Analysis of Synchronization Stability for Multi VSCs Parallel-Connected to Weak Grids by Improved Net Damping Criterion
Abstract
:1. Introduction
- (1)
- Building a reduced small-signal model suitable for synchronization stability analysis of multi-VSC systems. The used VSC module is much simpler than the detailed one in [3] for synchronization stability studies owing to the neglection of fast inner current controls, but more accurate compared to the models given in [9,14] for counting the interactions between PLL and outer power controls.
- (2)
- Comprehensively applying net damping criterion to investigate multi-VSC system’s synchronization stability. The used criterion is a combination of the classical criterion propose by Canay [20] and an alternative one in our recent work [24], where the later can serve as a complement to enlarge the application feasibility of the classical criterion. The effectiveness of this method is confirmed by eigenvalue analysis. Additionally, with this method, the impact of various system parameters, including different grid strengths, power outputs and bandwidths of controllers, on closed-loop system stability are quantitatively examined.
2. Review of the Net Damping Criterion
2.1. The Classical Net Damping Criterion
2.2. Limitations and the Complemental Criterion
- Step (1)
- Applying the classical criterion, if the judgment result is instability then the system should be unstable; otherwise, if stability is predicted, go to Step (2);
- Step (2)
- Checking the damping in the low frequencies close to zero, stability is guaranteed for positive damping, while negative damping indicates instability.
3. System Modeling
4. Net Damping Based Stability Analysis
4.1. Effect of Grid Strength
4.2. Effect of Loading Level
4.3. Effect of AC Voltage Control Tuning
4.4. Effect of PLL Tuning
4.5. An Unusual Case
5. Simulation Studies
6. Conclusions
Author Contributions
Funding
Conflicts of Interest
Nomenclature
Ut, θt | Terminal voltage magnitude and phase |
θpll, ωpll | PLL output angle and frequency |
Udc, C | DC-link voltage and capacitance |
PCC | Point of common connection |
X1, X2 | Reactance between terminal bus of VSC1 and VSC2 to the PCC |
Xg | Reactance between PCC and the infinite bus |
kp, ki | Proportion and integral control gain |
Lg | Transmission line inductance between the PCC and the infinite bus |
Li | Equivalent line inductance between VSC terminal bus and the PCC |
Subscripts: | |
0 | Steady-state value |
d, q | Synchronous rotating reference frame signal d-axis and q-axis components |
x, y | Global reference frame signal x-axis and y-axis components |
ref | Reference signal |
Superscript: | |
p | Components in PLL rotating frame |
Appendix A
Sbase = 1 MW | Ubase = 690 V(phase to phase RMS value) | ||
ωbase = 2 πfbase | fbase = 50 Hz | Udcref = 1100 V | C = 0.09 F |
Lf = 0.1 p.u. | L1 = 0.1 p.u. | L2 = 0.1 p.u. |
DC-link voltage control | kp1 = 5 | ki3 = 60 |
AC voltage control | kp2 = 1 | ki2 = 100 |
Phase-locked loop | kp3 = 40 | ki3 = 1400 |
Current control | kp4 = 1 | ki4 = 50 |
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Wang, D.; Zhang, X.; Yang, L.; Huang, Y.; Huang, W.; Wu, C.; Li, S. Analysis of Synchronization Stability for Multi VSCs Parallel-Connected to Weak Grids by Improved Net Damping Criterion. Energies 2020, 13, 3316. https://doi.org/10.3390/en13133316
Wang D, Zhang X, Yang L, Huang Y, Huang W, Wu C, Li S. Analysis of Synchronization Stability for Multi VSCs Parallel-Connected to Weak Grids by Improved Net Damping Criterion. Energies. 2020; 13(13):3316. https://doi.org/10.3390/en13133316
Chicago/Turabian StyleWang, Dong, Xiaojie Zhang, Lei Yang, Yunhui Huang, Wei Huang, Chen Wu, and Shengnan Li. 2020. "Analysis of Synchronization Stability for Multi VSCs Parallel-Connected to Weak Grids by Improved Net Damping Criterion" Energies 13, no. 13: 3316. https://doi.org/10.3390/en13133316