In a word, the relationship between the layers is as follows:
the upper issues orders to the lower layer executing
instructions, the lowest layer directly control the devices and
object. But it is not allow conveying instructions from lower
layer to upper. Otherwise, the operation information and
parameters is provided from the lower to the upper. The
control function is separate as far as possible in the same level
of control layer to avoid influence each other. The control
information is adjusting dynamically between layers to timely
meet the target of upper control instruction. Whole
VSC-HVDC control system is a unified coordination system.
Executing the control instruction is top-down process, while
control instruction dynamic adjustment needs corresponding
operation information from next control layer.