Regenerative Braking

[W@ttson]

If I have a motor that is connected to a drive that has the ability to regenerative brake and in one direction I am using the motor as a motor and sending real and reactive power to the motor to move my load. If in the reverse direction, my load is such that it begins overhauling the motor. Since it is connected to the regenerative drive, the drive pumps current back to the grid to slow down the motor and lower the DC bus voltage so it doesn't trip out.

Suppose the drive/motor are the only load connected to the service and I put a power/energy logger on it. I suppose the energy logger will show that one way the KW are positive and in the other direction the KW are negative. I assume that in both directions the reactive power is Positive. I assume that in both directions it will show that the apparent power is positive.

How would you calculate total energy billed to the customer? If apparent power is + in both directions how do you know what to subtract?

I assume the following, that the real power from the overhauling segment can be subtracted from the motoring segment. I assumed the reactive power is the same in both cases. I guess this is like a real world case of complex algebra: Smotoring = Pmotoring + jQmotoring; Sov.haul = Pov.haul + jQov.haul

Smotoring + Sov.haul = (Pmotoring+ Pov.haul) + (jQmotoring+ jQov.haul)

 

[Hv&Lv]

If I have a motor that is connected to a drive that has the ability to regenerative brake and in one direction I am using the motor as a motor and sending real and reactive power to the motor to move my load. If in the reverse direction, my load is such that it begins overhauling the motor. Since it is connected to the regenerative drive, the drive pumps current back to the grid to slow down the motor and lower the DC bus voltage so it doesn't trip out.

Suppose the drive/motor are the only load connected to the service and I put a power/energy logger on it. I suppose the energy logger will show that one way the KW are positive and in the other direction the KW are negative. I assume that in both directions the reactive power is Positive. I assume that in both directions it will show that the apparent power is positive.

How would you calculate total energy billed to the customer? If apparent power is + in both directions how do you know what to subtract?

I assume the following, that the real power from the overhauling segment can be subtracted from the motoring segment. I assumed the reactive power is the same in both cases. I guess this is like a real world case of complex algebra: Smotoring = Pmotoring + jQmotoring; Sov.haul = Pov.haul + jQov.haul

Smotoring + Sov.haul = (Pmotoring+ Pov.haul) + (jQmotoring+ jQov.haul)

Using a revenue meter of some kind for this?
Use one that does four quadrant metering

 

[W@ttson]

Using a revenue meter of some kind for this?
Use one that does four quadrant metering

I want to do it using a Fluke 1738.

 

[W@ttson]

Basically someone asks me to tell them how much energy is used in one direction and how much is produced in the other direction and then what is the net for the system. I have a Fluke 1738, how can I set it up to give that info.

 

[TwoBlocked]

If I have a motor that is connected to a drive that has the ability to regenerative brake and in one direction I am using the motor as a motor and sending real and reactive power to the motor to move my load. If in the reverse direction, my load is such that it begins overhauling the motor. Since it is connected to the regenerative drive, the drive pumps current back to the grid to slow down the motor and lower the DC bus voltage so it doesn't trip out.

...

I know of drives that use a braking resistor in such cases, but not ones that send power back to the AC grid. What brand/model drive is this please?

 

[petersonra]

I know of drives that use a braking resistor in such cases, but not ones that send power back to the AC grid. What brand/model drive is this please?

There are a bunch of companies that make Regen drives.

DC drives commonly do it. They are called four quadrant drives if designed to do so.

You can also buy devices that attach directly to the DC bus that send excess energy back to the line. Bonitron makes one among others.

Here is an old video from ABB

To be honest, the economics make it rarely cost effective. Basically you are buying an extra drive.

 

[petersonra]

I meant to ask this.

You can probably program a digital output to close when a kwhr has been used and another to pulse when a kwhr has been recycled and just count them.

 

[winnie]

@W@ttson

Your assumptions about the direction of reactive power flow are correct for an across the line induction motor, but not for a drive connected induction motor.

An induction motor always consumes reactive power even when operating as a generator. You would see this if you put a suitable meter between drive and motor.

But what the meter would see between drive and service totally depends on the rectifier front end of the drive. If the drive has a power factor correcting rectifier then there might be no reactive power at all. Or you might have zero displacement power factor but huge distortion power factor (no reactive power but lots of harmonics). The waveform or displacement could be entirely different motoring vs regeneration.

Regarding metering, the meter should report real energy without including reactive components, and then report reactive power or kVA demand or power factor or whatever apparent power metric _separately_.

For real energy simply subtract regeneration (less any efficiency losses) from consumption. It is that simple.

For the reactive components you could have a mess depending on what exactly is being billed for.

 

[Hv&Lv]

I want to do it using a Fluke 1738.

That should be able to be done with that meter. Setting it up I would think it needs to be a power study when you use the wizard.
I only looked at and played around with the 1738 while making the decision to purchase a PQM. I purchased the Dranetz PQM but they are similar in function

 

[petersonra]

Now that I am more awake, I seem to recall the AB regen drives keep track of both the used energy and the recycled energy. There might already be parameters you can look at in the drive that give you this information without needing any external device.

 

[Jraef]

How would you calculate total energy billed to the customer?

Simple.
They are billed for the energy when motoring.
They are NOT billed for the energy when it is regenerating.
They are also NOT going to get money back from the utility for the regenerated power. Here's why:

Do get money / credit back for regenerated energy, you must have a "Net Energy Metering" (NEM) contract with the utility. In the process of attaining an NEM, the utility reviews ALL of the equipment involved. A key aspect of that process is that you MUST use equipment that is listed under UL-1741 standard for "Inverters, Converters, Controllers and Interconnection System Equipment for Use With Distributed Energy Resources". This is a separate UL listing that goes way above and beyond what VFDs are listed under, because, among other things, it is necessary to ensure that if the grid side fails, the equipment will immediately shut down and not allow the regenerated power to flow back into the grid and kill a Lineman. Solar power inverters are ALL required to be UL-1741 listed for example. But as far as I know, there are NO VFDs on the market that are listed under UL-1741. The reason is, for the SMALL number of potential sales they might get, it is way too expensive not only for the listing, but for the liability insurance that they must carry along with it.

But to be clear, Line Regen VFDs CAN absolutely regen into the grid, and if you are using power elsewhere in your facility, that regenerated energy is used by other things and REDUCES the demand from the utility grid. So they do recoup energy costs (for that regen time, however brief) under those circumstances. The only scenario that this issue addresses is where there is NOTHING ELSE on the service that is able to consume that energy, and you end up putting it back on the grid. It WILL go back to the grid, and if the line fails, it will (should) shut the drive down and not kill a lineman. But to get the NEM so that the pennies go back into your pocket, it must have that UL-1741 listing, which it will not have.

By the way, this is a fact that is FREQUENTLY overlooked by marketing departments at many VFD manufacturers who offer Line Regenerative VFDs. I have seen plenty of brochures and blurbs saying things like "spin your meter backward", that is NOT true...

 

[W@ttson]

Very interesting responses all.

Here is some data that I took with a different power meter on a similar load (motoring up one way, overhauling other way). You can see the direction of the P, Q, S.

Motoring:




Overhauling:



As winnie was saying, this is between drive and motor. I do not know how it would look like from font end of drive, in anycase this particular location had DBRs so not applicable. Just wanted to share the direction of the powers.


At my original location, its actually a large facility where they happily take the power pumped back into their common electrical service as brief as it may be.

 

[GoldDigger]

Simple.
They are billed for the energy when motoring.

...The only scenario that this issue addresses is where there is NOTHING ELSE on the service that is able to consume that energy, and you end up putting it back on the grid. It WILL go back to the grid, and if the line fails, it will (should) shut the drive down and not kill a lineman. But to get the NEM so that the pennies go back into your pocket, it must have that UL-1741 listing, which it will not have.

If the meter itself is not configured for Net Metering, chances that it is configured so that energy transfer in either direction will add to the billed energy usage.
It is unlikely that a Regen VFD will produce sufficient power and see a low enough local load, but if that happens, POCO will bill you for the energy you pump back into the grid.