Transformer Tap moving time 1

[QBplanner]

Hi All:
I am not a field service engineers or testing engineers so I have no ideas how long it will take a typical distribution LTC transformer (230/138/66 down to 12/25kV level) tap move from neutral position to the maximum boost tap position assuming 5 taps up and 5 taps down.
I read some Transformer notes that it may take 30 -45 seconds for a tap moving including mechanics and protection relay times to avoid mis action. However, what if the tap has to keep moving to the next say 4 steps untill the tap tap position. then will it still be 30-45 seconds for each tap or only 5 seconds for the rest taps without resetting voltage relay and timer mechanism for next tap moving.

One of our best guys told me after first tap moving with the voltage relay timer and tap energizing and mechanism, the Trx tap will only take 5 seconds for the rest taps which is pure tap energizing and mechanism time.

I know there are a lot of transformer experts here in this forum. I hope some experts can explain how the voltage relay timer settings were set in your utilities for the LTC plus if anybody has the recorded data would be appreciated.
THanks in advance!

 

[slavag]

Hi.
I think you told not about voltage relay, you told about automatic tap changer controller ( or as many guys say- AVR).
Time setting is very variable, depend on type of loads and status of grid. You also have voltage regulation according to curve or DT. You have also two steps :, first moving and others. But your data and what your guy say is avarage data and suit for lot of cases.
Best Regards.
Slava

 

[cranky108]

I think on a typical LTC there are 16 taps up, and 16 taps down. And there are typically 4 or 5 no load taps.

I don't recall that there is an intential delay after a tap change, and the continuion to another tap change. However, there would be a delay if the LTC stops at the set voltage level, before it will change for another voltage change.

The delay settings are important to coordinate with other events, or voltage regulating devices in the system.

 

[dpc]

As cranky says, the tap changer will continue through the taps until the voltage is within the setpoint range. The LTC controller has an adjustable time delay to prevent another tap operation too quickly, even if the voltage changes. This is reduce wear on the tap changer.

Time between tap positions when the LTC is moving is probably much less than 30 seconds, more like 3 to 5 seconds.

 

[slavag]

Hi, maybe, I don't understand something.
I wrote 30-40sec for the 5 taps moving. It's include long first delay about 12-15sec.
Best Regards.
Slava

 

[dpc]

I don't know why there would be such a long delay for the first tap. I don't recall that, but I'm no expert on LTCs.

 

[slavag]

Hi Dave. I'm no expert too. It's according to lot of our customers. We only insert a settings .
Actually isn't LTC issue. For my understanding it's depend on the grid, main trafos LTC, generators, capasitor banks.
Regards.
Slava

 

[QBplanner]

Thanks for the replies and I expect someone will explain to me if the first tap moving has a delay why not the following taps?
No delay may cause a mis action for some spikes in the system since when system operates the voltage is not constant at all but always changes . The LTC can not always chase the system voltage. A band with is set up with a timer for the tap moving. that is one of the main reason why a delay is required. Otherwise, tap haunting will be problems.

It may have something to do with the type of the loads but not much to do with the status of the system.
When the LTC voltage ranges is set up the first time, all possible system conditions have to be considered in order to have a right settings.
Expect more...

 

[ScottyUK]

The long delay is typical behaviour around the mid tap for a reversing tapchanger. It it moves through a couple of mechanical positions without stopping to effect the reversal of the winding from a bucking to a boosting configuration.


----------------------------------

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

 

[dpc]

Thanks, Scotty - that makes sense.

 

[slavag]

Hi Guys.
Please don't forget, not always first delay in this tap.
For this problem ( middle point) you have anothe solution, long delay for the TC alarm
Best regards.
Slava

 

[isquaredr]

Some modern voltage control relays have a 2-stage timer. When the system voltage drifts outside limits a timer set to say 45-seconds begins to count, after the 45-seconds a tap change is initiated, if the voltage is still outside limits a second timer say set to 10-seconds begins to count, it initiates a 2nd, 3rd, 4th tap change until voltage is back within limits.
Extended tap change delay at the tap changer itself can be caused as scottyuk says by an intertap or transition tap at the tap midpoint going from buck to boost, check rating plate to see if this appies, you may also have taps....7-8-9-9a-9b-9c-10-11-12 etc, the taps suffixed with a letter are transition taps.
Modern tap changers take around 4-seconds to complete a tap whilst some older models can take upto 30-seconds.

 

[QBplanner]

Thanks everybody!
If as Isquaredr said, then the LTC transformer moves from mid tap to the maximum boost tap, it will averagely take let's say first tap 45 seconds for the first step then 11 X 10 seconds for the rest 11 taps so it will be 155 seconds total.
Isquaredr:
I don't quite get in your last sentence, you mentioned 4 seconds to complete a tap transition for modern and 30 seconds for 30 seconds for older ones. Is this 4 or 30 seconds pure mechanism times ?
THanks!

 

[slavag]

Hi QBplanner.
I work now on the some automtic voltage control.
Please see attached document. Of course every company have
other parameters and options, but principles are same.
I found this document very helpful.
Again, long time delay for the middle point isn't same as first and second timers in the newer sytems. In old versions
taps as 9a,9b, was w/o voltage changes and this inpact on the voltage control operation.
Best Regards.
Slava

 

[prc]

I dont know any scheme where time for the first tap changing will be more.As ScottyUk said in most of the control scheme there will be a three middle taps with same voltage ( say 9a,9b,9c for 17 tap OLTC)where the tap changing will take double time for two tap positions.ie from 8 to9b and then 9b to 10 it will take 10 seconds each.For all other taps it will be 5 seconds only.But please dont generalise it. This will be there both for reversing switch type and also for coarse-fine arrangement.We can eliminate this if we provide an additional tap winding section (ie instead of 8 winding loops,provide 9 winding loops)in case of above two arrangments or by providing a linear tap winding arrangement.But normally we save that additinal one section of copper and provide two run through taps at middle!

 

[Marmite]

In the Company I work for the initial tapchange delay is set at 90 seconds on the 33kV system (and 120 seconds on the 132kV system) so the voltage has to go out of limits (as defined by the AVC settings) for 90s before a tap is initiated. There is then an intertap delay of 10 seconds. If the AVC calls for more than one tapchange the next tap change will be initiated 10 seconds after the first completes. Delaying the tap change reduces operations, which reduces wear and maintenance requirements.
Regards
Marmite

 

[QBplanner]

Thanks for slavag's information. Very useful. Also appreciate Prc and Marmite's comments.

It seems every Utility has its own delay settings for LTC transformers tap changing.

Again thanks for everybody's inputs!
QB

 

[isquaredr]

Marmite
I to work in the uk, our 132kv oltc time delay is usually set to 45s whilst 33kv set to 120s. That way any grid voltage drifts are quickly corrected by the grid oltc ie 1 or two transformers tapping instead of say 12. Funny how companies operate to different policy even within the uk, how long have we been in private ownership ?

Obplanner, my comment on old & new tap changers having different operate times is to do with oltc type, most if not all modern oltc are extremely fast in operation ie 40ms/tap, the additional time is to charge the stored energy device which provides the energy to tap, overall typically 4.5s, these usually combine both selector & diverter within the same compartment.
Older slow speed tap changers are designed differnt usually having seperate selector / diverter compartments can take considerably longer.

 

[QBplanner]

Thanks Isquaredr!
When multiple contingencies happen, LTC tap will drift to the top tap which will have reserved impact to the system voltage stability.
That is why in the company I work, we blocked some major system LTC transformer moving and to have certain delay to avoid tap changing.

 

[QBplanner]

Sorry I mean "reversed" impact