Fault ride-through control of grid-connected photovoltaic power plants: A review

Solar Energy 180 (2019) 340–350

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Fault ride-through control of grid-connected photovoltaic power plants: A review

T

Ali Q. Al-Shetwia,b, , Muhamad Zahim Sujoda, Frede Blaabjergc, Yongheng Yangc ⁎

a

Faculty of Electrical and Electronic Engineering, University Malaysia Pahang (UMP), 26600 Pekan, Pahang, Malaysia Department of Electronic Engineering and Automatic Control, University of Science and Technology, Sana'a, Yemen c Department of Energy Technology, Aalborg University, Aalborg DK-9220, Denmark b

ARTICLE INFO

ABSTRACT

Keywords: Fault ride-through (FRT) requirements Grid codes low voltage ride through (LVRT) Photovoltaic (PV) system Solar energy

Over the recent years, the photovoltaic (PV) system generation and integration with utility grid became the most widely used energy resource among other renewable energies worldwide. Thereon, the integration of PV power plants (PVPPs) to the power grid and their dynamics during grid faults had become a critical issue in the new grid codes requirements. In line with this, the fault ride through (FRT) capability control of grid-connected PV power plants (GCPPPs) became the most important issue related to grid codes. In order to fulfill the FRT requirements imposed by grid codes, various approaches have been proposed in the last years. This paper presents an overview and comparison of several FRT capability enhancement approaches during grid fault conditions. A novel feature of this paper is to categorize FRT capability enhancement methods into two main groups depending on the control type and connection configuration including external devices based methods and modified controller based methods and then discuss their advantages and limitations in detail. A comparison between these methods in terms of grid code compliance, controller complexity and economic feasibility are also analyzed in this paper. According to the literature study, the FRT strategies based on external devices can be more effective. However, some of these methods come with significant increased cost. On the other hand, the modified controller-based FRT methods can achieve the FRT requirements at a minimal price. Among various types of control approaches, the modified inverter controller (MIC) is the highly efficient FRT capability approach.

1. 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, 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

Corresponding author at: Faculty of Electrical and Electronic Engineering, University Malaysia Pahang, 26600 Pekan, Pahang, Malaysia. E-mail address: [email protected] (A.Q. Al-Shetwi).

https://doi.org/10.1016/j.solener.2019.01.032 Received 1 November 2018; Received in revised form 28 December 2018; Accepted 9 January 2019 0038-092X/ © 2019 Elsevier Ltd. All rights reserved.

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Nomenclature ADL AC BESS BCC BDEW SCESS DC DCL DVR ESS FRT FCL FLS FACTS FLC

GC grid code GCPPP grid-connected Photovoltaic power plant GW gigawatt IGBT insulated-gate bipolar transistor LVRT low voltage ride through MPPT maximum power point tracking MIC modified inverter controller PSO particle swarm optimization PV photovoltaic PVPP photovoltaic power plant PCC point of common coupling RMS root-mean-square SDBR series dynamic breaking resistor STATCOM static synchronous compensator SSSC static synchronous series compensator SVC static VAR compensator TCSC thyristor controlled series compensator

adaptive DC-link alternating current battery energy storage system brake chopper circuits German Association of Energy and Water Industries capacitor energy storage system direct current dynamic current limitation dynamic voltage restorer energy storage system fault ride through fault current limiter feedback linearization strategy flexible alternating current transmission system fuzzy logic control

Fig. 1. Photovoltaic (PV) total capacity and annual increment 2007–2017.

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; CabreraTobar 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 ACside 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 341

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Vgrid

p.u

A

1

Vnominal 90%

Iq/In

B 0.5

Area (A)

50%

0.2

C

Area (B)

0

0% 0.15

1.5

3

Time [s]

0.5 Vg/Vgn

0.9 1 p.u

Fig. 3. Reactive current support requirement in the German grid code.

Fig. 2. Fault ride-through requirement in the German grid code.

must be fed into the power grid in order to maintain the power system stability and to assist the voltage recovery (Obi and Bass, 2016; Hasanien, 2016; Shah et al., 2015). Fig. 2 illustrates an example of the FRT curve enforced by the German grid code (BDEW) for any PVPP connected to the medium voltage of utility grid (Troester, 2009). Based on this requirement, if the voltage at point of common coupling (PCC) decreases to zero for a duration of less than 0.15 s, the PVPP must also stay connected to the grid, referred to as zero voltage ride-through operation. PVPPs can disconnect from the power grid in case the voltage drops into Area B. As an additional FRT requirement, certain grid codes require large-scale PVPPs to make contributions to the voltage recovery in the power system via reactive currents injection (Troester, 2009; BDEW, 2008). As part of the German grid code, Fig. 3 indicates the amount of reactive currents injected when the voltage at PCC decreases. As depicted in Fig. 3, according to the voltage sags’ depth, the ratio of the active current to the rated current (Iqr) is represented by three regions as follow: (a) Area C represents the normal grid operation condition as long as the amplitude of the present grid voltage is higher than the 0.9 p.u of the nominal value and therefore eliminating the need for reactive current injection (Iqr); (b) Area B defines the amount of Iqr when the voltage during grid fault is less than or equal to 0.9 p.u and higher than 0.5 p.u; and (c) Area A is the most critical area since the voltage drops under 0.5 p.u from its nominal value which requires the amount of injected reactive current Iqr to be equal to the value of nominal current (BDEW, 2008; Afshari et al., 2017; Neumann and Erlich, 2012). In other words, when the grid voltage amplitude varies in the range of 1.1 p.u and 0.9 p.u, the system should operate in normal mode whereby only active current will be injected by the inverter (no injection of reactive current is needed). Whereas, when the grid voltage amplitude falls under 0.9 p.u, the inverter control should switch to FRT control mode and consequently the required amount of injected reactive current must follow the curve shown in Fig. 3. It is important mentioning that, the Vg and Vgn are the amplitude values of the present voltage during the fault and the normal grid voltage, respectively. Fig. 4 explains the FRT requirement in various grid codes regarding the PVPP penetration to the utility grid, which vary from country to country and from one operator to another (Al-Shetwi and Sujod, 2018). It is evident from Fig. 4 that the German, Italian, and Australian grid codes are more stringent, which require the PVPPs to stay connected even though the voltage drops to zero.

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. 2. 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 some grid codes, the PVPP is expected to perform like the conventional synchronous generators in which reactive currents

3. Inverter controller-based GCPPPs GCPPPs mainly have two configurations, i.e., single-stage and twostage 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 342

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(a)

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 available DC power to AC power (Islam et al., 2014). In both topologies, the inverter control plays an important role to achieve the control process of the input side and grid side. The primary mission of the input controller is to obtain the maximum available power from the PV energy systems, while the grid side controller deals with the active and reactive power that is transferred from the PV system to the grid. Another responsibility of the grid-side controller is to ensure the grid synchronization as well as the quality of the power delivered into the network (Sinha et al., 2018; Hassaine et al., 2014; Al-Shetwi and Sujod, 2018). Inverter control techniques can either be current-controlled or voltage-controlled. However, the current-controlled inverters are more popular and utilized in grid-connected PV systems when compared to voltage-controlled inverters, as depicted in Fig. 6. This is because the current control strategies can achieve a high power factor and mitigate the harmonic current distortion (Hassaine et al., 2014; Hojabri and Soheilirad, 2014; Parvez et al., 2016). A review on the inverter control techniques for GCPPPs is carried out in Hassaine et al. (2014), Parvez et al. (2016). The inverter mainly affects GCPPP transient characteristics, and the FRT capability of a GCPPP is dominated by the inverter related control. A double-loop control mode having both outer and inner loops is adopted in the previous studies to achieve the PV system integration (Wang et al., 2011).

Filter

PV Array +

C _Vdc

DC-AC inverter

Transformer

Grid

R

L

R

L

+

~

R

L

+

~

+~ N

(b) Filter

PV Array DC-DC + DC-AC convert V C _ dc inverter er

Grid

Transformer

R

L

+

R

L

+

R

L

+

~ ~

N

~

Fig. 5. Configurations of typical GCPPPs: (a) single-stage and (b) two-stage.

3.1. Inner loop control To simplify the controller design, a feed-forward decoupling strategy is adopted in the internal loop control model to decouple the active (Id) and reactive currents (Iq). The time constant of the inner loop control is normally small (less than a millisecond), and thus the inner control model must be simplified to perform various simulation step sizes. The inner loop takes the reference active current generated by the outer loop control. Meanwhile, the reactive current reference is typically set to zero to achieve unity power factor during the normal operation. However, during fault operation, the reactive current should be injected based on the fault depth (Timbus et al., 2009). The simplification technique of the inner loop control is provided in Fig. 7.

Fig. 6. Share of the current-controlled and voltage-controlled inverters in GCPPPs (Hojabri and Soheilirad, 2014).

regulation that provides active current reference for the inner loop control and stabilize DC-link voltage (Vdc) to its rated value. The DClink voltage is regulated using a typical proportional-integral (PI) controller. The schematic diagram of the outer loop is simplified as shown in Fig. 8. The Vnom-dc, Id-ref, Kp-vdc, and Ki-vdc are the nominal DC

3.2. Control of the outer loop The outer loop of the inverters is known as the DC-link voltage

Fig. 4. FRT requirements in several grid codes (Al-Shetwi and Sujod, 2018). 343

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mode during grid steady-state conditions and the transient-state of operation with the FRT capability control once the voltage sag caused by grid faults occurs (Yang and Blaabjerg, 2013; Lin and Ou, 2011). Therefore, the sag detection technique has a significant effect on the overall performance of GCPPPs under grid faults. Consequently, the quick and precise detection method is an important feature for efficient FRT control (Yang et al., 2014). In the literature, most fault detection strategies are built up within the inverter in order to disconnect PVPPs from the utility grid during disturbances or faults to prevent islanding or activate the protection of the PV arrays from damage (Pigazo et al., 2009; Chine et al., 2014; Silvestre et al., 2013). However, with the new grid codes, the FRT requirement imposes the inverter to stay connected throughout the grid faults. Therefore, many sag detection strategies have been introduced, such as the positive sequence voltage method (Al-Shetwi and Sujod, 2018), root mean square (RMS) technique (AlShetwi and Sujod, 2018), peak value technique (Alepuz et al., 2009), the missing voltage method (Yang and Blaabjerg, 2013), and the wavelet transform technique (Sadigh and Smedley, 2016). All these methods have advantages and disadvantage in terms of efficiency, complexity, and detection speed. However, the RMS detection method is considered as the most used technique due to its simplicity.

Fig. 7. Inner loop control model of the PV inverters.

voltage, active current reference of the inverter, proportional and integral gain of the PI controller in voltage loop, respectively. 3.3. Other control and protection models An additional control and protection capabilities have to be added to the inverter for both single and two-stage topologies to enhance the PVPP overall performance concerning the following capabilities: multipeak maximum power point tracking control, flexible reactive power support, islanding protection, integration requirements, and power quality management (Eltawil and Zhao, 2010; Khadem et al., 2010; Batarseh and Za'ter, 2018). In addition, the FRT control strategies which are considered as the state-of-the-art regarding a high PV penetration (Yang et al., 2015) will be discussed in detail in the next section. It is important to mention that, the PVPP modelling especially with FRT capability should be established totally based on the control and protection techniques presented (Tan et al., 2004).

4.3. Maximum power point tracking (MPPT) and FRT strategies During the advancement of PV systems, the MPPT techniques are used to extract maximum available power from the PV modules and thus increases the system efficiency. Therefore, it is of great significance in GCPPP systems for economic benefits. There are several MPPT methods which are introduced, studied and compared in Ram et al. (2017). During grid faults (FRT mode), mostly all FRT control basedexternal devices require the MPPT to work regularly while, FRT basedmodified controllers require the PV array to switch to Non-MPPT operation mode. However, as long as the overvoltage is addressed, the MPPT should stay in operation mode and generate the appropriate active power according to the fault depth to keep the power balance of the system, as proposed in Al-Shetwi et al. (2018).

4. Fault ride-through approaches for GCPPPs 4.1. Overview of FRT capability control

4.4. Different FRT enhancement strategies

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 inverter; (c) protect the capacitor of the DC-link and the inverter from the DC-link overvoltage at DC side of the inverter; (d) inject the desired quantity of reactive currents to assist the grid and voltage recovery based on the standard requirements according to the sag depth; and (e) ensure that the PVPP stays connected to the power grid for a stable operation of power systems. These measures may be classified based on their strategies for improving the FRT capability of the GCPPPs, e.g., by applying:

The major strategies proposed in the literature that have addressed the FRT capability of PVPPs connected to the power grid are illustrated in Fig. 9. The two fundamental approaches of FRT capability control can be divided into the FRT control using external devices and modified controller approaches. Energy storage systems (ESSs) including battery energy storage system (BESS) (Ota et al., 2016; Manikanta et al., 2017; Saadat et al., 2015) and super capacitor energy storage system (SCESS) (Worku and Abido, 2015), brake chopper circuits (BCC) (Al-Shetwi and Sujod, 2018; Yang et al., 2017), flexible alternating current transmission system (FACTS) devices (Yang et al., 2016; Ayvaz and Özdemir, 2016), in addition to some other methods such as fault current limiters (FCLs) (Sadeghkhani et al., 2017) and series dynamic breaking resistor (SDBR) (Hossain and Ali, 2014), are the FRT capability control strategies using external devices. The modified inverter controllers (MIC) (AlShetwi et al., 2018; Huka et al., 2018; Merabet and Labib, 2017; Mirhosseini et al., 2015; Oon et al., 2018) and Computational methods (Saad et al., 2016; Prakash and Devaraju, 2017; Hossain and Ali, 2017) are the improved controller strategies without additional devices.

• Sag detection unit. • Protection circuits/devices only during grid faults. • Reactive current injecting controllers/devices during grid faults. • Suitable control structures during both grid faults and steady state operation.

Vdc*

4.2. Sag detection methods

+

Vdc

To be able to achieve FRT requirements, the PV system needs to have two modes of operations. The two modes are normal operation

-

-1/Vnom-dc

Kp

Vdc

Ki

Vdc

Id-ref.

s

Fig. 8. Outer loop control model of the PV inverters. 344

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Fig. 9. Categorization of prior-art control methods to enhance the FRT performance for grid-connected PV systems.

5. FRT control based-external devices 5.1. 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 overvoltage 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 restrain the DC side voltage. After the grid fault, the energy stored is injected to the grid. The ESSs used in the FRT capability improvement is currently limited to the BESS (Ota et al., 2016; Manikanta et al., 2017), and the SCESS (Worku and Abido, 2015). As a result, the ESSs improve the FRT capability by protecting the DC-link and inverter from a high voltage during grid faults. However, the main disadvantages of ESSs method are the high initial and maintenance cost of these devices. In addition, it can cause fluctuation to DC parameters before and after the fault. Although these strategies have been used the ESS devices to suppress the excessive energy and thus protected the inverter and ridethrough the faults, the injection of reactive current has not been addressed.

Fig. 11. Brake chopper protection in the case of the FRT operation (Al-Shetwi and Sujod, 2018).

over-voltage that happens due to the increase in the DC-link voltage during faults. It will be activated when the fault is detected, as discussed in Al-Shetwi and Sujod (2018). Therefore, the gate pulse of the IGBT will be switched on, whereby the excess energy generated by PV generators will be absorbed by the high-power resistor. The resistor in this scheme is calculated as , where Pdc is the power generated by the PV system. The results shown by studies introduced in Al-Shetwi and Sujod (2018); Yang et al. (2017) demonstrate the effectiveness of the braking chopper to solve the issue of over-voltage at the inverter DCside. However, to enhance the overall FRT performance, this technique is combined with other techniques (Al-Shetwi et al., 2018; Yang et al., 2014).

5.2. Brake chopper protection As mentioned previously, during grid faults, there is an imbalance between the grid and the PV array. This will lead to an increase in the DC-link voltage and may damage the power electronic devices. As a solution, some papers suggested that a brake chopper could be installed in parallel with the DC-link capacitor (Al-Shetwi and Sujod, 2018; Yang et al., 2017). It comprises of a switch such as the insulated-gate bipolar transistor (IGBT) with a series of high power resistor as shown in Fig. 11. This braking chopper is effective to protect the inverter against PV Array

Inverter dc-link +

Vdc _

DC-AC inverter

RL Filter R L R L R L

DC-DC converter

Transformer

∆

5.3. FACTS devices FACTS devices are alternative solution to maintain PV systems to be connected to the grid during the faults and to inject reactive power upon demand. It has been used in the literature to enhance the FRT capability for grid-connected wind turbines as a solution for improving the voltage stability and injection of reactive currents. For instance, static VAR compensator (SVC) has been used to enhance the FRT capability control as proposed in Döşoğlu et al. (2017). In HeydariDoostabad et al. (2016) the static synchronous compensator (STATCOM) was used to improve FRT capability of fixed speed wind turbines in presence of grid fault. Another FACTS device which called static synchronous series compensator (SSSC) has been controlled for enhancement of FRT capability and voltage stabilization as proposed in Mahfouz and El-Sayed (2014). Dynamic voltage restorer (DVR), and thyristor controlled series compensator (TCSC) which belong to the FACTS family have also been developed to enhance the FRT of gridconnected wind turbine (Azizi et al., 2017; Mohammadpour and Santi, 2015). However, for FRT-based GCPPPs, only the STATCOM and SVC have been used to enhance the performance (Yang et al., 2016; Ayvaz

Grid

Y

ESS (Batteries Supercapacitor)

Buck-Boost Fig. 10. Fault ride-through improvement of grid-connected PV systems using energy storages. 345

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DC/AC

DC/AC

Rf

c

Lf

Vgrid

~

c

PCC Rf

Lf

∆

Y

Grid

SDBR

~

Fig. 15. Dynamic breaking resistor protection for the FRT operation.

limiter (FCL) as well as a series dynamic breaking resistor (SDBR). In AlShetwi and Sujod (2018), a current limiter was used during grid faults to protect the inverter from over-current and therefore enhancing the FRT during disturbances. The FCL reduces the increase in active (Id) and reactive (Iq) current components of the inverter during the time of faults using the FCL circuit shown in Fig. 14. The work presented in Sadeghkhani et al. (2017) proposes a dynamic current limiting approach implemented in inverter-based islanded microgrids to enhance fault FRT capability. The effectiveness of this strategy to limit both inverter current and voltage using only a current limiter is explained. The SDBR is a resistor connected in series between the PVPP and PCC to improve the FRT capacity by protecting the system against the excess voltage during the fault. SDBR protection scheme is depicted in Fig. 15. In Hossain and Ali (2014), the SDBR was employed in GCPPPs to contribute to the system balance as a part of the FRT control. During grid faults, the voltage increases in the inverter DC side. Subsequently, the energy is dissipated in the SDBR, preventing the DC-link voltage to increase sharply and to overcome the overvoltage incident. As a conclusion, in order to enhance the FRT the FCL and SDBR address the over-current and over-voltage, respectively.

STATCOM

Fig. 12. Typical configuration of the STATCOM to improve the FRT performance for grid-connected PV system.

DC/AC

c

Rf

Lf

Vgrid

~

SVC

Fig. 13. Typical structure of the SVC to enhance the FRT performance for gridconnected PV systems.

6. FRT control based-modified controller

and Özdemir, 2016), and are connected to the connection point between PV systems and the power grid as illustrated in Figs. 12 and 13, respectively. STATCOM and SVC have the ability to support the voltage and supply reactive power of the hybrid power systems and therefore increase the ability of reactive power control (Ou and Hong, 2014; Ou et al., 2017). For this reason, these devices are used to inject reactive currents to the grid in order to meet the FRT requirements. For instance, in Yang et al. (2016), the coordination between the PV system and the STATCOM to deal with the grid fault had been introduced. Although the conventional STATCOM includes only one capacitor-based storage and it has restricted energy storage ability, it supplies the anticipated reactive currents to assist the voltage recovery during grid faults. The SVC also has the ability to inject reactive power and consequently compensate voltage sags as introduced in Ayvaz and Özdemir (2016). In general, these devices are effective to inject reactive currents and to enhance the FRT capability. However, it increases the complexity and cost due to the addition of an external hardware to the system.

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: 6.1. Modified inverter controllers (MIC) A modified inverter controller is presented in Huka et al. (2018). In this method, a comprehensive FRT strategy for GCPPPs contains the calculation of power references to inject the desired reactive currents during different faults as stated by modern grid codes. In addition, the overvoltage and overcurrent are addressed by the active power reduction and peak current limiting strategies, respectively. Another study proposed in Merabet and Labib (2017) used a dual current controller of the inverter to control the negative- and positive-sequence components under fault events. It allows the reactive current injection to the utility grid based on the new requirements of grid codes during symmetrical and unsymmetrical grid faults. The method also protects the inverter during fault conditions, in which the current does not exceed the inverter rating current. Furthermore, according to Al-Shetwi et al. (2018), a comprehensive control strategy of the inverter achieved the FRT requirements based on the Malaysian grid code by operating the system in two different modes. The two modes are the steady-state operation mode and the FRT mode. In this strategy, once the fault is detected using an efficient detection unit, the inverter will switch to the FRT mode that is designed in such a way to address the issues of excess ACcurrent and excess DC-voltage as well as the injection of reactive current efficiently, as stated by the grid code. This method is tested under all types of grid faults, either symmetrical or asymmetrical grid faults. Moreover, in Mirhosseini et al. (2015), the FRT requirements concerning single- and two-stage-inverters-based GCPPP were addressed in

5.4. Other methods Other techniques can also improve the FRT capability in GCPPPs with the use of external devices. For instance, the use of a fault current

Fig. 14. Fault current limiter for the grid-connected PV systems during FRT operation. 346

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this study. A few changes were introduced for the inverter controller to allow the PV system to properly ride-through any kind of faults consistent with the GC requirements. These adjustments contain current limiters and an anti-wind-up method controlling the DC-link voltage and reactive current injection. Finally, the method proposed in Oon et al. (2018) set up different possible fault current characteristics, specifically focusing on the compliance of today's FRT requirements with the injection of reactive current using five special kinds of reactive current injection controls. As a conclusion, majority of these methods meet the FRT requirements as stated by the modern grid codes efficiently without additional hardware and at a lower cost.

fulfill the GC requirements using FLC. Although these strategies meet some or all FRT requirements, the complexity of the system control is increased. 6.3. Other methods Other techniques can also be utilized in order to enhance the capability of FRT in GCPPPs without external devices. For instance, the dynamic current limitation (DCL) strategy was adopted in Benz et al. (2010) in order to limit the current in small-scale PVPPs and protect the inverter and other devices from damage. A new control on the basis of feedback linearization strategy (FLS) was proposed in Zhang et al. (2011) to ensure that the inverter has the ability to ride-through the fault by remaining current levels within the limits. However, the injection of reactive currents and the DC overvoltage issues were not addressed in the two techniques. A novel method using an adaptive DClink (ADL) voltage control strategy to reduce the excessive voltage during faults was applied in Ding et al. (2016). Although this method used a bidirectional DC-DC converter to change the voltage reference of the DC-link by the MPPT control, there is a fluctuation in the DC-link voltage during unsymmetrical faults. Additionally, this control does not deal with the overcurrent issue in the AC-side as well as the injection of reactive currents as imposed by certain GCs.

6.2. Computational methods One possible solution adopted to enhance the FRT capability in gridconnected PV systems is the computational methods using the particle swarm optimization (PSO) (Saad et al., 2016) and fuzzy logic control (FLC) (Prakash and Devaraju, 2017; Hossain and Ali, 2017). The PSO method presented in Saad et al. (2016) improved the FRT using a nonlinear control technique based on the PSO for the full bridge converter and addressed the transient behavior using a chopper circuit. However, the sag was mitigated without the reactive current injection, and an oscillation and overshooting appeared in the results. The FLC-based adaptive control strategy in order to improve the FRT capacity of GCPPPs had been also introduced in Prakash and Devaraju (2017). In this methodology, a vector control plot was utilized for the DC-link voltage control and ride-through the fault safely. However, the injection of reactive current is not discussed by this study. In the same manner, the study presented in Hossain and Ali (2017) improved the FRT along with the injection of reactive current during grid faults to

7. 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

Table 1 A technical comparison of the FRT enhancement strategies for GCPPPs. Methods BESS (Ota et al., 2016; Manikanta et al., 2017)

SCESS (Worku and Abido, 2015)

BCC (Al-Shetwi and Sujod, 2018; Yang et al., 2017) STATCOM (Yang et al., 2016)

DVR (Azizi et al., 2017)

FCL (Al-Shetwi and Sujod, 2018; Sadeghkhani et al., 2017) SDBR (Hossain and Ali, 2014) MIC (Al-Shetwi et al., 2018; Huka et al., 2018; Merabet and Labib, 2017; Mirhosseini et al., 2015; Oon et al., 2018) PSO (Saad et al., 2016) & FLC (Prakash and Devaraju, 2017; Hossain and Ali, 2017)

Main advantages

Main limitations

- Excessive energy can be stored in the BESS - Reduce the amplitude of AC current - Suppress the overvoltage - Injection of reactive current is possible - Long cycle life - Effective to protect the inverter against over- voltage - Control the reactive current efficiently - Fast response during disturbances - Reduce the voltage Negativesequence - Injection of reactive current - Voltage stability especially in weak system - Constant voltage control - High ability to restrict the excessive AC- current - Enhance the grid transient stability - Low maintenance and high reliability - Efficient to meets the FRT requirements - No additional hardware - Less cost - Fast response during faults - High efficiency in MPPT - Simplicity and flexibility - No overlapping or mutation calculation

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- DC parameters fluctuation - Short life cycle - Require regular inspection and maintenance - Relatively low specific energy

Note(s) - A costly solution

- Has short term voltage stability than batteries

- Should mixed with other techniques to enhance the overall FRT performance - Incapable to supply active power - Occupied with coupling transformer and include many switches

- Simplest protection device

- Reactive control depend on the voltage - The fast response cause unstable voltage oscillations - Should mixed with other techniques to enhance the overall FRT performance

- Effective in reactive power injection

- Weak in reactive power control - Incapable to voltage fluctuations - loses some power during the grid fault period

- Has less switches as compared with FACTS - Most efficient among all other strategies

- Bring some oscillation and overshooting

- Operate in a more intuitive way complex dynamic systems

- Less disturbances and provide faster response in comparison to SVC

- Confined to the suppression of excessive current

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Table 2 Economy & complexity comparison of FRT methods. Techniques

BESS SCESS BCC STATCOM DVR FCL SDBR MIC PSO FLC DCL FLS ADL

The Cost

High High Medium High High Low Medium Low Low Low Low Low Medium

The Complexity

Medium Medium Medium High High Medium Medium Low Medium Medium Low Low Low

Fulfillment of grid code requirements

Extra devices

Overvoltage in DC-link

AC Over-current

Reactive current injection

Yes Yes Yes Yes No No Yes Yes Yes Yes No No Yes

No No No No Yes Yes No Yes No No Yes Yes No

Low Low No Good Good No No Good No Low No No No

Yes Yes Yes Yes Yes Yes Yes No No No No No No

point of view. With regards to grid code compliance, the MIC has proven to be the best method, followed by the FACTS and ESS solutions. According to the literature, the MIC method has the ability to keep the inverter connected, to ensure the safety of the system equipment, to ensure all values return to pre-fault values as soon as the fault is cleared within almost zero second as compared to the other strategies such as ESS which needs around 0.20 s, and to provide grid support through active and reactive power control at different types of faults. In addition, it has a high response speed and produced smooth results without under/overshoot. In general, the MIC FRT method is the best among all other methods because it achieves all grid code requirements efficiently at the lowest possible cost. The review also concludes that more investigations should be carried out to completely fulfill the grid codes' requirements. In particular, reactive and active power requirements of grid codes should be taken into account in more depth in the future FRT solutions.

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 issues in order to fulfill the FRT requirements, are compared and summarized in Table 2. The two main issues include the protection from overvoltage and overcurrent during faults as well as the injection of reactive current based on grid codes requirements. It is important to mention that some FRT strategies are more expensive than others. In line with this, the cost of these strategies is classified into high, medium, and low. FACTS devices, either STATCOM or SVC usually require a coupling transformer and include many switches, and therefore are considered as the most expensive strategy. In addition, the difficulty in controlling their switches will increase the complexity of these FRT techniques. Another high-cost FRT strategy is the ESS such as the BESS and SCESS because it is equipped with batteries and super capacitors, respectively, which are expensive, require periodic inspection, and regular maintenance. Since the BCC and SDBR strategies have less switches as compared to FACTS and utilize high power resistors, they are the less expensive techniques among the FRT control based on external devices. Finally, the modified controller-based techniques are more economical FRT strategies, because they do not use additional devices in their structures. Regarding the grid code compliance, the modified inverter controller method is the most efficient among all other strategies.

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8. 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 study. Finally, a comparison of these FRT strategies in terms of economic feasibility, controller complexity, additional device and fulfillment of FRT requirements was summarized. It can be concluded that the overall cost and complexity of FACTS-based methods are the highest among the others. The ESS-based methods are also expensive due to the high investment cost and short life cycle of these units, but are of less complexity than FACTS devices. The BCC, SDBR, and FCL strategies were relatively the simplest and cheapest control structures among other FRT using external devices, but have less capability concerning the compliance with GC requirements. On the other hands, all modified controller FRT methods have lower cost when compared to external device-based FRT methods from the economic 348

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