JPS644433B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a self-excited inverter power generation system, and more particularly to a shutdown protection method for a power generation system that includes a DC intermediate circuit and a self-excited inverter and performs grid-connected operation with an AC system.

In recent years, in order to effectively use energy resources,
Various power generation systems have been developed that supply power to commercial grids from DC power sources such as solar cells, fuel cells, and storage batteries. Since such power generation systems perform grid-connected operation between the DC power source and the AC commercial grid, consistency is required for the operation, and in particular, system stoppage must not cause disturbance to the commercial grid.
In addition, it must be done safely and reliably to avoid applying overvoltage or overcurrent to the inverter or other connected equipment/devices.

The conventional method for stopping this type of power generation system is to first open a mechanical switch between the inverter and the utility grid to disconnect the system from the power grid, and then to stop the operation of the inverter. Therefore, according to this method, the main circuit is mechanically disconnected during operation of the inverter. However, such phylogenetic separation firstly requires
Since the current that had been flowing between the systems is forcibly cut off, equipment such as switches deteriorates quickly. Furthermore, such sudden interruptions or interruptions may cause surges to enter the inverter circuit, and this surge noise may turn into erroneous gate pulses and cause the thyristor, which should normally be in the extinguished state, to ignite, and in some cases, The thyristor becomes conductive while a reverse voltage is being applied, and an excessive reverse leakage current flows, damaging or destroying the thyristor and other elements.

As a method to solve this problem, for example, a shutdown protection method can be considered in which the output of the thyristor is first reduced to zero, then the operation of the inverter is stopped, and then the mechanical switch between the inverter and the commercial power system is opened. In this case, there is usually a certain waiting time from stopping the inverter to opening the switch. That is, even if the inverter is stopped, power supply is not immediately stopped and a transient current flows for a while, so in order to perform safe system separation, the switch is opened after such a transient current has sufficiently attenuated. However, during this waiting time, if the voltage on the utility grid side rises above the voltage on the inverter side, power from the grid side (self-excited) flows into the main circuit through the inverter's return diode, and as a result, the inverter circuit elements and DC Excessive voltage may be applied to intermediate capacitors, etc., which may damage or destroy these devices.

The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to more safely and reliably stop a power generation system that is equipped with a DC intermediate circuit and a self-excited inverter and operates in connection with the power grid. .

According to the present invention, first, the output of the inverter is reduced to zero, and then the operation of the inverter is stopped, and the DC intermediate circuit voltage is monitored. If the DC intermediate circuit voltage is below the reference value during a predetermined time period after the inverter is stopped, the system is separated using, for example, a disconnection switch when this time period has elapsed. Furthermore, if the DC intermediate circuit voltage exceeds the reference value during the elapse of this time period, system separation is performed at the time when the DC intermediate circuit voltage exceeds the reference value without waiting for the elapse of the time period. Such a sequence achieves a safe shutdown of the power generation system without generating false gate pulses or overloading the equipment. Hereinafter, one embodiment of the present invention will be described with reference to the accompanying drawings.

FIG. 1 shows a circuit diagram of a self-excited inverter power generation system to which the present invention is applicable. In FIG. 1, 1 is a DC power source such as a solar cell, a fuel cell, or a storage battery, and 2 is a blocking diode for preventing backflow of power to the DC power source 1. In FIG. A DC power supply 1 is connected to a self-excited inverter 5 via a diode 2 and a DC filter composed of a DC reactor 3 and a DC intermediate capacitor 4. The self-excited inverter 5 includes four thyristors 6,
7, 8, 9, with high frequency switching so that a control circuit (not shown) alternately repeats the first conduction cycle of the thyristors 6, 9 and the second conduction cycle of the thyristors 7, 8. Controlled (PWM control). Between the connection point of thyristors 6 and 8 and the connection point of thyristors 7 and 9,
A series resonant circuit consisting of a commutating capacitor 10 and a commutating reactor 11 is provided. This series resonant circuit provides commutation between the first conduction cycle and the second conduction cycle and at the same time serves as a means for supplying reactive power to the load. The diodes 12 and 13 are diodes for returning reactive power. The output of the self-excited inverter 5 is connected to the transformer 14 and the coupling reactor 1.
5. Connect to the commercial grid line 17 via the mechanical switch 16 for joining/disconnecting the grid.

Next, a sequence for stopping the power generation system shown in FIG. 1 according to this embodiment will be explained. When a command to stop the power generation system is issued, the first step is to reduce the inverter output. This is done by gradually varying the firing angle of each thyristor 6, 7, 8, 9 by means of a control circuit. When the output of the inverter 5 becomes zero, the supply of gate pulses is completely stopped to stop the operation of the inverter.
If the normal transient state continues thereafter, the switch 16 is released to perform system separation after a certain time period _T1 has elapsed since the inverter was stopped. The time period T ₁ is chosen to be long enough for the transient current to decay. The means for operating the switch 16 when the time period _T1 has elapsed includes a timer that is activated by stopping the inverter and generates an ON signal when the time period _T1 has elapsed, and an ON signal from this timer.
It may be constructed from any conventional switch actuator that opens the switch 16 in response to a signal.

The above is a stop operation for a normal transient phenomenon, but in reality, as mentioned above, during the transient period after the inverter stops, the commercial grid voltage rises above the inverter output, and the grid power passes through the return diode 12 to the main circuit. DC intermediate circuit capacitor 4 due to peak charging from the grid
The charging voltage may rise abnormally. According to the invention, it is monitored whether the terminal voltage of the capacitor 4 exceeds the reference value VT during the time period _T1 after the inverter is stopped. When the reference value VT is exceeded, priority is given to this, that is, the switch 16 is opened even before time _T1 has elapsed. This prevents excessive voltage from being applied to elements such as the capacitor 4 and the thyristors 6, 7, 8, 9, etc. even if power from the grid flows into the main circuit. Reference value
VT may be selected as a protection voltage that takes into account the characteristics and ratings of each element. The monitoring means includes, for example, a conventionally known voltage detection device for detecting the terminal voltage of the capacitor 4, and a conventionally known comparator that compares the detected voltage with a reference voltage and generates a comparison signal when the former is larger than the latter. and switch 1 in response to the comparison signal.
6 may be constructed from a conventionally known switch operating circuit for opening the switch. Further, such a monitoring device may be activated when the inverter is stopped, or may be in a constantly active state (standby state). In other words, under normal operation when power is supplied from the DC power source to the commercial grid, it is unlikely that the terminal voltage of capacitor 4 will rise abnormally, but if such an abnormal voltage occurs, the monitoring device operates to open switch 16, thereby ensuring safe grid connection.

FIG. 2 is a flowchart diagram schematically showing the above-described sequence. That is, when a stop command is issued (20), the inverter output is throttled down to zero (21) and the gate pulse is turned off to stop the inverter operation (22). The timer starts at the same time as the inverter stops. On the other hand, the DC intermediate circuit voltage (terminal voltage of the capacitor 4 in FIG. 1) is monitored 23 by a monitoring device. and 24 if the DC intermediate circuit voltage is below the reference value VT during the time period T ₁ ;
A first switching signal S ₁ (for example the aforementioned ON signal) is generated from the timer at the expiration of the time period T ₁ ;
This first switching signal _S1 causes the mechanical disconnection switch to open 26 via the OR circuit 25. Also, if the DC intermediate voltage exceeds the reference value VT during the elapse of the time period T ₁ due to power flowing into the inverter from the commercial system, etc., the monitoring device outputs the second switching signal S ₂ (for example, the above-mentioned comparison signal). is generated, and this second switching signal _S2 causes the mechanical switch for decoupling to be opened via the OR circuit 25 (26). Therefore, although the switch is opened by both the first switching signal _S1 and the second switching signal _S2 , the second switching signal _S2 works preferentially. In other words, the monitoring device takes priority over the timer operation.

Although one embodiment of the present invention has been described above, the above embodiment is merely an illustration and does not limit the present invention. For example, although a single self-excited inverter is illustrated in the above-described embodiment, the present invention is also applicable to a polyphase inverter in which a plurality of self-excited inverters are combined. Also, not only for thyristor inverters but also for transistor inverters and GTO inverters.
Alternatively, the present invention is applicable not only to parallel type self-excited inverters but also to series type self-excited inverters.

Further, in the above embodiment, the terminal voltage of the capacitor 4 is monitored, but the object of monitoring is not limited to this. Any suitable DC intermediate circuit voltage may be monitored depending on the circuit configuration of the various self-excited inverters. The monitoring device may be of any configuration as long as it reliably detects abnormal voltage and activates the mechanical switch for disconnection. Furthermore, although in the embodiments described above the system separation is effected by a mechanical disconnection switch 16, other suitable system disconnection means may also be used.

As described above, in the present invention, the inverter output is first reduced to zero, then the inverter is stopped, and then the parallel disconnecting mechanical switch is released. Therefore, both systems can be disconnected without much effort, and there is no possibility that a surge will jump into the inverter circuit and generate an erroneous gate pulse. Moreover, since the DC intermediate circuit voltage is monitored, even if power flows into the inverter circuit from the commercial system, an excessive voltage will not be applied to the main circuit. Therefore, a safe and reliable shutdown of the power generation system is achieved without damaging, deteriorating, or destroying the equipment.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a self-excited inverter power generation system to which the present invention can be applied, and FIG. 2 is a flowchart showing an outline of a power generation system stop sequence according to an embodiment of the present invention. 1...DC power supply, 4...DC intermediate capacitor,
5...Self-excited inverter, 16...Mechanical switch for joining/disconnecting the grid, 17...Commercial grid line.

Claims (1)

[Scope of Claims] 1. In a shutdown protection method for a power generation system that is equipped with a DC intermediate circuit and a self-excited inverter and operates in connection with an electric power system, the operation of the inverter is stopped after the output of the inverter is reduced to zero. At the same time, the voltage of the DC intermediate circuit is monitored, and if the DC intermediate circuit voltage is below the reference value for a predetermined time period after the inverter stops operating, system separation is performed at the end of the time period, and the system is separated. A stop protection system characterized in that, when the DC intermediate circuit voltage exceeds the reference value during the elapse of a time period, system separation is performed at the time when the DC intermediate circuit voltage exceeds the reference value.