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Reactive power

Reactive power

Reactive power

Hi.Can someone please help explain how it is that in a power system it is possible to have real and reactive power moving in opposite directions? Or is this just to do with nomenclature and ways of describing leading and lagging power factor?
Also how exactly does a generator exporting with a leading or lagging power factor affect how its impact on the voltage of the network, ie a lagging pf can support the voltage on a network?


RE: Reactive power

I promised myself never to use the bear/froth analogy. But it can't be helped.
Bear can go down the throat. But, as it may happen, it can also go the other way. cheers

Actually, is is all about phase angles. Positive flow: voltage and angle are both at 0 degrees. Negative flow: voltage and current are at 180 degrees angles. Reactive Power: voltage and current are 90 or 270 degrees apart. Direction is a definition thing, but mostly consuming is 90 degrees and producing is -90 (or 270) degrees apart.

Gunnar Englund
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Reactive power

Watts flow to where they are needed, from generator to load. Vars flow from high voltage to low voltage. A generator operating at 95% effective voltage will suck vars from the system and depress voltage in the area. At 105% voltage the generator will supply vars to the system and raise the system local voltage. (Assumes the system is at 100%).

Like Skogsgurra implied, at any location in the power system the operating point can be plotted on the MW/MVAr diagram with MW "out" on the positive X axis and MVAr's out on the positive Y axis. A line from the origin to the operating point will have a length = MVA and the cosine of the angle from the X axis to the line will be the power factor. MW are = cos angle, MVAR = sin angle. As the operating point goes quadrant to quadrant on the plot the sign changes and MW or MVAR go the opposite direction.

Imagine that curve on an Etch-A-Sketch, the old kid's toy that draws lines by turning the left knob to drive the pen vertical, the right knob for horizontal. Label the X knob "Throttle or Fuel (MW) " and the Y knob "Voltage or Exciter (MVAR)".

If you open up the throttle on the generator's turbine or engine, more MW's go "out" to the load and your operating point is in the fight half of the plot (+MW). Raise voltage or excitation and your pushing MVArs in the upper half of the plot. The controls for MW & MVAR (X & Y position)only have minor interactions so the two flows can be independently controlled.

The Etch-A-Sketch analogy starts getting complicated when transmission lines and other elements are added. (The analogy does help explain why power factor control is so difficult - try to draw a straight diagonal line by turning two knobs).

RE: Reactive power

Real power is power that is actually doing work. Reactive power is just magnetic fields around your cables and conductors increase and decreasing 120 time a second for a 60 hz system. No work can be done with reactive power and you can't move real power without generated magnetic fields. When you inject reactive power into the system, you are sort of paying the tolls to move real power.

RE: Reactive power

Reactive loads cause the current to get out of step or phase with the voltage.
Power factor and reactive power are imaginary concepts that may be used to describe and calculate the effects of the current being out of phase with the voltage.
The use of power factor and reactive power saves quite a bit of calculation. You can easily use the power factor to find the phase angle and use the phase angle to calculate the power factor.

"Why not the best?"
Jimmy Carter

RE: Reactive power

Ok. Thank you for all the replies. Please correct me in anything that I say below.

If a generator exports with unity power factor onto a network, if the power factor of the network is 0.95 for example, then even though the generator is exporting at unity power factor the current will lag the voltage because the inductive network will create a phase shift between the voltage and the current.

For a generator to export on to the network it needs to operate at a voltage higher than the network voltage is this correct? So by definition it will also be exporting VARs as it is operating at a higher voltage than the network?

I feel like the whole import/export of VARs does not make it easier to visualise or discuss, is it not better to view it as, there is the voltage waveform and the current could leading or lagging, if lagging then the load is inductive,if leading the load is capacitive assuming the the power is exported at unity pf?

Just trying to understand it all.

RE: Reactive power

The load determines the power factor.
The generator supplies what the load demands.
When a single generator supplies the network then:
If the network power factor is 0.95 then the generator power factor will be 0.95
The generator does not operate at a higher voltage that the network.
If you raise the voltage of the generator then the network voltage will rise.

If you have generators in parallel things change a little.
Changing the voltage of individual generators will change the sharing of the VARs between the generators.
The load will always determine the power factor.
If you raise the excitation of one generator, that will cause that generator to supply more than its equal share of VARs, but the sum of the VARs supplied by all generators will equal the VARs demanded by the load.
When generators are running in parallel the action of some of the control adjustments is completely different than when a generator is islanded.
You should first understand whether you are concerned with:
Islanded generation.
Parallel generation.
Distributed generation.
A combination of parallel and distributed generation.
You may get different answers depending on the mode of operation.

"Why not the best?"
Jimmy Carter

RE: Reactive power


What about inverter based generators that have a wide range of power factors that they can actively control?

Regarding the voltages of the network and the voltage of the generator, I am referring to a network with multiple sources, if a given generator is operating at the same voltage then there is no potential difference how will it export on to the network?


RE: Reactive power

Real power flow is a function of the sin of the difference in rotor angle. If your rotor angle leads by a little you export a little. If your rotor angle is more you export more. The power is also proportional to the product of the sending and receiving voltages, you can export real power from a lower voltage to a higher voltage.

Reactive power, on the other hand, is a function of the difference in voltages.

RE: Reactive power

Most of it happens automagically and one doesn't need to think much about it.

I had to when I was designing protection for power lines and also reverse power flow relays.
But it is very simple, actually. It is when you try to apply non-Electric phenomena to it that it may become more difficult. All you need is to sit down and draw a single-phase system with its voltage and current Arrows. Change the phase-angle (all 360 degrees) and see what the resulting Active Power and reactive Power and their direction is.

Do that first. It may take some time to get the feel of it. Then use exactly the same diagram, but with Three systems 120 degrees apart, to create a Three-phase system and observe that Power and reactive Power now is Three times higher. Then, as a final step, use line-line voltage to get the sqrt(3) factor instead of factor 3.

There is no "Ceasar Road" to understanding. You simply have to do it yourself.

Gunnar Englund
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Reactive power


What about inverter based generators that have a wide range of power factors that they can actively control?

Such generating sites are often constrained by the AHJ to have active power factor control in place that maintains the PF at the POCC at unity so as to not burden the adjacent grid with unnecessary reactive load.


"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

RE: Reactive power

Will the generator voltage be different from the network voltage? Depends on where the network voltage is measured. The network voltage at the generator connection will equal the generator voltage. They are the same point; how can they be different voltages? The network voltage measured somewhere else will be different from the generator voltage, based on the current times the reactance to that other point. Generally, with reactive power flow from the generator, the voltage magnitude will be higher at the generator. You can visualize this as the inductive current flowing through the line impedance causing a voltage drop. Or you can visualize this as the higher generator voltage causing a reactive power flow.

RE: Reactive power

Remember, we are talking about names given to the solution to formulas.
All that is happening is that the current waveform is displaced from the voltage waveform. (The cosine of the angle of displacement is called the "Power Factor".
Exporting reactive power?
This is adjusting or correcting the phase angle of the offset between the voltage waveform and the current waveform.
Power Factor
The cosine of the angle by which the voltage and the current waveform differ.

Exporting VARs is changing the angle between the voltage and the current, most often in a favourable direction.
Importing VARs is changing the angle between the voltage and the current, most often in an unfavourable direction.

The capacity of a transmission line is often limited by the amount of voltage drop that the online tap changers can correct.
On a long transmission line, a large part of the voltage drop may be reactive, caused by the inductive reactance of the line itself.
In effect, the transmission line is a series reactance.
If this voltage drop can be compensated, the capacity of the transmission line may be increased.
This may be done at the receiving end, or by several installations distributed along the line.
The correction may be done by capacitors, synchronous condensers, or by over over-excited generators.
Also, a poor power factor load will cause excess reactive voltage drop on the transmission line.

I have seen capacitors used in two series capacitor stations spaced along a long 500 kV transmission line.
I have seen a large mine mill that used large synchronous condensers to correct the power factor of the mill, with the possibility of over-correcting the power factor of the mill to reduce the voltage drop of the transmission line feeding the mill.
Note: Over-correcting will affect the power factor penalties and would only be done with the consent of the supply authority.
The most interesting correction was a large city fed by a long transmission line. The city was originally supplied by diesel generators. When the transmission line was built, the generation plant was mothballed.
Many years later, the growing city reached the limit of the capacity of the transmission line due to voltage drop issues.
The diesel plant was re-commissioned and put online.
The generators produced very little real power. This is controlled by the throttle position of the prime movers. The generators were over-excited to compensate for part of the transmission line voltage drop. An added advantage to correcting the phase angle of the transmission was a reduced transmission line current and the associated reduction in transmission line losses.
It would not be unusual for the voltage at the load end of the line to be higher than the voltage at the supply end of the line.

"Why not the best?"
Jimmy Carter

RE: Reactive power

Hi Bill/Waross
I'm surprised those diesels were ok with that. Most conventional diesels don't like prolonged running at low load (and running at higher load means expensive diesel fuel rather than presumably cheaper generation from the far end of the line). Could they not have removed the engines & used a pony motor to get them up to speed, or put a clutch so they engine could serve only for starting? The clutch is done on large, gas turbine based synchronous condensers.

RE: Reactive power

Good points John.
Not much money available in the poorest country in Central America.
They used what they had.
These were single bearing generators. They don't turn well when not connected to the engine.
I have no idea if they are still in use.

"Why not the best?"
Jimmy Carter

RE: Reactive power

One thing to keep in mind to with the transmission lines waross is talking about is that the line can act as reactive or capacitive depending if it is loaded over or under its SIL rating. There is positive and negative sequence capacitance between the three phases on a transmission line. If the loading is above the SIL rating, the vars produced by the shunt capacitance is consumed by the vars needed by the series inductance and addition vars flow in from the system. When the loading is below the SIL, the vars produced by this shunt capacitance actually leaks into the system and raises the system voltages. Usually you don't care about this but sometimes it is enough to cause problems and shunt reactors are installed at the end of these long segments to consume vars to prevent the system voltages from rising above acceptable levels under low loading conditions. Sometimes, reactors are needed just for open ended conditions, zero load. So, basically the transmission line consumes vars through its series inductance (I^2*Xl) and produces vars through shunt capacitance between the phases (V^2/Xc) and will import or export vars into the system depending on its loading.

RE: Reactive power

^^^ My nominee for post of the year. Best and most simple explanation I've heard thus far smile

RE: Reactive power

Another example is an induction generator.

The induction generator exports real power but it needs an external excitation to be able to operate. Thus, it exports watts but imports vars.

RE: Reactive power

Why does voltage drop as the reactive load is increased?

RE: Reactive power

You will see this if you draw the phasor diagrams.

Let's just model your transmission line as an inductor with an impedence of Xl*i.

Zt = Transmission line impedence = Xl*i= 1<90 deg pu. Completely reactive transmission line.

Vs = source voltage. = 1<0 deg pu for this example.

Il = load current. = 0.1<-90 deg pu for this example. A completely reactive load. Current lagging voltage by full 90 deg at 0.1 pu.

Vl = Voltage at load = Vs - Il*Zt = 1<0 - (0.1<-90)*(1<90) = 0.9 < 0 deg pu.

If you draw you out that phasor diagram for Vl = Vs-Il*Zt, what you will see is that the lagging current puts the voltage drop more inline with the source voltage resulting in a greater magnitude drop. If the load had been purely resistive, the voltage drop across the transmission line would have been 90 degree out of phase with the source voltage and the voltage drop would have been magnitudely less. Let's say that load was capacitive, like a capacitor bank, the voltage drop would be 180 degrees out of phase with the source and the voltage would actually be higher at the cap bank than at the source. The voltage drop will always be the greatest when the angle of the current is equal and opposite of the angle of the transmission line impedance. It just happens to be that transmission lines generally are very reactive. This is also why the voltage drop across a transformer depends on the power factor of the load.

To further answer your question of real and imaginary power flows, the angle differences between buses tends to be small. This allows the power flow equations to be decoupled for iterative power flow solution methods. When you decouple the equations, vars flow from high voltage to low voltage (on a pu basis) and real power flows based on voltage angle differences between buses. If you ever use do transmission studies with PSS/, Powerworld, or the like, it becomes very obvious that capacitor banks hardly impact real power flows and you need real power generation or load to change real power flows, excluding outages.

Look under 'Power-flow problem formulation' to see how the equations can be decoupled

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