Real Power Transferred to Receiving End

Hi

I have a question that I am having trouble doing. I will tell you how I have attempted it to help you know what I'm thinking. Don't solve it, just help me understand what I have to do or what it's asking. Thanks

It is part d) to a lossless long transmission line question and goes "what is the maximum amount of real power that can be transferred to the load at unity power factor if the required load (receiving end) voltage should always be greater than 0.9 p.u. For this case assume |Vs| = 500 kV.

First I thought about the theoretical maximum real power transfer equation:
Prmax = (VrVs/X`) - (AVr^2/x`)cos(θB - θA) but this seems independent of power factor and so I'm not sure if this is the right way to go considering they have specified the unity power factor in the question.

From an earlier part I am assuming that the |Vr| = 500 kV x 0.9. Also that the load is 800MW.

So then I used the equation Pr = (VsVr/B) sin(δ) to find the load angle.
then I plugged it into this equation, Pr = (VsVr/B)cos(θB - δ) - (AVr^2/B)cos(θB - θA) and the second term will become zero because it's a lossless line. And then I just go around in circle and find that it's 800MW, which is what I started with.

I'm missing some fundamental understanding here and how the 0.9 pu fits into this.

Can anybody help me?

David.