Negative Sequence Voltage 3

[rockman7892]

I understand that positive and negative sequence currents are represented by the following equations:

Pos - I1 = Ia + aIb + a^2Ic
Neg - I2 = Ia + a^2Ib + aIc

Can these same equation sets be used for calculating negative sequence voltages? If so I am assuming that the voltage magnitudes and angles are used in the equations just like those for currents are used.

We had an event last week where due to a fault on the utility transmission line, we lost our 4.16kV power to our plant. In the process of power being interrupted to our plant our main breaker tripped on a negative sequence voltage. I was trying to figure out why this was, and I am guessing that it has something to do with the way the voltage or current was interrupted during the utility circuit switcher opening? During the pickup of the event the current phasors all reversed their angles for a complete opposite rotation.

As I scrolled through the event history of the relay I found two other instances where the relay picked up on negative sequence voltages but then droped out or returned to normal before a trip occured. Looking at the phasors for these events I notice that the voltage magnitudes are below nominal voltage and appear to be about 5% unbalanced however have close to a 120deg displacement. The currents during these events appear to be either unbalanced or have a phase displacement of larger than 120deg. Are these negative sequence voltages usually a result of something occuring on the utility side of things such as a voltage drop of something else?

I have attached the phasors that were recorded during these events for viewing. The pickup for the Neg Sequence Voltage is .05VT (VT=4.16kV - 120V). Our system normally has pretty balanced voltage and has a pf of .69LAG(new plant slowly being brought online)

 

[cranky108]

During many faults it is possible to see negative sequence voltage and current. And if you are between the fault and the source you will see the negative sequence voltage.

For a phase to phase fault: V1=V2, or said another way you could see 50% negative sequence voltage. This is also a concern for three phase motors, because if they remain with that they will over heat from the excessive current draw on the remaining phase.

For a phase to ground fault: V1=V2=V0, or said another way you could see 33% negative sequence voltage. And there is the same concern for motors.

Also, if you were to lose a phase, such as one phase of a switch opening, you will also see V1=V2.
However, if one phase of a switch opens on the high side of a delta-wye transformer: V1=V2=V0, because of the lack of a neutral reference of the delta winding.

And there are other factors, which are more complicated.

pf of 0.69 lag? Maybe some capacitors are in order.

 

[slavag]

Hi Rockman.
Yep, positive, negative and zero sequence operators are same for the voltages and currents. Is only math, please dont connect it to some natural effects.
It's only make our live eazy .

Second, WHY you use this protection as protection function for the main breaker? Are you have generator with co-generation?
Im strognly recommend to you use this function as alarm only.
As undervoltage function too.
What is a setting of NPS voltage function?
Best Regards.
Slava
HAPPY A NEW YEAR

 

[davidbeach]

Yep, same formula. 0.05V secondary for negative sequence overvoltage is a bit tight perhaps, what time is associated with that voltage level? You can get some really weird sequence voltages (and currents) as a breaker is opening and some poles have cleared while others have not. Relays can get all sorts of things "wrong" during that time and you have to account for it.

 

[cranky108]

davidbeach, that's an under statement. Always, but always use a time delay for negative sequence relay (Lesson learned the hard way, and not tought very well in books).

 

[rockman7892]

The time delay set for the negative sequence voltage is 1s. This seems a bit short as others mentioned. This setting was given by a firm that perfromed the power study and relay coordination/protection study. Should this be increased?

I am fairly new to the whole pos/neg sequence thing and although I understand all of the math behind it I am wanting to find out what the causes are for neg seq voltage anc current. Back to the old saying "what came first the chicken or the egg?" i'm trying to learn weather or not neg seq currents cause neg seq voltages or vise versa. I'm hoping that understanding these causes will give me a better understanding.

I'm not sure why the firm set this neg sequence voltage setting (no neg sequence current setting) on the main breaker. We do not have a generator with co-generation only the one incoming utility feed. What are some reasons why you would want to have this setting?

 

[dpc]

Current comes from voltage. There can be negative sequence voltage with no current, but where's the current, there is voltage.

 

[davidbeach]

Think I have to disagree at bit there dpc. You drive a circuit with a positive sequence voltage; the sequence network connections determine what currents flow; the flow of those currents cause voltage drops. So, positive sequence voltage at the infinite bus and all impedances are givens. From them all currents can be derived. Once all currents are known the rest of the voltages can be derived.

 

[alehman]

As for why you would trip on negative sequence voltage... negative sequence voltage is used to detect phase unbalance. Sometimes it is desired to disconnect rather than running on significantly unbalanced voltage (can burn out motors). If that is the purpose, 1 second does seem a bit short. Some negative sequence relays have inverse time delay functions to roughly model the heating effects of unbalanced voltage in motors. This function is more commonly installed at individual motors or MCC's.

 

[sibeen]

I understand that positive and negative sequence currents are represented by the following equations:

Pos - I1 = Ia + aIb + a^2Ic
Neg - I2 = Ia + a^2Ib + aIc



Just to be pedantic, shouldn't the equations be:


Pos - I1 = Ia + aIa + a^2Ia
Neg - I2 = Ia + a^2Ia + aIa

 

[alehman]

I1 = 1/3(Ia + aIb + a^2Ic)
I2 = 1/3(Ia + a^2Ib + aIc)

Unless I'm missing something?

 

[sibeen]

Sorry, what I meant was:

Pos sequence = Ia + aIa + a^2Ia
Neg sequence = Ia + a^2Ia + aIa

If you have :

Pos = Ia + aIb + a^2Ic

this breaks down to:

First term = Ia

Second term = aIb, which is a 120 degree shift of Ib, which equals Ic, so then aIb = Ic = a^2Ia

Third term = a^2Ic, which is a 240 degree shift of Ic, which equals Ib, so then a^2Ic = Ib = aIa.

So, the positive sequence shown in the OP, is actually negative.

 

[alehman]

Are you assuming |Ia| = |Ib| = |Ic| ?
Symmetrical components is only useful where this is not the case.

 

[davidbeach]

alehman is correct, and yes the original post was missing the 1/3; it calculated 3I1 and 3I2.

sibeen, you are rotating things the wrong way around. In a balanced system Ib lags Ia by 120[°], so when you have aIb you bring it forward 120[°] and it aligns with Ia. You could certainly never come up with anything useful by your equations:

Pos sequence = Ia + aIa + a^2Ia
Neg sequence = Ia + a^2Ia + aIa

as you have lost all information about phases b and c. You can get away without considering b and c only in a completely balanced system, and then you don't need symmetrical components.

 

[VTer]

Please see attached. It’s going to put you in the right direction.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[sibeen]

sibeen, you are rotating things the wrong way around.


*shuffles feet in the sand*

*looks away*

*eventually bangs head on desk*

OMG, what the hell was I doing posting new years eve. Apologies all. I definitely screwed that one up...I blame the drink

 

[rmw]

And.... you've heard the warnings about drinking and posting...

rmw

 

[alehman]

I've had a few "what was I thinking?" posts myself.

 

[slavag]

Sibeen is OK.
small mistake, so ????.
btw, drinking and posting,...good idea.
Alehman, only here all of us had "what was I thinking" .

Please, explain me, for what I need this type of protection on the main breaker. Setting OK, possible change.and...
It's protection for the motors, for the generators, detection of problems on the back up feeder, motor bus, but sorry, not for the main preaker of plant.
Just my opinion.
Best Regards.
Have a nice vacation, guys.
Happy a New Year

 

[alehman]

I agree. The only time I have used negative sequence voltage on a main is where it is part of a transfer scheme to switch to an alternate source. Otherwise, it is better to protect nearer the load - why turn off loads, like lights, that are not sensitive to unbalance?