JPH0216658B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an overcurrent protection system for a DC power transmission system, and more particularly to an overcurrent protection system suitable for use in digitally controlled protection for a DC power transmission system.

Conventionally, overcurrent protection in the event of an accident such as a ground fault in a DC power transmission system has been carried out by the function of a constant current control system. However, when direct current power transmission systems are operated by direct digital control using a digital processing unit, overcurrents cannot be sufficiently suppressed in the event of an accident by the constant current control system alone. The reason for this will be explained with reference to a diagram.

FIG. 1 is a system diagram showing an example of a two-terminal DC/DC power transmission system. Power converters 3 and 4 are connected to each of three-phase AC systems 1 and 2, respectively. A constant current of electric power from the three-phase AC system 1 is transmitted to the three-phase AC system 2 via a DC reactor 5, a DC power transmission line 7, and a DC reactor 6. power converter 3,
4 are operated as a forward converter and an inverse converter, and the DC current I is kept constant by phase control of the power converter 3.

FIG. 2 is a circuit diagram showing an example in which the constant current control system in the power transmission system shown in FIG. 1 is implemented by a digital arithmetic processing device. The constant current control system in Figure 2 is
DC current detection circuit 8, digital arithmetic processing device 9
and a phase control circuit 10. The DC current detection circuit 8 inputs the DC current I, outputs the DC current detection value I _D , and outputs the DC current detection value I D.
is from the DC current detection value I _D and the DC current setting value I _R ,
A manipulated variable C for keeping the DC current I constant is created through arithmetic processing. In addition, the phase control circuit 10
generates an ignition pulse for controlling the phase of the power converter 3 according to the manipulated variable C.

FIG. 3 is a block diagram showing the configuration of the digital arithmetic processing device 9. As shown in FIG. Digital arithmetic processing device 9
These include a pulse generation circuit 11 that generates a pulse P, an arithmetic processing circuit 12 that reads input information, pulses P, and memory contents or outputs a control signal, and an input circuit 13 that takes in a DC current detection value I _D and a DC current set value I _R. , a memory circuit 1 in which a program is stored.
4 and an output circuit 15 that outputs the manipulated variable C according to the processing of the arithmetic processing circuit 12. Every time the pulse generation circuit 11 generates a pulse P, the arithmetic processing circuit 12 takes in the DC current detection value _ID and the DC current setting value I _R as digital quantities via the input circuit 13, and stores them in the storage circuit 14 in advance. The manipulated variable C is created in accordance with the arithmetic processing procedure and outputted via the output circuit 15. Pulse generation circuit 1
1 generates a pulse P in synchronization with the ignition pulse of the power converter 3. When the output voltage waveform of the power converter 3 with respect to time t is as shown in FIG. 4a, the pulse P is as shown in Figure 4b.
At this time, the arithmetic processing for creating the manipulated variable C is performed every time the pulse P occurs at the time indicated by hatching in FIG. 4c. FIG. 5 is a flowchart showing the arithmetic processing contents of the arithmetic processing circuit 12. In step 16, a function f(I _D , I _R ) of the DC current detection value I _D and the DC current set value I _R is calculated, and the manipulated variable The change amount ΔC of C is set as the change amount ΔC, and in step 17, the change amount ΔC is added to the operation amount C and output as a new operation amount C.

If a ground fault occurs in the DC transmission line at time t = t ₁ during rated operation of such a DC transmission system, the waveform of DC voltage V _d with respect to time t is as follows:
The waveform of the output voltage _Ed of the power converter 3 is as shown by the broken line in FIG. 6a, and the waveform of the output voltage Ed of the power converter 3 is as shown by the solid line in FIG. 6a. In addition, the output pulse P of the pulse generation circuit 11
The waveform of the DC current Id is as shown in FIG. 6b, and the waveform of the DC current _Id is as shown in the solid line in FIG. 6c. In Figure 6a,
The reason why the waveform of the output voltage E _d of the power converter 3 does not look like the one-dot chain line is because the value of the manipulated variable C immediately after the ground fault accident is determined in the arithmetic processing for the pulse P ₁ in Fig. 6b. It is. If power converter 3
If the output voltage waveform of can be made to look like the dashed-dotted line in Figure 6a, the DC current waveform will look like the dashed-dotted line in Figure 6c, which is more accurate than the waveform shown by the solid line in Figure 6c. Overcurrent is significantly reduced. However, it is difficult to make the output voltage waveform of the power converter 3 like the one-dot chain line in FIG. 6a only by constant current control by the digital arithmetic processing device.

SUMMARY OF THE INVENTION An object of the present invention is to provide an overcurrent protection system capable of sufficiently suppressing overcurrent in the event of a fault in a DC power transmission system in digital control protection using a digital arithmetic processing device.

The present invention changes the constants in the digital arithmetic processing device under the condition that the voltage between the terminals of the DC reactor is greater than a certain value, or that the DC voltage of the grid and the phase delay of the power converter are smaller than the set values. The overcurrent in the event of an accident is suppressed by changing the phase delay angle of the power converter in steps.

FIG. 7 is a block diagram showing an embodiment of the present invention. In the drawings, the same reference numerals are given to the same members as already described members, and illustration of parts not particularly relevant to the present invention is omitted. This embodiment detects overvoltage between the terminals of a DC reactor, which is comprised of DC voltage detection circuits 18 and 19, a differential voltage detection circuit 20, a comparison circuit 21, and a pulse generation circuit 22 in the constant current control system shown in FIG. This is an additional circuit. That is, DC voltage detection circuits 18 and 19 are connected to both ends of the DC reactor 5, and the detected voltages V ₁ and V ₂ are taken into the differential voltage detection circuit 20 to find the deviation thereof. The deviation and set value V ₃
is sent to the comparison circuit 21, and based on the comparison result, the pulse generation circuit 22 generates an output signal and outputs it to the arithmetic processing circuit 12. The pulse q is generated under the condition that the voltage between the terminals of the DC reactor 5 (V ₁ −V ₂ ) changes from the state below the set voltage V ₃ to the state above the set voltage V ₃ , and the pulse q is activated by the pulse q. This system is provided with an arithmetic processing function as shown in the flowchart of FIG. However, in the process in step 23 in FIG. 8, _the operation _amount C is
This shows that the value of _C1 is changed by a step. Furthermore, the arithmetic processing shown in FIG. 8 is given priority over the arithmetic processing shown in FIG. 5, and when pulses P and q occur simultaneously, the arithmetic processing shown in FIG. shall be carried out. Furthermore, if a pulse q occurs during the execution of the arithmetic processing shown in FIG. 5, the arithmetic processing shown in FIG. 5 is temporarily interrupted;
It is assumed that the arithmetic processing shown in FIG. 8 is performed.

FIGS. 9a, b, c, d, and e are explanatory diagrams of the operation of the present invention when a ground fault occurs.

If a ground fault occurs at time t= _t1 , the DC voltage _Vd changes as shown by the broken line in Figure 9a, and the pulses P and q change as shown in Figure 9b and c, respectively.
become that way. Furthermore, the arithmetic processing in FIGS. 5 and 8 is performed at the times indicated by hatching in FIG. 9 d and e, respectively, and the output voltage waveform of the power converter 3 is as shown by the solid line in FIG. 9 a. Become. Therefore,
In such overcurrent protection schemes, the voltage level
By appropriately selecting V ₃ and the value C ₁ after the step change of the manipulated variable C, it is possible to quickly suppress overcurrent when a ground fault occurs. Furthermore, overcurrent caused by an inverter commutation failure accident can be suppressed at high speed, and after the inverter recovers, the constant current control system quickly returns to a steady operating state. moreover,
The present invention is applicable not only to two-terminal DC power transmission systems but also to digital control protection of multi-terminal DC power transmission systems having three or more terminals.

FIG. 10 is a block diagram showing a modification of the present invention. In FIG. 10, the same members as shown in FIG. 7 are given the same reference numerals. Also,
The configuration of the constant current control system is as described above, so a description thereof will be omitted. This embodiment focuses on the fact that the average value of the output voltage of the power converter 3 has a one-to-one correspondence with the magnitude of the manipulated variable C, and does not use V ₁ in FIG.
The pulse q is generated based on the magnitude of the manipulated variable C and the voltage _V2 at the end of the DC transmission line.

The output voltage V ₂ of the DC reactor 5 is detected by the DC voltage circuit 19 and sent to the comparison circuit 24 . A set voltage V ₄ is further applied as an input signal to the comparator circuit 24, and an output signal is generated when V ₄ >V ₂ . The pulse generation circuit 26 sends a pulse signal to the AND circuit 27 based on the output signal of the comparison circuit 24. The output signal of the comparator circuit 25 is also applied to the AND circuit 27 . The comparison circuit 25 compares the manipulated variable set value _C2 and the manipulated variable C output from the output circuit 15.
and output the deviation amount. The AND circuit 27 generates a pulse q on the condition that both input signals are established, and outputs it to the arithmetic processing circuit 12.
In the example of FIG. 10, the condition that the magnitude of V ₂ has decreased from V ₄ or more to V ₄ or less, and the manipulated variable C
A pulse q is generated by ANDing with the condition that the value of is such that the phase delay angle is smaller than a certain value. Therefore, if the values of the reference values V ₄ and C ₂ with which the voltage V ₂ and the manipulated variable C are compared are appropriately selected, the response of the configuration shown in FIG. 10 will be slightly lower than that of FIG. The responsiveness can be sufficiently absorbed by the reactor 5, and the effect is almost the same as that shown in FIG.

As is clear from the above, according to the present invention, even with digital control protection using a digital arithmetic processing device, overcurrent in the event of a fault in a DC power transmission system can be sufficiently suppressed.

[Brief explanation of drawings]

Figure 1 is a system diagram showing an example of a two-terminal DC power transmission system, Figure 2 is a main block diagram showing a conventional overload protection method for a DC power transmission system, and Figure 3 is the digital calculation process shown in Figure 2. Detailed block diagram of device 9,
Figures 4a, b, and c are explanatory diagrams of the operation of the device shown in Figure 3, Figure 5 is a processing flowchart of the arithmetic processing circuit 12, and Figures 6a, b, and c are conventional diagrams for when a ground fault occurs. FIG. 7 is a block diagram showing an embodiment of the present invention, FIG. 8 is a processing flowchart according to the embodiment of FIG. 7, and FIGS. Diagram explaining the operation when a ground fault occurs, No. 10
The figure is a block diagram showing a modification of the present invention. 1, 2... 3-phase AC system, 3, 4... Power converter, 5, 6... DC reactor, 7... DC transmission line, 8... DC current detection circuit, 9... Digital arithmetic processing circuit, 10... Phase control circuit, 11...
Pulse generation circuit, 12... Arithmetic processing circuit, 13...
...Input circuit, 14...Memory circuit, 15...Output circuit, 18, 19...DC voltage detection circuit, 20...
Differential voltage detection circuit, 21, 24, 25... Comparison circuit, 22, 26... Pulse generation circuit, 27... AND circuit.

Claims (1)

[Scope of Claims] 1 DC power transmission in which AC power is converted to DC power by a power converter driven by a phase control signal generated by a constant current control system including an arithmetic processing unit, and the power is transmitted via a DC reactor. In an overcurrent protection system for a DC power transmission system in which overcurrent in the event of a failure in the system is suppressed by the constant current control system, when the voltage between the terminals of the DC reactor exceeds a predetermined set value, 1. An overcurrent protection method for a DC power transmission system, characterized in that a manipulated variable for constant current control calculation in an arithmetic processing unit is changed in steps, and current control is performed by continuing the constant current control calculation. 2. In the event of a failure in a DC power transmission system that converts AC power into DC power using a power converter driven by a phase control signal generated by a constant current control system including an arithmetic processing unit and transmits the power via a DC reactor. In an overcurrent protection method for a DC transmission system in which overcurrent is suppressed by the constant current control system, the DC transmission voltage of the DC transmission system is lower than a predetermined setting value, and the phase delay angle of the power converter is When the current value becomes smaller than a predetermined set value, the operation amount for constant current control calculation in the arithmetic processing unit is changed in steps, and the constant current calculation is continued to perform current control. Overcurrent protection method for DC power transmission systems.