Anti Condensation Heaters / General Electrical Loads - Medium Voltage Switchgears (PV Plants) 2

[mcm00048]

Good Morning all,

I am working now in a Grid Connected PV Project located in Spain. In the MV Switchgear Arrrangement, it is requested to provide space anti condensation heaters to avoid condensation. Such heaters shall be fed from an auxiliary power supply of 230 Vac or 125 V ac

During night, there is no power available from Inverters. In those cases, from which power supply the Space heaters are fed?, I guess it is made by the external grid supply. But, it comes to my mind the following question. In case of shutdown of the plant and also Grid is not available for feeding the loads of the Plant ( Heaters in this case) due to maintenance or replacement of Step-Up Transformer, what could be the power supply for these electrical loads? Here below:

1. Diesel Generator System , feasible?
2. Oversizing of battery rack
3. Shutdown of all equipment including heaters could cause malfunctioning or failure of MV Switchgear?

Thank you in advance,

In addition to above, I would like you to tell me names of books / literature of operation related with PV Plant operation, commisioning, scheduled shutdowns as well as emergency operation of this PV Plants It would be appreciated!

Hope all of you have a nice week,

Miguel.

 

[RRaghunath]

You are right, the anti-condensation (space)heaters are required when the plant is not working / shutdown.
I suppose the plant will have LV power supplies available all the time for supplying plant lighting and small power load.
The space heaters also should be supplied from the same switchgear.

Rompicherla Raghunath

 

[mcm00048]

Thank you RRaghunath for your prompt reply.

I assume the Loads from the PV Plant will be supplied from grid in case of shutdown of the plant.Of course, heaters are supplied from the Auxiliary Power Supply of the MV Switchgear Arrangement, but this power would come from the Grif. But, in case the PV Plant shall be brought to shutdown and grid power is not available, it is normal to feed these heaters from the UPS or Diesel Generator is common practice?

As far as I know, Batteries and UPS shall be sized to provide the first two hours of electrical consumption for most critical consumers ( emergency lighting, control voltage for commands of CB, SCADA and essential services)

Space heaters are also considered essential loads of the Plant? Can be they supplied from UPS as well?

Thanks in advance.

 

[itsmoked]

A UPS is not viable for this use. If the plant is truly isolated from the grid when shut down and has no alternative grid power for black-start type operations or plant functions during long scheduled maintenance you need a generator.

How are you able to do any work on the plant if you have no grid or local power company power? How is lighting provided? How do you charge your radios? How do you run a power drill? Seems short sighted not knowing the actual situation. Is the plant 50km from nowhere and the only power nearby is 100kV?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Marmite]

Switchgear panel heaters are not normally considered essential loads. Protection, SCADA, Comms, Security, emergency lighting and fire systems are essential loads. If you only have battery capacity to sustain essential loads for 2 hours after loss of mains power, then I wouldn't be worrying about the panel heaters going off. In my experience loss of panel heating is not even alarmed via SCADA. It's usually picked up by inspection.
Regards
Marmite

 

[crshears]

What Marmite said; loss of supply to anti-condensation heaters for a time [say, one or two weeks] would not normally lead to significant equipment lifespan degradation.

That being said, after such a shutdown, I'd recommend returning the heaters to service for a period of 24 or 48 hours to drive out the lioness' share of any moisture that may have infiltrated the windings / switchgear, after which the equipment could be livened up and generation could resume - and in my experience, such site outages are normally planned and scheduled in such a way that all work is completed and all equipment re-assembled by noon on a Friday, whereupon grid power is restored, auxiliaries are returned to service, batteries fully recharged, communications verified, anti-condensation heaters placed in service, etc., before everyone goes home for the weekend. Dry-out can therefore take place for ~60 hours before the balance of the plant is livened up and generation resumes on Monday morning.

In the event there is only one grid supply, which is interrupted for the duration of the outage so that maintenance can be performed, either the permanent standby generator or a temporary one brought in for this express purpose will be run during normal working hours to provide working power; the sizing of such a unit is a matter of judgment, and since under-loading is as bad as over-loading, anti-condensation heaters are commonly left out of service, batteries may or may not be topped up / floated for the day, etc., etc.; operator discretion is advised.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

Generally condensation may occur on any day.
The heaters in the island utility were alarmed and in the event of a failed heater alarm I would be on the next available flight to the island.
Every morning was a condensation threat.
That said, I will concede that rotating equipment and dry type transformers are at more risk than switchgear due to the greater thermal mass.
Anti-condensation heaters are often supplied at one half of rated voltage. eg: 230 volt heaters supplied with 115 Volts.
Condensation can form at any ambient temperature.
Condensation is most likely to form when the dew point is rising.
Equipment with a large thermal mass is most likely to be subject to condensation. The iron cores of the stator and of the rotor of a motor or of a generator are at particular risk. In conditions when both the temperature and the dew point are rising the temperature of the cores will often lag below the dew point due to the inertia of the thermal mass.
Most equipment will generate enough heat during normal operation to avoid condensation.
In the case of de-energized equipment, when dew forms on the ground you are at risk of condensation.
If the equipment is left de-energized without anti-condensation protection it may be well to consider a method of ensuring that the equipment is safe to energize every time that it is left de-energized during any time of rising temperature.

BUT Consider redefining your issue.
Rather than looking for a solution to powering heaters look for a solution to avoid condensation damage.
Consider a desicant pack with a heater to keep the equipment dry.
The desicant pack will absorb moisture from the air in the equipment enclosure and so lower the internal dew point.
Desicant packs are often used to protect new MCCs during new construction. The MCCs may be placed long before the room is heated and long before power is available. The desicant packs give a visual indication when they are becoming waterlogged. They are typically checked weekly or bi-weekly and changed as required.
In your case it may be possible to provide heating of the packs when power is available so that the packs are regenerated daily.
In this case the interval between checks may be greatly extended and may possibly be included with other regularly scheduled maintenance tasks.
Note: This method is most effective if the flow of air into the enclosure is restricted. It may be well to delay the application of heat to the desicant pack for enough time to allow the equipment to generate enough heat internally to raise its temperature rather than powering the desicant heater at the same time as cold equipment is powered. But that is a judgement call based on local conditions.

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

 

[crshears]

Good points all, Bill.

Now that you mention desiccant packs, I've also seen replaceable desiccant packs used for the purpose you describe; a schedule was established whereby the DPs in each piece of equipment were swapped out one at a time, with the wet pack being baked in an oven for a prescribed time and temperature to dry it out, whereupon it would be swapped out with the next one on the roster...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[prc]

In air filled cable boxes of large power transformers I used to provide normal silicagel breathers (type used with conservators of transformers).Cable box will be made air tight (esp at cable glands of power cables)and the breather in take will be from top of cable box. This will avoid condensation and during energized condition, heating of bus bars will send out dry hot air through silicagel, driving away the moisture absorbed during de-energised period. Something similar can be tried in your situation. Use a small sized silicagel breather.

 

[edison123]

Great simple solution by Bill. Cost effective and reliable.


Muthu

http://www.edison.co.in

 

[waross]

Nice suggestion prc. There may be a problem if the enclosure has breather holes or slots.
But I could have used that solution a couple of times in the past.

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

 

[prc]

waross, with replaceable desiccant packs also, air tightness is essential. Otherwise the pack will get saturated soon and become useless. Another alternative is to provide ventilation louvers (at bottom and top of cubicle) This is a common solution with large ,outdoor, cast resin transformers and quite effective. I had successfully convinced users with these alternate solutions who were insisting for space heaters.

 

[waross]

Hi prc.
I accept your solution for cast resin equipment.
However I see rotating equipment as a different issue. My concern with rotating equipment is the temperature lag of the thermal mass of the iron and the windings. In times of rising dew point, the temperature of the thermal mass may not rise fast enough to stay above the dew point.
I see internal condensation in a rotating machine as a much more serious problem than possible surface condensation on a cast resin transformer.
I will admit that most of my anti-condensation experience has been with rotating machinery which may one of the more critical situations.
I think that we are looking at opposite ends of the anti-condensation spectrum with cast resin transformers being the best and rotating transformers being the worst.
Going back to the original poster, he will have to evaluate his equipment and the susceptibility to condensation and decide where on the hazard spectrum it falls.
Respectfully.
Bill

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