CN106803679B - Flexible ring network controller for ring opening operation of electromagnetic ring network and control method - Google Patents
Flexible ring network controller for ring opening operation of electromagnetic ring network and control method Download PDFInfo
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Abstract
The invention relates to a flexible looped network controller and a control method for the operation of loop opening of an electromagnetic looped network. The control method comprises the following steps: the control mode when the single-phase earth fault of the alternating-current system without the transformer exists and the control mode when the single-phase earth fault of the alternating-current system with the transformer exists. The technical scheme provided by the invention adopts a back-to-back flexible direct current transmission technology, namely a flexible ring network controller, realizes the functions of urban electromagnetic ring network splitting, power flow control, reactive power regulation, short-circuit current suppression and the like, increases the flexibility and reliability of the operation of the power grid without increasing the short-circuit current level of the power grid and reducing the stability of the power grid, and realizes the soft partition operation of the alternating current power grid.
Description
Technical Field
The invention relates to a controller and a control method in the technical field of flexible direct current transmission (VSC-HVDC), in particular to a flexible ring network controller and a control method for ring-opening operation of an electromagnetic ring network.
Background
With the development of electric power systems, the power transmitted by urban power grids is rapidly increased, and the urban power grids must build power transmission lines with higher voltage levels. In the process, the power grid in China has two typical characteristics: firstly, the power grid development speed is high, and the problem of standard exceeding of short-circuit current is prominent; secondly, a large number of electromagnetic ring networks exist, and the power transmission capacity is limited.
The electromagnetic ring network is a parallel loop formed by connecting lines with different voltage grades through electromagnetic loops of transformers at two ends of the lines. A schematic diagram of which is shown in fig. 1. The operating voltage U1 of the line L1 is greater than the operating voltage U2 of the line L2, T1 and T2 are transformers at two ends of the line, and the lines L1 and L2 are connected through magnetic circuits of the transformers T1 and T2 to form an electromagnetic ring network.
At the initial stage of the development of a high-voltage primary-voltage power grid, the high-voltage and low-voltage electromagnetic ring network has small current, and the existence of the high-voltage and low-voltage electromagnetic ring network can improve the reliability and the flexibility of power supply of the power grid. Generally, the electromagnetic ring network can be maintained to operate as long as the safe and stable operation of the power grid is not influenced and the short-circuit capacity is not limited. However, with the development of a high-voltage power grid, the transmission load is continuously increased, and the electromagnetic ring network becomes a serious accident potential of a power system, so that a plurality of problems are brought to the operation of the power grid.
In order to eliminate the conventional method for operating the ring network, the ring network is disconnected, and the partitioned operation of the power grid is realized. Due to the complexity and diversity of power grid development, the power grid operation mode is greatly changed after the existing electromagnetic ring network is looped off, the power grid operation has new characteristics, and various factors need to be comprehensively considered before the loop is looped off, such as: when to unlink the ring, where to unlink the ring, how to partition, etc. Aiming at the problems, calculation analysis in many aspects such as trend, stability, short circuit and the like must be carried out, and meanwhile, the problems of influence on operation and the like are also considered, so that a decision is made on comprehensive advantages and disadvantages. Although the conventional ring-opening method can solve the problems of overproof short-circuit current, reduced stability and the like caused by the operation of a ring network, the reliability and the flexibility of the operation of a power grid can be reduced.
The flexible direct-current transmission technology is mainly applied to the fields of wind power integration, urban power supply, power grid interconnection, offshore platform power supply and the like, and has no application in the aspect of alternating-current power grid ring opening. The flexible direct current transmission technology can flexibly control active and reactive power transmission, and has the advantages of not influencing the short-circuit current level of an alternating current system, isolating faults of the alternating current system and the like. When the system breaks down, the flexible direct current can quickly isolate the power grid, so that the expansion of accident influence is avoided, the system damping can be improved, and low-frequency oscillation possibly brought by weak alternating current interconnection is inhibited.
Disclosure of Invention
In order to solve the defects in the prior art, the invention aims to provide a flexible ring network controller for the ring-opening operation of an electromagnetic ring network and a control method.
The purpose of the invention is realized by adopting the following technical scheme:
the invention provides a flexible looped network controller for the operation of ring opening of an electromagnetic looped network, which is improved in that the flexible looped network controller is formed by connecting two symmetrical unipolar modular multilevel converters back to back, wherein one side of the modular multilevel converter is connected into an alternating current system through a connecting transformer, and the other side of the modular multilevel converter is directly connected into the alternating current system.
Further, for the transformerless side converter, the transformerless side converter is composed of six three-phase bridge arms, each bridge arm is composed of submodules in series, and if the number of the submodules of the bridge arm of the converter is N, N/2 submodules are in a full-bridge structure to form a full-bridge submodule valve section; in addition, N/2 sub-modules are in a half-bridge structure to form a half-bridge sub-module valve section; and the half-bridge submodule valve section and the full-bridge submodule valve section form a hybrid submodule bridge arm.
Furthermore, the side converter with the transformer consists of six three-phase bridge arms, each bridge arm is formed by connecting half-bridge submodules in series, and the number of the half-bridge submodules of each bridge arm is N.
Further, the flexible ring network controller utilizes an alternating current power system ground connected with the flexible ring network controller as a reference ground potential for the operation of the converter.
The invention provides a control method of a flexible ring network controller for the ring-opening operation of an electromagnetic ring network, which is improved in that the control method comprises the following steps: the control mode when the single-phase earth fault of the alternating-current system without the transformer exists and the control mode when the single-phase earth fault of the alternating-current system with the transformer exists.
Further, under the normal operation condition of the flexible ring network controller, let one phase in A, B, C phases be i-phase, and the output voltages of the upper and lower bridge arms thereof respectively be:
in the formula ucFor the AC side of the flexible ring network controller to output a voltage, UdcFor the DC side voltage, u, of the flexible ring network controlleri1And ui2Respectively outputting voltage for an upper bridge arm and a lower bridge arm;
the control mode when no transformer side AC system single-phase earth fault includes: when the single-phase earth fault of the transformer-free side AC system occurs, the voltage of the non-fault phase is increased to 1.4 times of the rated voltage and is 1.4UcmAnd if the bridge arm submodules are in a half-bridge structure, the bridge arm output voltage ui1And ui2Will exceed the upper limit of the dc voltage, the valley will take on a negative value, and the distortion of the output ac voltage will cause current distortion, which temporarily reduces the dc voltage to Udc'/2, such that ui1＝Udc’/2-UcmAnd ui2＝Udc’/2+UcmThe output voltage is reduced to be within the range of the output capacity of the bridge arm, and the distortionless output of the alternating voltage is realized; u shapecmThe peak value of the output voltage at the AC side of the flexible ring network controller is referred to; u shapedc' denotes the direct voltage ratio UdcSmall, it is the maximum dc voltage when realizing the ac voltage undistorted output.
Furthermore, the control mode when there is single-phase earth fault of transformer side alternating current system includes: when the alternating current system on the transformer side has a single-phase ground fault, the connecting transformer has the function of isolating the zero sequence, the alternating current is controllable, and the continuous operation of the converter is realized through a fault ride-through control strategy of the flexible ring network controller, wherein the fault ride-through control strategy refers to the function of the corresponding control strategy of the control system of the flexible ring network controller when the alternating current system connected with the ring network controller has a fault, so that the flexible ring network controller can ride through the fault to continuously operate.
Compared with the closest prior art, the technical scheme provided by the invention has the following excellent effects:
1) the mode that one side is connected with the alternating current system through the connecting transformer and the other side is directly connected with the alternating current system is adopted, the structure is compact, and the economical efficiency and the technical performance of the flexible ring network controller are considered;
2) the system ground is used as the reference ground potential of the ring network controller, so that a special grounding device is omitted, and the problem that the special grounding device is difficult to realize is solved.
3) When a direct current fault occurs, the ring network controller topology provided by the invention has the fault clearing capacity, an alternating current circuit breaker is not required to participate, and the fault recovery is fast.
4) The invention adopts back-to-back flexible direct current transmission technology, namely a flexible ring network controller, realizes the functions of urban electromagnetic ring network disconnection, power flow control, reactive power regulation, short-circuit current suppression and the like, increases the flexibility and reliability of the operation of the power grid while not increasing the short-circuit current level of the power grid and not reducing the stability of the power grid, and realizes the soft partition operation of the alternating current power grid.
Drawings
FIG. 1 is a schematic diagram of an electromagnetic looped network;
FIG. 2 is a flexible ring network controller topology (single-phase schematic diagram) provided by the present invention;
FIG. 3 is a schematic diagram of a hybrid sub-module bridge arm connection provided by the present invention;
FIG. 4 is a graph of waveforms of output voltages of upper and lower bridge arms and AC output voltages when the converter provided by the present invention operates normally;
FIG. 5 is a waveform diagram of the response of a half-bridge submodule bridge arm converter provided by the invention in the case of single-phase earth fault;
fig. 6 is a waveform diagram of the response of the hybrid sub-module bridge arm converter provided by the invention when the single-phase ground fault occurs.
Detailed Description
The following describes embodiments of the present invention in further detail with reference to the accompanying drawings.
The following description and the drawings sufficiently illustrate specific embodiments of the invention to enable those skilled in the art to practice them. Other embodiments may incorporate structural, logical, electrical, process, and other changes. The examples merely typify possible variations. Individual components and functions are optional unless explicitly required, and the sequence of operations may vary. Portions and features of some embodiments may be included in or substituted for those of others. The scope of embodiments of the invention encompasses the full ambit of the claims, as well as all available equivalents of the claims. Embodiments of the invention may be referred to herein, individually or collectively, by the term "invention" merely for convenience and without intending to voluntarily limit the scope of this application to any single invention or inventive concept if more than one is in fact disclosed.
The topology of the flexible ring network controller provided by the invention is shown in fig. 2. The basic structure is that two symmetrical monopole modularized multi-level converters are connected back to back, wherein one side converter is connected into an alternating current system through a connecting transformer, and the other side converter is directly connected into the alternating current system. For the transformerless side converter, each bridge arm is formed by connecting a plurality of submodules in series, and if the number of the submodules of the bridge arm of the converter is N, N/2 submodules are in a full-bridge structure to form a full-bridge submodule valve section; in addition, N/2 sub-modules are in a half-bridge structure to form a half-bridge sub-module valve section. And the half-bridge submodule valve section and the full-bridge submodule valve section form a hybrid submodule bridge arm. The wiring diagram of the hybrid sub-module bridge arm is shown in fig. 3. For the side converter with the transformer, each bridge arm is formed by connecting half-bridge submodules in series, and the number of the half-bridge submodules of each bridge arm is assumed to be N.
Under normal operation, taking one of A, B, C phases as an example, for example, i phase, the output voltages of the upper and lower arms are:
in the formula ucFor the AC side of the flexible ring network controller to output a voltage, UdcIs a DC side voltage ui1And ui2Respectively, the upper and lower bridge arms output voltages. The waveforms of the voltages in the case of normal operation are shown in fig. 4.
When the single-phase earth fault of the transformer-free side AC system occurs, the voltage of the non-fault phase is increased to 1.4 times of the rated voltage and is 1.4UcmAnd if the bridge arm submodules are in a half-bridge structure, the bridge arm output voltage ui1And ui2Will exceed the dc voltage upper limit and the valley will take on a negative value and its output ac voltage will be distorted and will cause current distortion as shown in fig. 5. In the bridge arm structure of the hybrid submodule provided by the invention, the bridge arm can output negative voltage, so that the requirement of the bridge arm output voltage under the single-phase ground fault can be met, as shown in fig. 6. Meanwhile, if the peak value exceeds the upper limit and special treatment is not carried out, more sub-modules are needed to finish voltage output, and the method provided by the invention comprises the following steps: temporarily reducing the DC voltage to U under fault conditionsdc'/2, such that ui1＝Udc’/2-UcmAnd ui2＝Udc’/2+UcmAnd the output voltage is reduced to be within the range of the output capability of the bridge arm, so that the distortionless output of the alternating-current voltage is realized. The control mode not only can effectively inhibit zero sequence components, but also can realize a larger reactive power output range during fault.
When the single-phase earth fault of the transformer side alternating current system exists, the connecting transformer has the function of isolating the zero sequence, the alternating current is controllable, and the continuous operation of the converter can be realized through the fault ride-through control strategy of the controller. The fault ride-through control strategy means that when an alternating current system connected with the ring network controller fails, the flexible ring network controller can ride through the fault to continuously operate through the action of the corresponding control strategy of the control system of the flexible ring network controller.
When the direct current single pole is in ground fault, the transformer-free side bridge arm mixed structure can output negative voltage, so that the transformer-free side bridge arm mixed structure can block the direct current fault immediately due to the flexibility of the transformer-free side bridge arm mixed structure, and the fault can be isolated quickly. The connecting transformer at the transformer side can isolate the direct current bias voltage and the bias current, and the normal operation of the alternating current system is ensured.
The flexible ring network controller topology provided by the invention can utilize the system ground as the reference ground potential for the operation of the converter, thereby saving an additional grounding device and the occupied area of the flexible ring network controller.
Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art can make modifications and equivalents to the embodiments of the present invention without departing from the spirit and scope of the present invention, which is set forth in the claims of the present application.
Claims (4)
1. A control method of a flexible looped network controller for the loop opening operation of an electromagnetic looped network is characterized in that the flexible looped network controller is formed by connecting two symmetrical unipolar modular multilevel converters back to back, wherein the modular multilevel converter on one side is connected into an alternating current system through a connecting transformer, and the converter on the other side is directly connected into the alternating current system;
for the transformerless side converter, the transformerless side converter is composed of six three-phase bridge arms, each bridge arm is composed of submodules in series, and if the number of the submodules of the bridge arm of the converter is N, N/2 submodules are in a full-bridge structure to form a full-bridge submodule valve section; in addition, N/2 sub-modules are in a half-bridge structure to form a half-bridge sub-module valve section; the half-bridge sub-module valve section and the full-bridge sub-module valve section form a hybrid sub-module bridge arm;
the control method of the flexible ring network controller for the ring opening operation of the electromagnetic ring network comprises the following steps: the control mode when the single-phase earth fault of the alternating-current system at the transformer-free side occurs and the control mode when the single-phase earth fault of the alternating-current system at the transformer side occurs;
under the normal operation condition of the flexible ring network controller, let one phase in A, B, C phases be i-phase, and the output voltages of its upper and lower bridge arms respectively be:
in the formula ucFor the AC side of the flexible ring network controller to output a voltage, UdcFor the DC side voltage, u, of the flexible ring network controlleri1And ui2Respectively outputting voltage for an upper bridge arm and a lower bridge arm;
the control mode when no transformer side AC system single-phase earth fault includes: when the single-phase earth fault of the transformer-free side AC system occurs, the voltage of the non-fault phase is increased to 1.4 times of the rated voltage and is 1.4UcmAnd if the bridge arm submodules are in a half-bridge structure, the bridge arm output voltage ui1And ui2Will exceed the upper limit of the dc voltage, the valley will take on a negative value, and the distortion of the output ac voltage will cause current distortion, which temporarily reduces the dc voltage to Udc'/2, such that ui1＝Udc’/2-UcmAnd ui2＝Udc’/2+UcmThe output voltage is reduced to be within the range of the output capacity of the bridge arm, and the distortionless output of the alternating voltage is realized; u shapecmThe peak value of the output voltage at the AC side of the flexible ring network controller is referred to; u shapedc' denotes the direct voltage ratio UdcSmall, it is the maximum dc voltage when realizing the ac voltage undistorted output.
2. The method for controlling a flexible network controller according to claim 1, wherein the converter with transformer side is composed of six three-phase arms, each arm is composed of half-bridge sub-modules connected in series, and the number of the half-bridge sub-modules of each arm is N.
3. The method as claimed in claim 1, wherein the flexible ring network controller utilizes an ac power system ground to which the flexible ring network controller is connected as a reference ground for converter operation.
4. The method for controlling the flexible looped network controller according to claim 1, wherein the control manner in the case of the single-phase ground fault of the ac system with the transformer side includes: when the alternating current system on the transformer side has a single-phase ground fault, the connecting transformer has the function of isolating the zero sequence, the alternating current is controllable, and the continuous operation of the converter is realized through a fault ride-through control strategy of the flexible ring network controller, wherein the fault ride-through control strategy refers to the function of the corresponding control strategy of the control system of the flexible ring network controller when the alternating current system connected with the ring network controller has a fault, so that the flexible ring network controller can ride through the fault to continuously operate.
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| CN107017635B (en) * | 2017-06-09 | 2019-08-13 | 广东电网有限责任公司电力调度控制中心 | A kind of adaptive islanding control method and system |
| CN112398135B (en) * | 2020-11-30 | 2023-12-05 | 国网冀北电力有限公司电力科学研究院 | Control method and device for multi-port power electronic transformer of anti-electromagnetic ring network |
| CN113922365B (en) * | 2021-09-30 | 2024-01-30 | 西安理工大学 | Power grid self-healing circuit and method for multiple connecting lines |
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| CN104269847A (en) * | 2014-10-11 | 2015-01-07 | 国家电网公司 | Flexible looped network control system operation and power flow optimization method |
| CN104269861A (en) * | 2014-10-13 | 2015-01-07 | 国家电网公司 | Electromagnetic looped network reactive power ring current optimal control method based on flexible looped network controller |
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| CN104269847A (en) * | 2014-10-11 | 2015-01-07 | 国家电网公司 | Flexible looped network control system operation and power flow optimization method |
| CN104269861A (en) * | 2014-10-13 | 2015-01-07 | 国家电网公司 | Electromagnetic looped network reactive power ring current optimal control method based on flexible looped network controller |
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