Definately ! 90 % of pq issues are about VQ only

Certainly not.

E.g. deviations of frequency being one of the most important indicators of PQ depend on active powers balance in system not on voltage.

Besides distortions of sine-wave form (appearance of higher harmonics) take place mostly due to consumers parameters i.e. distortion of voltage occurs via distortion of current. In other words quality of voltage may depend on current quality.

I think there may be shown other reasons for not to equalize Power quality and Voltage quality.

certainly not. Power Quality is not only related to voltage but also other parameters like frequency. Please see various power quality problems occurring at the distribution side of power system, then you will be find the difference  

The answer is no. Indeed, the quality of the voltage depends on the connected consumer (load). It makes no sense to speak only about Power quality, without analyzing the consumer.

I agree with all aformentioned answers. Power Quality does NOT only depend on voltage stability; you also need to look at system frequency, harmonics etc. On top of that we should also look at reliability which is interconnected.

Anyways, I just want to make one final commend on something Andrii Goroshko said; Voltage stability does not only depend on the connected loads but also on the generated power. If generated power can follow the demand and the transformers are oversized then voltage will always be stable.

Power quality is such a big area and can not be analyzed with just one sentence. Everything is interconnect: RPM off generator, generated power, frequency, voltage, transmission lines loading, consumer load etc.......

I was going through a book ELECTRICAL POWER SYSTEM QUALITY by Roger C. Dugan and i found this information about PQ = VQ. please see this page (snapshot/) from book.

I see his point of view. Reading the short passage, I see that the author indeed takes into consideration everything we all have mentioned earlier. However, in his view Harmonics, Overload, Lightning, etc cause voltage disturbances in pure sinusoidal voltage waveform. Therefore, power quality and voltage quality are the same.

In a book it might be ok to say that because you have several pages afterwards to explain your thoughts; as he did. This also works as a catching phrase to draw your audience attention, as you did with ours :) some will agree and some will disagree.

Bottom line there is still something that bothers me about it. It would have been more complete for me if it mentioned reliability as well; Power Quality Reliability related to Voltage Stability. And then explain all sources of disturbances and the various techniques / solutions / practices.

We can  not restrict power quality only for voltage.

It also includes loss, current  and power factor 

Definately ! 90 % of pq issues are about VQ only