Effect of negative and positive reactive power on the generator 2

[RASitepu]

Dear all,
can you please explain to me about the reactive power, especially on what it does to the generator when the value is positive or negative. And also about the capacitive and inductive load.
Thanks alot.

 

[krisys]

The flow of inductive power is normally opposite in the case of loads and generators. For the generators, if the generator is supplying the inductive power, it is considered to be positive. In the case of loads, if the load is absorbing the inductive power, it is considered to be positive flow. It is only the convention. So when the generator is supplying the reactive power, the terminal voltage tend to drop. When the generator is absorbing the reactive power, the terminal voltage tend to rise.

Initially, I found it a little confusing to understand. Now I am fine with this convention.

 

[Power0020]

well, so if I will run the generator as a condenser, it will be overexcited supplying vars to grid (working as a capacitor), but, if the generator is supplying a capacitive load, it will be absorbing power, must be underexicted then.

the photo here says it all.

 

[waross]

With an islanded set you don't have control over the VARs. The load determines the VARs.
A capacitive load supplies extra excitation and tends to drive the voltage up. The AVR will cut back its output to compensate for this.
An inductive load tends to draw the voltage down. The AVR will increase its output to compensate for this.
With a paralleled set, the load still determines the total VARs but you may adjust the VAR sharing between sets by adjusting the excitation.
Generally performance is best when all sets are run at the same PF.
There are important exceptions.
I have seen diesel sets run at the end of a long heavily loaded transmission line run with very little kW output to save fuel, but heavily overexcited to supply the VAR losses in the transmission line and raise the voltage at the terminal end of the line.
The capacity of many transmission lines is limited not by the ampacity of the line but by the ability of the On Load Tap Changers to compensate for the voltage drop in the transmission line.
By supplying VARs at the terminal end of the line to compensate or offset the reactive part of the line losses, the capacity of the line may be increased.
One of the limits on generator output is the heating of the stator by I[sup]2[/sup]R losses. The more reactive current in the stator the less capacity for real power or kW, until the set reaches the rotor heating limit.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[HamburgerHelper]

The effect is that it has to operate at a different power output to not overheat the rotor (for putting vars into the system) or the stator (for absorbing vars). How much of a difference this makes depends on the MVAR vs MW curves of the generator. Some generator owners complain about needing to support the system voltage at the expense of MW generation. So, some ISOs have items in place to compensate generator owners for supplying vars.

For stability, generators are locked my tightly to the system when they are supply vars, which means they are generating at a high voltage.

 

[dpc]

The excitation of the generator controls its output voltage and hence its reactive power. By convention, positive var flow in power system represents inductive (lagging) vars. For ANY machine: It is LAGGING when real power and vars are flowing in same direction. It is LEADING when real power and vars are in opposite directions. Overexcited generator is LAGGING. Underexcited generator is LEADING.

Cheers,

Dave

 

[ScottyUK]

There is a lower limit to the leading vars a generator can absorb. Have a look at capability charts. A while ago I uploaded a superb reference paper on the co9nstruciton of capability digrams which was pubslihed many years ago by the CEGB.

Wolf posted a couple of good links in thread238-255286 which are still alive, and the one I mentioned above is available at

http://files.engineering.com/getfil...240&file=CEGB_Generator_Performance_Chart.pdf