Control method for phase-locked loop-network type MMC converter station parallel power supply system

Abstract

The application discloses a control method for a phase-locked loop-network type MMC converter station power supply system. The control method adopts a constant AC voltage amplitude and active power, the MMC is kept synchronous with an AC power grid through a phase-locked loop, the q-axis voltage control link adjusts the d-axis current reference value according to the q-axis voltage and the active current reference value, so as to adjust the active power and keep the angular frequency and power angle stable; the d-axis voltage control link adjusts the q-axis current reference value according to the d-axis voltage, so as to adjust the reactive power and keep the PCC voltage stable. The application introduces a frequency deviation to adjust the active current reference value, so that the MMC output active power can respond to the frequency change, and the MMC has a primary frequency modulation characteristic; the phase-locked loop-network type MMC converter station is suitable for both a strong power grid with a relatively large short circuit and a weak power grid with a relatively small short circuit, and can delay the power output when the power instruction value or the power grid frequency changes, so that the application has good steady-state performance and transient performance.

Claims

1. A control method for a power supply system of a phase-locked loop-grid type MMC converter station, characterized in that, The phase-locked loop-grid-type MMC in the power supply system adopts constant AC voltage amplitude and active power control; the MMC keeps synchronized with the AC grid through the phase-locked loop, and its q-axis voltage control link adjusts the d-axis current reference value according to the q-axis voltage and active current reference value, thereby adjusting the active power to maintain the stability of angular frequency and power angle; The d-axis voltage control loop of the MMC adjusts the q-axis current reference value according to the d-axis voltage, thereby regulating the reactive power to maintain the PCC voltage stability; The active current reference value is dynamically adjusted according to the frequency deviation, so that the active power output of the MMC can respond to frequency changes. The q-axis voltage control loop of the phase-locked loop-grid MMC outputs the d-axis current reference value based on the q-axis voltage and the active current reference value. Since the d-axis current is coupled with the active power output of the MMC, the MMC can adjust the output active power through the q-axis voltage control loop, thereby stabilizing the power angle and frequency. In steady state, the d-axis current closely follows the active current command value. The transfer function expression for the d-axis current reference value is as follows: ， in, i dref and u sq These are the d-axis current reference value and the q-axis voltage measurement value, respectively. I pref This is the active current command value. B ad These are the d-axis current control parameters. B ad The value of and PLL control parameters B a The values ​​are the same. T q Let be the inertia constant of the q-axis voltage control element; Active current reference value for phase-locked loop-networked MMC I pref It needs to respond to changes in the power grid frequency; when the power grid frequency is at its rated value, I pref Control the MMC output rated active power; when the grid frequency drops... I pref Controlling the MMC to output more active power provides frequency support for the power grid; when the power grid frequency rises... I pref Control the MMC to output less active power; The aforementioned I pref Provided by the active current feedback loop, I pref Calculate using the following formula: ， in, P ref This is the active power command value. V ref The PCC voltage command value, Δ For power grid frequency deviation, T w The inertial time constant, K w This is the frequency modulation coefficient.

2. The control method according to claim 1, characterized in that, The phase-locked loop-grid type MMC achieves synchronization with the AC power grid through a phase-locked loop (PLL). The input of the PLL is the q-axis voltage. u sq The output is the relative angular velocity Δ of the MMC. ω ; During power grid disturbances u sq The value determines the PLL angular velocity Δ of the converter station. ω The changing situation, its transfer function The expression is as follows: in, K p , K i These are the proportional and integral constants of the PLL controller. B a For PLL controller parameters, s For the Laplace operator, u sd This is the reference value for the d-axis voltage of the MMC. i sq This is the q-axis current measurement value for the MMC.

3. The control method according to claim 1, characterized in that, The d-axis voltage control loop of the phase-locked loop-grid MMC outputs the q-axis current reference value based on the d-axis voltage. Since the q-axis current is coupled with the reactive power output of the MMC, the MMC adjusts the reactive power output through the d-axis voltage control loop, thereby stabilizing the PCC voltage.

4. The control method according to claim 3, characterized in that, In steady state, the d-axis voltage closely follows the voltage command value, and its transfer function expression is as follows: ， in: u sd and i qref These are the measured d-axis voltage value and the commanded q-axis current value, respectively. V ref This is the PCC voltage command value. K o and T o These are the proportional and integral constants of the PI controller in the d-axis voltage control loop, respectively.

5. The control method according to claim 1, characterized in that, The d-axis current reference values ​​and q-axis current reference values ​​output by the d-axis voltage control and q-axis voltage control still need to pass through a limiting circuit to achieve fault ride-through.

6. The control method according to claim 1, characterized in that, When an AC system fault occurs, the MMC monitors the PCC voltage. u s Falling to the set threshold U thr The output dq-axis current switches to the given reference value at the following time. I dref and I qref And satisfy: ， in, I max This represents the maximum allowable current on the MMC valve side.

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