Home › Electrical Engineering Forum › General Discussion › Stability Factor: KFactor
 This topic has 5 replies, 3 voices, and was last updated 11 years, 3 months ago by Spir Georges GHALI.

AuthorPosts

2013/01/25 at 2:56 pm #10955Steven MillParticipant
KFactor determines the total harmonic current which a transformer can withstand without going beyond its specified temperature threshold limits. Under normal circumstances the value of KFactor ranges from 150. It is the load that determines the KFactor of the specific transformer. For example in the case of linear loads KFactor of 1 is used whereas in worst harmonic conditions KFactor of 50 is advisable. KFactor rating indicates the tendency of a transformer to supply rated KVA output to a load of specified harmonic content.
Most harmonic systems use transformers with KFactor of 13. Transformers using KFactor values are said to be Krated. The purpose of Krated transformers is to tackle the problems associated with inductive and nonlinear loads which include industrial loads such as motors. These types of loads are known as harmonic generating loads. Harmonics; are multiples of fundamental frequency which when increased beyond ordinary limits result in overheating of the system. Likewise, in a transformer harmonics produce excessive heat which could result in fire or in some extreme cases violent explosions. Most of these hazardous cases occur in non Krated transformers which lack Harmonic Protection. Harmonics result in decreased life of any electrical system. In order to tackle overheating issues in conventional non Krated transformers, Krated transformers are designed in such a way to mitigate the effects of harmonics and to dissipate the heat from the transformer to the environment as quickly as possible. The image underneath delineates the effects of harmonics on a conventional sine wave.Working Of Krated Transformer
The working principle of Krated transformers involves the use of a double sized neutral conductor with their alignment changed. Some Krated transformers use more than one conductor. Krated transformers could be used at places where the loads are either inductive or nonlinear. Most of these places include heavy industries, offices, automation factories and many other places where harmonic generating loads are present. Normally at consumer ends where the loads have harmonic content less than 75%, a transformer with KFactor of 13 could be used. For loads with Total Harmonic Distortion greater than 75%, a transformer with KFactor of 20 is used.
Need For Krated Transformers
Due to continuous increase of consumer end loads such as computers, Uninterrupted Power Supplies, Switch Mode Power Supplies, VFD’s and various small household motors the problem of harmonics has grown much severe. All of these loads are nonlinear and cause unnecessary loading of the Power System due to irregular demand of current and variable impedance during their voltage cycle. As a result these loads result in the generation of increased harmonic currents which at the end result in producing more heat in power distribution equipment which include distribution transformers.
This is where the importance of using Kfactor transformers arises. KFactor is a measure to determine how much a transformer could be overrated or derated to sustain a specified load.
Rules to Follow
There are some specific rules set by IEEE which must be followed before choosing a specific KFactor value for a specific load. These include:
 For loads with Harmonic Currents less than 15% a standard non Krated transformer could be used
 For loads with Harmonic Currents up to 35% a K4 rated transformer should be used
 For loads with Harmonic Currents up to 75% a K13 rated transformer should be used
 For loads with Harmonic Currents greater than 75% a K20 rated transformer should be used
Advantages
KFactor transformer or Krating is used to tackle the overheating of transformers due to excessive harmonic currents produced by nonlinear loads. KFactor neither improves the quality of Power System nor does it increase the efficiency of Power System. An added advantage of KFactor transformers is that these transformers prevent insulation breakdown by uniformly distributing the stress across the transformer’s winding. This results in longevity of transformer.
Did this introduction to KFactor draw your interest? Please let us know by posting a comment.
2013/01/27 at 12:20 pm #13223Spir Georges GHALIParticipantDear;
It’s very good and important to mention the effects of Harmonics on the Transformers, but I have some remarks that are :
– I think the percentages of Harmonics currents mentioned in this topic mean the percentage of the “ THDI ” ( Total Harmonic Distortion of Current ). If not, please clarify.
– It’s known that the effect of KFactor is to oversize the Transformer’s power, but what is the formula between the KFactor and the Transformer’s power ?
– You mention that “ the inductive loads like Motors are known as harmonic generating loads ”, but most of motors especially “ Squirrel Cage Motors ”, and after the transient running case are always “ Linear Loads ”, but for these loads the Harmonic currents can be generated or not depending on the kind of running equipment used to run these motors. ( for exp. Contactors with Thermal relays doesn’t generate any harmonic, VFD equipment generates harmonics ).
– It’s mentioned that “ the working principal of Krated Transformers involves the use of a double sized neutral conductor ”, but in general, depending on the Harmonics Currents value or percentage, we oversize “ double or even more ” the cables’ sections of Phases & Neutral “, and also depending on the value or percentage of the “ Third multiple Harmonics ” and the “ Unbalance current’s value ” we decide the final section of neutral conductor.
– It’s mentioned that “ Krated Transformers could be used at places where the loads are either inductive or nonlinear ”, and as known, some inductive loads generate harmonics but not all inductive loads.
– As mentioned, the KFactor Value’s range is from “ 1 ” to “ 50 ”, and also, the rules mentioned the KFactor Transformers that should be used depending on the Harmonic Current, ( exp. for Harmonics Currents more than 75% the “ K20 Transformers ” should be used ) but, at which levels or where the values “ K30 or K35 Transformers ” or more should be used ?
– The KFactor value is the percentage of Harmonics current’s percentage to fundamental current, but this percentage’s value can be only known after industries’ running, so, what the Electrical Engineers Designers should do at designing step ? For this point, the IEC determines a curve that helps the Electrical Engineers Designers to define the transformer’s powers that should be used depending on the percentage of “ Non Linear Loads ”.
2013/02/02 at 6:25 am #12791AnonymousGuestIT IS VERY VITAL PHENOMINON FOR THE SUBJECT MATTER OF KFACTOR TO OVER SIZE TRANSFORMERS.
2013/02/11 at 10:29 am #13000Steven MillParticipantThank you for your feedbacks!
Spir Georges GHALI, sorry for the late reply and thank you for your interesting remarks.
@Spir Georges GHALI said:
Dear;It’s very good and important to mention the effects of Harmonics on the Transformers, but I have some remarks that are :
– I think the percentages of Harmonics currents mentioned in this topic mean the percentage of the “ THDI ” ( Total Harmonic Distortion of Current ). If not, please clarify.
Yes these percentages are of THD. Sorry If that confused you.
– It’s known that the effect of KFactor is to oversize the Transformer’s power, but what is the formula between the KFactor and the Transformer’s power ?
Normally in order to calculate the KFactor you keep into account two things the KVA & load Harmonic Current. So accordingly the KFactor would be:
K – Factor = KVA * I_{L}
As I_{L} is the load harmonic content so you can easily calculate the KFactor value to be used.
– You mention that “ the inductive loads like Motors are known as harmonic generating loads ”, but most of motors especially “ Squirrel Cage Motors ”, and after the transient running case are always “ Linear Loads ”, but for these loads the Harmonic currents can be generated or not depending on the kind of running equipment used to run these motors. ( for exp. Contactors with Thermal relays doesn’t generate any harmonic, VFD equipment generates harmonics ).
I was specifically referring to induction motors which are widely used in industry as compared to squirrel cage motors which are most suited for small tasks. Very rare have I seen squirrel cage motors used in industry. Induction motors generate harmonics and come in the domain of nonlinear loads. Also in industries the Switch mode power supplies & DC rectifiers also add to the nonlinearity of the system.
– It’s mentioned that “ the working principal of Krated Transformers involves the use of a double sized neutral conductor ”, but in general, depending on the Harmonics Currents value or percentage, we oversize “ double or even more ” the cables’ sections of Phases & Neutral “, and also depending on the value or percentage of the “ Third multiple Harmonics ” and the “ Unbalance current’s value ” we decide the final section of neutral conductor.
Yes you are right about conductor size. It depends on the harmonic currents value. I also wrote in the article that Some Krated transformers use more than one conductor. Normally you don’t need a transformer with Kfactor rating more than K4 or K13 in industries which are coherent to the design specs presented in the article.
– As mentioned, the KFactor Value’s range is from “ 1 ” to “ 50 ”, and also, the rules mentioned the KFactor Transformers that should be used depending on the Harmonic Current, ( exp. for Harmonics Currents more than 75% the “ K20 Transformers ” should be used ) but, at which levels or where the values “ K30 or K35 Transformers ” or more should be used ?
Transformer size increase as you increases the value of KFactor & so does the heat tolerance due to harmonic currents. Normally transformers of KFactor 30, 40 & 50 are used where harmonic currents are somewhere b/w 125150%. These are worst harmonic conditions. As mentioned before the size of transformer increases with the increase in KFactor so the optimal KFactor should be chosen which would be a tradeoff between transformer size & its heat tolerance. Normally the K50 transformers are very expensive & much larger in size so manufacturers recommend a transformer of K30 with modified designs as a replacement.
2013/02/11 at 11:24 am #13002Spir Georges GHALIParticipantDear Mr. Steven ;
first, thanks for your reply, and be sure for me there’s no any confusion.
Please let me clarify the following :
– The percentages of Harmonics are certainly ” THDI “ ( Total Harmonics Distortion of Current ) and not ” THDV
– The mentioned formula between ” KFactor ” and ” the Transformer’s Power ” is ” KFactor = S ( kVA ) * IL ” as IL is the load harmonic, please let me clarify the following points :
 This formula is ” linear “, but the relation between KFactor and the Transformer’s Power is not a linear.
 As I mentioned, there’s a Curve ( not linear ) between ” the Percentage of Nonlinear Loads compared with the total installed loads ” and the ” Derating Factor “, when we obtain this factor, we use the following formula : S(new) = S / Derating Factor. Noting that I use this curve during designing step of any site if the derating of transformer is necessary, and if you want, send me your email to send it you.
– About the type of used motors : from my experience, most of LV motors even till 300kW are Squirrel Cage Motors. noting that all Motors are Induction Motors. Please clarify if there’s another kind, and what?
2013/05/13 at 3:48 pm #13049AnonymousGuesthow to calculate load harmonic content
please explain with example. 
AuthorPosts
 You must be logged in to reply to this topic.