JPH0126253B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION In a DC multi-terminal power transmission system consisting of a plurality of converters, when all forward conversions are power transmitting terminals and all inverse conversions are power receiving terminals, one or more voltage determining terminals are used. The present invention relates to a highly reliable DC multi-terminal power transmission system in which all terminals other than that terminal can freely transmit and receive power completely independently.

In conventional DC multi-terminal power transmission, load distribution for each terminal,
Starting, stopping, etc. all depend on control via communication lines etc. from a central control unit that is centralized at one location. Therefore, for example, when a ground fault occurs at a certain point in a DC power transmission line, all the converters to be stopped, the load redistribution, etc., are removed by commands from the central control unit. For this reason, communication lines are required to have extremely high reliability, while the functions of the central processing unit become complicated, significantly reducing the degree of freedom in system configuration.

An object of the present invention is to improve such a conventional system and provide a DC multi-terminal power transmission system that has a degree of freedom comparable to that of AC power transmission.

Below, the present invention will be explained in the case where it is applied to, for example, the multi-terminal system shown in FIG.

If the central control function were to simply specify the steady-state electrical output value of the alternating current generator on the power transmission side, it would be possible to make it as dependent on communication lines as current AC power transmission. In a DC multi-terminal system, if the forward converters included in this system are all power transmission terminals connected to the generator, and the inverse converters are power receiving terminals connected to the load system, then these power transmission terminals are as follows: It is sufficient to control the power supply so that it has the function of constantly transmitting the specified output. In other words, when these power transmission terminals are collected to form a DC power transmission network, if the voltage at at least one point on the transmission line is kept constant, each forward converter will have a line voltage drop less than that voltage. By operating at a higher output voltage, it becomes possible to transmit the required power. Each inverter serving as a power receiving terminal may perform well-known constant current control operation so as to obtain a predetermined alternating current output, and the specified current value can be selected completely independently for each inverter. However, it is impossible to constantly obtain received power that exceeds the total transmitted power of the forward converter, and this is not allowed even in an AC power transmission system. Therefore, during steady operation, the central control device specifies the power to be transmitted to each forward converter and the power to receive to each inverse converter.

In the DC multi-terminal power transmission system configured in this manner, it is necessary to maintain the voltage of the power transmission line at a reference point at a certain point. The following two methods can be applied to this.

One of them is at least one forward converter that is voltage controlled so as to maintain a constant DC output voltage regardless of fluctuations in the transmitted power of one forward converter.
One method is to provide at least one forward converter, and the other method is to provide at least one inverter whose voltage is controlled so as to maintain a constant DC input voltage completely independent of fluctuations in the received power of one inverter. This is a method of installing an inverse conversion device.

Now, assume that the power transmission terminal that regulates the voltage of the DC transmission line is REC1 in Fig. 1. REC1 can be considered to be equivalent to a load adjustment generator for frequency maintenance in AC power transmission. As is well known, in an AC system, active power fluctuations result in frequency changes, so in order to maintain a constant frequency, a load adjustment generator that absorbs active power fluctuations and suppresses frequency changes requires several percent of the total AC power transmitted. Approximately 10% or so. Even in a DC system, for example, as shown in Figure 1.
If the relationship between the DC output Ps ₁ of REC1 and the DC output voltage Vd is controlled as shown in Figure 2, the unbalance between the transmitted power and received power of the entire system will be reduced to the power fluctuation Pc within the control range of REC1. If the voltage remains constant, the line voltage can be held at a constant voltage Vdn at the point in FIG. The reason why the load power is suppressed from a certain amount in FIG. 2 is for the purpose of protecting the converter, and will be explained in detail later.

For example, in the case where the inverse converter INV4 is cut off due to an accident, if the transmitted power of each forward converter shown in Fig. 1 is Ps ₁ to _{Ps 4} , then the sum of them is Ps ₁ + Ps ₂
+Ps ₃ +Ps ₄ represents the power received by each inverter from Pr ₁ to _{Pr 5}
By automatically changing Ps ₁ so that it becomes equal to the sum Pr ₁ + Pr ₂ + Pr ₃ + Pr ₅ , the voltage at the point is kept constant, and the entire system continues to operate without disturbance. , REC2 to REC4 on the power transmission side
No load redistribution is necessary.

In order for the grid to operate stably, the power fluctuation Pc at the voltage determining terminal mentioned above and the total transmitted power 〓 ⁿ Psn
Between ≡Ps and total received power 〓 ⁿ Prn≡Pr｜Ps｜
It is clear from the above that it is a necessary and sufficient condition that the relationship −|Pr|≦|Pc| is satisfied.

The generator and forward converter, which serve as voltage determining terminals, are required to be able to respond to rapid load changes. An example of this control is shown in FIG. In Fig. 3, 1 is a prime mover, 3 is a governor for controlling the output of the prime mover 1, 4 is a frequency control measurement, f _det is a detected frequency, f _ref is a reference frequency, 2 is an alternator, 5
is a frequency detection device, 6 is a forward conversion device, 7 is a DC voltage detector, 8 is a phase control device for the conversion device, 9 is an amplifier, 10 is an AC voltage automatic controller, 11 is a field thyristor device, 12 is an AC Each field winding of the generator 2 is shown.

The operation of this control circuit will be explained. FIG. 3 A control circuit consisting of a DC voltage detector 7, a phase control device 8, and an amplifier 9 controls each thyristor of the forward conversion device 6 in order to make the output DC voltage Vd of the forward conversion device 6 match a constant reference value Vref. This is for phase control.
Now, even if the load on the forward converter 6 decreases and the voltage Vd tries to increase accordingly, the forward converter 6 will keep the voltage constant through the above-mentioned phase control, but since the load is decreasing, The speed of the alternator 2 is about to increase. This is detected by the frequency detection device 5, and the output of the prime mover 1 is reduced by the circuits of the frequency control device 4 and speed governor 3 so that the frequency becomes a constant reference value _fref , and its speed is adjusted. Keep it at a constant value. This control makes it possible to respond to output fluctuations. In order to keep the output AC voltage of the AC generator 2 constant regardless of the operation of the forward converter 6,
The current in the field winding 12 is controlled by controlling the 11 field thyristor devices via the 10 automatic voltage regulators. The prime mover 1 is preferably one that can change its output significantly and quickly, and hydraulic power, peak thermal power, etc. are applied.

The other forward converters REC2 to REC4 and their AC power supplies _G2 to _G4 in FIG. All you have to do is control it.
An example is shown in FIG. 1 in Figure 4 is the prime mover,
2 is an alternating current generator, 3 is a prime mover output control device, 5
is the output frequency detection device of the AC generator 2, f _det is the detected frequency, f _ref is the frequency reference value, 4 is the frequency control device, 13 is the signal conversion device, 6 is the forward conversion device, 7 is the DC voltage detection 14 is a DC current detection device, 15 is a DC power detection device, Pdet is a detected DC output, Pref is a specified value of power given from the central control device, 16 is a power control device, 17 is a field thyristor phase control 11 is a field thyristor device, and 12 is a field winding.

The operation of this control circuit will be explained below. The forward conversion device 6 shown in FIG. 4 operates with a fixed minimum phase control angle during steady state. Therefore, if a DC circuit or a disconnector is provided in the DC circuit for failure protection as described later, there is no problem even if the forward converter is simply a diode. The purpose of using a thyristor in this forward conversion device is to protect against DC line failures, inverter commutation failures, etc., as will be described later, and in consideration of cases where DC current or disconnectors cannot be applied. The specified power value Pref from the central control device is given to both the output control circuit of the prime mover 1 and the field control circuit of the alternator 2 in parallel. As is well known, in DC power transmission, output power can be controlled by controlling DC voltage. Therefore, the voltage detection circuit 7 and current detection device 14 provided in the DC circuit detect the DC voltage and current, and the power detection device 15 synthesizes them and outputs the output signal Pdet as the specified value Pref. comparison,
The power control circuit 16 performs amplification and signal level conversion, controls the field thyristor device 11 via the phase control device 17, adjusts the current in the field winding 12 of the alternator 2, and achieves a predetermined value. generate voltage,
A predetermined DC power is transmitted.

On the other hand, since a prime mover output corresponding to this DC power is required, a predetermined output is obtained by the output control device 3 of the prime mover 1 via the device 13 for signal level conversion of the central control device Pref. adjust. Since the output control of the forward converter 6 and the output control of the prime mover 1 are performed separately, errors occur in both, and the rotational speed, that is, the frequency of the alternator changes. Since this variation is undesirable, the output control of the prime mover 1 is slightly corrected via the devices 5 and 4 for constant frequency control.

As described in detail above, since the control illustrated in FIG. 4 leaves the forward converter 6 uncontrolled, there is no need to send and receive high-speed control signals through communication lines in conventional DC multi-terminal control. It has many features. In addition, generators G ₂ to _{G 4} are
It is also applicable to large thermal power plants, nuclear power plants, and other plants that cannot tolerate large load fluctuations.

Although the case where the voltage determining terminal is placed on the forward conversion side has been described above, it is of course possible to provide the voltage determining terminal on the reverse conversion side. In this case, for example, the inverter INV1 shown in FIG. 1 is controlled to have a constant voltage at the DC input point by well-known constant voltage control. Of course, no matter which inverter you choose, the DC transmission system voltage will be maintained close to the specified value, but if line voltage drop is taken into account, the voltage determining terminal should be selected at a point where it is least affected as much as possible. It is preferable.
For example, it is clear that a point on a main power transmission line is preferable to a point at the end of a line.

Although the above description has been made in detail regarding steady operation, protection in the event of an abnormality, such as when a ground fault occurs in a DC power transmission line or a commutation failure occurs in the inverter, will be described. In a DC multi-terminal system, if a DC or disconnector that can cut off the fault current (at least 5 times the steady current) is applicable, or if it can cut the fault current (at least about 5 times the steady current) or The protection methods are different depending on the case where the load switchgear is used or where no current switchgear is applicable.

If a current switchgear cannot be applied at all, when an overcurrent is detected in the forward converter, the current will be reduced to zero using well-known gate block protection, and the entire system will be temporarily stopped and then restarted. , depending on the type of prime mover, this is an extremely undesirable protection method. Therefore, in the DC multi-terminal power transmission of the present invention, it is necessary that at least the DC load switching device is connected to both forward and reverse converters.

Since the load switching device cannot withstand or cut off an excessive fault current, protection becomes impossible if, for example, most of the total short-circuit current of a forward converter flows into one reverse converter in which commutation has failed. Furthermore, even if a line failure occurs, it is undesirable that the entire short-circuit current of the forward converter is concentrated at that point. In view of this point, in the multi-terminal power transmission system of the present invention, the well-known voltage-dependent current reduction control (CIGRE Report 14-10, 8/30-9/
7, 1978) is applied to forward and reverse converters to suppress the fault current to a low level and make it possible to interrupt the fault current using a DC switch. The voltage-current characteristics of the forward and inverse converter when the voltage-dependent current reduction control is performed are as shown in FIGS. 5a and 5b. FIG. 5a shows the case of forward transformation, and FIG. 5b shows the case of inverse transformation. The sum of the short circuit currents I _SRo of each forward converter,
The system may be configured so that the difference between the short-circuit current _ISIo of each inverter and the sum total does not exceed, for example, the current that can be cut off by the load switching device.

If it is applied to DC or disconnection devices, there is no need to control the voltage-current characteristics of each forward and reverse conversion device as shown in Figure 5, and it is possible to control the necessary parts at the same time as failure detection. The fault can be removed by operating the disconnection device. Therefore, the forward conversion device except for the voltage determining terminal can be configured with diodes instead of thyristors, which increases the reliability of the entire system and at the same time allows the DC multi-terminal power transmission system to have the same degree of freedom as AC power transmission. It is now possible, and the effects are enormous.

[Brief explanation of drawings]

Fig. 1 is a diagram showing an example of DC multi-terminal power transmission to which the present invention can be applied, and Fig. 2 is a diagram showing the relationship between the load power and DC voltage of one forward converter when used as a voltage determining terminal. , Fig. 3 is a control block diagram of a forward converter that serves as a voltage control terminal, Fig. 4 is a control block diagram of a forward converter that serves as a terminal other than the voltage determining terminal, and Fig. 5 shows a control block diagram of a forward converter that serves as a terminal other than the voltage determining terminal. FIG. 3 is a characteristic diagram a of the forward transform device and a characteristic diagram b of the inverse transform device when REC1 to REC4...Forward conversion device, _G1 to _G4 ...
AC generator, INV1 to INV5...inverse conversion device,
AC1 to AC5...AC power receiving system, 1...prime mover,
2... AC generator, 3... Prime mover output control device, 4... Frequency control device, 5... Frequency detection device, 6... Forward conversion device, 7... DC voltage detection device, 8... Phase control Device, 9...Amplifier, 10...
... Automatic voltage adjustment device, 11 ... Field thyristor device, 12 ... Field winding, 13 ... Signal conversion device, 14 ... DC current detection device, 15 ... DC power detection device, 16 ... Electric power Control device, 17... Field thyristor phase control device.

Claims (1)

[Claims] 1. In a DC multi-terminal power transmission system consisting of a plurality of converters, all forward converters being connected to one or more alternating current generators, at least one of the forward converters is connected to one or more alternating current generators. The converter is operated as a voltage determining terminal for the transmission line by controlling the DC output voltage to be constant, and all forward converters other than this voltage determining terminal have their phase control angles fixed. The DC output voltage of these forward converters is controlled by the field current of the alternator connected to the converter, so that the specified power is transmitted, and the total inverter is is controlled to supply the specified AC load power, and the total transmitted power Ps
and between the total received power Pr and the transmitted power Pc at the voltage determination terminal, which is a part of the total transmitted power, |Ps|−
A DC multi-terminal power transmission system characterized in that |Pr|≦|Pc| is established. 2. In a DC multi-terminal power transmission system consisting of a plurality of converters, in which all forward converters are connected to one or more alternating current generators, at least one of the inverse converters is connected to a DC converter. By controlling the input voltage to be constant, it is operated as a voltage determining terminal for power transmission lines, and all forward converters are operated with their phase control angles fixed, and the DC voltage of these converters is specified. so that you can get the power
It is controlled by the field current of the alternator connected to each converter, and all inverters other than the voltage determining terminal are controlled so that they each supply the specified AC load power, and all power transmission Between the power Ps, the total received power Pr, and the received power Pc at the voltage determining terminal, which is a part of the total received power, |Ps|−|Pr|≦|
A DC multi-terminal power transmission system characterized by ensuring that Pc| holds true.