External Control
This type of control is used to cut off the power from the load (EWH) directly when the frequency goes below a certain set point frequency and turn it on again when the frequency recovers to the normal range.
One of the biggest advantages of this control is its guarantee that all loads (EWHs) will be turned off when the control becomes active. However, this type of control has aslo disadvantages. One of which is that this control approach may lead to loss of consumer comfort if the load (EWH) is turned off for long and the hot water temperature falls below certain values. Furthermore, an external control can only provide frequency regulation in case of under frequency operation conditions [51]. In other words, it is used only to reduce the load (peak shaving of load demand) to increase the system frequency.