This invention relates to a thyristor starter, and particularly to a thyristor starter starting a synchronous machine.
A thyristor starter includes a converter converting three-phase AC power to DC power, a DC reactor smoothing DC power, and an inverter converting DC power provided from the converter through the DC reactor to three-phase AC power at a desired frequency and providing the three-phase AC power to a synchronous machine. By controlling three-phase AC power provided to the synchronous machine, the synchronous machine which has been stopped can be started and rotationally driven at a prescribed rotation speed.
A method of controlling an inverter includes a first method of setting a phase control angle to a constant value and a second method of increasing a phase control angle from a minimum value to a maximum value at a constant rate of increase in accordance with a rotation speed of a synchronous machine (see, for example, Japanese Patent Laying-Open No. 2003-61380 (PTD 1)).
With the conventional first method, however, with increase in rotation speed of the synchronous machine, a DC voltage which appears on a side of an input terminal of the inverter disadvantageously increases. Since the converter should supply a DC voltage higher than a DC voltage which appears on the side of the input terminal of the inverter, the first method requires a large converter.
With the conventional second method, when a rotation speed of the synchronous machine attains to a certain value, a DC voltage which appears on the side of the input terminal of the inverter attains to a peak value. In this case, a converter having a capacity allowing supply of DC power at the time when the DC voltage attains to the peak value should be mounted and output of the converter should be lowered during a period in which a DC voltage does not attain to the peak value, which results in lowering in efficiency in use of the converter.
Therefore, a primary object of this invention is to provide a thyristor starter capable of achieving reduction in size and improvement in efficiency in use of a converter.
A thyristor starter according to this invention is a thyristor starter for starting a synchronous machine, including a converter converting first AC power to DC power, an inverter converting the DC power to second AC power at a variable frequency and supplying the second AC power to the synchronous machine, a speed operation portion finding a rotation speed of the synchronous machine, a control angle operation portion including a function or a table showing relation between the rotation speed of the synchronous machine and a phase control angle of the inverter and finding a phase control angle having a value in accordance with the rotation speed of the synchronous machine found by the speed operation portion, and a control unit controlling the inverter based on the phase control angle found by the control angle operation portion. Here, the phase control angle varies from a minimum value to a maximum value in accordance with the rotation speed of the synchronous machine, and a rate of increase in phase control angle relative to the rotation speed of the synchronous machine is varied in a plurality of steps in accordance with the rotation speed of the synchronous machine.
Preferably, the rate of increase in phase control angle relative to the rotation speed of the synchronous machine is varied in the plurality of steps in accordance with the rotation speed of the synchronous machine such that a DC voltage which appears at an input terminal of the inverter is constant in spite of variation in rotation speed of the synchronous machine.
Further preferably, the rate of increase in phase control angle relative to the rotation speed of the synchronous machine decreases in a plurality of steps in accordance with the rotation speed of the synchronous machine.
Further preferably, a voltage detector detecting a three-phase AC voltage output from the synchronous machine is included, and the speed operation portion finds a rotation speed of the synchronous machine based on the three-phase AC voltage detected by the voltage detector.
Further preferably, a position detector detecting a position of a rotor of the synchronous machine is included, and the speed operation portion finds a rotation speed of the synchronous machine based on a result of detection by the position detector.
Further preferably, the synchronous machine is a generator of a thermal power station.
In the thyristor starter according to this invention, a phase control angle of the inverter varies from a minimum value to a maximum value in accordance with a rotation speed of the synchronous machine, and a rate of increase in phase control angle relative to the rotation speed of the synchronous machine is varied in a plurality of steps in accordance with the rotation speed of the synchronous machine. Therefore, variation in DC voltage which appears on the side of the input terminal of the inverter can be suppressed to be less, and reduction in size and improvement in efficiency in use of the converter can be achieved.
In a thermal power station, a generator implemented by a synchronous machine is coupled to a gas turbine, and the generator is rotationally driven by the gas turbine. A thyristor starter is an apparatus starting the generator and the gas turbine which have been stopped, by rotating the generator which has been stopped to a prescribed rotation speed from which the gas turbine can speed up in a self-sustained manner. The thyristor starter is generally called a load commutated inverter (LCI).
A thyristor starter according to one embodiment of the present invention includes a transformer 1, a converter 2, a DC reactor 3, and an inverter 4, as shown in
Converter 2 is a three-phase full-wave rectification circuit including at least six thyristors and converts three-phase AC power from transformer 1 to DC power having a variable voltage. DC reactor 3 is connected between a positive output terminal 2a of converter 2 and a positive input terminal 4a of inverter 4 and smoothes a DC current. A negative output terminal 2b of converter 2 and a negative input terminal 4b of inverter 4 are connected to each other. Another DC reactor 3 may be connected between negative output terminal 2b of converter 2 and negative input terminal 4b of inverter 4.
Three output terminals 4c to 4e of inverter 4 are connected to an R-phase terminal 21a, an S-phase terminal 21b, and a T-phase terminal 21c of a synchronous machine 21, respectively. Inverter 4 is a three-phase full-wave rectification circuit including at least six thyristors and converts DC power supplied from converter 2 through DC reactor 3 to three-phase AC power having a variable frequency and a variable voltage, and provides the three-phase AC power to a stator of synchronous machine (generator) 21.
This thyristor starter includes voltage detectors 5 and 7, a current detector 6, a speed operation portion 8, a control angle operation portion 9, and a control unit 10. Voltage detector 5 detects an instantaneous value of a three-phase AC voltage supplied from AC power supply 20 and provides a signal indicating a detection value to control unit 10. Current detector 6 detects a current which flows from transformer 1 to converter 2 and provides a signal indicating a detection value to control unit 10. Voltage detector 7 detects instantaneous values of three-phase AC voltages VR, VS, and VT which appear at respective R-phase terminal 21a, S-phase terminal 21b, and T-phase terminal 21c of synchronous machine 21, and provides signals indicating detection values to control unit 10 and speed operation portion 8. Speed operation portion 8 operates a rotation speed N (rpm) of synchronous machine 21 based on a signal from voltage detector 7 and provides a signal indicating an operation value to control angle operation portion 9 and control unit 10. A position detector detecting a position of a rotor of synchronous machine 21 may be provided instead of voltage detector 7 and a rotation speed N (rpm) of synchronous machine 21 may be operated by speed operation portion 8 based on a result of detection by the position detector.
Control angle operation portion 9 stores a function (or a table) showing relation between rotation speed N of synchronous machine 21 and a phase control angle γ of inverter 4, finds phase control angle γ having a value in accordance with rotation speed N based on the function (or the table) and on a signal indicating rotation speed N provided from speed operation portion 8, and provides a signal indicating phase control angle γ to control unit 10.
According to the thyristor starter in the present application, with increase in rotation speed N, phase control angle γ continuously increases from a minimum value to a maximum value and a rate of change Δγ/ΔN in phase control angle γ varies two or more times. Thus, reduction in size and improvement in efficiency in use of converter 2 can be achieved, which will be described in detail later.
Control unit 10 controls converter 2 based on signals from voltage detector 5, current detector 6, and speed operation portion 8. Control unit 10 controls inverter 4 based on signals from voltage detector 7 (or the position detector above), speed operation portion 8, and control angle operation portion 9.
A method of controlling phase control angle γ characterizing this thyristor starter will now be described in detail. At the time of start, as shown in
During a period until synchronous machine 21 is rotated at a prescribed rotation speed Na from a completely stopped state, phase control angle γ is maintained at a sufficiently small constant value regardless of rotation speed N, and effective value VE of the terminal voltage of synchronous machine 21 increases linearly from 0 V to rated voltage VC.
As synchronous machine 21 is rotationally driven, three-phase AC voltages VR, VS, and VT appear at R-phase terminal 21a, S-phase terminal 21b, and T-phase terminal 21c of synchronous machine 21, respectively. These three-phase AC voltages VR, VS, and VT are converted by inverter 4 to DC voltage VDC2 and appear across input terminals 4a and 4b of inverter 4. Namely, as shown in
Thyristors U, V, and W have anodes all connected to positive input terminal 4a and cathodes connected to output terminals 4c to 4e, respectively. Thyristors X, Y, and Z have anodes connected to output terminals 4c to 4e, respectively, and cathodes all connected to negative input terminal 4b.
In synchronization with three-phase AC voltages VR, VS, and VT, one thyristor of thyristors U, V, and W and one thyristor of thyristors X, Y, and Z are rendered conducting, so that three-phase AC power can be supplied to synchronous machine 21 and rotation speed N of synchronous machine 21 can be increased.
For example, as shown in
In
With transition of conducting thyristors, line voltages VR-VS, VS-VT, and VT-VR of synchronous machine 21 successively appear as DC voltage VDC2 across input terminals 4a and 4b of inverter 4. Control unit 10 controls a path for a current which flows through synchronous machine 21, by igniting two of six thyristors U, V, W, X, Y, and Z in an orderly sequence with rotation of synchronous machine 21, and achieves synchronization with rotation of synchronous machine 21.
In an ideal case where an inductance of synchronous machine 21 is ignorably low, three-phase line voltages VR-VS, VS-VT, and VT-VR of synchronous machine 21 continue to successively appear across input terminals 4a and 4b of inverter 4 as shown in
Actually, however, arc suppression of a conducting thyristor does not instantaneously occur. Owing to an inductance component in synchronous machine 21 or the like, for a finite period, there is a period during which both of a thyristor which has been conducting until just before and a thyristor which will be conducting are rendered conductive. This period is called a commutation overlap period, and an angle corresponding to the commutation overlap period is called a commutation overlap angle u.
In order to address this, a method of linearly increasing phase control angle γ at a constant rate of increase Δγ/ΔN from a minimum value to a maximum value with increase in rotation speed N is possible, which is referred to as a γ linear control scheme here.
Though description has also been given with reference to
While rotation speed N is between 0 and Na, DC voltage VDC2 linearly increases. While rotation speed N is between Na and Nb, DC voltage VDC2 has a peak value at a certain rotation speed Np. When rotation speed N exceeds Nb, DC voltage VDC2 increases in accordance with rotation speed N.
As shown in
Then, according to the invention of the subject application, rate of increase Δγ/ΔN in phase control angle γ is varied two or more times in accordance with rotation speed N such that DC voltage VDC2 is close to a constant value. Here, this method is referred to as a γ variable control scheme. In
While rotation speed N is between 0 and Na, DC voltage VDC2 linearly increases. While rotation speed N is between Na and Nm, DC voltage VDC2 has a peak value at a certain rotation speed Np1. While rotation speed N is between Nm and Nb, DC voltage VDC2 has a peak value at a certain rotation speed Np2. When rotation speed N exceeds Nb, DC voltage VDC2 increases in accordance with rotation speed N.
Based on comparison between the γ linear control scheme in
Since rate of increase Δγ/ΔN in phase control angle γ is varied a plurality of times in accordance with rotation speed N in this embodiment, DC voltage VDC2 can be close to a constant value. Therefore, reduction in size and improvement in efficiency in use of converter 2 can be achieved.
Though a case that synchronous machine 21 is a generator rotationally driven by a gas turbine in a thermal power station has been described in this embodiment, the embodiment is not limited thereto and synchronous machine 21 may be a synchronous machine used in a general industrial field. For example, synchronous machine 21 may be a synchronous machine for a cooling blower in ironworks.
It should be understood that the embodiment disclosed herein is illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.
1 transformer; 2 converter; 3 DC reactor; 4 inverter; 5, 7 voltage detector; 6 current detector; 8 speed operation portion; 9 control angle operation portion; 10 control unit; 20 AC power supply; and 21 synchronous machine.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP2012/071792 | 8/29/2012 | WO | 00 | 2/19/2015 |