JPH0126257B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a DC multi-terminal power transmission system that can maintain the system stably even if any converter is disconnected from the system due to an accident or the like in a DC multi-terminal power transmission system consisting of a plurality of converters. Regarding terminal power transmission equipment.

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, 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, 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
43-8641 and JP-A-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. Hereinafter, for convenience of explanation, the forward conversion device 9 in FIG.
The forward conversion device 10 is set to REC2, and the inverse conversion device 11 is set to REC2.
INV1, inverse conversion device 12 as INV2, REC1,
The current setting values of the constant current control circuit of REC2 are Idpr ₁ and Idpr ₂ , the current values that actually flow to REC1 and REC2 are Idr ₁ and Idr ₂ , and the current setting values of the constant current control circuit of INV1 and INV2 are Idpi _1. , Idpi ₂ , and the current value actually flowing through INV1 and INV2 as Idi ₁ ,
Abbreviated as Idi ₂ . Now, in Fig. 2, the operating points of each converter in the steady state are P ₁ , P ₂ , P ₃ ,
It is _P4 . 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.

(Idpr ₁ + Idpr ₂ ) - (Idpi ₁ + Ipi ₂ ) = ΔI≧0 ... (Idr ₁ + Idr ₂ ) = (Idi ₁ + Idi ₂ ) ... 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 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. .

Now, 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, a characteristic diagram of JP-A-51-66455 is shown in FIG.

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, Japanese Patent Application Laid-Open No. 51-66455 proposes that a central control device be provided, and the information of all conversion devices can 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, so commutation failures are usually minor. It is treated as a failure 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 for a short period of time.) In other words, for example, if the previous commutation failure is a temporary AC system voltage 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. In other words, since 200% current is flowing,
The commutation overlap angle increases significantly and the margin angle becomes insufficient, making commutation impossible. 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.

The purpose of the present invention was to eliminate such drawbacks, and to reduce the dependence on transmission lines and centralized control equipment as much as possible without causing a system stoppage due to an accidental stoppage of a certain converter. Furthermore, it is an object of the present invention to provide a new DC multi-terminal power transmission equipment that can prevent system stoppage due to commutation failure.

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, numerals 21 to 25 are adders, into which respective set values and detected values are input. 26 is an inverting amplifier, 27 is a limiter circuit, 28 is a constant voltage control circuit (hereinafter abbreviated as AVR), 29 is a first constant current control circuit (hereinafter abbreviated as ACR1), 3
0 is a second constant current control circuit (hereinafter abbreviated as ACR2), 31 and 32 are minimum value selection circuits, and control delay angle (hereinafter abbreviated as α).
This circuit automatically selects the output signal of the control circuit with the smaller value. FIG. 5 also has almost the same configuration as FIG. 4, with 33 to 35 being adders, 36 being an AVR,
37 is ACR1, 38 is 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. In addition, Vdpr and Vdr respectively indicate the voltage setting value and voltage detection value of the forward conversion device, and Vdpi,
Vdi indicates the voltage setting value and voltage detection value of the inverter, respectively. Idpr, Idpr′, and Idr respectively indicate the current setting value of ACR1, the current setting value of ACR2, and the current detection value of the forward conversion device, and Idpi, Idpi′, and Idi are
The current setting value of ACR1 of the inverter, respectively.
Shows the current setting value and current detection value of ACR2. V _AC
is the AC system voltage. The output signal of the minimum value selection circuit 32 in FIG. 4, the minimum value selection circuit 40 in FIG.
The output signal of is 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 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, α becomes very large (in actual equipment, the maximum value of α is determined by the output limiter), and the minimum value selection circuit 31 selects the Output is selected. Next, as for the output of ACR2, since the detected value is 1000 A compared to the set value of 100 A, α becomes large (in actual equipment, α becomes the maximum value due to the output limiter), and eventually, As the output of the minimum value selection circuit 32, the output of ACR1 is selected.

Now, in steady state (rated voltage), the voltage reference value
Since the detected value Vdr is larger than Vo, the output voltage of the limiter circuit 27 is zero. Therefore, the device is being operated by ACR1.
In such a state, if some kind of accident occurs and the DC voltage drops significantly, the voltage reference value Vo becomes larger than the detected value Vdr, so the output voltage of the limiter circuit 27 becomes positive, and this positive Since the output voltage is applied to the adders 22 and 24, it is as if the detected values of AVR and ACR1 have increased, and the outputs of AVR and ACR1 become larger. As a result, ACR2 operates, and in the previous example, the DC current increases.
Controlled to 100A.

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

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. Steady state operating points in FIG. 6 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 stable conditions, but the above formula was essential for stability. In other words, when the above equation no longer holds true, it means that the forward converter is no longer able to supply the current value required by the inverse converter, and the inverse converter automatically lowers its own voltage to obtain the required current. This is because the system's DC voltage eventually drops to zero in an attempt to ensure the same. 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 comparing the control characteristic diagrams 2 and 3, essentially It's 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 voltage setting value, the system in JP-A-51-66455 also becomes unstable for the same reason as in JP-A-43-8641. The difference between JP-A-51-66455 and JP-A-43-8641 is that the normal tap control is 1
The assumption is that while it takes several seconds to perform a tap operation, high-speed control can be achieved by centralizing control using a transmission line.

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,
It is stable because all the converters do not perform current control based on completely uncoordinated current setting values, as in JP-A-8641 and JP-A-51-66455.

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 such thing.

Next, commutation failure will be considered.

Now, in FIG.
Suppose that the voltage of the AC system 3 connected to (INV1) drops 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 Publication No. 51-66455, REC is
The currents of 1 and REC2 all flow into INV1, and INV1 becomes an overcurrent, even if AC system 3
Even if the voltage recovers, INV1 will not be able to start.
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 to commutate, the currents of REC1 and REC2 become
All the current flows into INV1, but since commutation failure is the same as a DC short circuit, the DC voltage is almost zero. This means that in the control characteristic diagram of Fig. 6,
All converters shift to ACR2 operation, the DC voltage becomes almost zero, and INV1 has the maximum
This means that only a current equal to the sum of Idpr' ₁ and Idpr' ₂ flows. Therefore, Idpr′ ₁ and Idpr′ ₂
By setting in advance a small value that does not cause intermittent current, the
INV1 can be started. In other words, since the current value is small, the commutation overlap angle is small, and therefore,
A sufficient margin angle can be secured. This also applies to INV2. This means that
Except for the condition that a small current is flowing, the system can be started without any problem since it is the same as starting the system from a state where no current is flowing, that is, starting the system for the first time.

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.

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.

As explained above, according to the present invention, it is possible to prevent a system stoppage due to an accidental stoppage of a certain conversion device.

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 control characteristic diagram of a conventional device, Figure 3 is another control characteristic diagram of a conventional device, and Figures 4 and 5 are diagrams of an embodiment of the present invention. The control block diagrams shown in FIGS. 6, 7, and 8 show control characteristic diagrams of the present invention. 1-4...AC system, 5-8...Conversion transformer, 9-10...Forward converter, 11-12...Inverse converter, 13-20...DC or disconnector, 21-25
... Adder, 26 ... Inverting amplifier, 27 ... Limiter circuit, 28 ... Constant voltage control circuit, 29-30
... Constant current control circuit, 31 ... Minimum value selection circuit,
32... Minimum value selection circuit, 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 in which a plurality of converters are connected by a DC power transmission system, each of the plurality of converters has a constant voltage control circuit in which a different constant voltage setting value is set, and a constant voltage control circuit in which a different constant voltage setting value is set; a first constant current control circuit that operates when the voltage of the power transmission system reaches a voltage equal to or higher than the constant voltage setting value set by the constant voltage control circuit; Each of the second constant current control circuits is provided with a second constant current control circuit that operates when the voltage is equal to or lower than the constant voltage setting value, and the current setting value of the second constant current control circuit is determined by the current setting value of the first constant current control circuit. DC multi-terminal power transmission equipment characterized by having a current value smaller than the current setting value.