A circuit to operate high and low trips for frequency?
The basic component is a voltage to frequency converter. This can be a frequency sensitive circuit to operate a relay, using inductors or capacitors, often in a bridge arrangement. The description below is for a low cost and accurate analogue electronic approach. There are also more digital approaches measuring the period between zero crossings.
Use a voltage transformer to provide a few volts that is a sample of the generator voltage. It can be filtered to reduce higher frequency interference using a simple RC filter. A threshold detector (voltage comparator or schmidt trigger) converts the sine wave to a square wave. The trigger threshold reference can be 0V for a zero crossing detector, or maybe half the peak voltage. This can be used to trigger a monostable that generates pulses of constant width, one for each cycle. For 50Hz the monostable could be around 15ms of the 20ms duration, considering 55Hz is probably the high threshold for a generator. The output of the monostable is a series of constant width pulses that can be integrated (filtered) to provide close to a DC voltage that is proportional to frequency. This voltage is then used to operate separate voltage comparators for high and low frequency. The comparator outputs operate alarm relays. There may need to be a delay so temporary overshoots during a load change are ignored. Each of these stages can be provided by a 555 timer and a few other components. Use a measured frequency source to calibrate this.
There are commercial frequency monitoring relays that perform this. First link is one example.
The second link is a very basic form of the circuit described above, using a single 555 timer IC. It provides a DC voltage proportional to frequency. The 555 timer is used as a monostable, should have a regulated supply voltage (5 to 12V) and the period should be set to 15ms. Look up 555 timer for more info. There are numerous ways to do this. You might find other examples by searching for tachometer circuits.