I have an energy reading of 50 kWh. The duration of this reading is from 09:00 to 10:30 (or 1.5 hours).
Is it '50 kWh / 1.5 hours?'
That's energy / time, which is power. 50/1.5 = 33.33 kW. That's the average power over your interval.
This would give 33.33 kWh/2 which seems wrong.
Yes, it is wrong. kWh/2 is not a unit. If that /2 is meant to scale the number, then it makes the number wrong.
If I want to make a 30-minute time series, how could I convert 50 kWh into 3 half-hourly intervals?
Do you want an energy time series, or a power time series? Energy would be easier, but power is only a scaling by time if you want that. Pay careful attention to whether the unit is kW (power) or kWh (energy).
Here you have to make an assumption. A common assumption is that the power constant over whole period, so the energy consumed in each of the three intervals is 16.67, 16.67 and 16.67 kWh. This is why an energy series is easier, you just have to make sure the three data points add to your original total energy of 50 kWh. But if you want the average power in each 30 minute interval, then divide each by 0.5 hours to get 33.33 kW. That's not only the power averaged over the 30 minute interval, but also the instantaneous power at any point during the interval. You would get this power consumption from 11 3 kW kettles.
If the power was not constant, then you might have different amounts of energy consumed in each interval. A series of consumption figures of 20, 10, 20 kWh, or even 0, 50, 0 kWh, would still give you a total consumption of 50 kWh.