# Zero-Voltage Ride-Through Capability of Single-Phase Grid-Connected Photovoltaic Systems
## 1 Introduction  
Being an important part of the modern energy infrastructure, distributed renewable energy (DRE) systems have been developed at a fast rate. For instance, in recent years, due to the continuous reduction of the photovoltaic (PV) module price and the strong global demand for environment-friendly energy conversion systems, the solar PV markets have been particularly booming. The capacity of solar PV was increased by 25% in 2014 (i.e., approximately 50 GW), bringing the global total to 227 GW. The annual market in 2015 was nearly 10 times the world's cumulative solar PV capacity of the last decade. In future, the DRE systems satisfy the requirements for the generation closer to the consumption points [1; 2; 3]. However, with the fast development of distributed renewable power generations, stability and security have been attracting extensive attention [2]. To cope with the challenges due to a high penetration level of PV systems, many research activities have been conducted recently to improve the integration of PV systems. In summary, it is expected that the grid-friendly PV systems should be multiple-functional. That is, for instance, reactive power control, maximum power point tracking (MPPT), fault islanding detection, harmonic compensation, and fault ride-through (FRT) operation are required for PV systems [4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14]. Actually, some PV power systemson today's market are already able to provide such services. Nevertheless, the PV systems should be more intelligent in consideration of grid stability, reliability, and fault protection at a high penetration level. A direct reflection of the increasing demands for PV systems is that many countries have revised or have been revising the national grid regulations, where the distributed generations are required to provide advanced grid fault-handling functionalities [15; 16; 17]. According to the requirements, the PV systems should remain connected under grid faults, and also provide reactive power if demanded. This is also referred to as the low-voltage ride-through (LVRT) capability [13; 14; 15; 16; 17]. In extreme cases, i.e., the grid voltage dips to zero, and the disconnection from the grid is also not allowed within a predefined short-time interval (e.g., 150 ms) [15; 16], known as the zero-voltage ride-through (ZVRT) capability [18; 19; 20]. Similarly, in zero-voltage conditions, the PV systems should also support the grid recovery by means of reactive current injection. Although the ZVRT operation can be taken as a special case of LVRT, a more dedicated control strategy should be performed during the FRT operation. Especially in single-phase grid-connected PV systems, when the fault occurs, the systems still inject sinusoidal reactive current to support the grid without grid information. Challenging issues for the ZVRT operation in the single-phase PV system include how to detect the grid voltage sags quickly, how to switch to the ZVRT operation mode with no grid information, and after the fault, how to resynchronize rapidly without triggering the overcurrent protection. As aforementioned, the single-phase grid-connected PV system is required to operate in different modes accurately and rapidly in complicated situations. In order to implement the ZVRT operation, the following should be considered:  
* Grid voltage sag detection--grid monitoring and synchronization control strategies with high robustness and fast dynamics;
* Power calculation and power profiles--the power and current references are adjusted fast and accurately in both normal and FRT operation modes. The grid synchronization is a fundamental task but a crucial technique in the connection of the PV system to the grid. The capability of instantaneous responses of the synchronization technique has a direct impact on the performance of FRT in terms of accuracy and dynamics.