answer this

[karl riley]

Derck,
Without periodic ground rods, as the primary distribution line gets farther from the substation the neutral/earth voltage will build up due to neutral impedance increase. True or not?

Karl

 

[wirenut1980]

Charlie,

I read your explanation and I (believe) understand what you said. But to what Bob said, it seems like the voltage stabilization you described is just in reference to earth, and it does not stabilize the actual voltages between the conductors of the power system.

True or no?

But without starting with a known reference point (ground, ~0 V), how do we know what the line potential is? And if we don't know what the line potential is to ground, how do we know how they relate/compare to each other (line to line)? I believe by stabilizing voltage to ground for each ungrounded line, it stabilizes voltage line to line in a way. I'm in too deep here.

 

[hardworkingstiff]

But without starting with a known reference point (ground, ~0 V), how do we know what the line potential is? And if we don't know what the line potential is to ground, how do we know how they relate/compare to each other (line to line)? I believe by stabilizing voltage to ground for each ungrounded line, it stabilizes voltage line to line in a way. I'm in too deep here.

An ungrounded Delta has stable voltage between phases. It's just does not have a stable ground reference.

I believe the comment of grounding stabilizes voltage is somewhat misleading without additional comments.

 

[al hildenbrand]

I believe the comment of grounding stabilizes voltage is somewhat misleading without additional comments.

I look at the idea of voltage stabalization vs. additional comments just the opposite.

Earthing stablizes the voltages.

No Earthing leaves some voltages stable and other voltages fluctuate over time.

 

[wirenut1980]

An ungrounded Delta has stable voltage between phases. It's just does not have a stable ground reference.

I believe the comment of grounding stabilizes voltage is somewhat misleading without additional comments.

I was talking about a grounded system, and I agree that an ungrounded delta is a different animal altogether.

 

[don_resqcapt19]

Carl,

Without periodic ground rods, as the primary distribution line gets farther from the substation the neutral/earth voltage will build up due to neutral impedance increase.

It builds up in both cases, but maybe not as much with multiple grounds as you are using the earth in parallel with the neutral to reduce the voltage drop.
don

 

[dereckbc]

Derck,
Without periodic ground rods, as the primary distribution line gets farther from the substation the neutral/earth voltage will build up due to neutral impedance increase. True or not?

Karl

Absolutely true statement Karl, that is one reason utilities bond the neutral every 1/4 mile or so. I think what is being overlooked even though a delta is not planned or intentionally grounded doesn’t mean it is not grounded by some means. In fact it is via capacitance between phase and ground. It is this unknown capacitance that causes the instability and deterioration problems between L-G and L-L.

By simply grounding the system either solidly or resistance/impedance, you short out the capacitance thereby minimizing the problems.

I keep reading posters say there is no stability problems L-L on a delta system which is not true. The unknown capacitance coupled with the distributed inductance causes resonances ringing effect when switching operations occur like loads turning off/on. The resonances can cause the transient voltage to spike as much as 3 to 10 times system voltage. Granted that is not much of a problem for a motor, but can reek havoc on electronics.

 

[iwire]

But if you connect the neutral point to a ground rod, the answer would become item (1), and it would stay that way forever. That is what is meant by stabilizing the voltage to earth during normal operation.

I disagree, I know that is a surprise.

The ground rod can not 'hold' the potential between the neutral and earth at 0V, it may do it in the immediate vicinity of the rod but not once you move away.

Example.

You have 100 amp 120/240 service in the middle of a field with only two ground rods for electrodes.

The electrician for whatever reason puts all 100 amps of 120 loads on one leg and 0 on the other.

As the the neutral drops voltage due to load and distance will the two rods in the ground keep that neutral at 0V potential to earth?

 

[hardworkingstiff]

I keep reading posters say there is no stability problems L-L on a delta system which is not true. The unknown capacitance coupled with the distributed inductance causes resonances ringing effect when switching operations occur like loads turning off/on. The resonances can cause the transient voltage to spike as much as 3 to 10 times system voltage. Granted that is not much of a problem for a motor, but can reek havoc on electronics.

dereck,

Would you be so kind as to explain (in layman?s terms as best as possible) how the grounding (and where would you ground a delta) of a system prevents/dissipates the inductance and capacitance?

Are PoCo distribution lines Delta or Wye (I always thought Wye)?

Thanks,
Lou

 

[dereckbc]

dereck,

Would you be so kind as to explain (in layman?s terms as best as possible) how the grounding (and where would you ground a delta) of a system prevents/dissipates the inductance and capacitance?

Are PoCo distribution lines Delta or Wye (I always thought Wye)?

Thanks,
Lou

Lou, I will try. First let me set the stage.

Deltas systems are primarily used in industrial applications where outages and interruptions cannot be tolerated like chemical process, process manufacturing, water treatment, oil refineries, communication facilities, etc. These are large square footage facilities with lots of cable and distributed cable capcitance

Ok with that said, I am not advocating grounding a Delta system, they have their place, just not your normal commercial or residential applications. However there is a gray area that uses a resistive or impedance grounded system that gives the best of both worlds.

A resistive or impedance grounded system will behave similarly of both delta and wye systems. For example if a fault occurs on one of the supply phases, nothing happens except an alarm goes off to alert maintenance personnel. Sure some fault current flows, but not enough to operate the primary OCPD because of the Resistance/Impedance placed in series on the common conductor we know as the neutral between GEC and Xo. However we also get the benefit of a wye system and short out the cable capacitance to ground.

Hope that shines a little light on the subject.

 

[hardworkingstiff]

Sorry, but I still don't understand how grounding a system takes out the capacitance. I'm not arguing against the statement, just asking if anyone can explain it.

What is it about the grounding that takes out the spikes between phases?

 

[dereckbc]

Lou, sorry I misunderstood the question. The answer is really simple, sort of  . A capacitor is a very simple device of two plates separated/insulated by a dielectric material just like a battery. Imagine two pieces of wire side by side, or ran in conduit Capacitors will pass AC current just like a resistor with an impedance equal to 1/(6.24 * F * C). All you really need to get from the equation is the larger the capacitance or the higher the frequency is, the lower the impedance becomes.

OK take away from this thought that a cable ran with another cable, or a cable in a raceway forms the two plates of a capacitor, will couple together and pass current,, even though they are insulated from one another. If the system is large (long lengths of cable), you form large plates of a capacitor. So in effect you are coupled by capacitance to ground and adjacent conductors.

So if you are with me and understand how a capacitor works, then you know a capacitor will pass AC currents. This means in a ungrounded system you are passing AC current through your cable insulation which deteriorates the insulation over time. So if you ground the system, you short out the capacitance.

Ringing is a little more difficult to explain because it involves the interaction between inductance and capacitance as they are exact opposites of each other. The easiest way I can explain it is an analogy of of putting yourself in a barrel that is sealed up, or you singing in the shower. If you hit the barrel with a hammer (an impulse like a switch on and off) you get a very loud amplified sound that rings and decays. Or in a shower when you hit a certain note it resonates and is far louder than any other note. Hope that makes sense because otherwise it takes a lot of math to explain. In short, series and parallel capacitive/inductive circuits form oscillators that will generate a signal when excited. All you have to do is remove either the capacitance or inductance to solve the problem. Since we cannot remove the inductance of the cable, we can remove the majority of capacitance.

 

[hardworkingstiff]

Thanks Dereck!

That makes sense.

 

[dereckbc]

You are welcome. Glad you understood it, after I wrote it and re-read, I was worried.