Control circuit for an invertor operable in an individual mode and an interconnection mode

Abstract: A control circuit for invertor, which is capable of compensating an instantaneous variation of the power system without decreasing of the stability of the control circuit upon the synchronous-interconnection mode being in advance of interconnection mode. The control circuit for an invertor converting DC voltage to AC voltage operable in an individual mode and an interconnection mode, includes a synchronous compensating circuit in which when the invertor is operating in a power system synchronous operation mode under control of the synchronous-interconnection control system, a quantity of an instantaneous voltage variation occurring in the power system is separated into a quantity of the variation of the component in same phase with a reference voltage vector of the output voltage of the invertor and a quantity of the variation of the component orthogonal to the reference voltage vector in order to detect respectively, and the detected variation quantities are respectively added to the outputs of a voltage adjuster and a phase adjuster in the synchronous-interconnection control system.

Description: BACKGROUND OF THE INVENTION The present invention relates to a control circuit for a voltage invertor operable in an individual mode in which the invertor converts a DC voltage from a DC source, such as a fuel cell, a solar cell, and a battery, to an AC voltage, and individually transfers the converted AC voltage to an AC load, and in an interconnection mode in which the invertor power system is interconnected with or incorporated into a power system, the converted AC voltage, together with the power voltage of the power system, is transferred to the load. More particularly, the present invention relates to a control circuit for the voltage invertor, which includes a compensating means for compensating an instantaneous variation of the power system voltage. The control circuit for the voltage invertor operable in the individual and interconnection modes, comprises a constant voltage control system provided for use in the individual mode, and an effective power control system and a reactive power control system both provided for use in the interconnection mode. Generally, the invertor control circuit is provided with a synchronous-interconnection control system including phase and voltage (amplitude) adjusters, or an adjuster for adjusting those items of the output voltage. The synchronous-interconnection control system is used for a power system synchronous operation, which is performed when the invertor changes its operation mode from the individual mode to the interconnection mode. The synchronous-interconnection control system, including the phase and voltage (amplitude) adjusters, stops those adjusters after the invertor power system is interconnected with the power system, and operates an effective power adjuster of the effective power control system and a reactive power adjuster of the reactive power control system. In the interconnection mode, the synchronous-interconnection control system and the effective and reactive power control systems are selectively changed over before and after the invertor power system is interconnected with the power system. Fig. 2 is a block diagram showing a general circuit arrangement of the control circuit for voltage invertor operable in the individual and interconnection modes. In Fig. 2, reference numeral 1 designates a self-exciting voltage invertor; 2, an output reactor of the invertor 1; 3, a power system with which the invertor power system is interconnected; 4, a load; 5 a breaker for the interconnection mode; 6, a load breaker; 7, a voltage detect transformer (PT); and 8 a current detect transformer (CT). An individual operation control system 10 includes a voltage adjuster 10B for adjusting a detection voltage Vid detected by a voltage detector 9 to approach to a value set by a voltage setter 10A. The control system applies the output voltage of the voltage adjuster 10B through a selector 41 to a pulse generation-amplifier (PA) 30, which is provided for applying a striking pulse to the invertor 1. The same applies a fixed frequency signal generated by an oscillator 11 through another selector 45 to the pulse generation-amplifier PA. Consequently, the invertor 1 transfers an AC power of which the reference frequency is the fixed frequency of the oscillator 11, to the load 4 through the load breaker 6. An interconnected power system in which the power system of the invertor 1 is interconnected, for AC power transmission, with the power system 3, through the breaker 5, comprises an effective power control system 12 including a effective power adjuster 12B, and a reactive power control system 15 including a reactive power adjuster 15B. An effective-reactive power detector 14 receives and separates the AC power into an effective component P and a reactive component Q. The effective power adjuster 12B controls the output voltage of the invertor so as to make the detected effective component P approach to a set value set by an effective power setter 12A. The reactive power control system 15 controls the output voltage of the invertor so as to make the detected reactive component Q approach to a set value set by a reactive power setter 15A. A current/voltage (I/V) converter 31 converts the outputs of each control system into voltages corresponding to the output voltage of the invertor 1. The output voltages of the converter 31 are applied respectively through the selectors 41 and 42, to the pulse generation-amplifier (PA) 30. In this way, the invertor 1 is placed to the interconnection mode. The selector 45 is turned to a contact 45B so that the reference output frequency of the invertor 1 is controlled so as to be equal to the frequency of the power system 3. A synchronous-interconnection control circuit 20, which is operated when the operation mode of the invertor is changed from the individual mode to the interconnection mode, is made up of the voltage detector 9, the voltage setter 10A, and the voltage adjuster 10B, which are common used with the individual control system 10. The output control signal of the voltage setter 10A is supplied through the contact 41B to the pulse generation-amplifier 30. A phase difference  between the invertor output voltage and the power system voltage is detected by a phase difference detector 21. A phase-difference adjuster 20B, contained in the synchronous-interconnection control circuit, controls the reactive component of the invertor 1 so as to make the phase difference  approach to a value that is set by a phase-difference setter 20A (normally, it is set at 0). The output signal of the phase-difference adjuster 20B is supplied to the pulse generation-amplifier 30, through the contact 42B of the selector 42. In the individual mode, the breaker 5 is opened, and the respective selectors are turned to the contacts 41B, 42B, and 45A. Under this condition, an AC power of which, the voltage is controlled by the voltage adjuster 10B and the frequency is controlled by the oscillator 11, is supplied through the load breaker 6 to the load. The synchronous-interconnection control is performed in a manner that the selector 45 is turned to the contact 45B, the amplitude (instantaneous value) and the phase of the output voltage of the invertor 1 are made to approach to those values of the voltage of the power system 3, by the synchronous-interconnection control circuit 20, and thereafter, the breaker 5 is closed. The interconnection power system operation is performed by activating the effective power control system 12 and the reactive power control system 15 and turning the selectors to the contacts 41A and 42A. Most of the interconnected power systems in which the invertor 1, which converts a DC voltage from a DC source, such as a fuel cell, a solar cell, and a battery, to an AC voltage, is operated while being interconnected with or incorporated into the power system, have each a relatively small power capacity. Because of this fact, an instantaneous variation of the voltage vector of the power system is large when the load is connected to and disconnected from the power system. The control circuit of the invertor, containing a proportional integration adjuster (PI adjuster) as a main component, is based on the fixed value control technique which operates to keep a set value at a fixed value against incoming disturbance. In the invertor control circuit, if, to improve the follow-up performance, the proportional compensation quantity is excessively increased by increasing the feedback quantity of the PI adjuster, the fixed value control becomes instable, so that the output of the invertor constantly varies. In a prior art (DE-A-32 36 071) control circuit as indicated in the precharacterizing part of claim 1 uses as detecting device in connection with the compensating circuit a vector analyzer, a three-phases or two-phases converter for sensing the output of the voltage invertor in order to control response to the transient variation of the power system. It is the object of the invention to provide a control circuit for invertor, which is capable of compensating an instantaneous variation of the power system without decreasing of the stability of the control circuit upon the synchronous-interconnection mode being in advance of interconnection mode. To solve the above-mentioned problems, the present invention provides a control circuit for an invertor operable in an individual mode and an interconnection mode, having an individual operation control system in which a control system for a voltage invertor for converting DC voltage to AC voltage, controls AC voltage in a constant-voltage and individually supplies the converted AC voltage to a load, a synchronous-interconnection control system for controlling the amplitude and phase of the output voltage of the invertor for synchronization them with those in a power system to incorporate the invertor into the power system; and effective and reactive power control systems for controlling the effective component and the reactive component of an electric power transferred between the power system and the invertor interconnected with the power system, thereby to perform the interconnection mode operation; the control circuit serving also as a voltage adjuster for making a constant-voltage control or a synchronous control of the AC voltage, wherein the control circuit comprises a synchronous compensating circuit in which when the invertor is operating in a power system synchronous operation mode under control of the synchronous-interconnection control system, a quantity of an instantaneous voltage variation occurring in the power system is separated into a quantity of the variation of the component in same phase with a reference voltage vector of the output voltage of the invertor and a quantity of the variation of the component orthogonal to the reference voltage vector in order to detect respectively, and the detected variation quantities are respectively added to the outputs of a voltage adjuster and a phase adjuster in the synchronous-interconnection control system. For the interconnection mode operation, the control circuit for the invertor comprises an interconnection mode compensating circuit in which the output voltage of the invertor, which is operating in the interconnection mode under control of the effective power control system and the reactive power control system, is stored as an internal vector, a quantity of an instantaneous variation in the voltage of the power system is detected in terms of a quantity of a variation of the component in phase with the internal vector and a quantity of a variation of the component orthogonal to the internal vector, and the detected variation quantities are respectively added to the voltage-converted outputs of the effective power control system and the reactive power control system. The control circuit for the invertor may comprise a synchronous-interconnection mode compensating circuit in which the output voltage of the invertor, which is operating in the power system synchronous-interconnection mode or the interconnection mode, is stored as an internal vector, a quantity of an instantaneous variation in the voltage of the power system is detected in terms of a quantity of a variation of the component in phase with the internal vector and a quantity of a variation of the component orthogonal to the internal vector, and the detected variation quantities are respectively added to the outputs of the effective power control system and the reactive power control system or the outputs of the voltage adjuster and the phase-difference adjuster in the synchronous interconnection control system. The compensating circuit thus arranged controls the output voltages of both the control systems according to an instantaneous variation in the power system voltage. That is, the control circuit can control, in accordance with an instantaneous variation of the power system voltage, both the invertor output voltage at the time of setting up the synchronous operation mode, and the invertor output voltage at the time of setting up the interconnection mode. Accordingly, a stable fixed-value control can be secured while lessening the control processing loads of both the control systems. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram showing a control circuit according to an embodiment of the invention; and Fig. 2 is a block diagram showing a general circuit arrangement of a conventional control circuit for voltage invertor operation in the individual/interconnection mode. In Fig . 1 like reference symbols are used for designating like or equivalent portions in the drawing showing the conventional circuit, for simplicity. It is assumed that the load breaker 6 is open, the selectors 41, 42, and 45 are turned to the B contacts, and the power system synchronous operation is being performed under control of the voltage adjuster 10B and the phase-difference adjuster 20 B, which serve as the PI adjuster in the synchronous-interconnection control system. In Fig. 1 a synchronous-interconnection mode compensating circuit 70 includes a vector analyzer 71, a subtractor 72, a 3-phase/2-phase converter 61, a power system synchronous signal generator 62, and a vector rotator 63. In the control circuit thus arranged, the output voltage of the vector analyzer 71 is subtracted from the in-phase component detected by the vector rotator 63. A variation in the power system voltage is detected in terms of a quantity of a variation of the in-phase component and a quantity of a variation of the orthogonal component. Only the variation quantities of the in-phase component and the orthogonal component are added to the output voltage of the control circuit at the input of the pulse generating-amplifier 30. The compensating circuit is applicable for both the synchronous-interconnection operation mode and the interconnection mode. In the interconnection mode, the output voltage Vid of the voltage detector 9 is compensated by an adder 73 provided at the output of the I/V converter. In the synchronous/interconnection compensating circuit, an instantaneous voltage variation occurring in the power system is detected in terms of the component in phase with a reference voltage vector as the vector of the output voltage of the invertor, and the orthogonal component orthogonal to the reference voltage vector. The detected variations are respectively added to the outputs of a voltage adjuster and a phase adjuster in the synchronous interconnection control system. With such an arrangement, the output voltage of the voltage invertor can be controlled following instantaneous variations of the amplitude and phase of the power system voltage. The synchronous interconnection can be smoothly performed without disturbing the power system. The voltage adjuster and the phase adjuster, which make up the PI adjuster, when operate, do not need to follow the instantaneous variation. Therefore, the synchronous interconnection operation can be stably performed under the fixed-value control which is the control characteristic essential to the PI adjuster. In the second aspect of the invention. The control circuit includes a interconnection compensating circuit. In the compensating circuit, a quantity of an instantaneous variation in the voltage of the power system, when the invertor is operating in the interconnection mode, is detected in terms of a quantity of a variation of the component in phase with the internal vector and a quantity of a variation of the component orthogonal to the internal vector, and the detected variation quantities are respectively added to the I/V converted outputs of the effective power control system and the reactive power control system. Therefore, the effective power and the reactive power the invertor receives and transmits can be compensated at the input of the pulse generation-amplifier, which generates a firing pulse. With the compensation, the variation in the power system voltage is suppressed to lessen the adjust processing load of the PI adjuster in the reactive power control system. The operation of the interconnection mode can stably be performed by making full use of the fixed-value control of the PI adjuster. The compensating circuit is arranged such that a quantity of an instantaneous variation in the voltage of the power system is detected in terms of a quantity of a variation of the component in phase with the internal vector and a quantity of a variation of the component orthogonal to the internal vector, and the detected variation quantities are respectively added to the outputs of the synchronous-interconnection control system and the interconnection operation control system. Accordingly, the single compensating circuit is applicable for both the synchronous operation mode and the interconnection mode, thereby simplifying the circuit construction.

Claims: A control circuit for a voltage invertor (1) for converting DC voltage to AC voltage operable in an individual mode and an interconnection mode, said control circuit comprising an individual control system in which the AC voltage is controlled in accordance with a constant voltage and individually supplied to a load (4); a synchronous-interconnection control system for controlling an amplitude and a phase of an output voltage of said voltage invertor (1) to sychronize them with those in a power system (3) and incorporating said invertor (1) into the power system (3); and an effective power control system (12) and a reactive power control system (15) for controlling the effective component and the reactive component of an electric power transferred between the power system (3) and said invertor (1) interconnected with the power system (3), thereby to perform the interconnection mode operation; said control circuit serving also as a voltage adjuster for making a constant-voltage control or a synchronous control of said AC voltage, said control circuit further comprising: a synchronous-interconnection compensating circuit (70) in which the output voltage of said invertor (1), which is operating in the power system synchronous-interconnection mode or the interconnection mode, is stored as an internal vector, and a quantity of an instantaneous variation in the voltage of the power system (3) is detected in terms of a quantity of a variation of the component in phase with said internal vector and a quantity of a variation of the component orthogonal to the internal vector, characterized in that the detected variation quantities are respectively added to the outputs of a voltage adjuster (10) and a phase adjuster (20) in said synchronous-interconnection control system or to the voltage-converted outputs of said effective power control system (12) and said reactive power control system (15) in said interconnection mode. A control circuit as claimed in claim 1, wherein said synchronous-interconnection compensating circuit (70) comprises: synchronous signal generation for temporarily storing a power system voltage to generate an internal reference vector with a predetermined delay time; 3-phase/2-phase converter (61) for separating the voltage of the power system (3) into an in-phase component and an orthogonal component; vector rotator (63) for separately outputting a quantity of a variation of the output voltage of the converter (61) in the in-phase component and the orthogonal component; vector analyzer (71) for preparing a reference vector of the invertor voltage on the basis of an output voltage of the 3-phase/2-phase converter (61); subtractor (72) for subtracting the output voltage of the vector analyzer (71) from the in-phase component of the output of the vector rotator (63); first adding means (65) for adding an output of the subtractor (72) to the output voltage of the effective power control system (12) in said synchronous-interconnection control system or the output voltage of said voltage adjuster (10) in said interconnection mode; second adding means (66) for adding an output of the vector rotator (63) of the orthogonal component to the output voltage of the reactive power control system (15) in said synchronous-interconnection control system or the output voltage of said phase adjuster (20) in said interconnection mode.

Inventors: EGUCHI NAOYA C O FUJI ELECTRIC, HATTA YASUNORI C O FUJI ELECTR, EGUCHI, NAOYA, C/O FUJI ELECTRIC CO., LTD., HATTA, YASUNORI, C/O FUJI ELECTRIC CO., LTD.
Applicants: FUJI ELECTRIC CO LTD, FUJI ELECTRIC CO., LTD.
Application year: 1991
Main classification: H02M7
IPC codes: H02J3, H02M7, G05F1