Generator unbalanced current 4

[bdf5526]

Hi guys,

We have a problem with the generator line current unbalanced at +6MW output and -2MWar.

Measured current at 14.5 kv (Gen Nom. Voltage); L1 = 216 A
L2 = 364 A
L3 = 286 Max and min current difference is about 150A. I am not sure if this is normal.

Clients are chasing us since the plant trip (two times when reacing 20 MW) on GSU Trafo differencial protection. The type of reley we are using here is the areva P633. I have inserted the print shot from the relay the one i got from my friend.

When I measured the current at the HV side of UAT Trafo the current are balanced. I measured unbalance current at the HV side of the step up trafo (225kv). Is it possible the grid could draw unbalance curred for what ever reason?

Thanks in advance guys.

 

[waross]

The unbalanced currents and the transformer differential trip may be indications of a transformer differential issue.
Unbalanced line voltages may result in unbalanced currents, A small voltage unbalance may result in a large current unbalance particularly with some wye/delta configurations and some phantom delta effects.
If you have a tap changer check for mechanical damage or misadjustment. A hanging tap may result in differential issues.

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

 

[bdf5526]

hi warros,

we have solved the diff prot. trip. (the CT polarity was GSUT, now is ok)

Our GSU Trafo is delta (14.5kv) on generator side and Star on grid side(225kv).

Please explain me what is hanging tap.

Thanks

 

[waross]

A tap changer has a section for each phase. Some times a mechanical failure may allow one phase to "Hang" on the same tap when the other two phases change to other tap positions.
Thanks for the update. Glad to hear it is good now.


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

 

[bdf5526]

the unbalance current still there even at 30%(50MW)load. we checked the tap changer and seems fine(only visual inspection). You guys have any idea about the sudden load decrease from 50MW till the generator trip on reverse power (low set)? We experienced this today.

We had few times the same fenomena, where hte active power decrease very fast from 50MW to about 10MW. The strange thing I dont understand is the the generator current only start decreasing after the power decrease to 15 MW. Any idea about this?..I am suspecting the Step up trafo tap changer.

 

[waross]

The two things that will cause a drop in generator output real power are a drop in energy input (fuel,steam, hydro) or a drop in load. As the load drops, the generator requires less excitation. It sounds as if you are generating more reactive current due to over excitation when the load drops.

Our GSU Trafo is delta (14.5kv) on generator side and Star on grid side(225kv).

Is your star point low resistance or solidly grounded?

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

 

[slavag]

Hi bdf5526.
It's seems not good.
Are you check unbalnce current on the generator terminal side, on the star point side? Where you see unbalnce -on the protective relay, meters?

About reverse power trip-only one thing, maybe setting is underpower protection and not reverse power.
Before one month, we check load sudden from 45MW to zero, no reverse power trip.


More info needed

Best Regards.
Slava

 

[slavag]

Sorry, some additionals.
What about voltage wiring?
Are you provide stability tests, sorry, but not seems.
Im strongly recommned back to standard tests.
1-ph-SC, 3-ph SC , voltages tests. In 3-ph tests ( one before GSU, one after, one after UAT ) check unbalnce/meas current on all CT's.
Best Regards.
Slava

 

[ScottyUK]

Regarding the sudden de-loading of the unit: look at the trend of the governor valve position demand from the turbine control system. What machine parameter caused the governor to close? My guess if that the unit was in isochronous mode or possibly a frequency correction mode, both of which can throw the load around as the unit tries to regulate grid frequency.


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If we learn from our mistakes I'm getting a great education!

 

[odlanor]

We have been experience with generator 100MA-13,8kV with CT at phase terminal side in bus duct with exit in a angle of 90 degree. Induced currents distort completely final measuring....

 

[ProtectionEngineer]

Please attach the P633 event and disturbance file if you have. I would like to see the disturbance waveform.

Thanks.

Alex

 

[bdf5526]

SOrry for the late reply,

Hi protection engineer,

I have attached the print screen of the Micom P633 in my fist post. Luckly we have solved the problem since the CT connection was wrong at the gen. step up trafo.

For the plant tripping on sudden deloading we have found the cause( hope). Plant trip on PLST since there is a loose connection on the HV breaker open feedback when in reality the breaker was closed. Looks like the open feedback come only fraction of second so the operater can see in the recorder. When the last time trip we found the feed back open and never went off, so we managed to traced it.

For the unbalanced current we still have no idea. I have measured from the CT closer to genorator and indication form the operator screen, bouth showing the same unbalanced current.

We skipped the short circuit test here. I shall checked the GSUT enutral current. GSUT solidly grounded.

Will come back with more output guys.

 

[waross]

In your first post you asked
"Is it possible the grid could draw unbalance curred for what ever reason?"
Yes under certain conditions.
If the grid is feeding a Wyeelta transformer with a return path for the neutral current from the primary wye point.
I have seen this often on distribution circuits in an area where four wire connections are used to the primary of wye:delta transformer banks.
A delta winding is a great equalizer. By virtue of the vector diagram and the closed delta connection, the vector sum of the voltages on a delta secondary must be zero.
Now if the phase to ground voltages are not equal and the neutral is connected to a return path on the primary of the wye:delta transformer, unequal voltages willbe induced in the delta. This cannot be. The result is that a heavy circulating current will flow in the delta. This current will cause voltage drops in the higher voltage phases and voltage rises in the lower voltage phases. This circulating current will be limited by the transformer impedance so a voltage unbalance in the order of the percent impedance of the transformer may cause full load current to circulate in an unloaded transformer.
This is easy to check. Put a clamp Ammeter on the grounding conductor of the transformer. A current that is significant in comparison to the normal transformer primary current may indicate a circulating current in the delta. Another sign is unusually high transformer temperatures.
[RED]CAUTION CAUTION CAUTION CAUTION CAUTION CAUTION [/RED]
LETHAL VOLTAGES MAY BE DEVELOPED IF THE GROUND CONNECTION BECOMES OPEN AT ANY TIME.
AVOID PHYSICAL STRESS ON THE GROUND CABLE THAT MAY CAUSE A BAD CONNECTION TO COME APART.
USE HIGH VOLTAGE PERSONAL PROTECTION EQUIPMENT.
A MINIMUM WILL BE INSULATED GLOVES.
CONSIDER A FULL FACE SHIELD AND COTTON CLOTHING.
USE A FLASH SUIT IF ONE IS AVAILABLE.
[RED]CAUTION CAUTION CAUTION CAUTION CAUTION CAUTION [/RED]
YOU HAVE MENTIONED SEVERAL INSTANCES OF POOR WORKMANSHIP.
PLEASE DON'T TAKE ANY CHANCES AROUND A TRANSFORMER GROUND ON A SITE WITH ISSUES WITH WORKMANSHIP.
WE DON'T WANT TO LOSE ANY FORUM MEMBERS.
[RED]CAUTION CAUTION CAUTION CAUTION CAUTION CAUTION [/RED]

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

 

[bdf5526]

Hi warros,

I totaly agree with you. The workmanship here is quite poor.
I am aware of danger working on GSUT when it is loading. I have to be carefull.

Thanks for your advise.

Thanks alot guys.

 

[bdf5526]

Hi warros,

I measured the current and I am scared. Neutral point of the Step up has a surge arrester rated for 132kv connected in parellel. No current via the surge arrester grounding when I measure. If I measure at the grounding of the Neutral point I have about 4 Amps (50Hz).

Transformer body is grounded in 3 points and the measured currents are
point one : 104.6 Amps
point two : 34.5 Amps
point three: 95 Amps

I believe this curculating current is not normal. What could be wrong?. The earthing grid?.. I measure at least 5 amps at steel structures (isolator, surge arrester, CVT steel structures) earthing cable inside area of the step up trafo.

At the point of measuring the circulating current at trafo, the Generator output currents (at 14.5kv) are:
6135 Amps: L1
6276 Amps: L2
6227 Amps: L3

The site is at north africa. Trafo temperature appears normal even at almost full load (below 80 degree C).

 

[ScottyUK]

Suggestion: Look for a problem with the isolated phase bus. There should not be any metallic connection between the main bus structure and the transformer body, but it's not exactly unusual to find an earth strap or a scaffold bar bridging between the bus structure and earthed steelwork. If there's an unintentional earth connection then some disturbingly large image currents can flow where they shouldn't be.

If the transformer is completely offline and isolated from all power then check the isolation between the core and the tank. There should be an accessible bond which provides a single point earth between the core and the tank - when you remove the bond you should get an open circuit with a 500V megger. It is likely to be behind an inspection cover located low on the tank, but could essentially be anywhere: refer to the OEM manual. Please don't do this test with the transformer energised. Don't even think about it.


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If we learn from our mistakes I'm getting a great education!

 

[Kiribanda]

bdf5526,

You have mentioned that your gen step transformer is WYE(225 kV) and DELTA(14.5 kV). Also you have said that N-point of the transformer is grounded through a 132 kV (225 kV/1.732)surge arrester.

From the above does it mean that your 225 kV side is ungrounded?

 

[pashwuee]

I have never seen surge arrester in GSU Transformer neutral . Surge arresters are always tapped from phases.Since Transformer is Wye on HV side no use of surge arrester in neutral. Regarding current flowing in all the metallic structures it seems strange , but check the resistance values of plant system earthing and equipment earthing.

As far as current in GSUT body is concerned , such magnitude of current shall not flow . Viewing varying currents in body earthing it is sure that your plant earthing is very poor. Check I differential at micom terminals , and I spill current at micom terminals , this will give you idea whether there is earth fault or not. chek settings also.

 

[ScottyUK]

I have never seen surge arrester in GSU Transformer neutral. Surge arresters are always tapped from phases.

Just because you have not seen it does not mean that it is never done. Some grid operators require that the neutral is equipped with a single pole circuit breaker and surge arrestor to allow the neutral to be disconnected from earth in order to limit the overall system earth fault level. Kuwait is one example of where this takes place.


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If we learn from our mistakes I'm getting a great education!