hi,

i require some advice on a problem we are having with an I.T server bank based in uk.

we have a 300 amp 415volt incoming supply. we are using 220 amp of power, only powering data servers and it equipment.

however the 200mm2 4 core swa cable is red hot constantly we have been told this is due to harmonics and we have been told to upgrade the neutral inconductor as this is the one that's being overloaded due no non linear loads. this to me seems like it will just mask the problem. has anyone any solution to our problem? i have heard of some active harmonic filters, does anyone have any experience with these on systems using as much power as we do????

any help appreciated
richard

 

Richard... the problem you are experiencing (if not new) is not unusual for the load you described.

The most likely cause is that the neutral conductor is too small.

Q1) Are you sure the 4th conductor is 200mmq and not a smaller size?

Q2) Was the 220A value a reading of the phase conductors?

Q3) Do you have a deg-c value to describe "red hot?"

I suggest you take a current reading with a true RMS reading ammeter. Also monitoring the neutral current on a scope or P-Q instrument will confirm the presence of harmonics!

Regards, Phil

 

Well yes you can try resizing the neutral conductor - but you do realize that has nothing to do with the root cause of your Harmonics problem at all. Additionally once you re-size the neutral the problem may resurface some where else and of course all the while you'll be paying a hefty Electricity Bill for running your facility inefficiently on a Low .pf electricity

Being a Data Bank you probably also have a series of Manufactured or "Built Up in Place" UPS systems protecting your Servers. This coupled with Switching Power Supplies of the Servers them selves is the root cause of your problem.

Additionally your Utility provider can own a contributing percentage of the problem in the fact many Utilities often "Buy" KVAR from other Utilities (which is a really bizarre term) and the Grid supplying your facility can be partly to blame as well.

As I said most likely the problem resides within your UPS systems and you need to correct it there through the use of Harmonics Traps. But of course you'll need a thorough audit of your Power Quality to identify the sources and properly size the Harmonics Traps

Sorry for the bad news but you can always present it to the "Bean Counters" as Energy Efficiency Measures as once you correct these issues you should realize a significant reduction in your electricity consumption.

Good Luck

 

Hi ,

the 4th conductor is definitely 200mm2

61 degrees Celsius is the outside temperature of the cable

all the phases are balanced
red phase =218 amps
yellow phase =222 amps
blue phase is 215 amps

 

Richard... what about the neutral-current?

Phil

 

Richard... the line-current values yield an Ampere Unbalance Factor (AUF) of about 1.8%, which is considered low. Two additional thoughts:

(A) Can you confirm the 4 core cable has a non-metallic sheath!

(B) What are the line-to-line voltages?

(C) Ehat are the line-to-neutral voltages?

(C) What are the line-to-earth voltages?

Phil Corso

 

Richard... what is the ambient temperature?

Phil

 

can you confirm one thing....
what is your load i.e. how many load point is there.

The idea is if it is possible try to use your R,Y,B phases interchanged at different load points. This will give you max possible balanced condition.

 

You indicate SWA: Steel Armored Cable. is this the right cable for your application?

Power cables can run hot, that's why they have insulation ratings

sounds like you need a IR thermal survey to insure the installation is up to par

 

Richard... Good pickup by d-! If installed as a shield, then increased temperature of the shield could be caused by negative sequence currents being induced.

The root cause could be the earthing method used!

Regards, Phil Corso

 

> installed as a shield, then increased temperature of the shield could be
> caused by negative sequence currents being induced.

Only problem being ALL High Voltage cables are metal sheathed and if grounded and installed appropriately do not have Inductive Heating problems as you described.

The Op stated he was told he has a "Harmonics Problem" so I am assuming he had qualified personnel look over the installation and check for the obvious. Load Imbalance / Proper Grounding (to include the metallic sheath)/ Proper Installation and or Application of materials

Their is 1 more scenario and that is this is a Cable Tray installation and cables were not properly bundled together. This WILL cause hysteresis to the sheath/ground and result in over heating.

But really Harmonics content surrounding Switching Power Supplies is very common so I'll take the Op at his word and assume he has his information correct. That being said All the equipment on the same Service Transformer as the Data Servers will be effected and draw significantly more electrical current to meet their demands. Additionally the power supplies of the servers themselves will run a hotter and shorter service life span.

The whole Neutral Conductor resizing scheme (1992-1994) was merely the electrical industry buying time for the Underwriting Authority to regulate Harmonics content. Which they did and basically told the manufactures to clean up the content or loose the U.L. rating. Ever see the inside of Medium Voltage VFDs - they come with Harmonic Traps installed at the factory to meet these guide lines.

You can play with the conductor sizing but you will not change the adverse effects of the lower power quality

 

Freakindj... I accept your thesis regarding MV cable installations, but this is a LV one. And the cable-size is such that induction heating of the sheath/shield is a possibility. And, if correctable, then discussion of cable change is moot.

Regards, Phil Corso

 

R. Hoppy, I suggest you go back to the design of the system and check the following:

1. Was the supply system to the servers designed for a system operating at fundamental frequency only or were harmonics taken into consideration?

2. Was information about the amount of harmonic content fed back to the supply system from the servers, requested from the suppliers of the servers' power supplies?

3. Were the ratings of cables corrected for harmonic content, or conversely was the magnitude of the current passing through cables corrected for harmonic content?

4. Was the 3rd harmonic effect on neutral connections (addition of 3rd harmonic content at points where single phase feeders are connected to a 3-phase distribution board) taken into consideration in the design?

Food for thought?

 

There was a request by Phil Corso for the neutral current value which could confirm that the neutral current is the problem. 61 degrees C is close to the typical minimum power wire temperature of 60 degrees C but if you have 75 or 90 degrees C wire your current rating may be a bit higher but you may still have an issue with terminal ratings or other electrical equipment temperature ratings.

I agree with d-'s recommendation of doing a thermal scan for all your circuits (particularly I wonder about terminations, circuit breakers, and transformers in the circuit).

What can also be done to troubleshoot harmonic problems is that you can get a rough indication of the level of harmonics by using an averaging ammeter and a true RMS ammeter to read the current and divide the average current value by the true RMS current value (ratio = 1 =no distortion while a ratio of 0.5 would indicate significant distortion) or you can use an Agilent U1242B DMM which does the harmonic ratio or there are smart DMM like meters (e.g. Fluke 41B or 345; Amprobe ACD-50NAV, etc.) which are available to help you troubleshoot a harmonic problem. Using these you can verify the problem and you can also find out which of your equipment are the major offenders. Once you know what is going on, you can address any problems.


William (Bill) L. Mostia, Jr. PE
Sr. Consultant
SIS-TECH Solutions, LP

Any information is provided on Caveat Emptor basis.

 

If one is unable to procure an expensive instrument to determine Harmonics, it can be easily done with an oscilloscope, a piece of tissue paper, and an amazingly simple Fourier Series (mathematical) Analysis!

Anyone interested can contact me for the method.

Phil Corso (cepsicon[at]AOL[dot]com)

 

If you have spare ways on your PDU's, why not just add an active harmonic filter will take care of all your needs and all the harmonics?

 

Hi

1.- The problem is the power supply of your servers.
Normally this power supplies are rectifiers. This is a non-linear load. So large harmonic content deploys through the neutral wire.

2.- Even if you have balanced currents in the three phases, the 3rd harmonic from these currents don´t cancel themselves in the neutral. These currents reinforces themselves to produce an abnormal 150 Hz current through this wire. This frequency produce a accentuate skin effect and encounters more resistivity and the result is more heat in the wire.

3.- Solutions are many but depending on the costs you may take the more economic depending on your particular case. Here they are:

3.1.- Change the neutral wire for a lower resistivity one (bigger cross section)

3.2.- When installing new equipment (servers) select the ones that use a enhanced power supply (with power factor correction). This type of power supply don't "inject" harmonics to the neutral wire.

3.3.- If possible replace older technology power supply with power correction power supplies.

3.4.- If you have to pay an increased bill due to power factor penalization (due to harmonics) consider the installation of an UPS with unity power factor correction. Besides the indications in 3.1, 3.2 and 3.3

4.- If you are involved a new data center project, consider the idea of a centralized (unity power factor) power supply for the servers or server with individual non contaminating power supply