New energy diode rectification direct current sending-out system based on step wave voltage

Abstract

The invention discloses a new energy diode rectification direct current sending-out system based on step wave voltage, and belongs to the technical field of power generation, power transformation or power distribution. The system comprises a new energy collection branch, a collection transformer, a diode rectifier bridge, an auxiliary converter and a direct current cable. The auxiliary converter uses a power frequency fixed duty ratio control strategy, and assists a diode rectifier bridge to realize commutation by generating amplitude-variable three-phase step wave voltage at a common coupling point. The step wave voltage is adopted for commutation, a smoothing reactor is not needed on the direct current side of the diode rectifier bridge, meanwhile, compared with a traditional sine wave voltage commutation scheme, the needed reactive compensation capacity is greatly reduced, and the size, the weight and the cost of a sending end converter station are all effectively reduced.

Description

Technical Field

[0001] The present invention relates to a power system, and specifically discloses a new energy diode rectifier DC transmission system based on a stepped wave voltage, belonging to the technical field of power generation, transformation or distribution. Background Art

[0002] In China, new energy reserves are abundant. With the gradual increase in the single-unit capacity, scale, and collection radius of new energy farms, the advantages of DC transmission strategies have become increasingly prominent. Currently, flexible DC transmission is mostly adopted for new energy DC transmission. The converter station is the core equipment in flexible DC transmission. The modular multilevel converter topology is mostly used in the converter station. This topology has a large number of sub-modules. In particular, the large-volume capacitors in the sub-modules result in relatively large device volume, weight, and high cost, which is not conducive to the construction of the converter station platform.

[0003] In order to improve the problem of difficult construction of converter stations in flexible DC transmission systems, a DC transmission system based on diode rectification has been proposed. The converter station uses a diode rectifier bridge instead of a modular multilevel converter, and an auxiliary converter provides the three-phase AC voltage required for commutation for the diode rectifier bridge, and at the same time provides black start power for equipment such as wind turbines in new energy farms. However, affected by factors such as transformer leakage inductance, the auxiliary converter in this system needs to undertake a large amount of reactive power and harmonic compensation. Among them, the reactive power capacity accounts for about 30% of the active power transmitted by the system. Therefore, from the perspective of reducing the capacity of the auxiliary converter, there is still room for further improvement in the volume, weight, and cost of the converter station.

[0004] In summary, the present invention aims to propose a new energy diode rectifier DC transmission system based on a stepped wave voltage to overcome the above defects. Summary of the Invention

[0005] The invention object of the present invention is to overcome the deficiencies of the above background art, provide a new energy diode rectifier DC transmission system based on a stepped wave voltage, solve the technical problem that the auxiliary converter in the existing diode rectifier DC transmission system needs to undertake large-capacity reactive power compensation, and achieve the invention object of diode rectifier DC transmission with light weight, small volume, and low cost.

[0006] The present invention adopts the following technical solutions to achieve the above invention object:

[0007] A new energy diode rectifier DC transmission system based on a stepped wave voltage, comprising: at least one new energy collection branch, a collection transformer, a diode rectifier bridge, an auxiliary converter that forms a sending end converter station with the diode rectifier bridge, and a DC cable; each new energy collection branch outputs a stepped wave current with the same phase as the stepped wave voltage at the common coupling point; the collection transformer is used to collect the stepped wave currents output by each new energy collection branch and output them to the common coupling point; the AC side of the diode rectifier bridge is connected to the common coupling point, and the diode rectifier bridge commutes under the action of the stepped wave voltage at the common coupling point, rectifies the alternating current at the common coupling point and sends it out; the AC side of the auxiliary converter is the common coupling point, the DC side of the auxiliary converter is connected in parallel with the DC side of the diode rectifier bridge, does not work during the steady state stage of the stepped wave current, forms a stepped wave current freewheeling loop with the diode rectifier bridge during the commutation stage of the stepped wave current, and establishes a stepped wave voltage at the common coupling point; the DC cable is connected to the DC side of the diode rectifier bridge.

[0008] As a further optimized solution of a new energy diode rectifier DC transmission system based on a stepped wave voltage, the diode rectifier bridge is a three-phase six-pulse diode rectifier bridge topology or other three-phase multi-pulse diode rectifier bridge topologies.

[0009] As a further optimized solution of a new energy diode rectifier DC transmission system based on a stepped wave voltage, the auxiliary converter is a three-phase voltage source inverter based on unidirectional-conducting fully controlled switching devices or bidirectional-conducting fully controlled switching devices; the three-phase voltage source inverter includes, but is not limited to, three-phase two-level converter topologies, three-phase three-level converter topologies, three-phase modular multilevel converter topologies; the unidirectional-conducting fully controlled switching device is an IGBT switching device without an anti-parallel freewheeling diode; in the three-phase voltage source inverter based on bidirectional-conducting fully controlled switching devices, a current reverse blocking diode is connected in series with each phase bridge arm.

[0010] As a further optimized solution of a new energy diode rectifier DC transmission system based on a stepped wave voltage, the collection transformer is a three-phase step-up transformer, which not only collects the stepped wave currents output by each new energy collection branch and outputs them to the common coupling point, but also matches the voltage levels output by each new energy collection branch with the voltage level at the common coupling point.

[0011] As a further optimized solution of a new energy diode rectifier DC transmission system based on a stepped wave voltage, the voltage on the AC side of the auxiliary converter is clamped to a three-phase four-level stepped wave voltage under the power frequency fixed duty ratio modulation strategy, and each new energy collection branch outputs a three-phase four-level stepped wave current with the same phase as the three-phase four-level stepped wave voltage at the common coupling point.

[0012] As a further optimization solution for a new - energy diode - rectifier DC transmission system based on a stepped - wave voltage, during the steady - state stage of the stepped - wave current, the switches in each phase arm of the diode rectifier bridge conduct or turn off according to the voltage amplitude at the point of common coupling, and the three - phase four - level stepped - wave current at the point of common coupling all flows through the diode rectifier bridge.

[0013] As a further optimization solution for a new - energy diode - rectifier DC transmission system based on a stepped - wave voltage, under the action of the three - phase four - level stepped - wave voltage and the three - phase four - level stepped - wave current, during the commutation stage of the stepped - wave current, only the polarity of one phase current will reverse.

[0014] Before the current passes through zero and its polarity reverses, the commutation current is free - wheeled by the diodes that are conducting during the current steady - state stage in the diode rectifier bridge; when the current passes through zero and its polarity reverses but the three - phase four - level stepped - wave voltage at the point of common coupling has not commutated, the commutation current is free - wheeled by the switching devices that are conducting during the current steady - state stage in the auxiliary converter; when the three - phase four - level stepped - wave voltage at the point of common coupling commutates, the diodes in the corresponding arm where the current polarity in the diode rectifier bridge reverses conduct, and it enters the next steady - state stage.

[0015] The present invention adopts the above - mentioned technical solutions and has the following beneficial effects:

[0016] A new - energy diode - rectifier DC transmission system based on a stepped - wave voltage proposed by the present invention, compared with the traditional diode - rectifier DC transmission system based on a sine - wave voltage:

[0017] (1) Using a three - phase four - level stepped - wave voltage for commutation, the AC - side current of the diode rectifier bridge is also a three - phase four - level stepped - wave current. There is no need for harmonic - current compensation. The three - phase four - level stepped - wave voltage and the three - phase four - level stepped - wave current generate a stable DC transmission power, and there is no need for a large - inductance smoothing reactor on the DC side of the diode rectifier bridge.

[0018] (2) The diode rectifier bridge uses a stepped - wave voltage for commutation. Compared with the traditional sine - wave voltage commutation scheme, the time required for current commutation is significantly reduced, and the reactive - power compensation requirement brought about by the commutation process is also greatly reduced.

[0019] (3) The control strategy of the auxiliary converter is simple. Adopting a power - frequency fixed - duty - cycle control strategy, it adaptively adjusts the amplitude of the three - phase four - level stepped - wave voltage at the point of common coupling according to the system transmission power. Without a step - up transformer, the AC side of the auxiliary converter can be directly connected to the point of common coupling.

[0020] (4) The auxiliary converter only provides a free - wheeling path for the commutation current during the commutation process of the stepped - wave current. There is no current flowing through it during the steady - state stage, and the capacity required for the converter is extremely low.

[0021] (5) The voltage waveform at the common coupling point is a three-phase four-level stepped wave voltage, and the effective value of this voltage is higher than that of the sine wave voltage at the common coupling point in the traditional scheme. Therefore, when transmitting the same level of power, the effective values of the AC-side current of the diode rectifier bridge and the current of the new energy collection branch are both lower than those in the traditional scheme, and the conduction category loss of the system is effectively reduced. Description of the Drawings

[0022] Figure 1 is a simplified block diagram of a new energy diode rectifier DC transmission system based on stepped wave voltage according to the present invention.

[0023] Figure 2 is the topology of the sending converter station of the present invention including a diode rectifier bridge and an auxiliary converter. Among them, the diode rectifier bridge adopts a three-phase six-pulse diode rectifier bridge topology, and the auxiliary converter adopts a three-phase two-level voltage source converter topology.

[0024] Figure 3 is a waveform diagram of the four-level stepped wave voltage of a certain phase at the common coupling point of the present invention and the waveform diagram of the corresponding four-level stepped wave current output by the new energy collection branch.

[0025] Figure 4 is a current path diagram of a certain steady state stage of the sending converter station of the present invention.

[0026] Figure 5 is the sending converter station of the present invention from Figure 4 The current path diagram of the next steady state stage after commutation of the shown steady state stage.

[0027] Figure 6 is the sending converter station of the present invention from Figure 4 During the commutation from the shown steady state stage to Figure 5 The current path diagram when the current passes through zero and the auxiliary converter conducts continuous current during the commutation process to the shown steady state stage.

[0028] Figure 7 is the sending converter station of the present invention from Figure 4 During the commutation from the shown steady state stage to Figure 5 Schematic diagrams of the current waveforms of the A, B, and C phase new energy collection branches, the AC-side current of the diode rectifier bridge, the AC-side current of the auxiliary converter, and the DC transmission current during the commutation to the shown steady state stage.

[0029] Figure 8 is a schematic diagram of the voltage at the common coupling point, the current of the new energy collection branch, the AC-side current of the diode rectifier bridge, the AC-side current of the auxiliary converter, and the DC transmission current taking phase A as an example on the time scale of the power frequency cycle of the sending converter station of the present invention.

[0030] Description of the reference numerals in the figure: D 1 ~D6 , the first to sixth diodes, S 1 ~S 6 , the first to sixth switches, C d , filter capacitor, R d , DC cable equivalent resistance. Specific embodiments

[0031] Next, the technical solutions of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative efforts belong to the scope of protection of the present invention.

[0032] In the description of this specification, the descriptions referring to terms such as "an embodiment", "example", "specific example", etc. mean that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in a suitable manner in any one or more embodiments or examples.

[0033] As Figure 1 shown, a new energy diode rectifier DC transmission system based on a stepped wave voltage includes: a new energy collection branch, a collection transformer, a diode rectifier bridge, an auxiliary converter, and a DC cable. The output end of the new energy collection branch is connected to the input end of the collection transformer. Taking the output end of the collection transformer as a common coupling point, the AC side of the diode rectifier bridge and the AC side of the auxiliary converter are connected to the common coupling point. The DC side of the diode rectifier bridge and the DC side of the auxiliary converter are connected in parallel, and the DC side of the diode rectifier bridge is connected to the DC cable.

[0034] The sending end converter station includes a diode rectifier bridge and an auxiliary converter. As Figure 2 shown, in the figure, the diode rectifier bridge adopts a three-phase six-pulse diode rectifier bridge topology, and the auxiliary converter adopts a three-phase two-level voltage source converter topology. The three-phase six-pulse diode rectifier bridge topology includes: the A-phase bridge arm composed of the first diode D 1 and the second diode D 2 connected in series, the B-phase bridge arm composed of the third diode D 3 and the fourth diode D 4 connected in series, and the C-phase bridge arm composed of the fifth diode D 5 and the sixth diode D 6 connected in series. The three-phase two-level voltage source converter topology includes: the first switching device S 1 and the second switching device S2 The phase-A bridge arm formed by series connection, the third switching device S 3 and the fourth switching device S 4 The phase-B bridge arm formed by series connection, and the fifth switching device S 5 and the sixth switching device S 6 The phase-C bridge arm formed by series connection, and a filtering capacitor C is connected in parallel on the DC side d . The negative-polarity bus of the diode rectifier bridge and the negative-polarity bus of the DC side of the auxiliary converter are jointly connected to point N.

[0035] In the embodiment of the present invention, the diode rectifier bridge can adopt a three-phase six-pulse diode rectifier bridge topology or other multi-pulse diode rectifier bridge topologies. When it adopts a three-phase six-pulse diode rectifier bridge topology, the voltage waveform at the common coupling point should be a three-phase four-level stepped wave voltage, and the voltage waveform u of a certain phase AO such as Figure 3 shown, the diode rectifier bridge commutates with this voltage.

[0036] In the embodiment of the present invention, the auxiliary converter controls the voltage waveform at the common coupling point to be a three-phase four-level stepped wave voltage, and can adopt a three-phase two-level converter topology or other three-phase multi-level converter topologies. When adopting a three-phase two-level converter topology, the switching devices of the auxiliary converter can adopt unidirectional-conducting fully-controlled devices or other bidirectional-conducting fully-controlled switching devices. The unidirectional-conducting fully-controlled device can be an IGBT switching device without an anti-parallel freewheeling diode; the bidirectional-conducting fully-controlled switching device can be an IGBT switching device or a MOSFET switching device including an anti-parallel freewheeling diode, but at this time, a current reverse-blocking diode needs to be connected in series in the bridge arm of the auxiliary converter.

[0037] The auxiliary converter adopts a power-frequency fixed-duty ratio control strategy and operates in a two-level mode, specifically manifested as the upper and lower bridge arms in the same phase of the converter conducting complementarily at 180 degrees, and the conduction angles of each phase in the three phases differ by 120 degrees. Adopting this control strategy, the voltage waveform at the AC side output of the auxiliary converter, that is, the voltage waveform at the common coupling point, is clamped to a three-phase four-level stepped wave, and the four-level stepped amplitudes are 2U Cd / 3, U Cd / 3, -U Cd / 3 and -2U Cd / 3, where U Cd is the amplitude of the DC side voltage of the auxiliary converter.

[0038] In the embodiment of the present invention, the new energy collection branch injects three-phase four-level stepped wave currents with the same phase into the collection transformer based on the three-phase four-level stepped wave voltage at the common coupling point, and the current waveform i of a certain phase A such as Figure 3As shown, this current generates active power with the three-phase four-level stepped wave voltage at the common coupling point. Further dividing the stepped wave current waveform, it can be divided into a steady state stage and a commutation stage. The commutation stage refers to the process in which the current waveform transitions from the current steady state stage to the next steady state stage, and the duration of this process is Δt. During the commutation process, the current approximately undergoes a linear proportional transformation, and the current commutation process is ahead of the voltage commutation. When the current commutation is completed, the voltage undergoes commutation to reach the next steady state stage.

[0039] In the embodiment of the present invention, taking a certain steady state stage as an example, such as Figure 4 shown, at this time, the voltage of phase A in the three-phase four-level stepped wave voltage at the common coupling point is 2U Cd / 3, the voltages of phases B and C correspond to -U Cd / 3, the current of phase A is I d , and the currents of phases B and C correspond to -I d / 2. Constrained by the magnitude relationship of the voltage amplitudes, at this time, the first diode D 1 in the phase A bridge arm of the diode rectifier bridge conducts, and the current I d flows through it. The fourth diode D 4 in the phase B bridge arm and the sixth diode D 6 in the phase C bridge arm conduct, and the current -I d / 2 flows through them; at this time, the first switching device S 1 in phase A of the auxiliary converter and the fourth switching device S 4 S 4 in the phase B bridge arm and the sixth switching device S 6 in the phase C bridge arm receive turn-on signals, but they are all unidirectional conducting devices, and considering the current direction, no current flows through the auxiliary converter.

[0040] As Figure 4 shown, in this steady state stage, ignoring the influence of the switching frequency sub-ripple, then there is

[0041] I dr =I d (1)

[0042] In formula (1), I d is the amplitude of the final DC output current, and I dr is the amplitude of the DC side current of the diode rectifier bridge. This formula indicates that the DC side current of the diode rectifier bridge is the final output DC current. Considering that no current flows through the auxiliary converter, the output power of the sending end converter station can be calculated as

[0043] P o =I d U Cd (2)

[0044] Then, according to the power conservation, the total input power of the sending end converter station is

[0045] P in = P o = I d U Cd (3)

[0046] Meanwhile, in the auxiliary converter, there is

[0047] U AN = U Cd , U BN = U CN = 0 (4)

[0048] In formula (4), points A, B, and C are the common coupling points of three phases A, B, and C, and point N is the negative polarity point of the auxiliary converter. At this time, the amplitudes of the three-phase four-level stepped wave voltages at the common coupling point are respectively

[0049]

[0050] In formula (5), point O is the midpoint of the three phases. At the amplitude of this three-phase four-level stepped wave voltage, the power that the diode rectifier bridge can send out is

[0051]

[0052] It corresponds exactly to formula (3), that is, the auxiliary converter can adaptively adjust the amplitude of the DC side voltage according to the power transmitted by the new energy collection branch, and then adjust the amplitudes of the three-phase four-level stepped wave voltages at the common coupling point to ensure that all the power transmitted by the new energy collection branch passes through the diode rectifier bridge in the sending-end converter station for rectification to achieve DC transmission.

[0053] In the embodiment of the present invention, taking a certain commutation stage as an example, assume that the current path in the current steady state stage is as Figure 4 shown, that is, the voltage of phase A in the three-phase four-level stepped wave voltage at the common coupling point is 2U Cd / 3, the voltages of phases B and C correspond to -U Cd / 3, the current of phase A is I d , and the currents of phases B and C correspond to -I d / 2. The current path of the sending-end converter station in the next steady state stage is as Figure 5 shown, that is, the voltages of phases A and B in the three-phase four-level stepped wave voltage at the common coupling point are U Cd / 3, the voltage of phase C is -2U Cd / 3, the currents of phases A and B are I d / 2, and the currents of phase C correspond to -I d . Therefore, during the commutation process, the current of phase A needs to be reduced by I d / 2, the current of phase C needs to be reduced by I d / 2, and the current of phase B needs to be increased by Id Meanwhile, it is worth noting that the current of phase B changes from negative to positive, and the polarity is reversed.

[0054] The commutation current approximately linearly scales. Therefore, before Δt / 2, the current paths of the sending-end converter station are as Figure 4 shown, and no current needs to flow through the auxiliary converter. At the moment of Δt / 2, the current of phase B is zero and the polarity is reversed. However, at this time, since the three-phase four-level stepped wave voltage at the common coupling point has not been commutated yet, the third diode D in the phase B bridge arm of the diode rectifier bridge 3 cannot conduct and continue the current. Therefore, at this time, the fourth switching device S in the auxiliary converter that has received the turn-on signal 4 continues the current, and the current path diagram of this continuous current process is as Figure 6 shown. This process continues until the moment of Δt. At this time, the three-phase four-level stepped wave voltage at the common coupling point is commutated, and the third diode D in the phase B bridge arm of the diode rectifier bridge 3 conducts, and the commutation process of the system ends smoothly and transitions to the steady state stage as Figure 5 shown.

[0055] During the above commutation process, the current waveforms of the new energy collection branches of phases A, B, and C are i A , i B , i C ; the current waveforms of the AC sides of phases A, B, and C of the diode rectifier bridge are i DRUA , i DRUB , i DRUC ; the current waveforms of the AC sides of phases A, B, and C of the auxiliary converter are i VSCA , i VSCB , i VSCC ; the schematic diagram of the DC output current waveform I d is as Figure 7 shown.

[0056] In the embodiment of the present invention, other steady state stages and commutation processes are the same as the above steady state stages and commutation processes. On the time scale of the power frequency cycle, taking phase A as an example, the four-level stepped wave voltage waveform u AO , the new energy collection branch current waveform i A , the AC side current waveform i DRUA of the diode rectifier bridge, the AC side current waveform i VSCA of the auxiliary converter, and the DC output current waveform I d are as Figure 8 shown.

[0057] The foregoing has shown and described the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments, and what is described in the above embodiments and the specification only illustrates the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and all these changes and improvements fall within the scope of the present invention claimed.

Claims

1. A new energy diode rectifier DC transmission system based on step wave voltage, characterized in that: include: At least one new energy collection branch, each new energy collection branch outputs a step wave current with the same phase as the step wave voltage at the common coupling point; The collection transformer is used to collect the step wave current output by each new energy collection branch and output it to the common coupling point; The diode rectifier bridge has its AC side connected to the common coupling point, and changes phase under the action of the step wave voltage at the common coupling point, and rectifies the AC power at the common coupling point and sends it out; An auxiliary converter of the sending-end converter station is formed with the diode rectifier bridge, the AC side of which is a common coupling point, the DC side of which is connected in parallel with the DC side of the diode rectifier bridge, does not work in the steady-state stage of the step wave current, and forms a step wave current freewheeling loop with the diode rectifier bridge in the commutation stage of the step wave current, and establishes a step wave voltage at the common coupling point; and, A DC cable is connected to the DC side of the diode rectifier bridge.

2. According to claim 1, a new energy diode rectifier DC transmission system based on step wave voltage is characterized in that: The diode rectifier bridge is a three-phase six-pulse diode rectifier bridge topology or other three-phase multi-pulse diode rectifier bridge topology.

3. According to the new energy diode rectification DC transmission system based on step wave voltage as claimed in claim 1, it is characterized in that: The auxiliary converter is a three-phase voltage source inverter based on a unidirectional conductive fully controlled switching device or a bidirectional conductive fully controlled switching device; the three-phase voltage source inverter includes but is not limited to a three-phase two-level converter topology, a three-phase three-level converter topology, and a three-phase modular multi-level converter topology; the unidirectional conductive fully controlled switching device is an IGBT switching device without an anti-parallel freewheeling diode; in the three-phase voltage source inverter based on a bidirectional conductive fully controlled switching device, each phase bridge arm is respectively connected in series with a current reverse blocking diode.

4. According to claim 1, a new energy diode rectifier DC transmission system based on step wave voltage is characterized in that: The collection transformer is a three-phase step-up transformer, which collects the step wave currents output by each new energy collection branch and outputs them to a common coupling point, while matching the voltage level output by each new energy collection branch with the voltage level at the common coupling point.

5. A new energy diode rectifier DC transmission system based on step wave voltage according to any one of claims 1 to 4, characterized in that: The AC side voltage of the auxiliary converter is clamped to a three-phase four-level step wave voltage under the industrial frequency fixed duty cycle modulation strategy, and each new energy collection branch outputs a three-phase four-level step wave current with the same phase as the three-phase four-level step wave voltage at the common coupling point.

6. According to claim 5, a new energy diode rectifier DC transmission system based on step wave voltage is characterized in that: In the steady-state stage of the step wave current, the switches in the bridge arms of each phase of the diode rectifier bridge are turned on or off according to the voltage amplitude at the common coupling point, and the three-phase four-level step wave current at the common coupling point all flows through the diode rectifier bridge.

7. According to claim 6, a new energy diode rectifier DC transmission system based on step wave voltage is characterized in that: Under the action of the three-phase four-level step wave voltage and the three-phase four-level step wave current, only one phase current polarity will be reversed during the step wave current commutation stage. Before the current reverses its polarity at zero crossing, the commutation current is freewheeled by the diodes in the diode rectifier bridge that are conducting in the current steady-state phase; When the current zero-crossing polarity is reversed but the three-phase four-level step wave voltage at the common coupling point is not commutated, the commutation current is continued by the switch device that is turned on in the current steady-state stage in the auxiliary converter; When the three-phase four-level step wave voltage at the common coupling point changes phase, the current polarity in the diode rectifier bridge is reversed, and the diode in the corresponding bridge arm is turned on, entering the next steady state stage.

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