OLTC for Unidirectional Power Flow, Application!
OLTC for Unidirectional Power Flow, Application!
(OP)
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.
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.






RE: OLTC for Unidirectional Power Flow, Application!
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RE: OLTC for Unidirectional Power Flow, Application!
RE: OLTC for Unidirectional Power Flow, Application!
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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RE: OLTC for Unidirectional Power Flow, Application!
RE: OLTC for Unidirectional Power Flow, Application!
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.
Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon
Make the best use of Eng-Tips.com
Read the Site Policies at FAQ731-376
RE: OLTC for Unidirectional Power Flow, Application!
Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon
Make the best use of Eng-Tips.com
Read the Site Policies at FAQ731-376
RE: OLTC for Unidirectional Power Flow, Application!
RE: OLTC for Unidirectional Power Flow, Application!
RE: OLTC for Unidirectional Power Flow, Application!
RE: OLTC for Unidirectional Power Flow, Application!
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RE: OLTC for Unidirectional Power Flow, Application!