Understanding closed loop Power Flow

[philly]

Can someone please help me understand closed loop power flow given the following setup:

A 225kVA transformer feeds a bus (say 13.8kV) and fed from bus are two 1MW bi-directional AC/DC power supplies that are connected together to a common 1000V DC bus on their secondary's. I am told that the 225kVA transformer is only there to make up the losses in the system and that these power supplies can be ramped up to flow 1MW worth of power in the closed loop system. I'm trying to understand how with only a 225kVA supply 1MW can flow in system with transformer only making up for losses.

The power flow as I understand it comes from transformer secondary onto 13.8kV bus then goes through 1st AC/DC converter onto common DC bus which then feeds back through 2nd AC/DC converter (converting back to AC) to the primary of the 1st converter (via common 13.8kV bus). I am told that this power can keep circulating in this closed loop fashion with the first power supply being capable of being ramped up to 1MW to circulate the 1MW in the closed loop and 225kVA transformer will only make up for losses.

Can someone help explain how with only a 225kVA input this 1MW circulating power can be generated? I'm just not seeing it for some reason.

 

[synchro]

Can someone help explain how with only a 225kVA input this 1MW circulating power can be generated? I'm just not seeing it for some reason.

It sounds like you are describing a closed loop system with 1MVAr of reactive power which is circulating around and the 225kVA transformer is necessary to make up for losses in the converters, etc. So energy can be circulating within the loop as it is transferred between energy storage elements of the converters such as inductors and capacitors. But to build up and then maintain the required energy levels for 1MVAr of circulating reactive power, it is necessary to supply real power from the 225kVA transformer.

What you describe is analogous to resonant systems that exchange energy between storage elements such as an inductor and capacitor, mass and spring, etc. Low loss resonant systems have higher "Q" and therefore require less external power to keep going.

I wound not expect the closed loop system you mentioned to be trivial to implement because you have to take measures to make sure that the loop is stable, and that loop is likely to have a complicated transfer function.

 

[philly]

It sounds like you are describing a closed loop system with 1MVAr of reactive power which is circulating around and the 225kVA transformer is necessary to make up for losses in the converters, etc. So energy can be circulating within the loop as it is transferred between energy storage elements of the converters such as inductors and capacitors. But to build up and then maintain the required energy levels for 1MVAr of circulating reactive power, it is necessary to supply real power from the 225kVA transformer.

What you describe is analogous to resonant systems that exchange energy between storage elements such as an inductor and capacitor, mass and spring, etc. Low loss resonant systems have higher "Q" and therefore require less external power to keep going.

I wound not expect the closed loop system you mentioned to be trivial to implement because you have to take measures to make sure that the loop is stable, and that loop is likely to have a complicated transfer function.

Yes that analogy makes sense

Another analogy I recall hearing is one involving 2 generators (or an MG set, I don’t recall) in which the first generator is used to control and ramp the load up to 1MW with the transformer making up the losses.

 

[winnie]

The mg setup is exactly what we use in our lab for motor tests.

We have 2 variable frequency drives running two motors on a common shaft with a torque and speed transducer between them. The two vfds have a common dc bus.

To test a motor we drive in with one vfd. The other motor is operates as a regenerative brake.

So the first vfd is consuming electrical power and sending it to the motor which is converting it to mechanical power. The mechanical power goes down the shaft to the second motor and gets converted back to electrical power. The regenerated electrical power flows on the common dc bus back to the first vfd.

If there were no losses in the system then this power could just circulate forever. But of course there always are losses. An external supply is needed to make up for the losses.

If the losses are small then the circulating power can be much greater than the make-up power.i

Jon

 

[philly]

The mg setup is exactly what we use in our lab for motor tests.

We have 2 variable frequency drives running two motors on a common shaft with a torque and speed transducer between them. The two vfds have a common dc bus.

To test a motor we drive in with one vfd. The other motor is operates as a regenerative brake.

So the first vfd is consuming electrical power and sending it to the motor which is converting it to mechanical power. The mechanical power goes down the shaft to the second motor and gets converted back to electrical power. The regenerated electrical power flows on the common dc bus back to the first vfd.

If there were no losses in the system then this power could just circulate forever. But of course there always are losses. An external supply is needed to make up for the losses.

If the losses are small then the circulating power can be much greater than the make-up power.i

Jon

OK This analogy makes sense to me so lets continue with it.....

So lets say for the sake of example the motor and generator in this example at rated at 1MW and fed from a 225kVA source. When the system is first energized the 225kVA will be applied to the MG set through the VFD with both the motor and drive consuming and converting 225kVA.

What I'm trying to see is how this 225kVA can not be circulated with minimum losses to develop a much larger power. Is it as simple as looking at it from the perspective that if this initial 225kVA circulates and only has very minimal losses in system (lets say 5kVA) then this remaining 220kVA will flow back into the DC bus of the first drive an combine with the 225kVA coming from the system source to now build up to 445kVA in the system? If this is correct then I assume the power build-up can continue in this fashion adherent to the limitations of the equipment and system losses?

 

[winnie]

So you understand the steady state condition where everything is running at full steam with the external power input making up the losses, and now you are asking about the build up to that condition.

Is might be easier to think of this in terms of energy stored. So much energy in the DC capacitor banks, so much in the inertia of the system, so much in the inductance of the motors. Ignore for a moment the circulating aspects, you need to store so many watt seconds in the system. You have so much power going in, and every second you store some energy and dissipate some as losses.

Once the stored energy builds up to the state levels then you are at full power.

 

[philly]

So you understand the steady state condition where everything is running at full steam with the external power input making up the losses, and now you are asking about the build up to that condition.

Is might be easier to think of this in terms of energy stored. So much energy in the DC capacitor banks, so much in the inertia of the system, so much in the inductance of the motors. Ignore for a moment the circulating aspects, you need to store so many watt seconds in the system. You have so much power going in, and every second you store some energy and dissipate some as losses.

Once the stored energy builds up to the state levels then you are at full power.

I'm sort of following you hear but not 100%. I believe I understand the steady state concept (assuming my example above is correct). Just trying to tie my example and your explanation above together and I think it will click.

Thanks for the help!

 

[winnie]

imagine a conveyor belt that is rated to carry 1000000 sugar cubes per second. You load sugar cubes at one end, some 2.5% fall off in the middle, and lots of cubes get delivered to the other end.

You need to test this conveyor belt, so you get another one going in the opposite direction. Cubes are loaded at the start of conveyor 1, some % fall off in the middle, and get delivered at the end of conveyor 1 to the start of conveyor 2. Some fall off in the middle of conveyor 2 and get delivered to the start of conveyor 1. An external supply also delivers 50000 cubes per second to the start of conveyor 1.

Start the conveyors, and start external supply. Now what happens?

Jon

 

[GoldDigger]

You eventually get buried in fallen sugar cubes.