Back charging of a transformer 3

[vipineng]

Hello,
I have an issue with my transformer charging.Transformer ratings are 110 MVA,132/11 kV Three phase.This transformer has to be commissioned in a short period .But the problem is there is no 132 kV supply line from the switchyard which is in under construction.My question is ,can we charge the transformer by 11kV?....Please provide valuable suggestions.
.I heard about back charging concept.But i don't know the process.

 

[waross]

The energization surge may be 25 times rated current. You may wish to review the expected reaction of the 11 kV system to the energization surge.

Delta:wye transformers may present issues;
When back-fed, these become wye:delta transformers.
In the case where your 11 kV line is a four wire connection, the neutral connection to the transformer should be opened.
If a wye:delta transformer is energized with the wye neutral point connected to the supply system, then any inequality of applied voltages or supply phase angle errors will cause circulating currents. The voltage difference (or the voltage difference plotted from a phase angle error) will cause a circulating current that will be limited by three times the transformer impedance.
If you are testing the transformer, then any circulating current may cause errors in your benchmark measurements.
Circulating currents will be accompanied by zero sequence currents in the neutral.
If your 11 kV system is well balanced you may not have any issues, but is well to be aware of the possibility.

When the wye point neutral is left floating there will be no circulating currents.
However, leaving the neutral open may result in over-voltage switching transients.





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

 

[vipineng]

Thanks ...

My transformer is a Wye:wye transformer with HV neutral,LV1 &LV2 neutral.If i have to feed 11 kV ,then which point(LV1 or LV2) should be connected and all the neutrals should be opened in condition?..

 

[waross]

The circulating current issue only arises with wye:delta transformers.
Your only issue may be the inrush current. It will probably be OK
Back charging is applying rated secondary voltage to the secondary terminals. Your secondary switchgear should be adequate to handle the fault current. It should be able to handle the inrush. There may be an objectionable voltage dip on the 11 kV circuit due to the inrush current.


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

 

[prc]

Back charging of generator transformer(YNd connection) is usual in Power Stations and protocol is well established. In your case, the problem is if you are using a 10 MVA transformer to energise 110 MVA unit it can sometimes be a bit tricky. The inrush current of 110 MVA will be 5-8 times full load current. This will be seen as 50 to 80 times full load current as far as 10 MVA unit is concerned. No transformer will with stand such over currents. In reality, such a scenario may not happen. The high inrush in 10 MVA will dip secondary voltage coming on 110 MVA unit and that will mitigate the inrush current. Better to have controlled switching. The energized transformer must be on highest voltage tap (ie maximum turns with out part of winding floating) to avoid part winding resonance. Some high voltage GSUs had failed from such part winding resonance during switching in.

 

[MatthewDB]

ack charging of generator transformer(YNd connection) is usual in Power Stations and protocol is well established.

I'm guessing you're not in North America? I haven't seen a scheme of energizing a GSU from the low side by closing a breaker. For steam TG's or hydro, I've always seen the transformer brought up with the generator. When there are multiple units on one GSU, the first unit online will bring it up and the pair is syncronized on the high side. For gas turbines, they'll energize from the grid.

I've also never seen taps on a GSU. The generator runs off nominal of the grid is off nominal. The UAT tap changer keeps the voltage to auxiliaries in specification.

 

[prc]

I am not in North America. I do agree that the tap range in GSU shall be minimum or nil. Unfortunately in many regions, this is not followed. Some years back I had to handle a failure in a 600MVA 420KV GSU bank during first back charging from HV side with LV open. This GSU had +-10 % (total 20%) DETC tap range on HV side. Charging was done on rated tap with 10 % winding floating. Failure was from part winding resonance in floating tap winding. Utility later changed tap rage to total of 10 %. It was mandated that back charging shall be at highest tap ( to avoid floating of winding + minimize inrush current) with LA & capacitor of generator connected to LV side of transformer( to take care of excessive transferred surges)

Back charging of GSU will be necessary in following cases:
1) In thermal and hydro - the power station (generator + turbine) work will not be complete, but the project managers want to ensure that GSU is in sound condition. If they wait for PS work completion and when they energise from LV side and if they meet with a failure in GSU, the whole project will get delayed.
2) Where ever a generator circuit breaker is used.
3) In pumped storage plants.

 

[edison123]

prc - lps

Muthu

http://www.edison.co.in

 

[prc]

edison123, Will you be kind enough to tell me what is lps? I know only litre per second !

 

[edison123]

prc - little pink star

Muthu

http://www.edison.co.in

 

[prc]

Thank you Edison !!!

 

[prc]

Engineers face with many issues during the first charging of large transformers, including back charging. Sometime back,I listed up these as per attached paper. May be useful to some of you.

 

[waross]

Thanks for the paper, prc.
All of our replies have been to do with full voltage and frequency energization.
Another option for Generator Step-up Units may be a rented generator and simultaneous ramped voltage and frequency. The generator need only be large enough to supply the magnetizing current.
This will not be a complete test of transformers that are normally energized by applying full voltage.


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

 

[crshears]

From an instruction in force on this topic where I am employed [company identification redacted]:

1. For planned work, Operating Planning shall confirm:
• That there is no protection blind spot between the transformer HV winding and the HV disconnect switch, and
• That the Line Backup or equivalent protection is in-service prior to approving back-energizing.
• If adequate protection within the blind spot cannot be confirmed, then the operating procedure in step 3 below must be followed.
2. For unplanned work, the Control Room must follow the procedure in step 3 below.
3. If adequate protection within the blind spot cannot be confirmed, then to minimize the risk associated with back-energizing a Dual Element Spot Network [DESN] transformer, the operating procedure below must be followed:
a. Removing a transformer from service is an acceptable risk provided if it is performed promptly (i.e. immediately after opening the HV switch, open the LV switch/breaker).
b. When returning the transformer to service, close the High Voltage Transformer disconnect switch. (if there is a fault between the transformer HV winding and the HV transformer switch, then the line protection from the terminal(s) will clear the fault first, with the LV switch (or breaker) already in the open position).
c. If no fault present, open the HV switch.
d. Immediately close the transformer LV switch (or breaker) – back- energizing the transformer.
e. Immediately close the transformer HV switch.
Back-energizing is defined as placing the transformer on potential from the secondary side with the transformer’s primary disconnect switch in the open position. This process has the potential to result in a protection blind spot on the bus section between the transformer and the primary disconnect.
2.2 BLOCKING LINE BACKUP PROTECTIONS
2.2.1 PLANNED OUTAGE
When a transformer is removed from service for planned work or as part of a switching plan and the planned outage window extends beyond 2 hours, the Line Backup Protection (both phase and ground) shall be blocked on the companion transformer.
2.2.2 UNPLANNED OUTAGE
When a transformer is forced from service or is automatically removed from service, normal dispatch procedures shall be implemented to ascertain the readiness of the unit. If the transformer is required to remain out of service for longer than 2 hours to affect repairs then the Line Backup Protection shall be blocked on the companion transformer.
2.2.3 RETURN TO SERVICE
When the transformer returns to service, the Line Backup Protection (both phase and ground) on the companion transformer shall be immediately restored to normal.

Hope this helps.

CR

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

 

[davidbeach]

I think your procedure is for a rather much smaller transformer; something in the up to 2MVA realm with a 15kV class high side and a less than 600V low side. I think that the OP has a somewhat different problem to resolve. That reads like a procedure for a area (grid) network or spot network transformer in an urban core area.

 

[crshears]

Went back to the source instruction; used for transformers up to 125 MVA with 115 and 230 kV primaries, secondaries @ 14, 28 and 44 kV.

CR

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

 

[davidbeach]

Ok, then you use certain terms very differently than I'm used to. When you said "Dual Element Spot Network [DESN] transformer" I pictured one, very specific thing, that doesn't match the range of sizes listed. My response was based on that picture.

So, another example of not being able to assume any definition. Oh, well.

I wonder how many (how few?) terms there are that everybody on this forum has the same working definition?

 

[prc]

As Davidbeach said, even though all of use the same English, it takes several styles! It took me several minutes to grasp anything out of the Instructions. Normally back charging from LV side creates less problems than from HV side, that too in GSUs where the voltage ratio is very high. Waross, the method suggested by you is difficult in power stations as the GSU voltage is 11 or 24 kV. I don't know commercial generators are with such out put voltages. Probably you have to put a distribution transformer in between. I don't know whether any body has tried it,

 

[crshears]

So, another example of not being able to assume any definition. Oh, well.

I commiserate, davidbeach; indeed there have been many, many threads within these fora that seem in my view to descend from discussion to debate to heated argument to flame-throwing, with one significant cause being differentiation in the way terminology is used. As you say, "Oh, well"; and as per my trailer text, I've just had another burr taken off...

CR

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