Enhancing the Power Quality of Grid Connected Photovoltaic System during Fault Ride Through: A Comprehensive Overview

Abstract

Mitigation of harmonics and enhancement of power quality (PQ) in grid connected solar photovoltaic (SPV) system during fault ride through (FRT) needs to concentrate in power system research area. A comprehensive overview of FRT capability enhancement considering study of various power quality issues associated with grid connected solar systems is done here. Mitigation and capability enhancement strategies are also discussed here. This survey will help analysts in line with FRT capability enhancement for grid connected solar PV power conditioning units.

Introduction

The society knows the limitations of fossil fuel, which have impelled climate changes and its depletion in the days to come, so PV generation systems have been broadly explored throughout the world [1–4]. SPV systems are rapidly developing energy sources within the universe, per year annual increase rates of twenty-five to thirty-five percent over last 10 years. The potential of SPV in Ontario, Canada also considered [5]. Markets for SPV have borne an impressive change in last 5 years. Before 1999, prime marketplace for PV was for off-grid utilities. Currently, more than seventy-eight percent of worldwide market is for grid-connected operational systems.

The share of renewables in global electricity generation expanded from 29 to 30% in 2021 than in 2020. The fast-growing use of renewable sources has reduced just about 900 Mega-tone of carbon dioxide production and emission within the world. The worldwide electricity production from renewable excluding large hydro dams increased from 12.4 to 13.4% worldwide in 2019 than in 2018 as per sources mentioned in following figures. This percentage can likely increase up to near about 20% at the end of year 2021 if COVID-19 situation will be normal. Renewable electricity generation in 2021 is set to expand by more than 8% to reach 8300 TWh, the fastest year-on-year growth since the 1970s as per IEA global energy review [6, 7]. The electricity generation from fossil fuels, nuclear and renewable of the world is as shown in Fig. 1. The measure of worldwide electricity generation from renewable at the end of year 2020 is as shown in Fig. 2 in which renewable include electricity production from hydropower, solar, wind, biomass, geothermal, wave, tidal sources and waste.

Fig. 1.

Electricity generation from Fossil fuels, Nuclear and Renewable in the world

Fig. 2.

Share of worldwide electricity generation from renewable, 2020

 

Renewable sources are low-carbon technologies, which offer countries around the world to improve their energy security and trigger economic development [8, 9]. Tracking clean energy progress (TCEP ) checked the progress in the evolution and distribution of main clean energy technologies. Therefore, it is a smart option to invest in these technologies [10].

The impact of fast progression of PV systems on the grid is becoming increasingly credible. With fast-growing capacity of PV systems brings serious challenges for grid service stability in fault conditions [11].

Moreover, the majority of the most recent PV capability has been integrated into the network as distributed generation (DG). Concerns about SPV's possible influence on grid stability and operation are growing with increased integration of DG sources. Utility and power system operators are preparing for improvements in order to incorporate and control a large portion of this renewable energy supply into their networks. This survey examines the applications of a high distribution grid integrated PV systems for controlling the voltage and flow of reactive power.

The electricity generation for major countries in the world up to 2021 is as shown in Fig. 3. The worldwide measure of nuclear, renewable in total electricity generation is as shown in Fig. 4 in which renewable include hydropower, biomass, wind, geothermal, marine and solar generation; it does not entail nuclear and traditional biomass.

Fig. 3.

Electricity generation in major countries of the world, 2020

Fig. 4.

The worldwide share of nuclear and renewable in total electricity generation

A commercial and domestic distribution feeder with on-load tap changing transformers (OLTC) and switching capacitors those are well equipped with voltage control were chosen. For representation of on-duty transformers and inferior circuits to PCU connection point, model was further developed [12]. In distribution grid connected PCU, as shown in block diagram of Fig. 5, power generated from PV is directly fed to a transmission line through PCU and then distributed. Energy storage devices are not required so that less space is required for the set up and investment & maintenance costs are lower than with a standalone system. As shown in Fig. 6, the evolvement of technology of inverter with control technique has brought a standard grid connected PV systems.

Fig. 5.

Grid connected PCU block diagram

Fig. 6.

Grid connected solar PV system in detail

PCU topography and control method are contrived powerfully with assuring control design due to fluctuation in supply input at inverter side. DC link voltage is settled to supply stable voltage to PCU [13–15]. High power grid-connected PCUs commonly uses single stage topography along with LC filter. In consideration of isolation transformer’s leakage inductance, an output filter is identical to LCL filter. PCU control scheme and block diagram representation of grid interfacing inverter control is as shown in Figs. 7 and 8.

Fig. 7.

PCU control scheme

Fig. 8.

Block diagram representation of grid-interfacing inverter control

As per standards of State Grid Corporation of China, controlling purpose of PV generation systems is to provide stable active and reactive power supply for main grid in assistant of FRT. It is applicable for the sinusoidal current to be injected and voltage dips giving low harmonic distortion and without over current. Harmonic distortion limit & over current are very important for developing control strategy for FRT in PCU. Grid faults are classified as symmetrical and asymmetrical faults leading to voltage sags in the distribution system [16].

In area of renewable energy interfacing conversion, there is a significant global trend in developing grid-friendly converters whose aim is to enhance performance during normal and grid fault conditions. Many countries in the world are recommending and updating FRT requirements of PV generation system due to a rapid integration of Photo Voltaic Solar System (PVSS) with the grid [17]. At present, research of FRT technology of wind power is more and has made certain achievements, which can provide reference for FRT technology of a grid connected PV system [18].

Initiation of FRT has benefits to grid while increases difficulty in design of PCUs. Surge of imbalance, over current & overvoltage on two sides may occur during any grid faults or when voltage drops [19–22]. PV generation system’s general FRT specifications rely on maintaining PCUs connected to grid without causing over current while also providing support to reactive power for helping grid recovery in the event of faults. FRT standards are commonly approved and extensively used due to their uses in voltage recovery [23, 24] and stable frequency while avoiding effects on grid transient stability [25]. In addition, a comparison of previously adopted methodologies for mitigation of harmonics is shown in Table 1.

Table 1.

Comparison of previously adopted methodologies for mitigation of harmonics (PQ Event: FRT)

Method	Limitation of the method	Fulfilment of the Standards	Contribution
Energy Storage System	High fluctuation and overshooting; High investment price and short lifestyles cycle; Increase the complexity; Require regular inspection and maintenance	Yes. Grid is supported by reactive power	Zero voltage ride through capability is verified
STATCOM and SVC	Increase the complexity and cost Did not address the increasing of DC-link voltage during grid faults; Do not deals on inverter protection	Inject reactive currents and enhance FRT capability	Voltage support control strategies & strand dynamic performance of a VSC based PV-STATCOM for power quality enhancement in grid integrated system
DDSRF, LSRF, EPLL and MAF	Complexity of the circuitry, dc voltage on the capacitor continues to shift the frequency of VCO, till it picks up the signal again	PLL goes through three stages (i) free running, (ii) capture and (iii) locked or tracking	To improve amplitude measurement efficiency under polluted grid conditions
SSM and Quadrate methods	Conscious to the harmonics due to the derivation point, and the T/4 delay method requires memory space	Sequence operation	Filtering the negative sequence components
SOGI	Calculating voltage dip depth, integrator factor will inevitably introduce further time delay, which may cause PCU to fail to meet the strict grid code to some degree	SOGI with frequency locked loop (SOGI-FLL) available for grid synchronization	Beneficial in phase angle measurement because it does not require extra memory space and is not conscious to harmonics

The objectives of this paper are to understand different FRT capabilities in power systems, to study, review and analyze FRT techniques in grid connected SPV systems, as well as to study the methods of mitigating harmonics involved in the system. This comprehensive overview paper contains previously adopted and latest methodologies up to 2021 for mitigation of harmonics and enhancement of PQ in a grid connected SPV system during FRT.

Electrical Power Quality Problems and Solutions

Electrical Power Quality Problems

The suggested electrical power and conditioning architecture, operation and service intervals for sensitive electronic processing equipment are provided in IEEE 1100-1999 Standard, which has been superseded by IEEE 1100-2005 Standard.The approaches used in this article address electronic equipment performance problems while ensuring a safe installation. Type of power quality issues, potential remedies and available resources for service in dealing with issues are briefly listed.

When exposed to power quality issues, all electrical equipments are susceptible to failure or malfunction. Electrical power quality grows into a major matter for electric utilities. The power quality research is becoming increasingly common. Electrical equipment malfunctions, instabilities, and failure are caused by power line disturbances such as voltage sag/swell, momentary disruption, harmonic distortion, flicker, notch, spike, and transients, among other things.

Voltage sags or brief outages can be caused by faults in an electric distribution network whereas voltage swells by turning off an extensive load. A voltage dip is a temporary drop in root mean square (R.M.S.) voltage. It is normally expressed as a percentage of nominal R.M.S. voltage that stays at its lowest point during the dip and is calculated in terms of time and voltage sustained. A voltage drop indicates that load is not getting enough capacity, which can have serious consequences depending on the load type.

Large loads being started on distressed site or by a customer on same circuit, as well as faults on other networks are two important reasons of voltage dips. Voltage swell is transient increase in R.M.S. value of alternating voltage with magnitudes varying from 110 to 180% of the rated voltage.

When arc furnaces are used, flickers can occur. Transients are caused by ferroresonance, transformer energization, or capacitor switching, while spikes are caused by lightning strikes. Total harmonic distortion (THD) can be used to measure harmonic pollution on a power line. High harmonic distortion can have detrimental effect on electric distribution system and cause extra heat in motors, which can lead to premature failure.

Heat can also cause wire insulation to break down and fail. Increased operating temperatures may also have an effect on other equipment, causing malfunctions and premature failure. Similarly, harmonics in line causes restarting of computers and destroys another perceptive analogue circuit. Table 2 summarizes the various types of power quality issues. It is pertinent to identify the power quality issues in the system before initiating the mitigating measures.

Table 2.

Power quality types

Category	Specific types
Events	Transients
	Interruption
	Sag
	Swell
	Phase angle jump
Variations	Magnitude variation
	Frequency variation
	Phase variation
	Unbalance
	Flicker
	Harmonic distortion
	Inter-harmonics
	Notching
	Noise

Power Quality Solutions

In order to mitigate power quality problems, it is important to collect consequential data of best locations and in a timely manner. Instruments best suited for a specific application should be used to collect useful and relevant data. Table 3 categorizes the various power quality solutions currently in use and found in the literature.

Table 3.

Power quality solutions

Hardware	Software	Measuring and monitoring
Active harmonic filters	Wavelet theory	Artificial intelligence instruments
Micro SMES for power quality	Expert systems	remote access
Large SMES for transmission distribution	Fuzzy logic Genetic algorithm	Integrated diagnostic
PWM based higher power compensators	Neural network	Comprehensive system monitoring
FACTS Controller, custom power devices, transfer switches	–	Centralized monitoring (GPS)

FRT Capability

FRT refers to an electric generators ability to remain connected during short periods of lower electric network voltage (voltage dip). FRT also known as low voltage ride through (LVRT) or under-voltage ride through (UVRT). FRT required at distribution level (PV systems) to prevent a widespread loss of generation due to a short circuit at high or extra high voltage level. Computer systems and industrial processes are often met by using a capacitor bank or an uninterruptible power supply (UPS) to supply makeup power throughout events. FRT refers to an electrical device's ability to operate normally when the voltage is minimal. Many electrical devices, in general, need a certain amount of voltage to function properly. If the voltage is too low, the equipment will not function properly or will have a lower performance [26].

Grid stability and supply security are two critical conditions of energy supply. Power generation plants must have control capacities and safety measures in order to prevent power outages. Previously, traditional power plants were primarily responsible for meeting these needs.

Integration of renewable energy resources with the grid has increased the potential of maintaining the grid stability. As a result, operators of transmission system have well-settled grid codes (GCs) which mandate that generating plants follow certain basic values and control characteristics. A key element of these criteria is the FRT ability of generating plants, which states the requirement that generating plants keep working during short periods of low grid voltage and do not disconnect from the grid.

Short-term voltage dips can be caused by grid faults such as lightning strikes, short-circuits, and whenever massive loads are switched on and off instantly. Wind turbines and other renewable energy plants have traditionally been permitted to detach from the grid during a power outage and reconnect after a specific duration.

Such a technique would be fatal today given the large share of renewable. If so many generation systems shut down the same way, the whole grid falls down in a "blackout." As a result, the FRT requirement was created to ensure that generation systems remained grid connected. During voltage decreases, many grid codes require that the grid be supported. The grid will benefit from generation sources feeding reactive current into the grid, which raises the voltage. In certain duration after the fault has been cleared, the real output power should be restored to its magnitude before the fault. These standards, which were previously only applicable to wind turbines, now are to be met by the PV also. Figure 9 depicts the impact of a voltage drop on a PV device. In this test, voltage falls to about 20% of nominal voltage for about 550 ms. PCU senses voltage drop and feeds a reactive current into system equal to around 100% of the nominal voltage to keep the grid running for the duration of the fault. Within 160 ms after the fault has been cleared, the real power output is raised to the value prior to fault occurrence. A type certificate is usually required by the transmission system operator before a renewable generation system can be connected to grid. The assessment of electrical properties, which requires FRT capability test, is one of the certification criteria. Voltage drops are controlled during a test as well as the plant’s performance is determined and assessed using voltage, reactive current and active power [27]. Figure 10 shows the E.ON Netz GmbH Code for FRT, which has been used in various publications on FRT capability [28–33], as well as the percent voltage sag vs. time of an actual working curve [34].

Fig. 9.

Example of the results of a voltage drop test

Fig. 10.

E.ON Code for FRT Curve between % voltage sag and time of operation

The region just above the curve in red denotes that grid is still connected to the distributed generator (DG). The dark region beneath the red curve denotes that DG being tripped to security for safety. PCU, for example, must remain associated with a grid for 1.5 s during 90 percent voltage sag. DG will have a linearly proportional active/reactive current output over entire voltage sag range of 90–50% in accordance to E.ON code specifications. DG can output 100% reactive current at a fifty percent line voltage drop. The generation unit is allowed permitted to continue a grid connected once the intervals and levels of voltage sags appear under the bold line in Fig. 10.

When the grid is connected, a generation system is used to supply reactive power in proportion to the reactive current delivered in the grid, which can be seen in Fig. 11, which shows required rate of reactive current throughout FRT [35, 36].

Fig. 11.

Needed rate of reactive current throughout FRT

FRT Detection and Phase-Locked Loops

While some country’s grid codes do not explicitly limit FRT response time, the Chinese standard, which was recently revised, requires that necessary reactive power being introduced throughout 30 ms following voltage dips. Under ideal operating conditions, several R.M.S. value calculation methods, which will not be illustrated in detail, will produce same voltage dip level in halves the basic time. However, due to harmonics and an irregular grid state these amplitude measurement methods are insufficient to deal with conditions. A few of methods based on their own PLLs were presented, including Decoupled Double Synchronous Reference (DDSRF) [37], Synchronous Reference Frame with Low Pass Filter (LSRF) [38], Enhanced Phase-Locked Loop (EPLL) [39] and Moving Average Filter (MAF) [40] to improve amplitude measurement efficiency under polluted grid conditions. The amplitude detector systems are strong contenders for detecting low voltage faults. Everyone’s exact configuration will not be shown here, but their performance comparison is shown in Fig. 12.

Fig. 12.

Performance comparison of amplitude detection methods

Furthermore, since different standards use different FRT reference voltages, the self-reliant alert system should not only obtain maximum value of every step but also convert it to required values in accordance with the criteria [28, 41].Additional processing, such as string splitting, should be applied to PLLs to overcome negative sequence and harmonic disturbances and achieve the correct phase angle components.

The unbalanced grid voltage interferes with PLL and degrades output when Sequence Separation Method (SSM) is not used in control systems as shown in Fig. 13. It is supported by evidence that negative sequence is a double line frequency signal under the d-q frame; several methods for filtering the negative sequence are shown in [42].

Fig. 13.

Performance of PV inverter without applying SSM: a Grid voltage with Phase C drops to zero, b Deteriorated output current, c Polluted PLL result (sin)

According to [43], positive and negative sequences are easily measured in a frame using 90° lagging waveform in αβ structure, as mentioned in Eqs. (1–3).

$$V_{\alpha \beta }^{+}=\frac{1}{2}\left[\begin{array}{c}1-q\\ q1\\ \end{array}\right]V_{\alpha \beta }$$ 1

$$V_{\alpha \beta }^{-}=\frac{1}{2}\left[\begin{array}{c}1q\\ -q1\\ \end{array}\right]V_{\alpha \beta }$$ 2

$$q=e^{-j\frac{\pi }{2}}$$ 3

Here, positive sequence of voltage in αβ structure is denoted by $V_{\alpha \beta }^{+}$ while the negative sequence is denoted by $V_{\alpha \beta }^{-}$. Quadrate methods such as T/4 delay method [44], the differentiation method [42] and Second-Order-Generalized-Integrator (SOGI) method [45] are obviously necessary and important methods for sequence separation.

Figure 14 depicts everyone’s performance during single phase to ground fault conditions with grid voltage intervention from three percent seventh order harmonics. Here d⁺ means positive sequence in the d-axis and d⁻ represents the negative sequence in the d-axis. Differentiation method is quick but conscious to harmonics due to the derivation point, and the T/4 delay method requires memory space to make a stock of T/4 time signals and can only extract harmonics in a definite order and phase angles; the SOGI method is beneficial in phase angle measurement because it does not require extra memory space and is not conscious to harmonics. Figure 15 shows a comprehensive structure of SSM using the SOGI. However, when calculating voltage dip depth, integrator factor will inevitably introduce further time delay, which may cause PCU to fail to meet the strict grid code to some degree. Furthermore, a single three-phase PLL is incapable of determining the exact amplitude per phase. As a result, calculating voltage reference to map necessary reactive power cannot be assumed directly. To get around these limitations, [46] shows how to measure amplitude per step depending on the voltage distortion conditions using a quick voltage detection system.

Fig. 14.

Comparison of different SSMs

Fig. 15.

Structure of SSM with SOGI

FRT Control Techniques in PV System

Current References

To achieve reactive power injection, which is a basic consideration of FRT, PCU control device must be able to decouple actual and reactive components. Most applications do not require real current during FRT, allowing it to be set flexibly depending on the inverter design specifications and operating conditions. The control strategy has to ensure the limitation voltage less than or equal to 1.1 p.u. consequent to injection of reactive current. So formation of reactive current should be limited by amplitude of peak grid voltage. In the meantime, average output current does not exceed 1.1 per unit that can only be considered if active current is also introduced throughout FRT. A good example of how to measure proper actual and reactive current references can be found in reference [47] and droop regulation is also a viable option [48].

Dynamic Performance

General control methods like proportional + resonance (PR) controller in stable coordinates, PI controller in synchronous coordinates, deadbeat (DB) controller are used in the FRT control techniques [49]. On the other hand, sudden increase in output current leading to immediate decrease in voltage will cause over current and trigger integrated circuit breakers, leading to FRT malfunction.

Voltage input forwarding method in control system of a Dynamic Voltage Restorer (DVR) [42] to achieve rapid dynamic response and eliminates a sudden current change that is an important measure used in FRT control system [50].

Grid impedance changes occurred in tandem with grid faults, in addition to voltage amplitude variations. Effects of grid impedance variation should be given more attention since variations in grid impedance may affect the LCL filter's resonance frequency [51] and control stability [52], as well as mislead the generation of compensated current [53]. The respective remedy, proportional normalized averaged current feedback control [51] and H∞ [52] method can be used to oppose the distortion of variance in impedance of a grid. Grid impedance properties like cost and R/X ratio can also have an effect on reactive power support performance. As suggested in [53] grid impedance, detection via the online process and [54] introduces a control method that takes fluctuating R/X ratio considerations into account.

Control Under Unsymmetrical Faults

The grid connected voltage source inverters (VSI) are vulnerable to changes in voltage of a grid, particularly extreme and instantaneous distortions. When distortions are asymmetrical, effects are amplified by presence of a negative sequence variable leading to interfere in the PLL performance, causing the output current to degrade and the dc-link voltage to fluctuate, as seen above.

The factors responsible for unsymmetrical faults leading to negative consequences could be evaluated as follows. Whenever unsymmetrical fault detects, output power is calculated by using Eq. (4).

$$S_{\mathit{out}}=\frac{3}{2}{v}_{\alpha \beta }.i_{\alpha \beta }^{\ast }=\frac{3}{2}\left(e^{j0X}.v_{\mathit{dq}}^{+}+e^{-j0X}.v_{\mathit{dq}}^{-}\right).{\left(e^{j0X}.i_{\mathit{dq}}^{+}+e^{-j0X}.i_{\mathit{dq}}^{-}\right)}^{\ast }$$ 4

Equation (4) can be extended further as (5) and (6).

$$S_{\mathit{out}}=P_{\mathit{out}}+S_{oc1}.sin(2\omega t)+S_{oc2}.cos(2\omega t)+Q_{\mathit{out}}$$ 5

$$\left[\begin{array}{c}{P}_{\mathit{out}}\\ Q_{\mathit{out}}\\ S_{oc1}\\ S_{oc2}\\ \end{array}\right]=\frac{3}{2}\left[\begin{array}{c}{v}_d^{+}{v}_q^{+}{v}_d^{-}{v}_q^{-}\\ v_q^{+}-v_d^{+}{v}_q^{-}-v_d^{-}\\ v_q^{-}-v_d^{-}-v_q^{+}{v}_d^{+}\\ v_d^{-}{v}_q^{-}{v}_d^{+}{v}_q^{+}\\ \end{array}\right]\left[\begin{array}{c}{i}_d^{+}\\ i_q^{+}\\ i_d^{-}\\ i_q^{-}\\ \end{array}\right]$$ 6

Consequently, the output power includes steady real power P_out, steady reactive power Q_out and double line frequency variation elements Soc₁ and Soc₂. The existence of a negative sequence portion obviously complicates controlling power output; quality of output current and dc link voltage all at the same time. Unsymmetrical faults are more common in practice than symmetrical faults. Due to the negative sequence elements, commonly used control techniques are unprepared to handle double line frequency harmonics in synchronous frame; some changes should be made intended to prevent weak performance parameters during FRT.

To boost output during unsymmetrical faults [55, 56] proposed vector current control with feed forward of negative-sequence grid voltage (VCCF) was used. The control block is depicted in Fig. 16 where positive sequence output current is segregated and regulated whereas negative-sequence grid voltage is spotted and directly fed to the modulated signal. The feed forward negative series effectively compensates for unbalanced grid voltage by aligning the output voltage with grid voltage.

Fig. 16.

A typical VCCF Control block

With the aid of the SSM, the converter will insert symmetrical and sinusoidal current in a grid, preventing double line frequency variation in the d-q frame.

Dual vector current control (DVCC) [57–59] is another widely used control technique that improves on the traditional control approach by considering negative sequence current. This current is collected and regulated in the synchronous reference frame in addition to positive sequence current as shown in Fig. 17. Since the structure is sequential to relate sequence elements, current of output can be converted to dc signals that could be regulated with a traditional control system built for symmetric systems.

Fig. 17.

A typical DVCC control block

Two control strategies listed above and their results can be found in [55, 60]. Table 4 shows the outcome of the comparison. Since positive sequence current only is injected in a grid, VCCF will produce symmetric output current, according to this table. The control goal for DVCC can be versatile but complex. A symmetrical output current is obtained by setting the negative sequence current to zero [60].

Table 4.

Comparison of VCCF and DVCC

	Output current	DC-Link voltage	Reactive power	Current stress
VCCF	Symmetrical	2-order fluctuation	2-order fluctuation	Increased
DVCC	Unsymmetrical	smooth	Constant	Increased

Furthermore, as DVCC inferred, both negative and positive sequence elements can inject reactive power, which means output current could be asymmetrical; however, the power output variation is reduced. The real and reactive current references will be developed as per (5), and the control goal becomes to reduce power variation, assuming Soc₁ = Soc₂ = 0 [61]. Clearly, output current accuracy and output power consistency cannot be assured at the same time. Both strategies can result in an increase in output current stress resulting in over current and FRT failure.

To prevent over current, [62] proposed a strategy for reducing current stress by calculating the positive and negative current references using the largest maximum value for every step at a specific instant. Furthermore, as shown in [63], once the voltage decreases dramatically, DVCC with reactive current mitigation can have a negative impact on the dc link voltage especially with the current due to the current limitation requirement. The control scheme in [64] is more complicated than VCCF, DVCC, and their updated versions, since it regulates three phase output currents separately and attains symmetrical current rather than using sequence separation technique. Fortunately, real power is not needed in most FRT regulations, allowing for the development of various control techniques.

A power balance system for single-stage inverters was proposed in the literature [65] by designing PV array operating point in the control method. By setting controlled dc link voltage reference as per needed real power, this scheme defines both the necessary reactive and real current references.

When setting dc link voltage, however, real-time properties of PV array must be taken into account. To combat this issue, [66] regulates input power rather than dc link voltage for balancing of input and output true power, as shown in Fig. 18. By compromising constant dc link voltage but keeping it essentially steady, output current can be symmetrical and sinusoidal much beneath unsymmetrical faults.

Fig.18.

Control block explained in reference [55]

Two-stage converters with an additional input current control variable may have a more flexible control strategy. Maximum current amplitude control (MCAC) is a technique described in [67] that injects desired real power while limiting peak current amplitude. Multiple modes for the first stage of a grid connected PCUs are described in [68] in order to retain equitable power.

When voltage drops, effect of reactive power from PV system on the PCC voltage is evaluated using the German grid code and as per IEA report, a method to regulate PCU to inject reactive current [69–71], and an adaptive voltage support control technique [72] is defined to increase PCU’s FRT capability. Novel photovoltaic inverter mathematical models for balanced and unbalanced grids are proposed and analyzed.

The authors related to photovoltaic inverter control strategies focuses to solve complex issues in unbalanced conditions [73–76]. A comprehensive approach that relies on PR controller was specified to allow control of grid code of PCUs during FRT conditions [77].

After that, a new PV grid-connected system with FRT capability [78] was defined. The following are the key features of that system [79–81]:

• A)To keep active power constant while increasing reactive power injection at a predetermined interval
• B)To maintain steady active and reactive currents during voltage sags
• C)No new components are added to the system; instead, the inverter control parameters that are not used by maximum power point tracking (MPPT) technique are exploited.

Present harmonics injection sources from single-phase grid-connected inverter systems in various operating modes have been investigated [82–84].

The study shows how the injected current distortion differs with feed-in grid current level, power factor, and grid voltage level (e.g., in different PV system locations and under different environmental conditions) [85].

The traditional controller system with defined (PI) controller elements was tested using error strategies and performed well in normal conditions, but it was unable to handle the normal condition. By monitoring needed power ride through voltage sags, as well as harmonic mitigation for voltage and current droop control techniques demonstrate high performance [86, 87]. This is capable of maintaining voltage in distribution grids during voltage sags. To maximize benefits and minimize errors, the new researcher can investigate adjusting PI and droop controller parameters using artificial intelligence techniques instead of trial and error [88].

A FRT control technique of 3-phase, 2-stage grid-connected PV systems was mentioned in [89]. The control technique recommends two dc bus voltage controllers. In typical grid voltage mode, first one is then used to give power to grid side converter. The extra controller is connected to MPPT in series [90, 91].

Based on the current grid requirements, [92] addressed the ability of FRT control technique for a single stage photovoltaic power plant (PVPP) functioning in grid system disturbances. Since the modifications are mostly in the length and depth of voltage sag, the control technique listed few changes to allow PVPP to ride through all grid faults in compliance with Malaysian guidelines and various grid codes (GCs) and countries in the world.

Use of dc break chopper, insertion of reactive power control and current limiter, are among the modifications [93–98]. The effects of FRT and dynamic voltage support (DVS) capacity, active current survival rate, regional & plant level voltage control on short-term (ST) voltage stability, and also transient and frequency stability is investigated in large-scale PV system.

Moreover, voltage recovery index (VRI) and critical clearing time (CCT) were used to assess dynamic output [99–101]. In order to attain better operation under grid faults, a novel FRT control technique for 2-stage 3-phase SPV system was investigated. DC link voltage control technique, with and without MPPT controller were investigated during asymmetrical and symmetrical fault conditions. Under every fault condition, control strategy described will effectively protect the device against dc-link over-voltage events [102–106].

The assessment and management of power quality of a single stage, 3-phase photovoltaic power plant (PVPP) linked to medium voltage (MV) side of an electrical grid were investigated. Voltage sag, harmonics, power factor, voltage unbalance frequency and voltage flicker are taken into account for latest power quality integration requirements and regulations. Voltage sag was reduced.

Thanks to the injection of reactive current through enhanced PCU’s FRT functionality of controlling. The control was created by modifying a conventional inverter control to operate in two modes during a sag event: normal and faulty (FRT). The controller can be switched between two modes quickly and precisely using the R.M.S. detection method. The dynamic voltage regulator (DVR) demonstrated its capability to improve voltage stability by considering voltage flicker and unbalance.

To reduce voltage and current harmonics, control strategies and RLC filter were developed [107–111]. A single phase, single stage transformer-less grid connected PCU with FRT capability was suggested. That is why, Cuk derived transformer-less inverter (CDTI) was developed. CDTI would introduce reactive power in a grid without distorting injected current under conditions of voltage sag. In a transformer-less interface of a grid and PV, PCU injects reactive and active power components into a grid without impacting elimination of leakage current, which is a main consideration. Apart from that, during FRT, there was no decrease in performance or additional strain on the IGBTs used as switching devices in PCU. Losses in IGBTs for SPWM activity during FRT were determined to show that efficiency is unaffected [112–114].

Power Balance Control

DC link voltage control is also known as power balance control. It is a critical issue for PV generation systems during FRT. If the output current is not adequately controlled, output real power drops immediately as grid voltage drops. As studied in introduction section, the behaviors of SPV systems and wind power systems differ. The output power of a wind power generation system cannot be quickly reduced due to inertia of the wind turbine, allowing input power of integrating back-to-back converters to surpass power output. As a result, dc link voltage may increase remarkably. SPV systems will not experience from extreme dc link voltage uprush due to inherent characteristics of PV arrays, even if the interfacing inverters are single-stage converters. As voltage of PV system departs from MPP, power input decreases inherently until it hits a new stable operating point. Purposeful dc link voltage control has still been needed, particularly with common high power single stage PCUs, that are frequently used in centralized PV stations governed with GCs requiring FRT [115, 116].

Technical and Economical Consideration

As shown above, some inconsistencies such as quality of output current with dc-link fluctuation attenuation cannot be resolved simultaneously. Meanwhile, the demand for rapid real-time recovery after FRT, which in Chinese standards is 30 percent of rated power per second ramping rate, is a challenge for MPPT speed and stability. In fact, these differences are caused primarily by the coupling of dc link voltage and output current, resulting in trade-offs rather than complete solutions.

As a result, configuring operational requirements while rising cost per unit is impressive option for a single as well as two stage PCUs. Furthermore, increased storage can provide additional benefits such as enhanced real-time power dispatch response and grid demand adjustment in response to changing electricity prices. Figure 19 depicts the topography of a single-stage PCU with a super-capacitor. The general scheme of a distributed generation (DG) unit connected to the grid by the state distribution company for single-phase local load is shown in Fig. 20. The V/F power of the motor load is also shown in Fig. 21.

Fig.19.

Single-stage grid-tied PV inverter with super-capacitor configured

Fig. 20.

General scheme of DG unit connected to the grid

Fig. 21.

V/F Control of motor load

A bidirectional DC-DC converter (BDC) connects the super-capacitor to dc link, acting just as an additional energy absorber. As a consequence, dc link voltage control and current control output can be segregated during FRT. As a consequence, dc link voltage can be kept constant at the PV array's MPP to achieve adequate harvested energy while the PCU provides necessary reactive current.

FRT recovery mechanism is greatly improved by preventing the super-capacitor from consuming power input from SPV system and injecting entire power to grid without re-tracking the MPP.

Similarly, to enhance power quality of grid connected photovoltaic system as shown in Fig. 6; this needs to control dc-link voltage of PCU during FRT. Hence, it is required to focus the attention on controlling dc link voltage of PCU during FRT and develop a control strategy for controlling dc link voltage [117, 118].

Future Trends in FRT Requirement

The criteria will be studied in greater depth as FRT techniques grow and mature. Some published grid codes do not specify the THD requirement. A few methods try to introduce symmetrical and sinusoidal current into the grid, whereas remnant completely overlooks it. Efficiency of reactive power support under unbalanced faults is still unclear– whether or not PCUs will inject reactive power into regular operating phases. In addition, only medium and high voltage grid connected PV stations must comply with the FRT requirements. FRT regulation on the distribution grid is inevitable as most DG (PV) systems are grid connected. FRT technologies used in distribution systems would be specific due to the complex grid layout and must be researched in deep.

Conclusion

A comprehensive overview of FRT techniques for PV generation systems, grouped by the challenges that have been encountered in the field is provided here. The techniques for fault identification, current control, and balance of power in a PCU are defined using a review of some recently revised grid codes with FRT specifications. The sequence separation method is illustrated in detail, with existing control methods such as VCCF and DVCC with technological and economic considerations.

Funding

The authors declare that no funds, grants or other support were received during the preparation of this manuscript.

Declarations

Conflict of interest

The authors have no relevant financial or non-financial interests to disclose.