Negative Phase Sequence currents 1

[Bloozntooz7868]

Searched the thread archive for this one, but didn't find what I was looking for.

After showing someone how to calculate NPS currents from Generator data, they asked my a question that I had never even asked myself, and couldn't answer - What makes NPS currents rotate counter to the rotating magnetic field? I know they do, I can calculate them, but I don't know why.
All the internet searches come up with a plethora of people saying what happens, but just float over 'why' the question. They show Phasor diagrams reflecting the reversal, but no explanation why.
Why does an unbalanced system create a counter rotating current? Inclusion of a vector diagram would maybe help - something to relate to.

Thanks in advance for a clear explanation.

 

[Hoxton123]

We use all these imaginations to rationalise what we see.

It’s the load what does it.

When the (three phase) load is balanced then there are only balanced currents, when the loads are unbalanced then strange things happen.
This can be explained by imagining that the currents are split into positive (forward) sequence currents and negative (reverse) sequence currents.
Negative sequence currents do not exist per se.

Think about transient and sub-transient reactance. These do not exist. What exists is fault current. After a fault the generator current rises rapidly then decays. We use transient and sub transient reactance to help us understand what is happening , then as a tool to help us protect against the effects of the fault.

 

[Bloozntooz7868]

Hi Hoxton,

Thank you for the reply, but to my knowledge these currents do actually exist, and they counter rotate and therefore cut the rotor at 2x fundamental frequency. This causes rapid overheating of the rotor ends (in particular) and eventually ruins the machine.
I don't understand the notion of them not existing. What am I missing?

 

[Gr8blu]

Bloozntooz: When phase relationships (specifically current) are balanced, the result is a pretty smooth magnetic field, all things considered. The field rotates in the same direction as the phase sequence of the stator winding. However, when the phase relationships are unbalanced, the resulting magnetic field is less symmetrical - or, to put it another way, distorted. How do we get from smooth to distorted? Remember, zero sequence currents are just that - they only flow in the neutral conductor, and thus supply nothing to distort the field. That means there has to be one or more phase currents flowing in opposition to the main positive sequence currents. If they are flowing in opposition, then they could be called "negative sequence". They do create a magnetic field that runs counter to the way the rotor is turning - which gives us flux-cutting at 2x line frequency ... and correspondingly higher thermal stress.

Converting energy to motion for more than half a century

 

[xnuke]

Think of this the same way as a vector being resolved into components on the x, y, and z axes. The components exist to describe pieces of the actual vector. They are a mathematical change of representation to make difficult math easier to solve by looking at the components independently. The math we apply is known as a transform.

We also do a change of representation, i.e., apply a transform, to make the difficult math of unbalanced three-phase system analysis easier to solve by breaking the unbalanced phasors down into balanced components which we can then look at independently. Three-phase unbalanced current phasors can be resolved into three balanced component three-phase phasor sets on three different "axes": positive sequence, negative sequence, and zero sequence. These can be known as the ZPN domain. The phasor component axes in this domain are defined as all rotating in the same direction - counterclockwise. The positive sequence axis is defined as having phasors of equal magnitude separated by 120 degrees with the same phase sequence as the total current (ABC), the negative sequence axis is defined as having phasors of equal magnitude separated by 120 degrees with the opposite phase sequence (ACB, hence opposite phase sequence to the rotor rotation which is in the ABC direction), and the zero sequence axis is defined as having phasors of equal magnitude separated by 0 degrees.

When you calculate the NPS, you are calculating the projection of the unbalanced three-phase current onto the negative sequence "axis," much like you can calculate the projection of a vector in the xy plane onto the x-axis to determine its component along that axis. The negative sequence current does not exist independently, it is just a component of it.

Are the sequence components real? That begs the question of causality. Is it a vector that causes its components, or do the components cause the vector? I prefer to think of it the first way, though just for my peace of mind because I know that causality doesn't exist in the relationship of a vector and its components since they cannot exist without each other. Theoretically, if you hooked up three three-phase generators in parallel, one with a positive sequence current output, one with a negative sequence current output, and one with a zero sequence current output, you could create an unbalanced set of currents. We don't normally do that. Instead, we have unbalanced load creating unbalanced currents. When a meter presents the magnitude and angle of a negative sequence current, it's measuring the actual current and doing the mathematical analysis.

Can we manipulate certain components of current independently? Yes, and that is the foundation of vector control in drives, inverters, etc., but that is a story for another time and involves resolution into the dq0 and αβγ domains instead of the ZPN domain.

I hope that clears things up.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Bloozntooz7868]

Gr8blu / xnuke,
Thank you for your time replying. It is certainly more clear to me now, but I need to fully absorb the depth of what has been said to finally get there. I am always thinking of how to relate topics (any topic) to someone who asks me, and I need to visualize things to be able to do that.
These are the most comprehensive explanations I have had (or found) but I do need to put in work to get them to click - am I will get there with the knowledge given here

Thank you very much for your clear and expert input!