Transformer‐less dynamic voltage restorer based on buck‐boost converter
2017; Institution of Engineering and Technology; Volume: 10; Issue: 13 Linguagem: Inglês
10.1049/iet-pel.2016.0441
ISSN1755-4543
AutoresDariush Nazarpour, Mohammadreza Farzinnia, Hafez Nouhi,
Tópico(s)Magnetic Properties and Applications
ResumoIET Power ElectronicsVolume 10, Issue 13 p. 1767-1777 Research ArticleFree Access Transformer-less dynamic voltage restorer based on buck-boost converter Dariush Nazarpour, Corresponding Author Dariush Nazarpour d.nazarpour@urmia.ac.ir Faculty of Engineering, Urmia University, Urmia, IranSearch for more papers by this authorMohammadreza Farzinnia, Mohammadreza Farzinnia orcid.org/0000-0003-1330-1358 Faculty of Engineering, Urmia University, Urmia, IranSearch for more papers by this authorHafez Nouhi, Hafez Nouhi Faculty of Engineering, Urmia University, Urmia, IranSearch for more papers by this author Dariush Nazarpour, Corresponding Author Dariush Nazarpour d.nazarpour@urmia.ac.ir Faculty of Engineering, Urmia University, Urmia, IranSearch for more papers by this authorMohammadreza Farzinnia, Mohammadreza Farzinnia orcid.org/0000-0003-1330-1358 Faculty of Engineering, Urmia University, Urmia, IranSearch for more papers by this authorHafez Nouhi, Hafez Nouhi Faculty of Engineering, Urmia University, Urmia, IranSearch for more papers by this author First published: 09 August 2017 https://doi.org/10.1049/iet-pel.2016.0441Citations: 9AboutSectionsPDF 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 In this study, a new topology for dynamic voltage restorer (DVR) has been proposed. The topology is inspired by the buck-boost ac/ac converter to produce the required compensation voltage. This topology is able to compensate different voltage disturbances such as sag, swell and flicker without leap of the phase angle. The mass of the proposed topology has been reduced due to lack of injection topology. In addition to, the required compensation energy is directly delivered from the grid through the grid voltage. Therefore, the massive dc-link capacitors are not required to implement. To verify the qualification of the topology, the simulation results by MATLAB/SIMULINK software have been presented. Moreover, an experimental prototype of the case study has been designed and tested. 1 Introduction Nowadays, widespread use of voltage-sensitive equipments cause ‘voltage quality (VQ)’ has been emerged as an important issue in the power quality problem [1]. There are several irrepressible factors which affect the VQ such as, starting the heavy loads, switching the capacitors, grid faults and non-linear loads [1, 2]. Therefore, an efficient approach has been needed to deal with the voltage disturbance. Dynamic voltage restorer (DVR) is a series connected device which protects the sensitive loads by injecting the required voltage when disturbance is occurred [3, 4]. The conventional types of DVRs consist of energy storage element (ESE), inverter and injection transformer. In this type of DVR, the required compensation voltage is generated by the inverter and injected through injection transformer. The ESE includes massive dc-link capacitors, fly-wheels and battery, is responsible to provide the needed energy and voltage for inverter while restoring [5, 6]. In addition of the excessive weight of ESE, the limitation of stored energy by ESE causes the traditional topologies of DVR are unable to compensate severe disturbances for long time durations [6, 7]. To overcome the problem, the improved structures based on ac/ac converters have been presented [8-12]. In this kind of topologies, the DVR is directly connected to the grid and the ESE can be eliminated from the structure. Lozano-Garcia and Ramirez [9] proposed the control method for voltage compensator based on direct matrix converter without storage element. In [10], the proposed single-phase sag compensator based on direct ac/ac converter is supplied with two other phases to generate the required voltage. In [5], the presented topology based on direct three-phase converter is able to compensate both kind of symmetrical and asymmetrical disturbances. The compensation capability of the second type of DVR is directly dependent on the voltage amplitude of the grid. Therefore, the second type of DVRs are unable to compensate the voltage profile under severe sag condition unless, by increasing the turn ratio of the injection transformer. The third type of DVR includes the topologies which are inspired by the operation of the conventional dc–dc converters with the difference that the both of input and output voltages are alternative [13]. Owing to the adjustable voltage gain, the compensation capability of the third type topology is significantly improved in comparison with the second type. In this paper a new topology for DVR based on buck-boost ac/ac converter has been proposed. The proposed topology is able to compensate any voltage disturbances such as sag, swell and flicker. Unlike the presented topology, the injection transformer is eliminated in the proposed topology. For this reason the volume and the weight of the proposed topology is impressively reduce. In the first section, the operation principle of the structure is expressed. Then, the implemented control method for the topology is described. Finally, the simulation and experimental results are presented to verify the effective operation of the proposed topology. 2 Proposed topology The proposed topology for DVR is illustrated in Fig. 1. According to the schema, the topology is formed by a buck-boost ac/ac converter to generate the required compensation voltage in both directions. The structure contains five bidirectional switches, an inductor and a capacitor to regulate the output voltage. To provide bidirectional power transmission, the applied switches are used in bidirectional arrangement. The output capacitor C is placed in series with the grid line and its voltage is marked by vinj. The line and the load impedances are indicated by Zg and Zl, respectively. vg and vl denote the grid and the load voltages to the neutral point, respectively. The inductor of the buck-bust converter is showed by L and the voltage of the inductor is indicated by vind. Lack of the injection transformer is a prominent feature of the proposed topology. Transformers cause the increment of the weight and volume of the DVR. Moreover, the operation frequency is limited to the transformer design parameters. In the other word, the DVRs with the injection transformers are not capable to operate in low-frequency conditions such as dc voltages. Moreover, transformers increase the power losses due to eddy current and hysteresis in the magnetic core. Test and maintenance of the transformers spends more time and costs in comparison with capacitors and inductors. The other important feature of this topology is lack of dc-link capacitors. In the conventional DVR structures, the massive dc-link capacitors are used to store the required energy. For this reason, the traditional topologies are not able to compensate the severe disturbances especially for long time durations. In addition to, this kind of capacitors are heavy and also expensive. Based on the mentioned properties, the proposed DVR can operate in different operation frequency such as dc voltage. Moreover, the long-time disturbances can be compensated by the proposed topology because the required energy is directly provided from the grid. As mentioned previously, DVR restores the voltage profile by injecting the required voltage. According to the schema, the load voltage is the sum of the compensation (or injection) and the grid voltage: (1) Fig. 1Open in figure viewerPowerPoint Proposed topology The DVR is responsible to generate the required compensation voltage according to the reference signal of compensation which is obtained as follows: (2)where, denotes the reference signal of the load voltage under normal condition. The prerequisite of the proper operation of a converter is an efficient switching method. The switching configurations in each mode are presented in Table 1. In addition the current path for each switching mode is presented in Fig. 2. Owing to the alternative power flow between the grid and the inductor, a bidirectional circuit should be created. In the first mode, the inductor is connected to the phase and the neutral point by mean of the S1 and S3 switches. Same as first mode, the inductor is connected to the grid in the second mode but, in opposite direction. According to the switching modes, in modes 1 and 2 the inductor voltage is equal to vg and −vg, respectively, and the DVR transfer the energy from or to the grid according the situation. In the third mode, the inductor is placed in parallel with the injection capacitor and concedes (or receives) the required power to balance the output voltage. In addition of the mentioned triple modes, there are two additional modes which cause zero voltage on the inductor. Table 1. Possible switching modes Mode On switches vind Application Sag Swell Flicker 1 S1 and S3 vg ✓ ✓ 2 S2 and S4 −vg ✓ ✓ 3 S1 and S5 −vinj ✓ ✓ ✓ 4 S1 and S2 0 ✓ ✓ ✓ 5 S3 and S4 0 Fig. 2Open in figure viewerPowerPoint Current pathin the switching modes In the proposed topology, the switching process has been constituted by three time durations. The considered switching period along with the duty cycles and switching modes is shown in Fig. 3. As depicted the fives possible switching modes (M1–M5) and three duty cycles (d1–d3) are considered in the proposed control method and the switching modes are selected according to the voltage disturbance type. Fig. 3Open in figure viewerPowerPoint Switching period To generate the required injection voltage, the proper amounts of triple duty cycles should be calculated and applied in the switching period. Under the steady condition, the average of inductor voltage (vL) is: (3)Moreover, the average current of the capacitor is equal to zero: (4)As shown in Fig. 3, the duty cycles cover all the switching period which, can be expressed by following equation: (5)Considering (3)–(5), the voltage gain of the topology by importing into Laplace domain is obtained as follows: (6)In the above equation, ZL denotes the load impedance. By considering some design conditions (which is discussed in the next section), voltage gain is obtained as follows: (7)According to the above equation, amount of the voltage gain is a function of d1 and d2. The diagram of the voltage gain has been illustrated in Fig. 4. As evident, the gain can be a positive or negative value by changing the amounts of d1 and d2. Under the condition that the amount of d1 is greater than d2, the voltage gain is positive. Otherwise, the gain will be negative by choosing d2 greater than d1. As depicted in Fig. 4b a specific amount of voltage gain can be obtained by the set of different duty cycles which satisfy the following equality: (8)Therefore, the answers of the above equation are placed on a straight line. Fig. 4Open in figure viewerPowerPoint Diagram of the voltage gain To obtain the proper amounts of duty cycles two methods are proposed which are explained in two following subsections: 2.1 Reduced switching method (basic method) As mentioned, Gv varies between positive and negative values depending on the amounts of d1 and d2. To simplify the switching and the reduction power losses, one of the considered duty cycles can be eliminated according to sign of the Gv. When the voltage gain is positive, the amount of d2 is set to zero and d1 is calculated as follows: (9)In a similar way, when the voltage gain is negative d1 is set to zero and d2 is obtained as follows: (10) 2.2 Dynamic switching method This method is planned for dynamic control of proposed topology which, is based on the dynamic model of the device. As shown in Fig. 1, the topology contains two passive elements C and L, obviously, the duty cycles d1 and d2 are the input parameters to control the system. Moreover, the main aim of the DVR is efficient voltage compensation then, the injection voltage vinj is the output, exclusively. Therefore, the system has two state variables, two input and single output. By using (1)–(5), the average state space equation is obtained: (11)It should be noted in the above equation, iL and vinj are the controllable variables, but the variables vg and ig cannot be controlled directly. Because of the non-linear equality between the variables in (9), it is hard to obtain a straight forward equality between the inputs and outputs. The state variables (x) along with the control variables (u) contain dc and ac components. The dc part (X, U) denotes the operation point of the DVR in accordance with the grid condition. However, the ac part (, ) expresses the perturbation around the operation point. Based on the mentioned points the variables are written as follows: (12)By substituting (10) into (9), the small signal model of the proposed system is obtained. It should be noted the obtained equality is still non-linear, but it can be converted to linear form by neglecting the second order terms of the perturbation variables (). Because the amount of dc component is much greater than the ac, the mentioned estimation does not cause a significant error. Therefore the normalised form of the state space equation of the small signal is obtained as follows: (13) (14)where A, B, C and D denote the matrices of the state, input, output and feed-forward, respectively. As expressed in (14), the output is completely controlled by the state variables (D = 0). The main aim of the system is regulating the output by manipulating the duty cycles ( and ). The ac component should cover the error between the measured voltage and the reference signal. For this reason, an additional state variable is considered as follows: (15)Finally the reformed matrices are obtained as follows: (16)where the state variable matrix is considered as follows: (17)According to the desired situations, an efficient control method is required to achieve the mentioned goals in addition of the system stability. In this paper, the pole-placement approach (PPA) is employed to dynamically control the proposed topology. By implementing the PPA, the close-loop poles of the system can be chosen significantly far from the jω axis. For this purpose, the controllability of the system should be examined. The controllability matrix of the system with n loop is obtained as follows: (18)If the rank of the M be smaller than n, it means the system is uncontrollable. The rank of the M for the case study is equal to three while the vg voltage has non-zero value. Therefore it ensures that the system is controllable. In accordance with the PPA, the control signals are instantaneously determined by the state variables: (19)By substituting (19) into the state equations, the following equation is obtained: (20)Finally, by choosing the proper K matrix the eigenvalues of the system are placed at expected locations. The diagram of the proposed control method is shown in Fig. 5. In the first stage, the reference signal of the grid voltage is generated. Then, the difference of the reference signal and grid voltage is calculated to obtain the voltage disturbance and generating the reference signal of the injection voltage. If the instance amount of the required injected voltage is more than the threshold, the DVR start to compensate. In the next step, duty cycles can be obtained by (9) and (10). Finally, by using the calculated eigenvalues, the feedback of the injected voltage insures the stability of the proposed topology and control method to compensate different kind of voltage disturbances. After the calculating the duty cycles, the controller controls the switches by pulse width modulation method. Fig. 5Open in figure viewerPowerPoint Diagram of the control system 3 Design consideration In this section, the amounts of the required capacitance and inductance for proper operation of the DVR are determined. As noted, the ripple of the output voltage should be decreased as much as possible to increase the compensation quality. On the other hand, effect of inductor current ripple is revealed in the input current. Moreover, the current ripple can cause damage on the power electronic switches when the inductor current exceeds the nominal values. Because most of the disturbances are occur in short time intervals, hence the adverse effect on input current while compensation can be ignored. The amount of voltage ripple can be written as follows: (21)where f denotes the switching frequency. As mentioned in Section 2, to convert (6) into an efficient equality between the voltage gain and the duty cycles, the following inequalities should be satisfied: (22)where denotes the angular frequency of the grid. According to (19) and (20) by setting the maximum amount of d1 + d2, the values of the required inductance, capacitance and switching frequency will be obtained. The implemented switches should be capable to carry the maximum current flow: (23)In the above equation, ΔIL denotes the current ripple of the inductance which is obtained as follows: (24)In the above equation, the second order term of duty cycle can be neglected therefore the current ripple is simplified as follows: (25) 4 Experimental and simulation results In this section, to examine the compensation capability of the control method and the proposed topology, an experimental prototype of this device according to Fig. 1 is implemented. To create voltage turbulence, a transformer with three taps has been used. As shown in Fig. 6, each tap is controlled by power electronics switches. The employed switches are in bidirectional arrangement to provide feasibility of two-way power flow. The switching modes of the test transformer with the generated output voltage are presented in Table 2. For instance, when Si,3 is turned on and the other switches are turned off the voltage amplitude of the secondary is 0.6 time of the grid voltages amplitude. Table 2. Generated test voltage for different switching states Switching states vs, p.u. Description Si,1 Si,2 Si,3 On Off Off 1.4 40% voltage swell Off On Off 1 Normal condition Off Off On 0.6 40% sag voltage Fig. 6Open in figure viewerPowerPoint Voltage sag/swell generation transformer The applied system parameters in the experimental and simulation cases are presented in Table 3. Because the nature of the used converter is similar to the current source, the single capacitor can be used as the output filter instead of LC low-pass filter. As mentioned initially, the injection transformer is not required in the proposed topology. The prototype of the proposed topology is shown in Fig. 7. The used MOSFET switches in the experimental case are IRFP23N50L with voltage and current ratings equal to 500 V and 23 A, respectively. The injection capacitor and the inductor of the converter have been set as 20 µF and 7 µH, respectively. The switching frequency in both of the simulation and experimental cases is equal to 100 kHz. In this case study, the maximum allowed voltage ripple has been considered equal to 1 V. To calculate the capacitance of output capacitor, the maximum amount of (d1 + d2) is assumed equal to 0.8 and the amount of Ig is considered equal to the amplitude of the load current. According to (21), by considering the capacitance of Cf equal to 20 µF the desired voltage stability is obtained (Δvc ≤ 0.543). Table 3. System parameters Parameters Symbol Values grid amplitude, V vg frequency, Hz f 50 load resistance, Ω RL 50 inductance, mH LL 60 frequency, Hz fL 50 filter capacitance, µF C 20 switching frequency, KHz fs 100 Fig. 7Open in figure viewerPowerPoint Experimental prototype In the first case of experimental study, the capability of sag compensation by the proposed topology is evaluated. The results of the first experimental study are depicted in Fig. 8. As shown in Fig. 8a, 40% voltage sag for the time interval of nine cycles has been applied for grid voltage (vg). The required compensation voltage (vinj) which has been generated by the DVR is illustrated in Fig. 8b. The compensation process started at the same time with the disturbance detection by the controller. Fig. 8c shows the compensated load voltage (vl) which is sum of the grid voltage and injected voltage. To prevent the probable damage, the measuring has been accomplished by the voltage divider. For this reason, the presented experimental waveforms are ten times smaller than the real amounts. According to these results, the proposed DVR is capable to prevent the spreading of voltage sag to the load voltage. Moreover the simulation result for sag compensation is shown in Fig. 9. Same as the experimental results, the simulation result confirms the effective operation of the proposed topology. Moreover, the waveform of vind under sag condition is shown in Fig. 10. Fig. 8Open in figure viewerPowerPoint Experimental results for sag compensation (a) Grid voltage, (b) injected voltage, (c) load voltage Fig. 9Open in figure viewerPowerPoint Simulation results for sag compensation Fig. 10Open in figure viewerPowerPoint Waveform of vind The second experimental study was accomplished to evaluate the compensation capability of DVR under swell condition. In this case, 40% voltage swell for the period of eight cycles has been applied. The results of the second experimental study are presented in Fig. 11. According to Fig. 11b the injected voltage has 180° phase difference with the grid voltage (Fig. 11a). According to the results, the compensation capability of the proposed DVR under the swell condition is confirmed. In addition of the experimental results, the simulation result for swell turbulence is shown in Fig. 12. Fig. 11Open in figure viewerPowerPoint Experimental results for swell compensation (a) Grid voltage, (b) Injected voltage, (c) Load voltage Fig. 12Open in figure viewerPowerPoint Simulation results for swell compensation In the next case, the compensation capability of the proposed topology under harmonic distortion has been evaluated by experimental results and MATLAB/SIMULINK software. According to Fig. 13, the grid voltage has disturbed for seven cycles. the main harmonic of the grid voltage has been dropped to 40% along with the fifth harmonic of 0.1 p.u. Simultaneous with the onset of turbulence, DVR started to inject the required voltage to compensate. In addition, in the simulation case the grid voltage is under harmonic disturbance as shown in Fig. 14. According to the figure, the grid voltage initially is under normal condition until 0.1 s. The load voltage has been efficiently corrected and the sensitive load has been protected from disturbance. Fig. 13Open in figure viewerPowerPoint Experimental results for harmonic compensation (a) Grid voltage, (b) Injected voltage, (c) Load voltage Fig. 14Open in figure viewerPowerPoint Simulation results Generally, the losses of the converter can be considered sum of the switching and induction losses of power switches. According to the obtained formula in [5], the switching losses can be obtained as follows: (26)where, Econd,loss indicate the conduction losses of the switches. Eon,loss and Eoff,loss explain the losses switch while turning on and off, respectively. Each of the mentioned losses is expressed for this case study, separately. The conduction losses include the wasted power due to the voltage drops on the MOSFET. The voltage drop in a bidirectional switch is caused by a transistor and a diode. Therefore, the power loss of conduction can be written as follows: (27)where the amounts of Vd, RDS and Rd are the constant in a fixed temperature and can be obtained from the datasheet of the switch. Moreover, ton express the time interval of switch conductance. Fig. 15 shows the waveform of the voltages of transistor while switching. As shown in the figure the switching process will not happen instantly and it takes a time duration which are indicated with tr and tf. Therefore, the energy losses during the switching can be expressed as follows: (28) (29) Fig. 15Open in figure viewerPowerPoint Voltage waveforms of the MOSFET during the switching Finally by considering (26)–(29), amount of total losses is obtained as follows: (30)According to the above equation, the total loss per each switch is the function of current, blocked voltage and the temperature. However, as clear in the equation, the current effect on the total losses is more than VDS. In the most of the transformer-based DVRs, the used injection transformer is step-up and it causes the increment of the current in converter side. In addition the transformer has its own losses which are known as eddy and hysteresis losses. There are two on switches and two switching states while operating in sag condition: (31)Same as previous there are two conducting switches when the proposed topology operates under swell condition but, the number of switching equal to four. Hence, the losses under swell condition can be written as follow: (32)Using the above equation and considering the resistant losses in the topology, amount of average efficiency obtained by MATLAB is equal to 87.53%. 5 Conclusions In this paper a new topology for DVR using buck-boost ac/ac converter was proposed. This topology contains five bidirectional switches, an inductor and a capacitor. Unlike the conventional topologies, the proposed DVR does not have any injection transformer due to the structural features. Because of direct connection to the grid, the storage elements are not required in the proposed topology. Therefore, this topology has less physical volume, mass and price in comparison with traditional topologies. 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