WO2015070493A1 - Island switching control method for modular multi-level converter - Google Patents

Abstract

An island switching control method for a modular multi-level converter. The method comprises: when an island occurs, switching off a grid-connected interconnection circuit breaker between a converter and an external power grid (S130); switching a grid-connected control outer loop of the converter into an off-grid AC voltage outer loop; when a power grid returns to normal, adjusting an output AC voltage of the converter to allow same to follow a power grid voltage (S230); after an amplitude difference and a phase difference of the two are less than or equal to respective preset values, and as situation lasts for a specified period of time, switching on the grid-connected interconnection circuit breaker (S250); and switching the control outer loop back to an original grid-connected control loop. The method guarantees the smooth switching of a modular multi-level converter between an off-grid mode and a grid-connected mode without causing a supply interruption of the local load, which satisfies the requirements for a safe, reliable and high-quality electric energy supply of an electric power system.

Description

 An islanding switching control method for modular multilevel converter

TECHNICAL FIELD The present invention relates to a modular multilevel converter island switching control method. Background technique

 Based on the modular multi-level converter 匪 c, the flexible DC transmission technology uses fully-controlled power electronic devices and modular structures to improve DC voltage level and system capacity, and to expand control flexibility.

It has strong scalability and fault tolerance, and can directly supply power to passive loads. It is the development trend of DC transmission technology. In view of the advantages of large capacity and high control flexibility of the 匪C transmission system, it is sometimes necessary to continue to supply the regional load without leaving the transmission system in the event of an external power grid failure, and keep the off-grid operation. When the grid is normal again, the system needs to be timely. Exit the off-grid mode, re-integrate into the grid and resume the grid operation. In order to reduce the impact of grid faults on regional load and ensure reliable power supply, 匪c can quickly and reliably perform island switching between grid-connected mode and off-grid mode, and the switching process is as smooth and stable as possible. There is no stable and reliable island switching control strategy in the prior art. SUMMARY OF THE INVENTION An object of the present invention is to provide a modular multi-level inverter island switching control method for solving the problem that the prior art does not have a reliable island switching control method. To achieve the above object, the solution of the present invention includes: a modular multilevel converter island switching control method, the inverter controller adopts a double loop dQ axis decoupling control, and the inner loop is an inverter output current loop. In the grid-connected control mode, it is a DC voltage loop or an AC active and reactive loop connected to the outer loop. In the off-grid control mode, it is an AC voltage loop. The island switching method includes the following steps: 1) The inverter is connected to the grid, when the island occurs: disconnect the inverter from the external grid and connect the circuit breaker; switch the outer loop to the off-grid AC voltage outer loop, the current inner loop command value before the memory switch and The grid voltage amplitude and phase are respectively used as the initial value of the outer loop regulator output and the initial value of the AC voltage command during switching; the AC voltage command is gradually adjusted to operate the inverter in a control mode in which the output voltage is equal to the rated grid voltage;

 2) The inverter is operated in off-grid mode. When the grid returns to normal: Adjust the converter output AC voltage to track the grid voltage; When the converter output AC voltage and grid voltage are both amplitude and phase difference less than or equal to corresponding After the predetermined value and for a certain period of time, close the grid connection breaker; switch the outer loop to the grid-connected outer loop, and store the inverter output power/DC voltage and current inner loop command value before the memory switch, respectively. The loop command value and the outer loop regulator output initial value; gradually adjust the outer loop command to the reference value to make the inverter run on the grid.

 In 1), the AC voltage control loop D-axis command is the initial value of the grid voltage before the switching, and then gradually adjusted to the rated grid voltage amplitude with a certain length; the AC voltage control loop Q-axis command remains unchanged at 0V. The angle to be referenced when performing the coordinate transformation is the initial value of the grid voltage before the switching, and then integrated at a constant angular velocity of 314.16 rad/s.

 After the grid returns to normal, it is necessary to first detect the magnitude and phase of the grid voltage, and then gradually adjust the D-axis command of the inverter AC voltage loop and the angular velocity of the control according to the difference between the grid voltage and the amplitude and phase of the inverter output voltage. Ensure that the voltage amplitude and phase angle on both sides of the grid-connected circuit breaker are the same.

 In 2), the grid-connected outer loop D-axis command is used to store the pre-switching pre-converter output active power or DC bus voltage as the initial value, and then gradually adjust to the reference value with a certain length; the grid-connected outer loop Q-axis The command outputs the reactive power as the initial value before the switching of the memory, and then gradually adjusts to the reference value with a certain length; the reference angle required for the coordinate transformation is the angle obtained by phase-locking the grid voltage.

The invention adopts constant power/DC voltage control when grid-connected, and adopts constant AC voltage control when off-grid, and realizes mode switching by changing the converter control structure. The grid-connected operation control method and the off-grid operation control method of the converter involved in the present invention are relatively mature control methods, and a reasonable control structure is adopted. Switching can achieve island switching of the converter. The method is simple in principle and simple in implementation, and ensures smooth switching between the modular multi-level converter in off-grid and grid-connected mode, and does not cause power interruption to the local load, and satisfies the safe and reliable power supply of the power system. Requirements.

 DRAWINGS

 Figure 1 is a control structure diagram of the present invention;

 2 is a schematic diagram of an operation mode and a control target of the present invention;

 Figure 3 is a schematic diagram of coordinate transformation;

 Figure 4 is a flow chart showing the implementation of the present invention.

 detailed description

 The present invention will be further described in detail below with reference to the accompanying drawings.

 The control structure of the modular multilevel converter system is shown in Figure 1. The DC side of the converter is connected to the DC bus of the flexible DC transmission system, and the AC is connected to the local load. The inverter is connected to the grid through the grid connection breaker. When the breaker is closed, the converter is connected to the grid, and the local load connected to it can be simultaneously powered by the inverter and the grid; when the breaker is disconnected The converter is switched to the off-grid mode, and the local load connected to it is independently powered by the inverter.

 Figure 1 also shows the main circuit structure of the modular multilevel converter. The six bridge arms of the main power module each contain the same number of submodules and one bridge arm reactor, which are connected by a three-phase inverter bridge structure. The voltage level is adjusted to the grid voltage via a power transformer. When the converter is connected to the grid, if it is necessary to maintain the DC bus voltage constant, the control mode of the fixed DC bus voltage combined with the reactive power is adopted. To transmit the power, the control mode of the fixed power and the fixed reactive power is adopted. When the inverter is running off-grid, the AC bus voltage must be stabilized, so the constant AC bus voltage control is used.

The operating mode and control objectives are shown in Figure 2. The modular multi-level converter controller must be designed with a suitable coordinate system. The present invention selects the coordinate system shown in Figure 3, where abc is a three-phase stationary coordinate system, which is a two-phase stationary coordinate system, and dQ is two identical.歩 Rotate the coordinate system, and the direction of rotation is counterclockwise. When the modular multi-level converter works in the grid-connected mode, as shown in Figure 1, a voltage/power, current double closed-loop control structure is used. The outer ring D-axis command is DC bus voltage reference command Udc_ref or active power command P-ref as required, Q axis command is reactive power reference command Q_ref, outer ring D, Q axis output is used as inner ring D, Q Axis instructions. The feedback amount of each control loop is obtained by changing the measured phase and the phase of the grid voltage obtained by phase-locking. The inner loop output D and Q axis components are combined with the grid voltage phase by ipark transformation to obtain the three-phase modulation degree. Finally, the nearest level modulation (NLM) and the voltage equalization strategy are performed, and the driving signals of the respective submodules are obtained to realize the converter. control.

 When the modular multi-level converter works in the off-grid mode, as shown in Figure 1, the double-closed-loop control structure is still used, the control inner loop remains unchanged, and the outer-loop D-axis command is the desired peak value of the AC bus voltage. The Q-axis command is set to 0, and the feedback amount of each control loop is obtained from the measured voltage and current value using the reference angle obtained by integrating the grid voltage rated frequency of 50 Hz. The angle is obtained as the inner loop output D and Q-axis commands. The ipark transform angle. Control allows the AC voltage vector to always be on the D-axis.

 When the grid fails, the modular multilevel converter needs to switch from the grid-connected mode to the off-grid mode; when the grid returns to normal, the modular multilevel converter needs to adjust its AC voltage amplitude and phase. Move closer to the grid voltage and switch back to the grid-connected mode of operation from the off-grid mode of operation. Next, the operation of switching the inverter from the grid-connected mode to the off-network mode and from the off-network mode to the grid-connected mode will be described in detail with reference to FIG.

 In step S110, the converter is in the grid-connected operation mode;

 In step S120, if the inverter detects that an island is generated, step S230 is performed, otherwise the S110 is maintained unchanged;

In step S130, the grid-connected circuit breaker is disconnected, the converter and its local load are disconnected from the grid at step S140, and the inverter grid-connected outer loop is switched to the off-grid AC voltage outer loop. The off-grid AC voltage outer ring D-axis command is the grid voltage amplitude stored before switching, and the off-grid AC voltage Q-axis command 0, the initial value of the voltage outer loop D-axis output is the current inner loop D-axis command value stored before switching, and the initial value of the voltage outer loop Q-axis output is the current inner loop Q-axis command value memorized before switching. At the same time, the phase value of the grid voltage stored before the switching is taken as the initial value of the rotation coordinate transformation angle when the network is controlled;

 In step S150, the off-grid AC voltage outer loop D-axis command is adjusted to slowly change from the initial value to the grid voltage rated amplitude, and the Q-axis command remains unchanged at 0V. At the same time, the grid voltage rated frequency is integrated, and the integral value is added to the grid voltage phase value memorized before the switching, as the rotation coordinate transformation angle value after the off-grid operation;

 In step S210, the converter is in an off-network operation mode;

 In step S220, if the inverter detects that the power grid is back to normal, step S230 is performed, otherwise the maintenance S210 is unchanged;

 In step S230, the voltage amplitude and phase outputted by the converter are made to be different from the detected grid voltage amplitude and phase, and the amplitude of the output voltage of the converter is adjusted correspondingly according to the difference between the phase and the amplitude. frequency. When the amplitude difference between the two is less than 0, slowly increase the inverter AC voltage D-axis command until it is the same as the grid voltage amplitude, otherwise slowly reduce the inverter AC voltage D-axis command until it is the same as the grid voltage amplitude; When the phase difference between 0 ° and 180 ° is low, slowly reduce the frequency at which the inverter is used to integrate the reference angle. Otherwise, if the phase difference is between -180 ° and 0 ° °, increase the frequency slowly. .

 In step S240, if the amplitude and phase difference values of the converter output voltage and the grid voltage are both less than the corresponding predetermined value and continue for a certain time, proceed to step S250, otherwise the maintenance S230 is unchanged; the predetermined value is Depending on the impact of different island switching shocks, both are small values.

 In step S250, closing the grid connection breaker;

In step S260, the inverter off-grid AC voltage outer loop is switched to the grid-connected outer loop. The D-axis command of the grid-connected outer ring is the DC bus voltage or the output active power memorized before switching. The Q-axis command of the grid-connected outer loop is the output reactive power value of the memory before switching, and the initial value of the outer-axis D-axis output is The current inner loop D-axis command value memorized before switching, and the initial value of the outer loop Q-axis output is the current inner loop Q-axis command value memorized before switching. At the same time, the reference angle of the rotation coordinate change after switching is switched to the grid voltage lock The angle obtained by the phase;

 In step S270, the D-axis command of the grid-connected outer ring is adjusted to slowly change from the initial value to the predetermined DC bus voltage command or the output active power command, and the Q-axis command of the grid-connected outer loop is adjusted to slowly change from the initial value. To the predetermined output reactive power command. At the same time, the phase of the grid voltage obtained by phase locking is the angle value of the rotation coordinate change;

 As another implementation manner, when the power grid does not fail but is manually scheduled, and the converter needs to be switched from the grid-connected operation mode to the off-network operation mode, the output power of the converter can be reduced to 0 by scheduling, and then Step S130 starts the operation.

 A specific embodiment has been given above, but the invention is not limited to the described embodiments. The basic idea of the present invention resides in the above-described aspects, and it is not necessary for those skilled in the art to design various deformed models, formulas, and parameters in accordance with the teachings of the present invention. Variations, modifications, alterations and variations of the embodiments may be made without departing from the spirit and scope of the invention.

Claims

1, a modular multilevel converter island switching control method, characterized in that: the converter controller adopts double loop dQ axis decoupling control, and the inner loop is an inverter output current loop, in the grid connection control mode It is a DC voltage loop or an AC active and reactive loop connected to the outer loop. In the off-grid control mode, it is an AC voltage loop. The island switching method includes the following steps:  1) The inverter is connected to the grid, when the island occurs: disconnect the inverter from the external grid and connect the circuit breaker; switch the outer loop to the off-grid AC voltage outer loop, the current inner loop command value before the memory switch and The grid voltage amplitude and phase are respectively used as the initial value of the outer loop regulator output and the initial value of the AC voltage command during switching; the AC voltage command is gradually adjusted to operate the inverter in a control mode in which the output voltage is equal to the rated grid voltage;  2) The inverter is operated in off-grid mode. When the grid returns to normal: Adjust the converter output AC voltage to track the grid voltage; When the converter output AC voltage and grid voltage are both amplitude and phase difference less than or equal to corresponding After the predetermined value and for a certain period of time, close the grid connection breaker; switch the outer loop to the grid-connected outer loop, and store the inverter output power/DC voltage and current inner loop command value before the memory switch, respectively. The loop command value and the outer loop regulator output initial value; gradually adjust the outer loop command to the reference value to make the inverter run on the grid.  2. The modular multilevel converter island switching control method according to claim 1, wherein in the 1), the AC voltage control loop D-axis command uses the stored pre-switching grid voltage amplitude as an initial value. Then, gradually adjust to the rated grid voltage amplitude with a certain length; the AC voltage control loop Q-axis command remains unchanged at 0V; the reference angle required for coordinate transformation is the initial value of the pre-switched grid voltage phase, after which 314. A constant angular velocity integral of 16 rad/s is obtained. 3. The modular multilevel converter islanding switching control method according to claim 1, wherein after the power grid returns to normal, it is necessary to first detect the grid voltage amplitude and phase, and then according to the grid voltage and the converter output. The relationship between the voltage amplitude and the phase, gradually adjust the inverter AC voltage loop D The angular speed value of the axis command and control ensures that the voltage amplitude and phase angle on both sides of the grid-connected circuit breaker are the same. 4. The modular multilevel converter island switching control method according to claim 1, wherein in 2), the grid-connected outer loop D-axis command outputs a pre-switching pre-converter to output active power. Or the DC bus voltage is the initial value, and then gradually adjusted to the reference value with a certain length; the grid-connected external loop Q-axis command is used to record the pre-switching pre-switching output reactive power as the initial value, and then gradually adjusted with a certain length To the reference value; the angle to be referenced when performing coordinate transformation is the angle obtained by phase-locking the grid voltage.