Flexible power transformer control method and system

Abstract

The invention discloses a flexible power transformer control method and system. The flexible power transformer control system comprises a main transformer, an energy taking converter control part and a voltage regulation and control converter control part. By applying the technical scheme provided by the invention to control the flexible power transformer, the voltage of the controlled high-voltage side can be kept at an instruction value, and safe and stable operation of a power system is guaranteed. In addition, the voltage of the high-voltage side can be controlled to realize rapid and smooth voltage regulation, the defect of step-type voltage regulation of a traditional power transformer is overcome, and a technical means is provided for meeting the requirement of continuous voltage regulation of a novel power system. And active power and reactive power of the medium-voltage side and the high-voltage side of the flexible power transformer can be controlled to be kept stable, and the flexibility and controllability of system power flow control are improved.

Description

Technical Field

[0001] The present invention relates to the field of power system control, and in particular to a flexible power transformer control method and system. Background Art

[0002] Driven by the "dual carbon" goals, the proportion of renewable energy power generation in my country's power system has gradually increased, the installed capacity of renewable energy power generation has continued to grow, and the installed capacity has maintained rapid growth. However, the randomness, volatility, and intermittency of renewable energy power generation have impacted the stability of the power grid, posing challenges to continuous and reliable power supply and rapid and continuous voltage regulation. The high proportion of renewable energy grid connection and load changes have caused huge changes in the characteristics of the power system, and the problem of voltage stability has become increasingly prominent. The safe, stable, and economical operation of the power system is facing tremendous pressure, and there is an urgent need to improve the operational flexibility and regulation capabilities of the power grid.

[0003] The power transformer is the central node of electric energy transmission. It has the functions of stabilizing the voltage of the load center, regulating the load current and connecting the power grid. By adjusting the transformer voltage, the regional voltage can be affected. The flexible power transformer combines the technical advantages of traditional power transformers and power electronic transformers. The low-cost and reliable conventional electromagnetic power transformer is responsible for full power conversion, and the power electronic voltage source converter, which only accounts for about 10% of the transformer capacity, is responsible for fast and continuous regulation. Therefore, the flexible power transformer has the advantages of arc-free switching, flexible control, fast response speed, low cost, low overall loss and small footprint. It can meet the voltage regulation requirements of the new power system with an excellent economic and technical ratio.

[0004] For the control of flexible power transformers, PI control, hysteresis comparison control, space vector control and deadbeat control are generally used. Among them, PI control is controlled according to the deviation between the output and the reference. The control system has strong robustness, good stability and high steady-state accuracy. It is an engineering method that is easy to implement. However, the ability of PI control to adapt to changes in working conditions is limited. If the control of flexible power transformers relies on simple PI control, it can no longer meet the development needs of future smart distribution networks. Hysteresis comparison control has the advantages of fast response speed, high control accuracy, easy implementation and no load characteristics. The main disadvantages are that the switching frequency is fixed, the control is relatively complex, and there is serious phase interference when used in three-phase three-wire systems. Space vector control transforms the reference system of the three-phase system into a dq0 rotating coordinate system through Park transformation to achieve decoupling control. Its advantages are good steady-state performance and transient performance. The disadvantage is that it requires complex sine and inverse tangent calculations, which requires the system to have a certain computing power. The main idea of ​​deadbeat control is to deduce the switch control quantity of the next cycle based on the state equation of the system and the current state information, and finally achieve the purpose of making the output quantity track the input quantity. The use of deadbeat control can eliminate steady-state errors and end the transition process in the shortest time; however, it also has disadvantages such as poor robustness, large transient response overshoot, strong real-time calculation and therefore high hardware requirements. Summary of the invention

[0005] In order to solve the problems existing in the current control of flexible power transformers, the present invention provides a flexible power transformer control method, which is special in that it includes the following steps:

[0006] S1, construct the mathematical model of the main transformer;

[0007] S2, constructing a mathematical model of the energy-harvesting converter;

[0008] S3, constructing a mathematical model of the voltage-regulated converter;

[0009] S4, collecting the grid connection point voltage of the energy converter, the grid connection line current of the energy converter, and the DC side voltage of the energy converter;

[0010] The grid-connected point voltage of the energy-taking converter is input as an input quantity into the phase-locked loop module of the energy-taking converter to obtain the phase angle of the DC voltage reference quantity of the energy-taking converter;

[0011] In the energy-taking converter abc-dq coordinate transformation module, the energy-taking converter grid-connection point voltage and the energy-taking converter grid-connection line current are subjected to Park transformation to obtain the energy-taking converter grid-connection point voltage d-axis component, the energy-taking converter grid-connection point voltage q-axis component, the energy-taking converter grid-connection line current d-axis component and the energy-taking converter grid-connection line current q-axis component;

[0012] In the energy-taking converter dq-axis calculation module, the actual value of the DC side voltage of the energy-taking converter is subtracted from the DC side voltage command value of the energy-taking converter and then transmitted to the energy-taking converter PI controller. The difference between the d-axis component of the grid-connected line current of the energy-taking converter and the reference value of the d-axis component output by the PI controller of the energy-taking converter is used as the input of the inner-loop PI controller of the energy-taking converter. The voltage signal output by the inner-loop PI controller of the energy-taking converter is added with the cross-coupling term and the d-axis component of the grid-connected point voltage of the energy-taking converter to obtain the d-axis reference of the voltage of the energy-taking converter. The q-axis reference value of the grid-connected line current of the energy-taking converter is set to 0, and the difference between the q-axis component of the grid-connected line current of the energy-taking converter is input into the inner-loop PI controller of the line current of the energy-taking converter. The sum of the d-axis component of the grid-connected line current of the energy-taking converter and the output of the inner-loop PI controller is used as the q-axis reference of the voltage of the energy-taking converter.

[0013] Input the d-axis reference quantity of the energy-taking converter voltage and the q-axis reference quantity of the energy-taking converter voltage into the dq-abc coordinate transformation module of the energy-taking converter, use the output value of the dq-abc coordinate transformation module of the energy-taking converter as the input quantity of the SVPWM modulation module of the energy-taking converter, and the SVPWM modulation module of the energy-taking converter obtains the control signal of the IGBT in the energy-taking converter;

[0014] S5, collecting the effective value of the AC bus phase voltage controlled by the voltage control converter, the voltage at both ends of the line connected in series by the H-bridge submodule of the voltage control converter, and the current of the line connected in series by the voltage control converter;

[0015] The current of the series connection line of the voltage-regulated converter is used as the input quantity of the phase-locked loop module of the voltage-regulated converter to obtain the phase angle of the output voltage of the H-bridge submodule of the voltage-regulated converter;

[0016] In the voltage control converter abc-dq coordinate transformation module, the voltage at both ends of the series connection line of the voltage control converter H bridge submodule is subjected to equal amplitude Park transformation to obtain the d-axis component of the voltage at both ends of the series connection line of the voltage control converter and the q-axis component of the voltage at both ends of the series connection line of the voltage control converter;

[0017] In the voltage control converter dq axis calculation module, the effective value of the AC bus phase voltage controlled by the voltage control converter is collected as the d-axis reference value of the voltage control converter control loop, and the difference between it and the d-axis component of the voltage at both ends of the line connected in series with the voltage control converter is input into the voltage control converter PI controller to obtain the d-axis reference value of the output voltage of the voltage control converter PI controller;

[0018] The d-axis reference value of the voltage control converter PI controller output voltage is input into the dq-abc coordinate transformation module of the voltage control converter, the q-axis reference value of the voltage control converter control loop is set to 0, the difference of the q-axis components of the voltage at both ends of the line connected in series with the voltage control converter is input into the PI controller of the voltage control converter, and the q-axis reference value of the voltage control converter PI controller output voltage is input into the dq-abc coordinate transformation module of the voltage control converter;

[0019] The voltage control converter dq-abc coordinate transformation module performs coordinate transformation on the voltage control converter PI controller output voltage d-axis reference quantity and the voltage control converter PI controller output voltage q-axis reference quantity, outputs the voltage control converter series connection line voltage reference value at both ends, and inputs the voltage control converter SPWM module;

[0020] The voltage-regulated converter SPWM modulation module obtains the IGBT trigger pulse signal in the voltage-regulated converter H-bridge submodule through modulation.

[0021] Furthermore, the method for constructing the mathematical model of the main transformer in S1 comprises the following steps:

[0022] S101, the main transformer is on the grid voltage side,

[0023]

[0024] Where U sa , U sb , U sc Respectively represent the three-phase voltages of the system a, b, and c; U 1a , U 1b , U 1c Respectively represent the three-phase voltages of primary winding a, b, and c of the main transformer; U sea , U seb , U sec Respectively represent the three-phase voltages of the secondary winding a, b, and c of the isolation transformer;

[0025] S102, on the tertiary winding of the main transformer, there is

[0026]

[0027] Where U ta , U tb , U tc Respectively represent the voltages of the three-phase outputs of the energy-taking converter a, b, and c; U 3a , U 3b , U 3c Respectively represent the three-phase voltages of the main transformer's tertiary windings a, b, and c; L t Represents the filter inductance on the energy-taking converter side; i ga、i gb 、i gc They represent the three-phase grid-connected point currents of energy-taking converters a, b, and c respectively;

[0028] S103, on the primary winding of the isolation transformer, there is

[0029]

[0030] Where U ca , U cb , U cc Respectively represent the voltages of the three-phase outputs of the voltage-controlled converter a, b, and c; U 4a , U 4b , U 4c Respectively represent the three-phase voltages of the primary winding a, b, and c of the isolation transformer; L c represents the filter inductance on the voltage control converter side; i sea 、i seb 、i sec They represent the currents of the three-phase grid-connected points of voltage-controlled converters a, b, and c respectively;

[0031] S104, according to the voltage ratio of the main transformer,

[0032]

[0033] According to the magnetomotive force balance of the main transformer, we can get

[0034]

[0035] Where N 1 、N 2 、N 3 Respectively represent the number of winding turns on the three sides of the main transformer; N 4 、N 5 Respectively represent the number of turns of the primary and secondary windings of the isolation transformer; i sa 、i sb 、i sc Respectively represent the three-phase currents of the AC system a, b, and c; i 2a 、i 2b 、i 2c Respectively represent the three-phase currents of the secondary winding a, b, and c of the main transformer; i 3a 、i 3b 、i 3c They represent the three-phase currents of the tertiary winding a, b, and c of the main transformer respectively.

[0036] Furthermore, the method for constructing a mathematical model of the energy-extracting converter in S2 comprises the following steps:

[0037] S201, the output voltage of the energy-taking converter is

[0038]

[0039] In the formula, u 1a (t),u 1b (t),u 1c (t) are the three-phase voltages of converter a, b, and c respectively; K is an indicator of the proportional coefficient; δ is the angle between the system voltage and the STATCOM output voltage;

[0040] S202, the system three-phase voltage expression is:

[0041]

[0042] In the formula, u sa (t),u sb (t),u sc (t) are the phase voltages of the three-phase AC system; U s is the effective value of the AC system line voltage;

[0043] The mathematical equation of the three-phase a, b, c energy converter is:

[0044]

[0045] In the formula, i a (t), i b (t), i c (t) are the phase currents of phases a, b and c respectively; R is the equivalent resistance of the line; L is the equivalent inductance of the line;

[0046] Arrangement available

[0047]

[0048] S203, according to the energy relationship, the DC side capacitor voltage u of the energy-taking converter can be obtained. dc The equation is

[0049]

[0050] Where, C is the DC side capacitance of STATCOM; u dc (t) is the DC side capacitor voltage;

[0051] Simplified, we can get

[0052]

[0053] S204, the mathematical model of the energy-taking converter is:

[0054]

[0055] Furthermore, the method for constructing the mathematical model of the voltage control converter in S3 is:

[0056]

[0057] In the formula, i a 、i b 、i c are the three-phase currents of a, b, and c respectively; u 1a 、u 1b 、u 1c are the voltages of the three-phase sending end a, b, and c respectively; u 2a 、u 2b 、u 2c are the three-phase receiving terminal voltages a, b, and c respectively; u ina 、u inb 、u inc are the voltages injected by the voltage regulating converter into the three-phase AC systems a, b, and c respectively; R represents the line resistance; and L represents the line inductance.

[0058] Furthermore, the Park transformation expressions of the energy-extracting converter abc-dq coordinate transformation module and the voltage-regulating converter abc-dq coordinate transformation module are:

[0059]

[0060] Furthermore, the Park inverse transformation expressions of the energy-extracting converter dq-abc coordinate transformation module and the voltage-regulating converter dq-abc coordinate transformation module are:

[0061]

[0062] On the other hand, the present invention also provides a flexible power transformer control system, which is special in that it includes a main transformer, an energy extraction converter control part and a voltage regulation converter control part;

[0063] The energy-taking converter control part includes an energy-taking converter phase-locked loop module, an energy-taking converter abc-dq coordinate transformation module, an energy-taking converter dq axis calculation module, an energy-taking converter dq-abc coordinate transformation module and an energy-taking converter SVPWM modulation module;

[0064] The energy-taking converter dq-axis calculation module also includes an energy-taking converter PI controller and an energy-taking converter inner loop PI controller.

[0065] The voltage control converter control part includes a voltage control converter H-bridge submodule, a voltage control converter phase-locked loop module, a voltage control converter abc-dq coordinate transformation module, a voltage control converter dq axis calculation module, a voltage control converter dq-abc coordinate transformation module and a voltage control converter SPWM modulation module;

[0066] The voltage-regulated converter dq-axis calculation module also includes a voltage-regulated converter PI controller.

[0067] The flexible power transformer control method and system of the present invention can keep the controlled high-voltage side voltage at the command value, ensuring the safe and stable operation of the power system. In addition, the high-voltage side voltage can be controlled to achieve fast and smooth voltage regulation, making up for the shortcomings of the traditional power transformer step-type voltage regulation, and providing technical means to meet the needs of continuous voltage regulation in new power systems. The active power and reactive power of the medium-voltage side and high-voltage side of the flexible power transformer can also be controlled to remain stable, improving the flexibility and controllability of the system flow control. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] The drawings in the specification, which constitute a part of this application, are used to provide a further understanding of the present invention. The illustrative embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0069] Figure 1 Flexible power transformer system voltage stabilization effect simulation topology;

[0070] Figure 2 Voltage waveform at point A when there is no voltage fluctuation in the flexible power transformer;

[0071] Figure 3 Flexible power transformer voltage stabilization effect simulation waveform;

[0072] Figure 4 Flexible power transformer system voltage regulation characteristics simulation topology;

[0073] Figure 5 Flexible power transformer voltage regulation characteristic simulation waveform;

[0074] Figure 6 Flexible power transformer voltage regulation upper limit simulation waveform;

[0075] Figure 7 Flexible power transformer voltage regulation lower limit simulation waveform;

[0076] Figure 8 Topological structure for simulation of power flow control effect of flexible power transformer system;

[0077] Fig. 9 Active power waveform at the inlet of flexible power transformer;

[0078] Fig.10 Flexible power transformer inlet reactive power waveform;

[0079] Fig.11 Flexible power transformer export active power waveform;

[0080] Fig.12 Flexible power transformer outlet reactive power waveform;

[0081] Fig.13 A schematic diagram of the structure of the flexible power transformer control system of the present invention;

[0082] Fig.14 A schematic diagram of the control part of the energy-taking converter of the present invention;

[0083] Fig.15 A schematic diagram of the structure of the control part of the voltage regulation converter of the present invention. DETAILED DESCRIPTION

[0084] In order to deepen the understanding of the present invention, the present invention will be further described in detail below with reference to embodiments and drawings.

[0085] The flexible power transformer control system used in this embodiment includes a main transformer, an energy extraction converter control part and a voltage regulation converter control part;

[0086] The energy-taking converter control part includes an energy-taking converter phase-locked loop module, an energy-taking converter abc-dq coordinate transformation module, an energy-taking converter dq axis calculation module, an energy-taking converter dq-abc coordinate transformation module and an energy-taking converter SVPWM modulation module;

[0087] The energy-taking converter dq-axis calculation module also includes an energy-taking converter PI controller and an energy-taking converter inner loop PI controller.

[0088] The voltage control converter control part includes a voltage control converter H-bridge submodule, a voltage control converter phase-locked loop module, a voltage control converter abc-dq coordinate transformation module, a voltage control converter dq axis calculation module, a voltage control converter dq-abc coordinate transformation module and a voltage control converter SPWM modulation module;

[0089] The voltage-regulated converter dq-axis calculation module also includes a voltage-regulated converter PI controller.

[0090] The method for controlling a flexible power transformer using the system includes the following steps:

[0091] S1, construct the mathematical model of the main transformer;

[0092] S2, constructing a mathematical model of the energy-harvesting converter;

[0093] S3, constructing a mathematical model of the voltage-regulated converter;

[0094] S4, collecting the grid connection point voltage of the energy converter, the grid connection line current of the energy converter, and the DC side voltage of the energy converter;

[0095] The grid-connected point voltage of the energy-taking converter is input as an input quantity into the phase-locked loop module of the energy-taking converter to obtain the phase angle of the DC voltage reference quantity of the energy-taking converter;

[0096] In the energy-taking converter abc-dq coordinate transformation module, the energy-taking converter grid-connection point voltage and the energy-taking converter grid-connection line current are subjected to Park transformation to obtain the energy-taking converter grid-connection point voltage d-axis component, the energy-taking converter grid-connection point voltage q-axis component, the energy-taking converter grid-connection line current d-axis component and the energy-taking converter grid-connection line current q-axis component;

[0097] In the energy-taking converter dq-axis calculation module, the actual value of the DC side voltage of the energy-taking converter is subtracted from the DC side voltage command value of the energy-taking converter and then transmitted to the energy-taking converter PI controller. The difference between the d-axis component of the grid-connected line current of the energy-taking converter and the reference value of the d-axis component output by the PI controller of the energy-taking converter is used as the input of the inner-loop PI controller of the energy-taking converter. The voltage signal output by the inner-loop PI controller of the energy-taking converter is added with the cross-coupling term and the d-axis component of the grid-connected point voltage of the energy-taking converter to obtain the d-axis reference of the voltage of the energy-taking converter. The q-axis reference value of the grid-connected line current of the energy-taking converter is set to 0, and the difference between the q-axis component of the grid-connected line current of the energy-taking converter is input into the inner-loop PI controller of the line current of the energy-taking converter. The sum of the d-axis component of the grid-connected line current of the energy-taking converter and the output of the inner-loop PI controller is used as the q-axis reference of the voltage of the energy-taking converter.

[0098] Input the d-axis reference quantity of the energy-taking converter voltage and the q-axis reference quantity of the energy-taking converter voltage into the dq-abc coordinate transformation module of the energy-taking converter, use the output value of the dq-abc coordinate transformation module of the energy-taking converter as the input quantity of the SVPWM modulation module of the energy-taking converter, and the SVPWM modulation module of the energy-taking converter obtains the control signal of the IGBT in the energy-taking converter;

[0099] S5, collecting the effective value of the AC bus phase voltage controlled by the voltage control converter, the voltage at both ends of the line connected in series by the H-bridge submodule of the voltage control converter, and the current of the line connected in series by the voltage control converter;

[0100] The current of the series connection line of the voltage-regulated converter is used as the input quantity of the phase-locked loop module of the voltage-regulated converter to obtain the phase angle of the output voltage of the H-bridge submodule of the voltage-regulated converter;

[0101] In the voltage control converter abc-dq coordinate transformation module, the voltage at both ends of the series connection line of the voltage control converter H bridge submodule is subjected to equal amplitude Park transformation to obtain the d-axis component of the voltage at both ends of the series connection line of the voltage control converter and the q-axis component of the voltage at both ends of the series connection line of the voltage control converter;

[0102] In the voltage control converter dq axis calculation module, the effective value of the AC bus phase voltage controlled by the voltage control converter is collected as the d-axis reference value of the voltage control converter control loop, and the difference between it and the d-axis component of the voltage at both ends of the line connected in series with the voltage control converter is input into the voltage control converter PI controller to obtain the d-axis reference value of the output voltage of the voltage control converter PI controller;

[0103] The d-axis reference value of the voltage control converter PI controller output voltage is input into the dq-abc coordinate transformation module of the voltage control converter, the q-axis reference value of the voltage control converter control loop is set to 0, the difference of the q-axis components of the voltage at both ends of the line connected in series with the voltage control converter is input into the PI controller of the voltage control converter, and the q-axis reference value of the voltage control converter PI controller output voltage is input into the dq-abc coordinate transformation module of the voltage control converter;

[0104] The voltage control converter dq-abc coordinate transformation module performs coordinate transformation on the voltage control converter PI controller output voltage d-axis reference quantity and the voltage control converter PI controller output voltage q-axis reference quantity, outputs the voltage control converter series connection line voltage reference value at both ends, and inputs the voltage control converter SPWM module;

[0105] The voltage-regulated converter SPWM modulation module obtains the IGBT trigger pulse signal in the voltage-regulated converter H-bridge submodule through modulation.

[0106] Relying on the PSCAD / EMTDC simulation platform, a simulation model of the flexible power transformer system is constructed, including: main transformer simulation model, voltage regulation converter main circuit simulation model, energy extraction converter main circuit simulation model, voltage regulation converter control circuit simulation model and energy extraction converter control circuit simulation model. The above simulation models are connected with the AC power grid simulation model containing three-phase AC power supply and load to form a flexible power transformer (FPT) simulation model put into operation in the AC power system.

[0107] Table 1 shows the main parameter settings of the main transformer simulation model.

[0108] Table 1 Main parameter settings of the main transformer simulation model

[0109]

[0110]

[0111] The construction of the main circuit simulation model of the energy-harvesting converter first constructs a three-phase bridge converter, then connects the three-phase bridge converter to a two-winding transformer, and connects it in parallel to the tertiary winding #3 of the main transformer, i.e., the energy-harvesting winding, to form a main circuit simulation model of the energy-harvesting converter.

[0112] The main parameter settings of the main circuit of the energy-harvesting converter are shown in Table 2.

[0113] Table 2 Main parameter settings of energy-taking converter

[0114]

[0115] The construction of the voltage-regulated converter main circuit simulation model first constructs the H-bridge sub-module, and then connects the H-bridge sub-module interface in series to the three-phase AC system. The three H-bridge sub-modules A, H-bridge sub-module B and H-bridge sub-module C connected in series to the three-phase AC system constitute the voltage-regulated converter simulation model.

[0116] The main parameter settings of the voltage control converter main circuit are shown in Table 3.

[0117] Table 3 Main parameter settings of voltage control converter

[0118]

[0119] A simulation model of flexible power transformers being put into AC power grid operation is constructed. An equivalent AC power grid system is connected at the commutation bus of the flexible power transformer system, and the AC power grid system includes three-phase AC power source, overhead transmission line, flexible power transformer and load.

[0120] The main parameter settings of the AC power grid simulation model are shown in Table 4.

[0121] Table 4 Main parameter settings of AC power grid simulation model

[0122]

[0123] In order to verify the voltage stabilization effect of the electromagnetic transient simulation model of the flexible power transformer system, based on the PSCAD / EMTDC simulation software and the simulation parameter settings in Tables 1 to 4, the voltage stabilization effect of the voltage at point A on the high-voltage side of the constructed flexible power transformer simulation model is simulated and analyzed. The simulation topology is as follows: Figure 1 shown.

[0124] Figure 2 The figure shows the voltage waveform of point A when there is no voltage fluctuation in the flexible power transformer. It can be seen from the waveform that after 0.15s, the voltage of point A on the high-voltage side of the flexible power transformer stabilizes at 345kV.

[0125] In order to further verify the voltage stabilization effect of the flexible power transformer when voltage fluctuation occurs, the simulation command value and simulation time are set as follows: the voltage command value of point A on the high-voltage side of the flexible power transformer is 345kV. The simulation time is 2s, of which the main circuit of the energy-taking converter is put into operation at 0.2s, the control circuit of the energy-taking converter is put into operation at 0.3s, the control circuit of the voltage-regulating converter is put into operation at 0.5s, and the voltage command value of point A is increased by 5% at 0.7s.

[0126] like Figure 3 The figure shows the simulation waveform of the voltage stabilization effect of the flexible power transformer. It can be seen from the waveform that after 0.5s, the energy-taking converter and the voltage-regulating converter are all put into operation. During the time period of 0.5-0.7s, the voltage at point A is stabilized at the original command value of 345kV. At 0.7s, the voltage-regulating converter starts to control the voltage at point A to increase by 5%. After 0.08s, the voltage at point A is stabilized at 362.25kV.

[0127] Based on simulation analysis, the constructed flexible power transformer simulation model can keep the controlled high-voltage side voltage at the command value, ensuring the safe and stable operation of the power system.

[0128] In order to verify the voltage regulation effect of the electromagnetic transient simulation model of the flexible power transformer system, based on the PSCAD / EMTDC simulation software and the simulation parameter settings in Tables 1 to 4, the constructed flexible power transformer simulation model is simulated and analyzed for the voltage regulation effect of the flexible power transformer system. The simulation topology is as follows: Figure 4 shown.

[0129] The simulation command value and simulation time are set as follows: the voltage command value of point A on the high-voltage side of the flexible power transformer is 345 kV. The simulation time is 2 seconds, of which the voltage command value of point A is increased by 2% at 0.7 seconds, by 3% at 0.9 seconds, by 7% at 1.1 seconds, and by 2% at 1.3 seconds.

[0130] like Figure 5The figure shows the simulation waveform of the voltage regulation characteristics of the flexible power transformer. It can be seen from the waveform that the voltage at point A is stable at the original command value of 345kV in the time period of 0.5-0.7s. At 0.7s, the voltage regulating converter starts to control the voltage at point A to increase by 2%. After 0.08s of voltage rise, the voltage at point A is stable at 351.9kV in the time period of 0.78-0.9s; at 0.9s, the voltage regulating converter controls the voltage at point A to increase by 3%. After 0.08s of voltage rise, the voltage at point A is stable at 362.25kV in the time period of 0.98-1.1s; at 1.1s, the voltage regulating converter controls the voltage at point A to decrease by 7%, after 0.1s of voltage drop, and the voltage at point A is stable at 338.1kV in the time period of 1.2-1.3s; at 1.3s, the voltage regulating converter controls the voltage at point A to decrease by 2%. After 0.08s of voltage drop, the voltage at point A is stable at 331.2kV in the time period of 1.38-2s.

[0131] In order to verify the upper limit of the voltage regulation range of the flexible power transformer, the simulation command value and simulation time are set as follows: the voltage command value of point A on the high-voltage side of the flexible power transformer is 345 kV. The simulation time is 2 seconds, of which the voltage command value of point A is increased by 10% at 0.7 seconds, the voltage command value of point A is increased by another 10% at 0.9 seconds, and the voltage command value of point A is increased by another 10% at 1.1 seconds.

[0132] Figure 6 This is the simulation waveform of the upper limit of the voltage regulation of the flexible power transformer. In the time period of 0.15-0.7s, the voltage at point A of the flexible power transformer is stable at the original command value of 345kV. At 0.7s, the voltage command value of point A is increased by 10%, and the voltage at point A rises to 379kV at 0.8s. In the time period of 0.8-0.9s, the voltage at point A is stabilized at 379.5kV. At 0.9s, the voltage command value at point A is increased by another 10%, and the voltage at point A stabilizes at 400kV after 0.98s. Although the command value is increased by another 10% at 1.1s, the voltage at point A is still stable at 400kV. From the above analysis, it can be seen that under the conditions of the flexible power transformer parameter settings in Tables 1 to 4, the upper limit of the voltage regulation of the flexible power transformer is about 16%.

[0133] In order to verify the lower limit of the voltage regulation range of the flexible power transformer, the simulation command value and simulation time are set as follows: the voltage command value of point A on the high-voltage side of the flexible power transformer is 345 kV. The simulation time is 2 seconds, of which the voltage command value of point A is reduced by 10% at 0.7 seconds, the voltage command value of point A is reduced by another 10% at 0.9 seconds, and the voltage command value of point A is reduced by another 10% at 1.1 seconds.

[0134] Figure 7This is the simulation waveform of the upper limit of the voltage regulation of the flexible power transformer. During the time period of 0.15-0.7s, the voltage at point A of the flexible power transformer is stable at the original command value of 345kV. At 0.7s, the voltage command value of point A is reduced by 10%, and the voltage at point A drops to 310kV at 0.8s. During the time period of 0.8-0.9s, the voltage at point A is stabilized at 310kV. At 0.9s, the voltage command value at point A is further reduced by 10%, and the voltage at point A stabilizes at 290kV after 1.00s. Although the command value is further reduced by 10% at 1.1s, the voltage at point A is still stable at 290kV. Under the conditions of the flexible power transformer parameter settings in Tables 1 to 4, the lower limit of the voltage regulation of the flexible power transformer is about 16%.

[0135] Based on simulation analysis, the constructed flexible power transformer simulation model can control the high-voltage side voltage to achieve fast and smooth voltage regulation, make up for the shortcomings of traditional power transformer step-type voltage regulation, and provide technical means to meet the needs of continuous voltage regulation in new power systems.

[0136] In order to verify the power flow control effect of the electromagnetic transient simulation model of the flexible power transformer system, the power flow control effect of the constructed flexible power transformer simulation model at point A on the high voltage side and point B on the medium voltage side is simulated and analyzed. The simulation topology is as follows: Figure 8 shown.

[0137] Fig. 9 The figure shows the active power waveform at the inlet of the flexible power transformer. Fig.10 The figure shows the reactive power waveform at the inlet of the flexible power transformer. At 0.3s, the energy-taking converter was put into operation, and the DC capacitor of the energy-taking converter was charged to supply energy to the voltage-regulating converter. The active power at the inlet of the flexible power transformer increased from 1990MW to 2051MW, and the reactive power at the inlet increased from 691.75MVar to 715MVar. After 0.5s, the energy-taking converter and the voltage-regulating converter were all put into operation, and the voltage-regulating converter controlled the voltage at point A to be stable. After 0.52s, the active power at the inlet of the flexible power transformer was stable at 1993MW, and the reactive power at the inlet was stable at 710MVar.

[0138] Fig.11 The figure shows the waveform of the active power at the outlet of the flexible power transformer. Fig.12 The figure shows the reactive power waveform at the outlet of the flexible power transformer. After 0.6s, the active power at the outlet of the flexible power transformer stabilizes at 1977MW, and the reactive power at the outlet stabilizes at 152.94MVar.

[0139] Based on simulation analysis, the constructed flexible power transformer simulation model can control the active power and reactive power on the medium voltage side and high voltage side of the flexible power transformer to remain stable. The application of flexible power transformer technology in AC power grid will enhance the flexibility and controllability of system power flow control.

[0140] The above are only specific embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A flexible power transformer control method, characterized in that: The following steps are involved: S1, construct the mathematical model of the main transformer; S2, constructing a mathematical model of the energy-harvesting converter; S3, constructing a mathematical model of the voltage-regulated converter; S4, collecting the grid connection point voltage of the energy converter, the grid connection line current of the energy converter, and the DC side voltage of the energy converter; The grid-connected point voltage of the energy-taking converter is input as an input quantity into the phase-locked loop module of the energy-taking converter to obtain the phase angle of the DC voltage reference quantity of the energy-taking converter; In the energy-taking converter abc-dq coordinate transformation module, the energy-taking converter grid-connection point voltage and the energy-taking converter grid-connection line current are subjected to Park transformation to obtain the energy-taking converter grid-connection point voltage d-axis component, the energy-taking converter grid-connection point voltage q-axis component, the energy-taking converter grid-connection line current d-axis component and the energy-taking converter grid-connection line current q-axis component; In the energy-taking converter dq-axis calculation module, the actual value of the DC side voltage of the energy-taking converter is subtracted from the DC side voltage command value of the energy-taking converter and then transmitted to the energy-taking converter PI controller. The difference between the d-axis component of the grid-connected line current of the energy-taking converter and the reference value of the d-axis component output by the PI controller of the energy-taking converter is used as the input of the inner-loop PI controller of the energy-taking converter. The voltage signal output by the inner-loop PI controller of the energy-taking converter is added with the cross-coupling term and the d-axis component of the grid-connected point voltage of the energy-taking converter to obtain the d-axis reference of the voltage of the energy-taking converter. The q-axis reference value of the grid-connected line current of the energy-taking converter is set to 0, and the difference between the q-axis component of the grid-connected line current of the energy-taking converter is input into the inner-loop PI controller of the line current of the energy-taking converter. The sum of the d-axis component of the grid-connected line current of the energy-taking converter and the output of the inner-loop PI controller is used as the q-axis reference of the voltage of the energy-taking converter. Input the d-axis reference quantity of the energy-taking converter voltage and the q-axis reference quantity of the energy-taking converter voltage into the dq-abc coordinate transformation module of the energy-taking converter, use the output value of the dq-abc coordinate transformation module of the energy-taking converter as the input quantity of the SVPWM modulation module of the energy-taking converter, and the SVPWM modulation module of the energy-taking converter obtains the control signal of the IGBT in the energy-taking converter; S5, collecting the effective value of the AC bus phase voltage controlled by the voltage control converter, the voltage at both ends of the line connected in series by the H-bridge submodule of the voltage control converter, and the current of the line connected in series by the voltage control converter; The current of the series connection line of the voltage-regulated converter is used as the input quantity of the phase-locked loop module of the voltage-regulated converter to obtain the phase angle of the output voltage of the H-bridge submodule of the voltage-regulated converter; In the voltage control converter abc-dq coordinate transformation module, the voltage at both ends of the series connection line of the voltage control converter H bridge submodule is subjected to equal amplitude Park transformation to obtain the d-axis component of the voltage at both ends of the series connection line of the voltage control converter and the q-axis component of the voltage at both ends of the series connection line of the voltage control converter; In the voltage control converter dq axis calculation module, the effective value of the AC bus phase voltage controlled by the voltage control converter is collected as the d-axis reference value of the voltage control converter control loop, and the difference between it and the d-axis component of the voltage at both ends of the line connected in series with the voltage control converter is input into the voltage control converter PI controller to obtain the d-axis reference value of the output voltage of the voltage control converter PI controller; The d-axis reference value of the voltage control converter PI controller output voltage is input into the dq-abc coordinate transformation module of the voltage control converter, the q-axis reference value of the voltage control converter control loop is set to 0, the difference of the q-axis components of the voltage at both ends of the line connected in series with the voltage control converter is input into the PI controller of the voltage control converter, and the q-axis reference value of the voltage control converter PI controller output voltage is input into the dq-abc coordinate transformation module of the voltage control converter; The voltage control converter dq-abc coordinate transformation module performs coordinate transformation on the voltage control converter PI controller output voltage d-axis reference quantity and the voltage control converter PI controller output voltage q-axis reference quantity, outputs the voltage control converter series connection line voltage reference value at both ends, and inputs the voltage control converter SPWM module; The voltage-regulated converter SPWM modulation module obtains the IGBT trigger pulse signal in the voltage-regulated converter H-bridge submodule through modulation.

2. The flexible power transformer control method according to claim 1, characterized in that: The method for constructing the mathematical model of the main transformer in S1 comprises the following steps: S101, the main transformer is on the grid voltage side, Where U sa , U sb , U sc Respectively represent the three-phase voltages of the system a, b, and c; U 1a , U 1b , U 1c Respectively represent the three-phase voltages of primary winding a, b, and c of the main transformer; U sea , U seb , U sec Respectively represent the three-phase voltages of the secondary winding a, b, and c of the isolation transformer; S102, on the tertiary winding of the main transformer, there is Where U ta , U tb , U tc Respectively represent the voltages of the three-phase outputs of the energy-taking converter a, b, and c; U 3a , U 3b , U 3c Respectively represent the three-phase voltages of the main transformer's tertiary windings a, b, and c; L t Represents the filter inductance on the energy-taking converter side; i ga 、i gb 、i gc They represent the three-phase grid-connected point currents of energy-taking converters a, b, and c respectively; S103, on the primary winding of the isolation transformer, there is Where U ca , U cb , U cc Respectively represent the voltages of the three-phase outputs of the voltage-controlled converter a, b, and c; U 4a , U 4b , U 4c Respectively represent the three-phase voltages of the primary winding a, b, and c of the isolation transformer; L c represents the filter inductance on the voltage control converter side; i sea 、i seb 、i sec They represent the currents of the three-phase grid-connected points of voltage-controlled converters a, b, and c respectively; S104, according to the voltage ratio of the main transformer, According to the magnetomotive force balance of the main transformer, we can get In the formula, N1, N2, and N3 represent the number of turns of the three-side windings of the main transformer; N4 and N5 represent the number of turns of the main and secondary windings of the isolation transformer; i sa 、i sb 、i sc Respectively represent the three-phase currents of the AC system a, b, and c; i 2a 、i 2b 、i 2c Respectively represent the three-phase currents of the secondary winding a, b, and c of the main transformer; i 3a 、i 3b 、i 3c They represent the three-phase currents of the tertiary winding a, b, and c of the main transformer respectively.

3. The flexible power transformer control method according to claim 1, characterized in that: The method for constructing a mathematical model of the energy-extracting converter in S2 comprises the following steps S201, the output voltage of the energy-taking converter is In the formula, u 1a (t),u 1b (t),u 1c (t) are the three-phase voltages of converter a, b, and c respectively; K is an indicator of the proportional coefficient; δ is the angle between the system voltage and the STATCOM output voltage; S202, the system three-phase voltage expression is: In the formula, u sa (t),u sb (t),u sc (t) are the phase voltages of the three-phase AC system; U s is the effective value of the AC system line voltage; The mathematical equation of the three-phase a, b, c energy converter is: In the formula, i a (t), i b (t), i c (t) are the phase currents of phases a, b and c respectively; R is the equivalent resistance of the line; L is the equivalent inductance of the line; Arrangement available S203, according to the energy relationship, the DC side capacitor voltage u of the energy-taking converter can be obtained. dc The equation is Where, C is the DC side capacitance of STATCOM; u dc (t) is the DC side capacitor voltage; Simplified, we can get S204, the mathematical model of the energy-taking converter is:

4. The flexible power transformer control method according to claim 1, characterized in that: The method for constructing the mathematical model of the voltage control converter in S3 is: In the formula, i a 、i b 、i c They are the three-phase currents of a, b and c respectively; u 1a 、u 1b 、u 1c are the voltages of the three-phase sending terminals a, b, and c respectively; u 2a 、u 2b 、u 2c are the three-phase receiving terminal voltages a, b, and c respectively; u ina 、u inb 、u inc are the voltages injected by the voltage regulating converter into the three-phase AC systems a, b, and c respectively; R represents the line resistance; and L represents the line inductance.

5. The flexible power transformer control method according to claim 1, characterized in that: The Park transformation expressions of the energy-taking converter abc-dq coordinate transformation module and the voltage-regulating converter abc-dq coordinate transformation module are:

6. The flexible power transformer control method according to claim 1, characterized in that: The Park inverse transformation expressions of the energy-taking converter dq-abc coordinate transformation module and the voltage-regulating converter dq-abc coordinate transformation module are as follows:

7. A flexible power transformer control system, characterized in that: It includes a main transformer, an energy extraction converter control section and a voltage regulation converter control section; The energy-taking converter control part includes an energy-taking converter phase-locked loop module, an energy-taking converter abc-dq coordinate transformation module, an energy-taking converter dq axis calculation module, an energy-taking converter dq-abc coordinate transformation module and an energy-taking converter SVPWM modulation module; The energy-taking converter dq-axis calculation module also includes an energy-taking converter PI controller and an energy-taking converter inner-loop PI controller; The voltage control converter control part includes a voltage control converter H-bridge submodule, a voltage control converter phase-locked loop module, a voltage control converter abc-dq coordinate transformation module, a voltage control converter dq axis calculation module, a voltage control converter dq-abc coordinate transformation module and a voltage control converter SPWM modulation module; The voltage-regulated converter dq-axis calculation module also includes a voltage-regulated converter PI controller.

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