Open Delta Resistors

G

Thread Starter

George Laloumis

Does anyone know how it is done the sizing of the VT's open delta winding dumping resistor?
 
My interpretation:
You're concerned an ungrounded power system could have the potential for a ferroresonance condition because of the existence of PTs (or VTs) used to power indication, annunciation, metering, or protection circuits. Their primaries are connected as earthed-wye. Their secondaries are open-delta. You also feel that the addition of a resistor in the secondary will dampen (excuse the pun) the threat of ferroresonance. Lastly, you want to know how to "size" it.

Dumping-Resistor.
I must admit that I have never heard the term "dumping" resistor, but then I ain't dead yet. I am familiar with terms like secondary-shunting, secondary-loading, secondary-burden, and even oscillation-killer.

Purpose of Resistor.
The purpose of said resistor is to prevent the transformers' unsaturated reactance to decrease enough to where an oscillatory L-C tank-circuit is produced!

Resistor Placement.
Before providing sizing criterion, a word about location of the dumping-resistor! A resistor can be certainly be connected in parallel with each secondary, but they would then dissipate power. Placing it across the open-delta eliminates power consumption by the resistor.

Sizing Criterion.
While there are several solutions the dumping-resistor is most popular. Its value should equal 30-40% of the PT's magnetizing reactance!

This concludes Ferroresonance-101!

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])
 
G

George Laloumis

I have three PTs connected in Wye connection with ungrounded neutral point at primary side. The secondary side is Wye connection with grounded neutral point and there is a secondary winding connected as open delta. I noticed that with the open delta I have unbalanced measurment with about 20% differences between the phases and I am considering of connecting the terminals of the open delta to a resistor. I need to know if there is any folmula of calculating these kind of resistors and if it is needed or not. How can I know every time that I need a resistor or not?
 
Responding to G. Laloumis' Mar 1, 1:02am query:

Q1) Are observed voltage variations ph-ph, ph-earth, or across delta?

Q2) Are observed voltage variations random occurrences? Or, do they occur at times coincident with a specific plant condition? For example, when operating a very large motor!

Q3) What is present "burden" or load connected to xfmr secondary?

Q4) A parametric approach can predict exposure to ferroresonance. But more information is required! Can you send a schematic showing pertinent primary system parameters as well as PT specs?

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])
 
M

Muhammad Zafar Sajid

> Does anyone know how it is done the sizing of the VT's open
> delta winding dumping resistor?

REPLY/ANSWER:

The sizing of DUMPING RESISTOR for the V.T open delta winding is done in the way that the value of Dumping resistor should be equal to 30 -- 40 % (percent) of P.T magnetizing Reactance.
 
> Does anyone know how it is done the sizing of the VT's open
> delta winding dumping resistor?

First, there is a definition problem: with two PT's (VT's) you can construct an open delta connection. To get a "broken delta" connection for ferroresonance damping, you must have 3 PT's. Then on the secondary side, one corner of the "full" delta is "broken" and a resistor is inserted, along with an optional voltage relay to activate a protection function or alarm.

Size the resistor at least large enough to load the PT's to half of the VA capacity. Use VA/E/0.5 to calculate using half load Amps. Then R = E/I. Remember the broken delta, when carrying the zero-sequence currents, sum the Io current.
 
Top