New Fault-Tolerant Control Approach for a Reconfigurable Grid-Connected PV System
Authors: Trabelsi Mohamed , Abu-Rub Haitham 
Published: July 2019

Background & Objectives: Power converters have become over the last decades an enabling technology for PV applications. As demonstrated by scientific literature on the subject, current research efforts are directed toward the use of intermediate solutions between fully centralized and totally distributed (one inverter for each panel or strings of panels) PV grid connection architectures, for cost, reliability and maintenance concerns. Within this trend, topologies of utility scale PV inverters are moving towards Multi-Level Inverters (MLI), which provide better power quality, lighter passive filtering components, and potential to eliminate bulky line frequency transformers. However, in high performance grid-connected PV systems, the failure of the power electronics converters has very serious consequences on the overall system operation. In view of an optimal utilization of the generated electrical power and as per the general fault-tolerance requirements, deploying a power electronics converter capable of continuing to operate effectively in the presence of any single point failure is essential for such as systems. A large-scale solar plant needs to tolerate short-term malfunctions while maintaining the inverter connected to the grid and eventually provide grid support.

One of the main advantages of Cascaded H-Bridge (CHB) MLI is the modularity. A CHB inverter employs many partially separate power modules (cells). If these cells equipped with a bypass switching device (external switch), then if one of the power modules fails it can be bypassed and operation can continue at reduced capacity. This even allows the faulty cell to be replaced by a new one without turning off the system. However, bypassing a module reduces the voltage and power available from the inverter. Then, the problem becomes how to obtain the highest power level with the remaining operative cells.

Moreover, the association of a quasi Z-Source (qZS) network with a CHB MLI was deeply investigated in the last decade for grid-tied PV systems. This single-stage mix-topology is characterized by high-quality staircase output voltage with lower harmonic distortions, independent DC-link voltage compensation with the special voltage step-up/down function in a single-stage power conversion, and independent control of the power delivery with high reliability.

By taking advantage of the high CHB inverter's modularity and flexibility of the qZS network in controlling the DC-link voltage, this research work proposes a new fault-tolerant control strategy for a reconfigurable grid-connected PV system. Methods: The system under study consists of a three-phase sixteen-cell CHB inverter where each module is fed by a qZS network (Fig. 1). The proposed combined controller achieves grid-tie current injection, DC-link voltage balance for all qZS-CHB inverter modules, anti-islanding protection, and fault-tolerant operation. The fault-tolerance feature is explored and discussed for two modulation techniques, which are the Level-Shifted Pulse Width Modulation (LSPWM) and Pulse Width Amplitude Modulation (PWAM). The proposed strategy can be easily implemented without extra hardware requirements. It takes into account key crucial factors for high-efficiency and reliability grid-connected PV systems such as; cost reduction (selection of high efficient and high performing qZS-CHB MLI topology), high power quality (grid current injection with unity power factor and low harmonics distortion), active anti-islanding protection (according to grid codes), and fault-tolerance (continuous operation during malfunction of some system components, which leads to the system reconfiguration). The fault-tolerant design is taking advantage of the large number of redundant switching states for the same output voltage level, which characterizes the selected cascaded topology. However, one can note that this redundancy is effective only for the intermediate levels output voltages, while the extreme levels (highest and lowest levels) are achieved by only one switching state. Accordingly, the proposed approach offers circuit reconfiguration (based on a measurement based fault detection strategy) and voltage stress adjustment to achieve a balanced line-to-line voltage when a fault occurs.

Moreover, Battery Energy Storage Sources (BESS) are used as additional source of energy to support the grid at fault times. At normal operating conditions, the BESS are used to store the excess power available from the PV to avoid the over voltage state. At fault conditions, the BESS could be used to provide the amount of power lost because of the failure of one or more of the inverter modules.