about reactive power on synchronous generators

I wonder if there is a reactive power when we have a resistive load in isolated operation how reactive power circulates,

If you have AC power, there is no reactive component when you have only a resistive load.

i mean if load has no a capacitive component how this reactive power goes and comes, also when we feed a d.c current to the excitation windings so what about the power consumption of this unit (excitation unit)reactive or active power?

Reactive load generally comes from motors and an other load that has a reactive component.

İn essence i want to learn the relation between excitation and reactive power of syn.generators. how a d.c current can create reactive power. any way thanks in advance my friends...

DC current does not have a reactive component. What makes you think it does.? What kind of problem are you having?

1. The DC current do not have any reactive power. The eractive power is the prerogative of AC.

2. What is a reactive power ?

When the voltage and current are not in phase (that means imagine the sinusoids) if the peak of the Voltage is shifted (in time) from the current, the corresponding shift results in the reactive power.

In case of vector diagram. if I(φ) is decomposed, in two 90^o components, in phase and perpendicular to V. You have I sinφ (at 90^o) and I cosφ (in phase)

The one in phase with V is the active component, and perpendicular to it is the reactive one.

(This can also be found by a bit tedious method ∫V Sin(ωt) Isin(ωt+φ) dt over one cycle)

3. At generator level, there is a pole rotating created by the DC current. The EMF is generated of course by the pole and moves along with this. When the current flows in the circuit, there is a magnetising current flows (or demagnetising one? flows, which tries to counter the magnetising current of the poles) to maintain the status quo.

4. This current may be a bit lagging the original DC poles, or exactly in front of it. This lag is what creates the reactive current.

5. If you increase the DC current (ie excitation) - you are increasing the strenght of the pole, and it will try to dra in the other pole nearer, and that is the compensation by field excitation.

This is in brief, and simple terms, But as usual the life is not so simple and the total system needs a bit of complex maths and more.

for some explanation on ac circuits please see my note http://cr4.globalspec.com/thread/17039#comment176148

Dear,

pl. study the equivalant vector circuit of a generator. You shall find the reactive component of the power.

It is synchronous reactance X which is used in the creating the magnetic field. this is of the winding it self.

What resistive portion you are talking about is on the load side, which can be any thing.

If it is pure resistance, no harm... generator works ideally, assuming no negative sequence currents.

If it is inductive, say motor, transformers , no harm, more stable.

If it is capacitive, definitely negative sequence power reverses the direction of power flow, that is if you connect purely the capacitance, generator will act like a motor. in result, instability occurs , leading to voltage hunting with very high voltages etc...

As others have replied, a steady state DC current has no "reactive" component. However, there is no such thing as a purely resistive load; they all have stray capacitance and inductance. These strays will play a part in the transient behavior of the current when it is switched on or off.

Hi Jinxnao,

Synchronous machines are doubly fed machines. Their behaviour is different when operated in isolation or when connected to grid. In general speed of rotation determines the frequency , magnitude of excitation current determines the ac voltage produced in the stator if machine is operated in isolation and it absorbs or produce vars if machine is connected to grid. In most cases machine connected to grid is weaker than grid hence not able to change the voltage but it changes the vars.

For your first question, if the load is resisitive it will not demand any vars, machine will operate at unity.Considering your machine operates in isolation,if you try to change the excitation current voltage will increase/decrease and vars will be zero at all times and pf will be unity.Only assumption here is that generator is connected to load by a negligible line lenght and line itself do not absorb or produce reactive power.If line produces or absorbs reactive power then we can say that total load on the generator is not resistive and machine will operate at leading or lagging pf depending on the type of line ( cable or overhead line).

For your second question it can be said that power consumption in the excitation winding is active . DC power is consumed in excitation winding,which is product of voltage and current.

For understanding the behaviour of synchronous machine please try to understand the phasor diagram of synchornous machine.That will make everything clear to you.

First, the phase terminals on the generator are your source and reference thereby your resistive load is your ONLY load. There is inductance in the Armature windings that are these phase terminals and with no other reference there can be no measurement.

Second, the DC power supplied to the generator "excites" the magnetic field, ideally a constant one, that the armature turns within and thereby generate AC power at the phase terminals. Controlling the excitation field will regulate the output voltage at the phase terminals.

Lastly, The frequecey of the generated AC is directly related to the rotational speed and the number of poles that make up the armature.

A simple explanation is that if the voltage of one generator is set higher, current flows from one generator to the other and this current is 90 degrees out of phase with the voltage. Actual power is generated with the voltage and current in phase, reactive power is the product of current and voltage 90 degrees out of phase.

1> If you have two or more AC generators connected together they are connected in parallel. As such they see the same load reactive or not

2> If all phases don't match when an AC generator is connected in Parallel with another or other(s) in the circuit then will try to synchronize and the instantaneous differential torque can be very distructive. Phase control circuits and contactors are worth knowing about in advance.