**1. Introduction**

With the increasing demand for conversion e fficiency during recent years, multilevel converter topologies have become more and more applied to the grid-connected PV generation system [1–4]. Among all kinds of multilevel inverters, the cascaded H-bridge (CHB) has many advantages, such as modularization, simplicity, and high reliability, which makes it become the most attractive topology [5–9]. In addition, the CHB topology makes it possible to, respectively, regulate each DC-side voltage and, thus, realizing the maximum power point tracking (MPPT) of each PV module. Therefore, the CHB inverter is considered to be one of the most suitable candidates for next generation PV inverters [10,11].

The energy loss caused by non-uniform solar radiation, degradation of PV modules and di fferent types of partial shading is greatly decreased by module level MPPT. However, due to the unequal temperature and irradiance of PV modules, the output power of H-bridge unit varies greatly in the case of severe mismatching. This may lead to over-modulation of H-bridge units with high output power, thus, resulting in system instability and injection current distortion [12].

For the sake of expanding the stable operation range of the CHB inverter, some methods have been proposed. The power balance control strategy based on active component modification of duty cycles, and its e ffective power balance area are proposed in [13–15]. Although the system can operate stably by utilizing this method under slight mismatch conditions, there are still instability problems under severe mismatch conditions. In [16,17], a control strategy of reactive power compensation is proposed, which utilizes the power factor as the degree of freedom to stabilize the operation of the system. However, an increase in reactive power injection will lead to a decrease of the system power factor, which may be undesirable from the perspective of electric dispatchers. A new, improved MPPT method is presented in [18], which changes the working point of the over-modulated H-bridge unit to ensure that all H-bridge units work in the stable operation region to improve the stability margins of the system. However, this will result in lower output power and make the system less-e fficient. This is contrary to the original intention that all PV modules operate at a maximum power point to realize higher e fficiency in energy harvesting.

Hopefully, In [19–21], the hybrid modulation strategy (HMS) is proposed, which utilizes a mixture of low-frequency PWM and high-frequency PWM methods to regulate the DC-side voltage and control the AC current separately. The HMS has been proved to maintain the stabilization of power rectifiers effectively even under critical operating conditions, because it provides higher DC-side utilization (the maximum modulation index of square wave can reach 4/π) compared with conventional sinusoidal pulse width modulation (SPWM). In [22,23], a control method of a grid-connected PV CHB inverter is proposed, which is based on the hybrid modulation strategy containing the zero state (HMSCZS). With HMSCZS, the system can operate stably under heavy mismatching conditions. Once the CHB inverter is operating in the fault mode, owing to failing solar panels, the HMSCZS fails to regulate the DC-side voltages to the references and may lead to low output power of the inverter. In [24], the hybrid modulation strategy without the zero state (HMSWZS) is proposed to maximize the range of stable operation of system. However, the HMSWZS cannot control the DC-side voltage of each module accurately. This may aggravate the fluctuation of modules' DC-side voltages, thus resulting in the decrease of the generated energy of PV modules.

Therefore, a switching hybrid modulation strategy (SHMS) is proposed in this paper. The HMSCZS is selected to suppress DC-side voltages fluctuation when the CHB inverter is operating in normal mode. Once the CHB inverter is operating in fault mode, owing to failing solar panels, the HMSWZS is utilized to control the DC-side voltages to track the references, thus maintaining a higher energy yield under fault condition. With this method, the average output power of the PV modules will be improved both in the normal and fault modes.

The paper is arranged as follows: In Section 2, the system configuration and control method are introduced. In Section 3, the existing problem of the HMSCZS and the HMSWZS is shown. In Section 4, the switching hybrid modulation strategy is put forward. In Sections 5 and 6, the performance of the proposed strategy is verified by simulation and experimental results. Finally, in Section 7, a conclusion is drawn to summarize the paper.
