双馈异步风力发电机组的主制动器是空气动力学制动,即通()。二级制动是安装在齿轮箱高速轴输出端的常开式(),该制动器执行的为()保护。
参考答案:过叶片变桨制动;机械制动器;失压有效
职业病危害因素的强度或者浓度应符合()。
A、国家职业卫生标准
B、国际职业卫生标准
C、地方职业卫生标准
D、企业职业卫生标准
Frame Relay is simplified form of (71) ,similar in principle to (72) , in which synchronous, frames of data are routed to different destinations depending on header information. Packets are routed throught one or more Virtual Circuits known as (73) . Most Virtual Circuits are (74) ,which means that the network provider sets up all DLCI connections at subscription time. (75) are also part of the Frame Relay specification. They privide a link that only lasts only as long as the session.
A.X.21
B.X.25
C.X.28
D.X.29