Sequence Angles Imbalance 1

[sky2020]

My measured generator sequence currents
I1 = 18.2A -4 deg
I2 = 0.63A 55 deg
I0 = 0.63A 79 deg
Voltage is good. Will this be load unbalance or grounding issue?

 

[xnuke]

Why do you think these are problematic? Have you calculated the equivalent phase values? Do they look bad?

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.

 

[sky2020]

Yes it looks bad. What is the meaning of unbalanced sequence angles?

 

[davidbeach]

I’m not aware of any meaning there. Text book systems are all positive-sequence. Real world systems have some standing zero- and negative-sequence values. Angles mean nothing at those levels.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[sushilksk]

This translates to current values:
IL1 = 18.6A -0.4 deg
IL2 = 17.9A -126.6 deg
IL3 = 18.1A 114.8 deg

Minor imbalance in IL1, may be due to loading condition.

 

[xnuke]

The reason for my above post was stated by davidbeach: there is no physical meaning to sequence angles. Sequence values are merely components of the phase values. If you had calculated the phase quantities (thanks for posting them, sushilksk) and the current imbalance from them (2.2%) you probably would have thought there was no significant issue.

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.

 

[stevenal]

By definition, sequence components are balanced sets of phasors. The angles shown are likely intended to be the A phase component angle referenced to the A phase voltage. The other phase I2 angles will be displaced by 120 degrees, and the other I0 phase angles will be displaced by 0 degrees. I see no cause for concern.

 

[7anoter4]

Percent voltage unbalance is defined by the National Electrical Manufacturers Association (NEMA) as 100 times the absolute value of the maximum deviation
of the line voltage from the average voltage on a three-phase system, divided by the average voltage. For example, if the measured line voltages are
462, 463, and 455 volts, the average is 460 volts. The voltage unbalance is: (460 – 455) 460 x 100 = 1.1%
If currents unbalance is due to the voltage unbalance the maximum voltage unbalance recommended by Figure 20-2 of NEMA MG-1-2011 is 1%.
It is recommended that voltage unbalances at the motor terminals do not exceed 1%. Unbalances that exceed 1% require derating of the motor, per Figure 20-2 of NEMA MG-1-2011, and will void most manufacturers’ warranties. If the current is proportional with the voltage then the current average is 18.2 A and the unbalance is (18.2-17.943)/18.2*100=1.4%

 

[stevenal]

Motor current unbalance is 6 to 10 times the voltage unbalance. See ANSI C84.1. 1.4/6=0.23% V unbalance, well under 1%.

 

[waross]

Voltage unbalance causes reactive currents to circulate between the motor and the source transformer.
Most of the I[sup]2[/sup]R losses of the extra current are shared between the motor, and the supply transformer.
The sharing ratio depends on the relative characteristics of both the motor and the transformer.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[7anoter4]

I agree with you Stevenal if the current unbalance is due load impedances' unbalance. Then if 0.76 ohm-for instance-is balanced system impedance the voltage unbalance will be 0.12%.However the same unbalance current may be if the load impedances are balanced but the supply voltages are unbalanced. Then the voltage unbalance is 1.5%.

 

[stevenal]

7,
I was speaking of motor loads as you were above. I don't believe they exhibit the nice linear wye impedance you drew. The 6 to 10X is commonly quoted. See ANSI.

 

[waross]

An old rule of thumb for servicemen was that the current unbalance is roughly the square of the voltage unbalance.
Accurate? Not at all.
Useful? Yes.
It was a good illustration that a small voltage unbalance will cause a large current unbalance.
It indicated to servicemen that the issue was a supply issue and not a motor fault.
A typical service call may be initiated by an overheated motor.
Step one, measure the current and see a large current unbalance.
Step two, measure the voltage and see a relatively small voltage unbalance.
Step three diagnose the problem as a supply issue and look for the cause of the voltage unbalance.
Note that actually calculating the percent voltage unbalance was not one of the trouble shooting steps.
The rule served as a good indication that a little cause (voltage) resulted in a large effect (current).

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[sky2020]

The voltage is balanced but the current waveform is unbalanced.

 

[7anoter4]

I am sorry stevenal , but, as far as I know the national standard for utility voltage tolerance in North America is ANSI C84.1. This standard establishes nominal voltage ratings and operating tolerances for 60Hz electric power systems above 100 volts. There is no mention of voltage unbalance here.
It is very interesting Sky2020, indeed, what wave-form of diamond shape is it?

 

[davidbeach]

May mean nothing. 4 samples per cycle and straight line fitting will produce diamonds where a curve fit will produce something more sine-like. But it could also be something else. Not enough info.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[stevenal]

7,
V unbalance is part of the tolerances. See page 9. Derating from MG1 is shown.

Here's another (DOE) source