JPH0127654B2 - - 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 system due to an accident etc. This invention relates to a constant current control device for a DC multi-terminal power transmission system that can maintain stable current.

(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 4-terminal power transmission system diagram to which the present invention can be applied, in which 1 to 4 are tributary 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, the control method must be such that a stable operating point for the DC voltage and current of each converter can be obtained without relying on the central control unit and transmission system. In particular, as direct current and circuit breakers are developed, 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. Japanese Patent Publication No. 43-8641 expands the conventional two-terminal power transmission control method to multi-terminal power transmission, and its characteristic diagram is shown in FIG. For convenience of explanation, the forward conversion device 9 in FIG. 1 will be referred to as REC1, the forward conversion device 10 as REC2, the inverse conversion device 11 as INV1, and the inverse conversion device 12 as INV2, and current settings of the constant current control circuits of REC1 and REC2. Values I _dpr1 , I _dpr2 and actually REC1, REC2 respectively
The current values flowing through INV1 and INV2 are abbreviated as I _dr1 and I _dr2 , the current setting values of the constant current control circuits of INV1 and INV2 are I _dpi1 and I _dpi2 , and the current values actually flowing through INV1 and INV2 are abbreviated as I _di1 and I _di2 . 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 and INV2 perform constant current control.
In order for this control method to operate stably, the following conditions must be satisfied.

(I _dpr1 + I _dpr2 ) - (I _dpi1 + I _dpi2 ) = ΔI≧0 ... (I _dr1 + I _dr2 ) = (I _di1 + I _di2 ) ... In other words, the sum of the current setting values of the forward conversion device is equal to the current of the inverse conversion device Must be greater than the sum of the settings. 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 the voltage determining terminal REC2 were to stop due to an accident, all converters would have to stop because the stability condition would no longer hold. 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 causes 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, - Honshu DC power transmission equipment operates the inverter in bypass pair (hereinafter abbreviated as BPP) for a short period of time and automatically returns to normal operation. It is being treated as a light or medium failure. However, commutation failure in multiple DC terminals may lead to system stoppage.
This is because, for example, in Fig. 1, when all the converters are currently operating at their rated values, if INV1 (inverse converter 11) fails in commutation, REC1,
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 if it is for a short period of time.) If it is due to a decline,
After the AC system voltage is restored, we would like to immediately return to a steady state, 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 a DC multi-terminal power transmission system that can reduce the dependence of the system to the maximum extent 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 set values and detected values are respectively 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 abbreviated as ACR2), and 27 is a maximum value selection circuit, which is a control circuit with a larger control delay angle (hereinafter abbreviated as α). 28 is a minimum value selection circuit that automatically selects the output signal of the control circuit with smaller α; 29;
is the same maximum value selection circuit as 27, and 30 is a level detector which receives a DC voltage as an input and detects when the value becomes below a predetermined value. Also, 31, 32
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, and 3
3 to 35 are adders, 36 is AVR, 37 is ACR1,
38 is an ACR2, 39 is a maximum value selection circuit, and 40 is a minimum value selection circuit. The inverse conversion device side is further provided with a constant margin angle control circuit 41 (hereinafter abbreviated as CER) that controls the margin angle to be constant. or,
V _dpr and V _dr indicate the voltage setting value and voltage detection value of the forward conversion device, respectively, and V _dpi and V _di indicate the voltage setting value and voltage detection value of the inverse conversion device, respectively. I _dpr ,
I _dpr ′ and I _dr indicate the current setting value of ACR1, the current setting value of ACR2, and the current detection value of the forward conversion device, respectively, and I _dpi , I _dpi ′, and I _di indicate the current setting value of the inverse conversion device, respectively.
Shows the current setting value of ACR1, current setting value of ACR2, and current detection value. V _AC is the alternating current grid voltage. The output signal of switch 31 or 32 in FIG. 4 and the output signal of minimum value selection circuit 40 in FIG. 5 are sent to a phase control circuit to determine the firing 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 vehicle is operated by ACR1 in a steady state. For example, if the current is 1000A, ACR1
I _dpr will be set to the optimal value for 1000A flow. At this time, the V _dpr of the AVR is set, for example, to perform constant voltage control of 230 kV with respect to the rated voltage of 250 kV. Furthermore, I _dpr ' of ACR2 is set to perform constant current control of, for example, 100A. If the settings are made as above, the output of the AVR will be negative because the detected value is 250 kV compared to the set value of 230 kV (in actual equipment, the output limiter will set the minimum value of α), The maximum value selection circuit 27 selects the output of ACR1. Next, ACR2
For the output of 100A, the detected value is
1000A, α becomes large (in actual equipment, the maximum value of α is determined by the output limiter), and in the end, the output of ACR1 is selected as the output of the minimum value selection circuit 28, and the maximum Value selection circuit 2
As the output of 9, 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 this 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. In this case, 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 REC2 is stopped by cutting off the DC current or the disconnector 15. At this time, the operating points are P ₁ , P ₃ , and P ₄ shown in the characteristic diagram of FIG. 7. That is, the seventh
In the figure, the voltage determining terminal moves 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. That is,
If the above equation no longer holds true, it means that the forward converter cannot supply the current value required by the inverse converter, and the inverse converter automatically lowers its own voltage to secure the necessary current. This is because the DC voltage of the system will eventually drop to zero. In other words, all converters, whether forward or inverse converters, perform constant current control and are operated with completely uncoordinated current setting values.

Next, JP-A-51-66455 added the concept of voltage margin to the concept of current margin, but as is clear from the comparison of control characteristic diagrams 2 and 3, the essential are the same. 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 terminal (REC2) that determines the voltage to that of all other terminals. Since the voltage determining terminal is determined by making the voltage smaller than the voltage setting value, the system in Japanese Patent Laid-Open No. 51-66455 also becomes unstable for the same reason as in Japanese Patent Publication No. 43-8641. Unexamined Japanese Patent Publication 1973
The difference between Publication No. 66455 and Japanese Patent Publication No. 43-8641 is that normal tap control takes several seconds to operate one tap, but it is assumed that high-speed control can be achieved by centralizing control using a transmission line. This is what we are doing.

However, in the present invention, since the voltage setting values of 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 To act 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 method 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.

Next, commutation failure will be considered. 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, Japanese Patent Publication No. 43-8641,
In the method of JP-A No. 51-66455, REC1 and
All current of REC2 flows into INV1, and INV1
becomes an overcurrent, and even if the voltage of the AC system 3 is restored, INV1 will be unable to start. however,
In the present invention, INV1 can be started without any problems. The reason for this will be explained using the control characteristic diagram shown in FIG.

When INV1 fails in commutation, the currents of REC1 and REC2 all flow into INV1, but when commutation fails,
Since this is the same as a DC short circuit, the DC voltage is almost 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
This means that only a current equal to the sum of I _dpr ′ ₁ and I _dpr ′ ₂ flows. Therefore, if I _dpr ′ ₁ and I _dpr ′ ₂ are set in advance to small values that do not cause intermittent currents, INV1 will increase as the voltage of AC system 3 recovers.
can be started. 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
The same applies to INV2. 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 straight line downward to the right,
is due to a minimum value limiter of α, not shown in FIG. 4, and the straight line is due to constant margin angle control.

Now, let's consider this in more detail. Now, in FIG. 6, it is assumed that the following equation holds true.

I _dpr ′ ₁ + I _dpr ′ ₂ < I _dpi ′ ₁ + I _dpi ′ ₂ … In this state, as in the previous example, AC system 3
Assume that the voltage of INV1 decreases and commutation of INV1 fails as a result. At this time, as mentioned earlier,
The DC voltage becomes almost zero and all converters shift to ACR2 operation, but since the conditions in the formula are met, INV1 and INV2 shift to the REC region and remain in a stable state. . In this state, even if the voltage of the AC system 3 is restored, it is no longer possible to automatically escape from this stable state; for example,
Generally, it is not possible to return to the original state unless I _dpr ′ ₁ and I _dpr ′ ₂ are increased. However, the method of temporarily increasing I _dpr ′ ₁ and I _dpr ′ ₂ as described above is not preferable because the timing of the increase is very difficult and the method has to depend on the transmission system. Therefore, I _dpr ′ ₁ , I _dpr ′ ₂ ,
Set I _dpi ′ ₁ and I _dpi ′ ₂ .

I _dpr ′ ₁ + I _dpr ′ ₂ > I _dpi ′ ₁ + I _dpi ′ ₂ ... In this way, when the previous commutation fails, the original state can be restored without any problem. however,
For example, as shown in Figure 7, the above formula is insufficient when REC2 is stopped due to an accident.
In other words, if REC2 is stopped due to an accident, there is a possibility that I _dpr ′ ₁ < I _dpi ′ ₁ + I _dpi ′ ₂ . Therefore, I _dpr ′ ₁ , I _dpr ′ ₂ , I _dpi ′ ₁ , and I _dpi ′ ₂ are set so that the following equations hold true.

I _dpr ′ ₁ ≧I _dpi ′ ₁ +I _dpi ′ ₂ … I _dpr ′ ₂ ≧I _dpi ′ ₁ +I _dpi ′ ₂ … If you do this, there will be no problem at all even when commutation fails or when the REC terminal stops due to an accident. .

(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.

(g) Integrated 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 to the constant voltage. The voltage setting value is set to be smaller than the voltage setting value of the converter that performs control, and each voltage setting value is set to a different value, and the second constant current control circuit at least maintains a DC voltage lower than the DC voltage value in a steady state. The constant current setting value of the second constant current controller of each forward converter is greater than the sum of the constant current settings of the second constant current controller of each inverse converter. By setting a large value, it is possible to prevent the system from stopping due to an accidental stop of a certain converter.

Direct current and disconnectors can be used effectively.

Dependency on transmission lines can be reduced.

Dependency on a centralized 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, Figure 3 is another conventional control characteristic diagram, and Figures 4 and 5 are an embodiment of the present invention. 6, 7, and 8 show control characteristic diagrams 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 system 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 constant voltage setting value of the operating converter and operates 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 are different from the above and control the current so that it does not exceed a first constant current setting value different from the above for a voltage higher than the constant voltage setting value. a first constant current control device, and a second constant current control device whose current is different from the above for a voltage lower than the constant voltage setting value;
A second constant current control device different from the above is provided for controlling the constant current so that the constant current does not become less than the constant current setting value of the A constant current control device for a DC multi-terminal power transmission system, characterized in that the constant current setting value is set larger than the sum of the constant current setting values of the second constant current control device of the inverter.