Fast inrush voltage and current restraining method for droop controlled inverter during grid fault clearance in distribution network
2017; Institution of Engineering and Technology; Volume: 12; Issue: 20 Linguagem: Inglês
10.1049/iet-gtd.2017.1209
ISSN1751-8695
AutoresZhikang Shuai, Jun Ge, Wen Huang, Yaojing Feng, Jie Tang,
Tópico(s)Power Systems and Renewable Energy
ResumoIET Generation, Transmission & DistributionVolume 12, Issue 20 p. 4597-4604 ArticleFree Access Fast inrush voltage and current restraining method for droop controlled inverter during grid fault clearance in distribution network Zhikang Shuai, Zhikang Shuai College of Electrical and Information Engineering, Hunan University, Changsha, 410082 People's Republic of ChinaSearch for more papers by this authorJun Ge, Jun Ge College of Electrical and Information Engineering, Hunan University, Changsha, 410082 People's Republic of ChinaSearch for more papers by this authorWen Huang, Wen Huang College of Electrical and Information Engineering, Hunan University, Changsha, 410082 People's Republic of ChinaSearch for more papers by this authorYaojing Feng, Corresponding Author Yaojing Feng fyj_hnu@126.com College of Electrical and Information Engineering, Hunan University, Changsha, 410082 People's Republic of ChinaSearch for more papers by this authorJie Tang, Jie Tang College of Electrical Engineering, Shaoyang University, Shaoyang, 422000 People's Republic of ChinaSearch for more papers by this author Zhikang Shuai, Zhikang Shuai College of Electrical and Information Engineering, Hunan University, Changsha, 410082 People's Republic of ChinaSearch for more papers by this authorJun Ge, Jun Ge College of Electrical and Information Engineering, Hunan University, Changsha, 410082 People's Republic of ChinaSearch for more papers by this authorWen Huang, Wen Huang College of Electrical and Information Engineering, Hunan University, Changsha, 410082 People's Republic of ChinaSearch for more papers by this authorYaojing Feng, Corresponding Author Yaojing Feng fyj_hnu@126.com College of Electrical and Information Engineering, Hunan University, Changsha, 410082 People's Republic of ChinaSearch for more papers by this authorJie Tang, Jie Tang College of Electrical Engineering, Shaoyang University, Shaoyang, 422000 People's Republic of ChinaSearch for more papers by this author First published: 17 January 2018 https://doi.org/10.1049/iet-gtd.2017.1209Citations: 4AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract Control mode switching strategy of droop controlled inverter can effectively avoid overcurrent during grid fault, but it is easy to cause inrush voltage and current during grid fault clearance, which leads to the failure of fault ride through process of the distribution network. In this study, a new control mode re-switching method is proposed on the basis of restricting the outputs of the off-line controllers, which can make sure that the inrush voltage and current are suppressed and return to the normal condition quickly. First, the dynamic characteristics of fault current of droop controlled inverter are analysed. Then, the instantaneous inrush voltage and current caused by the output saturations of voltage controller and power controller before the re-switching process are mainly discussed. This method takes full advantage of integration of the existing off-line controllers to limit the output saturations, which can make the inrush problems well solved, and reduce the influence on grid connected inverters and the distribution network. Simulation results verify the validity of the theoretical analysis. 1 Introduction Nowadays, more and more renewables and flexible loads, such as photovoltaic, wind power, electric vehicles and so on, access to the distribution network with the power electronic devices [1, 2]. Thus, the distribution network has gradually become a weak grid and its operation characteristic has been evidently changed. Due to having complex topological structures with inverter interfaced distributed generators (IIDGs), various faults often occur and spread fast, the stability and security of power electronic based distribution system need to be highly concerned [3–6]. Similar to the role of synchronous generators in the distribution grid, IIDGs are also the main power supplies to the load, which can affect the operation of distribution network to a great extent. Due to the small overcurrent ability of the inverters, it is easy to result in the relay protection action or burning down devices, which can disconnect the inverters from the distributed grid and eventually affect the system stability. As an important characteristic of grid connected inverters, fault ride through capacity determines whether the distribution grid and devices can operate reliably or not [7]. Due to the external characteristic of voltage source, grid connected inverters with droop control or virtual synchronous generator (VSG) control [8–10], will produce larger transient inrush current than current controlled inverters [11, 12]. At present, researches on fault current suppression of voltage controlled inverter during grid fault are mainly focused on reducing the internal potential of the inverter [13, 14] and switching current control mode [15–17]. Compared with the former, current control mode switching strategy can be better applied to the fault ride through process, because it increases the control degrees of freedom and the rapidity of current response. Thus, control mode switching strategy is a hot research topic currently. Some literatures about control mode switching of voltage controlled inverter during grid fault have been discussed. In [15], control mode switching method of V/f controlled inverter is used to avoid inrush fault current and harmonic in the natural reference frame (ABC-frame). In [16], a fault through method of droop controlled inverter based on control mode switching strategy is proposed, but the power sharing during grid fault and its re-switching process are not considered. In [17], a fault through method of VSG based on switching to fast hysteresis current control is proposed, but the idea of current pre-synchronisation during fault clearance will increase the recovery time to the normal condition. According to the overview above, control mode switching is widely used to suppress the fault current contribution of the inverter with different control strategies, but less attention is paid to the inrush voltage and current during the re-switching process. In addition, due to droop controlled inverter has multi-loop cascade control system, the process of control mode switching is more complex than V/f control and VSG control, which leads to its inrush phenomenon being more serious, especially in the re-switching process when distribution grid fault is cleared. In this paper, we propose a novel control mode re-switching method based on restricting the output saturations of the off-line voltage controller and power controller. It focuses on the safe operation of IIDGs during distribution grid fault clearance, which is a supplement to the fault ride though process of grid connected inverters and the distribution grid. This paper is organised as follows. Section 2 analyses the characteristics of the inverter fault current during a grid symmetric fault. Section 3 discusses the forming reasons of inrush voltage and current of the inverter during the re-switching process. Section 4 presents a fast inrush voltage and current restraining method and describes its detailed implementation process. Section 5 shows simulation results to validate the proposed method. Section 6 concludes this paper. 2 Dynamic characteristics of droop controlled inverter during grid fault As the transient analysis and control of grid connected inverters are the basis of fault ride through process, the inrush fault current is necessary to be analysed. Fig. 1a shows the typical topology of stored-energy distributed generator (DG) with droop controlled inverter connected to the distribution grid. In Fig. 1a, Vdc is the output dc voltage of the DG, which can be considered as a constant value; L and C are the inductance and capacitance of the output filter, respectively; Zg ∠θ and Zs ∠φs are the line impedance and grid equivalent impedance, respectively; Zl ∠φl is the feeder impedance between the fault point and point of common coupling (PCC); vo and io are the output voltage and current of the inverter, respectively; vp and vs are the voltage of PCC and the grid, respectively; δ is power angle difference between the output voltage and PCC voltage. Generally, according to the power transmission relationship between points A and B, the output active and reactive power of the inverter can be obtained as follows: (1) (2) Fig. 1Open in figure viewerPowerPoint Droop controlled inverter (a) Topology of the inverter connected to the distribution grid, (b) Diagram of the external characteristic Since the line impedance has both resistance and inductance, the output active and reactive power are often highly coupled. Consequently, a concept of virtual power is reconstructed by introducing the orthogonal transformation matrix T with the line impedance angle θ, and then the output active and reactive power are effectively decoupled [18]. The droop control method which uses the transformation T is shown as follows: Droop equation (3) (4) where ω0 and V0 are the references of the angular frequency and voltage amplitude, respectively; m and n are the droop frequency and amplitude coefficients, respectively; P* and Q* are the references of the active and reactive power, respectively; and are the actual active and reactive power after low pass filter (LPF), respectively; and the cut-off frequency of LPF is ωc. The external characteristic of droop controlled inverter, which is based on the virtual power decoupling, is shown in Fig. 1b, where Pc*, Qc*, Pc and Qc are the corresponding virtual power. The inverter grid connected system is generally high order, non-linear and coupled, and mainly consists of circuit model and control model. Thus, the precise solution of the transient process is often complicated, and needs to be demonstrated by computer simulation or iterative procedure. However, considering that power electronic based power system often has large time scales, the response time can vary from microsecond to minute. Therefore, it can be simplified and analysed at different time scales [19–21]. For the droop controlled inverter, the cascade control of power loop, voltage loop and current loop is often adopted. Each loop has different bandwidth because the different functions and the time scales of power loop, voltage loop and current loop are about 15, 1 and 0.2 ms, respectively. From the voltage loop and current loop of the inverter connected to the grid, the output voltage can be described as follows: VSC equation (5) Grid equation (6) where H (s) and Zo (s) are the voltage transfer function and equivalent output impedance of the inverter, respectively; Rg and Lg are the resistance and inductance of the line, respectively. By combining (5) and (6), the output current can be achieved as follows: (7) Fig. 2 shows the bode and zero-pole diagrams of the equivalent admittances when voltage and current controllers are considered, where Go (s) and Gp (s) are the equivalent admittances corresponding to the reference voltage and PCC voltage, and Gn (s) = 1/(Rg + sLg) is the line admittance. Fig. 2a shows that the gains of Go (s), Gp (s) and Gn (s) near the fundamental frequency are basically the same. Fig. 2b shows that Go (s) and Gp (s) have a dominant zero and a pair of conjugate poles, which leads to their dynamic performance similar to the first-order inertial response of Gn (s). Therefore, when analysing the time scale of inrush fault current, the influence of voltage loop and current loop can be neglected. The dq- axis components of the output current of droop controlled inverter can be calculated by the following ordinary differential equations: (8) (9) Fig. 2Open in figure viewerPowerPoint Response characteristics of Gn (s), Go (s) and Gp (s) (a) Bode diagrams, (b) Zero-pole diagrams Due to the relational expression , the influence of reference phase variations caused by virtual active power can be neglected. In addition, considering the LPF in the power loop, the dq- axis components of the output current can be approximately obtained with the Laplace expression as follows: (10) According to the analysis above, the amplitude of PCC voltage decreases when grid fault occurs, which can result in the output current having two parts: a steady ac component and a decaying dc component. Voltage controlled inverter with different strategies will have different inrush fault current characteristics. A comparison of inrush current amplitude between V/f controlled inverter and droop controlled inverters with different cut-off frequencies is shown in Fig. 3. On the one hand, since the amplitude of reference voltage decreases when output power increases, the inrush current peak value of droop controlled inverter is smaller than V/f controlled inverter. On the other hand, there is control delay of LPF in the power loop, so that the higher the cut-off frequency, the smaller the inrush current peak value will be. Fig. 3Open in figure viewerPowerPoint Comparison of inrush current amplitude between V/f controlled inverter and droop controlled inverter (p.u.) (a) Inrush currents during grid fault, (b) Zoom in the peak of inrush currents In summary, because the transient inrush current of droop controlled inverter is too large, the safe operation of the inverter will be seriously affected. However, control mode switching can suppress the inrush current directly by converting the external characteristic of the voltage source into the current source, which makes it a promising and effective method. 3 Voltage and current inrush during control mode switching As analysed above, the control system of droop controlled inverter consists of power loop, voltage loop and current loop, and the specific implementation method of traditional control mode switching is shown in Fig. 4, where ILdr, ILqr, Idset, Iqset, ILd* and ILq* are the dq- axis components of the output reference current of voltage loop, the given reference current and the input reference current of current loop, respectively; Vod, Voq, Iod and Ioq are the dq- axis components of the output voltage and current, respectively. Fig. 4Open in figure viewerPowerPoint Traditional switching current control mode of droop controlled inverter during grid fault Control mode switching strategy will cause inrush voltage and current while improving the flexibility of its control system. When grid fault occurs, the inverter will switch droop control mode into current control mode. At this time, the power loop and voltage loop are disconnected, and new given reference values Idqset will be added to the current loop (S1, 2 are switched from logic '0' to '1'). However, during current control mode, because both the power loop and voltage loop are in the off-line states, the input errors of the power controller and voltage controller will make their outputs continue to accumulate, and this phenomenon will eventually affect the re-switching process back to the droop control mode (S1,2 are switched from logic '1' to '0') when grid fault is cleared. 3.1 Influence of off-line voltage loop on the re-switching process Voltage loop of droop controlled inverter usually adopts proportional–integral (PI) controller in the synchronous reference frame (DQ-frame) to reduce the steady-state errors of control variables. As shown in Fig. 4, when grid fault occurs, the inverter is operated in current control mode, the switches S1,2 are activated immediately and the voltage loop is out of control. The dq- axis components of output reference current of off-line voltage loop can be expressed as follows: (11) where Kvp and Kvi are proportional and integral coefficients of voltage controller, respectively. The fault characteristics are analysed below in three cases: (i) without current limit, (ii) using the current limiter, (iii) using control mode switching strategy. Fig. 5 shows the amplitudes of output voltage Vom, output current ILm and output reference current of voltage loop ILr during a serious fault process. When grid fault occurs, for the absence of current limit, Vom decreases and then return to normal value quickly with the regulation of voltage controller, ILm increases to the steady-state value but exceeds the saturation value Isat, so an appropriate fault current limiting strategy is needed. For traditional current limiter, when ILr is larger enough, it will be limited to Isat and the regulating ability of voltage loop has reached the limit position. So the input of the voltage controller has a steady-stage error and causes its output to accumulate until it is saturated. Similarly, for control mode switching strategy, because ILm is controlled to a constant value, such as Isat, and the output voltage Vo2 will be smaller than the reference voltage Vo* (1 p.u.), which results in a gradual increase in ILr. Thus, its final current value ILr2 will be large enough and can be calculated by (12), where Tfault is the fault duration. It is worth noting that, control mode switching strategy has active control flexibility to limit current, which makes the voltage controller easy to trigger positive or reverse saturation (12) (13) Fig. 5Open in figure viewerPowerPoint Magnitudes of related voltage and current during the whole fault process (a) Overcurrent without the current limiting strategy, (b) Inrush voltage by using the current limiter, (c) Inrush voltage and current by using control mode switching When grid fault is cleared, compared with the method of using the current limiter, droop controlled inverter with control mode switching strategy has different voltage and current response characteristics because of the output saturation of the voltage controller. On the one hand, the output current of droop controlled inverter is easy to overcurrent (ILm3) due to the lack of current limitation. On the other hand, from (13), in order to reduce the output current from the value ILr2 to the normal value ILr4 (approximately equal to ILr1), Vo3 must be greater than the Vo*, and it will last a period of time Trecovery to return to the normal condition. Meanwhile, the output voltage harmonic (Vo3) is induced due to the limitation of modulation voltage. Thus, this is the reason for inrush output voltage and long recovery time of droop controlled inverter during grid fault clearance. The recovery time of output voltage can be approximately obtained as follows: (14) According to the analysis above, we can conclude that the magnitude of inrush output voltage and its recovery time are determined by the severity degree and duration of grid fault. In addition, the addition of output current feedforward can improve the dynamic performance of the inverter, but its influence is limited. 3.2 Influence of off-line power loop on the re-switching process Compared with the certain references of voltage and phase in V/f controlled inverter, droop controlled inverter with power loop can regulate the amplitude and phase of the output voltage. As shown in Fig. 4, when grid fault occurs, the inverter is in current control mode, power loop is also out of control. The output reference phase of off-line power loop can be expressed as follows: (15) In general, the priority principle is to support the fault voltage based on reactive power injection strategies during grid fault. For the off-line power loop, the increase of reactive power will reduce the magnitude of the reference voltage, and the decrease of active power will produce a positive frequency deviation. The above situations will lead to the output phase that is more advanced than the phase of PCC voltage by integrating the integrator during grid fault. Therefore, when grid fault is cleared and then the re-switch is activated, the output power will increase and lead to the inrush current. More seriously, the reference phase will be out of synchronisation and cannot return to the normal state. 4 Fast and little impact control mode switching strategy Compared with the grid fault process, the grid fault clearance process of grid connected inverters is equally important [22–24], but it is often overlooked. As an effective method in the grid fault process, control mode switching is widely adopted to avoid overcurrent, but it will cause the inrush problems during the re-switching process after grid fault is cleared, and also increase the re-synchronisation time of grid connected inverters. As shown in (12), if its relationship is broken and the final current value ILr2 is limited in a low level, such as the saturation value Isat, then the inrush voltage Vo3 will not appear from (13), and the inrush current ILm3 will also not appear because of the control of the inner current loop. The power loop can also be considered similarly. Based on this idea, a little impact control strategy based on control mode switching is proposed, which can suppress inrush voltage and current during the re-switching process. 4.1 Control principle of the proposed strategy The control block diagram of the proposed strategy is shown in Fig. 6a, where kc is the closed-loop gain of dq- axis output reference currents of voltage loop, and kp is the closed-loop gain of output reference phase of power loop. Compared with the traditional control mode switching strategy in Fig. 4, the proposed strategy adds two kinds of units to keep the control system operate better. On the one hand, the reference phase switching unit (S3) is added to ensure the certain output power relation during the fault process, and PCC voltage phase φp is detected by second-order generalised integrator phase locked loop. On the other hand, the closed-loop tracking units of the output current of off-line voltage loop (S4,5), and the output phase of off-line power loop (S6) are also added. Fig. 6Open in figure viewerPowerPoint Proposed control mode switching strategy (a) Overall control block diagram, (b) Droop control mode before control mode switching and after control mode re-switching, (c) Current control mode during control mode switching The control principle is as follows. In normal operation, the inverter operates in the droop control mode (S1– 6 are at the position of logical '0'), and its control block diagram is shown in Fig. 6b. When grid fault occurs, the fast fault detection unit will be activated, then the inverter will switch from droop control mode to current control mode (S1– 6 are switched from logic '0' to '1'), and then its control block diagram is shown in Fig. 6c. When grid fault is cleared, the fast fault detection unit will be reset, then the inverter will switch from current control mode to droop control mode (S1– 6 are switched from logic '1' to '0'), and then its control block diagram is also shown in Fig. 6b. From the analysis of previous section, based on the idea of limiting output saturations of off-line controllers, the output reference phase φd of power controller is regulated by tracking PCC voltage phase φp with its own integrating unit, and the output reference currents ILdqr are regulated by tracking the given reference currents Idqset with their own PI control units, as shown in Fig. 6a. This proposed method takes full advantage of the closed-loop regulation of these two existing controllers without adding additional complex algorithms, and their performance mainly depends on two aspects: (i) the rapidity of control mode switching; (ii) the tracking ability of the closed-loop controllers. For the former, in order to detect the grid fault occurrence and clearance as quickly as possible, the faster hardware circuit unit is adopted instead of the software algorithms [17]. For the latter, the key factor that will affect the performance is the controller parameters, which mainly refer to close-loop gains kp and kc, and their design methods are described in the next section. 4.2 Parameters design of the proposed strategy In order to achieve rapid control effects of the phase and current tracking units, the close-loop gains can be easily designed according to the first-order control relationship from Fig. 6c. The implementation will be divided into two steps: (i) design of phase tracking unit; (ii) design of current tracking unit. Step 1: When in the closed-loop tracking mode, the input of off-line power controller is composed of the difference between φp and φd by the multiplication factor kp. Since φp is a ramp function with time, then there are φp (t) = 100πt, ω0 = 100π and φd (0−) = A, respectively. The relationship of phase tracking unit and the time-domain solution of φd can be described as follows: (16) (17) From (17), it can be seen that the final steady-state value of φd is controlled to φp, and its time constant T1 is 1/mkp. Considering the grid fault duration is short (such as Tfault is 500 ms), and the steady-state error of φd is <2%, we can get a relationship that the setting time 4T should be less than Tfault, and then the value of kp can be calculated as follows: (18) Step 2: When in the closed-loop tracking mode, the inputs of dq- axis off-line voltage controllers are composed of the current difference between Idqset and ILdqr by the multiplication factor kc. The relationship of current tracking units can be described as follows: (19) Unlike (16), since it has feedforward components Iodq, from (19), ILdqr can be controlled by both the current tracking unit and the inner current loop. Considering the general situation and Iodq is not added, the time constant T2 is (1 + kc kvp)/kc kvi, and the value of kc can be calculated through the above analysis, as shown below: (20) 4.3 Optimal power control during the grid fault In addition, in order to support the voltage during the fault process, a power sharing strategy based on supporting the local voltage is adopted. Vo ∠φo, Io ∠φio and Vp ∠φp are the phasors of the inverter output voltage, output current and PCC voltage, respectively. Considering the different line impedance in different voltage levels, the vector expression of local output voltage Vo ∠φo is shown as follows: (21) It can be seen that the magnitude of output voltage Vom is related to the magnitude and phase of the output current. When the amplitude of output current Iom is constant, there will be a maximum value of output voltage at the phase of output current φio, which is shown in (22). Thus, the maximum value of output voltage can be obtained as (Vpm + Iom Zg). In addition, the line impedance and PCC voltage can be approximately measured and calculated in the distribution system. Considering output current constraint of the inverter, the maximum support of output voltage is achieved and the amplitude of reference voltage is set as the amplitude of rated voltage Vnm. So the amplitude of output current Igm can be shown as follows: (22) (23) where Isat is two times of the amplitude of rated current Inm. The relationship of active and reactive output power of the inverter can be shown as follows: (24) 5 Simulation results In order to verify the effectiveness of the proposed control mode switching strategy in the fault ride through process, two grid connected models of droop controlled inverters with different capacities have been established on the PSCAD/EMTDC simulation, as shown in Fig. 7. The inverters connect to grid at PCC with different lines zg1 and zg2. The grid is composed of an ideal voltage source and its equivalent impedance zs, which supplies power to the load through the feeder zl. The parameters of droop controlled inverters and the grid system are shown in Tables 1 and 2. Fig. 7Open in figure viewerPowerPoint Distribution system with two parallel DGs Table 1. Parameters of droop controlled inverters Parameters of DG1 Values Parameters of DG2 Values P1 * 50 kW P2 * 14.14 kW Q1 * 0 kvar Q2 * 14.14 kvar m1 2π × 10−5 m2 5π × 10−5 n1 4 × 10−4 n2 1 × 10−3 ω01 100π rad/s ω02 100π rad/s V01 325 V V02 355 V kvp1, kvi1 1, 100 kvp2, kvi2 1, 100 kip1, kii1 30, 100 kip2, kii2 30, 100 L1 5 mH L2 4 mH C1 10 µF C2 10 µF zg1 0.2 Ω + 2 mH zg2 0.4 Ω + 4 mH Inm1 102 A Inm2 37.5 A Isat1 204 A Isat2 75 A kp1, kc1 5 × 105, 0.5 kp2, kc2 5 × 105, 0.5 Table 2. Parameters of the distribution system Parameters Values Parameters Values Vs(L-L) 400 V fs 50 Hz zs 5 mΩ + 0.07 mH zl 0.02 Ω + 0.06 mH In this case, three-phase symmetrical fault occurs in the time of 0.6 s, then the distribution grid returns to the normal condition after circuit breaker works to clear the fault in the time of 1.1 s. According to the basic circuit principle, the normal or fault conditions of grid side can be equivalent to a PCC voltage with variable amplitude and phase. Thus, compared with PCC rated voltage in the normal operation, the amplitude will drop to 60%UN and the phase will lag 10° during grid fault. Fig. 8 shows the fault through process of droop controlled inverter with DG1 by using conventional control mode switching. From Fig. 8, it can be seen that the output currents are limited to the maximum current Isat1 and the output voltages drop during grid fault. Meanwhile, both the output currents and the output voltages increase during grid fault clearance, which cause the inrush phenomenon and harmonic problem, and the output active and reactive power are large enough to recover slowly. In addition, DG2 also has the inrush phenomenon, which is similar to that of DG1, so the waveforms are not listed due to the space limit. Fig. 8Open in figure viewerPowerPoint Fault process of DG1 by using conventional control mode switching (a) Three-phase output voltages, (b) Three-phase output currents, (c) Output active and reactive power Figs. 9 and 10 show the fault through process of droop controlled inverters with DG1 and DG2 by using proposed little impact control mode switching. From Fig. 9, it can be seen that the output voltages and currents are well controlled in the whole fault process. In detail, on the one hand, the output currents are limited to the maximum current Isat1 and the output voltages are supported close to the rated value Vo1 during grid fault. On the other hand, the output currents drop rapidly to the rated value Inm1 and the output voltages are still Vo1 during grid fault clearance. Thus, the fault recovery process of the voltage and current is very fast and smooth. From Fig. 10, it can be seen that the proposed method is equally effective for the fault process of DG2. It is worth noting that, as the rated active and reactive power of DG2 are different from that of DG1, DG2 has limited capacity to support its output voltage during grid fault, but this has reached its limit. Fig. 9Open in figure viewerPowerPoint Fault process of DG1 by using proposed method (a) Three-phase output voltages, (b) Three-phase output currents, (c) Output active and reactive power Fig. 10Open in figure viewerPowerPoint Fault process of DG2 by using proposed method (a) Three-phase output voltages, (b) Three-phase output currents, (c) Output active and reactive power The dynamic waveforms of DG1 and DG2 by using proposed method during grid fault clearance are shown in Fig. 11. It can be seen that the recovery time of DG1 and DG2 are 0.05 and 0.04 s, respectively, which are less than the recovery time with the current limiter (about 0.3 s). So it can verify the correctness of the above theoretical analysis. Fig. 11Open in figure viewerPowerPoint Dynamic waveforms of DG1 and DG2 in the re-switching process by using proposed method (a) Three-phase output voltages of DG1, (b) Three-phase output currents of DG1, (c) Three-phase output currents of DG2, (d) Three-phase output currents of DG2 6 Conclusion This paper has analysed the transient operation characteristics of droop controlled inverter during grid fault, and discussed the phenomena of inrush voltage and current during distribution grid fault clearance, then a little impact fault through method based on the control mode switching is proposed. This method consists of control mode switching unit and two closed-loop tracking units. The main contributions are as follows: (i) The dynamic characteristics of fault current of droop controlled inverter consist of two parts: a steady ac component and a decaying dc component, which are mainly determined by the grid equation. The peak values of inrush current are from large to small: ideal voltage source, V/f controlled inverter and droop controlled inverter. At the same time, the larger the cut-off frequency of LPF, the smaller the peak current value of droop controlled inverter will be. (ii) Output saturations of power loop and voltage loop are the forming reason of inrush voltage and current of droop controlled inverter during the re-switching process. Then, the duration and severity of grid fault will affect the magnitude and recovery time of inrush voltage and current during the re-switching process. (iii) The proposed control method can effectively suppress the inrush voltage and current during the re-switching process, and return to the normal state quickly and smoothly. Then, the maximum support of the output voltage is achieved under the maximum current constraint. 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