3 phase motor current imbalance test. 6

[dynets]

Does anyone know how I can easily create a phase current imbalance on a 3-phase AC motor? I am testing a product that claims to protect for motor current imbalances and I'd like to see the point (how much of an imbalance) at which the protection works. I would like to be able to create current imbalances from 5 to 60% while running an actual 3-phase 460V motor.

I was thinking about using a 3 phase variac and isolate one of the three coils. This will allow it to be independently adjusted from the other two to increase or decrease one phase voltage and create a current imbalance. But I'm not quite sure the best way to connect the coils. Also I thought I'd have a problem with inrush when I start the motor. PowerStats don't have much of an overload capacity unless they are oversized. I don't want to have to go buy a large 3-phase powerstat unless I have too (times are lean).

I was hoping there was a motor designer out there that might know of another way, perhaps with large power resistors or line reactors. Or maybe somehow manipulating or miss wiring the nine motor leads.

Any suggestions would be appreciated.

 

[electricpete]

busbar
Good questions. They are Probably beyond my expertise but that never stopped me before!

Do you have any feel for what should used be for supervision of the 46? Maybe a 32 device to inhibit 46 operation if watt flow is away from the stator? If no, in your opinion what would be the proper device to supervised the 46?

I don't have any experience with a directionally-supervised negative sequence protection scheme (I was only quoting the standard). I don't think reverse power is the proper relay... it probably wouldn't respond fast enough to block an instantaneous relay (in fact, can any relay change state fast enough to block an instantaneous relay). I'll bet there are some T&D types out there that have a good handle on this since directional sensing is common in the T&D world. Or how about peterb?

Based on what's in the standard, I would be more more comfortable with time delay.... remembering that the purpose of the relay is to prevent overheating which occurs over a period of time.

If yes, this leads to: Is stator heating more or less of a problem with changing power-flow direction?

I don't know that answer to that question.
I picture negative sequence falling in-line with overload protection (long-term overheating). In contrast I picture a power system disturbance severe enough to cause power reversal in the motor to be a fault which would likely be removed by external instantaneous-acting relays.

During the brief time while the fault is present, overload/negative sequence is not a concern. Also note that the motor's own instantaneous overcurrent and overload relay protection presumably would remain in effect during this scenario.

That's how I see it from here, but take it with a grain of salt... just shooting from the hip on this one.

 

[electricpete]

Thinking about it a little more: IF negative sequence truly is a long-term/overload type of issue, then why would we ever want an instantaneous-acting relay. Sure we could directionally-protect it for the one case of severe out-rush current feeding an external fault, but what about a less severe momentary voltage imbalance of several cycles ... it might not cause reverse current but could trip an instantaneous negative sequence relay unnecessarily.

All of the above makes me wonder what whether I am missing something. Why would there ever be a need for instantaneous negative sequence on a motor?

 

[stevenal]

Negative sequence voltage is used to polarize negative sequence current elements in order to make them directional. Think of a phase to phase fault somewhere on the system. Remote protection operates to clear this fault with no local outage. Your motor will feed negative sequence current to the fault until remote protection operates. As Pete has said, this brief current will not harm the motor. Directional control or a time delay on the 46 element will allow your motor to ride through this disturbance.

 

[peterb]

electricpete, I agree that that you would need to delay the "instantaneous" element when directionalizing a scheme like this. There are high speed directional relays available for this type of dut; as you said, they are usually used specifically for T&D directional comparison type schemes.
An instantaneous 46 could protect against starting a motor under single phasing conditions, but I can't think of any other reason not to have a time delay.

 

[SooryaShrestha]

electricpete, the true unbalance I had referred is given in IEEE Power Engineering Review on May, 2001 issue with the true definition of voltage unbalance factor. It also has got approximation to true definition.

 

[electricpete]

SooryaS

"True imbalance" remains an unknown term for me.

In contrast the term "percent voltage imbalance" is defined in NEMA MG-1-1998R1 section 14.36 as well as IEEE 141-1993 section 3.8.1 (same definition in both documents). It is the maximum deviation from the average (used as an approximation to negative sequence).

Also "negative sequence" is defined in IEEE 399-1997 section 3.2.9 as well as most power textbooks on symmetric component analysis and is used in IEEE141,399,242 among other standards. It is defined as
V2 = (1/3)*(Va+ alpha^2*Vb+alpha*Vc) where Va, Vb, Vc are phasors and alpha is exp(j*120 degrees)

I found no mention of "true imbalance" in these documents and I honestly don't know what it means. Can you describe it some more?
electricpete