Series resistors in PT circuit 5

[buzzp]

In an older design, they placed resistors (.1 ohm) in series with a fuse that feeds the primary of a PT for protection purposes (both leads are fused and have resistor in series). The secondary is also fused. I can not think of any reason to have the resistors in the circuit (sure they will limit the current but so will the fuse by opening). Anyone think of a reason for these resistors?

 

[GOTWW]

Interrupt rating of the fuses / cut-outs?

 

[scottf]

That's a new one on me. I've never heard of that.

What is the voltage class and arrangement of the VTs (i.e. L-G or L-L connection)?

Inserting a 0.1 ohm resistor on the primary of the VT would do little to nothing to limit the fault current, as the primary winding impedance of a VT is very high relatively (in the order of kohms for sure).

Normally, the problem with protecting VTs is that the fault current is too low for the fuse to operate until the VT fully fails to ground. Normally a 1A or 0.5A fuse is used.

Does the line terminate at the VT, i.e. is there anyting else downstream from the resisitor?

 

[buzzp]

The fuses are unknown at this time. The techs are finding out (no values-types on dwgs). The PTs (600V) are hooked directly to a 480V bus (station service bus for power plant) via these resistors and fuses. Two PT's are used L-L. There are fuses on the secondary side of these PT's as well. No fuse information is known yet. After the secondary fuses, it goes to protective relays (old electro-mechanical).
Its got me stumped if the configuration is really as shown in the drawings.

 

[TestBeforeTouch]

The resistors may have been installed to deal with ferroresonance.
Check out the link:

http://www.cadickcorp.com/download/TB004a_Ferroresonance.pdf

 

[scottf]

Test-

I don't think putting a resistor in the primary (especially one 0.1 ohm) would do anything for ferroresonance. To help mitigate FR, the resisitors would need to be on the secondary of the VT. Typically, this would be done with a broken delta arrangement, so as not to influence the VTs measuring accuracy.

This is typically not a problem on 600 V systems.

 

[buzzp]

For lack of a better reason, I am leaning towards the ferroresonance issue. This is the only thing that makes a little sense. The transformers are wired open delta. They feed an over/under voltage relay and a watt-hour meter. Will post any other relevant info. Thanks

 

[busbar]

ANSI/IEEE Std C57.105 §7 cites [in Table 2] open-delta ungrounded-primary / grounded secondary transformer sets grouped as susceptible to ferroresonance, where [in Table 3] grounded-wye primary / grounded-wye secondary transformers are not.

 

[scottf]

I agree that the unit maybe susceptable to FR...no doubt. However, since the L-G capacitances on a 600 V system are much much lower than in a MV or HV system, FR is not normally an issues.

However, I can't see any reason that adding a 0.1 ohm resistor on the primary of a VT, which probably already has a primary winding resistance of around 10 kOhms would make any difference in terms of FR.

 

[GOTWW]

You guyes have now have me worried, I just sent out a medium-voltage metering job with many FM-5 meters that use open delta PT's with grounded centers and fused disconnect switches that will be energized seperately. Is the FR a problem with 1000VA 3% VT's? Say no, please!!!

 

[scottf]

What do you mean by 1000 VA 3% VTs?

Do you mean that the thermal burden rating is 1000VA? If so, this is irrelevent for metering applications. Perhaps you mean 0.3% and not 3% for metering?

What do you mean by open dela VTs with grounded centers?

 

[GOTWW]

That would be Accuracy @ 60Hz - 0.3 @ O,W,X,M,Y
Thermal Rating- 1000 VA @ 30 degrees C.

The FM-5 KWH meters are connected for 3-wire delta service.

Sorry, my power terminology is rather crude. For a FM-5 meter there a two line-to-line PT's with primary connections Phase A connected to H1 of PT#1, Phase B connected to H2 of PT#1 and H1 of PT#2, and Phase C is connected to H2 of PT#2. Now X1 of PT#1 is connected to meter Va input, X2 of PT#2 is connected to meter Vb input. The PT#1 X2 and PT#2 X1 are connected together and grounded (Per the NEC) and connect to the "wild-leg" of the meter.

 

[GOTWW]

I thought the 1000VA would apply with regards to the ferro-resonance, as the thermal rating would be indicative of how much iron(ferro) is present in the transformers.

 

[scottf]

The thermal burden rating really doesn't tell you much in terms of how susceptable the VT would be to FR. The VT's chance to go into FR is a function of a number of parameters...mainly the nominal flux rating and how close the VT operates to saturation level under normal circumstances. It also depends on the winding resistance, among other things.

I understand what you mean now about your VT's connection. Not all VTs connected open-delta will be susceptable to FR. It really depends on your system. Unless you've had FR problems before or if this line has an especially high capacitance from line-to-ground, you probably will be ok.

I assume by your VT ratings, that this is a 15 kV class VT (or possibly 25 kV). FR problems generally don't occur on MV lines...they are more problematic on HV applications. Your VT application is quite common.

As far as thermal burden goes, it's mostly dependant on the size primary and secondary wire used in the coil and the coil design, i.e. how much heat is generated by the CU losses. The core losses do come into play too, but for the most part, it's based on the HV winding.

 

[busbar]

 
Aside — ANSI/IEEE Std C57.105 §7 cites [in Table 2] open-delta ungrounded-primary / open-delta grounded secondary transformer sets grouped as susceptible to ferroresonance, where [in Table 3] grounded-wye primary / grounded-wye secondary transformers are not.

 

[scottf]

busbar....point?

 

[GOTWW]

yey!

So for pole-line mounted instrumentation, the capacitance to ground is zip. I just got spooked from the cadicorp pdf file. Just never even considered the ferro-resonance being an issue here,... the perfect murphy blindside attack.

So to summerize my expectations: For polemounted 5kv and 34.5kv systems, very lightly loaded open-delta PT's feeding digital meters, the phenomena of ferro-resonance should be a very low probability event.


Thanks All

 

[buzzp]

I will have to check out that IEEE standard. The primary of the PTs is not grounded but the middle leg of the secondary is. In the same plant, we have 7200/120 PTs for protection and synchronizing configured as an open delta - ungrounded primary/grounded secondary. There is no resistors in series with either the primary or secondary of the transformer.

Busbar is merely pointing out that the IEEE recognizes that these type of xformer configurations are susceptible to ferroresonance.

I really need to push to get the IEEE subscription to make my life easier.

 

[busbar]

 
scottf, I was suggesting that ferroresonance may be possible with that VT configuration, if for however ill-chosen, may have been the reasoning behind hiside resistors (and something to rule out.) I apologize if I represented my comments or the reference as anything more than that.

buzzp, C57.105 is fairly general and not very new, although it was reaffirmed in 1999.
Desc:

http://shop.ieee.org/ieeestore/Product.aspx?product_no=SS6924

TOC:

http://standards.ieee.org/reading/ieee/std_public/description/dtransformers/C57.105-1978_desc.html

 

[buzzp]

I do have that IEEE document as part of our IEEE distribution standards collection. I read through it and it very well could be what they are for (although the voltage is lower than that of most susceptible systems).

Whether the plant had occurences to initiate installing these or if they were installed when the plant was built, I don't know. Ferroresonance is the only thing I have heard that makes any sense. I will be leaving them installed for the time being and, if time allows, will check the operators logs for any problems that may have warranted installing these resistors. Thanks for all the help.