this is how I would personally do it if you're plotting a rolling high and low of the day:

function run()
{

BarPeriod = 30; // you're using 30 minutes
vars Close = priceClose();

int period = 48; // 30 minutes * 48 is 24 hours
vars high = series(MaxVal(Close, period);
vars low = series(MinVal(Close, period);

plot("high", high, MAIN, BLUE);
plot("low", low, MAIN, RED);

}