Excessive reactive power on a generator/transformer set 5

[PMACP]

Hi,

What could be the reason that leads a synchronous generator connected to a 15 kV network through a 0,4 kV/15 kV transformer to produce excessive reactive power?

This effect tends to occur during periods of reduced load on the MV network.

The generator must operate in a situation such that Q = 0.4 P. In fact, most of the time, the generator works on such regime. However, at certain periods, the reactive power generated goes far beyond the active power generated.

Some help?

 

[PMACP]

Hi

Some more interesting information in attach

 

[PMACP]

Hi again

Is that possible that the generator works with symetrical active and reactive power on each phase and than the transformer presents unbalanced reactive powers on each phase? Is that a evidence of saturation?

Thanks

 

[waross]

That's an interesting question. I'll have to think about it. If the voltage is high on one phase of the utility, and the transformer goes into saturation on that phase, the transformer will draw more magnetizing amps or VARs from the line. I'm not sure what you would see on the generator instrumentation.
If voltage is high on one phase of the utility side, then the generator will be under excited on that phase and will be importing VARs. The PU current could be higher than normal on the utility side with the difference or loses producing heat in the transformer.

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

 

[PMACP]

Hi:

Bill: "That's an interesting question. I'll have to think about it."

Thank you. I wait for your opinion.

Now, can you tell me if there is some problem resulting from connecting a measurement equipment to the voltage transformers on the HV side of the power transformer. I'm thing to connect my measurement equipment to the terminals of a power relay. Please see the attach.

If I want also measure the currents, may I put my measuremnt circuit in serie?

Thanks

 

[PMACP]

Hi,

I have now some data about MV voltages:

Time UAB UBC UAC
7:00 16026 16065 16126
8:00 16050 16066 16120
9:00 15837 15871 15924
10:00 15897 15913 15958
11:00 15805 15838 15912
12:00 15882 15898 15987
13:00 16018 16045 16113
14:00 15799 15829 15880
15:00 15880 15880 15978
16:00 15922 15937 16018
17:00 16026 16059 16101
18:00 15969 16005 16075
19:00 15885 15843 15952
20:00 15945 15987 16011
21:00 16084 16087 16186
22:00 16194 16191 16259
23:00 16072 16083 16165
0:00 16162 16201 16266

Note that the tap is regulated for 15375 V. It has Ucc=6%, and it is a ynD05 with 2000 kVa of power.

There are some unbalances on voltages on the HV side of the transformer. Is that sufficient to produce problems?

The voltage is permanently above the regulated voltage. Problems?

Thanks.

 

[nawao]

There are some errors in the data you earlier supplied; you cannot add the values as you did in the columns R, V and Z.
Definitely, the AVR is not doing its job at all.
Did you check the following setting procedure in the card M50FA400A?

SETTING PROCEDURE WHEN MACHINE IS
CONNECTED WITH THE MAINS
After connection to the mains, check the correct operation
of current transformer (TA2).
Rotate the potentiometer P1 clockwise: the reactive power
(when BRI is in VAR position) must increase, or the power
factor (lagging) must reduce. If there is a different
behaviour, reverse the wires from the current transformer
TA2 and check if it is connected to the correct phase.

 

[PMACP]

Nawao,

Hi. Can you, please, explain why I can't add the values? I measured the power per phase. So, The total reactive power (for instance) should be the sum of the reactive power of each phase. Where is my mistake?

Thanks

 

[PMACP]

Nawao,

I forguet to say that in last post. The AVR only works when the machine starts. After it is connected to the main grid, the AVR is stopped and the control is made by the cos Fi regulator. Sorry for forguet say you that.

This means that the voltage is not controlled, right?

 

[nawao]

Is not right. Both cards work together. The procedure I’m talking about corresponds to the reactive power/power factor regulator M50FA400A. The voltage must be always controlled

 

[PMACP]

nawao:

The guy that isntalled the system said to me that the AVR card was set to 400 V. Also he said to me that this card only is used in order to do the excitation of the generator before the paralell. After the paralell, the AVR card doesn't any contro (He said), and only the cos fi card acts.

I agreed with your opinion that both cards should act at same time... however the guy said that is not the case. So, in your opiniom is impossible the kind of control the guy told me?


Please, can you give me more details about why I can't add the values you refereed in your previous post? I measured the power per phase. So, The total reactive power (for instance) should be the sum of the reactive power of each phase. Where is my mistake?

 

[nawao]

Sorry, the data is correct. I mistake lines to phasors.
Regarding what the guy says; forget about it.
Just applie the settings procedure and let us know the results.

 

[PMACP]

Hi,

nawao, note that the card M50FA400A is regulated in order to control de power factor, not the reactive power directly. That is, the card trys to maintain a specific cos fi not a constant value of reactive power. So, when the voltage goes up the generated reactive power tends to increase and, as a consequence, the cos fi tends to decrease. The card should reduce the excitation of the generator in order to reduce the reactive power generation, right?

Therefore, the voltage should be reduced.

However, because there is the obligation of injecting a value of reactive power corresponding to 40% of the generated active power, the generator cannot work importing reactive power. So, it can't reduce the voltage levels on its terminals... That is, if the AVR was working, we can control the voltage level but it will occur that the reactive power goes from exported by the generator to imported... right?

 

[davidbeach]

No 400V generator tied into the larger system should be trying to do voltage control, it can't. If it tries, one of the symptoms is outrageous var flows. The voltage will be what the voltage will be and your generator can't do anything about it, but if it is in voltage control mode it will try to produce or absorb vars to bring the voltage into range.

It is also impossible to do both voltage control and power factor control simultaneously. Trying to do both can easily result in a situation where the voltage controller is trying to absorb vars while the power factor controller is trying to produce vars. Pick one.

 

[nawao]

"nawao, note that the card M50FA400A is regulated in order to control de power factor, not the reactive power directly"
The card has both possibilities; the switch BRI allows COSfi or VAR control. Read the attached document again.

 

[PMACP]

Hi,


nawao, I read the datasheet, and I know what you said. However, the card is regulated to cos fi control, and i need to know what happens on that situation before do some other control. On other and, davidbeach said (previous post) that is not p+ossible to control the voltage.... what you think anout that?

Thanks.

 

[nawao]

PMACP,
This procedure verifies if the card and the associated current transformer are working properly.
Seems to me that you are not familiar with this kind of problems. In this case ask assistance to a qualified technician and perform the procedure.

SETTING PROCEDURE WHEN MACHINE IS
CONNECTED WITH THE MAINS
After connection to the mains, check the correct operation
of current transformer (TA2).
Rotate the potentiometer P1 clockwise: the reactive power
(when BRI is in VAR position) must increase, or the power
factor (lagging) must reduce. If there is a different
behaviour, reverse the wires from the current transformer
TA2 and check if it is connected to the correct phase.

 

[PMACP]

nawao:

I did the procedure. The card appears responding as expected.

Note that the excessive reactive generation only occurs at some specific periods of the week. In fact it tends to occur at saturday near 12:00-14:00 when the voltage on the grid increases. During the rest of the time the system works well.

As well, it is important to stress that the reactive power tends to appear unbalanced and, sometime there is a phase importing vars and the others exporting it

Assuming that the card is operating correctly, which shoul be its behaviour when the voltage level of the grid increases? if the voltage increases, the reactive power generated tends to increase and, assuming a constant value of active power, the cos fi tends to decrease. So, the card will act in order to decrease the excitation of the machine in order to reduce reactive power generated. Correct? So, why that is not happening? In fact, when the voltage goes beside a specific value, the generated reactive power increases so much.

Is there some possibility of transformer saturation?

 

[waross]

As well, it is important to stress that the reactive power tends to appear unbalanced and, sometime there is a phase importing vars and the others exporting it

I expect that the same load shedding that is causing the grid voltage to rise is also causing the phase voltages to become unbalanced.
Why, good question. It may be a sticking voltage regulator,it may be a faulty capacitor switching mechanism or a faulty capacitor in a stepped capacitor bank, it may be a large phase to neutral load still in place on the grid, it may be a combination.
It has been shown how the voltage relationship between the generator voltage and the grid voltage influences the magnitude and direction of VAR flow. If the voltage relationship varies from phase to phase (due to unbalanced grid phase voltages), so also may VAR production and direction vary.

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