Power generation systems and control devices

Abstract

To provide a power generating system that can be operated in a wider operation region by preventing a variation in neutral point potential.SOLUTION: A power generating system comprises: a neutral-point clamped power converter having a converter that converts AC power supplied from an electric power system into DC power, and an inverter that converts the DC power supplied from the converter into AC power; an induction generator that is connected to the inverter; and a control unit that, according to at least slip of the induction generator, switches a control mode of the power converter between a first control mode and a second control mode different from the first control mode in at least one of a modulation system of the power converter and an output method of reactive power to the electric power system.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] FIELD OF THE INVENTION An embodiment of the present invention relates to a power generation system and a control device. [Background technology]

[0002] Adjustable-speed pumped-storage power generation systems, which contribute to the stabilization of power grids through high-speed output control, have been developed and introduced worldwide.A known adjustable-speed pumped-storage power generation system is the secondary excitation converter system, in which a secondary excitation converter is connected to the secondary (rotor) winding of an induction generator.

[0003] The secondary excitation converter includes a power converter such as an inverter and a converter. The inverter supplies excitation current to the stator windings of the induction generator, while the converter operates to maintain the DC voltage of the inverter.

[0004] A neutral point clamped (NPC) type power converter circuit configuration may be adopted. In a neutral point clamped type power converter, six semiconductor switching elements are provided for each phase of the induction generator 30, and two capacitors are provided to divide the power supply voltage. In a neutral point clamped power converter, the potential at the neutral point between the two capacitors may fluctuate. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 7068015 [Patent Document 2] Patent No. 7005417 [Patent Document 3] Patent No. 7002985 Summary of the Invention [Problem to be solved by the invention]

[0006] When the neutral point potential fluctuation is large, a voltage exceeding the rated voltage may be applied to the semiconductor switching elements that make up the power converter. In this case, there is a risk of the semiconductor switching elements being destroyed. As a result, the converter can only operate under conditions where the neutral point potential fluctuation is small, limiting its operating range.

[0007] The problem to be solved by the present invention is to provide a power generation system and a control device that can suppress the potential fluctuation of the neutral point and operate in a wider operating range. [Means for solving the problem]

[0008] A power generation system according to one embodiment includes a neutral-point clamped power converter having a converter that converts AC power supplied from a power grid into DC power and an inverter that converts the DC power supplied from the converter into AC power; an induction generator connected to the inverter; and a control device that switches the control mode of the power converter between a first control mode and a second control mode in which at least one of the modulation method of the power converter and the method of outputting reactive power to the power grid differs from the first control mode, depending on at least the slip of the induction generator. [Effects of the Invention]

[0009] According to this embodiment, it is possible to suppress fluctuations in the neutral point potential and operate in a wider operating range. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a block diagram showing an example of the configuration of a power generation system according to a first embodiment. [Figure 2] FIG. 2 is a diagram illustrating an example of a circuit configuration of a secondary excitation converter. [Figure 3] 3 is a flowchart showing an operation procedure of the power generation system 1 according to the first embodiment. [Figure 4] 10 is a diagram for explaining the potential fluctuation of the neutral point NP. FIG. [Figure 5] FIG. 10 is a diagram illustrating an example of a system voltage and an allowable range of slip. [Figure 6] 10(a) shows the modulation method in the first control mode of the first embodiment, and FIG. 10(b) shows the modulation method in the second control mode of the first embodiment. [Figure 7] FIG. 10 is a diagram showing an example of a potential fluctuation at the neutral point when the secondary excitation converter is modulated in the second control mode. [Figure 8] 10(a) shows the supply path of active power in the first control mode of the second embodiment, and FIG. 10(b) shows the output path of reactive power in the second control mode of the second embodiment. [Figure 9] FIG. 10 is a diagram illustrating an example of an allowable range of slippage. DETAILED DESCRIPTION OF THE INVENTION

[0011] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. The present invention is not limited to the following embodiments.

[0012] (First embodiment) FIG. 1 is a block diagram showing an example of the configuration of a power generation system according to a first embodiment. The power generation system 1 shown in FIG. 1 includes a doubly excited converter 10, a control device 20, an induction generator 30, a water turbine 40, a transformer 50, and a circuit breaker 60. The power generation system 1 is an adjustable-speed pumped-storage power generation system that generates power by rotating the water turbine 40. However, the power generation system according to the present invention is not limited to hydroelectric power generation systems including adjustable-speed pumped-storage power generation systems, but can also be applied to other power generation systems, such as wind power generation systems. When the power generation system 1 is a wind power generation system, a wind turbine is installed in place of the water turbine 40. The configuration of the power generation system 1 will be described below.

[0013] The secondary excitation converter 10 is an example of a neutral point clamped power converter, and includes an inverter 11, a converter 12, a first capacitor 13a, a second capacitor 13b, and a transformer 14. The circuit configuration of the secondary excitation converter 10 will now be described with reference to FIG.

[0014] Fig. 2 is a diagram showing an example of the circuit configuration of the secondary excitation converter 10. The circuit configuration of the inverter 11 and converter 12 shown in Fig. 12 is a neutral point clamp type. Specifically, the inverter 11 has six semiconductor switching elements S for each of three AC terminals u1, v1, and w1 corresponding to each phase of the induction generator 30. 11 ~Semiconductor switching element S 16 Each semiconductor element is a power semiconductor element such as an IGBT (Insulated Gate Bipolar Transistor), and a diode is connected in antiparallel to each semiconductor element.

[0015] Semiconductor switching element S 11 ~Semiconductor switching element S 14 are connected in series between the high potential point P and the low potential point N. Therefore, the semiconductor switching element S 11 The collector terminal of the semiconductor switching element S is connected to the high potential point P as a DC terminal. 14 The emitter terminal of is connected to the low potential point N as a DC terminal.

[0016] In addition, the semiconductor switching element S 15 and semiconductor switching element S 16 are connected in series. At this time, the semiconductor switching element S 15 and semiconductor switching element S 16 The connection point is connected to a neutral point NP located midway between the high potential point P and the low potential point N. Furthermore, the semiconductor switching element S 15 and semiconductor switching element S 16 is the semiconductor switching element S 12 and semiconductor switching element S 13 is connected in parallel to

[0017] Semiconductor switching element S 11 ~Semiconductor switching element S 16Each of the semiconductor switching elements S is turned on and off based on a signal input from the control device 20 to the gate of each element. As a result, the DC power input to the DC terminals is converted into AC power. This AC power is supplied to the induction generator 30 from the AC terminals u1, v1, and w1. 11 ~S 16 2 as a single semiconductor switching element, multiple semiconductor switching elements may be connected in series and driven with the same gate signal to operate as a single element. Also, inverter 11 may be configured by connecting multiple identical circuits in parallel.

[0018] On the other hand, the converter 12 has six semiconductor switching elements S for each of three AC terminals u2, v2, and w2. 21 ~Semiconductor switching element S 26 Each semiconductor element is a power semiconductor element such as an IGBT (Insulated Gate Bipolar Transistor) as in the inverter 11. 21 ~Semiconductor switching element S 26 A diode is also connected in antiparallel to each of the semiconductor switching elements S 21 ~Semiconductor switching element S 26 The connection configuration of the semiconductor switching element S is the same as that of the inverter 11. 21 ~Semiconductor switching element S 26 Each of the semiconductor switching elements S is turned on and off based on a signal input from the control device 20 to the gate of each element. As a result, AC power input from the power grid 90 to the AC terminals u2, v2, and w2 via the transformer 50 is converted into DC power required to operate the inverter 11. 21 ~S 26 2 as a single semiconductor switching element, multiple semiconductor switching elements may be connected in series and driven with the same gate signal to operate as a single element. Also, converter 12 may be configured with multiple identical circuits connected in parallel.

[0019] The first capacitor 13a is connected between the high potential point P and the neutral point NP. The second capacitor 13b is connected between the neutral point NP and the low potential point N. The first capacitor 13a and the second capacitor 13b are connected to the power supply voltage V PN , voltage V p and voltage V N The voltage is divided into

[0020] 1, the transformer 14 is connected to the input side of the converter 12. The transformer 14 transforms the input voltage to a predetermined voltage. The transformed voltage is input to the converter 12.

[0021] Next, the control device 20 will be described. The control device 20 has an operation mode determination unit 21, a control mode setting unit 22, and a drive control unit 23. The operation mode determination unit 21 determines whether the operation mode of the secondary excitation converter 10 is a phase-modifying operation mode. The control mode setting unit 22 sets the control mode of at least one of the inverter 11 and the converter 12 to a first control mode or a second control mode based on the system voltage of the power system 90 and the slip of the induction generator 30. The first control mode and the second control mode will be described later. The drive control unit 23 controls the semiconductor switching elements S of the inverter 11 in the control mode set by the control mode setting unit 22. 11 ~S 16 and the semiconductor switching element S of the converter 12 21 ~S 26 respectively.

[0022] Induction generator 30 is a wound-rotor three-phase induction generator. The secondary excitation winding of induction generator 30 is connected to AC terminals u, v, and w of inverter 11. The stator winding of induction generator 30 is connected to transformer 50 via circuit breaker 60.

[0023] The water turbine 40 is connected to the rotating shaft of the induction generator 30. The water turbine 40 is rotated by the water flow generated by the release of water from the dam. The rotational force of the water turbine 40 is transmitted to the induction generator 30 and converted into electric power.

[0024] Transformer 50 transforms the output voltage of induction generator 30. The transformed voltage is transmitted to power grid 90 via transmission line 80 via circuit breaker 70. Transformer 50 also transforms the AC voltage supplied from power grid 90 via transmission line 80. The transformed AC voltage is supplied to converter 12.

[0025] Circuit breaker 60 is connected between induction generator 30 and transformer 50. Circuit breaker 60 cuts off the connection between induction generator 30 and transformer 50 when an abnormality occurs in power generation system 1.

[0026] The circuit breaker 70 is connected between the transformer 50 and the power transmission line 80. The circuit breaker 70 cuts off the connection between the transformer 50 and the power transmission line 80 when an abnormality occurs in the power system 90. In this embodiment, the transformer 14, the transformer 50, the circuit breaker 60, and the circuit breaker 70 do not necessarily have to be installed.

[0027] Next, the operation of the power generation system 1 according to this embodiment configured as described above will be described.

[0028] 3 is a flowchart showing the operation procedure of the power generation system 1 according to the first embodiment. Here, the operation of the control device 20 will be mainly explained.

[0029] First, the operation mode determination unit 21 of the control device 20 determines whether the power generation system 1 is in a phase-modifying operation mode (step S1). In step S1, the operation mode determination unit 21 divides the operation mode of the power generation system 1 into a phase-modifying operation mode and an operation mode other than the phase-modifying operation mode. The phase-modifying operation mode is an operation mode in which only reactive power is output from the induction generator 30 to the power grid 90 under the control of the secondary excitation converter 10. An example of an operation mode other than the phase-modifying operation mode is a power generation operation mode in which the rotational force of the water turbine 40 is converted into electric power by the induction generator 30.

[0030] The operation mode of the power generation system 1 is determined by an operation command 100 output from an external central control command device (not shown) by an operator. This operation command 100 is input to the control device 20. Therefore, in step S1, the operation mode determination unit 21 determines whether or not the operation mode is a phase-modifying operation mode based on the content of the operation command 100.

[0031] 4 is a diagram for explaining the potential fluctuation of the neutral point NP. In FIG. 4, the voltage V across the first capacitor 13a P , the voltage V across the second capacitor 13b N , power supply voltage V PN As shown in Figure 4, the waveform of the power supply voltage V PN is constant, whereas the voltage V P , voltage V N fluctuates with the charging and discharging of each capacitor. This causes the potential of the neutral point NP to fluctuate. The fluctuation in the potential of the neutral point NP becomes larger in the phase-modifying operation mode. Therefore, in this embodiment, first, the operation mode determination unit 21 determines whether the power generation system 1 is in the phase-modifying operation mode.

[0032] If operation mode determination unit 21 determines that the operation mode of power generation system 1 is the phase-modifying operation mode (step S1: YES), control mode setting unit 22 of control device 20 determines whether the system voltage of power system 90 and the slip of induction generator 30 are within allowable ranges (step S2). In step S2, the system voltage and slip values ​​are indicated in operation command 100. Note that control mode setting unit 22 may, for example, obtain the system voltage value from a measurement value of a voltage sensor installed in power system 90. Furthermore, control mode setting unit 22 may, for example, obtain the slip value from a measurement value of a speed sensor installed in induction generator 30, or may obtain the slip value from frequency information used to control inverter 11.

[0033] FIG. 5 is a diagram showing an example of the system voltage and the allowable range of slip. In FIG. 5, the horizontal axis represents the slip of induction generator 30, in other words, the rotational speed of induction generator 30. A slip of 0 represents synchronous speed with the system. Meanwhile, the vertical axis represents the system voltage, i.e., the voltage of power system 90. Region R1 shown in FIG. 3 corresponds to the allowable range. Region R2 outside region R1 is outside the allowable range.

[0034] In the power generation system 1 according to this embodiment, as the absolute value of the slip of the induction generator 30 increases, the potential fluctuation at the neutral point NP increases. Furthermore, in the power generation system 1, the power grid 90 and the induction generator 30 are connected via the transformer 50. Therefore, as the system voltage of the power grid 90 increases, the stator voltage of the induction generator 30 increases. As a result, the potential fluctuation at the neutral point NP increases.

[0035] Therefore, in step S2, if a coordinate point specified by the system voltage value of power system 90 and the slip value of induction generator 30 in the coordinate system shown in Fig. 5 is in region R1, control mode setting unit 22 determines that the system voltage and slip are within the allowable ranges. Conversely, if this coordinate point is in region R2, control mode setting unit 22 determines that the system voltage and slip are outside the allowable ranges. Note that control mode setting unit 22 may make this determination based on a function equation that uses the slip and system voltage values ​​as parameters. Alternatively, control mode setting unit 22 may make this determination using a data table that associates allowable values ​​of slip and system voltage with each other.

[0036] 3, if the operation mode of the power generation system 1 is not the phase-modifying operation mode in step S1 (step S1: NO), i.e., if it is the power generation operation mode, or if the system voltage and slip are within the allowable ranges in step S2 (step S2: YES), the control mode setting unit 22 sets the modulation method of the secondary excitation converter 10 to the first control mode (step S3). Also, if the system voltage and slip are not within the allowable ranges in step S2 (step S2: NO), the control mode setting unit 22 sets the modulation method of the secondary excitation converter 10 to the second control mode (step S4). Here, the first control mode and the second control mode will be described.

[0037] 6(a) shows a modulation method in the first control mode of the first embodiment. In FIG. 6(a), voltage command values ​​of each phase corresponding to AC terminals u1, v1, and w1 of inverter 11, which is a three-phase power converter, are expressed by command values ​​v u ,v v ,v w Each sinusoidal signal is included in the operation command 100 as a signal. The drive control unit 23 of the control device 20 compares each sinusoidal signal with a triangular carrier wave signal to generate a PWM (Pulse Wide Modulation) signal. The generated PWM signal is input to the gate of each semiconductor switching element of the inverter 11. Note that this PWM signal is input to the gate of each semiconductor switching element of the converter 12 as well as to each semiconductor switching element of the inverter 11 in a similar manner.

[0038] When generating the PWM signal, the drive control unit 23 uses the command value v u ,v v ,v wThe zero-phase sequence voltage of the inverter 11 or the converter 12 may be superimposed on each of the voltage command values. Specifically, if there is a voltage command value whose sign changes due to the superimposition of the zero-phase sequence voltage, the drive control unit 23 inverts the sign of the voltage command value and recalculates the zero-phase sequence voltage. Subsequently, the drive control unit 23 superimposes the recalculated zero-phase sequence voltage on the voltage command value of each phase. Even if there is no voltage command value whose sign changes due to the superimposition of the zero-phase sequence voltage, if the denominator of the calculation formula changes sign by crossing 0, the drive control unit 23 inverts the sign of the voltage command value and recalculates the zero-phase sequence voltage. This makes it possible to suppress potential fluctuations at the neutral point NP even in the first control mode.

[0039] FIG. 6(b) shows a modulation method in the second control mode of the first embodiment. In the first control mode described above, one voltage command value is used for each phase. On the other hand, the second control mode, as described in Japanese Patent No. 7002985, uses two command values ​​for each phase: an upper arm command value and a lower arm command value. FIG. 6(b) shows the modulation method in the second control mode of the first embodiment. In the first control mode described above, one voltage command value is used for each phase. On the other hand, as described in Japanese Patent No. 7002985, the second control mode uses two command values ​​for each phase: an upper arm command value and a lower arm command value. up and the command value for the lower arm v un Here, the upper arm semiconductor switching elements of the inverter 11 are three semiconductor switching elements S arranged above the neutral point NP. 11 , S 12 , S 15 On the other hand, the lower arm semiconductor switching elements of the inverter 11 are three semiconductor switching elements S 13 , S 14 , S 16 The upper arm of the converter 12 is made up of three semiconductor switching elements S 21 , S 22 , S 25 On the other hand, the lower arm of the converter 12 is made up of three semiconductor switching elements S 23 , S 24 , S 26 is.

[0040] The drive control unit 23 compares the upper arm command value signal with the upper carrier wave signal to generate gate signals to be input to the gates of the semiconductor switching elements in the upper arm. The drive control unit 23 also compares the lower arm command value signal with the lower carrier wave signal to generate gate signals to be input to the gates of the semiconductor switching elements in the lower arm.

[0041] For example, the upper arm command value signal and the upper carrier wave signal change between 0 and 1, and the lower arm command value signal and the lower carrier wave signal change between -1 and 0. The drive control unit 23 controls the upper arm command values ​​v for three phases. ip , lower arm command value v in (i=u,v,w) is calculated using the following formula (1).

number

[0042] FIG. 7 is a diagram showing an example of the potential fluctuation at the neutral point NP when the secondary excitation converter 10 is modulated in the second control mode. The graph shown in FIG. 7 is a calculated representation of the magnitude of the potential fluctuation at the neutral point NP due to the modulation rate and power factor of a certain NPC converter. As shown in FIG. 7, when the drive control unit 23 modulates the secondary excitation converter 10 in the second control mode, the potential fluctuation at the neutral point NP can be suppressed to almost zero in a certain operating range. The converter modulated in the second control mode may be only the inverter 11, only the converter 12, or both the inverter 11 and the converter 12.

[0043] Therefore, according to this embodiment, it is possible to suppress the potential fluctuation of the neutral point NP and operate the power generation system 1 in a wider operating range.

[0044] In the second control mode, the number of switching operations of the semiconductor switching elements increases compared to the first control mode. This raises concerns about increased switching loss. However, in this embodiment, the second control mode is set when the system voltage and slip are outside the allowable range, i.e., when they are in region R2 shown in FIG. 5. Therefore, the application range of the second control mode is limited to the region where the potential fluctuation of the neutral point NP increases. This makes it possible to suppress the potential fluctuation of the neutral point NP while minimizing the switching loss of each semiconductor switching element.

[0045] (Second embodiment) The second embodiment will be described below. The configuration of the power generation system according to this embodiment is similar to that of the power generation system 1 according to the first embodiment (see FIG. 1), and therefore a description thereof will be omitted.

[0046] The operation of the power generation system according to this embodiment is also performed in the same manner as in the flowchart shown in Fig. 3. That is, when the power generation system is operating in the phase-modified operation mode, the control device 20 sets the first control mode or the second control mode according to the system voltage and the slip of the induction generator 30. However, in this embodiment, the first control mode in step S3 and the second control mode in step S4 are different from those in the first embodiment. Therefore, the two types of control modes of the secondary excitation converter 10 in this embodiment will be described with reference to Figs. 8(a) and 8(b).

[0047] Fig. 8(a) is a diagram showing the supply path of active power in the first control mode of the second embodiment. In the first control mode of the present embodiment, as shown in Fig. 8(a), the converter 12 converts the active power Pc supplied from the power grid 90 into DC power, thereby maintaining the DC voltage supplied to the inverter 11.

[0048] Fig. 8(b) is a diagram showing the output path of reactive power in the second control mode of the second embodiment. In this embodiment, when control mode setting unit 22 of control device 20 sets the second control mode, drive control unit 23 causes converter 12 to bear part of the reactive power indicated by reactive power command value Q for the power generation system, as shown in Fig. 8(b). In other words, drive control unit 23 controls the reactive power output from the power generation system to be the sum of the reactive power output from induction generator 30 by controlling inverter 11 and the reactive power output from converter 12.

[0049] Specifically, the drive control unit 23 calculates the reactive power command value Q s The inverter 11 is controlled to output the reactive power shown in the figure from the induction generator 30, and the reactive power command value Q c The converter 12 is controlled to output the reactive power shown in the figure. Here, the reactive power command values ​​Q and Q s , Q c Between them, Q=Q s +Q c The reactive power command value Q c As described in Japanese Patent No. 7005417, the above can be calculated by determining an active current command value, an active voltage command value, a reactive current command value, a reactive voltage command value, etc. of the converter 12 and performing arithmetic processing on these values.

[0050] In general, the frequency of the output voltage of inverter 11 is lower than the frequency of the input voltage of converter 12. Therefore, inverter 11 has a greater effect on the potential fluctuation of neutral point NP than converter 12. Furthermore, the potential fluctuation of neutral point NP increases as the power factor decreases. In this embodiment, when induction generator 30 outputs reactive power to power grid 90, inverter 11 also outputs reactive power.

[0051] However, in this embodiment, part of the reactive power output from the power generation system is borne by converter 12. This reduces the reactive power of induction generator 30, which in turn reduces the reactive power of inverter 11 and improves the power factor of inverter 11. As a result, it becomes possible to suppress fluctuations in the potential of neutral point NP.

[0052] As described above, reducing the reactive power output value of induction generator 30 (and inverter 11) is effective in reducing the potential fluctuation of neutral point NP. Therefore, drive control unit 23 reduces the reactive power command value Q of converter 12. c The reactive power command value of the induction generator 30 is set to the reactive power command value Q s It is desirable to allocate the reactive power from inverter 11 to inverter 12 with priority over inverter 12. In this case, drive control unit 23 may perform control so that the reactive power obtained by subtracting the maximum reactive power allowable by converter 12 from the reactive power output from the power generation system is output from induction generator 30 (and inverter 11). Note that if converter 12 can provide all of the reactive power output from the power generation system, induction generator 30 does not need to output reactive power. In this case, the reactive power of inverter 11 becomes zero, making it possible to further suppress fluctuations in the potential of neutral point NP.

[0053] In the present embodiment described above, as in the first embodiment, it is possible to suppress the potential fluctuation at the neutral point NP and operate the power generation system in a wider operating range.

[0054] (Third embodiment) The third embodiment will be described below. The configuration of the power generation system according to this embodiment is similar to that of the power generation system 1 according to the first embodiment (see FIG. 1), and therefore a description thereof will be omitted.

[0055] The operation of the power generation system according to this embodiment is also performed in the same manner as in the flowchart shown in Fig. 3. That is, when the power generation system is operating in the phase-modifying operation mode, the control device 20 sets the first control mode or the second control mode according to the system voltage and the slip of the induction generator 30. However, in this embodiment, the contents of the first control mode in step S3 and the second control mode in step S4 are different from those in the first embodiment.

[0056] In the first control mode of this embodiment, the drive control unit 23 of the control device 20 controls the switching operations of the semiconductor switching elements of the inverter 11 and the converter 12 using the PWM signals described in the first embodiment.

[0057] On the other hand, in the second control mode of this embodiment, the drive control unit 23 controls the upper arm command value v ip and the command value for the lower arm v in As described in the second embodiment, the drive control unit 23 controls each semiconductor switching element of the converter 12 using the reactive power command value Q c The converter 12 is controlled to output reactive power using the above formula.

[0058] In the second control mode of this embodiment, the drive control unit 23 controls the upper arm command value v ip and the command value for the lower arm v in However, the upper arm command value v ip and the command value for the lower arm v in Therefore, the driving method using the upper arm command value v ip , lower arm command value v in , and the reactive power command value Q c When the voltage fluctuation at the neutral point NP can be sufficiently suppressed by driving the converter 12 using the ip and the command value for the lower arm v inIt is desirable to apply the driving method using the above to only the converter 12. This makes it possible to minimize the increase in switching loss due to the above driving method.

[0059] According to the present embodiment described above, the modulation method of inverter 11 or converter 12 and the reactive power output method are switched between the first control mode and the second control mode based on the operation mode of the power generation system, the system voltage, and the slip of induction generator 30. This makes it possible to suppress potential fluctuations at neutral point NP and operate the power generation system in a wider operating range, as in the other embodiments described above.

[0060] (Fourth embodiment) The fourth embodiment will be described below. The configuration of the power generation system according to this embodiment is similar to that of the power generation system 1 according to the first embodiment (see FIG. 1), and therefore a description thereof will be omitted.

[0061] The operation of the power generation system according to this embodiment is also performed in the same procedure as in the flowchart shown in Fig. 3. However, in this embodiment, the content of step S2 is different from that in the first embodiment.

[0062] In the first embodiment described above, the control mode setting unit 22 switches the setting of the control mode of the doubly excited field converter 10 based on both the system voltage and the slip of the induction generator 30. However, when comparing the system voltage and the slip, the slip has a greater effect on the potential fluctuation of the neutral point NP. Therefore, in step S2 of this embodiment, the control mode setting unit 22 determines whether the slip of the induction generator 30 is within an allowable range.

[0063] Fig. 9 is a diagram showing an example of the allowable range of slip. In Fig. 6, the horizontal axis represents the slip of induction generator 30, in other words, the rotational speed of induction generator 30. Meanwhile, the vertical axis represents the system voltage, i.e., the output voltage of power system 90. Region R1 shown in Fig. 9 is within the allowable range, and region R2 is outside the allowable range. If the slip is within the range from lower limit Sn to upper limit Sp, it is within the allowable range regardless of the value of the system voltage.

[0064] If the slip is within the allowable range, the control mode setting unit 22 sets the control mode of the secondary excitation converter 10 to the first control mode. Conversely, if the slip is out of the allowable range, the control mode setting unit 22 sets the control mode of the secondary excitation converter 10 to the second control mode. Any of the first to third embodiments described above can be applied to the first and second control modes.

[0065] According to the present embodiment described above, as with the other embodiments described above, it is possible to suppress the potential fluctuation of the neutral point NP and operate the power generation system in a wider operating range. Furthermore, in this embodiment, the control mode setting unit 22 sets the control mode of the doubly excited field converter 10 based only on the slip of the induction generator 30. Therefore, it is possible to reduce the processing load of the control mode setting unit 22 required for setting the control mode.

[0066] Although several embodiments have been described above, these embodiments are presented only as examples and are not intended to limit the scope of the invention. The novel system described in this specification can be embodied in various other forms. Furthermore, various omissions, substitutions, and modifications can be made to the forms of the system described in this specification without departing from the spirit of the invention. The appended claims and their equivalents are intended to cover such forms and modifications that fall within the scope and spirit of the invention. [Explanation of symbols]

[0067] 1: Power generation system 10: Secondary excitation converter (neutral point clamped power converter) 11: Inverter 12: Converter 20: Control device 22: Control mode setting section 23: Drive control unit 30: Induction generator S 11 ~S 16 , S 21~S 26 :Semiconductor switching element

Claims

1. a neutral point clamped power converter including a converter that converts AC power supplied from a power grid into DC power, and an inverter that converts the DC power supplied from the converter into AC power; an induction generator connected to the inverter; a control device that switches a control mode of the power converter between a first control mode and a second control mode in which at least one of a modulation method of the power converter and a method of outputting reactive power to the power grid is different from that of the first control mode, in accordance with at least a slip of the induction generator; A power generation system comprising:

2. the induction generator is a three-phase induction generator, the converter and the inverter include a plurality of semiconductor switching elements provided for each phase of the induction generator, 2. The power generation system according to claim 1, wherein the control device controls the plurality of semiconductor switching elements with one voltage command value in the first control mode, and controls an upper arm semiconductor switching element arranged above a neutral point of the power converter and a lower arm semiconductor switching element arranged below the neutral point among the plurality of semiconductor switching elements with different voltage command values ​​in the second control mode.

3. 3. The power generation system according to claim 1, wherein the control device causes the converter to convert active power supplied from the power grid into DC power to be supplied to the inverter in the first control mode, and causes the converter to output reactive power in the second control mode.

4. 2. The power generation system of claim 1, wherein the controller controls the power converter in the first control mode when the slip is within an acceptable range, and the controller controls the power converter in the second control mode when the slip is outside the acceptable range.

5. 2. The power generation system according to claim 1, wherein the control device controls the power converter in the first control mode when a grid voltage and the slip of the power grid are within an acceptable range, and the control device controls the power converter in the second control mode when the grid voltage and the slip are outside the acceptable range.

6. 2. The power generation system according to claim 1, wherein when an operation mode of the power generation system is a phase-modifying operation in which reactive power is output to the power grid, the control device switches the control mode of the power converter between the first control mode and the second control mode.

7. The power generation system according to claim 2 , wherein the control device superimposes a zero-phase sequence voltage of the power converter on the voltage command value in the first control mode.

8. 3. The power generation system according to claim 2, wherein the control device calculates the power command value for the upper arm semiconductor switching element based on a minimum value of the voltage command values ​​for each phase of the power converter, and calculates the power command value for the lower arm semiconductor switching element based on a maximum value of the voltage command values ​​for each phase of the power converter.

9. 4. The power generation system according to claim 3, wherein the control device controls the reactive power output to the power grid in the second control mode to be a sum of reactive power output from the induction generator by controlling the inverter and reactive power output from the converter.

10. The power generation system according to claim 9 , wherein the control device allocates the reactive power output from the converter with priority over the reactive power output from the induction generator.

11. A control device provided in a power generation system including a neutral point clamped power converter having a converter that converts AC power supplied from an electric power system into DC power and an inverter that converts the DC power supplied from the converter into AC power, and an induction generator connected to the inverter, a control mode setting unit that switches a control mode of the power converter between a first control mode and a second control mode in which at least one of a modulation method of the power converter and a method of outputting reactive power to the power grid is different from the first control mode, in accordance with at least a slip of the induction generator; a drive control unit that controls the power converter in the first control mode or the second control mode set by the control mode setting unit; A control device comprising:

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