Energy and the grid (part 4) Flashcards
What is the lopping threshold
When using EVs in the grid to meet peak demand, the battery storage will start to be used once power demand goes over the ‘lopping threshold’. For levels of power demand below that threshold, normal power stations will be used. The batteries will be charged up again later at off peak times. The threshold value is chosen by taking into account the peak demand value and the desired minimum SoC of the battery - don’t want to discharge each battery very low at all because it affects its SoH over time
What is the disadvantage of adding an isolation transformer
They are often tied to the grid and must operate at 50Hz, which is a low frequecy. That means the transformer would have to be big and heavy
What is an AC-DC Front-end converter
Like the LFT-based bidirectional battery charger but without need for an LFT. A ‘discrete’ line inductance is included for PFC
What is a DAB
A Dual Active Bridge. It is made of two full bridge circuits connected via a HFT. Can do step up or step down.
How do you control the direction of power flow in a DAB
Using the phase difference between each H-bridge. This is done using the leakage inductance
What are the benefits of using a HFT as the source of inductance used in a buck-boost converter (mixed with a flyback converter)
- Can do polarity inversion (like dots notation)
- Can step the voltage up/down. This scales the control delta to give it a more appropriate range
What are the two methods used for volt-second balancing of buck/boost converters?
- Snubber circuit
- Use a demagnetising winding (reset winding) on the HFT
What is ‘islanding’ of a grid-tied system
When a fault in the grid causes a Distributed Generator source (e.g. battery storage, wind farm) to exist on its own - an ‘island’. This can cause it to go out of phase (etc) with the rest of the grid. The engineers carrying out the repairs of the fault may also think that the power sources of the island are off when actually they aren’t
What would cause the grid frequency to decrease?
If net real-time demand is greater than generated capacity
