In this paper, a simple static var compensating scheme using a cascaded two-level inverter-based multilevel inverter is proposed. The topology consists of two standard two-level inverters connected in cascade through open-end windings of a three-phase transformer. The dc link voltages of the inverters are regulated at different levels to obtain four-level operation. The simulation study is carried out in MATLAB/SIMULINK to predict the performance of the proposed scheme under balanced and unbalanced supply-voltage conditions. A laboratory prototype is developed to validate the simulation results. The control scheme is implemented using the TMS320F28335 digital signal processor. Further, stability behavior of the topology is investigated. The dynamic model is developed and transfer functions are derived. The system behavior is analyzed for various operating conditions.
- DC-link voltage balance
- Multilevel inverter
- Power quality (PQ)
- Static compensator (STATCOM)
Fig. 1. Power system and the STATCOM model.
EXPECTED SIMULATION RESULTS
Fig. 2. Frequency response ∆Vdc1(s) /∆δ1(s) at i’q0 =1.02 p.u., δ1=-0.90,δ2=178.90,R1=80 p.u., R2=60 p.u.
Fig. 3. Root locus of the transfer function ∆Vdc1(s) /∆δ1(s) at i’q0 = – 0.75 p.u., δ1=-0.570,δ2=179.60,R1 =80 p.u., R2=60 p.u.
Fig. 4. Reactive power control. (a) Source voltage and inverter current. (b) DC-link voltages of two inverters.
Fig. 5. Operation during fault. (a) Grid voltages on the LV side of the transformer. (b) -axis negative-sequence current component i’dn. (c) -axis negative- sequence current component i’qn.
Fig. 6. Experimental result: Capacitive mode of operation. (a) Source voltage (50 V/div) and STATCOM current (5 A/div). (b) DC-link voltages of inverter-1 and inverter-2 (20 V/div). Time scale: 5 ms/div. (c) Harmonic spectrum of current.
Fig. 7. Experimental result: Mode change from capacitive to inductive. (a) DC-link voltages of inverter-1 and inverter-2 (20 V/div). Time scale: 100 ms/div. (b) Source voltage (100 V/div) and STATCOM current (5 A/div) in steady state. Time scale: 100 ms/div.
DC-link voltage balance is one of the major issues in cascaded inverter-based STATCOMs. In this paper, a simple var compensating scheme is proposed for a cascaded two-level inverter- based multilevel inverter. The scheme ensures regulation of dc-link voltages of inverters at asymmetrical levels and reactive power compensation. The performance of the scheme is validated by simulation and experimentations under balanced and unbalanced voltage conditions. Further, the cause for instability when there is a change in reference current is investigated. The dynamic model is developed and transfer functions are derived. System behavior is analyzed for various operating conditions. From the analysis, it is inferred that the system is a non minimum phase type, that is, poles of the transfer function always lie on the left half of the -plane. However, zeros shift to the right half of the -plane for certain operating conditions. For such a system, oscillatory instability for high controller gains exists.
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