CN111953221A - A modular multilevel converter and converter station - Google Patents

Abstract

The invention discloses a modular multi-level converter and a converter station. The modular multi-level converter includes three-phase bridge arms, and each phase bridge arm includes an upper bridge arm and a lower bridge arm. The upper bridge arm and the lower bridge arm are provided with a plurality of cascaded converter sub-modules, the converter sub-modules include a first fully-controlled switch module, a second fully-controlled switch module, a buffer circuit and a first capacitor , wherein the first fully-controlled switch module and the second fully-controlled switch module are connected in series to form a switch branch; the switch branch, the buffer circuit and the first capacitor are connected in parallel with each other; the first fully-controlled switch module and the second fully-controlled switch module includes an IGCT device and a first diode connected in anti-parallel with it. The converter is equipped with an IGCT device, which realizes the integration of the converter and the load switch in one converter station, reduces the overall volume and cost of the converter station, realizes the high integration of the converter station, and makes the converter station It has high reliability and can be widely used in flexible DC transmission systems.

Description

A modular multilevel converter and converter station

technical field

The invention belongs to the field of electric power transmission, and particularly relates to a modular multilevel converter and a converter station.

Background technique

Flexible DC transmission has become the most potential new power transmission method, and has been applied in large-capacity transmission systems. Modular multilevel converters are currently the most mainstream topology. Most of the flexible DC transmission projects that have been put into operation use modular multi-level converters based on IGBT (Insulated Gate Bipolar Transistor: Insulated Gate Bipolar Transistor), but there are great difficulties in implementing DC fault clearance. When a DC short-circuit fault occurs, a huge DC short-circuit fault current is often generated. For this reason, it is necessary to break the DC short-circuit fault current, and a DC circuit breaker with a large breaking current capacity is required. Such DC circuit breakers are expensive and huge. Therefore, as shown in Figure 1, the converter station adopts a modular multi-level converter based on IGBT, and the converter and the circuit breaker are placed in two independent converter stations.

Therefore, how to provide a modular multi-level converter and the functions that can realize the integrated commutation and switch integration of the converter station has increasingly become a technical problem to be solved urgently.

SUMMARY OF THE INVENTION

In view of the above problems, the present invention provides a modularized multi-level converter and a converter station, which improves the reliability of the converter and realizes the functions of integrated converter and switch integration of the converter station.

The purpose of the present invention is to provide a modular multilevel converter, which includes three-phase bridge arms, each phase bridge arm includes an upper bridge arm and a lower bridge arm, and the upper bridge arm and the lower bridge arm are provided with a plurality of stages The inverter sub-module is connected to the inverter, and the inverter sub-module includes a first fully-controlled switch module, a second fully-controlled switch module, a buffer circuit and a first capacitor, wherein,

The first fully-controlled switch module and the second fully-controlled switch module are connected in series to form a switch branch;

the switch branch, the buffer circuit and the first capacitor are connected in parallel with each other;

The first fully-controlled switch module and the second fully-controlled switch module both include an IGCT device and a first diode connected in anti-parallel with the IGCT device.

Further, the second fully-controlled switch module is connected in parallel with the output end of the converter sub-module.

Further, the cathode of the IGCT device in the first fully-controlled switch module in the converter sub-module is connected to the anode of the IGCT device in the second fully-controlled switch module.

Further, the buffer circuit includes a first inductor, a first resistor, a second diode and a second capacitor, wherein,

The first end of the first inductor is connected to the first end of the first capacitor and the first end of the first resistor respectively, and the second end of the first inductor is respectively connected to the first fully controlled switch module and the second diode positive connection;

The cathode of the second diode is respectively connected to the second end of the first resistor and the first end of the second capacitor;

The second end of the second capacitor is respectively connected to the second fully-controlled switch module and the second end of the first capacitor.

Another object of the present invention is to provide a converter station, comprising the above-mentioned modular multilevel converter, load switch and first reactor, wherein,

the modular multilevel converter is connected to one end of the load switch;

The other end of the load switch is connected to one end of the first reactor.

Further, the modular multilevel converter further includes a DC side and an AC side, wherein,

The positive pole of the DC side of the modular multilevel converter is connected to the DC line by connecting the load switch and the first reactor in sequence;

The AC side of the modular multilevel converter is connected to the AC power grid.

Further, the IGCT devices in all the first fully-controlled switching modules and the second fully-controlled switching modules in the modular multilevel converter are used to control the first fully-controlled switching module when a short-circuit fault occurs in the DC line. The IGCT device in the switch module is blocked, and the IGCT device of the second fully-controlled switch module is turned on, so that the AC grid is in a three-phase short-circuit state and the potential of the DC port of the DC line is zero, and finally the fault current stops rising.

Further, the first reactor is a smoothing reactor.

Further, the load switch is a rated load switch, and is used for opening and cutting off the fault current when the fault current of the DC line reaches the rated current value of the load switch.

Further, the rated load switch is a mechanical load switch, an electronic load switch or a hybrid load switch.

The first fully-controlled switch module and the second fully-controlled switch module in the modular multilevel converter of the present invention both use IGCT devices. Compared with IGBTs, IGCTs have very high surge Current withstand capability, on-state current capability, forward blocking voltage capability and reliability, and also have very low on-state voltage drop, making the modular multilevel converter with high current, high blocking voltage, reliable It has the advantages of high performance, compact structure and low conduction loss.

Further, the converter station can integrate the modular multi-level converter and load switch into one station, which greatly reduces the overall volume and cost of the converter station, realizes high integration and high economy, and can It will be widely used in future flexible DC transmission systems.

In addition, by controlling the IGCT device in the first fully-controlled switch module to be blocked, the IGCT device of the second fully-controlled switch module is turned on, which can effectively stop the rise of the fault current, so that the load switch only needs to break the rated current. DC current, which also greatly reduces the volume, amount and cost required for the load switch. And only a smoothing reactor is needed in the converter station, the inductance value of the smoothing reactor is small, and the current limiting function does not need to be considered.

Other features and advantages of the present invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure pointed out in the description, claims and drawings.

Description of drawings

In order to illustrate the embodiments of the present invention or the technical solutions in the prior art more clearly, the following briefly introduces the accompanying drawings that need to be used in the description of the embodiments or the prior art. Obviously, the drawings in the following description are For some embodiments of the present invention, for those of ordinary skill in the art, other drawings can also be obtained according to these drawings without creative efforts.

FIG. 1 shows a schematic structural diagram of a converter station and a circuit breaker in the prior art;

2 shows a schematic structural diagram of a modular multilevel converter in an embodiment of the present invention;

3 shows a schematic structural diagram of a converter station in an embodiment of the present invention;

FIG. 4 is a graph showing the change of U _MMC , U _DC and i _DC in the control process in the converter station in the embodiment of the present invention.

Detailed ways

In order to make the purposes, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments These are some embodiments of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

As shown in FIG. 2, an embodiment of the present invention introduces a modular multi-level converter, including three-phase bridge arms, each phase bridge arm includes an upper bridge arm and a lower bridge arm, the upper bridge arm and the lower bridge arm The bridge arm is provided with a plurality of cascaded converter sub-modules, among which, the number of converter sub-modules in the upper bridge arm of each phase bridge arm is n, respectively, in Figure 2: SMap1, SMap2...SMapn, SMbp1, SMbp2...SMbpn, SMcp1, SMcp2...SMcpn. The number of converter sub-modules in the lower bridge arm of each phase bridge arm is n respectively, which are respectively in FIG. 2: SMan1, SMan2...SMann, SMbn1, SMbn2...SMbpnn, SMcn1, SMcn2...SMcnn. Further, the converter sub-module includes a first fully-controlled switch module, a second fully-controlled switch module, a buffer circuit and a first capacitor. The first fully controlled switch module includes an IGCT (integrated Gate Commutated Thyristors: integrated gate commutated thyristor) device S _xi1 and a first diode D _xi1 connected in anti-parallel to it; the second fully controlled switch module includes an IGCT device S _xi2 and its anti-parallel first diode D _xi2 ; and the first fully-controlled switch module and the second fully-controlled switch module are connected in series to form a switch branch; the switch branch, the buffer circuit and the third A capacitor _Cxi is connected in parallel with each other. Further, when the first fully-controlled switch module and the second fully-controlled switch module are connected in series, the IGCT device S _xi1 and the IGCT device S _xi2 are in the same direction, that is, the cathode of the IGCT device S _xi1 and the IGCT device Anode connection of S _xi2 . The first fully-controlled switch module and the second fully-controlled switch module in the modular multi-level converter both use IGCT devices. On-state current capability, forward blocking voltage capability and reliability, and also have very low on-state voltage drop, so that the modular multilevel converter has high current, high blocking voltage, high reliability, structure compact and low conduction losses.

In this embodiment, each of the first diodes in anti-parallel with the IGCT device can effectively cut off the fault current in the reverse direction, thereby protecting the IGCT device.

More specifically, as shown in FIG. 2 , the second fully-controlled switch module is connected in parallel to the output end of the converter sub-module, and the plurality of converter sub-modules on the upper bridge arm and the lower bridge arm pass through. The respective outputs are connected. Further, the buffer circuit includes a first inductor L _xis , a first resistor R _xis , a second diode D _xis and a second capacitor C _xis , wherein the first end of the first inductor L _xis is connected to the The first end of the first capacitor C _xi is connected to the first end of the first resistor R _xis , and the second end thereof is respectively connected to the positive pole of the first fully controlled switch module and the second diode D _xis ; The negative pole of the pole tube is respectively connected with the second end of the first resistor R _xis and the first end of the second capacitor C _xis ; the second end of the second capacitor C _xis is respectively connected with the second fully controlled switch module and the first capacitor C The second end of _xi is connected. The first end of the first capacitor Cxi is the positive electrode, the second end is the negative electrode, and the voltage is represented by Vc. By adopting the buffer circuit, when the modular multilevel converter works normally, the rate of increase of current or voltage on the first fully-controlled switch module and/or the second fully-controlled switch module can be effectively suppressed. However, when the modular multi-level converter works under the short-circuit fault state of the DC line, because the IGCT device in the first fully-controlled switch module is blocked and the IGCT device in the second fully-controlled switch module is turned on, The snubber circuit is in a bypass state during fault handling.

In this embodiment, the upper bridge arm and the lower bridge arm of each phase bridge arm are further provided with a second reactor (not shown in FIG. 2-3 ), which is cascaded with a plurality of commutators on the corresponding bridge arm The sub-modules are connected in series.

As shown in FIG. 3 , a converter station is also introduced in the embodiment of the present invention, and the converter station includes the above-mentioned modular multi-level converter, a load switch and a first reactor, wherein all the The modular multilevel converter includes a DC side and an AC side; the positive pole of the DC side of the modular multilevel converter is connected to one end of the load switch, and the other end of the load switch is connected to the first end of the load switch. One end of a reactor is connected to the other end of the first reactor is connected to the DC line. Further, the negative pole of the modular multilevel converter and the other end of the first reactor form the DC port of the converter station, that is, the DC port of the converter station. The converter station is connected to the DC line. The AC side of the modular multilevel converter forms the AC port of the converter station and is connected to the AC power grid. In the embodiment of the present invention, the converter and the load switch are integrated into one converter station, which greatly reduces the overall volume and cost of the converter station, realizes high integration and high economy, and can be used in the future. It has been widely used in flexible DC transmission systems. In addition, the above-mentioned modular multilevel converter is applied to the converter station, so that the converter station based on IGCT can realize the function of self-clearing of the DC fault when facing the DC short-circuit fault, and can The rise of the DC short-circuit fault current is well limited.

In Fig. 3, the load switch is a rated load switch. It is used to switch off the fault current when the fault current of the DC line reaches the rated current value of the load switch. Since the IGCT device in the first fully-controlled switch module is controlled to be blocked, the IGCT device of the second fully-controlled switch module is turned on, which can effectively stop the rise of the fault current. The load switch only needs to break the DC current of the rated current, which greatly reduces the volume, amount and cost required by the load switch. Further, the rated load switch is a mechanical load switch, an electronic load switch or a hybrid load switch. The first reactor is a smoothing reactor. Since the above-mentioned modular multi-level converter can directly limit the rise of the fault current during the short-circuit fault of the DC line, the converter in the embodiment of the present invention does not need to be equipped with a reactor with a large inductance value for current limiting, Use chokes only for smoothing. Therefore, the inductance value of the first reactor in the embodiment of the present invention is small, and the current limiting function does not need to be considered. However, based on IGBT-MMC (Modular Multilevel Converter: Modular Multilevel Converter), since it cannot limit the rapid rise of fault current during fault, it needs to be equipped with a large inductance value reactor for current limiting, and at the same time A reactor is also used to smooth the wave.

The embodiment of the present invention also introduces a control method for the above-mentioned converter station, and the control process of the control method specifically includes the following steps:

1) Time t0: Assuming that a short-circuit fault of the DC line occurs at time t0, the fault current caused by the short-circuit fault of the DC line begins to rise rapidly.

2) Time t1: the time when a short-circuit fault is detected and the protection action of the converter sub-module occurs. When the short-circuit fault of the DC line is detected, the IGCT device S _xi1 of the first fully-controlled switch module in all the converter sub-modules in the modular multilevel converter is blocked, and the IGCT device of the second fully-controlled switch module is blocked. S _xi2 is turned on. Specifically, the length of the time period from t0 to t1 is the delay time after a short-circuit fault occurs in the DC line, wherein the length of the time period from t0 to t1 is 0.3 ms (milliseconds).

When a short-circuit fault occurs in the DC line, the IGCT device S _xi1 of the first fully-controlled switch module in all the converter sub-modules of the modular multilevel converter in the converter station is blocked, and the second fully-controlled switch module is blocked. The IGCT device S _xi2 is turned on, so that the AC grid connected to the AC side of the modular multilevel converter is in a three-phase short-circuit state. The second reactor is short-circuited in three phases, the potential of the DC port of the converter station is equal to zero, and at the same time, the potential of the DC side of the modular multilevel converter is also zero, and the fault current stops rising at this time.

3) Time t2: the moment when the load switch cuts off the fault current, at this time the fault current of the short-circuit fault of the DC line passing through the load switch reaches the maximum value. The length of the time period from t1 to t2 is the time when the load switch receives the action command and separates the switch, and the length of the time period from t1 to t2 is about 3ms. After the disconnection of the load switch, isolating the DC line and the modular multilevel converter, the fault current begins to decay, and since then the DC line begins to de-ionize. Further, the maximum value of the fault current is the rated current value of the load switch.

4) Time t3: the blocking time of the IGCT device S _xi2 of the second fully-controlled switch module in all the converter sub-modules in the modular multilevel converter. After the load switch breaks the fault current, it is no longer necessary to actively short-circuit the IGCT device to clamp the voltage of the DC side port of the modular multilevel converter. Therefore, the IGCT device can be controlled to turn off when the current of each phase bridge arm crosses zero or when the current is small, and ends the three-phase short-circuit state of the AC grid. The length of the time period from t2 to t3 is less than 10ms. In the embodiment of the present invention, the small current is that the current on the bridge arm of each phase is not greater than the preset value of the repeated turn-off current of the IGCT device, and the preset value of the repeated turn-off current of the IGCT device is the repeated turn-off current of the IGCT device. The maximum capacity of turn-off current, the maximum capacity of repeated turn-off current of the four-inch IGCT device with the highest market share can be 4.5kA (kiloampere), but not limited to 4.5kA, 5kA, 6kA, etc. are all applicable to the present invention.

5) Time t4: the time when the modular multilevel converter is unlocked, and the IGCT devices in all converter sub-modules are reopened at this time, that is, all IGCT devices S _xi1 and IGCT devices S _xi2 are turned on. The modular multilevel converter slowly recovers the voltage from the blocking state to the normal operating state. The process of restoring the voltage often takes one to two cycles, for a total of about 40ms. However, it is only necessary to ensure that the voltage recovery has been completed before the load switch is reclosed. The voltage is the rated DC voltage of the modular multilevel converter during normal operation.

6) Time t5: the time to control the reclosing of the load switch. At this point the modular multilevel converter has been unlocked and returned to the normal operating voltage. The length of the time period from t2 to t5 is the time when the DC line is freed, so the length of the time period from t2 to t5 is about 300 ms.

7) Time t6: when the load switch completes the reclosing, the modular multilevel converter resumes power transmission to the DC side. The length of the time period from t5 to t6 is the reclosing time of the load switch. For the mechanical load switch, the length of the time period from t5 to t6 is about 15 to 20 ms.

Through the above control process, the changes of the voltage U _MMC of the modular multilevel converter and the voltage U _DC and the current i _DC of the DC port of the DC line are shown in FIG. 4 . In the control process, the length of each time period involved effectively indicates that the converter station in the embodiment of the present invention can quickly process and recover the short-circuit fault of the DC line, and has higher reliability.

The modular multi-level converter and load switch provided with IGCT devices are arranged together in a converter station, which makes the structure more compact and enables the converter station to have the functions of commutation and switching at the same time.

Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: it is still possible to modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements to some of the technical features; and these Modifications or substitutions do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims (10)

1. A modular multilevel converter, comprising three-phase bridge arms, each phase bridge arm comprising an upper bridge arm and a lower bridge arm, and the upper bridge arm and the lower bridge arm are provided with a plurality of cascaded commutators The inverter sub-module is characterized in that the inverter sub-module includes a first fully-controlled switch module, a second fully-controlled switch module, a buffer circuit and a first capacitor, wherein, The first fully-controlled switch module and the second fully-controlled switch module are connected in series to form a switch branch; the switch branch, the buffer circuit and the first capacitor are connected in parallel with each other; The first fully-controlled switch module and the second fully-controlled switch module both include an IGCT device and a first diode connected in anti-parallel with the IGCT device. 2 . The modular multilevel converter according to claim 1 , wherein the second fully-controlled switch module is connected in parallel with the output end of the converter sub-module. 3 . 3 . The modular multilevel converter according to claim 2 , wherein the cathode of the IGCT device in the first fully-controlled switch module in the converter sub-module and the second fully-controlled switch are 3 . Anode connection of the IGCT device in the module. 4. The modular multilevel converter according to any one of claims 1-3, wherein the buffer circuit comprises a first inductor, a first resistor, a second diode and a second capacitor, wherein , The first end of the first inductor is connected to the first end of the first capacitor and the first end of the first resistor respectively, and the second end of the first inductor is respectively connected to the first fully controlled switch module and the second diode positive connection; The cathode of the second diode is respectively connected to the second end of the first resistor and the first end of the second capacitor; The second end of the second capacitor is respectively connected to the second fully-controlled switch module and the second end of the first capacitor. 5. A converter station, characterized in that it comprises the modular multilevel converter of claim 1, a load switch and a first reactor, wherein, the modular multilevel converter is connected to one end of the load switch; The other end of the load switch is connected to one end of the first reactor. 6. The converter station according to claim 5, wherein the modular multilevel converter further comprises a DC side and an AC side, wherein, The positive pole of the DC side of the modular multilevel converter is connected to the DC line by connecting the load switch and the first reactor in sequence; The AC side of the modular multilevel converter is connected to the AC power grid. 7. The converter station according to claim 6, wherein the IGCT devices in all the first fully-controlled switching modules and the second fully-controlled switching modules in the modular multilevel converter are used for When a short-circuit fault occurs in the DC line, by controlling the IGCT device in the first fully-controlled switch module to block, the IGCT device of the second fully-controlled switch module is turned on, so that the AC grid is in a three-phase short-circuit state and the DC port potential of the DC line is zero, and eventually the fault current stops rising. 8. The converter station according to claim 7, wherein the first reactor is a smoothing reactor. 9 . The converter station according to claim 8 , wherein the load switch is a rated load switch, which is used for opening and cutting off the fault current when the fault current of the DC line reaches the rated current value of the load switch. 10 . 10 . The converter station according to claim 9 , wherein the rated load switch is a mechanical load switch, an electronic load switch or a hybrid load switch. 11 .

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