Series resistors in PT circuit
Series resistors in PT circuit
(OP)
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?






RE: Series resistors in PT circuit
RE: Series resistors in PT circuit
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?
RE: Series resistors in PT circuit
Its got me stumped if the configuration is really as shown in the drawings.
RE: Series resistors in PT circuit
Check out the link: http://www.cadickcorp.com/download/TB004a_Ferrores...
RE: Series resistors in PT circuit
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.
RE: Series resistors in PT circuit
RE: Series resistors in PT circuit
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.
RE: Series resistors in PT circuit
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.
RE: Series resistors in PT circuit
RE: Series resistors in PT circuit
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?
RE: Series resistors in PT circuit
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.
RE: Series resistors in PT circuit
RE: Series resistors in PT circuit
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.
RE: Series resistors in PT circuit
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.
RE: Series resistors in PT circuit
RE: Series resistors in PT circuit
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
RE: Series resistors in PT circuit
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.
RE: Series resistors in PT circuit
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?produc...
TOC: http://standards.ieee.org/reading/ieee/std_public/...
RE: Series resistors in PT circuit
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.
RE: Series resistors in PT circuit
Note that for initiation of a resonant condition, there has to be a open phase resulting from something like a fuse operation—with hi-side capacitance in the right place. buzzp, what is the primary-bus voltage?
RE: Series resistors in PT circuit
RE: Series resistors in PT circuit
The primary resistors sound like a very misguided approach and a plain old fire hazard. Class-J 200- or 300-kA interrupting fuses are available by the ton.
If you really wanted to go crazy protecting the tap conductors above the fuses, strap on some Bussmann KQT cable limiters at the bus tap. www.bussmann.com/library/bifs/1042.pdf (Leave the fuse block as a handy isolation point.)
RE: Series resistors in PT circuit
The IEEE standard points out that LV PT's are generally not an issue regarding ferroresonance. It does not rule it out. My only conclusion is they were placed there for that reason. Until more information is know, I am sticking with this conclusion. Nothing else makes sense.
RE: Series resistors in PT circuit
RE: Series resistors in PT circuit
RE: Series resistors in PT circuit
RE: Series resistors in PT circuit