JPS6111531B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a protection system for DC power transmission lines, and particularly to a protection system suitable for facilitating restart of a DC power transmission system.

Conventionally, when a failure occurs in a DC power transmission line, it is common to cut off the DC current by operating a gate of a converter. For example, by operating the converters connected to both terminals of a DC transmission line as inverse converters, it is possible to quickly cut off the DC current by releasing energy from the DC system into the AC system. be.

However, such a protection scheme has the following drawbacks.

The first is that once the DC current is cut off,
If you try to restart after a certain amount of time,
In order to create a current loop through the forward and inverse converters, it is necessary to send and receive signals to synchronize both converters, which takes time. In addition, if this signal is sent and received incorrectly due to a transmission error, etc., the gate signal does not enter the inverse converter and the forward converter is activated in an open state, causing an overvoltage on the DC transmission line due to so-called full voltage activation. There is a danger that this may occur.

Second, if the arc at the fault point is extinguished during the process of inverting the converters at both ends of the transmission line and cutting off the current, the point at which the DC current is interrupted is Charges may remain in the capacitance, inhibiting the operation of the converter during restart, or even worse, the ground capacitance of the transmission line may be charged again after the arc has disappeared, causing the arc to continue until the capacitance charge disappears. There may be cases where this persists.

The present invention eliminates the drawbacks of the prior art as described above,
The purpose of this is to provide a protection method for DC transmission lines that can quickly extinguish only the arc at the fault point of the DC transmission line, while allowing DC current to circulate for stable restart operation. It is something to do.

In the present invention, when a failure occurs in a DC power transmission line, the bypass pair of one converter is ignited to create a short circuit, reduce the DC voltage to a very small value, and control the converter at the other end. In this way, by reducing the current at the failure point to zero to extinguish the arc, and then continuing to flow DC current, stable restart is possible.

Before explaining the embodiments of the present invention, the first
With reference to FIGS. 2 and 2, the current at the time of a power transmission line failure in a general DC power transmission system will be explained.

In FIG. 1, A and B are AC systems, B is a forward converter, I is an inverse converter, L _DC1 and L _DC2 are DC reactors, and DL is a DC transmission line.

Generally, in a DC power transmission system, the forward/reverse converter is equipped with a constant current control circuit, and the inverter is usually given a DC current setting value that is smaller than the forward converter by a current margin ΔId. be. Therefore, the regulation characteristics of both converters are as shown in FIG. In FIG. 2, the horizontal axis is the DC current id, the vertical axis is the DC voltage vd, R represents the regulation characteristic of the forward converter, and I represents the regulation characteristic of the inverse converter. As is well known, during normal times, operation is carried out at point A, but when a fault occurs in the power transmission line and the DC voltage becomes zero, the operating points become B and C, as shown in Figure 1. , the DC current on the forward converter side is Id, the DC current on the inverse converter side is (Id - ΔId), and the current at the failure point is ΔId.

FIG. 3 shows an embodiment of the invention. In the figure, the same reference numerals as in FIG. 1 are the same as those explained in FIG. DCCT1 and DCCT2 are DC current transformers, and FD1 and FD2 are DC transmission line fault detection devices. Although various types of failure detection devices are conceivable, a current differential type is shown here. This is because, as shown in Figure 1, when a fault occurs in a DC transmission line, a difference in ΔId appears between the currents at both ends of the transmission line, and by detecting this difference, it is possible to detect a fault in the DC transmission line. It is something to judge.

The other parts of FIG. 3 show the control circuit of the conversion device, and here, a constant current control circuit and a gate logic circuit that are related to the present invention are shown.

T ₁ and T ₂ are terminals for giving setting values,
_T1 is a terminal for giving a set value Idp corresponding to the DC current Id, and _T2 is a terminal for giving a predetermined value ΔIdp corresponding to the current margin ΔId. DA1, DA2
is an adder that adds multiple inputs with the polarity shown,
A1 and A2 are amplifiers, AP1 and AP2 are pulse phase shifters, GL1 and GL2 are gate logic circuits, PA1 and PA
2 is a pulse amplifier.

Next, the operation of this embodiment will be explained in detail using FIGS. 4 and 5.

When a fault occurs in a DC transmission line, the current distribution will be as shown in Figure 1.

The voltages at various parts of the power transmission line in this state are shown as Vdr, Vdf, and Vdi in FIG. In FIG. 4, point F is the fault point, and as ΔId flows through the arc resistance at the fault point, a voltage like Vdf appears. Vdr and Vdi are respectively in order,
Transmission line voltage at the connection point with the inverter, Vdf
The difference is the voltage drop due to the resistance of the power transmission line.
Therefore, voltages such as Vdr, Vdf, and Vdi have very small values compared to the rated DC voltage. The P ₀ point is the point where the voltage is zero, but as can be seen from the figure, this point is closer to the inverter than the failure point.

When FD1 and FD2 detect a fault in the power transmission line,
The output of FD1 is applied to GL1 and the gate signal of the bypass pair of R is applied, so that R becomes a DC short circuit. On the other hand, the switch SW shown in Fig. 3 is opened by the output of FD2, and the set value given to the constant current control circuit of the inverter increases by ΔIdp.
Becomes an Idp. The changes in I ₁ , I ₂ , and I ₃ shown in FIG. 3 in this case are as shown in FIG. 5.

T ₀ is the point in time when FD1 and FD2 operate. At this point, I ₁ , I ₂ , and I ₃ are each equal to the values shown in Figure 1, but since the electromotive force of R disappears, I ₁ begins to decrease, and I ₂ is the operation of the constant current control circuit of I. It starts to increase due to When I ₁ and I ₂ become equal, the fault point current I ₃ becomes zero. The voltage distribution at this point is
Vdr', Vdf', and Vdi' in Fig. 4, and Vdf' is zero. Therefore, the arc at the fault point disappears at this point. After this, I ₁ = I ₂ , increasing to a value equal to Id, and finally, as shown in Figure 4.
The voltage distribution will be Vdr″, Vdf″, Vdi″,
Vdr'' is zero.The voltage at the fault point becomes zero, and after the arc disappears, a voltage Vdf'' of the opposite polarity is applied, but its value is small and the arc will not occur again. The state of the DC transmission line is then maintained in this state where Id flows for a certain period of time.

As described above, according to the present invention, the voltage at the failure point
Since the current zero point is reliably created and the voltage at the fault point is kept at a very small value thereafter, the arc at the fault point can be reliably extinguished.

Furthermore, since the DC current is circulating through the forward and inverse converters until a restart is attempted, by bringing the forward converter, which is now in the bypass pair state, into a normal commutation state, Stable restart is possible without the need for complex signal exchange for synchronization between forward/inverter converters, and without risking full voltage startup or the like.

In the embodiment of FIG. 3, a bypass pair of forward conversion devices is used, but the same effect can be obtained even if a bypass pair of inverse conversion devices is used.

In addition, in the embodiment shown in FIG. 3, an example was shown in which the forward conversion device is placed in a bypass pair state and the DC current setting value of the inverse conversion device is also changed. Due to the decrease in I ₁ shown in , there is a point in time when I ₁ = I ₂ , so a similar effect can be expected.

In addition, in the embodiment shown in FIG. 3, even after the arc disappears,
Although an example has been shown in which a DC current Id equal to that before the failure flows until the restart, this current may be reduced to the minimum operating current during normal operation to wait for the restart. By doing so, Vdf'' shown in FIG. 4 can be made even smaller.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the current distribution when a transmission line failure occurs in a general DC transmission system, Figure 2 is a diagram to explain the current distribution in Figure 1, and Figure 3 is a diagram showing the implementation of the present invention. For example, FIGS. 4 and 5 are drawings for explaining the operation of the present invention. Explanation of symbols, DCCT1, DCCT2...DC current transformer, FD1, FD2...Fault detection device, A1,
A2...Amplifier, AP1, AP2...Pulse phase shifter, GL1, GL2...Gate logic circuit, PA1,
PA2...Pulse amplifier.

Claims (1)

[Claims] 1 forward converter controlled by a constant current control device,
An inverse converter that is normally controlled by a constant voltage controller or constant margin angle controller, and is controlled by a constant current controller when a fault occurs in the DC line and the DC voltage drops, and a DC converter that connects these converters. line,
A DC current transformer detects the current of the DC line, a constant current control circuit operationally amplifies the error signal between the output of the DC current transformer and the set value, and the pulse phase of the converter is determined by the output of the constant current control circuit. In a DC power transmission line constituted by the constant current control device equipped with a pulse phase control device for controlling bypass pair forming means for forming a bypass pair by this converter by controlling the pulse phase of one converter according to the output of the failure detection means; and the pulse phase of the other converter is controlled according to the output of the failure detection means. 1. A protection system for a DC power transmission line, comprising: control means for controlling according to the output of a constant current control circuit and causing the converter to perform constant current control.