Transformer-less photovoltaic (PV) inverters are more widely adopted due to high efficiency, low cost and light weight, etc. However, H5, HERIC, etc. transformer-less PV inverters do not have the bidirectional capability for solar energy storage system in the future. With topology derivation history reviewed from rectifier to inverter, the essence of bidirectional rectifier/inverter is revealed to find a reverse power flow approach. Therefore, this paper proposes an advanced bidirectional technique for a selected H6 inverter topology with only modulation strategy modified, while the others remain the same. For the H6 circuitry in both rectifier and inverter modes, excellent three level DM voltage feature is achieved, while leakage current issue is eliminated at the same time with improved modulation method. Simulations and experimental results verify the proposed single phase bidirectional H6 rectifier/inverter technique.
1. H6 inverter
3. Improved modulation
4. Bidirectional power flow
5. Leakage current
Figure 1 Selected H6 inverter
(a) Ac grid voltage, ac current, DM voltage and CM voltage in rectifier mode
(b) Switching pulses S1&S4, S2&S3, S5, S6 in rectifier mode
(c) Expanded switching pulses S1&S4, S2&S3, S5, S6 in rectifier mode
(d) Ac grid voltage, ac current, DM voltage and CM voltage in inverter mode
(e) Switching pulses S1&S4, S2&S3, S5, S6 in inverter mode
(f) Expanded switching pulses S1&S4, S2&S3, S5, S6 in inverter mode
Figure 2 Bidirectional H6 rectifier/inverter modulation method simulation Results
(a) Current stress
(b) Voltage stress
Figure 3 H6 rectifier device stress
Aiming solar energy storage system, this paper improves a grid-tied single phase H6 PV inverter from unidirectional power flow to bidirectional power flow. A unified hybrid modulation method is proposed for both rectifier and inverter modes. The main advantages of the proposed solution can be summarized as:
1) Compared with the traditional hybrid modulation method for power rejection to grid only, a simple modification in the switching patterns is just needed for solar energy storage system with H6 type topology.
2) Battery storage is adopted for emergency usage in small solar energy storage system. Therefore, a slight cost of efficiency decreases in rectifier mode due to the partly used body diodes is acceptable, and the excellent DM/CM voltage features of the H6 circuitry in both rectifier and inverter modes are totally achieved.
3) The improved hybrid modulation method would be easily modified and applied to other H6 and similar topologies.
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