Let me start this post by saying that I'm a protection and controls engineer with a good bit of automation experience and while I have a decent amount of industry experience with controls, I have little to no experience with intrinsic safety. As such, I am humbly asking for some clarification...
When we model short circuit capacity of a synchronous generator, we usually model it based on the subtransient reactance for the "worst-case" scenario, which would also include the increase related to the DC offset based on the X/R ratio. For our purposes, let's just talk about the symmetrical...
We use 27 relays for several applications. They are a permissive relay for certain breaker reclose conditions (Hot Bus / Dead Line). They are also a tripping relay in some instances. This is usually applied on a transmission bus that is connected to non-conventional generation. In these...
Without going into phasor diagrams, I'll give you the 10-cent version of this. If you have a high-current load that is primarily inductive, the high-magnitude current is going to cause the voltage to drop between the source and the load due to the fact that the lines supplying the load have...
I've recently moved jobs and am trying to come up to speed on SCADA and metering. Through an unfortunate series of events, it has happened that we are in a situation where we need to sum two meter accumulators (read via DNP/Serial) in our D20 RTU, once we've polled them. Unfortunately, I...
This has been an excellent discussion. I tend to agree with tinfoil's response. It doesn't make sense for a utility to specify a minimum, as it truly can be anywhere between the maximum and zero, depending on circumstances. Thank you all for your input.
We have a an installation where we have two antennas installed on the roof of a metal frame building. One antenna is a GPS antenna that barely protrudes above the roof line. The other is an electrostatic antenna that protrudes about 6 or 7 feet above the edge of the roof from the side of the...
I don't think this is a vacuum tube unit. The instruction manual shows a lot of schematics with BJT's and diodes. I don't see any tubes.
We think the oscillator circuit (maybe the crystal) has failed. It does not appear to be a catastrophic failure. We just don't have the expertise to...
Does anyone know of a facility that will repair a General Electric CT-61A power line carrier transmitter? Normally, we'd just upgrade the unit to something newer but we are in a bit of engineering/design time crunch and would like to get the unit repaired if possible to avoid having to modify...
Thank you all for your input. We've decided that whenever possible we will give the requestor a network equivalent reduction model at the delivery point, stating what the conditions of the model represent. We still ask what they are using the model for, but often they aren't able to give a...
Cranky, it varies per request. Usually, the customer who is requesting the data is doing so for use inside of their facilities, on the LV side of the substation transformer. We get asked about system impedance on the transmission bus. So, I'm assuming the customer is adding the step-down...
DPC, I do note where you speak of the largest single source being out. That does seem like a good idea, but are you aware of any guide or standard that spells this out?
Yes, I agree. I should have said "COULD" be worse. But, back to the original question. Any idea what the "standard method" amongst utilities would be for specifying minimal? I'm not aware of any guide or standard that spells this out.
I am not an arc-flash expert, but I believe the philosophy is that a lower system impedance will result in a lower fault current. A lower fault current will take longer to clear, as it is closer to the Y-axis on the Time/OC curve. The idea is that a longer exposure to lower-current arc-flash...
Cranky, I get what you are saying and I agree. Having said that, usually they are asking for the purpose of an arc-flash study. There are a variety of system conditions ranging from mass outages due to storms all the way up to N-1 contingencies. Of course each can give varying levels of fault...
I’m embarrassed to ask this, but can anyone tell me if there is a definitive guide to what is meant by “minimum available fault current” on transmission systems? Traditionally, when I’ve been asked for this and I do not get more information from the requesting party, I give the fault...
For what it is worth, the differential current has gone away, which leads to me to believe that this was the result of residual magnetism in the core.
Thank you all for your responses.
Unfortunately, we don't have a voltage reference tied into the circuit right now. Also, CT secondaries do a appear to be tight. We've made another observation. Viewing the mathematically derived differential currents on the fault recorders shows that the differential currents have a slight DC...
We did briefly consider that, but the fault recorder trace on the source winding looks pretty clean. The transformer is only loaded to about 21% of it's base rating. So, I guess it's possible there is not enough load current to mask the exciting currently completely. Let me subtract the...
