KW metering

[techzone12]

We have an old KWH meter that has been isnatlled more than 12 years ago. I was told that it's not easy to access the meter to look at it. But I was tasked to figure out if the exisiting calculations are correct for KW values.
The meter measures KWH. Every so many KWH it create a pulse which is connected to counter input on a PLC. The PLC senses the pulse and does some math to get the KW value.

Here is the inf that I was given about the meter:
Multiplier = 1000
KWC pulse value = 10
RVA pulse value = 4.8

I know that the usual formual is:
KW = 3600*N/(t*K) where N = number of revolutions, t = time in sec and K is the meter constant in rev/KWH. But they did not give me a single meter constant. I am sure it's related to the constants given above, I just don't know how?
In other words, how can I get the meter constant from the above privided constants?

Any help will be appreciated?

 

[davidbeach]

The best you could come up with is some sort of average kW, but you'd be far better off with a kW meter if you actually need to know kW. kW is an instantaneous value, no history, no future. kWH is an accumulation, an integration of kW. You'll never know peak kW if you try to work backward from kWH. A kW demand meter could provide more information, but it has a tendency to ignore sharp peaks and looks at the heating equivalent of the power flow.

 

[techzone12]

For a KWH meter:
What does KWC pulse value of 10 mean?
What does RVA pulse value of 4.8 mean?

How can the above values be used to calcualte KW?

 

[dpc]

Each pulse from the meter represents a unit of energy used (kilowatt-hours). kW is instantaneous power. Energy = the integral of power over time.

So, by itself, a single kwh pulse can tell you nothing about power. By taking an average of the pulses over a time period, you can compute an AVERAGE kW that has previously occurred over that time period.

As for the multipliers, I have no idea. If you know how many kWh is represented by each pulse, that is all you are ever going to get from this meter. These are energy meters, not power meters.

Also, older electro-mechanical meters actually generate two pulses on two separate contacts (Google "kyz pulse initiator"). Actually it's a Form C contact.

If you are taking only one contact into your PLC totalizer, you will probably need to double your count.

If this is a power company meter, their meter shop should be able to help you.

 

[techzone12]

Ok I am finding more info on this.
The meter is made by GE, model# VW63S.
Line K is attached to 2 single-pole single-throw switches "Y" and "Z". "Y" and "Z" open and close as the disk rotates. As the meter rotates in one direction, Y closes then Z closes, then Y opens, then Z opens. When it roates in the opposite direction (export of power) then the order is reversed.

So Basically I am getting 2 pulse inputs form each meter. Which I need to use to calculate KWH and KW. I am still trying to figure this out?

 

[jraef]

Are you not reading the responses?

You CANNOT get meaningful kW values from a kWH meter no matter how deeply you dissect it. Period. That meter is looking at ENERGY, kW is POWER. They are two different (albeit related) units of measurement. You can use kW integrated with time to determine kWH, but once integrated as kWH, you cannot extract a meaningful value of kW any longer. As davidbeach and dpc have said, the BEST you can hope for is an average kW over a period of time, but what value will that have in determining accuracy (unless you had another calibrated meter connected to compare it to)?

Think of this analogy: you are driving a car and you don't have a speedometer, but you do have an odometer and you want to know how fast you are going at any given moment. You can look at two mile markers on the side of the road and determine that between the two, you took 120 seconds, meaning you AVERAGED 30 MPH in that distance. But at any given moment you could have gone 60MPH for 29 seconds and 1MPH for the remaining 91 seconds and the AVERAGE would still be pretty close to 30MPH. So what did the average tell you about your speed at any given moment? Nothing useful.

Give it up and just have your meter calibrated by a metering company. What they do is have a standard certified meter to measure a given quantity of energy put through your meter and see if they match. You could do the same of course, but it means installing a new meter, and trusting that meter as well.

 

[waross]

Each pulse will indicate the consumption of a number of KWHrs.
The number of KWHrs divided by the time in hours since the last pulse will give the average KWs during the time interval.
Utilities base KW and KVA demand on time intervals that may range from 15 minutes to 30 minutes or more.
At normal loadings, the interval between pulses will normally be much less than the time interval used by the utility to determine demand values for billing purposes.
It is correct that you cannot determine instantaneous KW values from the pulse train.
However you can determine average KW values over time periods short enough to yield greater resolution than normal utility demand figures.
You can use software to impose a sliding time window on the pulse train and accurately track the utility's demand values.
The KWHr value represented by each pulse will be a constant of the meter multiplied by the metering multiplier.
According to the
Meter and Instrument Transformer Application Guide and Condensed Metering Course from Westinghouse;
The Kh of the meter represents the Watthours for a single revolution of the disk. Note: Watthours, NOT Kilo Watthours.
The Kh is written on the face of the meter. Your challenge is to determine the Kh without inspecting the meter.
One possible solution would be to install a portable KWHr meter on the service or feeder in question and check the WHrs consumed between meter pulses. It may be easier and more accurate to bite the bullet and find a way to access the meter.
Another solution may be to determine the make and model of the meter and look the constant up in a catalog.
respectfully

 

[ssn596]

OK so you take a Kw Hr reading from the meter and put it into the PLC.
You then divide by time to get average Kw.
What do you do with this data after it is in the PLC
- in other words why???
What are you controlling with this Kw value??
OR are you using the PLC as a recording device??

Dan Bentler

 

[itsmoked]

The weird thing is that each revolution of the meter results in the consumption of,say, 10WHrs. For maximum accuracy you actually need to time the exact time between each pulse,(revolution). Avg Watts = 10wHrs / Z seconds. Hence you have no idea how long you need to wait between switch contacts. Could be 10 minutes, could be,(like at my house), 1/2second.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Hi Keith;
I use the Kh several times a year. I always time at least 10 revolutions. I do several 10 revolution checks until I get either consistent or understandable results.
With very light loads, there may be a long time between pulses however for industrial installations the corresponding KW under light loads is so far below the demand threshold that it is unimportant.
When the load has increased to the point that demand charges are imminent the period between pulses will be much shorter than the interval over which the demand is measured for penalty charges.

One of my first steps in specifying a standby generator is to look at the Watt-Hour-Meter. The electronic meters typically cycle the reading through;
KWHrs, KVARHrs, Instantaneous KW, Maximum KW, Possibly power factor or Q factor, and possibly KVAHrs.
If there is no demand metering the meter will not have been reset and will indicate the maximum demand since the meter was installed.
If there is demand metering the meter will be reset every time the meter is read. Then I go to the accounting department and as to see the power bills for the preceeding two or three years.
Several times I have had the utility metering departments provide a history of parameters from an electronic meters memory.
Yours
Bill