One expert recommended that run generator on 0.8pf lag. Can you help if we will run on 0.95 lag of then what will be adverse effects on generator or turbine?
Power factor is a number between 0 and 1; it is never greater than one. (A power factor of 1.0 is also called "unity power factor.")
Power factor can be leading or lagging. At the generator, a lagging power factor means the generator is "producing" reactive power and supplying it to the load. At the generator a leading power factor means the generator is "consuming" reactive power from the grid.
Power factor is generally a measure of the efficiency of the production of AC power. It takes into account both the real power (Watts; kW; MW) and the reactive power. In general, power producers DO NOT get paid for producing reactive power (or for consuming it, either!). So, a power factor of 0.80 would say that real power being produced by the generator is only 80% of what it could be, because 20% of the energy is being used to produce real power. A power factor of 0.95 would mean that the real power being produced by the generator is 95% of what it could be, because 5% of the energy is being used to produce real power.
Synchronous generators have a power factor rating, and many people mistakenly believe that the generators should be operated at the power factor rating--but that's not being efficient, and unless the power producer is getting paid for producing reactive current it's also a waste of fuel, since 20% of the energy that's being produced by the generator when operating at a power factor of 0.80 is going into producing power the plant is not getting paid for.
The power factor rating value just states the maximum recommended reactive power that the generator is capable of producing while still maintaining proper generator cooling (because reactive current flowing in the generator produces additional heat in the generator which must be removed). (The generator rating for real power is also a measure of the amount of real power that can be produced by the generator while still maintaining proper cooling in the generator for long life.)
Without understanding why the expert made the recommendation it's very difficult to say if the recommendation was good or bad. Some power producers are required to produce power at a certain rate; most are not. It all depends on the situation, and some are very unique.
In general, unless the power producer is getting paid to produce (or consume) reactive current (which some ARE), it's best to operate a synchronous generator as close to unit power factor (1.0) as possible. This is for generators connected to a grid supplying lots of homes and businesses and factories that all share in the production of both real- and reactive power.
If the power plant is producing power for an isolated load (such as a cement plant or a steel plant or a refinery) then there could well be a need for producing reactive current IN ADDITION to real power. And, we don't know enough about the power plant and the load it is powering in question to say what's required or not.
The above discussion is for lagging reactive power, and lagging power factors, from the generator's perspective. Leading power factors are not generally encountered for most synchronous generators (but there are exceptions). Excessive leading power factor operation can be damaging to the generator and/or the prime mover and the coupling between them. EVERY synchronous generator is supplied with a reactive capability curve, sometimes called a "D" curve because of it's shape. It defines the limits of operation for both real and reactive power, and usually, the curve sets a very low limit on the amount of leading reactive power that can be produced to protect the generator and its prime mover and couplings. Every operator should have immediate access to the reactive capability curve for the generator(s) at their plant, and refer to them as necessary (which is usually when things are at some emergency point, and the curves are most valuable).
If you want to know what the effects of operating the synchronous generator in question at a power factor of 0.80 are you need to consult the reactive capability curve to see if the generator can safely support the high reactive current at the load (real power) being produced. As long as the total power is inside the curve, then the generator is probably safe. (There may be several curves--each one represents the temperature of the medium being used to cool the generator, usually air, but maybe hydrogen, or some other gas. So, it's important to choose the right curve based on current operating conditions, which can change with ambient temperature changes throughout the year.)
Hope this helps!!!