Study On Transient Overvoltage Of Operation Of Medium Collecting System In Offshore Wind Farm

Compared with onshore wind farms,the offshore environment is more special,The medium voltage collector system of offshore wind farms,which consists of a large number of cables of different lengths and lengths,has its own particularity.The randomness and intermittency of the wind will lead to the frequent opening of the vacuum circuit breaker in the pressure collecting system in the offshore wind farm and the transient overvoltage in the internal collector system.The high frequency oscillating overvoltage produced by the overvoltage and the cable capacitance and other electrical equipment components causes the insulation damage to the equipment of the offshore wind field,such as the end transformer of the wind field.Main factors.In order to make the offshore wind farm operate economically and reliably,reduce the fault of the power grid and improve the reliability of the power supply,it is necessary to study the transient overvoltage and obtain transient overvoltage,and to put forward transient overvoltage protection device on this basis.The main contents of this paper are as follows:(1)The transient overvoltage has the characteristics of high frequency,high steepness and uncertain time.The existing model in the software package of power system transient simulation software is not suitable for the study of transient overvoltage.Based on the PSCAD/EMTDC simulation software,a custom model can be developed to reflect the pre breakdown characteristics of the vacuum circuit breaker,which takes into account the two characteristic parameters of the dielectric insulation strength and the high frequency arc extinguishing capability of the vacuum circuit breaker,and proposes a real-time control of the closed circuit breaker with the controlled resistance value of the vacuum circuit breaker..The cable model is based on the frequency dependent model,and the parameters of the cable model are corrected and calculated based on the actual data of the physical structure parameters of the cable.It can correctly reflect the impedance of the cable and the propagation speed of the wave.The transformer model is based on the UMEC model of PSCAD,and the transformer's high voltage side,low voltage side and high voltage parallel capacitor are used to simulate the high-frequency characteristics of transformer.(2)The amplitude and steepness of transient overvoltage have a great influence on the insulation performance of the transformer,but there are few studies on transient overvoltage at present,especially the quantitative analysis of transient overvoltage steepness.Based on the custom model,this paper builds a simplified medium voltage cable collector system in PSCAD,and studies the effect of the initial phase angle,the length of the cable and the position of the terminal transformer on the transient overvoltage of the vacuum circuit breaker.The simulation results show that the overvoltage steepness is maximum when the terminal transformer is located in the engine room and the initial phase angle of the closing time is 15 degrees.(3)At present,the main measures to suppress the overvoltage in industry are the installation of surge protectors,including the Zinc Oxide lightning arrester(MOA),the RC protector and the new device choke coil,while the wind power field on the sea mainly uses MOA.This paper simulates and compares the effect of three kinds of protection devices which are installed MOA,RC protector and choke coil on the high voltage side of the machine end transformer.The suppression effect of the choke coil on transient overvoltage steepness is most obvious,which provides guidance for the selection and safe operation of the actual offshore wind farm equipment.