Electric boiler and zig-zag transformer

[moosewl]

We are looking at putting in a new electric fired boiler. Our understanding of how the boiler works is as follows. Water is sprayed from a nozzle onto energizered plates. The nozzle is connected to neutral and the plates are energized with 13.8KV. This vaporizes the water into steam.

Our distribution system is the plant in a 13.8KV delta system. We have a preliminary design to install a zig-zag transformer and neutral grounding resitor to give us an "artifical" neutral.

Two questions: 1)Is this a correct application for zigzag xfmr and ngR?
2) Is the neutral bus in the substation connected to the X0 point of the zig zag, or the grounded point of the NGR?

 

[waross]

If you are using line to neutral connections you MAY NOT use a grounding resistor.
Any ground fault anywhere on your system may raise the potential on the nozzle to over 7 kV with a resistance grounded system. This is completely unacceptable for safety.
The grounding transformer(s) will carry only harmonic currents and magnetizing currents during normal service.
The grounding transformers will have to withstand line to ground fault currents until the protection clears.
Your artificial neutral will provide a ground for not just your furnace but the entire 13.8 kV distribution system.
Consider using three identical 13.8 kV primary distribution transformers in a wye:delta configuration to develop your artificial neutral. These should be off the shelf and may be much cheaper than a special order zig-zag transformer. The secondary voltages are unimportant but must be equal.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

I am more worried about the boiler! How does spraying water on "energized" plate vaporizes? The rain water on 750 kV lines does not.


Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

Hi Rafiq.
I assumed conduction between the grounded nozzle and the energized plates. If it works, the R&D must have been exciting.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[moosewl]

Waross - Thanks for the information. We have changed paths a little and are looking at installing a 13.8KV 1:1 isolation transformer with a gounded wye secondary to give us a solid neutral point.

Thats correct waross, the water acts as a conductor between the nozzle and the plate. The water is continually being sprayed onto the plates and not all of it turns to steam at once. Definitely think the R&D was pretty interesting.

 

[Compositepro]

I can see boiling water in an electric arc, but not using 13 KV in the arc. Once the arc is established the resistance drops pretty low. The power dissipated in the arc is I squared R. So high current, not voltage, is needed. A balast is needed to limit crrent and power is lost in the ballast.

 

[waross]

I have boiled a lot of water with 240 Volts and with 120 Volts.
I imagine that with a greater spacing water could be boiled with 8000 Volts(approximate voltage from the nozzle to the plates).

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rcw retired EE]

I worked on 12 kV and 13.8 kV electric boilers, but it was >20 years ago adn don't remember much of the operational details.


But one problem was common to several installations. During operation the boiler acts as a grounded wye load and is a source of ground fault current to the system.

At one hospital, the 13.8 kV feeder had a zero sequence CT around all cables feeding the boiler with a sensitive 50/51G ground fault relay to detect faults in the boiler. The relay also tripped for any ground fault in the utility's 13.8 kV underground network creating a lot of issues for the hospital.

Using an isolation transformer prevents this problem.

 

[moosewl]

FYI, here's a brief description from the boiler vendor.

The model HVJ is a high voltage jet-type boiler in which water from the lower part of the boiler shell is pumped by either an internal or external circulation pump to the nozzle header and flows by gravity through the jets to strike the electrode, thus creating a path for the electrical current. As the unevaporated portion of the water flows from electrode to the counter electrode, a second path for current is created. Primary voltage connections are made directly to the electrode terminals often eliminating the need for a step-down transformer. At maximum rated conductivity, approximately 3% of the flowing water is evaporated.

Regulation of the boiler output is accomplished by variable frequency drive of the circulation pump, which controls the amount of water reaching the nozzle header. This pump speed is regulated by the boiler pressure and load control system either to hold the steam pressure constant or to stay within an adjustable KW limit.

Regulation is stepless between no-load and full-load, so that the boiler output is finitely responsive to demand variation.
No-load to full-load regulation may be accomplished as quickly as 5 seconds, and vice-versa.

 

[waross]

Two issues here:
If you run with a neutral point generated by a zig-zag transformer or by a wye:delta transformer this will provide a ground for your entire 13.8 kV distribution system. A ground fault anywhere in your 13.8 kV system will be grounded by your grounding transformers. The sizing of your grounding transformers will be dictated by considerations of your entire 13.8 kV distribution system.
If you run with impedance grounding, or if you use transformers to establish a neutral point but do not ground the neutral, then the nozzle and counter electrode may rise to almost 8000 Volts above ground potential in the event of any ground fault in your 13.8 kV system. This will not be allowed by either code or OSHA unless the boiler is specifically approved for operation with the nozzle and counter electrode ungrounded. Ungrounded operation may cause further complications if there is a potential drop in the piping between the counter electrode and the re-circulating pump, inside the piping. If the piping is metallic there may be voltage rise issues with the re-circ pump including the possibility of steam generation in the piping to the re-circ pump. Not good.

Bill
--------------------
"Why not the best?"
Jimmy Carter