Please explain why increase or decrease in system frequency leads to black out?

frequency has relation to impedance , the current flowing through the electrical net will be determined by these impedance's , so if the current is higher or lower than the set value  the over current and the overload protections will be activated and will send a signal to the circuit breaker to trip, this case is not a general blackout but a series of these shutdown may cause a general blackout. 

Mr. mansoor,

It is due to the protection devices (over frequecy/under frequency relay) that causes to trip a generator and eventually lead to power failure (or black out as you would say). If its only one generator supplying a system. The question however mr. mansoor is why do they need to set a setting in the protection device? Answer is, Because each country has its own frequency some countries have 50hz others have 60hz and so on. That is why some devices in other country can not work to another country not unless they have the same frequency. And maybe you would ask to yourself what is a frequency in real world.  And again answer is, frequency is just an up & down of an a/c (alternating currents) that is why it is alternating. And  i almost forgot frequency is discovered by henry hertz check him out he knows better when it comes to frequency.

 

Hope this it enlightens you in some way.

 

Dear Manzoor Umar! There are two kind of black out. 1) Partial blackout when one and are part of generators are isolated on fault condition. But grid still remains energized. 2) A complete shutdown of the power plant where grid is dead and need a black start to re-energize the grid. First condition happens frequently while second is rare. However frequency fluctuation is not particularly related to this fault. It can happen due to many factors. Frequency is on of that. There is only one situation in my experience that if frequency fluctuation causes high harmonic generation, these harmonics may trip the grid and due to total load loss, generators shall shut down by motor generator action. I hope you understand what i mean to say. Should you require any further information, please revert. Regards

One of the most important conditions that the utility must provid it to the consumers is to ensure that the frequency in the permissible limits, as any flactuation in the frequency leads to flactuation in the voltage because the speed of the synchronous generator related to its frequency; if the frequency decraced the speed of the machine will decrease also if the frequency increased the speed of the machine will increase; this 2 conditions lead to make the machine out of synchronization and it will be out of service which lead to total black out

Hi ...for abnormal situation that will happen (like system failure,or fault happened,,,)

that make the frequency increase and decrease instantaneously and some time that will Exceed the set point of protection system of the power plant (the generator)..

in some time in my work (wartsilla hfo power plant) that the frequency drop under the limit and activate the protection system of the generator because the the engine will slow down under the limit for that the protection system disconnect the generator from the grid....