JPH0126256B2 - - Google Patents

Description

[Detailed Description of the Invention] (a) Description of the Technical Field The present invention provides a DC multi-terminal power transmission system consisting of a plurality of converters, even if any converter is disconnected from the grid due to an accident, etc. The present invention relates to a control device for DC multi-terminal power transmission equipment that can stably maintain power.

(b) Description of Prior Art In order to further utilize the advantages of conventional DC two-terminal power transmission, there is a strong desire to develop technology for DC multi-terminal power transmission.

FIG. 1 is a DC four-terminal power transmission system diagram to which the present invention can be applied, in which 1 to 4 are AC systems, 5 to 8 are conversion transformers, 9 and 10 are forward conversion devices, and 11 and 12 are conversion transformers.
1 is an inverse converter, and 13 to 20 are direct current or disconnectors.

Compared to DC two-terminal power transmission, DC multi-terminal power transmission has
More advanced cooperative control between each conversion device is required. For this reason, in DC multi-terminal power transmission, information from each converter is collected and monitored by a central control unit via a transmission system (not shown), and based on this information, optimal operating command values are given to each converter. There is a need. On the other hand, in an emergency, a control system must be used that can obtain stable operating points for the DC voltage and current of each converter without relying on the central control unit and transmission system. In particular, with the development of direct current and disconnectors, there is an increasing need for control systems that allow stable operation to continue even if a faulty location is isolated at high speed.

Now, as a control method for DC multi-terminals, the
Publication No. 43-8641 and Japanese Unexamined Patent Publication No. 51-66455 are well known. The one published in Special Publication No. 43-8641 is
This is an extension of the conventional two-terminal power transmission control method to multi-terminal power transmission, and its characteristic diagram is shown in the second figure.
As shown in the figure. For convenience of explanation, the forward conversion device 9 in FIG. 1 is referred to as REC1, and the forward conversion device 10 is referred to as REC1.
2. Inverse transformation device 11 is INV1, inverse transformation device 12
is INV2, and the current setting values of the constant current control circuits of REC1 and REC2 are Idpr1 and Idpr2, respectively, and the current values that actually flow to REC1 and REC2 are Idr1, Idr2, and
The current setting values of the constant current control circuits of INV1 and INV2 are abbreviated as Idpi1 and Idpi2, and the current values actually flowing through INV1 and INV2 are abbreviated as Idi1 and Idi2. Now, in FIG. 2, the operating points of each converter in the steady state are P ₁ , P ₂ , P ₃ , and P ₄ . That is, REC2
determines the voltage, and other REC1, INV1, INV2
performs constant current control. In order for this control method to operate stably, the following conditions must be satisfied.

(Idpr1+Idpr2)-(Idpi1+Idpi2)=ΔI≧0...(Idr1+Idr2)=(Idi1+Idi2)...That is, the sum of the current settings of the forward converter must be greater than the sum of the current settings of the inverse converter. In other words, the value obtained by subtracting the sum of the current settings of the inverse conversion device from the sum of the current settings of the forward conversion device (hereinafter referred to as current margin, ΔI in the formula) must be positive or zero. .

However, this method has the following drawbacks. For example, if REC2, the voltage determining terminal, stops due to an accident, all converters must stop because the stability conditional expression no longer holds. In other words, an accident in a certain forward conversion device will cause the system to stop.

Next, the characteristic diagram of Japanese Patent Application Laid-Open No. 51-66455 is shown in the third figure.
As shown in the figure.

In this method, each converter is equipped with constant voltage control in addition to constant current control, and in addition to the concept of current margin described above, the concept of voltage margin is introduced. If the terminal is determined, the voltage setting value of the constant voltage control of the converter is
The voltage setting value of all other converters is set to be smaller by a predetermined voltage margin (hereinafter abbreviated as ΔV), and the current setting value is set so that the formula holds. be.

Now, as before, if we assume that REC2 has an accident and stops, the system will stop if some measures are not taken with this method. This is because the voltage determining terminal no longer exists. Also, as a matter of course, the formula no longer holds true. Therefore, in Japanese Patent Application Laid-open No. 51-66455,
It is proposed that a central control device be provided, and the information of all conversion devices be collected and processed in this central control device. That is, in the previous example, if REC2 stops due to an accident, that information will be transmitted to the central control device, and the central control device will use that information to set the new voltage setting value and current setting value to the remaining healthy converters. It is proposed that this command be given to prevent the system from stopping.

It is true that this method seems to have no problems in theory, but there are various problems when manufacturing actual equipment using this method.

The first problem is that transmission lines that can transmit large amounts of information at very high speeds are essential. Such transmission lines are not desirable in Japan in the future due to problems such as poor location. In addition, in other countries, the highly reliable micro lines currently used in Japan are not used, and unreliable and low-speed power line transmission is common. It is difficult to require high-speed transmission lines.

The second problem is related to the reliability of the central control device. That is, if a problem occurs in the central control device, there is a risk that the system will stop, so this central control device must have extremely high reliability. This results in extremely high costs.

The third problem is related to commutation failure.
There are many factors that can cause commutation failures, such as voltage drop and waveform distortion in the AC system connected to the inverter, and it must be kept in mind that commutation failures will always occur with a non-negligible probability. It won't happen.
This is clear from past operating results of two-terminal power transmission. According to literature from other countries, it occurs quite frequently. Commutation failures are not scary accidents in two-terminal power transmission, and are usually resolved as single commutation failures. Even if they occur continuously, the converter should not be stopped. For example, in Hokkaido, - At the Honshu DC power transmission facility, the inverter is operated in a bypass pair (hereinafter abbreviated as BPP) for a short period of time, and a system is adopted in which it automatically returns to normal operation. Normally, it is treated as a minor or medium failure. However, commutation failure in multiple DC terminals may lead to system stoppage. This is because, for example, in Figure 1, when all converters are operating at their rated values, INV
1 (inverter 11) fails in commutation, REC
1. All the current of REC2 flows into INV1, so INV1 becomes 200% overcurrent and INV2 becomes no-load operation. The problem is not the 200% overcurrent value. (It is sufficient to design the converter in advance so that it can be overloaded for a short period of time, and there is no problem in terms of cost for a short period of time.) In other words, for example, a previous commutation failure may cause a temporary AC system voltage drop. If this is the cause, we would like to return to the steady state immediately after the AC system voltage recovers, but INV1 is no longer able to start. That is, since 200% of the current is flowing, the commutation overlap angle increases significantly, resulting in insufficient margin angle and commutation cannot be performed. At this time, if the control lead angle is increased to ensure a sufficient margin angle, commutation may be possible, but this will result in so-called zero power factor operation where the voltage is almost zero, causing a problem. is not resolved at all. The above is based on the fact that a commutation failure, which was a minor failure in a conventional DC two-terminal system, becomes a serious failure in a DC multi-terminal system, and commutation failures occur quite frequently. This means that there is a risk that the system will shut down many times a year.

(c) Purpose of the Invention The purpose of the present invention was to eliminate such drawbacks, and to provide a system that allows transmission lines and central control equipment to be connected without causing a system shutdown due to an accidental shutdown of a certain converter. It is an object of the present invention to provide a new control device for DC multi-terminal power transmission equipment, which can reduce the dependence of the DC current as much as possible and further prevent system stoppage due to commutation failure.

(d) Structure of the invention Hereinafter, the present invention will be explained with reference to the drawings.

4 and 5 are control block diagrams showing one embodiment of the present invention, FIG. 4 is a control block diagram of a forward conversion device, and FIG. 5 is a control block diagram of an inverse conversion device.

In FIG. 4, 21 to 23 are adders, into which each set value and detected value are input. 24 is a constant voltage control circuit (hereinafter abbreviated as AVR); 25 is a first
constant current control circuit (hereinafter abbreviated as ACR1)
26 is a second constant current control circuit (hereinafter referred to as ACR2
It is abbreviated as ), 27 is a maximum value selection circuit, which automatically selects the output signal of the control circuit with a larger control delay angle (hereinafter abbreviated as α), and 28 is a circuit where α is smaller. a minimum value selection circuit that automatically selects the output signal of the one control circuit;
9 is the same maximum value selection circuit as 27, and 30 is a level detector that receives a DC voltage as an input and detects when the value becomes below a predetermined value. Also, 31,3
2 is a switch which responds to the output signal of the level detector, and when the output signal becomes logic level "1", that is, when the level detector 30 detects a drop in the DC voltage. , the switch 31 is turned off and the switch 32 is turned on. Figure 5 also has almost the same configuration as Figure 4,
33 to 35 are adders, 36 is AVR, 37 is ACR
1, 38 are ACR2, 39 is maximum value selection circuit, 4
0 indicates a minimum value selection circuit. On the inverse conversion device side,
Furthermore, constant margin angle control 41 controls the margin angle to be constant.
(hereinafter abbreviated as CER).
Further, Vdpr and Vdr indicate the voltage setting value and voltage detection value of the forward conversion device, respectively, and Vdpi and Vdi indicate the voltage setting value and voltage detection value of the inverse conversion device, respectively.
Idpr, Idpr′, and Idr are the ACR of the forward conversion device, respectively.
The current setting value of ACR1, the current setting value of ACR2, and the current detection value are shown, and Idpi, Idpi', and Idi indicate the current setting value of ACR1, the current setting value of ACR2, and the current detection value of the inverter, respectively. V _AC is an alternating current voltage. The output signal of the switch 31 or 32 in FIG. 4 and the output signal of the minimum value selection circuit 40 in FIG.
It is sent to a phase control circuit to determine the ignition pulse.

Now, the operation of the control circuit configured as described above will be explained below.

First, in FIG. 4, it is assumed that the ACR1 is operated in a steady state. For example, if the current is 1000A, ACR1
Idpr will be set to the optimal value for 1000A flow. At this time, the AVR's Vdpr is:
For example, set to perform constant voltage control of 230kV for a rated voltage of 250kV. Furthermore, Idpr′ of ACR2 is
For example, set it to perform constant current control of 100A.
If you set it as above, the output of AVR will be:
Since the detected value is 250 kV against the set value of 230 kV, it is negative (in actual equipment, it becomes the minimum value of α due to the output limiter), and the maximum value selection circuit 27
The output of ACR1 is selected. next,
For the output of ACR2, the set value is 100A and the detected value is 1000A, so α becomes large (in actual equipment, the maximum value of α is determined by the output limiter), and the minimum value is selected. As the output of the circuit 28, the output of ACR1 is selected, and as the output of the maximum value selection circuit 29, the output of ACR2 is selected.

In steady state (rated voltage), level detector 30
is inoperative, so switch 31 is on and switch 32 is off, so the converter is operated by ACR1.
In such a state, if some kind of accident occurs and the DC voltage drops, the level detector 30 will operate, the switch 32 will turn on, the switch 31 will turn off, and the ACR2 will then operate, reverting to the previous example. So,
The DC current is controlled to be 100A.

Since the same concept can be applied to FIG. 5, the explanation will be omitted.

(e) Effect of the invention Now, FIG. 6 shows a characteristic diagram when FIGS. 4 and 5 are applied to the DC 4-terminal power transmission shown in FIG. 1. That is, this corresponds to FIGS. 2 and 4. 6th
Steady state operating points in the figure are indicated by _P1 to _P4 .
From Figure 6, the following can be seen.

(1) The converter that determines the voltage is REC2. That is, REC2 performs AVR operation, and the remaining converters perform ACR1 operation.

(2) Regarding the current setting value and the current value flowing through each converter, the above equations and formulas hold true.

Now, let's consider a case where REC2 stops due to an accident. For example, if a ground fault occurs at point F in Figure 1,
This is a case where the REC 2 is stopped by cutting off the DC current or the disconnector 15. At this time, the operating point is the seventh
P ₁ , P ₃ , and P ₄ are shown in the characteristic diagram in the figure. That is, in FIG. 7, the voltage determining terminal shifts to INV1,
REC1 and INV2 will perform ACR1 operation. At this time, regarding the current setting value in FIG. 7, the above-mentioned formula holds true, but the formula no longer holds true. However, this system can be operated stably. The applicant has already demonstrated through digital simulation that a system having the control characteristic diagram shown in FIG. 7 can operate stably. The reason for this will be explained below.

Japanese Patent Publication No. 43-8641 introduced the concept of current margin in order to ensure stability conditions, but the above formula was essential for stability.
In other words, the fact that the above equation no longer holds true means that
When the forward converter is no longer able to supply the current value required by the inverter, the inverter attempts to lower its own voltage to secure the necessary current, eventually bringing the system's DC voltage to zero. This is because it becomes In other words, all converters, whether forward or inverse converters, perform constant current control and are operated with completely uncoordinated current setting values.

Next, in JP-A-51-66455, the concept of voltage margin is added to the concept of current margin, but as is clear from comparing the control characteristic diagrams 2 and 3, They are essentially the same thing. That is, in Figure 2, the secondary voltage of the converting transformer at the terminal (REC2) that determines the voltage is made smaller than the secondary voltages of the converting transformers at all other terminals by tap control. However, in Figure 3, instead of using tap control, a constant voltage control circuit is provided to adjust the voltage setting value of the voltage determining terminal (REC2) to all other terminals. Since the voltage determining terminal is determined by making the voltage smaller than the set value, Japanese Patent Application Laid-Open No. 51-66455 is also similar to Japanese Patent Publication No. 43-8641.
The system becomes unstable for the same reason as in the publication.
The difference between JP-A-51-66455 and JP-A-43-8641 is that normal tap control takes several seconds to operate one tap, whereas centralized control using a transmission line allows for high-speed control. This is a point that is assumed to be controllable.

However, in the present invention, since the voltage setting values for each terminal are different, as is clear from the characteristic diagram in Figure 7, even if the voltage determining terminal (REC2) stops due to an accident, for example, INV1 will automatically In order to function as a voltage determining terminal and perform constant voltage control,
Like Publication No.-8641 and Japanese Unexamined Patent Publication No. 51-66455,
It is stable because all the converters do not perform constant current control based on completely uncoordinated current setting values.

This means that in the control device of the present invention, an accidental stoppage of a certain conversion device will not cause a system stoppage, a high-speed transmission line is not necessarily required, and a system stoppage will not occur due to a malfunction in the central control device. It means that there is no such thing.

Next, we will consider commutation failure. Now, the first
In the figure, it is assumed that the voltage of the AC system 3 connected to the inverter 11 (INV1) decreases for some reason, and as a result, INV1 fails in commutation.
At this time, as mentioned above, in the method of Japanese Patent Publication No. 43-8641 and Japanese Patent Application Laid-open No. 51-66455, the currents of REC1 and REC2 all flow into INV1,
INV1 becomes overcurrent, and even if the voltage of AC system 3 is restored, INV1 cannot be started. However, in the present invention, INV1 can be activated without any problem. The reason for this will be explained using the control characteristic diagram shown in FIG. When INV1 fails in commutation, all of the currents in REC1 and REC2 flow into INV1, but since commutation failure is the same as a DC short circuit, the DC voltage is approximately zero. This means that in the control characteristic diagram in Figure 6, all converters shift to ACR2 operation, the DC voltage becomes almost zero, and INV1 has a maximum of Idpr′1.
This means that only a current equal to the sum of Idpr'2 flows. Therefore, if Idpr'1 and Idpr'2 are set in advance to small values that do not cause intermittent currents, the INV1 can be activated when the voltage of the AC system 3 is restored. That is, since the current value is small, the commutation overlap angle is small, and therefore a sufficient margin angle can be secured. This means that INV
The same applies to 2. This is the same as starting the system from a state where no current is flowing, that is, starting the system for the first time, except for the condition that a small current is flowing, so it will start without any problems. be able to.

In addition, in Fig. 6, the downward-sloping straight lines a and b
is due to a minimum value limiter of α, which is not shown in FIG. 4, and the straight lines c and d are due to constant margin angle control.

(f) Modification FIG. 8 shows the inverse transformation device 11 in FIG.
(INV1) performs constant voltage control (AVR) operation,
It is a control characteristic diagram when all the remaining conversion devices are operated by the first constant current control (ACR1). The operating points are points _P1 to _P4 . It is clear that even in such a case, the same effect as described above will be obtained.

Furthermore, although the explanation has so far been made using the DC 4-terminal power transmission system diagram shown in FIG. Furthermore, it goes without saying that it is also applicable to two-terminal power transmission with converters in parallel. Furthermore, it is also applicable to conventional two-terminal power transmission. This means that the present invention is very effective when considering the construction process of multi-terminals.

Furthermore, in FIG. 4, various methods can be considered as means for selecting ACR2 when the DC voltage drops below a predetermined value. For example, the switches 31, 32, the maximum value selection circuit 29, etc. are not provided, and when a drop in the DC voltage is detected, the adder 2
There is also a method of applying a bias in the direction of increasing α to 1 and 22. In any case, since the gist of the present invention is to realize the control characteristics as shown in FIG. 6, the means for realizing the same is not limited to the above embodiment.

(g) Overall effect As explained above, according to the present invention, all conversion devices include the first and second constant current control circuits,
A constant voltage control circuit is provided, and a constant margin angle control circuit is added to the inverse conversion device, and the voltage setting value of the constant voltage control circuit of all conversion devices other than the conversion device that performs constant voltage control is set according to the constant voltage control. The voltage setting value is set lower than the voltage setting value of the converter that performs the conversion, and each voltage setting value is set to a different value from each other,
By configuring the second constant current control circuit to operate at least at a DC voltage value that is lower than the DC voltage value in a steady state, system stoppage due to an accidental stoppage of a certain converter can be prevented.

Direct current and disconnectors can be used effectively.

Dependency on transmission lines can be reduced.

Dependency on the central control device can be reduced.

System stoppage due to commutation failure can be prevented.

It has many remarkable effects.

[Brief explanation of drawings]

Figure 1 is a DC 4-terminal power transmission system diagram, Figure 2 is:
A conventional control characteristic diagram, FIG. 3 is another conventional control characteristic diagram, FIGS. 4 and 5 are control block diagrams showing one embodiment of the present invention, and FIGS. 6, 7, and 8 The diagram is
FIG. 3 shows a control characteristic diagram of the present invention. 1-4...AC system, 5-8...Conversion transformer, 9-10...Forward conversion device, 11-12...Inverse conversion device, 13-20...DC or disconnector, 21-
23... Adder, 24... Constant voltage control circuit, 25
~26... Constant current control circuit, 27... Maximum value selection circuit, 28... Minimum value selection circuit, 29... Maximum value selection circuit, 30... Level detection circuit, 31, 32
...Switch, 33-35...Adder, 36...
Constant voltage control circuit, 37-38... Constant current control circuit, 39... Maximum value selection circuit, 40... Minimum value selection circuit, 41... Constant margin angle control circuit.

Claims (1)

[Claims] 1 In a DC multi-terminal power transmission facility consisting of multiple converters, all converters are each equipped with a constant voltage control device, and the minimum constant voltage setting value of the converter that operates as a forward converter is set as the inverse converter. The converter is set to be larger than the maximum value of the constant voltage setting value of the operating converter, and is operated as a forward converter, so that the current is equal to or higher than the first constant current setting value for a voltage lower than the constant voltage setting value. a first constant current control device that controls the current so that the current does not fall below a second constant current setting value for voltages higher than the constant voltage setting value and below a predetermined DC voltage value; The constant current control device and the conversion device that operates as an inverse conversion device include a constant current control device and a conversion device different from the above, which controls the current to a voltage higher than the constant voltage setting value so that it does not exceed a first constant current setting value different from the above. a first constant current control device; and a second constant current control different from the above for controlling a voltage lower than the constant voltage setting so that the current does not fall below a second constant current setting different from the above. 1. A control device for DC multi-terminal power transmission equipment, characterized by comprising the device.