Disclosure of Invention
The invention aims to provide a loop operation method based on a power electronic transformer power distribution network, which can effectively solve the problem of loop operation based on the power electronic transformer power distribution network and overcome the transient problem in the process of switching a radiation type power distribution network to a looped network operation.
The purpose of the invention can be realized by the following technical scheme:
a loop operation method based on a power electronic transformer power distribution network comprises a method for operating and synchronizing the power electronic transformer in different modes and a method for adjusting power flow during loop operation.
Furthermore, the loop operation method is composed of a phase detection module, a phase difference calculation module, a power point setting module, a power synchronization module, a voltage control module and a pulse width modulation module.
Further, the loop operation method comprises three operation modes: the system comprises a radiation type power grid independent operation mode, a power electronic transformer and other feeder line pre-synchronization mode and a power distribution network loop operation mode based on the power electronic transformer, wherein bus switches in the first two working modes are disconnected, and bus switches in the loop operation mode are closed.
Furthermore, the phase detection module measures the amplitude and the phase of the line voltage which needs to be synchronized with the feeder line of the power electronic transformer in real time, and the phase difference calculation module calculates the phase and amplitude difference between the voltage of the low-voltage side of the power electronic transformer and the voltage of other feeder lines.
Further, the power point setting module and the power synchronization module are matched with each other in different modes, in the independent operation mode of the radiation type power grid, the reference value of the output voltage of the power synchronization module is constant, and the output of the power point setting module is zero; under a power electronic transformer and other feeder line pre-synchronization mode and a power distribution network loop operation mode based on the power electronic transformer, the power point setting module provides corresponding power reference values for the power synchronization module, adjusts the alternating voltage reference values in real time, and achieves pre-synchronization and loop operation functions.
Further, the power point setting module outputs different power reference values according to different working modes based on a power electronic transformer distribution network, the output of the power point setting module is zero in a radiation type power grid independent operation mode, and no power is transmitted among feeders; under a pre-synchronization mode of the power electronic transformer and other feeders, the power point setting module outputs virtual power according to the phase difference and the amplitude difference of the two feeders, so that the power electronic transformer can smoothly enter a synchronization state in a loop to realize pre-synchronization, and no power is transmitted between the feeders in the mode; under the power distribution network loop operation mode based on the power electronic transformer, the output of the power point setting module is the difference between the output power of the power electronic transformer and a power reference value, the power reference value is determined by the actual condition of the power distribution network and the power flow regulation requirement, at the moment, a bus switch can be effectively closed, and the power transmission between feeders is controlled by the power electronic transformer.
Further, as a preferred embodiment of the present invention, the output of the power synchronization module can be used as a voltage reference value of the voltage control module, and the voltage control module can implement zero-static-error tracking of the voltage by using controllers such as proportional-integral control, proportional-resonant control, or repetitive control according to a selected reference coordinate system, such as a synchronization coordinate system (dq coordinate system) and a stationary coordinate system (α β coordinate system).
The invention has the beneficial effects that:
1. the loop operation method has simple control structure and easy parameter adjustment, and can flexibly and undisturbed switch between two power grid topologies of a radiation power grid and a loop operation;
2. the loop line operation method comprises a power electronic transformer and other feeder line presynchronization working mode, realizes the voltage synchronization of two feeder lines by controlling the voltage of the low-voltage side of the power electronic transformer, ensures that the voltage and current disturbance at the moment of closing a bus switch is small, and the switching process of a radiation type power grid to a looped network is smooth, thereby avoiding the instability of the system caused by transient impact;
3. according to the loop operation method, when the loop operation mode of the power distribution network of the power electronic transformer is based on, the transmission power of the power electronic transformer can be adjusted through the power point setting module and the power synchronization module, power interaction is realized through the bus switch, and the power flow of the power distribution network is balanced.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Referring to fig. 1, fig. 1 is a typical system configuration diagram of a loop operation method based on a power distribution network of a power electronic transformer, which includes a medium voltage ac bus and a medium voltage ac power grid (1), a power electronic transformer (2), a filter device (3), a conventional transformer (4), a low voltage ac bus and a low voltage ac power grid (5), various ac devices or loads (6), and a bus switch (7).
The power electronic transformer (2) is formed by cascading an AC/DC rectifier (2-1), an isolated DC/DC converter (2-2) and a DC/AC inverter (2-3), wherein the AC/DC rectifier (2-1) is connected with a medium-voltage alternating current bus (1), and the DC/AC inverter (2-3) is connected with a low-voltage alternating current bus (5).
The primary side and the secondary side of the traditional transformer (4) are respectively connected with a medium-voltage alternating-current bus (1) and a low-voltage alternating-current bus (5). The alternating current device and the load (6) are connected to a low-voltage alternating current network (5).
The direct current side of the power electronic transformer (2) is connected with a large capacitor to reduce direct current voltage ripples, and the low voltage side of the power electronic transformer (2) is connected with a filter device (3), and generally adopts LC type filter devices and the like to eliminate higher harmonics in the low voltage side voltage.
The bus switches (7) are usually two, one is connected between two alternating current buses, the other is connected to the tail ends of two feeder lines, and one bus switch (7) needs to be closed based on a power electronic transformer distribution network loop operation mode.
Fig. 2 is a control block diagram of a low-voltage side inverter of a power electronic transformer, and the control block diagram includes a phase detection module (8), a phase difference calculation module (9), a power point setting module (10), a power synchronization module (11), a voltage control module (12), and a Pulse Width Modulation (PWM) module (13).
The low-voltage side DC/AC inverter (2-3) of the power electronic transformer is driven by a switching signal generated by a pulse width modulation module (13) to generate a required voltage waveform, and the input voltage of the pulse width modulation module (13) is obtained by the loop operation method provided by the invention.
The phase detection module (8) measures the voltage amplitude and the phase of a feeder line connected with the traditional transformer (4) in real time, the phase difference calculation module (9) compares the measured value of the phase detection module (8) with the output value of the power synchronization module (11) to obtain the voltage amplitude and the phase difference of the two feeder lines connected with the power electronic transformer (2) and the traditional transformer (4), and the output signal of the phase difference calculation module (9) can be filtered by a low-pass filter to remove noise and disturbance.
The voltage control module (12) takes the output of the power synchronization module (11) as a reference value, can adopt a double-loop control scheme of a voltage outer loop and a voltage current inner loop, and can adopt controllers such as proportional-integral control, proportional resonance control or repetitive control to realize zero static error tracking of voltage according to a selected reference coordinate system.
According to different working modes, the power point setting module (10) outputs different power reference values:
under the independent operation mode of the radiation type power grid, the output value of the power point setting module (10) is zero;
under the presynchronization mode of the power electronic transformer and other feeders, the output value of the power point setting module (10) is obtained by calculating the voltage amplitude and the phase difference of the phase difference calculating module (9);
under the power distribution network loop operation mode based on the power electronic transformer, the output value of the power point setting module (10) is determined by the power transmission and power flow regulation requirements of a power grid.
The power reference value output by the power point setting module (10) will be used as the input value of the power synchronization module (11), as shown in fig. 3, which is a control block diagram of the power synchronization module (11).
The three modes of operation of the present invention are explained in detail below:
under the independent operation mode of the radiation type power grid, a feeder line connected with a power electronic transformer (2) and a feeder line connected with a traditional transformer (4) respectively and independently operate, at the moment, a bus switch (7) is opened, the output power of a power point setting module (10) and the input of a power synchronization module (11) are zero, the reference value of a voltage control module (12) is a rated voltage amplitude and frequency, such as 220V/50Hz, and no power is transmitted among the feeder lines.
When the power electronic transformer (2) or the traditional transformer (4) is overloaded or needs maintenance, loop operation can be realized by closing the bus switch (7), wherein the loop operation comprises a power electronic transformer and other feeder line presynchronization mode and a power distribution network loop operation mode based on the power electronic transformer, the bus switch (7) usually receives presynchronization signals but is still in an open state in the power electronic transformer and other feeder line presynchronization mode, the output reference value of the power point setting module (10) is obtained according to the voltage amplitude and the phase difference of the phase difference calculation module (9), taking an inductive line as an example, the active power reference value is approximately proportional to the phase difference, and the reactive power reference value is approximately proportional to the line voltage amplitude. The power reference value is input into the power synchronization module (11) to adjust the amplitude and the phase angle of the voltage at the low-voltage side of the power electronic transformer until the amplitude and the phase difference in the phase difference calculation module (9) become zero, so that the voltage synchronization of the two feeder lines is realized, and at the moment, no power transmission exists between the two feeder lines.
After the presynchronization is finished, the bus switch (7) receives signals and finishes the switch closing, and because the amplitude and the phase between the feeders are zero, the voltage and current disturbance is small at the moment of the switch closing, the radiation type power grid stably transits to the ring network operation, and the system instability can not be caused. Under a power distribution network loop operation mode based on a power electronic transformer, a bus switch (7) is in a closed state, power transmission exists between two feeders, power is regulated by a DC/AC inverter (2-3) at the low-voltage side of the power electronic transformer, power transmission is realized to follow a power reference value through a power synchronization module (11), and a power point setting module (10) at the stage can reasonably set the power reference value according to the requirements of a dispatching center, actual rate transmission and power flow regulation of a power distribution network, so that power regulation and power flow optimization of the power distribution network are realized.
The power reference value adopted in the power electronic transformer and other feeder line pre-synchronization modes is virtual power, is not related to actual power transmission, does not influence the actual power flow of a power grid, and the virtual power value and related parameters thereof determine the speed of the pre-synchronization process.
The power transmission in the loop operation mode of the power distribution network of the power electronic transformer is bidirectional, the adopted power reference value can be a positive value or a negative value, and the actual power transmission direction at the bus switch (7) is determined.
In the description herein, references to the description of "one embodiment," "an example," "a specific example" or the like are intended to mean that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, the schematic representations of the terms used above do not necessarily refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.
The foregoing shows and describes the general principles, essential features, and advantages of the invention. It will be understood by those skilled in the art that the present invention is not limited to the embodiments described above, which are described in the specification and illustrated only to illustrate the principle of the present invention, but that various changes and modifications may be made therein without departing from the spirit and scope of the present invention, which fall within the scope of the invention as claimed.