bus voltage estimator
24 Contents Introduction Parallel Inverters Synchronous Generator Inverter Conclusion
25 Microgrid: SG Inverter Standalone microgrid in remote area SG (10 kva): Roundrotor moved by gas engine DG (10 kva): Photovoltaic panels Load: Threephase loads and singlephase Loads
26 Issues of Small Rating SG SG Controller SG Parameters Parameter Value Parameter Value 200 V 1 pu 10 kva 0 pu 376.99 rad/s 20 pu 0.16 s 5 pu 20 pu 1 s 0.025 s AVR LPF cutoff frequency 20 Hz Poor inertia Impedance Model 0.219 pu 0.027 pu 0.01 pu 6.55 s 0.039 s 0.85 s 0.071 s Slow governor response
27 Simulation of Single SG Operation Initial loading : 3P 1 kw, 0.5 kvar Connected loading : 1P 4.8 kw, 2.1 kvar Ripples due to 3P unbalance introduced by 1P loading Slow governor response cannot restore this frequency drop immediately Rotor frequency decreased rapidly due to poor inertia
28 Control Requirements of DG 1. Reduce SG rotor frequency deviation If SG rotor frequency drop below 90%, SG may be unstable Virtual Synchronous Generator (VSG) Control Generate virtual inertia to provide transient frequency support Share active and reactive power with the SG 2. Eliminate NegativeSequence Current from SG Prevent SG from overheating and torsional stresses Active power filter (APF) 3. Both 12 should be realized in ONE inverter
.. 29 Proposed Modified VSG Control Q 0 V out_dg Q Droop Governor Model P 0 Distributed Generator P out_dg Q ref_dg P in_dg Swing Equation Function ω g_dg LPF Energy Storage PI ω m_dg 1/s E 0 I out _ sg( dq) Q out_dg E dg θ m_dg Z ls V comp(αβ) SG Neg.Seq. Compensation θ g_sg L f Stator Impedance Adjuster I out_dg DDSRF C f V pwm θ pwm Z line abc/ αβ PWM I out_dg(αβ) BUS I out_dg(abc) V out_dg(abc) I out_sg(abc) V out_sg(abc)
.... Q 0 V out_dg Q Droop Governor Model P 0 Distributed Generator P out_dg Q ref_dg P in_dg Swing Equation Function ω g_dg LPF Energy Storage PI ω m_dg 1/s E 0 I out _ sg( dq) Q out_dg E dg θ m_dg Z ls V comp(αβ) SG Neg.Seq. Compensation θ g_sg L f Stator Impedance Adjuster I out_dg DDSRF C f V pwm θ pwm Z line abc/ αβ PWM I out_dg(αβ) BUS I out_dg(abc) V out_dg(abc) I out_sg(abc) V out_sg(abc) Applied to both DG and SG voltage and current to extract positive and negative sequence components Output power, voltage and current of DG are calculated from only positive sequence components Prevent ripples due to negative sequence from entering the controller DDSRF Double Decoupled Synchronous Reference Frame (DDSRF) Decomposition V αβ T θ g T I αβ T θ g T θ g ω g T 2 T 2 T 2 T 2 V dqf V dqf I dqf I dqf PLL LPF LPF LPF LPF v q V dq V dq I dq I dq 30 LPF: 1 st order low pass filter (cutoff frequency 40 Hz)
.. SG Neg.Seq. Compensation 31 Q 0 V out_dg Q Droop Governor Model P 0 Distributed Generator P out_dg Q ref_dg P in_dg Swing Equation Function ω g_dg LPF Energy Storage PI ω m_dg 1/s E 0 I out _ sg( dq) Q out_dg E dg θ m_dg Z ls V comp(αβ) SG Neg.Seq. Compensation θ g_sg L f Stator Impedance Adjuster I out_dg DDSRF C f V pwm θ pwm Z line abc/ αβ PWM I out_dg(αβ) BUS I out_dg(abc) V out_dg(abc) I out_sg(abc) V out_sg(abc) Stator Impedance Adjuster SG Neg.Seq. Compensation Control SG Neg.Seq. to be 0
32 Parameter Design of Inverter Parameter Value Parameter Value 200 V 1 pu 10 kva 0 pu 376.99 rad/s 20 pu 0.16 s 5 pu 8.7 pu 0 s 1.122 mh 0.69 pu 1.836 mh 5 PI Controller for Reactive Power 0.05 pu PI Controller for SG Neg.Seq. Compensation 0.1 pu 0.01 s All parameters in red should be set equal to SG in per unit value to ensure proper transient and steady power sharing 20 Hz Design of Parameters in blue will be discussed in detail
33 Set equal to SG in order to share transient active power? Low frequency oscillation indicated by oscillatory conjugated eigenvalues
34 Constant Virtual Stator Inductance Output reactance of DG and SG should be set equal to share transient power, but how about the value? Connection of DG
35 Simulation of 1P Loading Transition SG only Initial loading : 3P 1 kw, 0.5 kvar Connected loading : 1P 4.8 kw, 2.1 kvar Improved SG rotor frequency deviation (Frequency Support of VSG) SG VSG Initial loading : 3P 2 kw, 1 kvar Connected loading : 1P 9.6 kw, 4.2 kvar (2x of SG only case)
36 Simulation of 1P Loading Transition w/o DDSRF and SG Neg. Seq. with DDSRF and SG Neg. Seq. Ripples due to unbalance have been eliminated
37 Phase Current w/o DDSRF and SG Neg. Seq. with DDSRF and SG Neg. Seq. Balanced SG current
38 Contents Introduction Parallel Inverters Synchronous Generator Inverter Conclusion
39 Conclusion Conclusion VSG control can provide inertia support for microgrids, leading to less fluctuant frequency Parallel inverters and SG inverter operations were established, and several related issues were solved Future Plan Operation of Multiple SGs Multiple inverters
Thank you for your kind attention! For more details, please refer to: J. Liu, Y. Miura, H. Bevrani, and T. Ise, Enhanced virtual synchronous generator control for parallel inverters in microgrids, IEEE Transactions on Smart Grid., doi: 10.1109/TSG.2016.2521405. J. Liu, Y. Miura, T. Ise, J. Yoshizawa, and K. Watanabe, Parallel operation of a synchronous generator and a virtual synchronous generator under unbalanced loading condition in microgrids, 8th International Power Electronics and Motion Control Conference (IPEMCECCE Asia), Hefei, China, 2016, pp. 37413748. J. Liu, Y. Miura, and T. Ise, Power quality improvement of microgrids by virtual synchronous generator control, 10th Electric Power Quality and Supply Reliability Conference (PQ2016), Tallinn, Estonia, 2016.