Voltage Drop Calculation with frequency converter

[Gilf]

Hi all,

I have inherited this project and when I look at the voltage drop calculations I see that the formula is always reset when there is a frequency converter.

As a general rule, maximum allowed voltage drop is taken as 5% in total which consists of 3% for branch circuit and 2% from source to panel.

For example, one of the frequency converters is on a 400/230V, 50 Hz system which starts at a transformer, has a MDP and distribution panel. Then we have a frequency converter which has 400V, 3P, 50 Hz input and 200/115V, 400 Hz output. The 5% allowed voltage drop is reset at this point. Normally everywhere else in the project the 5% start from the transformer but when there is a frequency converter the 5% limit is reset to start at the frequency converter.

I understand that at this point the frequency converter is taken as the source, however I could not find any technical documents that explain the physics behind it. Can someone explain the concept or shoot me in the right direction for some tech papers?

Thanks in advance!

 

[Ingenieur]

I doubt you will find any papers on it
it is as you said, it is considered a new source
an electric motor driven generator with voltage regulation

 

[steve66]

Hi all,

I have inherited this project and when I look at the voltage drop calculations I see that the formula is always reset when there is a frequency converter.

As a general rule, maximum allowed voltage drop is taken as 5% in total which consists of 3% for branch circuit and 2% from source to panel.

For example, one of the frequency converters is on a 400/230V, 50 Hz system which starts at a transformer, has a MDP and distribution panel. Then we have a frequency converter which has 400V, 3P, 50 Hz input and 200/115V, 400 Hz output. The 5% allowed voltage drop is reset at this point. Normally everywhere else in the project the 5% start from the transformer but when there is a frequency converter the 5% limit is reset to start at the frequency converter.

I understand that at this point the frequency converter is taken as the source, however I could not find any technical documents that explain the physics behind it. Can someone explain the concept or shoot me in the right direction for some tech papers?

Thanks in advance!

I assume your frequency converter is basically a UPS. It converts the input AC to DC, and then back to AC at 400 Hz. Since its basically a UPS, it will produce the correct output voltage even if the input voltage drops by a few percent.

So from a standpoint of providing the correct voltage to the load, the voltage drop in front of the converter wouldn't matter. (As long as its not excessive).

From a standpoint of efficiency, then yes, all the voltage drops contribute to the power losses. But you also have losses in the frequency converter, and these may be a lot higher than the losses due to voltage drop over the feeders. So as long as the voltage drop on the input feeder doesn't exceed 2 or 3%, and the drop on the output feeders and branch circuits doesn't exceed 5%, its probably not worth worrying too much about, unless someone reviewing energy code compliance is going to make an issue about it.

 

[Gilf]

I assume your frequency converter is basically a UPS. It converts the input AC to DC, and then back to AC at 400 Hz. Since its basically a UPS, it will produce the correct output voltage even if the input voltage drops by a few percent.

So from a standpoint of providing the correct voltage to the load, the voltage drop in front of the converter wouldn't matter. (As long as its not excessive).

From a standpoint of efficiency, then yes, all the voltage drops contribute to the power losses. But you also have losses in the frequency converter, and these may be a lot higher than the losses due to voltage drop over the feeders. So as long as the voltage drop on the input feeder doesn't exceed 2 or 3%, and the drop on the output feeders and branch circuits doesn't exceed 5%, its probably not worth worrying too much about, unless someone reviewing energy code compliance is going to make an issue about it.

It's not a UPS. It feeds directly from the main emergency panel. The main emergency panel has a feed from the transformer panel and second feed from a generator via an ATS. From the static frequency converter I feed a 400 Hz distribution panel, which eventually feeds the 400 Hz support equipment.

The issue here is that, the authority requests line drop compensators for the support equipment, yet previously they had said that if the voltage drop is within the allowed range there is no need. There fore, resetting the allowed voltage drop limit in the calculation does make a difference.

 

[Ingenieur]

It's not a UPS. It feeds directly from the main emergency panel. The main emergency panel has a feed from the transformer panel and second feed from a generator via an ATS. From the static frequency converter I feed a 400 Hz distribution panel, which eventually feeds the 400 Hz support equipment.

The issue here is that, the authority requests line drop compensators for the support equipment, yet previously they had said that if the voltage drop is within the allowed range there is no need. There fore, resetting the allowed voltage drop limit in the calculation does make a difference.

isn't that to your advantage?
what isthe issue?
the static converter puts out rated voltage even if the input is 5% low
so the v drop calc is 'reset'

 

[gar]

171110-0907 EST

Gilf:

Ingenieur gave you a very good simple answer (in his first post). I will add some detail.

Suppose you had a black box device at the end of a long distribution line. The voltage into the black box will change from 100% at no load to 80% at full load.

Inside the black box is a voltage regulator that produces an output voltage of 100% anywhere from 0 to full load.

Where do you start your voltage drop calculation for loads on the output side of the black box?

Next suppose your frequency converter consists of a synchronous motor, no speed variation with change in load, driving a generator that produces the new frequency.

Next suppose the generator (alternator) has a permanent magnet rotor, then the induced voltage in the stator is constant independent of output load because induced voltage is proportional to speed and speed did not change with load. This does not mean the terminal voltage was constant. The generator has an internal impedance so output terminal voltage will vary with load.

Change the driving motor to an induction motor and its speed is slightly dependent on load. Thus, both generated voltage and frequency will vary some with load.

If you add a voltage regulator, passive or active, to the generator, then output voltage can be corrected to some extent, or even over corrected, as a function of load.

Anytime you add voltage regulation to a source you have created a means to change the apparent internal impedance of the source.

Since I started writing this post the information you provided has changed. What is a static inverter? What are its characteristics?

The design of the static inverter will determine whether it can be considered the source or not.

If it is AC to DC to AC with internal voltage regulation, then it is the source with some internal impedance.

If it is AC to DC to AC without internal voltage regulation, then it is some form of series impedance added to the input and output line impedances.

.

 

[Gilf]

171110-0907 EST

Gilf:

Ingenieur gave you a very good simple answer (in his first post). I will add some detail.

Suppose you had a black box device at the end of a long distribution line. The voltage into the black box will change from 100% at no load to 80% at full load.

Inside the black box is a voltage regulator that produces an output voltage of 100% anywhere from 0 to full load.

Where do you start your voltage drop calculation for loads on the output side of the black box?

Next suppose your frequency converter consists of a synchronous motor, no speed variation with change in load, driving a generator that produces the new frequency.

Next suppose the generator (alternator) has a permanent magnet rotor, then the induced voltage in the stator is constant independent of output load because induced voltage is proportional to speed and speed did not change with load. This does not mean the terminal voltage was constant. The generator has an internal impedance so output terminal voltage will vary with load.

Change the driving motor to an induction motor and its speed is slightly dependent on load. Thus, both generated voltage and frequency will vary some with load.

If you add a voltage regulator, passive or active, to the generator, then output voltage can be corrected to some extent, or even over corrected, as a function of load.

Anytime you add voltage regulation to a source you have created a means to change the apparent internal impedance of the source.

Since I started writing this post the information you provided has changed. What is a static inverter? What are its characteristics?

The design of the static inverter will determine whether it can be considered the source or not.

If it is AC to DC to AC with internal voltage regulation, then it is the source with some internal impedance.

If it is AC to DC to AC without internal voltage regulation, then it is some form of series impedance added to the input and output line impedances.

.

thanks a lot! I was after the detailed explanation.