Transformer Auto paralleling 5

[ART007]

We are using an automatic closing system in 132/33kV substation, where we have 4 nos. of 132/33kV transformers which runs in 2 groups. T1||T2 and T3||T4.
Scheme is such that if one transformer trips, the left alone transformer from that group get paralleled with the other healthy group of transformers which are running in parallel by closing Sections / coupler as appropriate.

My question is that what will be the significance transformers' tap position. In our system, the tap difference of 4 taps is allowed and transformer can be paralleled. However, what will be the impact on Sections closing which have synchrocheck closing of CBs.
Also, will there be any special consideration when we talk about the sharing of load?

Thanks for your time and feed backs.

ART

 

[ART007]

Addition:
1. The difference in tap will result in voltage differences, which will affect the synch closing of Bus sections/Couplers.
I understood that we should adjust the synchrocheck settings accordingly in sec/coupler.

2. What should be the transformer voltage regulation mode after autoclosing? Auto or Manual?
Does it has any significance?

 

[waross]

The transformers will share the KVA load in the inverse proportion to their impedances.
Tap setting affects the sharing of the reactive load. If the difference in tap settings generates more KVARs than the system demands the result will be circulating currents between the transformers.
Where you normally have two outgoing lines each with independent voltage control, after paralleling the transformers the head end voltages of both circuits will be the same. It may be difficult to keep the voltages at the ends of the circuits within acceptable limits.
You may wish to investigate the feasibility of feeding the lessor loaded circuit from a single transformer.

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

 

[rasevskii]

I believe it can be said that if both transformers are identical, from the same manufacturer and have the same per cent impedance as well as identical tap changers with the same voltage change per step, then if paralleled on both sides, you must have the same tap positions at all times. The voltage regulating relays have to operate together, or preferably one relay must be connected in in such a way that it operates both tap changers simeltaneously. A circuit has to be installed to insure that both tap changers are actually on the same step, when paralleled.

Your remarks on synchro-check, etc cannot be understood without seeing the entire scheme.

rasevskii

 

[ART007]

Thanks Waross and rasevskii for helpful hints.
Any idea about keeping the transformer in Manual AVR mode (Raise/Lower in Manual) after the successful paralleling of left alone trafo with others. Why not keep it in Auto and let AVR regulates the voltage. This is the request from customer to keep it in manual.

 

[waross]

Are you controlling the voltage to compensate for voltage variations on the incoming transmission line or to compensate for voltage drop with load on the outgoing lines?
Either way transformers that are connected in parallel should be on the same tap.
Consider two similar generators in parallel.
The prime movers are adjusted so that the load is shared equally and the voltage on each machine is set equally.
The generators will share both the kW load and the KVAR load.
If the voltage setting of one generator is now raised that machine will take a greater proportion of the KVAR load.
As the voltage setting is raised further the machine with the higher voltage setting will supply all the reactive or KVAR load.
If the voltage is raised further circulating currents will circulate between the generators.
In the case of parallel transformers, the load is shared in inverse proportion to the transformer impedances.
If the tap is set so that the secondary voltage of one transformer is higher than the secondary voltage of the other transformer a similar effect will result. As the secondary voltage of one transformer is raised that transformer will take a greater share of the reactive or KVAR load.
When unequal phase voltages cause circulating currents in a three phase delta transformer bank, a voltage percent error of three times the transformer percent impedance voltage will cause full load current to circulate in the delta.
For example: With transformers of 3% impedance voltage, a 9% voltage imbalance will cause full load current to circulate in the delta.
I am not sure of the formula for three phase transformers in parallel but it will be related to the transformer percent impedance voltage.
You can probably accept a one step difference for a short time during tap changing operations but it is a good idea to get the taps on the same position as soon as possible. I would aim for under a minute as the maximum time.
YOU DO NOT WANT one transformer on manual control in parallel with one or more transformers on automatic control.

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

 

[ART007]

Thanks Waross for a good explanation.

Yes, you are right. As soon as the leftalone transformer comes in parallel with other two, it will adjust the tap automatically (for this, all the transformers has to be in AUTO Mode) and usually within 30 seconds the taps will be equal for all the 3 transformers in parallel.

Regarding Manual control, we have two options.

1. After successful autoclosure and tap adjustment, ALL transformer remains in Auto mode.
2. Ater successful autoclosure and tap adjustment, ALL transformer goes to Manual mode.

Customer asked for the 2nd option to be implemented.

However,it is not clear what would be the significance for keeping it in manual. Because keeping it in manual means there is no automatic voltage regulation. Station is ofcourse unmanned and there is a provision to control taps from NCC.


Thanks for your time.

ART

 

[ScottyUK]

If you are using a master-slave auto arrangement, be very certain how the scheme behaves in contingency states, such as loss of master unit, loss of tap position indicator, and so on. It is, shall we say, a little inconvenient when a master relay fails and the four slaves all ramp off to their bottom tap. Not that this has ever happened of course, but if the old Reinhausen MK30E is still on the market then beware of how it is configured. I'm sure it is not the only one which can cause this.

 

[stevenal]

http://www.beckwithelectric.com/docs/app-notes/appn-13.pdf

 

[slavag]

Master-Slave....is not a best solution today.
Im prefered MCC -- minimal circulating current.

 

[kssschsekhar]

Hi ART007

As per your technical details. The following option is essential to keep safe and meet long life of the transformers.

1. After successful autoclosure and tap adjustment, ALL transformer remains in Auto mode.

In fact while paralleling it is highly essential to ensure transformer tap setting should same as other two transformers.

After completion of three trasnformer in parallel. Tap position of each transformer shall be controlled through auto mode.

If not done then

1. Unneccessary circulating currents flow in the 132 and 33 kV switchgear and transformers. if those are in tolerable limits, then no problem. Otherwise their life starts reducing trend becasue of over loading condition (if all loads operating).

2. Transformers will over heat or burdened due to circulating current which is not required if AVR in auto mode.

Since three transformer are operating in an unmanned substation that means tap changing suppose to be in auto mode.

Why the customer interested to operate in taps in Manual mode with automatic paralleling. If you ask his real purpose. Then it will be easy to provide solution.

a master-follower method easily proivde automatic tap changing.

The following advantages can be shown to him

1. Avoids circulating current in bus bars and transformers which in turn increase the life expectancy of the equipment
2. Unneccessary interference of tap changing can be avoiable.
3. If substation operates in full load condition then transformers may operate in over load condition, it needs additional monitoring requirement to meet grid code requirement
4. A smooth Auto mode operation provides hassle free environment

Thank you

 

[Marmite]

A master/follower scheme that works satisfactorily for the loss of any one of the transformers would be complex to implement and is not the best scheme. A better scheme would be one that works to minimise circulating current using the negative reactance method, such as Supertapp. This system will also work with dissimilar transformers.

http://www.fundamentalsltd.co.uk/pdfs/supertapp_nplus_leaflet2.pdf

Regards
Marmite