JPH11289670A - Two-pole dc power transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To automatically adjust an AC voltage for ground detection, by judging the operating state of each DC power transmission system based on the switched state of a switch provided at the return pole DC line, and by adjusting the output voltage of an AC power supply. SOLUTION: When an AC power supply device 25 is connected only to a return line 6 on two-pole operation, the AC voltage of a capacitor 22 of the AC power supply device 25 is applied to the return line 6 via a capacitor 23 and a switch 54 and, furthermore, is applied to a return line 7 via a circuit breaker 13 and a switch 56. In this case, when a grounding accident occurs at F1 point of the return line 6, an AC current flowing to the grounding circuit of the return line 6 bypasses one part of the return-line 6, thus increasing the AC current, and hence detecting the occurrence of the grounding accident at the return line 6 by a grounding detection relay 16. At this time, a return line protection device 80 controls AC power supplies 20 and 20 based on the state signal of switches 14, 15, 53, 54, 55, and 56 and the circuit breaker 13 and constantly controls current being allowed to flow to each return line regardless of an operating state.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bipolar DC power transmission system, and more particularly, to a bipolar AC line having a plurality of return lines operated in parallel to detect a return line having a ground fault. The present invention relates to a bipolar DC power transmission system including an AC power supply for supplying current.

[0002]

2. Description of the Related Art A DC transmission system is formed by connecting AC / DC converters linked to an AC system to both ends of a DC line, and transmits and receives power between the two AC systems via the DC line. There is known a bipolar DC transmission system operated by dividing the two DC transmission systems into two positive and negative poles, and connecting the return lines on the neutral point side of the DC lines of each system in common. In the case of this bipolar DC transmission system, from the viewpoint of operation, return lines are individually provided, and when performing bipolar operation, both ends of a plurality of return lines are commonly connected by switches, and when performing unipolar operation, a plurality of return lines are provided. Both ends of the wire are separated by switches.

If the transmission power of each system of such a bipolar DC transmission system is balanced, no current flows on the return line. For this reason, even if a ground fault occurs on the return line, the ground fault current does not flow and cannot be detected. So, conventionally,
One end of the return line of each system is grounded, and the other end of the return line is connected to an AC power supply for ground fault detection. Is detected by a ground fault detection relay to detect a ground fault.

[0004]

However, the impedance seen from the AC power supply for detecting a ground fault differs between the bipolar operation and the unipolar operation, so that the AC current flowing through the ground detection relay changes. , The ground fault cannot be detected properly.

Therefore, it is necessary to adjust the current flowing through the ground fault detection relay to a constant value by adjusting the voltage of the AC power supply for ground fault detection in accordance with the operation state of the bipolar DC transmission system.

Conventionally, the operator judges the operation state, manually switches the voltage of the AC power supply for ground fault detection based on this, and adjusts the current flowing through the ground fault detection relay to a constant value. There is a problem that operability is poor because the time required for switching becomes longer and there is a possibility of erroneous operation.

The problem to be solved by the present invention is to solve the above problem, that is, to make it possible to automatically adjust the AC voltage for ground fault detection in accordance with the operation mode of the bipolar DC power transmission system. .

[0008]

SUMMARY OF THE INVENTION The present invention divides at least two sets of DC transmission systems having AC / DC converters at both ends, which are linked to an AC system, into two positive and negative poles and returns the DC power to the neutral point. Both ends of the line are connected in common through a switch, and the switch is opened and closed to form two sets of the power transmission system so as to be switchable between bipolar operation and monopolar operation, and one end of each return line is grounded. The other end is connected to an AC power supply for ground fault detection, and in a bipolar DC power transmission system including a ground fault detection relay for detecting a ground fault based on the AC current flowing through each return line, the switch of the switch An object of the present invention is to solve the problem by providing control means for judging an operation state of each of the DC power transmission systems based on an open / close state and adjusting an output voltage of the AC power supply.

That is, in the bipolar DC transmission system, 2
In the case where the return of the DC line of the pole is provided independently and the bipolar operation or the single-pole operation can be selected, both ends of at least the return of the two poles are switched (breakers if a breaking capacity is required). Connect through. Therefore, if all of those switches or circuit breakers are in the closed state, the operation is bipolar.

When switches are provided between the grounding end of the two-pole return line and the grounding pole, bipolar operation is possible when both switches are closed according to the open / closed state of the switches. When one of them is closed and the other is open, it is in the state of unipolar operation in which the open pole is stopped. Furthermore, if switches are provided at both ends of each return line so that the return line can be separated from the DC line, if the switches at both ends of either return line are open, the DC line Is not in use. Therefore, depending on the open / closed state of the switch that allows connection of both ends of the return line of the two-pole DC line, or in addition to this, the open / closed state of the switch provided on the return line of the two-pole DC line, The operation state of the transmission system can be determined.

Therefore, by detecting the open / closed state of a switch that enables at least both ends of the return line to be commonly connected,
Bipolar operation and unipolar operation can be automatically determined, so by adjusting the output voltage of the AC power supply according to the determination, the AC voltage for ground fault detection can be automatically adjusted according to the operation mode of the bipolar DC transmission system. Can be adjusted.

In this case, it is preferable that the control means adjusts the output voltage of the AC power supply so that the AC current flowing through each return line is maintained at a substantially set value. According to this,
Since the set value for the determination of ground fault detection can be set strictly,
Detection accuracy can be increased.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a configuration diagram of an embodiment of a bipolar DC power transmission system according to the present invention. In the figure, the bipolar DC transmission system is formed to have two DC transmission systems A and B. The DC transmission system A includes AC / DC converters 1A, 1B and AC / DC converters 2A,
2B is connected. The AC / DC converters 1A and 1B and the AC / DC converters 2A and 2B are connected to AC systems 40 and 41, respectively. The AC-DC converters 1A, 1B, 2A,
2B has a function of converting AC to DC or DC to AC, respectively, and is formed using a semiconductor switch such as a thyristor. The DC line of the DC transmission system A includes a main line 5 and a return line 6. One end of the main line 5 is connected to the positive electrode of the AC / DC converter 1A via a switch 51 and a DC reactor 9, and the other end is connected to a switch 52 and a DC reactor 10A.
To the negative electrode of the AC / DC converter 2A. Both ends of the return line 6 are connected to a negative electrode of the AC / DC converter 1B via a switch 53 and a positive electrode of the AC / DC converter 2B via a switch 54, respectively. One DC transmission system B, like the DC transmission system A, has AC / DC converters 3A, 3B, 4A, 4
B, and a DC line composed of a return line 7 and a main line 8. However, the difference is that the line connected to the positive electrode side of the AC / DC converter 3A is the return line 7 and the line connected to the negative electrode side of the AC / DC converter 3B is the main line 8. The switch 57, the DC reactor 11, the switch 58, and the DC reactor 12 are connected to the main line 8, similarly to the DC power transmission system A.

One end of the return line 6 on the AC / DC converter 1B side is grounded via a switch 14. Similarly, one end of the return line 7 on the AC / DC converter 3A side is grounded via a switch 15. The other ends of the return line 6 and the return line 7 can be commonly connected via a switch 54, a circuit breaker 13, and a switch 56.

An AC power supply 25 and an AC power supply 35 for detecting a ground fault are connected to the ends of the return lines 6 and 7 on the side opposite to the ground. The AC power supplies 25 and 35 are respectively
It is formed to include an AC power supply 20, an insulating transformer 21, and a capacitor 22 connected in parallel to the secondary side of the insulating transformer 21. One end of the capacitor 22 is grounded, and the other end is connected to the capacitor 23 and the switches 24, 54 (or the switch 34,
56) to the return line 6 (or 7). In the case of bipolar operation, only one of the AC power supply device 25 and the AC power supply device 35 is used, and the other is a standby device. For monopolar operation, use both. The switching is performed by switches 24 and 34.

Further, ground fault detection relays 16 and 17 are provided at ends of the return lines 6 and 7 which are remote from the grounding ends. The ground fault detection relays 16 and 17 are connected to the current transformers 1 and 198, respectively.
When the AC current of the return lines 6 and 7 detected by the above is equal to or greater than a predetermined reference value, a ground fault accident of the return lines 6 and 7 is detected.

Next, the configuration of the characteristic portion of the present invention will be described. The return line protection device 80 includes line switches 53, 54,
55, 56, line switches 14, 15 for 2-pole separation,
A state signal indicating the open / close state of the circuit breaker 13 for bipolar separation is input. The return line protection device 80 determines the operation state of the bipolar DC transmission system, particularly the operation state of the return lines 6 and 7 based on the input state signal under the following conditions.

(1) Line switches 53, 54, 55, 56
When all of the line switches 14, 15 for separating the two poles and the circuit breaker 13 for separating the two poles are in the closed state, the systems A, B
It is determined that both return lines 6 and 7 are in operation. (2) When only the line switch 14 of the two-pole separating line switches 14 and 15 is in an open state, it is determined that the system A is in a single-pole operation state in which the system A is stopped, and the line switch is determined. When only 15 is in the open state, it is determined that the system B is in the monopolar operation state in which the system B is stopped. When both the track switches 14 and 15 are in the open state, it is determined that both the systems A and B are stopped. (3) When the line switches 53 and 54 are in the open state, the return line 6 is in an unused state.
When 6 is in the open state, it is determined that the return line 7 is in an unused state.

The return line protection device 80 sends a command to change the output voltage to the AC power supply 20 or 30 based on the result of the determination, and sets the AC current for ground fault detection in advance according to the operation state. The set current is set to flow to the return line.

The operation of the embodiment constructed as described above will be described below. The DC transmission system A is, for example, an AC system 4
AC power supplied from 0 is converted into DC power by AC / DC converters 1A and 1B, and transmitted to AC / DC converters 2A and 2B via a main line 5 and a return line 6 which are DC lines. AC / DC converter 2
A and 2B convert the transmitted DC power into AC power and supply it to AC system 41. In addition, it is also possible to transmit electric power from the AC system 41 to the AC system 40 in the reverse direction.
Similarly, the DC transmission system B also includes an AC system 40 and an AC system 41
Send and receive power between

Here, the operation of the DC transmission systems A and B will be briefly described. First, the circuit breaker 13 and the line switch 1
Closing 4, 15, 53-56 results in bipolar operation. In this case, since the currents flowing into the return lines 6 and 7 are opposite to each other and cancel each other, the current flowing through the return lines 6 and 7 is reduced, and power transmission loss can be reduced. In particular, DC transmission systems A and B during bipolar operation
If the amount of transmitted power is equal, the current flowing through the return line 6 and the return line 7 can be reduced to zero, and such operation is called bipolar balance operation. On the other hand, when the circuit breaker 13 is operated in the open state, the DC power transmission systems A and B can be operated or stopped independently. When stopping, the line switches 14, 15 of the corresponding system are opened.

Here, the operation of ground fault protection during bipolar balance operation will be described. First, at the time of bipolar operation, one of the switches 24, 34 is opened, and one of the AC power supply devices 25, 35 for detecting a ground fault is connected to the return line 6, the return line 7, and the other is connected. Make a spare. Here, description will be given assuming that AC power supply device 25 is connected to return line 6. The capacitor 22 of the AC power supply 25 is charged by the AC power supply 20 via the insulating transformer 21. The AC voltage of the capacitor 22 is connected to the capacitor 23 and the switch 5.
4 to the return line 6, and further to the return line 7 via the circuit breaker 13 and the switch 56. This allows
Capacitor 22 → Capacitor 23 → Switch 24 → Switches 54 and 56 → Return 6,7 → Switches 53 and 55 → Switch 1
An alternating current flows through the route of 4, 15 → ground → capacitor 22. Here, for example, if a ground fault occurs at the F1 point of the return line 6, in the ground circuit including the return line 6, the capacitor 22
AC current flows through the route of the capacitor 23 → switch 24 → switch 54 → return line F1 point → ground → capacitor 22. On the other hand, in the ground circuit of the return line 7, an alternating current in a normal state where the above-mentioned ground fault has not occurred flows. FIG. 2 shows an equivalent circuit of the above-described ground circuit. The resistance 61 and the reactance 62 are the impedance of the return line 6, and the resistance 71 and the reactance 72 are the impedance of the return line 7.

The alternating current flowing in the ground circuit of each return line in a normal state is determined by reactances 62 and 72. Return 6
When a ground fault occurs at the point F1 of the above, since the AC current flowing through the ground circuit of the return line 6 bypasses a part of the reactance 62 of the return line 6, the AC current becomes larger than when no ground fault occurs. Become. Thus, the ground fault detection relay 16 detects that the AC current flowing through the return line 6 is equal to or greater than the determination reference value, and detects that a ground fault accident has occurred on the return line 6. On the other hand, the ground fault detection relay 17 does not operate because the same alternating current flows as in the normal state, so that the ground fault can be detected by selecting the return line.

Here, it is preferable that the AC current for detecting a ground fault flowing through the return lines 6 and 7 is as low as possible. The criterion values set in the ground fault detection relays 16 and 17 are set strictly in order to cope with the difference in the ground fault point position. Therefore, it is important that the AC current for ground fault detection flowing in each return line is kept at a constant value. However, when the state shifts from the bipolar operation to the unipolar operation or the unipolar stop, and conversely, when the state shifts from the unipolar operation or the unipolar stop to the bipolar operation, the load impedance viewed from the AC power supply device 25 greatly changes. , To keep the AC current of each return line constant
It is necessary to adjust the output voltage of the AC power supply 20.

Therefore, in this embodiment, the switch 14,
Based on the state signals of the open / closed state of the circuit breaker 13, 15, 53, 54, 55, 56, the return line protection device 80 determines the return line operation state in accordance with the above-described conditions. According to this determination, the AC power supply 20 is controlled so as to keep the current supplied to each return line constant. For example, in FIG. 2, the impedance viewed from the AC power supply device side when only one return line is used is Rc + jXc, and the operation voltage at this time is V1 = I (Rc + jXc). On the other hand, the impedance viewed from the AC power supply device side when two return lines are operated is (Rc + jXc) / 2, and when the current I flowing through the return line is constant, the operating voltage at this time is V2 = I × (Rc + jXc) / 2 = I (Rc + j
Xc) / 2 = V1 / 2.

As described above, the flyback protection device 80 controls the AC power supply 20 or 30 depending on the operation state of the flyback,
The current flowing through each return line is controlled to be constant regardless of the operation state.
As a result, the current value flowing through the operated return lines does not change irrespective of the number of operation of the return lines 6 and 7, so that the ground fault detection relay 1 that detects the presence or absence of a ground fault based on the determination reference value is provided.
Operations 6 and 17 can be made appropriate.

When the ground fault detection relay 16 operates, the return line protection device 80 detects that the return line 6 has a ground fault.
When the ground fault detection relay 17 is operated, it is determined that the return line 7 is grounded, the DC transmission system having the ground fault is separated and stopped, and operation with a healthy pole is enabled.

[0028]

As described above, according to the present invention,
In a bipolar DC transmission system having a plurality of return lines, the AC voltage for ground fault detection can be automatically adjusted in accordance with the operation mode of the bipolar DC transmission system.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an embodiment of a bipolar DC power transmission system according to the present invention.

FIG. 2 is an equivalent circuit of the ground fault circuit according to the embodiment of FIG.

[Explanation of symbols]

 1,2,3,4 AC / DC converter 5,8 Main line 6,7 Return line 9,10,11,12 DC reactor 13 Circuit breaker 14,15 Line switch 53,54,55,56 Line switch 16,17 Ground fault detection relay 20, 30 AC power supply 25, 35 AC power supply 40, 41 AC system 80 Return protection device

Claims (2)

[Claims] 1. At least two sets of DC power transmission systems having AC / DC converters at both ends linked to an AC system are divided into positive and negative poles, and both ends of a return line on the neutral point side are connected via switches. Connected in common, the switch is opened and closed to form two sets of the DC transmission system switchable between bipolar operation and monopolar operation,
One end of each return line is grounded, the other end is connected to an AC power supply for detecting a ground fault, and a ground fault detection relay for detecting a ground fault based on an AC current flowing through each return line is provided. A bipolar DC power transmission system comprising control means for judging an operation state of each of the DC power transmission systems based on the open / close state of the AC power supply and adjusting an output voltage of the AC power supply. 2. The bipolar DC power transmission system according to claim 1, wherein said control means adjusts said output voltage so as to keep an alternating current flowing through each of said return lines at substantially a set value.