JPS6026469A - Controller of ac/dc converter - Google Patents

Abstract

PURPOSE:To enhance the reliability of a power system by inserting an impedance element to an AC/DC converter side and shortcircuiting it when a defect occurs during the reverse conversion time. CONSTITUTION:An AC/DC converter 1 is connected to a DC transmission system 4, and connected to an AC transmission system 3 with the converter as a reverse or normal converting operation. In this case, an impedance element 10 is inserted between the AC terminal side of the converter 1 and the bulb winding side of a transformer 2, and a bidirectional semiconductor switch 15 is connected in parallel with the element. The switch 15 is conducted by a firing signal from a firing controller 300. The conduction conducts the switch 15 by inputting an operation mode signal IVN and a signal of a defect detector 200 for detecting the occurrence of a ground-fault defect F in the system 3 through an AND element A1 to the controller 300 when the converter 1 inverts. Thus, the reactor 10 is shortcircuited, and the commutation impedance is reduced from the forward converting operation time.

Description

[Detailed description of the invention] The present invention relates to a control device for an AC/DC converter.

FIG. 1 shows the main circuit of a conventional AC/DC converter used in a DC power transmission system. In the figure, the reference numeral ``■'' denotes an AC/DC converter, which is constructed by connecting semiconductor switching elements such as silice valves S1 to S6 in a bridge manner. 2 is a transformer for an AC/DC converter, 3 is a three-phase AC transmission system, and 4 is a DC transmission system. In this configuration, during inverter operation of the AC/DC converter 1, that is, during a reverse conversion operation to convert DC power to AC power, each thyristor valve S1 to S6 receives a firing signal in this order from a control device (not shown), and each It turns on every 120 degrees (electrical angle). Figure 2 shows the anode and
The voltage between the cathodes ■ and -K is shown, and T is the reverse voltage period required for the thyrisk valve to transition from the turn-off state to the turn-off state, which is generally called the commutation margin angle, and is usually Turn-off is possible if it is 8.66° (electrical angle equivalent to 400 μsec) or more, but 8.
When the angle is less than 64°, turn-off becomes impossible and commutation fails. This commutation margin angle γ is expressed by the following formula, where β: Control advance angle Xi: Commutation impedance d: Direct current Ei: Valve winding voltage As is clear from equation (1), the transformer It can be seen that as the valve winding voltage E1 of No. 2 becomes smaller, it decreases.

The relationship between this valve winding voltage Ei and commutation margin angle γ is expressed as
As shown in the figure.

In this way, when the valve winding voltage Ei decreases below a predetermined level, the commutation margin angle γ becomes less than 8.64°, and a commutation failure occurs. Conventionally, this commutation failure occurs. Since the operation of the AC/DC converter 1 is temporarily stopped, there is a problem that the power supply is interrupted and the power transmission system is disturbed, especially when large power is transmitted by independent DC power transmission. In such a case, the power system will experience significant fluctuations.

Of course, in order to prevent the above commutation failure, various methods have been used in the past.
For example, control methods have been developed that increase the control advance angle β or narrow down the DC current Id when the voltage drops due to a ground fault in the AC transmission system. %, it is unavoidable that commutation failure continues), so there is a problem that the reliability of a DC transmission system equipped with an AC/DC converter is extremely low compared to a normal AC transmission system.

This invention was made in view of the above-mentioned conventional problems, and an impedance element is inserted into the AC/DC converter side or the AC system side of an AC/DC converter transformer, so that when an accident occurs on the AC system side, the AC/DC converter By configuring the impedance element to be short-circuited when the converter is in the process of inverse conversion, it is more resistant to disturbances in the AC system than before, and improves the reliability of power systems equipped with AC/DC converters. It is an object of the present invention to provide a control device for an AC/DC converter that can be greatly improved.

An embodiment of the present invention will be described below with reference to the drawings.

Although FIG. 4 shows a single phase, the same reference numerals as in FIG. 1 indicate the same components as in FIG. Reference numeral 10 denotes an impedance element, that is, a reactor, which is inserted in series between each phase on the AC terminal side of the AC/DC converter 2 and each phase on the valve winding side of the transformer 2. Semiconductor switches 15 are connected in parallel. This bidirectional semiconductor switch 15 is constructed using anti-parallel circuits, gate turn-off circuits, etc., and includes an ignition controller 30.
It receives an ignition signal from 0 and becomes conductive or closed. 100
is an AC transformer, which is connected to the AC bus 50 of the AC power transmission system 3.
voltage is detected and input to the fault detector 200. The fault detector 200 transmits a fault signal to an AND element A when a ground fault F or the like occurs in the AC power transmission system 3 and the voltage of the AC bus 5° drops and the input to the AC transformer 1oo decreases below a predetermined value.
Send to l. An operation mode signal INV is supplied to this AND element A1 during the impark operation of the AC/DC converter 1.
The output is input to the ignition controller 300 as an ignition command signal.

Next, the operation of this device will be explained.

Now, suppose that the AC/DC converter 1 performs a forward conversion operation and supplies DC power to the DC transmission system 4. Since the operation mode signal INV is not supplied to the AND element AI, the AC power is transmitted. Even if a ground fault occurs in the system 3, the AND element will not output and the ignition controller 300 will not send out an ignition signal.For this reason, the bidirectional semiconductor switch 15
Since the circuit remains open, the commutation impedance Xi seen from the AC/DC converter 1 side is: . The commutation impedance Xi is usually selected to be, for example, about 20% of the self-capacity of the transformer 2, so
X so that the value of Xi in equation (2) above becomes this 20% value
By selecting t and XR in advance, even if an arm short circuit occurs in the AC/DC converter 1, it is possible to obtain the same short circuit current suppressing effect as before.

When the AC/DC converter 1 is operated by an inverter, this time,
By inputting the operation mode signal INV to the AND element A1, when a ground fault F occurs in the AC power transmission system 3 and the voltage of the AC bus 50 drops below a predetermined value, the AND element AI activates the fault detector 200. output after receiving the accident signal. Therefore, the controller 300 operates and sends out an ignition signal, so the bidirectional semiconductor switch 15 closes and the reactor 10 is short-circuited. As a result, the commutation impedance X+ becomes Xl-Xt-(3), which is smaller than when the rectifier is in operation.

In this manner, during inverter operation, the commutation impedance Xi decreases, so even if the valve winding voltage EiA<decreases, the decrease in the commutation margin angle γ is suppressed. That is, for example, the commutation impedance Xi is 1/2 that of the rectifier operation.
If XR is selected so that Ei becomes 1/2, the value of T will not change, and the voltage of AC power transmission system 3 will be (50 to 60%)/2 = 25 during normal operation. Even if the ratio drops to ~30%, the operation can be performed without causing the commutation failure.

In the above embodiment, the reactor 10, which is an impedance element, is inserted between the AC/DC converter 1 and the transformer 2.
It can also be inserted in series on the side.

Furthermore, in the embodiments described above, faults in the AC power transmission system are detected by changes in the AC voltage, but they may be detected by changes in the AC current or detection of traveling waves that occur when a fault occurs.

As described above, according to the present invention, an impedance element is inserted in series with the transformer for the AC/DC converter, and in the event of an accident in the AC power transmission system during the inverse conversion operation of the AC/DC converter, the impedance element is short-circuited and the AC/DC converter is By adopting a configuration in which the commutation impedance is reduced, it is resistant to disturbances in the AC system, and the reliability of the DC power transmission system including the AC/DC converter can be greatly improved.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing the main circuit part of a conventional AC/DC converter, Fig. 2 is a voltage waveform diagram of the silisk valve of the above AC/DC converter, and Fig. 3 is a diagram showing the commutation margin angle and valve winding voltage. FIG. 4, a diagram showing the relationship, is a circuit diagram of a control device for an AC/DC converter according to an embodiment of the present invention. In the figure, 1-AC/DC converter, 2-AC/DC converter transformer, 3-AC power transmission system, 4-DC power transmission system, 10-, -reactor, 15-bidirectional semiconductor switch, 50-AC bus, 1oo - AC transformer, 200 - Fault detector, 30
0-controller, AI-AND element. In the drawings, the same reference numerals indicate the same or corresponding parts. Agent, Masuo Oiwa = 337 VA- 0.5P, LJ 1. OP, LI

Claims (1)

[Claims] An impedance element inserted into the AC/DC converter side or the AC system side of the AC/DC converter transformer connected to the AC/DC converter, a bidirectional semiconductor switch connected in parallel to the impedance element, and an accident in the AC system. An AC/DC converter comprising a fault detector for detecting a fault, wherein the bidirectional semiconductor switch is closed on the condition that the AC/DC converter is in an inverse conversion operation when the fault detector outputs an output. control device.