CN111181427A

CN111181427A - Two-level AC/DC converter and DC fault clearing method

Info

Publication number: CN111181427A
Application number: CN202010021398.2A
Authority: CN
Inventors: 韩民晓; 曹文远
Original assignee: North China Electric Power University
Current assignee: North China Electric Power University
Priority date: 2020-01-09
Filing date: 2020-01-09
Publication date: 2020-05-19
Also published as: CN112332685A; CN112332685B

Abstract

The invention relates to a two-level AC/DC converter and a DC fault clearing method. Under the normal working state of the direct current side, T1 is triggered to be conducted, and T2 is locked. When the direct current short circuit is failed, the trigger pulse of the T1 is blocked, and the energy is blocked from being transmitted to a direct current system. And if the residual voltage on the direct current side is higher, controlling the ARM PLUS to work in a reverse Boost state. And a disconnecting switch is added on the direct current output side, so that permanent faults can be eliminated. And after the short-circuit fault is eliminated, when the short-circuit current is detected to reach a zero value, the ARM PLUS is recovered to a normal working state after the short-circuit arc is completely extinguished. The pulse of the three-phase bridge arm does not need to be blocked urgently in the fault process, and the circuit breaker on the alternating current side does not need to be tripped, so that the system can be quickly recovered to operate. The ARM PLUS topology provided by the invention has clear principle and simple implementation method, and can obviously improve the direct current fault processing and operation recovery speed.

Description

Two-level AC/DC converter and DC fault clearing method

Technical Field

The invention relates to the technical field of power electronics, in particular to a two-level AC/DC converter and a DC fault clearing method.

Background

With the increasing demand of renewable energy grid connection and the increasing severity of the problem of shortage of alternating current distribution resources, the direct current distribution technology has become the focus of domestic and foreign research due to the advantages of being beneficial to the access of clean energy, reducing transformation links, being strong in controllability, having few problems of electric energy quality, being low in line loss and the like. The two-level AC/DC converter has the advantages of simple topological structure, easiness in control, high reliability and low manufacturing cost, has a better cost-efficiency ratio in medium and low voltage occasions, and is widely applied to the field of direct-current power distribution. With the development of wide bandgap power electronic devices, the voltage endurance of the two-level AC/DC converter is further improved, and the two-level AC/DC converter is expected to be applied to higher voltage levels and wider working scenes.

The development of the advantages of the direct current power distribution technology depends on the networking of direct current, and the problems that the direct current short-circuit fault can cause the breakdown of a networked direct current system and the recovery period is long are also caused. Under the condition of direct-current short-circuit fault, the control system can quickly respond to lock the converter station, but the alternating-current power grid still provides short-circuit current for a short-circuit point through an anti-parallel diode of a traditional two-level AC/DC converter, the short-circuit fault can be cleared only by tripping off an alternating-current side circuit breaker, the requirements cannot be met in the aspects of action time and system power supply reliability guarantee, and the cycle of fault processing and operation recovery is increased by locking the converter station and tripping off the alternating-current side circuit breaker.

The direct current breaker can clear direct current faults in a short time, and is an effective solution, but the research and development of large-capacity direct current breakers at home and abroad are still in a starting stage, the direct current breaker which can be commercially applied at present has limited capacity, high price and poor reliability, and the coordination control after the faults occur is difficult.

The key point of the analysis of the serious damage caused by the direct current short circuit fault is that an AC/DC converter used for a direct current power grid is a Voltage Source Converter (VSC), and the direct current side short circuit is equivalent to the short circuit of a voltage source. The inverter does not have any control means, so that the short-circuit fault causes great impact on a direct-current system. Therefore, topology improvement and control of the conventional VSC to have the dc fault clearing capability is a valuable solution.

Disclosure of Invention

The invention aims to provide a two-level AC/DC converter and a DC fault clearing method, which can clear DC faults.

In order to achieve the purpose, the invention provides the following scheme:

a two-level AC/DC converter, the converter comprising:

the circuit breaker comprises an alternating current circuit breaker group, a three-phase rectifier bridge circuit, a first switch tube, a second switch tube, an inductor, a capacitor and an isolating switch; the first switch tube and the second switch tube form an ARM PLUS circuit;

the alternating current circuit breaker group is connected with the input end of the three-phase rectifier bridge circuit;

one end of the first switch tube and one end of the second switch tube are both connected with the output end of the three-phase rectifier bridge circuit;

the other end of the first switch tube is connected with one end of the inductor, and the other end of the inductor is connected with one end of the capacitor and one end of the isolating switch;

the other end of the second switch is connected with the other end of the capacitor.

Optionally, the ac circuit breaker set specifically includes: a first ac circuit breaker, a second ac circuit breaker and a third ac circuit breaker.

Optionally, the parameters of the first switching tube and the second switching tube are consistent with the parameters of the switching tubes in the three-phase rectifier bridge.

Optionally, the first switch tube includes a first IGBT and a first diode, a collector of the first IGBT is connected to a cathode of the first diode, and an emission set of the first IGBT is connected to an anode of the first diode;

the second switch tube comprises a second IGBT and a second diode, a collector electrode of the second IGBT is connected with a cathode of the second diode, and an emission set of the second IGBT is connected with an anode of the second diode.

The present invention further provides a DC fault clearing method, which is applied to the above two-level AC/DC converter with DC fault clearing capability, and the method includes:

detecting direct current or voltage measured by a direct current circuit to obtain first direct current or voltage;

judging whether a short circuit occurs on the direct current side or not based on the first direct current or voltage to obtain a first judgment result;

if the first judgment result shows that the short-circuit fault occurs, blocking the trigger pulse of the first IGBT;

judging whether the direct current residual voltage exceeds a set threshold value, and controlling the RAM PLUS to work in a reverse Boost state if the direct current residual voltage exceeds the set threshold value;

waiting for a fixed time interval until the short-circuit arc is completely extinguished;

continuously detecting the current or voltage of the direct current side to obtain a second direct current or voltage;

judging whether the direct current side is short-circuited or not based on the second direct current or voltage to obtain a second judgment result;

if the second judgment result shows that the short-circuit fault still exists, blocking the trigger pulse of the first IGBT and judging the short-circuit fault as a permanent fault;

and when the short-circuit current is reduced to 0, the direct-current side isolating switch is switched off.

Optionally, the current or voltage transformer is used for continuously detecting the direct current or voltage of the direct current line.

According to the specific embodiment provided by the invention, the invention discloses the following technical effects:

the invention provides a topology-improved and controlled converter with direct-current fault clearing capability based on a traditional two-level AC/DC converter, which is called an ARM PLUS mode two-level AC/DC converter. On the basis of a traditional two-level AC/DC converter, an ARM PLUS mode two-level AC/DC converter is additionally provided with a bridge ARM (ARM PLUS) with device parameters and structures consistent with three-phase bridge ARMs of the original converter behind a capacitor on the original direct current side, and an inductor and a capacitor are connected in series with an outlet on the ARM PLUS direct current network side, so that a Buck-Boost DC/DC circuit is additionally arranged on the direct current side of the original two-level AC/DC converter, and appropriate control is added, so that the control of short-circuit current can be realized, and the fault processing and operation recovery speed is improved.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings needed to be used in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings without inventive exercise.

FIG. 1 is a topology diagram of a two-level AC/DC converter with DC fault clearing capability according to an embodiment of the present invention;

FIG. 2 is a flowchart illustrating an ARM PLUS fault operation according to an embodiment of the present invention;

FIG. 3 is a schematic diagram of a capacitor discharge circuit after a DC short circuit according to an embodiment of the present invention;

fig. 4 is a schematic diagram of an inductor discharge loop after dc short circuit according to an embodiment of the present invention.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

The invention aims to provide a two-level AC/DC converter with a direct-current fault clearing capability, which can clear a direct-current fault.

In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in further detail below.

Fig. 1 is a two-level AC/DC converter according to an embodiment of the present invention, which includes:

the circuit breaker comprises an alternating current circuit breaker group, a three-phase rectifier bridge circuit, a first switch tube, a second switch tube, an inductor L, a capacitor C and an isolating switch; the first switch tube and the second switch tube form an ARM PLUS circuit;

one end of the first switch tube and one end of the second switch tube are both connected with the output end of the three-phase rectifier bridge circuit; the first switch tube and the second switch tube form an ARM PLUS bridge ARM.

Specifically, the ac circuit breaker group specifically includes: a first ac circuit breaker, a second ac circuit breaker and a third ac circuit breaker.

And the parameters of the first switch tube and the second switch tube are consistent with the parameters of the switch tubes in the three-phase rectifier bridge.

The first switch tube comprises a first IGBT 1 and a first diode D1, wherein a collector electrode of the first IGBT is connected with a cathode of the first diode, and an emission set of the first IGBT is connected with an anode of the first diode;

the second switch tube comprises a second IGBTT2 and a second diode D2, wherein a collector electrode of the second IGBT is connected with a cathode electrode of the second diode, and an emission set of the second IGBT is connected with an anode electrode of the second diode.

Compared with the traditional two-level AC/DC converter, the ARM PLUS type two-level AC/DC converter is additionally provided with a bridge ARM (namely ARM PLUS) with the device parameters and the structure consistent with those of a three-phase bridge ARM of the original converter after the capacitor of the original direct current side is arranged, and an inductor and a capacitor are connected in series at an outlet of the ARM PLUS direct current network side, so that the direct current side of the original two-level AC/DC converter is equivalently provided with a Buck-Boost DC/DC circuit. Under normal working conditions, energy flows from an alternating current side to a direct current side, at the moment, the T1 and the D2 form a Buck chopper circuit (Buck circuit), and the T2 and the D1 are in an off state. If the T1 is kept to be always conducted, the ARM PLUS gain is approximately 1, namely the working principle of the ARM PLUS type two-level AC/DC converter under the normal working condition is the same as that of the traditional two-level AC/DC converter; when short-circuit fault occurs on the direct current line side, the trigger pulse of T1 is blocked, and the energy on the alternating current side cannot flow to the direct current side through ARM PLUS. If the residual voltage of the direct current side is high, the ARM PLUS can be controlled to work in a reverse Boost state, at the moment, the T2 and the D1 form a Boost chopper circuit (Boost circuit), and the T1 and the D2 are in an off state, so that the energy of the direct current side can be fed back to the alternating current side, and the fault processing speed is accelerated. The isolating switch on the direct current output side can be used for being matched with the ARM PLUS to remove permanent faults.

Fig. 2 is a flowchart of an ARM PLUS fault action according to an embodiment of the present invention, which specifically includes the following steps:

detecting direct current or voltage measured by a direct current circuit to obtain first direct current or voltage; specifically, the direct current or voltage of the direct current side line is continuously monitored through a voltage or current transformer.

And judging whether the direct current side is short-circuited or not based on the first direct current or voltage to obtain a first judgment result.

If the first judgment result shows that the direct current rapidly rises to exceed the threshold value when the short-circuit fault occurs, the direct current voltage rapidly falls to be lower than the threshold value, the trigger pulse of the first IGBT is blocked, and the energy is blocked from being transmitted to the direct current system.

Judging whether the direct current residual voltage exceeds a set threshold value, and controlling the RAM PLUS to work in a reverse Boost state if the direct current residual voltage exceeds the set threshold value; when the residual voltage of the direct current side is higher, the RAM PLUS is controlled to work in a reverse Boost state, and energy is fed back to the alternating current side.

And after the short-circuit fault is eliminated, the short-circuit current is reduced to 0, at the moment, the short-circuit current waits for several ms until the short-circuit arc is completely extinguished, and the RAM PLUS is recovered to a normal working state.

And continuously detecting the current or voltage of the direct current side to obtain a second direct current or voltage.

And judging whether the direct current side is short-circuited or not based on the second direct current or the voltage to obtain a second judgment result.

And if the second judgment result shows that the short-circuit fault still exists, blocking the trigger pulse of the first IGBT, blocking the alternating current side to provide short-circuit current to the short-circuit point, and judging that the short-circuit fault is a permanent fault.

And if the second judgment result shows that the direct current side has no short-circuit fault, the direct current system is recovered to a normal operation state.

And when the short-circuit current is reduced to 0, disconnecting the direct-current side isolating switch and clearing the permanent fault.

Fig. 3 is a schematic diagram of the capacitor discharge circuit after dc short circuit. Since the trigger pulse of T1 is blocked quickly after the dc fault occurs, the energy on the ac side cannot flow to the dc side through ARM PLUS. If T2 is also always off at this time, the capacitor current flows through the capacitor, cable, and arc resistor as shown by the solid arrow in fig. 3. The capacitor discharge current does not pass through a power device, and the converter cannot be damaged; when the inductor current is reduced to 0, the ARM PLUS is enabled to work in a reverse Boost state, and then the capacitor current can not only flow through the capacitor, the cable and the arc resistor, but also flow to the three-phase bridge ARM side through the ARM PLUS, as shown by the arrow dotted line in fig. 3. Since the current in the reverse Boost state is in a controllable state, the current converter is not damaged.

Fig. 4 is a schematic diagram of an inductor discharge loop after dc short circuit. Similarly, since the trigger pulse of T1 is quickly blocked after the dc fault occurs, the energy on the ac side cannot flow to the dc side through ARM PLUS. If T2 is also always off at this time, inductor current flows through inductor, cable, arc resistor and freewheeling diode flow D2, as shown by the solid arrow in fig. 4. The current of the inductor cannot suddenly change, so that the current allowable value of the power device cannot be exceeded. When the inductive current is reduced to 0, the ARM PLUS is enabled to work in a reverse Boost state, energy on the direct current side is fed back to the three-phase bridge ARM side through the reverse inductive current, and a current path is shown as an arrow dotted line in fig. 4. Since the current in the reverse Boost state is in a controllable state, the current converter is not damaged.

The ARM PLUS type two-level AC/DC converter with the direct-current fault clearing capability can clear direct-current faults by adding a bridge ARM (ARM PLUS) and assisting appropriate control on the basis of the topology of the traditional two-level converter. Under the normal working state of the direct current side, a full control device T1 of the ARMPLUS triggers to be conducted, and T2 is locked. When a dc short fault occurs, the trigger pulse of T1 is blocked, thereby blocking the transfer of energy to the dc system. If the residual voltage of the direct current side is higher, the ARM PLUS can also be controlled to work in a reverse Boost state, and energy is fed back to the alternating current side. In addition, a disconnecting switch is added on the direct current output side to be matched with the ARM PLUS, and permanent faults can be eliminated. And after the short-circuit fault is eliminated, when the short-circuit current is detected to reach a zero value, waiting for a plurality of ms, and recovering the ARM PLUS to a normal working state after the short-circuit arc is completely extinguished. The pulse of the three-phase bridge arm does not need to be blocked urgently in the fault process, and the circuit breaker on the alternating current side does not need to be tripped, so that the system can be quickly recovered to operate. The novel ARM PLUS topology provided by the invention has clear principle and simple implementation method, and can obviously improve the direct current fault processing and operation recovery speed.

The embodiments in the present description are described in a progressive manner, each embodiment focuses on differences from other embodiments, and the same and similar parts among the embodiments are referred to each other. For the system disclosed by the embodiment, the description is relatively simple because the system corresponds to the method disclosed by the embodiment, and the relevant points can be referred to the method part for description.

The principles and embodiments of the present invention have been described herein using specific examples, which are provided only to help understand the method and the core concept of the present invention; meanwhile, for a person skilled in the art, according to the idea of the present invention, the specific embodiments and the application range may be changed. In view of the above, the present disclosure should not be construed as limiting the invention.

Claims

1. A two-level AC/DC converter, characterized in that it comprises:

2. The two-level AC/DC converter according to claim 1, characterized in that the group of AC circuit breakers comprises in particular: a first ac circuit breaker, a second ac circuit breaker and a third ac circuit breaker.

3. The two-level AC/DC converter according to claim 1 wherein the parameters of the first and second switching tubes are identical to the parameters of the switching tubes in the three-phase rectifier bridge.

4. The two-level AC/DC converter according to claim 1, wherein the first switching tube comprises a first IGBT and a first diode, a collector of the first IGBT is connected to a cathode of the first diode, and an emission set of the first IGBT is connected to an anode of the first diode;

5. A method of clearing a DC fault applied to a two-level AC/DC converter according to any of claims 1-4, the method comprising:

6. The dc fault clearing method of claim 5, wherein the dc line side dc current or voltage is continuously sensed by a current or voltage transformer.