I work in a hydroelectric generating station that feeds into an HVDC station that is about 30km away. The HVDC station converts our AC into DC for transmission.

We are often required to increase/decrease our inductive VAR output, but I'm not sure why. Surely there is a reason that we are asked to increase or decrease the VAR output. I understand the purpose of power factor correction when it comes to feeding customer loads, but this has me lost.

 

Hector C,

There is a saying in the power generation industry:

"Lagging VArs feed a Lagging load." (This is from the generator's perspective.)

The effect of VArs on any AC electrical power system is to shift the voltage and current sine waves out of phase with each other. Left unchecked, the efficiency of the electrical power distribution will be compromised.

Synchronous generators can "produce" VArs to offset the effect of VArs "required" or "consumed" by components of the electrical load--to restore the voltage and current sine waves closer to being "in phase" with each other, which improves the efficiency of power distribution.

Not knowing the exact particulars of your connection to the grid and the HVDC link and the nature of the HVDC link with respect to the generator, it's impossible to say much more than the above.

In substations, and on other parts of electrical grids, power factor correction capacitors are used to help with controlling the relationship between the voltage and current sine waves. On an electrical grid, VAr "flows" are relative to where they are being measured.

Hydroelectric generators are commonly used to produce VArs because they are good machines for doing so. The energy required to produce the VArs from the head of the water is "cheap", less expensive than steam or hydrocarbon fuels.

Hope this helps!

 

Hector... is your question, "How is the VAr component transmitted over the HVDC transmission system?" If so, the the answer is simple. It isn't!

Capacitors are used at the HVDC station offsetting the VAr, so that only power, i.e., Vdc x Adc is transmitted.

Sometimes it is easier to adjust the AC/DC conversion process by varying the VAr component rather than alter the capacitance.

Let us know if my presumption about your question is in error!

Regards, Phil Corso