how to specify power/ground nets

tried to run derive_pg_connection -power_net {VDD} -ground_net {VSS} -power_pin {VDD} -ground_pin {VSS}
and always got "Error: Net VDD is not a power net. (MWUI-714)"
also using report_pg_net, it reported "total 0 nets"
How do I make ICC think they are power or ground nets.
please help

global net应该是在导入design的时候定义好。encounter中是init_power_net，icc不知有没有类似的定义

remove_net VDD
create_net -power VDD
derive_pg_connection -power_net VDD -power_pin VDD

that works, thanks

可是這樣我在verilogOut會出現
module moduleName ( aa, bb, cc, VDD, VSS );
input VDD;
input VSS ;
supply0 n7 ;
supply1 n6 ;
DUMMY dummy1 (.clk ( n7 ),.VDD ( n6 ), .VSS ( n7 ) ) ;
這要怎麼解決啊