The Engineer's Place for News and Discussion®

 Previous in Forum: Suitable Project For Undergraduate Student Next in Forum: CNAOB Digital Ammeter Mounted Version!

Subscribe to Discussion:

CR4 allows you to "subscribe" to a discussion
so that you can be notified of new comments to
the discussion via email.

Rating Vote:

Participant

Join Date: Jun 2012
Posts: 3

Different Reading's Between kWH Meters on 11kV Switchgear

06/20/2012 6:11 AM

Dear friends,

I have 2 digital power meters connected to 2 11kV HT switchgears. One is rated 150/5A and the other is rated 200/5A. Both meters give different readings as below:-

150/5A - 200/5A

1609kWH - 41kWH

2119kWH - 53kWH

3474kWH - 87kWH

2426kWH - 61kWH

readings are taken on a daily basis.

the ratio between them is basically about 0.02548. Is there any calculations to calculate the ratios between them since they are on different CTs.

Thanks.

Interested in this topic? By joining CR4 you can "subscribe" to
Guru

Join Date: Aug 2010
Location: Under the Major Oak
Posts: 3065
#1

Re: Different Reading's Between kWH Meters on 11kV Switchgear

06/20/2012 1:31 PM

Are the two feeders in parallel?

Line impedance can create the diference.

__________________
Pessimists are rarely disappointed.
Participant

Join Date: Jun 2012
Posts: 3
#7

Re: Different Reading's Between kWH Meters on 11kV Switchgear

06/21/2012 3:45 AM

The 2 feeders are in series Sir.

Guru

Join Date: May 2010
Location: Liverpool, NY
Posts: 569
#2

Re: Different Reading's Between kWH Meters on 11kV Switchgear

06/20/2012 2:22 PM

Just because the CTs have ratios of 150/5 and 200/5 doesn't mean the load is split between the two feeders in proportion to the CT primary values. Unless the meters do not match the CT ratios to which they are connected, it looks to me like the second feeder has only 1/40 the load of the first (if they are continuous loads). Better take a look at what is on each of the feeders, and add up their kVA.

Besides, if you are looking at kWH, you have the time element involved. One of the loads may only operate intermittently, but at higher load. Again, add up the kVA loads on each feeder, and also correlate that with how often and how long they run.

__________________
To get the right answers, first you need to ask the right questions.
Guru

Join Date: Jun 2011
Location: Phnom Penh
Posts: 2371
#3

Re: Different Reading's Between kWH Meters on 11kV Switchgear

06/20/2012 11:48 PM

What is your concern? What are you trying to prove or disprove or hypothesise?

Are you concerned that they are different?

__________________
Difficulty is not an obstacle it is merely an attribute.
Power-User

Join Date: Jun 2010
Posts: 107
#4

Re: Different Reading's Between kWH Meters on 11kV Switchgear

06/21/2012 12:03 AM

I doubt very much that you know what you have in the way of metering.

1. You likely have 2 meters on the same feeder, rather than one meter on each of two feeders.

2. One meter (150/5) is likely set up with the multiplier of 30 and reads correctly.

3. The other meter (200/5) is not setup with the multiplier thus reading 1/40 of the power.

4. The readings of the 200/5 meter being in the 40-80 kW range are very small compared to the capacity of the 11 kV line and 200A CTs which is in the order of ~3800kVA You need be multiply these readings by 40 to obtain true power/energy.

Guru

Join Date: May 2006
Location: Placerville, CA (38° 45N, 120° 47'W)
Posts: 2695
#5

Re: Different Reading's Between kWH Meters on 11kV Switchgear

06/21/2012 12:07 AM

You need to provide a circuit showing how these units are connected! The only way you should expect two power meters to agree is if the energy goes first through one meter and then through the other on its way to a single load or set of loads.

If, as you seem to indicate, they are on two different circuits, then there is no way they should agree. Any constant ratio between the loads will be distorted by different inductances, capacitances, and resistances of the leads and loads, as well as (in most cases) different times of operation etc.

__________________
Teaching is a great experience, but there is no better teacher than experience.
Participant

Join Date: Jun 2012
Posts: 3
#6

Re: Different Reading's Between kWH Meters on 11kV Switchgear

06/21/2012 3:44 AM

The scenario is like this;

the first meter (meter 1) is connected to a switchgear (switchgear No.1) with a rating of 150/5A @ 11kV

the second meter (meter 2) is connected to a switchgear (switchgear No.2) with a rating of 200/5A @ 11kV

power travels from switchgear No.1 @ 11kV to switchgear No. 2 @ 11kV to a 2000kVA stepdown transformer and then to a 2500A Main Switch Board (MSB) with an output voltage of 240V (phase - neutral) and 415V (phase to phase) which then goes to various loads.

The question is why doesn't the 2 meters (meter 1 and meter 2) show the same reading when they are on the same circuit. Is it possible that they are not calibrated properly since meter 2 shows a reading of 1/40 of meter 1. By the way the meters that i'm using are Socomec Diris A40 digital power meters.

Yes there are some losses on the copper cable from switchgear No.1 to switchgear No. 2 as they are about 100m apart but the kWH readings between the 2 meters should not be so far off.

Power-User

Join Date: Jun 2010
Posts: 107
#8

Re: Different Reading's Between kWH Meters on 11kV Switchgear

06/21/2012 4:27 AM

The meter on the 200/5 CT is not applying the x40 multiplier.

Guru

Join Date: Jun 2011
Location: Phnom Penh
Posts: 2371
#9

Re: Different Reading's Between kWH Meters on 11kV Switchgear

06/21/2012 5:33 AM

Have you checked that yet giantleo?

__________________
Difficulty is not an obstacle it is merely an attribute.
Power-User

Join Date: Sep 2008
Posts: 197
#12

Re: Different Reading's Between kWH Meters on 11kV Switchgear

06/21/2012 12:33 PM

Check the meters for proper setup such as the following:

Scaling correct?

Range correct?

Multiplier correct and in use/active?

CT Ratio correct?

Meter display resolution set the same? (If the resolution is not set the same in both meters the number of decimal places will vary. The second appears to be 'clamped' at two (2) while the first appears to be set at four (4) which will cause even further disagreement and confusion due to "rounding up or down" of display output.

Associate

Join Date: May 2010
Posts: 45
#10

Re: Different Reading's Between kWH Meters on 11kV Switchgear

06/21/2012 7:17 AM

It appears that the 150/5 energy meter gives a direct reading with CT ratio considered while the 200/5 energy meter does not consider the CT ratio and hence the reading must be multiplied by 40 as already pointed out by a friend.

Guru

Join Date: Jun 2011
Location: Phnom Penh
Posts: 2371
#11

Re: Different Reading's Between kWH Meters on 11kV Switchgear

06/21/2012 7:26 AM

The meter should read kWh direct by applying the multiplier before display.

The reading multiplier must be settable for use with different CTs.

Meter needs a closer scrute. (it is 2012 and post processing a meter reading is just not on)

__________________
Difficulty is not an obstacle it is merely an attribute.