CN107134802B - Medium-voltage three-port flexible multi-state switch topology based on power electronic transformer - Google Patents

Medium-voltage three-port flexible multi-state switch topology based on power electronic transformer Download PDF

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CN107134802B
CN107134802B CN201710518154.3A CN201710518154A CN107134802B CN 107134802 B CN107134802 B CN 107134802B CN 201710518154 A CN201710518154 A CN 201710518154A CN 107134802 B CN107134802 B CN 107134802B
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刘进军
欧阳少迪
宋曙光
陈星星
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Xian Jiaotong University
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

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Abstract

The invention discloses a topological structure of a medium-voltage three-port flexible multi-state switch based on a power electronic transformer. The topology consists of three series output stages A, B and C and two high-frequency isolation stages X, Y; each series output stage comprises three phases, N single-phase AC-DC converters connected in series in each phase; each high frequency isolation stage comprises 3N two-port high frequency transformers and 6N single phase AC-DC converters. The series output stage A is connected with the series output stage C through the high-frequency isolation stage X, and the series output stage B is connected with the series output stage C through the high-frequency isolation stage Y. The topology can realize interconnection and power flow control of three medium-voltage alternating-current distribution lines, and can replace and exceed the functions of a traditional mechanical interconnection switch. The simulation result verifies the effectiveness of the topology, and provides a good reference value for engineering application.

Description

Medium-voltage three-port flexible multi-state switch topology based on power electronic transformer
Technical Field
The invention belongs to the field of flexible multi-state switches, and relates to a novel modular, medium-voltage and three-port flexible multi-state switch topology.
Background
In a future intelligent power distribution network, the uncertainty of system operation is aggravated by widely accessed distributed power supplies, and a series of problems are brought, such as voltage line crossing, line overload and the like. These problems have prompted power distribution networks to perform real-time network reconfiguration and active power management. At present, network reconstruction of a power distribution network mainly depends on a traditional mechanical interconnection switch, and due to the limitation of switching loss, impact current and the like, the traditional mechanical switch cannot be frequently switched on and switched off, so that the instantaneity of network reconstruction of the power distribution network is limited, and the requirement of a future intelligent power distribution network is difficult to meet.
The flexible multi-state switch is a power electronic device for connecting two or more distribution lines, can change the transmission power between the two or more distribution lines in real time, can adjust the on-off state in real time, can replace a traditional mechanical switch, and is helpful for a power distribution network to deal with a series of problems brought after a distributed power supply is connected.
Until now, the worldwide research on flexible multi-state switches has been still on the overall concept level and the power grid operation level. The power electronic device is still blank. Under different voltage levels, power levels and port numbers, the flexible multi-state switch can adopt specific topologies and analyze the advantages and disadvantages between different topologies, and also belongs to the research blank.
Disclosure of Invention
The invention aims to provide a medium-voltage three-port flexible multi-state switch topology based on a power electronic transformer, which is used for replacing a traditional mechanical interconnection switch in a medium-voltage distribution system and is called as a flexible multi-state switch. By using the flexible multi-state switch, the connection/disconnection control can be carried out on a plurality of medium-voltage power distribution lines so as to realize the quick reconstruction of a power distribution system network; power regulation can be performed on a plurality of medium voltage distribution lines to achieve power flow distribution optimization in a power distribution network. To achieve this, a suitable topology is required for the flexible multi-state switch.
The invention is realized by the following technical scheme:
a medium-voltage three-port flexible multi-state switch topology based on a power electronic transformer comprises three series output stages A, B and C and two high-frequency isolation stages X, Y, wherein the three series output stages are connected with the two high-frequency isolation stages.
Each series output stage is three-phase, each phase comprises N identical series AC-DC converters, N is a natural number, and a capacitor is connected in parallel between the direct current ends of each AC-DC converter; the total alternating current side of the AC-DC converter connected in series with each phase is connected with a reactor and then connected with one phase of an input power grid, and the phases are connected in a triangular or star shape;
each high frequency isolation stage comprises 3N two-port high frequency transformers and 6N AC-DC converters. Each high-frequency transformer and the two single-phase AC-DC converters connected with the high-frequency transformer form a high-frequency isolation level unit, and each high-frequency isolation level is provided with 3N high-frequency isolation level units.
Each AC-DC converter of the high frequency isolation stage is connected at the DC side to an AC-DC converter of a series input stage.
In the high-frequency isolation stage X, Y, each high-frequency transformer and the two single-phase AC-DC converters connected to it constitute a high-frequency isolation stage unit, and each high-frequency isolation stage has 3N high-frequency isolation stage units.
The N AC-DC converters of the A phase of the series output stage A are connected with the N AC-DC converters of the A phase of the series output stage C through the N high-frequency isolation level units of the high-frequency isolation level X;
the N AC-DC converters of the B phase of the series output stage A are connected with the N AC-DC converters of the B phase of the series output stage C through the N high-frequency isolation level units of the high-frequency isolation level X;
the N AC-DC converters of the C phase of the series output stage A are connected with the N AC-DC converters of the C phase of the series output stage C through the N high-frequency isolation level units of the high-frequency isolation level X;
the N AC-DC converters of the A phase of the series output stage B are connected with the N AC-DC converters of the A phase of the series output stage C through the N high-frequency isolation stage units of the high-frequency isolation stage Y;
the N AC-DC converters of the B phase of the series output stage B are connected with the N AC-DC converters of the B phase of the series output stage C through the N high-frequency isolation stage units of the high-frequency isolation stage Y;
the N AC-DC converters of the C phase of the series output stage B are connected with the N AC-DC converters of the C phase of the series output stage C through the N high-frequency isolation stage units of the high-frequency isolation stage Y;
the two high-frequency isolation level units connected with each AC-DC converter in the series output stage C and the two AC-DC converters from the series output stages A and B connected with the two high-frequency isolation level units form a flexible multi-state switch unit, and the whole system comprises 3N flexible multi-state switch units.
Compared with the prior art, the invention has the beneficial technical effects that:
1. the on/off control of three medium-voltage distribution lines can be realized, and the function of a traditional mechanical switch is realized;
2. the active power flow among the three distribution lines can be controlled at will, and the function of the traditional mechanical switch is surpassed;
3. reactive power and harmonic waves can be compensated;
4. the high-frequency transformer is adopted to realize the electrical isolation among the three series output stages, a multi-winding power frequency isolation transformer is omitted, and the size is small.
Drawings
Fig. 1 is a medium voltage three port flexible multi-state switching topology based on a power electronic transformer.
Fig. 2 is a basic unit diagram of a three-port flexible multi-state switch.
Fig. 3 is a diagram of a basic unit of a high-frequency isolation stage.
FIG. 4 is a graph of voltage waveforms of a basic cell of a high frequency isolation stage; phi is the phase difference between the alternating-current square wave voltages generated by the two AC-DC converters connected with the primary side coil and the secondary side coil of the high-frequency transformer; .
Fig. 5 shows a control strategy for the series output stage a.
Fig. 6 shows the control strategy of the high frequency isolation stage X.
Fig. 7 shows the control strategy of the high frequency isolation stage Y.
Fig. 8 shows the control strategy for the series output stages b and c.
Fig. 9 is a waveform of the port current of the series output stage c.
Fig. 10 is a waveform of the port current of the series output stage b.
Fig. 11 is a waveform of a port current of the series output stage a.
Fig. 12 shows the DC voltages of the AC-DC converters of the series output stage c.
Fig. 13 shows the DC voltages of the AC-DC converters of the series output stage b.
Fig. 14 shows the DC voltages of the AC-DC converters of the series output stage a.
Detailed Description
The medium-voltage three-port flexible multi-state switch topology provided by the invention adopts three output stages connected in series and two output stages connected in series. The present invention will now be described in further detail with reference to specific examples and figures, which are intended to be illustrative, but not limiting, of the invention.
Example (b):
the medium-voltage three-port flexible multi-state switch topology is shown in figure 1, and the medium-voltage three-port flexible multi-state switch topology is composed of an AC-DC converter and a three-port high-frequency transformer, so that a 10kV and 6MW three-port system per port is formed. In fig. 1, the left side is a series output stage a and a high-frequency isolation stage X, the upper side is a series output stage b and a high-frequency isolation stage Y, and the right side is a series output stage c.
All the AC-DC converters are composed of IGBT with the blocking voltage of 3.3kV, and the direct-current side voltage of the converters is 2000V. The operating frequency of the high-frequency transformer is 2000 Hz.
The basic unit of the medium voltage three-port flexible multi-state switch topology of fig. 2. The system has a total of 15 such basic units.
The first, second and third series output stages have the same structure, and each phase comprises 5 AC-DC converters and 15 AC-DC converters in total, taking A as an example. A capacitor is connected in parallel between the direct current ends of the AC-DC converters; 5 AC-DC converters in each phase are connected in series at an alternating current port and are connected with one of the medium-voltage distribution lines A through a reactor; the phases are connected in a star shape.
The series output stage B is connected with a medium-voltage distribution line B; and the series output stage C is connected with the medium-voltage distribution line C.
The high frequency isolation stage X, Y has the same structure, taking X as an example, and has 15 two-port high frequency transformers and 30 AC-DC converters. Two coils of each two-port high-frequency transformer are respectively connected with alternating current ports of two AC-DC converters.
Among 30 AC-DC converters of the high-frequency isolation level X, 15 AC-DC converters connected with the first series output level are correspondingly connected with the direct-current ports of the first series output level one by one, and 15 AC-DC converters connected with the second series output level C are correspondingly connected with the direct-current ports of the 15 series output level one by one.
And in the 30 AC-DC converters of the high-frequency isolation stage B, 15 AC-DC converters are connected with the 15 AC-DC converters of the series output stage B in a one-to-one correspondence mode at the direct-current ports, and 15 AC-DC converters are connected with the 15 AC-DC converters of the series output stage C in a one-to-one correspondence mode at the direct-current ports.
The topology provided by the invention can control the power flow among three alternating current medium voltage systems, and has more possible operation modes. In different operation modes, the control strategies of the system are different, and cannot be enumerated here. In this embodiment, a control strategy of the system is described by taking an operation mode as an example. The description is illustrative of the mode of operation of the present invention and its corresponding control strategy and is not limiting.
The operation mode is as follows: the first port and the second port provide active power to the third port, and the three ports do not provide reactive power.
Control strategy of the series output stage A:
the control strategy for the series output stage a is shown in fig. 5. The control targets of the series output stage A are two, one is to realize the current unit power factor, and the other is that all the capacitor voltages of the AC-DC converter are equal and equal to the reference value. The control of the series output stage A comprises three levels of average direct current voltage control, interphase direct current voltage balance control and direct current voltage balance control of each AC-DC converter in a phase. The average direct current voltage control is realized by DQ voltage current double closed loop control; the interphase direct-current voltage balance control is realized by adopting zero-sequence voltage injection for balancing the power of three interphase; and the direct-current voltage balance control of each AC-DC converter in the phase is realized by adjusting the amplitude of the modulation wave of each AC-DC converter by using a PI (proportional integral) controller.
The average direct current voltage control specifically comprises the following steps: the direct current voltage of all the AC-DC converters of the series output stage A is sampled, and the average direct current voltage reflects the active power required by the series output stage A. And comparing the average direct current voltage with a reference value, and obtaining the d-axis command current through a PI controller. Since the series output stage a does not provide reactive power in this example, the q-axis command current is set to 0.
The interphase direct-current voltage balance control specifically comprises the following steps: the A, B, C three-phase direct current voltage of the input stage A in series connection is sampled, the comparison result of each phase direct current voltage and the average direct current voltage reflects the zero sequence power value required by each phase, the respective average direct current voltage of A, B two phases is compared with the total average direct current voltage of the three phases according to the condition that the sum of the zero sequence power values of the three phases is 0, and the magnitude P of the zero sequence power value required by A, B two phases is obtained through a PI (proportional integral) controller0A、P0BThen, a zero sequence voltage command value is calculated through the following formula:
Figure BDA0001337080950000051
wherein, U0Is the amplitude of the zero sequence voltage, theta is the phase difference of the zero sequence voltage relative to the current of the distribution line A, ISThe current amplitude of the distribution line A is shown.
The DC voltage balance control of each AC-DC converter in the first phase of the series input stage specifically comprises the following steps: taking the phase a as an example, the direct current voltages of all the phase a AC-DC converters are sampled, and the average direct current voltage of all the phase a AC-DC converters is obtained. The difference between the direct current voltage of each AC-DC converter in the phase A and the average direct current voltage of the phase A reflects the active power regulating amount required by each AC-DC converter. And comparing the direct current voltage of each AC-DC converter of the A phase with the average direct current voltage of the A phase, obtaining the fine tuning coefficient of each AC-DC converter of the A phase through a PI regulator, and adjusting the amplitude of the modulation wave of each AC-DC converter through the fine tuning coefficient.
Control strategy of the high-frequency isolation stage X:
the control strategy for the high frequency isolation stage X is shown in fig. 6. Control target of the high frequency isolation stage X: and making the voltage of each direct current capacitor of the series output stage C equal to the reference voltage. In this case, the DC voltages of the individual AC-DC converters of the series output stage a are controlled by themselves, so that the high-frequency isolation stage X only needs to control the DC voltages of the individual AC-DC converters of the series output stage c.
Control strategy of the high-frequency isolation stage Y:
the control strategy for the high frequency isolation stage Y is shown in fig. 7. Control target of high frequency isolation stage Y: and making the voltage of each direct current capacitor of the series output stage B equal to the reference voltage. In this example, the DC voltages of the AC-DC converters of the series output stage c are controlled by the high frequency isolation stage X, so the high frequency isolation stage Y only needs to control the DC voltages of the AC-DC converters of the series output stage b.
Each two-port high frequency transformer and its two AC-DC converters in the high frequency isolation stage X, Y are considered as one high frequency isolation stage unit, as shown in fig. 2.
The AC-DC converter of the high-frequency isolation stage unit adopts high-frequency square wave modulation, and the voltage waveform of the high-frequency isolation stage unit is shown as the following figure: the two AC-DC converters output two-level square waves at the alternating current end, and the positive duty ratio and the negative duty ratio are both 50%. The square wave output by the primary side AC-DC converter is taken as a phase reference, and a certain phase difference exists between the square wave output by the secondary side AC-DC converter and the square wave output by the primary side AC-DC converter. The phase difference determines the power transferred between the primary side and the secondary side, and the power transferred between the primary side and the secondary side can be adjusted by adjusting the phase difference.
Each high-frequency isolation stage unit is independently controlled. In the control of each high-frequency isolation level unit of the high-frequency isolation level X, the amplitude of the C-side direct-current voltage reflects active power required by the C-side, the C-side direct-current voltage is compared with reference voltage, and the phase difference between the square wave output by the C-side AC-DC converter and the square wave output by the A-side AC-DC converter is obtained through the PI controller
Figure BDA0001337080950000061
The control of the third-side direct-current voltage is realized through the phase difference. Similarly, in the control of each high-frequency isolation stage unit of the high-frequency isolation stage Y, the amplitude of the direct-current voltage on the B side reflects the active power required by the B side, the direct-current voltage on the B side is compared with the reference voltage, and the phase difference between the square waves output by the AC-DC converter on the B side and the square waves output by the AC-DC converter on the C side is obtained through the PI controller
Figure BDA0001337080950000062
And the control of the DC voltage on the B side is realized through the phase difference.
And (3) controlling strategies of series output stages B and C:
the control strategy for the series output stages b and c is shown in fig. 8. The number of control targets of the series output stages B and C is 1, namely, the specified alternating current is output. In this example, the DC voltages of the AC-DC converters of the series output stages b and c are controlled by the high frequency isolation stage X, Y, so the series output stages b and c need only control the output AC current and do not need to control the DC voltages.
The d-axis current instruction is given to the power grid; the q-axis current command is set to 0. The amplitude of the modulation wave of each AC-DC converter in each phase is equal.
Simulation verification:
with reference to the system parameters in the examples, a simulation of a three-port flexible multi-state switch was constructed.
Before 0.2 second, the output power of the third port is 5.5MW, and the line current amplitude is 450A; let the input power of port B be 0, equivalently turn off; the A port completely bears the output power of the C port, the input power is automatically adjusted to 5.5MW, and the line current amplitude is 450A.
0.2-0.3 second, the output power of the third port is unchanged; the input power command of port B is changed into 2.45MW, and the line current amplitude is 200A; the input power of the A port is automatically adjusted to 3.05MW, and the amplitude of the line current is 250A.
The waveforms are shown in fig. 9-14. The initial start-up waveform is omitted from each figure, and only 0.1-0.3 seconds of waveform are shown.
Fig. 9 shows the output current waveform at the third terminal. The output current amplitude is always 450A.
Fig. 10 shows the input current waveform at terminal b. Before 0.2 second, the input current is 0, which is equivalent to disconnection; after 0.2 seconds, the input current amplitude was 200A.
Fig. 11 shows the input current waveform at the nail end. Before 0.2 second, the amplitude of the input current is 450A, and the first end completely bears the output power of the third end; after 0.2 second, the current amplitude of the A end is adjusted to be reduced to 250A along with the input of the B end.
Fig. 12-14 show the dc capacitor voltages at the series output terminals c, b, and a, respectively. It can be seen that the capacitor voltage is always stabilized at 2000V, except during the conditioning process.

Claims (2)

1. A medium-voltage three-port flexible multi-state switch topology based on a power electronic transformer is characterized by comprising three series output stages A, B and C and two high-frequency isolation stages X, Y, wherein the three series output stages are connected with the two high-frequency isolation stages, each series output stage is three-phase, each phase comprises N identical series AC-DC converters, N is a natural number, and a capacitor is connected in parallel between the direct-current ends of each AC-DC converter; the total alternating current side of each phase of the series AC-DC converters is connected with a reactor and then connected with one phase of an input distribution line, the phases are connected in a triangular or star shape, each AC-DC converter of the high-frequency isolation level is connected with one AC-DC converter of one series input stage at the direct current end, wherein: the N AC-DC converters of the A phase of the series output stage A are connected with the N AC-DC converters of the A phase of the series output stage C through the N high-frequency isolation stage units of the high-frequency isolation stage A; the N AC-DC converters of the B phase of the series output stage A are connected with the N AC-DC converters of the B phase of the series output stage C through the N high-frequency isolation level units of the high-frequency isolation level X; the N AC-DC converters of the C phase of the series output stage A are connected with the N AC-DC converters of the C phase of the series output stage C through the N high-frequency isolation level units of the high-frequency isolation level X; the N AC-DC converters of the A phase of the series output stage B are connected with the N AC-DC converters of the A phase of the series output stage C through the N high-frequency isolation stage units of the high-frequency isolation stage Y; the N AC-DC converters of the B phase of the series output stage B are connected with the N AC-DC converters of the B phase of the series output stage C through the N high-frequency isolation stage units of the high-frequency isolation stage Y; the N AC-DC converters of the C phase of the series output stage B are connected with the N AC-DC converters of the C phase of the series output stage C through the N high-frequency isolation stage units of the high-frequency isolation stage Y; the two high-frequency isolation level units connected with each AC-DC converter in the series output stage C and the two AC-DC converters from the series output stages A and B connected with the two high-frequency isolation level units form a flexible multi-state switch unit, and the whole system comprises 3N flexible multi-state switch units.
2. A medium voltage three-port flexible multi-state switching topology based on power electronic transformers, according to claim 1, characterized in that each high frequency isolation stage comprises 3N two-port high frequency transformers and 6N AC-DC converters, each 2 AC-DC converters are connected by one high frequency transformer to form one high frequency isolation stage unit, and each high frequency isolation stage has 3N high frequency isolation stage units.
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