Control Method and System of a Flexible Bus-tie Controller

Abstract

The present invention provides a control method and system for a flexible bus coupler controller. The control method includes: after receiving a power command, a DC voltage feedback, a three-phase AC voltage feedback, and a three-phase AC current, an outer loop controller outputs a current command; after receiving the current command, an inner loop controller outputs a modulation voltage command; the modulation voltage command is rectified by a PWM rectification module and then input into the flexible bus coupler controller to achieve modulation of the flexible bus coupler controller. The technical solution provided by the present invention can achieve compensation for reactive current only by using the reactive current feedback value obtained through coordinate transformation as the current command reference value for compensating reactive power.

Description

Technical Field

[0001] The present invention relates to the field of flexible power transmission, and particularly to a control method and system for a flexible bus coupler controller. Background Art

[0002] With the improvement of the penetration rate of distributed generation connected to the grid, it is necessary to make full use of the power distribution capacity of the existing grid topology. On the other hand, the intermittency and volatility of distributed generation output will cause rapid changes in the power flow of the distribution network. Therefore, new technical means need to be adopted. Installing tie switches and sectionalizing switches can achieve active and flexible power flow control of the distribution network.

[0003] However, traditional tie switches can only connect distribution networks with the same voltage level, and the network frequencies and phases at both ends of the distribution network need to be consistent. In addition, traditional tie switches also have problems such as slow switch action time and low automation level. Power electronic devices have a fast response speed and flexible control. A new type of flexible distribution network switch formed by them can replace traditional tie switches. However, in the power grid, due to the relatively high voltage level, traditional converters have certain limitations. The sub-modules of modular multilevel converters can operate at low voltage, and their switching devices have the advantages of small stress, low harmonics, easy expansion, and high reliability, and are widely used.

[0004] Traditional power quality compensation devices need to separately extract relevant information such as system reactive power, harmonic currents of each order, and negative sequence current, so as to calculate compensation commands for reactive power, harmonics, and negative sequence. Therefore, the extraction process is computationally complex and requires very high requirements for the processor.

[0005] Therefore, a bus coupler flexible control system is needed to simplify the extraction calculation process. Summary of the Invention

[0006] In order to overcome the above-mentioned deficiencies of the prior art, the present invention provides a bus coupler flexible control method and system for a distribution network. The control method includes the steps of:

[0007] After receiving a power command, DC voltage feedback, three-phase AC voltage feedback, and three-phase AC current, an outer loop controller outputs a current command. After receiving the current command, an inner loop controller outputs a modulation voltage command. The modulation voltage command is rectified by a PWM rectification module and then input into a flexible bus coupler controller to achieve modulation of the flexible bus coupler controller.

[0008] After receiving the power command, DC voltage feedback, three-phase AC voltage feedback, and three-phase AC current, the outer-loop controller outputs a current command, including: a power control command calculation unit in the outer-loop controller, which receives the power command and outputs a power current command; a DC voltage regulation unit in the outer-loop controller, which receives the DC voltage feedback and outputs a DC current command; an AC voltage regulation unit in the outer-loop controller, which receives the three-phase AC voltage feedback after coordinate transformation and outputs an AC current command; a power quality control unit in the outer-loop controller, which receives the three-phase AC current after coordinate transformation. After the compensation command is extracted and calculated from the three-phase AC current, the power quality control unit outputs a compensation current command; a control function selector in the outer-loop controller selects one of the power current command, DC current command, and the AC current command as the output quantity according to the control objective of the distribution network. If the output quantity is not the AC current command, after the output quantity is superimposed with the compensation current command, the current command is output. After receiving the current command, the inner-loop controller outputs a modulation voltage command, including: after the current command is input into the current feedforward decoupling control unit in the inner-loop controller, the modulation voltage command is output.

[0009] The compensation current command includes: the active current compensation command i dc * and the reactive current compensation command i qc * . When there is negative sequence current and harmonics in the distribution network, the active current i d superimposes the double-frequency pulsation and high-order pulsation. The active current pulsation value extracted by taking the inverse is used to obtain the active current compensation command i dc * ; the reactive current is inverted to be used as the reactive current compensation command i qc * .

[0010] The active current pulsation value includes: the DC component is separated from the active current i d by using a second-order generalized integrator and a DC integrator to obtain the active current pulsation value.

[0011] Its bus-tie flexible control system includes: an outer-loop controller module, which is used to output a current command after receiving the power command, DC voltage feedback, three-phase AC voltage feedback, and three-phase AC current; an inner-loop controller module, which is used to output a modulation voltage command after receiving the current command; a modulation module, which rectifies the modulation voltage command through a PWM rectifier module and inputs it into the flexible bus-tie controller to realize the modulation of the flexible bus-tie controller.

[0012] The outer - loop controller specifically includes: a power - control - instruction calculation unit in the outer - loop controller, which receives a power instruction and outputs a power - current instruction; a DC - voltage regulation unit in the outer - loop controller, which receives a DC - voltage feedback and outputs a DC - current instruction; an AC - voltage regulation unit in the outer - loop controller, which receives the three - phase AC - voltage feedback after coordinate transformation and outputs an AC - current instruction; a power - quality control unit in the outer - loop controller, which receives the three - phase AC - current after coordinate transformation. After the compensation instruction is extracted and calculated from the three - phase AC - current, the power - quality control unit outputs a compensation - current instruction; a control - function selector in the outer - loop controller, which selects one of the power - current instruction, DC - current instruction, and AC - current instruction as the output quantity according to the control objective of the distribution network. If the output quantity is not the AC - current instruction, after the output quantity is superimposed with the compensation - current instruction, the output - current instruction is output.

[0013] The inner - loop controller specifically includes: after the current instruction is input into the current feed - forward decoupling control unit in the inner - loop controller, a modulation - voltage instruction is output.

[0014] The above - mentioned flexible bus - tie controller includes: two back - to - back modular multilevel converters; the modular multilevel converter is a three - phase six - leg structure, and the upper and lower legs of each phase leg both include an inductor and n sub - modules connected in series with the inductor, where n is an integer greater than 1; the three - phase upper and lower legs of the two modular multilevel converters are interconnected through switches; the control - signal input terminal of the flexible bus - tie controller is arranged at the connection point of the upper and lower legs of each phase.

[0015] The sub - module includes: an insulated - gate bipolar - transistor (IGBT) branch and a capacitor connected in parallel with the IGBT branch; the IGBT branch includes series - connected IGBTs, and the IGBTs are respectively connected in parallel with diodes.

[0016] Compared with the closest prior art, the technical solution provided by the present invention has the following beneficial effects:

[0017] 1. When performing power - quality compensation, the present invention adopts a compensation - current extraction and calculation method, extracts the reactive power, harmonics, and negative - sequence to be compensated as a whole, and performs compensation. Compared with the traditional method of separately extracting the compensation amounts for each sequence, the extraction and calculation speed is accelerated, the burden on the processor is reduced, and it has a higher compensation accuracy.

[0018] 2. The present invention does not need to calculate the reactive power. Only by using the reactive - current feedback value obtained by coordinate transformation as the current - instruction reference value for compensating the reactive power, the compensation of the reactive current can be realized.

[0019] 3. The power grid current obtained by the present invention is subjected to coordinate transformation, and a simplified harmonic negative sequence extractor is used to calculate the current command for suppressing harmonics, thereby achieving the suppression of grid-side harmonic current. A PIR regulator is used to adjust it to achieve the suppression of grid-side negative sequence current. Description of the Drawings

[0020] Figure 1 is the circuit topology diagram of the bus-coupling flexible controller of the present invention;

[0021] Figure 2 is the structural schematic diagram of the sub-module in the bus-coupling flexible controller of the present invention;

[0022] Figure 3 is the schematic diagram of the outer-loop regulation controller of the present invention;

[0023] Figure 4 is the method block diagram of instruction extraction and calculation in the power quality control of the present invention;

[0024] Figure 5 is the schematic diagram of the double closed-loop control strategy of the single-side MMC in the bus-coupling flexible controller of the present invention. Detailed Embodiment

[0025] The technical solution of the present invention will be further described in detail below with reference to the drawings in the specification.

[0026] As Figure 1 shown, the bus-coupling flexible controller consists of two back-to-back MMCs (modular multilevel converter), and SM is the sub-module.

[0027] Each phase of the MMC is divided into upper and lower arms, and each arm is composed of an arm inductor and n sub-modules connected in series in sequence. The connection points of the two arm inductors correspond to the output terminals of the phase arms.

[0028] The present invention calculates the AC current by collecting the currents of the upper and lower arms of each phase of the two ends of the MMC, and the AC voltage is sampled by the AC voltage sensor in the bus-coupling flexible controller. The DC voltage and DC current are also sampled by the voltage and current sensors on their respective DC sides in the bus-coupling flexible controller. At the same time, in order to avoid large deviations in the sub-module voltage, it is also necessary to sample the voltages at both ends of each sub-module capacitor.

[0029] As Figure 2 shown in the SM sub-module, La is the arm inductor, T1 and T2 are IGBTs, D1 and D2 are diodes, and C0 is the DC-side capacitor of the sub-module. The equal voltage function of the sub-module is realized according to the sampled sub-module voltage.

[0030] The DC bus is generally first connected to a DC / DC converter that can realize step-up and step-down functions and whose power can flow bidirectionally to control the power supply of the secondary DC bus voltage to the DC load; the DC / DC converter is mainly used to stabilize the voltage of the secondary DC bus for the normal operation of various DC devices. Regarding the specific control of the DC / DC converter, the technology is quite mature and will not be elaborated here; the secondary DC bus generally supplies power to DC loads such as civilian devices (lighting devices such as lights, small power-consuming devices such as home energy storage, air conditioners, and refrigerators) and factory devices (large DC motors and large energy storage power stations, etc.). KM1 and KM2 are the DC circuit breakers of the left MMC and the right MMC respectively; the operation mode of the bus-coupling flexible control system is selected by opening and closing KM1 and KM2: double-end partition operation, double-end combined operation, and single-end power supply operation.

[0031] By connecting bus 1 where current i1 is located and bus 2 where current i2 is located through the bus-coupling flexible controller, double-end partition operation, double-end combined operation, and single-end power supply operation can be realized. Under the condition of double-end partition operation, the two substations supply power to the loads on their respective sides, and there is no power exchange between the two ends of the MMC. Both ends of the MMC operate in the constant DC voltage mode; under the condition of double-end combined operation, there is power exchange in the middle DC tie line at this time: one operates in the constant DC voltage mode and the other operates in the constant power mode; under the condition of single-end power supply operation, one end withdraws from operation due to maintenance or fault, and the load is supplied by the MMC at the other end. At this time, the MMC on that side operates in the constant DC voltage mode.

[0032] The controls on both sides of the bus-coupling flexible controller need to be coordinated with each other. Usually, one side of the converter maintains the stability of the DC voltage, and the other side maintains the control of the power flow or the stability of the AC output voltage. Both sides can complete the power quality control of the connected AC network at the same time.

[0033] As Figure 3 shown in the implementation method of the MMC outer-loop control, the control outer-loop consists of a power control unit, a DC voltage control unit, an AC voltage control unit, a power quality control unit, and a control function selector. After receiving the active power P * and the reactive power Q * through the power control unit, after calculating the power control command, the power current command i dq * is output; the DC voltage control unit receives the DC voltage U dc , and after being regulated by the DC voltage regulator, the DC voltage command i dqd * is output; the AC voltage control unit receives the AC voltage U abc , after U abc undergoes coordinate transformation to obtain u dq , and u dq passes through the AC voltage regulator to output the AC current command idqu * ; The power quality control unit receives the alternating current I abc After that, I abc Is transformed through coordinates to obtain i dq , i dq After extraction and calculation of the compensation instruction, the compensation current instruction i is obtained dqc * ; Through the control function selector, the operation mode of any one MMC can be flexibly switched, and whether to add the power quality compensation function during operation, i d’ * And i q’ * Are the active current instruction and the reactive current instruction output by the outer loop controller after passing through the function selector. The control function selector selects one of the outputs of the power control unit, the DC voltage control unit, and the AC voltage control unit as the output of the outer loop according to the needs of the control target. If the control target is the output quantity of the power control unit or the DC voltage control unit, the power quality control instruction can be superimposed on the output instruction of the outer loop according to the system requirements. Among them, the power control instruction calculation in the power control unit, the DC voltage regulator in the DC voltage unit, and the AC voltage regulator in the AC voltage unit are existing technologies.

[0034] Specifically, the DC voltage control unit compares the detected common DC bus voltage value with the given reference value, and through relevant PIR regulators and limiting links, obtains the reference value of the active current; the power control unit compares the detected active power and reactive power with the given active reference value and reactive reference value, and through the PIR regulator and limiting link, thus obtains the active power current instruction and the reactive power current instruction; the AC voltage control unit generates an AC current instruction according to the difference between the feedback AC voltage and the given reference voltage through the PIR regulator; the power quality controller uses the three-phase current signals of the substation AC bus to extract the negative sequence, reactive, and harmonic currents in the system, calculates the compensation current instruction, and superimposes it on the current instruction. According to the above-obtained reference value of the active or reactive current, the corresponding control target can be achieved through the current inner loop control.

[0035] Such as Figure 4 Shown, the extraction and calculation method of the power quality control instruction: i d And i q Are the feedback values of the active current and the reactive current in the rotating coordinate system after the grid current is transformed through coordinates. i dc * And i qc * Are the active compensation current instruction and the reactive current compensation instruction output by the power quality controller. System i dand i q The DC components of and i respectively represent the positive-sequence active current and the reactive current. When there are negative-sequence currents and harmonics in the system, there will be second-harmonic pulsations and higher-order pulsations superimposed on i d and i q . Therefore, by extracting the pulsation value of the active current, it represents the current total negative-sequence quantity and harmonic quantity in the grid active current. Taking the opposite of the extracted pulsation value gives the compensation command for the active current. Taking the opposite of the reactive current as the compensation command can completely eliminate the reactive power in the system. For the extraction of the active current fluctuation quantity, here a second-order generalized integrator and a DC integrator are used to first separate the DC component in i d , and then subtracting the DC component from i d gives the active current pulsation value in the active current feedback value i d . This method calculates quickly, has low requirements for the processor, and has high compensation accuracy.

[0036] Collect the AC current, and through transformation, obtain the line current in the rotating coordinate system; since the compensation target is to eliminate reactive power, harmonics, and negative-sequence currents, in the case of complete compensation, the line active current in the rotating coordinate system should be a DC quantity and the reactive current is 0.

[0037] Therefore, the DC component of the active current (the positive-sequence power of the load) can be extracted, subtracted from the load current, to obtain the sum of the negative-sequence current and harmonic current of the load. Taking the opposite can be used as the compensation quantity of the active current in the rotating coordinate system; taking the opposite of the load reactive current can be directly used as the compensation quantity of the reactive current in the rotating coordinate system. Superimposing the compensation quantity on the current (I dq * or I dqd * or I dqu * ) command forms a new active current command I d’ * and a reactive current command I q’ * .

[0038] Finally, as shown in Figure 5 , the active current command and the reactive current command are decoupled and controlled by the current feedforward of the grid voltage orientation in the inner-loop controller to obtain the three-phase modulation voltage, realizing the elimination of the reactive power, negative-sequence, and harmonics in the system.

[0039] Based on the same inventive concept, the present invention also provides a bus-coupling flexible control system, which will be described below.

[0040] The system provided by the present invention may include: an outer loop controller module, which is configured to receive a power command, a DC voltage feedback, a three-phase AC voltage feedback, and a three-phase AC current, and then output a current command; an inner loop controller module, which is configured to receive the current command and then output a modulation voltage command; and a modulation module, which is configured to rectify the modulation voltage command through a PWM rectification module and input it to a flexible bus coupler controller to implement modulation of the flexible bus coupler controller.

[0041] Those skilled in the art should understand that the embodiments of the present application can be provided as a method, a system, or a computer program product. Therefore, the present application can take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present application can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0042] The present application is described with reference to the flowcharts and / or block diagrams of methods, apparatuses (systems), and computer program products according to the embodiments of the present application. It should be understood that each flow and / or block in the flowchart and / or block diagram, and the combination of flows and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to the processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing devices to generate a machine, such that the instructions executed by the processor of the computer or other programmable data processing devices generate means for implementing the specified functions in Figure 1 one process or multiple processes and / or blocks Figure 1 one block or multiple blocks.

[0043] These computer program instructions can also be stored in a computer-readable memory that can direct a computer or other programmable data processing device to work in a specific manner, such that the instructions stored in the computer-readable memory generate a manufactured article including instruction means, and the instruction means implement the specified functions in Figure 1 one process or multiple processes and / or blocks Figure 1 one block or multiple blocks.

[0044] These computer program instructions can also be loaded onto a computer or other programmable data processing device, such that a series of operation steps are executed on the computer or other programmable device to generate a computer-implemented process, so that the instructions executed on the computer or other programmable device provide steps for implementing the specified functions in Figure 1 one process or multiple processes and / or blocks Figure 1 one block or multiple blocks.

[0045] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Those of ordinary skill in the art can still modify or equivalently replace the specific implementation manners of the present invention with reference to the above embodiments. Any modifications or equivalent replacements that do not depart from the spirit and scope of the present invention are within the scope of the claims of the present invention pending approval.

Claims

1. A control method for a flexible bus coupler controller, characterized in that, The flexible bus - tie control method includes the following steps: After receiving the power command, DC voltage feedback, three - phase AC voltage feedback, and three - phase AC current, the outer - loop controller outputs a current command; After receiving the current command, the inner - loop controller outputs a modulation voltage command; After being rectified by the PWM rectification module, the modulation voltage command is input into the flexible bus - tie controller to achieve modulation of the flexible bus - tie controller; After receiving the power command, DC voltage feedback, three - phase AC voltage feedback, and three - phase AC current, the outer - loop controller outputs a current command, including: The power control command calculation unit in the outer - loop controller receives the power command and outputs a power current command; The DC voltage regulation unit in the outer - loop controller receives the DC voltage feedback and outputs a DC current command; The AC voltage regulation unit in the outer - loop controller receives the three - phase AC voltage feedback after coordinate transformation and outputs an AC current command; The power quality control unit in the outer - loop controller receives the three - phase AC current after coordinate transformation. After the compensation command is extracted and calculated from the three - phase AC current, the power quality control unit outputs a compensation current command; the control function selector in the outer - loop controller selects one of the power current command, the DC current command, and the AC current command as the output quantity according to the control objective of the distribution network; If the output quantity is not the AC current command, then after the output quantity is superimposed with the compensation current command, the current command is output.

2. The control method according to claim 1, characterized in that After receiving the current command, the inner - loop controller outputs a modulation voltage command, including: The current command is input into the current feed - forward decoupling control unit in the inner - loop controller, and then the modulation voltage command is output.

3. The control method according to claim 1, wherein The compensation current command includes: active current compensation command i dc * and reactive current compensation command i qc * When there are negative sequence currents and harmonics in the distribution network, the active current i d superimposes double-frequency pulsation and high-order pulsation, and takes the inverse of the extracted active current pulsation value to obtain the active current compensation command i dc * ; Take the reactive current after inversion as the reactive current compensation command i qc * 。 4. The control method according to claim 3, characterized in that The active current pulsation value includes: Separating the DC component from the active current i using a second-order generalized integrator and a DC integrator gives the pulsation value of the active current. d ​ 5. A control system for a flexible bus coupler controller, characterized in that, The flexible bus - tie control system includes: An outer - loop controller module, which is used to output a current command after receiving the power command, DC voltage feedback, three - phase AC voltage feedback, and three - phase AC current; An inner - loop controller module, which is used to output a modulation voltage command after receiving the current command; A modulation module, which is used to input the modulation voltage command into the flexible bus - tie controller after being rectified by the PWM rectification module to achieve modulation of the flexible bus - tie controller; The outer - loop controller specifically includes: The power control command calculation unit in the outer - loop controller receives the power command and outputs a power current command; The DC voltage regulation unit in the outer - loop controller receives the DC voltage feedback and outputs a DC current command; The AC voltage regulation unit in the outer - loop controller receives the three - phase AC voltage feedback after coordinate transformation and outputs an AC current command; The power quality control unit in the outer - loop controller receives the three - phase AC current after coordinate transformation. After the compensation command is extracted and calculated from the three - phase AC current, the power quality control unit outputs a compensation current command; the control function selector in the outer - loop controller selects one of the power current command, the DC current command, and the AC current command as the output quantity according to the control objective of the distribution network; If the output quantity is not the AC current command, after the output quantity is superimposed with the compensation current command, the current command is output; Wherein, the flexible bus-tie controller includes: two back-to-back modular multilevel converters; The modular multilevel converter has a three-phase six-arm structure, and the upper arm and the lower arm of each phase arm both include an inductor and n sub-modules connected in series with the inductor, where n is an integer greater than 1; The three-phase upper arms and lower arms of the two modular multilevel converters are connected by switches respectively; The control signal input end of the flexible bus-tie controller is arranged at the connection point of the upper arm and the lower arm of each phase.

6. The control system according to claim 5, wherein The inner loop controller specifically includes: After the current command is input into the current feed-forward decoupling control unit in the inner loop controller, the modulation voltage command is output.

7. The control system according to claim 5, wherein The sub-module includes: An insulated gate bipolar transistor branch and a capacitor connected in parallel with the insulated gate bipolar transistor branch; The insulated gate bipolar transistor branch includes insulated gate bipolar transistors connected in series, and the insulated gate bipolar transistors are respectively connected in parallel with diodes.

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