Fault Ride-Through Capability Enhancement for Microinverter Applications

Due to the fast growth of single-phase grid-connected photovoltaic (PV) systems, the existing grid codes are expected to bemodified to guarantee the availability, quality, and reliability of the electrical system. Therefore, the future single-phase PV systems should become smarter and support low voltage ride-through (LVRT) capability, which are required for three-phase wind power systems. In this paper, the operation principle of a flyback inverter in a low-voltage ride-through operation is demonstrated in order to map future challenges. The steady state performance of the flyback inverter under voltage rise and drop conditions at boundary conduction mode (BCM) and discontinues conduction mode (DCM) is studied theoretically. The simulation results of the flyback inverter for various grid faults are presented to verify the theoretical analyses. The results indicate the fact that the flyback inverter at BCM condition can provide LVRT capability for photovoltaic microinverter applications in distributed generation (DG) systems, even though it does not need any auxiliary control branches and any limitations in components design.


Introduction
Recently, due to the matured PV technology and the declined cost of the PV module cells, the use of single-phase gridconnected PV systems has increased in the public grid [1,2].PV power systems can be categorized into centralized, string, and AC module systems.The use of a microinverter, which is also named AC-PV module, is becoming more popular due to its important benefits compared to the conventional centralized architecture.The AC-PV module which is shown in Figure 1 has numerous merits, namely, higher efficiency, individual maximum power point tracking (MPPT), alleviating the negative effects of partial shading, higher reliability and flexibility, friendly "Plug-N-Play" operation, and simple installation of modules [3][4][5][6].The microinverter consists of a single PV module and a single-phase inverter ranging from 50W to 400W.Therefore, these small scale systems can be forthrightly deployed in urban low voltage DG systems with relatively simple technical efforts and without occupying productive land and/or deteriorating urban environments.Nevertheless, the phenomenon of large scale DG systems which are connected to the public network increases some significant alarm about the availability, quality, and reliability of the public network.
Traditionally, since the grid-connected PV distributed generation systems were small scale at a residential level, they were designed to be disconnected from the grid under voltage rise and drop conditions in order to protect the interfacing inverter [7].However, because of the increasing penetration of grid-connected PV systems in the public grid, not only can the disconnection of a large power grid-connected PV systems disturb the stability of the main grid but also it can cause detrimental influence on the quality, availability, and reliability of the whole system.Accordingly, many grid requests are proposed for DG systems recently to control the interaction between DGs and the public network [8,9].The fact that the future grid-connected PV systems will be smarter to provide the LVRT capability is expected [ 10].The LVRT capability needs that the generation system remains connected to the grid in order to help the grid voltage and frequency remain stable during the grid faults.On this basis, a lot of research has been done on the gridconnected PV systems so that they provide LVRT capability [11][12][13][14].Also, various developments related to this concern have been already done for wind farms [15][16][17][18], since they provide higher power in comparison to the photovoltaic systems which are connected to the grid.It is common that the LVRT capability generally is used in medium-voltage networks.Because of the rapid rising of single-phase grid-connected PV systems, this feature is extended in DG units installed in the low-voltage networks.In [19][20][21][22][23], some investigations on the single-phase gridconnected PV systems have been presented in order to prove that the LVRT capability can be achieved.These papers studied various aspects of LVRT control strategies which include sequence separation techniques, phase locked loop (PLL), current controller, and control approaches.A vector current control technique which uses negative sequence voltage feed forward is introduced in [1] to enhance the single-phase inverter performances under various grid faults.A through control strategy which allows the PV power plant to tolerate grid faults is proposed in [24].Not only is the ac over-current as well as the dc-link over-voltage limited during voltage rise and fall, but also reactive power is injected in gird.However, since the control techniques in previous works are based on the concept of single-phase active and reactive power or the frequency and voltage droop control through active and reactive power, respectively, the complexity as well as cost of the control system of the grid-connected PV systems surges.
The aforementioned benefits of the microinverters and growing their power ranges make them a significant factor in the growth of the penetration of PV systems.Moreover, the microinverters are used in 5-10kW building PV applications since they provide higher energy in comparison with the centralized architecture of PV systems.Hence, in the future, the microinverters will be a significant part of the PV systems at the urban low-voltage networks; thus, they should be smarter and support LVRT capability to enhance the stability and reliability of the grid [25][26][27].
The various inverters which can be employed in singlephase grid-connected PV application are reviewed in [3,28].Among the numerous topologies, flyback inverter is widely used in microinverters because it has many advantages such as simple configuration, low manufacturing cost, high efficiency, no DC-DC step-up converter, and isolation between the input source and public grid [29][30][31].Hence, the flyback inverter should be modified to support the LVRT capability.In [26], the LVRT capability is implemented on the microcontroller so that the flyback microinverter remains connected to the grid during some prespecified voltage sag limits.In this topology, an auxiliary control branch is added to the main control system of the flyback inverter to support the LVRT capability which causes complexity of the control system and high manufacturing cost.Also, the inverter cannot support LVRT for various ranges of voltage sag.In [25], the LVRT capability for the flyback inverter is provided at DCM condition.Nonetheless, it suffers from some shortcomings, namely, numerous limitations for designing of the inverter's components, low quality, and low reliability under deep grid faults.Also, the THD analysis is missing in it and from its results, it seems that the THD is high during the voltage fault.
In this paper, the performance of the flyback inverter for microinverter applications is studied.The complete operation of the flyback inverter at BCM and DCM operations under voltage rise and drop conditions is presented.It will be indicated that the flyback inverter at BCM condition can provide LVRT capability, despite the fact that it does not need any auxiliary control branches and any limitations in components design of the inverter.Also, the output current THD is in appropriate range during voltage fault.The control system of the flyback inverter at BCM condition is simple since it only needs an open-loop control technique.The overall control system of the flyback microinverter based on the grid requirements which have been already defined for wind power systems while public grid is under faulty mode condition is presented.It is predicted that these requirements for wind power systems will be the basic requirements for grid-connected PV distributed generation systems.Finally, the unique feature of the flyback inverter at BCM condition under various grid faults is verified by simulation results.
The paper is organized as follows.In Section 2, there is an overview of selected grid requirements.The analysis of the flyback inverter at BCM condition is presented in Section 3. In Section 4, the analysis of the flyback inverter under voltage rise and drop conditions is discussed.In Section 5, design considerations of the flyback inverter at BCM condition are obtained.In Section 6, the simulation results of the flyback inverter at BCM and DCM operations under various grid faults are presented.Finally, a brief conclusion is provided in Section 7.

Grid Requirement
In some international regulations [32], it is mentioned that the grid-connected photovoltaic DG systems at a residential level have to be disconnected from the grid to protect the interfacing inverter.In these grid requirements, the basic demands are imposed such as the power quality of the gridconnected inverter (the output current THD) and the antiislanding condition.However, because of the fast growing installations of single-phase grid-connected PV systems, the grid requirements for DG systems are updated and modified based on the growth and penetration level of grid-connected photovoltaic DG systems.Like wind power systems which are connected to the medium and high voltages of the public grid, the future single-phase grid-connected PV systems should provide a contribution to the public grid by means of riding through grid faults, which is known as LVRT capability.
The LVRT capability is implemented in order that the DG system should remain connected to the grid for a short transition time and help the grid voltage and frequency stay stable during the public grid faults.Figure 2 depicts the various LVRT curves, derived by studying several energy markets, with defined remain-connected time under various grid faults which is named grid code [8,33].According to Figure 2, correspondingly to the depth of the voltage drop, the required time duration where DG system should remain connected to the grid is between t and t .For instance, if the grid voltage falls to V, the DG unit should remain connected to the grid for .seconds in the German grid code.Also, under voltage rise condition, the DG unit operates in normal condition.These grid codes are provided to guarantee the safety of utility maintenance personnel and equipment of the microinverter and to avoid the grid collapse because of grid faults.

Analysis of the Flyback Inverter at BCM Condition
The flyback inverter can operate with its magnetizing current in continuous conduction mode (CCM), DCM, or BCM.
In the flyback inverter, CCM operation requires a complex control system since the transfer function of the inverter in this mode has a right half plane (RHP) zero.However, DCM and BCM operations are simply achieved by peak-current control [34,35].In addition, a hybrid operation which mixes DCM and BCM operations in a half grid period has been adopted to enhance the efficiency and diminish the output current THD of the inverter [36,37].The schematic of the flyback inverter and its key waveforms under BCM condition are presented in Figures 3(a) and 3(b), respectively.As shown in Figure 3(a), the flyback inverter includes one high frequency switch S , input capacitance   , a high frequency transformer T with two secondary windings, two low frequency switches in the secondary S o and S o , two secondary diodes D o and D o , and output filter including a capacitor   and an inductance   .The turns ratio of the primary to the secondary winding is   :  , and the magnetizing inductance of the transformer is   .
At BCM condition, the output current i out can be directly controlled by the primary reference current i ref in each switching period.The gating pulses as well as current waveforms of the switches of the flyback inverter are depicted in Figure 3(b).From Figure 3(b), it can be observed that the pulse width of the main switch is created in a Sine Pulse Width Modulation (SPWM) pattern to generate a sinusoidal output current.When the main switch is turned on,   is charged by the input voltage source.Then, the main switch is turned off and the energy stored in   is transferred to the grid by the one of the secondary diodes.Under BCM condition, the main switch S is turned on when the current of the magnetizing inductance drops to zero.Also, the secondary switches S o and S o are turned on in the positive and negative half grid period, respectively, to change the semi-sinusoidal secondary currents of the secondary windings to a full-sinusoidal current.
According to Figure 3(b), the peak value of the primary winding current   is forced to follow the primary reference current i ref under BCM operation.In each switching period, when the secondary current   drops to zero, S is turned on, and   rises linearly with   .When   reaches i ref , S is turned off and   reduces linearly with   .Finally, the output current of the inverter i out is achieved by filtering the secondary current of the transformer.
In order to make the analysis simple, some assumptions are considered which are presented as follows: (i) All the components are considered ideal.(ii) The inverter operates in steady-state condition.(iii) Both the input and output voltages are constant in each switching cycle.(iv) The grid voltage is positive; thus, S o is on and S o is off in the analysis.It is worth mentioning that the operation of the flyback inverter during both the positive and negative half period of the grid voltage is analogous.
The primary and secondary currents of the flyback inverter in switching period under BCM condition are presented in Figure 4(a).As shown in this figure, the turn-on time   and turn-off time T off and the switching period   can be achieved as ( 1) and (2), respectively.The relationship between the maximum of the primary current in each switching period   and the maximum of the secondary current in each switching period   can be defined as (3).
Since i out , which injects to public grid, is achieved by filtering   , i out is approximately equal to the average value of   in each switching cycle.Hence, the output current can be achieved as (4).
After simplification, the reference current i ref is where Substituting ( 7) into (6), the primary reference current i ref of a single-phase grid-connected flyback inverter at BCM condition is calculated by (8).

Analysis of the Flyback Inverter under Voltage Rise and Drop Conditions
In Figure 4(b), the primary and secondary currents of the flyback's transformer at DCM condition in some switching period are presented.At DCM condition, according to Figure 4(b), it must be confirmed that the difference between the time interval of the switching period   and the maximum of the turn-on time interval  . is higher than the turn-off time interval T off , It can be found from Figure 4 that the turn-off time T off and turn-on time   have the same relations in DCM and BCM operations.
In voltage drop condition, according to the relation ( 1), the turn-off time interval T off rises, even though the switching frequency   and the maximum of the turn-on time interval  .remain constant.Hence, the condition (9) is not true in this condition.This effect leads the mode of the flyback inverter to the mixed conditions where CCM and DCM operations take place simultaneously during a half grid period.The output current of the flyback inverter at mixed operation under light and deep voltage drop conditions is shown in Figure 5.As shown in Figure 5, the output current during grid period includes numerous harmonics with high amplitude which is very detrimental for the microinverter system.Also, the previous section mentions that the flyback inverter at CCM operation has an RHP in its transfer function, which makes the inverter unstable and distorts the output current.Therefore, the flyback inverter in DCM operation has to be disconnected from the grid under voltage drop condition.Also, according to Figure 5, the flyback inverter at hybrid DCM-BCM operation cannot support LVRT capability under deep voltage drop condition because hybrid switching technique uses DCM operation at the beginning and end of each half line cycle and under deep voltage drop, inverter leads to CCM operation quickly before the operation mode of the inverter converts to BCM operation.
In BCM operation, according to the relation (2) and Figure 4(a), the interval T off is equal to the time interval between the intervals   and  .,   =   −  .(10) In BCM operation, according to the relations (1) and ( 10), since the switching period   continues until the current of the magnetizing inductance drops to zero,   is not constant in all switching periods.Hence, if the turn-off time interval T off surges, the operation mode of the converter does not change and inverter remains under BCM operation and only the inverter switching period   increases which does not have a detrimental effect on the performance of inverter.In addition, the output current does not rise too much, since the primary reference current is limited by the value specified in the MPPT system.Hence, the flyback inverter at BCM condition can stay connected to the grid under voltage drop condition and support LVRT capability.
In voltage rise condition, at both of the DCM and BCM operations, according to the relation (1), because of the fact that the turn-off time interval T off reduces, the mode of the flyback inverter does not change.Therefore, under the case of voltage rise, the operation mode of the flyback inverter remains unchanged.Consequently, this paper only concentrates on the case of voltage drop condition.

Parameters Design
A W microinverter at Vrms/ Hz utility condition is designed in this work.The voltage of the PV panel at the maximum power point is equal to V. The design of the flyback inverter's parameters is the same as conventional design procedure of flyback inverter at BCM condition, which is presented as follows.
. .Selection of the Magnetic Inductance.The primary inductance   can be calculated from (11), where, according to [38],  max is the maximum duty ratio of the main switch which is equal to .and   is the minimum value of the switching frequency which is equal to kHz.
. .Selection of the Input Capacitor.The flyback microinverter needs a gigantic DC input capacitance   to decouple the power pulsation which is caused by single-phase power generation to the utility line [39].The value of   can be calculated by ( 14) [30].Since the input voltage ripple Δu dc is considered equal to V, the minimum required value of the input capacitor is .mF.  . .Output Filter Design.The equivalent circuit of the output filter of the flyback inverter is illustrated in Figure 6.According to Figure 6, the relationship between i out ,   , and   can be obtained as (15).
The expression of the CL filter is This is a second-order low-pass filter with a resonant frequency   , which can be defined as (17).In this design, fr should comply with (18) [30].Finally,   and   are selected as H and nF, respectively.18) . .Design of the Reference Current.The value of   which is related to P out is shown as (19).The input power is W. Therefore,   is .A.
Finally, from ( 8), the reference primary current of the flyback microinverter for the introduced control system is obtained by (20).

Simulation Results
To validate the theoretical analyses and show the effectiveness of the flyback inverter at BCM condition for LVRT capability, the simulation results of a W flyback microinverter under various grid faults are given.The parameters' values which are calculated according to the design procedure mentioned in the section fifth are presented in Table 1.
The schematic of the flyback inverter and its control system at BCM condition is presented in Figure 7.The control system of the flyback inverter at BCM condition is simple since it only needs an open-loop control technique.Inasmuch as the inverter operates at BCM, its switching frequency   is variable.The frequency and phase angle  of the grid are sensed by phase locked loop (PLL).According to the grid voltage, the gating pulses of the secondary switches are created, where S o and S o are on in the positive and negative half grid period, respectively.The operation of the microinverter under normal condition of the grid in accordance with the grid code is guaranteed by islanding protection.The input energy is achieved by sensing the PV voltage and current in order that   can be regulated to obtain the MPPT based on the perturbation and observation (P&O) method [40].The conventional P&O algorithm uses fixed step-length, which is realized by adjusting   as shown in Figure 8.
The output current of the flyback inverter and grid voltage under DCM operation for two different voltage drops (0.6 p.u. and 0.1 p.u.) are depicted in Figure 9.According to Figure 9, the voltage sag starts at t=0.3s and continues for 100 ms.The primary current of the flyback inverter in grid period is presented in Figure 10.Also, the details of the primary and secondary currents during voltage fault are shown in Figure 11.According to Figures 9-11, for voltage drop condition, the mode of the flyback inverter leads to the mixed operation where CCM and DCM operations take place in a half grid period.Therefore, the output current u g u g  THD and amplitude of the output and the primary currents increase significantly, which is very detrimental for the microinverter.In real case, before the current exceeds the critical value, the protection system operates and the inverter is disconnected from the grid.Hence, a flyback converter in DCM operation cannot support the LVRT capability in normal condition.In addition, as shown in Figure 9(b), the output current at the beginning and end of each half line cycle is also very high; hence, the hybrid DCM-BCM operation method which is presented in [36,37] cannot support the LVRT capability in normal condition since this method uses DCM operation at the beginning and end of each half line cycle.
The output current of the flyback inverter and the grid voltage under BCM operation with the previous grid faults are presented in Figure 12.According to Figure 12, the amplitude of the output current is not high during the voltage sag and the inverter can tolerate this current for a short transient time.The primary current of the flyback inverter in grid period is presented in Figure 13.As presented in this figure, the output current does not rise too much since the primary reference current is limited by the value specified in the MPPT system.Also, the details of the primary and secondary currents of the flyback's transformer during voltage fault are shown in Figure 14.It can be observed that the operation of the flyback inverter remains under BCM during voltage fault.From the aforementioned results, it can be concluded that, in voltage sag duration, the flyback inverter under BCM operation can stay connected to the grid and support the LVRT capability.
Figure 15 shows the amplitude and THD of the output current of the flyback inverter at BCM condition for different voltage sags.According to this figure, decrease of the grid voltage will cause the THD and amplitude of the output current to increase.The THD of the output current during deep voltage sag (voltage drops to 0.1 p.u.) is about 6.8%.The harmonic spectrum of the output current is illustrated in Figure 16, which verifies that the harmonic components are in appropriate range even during deep voltage sags.The THD analysis is missing in [19-22, 25, 26, 41] and from their provided results, it seems that the THD is high during voltage sag.
The V-I and P-V curves of the input PV panel with the maximum power of 200W are depicted in Figure 17.Also, the power, voltage, and current of the PV panel during the normal and fault conditions are presented in Figure 18.Because of the fact that the grid voltage is dropped, the power of PV  panel reduces in the duration of the voltage sag.Also, the system returns to its normal operation and tries to achieve the maximum power point of the PV panel as soon as the fault is cleared.Nonetheless, as it can be observed from Figure 18, it may take some time due to MPPT process.
According to the presented simulation results, the flyback microinverter at BCM condition can provide LVRT capability.They confirm that, even under a deep voltage drop, the mode of the flyback inverter remains at BCM condition and the inverter contributes to helping the grid voltage and frequency stay stable during the grid faults.

Conclusion
Because of the fact that the microinverters have many merits compared to the conventional centralized architecture in DG applications, they will be a significant part of the photovoltaic systems at the urban low-voltage networks in the future.Hence, they should provide LVRT capability to enhance the grid reliability and stability.
Among various single-stage and two-stage inverters, the flyback inverter is widely used in microinverter applications because of its unique characteristics.Hence, in this paper, the

Figure 4 :
Figure 4: The primary and secondary currents of the flyback's transformer in switching period.(a) Under BCM operation.(b) Under DCM operation.

Figure 5 :
Figure 5: The output current of the flyback inverter in mixed operation mode.

Figure 6 :
Figure 6: Equivalent diagram of the output filter.

Figure 7 :
Figure 7: Control scheme of the flyback microinverter at BCM condition.

Figure 12 :Figure 13 :Figure 14 :
Figure 12: The output current and voltage of the flyback inverter at BCM condition under voltage drop.(a) Grid Voltage falls to 0.6 p.u.(b) Grid voltage falls to 0.1 p.u.

Figure 15 :Figure 16 :
Figure 15: Amplitude and THD of the output current for different voltage sags.

Figure 17 :
Figure 17: V-I and P-V curves of the PV panel.

Figure 18 :
Figure 18: Simulation results of the MPPT control during LVRT.

Table 1 :
Value of the microinverter's components.