Synchronous generators are rated in MVA where as when we refer to a power plant we refer to their MW rating? Why is this so?
Power plants capacity decided on the basis of how much active(useful) power(demand MW)could be supplied by the power plant to load area(turbine capacity), while the MVA decides the capacity of machines(generator/transformer) as the MVA=P +/- jQ, not only active power(P) but the supple of reactive power(Q) depending on power factor(P=W=VICos(phi), Q=VISin(phi))also, though Q is not the active(useful) power.
Because there are various generators, but turbine is single source (ie, turbine is a mechanical device which has nothing to do with reactive power). Hence, rating of turbine is in MW(active power) = power plant rating
In addition, MVA with a rated voltage of operation in kV, gives a direct indication of ampere based on which conductors are to be designed.
I agree with your answers. However, I would like to clarify a couple of things for students from different countries that might not be exposed to all terminologies.
1. The Power Plant has full access on the power factor. However, the power plant will generate as much power as the customer requires. So in the end the customer determines the power factor. This is why in many countries the Electricity Authority charges penalties for factories that do not have good power factor.
2. Terminology :
Complex Power, S is measure in VA. Complex Power has a magnitude that is called Apparent Power and a direction (angle). The angle is used to calculate the power factor.
Now, if we convert complex power from polar form (mag, direction) to rectangular (Real, Imaginary) then:
Real Part, is the Real or Average or Active Power, P measured in Watts
The imaginary part is the Reactive Power, Q measured in VARs. Q can be negative or positive according to the load.
3. I believe that MW is used so that it is easier to calculate the maximum amount of money the plant can generate. Electricity bills are always for the amount of consumed energy in kWh.
Thanks for the further clarification with the role of power factor, which differentiate MVA and MW, brought into picture. Poor power factor obviously draws attention for penalty, causing stress on the asset due to overloading as well deteriorating receiving-end voltage vis-a-vis incurring more losses. Ideally MW should be as close to MVA thus reducing the requirement of MVAR, the reactive power.
Generation units based on turbine capacity i.e MW where as Generator on MVA since it's type of and pf not known
VA is the rated 3 phase power before loss due to Power Factor of the generated line. W, on the other hand, is
the rated power after Power Factor is applied.
Considering that manufacturer had no idea of their customer's power factor, its best to use VA for stating the machine's Power Capacity.
As for Power Plants, since they're the ones supplying electricity for uses, it is important to know the losses due to Power Factor which is rated in W, which hence is the actual useable power ratings.