Modular Multilevel Converter for Wind Power Generation System connected to Micro-grid

Transcription

1 Modular Multileel Conerter for Wind Power Generation System connected to Microgrid Toshiki Nakanishi Nagaoka Uniersity of Technology Nagaoka, Japan Koji Orikawa Nagaoka Uniersity of Technology Nagaoka, Japan Junichi Itoh Nagaoka Uniersity of Technology Nagaoka, Japan Abstract This paper discusses a ACDC conerter which is constructed by a Modular Multileel Conerter (MMC) for a wind power generation system connected to Microgrid. The proposed system which is constructed the MMC with an H bridge achiees to conert from a generator oltage of 3.3 kv into DC bus oltage of 34 V. Moreoer, as a fundamental ealuation, the experimental result by miniature model of 7 W confirms that the proposed system achiees the stepdown operation from input oltage of 2 V into DC oltage of 65 V. Finally, the proposed system maintains the capacitor oltage of each to the oltage command. Furthermore, the maximum oltage error between the oltage command of the capacitor and the measured oltage is 1% or less. KeywordsModular Multileel Conerter;Hbridge ; Microgrid; wind power generation system; high power rectifier I. INTRODUCTION Recently, a microgrid and a DC power grid are actiely researched as a next generation power supply. Adantages of the microgrid are summarized as follows [1][4]; (i) the system achiees high efficiency operation because of the reduction of conersion losses in inerters between dc output sources and loads. (ii) it is not necessary to consider synchronization with utility grid and reactie power. (iii) when a blackout or oltage sag occurs in the utility grid, it does not affect the dc bus oltage of the microgrid directly because of the stored energy of the dc capacitor and the oltage control of ACDC conerter. (i) the microgrid is suitable to connect a battery energy storage system and a renewable energy source such as a photooltaic generation system and a fuel because the output oltage of many renewable energy sources is DC oltage. From adantages of the aboe, the microgrid has been applied to the power grid in isolated islands as a standalone power system [5][6]. Moreoer, a wind power generation is also applied to the power grid in isolated islands as one of the power source. Presently, many power grids in isolated islands are constructed by a diesel generation system. Howeer, this system is ery costly because the generation costs in the isolated island inole a high transportation cost to carry fuels for diesel generators [5]. Thus, applications of renewable energy sources such as a wind power generation are actiely researched and deeloped in order to reduce the fuel cost. Moreoer, locations of offshore and the isolated island are suitable for a wind power generation with high power capacity because wind is much stronger and much constant compared with wind conditions of onshore [8][1]. Therefore, the wind power generation is applied as one of power sources for the microgrid in isolated islands. Presently, a conentional wind power generation system is constructed by a transformer and a bidirectional conerter in order to connect to an AC grid [11][13]. Besides, a wind power generation system connected to the microgrid has a feature which the rated oltage of the wind power generator with high power capacity is higher than DC bus oltage of 34V in the microgrid [1], [8]. Therefore, it is necessary for a power system to conert from generator oltage into DC bus oltage when power is supplied from the wind power generator into the microgrid. Additionally, the system has to boost from DC bus oltage into generator oltage because starting torque is required while the wind power generator starts up. Thus, the bidirectional ACDC conerter operates as a rectifier when the system generates power. In contrast, the bidirectional ACDC conerter operates as an inerter when the generator starts up. Howeer, a size of the transformer is bulky because the transformer operates in low frequency because the generator operation frequency is low. In addition, the reasons which the transformer is bulky include that a high transformer ratio is required to conert from generator oltage into DC bus oltage. As one of solutions, employing a Modular Multileel Conerter (MMC) to the wind power generation system connected to the microgrid is an effectie method. Adantages of the MMC are as follows; (i) the circuit configuration is simple because of cascade connection of s. (ii) the MMC reduces the harmonic distortions because the MMC output the multileel waeform (iii) low oltage deices are applied to each because the output oltage of the is low by increasing a number of s. Thus, the system achiees the high response operation. From the adantages of the aboe, the MMC has been actiely researched as a next generation high power conerter without the bulky transformer [14][16]. Therefore, the wind power generation system achiees the size reduction by applying the MMC into the system. Howeer, in the general ACDC conerter using the MMC which consists of chopper s, it is difficult to achiee the stepdown rectified operation because chopper s cannot output negatie oltage. Moreoer, many control methods of the MMC hae been reported. Fundamental configurations of control system for the MMC are constructed from a current control system and a capacitor oltage control system. Howeer, most of conentional systems are complex and it is difficult to understand these control principle instinctiely.

2 Especially, the capacitor oltage control is constructed from many components. For example, a main control system in order to control aerage alue of all capacitor oltage is applied. In addition, many balancing control systems are applied as sub control system [16]. Therefore, it is also difficult to design control parameters in each controller. This paper discusses a wind power generation system connected to the microgrid using the MMC which consists of Hbridge s in order to conert from high generator oltage into DC bus oltage. Employing Hbridge is able to sole the oltage limit. Moreoer, the system is able to operate as a high power stepdown rectifier. Additionally, a proposed control system is ery simple because the proposed control system is constructed from the capacitor oltage control system and the arm current control system for each arm. Thus, it is able to control each arm independently. In addition, as oltage balancing control system in order to compensate the effort among s in the arm, the output DC oltage of the is aried depending on the capacitor oltage. Furthermore, it is easy to design the control parameters by applying the proposed balancing control system which is ery simple. As a fundamental ealuation, this paper proposes a control method in order to achiee the stepdown rectification. Moreoer, from an experimental result by miniature model of 7 W, the proposed system achiees to conert from three phase oltage of 2 V into the DC oltage of 65 V. II. WIND POWER GENERATION SYSTEM USING MMC A. Main circuit configuration Fig. 1(a) shows the main circuit configuration of the proposed wind power generation system using the MMC. Each leg consists of two buffer reactors L b and Hbridge s. Due to cascade connection of s, the conerter achiees a multileel oltage waeform and also reduces the rated oltage of each. Thus, many cascaded s are used in practical because it reduces harmonic distortion and utilize with a low oltage rating deices. On the other hand, in the MMC, the output DC oltage depends on the summation of the aerage alue of output oltage. Table 1 shows the configuration example of the MMC when the MMC is applied into the wind power generation system as the stepdown conerter connected to the microgrid. The power capacity of the system is 1 MW and the generator oltage is 3.3 kv. On the other hand, the DC oltage of the microgrid is 34 V [1] In addition, each parameter is calculated when the IGBTs which the oltage rating is 1,2V and the current rating is 2,4 A. are applied. Moreoer, the capacitor oltage is calculated by (6) when the modulation factor is set to.8. Additionally, the oltage rating of the IGBT is set 3% more than the capacitor oltage. The current rating of IGBT is set 5% more than the each arm current. The numbers of capacitor are calculated when each has one capacitor. Finally, examples of high power IGBTs which meet the conditions of the rating oltage and rating current are shown. Wind power generator G Ar1 car1 Ar(n/2) i Ar i r i s i t i Br Sb Br1 S C a S b Hbridge Br(n/2) car(n/2) L b L b cbr1 cbr(n/2) V mmc DC bus of Microgrid Fig. 1. Circuit configuration of the MMC for the wind power generation system connected to the microgrid. The MMC with Hbridge s is able to operate the stepdown rectifier. TABLE I. THE CONFIGURATION EXAMPLE OF THE WIND POWER GENERATION MW Voltage rating of IGBT 1,2 V Current rating of IGBT 2,4 A Capacitor =.8 86 V Numbers of leg 8 Total numbers of Cells 24 Numbers of Switching Deice 96 Numbers of Capacitors 24 1MBI24U4D12 (Fuji Electric Co., Ltd.) MBN24E17D (1,7 V, Hitachi, Ltd.) Examples of IGBTs FZ24R12HP4 E9 (Infineon) 5SNA 24E1735 (1,7 V, ABB) chopper chopper _ae pp C (a) Chopper C Hbridge S b Hbridge _ae pp S b (b) Hbridge Fig. 2. Circuit configuration and output oltage of each. The chopper has the output oltage limit because the chopper cannot output the negatie oltage. On the other hand, the Hbridge does not hae the output oltage limit. C C

3 B. Relationship between circuit configurations and output oltage Fig. 2 shows relationships between circuit configurations and output oltage. In the ACDC conerter using the MMC, a maximum alue of generator output oltage is applied to each arm of the MMC when the wind generator operates. Hence, a peaktopeak alue of the oltage pp is gien by (1) from generator output oltage and the number of s per leg 2 E p p 2 3 n where E is an effectie alue of the output line to line oltage in the wind generator, n is the number of s at each leg. In addition, the output DC oltage of the ACDC conerter using the MMC V mmc equals a summation of output aerage oltage at one leg. Hence, V mmc is gien by (2). Vmmc n _ ae where _ae is an aerage alue of the output oltage. Fig. 2(a) shows the relationships between the chopper and its output oltage. In the case of applying the chopper, a lower limit of output aerage oltage is uniquely determined from the peaktopeak alue of the oltage pp and the number of s at each leg n because the chopper cannot output negatie oltage. Therefore, in the chopper, the lower limit of output aerage oltage is gien by (3) _ ae 1 2 p p From (1), (2) and (3), the lower limit of the output DC oltage V mmc equals a maximum oltage of the wind generator phase oltage. From the principle of the aboe, the MMC with chopper s cannot achiee the stepdown rectified operation. Fig. 2(b) shows the relationships between the Hbridge and its output oltage. In the case of the Hbridge, there is no limit of output aerage oltage because it is possible for Hbridge to output negatie oltage. Hence, it is possible for Hbridge to control lower oltage than that of (3). Therefore, the MMC with Hbridge s achiees the stepdown rectified operation. III. CONTROL STRATEGY Fig. 3 shows the control block diagram of the proposed stepdown conerter. One of the features in the proposed system is to control each arm as shown in Fig. 1. The control block diagram is separated to the capacitor oltage control block and the input current control block. Moreoer, the capacitor oltage control block consists of a oltage aeraging control system and a oltage balancing control system. At first, the oltage aeraging control system corrects the error between the capacitor oltage command and the aerage alue of capacitor oltage in the arm. Additionally, the oltage balancing control system corrects the error among the capacitor oltage in the arm. A. Voltage Aeraging Control System The oltage aeraging control system is applied in order to control the aerage alue of all capacitor oltage in the arm. Therefore, the aerage alue of all capacitor oltage has to be calculated in each arm. Each aerage alue of the capacitor oltage is gien by (4). AVR c PI 1 2 K acr i Ar_ph car_ae r 2 2 n/ cmk _ ae, m = A, B k = r, s, t cmkx n x1 1/E i Ar_ph ACR where cmk_ae is the aerage alue of the capacitor oltage. m shows the index which is the upper side A or the lower side B. k is the index of each phase. Moreoer, m and k are matched in both side of the equation. The error between the capacitor oltage command and the aerage alue of capacitor oltage in the arm is corrected by a PI controller. Moreoer, the output alue of the PI controller is gien as the arm current command. In the proposed control system, the command of a positie phase current is generated depending on the fluctuation of the aerage alue of all capacitor oltage. Thus, the command of a positie phase current is multiplied by the sinusoidal wae in order to synchronize with the input phase oltage. Moreoer, the output the DC oltage in order to achiee the rectification. Therefore, the capacitor oltage includes the oltage ripple which the cycle same as the input oltage one. The control system may become instability due to the oltage ripple. Thus, in order to keep the stability of the control system, the response angular frequency of the PI controller would be designed into low or a filter in order to damp the frequency component of the input oltage cycle should be applied. B. Arm Current Control System Each arm current includes the DC component and the AC component. The DC component of the arm current flows in order to supply the DC power into a load. Moreoer, the DC current shunts in to the each leg of the MMC. Generally, the i Ar 2/n () Eq.(5) Ari A : Elimination of B : zero phase current V mmc_ari V mmc Fig. 3. Control block diagram for stepdown rectified operation of MMC. The proposed control system is applied to each arm of the MMC in order to control each arm. Moreoer, the ACR controls only the AC component of the arm current. i is the index of the number.

4 shunted DC current is constant and same alue among arms when s equally diide the output DC oltage of the MMC. Furthermore, the DC current is considered as the zero phase current because the DC current is temporally not changed. On the other hand, from the preious section, the AC component of the arm current flows as the positie phase current is generated by the oltage aeraging control system. From the aboe, the output power is controlled by the zero phase component of the arm current. The capacitor oltage is controlled by the positie phase component of the arm current. In other words, it is not necessary to compensate the zero phase component of the arm current. Therefore, in the proposed control system, the zero phase current is controlled by openloop control. On the other hand, in order to keep the capacitor oltage at constant, the system compensates only the positie phase current. C. Elimination System of Zero Phase Current Fig. 4 shows a schematic diagram of current waeform on of the arm. It is necessary to eliminate only the zero phase component from the arm current in order to control the capacitor oltage after the system detects all arm current. Howeer, it is difficult for a highpass filter to extract only the positie phase component because AC current cycle is ery low. Methods in order to extract the positie phase component are shown in the following; a) Addsubtract of three phase current b) Application of rotational coordinate transform First, a) is a method to calculate the zero phase component of the arm current by the addition and subtraction of three phase current. In the MMC, the each arm current is detected. On each of the upper side and the lower side, the zero phase current is extracted by addition of the three phase current because the addition alue of the positie phase current is zero. Moreoer, because the zero phase current which flows on each leg is equal. Thus, by subtracting the alue of the zero phase current which diides into three equal parts from the detection alue of each current, so that the positie current is extracted. Second, b) is a method to extract the positie phase component by using the rotational coordinate transform. On each of the upper side and the lower side, by the rotational coordinate transform and the inerse transform, the zero phase current is eliminated and only the positie phase current is extracted. In this paper, b) is applied. D. Output Voltage Control Systm From the aboe, the output DC oltage is controlled by the openloop control of the zero phase current. Thus, the command of the output DC oltage is added into the output alue of the controller in the arm current control system. Moreoer, the output DC oltage of the MMC is diided on each leg. Thus, the output DC oltage of a is fundamentally set V mmc /n when the command of the MMC output oltage control is V mmc and the number of s in a leg is n. E. Voltage Balancing Control System The oltage aeraging control system is used to keep the I Positie phase component of arm current (AC) Actual arm current (DC AC) oltage of all capacitor oltage. Howeer, an unbalance oltage which occurs among capacitors in same arm cannot be suppressed by oltage aeraging control system only, because the oltage aeraging control system corrects only the error between the oltage command and the aerage alue of the capacitors oltage in the arm. So far, some methods in order to balance each capacitor oltage hae been reported [14][16]. Howeer, in most of the conentional control systems, it is difficult to understand these control principle instinctiely and design control parameters. Additionally, the positie phase current in order to keep the capacitor oltage flows to all s which exist in 1 arm. Moreoer, the common alue of the positie phase current which flows each arm. Thus, it is difficult to adjust the arm current in order to balance on each. Against the problem, in the proposed control system, the output DC oltage of the is aried depending on the capacitor oltage. This control principle is gien by (5). 1 2 cmki Vmmc _ mki V n/ 2 mmc cmkx x1 Zero phase component of arm current (DC) where V mmc_mki is the output DC oltage command of each. i is the index of the number. Moreoer, m, k and i are matched in both side of (5). For example, the output DC oltage command of the s is set into high when its capacitor oltage is higher than the command of the capacitor oltage. Thus, the output power of the becomes high and the alue of the capacitor oltage discharges. In contrast, the output DC oltage command of the s is set into low when its capacitor oltage is lower than the command of the capacitor oltage. Thus, the output power of the becomes low and the capacitor oltage charges. Moreoer, the output DC oltage of a is set V mmc /n when the oltage alue of all capacitor in the arm is same. t To control capacitor oltage To supply power into DC load Fig. 4. schematic diagram of current waeform on of 1 arm. Each arm current includes an AC component and a DC component. In order to control the capacitor oltage, it is necessary to eliminate the AC components from the arm current.

5 F. Capacitor Voltage Determination The output DC oltage command of the MMC V mmc that uses to obtain the output DC oltage is added to the output block of the input current control. The change of oltage alue in each depends on the number of s at each leg since the s are connected to the load in parallel. In addition, each capacitor oltage also depends on the input and output oltage. The capacitor oltage command c is gien by (6). Note that V mmc_mki is a little different among s. Howeer, c is gien by the modulation factor and the modulation factor is set to.8 or less. Thus, the different of V mmc_mki among s is ignored because the oltage margin of the capacitor is fully large compared with the different of V mmc_mki among s. IV. c 1 2 n 2 E V 3 mmc FUNDAMENTAL EXPERIMENTAL RESULT In order to ealuate the effectieness of the proposed control system, a fundamental experiment result by using the prototype of the MMC is shown. In the experiment, the prototype is connected to the power grid of 2 V as substitute for the wind power generator in order to ealuate the stepdown rectification by the MMC. Moreoer, the resistance load is connected on the output DC side. Table I shows the experiment condition of the prototype. In the experiment, the output oltage command is 65 V. Moreoer, the leg is constructed by four s and the prototype has twele s in the three legs. Fig. 5 shows waeforms of the input phase oltage, the input current and the output DC oltage. Firstly, from the waeforms of the input phase oltage and the input current, it is confirmed that the unity power factor is obtained in the input stage. Moreoer, the total harmonic distortion (THD) of the input current is approximately 11.9%. Thus, as a future work, it is necessary to reduce the total harmonic distortion of the input current. Second, the waeform of the output DC oltage in lower side of Fig. (5) shows that the prototype conert from input oltage of 2 V into output DC oltage of 65 V. From this waeform, the output DC oltage is kept at constant. Therefore, the prototype of the MMC achiees the stepdown rectification. Fig. 6 shows the waeforms of the capacitor oltage which are connected to rphase leg. The capacitor oltage is controlled according to the capacitor oltage command c. As a result, the proposed system maintains the capacitor oltage of each Hbridge to the oltage command of 12 V. Furthermore, the maximum oltage error between the oltage command of the capacitor and the measured oltage is 1% or less. As a future work, it is necessary to consider the cause of the error between the oltage command of the capacitor. TABLE III. EXPERIMENTAL CONDITIONS AND CIRCUIT PARAMETERS Output power P O 7 W Input oltage rms E 2 V Input oltage frequency f 5 Hz Output oltage mmc 65 V Number of per leg n 4 DC capacitor C 13 F Load R 5.8 Carrier frequency f S 8 khz Buffer reactor L b Input Phase Voltage (Rphase) Input Phase Current (Rphase) MMC Output Voltage 4 mh 1 msec V. CONCLUSION 25 V 5 A 25 V Fig. 5. Waeforms of input oltage, input current and output oltage. The unity power factor is be obtained in the input stage. On the other hand, as a future work, it is necessary to reduce the total harmonic distortion of the input current. 12 V Capacitor oltage (Rphase) car1, car2, cbr1, cbr2, 25 msec 5 V Fig. 6. Waeforms of the capacitor oltage in rphase leg. The proposed system maintains the capacitor oltage of each Hbridge to the oltage command of 12 V. In addition, the maximum oltage error between the oltage command of the capacitor and the measured oltage is 1% or less. This paper discusses the stepdown conerter using a modular multileel conerter topology for the wind power generation system connected to the microgrid. The system conerts from a threephase AC oltage of the generator into DC bus oltage of 34 V by stepdown operation. Moreoer, the proposed control system is ery simple because the control system is applied to the each arm of the MMC. Finally, from the fundamental experiment, the proposed control maintains the capacitor oltage at constant. In the future work, the control method to reduce of the total harmonic distortion of the input current and will be discussed.

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