An H-bridge hybrid modular converter (HBHMC) is proposed for HVDC applications. It uses a wave-shaping circuit (WSC) consisting of series-connected full-bridge submodules (FBSMs) at the output of the main H-bridge converter (MHBC). For a three-phase system, three HBHMCs are connected either in series (series-HBHMC) or in parallel (parallel-HBHMC) across the dc-link. The operating modes of HBHMC, novel modulation strategies for voltage balancing of FBSMs, and control of HBHMC based HVDC system are presented in this paper. A detailed comparison between HBHMC and other hybrid topologies is performed on the basis of required number of switches and capacitors. The HBHMC has the features of dc fault blocking capability, lower footprint structure and extra degree of freedom for submodules capacitor voltage balancing. The efficacy of the HBHMC based HVDC system for three-phase balanced and unbalanced grid conditions and its fault tolerant capability are validated using PSCAD simulation studies. Further, the feasibility of proposed converter under normal, and dc fault conditions, and of the proposed capacitor voltage control scheme are validated experimentally by using a three-phase grid connected HBHMC laboratory prototype. The results demonstrate the effectiveness of the proposed HBHMC topology, control techniques, and satisfactory responses of the HBHMC based HVDC system.
Fig. 1 Block diagram of single-phase HBHMC
EXPECTED SIMULATION RESULTS:
Fig. 2. Waveforms of HBHMC using HCI method (a) Output voltage and current waveforms, and (b) Modulation signals for HCI method, (c) switching signals of MHBC, and (d) individual capacitor voltages of WSC.
Fig. 3 Waveforms of HBHMC using AZCI method (a) output voltage and output current waveform, (b) Modulation signals for ACZI method, (c) switching signals of MHBC, and (d) Individual capacitor voltages of wave shaping circuit.
Fig. 4. Series-HBHMC operating at mi = 1.2 (a) three-phase output voltage waveforms and, (b) three-phase output current waveforms (c) modulation signals for ACZI method.
Fig. 5. Responses of converter for HVDC system when active power flow is reversed from -150MW to 150MW at CS1 (a) active power at CS1 and CS2, (b) three-phase ac grid currents at CS1, (c) CS1 dc-link voltages, (d) CS2 dc-link voltages, (e) capacitor voltages of FBSMs of WSC of phase-a at CS1, and (f) capacitor voltages of FBSMs of WSC of phase-a at CS2.
Fig. 6. (a) Active and reactive power waveforms of CS1 when reactive power is changed from -100MW to 100MW, (b) dc-link capacitor voltages, and (c) capacitor voltages of WSC.
Fig. 7. (a) Active and reactive power waveforms of CS2 when reactive power is changed from 100MW to -100MW, (b) dc-link capacitor voltages, and (c) capacitor voltages of WSC.
Fig. 8. Responses of CS1 when dc fault occurs during 0.5 to 0.7 sec (a) active and reactive power at the time of dc side fault, (b) ac current during dc side fault, (c) dc-link voltage during dc fault, and (d) FBSM capacitor voltages of phase-a.
Fig. 9. (a) Unbalance in grid voltage at t = 2.0 s, (b) grid currents when positive and negative current control is activated at t = 3.0 s, and (c) grid currents when dc capacitor voltage control is activated at t = 4.0 s.
Fig. 10. Response of the series-HBHMC to ac side unbalance voltage: (a) active and reactive power, (b) dc-link capacitor voltages, and (c) average values of WSC capacitor voltages.
This paper proposes an H-bridge hybrid multilevel converter topology, HBHMC, for HVDC applications. The proposed converter is a dc fault tolerant hybrid topology, which uses cascaded FBSMs (i.e. WSC) connected to the output of the MHBC. The WSC helps in generating the multilevel voltage waveform at the HBHMC output. For a three-phase circuit, three such HBHMCs can be connected in series on the dc side to handle a high dc-link voltage. Similarly, they can instead be connected in parallel across the dc-link for high dc current. In this paper, the basic operation of HBHMC and new modulation techniques to balance the capacitor voltages of HBHMC by appropriately selecting an operating mode (isolation mode: HCI and AZCI methods) are presented. The suggested voltage control methods are simple and easy to implement. Moreover, both the HCI and the AZCI methods are designed in a way that the MHBC always operates at the fundamental frequency to reduce the switching losses of converter. Further, the AZCI method offers more advantages than the HCI method, such as, smaller value of submodules capacitance and ability to operate in the overmodulation mode. Some of the other prominent advantages of the proposed converter are: (i) extra degree of freedom for capacitor voltage balancing, (ii) fewer semiconductor devices and capacitors in series-HBHMC, (iii) higher dc-link utilization in parallel-HBHMC, and (iv) inherent dc fault current blocking capability. Simulation and experimental studies are performed to validate the proposed converter topology and capacitor voltage control methods.
The effectiveness of proposed HBHMC and its control strategies for HBHMC-based HVDC system are investigated using PSCAD/EMTDC simulations under various operating conditions. The simulation studies show that the HBHMC has good performance under normal, dc fault, and grid voltage unbalance conditions. Moreover, the effectiveness of proposed converter is verified experimentally by using a three-phase grid connected HBHMC porotype. The obtained experimental results demonstrated that the proposed control method is effective in controlling WSC capacitor voltages under different operating conditions. It is also demonstrated that the HBHMC provides the desired dc fault tolerant capability. The simulation and experimental results highlight excellent performance of the proposed converter topology and control schemes. Hence, the HBHMC can be a good alternative for HVDC applications where dc fault blocking capability is required.
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