Varying difference between constant generated KWH and utility side measured KWH 1

[edison123]

A 2100 KVA generator is constantly outputting 2000 KWH every hour but the utility side measured KWH is varying from 1300 KWH to 1800 KWH with difference between them varying from 700 KWH to 200 KWH on hour to hour basis.

There is very little power tapped off from the generator for lighting and a 5 HP fan for oil cooling. Utility says they checked both the meters and found ok.

My doubt is that the PT or CT secondary has a loose connection on the utility side metering which could account for the varying lost power.

What do you guys think?


Muthu

http://www.edison.co.in

 

[che12345]

Mr edison123 (Electrical)(OP)18 May 23 06:15
"....A 2100 KVA generator is constantly outputting 2000 KWH every hour but the utility side measured KWH is varying from 1300 KWH to 1800 KWH with difference between them varying from 700 KWH to 200 KWH on hour to hour basis.... Utility says they checked both the meters and found ok...My doubt is that the PT or CT secondary has a loose connection on the utility side metering which could account for the varying lost power".
I am of the opinion that:
a) If it is new installation, check:
the CT (polarity P1, P2, S1, S2), the PT phase sequence (primary ABC, secondary abc) and correct connection on the kWh meters.
b) If it is an existing installation and was measuring (kWh) correctly before and this happens just recently, it could be due to CT, PT faulty , including loose connection etc,
c) It is not too costly to own or rent a portable Power meter or out source a contractor to verify the actual kWh. It would be only a very small expenditure to the cost of the kWh that you are generating.
Che Kuan Yau (Singapore)
c) operation

 

[edison123]

che

This is old installation with refurbished generator.

Does PT phase sequence really matter in energy meters?

Even if CT polarity is reversed, for a constant generator KWH, wouldn't the utility side show the same reduced KWH, not a fluctuating KWH?

A loose connection in CT secondary could create a varying high contact resistance (so varying secondary current) thereby resulting in varying KWH meter reading? Possible?



Muthu

http://www.edison.co.in

 

[LionelHutz]

I ran into a utility metering issue where the metering would never go above a power factor of around 70% lagging. At times of heavy loading it'd show a higher kVA consumption but not the proper wattage consumption. The customer was paying high demand charges so the customer wanted power factor capacitors to correct their poor power factor. But, they didn't fix it so I did some more metering and confirmed their power was near unity as it should have been. I don't know the issue, I was told the customer had to sign an NDA with the utility as part of the settlement on the billing error. Anyways, strange things can happen with metering you wouldn't expect.

 

[cranky108]

What about your losses? KWH is a measure of energy for billing, so why are you using it and not KW?
I assume you have a step up transformer, and what are it's losses?
How are you measuring the output of the generator? Are you assuming something here, or do you have some other metering?
Metering using other than metering rated CT's, and PT's is not a good measure, but a rough indication.
I also doubt you have a lose CT connection as they tend to get very hot. Than can be seen with a thermal scan.
PT connections can be lose. But as you said this is an old installation, so what has changed?

 

[waross]

A 2100 KVA generator is constantly outputting 2000 KWH every hour

What is the basis for this assumption?
Check your own metering and controls.
There are several control methods that will cause the output kW to vary with the utility voltage.
Bad fuel may be an issue.
With an old engine and bad fuel, the injectors may be "dead racking". That is, full fuel position on the injector rack.
That's all there is and there ain't no more.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[edison123]

Just now, the utility identified their measurement CT secondary leads as the problem just as I suspected given the hourly fluctuations. They changed the CT secondary leads and now their metering agrees with the generator metering (sans their transformer loss) just as it was before the shut down. No more utility side KWH fluctuations for the past two hours.

Generator metering was tested before recommissioning and it was recording constant 2000 KWH after recommissioning, which is the only criteria here (not KW, KVA, PF etc) since they are a small hydro IPP and they get paid by the units exported.

Thanks for your inputs.

Muthu

http://www.edison.co.in

 

[waross]

Thanks for the come-back.
Do you know the nature of the problem with the CT leads?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[edison123]

Bill

Not yet. I am doing long distance troubleshooting. Client is in Africa and I am in India. Will update when I get more info.

Initially, the utility local guys said everything is fine with their metering. I forced them to re-check and to their credit, they sent an actual metering guy from their HQ to check this time around. Client was pissed off.

You can see the removed CT wires and their troubleshooting external red & black leads in the pic below.

Muthu

http://www.edison.co.in

 

[che12345]

Mr edison123 (Electrical)(OP)18 May 23 11:13
"...che. This is old installation with .... #1. Does PT phase sequence really matter in energy meters?...#2. Even if CT polarity is reversed, for a constant generator KWH, wouldn't the utility side show the same reduced KWH, not a fluctuating KWH? ...#3.A loose connection in CT secondary could create a varying high contact resistance (so varying secondary current) thereby resulting in varying KWH meter reading? Possible?"
1. I noticed in your later updating post confirmed that the problem was due to the CT loose connection. The problem is now solved.
2. I try to answer your questions, as a general information.
#1. Yes, it does and is very important. Observe it without failed.
#2. a) the CT polarity is very important. Observe it without failed.
b) Your CT is likely a different set of CT from that of the utilities. If any of your CTs is/are with wrong polarity, your metering would for sure with error.
#3. Yes, possible. CT with loose connection = higher burden which lower the accuracy...etc.
BTW: for correct kW or kWh metering, the CT polarity, the PT phase sequence and the correct termination on the measuring meter MUST be observed without failed. Extremely important!
Che Kuan Yau (Singapore)

 

[waross]

Wrong CT polarity will certainly give errors in measurement, but by a constant ratio.
Wrong polarity will not give an error that changes by the hour.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[edison123]

The CT loose connection was traced to a terminal block before the utility metering. Now all is normal.

Muthu

http://www.edison.co.in

 

[edison123]

che - I understand CT polarity is important but why is voltage phase sequence important in energy meters?

Muthu

http://www.edison.co.in

 

[waross]

For each phase there is a voltage measurement and a current measurement.
Each pair of V and I measurements must be for the same phase and in the correct polarity.
If the voltage is offset by 90 degrees, that element will meter KVAHrs instead of KWHrs.
Back when electro-mechanical meters were common, we used a phase shift coil in series with the voltage coil to convert a KWHr meter to a KVARHr meter.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[edison123]

Bill

Don't they measure line to line voltages and not phase voltage and only phase currents?

Muthu

http://www.edison.co.in

 

[che12345]

Mr edison123 (Electrical)(OP)19 May 23 01:13
" ....che - I understand CT polarity is important but why is voltage phase sequence important in energy meters?"
For kW, kWh, power factor, reverse power metering, the CT polarity, the PT phase sequence; the CT and voltage shall be of the same phase; the phase sequence ABC.... etc.; terminated on the meter MUST observe the meter terminal instruction/marking WITHOUT failed. There is NO two ways. Any deviations would certainly result to error.
Attention: on the meter side, observe the A-phase current in out, followed by A-phase voltage per the meter marking/instruction. For CT and PT relationship, observe the "dot" notation. It is essential, NO two ways.
Che Kuan Yau (Singapore)

 

[waross]

It's been years and I can't find my Metering book.
Some schemes use line to line voltage, some use line to neutral voltage.
Some metering schemes make assumptions about voltages.
Assuming unity PF;
If the current is metered on line A and the voltage is measured line A to B,
the meter will respond to the A to B component of the current in line A.
This reduces the metered amount by a factor of 1/1.73 to give the same result as would have been obtained with a line to neutral voltage.

Voltage assumption #1, The household meter so common in North America;
The standard meter assumes that the line to neutral voltages are balanced.
In the case of a long supply line with badly unbalanced loads and a voltage drop/rise on the neutral, the meter reading will become inaccurate.
Voltage assumption #2, metering three phase with a two element meter.
One element is used to meter A phase, line to neutral.
The second element is used to meter C phase, line to neutral.
Each phase may be expressed as components of the other two phases,
So if the B phase conductor is passed through the A and C CTs, the A phase element will meter the A phase component of B phase and the C phase element will meter the A phase component of C phase.
This scheme will meter unbalanced line to neutral loads.
This scheme makes the assumption that the voltages are equal.
There is an alternative arrangement where three CTs are used and the CT secondaries are connected in delta.
I used this scheme to meter several large consumers.
BUT:
Anecdote alert:
On of the installations was on a reef, about 1000 Feet offshore, fed by a 3/C submarine cable at 13200 Volts.
One conductor failed and was replaced with a single conductor cable running alongside the original cable.
The conductors were no longer balanced as to inductive reactance and the load end voltages became badly unbalanced.
The meter readings became inaccurate and the two element meter was replaced with a three element meter.
And;
A cannery was selling shore power to ships tied up for the winter.
No refrigeration was running and three phase was not needed.
The supply was 208/120 Volts, but the plant was metering with a 120/240 Volt KWHr meter.
While each line to neutral load was at 120 Volts, the meter assumed that each side was receiving 208/2 = 104 Volts.
I recommended that they increase their billing by a factor of 240V/208V.

In conclusion, I may have misspoke.
When the current in a meter element is not in phase with the voltage in the same meter element, only the in-phase component will be metered.
But to agree with Mr. Che, the connections and the polaritys of the connections are very important.

When I first started using unfamiliar three phase metering connections, I have hidden a single phase meter connected line to neutral.
The readings correlated closely with 1/3 of the three phase metering. (Close enough)
The correlation did not have to be exact.
Any misconnection would result in errors grossly exceeding unbalanced load errors.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

Not all metering connections comply with Blondel's theorem.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[edison123]

Turns out that during generator outage, the utility guys checked their CT's with secondary injection and then forgot to re-tighten all 3 CT leads.

Muthu

http://www.edison.co.in

 

[wcaseyharman]

Your client was lucky as it was an easy fix. Several times loose CT leads have caused collateral damage at my company - burnt terminal blocks and melted wire.