This discussion centers on the concepts of reactive power consumption and production within electrical systems, specifically addressing the relationship between instantaneous power and complex power. Participants clarify that reactive power is not consumed but "borrowed" and later "paid back," with complex power defined as S = V * I*, where P represents active power and Q represents reactive power. The conversation also highlights the role of power stations in generating reactive power to balance the grid, emphasizing that they provide reactive power rather than produce it.
PREREQUISITESElectrical engineers, power system analysts, and anyone involved in the management and optimization of electrical grids will benefit from this discussion.
That depends very much on the application. I have seen examples (CRT scanning) where the peak voltage and the peak current were both quite high - but never simultaneously.Passionate Eng said:And what is the relationship between instantaneous power and complex power?
In the framework of steady-state AC analysis, the OP's lingo is correct.Svein said:
- You do not consume reactive power, you "borrow" it and then "pays it back".
- You do not produce reactive power. You supply power, the load may be more or less reactive.
Assuming use of the passive sign convention, the complex power is given by:Passionate Eng said:Why does a load consume reactive power?
##Q## will, for instance, be negative for an ideal capacitor, i.e. it is said to produce reactive power.Passionate Eng said:How can we produce reactive power?
There is none. Complex power is defined in terms of phasors.Passionate Eng said:And what is the relationship between instantaneous power and complex power?
Svein said:
- You do not consume reactive power, you "borrow" it and then "pay it back".
- You do not produce reactive power. You supply power, the load may be more or less reactive.
Properly speaking: They introduce a reactive load in order to "balance the grid".William White said:Power stations generate reactive power to balance the grid
I am a mathematician - in my world there is no such thing as "everybody knows". A power station generates voltage and supplies current to the load. The phase of the load current is determined by the load, not by the generator.William White said:everybody knows what is meant by generating reactive power.
Svein said:I power station generates voltage and supplies current to the load. The phase of the load current is determined by the load, not by the generator.
William White said:like I said, you are being picky; everybody knows what is meant by generating reactive power.
Dont be pedantic - it does not help.
Power stations generate power. Some of it is VARS, some is VAs. The ratio can be changed by the power station manager.I understood the OPs question, and the manager of the power station would too.
That's not right. In a grid ( connected to several power stations ) you can control the production of reactive power in a single power station by over/under-magnetizing the generator, thereby distributing the production/consumption amongst the power stations in a way, that leads to minimum conducting power losses in power lines.Svein said:The phase of the load current is determined by the load, not by the generator.
Yes. I do not quite see how you control the current phase in that way, unless you introduce an output impedance in the generator that is (partly) reactive. What staggers me, however, is the problem of doing any control at all on a high voltage transfer line (think 700kV)...Hesch said:That's not right. In a grid ( connected to several power stations ) you can control the production of reactive power in a single power station by over/under-magnetizing the generator, thereby distributing the production/consumption amongst the power stations in a way, that leads to minimum conducting power losses in power lines.
Say you have this 700kV transfer line, and you magnetize a single connected generator to yield 800kV ( transformed ), the generator cannot change the voltage of the 700kV ( too many competiting power stations ), so the 800kV ( which is there as for the single power station ) must be vectorially folded, so that the sum of the folded vectors are kept within a radius of 700kV from 0V, and when you fold the 800kV vector, a transverse vector must be created, resulting in a reactive power.Svein said:I do not quite see how you control the current phase in that way, unless you introduce an output impedance in the generator that is (partly) reactive.
Well, the 700kV and the power is created by power stations. They ( and the consumers ) are the ones to control it.Svein said:What staggers me, however, is the problem of doing any control at all on a high voltage transfer line (think 700kV)...
Yes, or you connect a huge synchronous motor to the transmission line, doing no mechanical work, just over-magnetize it, and it will produce adjustable reactive power, because it acts as an adjustable capacitive load.Windadct said:And the generator are NOT the only source of reactive power - they can and do add significant bulk capacitance at the distribution level, preferably before reaching the transmission level where the added reactive current decreases the capacity of the system.