WO2014166129A1

WO2014166129A1 - Overvoltage protection device and protection method for direct current side of flexible direct current transmission system

Info

Publication number: WO2014166129A1
Application number: PCT/CN2013/074824
Authority: WO
Inventors: 荆平; 邱宇峰; 周飞; 陆振纲
Original assignee: 国家电网公司; 国网智能电网研究院
Priority date: 2013-04-09
Filing date: 2013-04-26
Publication date: 2014-10-16
Also published as: CN103187722B; CN103187722A

Abstract

An overvoltage protection device and protection method for a direct current side of a flexible direct current transmission system. The protection device (7) is connected to a positive electrode and a negative electrode of a direct current line (8) in parallel, and is used for conducting an action to draw away a fault current when the direct current line (8)is overvoltage because of an alternating current line I (4) or the direct current line (8) has failed or when a valve base controller detects that a short circuit fault has occurred at any electrode of the direct current line (8) has occurred, thereby restraining the overvoltage level of the direct current line (8) and protecting a valve body (9) of a converter valve. The protection device (7) uses an overvoltage or mandatory breakdown element for improving the overvoltage protection performance, thereby solving the problem of overvoltage caused by the fault of the flexible direct current transmission system.

Description

DC side overvoltage protection device and protection method for flexible direct current transmission system

The invention relates to the field of flexible direct current transmission systems, in particular to a DC side overvoltage protection device and a protection method for a flexible direct current transmission system. Background technique

Flexible DC transmission is a new type of high-voltage direct current transmission technology. It uses a voltage source type converter to convert traditional alternating current into direct current at a power source such as a hydropower plant or a thermal power plant, and sends the power through a transmission line such as an overhead line or a cable. To the far side of the load. Compared with the traditional high-voltage direct current transmission, it has the advantages of strong controllability, easy reversal of the current flow, and no need for reactive compensation.

The flexible DC transmission system consists of three parts, including two converter stations at the power supply and load, and a DC cable or overhead line connecting the two stations. The core equipment is a flexible DC converter valve, and its safety needs to pass through the protection system. Get a reliable guarantee.

The converter station has a converter valve 5 switchable between the AC line I and the DC line 8, comprising a power semiconductor valve 9 extending between the AC line I and the DC line 8, through which the power parameters are Conversion between AC and DC. In addition, there is a converter transformer 3 between the AC line I and the AC line II, which is star-connected on the I-valve side of the AC line, and the neutral point is grounded through the resistor 6.

The fault of the flexible DC converter station is generally divided into three types: AC side fault, DC side fault and converter valve fault. The AC and DC side fault protection is protected by the station control system (by voltage transformer, current transformer, AC/DC side filter). , transformer protection and other components are composed).

At present, the DC protection system in the flexible DC converter station mainly judges the fault condition of the DC system through DC undervoltage and overvoltage protection. When the AC line of the converter station is single-phase ground fault or the DC line has a single-pole ground fault, the other The voltage of the DC line 8 may increase to 1.5 to 2 times the normal operating voltage, and the power semiconductor valve may be damaged by excessive fault voltage.

The conventional power semiconductor valve 9 protection measures are mainly parallel with no gap MOV. The protection method generally connects the gapless MOV in parallel with the power semiconductor valve 9, and utilizes the nonlinear characteristic of the voltage and current of the MOV to consume the energy that may generate the overvoltage through the MOV, thereby protecting the protected device from the overvoltage. However, this method has defects such as difficulty in setting the operating parameters, excessive energy stress, and uncontrollable conduction. Other traditional line protection devices such as the side The thyristor switch has the disadvantages of complex structure, slow operation time and low overcurrent capability. In view of the above problems, adding the protection device of the present invention to the DC line can effectively reduce the overvoltage of the DC line and avoid damage to the inside of the valve body of the converter valve. Summary of the invention

In view of the deficiencies of the prior art, the object of the present invention is to provide a DC side overvoltage protection device and a protection method for a flexible direct current transmission system, and the protection device 7 uses an overvoltage or forced breakdown component to improve overvoltage protection performance. The problem of overvoltage caused by the fault of the flexible direct current transmission system is solved.

When the single-phase ground fault occurs on the AC line or the single-pole ground short-circuit fault occurs on the DC line, the voltage of the other pole DC line will rise. The grounding breakdown gap is adjusted according to a predefined voltage. When the voltage is less than the set breakdown voltage, the protection device is not turned on, the DC system can normally transmit power, and once the overvoltage reaches the set value, the protection device is broken down. An electric spark is generated, and current is introduced into the ground through the gap to suppress the DC side overvoltage, thereby preventing the converter valve body and the DC line from being damaged due to overvoltage.

The object of the present invention is achieved by the following technical solutions:

A DC-side overvoltage protection device for a flexible DC transmission system, the protection device being included in a flexible DC converter station 1, comprising a flexible DC converter valve 5 switching between an AC line I and a DC line 8, the AC A converter transformer 3 is disposed between the line I and the AC line II, and the converter transformer 3 is star-connected on the side of the AC line I, wherein the neutral point is grounded through the resistor 6; the converter transformer 3 is in the AC line II The mesh side is a triangular or star connection; the improvement is that the protection device 7 is connected in parallel with the positive and negative poles of the DC line 8 for causing a fault in the AC line I or the DC line 8 resulting in a DC line 8 overvoltage. Or when the valve base controller detects that a short circuit fault occurs at any pole of the DC line 8, the action is taken to open the fault current, the DC line 8 overvoltage level is suppressed, and the converter valve body 9 is protected.

Preferably, the protection device 7 employs a fast conduction element such as a spark gap; the spark gap is grounded. Preferably, the converter valve 5 is a three-phase six-bridge arm structure; each bridge arm is composed of a converter valve body 9; the converter valve body 9 is realized by a semiconductor switching element, including a switchable Semiconductor devices (such as IGBT, IGET,

IGCT or GTO) and its anti-parallel diodes.

The invention provides a method for protecting a DC side overvoltage of a flexible direct current transmission system based on another object, the improvement being that the protection device 7 for the method is connected in parallel to the positive and negative poles of the direct current line 8; Including the following steps:

A. Detecting whether the DC line is faulty, the fault includes an AC line I single phase ground fault, a DC line 8 single pole ground short fault, and a bipolar short fault; B. When a single-phase ground fault of the AC line I or a single-pole ground fault of the DC line occurs, the voltage across the spark gap rises to the breakdown voltage and breaks the spark gap;

Or when the control device of the spark gap detects a DC line short-circuit fault, the control device sends a trigger command to the freewheeling gap to force the freewheeling gap to break the spark gap;

After the spark gap breaks down, the fault current is led to the earth;

C. After the spark gap is broken down, the return status information is sent to the control device, and a blocking signal is sent to the converter valve 5 to protect the valve body 9 of the converter valve.

Preferably, in the step A, the protection method for detecting the DC line voltage overvoltage is: if the DC line voltage exceeds a preset value, and the duration also reaches a preset value, the protection device 7 is turned on, and the valve base is The controller blocks the trigger pulse of the flexible DC converter valve 5.

Preferably, the preset self-breakdown voltage is not broken down when the DC line voltage is not overvoltage, and is broken down when the DC line voltage is overvoltage, and the self-breakdown voltage is calculated by theoretical calculation, computer simulation or test. determine.

Preferably, the magnitude of the discharge current at the time of breakdown is determined by theoretical calculations, computer simulations or experiments.

Preferably, the amount of absorbed energy at the time of breakdown is determined by theoretical calculations, computer simulations or experiments. Compared with the prior art, the beneficial effects achieved by the present invention are:

1. The protection device proposed by the invention has rapid action, strong flow energy, large absorption energy, and can quickly protect the valve body.

2. The control protection device proposed by the invention does not require complicated structure and function.

3. The protection device proposed by the invention has the advantages of simple structure, low cost and easy realization.

4. Reduce the insulation cost of DC line and converter valve;

5. Ensure the safety of DC line and converter valve equipment;

6. Improve the anti-fault capability of the flexible DC converter station;

7. Guarantee the safe and stable operation of the flexible DC transmission system. DRAWINGS

1 is a schematic diagram of a protection device for a DC-side overvoltage of a flexible direct current transmission system provided by the present invention; wherein: 1- flexible DC converter station; 2-AC line II; 3-conversion transformer; 4-AC line I; 5-conversion valve; 6-resistor; 7-protection device; 8-DC line; 9-return valve body. detailed description

The specific embodiments of the present invention are further illustrated in detail below with reference to the accompanying drawings. The schematic diagram of the converter valve protection device of the flexible DC transmission system for DC line fault provided by the present invention is shown in FIG. 1 , and the protection device is included in the flexible DC converter station 1 , including the flexibility of switching between the AC line I and the DC line 8 a DC converter valve 5, a converter transformer 3 is disposed between the AC line I and the AC line II, and the converter transformer 3 is star-connected on the side of the AC line I valve, wherein the neutral point is grounded through the resistor 6; The flow transformer 3 is a triangular or star connection on the side of the AC line II network.

The converter valve 5 is a three-phase six-bridge arm structure; each of the bridge arms is composed of a converter valve body 9; the converter valve body 9 is realized by a semiconductor switching element, including an IGBT and a diode connected in anti-parallel thereto. Its structural form includes, but is not limited to, modular multi-level, H-bridge chain, IGBT series.

The protection device 7 is connected in parallel to the two poles (positive and negative) of the DC line 8 for performing an overvoltage at the fault of the DC line 8 or when the valve base controller detects a faulty short circuit of the DC line 8, performing an action to open the fault current and suppress the DC Line 8 overvoltage level protects the converter valve body 9.

The protection device 7 uses a spark gap conduction element; the spark gap is grounded.

The working principle of the spark gap adopted by the present invention is as follows:

The seal gap is automatically triggered when the spark gap is single-phase grounded by AC line I or a single-pole ground fault of DC line 10 causes its voltage to exceed the self-trigger voltage.

Or when the converter valve control protection device detects a short-circuit fault of the converter DC line 8 , a forced trigger command is issued through the fiber or cable to force the gap to be turned on, so that the short-circuit current flows quickly through the seal gap.

The preset breakdown voltage is required to ensure that the device will not be broken down when the DC line voltage is not over-voltage, and will be broken down when the DC line voltage is over-voltage. The size is configured by theoretical calculation, computer simulation or Test confirmed. The configuration of the discharge current at the time of breakdown is determined by theoretical calculation, computer simulation or experiment. The configuration of the amount of energy absorbed during breakdown is determined by theoretical calculations, computer simulations or experiments. The self-breakdown voltage is 1.3-1.8 times the rated voltage of the line. The breakdown current during breakdown is 1.5-20 line rated current. The energy absorbed during breakdown is the integral of the product of the above voltage and current in time.

(1) According to the theoretical calculation, flexible computer simulation and test of flexible DC short-circuit fault, the breakdown voltage, short-circuit current and absorbed energy of the spark gap are determined as follows:

The theoretical calculation process is: the converter transformer side AC bus 2 is equivalent to a voltage source t / _s and the internal impedance Z _s in series, according to the converter transformer 3 ratio, and consider the transformer valve side leakage reactance X, will The voltage source t/ _s and internal impedance are converted to the transformer valve side.

The power supply potential equivalent to the transformer valve side is: U _s2 = kU _s

The internal resistance is: Z _s2 = k + X _L ', In normal operation, the transformer valve side voltage t/ _s2 . As an initial condition, according to the characteristic that the voltage of the DC line (or AC line II single phase) at the fault ground is 0, a mathematical expression reflecting the voltage of the normal pole of the DC line in the ground fault process is established, and the DC is calculated according to the initial condition and the expression. The fault voltage expression of the line;

After the fault, the normal pole DC line voltage _dOT (t) is the initial voltage t/ _s2 of the transformer valve side. , the initial DC voltage line impedance Z _s2 and the ground impedance function: u _d J ) = FU _s2Q , U _dcQ , Z _s2 , Z _n , t) '' represents time.

The breakdown voltage of the spark gap is determined according to the voltage of each phase to ensure that the breakdown voltage is higher than the peak value of the DC line voltage during normal operation (preferably 1.2 times), which is lower than the maximum value of the DC line voltage at the time of failure. Combined with the mathematical model of the spark gap, the discharge current and absorbed energy of the spark gap under the fault voltage can be calculated.

The process of computer simulation is as follows: a simulation model including equivalent system, transformer, DC line, converter valve and ground fault is established in the computer simulation software. The single-phase and two-phase ground fault simulations at different times are used to obtain the fault time station. The amplitude of another DC line. The breakdown voltage of the spark gap is determined based on the voltage to ensure that the breakdown voltage is higher than the DC line voltage during normal operation (preferably 1.2 times), which is lower than the maximum value of the DC voltage at the time of failure.

A model of the spark gap is established in the simulation software to determine the maximum current allowed by the spark gap based on the maximum current flowing through the spark gap at the fault. Determine the energy that the spark gap needs to absorb based on the highest breakdown voltage and the maximum allowable current

(2) Determine the separation distance of the spark gap according to the breakdown voltage; the breakdown voltage U is a function of the gas pressure Ρ and the distance d between the electrodes and the absolute temperature T: U=f (pd / T).

(3) Install one end of the spark gap in parallel on the positive and negative poles of the DC line 8 in the station, and ground the other end;

(4) As shown in Figure 1, when AC line I single-phase grounding or DC line single-pole ground fault 10 occurs, the voltage across the spark gap rises to the breakdown voltage, thereby breaking the spark gap;

Or when a short circuit fault occurs in the DC line, the voltage measuring device or the protection device monitors the voltage across the spark gap, triggers a spark gap, and forces the spark gap to be triggered by the trigger gap to break the spark gap.

(5) The short-circuit fault current flows through the spark gap, suppressing the over-voltage level of the DC line, and avoiding the over-voltage of the valve body of the converter valve;

(6) After the spark gap is broken down, the state information is returned to the control device of the spark gap, and the control device detects the break of the spark gap and sends a blocking signal to the converter valve.

The invention further relates to a DC side overvoltage protection method for a flexible direct current transmission system, the protection device 7 for the method being connected in parallel to the two poles of the direct current line 8; the method comprising the steps of:

A. Detecting whether the DC line is faulty, the fault includes an AC line I single phase ground fault, a DC line 8 single pole ground short fault, and a bipolar short fault; B. When the AC line I single-phase ground fault or the DC line single-pole ground fault 10 occurs, the voltage across the spark gap rises to the breakdown voltage and breaks the spark gap;

Or when the control device detects a DC line bipolar short circuit fault, the control device sends a trigger command to the freewheeling gap to force the freewheeling gap to break the spark gap;

After the spark gap breaks down, the fault current is led to the earth;

C. After the spark gap is broken down, the return status information is sent to the control device in the spark gap, and a blocking signal is sent to the converter valve 5 to protect the valve body 9 of the converter valve.

It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not limited thereto. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the present invention can still be The invention is to be construed as being limited to the scope of the appended claims.

Claims

Rights request

1. A DC side overvoltage protection device for a flexible DC transmission system. The protection device is included in a flexible DC converter station (1) and includes a flexible switch between the AC line 1 (4) and the DC line (8). DC converter valve (5), a converter transformer (3) is provided between the AC line I (4) and the AC line II (2), the converter transformer (3) is in the AC line I (4) valve The converter transformer (3) has a delta or star connection on the grid side of the AC line II (2); the neutral point is connected to the ground through the resistor (6);

It is characterized in that the protection device (7) is connected in parallel to the positive and negative poles of the DC line (8), and is used to prevent overvoltage or overvoltage of the DC line (8) due to a fault in the AC line (4) or the DC line (8). When the valve base controller detects a short-circuit fault in any pole of the DC line (8), it takes action to divert the fault current, suppress the overvoltage level of the DC line (8), and protect the converter valve body (9).

2. The DC side overvoltage protection device according to claim 1, characterized in that the protection device (7) adopts a fast conduction element such as a spark gap; the spark gap is grounded.

3. The DC side overvoltage protection device according to claim 1, characterized in that the converter valve (5) has a three-phase six-arm structure; each bridge arm is composed of a converter valve body (9) Composition: The converter valve body (9) is implemented by a semiconductor switching element, including a turn-off semiconductor device and a diode connected in anti-parallel with it.

4. A DC side overvoltage protection method for a flexible DC transmission system, characterized in that the protection device (7) used in the method is connected in parallel to the positive and negative poles of the DC line (8); the method includes the following steps :

A. Detect whether there is a fault in the DC line. The fault includes AC line I (4) single-phase ground fault, DC line (8) single-pole grounding short-circuit fault and bipolar short-circuit fault;

B. When a single-phase ground fault in the AC line I (4) or a single-pole ground fault in the DC line occurs, the voltage at both ends of the spark gap rises to the breakdown voltage and breaks down the spark gap;

Or when the control device of the spark gap detects a bipolar short-circuit fault in the DC line, the control device sends a trigger command to the freewheeling gap to forcefully trigger the freewheeling gap and breakdown the spark gap;

After the spark gap breaks down, the fault current is directed to the earth;

C. After the spark gap breaks down, return status information is sent to the spark gap control device, and a locking signal is sent to the converter valve (5) to protect the converter valve body (9).

5. The DC side overvoltage protection method according to claim 1, characterized in that, in step A, the protection method for detecting DC line voltage overvoltage is: the DC line voltage exceeds a preset value, and the duration also reaches the preset value, the protection device (7) is turned on, and at the same time, the valve base controller blocks the trigger pulse of the flexible DC converter valve (5).

6. The DC side overvoltage protection method according to claim 1, characterized in that the preset self-breakdown voltage The self-breakdown voltage is not broken down when the DC line voltage is not overvoltage, but is broken down when the DC line voltage is overvoltage. The self-breakdown voltage is determined through theoretical calculation, computer simulation or experiment.

7. The DC side overvoltage protection method according to claim 1, characterized in that the size of the discharge current during breakdown is determined through theoretical calculation, computer simulation or experiment.

8. The DC side overvoltage protection method according to claim 1, characterized in that the amount of energy absorbed during breakdown is determined through theoretical calculation, computer simulation or experiment.