Low Forward Power means the output of Generator is sufficiently reduced. Low Forward Power Protection is basically a check not a protection i.e. Generator is tripped after having a check that its forward power has reduced. When the machine is generating and the CB connecting the generator to the system is tripped, the electrical load on the generator is cut. This could lead to generator over-speed if the mechanical input power is not reduced quickly. Large turbo-alternators, with low-inertia rotor designs, do not have a high over speed tolerance. Trapped steam in the turbine, downstream of a valve that has just closed, can rapidly lead to over speed.
To reduce the risk of over speed damage to Turbine, it is sometimes chosen to interlock non-urgent tripping of the generator breaker and the excitation system with a low forward power check.
“There are two types of tripping of Generator in broad sense, one is Non-urgent where immediate tripping of Generator is no required and another is Urgent tripping where immediate tripping is required. To be more accurate, if there is a fault in the Generator then it needs immediate tripping while if there is some problem in process side i.e. steam cycle then immediate tripping of Generator is not required. Here in this case it is better to utilize the trapped steam in between the Steam Stop Valve and Turbine. There is classification of Generator Protection as Class A, Class B and Class C which we will discuss latter.”
This ensures that the generator set circuit breaker is opened only when the output power is sufficiently low that over speeding is unlikely. The delay in electrical tripping, until prime mover input power has been removed is acceptable for non-urgent protection trips.
For urgent trips for example, stator current differential protection the low forward power interlock should not be used.
To prevent unwanted relay alarms and flags, a low forward power protection element can be disabled when the circuit breaker is opened.
The threshold setting of the low forward power protection function should be less than 50% of the power level that could result in a dangerous over speed transient on loss of electrical loading. Normally no time delay is provided for Low Forward Power Protection.
You may like to read Reverse Power Protection of Generator.