Full converter

Abstract

To provide a full converter that minimizes a stator copper loss of a generator across all wind speeds, provides stable AC output despite having an electrolytic capacitor-less structure, is robust against false turn-on, and further improves efficiency in an entire system at a low wind speed.SOLUTION: A full converter having a Z source circuit, the full converter having an AC / DC conversion circuit section including a converter for converting primary AC power into intermediate DC power, and a DC / AC conversion circuit section including an inverter for converting the intermediate DC power into secondary AC power, wherein the converter is controlled by an MTPA control system combined with a pulse width modulation (PWM) control system and an automatic current (ACR) control system.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a full converter having an AC / DC conversion circuit unit including a converter that converts primary AC power into intermediate DC power, and a DC / AC conversion circuit unit including an inverter that converts the intermediate DC power into secondary AC power, and having a Z-source circuit. [Background technology]

[0002] In power generation systems that use fluid energy, such as wind power generation, the AC power output from a synchronous generator has a frequency proportional to the rotor rotation speed, making it impossible to connect to a power grid as is. For this reason, a full converter is used, which first converts the DC power into DC power and then converts it into AC power synchronized with the power grid frequency using an inverter (see, for example, Patent Documents 1 and 2). However, because the full converter includes multiple active elements, it has the problem of increased switching loss. To avoid this switching loss, a wind power generation system has been developed that combines a power converter composed of diodes with a Z-source boost circuit that boosts the output instead of the converter in the full converter, as shown in Patent Document 3. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-40551 [Patent Document 2] Special Publication No. 06-505618 [Patent Document 3] Japanese Patent Application Publication No. 2019-30039 Summary of the Invention [Problem to be solved by the invention]

[0004] The device of Patent Document 3 merely combines a diode and a Z-source boost circuit as a power converter on the generator side, and therefore has a technical problem of reducing the efficiency of the entire system.

[0005] On the other hand, in full converters such as those disclosed in Patent Documents 1 and 2, in addition to the problem of reduced overall system efficiency, electrolytic capacitors are used as smoothing capacitors to smooth DC power (see, for example, Figure 7 of Patent Document 3), and these electrolytic capacitors are a major cause of full converter failure. Therefore, it is conceivable to reduce operation and maintenance costs by eliminating electrolytic capacitors, i.e., by replacing the electrolytic capacitors with long-life film capacitors or the like. However, using film capacitors or the like as smoothing capacitors makes it difficult to adequately smooth DC power and therefore results in poor AC output. Furthermore, while increasing the switching frequency allows film capacitors or the like to provide adequate smoothing and enable AC output, the increased switching frequency can lead to false turn-on of the inverter, making it vulnerable.

[0006] This invention addresses these problems. First, it aims to minimize generator stator copper loss across all wind speeds by controlling the converter using a maximum torque per ampere (MTPA) control system that combines a pulse-width modulation (PWM) control system and an automatic current control (ACR) control system. Second, it aims to provide a full converter with a Z-source structure that uses, for example, metal-oxide semiconductor field-effect transistors (MOSFETs) for high-speed switching, thereby increasing the switching frequency and eliminating the need for electrolytic capacitors. Third, it aims to further improve the overall system efficiency at low wind speeds by utilizing the boost operation of the Z-source to drive the converter and inverter with different DC voltages. [Means for solving the problem]

[0007] The full converter of the first aspect of the present invention comprises: This full converter has an AC / DC conversion circuit section including a converter that converts primary AC power into intermediate DC power, and a DC / AC conversion circuit section including an inverter that converts the intermediate DC power into secondary AC power, and also has a Z-source circuit, and is characterized in that the converter is controlled by an MTPA control system combined with a pulse width modulation (PWM) control system and an automatic current reduction (ACR) control system. This feature improves the overall system efficiency at all wind speeds.

[0008] A full converter according to a second aspect of the present invention is the full converter according to the first aspect, further characterized in that the Z source circuit is an X-connected Z source circuit.

[0009] A full converter of a third aspect of the present invention is the full converter of the first aspect, characterized in that at least one of the converter and the inverter performs high-speed switching using a metal-oxide semiconductor field-effect transistor (MOSFET).

[0010] A full converter according to a fourth aspect of the present invention is the full converter according to the third aspect, characterized in that the metal oxide semiconductor field effect transistor (MOSFET) that performs high-speed switching is provided only in the inverter.

[0011] A full converter according to a fifth aspect of the present invention is the full converter according to the fourth aspect, characterized in that the inverter is a SiC inverter.

[0012] A full converter according to a sixth aspect of the present invention is the full converter according to the third aspect, characterized in that the capacitor constituting the Z source circuit is a film capacitor.

[0013] A full converter of a seventh aspect of the present invention is the full converter of the third aspect, characterized in that it includes a short-circuit ratio control means for controlling the short-circuit ratio D of the Z source circuit so as to keep the capacitor voltage Vdc of the capacitor constituting the Z source circuit constant.

[0014] The full converter of the eighth aspect of the present invention is a full converter of the third aspect, characterized in that the drive voltage of the converter is lower than the drive voltage of the inverter, and although the efficiency of the system-side inverter decreases due to the increase in current caused by the lower voltage, the efficiency of the entire system is improved by operating at an optimal drive voltage point that takes this into consideration.

[0015] The present invention may have only the invention-specifying matters set forth in the claims of the present invention, or may have the invention-specifying matters set forth in the claims of the present invention as well as configurations other than the invention-specifying matters. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a diagram showing an example of an embodiment of a full converter of the present invention; [Figure 2] FIG. 2 is a diagram showing an equivalent block model of a control system of a full converter according to an embodiment of the present invention. [Figure 3] 10(a) to 10(c) are graphs showing simulation results when the parameter (d) is used in the full converter of the embodiment. [Figure 4] FIG. 10(a) is a diagram showing the contents of cases 1 to 4 to be compared, FIG. 10(b) is a diagram showing various parameters in the simulation comparison of FIGS. 5 to 7, and FIG. 10(c) is a diagram showing each parameter of the filter in the simulation comparison of FIGS. 5 to 7. [Figure 5] 1A is a graph showing simulated changes in the DC smoothing capacitor voltage in the Z-source full converter of the embodiment, FIG. 1B is a graph showing simulated waveforms of the system current in the Z-source full converter of the embodiment, and FIG. 1C is a graph showing the frequency distribution of the system current in the Z-source full converter of the embodiment. [Figure 6] (a) is a graph showing the simulated change in DC smoothing capacitor voltage in a full converter of Case 2, which is a comparative example; (b) is a graph showing the simulated waveform of the system current in the full converter of Case 2; and (c) is a graph showing the relationship between the switching frequency and magnitude in the full converter of Case 2. [Figure 7] (a) is a graph showing the simulated change in DC smoothing capacitor voltage in a full converter of Case 3, which is a comparative example; (b) is a graph showing the simulated waveform of the system current in the full converter of Case 3; and (c) is a graph showing the relationship between switching frequency and magnitude in the full converter of Case 3. [Figure 8](a) is a graph showing the simulated change in DC smoothing capacitor voltage in a full converter of Case 4, which is a comparative example; (b) is a graph showing the simulated waveform of the system current in the full converter of Case 4; and (c) is a graph showing the relationship between the switching frequency and magnitude in the full converter of Case 4. [Figure 9] FIG. 10 is a table showing the comparison results of cases 1 to 4. [Figure 10] 1(a) is a graph showing the relationship between wind speed and efficiency when variable DC link voltage control is performed in the full converter of the present invention, and FIG. 1(b) is a table showing annual loss, efficiency, and capacity factor. DETAILED DESCRIPTION OF THE INVENTION

[0017] An embodiment of a full converter according to the present invention will be described below.

[0018] An example configuration of a full converter equipped with a Z-source circuit of the present invention (hereinafter referred to as Z-source full converter) is shown in Fig. 1. The Z-source full converter of the present invention is mainly composed of an AC / DC conversion circuit unit 2 that is provided on the side of a fluid power generator (IPMSG) 1, such as a wind turbine generator that generates electricity by rotating a windmill using wind power, and that converts the AC power output from the fluid power generator (IPMSG) 1 by power generation into DC power, a DC / AC conversion circuit unit 4 that converts the DC power converted by the AC / DC conversion circuit unit 2 into AC power, which is grid power, and outputs it to the grid side, and a Z-source circuit 3 that is provided between the AC / DC conversion circuit unit 2 and the DC / AC conversion circuit unit 4.

[0019] The AC / DC conversion circuit unit 2 includes a converter that converts AC power output from the fluid power generator (IPMSG) 1 into DC power, a PWM control system that performs pulse-width modulation control of the converter, and a capacitor C1 that smooths the DC pulses generated by the converter. The PWM control system is further controlled by an automatic current limiter (ACR) control system, which in turn is controlled by an MTPA control system. However, to perform MTPA control in a full converter, both the upstream and downstream converters must be appropriately controlled. In a full converter with a Z-source circuit, only an MTPA control system combining PWM and ACR can stably achieve MPPT and MTPA control of the generator. Furthermore, the present invention is characterized by the fact that the MTPA control system can vary the converter-side voltage, thereby reducing generator losses.

[0020] The AC / DC conversion circuit section 2 of the present invention can be an AC / DC conversion circuit having a converter with a low switching speed that uses an inexpensive IGBT (Insulated Gate Bipolar Transistor) used in conventional normal full converters, but the capacitor C1 is not an electrolytic capacitor but a highly durable film capacitor or ceramic capacitor.

[0021] The DC / AC conversion circuit unit 4 includes an inverter that converts direct current to alternating current and a PWM control system that PWM-controls the inverter. It is preferable to use an inverter that uses metal oxide semiconductor field-effect transistors (MOSFETs) that can switch at higher speeds than the power transistors used in conventional full converters. These metal oxide semiconductor field-effect transistors (MOSFETs) should be high-voltage transistors such as SiC-MOSFETs that are as inexpensive as possible, but transistors other than SiC-MOSFETs can also be used as long as they are capable of high-speed switching. The DC / AC conversion circuit unit 4 is connected to the grid via an L filter inductor Lf.

[0022] For example, as shown in Figure 1, Z source circuit 3 can be configured with one diode D in the DC link of the full converter, and an impedance circuit in which two inductors L1 and L2 and two capacitors C2 and C3 are connected in an X configuration on the DC / AC conversion circuit 4 side of diode D. Also, by utilizing equivalent transformation of the circuit, other Z source circuits can be used to achieve the same effect as the circuit in Figure 1.

[0023] In this way, by providing the Z source circuit 3, which serves as an impedance circuit, in the DC link, even if a short circuit occurs in the inverter in the DC / AC conversion circuit section 4, the circuit will not be damaged, and therefore a short circuit in the inverter due to erroneous turn-on is permitted, making it possible to perform high-speed switching without regard to the constraints of erroneous turn-on in the inverter.

[0024] In other words, by using an inverter that uses MOSFETs or the like to significantly increase the switching speed compared to conventional inverters, even if capacitor C1 is replaced with a film capacitor that is more durable but has a smaller capacitance than conventional electrolytic capacitors, the low capacitance can be compensated for. As will be described later, it is possible to output grid power with a waveform that is equal to or better than that of a full converter using conventional electrolytic capacitors. By increasing the switching speed, the filter size can be further reduced, and the overall size of the full converter can be reduced. In other words, conventional circuits using electrolytic capacitors are too large to remove harmonics with an L filter, but the switching frequency of the structure of the present invention is high, so a small L filter is sufficient to remove harmonics.

[0025] As described above, the switching speed may be increased on the converter side rather than the inverter side, as in the present embodiment, or on both the inverter side and the converter side. However, if only the converter side switching speed is increased, the waveform performance of the system power will be degraded compared to when the inverter side switching speed is increased.

[0026] Furthermore, if MOSFETs or the like are used on both the converter and inverter sides to increase the switching speed, the cost of the resulting full converter may increase.In that case, by using MOSFETs or the like only on the inverter side to achieve high-speed switching, it is possible to prevent an increase in cost while reducing the harmonic content of the grid current.

[0027] In this embodiment, as described above, expensive metal oxide semiconductor field effect transistors (MOSFETs) are used only on the inverter side in order to reduce the harmonic content of the grid current while preventing an increase in the cost of the full converter. However, the present invention is not limited to this. For example, if a good waveform can be obtained by using a filter, as described above, metal oxide semiconductor field effect transistors (MOSFETs) may be used only on the converter side to achieve high-speed switching. For example, if the price of metal oxide semiconductor field effect transistors (MOSFETs) becomes cheaper in the future, metal oxide semiconductor field effect transistors (MOSFETs) may also be used on the converter side to increase the switching speed.

[0028] In Figure 1, the subscripts d and q represent the state quantities on the dq coordinate axis, while m and g represent the state quantities on the generator side and the grid side, respectively. The superscript ref represents the command value.

[0029] Next, the control system will be described. Although not shown in Fig. 1, it goes without saying that a control device (control means) such as a controller or computer that executes the following control based on a program may be provided separately.

[0030] As shown in Fig. 1, the converter performs MPPT operation to operate the generator at high efficiency. The active power command value is calculated using the rotation speed of the fluid generator (IPMSG) 1, maximum torque per current (MTPA) control is performed by independently controlling the dq-axis current, and losses in the fluid generator (IPMSG) 1 are reduced by adjusting the voltage on the generator side.

[0031] On the other hand, in the inverter, dg ;i qg In addition to controlling the transmission power by the sh Voltage V dc and the voltage v of the capacitors C2 and C3 in the Z source circuit 3 c Regarding power, the phase that makes the q-axis component of the grid voltage zero is obtained using a PLL, and this is used to perform a dq coordinate transformation, thereby controlling the active power and reactive power independently using the dq-axis current.

[0032] voltage v c is controlled by a PI controller in the outer loop of the d-axis current control (ACR; Auto Current Regulator). Here, the control system is expressed by an equivalent block model as shown in Figure 2. LL is the effective value of the line voltage of the grid. In this embodiment, the effective power P g to voltage v c The transfer function G(s) up to is derived by linear approximation, and a controller (control device) is designed by frequency shaping.

[0033] On the other hand, the voltage v dc is the voltage v c is controlled to a constant value, the short-circuit ratio D of the inverter of the DC / AC conversion circuit part 4 is sh This short circuit ratio D shis the voltage v dc The command value v dc ref and the feedback voltage v c From this, it is given by the equation shown in Figure 1.

[0034] Next, Figures 3(a) to 3(c) show the simulation results for the Z-source full converter of the present invention shown in Figure 1, using the parameters shown in Figure 3(d). The circuit parameters were analytically designed based on the current and voltage ripple conditions and system stability conditions. The inverter switching frequency was set to 50 kHz, and the capacitance was designed to match the available film capacitors. The simulation was performed using MATLAB (registered trademark of MathWorks, Inc., USA) / Simulink 2021b, and natural wind speed data for 300 seconds was used as the wind speed. These simulation results are accurate, including those that are not affected by individual differences between devices, and are identical to the results obtained when an actual circuit is created.

[0035] In Fig. 3(a), i dm and i qm The time response of both is shown. It can be confirmed that both follow the command value and achieve MTPA control for the fluid power generator (IPMSG) 1 while tracking the maximum power point.

[0036] Also, in Figure 3(b), i dg ;i qg The time response of i, which controls the active power, is shown. dg changes depending on the wind speed, while i qg can always be controlled to 0, and the power factor can be maintained at 1.

[0037] Also, in Figure 3(c), the voltage v cThe time response of the full converter is shown in Fig. 1. Even when using a 400 μF film capacitor as capacitor C1 and 300 μF film capacitors as capacitors C2 and C3, totaling a low capacitance of 1000 μF, the voltage ripple is kept within 2%. This demonstrates that the Z-source full converter of this embodiment, which uses film capacitors, has sufficient performance and is fully capable of using film capacitors. In other words, even if the reliability of the full converter is improved by replacing the conventional electrolytic capacitors with highly durable but low-capacity film capacitors, a stable AC output Pg can be transmitted to the grid with little voltage ripple, just like a full converter using conventional electrolytic capacitors.

[0038] Here, the Z-source full converter using the film capacitor of this embodiment shown in FIG. 1 can reduce voltage ripple and output a good AC output Pg with little noise, as will be explained below by comparing it with the AC output waveform of a conventional full converter that does not have the Z-source circuit 3.

[0039] As shown in Figure 4(a), for comparison, case 1 is the Z-source full converter of this embodiment shown in Figure 1, which has a Z-source circuit 3, uses a film capacitor as the capacitor, and uses an L filter as the AC output filter; case 2 is a converter without the Z-source circuit 3, uses an electrolytic capacitor as the capacitor, and uses an L filter as the AC output filter; case 3 is a converter without the Z-source circuit 3, uses an electrolytic capacitor as the capacitor, and uses an LCL filter as the AC output filter; and case 4 is a converter using a film capacitor as the capacitor and an LCL filter as the AC output filter.

[0040] The parameters used in the above comparison are as shown in Fig. 4(b), with the film capacitor having a capacitance of 450µF and the electrolytic capacitor having a capacitance of 30mF. The other parameters are the same for Cases 1 to 4.

[0041] The parameters of the L filter and LCL filter are as shown in Figure 4(c). In both case 1 of the Z-source full converter of this embodiment and case 2 of the normal full converter, an L filter is used, but the capacitance of the inductor used is 0.15 mH in case 1, which is significantly smaller than the 20 mH in case 2.

[0042] FIG. 5 shows the voltages of the capacitors C2 and C3 in the Z-source circuit 3 of the Z-source full converter of Case 1 of this embodiment (voltage v c ) over time, the waveform of the grid current, and the frequency distribution of the AC output Pg. The magnitude (%) in the frequency distribution is calculated by dividing the amplitude of the signal at each frequency by the amplitude of the 60 Hz fundamental wave signal of the grid power.

[0043] As shown in Figure 5(a), the voltage (voltage vc) across capacitors C2 and C3 shows periodic fluctuations, but remains within a range of approximately ±35V from the command value of 1000V. Furthermore, as shown in Figure 5(b), the waveform of the grid current also shows a smooth, regular curve, demonstrating a good waveform. Furthermore, the frequency distribution shown in Figure 5(c) shows that some noise is generated at frequencies near integer multiples of the switching frequency, but the filter in Case 1 is designed to keep harmonics of the switching frequency below 0.3%, satisfying the 0.3% specification for a single L filter alone.

[0044] FIG. 6 shows the time change in the voltage of the electrolytic capacitor, the waveform of the system current, and the frequency distribution of the AC output Pg in Case 2, which does not have the Z source circuit 3, uses an electrolytic capacitor as the capacitor, and uses an L filter as the filter.

[0045] As shown in Figure 6(a), the electrolytic capacitor voltage remains within a range of approximately ±1.5V from the command value of 1032V, indicating very little voltage change in the DC link. However, because the inverter switching speed is low at 3kHz and the filter is an L filter, as shown in Figure 6(b), the grid current waveform is more distorted than in Case 1, indicating that the grid current has a lot of noise and poor waveform performance. Furthermore, as shown in Figure 6(c), the magnitude units of the frequency distribution are much larger than in Case 1, indicating that large noise is generated near frequencies that are integer multiples of the switching frequency.

[0046] Figure 7 shows the time change in the voltage of the electrolytic capacitor, the waveform of the AC output, and the frequency distribution of the AC output for Case 3, which does not have a Z source circuit 3, uses an electrolytic capacitor as the capacitor, and uses an LCL filter as the filter.

[0047] As shown in Figure 7(a), the change in the voltage of the electrolytic capacitor is very small, just like in Case 2, and it can be seen that the voltage change in the DC link is very small. Also, because the filter is a high-performance LCL filter, as shown in Figure 7(b), the AC output waveform is very good, unlike in Case 2, and it can be seen that the harmonics in the grid current are sufficiently suppressed. Also, as shown in Figure 7(c), the magnitude units of the frequency distribution are small, just like in Case 1, and although there is some noise at low frequencies and near frequencies that are integer multiples of the switching frequency, the 0.3% specification is met, just like in Case 1 above.

[0048] FIG. 8 shows the time change in the voltage of the film capacitor, the waveform of the system current, and the frequency distribution of the AC output in Case 4, which does not have the Z source circuit 3, uses a film capacitor as the capacitor, and uses an LCL filter as the filter.

[0049] As shown in Figure 8(a), the change in the film capacitor voltage is much larger than in Cases 1 to 3, indicating that the DC power is not sufficiently smoothed. Furthermore, despite the fact that the filter is a high-performance LCL filter, the waveform of the grid current is more severely distorted than in Case 1, as shown in Figure 8(b). Furthermore, as shown in Figure 8(c), although the magnitude units of the frequency distribution are the same as in Cases 1 and 3, it is clear that a lot of low-frequency noise is occurring below 3 kHz, the switching frequency.

[0050] Although not shown, a simulation was also performed on Case 5, which did not include the Z source circuit 3, used a film capacitor as the capacitor, and used an L filter as the AC output filter, but the effect of noise was too great, making it unstable and uncontrollable, and so the AC output waveform could not be obtained through simulation. In other words, a conventional full converter, which does not have the Z source circuit 3 and cannot speed up switching, becomes unstable, and cannot be operated with just an inexpensive, small-sized L filter.

[0051] From the above, it can be seen that in Case 1 of this embodiment, like Case 3 which is a conventional full converter using an electrolytic capacitor, a system current with a good waveform and little distortion can be obtained, and that this system current with a good waveform can be obtained even if an inexpensive L filter with a simple structure and small inductance is used, rather than a high-performance LCL filter with a complex structure.

[0052] The results of the above comparison are summarized in Figure 9. As described above, Case 1 of this embodiment can use an L filter with small inductance, and it can be seen that the filter loss is very small and the filter efficiency is high compared to the conventional normal full converters in Cases 2 to 4.

[0053] Furthermore, the THD (total harmonic distortion) was "0.03%," which is one to two orders of magnitude smaller than those in cases 2 to 4, and it can be seen that the AC output characteristics are excellent.

[0054] Next, the DC link voltage v dc The efficiency of the wind power generation system can be improved by controlling the variable

[0055] As described above, the Z-source full converter of this embodiment is capable of high-speed switching, so it is possible to obtain a system current with a good waveform with little distortion even when a film capacitor is used. However, the increased loss that accompanies high-speed switching may lead to a decrease in system efficiency.

[0056] Therefore, in order to suppress the increase in loss that accompanies this high-speed switching, the boost function of the Z source circuit 3 is utilized, and the converter side and the inverter side are driven with different DC voltages, thereby improving the efficiency of the entire system, especially at low wind speeds.

[0057] Specifically, in the Z source circuit 3, by incorporating upper and lower short-circuit operation into the inverter operation, the voltage v dc With respect to voltage v c This allows the inverter drive voltage v c The converter drive voltage v dc It becomes possible to variably control the

[0058] The iron loss of the hydrodynamic generator (IPMSG) 1 and the switching loss of the converter are calculated by the converter drive voltage v dc The lower the wind speed, the lower the voltage v dc However, the efficiency of these can be improved by lowering the voltage v dc A decrease in V increases the current flowing through the inductor, leading to an increase in the parasitic resistance of the inductor and the conduction loss in the inverter. Therefore, the optimum voltage V is determined by taking these trade-offs into account. dc optBy providing this as a command value, the efficiency of the entire system can be improved.

[0059] When space vector pulse width modulation (SVPWM) is used as the modulation method, the minimum value of the converter drive voltage required to control the fluid generator 1 is v dc min Considering overmodulation, it becomes the following equation (1).

[0060]

number

[0061] Here, Vm is the output phase voltage amplitude of the fluid generator (IPMSG) 1, and can be calculated using equation (2).

[0062]

number

[0063] On the other hand, the voltage v dc The maximum voltage that can be taken by dc max is the inverter drive voltage v c Therefore, the optimum voltage v that maximizes the system efficiency within this range is dc opt By driving the converter with this, the loss in the entire system can be reduced. dc is controlled using a state feedback controller K.

[0064] As shown above, the converter drive voltage v dc The optimum voltage V dc opt For comparison, the converter drive voltage v dc Figure 10(a) shows the results of a simulation comparing the efficiency of Case 2, which is a configuration in which Z is kept constant, and Case 3, which is a configuration of a conventional normal full converter that does not have the Z source circuit 3 and uses an electrolytic capacitor as the capacitor.

[0065] The simulation parameters were a 750kW fluid generator (IPMSG) 1, an IGBT module (2MBI650XXA170-50) for the converter, and a full SiC module (FMF600DXE-34BN) for the inverter, all connected in parallel; the converter switching frequency was set to 5kHz, and the inverter switching frequency was set to 30kHz in cases 1 and 2, and 5kHz in case 3.

[0066] As shown in Fig. 10(a), comparing Case 1 and Case 2, the converter drive voltage v dc By variably controlling the inverter, efficiency at low wind speeds is greatly improved, and it can be confirmed that it is possible to reduce system losses without requiring hardware changes. Furthermore, comparing Case 1 and Case 3, it can be seen that Case 1 is able to operate with higher efficiency at low wind speeds of 10 m / s or less. On the other hand, at high wind speeds, the current flowing increases as the amount of power generation increases, and inverter losses become dominant, so the difference in efficiency is small.

[0067] That is, as mentioned above, the converter drive voltage v dc , the optimum voltage v dc opt By providing a control circuit (control means) that controls the wind speed in this way, the efficiency can be significantly improved, especially at low wind speeds.

[0068] Here, the Weibull distribution function of equation (3) is used as the probability distribution density function for wind speed Vw to calculate annual losses, efficiency, and capacity factor.

[0069]

number

[0070] The scale factor k and shape factor c were determined based on the local wind speed map provided by NEDO (New Energy and Industrial Technology Development Organization), with the parameters of Cape Soya (A: k = 1:99; c = 9:50, average wind speed 8.69 m / s) as the favorable wind area and Chiba City (B: k = 1:86; c = 6:30, average wind speed 5.30 m / s) as the inland area. The cut-in and cut-out wind speeds were set at 3 m / s and 25 m / s, respectively, and the generator pitch was controlled to operate at rated output when the rated wind speed of 12 m / s was exceeded.

[0071] The calculation results are shown in Fig. 10(b). As mentioned above, the converter drive voltage v dc , the optimum voltage v dc opt By controlling the wind speed to this value, Case 1 achieved the highest total efficiency at both points A and B. In particular, when comparing Case 1 with Case 3, an efficiency improvement of more than 3% was achieved at point B, where the average wind speed is low. Note that these are calculation results for one hydrodynamic generator (IPMSG), so even greater improvements in facility utilization rate are expected when introducing a large number of wind turbines, such as in a wind farm.

[0072] In addition, the calculation results shown in Figure 10(b) do not take into account system downtime, such as downtime caused by replacing electrolytic capacitors, so if the downtime caused by replacing these electrolytic capacitors were included, the capacity factor in Case 3 would be even lower. Therefore, it is thought that the difference in capacity factor between Cases 1 and 2, where the use of film capacitors can significantly reduce system downtime, would actually be even greater.

[0073] As described above, according to the Z-source full converter of this embodiment, the short-life electrolytic capacitors (capacitors), which are the main cause of failures in conventional full converters, can be replaced with long-life, highly reliable film capacitors (capacitors), and even when replaced with these film capacitors (capacitors), a good system output (AC output Pg) can be obtained.

[0074] Furthermore, the Z-source full converter of this embodiment has the Z-source circuit 3, which enables high-speed switching of the inverter. That is, even if an erroneous turn-on occurs in the inverter, the Z-source full converter will not be damaged.

[0075] Furthermore, because the characteristic of not being damaged even if a false turn-on occurs makes it possible to significantly reduce the impedance of the harmonic filter Lf used for grid interconnection.Furthermore, since the above-mentioned features can be obtained without increasing the switching speed of the converter on the generator side, losses do not increase more than necessary and a highly efficient full converter can be obtained.

[0076] In addition, the Z source circuit 3 allows independent control of the applied voltages of the converter and inverter, so the optimum voltage v dc opt Therefore, it is possible to operate the converter at a higher efficiency than conventional full converters.

[0077] Although the embodiments of the present invention have been described above with reference to the drawings, the specific configuration is not limited to these embodiments, and the present invention also includes changes and additions that do not deviate from the gist of the present invention.

[0078] For example, in the above embodiment, in order to improve maintainability, an example is given in which the capacitors C2 and C3 of the Z source circuit 3, along with the capacitor C1 of the AC / DC conversion circuit unit 2, are made into highly durable film capacitors, but the present invention is not limited to this. For example, it is also possible to make only the capacitor C1 a film capacitor, and the capacitors C2 and C3 of the Z source circuit 3 into electrolytic capacitors.

[0079] In addition, in the above embodiment, a configuration in which a metal oxide semiconductor field effect transistor (MOSFET) that performs high-speed switching is provided only on the inverter side is exemplified, but the present invention is not limited to this, and a configuration in which a metal oxide semiconductor field effect transistor (MOSFET) that performs high-speed switching is provided only on the converter side, or on both the converter side and the inverter side may also be used.

[0080] Furthermore, in the above embodiment, an example is given in which the Z source circuit 3 is provided in the DC link. However, for example, if a field effect transistor that does not erroneously turn on even during high-speed switching is used as the field effect transistor capable of high-speed switching, the inverter may be switched at high speed by using a film capacitor for the capacitor C1 without using the Z source circuit 3.

[0081] Furthermore, in the above embodiment, a wind power generator that generates power by rotating a windmill is exemplified as the fluid power generator (IPMSG) 1, but the present invention is not limited to this. These fluid power generators may also be fluids other than air, such as hydroelectric power generators that generate power using flowing water or wave power generators that generate power using seawater. Any power generator that generates power by operating a power generation device using the movement of a fluid can be applied.

[0082] Furthermore, in the above embodiment, the Z-source full converter is exemplified as a form in which the AC power generated from the fluid generator 1 is converted into grid power, but the present invention is not limited to this, and can also be used as a full converter for power generation using the energy of other fluids, for example, in pumped-storage power generation, in which grid power is converted and supplied to a hydroelectric generator.In this way, by applying the present invention to pumped-storage power generation, it is possible to significantly improve the conversion efficiency and equipment efficiency.

[0083] In addition, in the above embodiment, the Z-source full converter is exemplified as a form in which AC power generated from a generator is converted into grid power, but the present invention is not limited to this, and can also be used as a full converter that converts AC power to AC power, for example, in grid interconnection from AC to AC. [Explanation of symbols]

[0084] 1 Fluid power generator (IPMSG) 2 AC / DC conversion circuit section 3 Z Source Circuit 4 DC / AC conversion circuit section

Claims

1. A full converter having an AC / DC conversion circuit section including a converter that converts primary AC power into intermediate DC power, and a DC / AC conversion circuit section including an inverter that converts the intermediate DC power into secondary AC power, and having a Z-source circuit, characterized in that the converter is controlled by an MTPA control system combined with a pulse width modulation (PWM) control system and an automatic current reduction (ACR) control system.

2. 2. The full converter according to claim 1, further characterized in that the Z source circuit is an X-connected Z source circuit.

3. 2. The full converter according to claim 1, wherein at least one of the converter and the inverter performs high-speed switching using a metal oxide semiconductor field effect transistor (MOSFET).

4. 4. The full converter according to claim 3, wherein the metal oxide semiconductor field effect transistor (MOSFET) that performs high-speed switching is provided only in the inverter.

5. 4. The full converter according to claim 3, wherein the inverter is a SiC inverter.

6. 4. The full converter according to claim 3, wherein the capacitor constituting the Z source circuit is a film capacitor.

7. 4. The full converter according to claim 3, further comprising a short-circuit ratio control means for controlling the short-circuit ratio D of the Z source circuit so as to keep the capacitor voltage Vdc of the capacitor constituting the Z source circuit constant.

8. 4. A full converter according to claim 3, characterized in that the drive voltage of the converter is set lower than the drive voltage of the inverter, and the converter is operated at an optimum drive voltage point taking into consideration an increase in current caused by the lowered voltage, thereby controlling the drive voltage to maximize the efficiency of the entire system.

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