An Overview of Carrier-based Modulation Methods for Z-Source Inverter <s> Two-level ZSI <s> This paper presents an impedance-source (or impedance-fed) power converter (abbreviated as Z-source converter) and its control method for implementing DC-to-AC, AC-to-DC, AC-to-AC, and DC-to-DC power conversion. The Z-source converter employs a unique impedance network (or circuit) to couple the converter main circuit to the power source, thus providing unique features that cannot be obtained in the traditional voltage-source (or voltage-fed) and current-source (or current-fed) converters where a capacitor and inductor are used respectively. The Z-source converter overcomes the conceptual and theoretical barriers and limitations of the traditional voltage-source converter (abbreviated as V-source converter) and current-source converter (abbreviated as I-source converter) and provides a novel power conversion concept. The Z-source concept can be applied to all DC-to-AC, AC-to-DC, AC-to-AC, and DC-to-DC power conversion. To describe the operating principle and control, this paper focuses on an example: a Z-source inverter for DC-AC power conversion needed in fuel cell applications. Simulation and experimental results are presented to demonstrate the new features. <s> BIB001 </s> An Overview of Carrier-based Modulation Methods for Z-Source Inverter <s> Two-level ZSI <s> This paper presents the development of two three-level cascaded Z-source inverters, whose output voltage can be stepped down or up unlike a traditional buck three-level inverter. The proposed inverters are designed using two three-phase voltage-source inverter bridges, supplied by two uniquely designed Z-source impedance networks and cascaded at either their DC sides to form a DC-link-cascaded Z-source inverter or AC outputs using single-phase transformers to form a dual Z-source inverter. For controlling both inverters, various modulation schemes are designed with their performances verified experimentally using an implemented laboratory prototype <s> BIB002
The topology of ZSI consists of two inductors and two capacitors connected in X-shape forming an impedance network. Therefore, ZSI can also be considered as an impedance inverter. The two-level and three-level topologies of ZSI are portrayed in Fig. 3 . The impedance network is fed by either a dc voltage source or a dc current source. The inductors used in the impedance network can be a split inductor or two different inductors. The inverter circuitry for the voltage-fed and current-fed ZSI follows the same configuration as the traditional VSI and CSI, respectively. ZSI is considered an attractive option superior to traditional converters due to its capability of delivering any value of output voltage regardless of the input, theoretically BIB001 . As mentioned earlier, single-stage converters consist of an additional state, which is called the ST state. This state is introduced into the inverter operation by shorting the switches in any one-phase leg, or by shorting the switches in any two-phase leg, or by shorting the switches in all the three-phase legs. This state causes damage to the circuit in the traditional inverter, whereas it provides the unique buck-boost characteristic to the ZSI. The ST state is introduced in both the two-level and threelevel operations of ZSI to ensure output voltage boosting. For the two-level operation, ZSI operates in nine switching states (eight states such as traditional inverter and the additional ST state). The operation involves the charging of the two inductors in the impedance network that results in voltage boosting. In the two-level configuration as shown in Fig. 3(a) , the ST state is achieved by shorting two switches in the same leg. This shorting causes damages to the circuit in the traditional inverter. However, due to the restricting action by the inductors, damages are prevented in ZSI. Furthermore, this aids in the boosting of the output voltage. As this gating of switches 'on' and 'off' in the same leg does not affect the circuit, ZSI can be operated without any dead time protection BIB002 . The operation of ZSI in nine switching states is classified into two modes: N-ST and ST states. N-ST state represents the operation of the inverter by comparing the three-phase reference sinusoidal signals with the triangular carrier signal. ST state represents the operation of the inverter due to an additional comparison with the carrier signal. Various modulation methods have been proposed to perform this additional comparison for the ST operation of the inverter. The objective of these modulation methods is to achieve a wide range of modulation, lower stress on the devices, minimum commutation per switching cycle and simpler implementation. Among the several modulation methods, ST insertion by carrier-based comparison methods is reviewed in Section 4. The modulation methods for a two-level ZSI are described as follows.