CN110611421A - A bypass mechanism and working method of a modular multilevel converter - Google Patents

Abstract

The invention provides a bypass mechanism and working method of a modular multilevel converter, including: an AC circuit breaker QF and six converter components; the ability of the diodes at the bottom of each sub-module to withstand the impact current is improved, which is beneficial to The entire converter changes the way of connecting thyristors in parallel with each sub-module in the traditional scheme to parallel thyristors at both ends of all sub-modules in each bridge arm. High-power and high-voltage thyristors can be preferred, which reduces the amount of thyristors and is more economical. , the control is also simpler, and it is also conducive to the design of the overall structure; in the inverter mode, the inrush current passing through the diodes at the bottom of each sub-module can be greatly reduced, and the reliability is higher.

Description

A bypass mechanism and working method of a modular multilevel converter

technical field

The invention relates to the technical field of level converters, in particular to a bypass mechanism and working method of a modularized multi-level converter.

Background technique

With the continuous advancement of power semiconductor technology, the voltage source converter-based flexible direct current transmission (VSC-HVDC) technology has become a new type of power transmission system recently developed, which uses pulse width modulation technology (PWM) to directly convert DC voltage Inverted to an AC voltage with controllable amplitude and phase, without the need for phase commutation with the receiving end grid voltage, and there is no limit to the system capacity of the receiving end grid, it can be used to isolate the connection between small systems and the main grid. Modular multilevel converter MMC is widely used in VSC-HVDC. Its basic unit is a half-bridge or full-bridge converter unit; similar to the chain-link converter structure, multiple sets of converter units are cascaded in together form a converter bridge arm; 6 sets of cascaded converter bridge arms are combined to form a three-phase converter.

When a DC short-circuit fault occurs in the converter, if the converter works in the rectification mode, the bypass thyristors connected in parallel of each sub-module need to be turned on to realize the bypass of each bridge arm of the converter. For the high-voltage converter valve, the sub-module The number of modules is large, the structure is complex, the cost is high, and the control difficulty is also increased; if the converter works in the inverter mode, the fault current will form a loop through the diodes at the bottom of each sub-module, and the large inrush current may cause damage to the diodes. It is not conducive to the stability of the system.

Contents of the invention

In order to overcome the deficiencies in the above-mentioned prior art, the present invention provides a modular multilevel converter bypass mechanism that improves the ability of the diodes at the bottom of each sub-module to withstand the surge current and is beneficial to the structural optimization of the entire converter , including: AC circuit breaker QF and six inverter assemblies; the six inverter assemblies are divided into three groups, and each group has two inverter assemblies;

The first phase electrical output terminals of the AC circuit breaker QF are respectively connected to the two converter assemblies to form a loop;

The second phase electrical output terminals of the AC circuit breaker QF are respectively connected to the two converter assemblies to form a loop;

The third phase electrical output terminals of the AC circuit breaker QF are respectively connected to the two converter assemblies to form a loop.

It should be further noted that the converter assembly includes: a bypass unit, a converter sub-module, a bridge arm resistance and a bridge arm inductance;

The bypass unit and the commutation sub-module are connected in parallel, and the parallel circuit of the bypass unit and the commutation sub-module is connected in series with the bridge arm resistance and the bridge arm inductance.

It should be further noted that the converter sub-module is provided with multiple converter units, and the multiple converter units are connected in series.

It should be further explained that one end of the parallel circuit between the bypass unit and the converter sub-module is connected to the loop;

The other end of the parallel circuit of the bypass unit and the commutation sub-module is connected to the first end of the bridge arm resistor;

The second end of the bridge arm resistor is connected to the first end of the bridge arm inductance;

The second end of the bridge arm inductor is connected to the phase power output end of the AC circuit breaker QF.

It should be further noted that the bypass unit is provided with a first bypass thyristor and a second bypass thyristor;

The first bypass thyristor and the second bypass thyristor are connected in parallel, and conduction directions of the first bypass thyristor and the second bypass thyristor are opposite.

The present invention also provides a working method of a bypass mechanism of a modular multilevel converter, the method comprising:

When the mechanism is in normal operation, the bypass unit does not operate;

When a short-circuit fault occurs on the DC side, the converter unit of the converter sub-module is blocked, and at the same time, one of the bypass thyristors in the bypass unit is turned on.

When the mechanism works in the rectification mode, while the commutation sub-module is blocked, the second bypass thyristor is triggered to conduct, and the second bypass thyristor of the bypass unit conducts with the bridge arm resistance and the bridge arm inductance.

When the mechanism works in the inverter mode, while the commutation sub-module is locked, the first bypass thyristor in the bypass unit is turned on, and the fault current forms loop 2 through the bridge arm resistance, bridge arm inductance and the commutation sub-module; Loop 1 is formed through the first bypass thyristor, and the first bypass thyristor and the commutation sub-module form a shunt, which reduces the impact of short-circuit current on the diode;

After the first bypass thyristor forms a loop, the AC circuit breaker QF trips and recloses after the DC side current decays. After successful reclosing, the commutation sub-module is unlocked and the mechanism restarts.

As can be seen from the above technical solutions, the present invention has the following advantages:

In the traditional scheme, the method of connecting thyristors in parallel with each sub-module is changed to parallel thyristors at both ends of all sub-modules of each bridge arm. High-power and high-voltage thyristors can be preferred, which reduces the amount of thyristors. It is more economical and more controllable. In order to be simple, it is also conducive to the design of the overall structure; in the inverter mode, the impact current passing through the diodes at the bottom of each sub-module can be greatly reduced, and the reliability is higher.

Description of drawings

In order to illustrate the technical solution of the present invention more clearly, the accompanying drawings that need to be used in the description will be briefly introduced below. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. As far as people are concerned, other drawings can also be obtained based on these drawings on the premise of not paying creative work.

Fig. 1 is a schematic diagram of a modular multilevel converter with the bypass structure;

Fig. 2 is a schematic diagram of a discharge circuit when the converter works in an inverter mode.

The meanings of the symbols in the figure are: 1 is the bypass unit, 1-1 is the first bypass thyristor, 1-2 is the second bypass thyristor, 2 is the commutation sub-module, 3 is the commutation unit, and 4 is the bridge arm Resistor, 5 is bridge arm inductance, 6 is converter assembly, 8 is loop one, 9 is loop two, QF is AC circuit breaker.

Detailed ways

In order to make the purpose, features and advantages of the present invention more obvious and understandable, the technical solutions protected by the present invention will be clearly and completely described below using specific embodiments and accompanying drawings. Obviously, the implementation described below Examples are only some embodiments of the present invention, but not all embodiments. Based on the embodiments in this patent, all other embodiments obtained by persons of ordinary skill in the art without creative efforts fall within the protection scope of this patent.

The present invention provides a modular multi-level converter bypass mechanism, as shown in Figure 1 and Figure 2, comprising: an AC circuit breaker QF and six converter assemblies 6; the six converter assemblies 6 are divided into Three groups, each group is provided with two converter assemblies 6;

The first phase electrical output terminals of the AC circuit breaker QF are respectively connected to the two converter assemblies 6 to form a loop; the second phase electrical output terminals of the AC circuit breaker QF are respectively connected to the two converter assemblies 6 to form a loop; the AC circuit breaker The third phase electrical output terminals of the converter QF are respectively connected with the two converter assemblies 6 to form a loop.

The converter assembly 6 includes: a bypass unit 1, a commutation sub-module 2, a bridge arm resistance 4 and a bridge arm inductance 5; the bypass unit 1 is connected in parallel with the commutation sub-module 2, and the bypass unit 1 and the commutation sub-module The parallel circuit of the module 2 is connected in series with the bridge arm resistor 4 and the bridge arm inductance 5 .

The converter sub-module 2 involved in the present invention is provided with a plurality of converter units 3, and the plurality of converter units 3 are connected in series.

One end of the parallel circuit between the bypass unit 1 and the commutation sub-module 2 is connected to the loop; the other end of the parallel circuit between the bypass unit 1 and the commutation sub-module 2 is connected to the first end of the bridge arm resistor 4; the second end of the bridge arm resistor 4 is connected to the The first end of the bridge arm inductor 5 is connected; the second end of the bridge arm inductor 5 is connected to the phase power output end of the AC circuit breaker QF.

The bypass unit 1 is provided with a first bypass thyristor 1-1 and a second bypass thyristor 1-2; the first bypass thyristor 1-1 and the second bypass thyristor 1-2 are connected in parallel, and the first bypass thyristor 1-1 and the conduction direction of the second bypass thyristor 1-2 are opposite.

The technical solution adopted by the present invention is realized by connecting a bypass structure in parallel at both ends of all sub-modules of each bridge arm of the modularized multilevel converter, that is, the bypass structure is first connected with each bridge arm of the multilevel converter Both ends of the sub-module are connected in parallel, and then connected in series with the bridge arm inductance and the bridge arm inductance.

Moreover, the method of paralleling thyristors for each sub-module in the traditional scheme is changed to parallel thyristors for all sub-modules of each bridge arm. High-power and high-voltage thyristors can be preferred, which reduces the amount of thyristors and is more economical. It is also simpler and is also conducive to the design of the overall structure.

The multi-level converter bypass structure provided by the present invention can realize reliable bypass of the entire converter valve when a DC short-circuit fault occurs, and has a simple structure and high economic efficiency, and improves the impact resistance of the diodes at the bottom of each sub-module The current capacity is conducive to the structural optimization of the entire converter.

The present invention also provides a working method of a bypass mechanism of a modular multilevel converter, the method comprising:

When the mechanism is running normally, the bypass unit does not operate; when a short-circuit fault occurs on the DC side, the converter unit of the converter sub-module is blocked, and at the same time, one of the bypass thyristors in the bypass unit is turned on.

When the mechanism works in the rectification mode, the commutation sub-module is blocked, and at the same time, the second bypass thyristor is triggered to conduct, and the second bypass thyristor of the bypass unit conducts with the bridge arm resistor 4 and the bridge arm inductance 5 .

When the mechanism works in the inverter mode, while the commutation sub-module is locked, the first bypass thyristor in the bypass unit is turned on, and the fault current forms a circuit 29 through the bridge arm resistance, bridge arm inductance and the commutation sub-module; That is, the fault current passes through the bridge arm resistance, the bridge arm inductance and the diode of the commutation sub-module to form a loop 2 9 .

Loop one 8 is also formed through the first bypass thyristor, and the first bypass thyristor and the commutation sub-module form a shunt, that is, the first bypass thyristor and the diodes at the bottom of each sub-module form a shunt, which reduces the impact of short-circuit current on Diode strike.

After the first bypass thyristor forms a loop, the AC circuit breaker QF trips and recloses after the DC side current decays. After successful reclosing, the commutation sub-module is unlocked and the mechanism restarts.

In the traditional scheme, the method of connecting thyristors in parallel with each commutation sub-module is changed to parallel thyristors at both ends of all commutation sub-modules in each bridge arm. High-power and high-voltage thyristors can be preferred, which reduces the amount of thyristors and is more economical. Higher, the control is simpler, and it is also conducive to the design of the overall structure; in the inverter mode, the inrush current through the lower diodes of each sub-module can be greatly reduced, and the reliability is higher.

The terms "first", "second", "third" and "fourth" in the description and claims of the present invention and the above drawings, if present, are used to distinguish similar objects and not necessarily to describe specific sequence or sequence. It is to be understood that the data so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein can be practiced in sequences other than those illustrated or described herein. Furthermore, the terms "comprising" and "having", as well as any variations thereof, are intended to cover a non-exclusive inclusion.

The above description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention will not be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (8)

1. A modular multilevel converter bypass mechanism, characterized in that it comprises: an AC circuit breaker QF and six converter assemblies (6); the six converter assemblies (6) are divided into three groups , each group is provided with two converter assemblies (6); The first phase electrical output terminals of the AC circuit breaker QF are respectively connected to the two converter assemblies (6) to form a loop; The second phase electric output end of the AC circuit breaker QF is respectively connected with two converter assemblies (6) to form a loop; The third phase electrical output terminals of the AC circuit breaker QF are respectively connected with the two converter assemblies (6) to form a loop. 2. The bypass mechanism of modular multilevel converter according to claim 1, characterized in that, The converter assembly (6) includes: a bypass unit (1), a converter sub-module (2), a bridge arm resistance (4) and a bridge arm inductance (5); The bypass unit (1) is connected in parallel with the converter sub-module (2), and the parallel circuit of the bypass unit (1) and the converter sub-module (2) is connected in series with the bridge arm resistance (4) and the bridge arm inductance (5) connect. 3. The bypass mechanism of modular multilevel converter according to claim 2, characterized in that, The converter sub-module (2) is provided with multiple converter units (3), and the multiple converter units (3) are connected in series. 4. The bypass mechanism of modular multilevel converter according to claim 2, characterized in that, One end of the parallel circuit between the bypass unit (1) and the converter sub-module (2) is connected to a loop; The other end of the parallel circuit of the bypass unit (1) and the commutation sub-module (2) is connected to the first end of the bridge arm resistor (4); The second end of the bridge arm resistance (4) is connected to the first end of the bridge arm inductance (5); The second end of the bridge arm inductor (5) is connected to the phase power output end of the AC circuit breaker QF. 5. The bypass mechanism of modular multilevel converter according to claim 2, characterized in that, The bypass unit (1) is provided with a first bypass thyristor (1-1) and a second bypass thyristor (1-2); The first bypass thyristor (1-1) and the second bypass thyristor (1-2) are connected in parallel, and the conduction direction of the first bypass thyristor (1-1) and the second bypass thyristor (1-2) on the contrary. 6. A working method of a bypass mechanism of a modular multilevel converter, characterized in that the method comprises: When the mechanism is in normal operation, the bypass unit does not operate; When a short-circuit fault occurs on the DC side, the converter unit of the converter sub-module is blocked, and at the same time, one of the bypass thyristors in the bypass unit is turned on. 7. The working method of the bypass mechanism of the modularized multilevel converter according to claim 6, characterized in that the method further comprises: When the mechanism works in the rectification mode, while the commutation sub-module is blocked, the second bypass thyristor is triggered to conduct, and the second bypass thyristor of the bypass unit conducts with the bridge arm resistance (4) and bridge arm inductance (5) . 8. The working method of the bypass mechanism of the modularized multilevel converter according to claim 6, characterized in that the method further comprises: When the mechanism works in the inverter mode, while the commutation sub-module is locked, the first bypass thyristor in the bypass unit is turned on, and the fault current forms loop 2 through the bridge arm resistance, bridge arm inductance and the commutation sub-module; Loop 1 is formed through the first bypass thyristor, and the first bypass thyristor and the commutation sub-module form a shunt, which reduces the impact of short-circuit current on the diode; After the first bypass thyristor forms a loop, the AC circuit breaker QF trips and recloses after the DC side current decays. After successful reclosing, the commutation sub-module is unlocked and the mechanism restarts.