JPH0130370B2 - - Google Patents

Description

[Detailed Description of the Invention] (a) Description of the Technical Field The present invention is directed to a DC interconnection system such as a frequency converter connected to a weak AC system or a DC power transmission equipment, in which an AC/DC converter is damaged due to some kind of accident. The present invention relates to a method for stopping an AC/DC converter, which stops the AC/DC converter without generating an overvoltage in the AC system when the AC/DC converter is stopped.

(b) Description of prior art Figure 1 shows a schematic configuration diagram of a DC power transmission device,
The AC buses 1, 1' are connected via converting transformers 2, 2' to converters 3, 3' consisting of, for example, a series-parallel connection of a large number of thyristors, and control the firing phase of each thyristor. This converts alternating current into direct current or direct current into alternating current. 4 and 4' are smooth-sliding actuators, 5 is a DC transmission line, 6 and 6' are potential transformers (PT), and 7 and 7' are instrument alternators (CT). As a control device in such a main circuit configuration, constant current control circuits (ACR) 8, 8' and constant voltage control circuits (AVR) 9, 9' each have a reference value.
The deviation between Idp, Edp and the detected values Id, Ed is controlled by the voltage Ec.
This control voltage Ec is input to the control voltage selection circuits 10 and 10'. The control voltage selection circuits 10 and 10' automatically select the control system that advances the control angle most among various types of control, and the control voltage Ec selected here is used to select the control system that advances the control angle most. The signal is subjected to upper and lower limit limiters at '' and is input to the ignition phase control circuits 12, 12'. The ignition phase control circuits 12, 12' determine an ignition phase proportional to the control voltage Ec and output an ignition command to the thyristor. As is well known, in the AC/DC converter configured in this way, by switching the current margin (ΔI), one is operated as a forward converter under constant current control, and the other is operated as an inverse converter under constant voltage control. . 13 to 16 are line breakers, 17 and 18 are power capacitors (hereinafter abbreviated as SC) that supply phase-advanced reactive power;
19 and 20 are AC filters (hereinafter abbreviated as ACF) for absorbing harmonics.

Now, in FIG. 1, when the converter 3 is being operated as a forward converter and the converter 3' is being operated as an inverse converter, if there is a ground fault or short circuit on the AC line on the AC system 1' side, Suppose that an accident occurs. In such a case, the conventional method for stopping the AC/DC converter is to instantly put the inverter 3' into a gate block (hereinafter abbreviated as GB) or a bypass pair (hereinafter abbreviated as BPP). At the same time, the information is transmitted to the forward converter 3, and upon receiving the information, the forward converter 3 instantaneously performs a gate shift (hereinafter abbreviated as GS) to immediately reduce the DC current to zero. A method was adopted in which the data was then converted to GB.

However, if the AC system 1 is connected to a weak system, such as a strong AC system, through a line with large impedance, the conventional shutdown method may cause an overvoltage in the AC system at the time of shutdown. As a result, the arrester will be destroyed and the converter equipment will suffer dielectric breakdown.

The reason for this is that while the converter is transmitting power, it consumes lagging reactive power corresponding to the transmitted power value, but as the converter rapidly stops, the leading phase reactive power becomes excessive. This is because if there is a large fluctuation in reactive power and the AC system is weak, the large fluctuation in reactive power will cause a large voltage fluctuation in the AC system. Furthermore, it is well known that at this time, voltage fluctuations in the AC system tend to have an overvoltage tendency. In the above example, an accident on an AC line is assumed, but the same holds true even if other accidents, such as a ground fault on the DC transmission line 5, are assumed. That is, when a ground fault occurs in the DC power transmission line 5, the conventional shutdown method adopts a method in which when the ground fault is detected, the forward converter 3 and the reverse converter 3' are set to GS, and then to GB. Therefore, the above problem is not solved at all.

As explained above, the conventional accident shutdown method for converters when some kind of accident occurs is that in systems where the converter is connected to a weak AC system, overvoltage may be generated in the AC system, leading to an expanded accident. There was a big drawback.

(c) Object of the present invention Therefore, the object of the present invention has been made to eliminate such drawbacks, and particularly to avoid AC line accidents in systems where the converter is connected to a weak AC system. An object of the present invention is to provide a new method for stopping an AC/DC converter, which can quickly stop the converter without generating an overvoltage in an AC system.

(d) Structure of the Invention FIG. 2 is a control circuit diagram showing one embodiment of the present invention, and FIG. 3 is a time chart for explaining the operation of FIG. 2. Elements that are the same as those are indicated by the same reference numerals.

Now, in Fig. 2, 21 is a control voltage setter corresponding to a desired control delay angle, and 22 and 23 are switches. During operation, switch 22 is on and switch 23 is off, and the converter is stopped in case of an accident. When the switch 22 is turned off, the switch 23 is turned on. Further, 24 is a level detector for detecting DC overcurrent, and 25 and 26 are AND elements.

(e) Effect of the invention Now, as described above, suppose that the converter 3 is a forward converter and the converter 3' is an inverse converter, and a ground fault or short circuit accident occurs on the AC line on the AC system 1' side. This fault is detected by, for example, an AC undervoltage relay, and the fault detection signal causes the inverter 3' to enter BPP. On the other hand, the accident detection signal is transmitted to the forward conversion device 3 side via a transmission system. At this time, when the inverter 3' becomes BPP, the DC current rapidly increases because this is the same phenomenon as a DC short circuit. That is, in FIG. 2, the level detector 24 operates and the logic level is "1".
Therefore, the output signal of the AND element 25 also becomes logic level "1". This AND element 2
When the output signal of the switch 5 becomes logic level "1", the switch 22 is turned off and the switch 23 is turned on in conjunction with this signal, and the forward converter 3 is operated at a certain fixed control delay angle. Become. This operating state is a so-called zero power factor operating state since the inverse converter 3' is in the BPP state, and is an operating mode in which the forward converter consumes the slowest phase reactive power.
At this time, the voltage of the AC system 1 decreases. Therefore,
In this state, that is, when the output signal of the AND element 25 reaches logic level "1",
Based on the conditions under which the SC17 and ACF19 are disconnected after generating a disconnection command and the SC17 and ACF19 are disconnected, that is, when the output signal of the AND element 26 becomes logic level "1". If the forward converter 3 is set to GS and GB, phase-advanced reactive power will not become excessive, so no overvoltage will occur in the AC system.

In addition, in the time chart of Figure 3, the time t ₀ is. When the inverse conversion device 3' is introduced into the BPP,
The time _t1 is the time when the accident detection signal detected on the reverse conversion device 3' side is received on the forward conversion device 3 side,
t ₂ is the time when the output signal of the AND element 25 becomes logic level "1", and t ₃ is the time when the output signal of the AND element 26 becomes logic level "1", i.e.
At _t4 , when the SC17 and ACF19 are disconnected and the forward converter 3 enters GS operation, the forward converter 3
Indicates the time when GB was added.

(f) Modification In the above embodiment, the forward conversion device 3 is continuously operated at a desired fixed control delay angle, but operation is allowed only at a control delay angle within a predetermined range. For example, a method may be considered in which the control voltage limiter circuit 11 shown in FIG. 1 is compressed at a predetermined speed.

In addition, in the above explanation, an accident on the AC system 1' side was assumed with the converter 3 as a forward converter and the converter 3' as an inverse converter.
The same effect can be obtained by assuming an accident on the AC system 1 side by using the converter 3' as an inverse converter and the converter 3' as a forward converter. That is, in the embodiment shown in FIG. 2, the control voltage setter 21 is originally set to operate in the forward conversion control angle region.
Furthermore, even if the embodiment shown in FIG. 2 is not used, it is sufficient to allow the control delay angle to shift to the operating range as the forward conversion control angle range. for example,
The upper limiter of the control voltage limiter circuit 11 in FIG. 1 is set at 160 degrees in electrical angle, and the lower limiter is set at 80 degrees in electrical angle, so that the control voltage limiter circuit 11 is configured so that a sufficient DC current can flow even in the single terminal BPP state. Bye.

(g) Overall effect As explained above, according to the present invention, when an accident occurs at one end of an AC/DC converter connected to a weak AC system, the converter at the faulty end is switched to BPP.
, perform zero power factor operation, and then
By cutting off the ACF and promptly stopping the converter, overvoltage in the AC system is suppressed during an accidental stop, and furthermore, it is possible to prevent further accidents such as arrester destruction and insulation breakdown of converter station equipment. It has the remarkable effect of being able to.

[Brief explanation of drawings]

FIG. 1 is a schematic configuration diagram of a DC power generator, FIG. 2 is a control circuit diagram showing an embodiment of the present invention, and FIG. 3 is a time chart thereof. 1, 1'... AC system, 2, 2'... Conversion transformer, 3, 3'... Conversion device, 4, 4'... Smooth sliding actor, 5... DC transmission line, 6, 6' ...Instrument transformer, 7,7'...Instrument current transformer, 8,8'...
...constant current control circuit, 9,9'...constant voltage control circuit,
10, 10'... Minimum value selection circuit, 11, 11'...
...Control voltage limiter circuit, 12,12'...Ignition phase control circuit, 13-16...Shin breaker, 17,
18... Power capacitor, 19, 20... AC filter, 21... Control voltage setting device, 22,
23...Switch.

Claims (1)

[Claims] 1. In an AC/DC converter that converts alternating current to direct current and direct current to alternating current, when the AC/DC converter is stopped, one terminal is put into a bypass pair, and the other terminal is set at a desired control delay angle. A method for stopping an AC/DC converter, characterized in that the AC/DC converter is stopped after the AC filter and the power capacitor are cut off during operation. 2. In an AC/DC converter that converts alternating current to direct current and direct current to alternating current, when the AC/DC converter is stopped, one terminal is placed in a bypass pair, and the control delay angle of the other terminal is limited within a desired range. drive,
A method for stopping an AC/DC converter, characterized in that the AC/DC converter is stopped after an AC filter and a power capacitor are cut off.