OLTC for Unidirectional Power Flow, Application!

[switchnlight]

Hallo forum members,

We have recently procured two 25MVA 66/33kV power transformers from a supplier in India. The transformers have an OLTC of single resistor type and they have put a caution that it is suitable for unidirectional power flow i.e. from HV to LV and not in the reverse. We however want to use this unit as a step up transformer supplying a radial feeder to a load at the 66kV end where power flow will always be in one direction. We therefore contacted the manufacturer to clarify this limitation in application of the tap changer. They informed us that it can only be used in the step down mode and not vice versa. They further said that the transformer could only be used as a step up unit with the tap changer utilized as an off load type or locked at one suitable tap position.

We consulted the J & P Transformer Book 1986 Ed. which says that “Tap changers constructed with one transition resistor are suitable for power flow in one direction only…… When two resistors are employed, the duty imposed on the diverter switch is unchanged by a change in the direction of power flow”. To our understanding the key words are "change in direction of power flow". This seems to imply that the restriction to unidirectional flow is when the transformer is used in bi-directional power flows such as when the transformer is connected in a ring or to generating sources on both HV and LV, which means that the flow of power may reverse. We see no reason why the magnitude and distribution of the circulating current in the diverter switch should be different for step-up and for step-down use of the tap changer. Can the experts in our forum kindly study our reasoning and advise if it is correct. Also kindly shed more light on this unidirectional and bidirectional designs for OLTCs.

 

[cuky2000]

Enclose is one of many companies that you may contact for information regarding this topic.

http://www.icmiinc.com/contact_us.htm

 

[switchnlight]

Thankyou Cuky200. This site however has little info on OLTC's. It seems they more into voltage regulators, i.e. automatic (or universal) voltage regulators (AVR or UVR). The info I am requesting for is to do with the actual on load tap changer itself as an intergral part of the tranformer and in particular the single resistor type. How does this particular design limit power flow in "one direction and how is this problem overcome viz a viz, the two resistor type?" And does this flow in one direction satisify the scenario which I described in my introduction?

 

[RalphChristie]

From the J&P Transformer Book 1998 (Twelfth Edition):

High-Speed resistor tapchangers can be devided into two types, those which carry out selection and switching on the same contacts and generally use one resistor, and other which have selectors and seperate diverter switches and which normally use two resistors.
With a single resistor, load current and resistor circulating current have to be arranged to be subtractive, which dictates use with unidirectional power flow or reduced rating with reverse power flow.
When two resistors are employed the duty imposed on the diverter switch is unchanged by a change in the direction of power flow. Recently versions of the combined diverter/selector types have been developed having double resistors and thus overcomming the unidirectional power flow limitation.

In the book is also schematic and sequence diagrams to illustrate the difference between the two methods.

Have to say, I was also not aware of this limitation in the single resistor-type tapchangers.

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[stevenal]

Since the intended power flow is unidirectional, I wonder if the LTC could be modified to be subtractive in the step up mode. Bring the source to the opposite end of the resistor.

 

[RalphChristie]

Steve:

I think you are correct. If it is done, one has just to remember that the tapchanger is still unidirectional - now just for use in a step up mode and not in a step down mode.


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[RalphChristie]

Remember, before you try it, please consult the manufacturer first.

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[switchnlight]

Thanx RalphChristie and Stevenal for your contributions. We have contacted the suppliers about the possibility of modifying the tap changer wrt the resistor connection to see if it can work in the step up mode. But just following up on RalphChristie's contribution, how does reversal of the power flow reduce the rating of the transformer, because I thought that by reversing the source the circulating currents would be additive and not subtractive? Iam not sure if my thinking is correct, but kindly shade more light if you're able to. Is the worry more to do with the resistor or the contact sizing for the oltc? What will really get damaged if power was allowed to flow in the reverse direction?

 

[stevenal]

Sounds to me like they sized the resistor to handle circulating current only. No reason to go any bigger if transformer is to be used for step down only. Also, one of the movable contacts may be smaller as well. If so, my suggestion above will not work without further modification.

 

[switchnlight]

Well, you could be right Stevenal about the sizing of the resistor and contacts. But looking at the statement from the J & P transformer book, they're also taking about reducing the rating of the tranformer if power flow reverses, is someone able to explain how this comes about because they dont seem to explain further in this book. Or do you know of any other book or literature that covers this topic extensively?

 

[ScottyUK]

In my copy of the J&P book I thought the transformer derating comment was intended to protect the diverter resistor applied IF the tapchanger was not modified as stevenal has suggested.


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[stevenal]

I think the message is that unidirectional power flow in the assumed forward direction only is allowed. Unidirectional in the reverse or bi-directional power flows are not allowed, at least during tap changer operation. These limits are set by the resistors and maybe the contacts. Note that full capacity in the reverse direction is allowed with the tap changer locked off, so there is other limiting factor but the ability of the tap changer to operate under load. Many tap changer controllers can be set to block automatic operation during reverse power flow. (They'd be sensing the wrong voltage anyway). Still have the risk of a manual operation taking place, though.