A Grid Frequency and Voltage Agnostic Startup Technique for Cascaded H-bridge Converters

This paper presents a decentralized startup technique for bidirectional series-connected inverters with grid-forming capability. We control each converter module as a virtual resistor that absorbs power from the grid and transfers energy to the dc side. Owing to the passive nature of the virtual resistors, the series ac-side current automatically synchronizes the cascaded ac-side module voltages to the grid voltage. The virtual resistors not only prevent over-currents, but they are also agnostic to changes in grid frequency and voltage. This is extremely useful for startup during off-nominal grid conditions. Furthermore, unlike existing grid-connected startup methods, this technique does not require external charging resistors or a centralized phase-locked loop (PLL) to synchronize the series-connected modules. We demonstrate that stable rectifier operation is achieved if the emulated resistances are lower than a critical value. Once steady-state operation is reached, a local decentralized PLL within each module is able to decipher the frequency, phase angle, and magnitude of the grid voltage. This information is then passed to the primary control (droop, virtual oscillator, or virtual synchronous machine) and we flip the direction of power flow as the system begins normal operation. We verify the proposed startup technique for a 1.5 kVA system with three cascaded converters connected across a grid.