Permissible loading for transformer 2

[vandax]

We have been operating 90/150(FOA) MVA transformer for 230/115 kV for 3 years. I have no accurare refference, how many % the transformer can be loaded exceed its maximum capacity? The fact that..sometimes we loaded it 125% of maximum cap for a 15-20 menutes due to operation need.
Present setting for over load is 150%?? Is it right?
Please your help and opinion.

rgds

 

[sparky1976]

try this USBR site, they have good information.

http://www.usbr.gov/power/data/fist_pub.htm

 

[SidiropoulosM]

A transformer of these specifications can be loaded to 125% for up to 30 minutes without loss of life expectancy. If this overload occurs several times a day at ambient temperatures over 30 degrees C, some life expectancy will be sacrificed. Keep in mind that thermal aging is a cumulative process. If you need to develop a comprehensive overload strategy, the transformer manufacturer should be able to supply you with overload capabilities for different time durations.
Michael Sidiropoulos

 

[RRaghunath]

Vandax,

Overload capability of a transformer is a complex thing to decide. IEC 60356 provides guidance in this respect.

The capability takes in to account loading prior to the start of overload, ambient temperature, loading pattern during other than overload periods.

The impact on life of the transformer has to seen after studying the above parameters over a period of one year. It is confirmed by all the manuacturers that if the transformer is operated within the guidelines of the IEC 60356, the design life of the transformer stays unaffected.

It is also said that the transformer overload should not exceed 150%, gnerally speaking, to prevent failure due to localised hotspots.

It is an accepted practice to set Overcurrent protection pick up at 150 to 175% in order to allow short time over loading, especially in cases involving parallel connected transformers and one of them tripping. Raghunath

 

[peebee]

The IEEE Red Book also has some information.

The greater the magnitude & frequency of overloads, the shorter the transformer life. As previously mentioned, temperature has a big impact too.

Utilities commonly run transformers in overload conditions. My local utility won't change a tx out until it's at least 40% overloaded. In warm areas (such as the southwest US), though, utilities are generally more careful not to load transformers beyond nameplate rating.

 

[jwerthman]

Check IEEE/ANSI C57.92 - Guide for Loading Mineral-Oil-Immersed Power Transformers.

 

[DanDel]

I would check with the transformer manufacturer before I ran the transformer for extended periods at values above 100% nameplate rating. This is also my recommendation for your overcurrent device setting, which will allow a 50% overload with no time limit. I've been providing coordination and protection settings for over a decade, and have never set an overcurrent device to more than the manufacturer's nameplate rating of the device being protected(especially for this size transformer).

 

[cuky2000]

Under emergency loading conditions, many utilities loading practice could operate units at 130% of nameplate rating in summer and 150% of nameplate rating when ambient air temperature is les than 10oC (50oF). For short-term loading, the top capacity is limited to 200% of nameplate rating.

For continuous loading, the ANSI Standard C57. 92 permit to load the transformer increasing the rating 1% for 24 hr air temperature bellow 0oC<T< 30 oC. Also decrease the nameplate rating 1.5% for 24 hr average air temperature between 30oC>T>50oC.

For short time rating of 15 to 20 minutes per day the transformer should be operating OK within the normal life expectancy. However, to minimize the guessing, please consider the following suggestions:

- Estimate the daily loaded cycle and the site ambient seasonal temperatures.
- Check the data and graph on C57.91 & 92, for short-term overload operation.
- Consult the transformer manufacture for recommendation.
- Monitoring closely the top oil temperature, hot-spot and oil characteristics Typical values are as follow:
o Top oil max…………………….110 oC
o Hottest Spot conductor temp….180 oC
Taking temperature reading is OK . However, having online oil monitoring could be a lot easy. Check the following site:

http://www.gepower.com/dhtml/substation/en_us/products/monitoring_diagnostics/faraday_ism_d.jsp

- Explore supplementary cooling to operate the unit within the hotspot temp allowance.
- Explore if is cost effective use a loading software. See sample attached

 

[jghrist]

The overcurrent relay is to protect against faults, not overload. You don't want to knock 150 MVA of load offline for a temporary overload. I would set the pickup of the overcurrent relay above the maximum expected temporary overload, normally at least 130%. This can be done with inverse time-overcurrent relays while still staying below the transformer damage curve.

If loading can be this high, it can be monitored by SCADA and decisions made depending on the circumstances. The decisions could well include switching part of the load to other sources. Some utilities use high temperature for tripping, which will not cause tripping on short overloads.

 

[RRaghunath]

Cuky,

You mentioned allowable hot spot temperature as 180 deg.C. I read that the transformer oil (Petroleum crude based mineral oil) flash point is 140 degC and fire point is 160 degC. My understanding is that the hot spot temperature shall in no case exceed 140 degC.

The winding temperature trip is set at 115degC, which is supposed to reflect the winding hotspot temperature, in almost all the cases that I have come across and seems to be the transformer manufacturer's recommendation too.

I would be grateful if you could elaborate.

Thanks. Raghunath

 

[jlazucena]

See previous posts use key word search:transformer loading
Please see IEEE Std C57.91-1995
Specially 4.1 and 8.1, then Table 1
In short make sure that you do not exceed the cooling capacity of the transformer oil,
Basically is a 65º Celsius raise from an ambient temperature of 30º Celsius. You can exceed this value but you will accelerate XFMR aging. (Make sure that the current overload relay and tem raise relay do not operate for that loading)
If you want to load the XFMR during winter you have extra capacity (Ambient temp less than 30º C), but in a hot summer week you have less capacity (Ambient temp more than 30º C).

The industry recommendation for a &quot;65° C raise transformer&quot; is that during rated load, the temperature of the winding hot spot should not surpass 110°C or rise 80°C above a 30°C ambient temperature, as follows: 30°C ambient temperature, plus 65°C allowed oil temperature raise, plus 15°C that is the expected difference between the oil temperature and the winding hottest spot. To determine operating temperatures, the oil temperature rises for any loads are added to the ambient. This makes the ambient temperature a significant element in determining the loading capability of a transformer.

 

[cuky2000]

Bubble evolution is a probability that happen not only on the oil but in the solid insulation as well. Overloading the transformer beyond the nameplate is not a free ride; there is a potential risk of degradation of the dielectric strength integrity of the transformer that should be considered.

The 180 oC hottest spot operations is the standard policy recommended for several major utilities in the Northeast of the US that had over 30 year of experiences dealing with transformer loading capability beyond the nameplate.

There are operators with more conservative’s approach and others with more aggressive loading practice as a function of loading time as follow:

- Normal Life expectancy…………………………140 oC
- Planned beyond nameplate………………………150 oC
- Long term contingency…………………………. 160 oC
- 12 hours contingency…………………………… 180 oC
- 2 hours contingency…………………………….. 200 oC

Beware that there are other limiting factors to load transformers beyond the nameplate some related with ancillary equipment such as bushing, tap changer (if installed), etc and others with design limits such as Eddy current circulation, stray flux heating, noise, cooling, regulation, etc.

 

[DanDel]

jghrist, I would agree with you that the Primary O/C device is typically used for faults, but the Secondary O/C device is for overloads(per IEE 242). It all comes down to heat damage in the transformer( as detailed by the other posts here), and the manufacturer has determined the load ratings by the installed nameplate. I'll stand by my original recommendations.

 

[jstickley]

DanDel --

I've gotta agree with jghrist on this one -- the bottom line is that transformers typically overload when they're most critically needed (during periods of high load). I wouldn't want to trip a transformer automatically in that circumstance (unless the transformer were severely overloaded and in imminent danger of failure). Tripping transformers due to overload can subject a system to even worse conditions (other transformer overloads, low voltages, loss of load, etc.), which is unacceptable.

Maybe I'm mis-interpreting your posts, but setting protective relays to trip a transformer immediately after it exceeds its nameplate sounds like a dangerous practice to me.

 

[peterb]

For overload protction, the winding temperature device is usually set to trip, after a prior alarm. I fully agree that the overcurrent relay is there for fault protection only - typical settings are 150-300% of ON rating. With an extremely inverse time curve, this will allow setting below the short time overload curve.

 

[DanDel]

All overcurrent devices(51, not 50) have a time delay curve. The pickup of the 51 device should be set at the maximum rating of the device being protected from overload. A transformer has a Damage Curve which is based on it ratings. If you follow this damage curve on a time-current plot, will end up at the FLA of the transformer at somewhere near the 10000s point, though this is not usually drawn on a typical TCC. The secondary 51 device should always be set to trip open the secondary C/B before the Damage Curve is reached. The proper setting could allow FLA forever, and a trip at 120% load in maybe 10-15 minutes. Higher loads will cause a quicker trip. This is called an inverse-time trip curve. The 50 (Instantaneous) device is used for fault protection, has no inverse-time characteristic, and will trip with no intended time delay at its setting.
Typically, the secondary device for low-voltage applications(600V and under) does not have an Inst trip if there are feeder C/Bs present. The feeder C/Bs will have Inst trips for any secondary faults. The primary device will have an Inst trip, set relatively high, to protect the transformer and switchboard bus from a fault.
A &quot;90/150(FOA) MVA transformer for 230/115 kV &quot; as posted in the original question will have a MV secondary C/B with 50/51 protection, along with primary 50/51 protection. The IEEE 242 Standard, 10.8.3.2, which lists this size transformer as Cat IV, explains how transformer protection should be used.
Again, I will stand with my original recommendations.

 

[jstickley]

DanDel --

Here are some links to some fairly well accepted transformer protection guides (note that this discussion over overcurrent relaying neglects the fact that a far better primary protective scheme for a 150 MVA transformer is differential relaying).

See page 9, short-circuit protection with overcurrent relays at:

http://www.geindustrial.com/industrialsystems/pm/notes/artsci/art11.pdf

&quot;The overcurrent relays should have an inverse-time element whose pickup can be adjusted to somewhat above maximum rated load current, say 150% of maximum...&quot;

From Blackburn's book, Protective Relaying, Principles and Applications, 2nd Edition, page 310.

&quot;It is desirable to set the protective devices as sensitive as possible, but the fuses and phase-overcurrent relays must not operate on any tolerable condition...&quot;

The bottom line to me is this: what's more important, preventing any loss of transformer life, or ensuring that your system is secure? Personally, I'll take keeping the lights on any day over possibly needing to replace a transformer a couple years earlier due to loss of life from overloading. You simply have to consider the big picture of why your transformer is there in the first place and let that guide your protective settings. I'm not saying that your settings are wrong, but simply that they may not be appropriate for every situation (I believe this to be one, given that the thread author stated this transformer has occasionally been loaded to 125% of maximum capacity).

 

[DanDel]

jstickley, I agree, the same settings are not appropriate for every application.
If you look at the IEEE 242, and ANSI/IEEE C57.92, you can calculate just how much of this particular transformer insulation life you will lose by running into overload situations.
All transformers must have a short-term overload capability, and this is confirmed by the manufacturer. However, let's go back to my original recommendation(it seems so long ago...) and contact the manufacturer of the transformer to ask their opinion. While they probably don't have a problem with a 125% overload for 15-20 minutes, I'll bet they do have a problem with the overcurrent device set at 150%, which will (theoretically) allow a 149% continuous overload.
You have to look at several things which we aren't aware of: the age of the transformer, the results of the regularly scheduled maintenance testing and oil sampling of this transformer, the possibility of downstream PF correction to open up some extra kVA, etc. The main thing for vandax to understand is that he is shaving service time off of this transformer with every overload, and considering loading up to and above the 150% setting of his overcurrent device, as recommended by other people in this thread, isn't helping that situation.

 

[jstickley]

DanDel --

I don't think people are suggesting that 150% overload of his transformer is necessarily acceptable, but that the transformer should not automatically trip until at least that level. Operator intervention is a necessity to alleviate some transformer overloads, simply because you can't afford to just allow relaying to trip the unit due to other concerns (voltages, other overloads, etc.).

 

[SidiropoulosM]

I believe we all agree that it is not sensible to trip the transformer automatically. You would be leaving money on the table, because transformers do have some overload capability that comes free, without reduction of life expectancy. Whether this capability is 125% or 150% or some other number depends on the ambient temperature and on the duration of the overload. In many cases this is a preferential issue. If the transformer is critical for reliability, then some loss of life will be acceptable. In many other cases, your system operators will have 30 minutes or so to find a solution. For example, switching loads to other circuits may be an option. The bottom line is that you must co-ordinate your protection system with the transformer damage curves. This has to be the basis for any transformer overload strategy. Michael Sidiropoulos