Rules to only fire when its :00 or :30

[Mihai_Badea]

Take a look at the documentation.
You’re rule should look like this:

rule “PVisland Voltage below 23V Alert - Notification”
when
    Time cron "0 0/30 * 1/1 * ? *"
then
    if (PVislandVolt.state instanceof DecimalType) {
.........................................................

Also, for better visibility in forum, you should use code fences…