Offshore wind farm low-frequency alternating-current uncontrolled rectification electric power transmission system

An offshore wind farm low-frequency alternating-current uncontrolled rectification electric power transmission system, comprising an onshore converter station and an offshore alternating-current (AC) system, wherein:

• the offshore AC system comprises:
• wind turbine generators, AC submarine cables, and offshore booster stations;
• wherein the wind turbine generators, each having a grid-side converter, are respectively connected to low voltage sides of the offshore booster stations through respective AC submarine cables, and high voltage sides of the offshore booster stations are connected to a wind farm side AC bus of the onshore converter station;
• the onshore converter station comprises a wind farm side AC bus, a converter transformer, a grid-side AC bus, an AC filter, an energy dissipation device, a rectifier and an inverter;
• wherein the wind farm side AC bus is connected to an AC side of the rectifier through the converter transformer, a direct-current (DC) side positive port of the rectifier is connected to a DC side positive port of the inverter, a DC side negative port of the rectifier is connected to a DC side negative port of the inverter, and an AC side of the inverter is connected to the grid-side AC bus through the converter transformer;
• a rated frequency of the offshore AC system and a rated frequency of the onshore converter station are both 8-20 Hz, and a rated frequency of the booster transformer in each of the offshore booster stations and a rated frequency of the rectifier side converter transformer in the onshore converter station adapt a rated frequency of the offshore AC system and a rated frequency of the onshore converter station;
• and
• the grid-side converter of each wind turbine generator comprises a control system;
• the control system comprising a power controller, an outer loop controller, an inner loop controller, and a trigger link;
• the power controller, the outer loop controller, the inner loop controller and the trigger link all include two control dimensions of d-axis and q-axis;
• the input of the d-axis component of the power controller is an active power reference value Pref and an active power measured value P of the wind turbine generator, and a difference between the active power reference value Pref and the active power measured value P is output as ΔUdref after being controlled by proportional-integral (PI) controller;
• ΔUdref is added with a d-axis voltage rated value Udref0 to obtain a d-axis voltage reference value Udref;
• an input of the q-axis component of the power controller is a reactive power reference value Qref and a measured reactive power value Q of the wind turbine generator, a difference value between the reactive power reference value Qref and a reactive power measure value Q is output as a q-axis voltage reference value Uqref after proportion control;
• an input of the outer loop controller is the grid-side converter AC outlet voltage d-axis component Ud, voltage q-axis component Uq, the d-axis voltage reference value Udref and the q-axis voltage reference value Uqref;
• a difference value between the d-axis voltage reference value Udref and the voltage d-axis component Ud are output as d-axis current reference value idref after being controlled by PI controller;
• a difference value between the q-axis voltage reference value Uqref and the voltage q-axis component Uq is output as q-axis current reference value iqref after being controlled by PI controller;
• an input of the inner loop controller is the grid-side converter AC outlet current d-axis component id, the current q-axis component iq, the d-axis current reference value idref, the q-axis current reference value iqref, the voltage d-axis component Ud and the voltage q-axis component Uq;
• an output result of a difference value between the d-axis current reference value idref and the current d-axis component id after being controlled by PI controller is added with Ud−iq*X, to obtain a d-axis voltage modulation wave Uvdref;
• an output result of a difference value between the q-axis current reference value iqref and the current q-axis component iq after being controlled by PI controller is added with Uq+iq*X, to obtain a q-axis voltage modulation wave Uvqref;
• wherein X is a connect reactance of the grid-side converter;
• the input of the trigger link is the d-axis voltage modulation wave Uvdref and the q-axis voltage modulation wave Uvqref;
• the d-axis voltage modulation wave Uvdref and the q-axis voltage modulation wave Uvqref are subjected to dq/abc conversion and PWM modulation, and output trigger signals of each switching device in the grid-side converter.