power quality

Status
Not open for further replies.

JdoubleU

Senior Member
We got a new metering system that monitors power quality on campus. My question is when I am looking at kvar vs kw what do I look for as being a problem? for example, one of our buildings is using in real time 11kw and has 1kvar. Also when is harmonics a problem? Another one of our buildings has a total harmonic distortion in volts of 1.7% and in current 17.5 % on 1 phase in a 3 phase system. This meter also measures harmonic in each level. What is exceptible in each level? I am in a learning process in understanding power quality so any help will be great.
 

StephenSDH

Senior Member
Location
Allentown, PA
KW = Real (Billable) Power
KVAR = Real + Inductive (Imaginary/Reactive) Power

If you have motors or other inductive loads then you will have a larger KVAR then KW. I'm sure the meter records PF also, which is just the relationship between KVAR and KW. I believe you had the meter hooked up wrong to read 1 KVAR and 11KW. KVAR is always greater or equal to kw. There is nothing to be alarmed about, when you do see a low powerfactor. Depending on your loads your power factor somewhere between .6 and 1. KW is what you will be billed for, and KVAR is what you have to size your system for.

Harmonics are also part of every electrical system, and you shouldn't be alarmed. I wrote a little article to regurgitate some books I read. Anyone can feel free to proof read it for me. :) Link

Have fun with your meter.
 

StephenSDH

Senior Member
Location
Allentown, PA
I'm sorry, I just realized I switched KVAR with KVA. You did have the meter hooked up right. I have been away from it for 3 years.

KW = Real (Billable) Power
KVAR = Inductive (Imaginary/Reactive) Power
KVA = Real + Inductive
 
Last edited:

mivey

Senior Member
See IEEE Std 519

For I_short_ciruit / I_maximum_demand < 20:
Odd harmonics (even limited to 25% of odd):
<11th order: 4.0%
11th to<17th order: 2.0%
17th to <23rd: 1.5%
23rd to <35th: 0.6%
35th + : 0.3%
Total: 5.0%

For I_short-ciruit / I_load = 20 to <50:
Odd harmonics (even limited to 25% of odd):
<11th order: 7.0%
11th to<17th order: 3.5%
17th to <23rd: 2.5%
23rd to <35th: 1.0%
35th + : 0.5%
Total: 8.0%

Individual voltage harmonics : 3.0%
Total voltage distortion: 5.0%
 

mivey

Senior Member
We got a new metering system that monitors power quality on campus. My question is when I am looking at kvar vs kw what do I look for as being a problem? for example, one of our buildings is using in real time 11kw and has 1kvar. Also when is harmonics a problem? Another one of our buildings has a total harmonic distortion in volts of 1.7% and in current 17.5 % on 1 phase in a 3 phase system. This meter also measures harmonic in each level. What is exceptible in each level? I am in a learning process in understanding power quality so any help will be great.
If the load is low, you need to adjust the percentage because the current distortion is should be relative to the maximum demand. You can get some pretty high harmonics with relatively low current levels but their impact is minimal. You can adjust the current levels or just focus on the voltage distortion. The current distortion is driving the voltage distortion anyway.

Add:
As for kvar vs kW: It can depend on what the supplier is charging for excess kvar. If the kvar is more than about 1/2 of the kW (90% power factor), then it can start becoming an issue if you are charged for excess vars. Note that some utilities may start charging you if the kvar is more than about 1/3 of the kW (95% power factor).

If you have no penalty, I would probably consider investigating if the kvar is more than about 60% of the kW value (85.7% power factor).
 
Last edited:

robbietan

Senior Member
Location
Antipolo City
We got a new metering system that monitors power quality on campus. My question is when I am looking at kvar vs kw what do I look for as being a problem? for example, one of our buildings is using in real time 11kw and has 1kvar. Also when is harmonics a problem? Another one of our buildings has a total harmonic distortion in volts of 1.7% and in current 17.5 % on 1 phase in a 3 phase system. This meter also measures harmonic in each level. What is exceptible in each level? I am in a learning process in understanding power quality so any help will be great.

First thing, if you have a low power factor (in our case less than 0.8 lag) the utility will penalize you until you bring your pf up to standard. Correct me if I am wrong but my calculations say that you have a high pf of 91% if you have an 11 kW and 1 kVAR reading.

On harmonics, we have a maximum limit of 5% Voltage THD (total harmonic distortion). A reading of 1.7% VTHD is nothing to worry about. A 17.5% current THD is not bad, IF the current total demand distortion is less than 5%. This is to say having a small amount of current harmonics is not a big deal if you have a total load that is big compared to the amount of current harmonics.
 

mivey

Senior Member
First thing, if you have a low power factor (in our case less than 0.8 lag) the utility will penalize you until you bring your pf up to standard. Correct me if I am wrong but my calculations say that you have a high pf of 91% if you have an 11 kW and 1 kVAR reading.
That would be 99.6%
 

PowerQualityDoctor

Senior Member
Location
Israel
It is indeed 0.996. kVAr/kW = Tan phi = 1/11
Cos (tan-1 (1/11)) = 0.996

1.7% THD of voltage is acceptable value, the problem lies with the extra losses that it incorporates. 17.5% current THD should be checked, providing that the load is high enough. If your meter has TDD measurement - check it.

You mentioned a monitoring system, so I guess there is more than one meter. What are the values on other circuits?
 

wptski

Senior Member
Location
Warren, MI
It is indeed 0.996. kVAr/kW = Tan phi = 1/11
Cos (tan-1 (1/11)) = 0.996

1.7% THD of voltage is acceptable value, the problem lies with the extra losses that it incorporates. 17.5% current THD should be checked, providing that the load is high enough. If your meter has TDD measurement - check it.

You mentioned a monitoring system, so I guess there is more than one meter. What are the values on other circuits?
What meter has TDD? I looked for that sometime ago and couldn't find any that did!
 

PowerQualityDoctor

Senior Member
Location
Israel
What meter has TDD? I looked for that sometime ago and couldn't find any that did!

I had enough with the forum moderators, if they explicitly tell me to post names, I can give you more than one (including companies that are not related to me...). Personally, I currently use three manufacturers, one of them has TDD and I know about another one which has. Some data logger even record the amount of harmonic current in amperes, not in percents, which allow you to track problems, not just know they exist.
 

roger

Moderator
Staff member
Location
Fl
Occupation
Retired Electrician
I had enough with the forum moderators, if they explicitly tell me to post names, I can give you more than one (including companies that are not related to me...).

If another member is interested, you can use the PM feature to exchange information concerning a manufacturer you may or may not be associated with.

Roger
 
Status
Not open for further replies.
Top