Power generation system

Abstract

To suppress an inrush current, which is generated when an induction generator is used, and secure a power source, which is required when starting operation of the induction generator, without reducing a power factor of a power system.SOLUTION: In a power generation system connected to a power system via a rectifier and an inverter using an induction generator, a voltage applied to the induction generator is set to zero in the beginning and made rise gradually to a rating voltage thereafter. A control power source which is supplied from a system control panel is stepped up and full-wave rectified and an electrolytic capacitor C is charged with the DC power, thereby preparing a DC voltage which is required when starting operation of an induction generator 1. Further, a shaft voltage of the induction generator 1 is controlled by a common mode filter, such that a bearing is prevented from being damaged by discharge.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a power generation system using an induction generator that is connected to a power grid. [Background technology]

[0002] In recent years, power generation projects using renewable energy have become important as a measure against global warming. Demand for these power generation projects has increased dramatically since the implementation of the Feed-in Tariff (FIT) system in 2012.

[0003] Generators are used in power generation systems that utilize renewable energy sources such as wind, hydroelectric, and biomass power generation. The simplest configuration is to directly connect an AC generator to the power grid, as shown in Figure 3. This method is widely used in 6.6kV high-voltage interconnection facilities.

[0004] On the other hand, relatively small-scale electricity sellers choose low-voltage interconnection for their power generation systems. Low-voltage interconnection handles low voltages such as 200V, and has many advantages over high-voltage interconnection in terms of lower initial costs and easier procedures and maintenance.

[0005] However, in Japan, when connecting at low voltage, it is prohibited to directly connect the output of an AC generator to the power grid. This is because AC generators generally have a slow response time, making it difficult to respond when a system failure occurs and a rapid shutdown or cutoff is required.

[0006] Therefore, a system has been adopted in which an inverter (reverse conversion device) is combined with an AC generator to connect to the grid, as shown in Figure 4.

[0007] The inverter uses semiconductor valve devices, which allows it to be stopped or cut off quickly. A power conditioner is a device that adds various functions, such as a parallel-off switch and protective relay required for grid connection, to the inverter's functions.

[0008] In the power generation system shown in Figure 4, AC power generated by an AC generator is first converted to DC power by a converter, then converted back to AC power by a power conditioner, and then the power is reverse-flowed to the connected power grid.

[0009] AC generators are broadly divided into two types: synchronous and induction. The general equipment configuration of a synchronous generator is shown in Fig. 5. Synchronous generators have the advantage that the output voltage can be controlled externally, allowing power factor adjustment when connected to the grid, and that they can operate independently without being connected to the grid.

[0010] On the other hand, to control the output voltage in this way, it is necessary to supply a controlled DC current from outside to the rotor field winding, which requires a DC power supply and AVR (automatic voltage regulator) for the field.

[0011] In addition, generators that use slip rings and brushes require periodic part replacement, and the number of parts makes the system complicated.

[0012] Induction generators have a simpler structure than synchronous generators, and are characterized by their small size and robustness. Another advantage is that they do not require a field magnet device like synchronous generators, which simplifies the system.

[0013] Furthermore, when a squirrel-cage rotor is used, there are no slip rings or brushes, which not only simplifies the structure and makes it less expensive, but also eliminates the need for periodic part replacement.

[0014] As a publicly known document relating to a power generation system using an induction generator, for example, Patent Document 1 listed below can be mentioned. [Prior art documents] [Patent documents]

[0015] [Patent Document 1] Patent Publication No. 2006-87221 Summary of the Invention [Problem to be solved by the invention]

[0016] When an induction generator is used in a power generation system, it is necessary to supply excitation current from a power source (electrical power system) connected to the output terminals of the generator, which can lead to problems such as a decrease in the power factor of the power source (electrical power system) and the inability to operate independently.

[0017] Furthermore, when the generator is connected to a power source (electric power system), an inrush current several times the rated current occurs between the generator and the rectifier.

[0018] Possible measures to suppress inrush current include inserting a current-limiting reactor between the converter and the induction generator, or increasing the apparent capacity of the converter, but both of these methods result in larger equipment and higher costs.

[0019] Therefore, the present invention provides a power generation system that can ensure a power supply when an induction generator starts operating without reducing the power factor of the power system, and can reliably suppress the inrush current that flows when the induction generator is turned on, without increasing the size or cost of the equipment. [Means for solving the problem]

[0020] The invention described in claim 1 is a power generation system that uses an induction generator and connects to a power grid via a converter and an inverter, characterized in that the voltage applied to the induction generator is initially set to zero and then gradually increased to the rated voltage.

[0021] The invention described in claim 2 is characterized in that the control power supply is boosted and full-wave rectified, and the DC power is charged into an electrolytic capacitor, thereby preparing the DC voltage required when the induction generator described in claim 1 starts operating.

[0022] The invention as recited in claim 3 is characterized in that the shaft voltage of the induction generator as recited in claim 1 or claim 2 is suppressed by a common mode filter. Effect of the Invention

[0023] According to the invention of claim 1, the current when an AC voltage is applied to the induction generator can be suppressed to within the rated current.

[0024] According to the invention of claim 2, the DC voltage required to be supplied to the induction generator is supplied from the control power supply, thereby making it possible to prevent a drop in the power factor of the power supply (electric power system).

[0025] According to the invention as recited in claim 3, it is possible to prevent the shaft voltage of the induction generator from becoming excessive, and to prevent the bearings of the induction generator from being damaged by discharge. [Brief description of the drawings]

[0026] [Figure 1] 1 is a system configuration diagram of a power generation system according to the present invention. [Diagram 2] 2 is an equivalent circuit for the shaft voltage of the induction generator according to the present invention. [Diagram 3] FIG. 1 is a configuration diagram of a power generation system in which an AC generator is directly connected to a power grid. [Figure 4] FIG. 1 is a configuration diagram of a power generation system in which an AC generator is connected to a power grid via an inverter. [Diagram 5] FIG. 1 is a diagram showing a typical equipment configuration of a synchronous generator. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] Hereinafter, an embodiment of the present invention will be described with reference to Figures 1 and 2. Figure 1 is a configuration diagram of a power generation system A of the present invention, in which 1 is, for example, a three-phase AC 200V, 50Hz or 60Hz induction generator.

[0028] Reference numeral 2 denotes a converter for induction generators, with induction generator 1 connected to its AC input terminal, which converts the AC power generated by induction generator 1 into DC power and outputs it. Reference numeral 3 denotes a power conditioner, which is connected to the DC output terminal of converter 2, which converts the DC power output by converter 2 back into AC power and allows it to flow in reverse to power grid 4, which is connected to it.

[0029] Next, a description will be given of the configuration of the converter 2, which is a characteristic part of the present invention. The converter 2 according to the present invention is generally composed of circuit breakers MCCB1 and MCCB2, a switch MC1, a reactor L, a semiconductor valve device 5, an electrolytic capacitor C, a control unit 6 that drives these components, and an initial charging circuit 7.

[0030] The circuit breakers MCCB1 and MCCB2 open the electric circuit when an abnormal overcurrent flows in the secondary circuit due to an overload, short circuit or other cause, and cut off the power supply from the primary side to protect the secondary circuit and wires from damage.

[0031] Switch MC1 is a control device that starts / stops induction generator 1 by opening and closing the current using control unit 6, and reactor L suppresses electrical stress applied to induction generator 1, such as a surge voltage.

[0032] An IGBT (Insulated Gate Bipolar Transistor) is used as a power semiconductor constituting semiconductor device 5. Electrolytic capacitor C is charged with a DC voltage for starting induction generator 1 by initial charging circuit 7.

[0033] The control unit 6 receives an operation permission signal from a system control panel (not shown) to close / open the switch MC1 and to turn on / off the power semiconductor of the semiconductor valve device 5. It also has a function of outputting a closing command to the initial charging circuit 7 and outputting the state of the converter 2 to the system control panel.

[0034] The initial charging circuit 7 is composed of a switch MC2, a step-up transformer Tr, a rectifier 8, and a resistor R, and is supplied with a control power supply (for example, single-phase AC 200V) from a system control panel (not shown) via a circuit breaker MCCB2.

[0035] In the power generation system A configured as described above, the induction generator 1 constituting the power generation system A will not output power unless an AC voltage is applied. Therefore, in the present invention, a closing command is output from the control unit 6 to the switch MC2 of the initial charging circuit 7, thereby closing the switch MC2, and the control power from the system control panel is boosted by the transformer Tr of the initial charging circuit 7 and full-wave rectified by the rectifier 8, thereby charging the electrolytic capacitor C with the DC power.

[0036] The DC voltage obtained by this charging is converted by semiconductor valve device 5 with high frequency PWM (Pulse Width Modulation) and applied to induction generator 1. This allows induction generator 1 to start operating.

[0037] That is, in the present invention, since there is no need to supply an excitation current from the power system 4 to start up the induction generator 1, the power factor of the power system 4 does not decrease.

[0038] Furthermore, when applying an AC voltage to induction generator 1 as described above, the present invention uses a method in which, when connecting induction generator 1 and converter 2, the output voltage of converter 2 is initially set to zero, and after connection, the output voltage is gradually boosted to the rated voltage.

[0039] This makes it possible to suppress the inrush current that flows when induction generator 1 is powered on (when switch MC1 is closed) to within the rated current.

[0040] In this way, inrush current can be suppressed by software, and unlike when measures are taken using hardware, there is no increase in the number of parts or the complexity of the circuitry, and it is possible to prevent the device from becoming larger and more expensive.

[0041] In the present invention, IGBTs are connected in a three-phase full bridge configuration as power semiconductors constituting semiconductor valve device 5, and as described above, a voltage obtained by converting a DC voltage with high frequency PWM is applied to induction generator 1. In this case, a common mode voltage is applied to the stator windings of induction generator 1.

[0042] Since the PWM inverter switches at a high frequency, the common mode voltage is also at a high frequency. The shaft of the induction generator 1 is electrically coupled to the stator winding via floating capacitance, so a voltage is generated on the shaft relative to the ground (hereinafter referred to as the shaft-to-ground voltage).

[0043] Figure 2 shows an equivalent circuit for the shaft voltage. If the shaft voltage becomes excessive, the bearings will corrode electrically, leading to failure of the induction generator 1. The guideline for the magnitude of the shaft-to-ground voltage is a peak value of 1.0 V or less. A common mode filter is effective in reducing the shaft-to-ground voltage.

[0044] From the equivalent circuit shown in Figure 2, the shaft voltage can be roughly calculated using the following [Equation 1].

[0045]

number

[0046] where Vb is the shaft voltage peak value, Vc is the common-mode voltage peak value, X1 is the reactance between the stator winding and the stator core, X2 is the reactance between the stator core and the shaft, X3 is the reactance between the shaft and the stator, R1 is the resistance between the stator core and the ground, and Xc is the reactance between the converter and the ground.

[0047] By designing a common mode filter using the above [Equation 1] and keeping the peak value of the shaft-to-ground voltage at or below 1.0 V, it is possible to suppress electrolytic corrosion of the bearings of the induction generator 1.

[0048] As described above, the power generation system according to the present invention can suppress the inrush current that occurs when an induction generator is used, and can ensure the power supply required when the induction generator starts operating without reducing the power factor of the power grid. [Industrial Applicability]

[0049] This can be applied to power generation systems that use renewable energy. [Explanation of symbols]

[0050] 1. Induction Generator 2. Converter 3. Reverse conversion device (power conditioner) 4 Power system 5. Semiconductor valve devices 6. Control Unit 7 Initial charging circuit 8 rectifier MCCB1, MCCB2 circuit breaker MC1,MC2 switch L Reactor C Electrolytic capacitor R resistance A Power Generation System

Claims

1. A power generation system that uses an induction generator and is connected to a power grid via a converter and an inverter, characterized in that the voltage applied to the induction generator is initially set to zero and then gradually increased to a rated voltage.

2. 2. The power generation system according to claim 1, wherein the induction generator prepares a required DC voltage at the start of operation by boosting and full-wave rectifying a control power supply and charging an electrolytic capacitor with the DC power.

3. 3. The power generation system according to claim 1, wherein the shaft voltage of the induction generator is suppressed by a common mode filter.

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