Fault ride-through control of grid-connected photovoltaic power plants: A review
Introduction
The Solar photovoltaic (PV) technology is currently significant in many areas and its usage is expected to increase globally. The PV technology is considered to be the most vital and promising renewable energy resource (Obeidat, 2018). Recently, a continuous sharp growth is observed in the PV renewable energy sector, whilst other renewable sectors grew relatively slowly. The PV capacity installations had been remarkable - almost twice the ones of wind energy (the second largest renewable energy) - adding extra net capacity than natural gas, nuclear power, and coal combined (Renewable Energy Policy Network, 2018, Kabir et al., 2018). The year 2017 was a phenomenal year for PV power generation as the PV plants generated more power than any other kind of renewable energy technology. The PV system was the primary renewable energy provider, representing almost 55% of renewable power capacity that was newly installed. The remaining capacity additions were represented by wind (29%) and hydropower (11%) energy. Globally, at least 98 GW of solar PV capacity was installed, increasing total capacity by nearly one-third, for a cumulative total of approximately 402 GW, as depicted in Fig. 1 (Renewable Energy Policy Network, 2018). It is well known that the most frequent cause of the instability in power system is the grid faults. In the literature, some existing analyses and solutions of balanced and unbalanced grid faults concerning the connection of hybrid renewable energy sources and microgrids have been proposed (Ou, 2012, Ou, 2013, Almeida et al., 2016). The studies proved that the grid fault is one of the important issues that should be addressed, however the fault analysis regarding FRT capability has not been discussed in all the references mentioned above.
A great part of PV plants are connected to the power grid known as the grid-connected photovoltaic power plants (GCPPPs) (Al-Shetwi and Sujod, 2018). As the GCPPPs capacity increases, the need for these plants to be more effective contributors to keep the stability, operability, reliability, and quality of the power grid increases. Therefore, it is essential to require PV power plants to act as much as possible like conventional power plants. For that reason, several new requirements and rules regarding the operation of GCPPPs were imposed by some nations, which are known as the modern grid codes (GCs) requirement. In the past, GCs required PV systems to disconnect from the grid after a fault occurrence. However, recently, with this remarkable increase in the integration of solar PV plants into the power grid, the interruption of these plants at the same time of grid disturbances may cause operational and stability problems to the grid and customers, and may lead to blackouts (Honrubia-Escribano et al., 2018). To solve this issue, one of the most essential requirements is the low voltage ride through (LVRT) or fault ride through (FRT) capability that should be met by GCPPPs via the PV inverters (Rodrigues et al., 2014). Thus, it is important to analyze PV power's impacts on power grid and impacts of grid disturbances such as grid faults on PV farm generators (Obi and Bass, 2016). As a result, for PV system-grid integration, the FRT capability control becomes an important aspect regarding the control system design and manufacturing technology (Lammert et al., 2017). The FRT capability indicates that the PV inverter need to behave like traditional synchronous generators to tolerate voltage sags resulting from grid faults or disturbances, stay connected to the power grid, and deliver the specified amount of reactive current at the time of grid faults, respectively (Al-Shetwi et al., 2015). In the recent literature, various studies have been documented in terms of FRT requirements in modern grid code (Al-Shetwi et al., 2015, Yang et al., 2015, Cabrera-Tobar et al., 2016).
In order to achieve the FRT operation required by GCs for GCPPP, the PV inverter should be properly controlled to deal with grid voltage disturbances. Therefore, the PV system must manage the problems of inverter disconnection and supply reactive currents to the power grid at the time of disturbances (Al-Shetwi et al., 2018). Looking into the growing share of the PV energy in power systems and the updated technical necessities for grid connection and operation, variable methods have become the point of interest in the GCPPPs studies. Once the fault occurs, there are two main problems that should be addressed and managed via the PV system in order to fulfill the FRT standard requirements. The first is the overcurrent which may arise at the AC-side of the inverter in addition to the overvoltage of the DC-link in the DC-side. This issue occurs because of the inequality between the incoming energy from the PV side and the energy delivered into the electric grid (Perpinias et al., 2015). The second problem is the injection of reactive currents, which is considered important for voltage recovery as well as to assist the power system to overcome the fault incidents (Jaalam et al., 2017).
It is well-known that the FRT capability was applied to wind energy before the PV system due to the high integration of wind farms to the utility grid (Mohseni and Islam, 2012). However, recently the FRT applied for the PV system, as the PV generation almost doubled when compared to that of the wind energy (Renewable Energy Policy Network, 2018, Al-Shetwi and Sujod, 2018). Many literature studies have reviewed the FRT control methods for different types of wind energy systems (Howlader and Senjyu, 2016, Moghadasi et al., 2016, Justo et al., 2015, Nasiri et al., 2015). Regarding PV system, although most of the recent studies focus on the FRT requirements imposed by different grid codes in many countries as discussed and summarized in Al-Shetwi and Sujod, 2018, Al-Shetwi et al., 2015, Perpinias et al., 2015, Badrzadeh and Halley, 2015, El Moursi et al., 2013. However, no comprehensive review has yet been made for FRT control methods applied to PV systems in order to fulfil these requirements. In the recent literature, various approaches have been individually documented to study and improve the FRT capability control of GCPPPs during faults, which need to be properly reviewed and discussed. Therefore, the main objective of this study is to introduce a comprehensive review on the FRT strategies and controllers which have been already developed and employed in the GCPPPs systems. In addition, a comparative study in terms of dynamic performance, grid code compliance, controller complexity, and cost evaluation of these LVRT methods is carried out. Moreover, an in-depth and comprehensive review is needed to reflect the most recent updates of FRT researches.
The paper is structured as follows: Section 2 introduces the FRT requirements in modern grid codes concerning the penetration of PV system to the power grid. Subsequently, Section 3 presents a brief review of the inverter controller-based GCPPPs including the controller designs. Next, an overview of the recently published FRT approaches along with the fault detection methods are discussed in Section 4. The two main approaches to realize FRT capability in PV systems are also classified in this Section. Further, Section 5 reviews the FRT control methods based-external devices and Section 6 presents the review of FRT control strategies based-modified controller. Moreover, a comparative study in terms of dynamic performance, technical pros and cons, controller complexity, and cost evaluation of these FRT methods is carried out in Section 7. Finally, the conclusions and recommendations are summarized in Section 8.
Section snippets
FRT requirements in modern grid codes
In case of grid faults, the act of quickly disconnecting PV power plants may effect on the power grid stability, especially with large-scale PVPPs. Thus, FRT or LVRT requirements that are imposed by modern grid codes require the PV system to remain connected when the grid voltage sags occur and cause the grid voltage to decrease to a specific percentage of the normal voltage for a specific period. This is required to make sure there is no loss of power generated due to commonly voltage sags. In
Inverter controller-based GCPPPs
GCPPPs mainly have two configurations, i.e., single-stage and two-stage systems, depending on the inversion systems and power ratings (Zhu et al., 2011), as seen in Fig. 5. The direct connection from PV system array to the DC side of the inverter is called single stage conversion. The two-stage conversion system consisting of DC-DC converter part as a first stage exists between the PV array and the inverter, and then followed by the second stage, which is the inverter part to invert the
Overview of FRT capability control
In order to fulfill the FRT requirements enforced by modern grid codes concerning the penetration of large-scale PVPPs into power grid mentioned above, once the voltage sag occurred, the control system should have the ability to take the following measures: (a) fast and precise fault detection to inform the system to switch from steady-state operation mode to the faulty state; (b) protect the PV inverter and other semiconductor devices from the overcurrent that occurs at AC side of the
Energy storage systems (ESSs)
The FRT capability improvement utilizing ESSs has been proposed in the literature for GCPPPs, as shown in Fig. 10. Typically, the ESSs can be connected to the DC-link through a buck-boost DC-DC converter (Saadat et al., 2015). Once a grid fault occurs, the ESSs will absorb extra energy from the DC-link at the inverter DC side to overcome the over-voltage incident. During this period, the duty cycle of the DC-DC converter is adjusted to reduce the output power of the PV battery in order to
FRT control based-modified controller
The previous section introduces the methods which require extra devices in order to improve the FRT capability. It is evident that, the additional equipment will increase the overall cost of the GCPPP system. However, it is preferred to improve the FRT at the lowest possible additional cost. Therefore, some studies resort to modifying the inverter control itself to achieve the FRT without extra devices. Those strategies are described in the following:
Technical, economic, and complexity comparison of FRT enhancement methods
Table 1 summarizes the technical pros and cons of the all types of the FRT improvement strategies mentioned previously. Although the goal of this summary is not to prioritize the FRT improvement strategies depending on technical capabilities, it presents clear and simple evaluation for the most popular methods in the field that might be utilized for decision-making purposes.
The comparison of different FRT strategies in terms of complexity, economy, additional device, and addressing the two main
Conclusion
This paper reviewed the state-of the-art FRT enhancement methods, which are still an active research area for GCPPPs. Firstly, the FRT requirements in modern grid codes concerning a high PV penetration level and the inverter controller-based GCPPPs were discussed. Next, all the reviewed strategies were categorized into two main groups using external controller and modified devices-based strategies. The performance, advantages, and limitations of various strategies are also discussed in this
Conflict of interest
The authors declared that there is no conflict of interest.
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