Allowable number of startups for a 132KV transformer 2

[Sofistioelevib]

Hello to everyone,
i have a transformer ABB ADDA, 35MVA, ONAN TYPE, pri:138KV sec:11KV with only primary and secondary winding oil radiant cooling system.
132KV line is a star connection and secondary is delta connection.
The connection is 132KV line >> trafo >> generator 11KV
Consider generator disconnected so no load on transformer.
Going to energize transformer from 132KV and consider the magnetization current, how many energization i can make in a day?how many in hour?
Someone says no problem to energize many time, some other 2 energization in a day is too much. Who have right? and why?
Thank you

 

[marks1080]

I've never heard of a hard limit. Doesn't sound like a particularly large unit. I think you're fine as long as you give a reasonable amount of time between switching... say 10 minutes?

The magnetization current shouldn't be degrading to the unit... but I'm not sure to say that. Others here will know for sure. But I think you're fine switching it in and out at will.

 

[rcw retired EE]

Is the delta winding referenced to ground in some way, such as a set of wye connected voltage transformers and/or surge arrestors on the 11kV bus? If not, it is possible for a high voltage surge to be passed to the 11 kV winding by the inter-winding capacitance creating a much higher voltage than expected on the 11kV winding. The winding's H-L & L-G capacitance act like a capacitive connected voltage divider (like a CCVT) that passes high frequency surges to the floating LV winding. Don't float the delta winding.

We have seen issues when first energizing a generator transformer with the generator isolated if the isolation also disconnects the generator side VT's from the winding, leaving the delta winding free to float to any voltage it wanted. In one case the wye VT's were connected but had insufficient damping. The resulting ferro-resonance destroyed the VT's but fortunately did not damage the transformer.

I have not heard of a limitation on Energization, like there is on motor starting. I know we energized a new 230 kV, 250 MVA generator transformer about 12 times in 8 hours , trying to adjust the 230 kV breakers' Point on Wave controller that was supposed to limit the inrush current by closing at the peak of the voltage wave. Ten years later, there are no known issues with the transformer.

 

[edison123]

Transformer no load steady state current is very low at around 1 to 2% of rated current. The switching inrush current is high but lasts only a few cycles. So, transformer can handle many starts. The question is can the 132 KV breaker handle so many operations.

Muthu

http://www.edison.co.in

 

[HamburgerHelper]

I have heard of a limited number of starts for a motor during in day but that was I believe more so due to the inrush causing a voltage dip that would be seen by its neighbors. Repeated starting of motor will heat it up fast, though.

Due to how much thermal inertia a transformer has with all that oil, I believe that any utility requirement or rule of thumb is just about not exposing a bunch of customers to too many voltage dips in a day.


------------------------------------------------------------------------------------------
If you can't explain it to a six year old, you don't understand it yourself.

 

[waross]

Hello rcwilson;
In regards to the transformers with which you had issues, Was the primary wye point floating?


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

 

[bacon4life]

Switching it every day sounds like an unusual service condition. I haven't heard of a specific limit to the number of energizations, but every energization has the potential to create surges as rcwilson explained. With 15,000 energizations, you will have a pretty good chance of repeatedly hitting the worst case timing for the energization transient.

Sometimes GSUs are directly tied to the generator terminals without a generator breaker. In this directly connected configuration the transformer is slowly energized as the generator voltage is raised, and thus the transformer is not subjected to the same kinds of inrush transients as your case.

 

[crshears]

The question is can the 132 KV breaker handle so many operations.

Breakers are not always the potting device of choice...

Our 500 kV high-side trafo's are indeed switched with breakers, but only because the high-side disconnects are three separate motor-operated devices, and there are timing concerns with them; our 230 kV and lower high-side trafo's are almost universally provided with disconnects having mechanically ganged blades driven by one motor, and we routinely use these switches to place trafo's on potential and / or remove them from same [although some switches are provided with whips or the like to facilitate breaking the arc during opening operations].

Stated rationale is that as the arc strikes during closing operations the bank is eased into service, so to speak, rather than going straight to grid potential. Likewise when being removed from potential.

Lower cost is also a factor, naturally, particularly at tapped stations partway along a circuit...

CR

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

 

[davidbeach]

Energize the transformer with the generator, the transformer will love you. Synchronize across the 132kV breaker. You’ll beat the breaker to death much quicker than you’ll wear out other 132kV breakers. Accept that and all is well. Expect to get a “normal” life and you’ll have surprise issues before you know it.

 

[crshears]

Hi davidbeach, I totally agree with your post in the normal course of events; however the OP appears to be referencing the situation with the LT isolated phase bus links open, iow back-potting the trafo from the grid.

CR

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

 

[davidbeach]

That’s done a half dozen times at most during commissioning and then never worried about again.

 

[edison123]

@OP

In transformers, the side which is energized is called the primary and the other side is called secondary. In case of GT's, the LV side is the primary and the HV side is the secondary.

@davidbeach - OP says it is an open circuit backcharging.


Muthu

http://www.edison.co.in

 

[crshears]

That’s done a half dozen times at most during commissioning and then never worried about again.

Agreed, David; unfortunately the OP does not state the circumstances surrounding why the question is being asked in the first place...

Like someone else's tag-line says: "more information = better answers"

CR

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

 

[prc]

When a 30 MVA transformer is energized, there is a possibility of the inrush current peak reaching 8-10 times the full load current peak. Even though this is only for a few cycles, the winding where voltage is applied,will be subjected to mechanical forces and oscillations that can weaken the winding if subjected to frequent energizations. Of course this peak will happen only in case switching is done at zero point of voltage wave. Hence the possibility of chance. Thermally this is not an issue as the current peak is only a small part of the current wave.
As Wilson said in case of EHV generator transformer back charging with LV open, there are chances of a) ferro-resonance b) the transferred surge exceeding the BIL of LV bushings esp in UHV units (c) In case there is a large tap range on HV winding and the same is floating, there is chance of part winding resonance in the open part of tap winding causing a failure.( chance more with 400kV and above)
To take care of above I used to give the following protocol in case of back charging EHV GTs. 1) Put the tap on No1 (ie full tap winding in circuit) 2) Keep the LA and surge absorber of the generator in the circuit. Where generator breaker is used, insist for LV and surge capacitor on both sides of contacts.
Just out of curiosity- why OP is planning such abnormal frequent switching? if it is going to be a frequent operation, better to provide inrush current mitigators- point on wave controlled switching, closing resistors with breaker,LV premagnetisation etc,

 

[itsmoked]

Is point-of-wave always just the peak voltage? I thought it was the point opposite of the remnant magnetization.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[FPelec]

We generally perform an "energization withstand test", by switching on the transformers 10 times in a few hours, at the maximum operating voltage and with the most unfavourable tap changing position.
This however is only done as a type test, and it should not be allowed during normal operation.
Such an high number of switching manoeuvres definitively represents, as per IEC 60076-1, an unusual service condition.
As already stated, this condition can be technically feasible only adopting some "soft" switching method as Point-On-Wave switching.
By the way, performing several manoeuvres each day, in a few years you will also need to replace the circuit breaker.

Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[MatthewDB]

Whenever I hear about using a delta-wye generator step up transformer in reverse to feed plant service, I think of the Detroit Dam Powerhouse fire that occured in 2007. Detroit Dam Powerhouse is a 100MW plant in Oregon, USA, owned and operated by the US Army Corps of Engineers.

The plant has two 40MW generators and two 10MW generators 3 miles downstream at a re-regulating dam. All are 13.8kV, with two step up delta-wye transformers to 230kV at the upstream powerhouse. Each transformer has a bus with one 10MW and one 40MW generator. Station service is from two 13.kV:480V delta-wye transformers, one off each bus. Each station transformer feeds 1/2 the station service, with the ability to transfer all to one transformer. There is no interconnection between the two 13.8kV busses.

Each generator has an independent neutral grounding resistor. There is ground fault relaying triggered by each generators' neutral resistor and bus differential, either one tripping both the 230kV breaker and both generator breakers. There is also ground fault monitoring on the bus to alarm on a ground fault if all generators are offline. There are 14.4kV (L-N) rated MOV arresters at multiple locations on the 13.8kV system.

They experienced a broken insulator on the connection to the downstream power house, and the bus tripped on ground fault and differential. The operators of the plant decided to re-energize the 13.8kV bus to restore 1/2 of the station service, rather than transferring it all to the other bus. They thought that since the system is floating, it would be OK to leave it energized with a persistent ground. The problem was that it wasn't a solid ground fault, but a restriking arc. (A restriking arc is where there is an air gap in the ground fault. System capacitance will attempt to keep the system centered with respect to ground, but as the phase to earth voltage builds, it will arc over. The arc drops the phase to near zero abruptly. Without current to sustain the arc, the arc stops and then the voltage will build again. It is quite common for restriking arcs to arc at twice the line frequency.) The restriking arc created over-voltages on the other two phases, and after a while they overwhelmed the MOV arresters and those arresters failed violently. (Re-striking arcs create over-voltages on the other phases because the transformer leakage inductance and phase stray capacitance create a resonant circuit that rings every time the arc strikes.) When the arrestor failed, it created a phase to phase fault before the differential picked up. The fault managed to start a fire in the switchgear that spread throughout the cabling in the powerhouse.

It created $9 million in damage, created chaos as they had to rush to bring in generators to restore power to the spillway gates to prevent over-topping the dam, and took more than a year to recover.

All of that has led me to the opinion that if you are going to back feed a delta-wye generator step up transformer as a regular practice, put a grounding transformer on the bus.

 

[davidbeach]

And keep that delta system as small, physically, as possible.