in electric power station we have posistive Mvar
and negative Mvar. I want to know how it decreases and increases. and how it generate? and what is its effect on the generator and the grid?


my email
aboforat_2005 @ yahoo.com

 

Try wikipedia.org for a generic definition of VAr, Volt-Ampere Reactive, or, reactive power.

There have been many posts to control.com about reactive power; probably the easiest way to search is to use kvar or mvar or var and go through some of the threads from the last couple of years or three.

Positive MVArs are "generated" ("produced") when excitation is increased above that required to make the generator terminal voltage equal to grid voltage. Sometimes, this is referred to as "over-excitation."

Negative MVArs are "generated" ("produced") when excitation is decreased below that required to make the generator terminal voltage equal to grid voltage. Sometimes this is referred to as "under-excitation."

In the power generation industry, most (but *not* all) generators are not made to be operated for extended periods of time in an under-excited condition (negative MVArs) because of unwanted heating which can damage the generator. The amount of negative MVArs the generator is capable of "producing" is also usually much less than the amount of positive MVArs.

The generator manufacturer usually provides some kind of "reactive capability" graph or curve which depicts how the generator can be operated with repect to negative MVArs, positive MVArs, and MW (megawatts). You should locate the graph or curve for your site and study it closely; write back if you have any questions.

After you've had a chance to search the control.com archives and read through some of the threads, write back for an further information or clarifications.

 

The worm has been taken out yet again, hehe... Here is what I think...

Actually the +ve or -ve VArs that you may see at the HMI shows you whether you are importing or exporting VArs. By right, all generators should be exporting VArs to the system as importing VArs from the system to the generator will make the generator run as an induction generator. This may, in my opinion, have severe effect on the life span of the generators due to the stresses on the windings.

You can look at whether the generator is running at lagging PF (exporting) or leading PF (importing) by looking at the capability curve of the generator.

As for the 2nd question, VArs are generated by exciting the rotor windings with DC current. This can be seen at the generator side where the DC current are injected to the slip rings of the rotor via carbon brushes (most of the case).
The more DC current injected, the more VARs will flow out from the generator until a certain limit to prevent overfluxing of the windings. This overfluxing is usually limited by the V/Hz relay.
The less DC current will result in a condition otherwise known as underexcited, whereby at a certain point where the VARs are actually taken from the system to the generators.

For the last question, I would like to make it simple because a lot of people will have different points of view regarding it. As to what I would like to say, from the power station point of view, VArs are exported to the system to maintain the voltage level of the grid and to satisfy the reactive power requirement of most of the induction machines on the grid.

 

VARS are controlled by adjusting the generator field voltage. Increasing the field voltage increases VARS and decreasing the field voltage decreases VARS. Basically you are changing the power factor of the generator. VARS are sometimes used to correct for variances in the type of load that your system is running (inductive or capacitive).

A high VAR loading will increase field winding temperature. Your generators should come with a capability curve that tells what VAR levels you can safely run depending on the temperature of the cooling media in the generator. Whether air or hydrogen cooled, the cooling capacity sets the ratings of the generator. You do not want to exceed the rated temperature of the windings due to the possibility of insulation breakdown. You can significantly reduce the life of your generator by exceeding the temperature rating. I've heard that as little as 10 degrees above the rated temperature will reduce the generator life by 50%.

High voltages lines are dependent on a particular voltage level to allow them to carry power to their end users. Dispatchers will call upon the power plants on their system to raise and lower VARS to adjust the high voltage line voltages (higher VARS = higher voltage). During high load times, they may want extra VARS to keep the line voltage higher allowing more power to be transmitted down the line to end users while reducing the line losses due to heat.

 

Mr Kalil,

You have been given very good response by CSA and SD. Up to the mark.

Turbine controls the MW(Active Power).

Reactive Power (MVAR) by the amount of excitation. Exporting more MVARs - increase excitation. Importing more MVARs - decreasing excitation. MVARs are produced by the magnetic loading in the AC system.

In a unit connected generator MVARs control the power factor. Lower power factor(for constant MW output, increase in MVARs) can seriously heat the transmission lines.

Lower excitation (under-excitation) causes stator end-winding heating and under-excitation with low active power has a possible chance of generator going out of synchronisation.

Higher excitation causes field winding heating.

This is just the brief of the explnation. As indicated by other authors, a lot other threads exist on MVAR related topics. They will provide a deeper insight in the topic.