Why is there a solidly grounded Neutral requirement for Electrode Boiler by NEC?

[TM519]

According to NFPA 70 (NEC) 490.71, for an Electrode-Type Boiler it states that it must only be supplied from a 3-phase, 4-wire solidly grounded wye system, or from isolating transformers arranged to provide such a system. What I have been unsuccessful in finding out is why for an electrode-type boiler is there the specific requirement for the grounding to be a solid ground?

We are installing an electrode boiler on a 6.9kV 3-wire system, so we need to implement a 4-wire solution in order to ground the boiler. This can be done with an isolation transformer and solidly grounding the secondary side.

With other equipment that do not say to exclusively use a solidly grounded system, another option would be to install an NGR with a zig-zag transformer, which accomplishes the same thing, at a significantly cheaper price.

We will be installing an Isolation Transformer in order to achieve a solidly grounded system to meet NEC, but I would just like to know why the specific requirement for an Electrode-Type Boiler to be solidly grounded? I have an electrical background, but I don't know much about Electrode-Type Boilers other than the basics of how they work.

 

[LarryFine]

I take "electrode-type" to mean the line conductors are directly "connected" to the liquid, like in an arc furnace. I can see why they would want and need the exposed portions of the tank well grounded, and that the neutral may well be a current-carrying conductor.

Is the 6.9kV not from a grounded source, or is the voltage wrong, too?

Sounds like a typical delta-wye transformer would work perfectly.

 

[StarCat]

I take "electrode-type" to mean the line conductors are directly "connected" to the liquid, like in an arc furnace. I can see why they would want and need the exposed portions of the tank well grounded, and that the neutral may well be a current-carrying conductor.

Is the 6.9kV not from a grounded source, or is the voltage wrong, too?

Sounds like a typical delta-wye transformer would work perfectly.

I am curious what TM's interpretation of a solidly gorunded system exactly is:
https://electrical-engineering-portal.com/an-overview-of-wye-and-delta-solidly-grounded-systems
The first WYE diagram on the page is what I am familiar with as LF has stated.
A lot of these units are being made in small capacities for small scale steam applications.

 

[tortuga]

490.71 was proposed in '75 then added to the 1978 NEC as 710.71, my understanding is there are 3 electrodes one for each phase in contact with water in a metal tank, all the electrodes flow Line -> Neutral, there is no line to line electrode boiler that I know of. Utilities have used them for ever to supplement steam plants, they became popular during the energy crisis of the 1970's in heavy industry like paper pulp mills and stuff that needs lots of steam. If the tank runs dry there is no runaway effect as it faults to an open circuit so they are safer than combustion boilers and can operate with less labor cost.

 

[mbrooke]

I'm just as confused.

When we say electrodes, do we mean probes in water or a sheathed heating element?

 

[kwired]

I'm just as confused.

When we say electrodes, do we mean probes in water or a sheathed heating element?

I think probes in water, and water is part of the circuit path. Sheathed heating element is what typical household water heater has and they don't call them electrode boilers, same for any other similar immersion type heaters.

 

[mbrooke]

I think probes in water, and water is part of the circuit path. Sheathed heating element is what typical household water heater has and they don't call them electrode boilers, same for any other similar immersion type heaters.

Makes sense.

 

[winnie]

If, as tortuga describes, the tank is part of the circuit, then the reason for a grounded neutral is clear. For safety reasons the tank would need to be grounded, and for symmetry reasons it would need to be a neutral.

Consider the relative sizes of the tank and the electrodes. If you built the system using say a corner grounded delta (say B phase grounded) then you would see high current from A to B and from C to B but I suspect lower current from A to C.

-Jon

 

[kwired]

If, as tortuga describes, the tank is part of the circuit, then the reason for a grounded neutral is clear. For safety reasons the tank would need to be grounded, and for symmetry reasons it would need to be a neutral.

Consider the relative sizes of the tank and the electrodes. If you built the system using say a corner grounded delta (say B phase grounded) then you would see high current from A to B and from C to B but I suspect lower current from A to C.

-Jon

Ungrounded delta should work, but if anything develops that would shift the neutral point then you are unbalanced, by using a solidly grounded wye you are forcing a fixed neutral point.

 

[iceworm]

I'm up for some education.
How do these work? Do they depend on the conductivity of the fluid (water?)? Boiler grade water is pretty non-conductive - or maybe not?
Do they just draw an arc between the electrodes?
Just curious

 

[kwired]

I'm up for some education.
How do these work? Do they depend on the conductivity of the fluid (water?)? Boiler grade water is pretty non-conductive - or maybe not?
Do they just draw an arc between the electrodes?
Just curious

I'm guessing majority of them are using medium voltages, 600 or less I wouldn't think would work so well for a boiler in this manner.

 

[winnie]

We are installing an electrode boiler on a 6.9kV 3-wire system, so we need to implement a 4-wire solution in order to ground the boiler. This can be done with an isolation transformer and solidly grounding the secondary side.

With other equipment that do not say to exclusively use a solidly grounded system, another option would be to install an NGR with a zig-zag transformer, which accomplishes the same thing, at a significantly cheaper price.

If you don't mind grounding the entire 6.9kV system, you could use a large enough zig-zag transformer and solidly ground it, rather than using a smaller zig-zag with an NGR. I don't work with MV systems, but a grounding autotransformer would presumably be cheaper than an isolation transformer.

-Jon

 

[TM519]

Really appreciate all of the responses, it is definitely helping me piece this together. The reasoning for grounding is clear to me, where I am still failing to make a connection is why the NEC is specific about the grounding being a solidly grounded system. I don't completely understand why it would not be ok to do a zig-zag transformer when in theory it would accomplish the same thing. The plant is now pushing back looking for this answer and I don't really have a clear answer. I am guessing it has something to do with the way that an electrode boiler works?

 

[winnie]

A zig-zag transformer with a neutral grounding resistor is one thing.
A zig-zag transformer with a solidly bonded neutral is another.

What does a zig-zag transformer do: It is an autotransformer that derives a neutral from 3 phase lines. You can use it to change a transformer with a delta secondary into what is effectively a wye secondary.

AFIK if you have a delta source, and connect a suitably sized zig-zag transformer, and then solidly ground the neutral, then you have created a real solidly grounded system.

The key is 'suitably sized'. A solidly grounded zig-zag transformer is expected to carry neutral load current in a wye system and must therefore be sized for this application. A zig-zag transformer connected to a neutral grounding resistor is only expected to carry fault current limited by the resistor.

As I said, I do not work with medium voltage systems and I don't design system grounding. I do motor stuff. However my _guess_ is that you can use a suitably designed zig-zag to turn your 6.9kV system into a solidly grounded system. You would need to consider all other connected loads, and might need to change all of your protective relaying.

 

[texie]

While I'm not an expert on zig-zag transformers as they are mostly in the POCO world, I think the issue here is that it would have to be sized to carry the full load. As I understand them most are used and sized to only carry short duration fault currents to open OCPDs. I would seem that by the time you size it to carry full load continuously you might just as well use a transformer with a Y secondary.
Also as Winnie alluded to, a separate winding Y transformer would not affect your 6.9 KV system up stream. And of course there is the issue that the NEC says it must be connected to a grounded Y system.

 

[TM519]

A zig-zag transformer with a neutral grounding resistor is one thing.
A zig-zag transformer with a solidly bonded neutral is another.

What does a zig-zag transformer do: It is an autotransformer that derives a neutral from 3 phase lines. You can use it to change a transformer with a delta secondary into what is effectively a wye secondary.

AFIK if you have a delta source, and connect a suitably sized zig-zag transformer, and then solidly ground the neutral, then you have created a real solidly grounded system.

The key is 'suitably sized'. A solidly grounded zig-zag transformer is expected to carry neutral load current in a wye system and must therefore be sized for this application. A zig-zag transformer connected to a neutral grounding resistor is only expected to carry fault current limited by the resistor.

As I said, I do not work with medium voltage systems and I don't design system grounding. I do motor stuff. However my _guess_ is that you can use a suitably designed zig-zag to turn your 6.9kV system into a solidly grounded system. You would need to consider all other connected loads, and might need to change all of your protective relaying.

This comment especially, combined with some of the other comments, has really helped me understand a lot more. So a zig-zag transformer connected to a neutral grounding resistor is only rated to carry the current produced by the line to neutral voltage, which is ~1.73 less than that from a wye connected line-line. Since the electrode-type boiler heats the water by current flow from line-neutral voltage then significant damage could occur to the boiler if a ground fault occurred in the system and the NGR was only sized for current produced by line to neutral voltage. And from other comments made, if you solidly ground a zig-zag transformer then you might as well just go with an isolation transformer because the zig-zag transformer would have to be oversized to the point that it would not longer become the cost-effective option, which is really the whole point.

I really appreciate all the input here.

 

[winnie]

Thanks for doing the readback, since I think you sort of misunderstood me.

A zig-zag tranformer creates a neutral.

When this derived neutral is connected to a NGR it is sized for the current limited by that _resistor_, and then only during fault conditions.

When this derived neutral is solidly grounded and connected to a line-neutral load, the the transformer needs to be sized for the _load_ in addition to any fault current.

This has nothing to do with the line to line vs line to neutral voltage, and simply the current that the transformer needs to carry.

I think though that the take-away is correct, the zig-zag transformer for solid grounding is significantly larger than one used for a NGR, and thus may not be much of a win versus an isolation transformer. It is still an autotransformer, so might actually be smaller/cheaper or more efficient, but that is in the detailed design.

Is there any other equipment connected to this 6.9kV supply? Can it tolerate solid grounding?

-Jon