Zero-Sequence
- Thread starter Mike01
- Start date
- Status
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
This was the subject of a graduate level course that I took during my masters degree program. I might give you a notion of what the terms mean, but I can’t teach you how to use them, or at least not without getting a serious case of carpal tunnel syndrome.
To start with, we tend to presume that all loads are balanced among the three phases. That allows us to use simple methods of adding loads. If the loads are not balanced, the math becomes very tricky. If you look at a non-balanced fault (e.g., single line to ground, line to line, or double line to ground, anything other than a bolted three phase fault), the math is also very tricky.
But some brilliant guy (who, by the way, I understand was awarded a PhD on the basis of about 20 pages of hand-written text), came up with a simple way to model unbalanced loads (including unbalanced faults). He created a model that uses three sets of data. One set of data (meaning current and voltage data) would be balanced within itself, and would have the same phase sequence (A-B-C) as the original (unbalanced) load. The second set of data (again meaning current and voltage data) would also be balanced within itself, but it would have the opposite phase sequence (A-C-B). Those two sets of data are called the "Positive Sequence" and the "Negative Sequence." The third set of data is also balanced within itself, but there is no phase sequence. All three of its voltages are in phase with each other, as are all three of its currents. That set is called the “Zero Sequence.”
What you are left with are three simple problems, instead of one complex one. You begin the analysis by converting the unbalanced problem into three balanced problems. Then you do the three problems separately. Finally, you combine them back by converting to a single set of data. The mathematical tool used to perform these manipulations is matrix algebra.
There. That’s about all my wrists can handle for now. Did this help?
To start with, we tend to presume that all loads are balanced among the three phases. That allows us to use simple methods of adding loads. If the loads are not balanced, the math becomes very tricky. If you look at a non-balanced fault (e.g., single line to ground, line to line, or double line to ground, anything other than a bolted three phase fault), the math is also very tricky.
But some brilliant guy (who, by the way, I understand was awarded a PhD on the basis of about 20 pages of hand-written text), came up with a simple way to model unbalanced loads (including unbalanced faults). He created a model that uses three sets of data. One set of data (meaning current and voltage data) would be balanced within itself, and would have the same phase sequence (A-B-C) as the original (unbalanced) load. The second set of data (again meaning current and voltage data) would also be balanced within itself, but it would have the opposite phase sequence (A-C-B). Those two sets of data are called the "Positive Sequence" and the "Negative Sequence." The third set of data is also balanced within itself, but there is no phase sequence. All three of its voltages are in phase with each other, as are all three of its currents. That set is called the “Zero Sequence.”
What you are left with are three simple problems, instead of one complex one. You begin the analysis by converting the unbalanced problem into three balanced problems. Then you do the three problems separately. Finally, you combine them back by converting to a single set of data. The mathematical tool used to perform these manipulations is matrix algebra.
There. That’s about all my wrists can handle for now. Did this help?
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
Search for zero sequence impedance in yahoo or google for an earfull.
Simply, this information will be used for short circuit calculations.
Simply, this information will be used for short circuit calculations.
rcwilson
Senior Member
- Location
- Redmond, WA
Zero Sequence Impedance
Zero Sequence Impedance
The symmetrical components method of calculation uses Positive, Negative and Zero sequence impedance networks to calculate unbalanced currents and voltages that occur during faults or normal operation. It reduces a complex math to simple, symmetrical math. (It is similar to thinking about watts and vars and RMS currents to simplify the vector math of sinusoidal circuits.)
One useful thing that drops out with this math analysis is that for a phase-ground fault, total fault current is = 3 x I0 at the fault = three times the Zero Sequence Current calculated at the fault. No positive or negative sequence current is present in the fault current.
In a normal balanced load, there are only positive sequence currents, no Negative or Zero Currents. A fault will produce negative or zero sequence currents, unless it is a balanced three-phase fault. So relay systems that measure negative sequence or zero sequence currents can distinguish between "good" load current (positive sequence) and "bad" negative or zero currents better than a breaker that just looks at amps.
A "Zero Sequence" CT is a doughnut CT encircling all current carrying conductors so it only reads the zero sequence current. (Actually 3 x the I0 current). And all the time you thought this was a ground fault CT. A ground fault relay is looking for zero sequence currents.
When a utility gives the system impedance at your location it will usually provide the positive sequence and zero sequence impedance, or the three-phase fault and ground fault current data. You can calculate one from the other. Negative sequence impedance is usually = to positive sequence so it is not normally given.
We use these values to calculate fault current levels in our systems.
The above explanation is not technically rigorous. Google ?Symmetrical Components training aids? to find some neat websites that show how this mathematical analysis works.
Zero Sequence Impedance
The symmetrical components method of calculation uses Positive, Negative and Zero sequence impedance networks to calculate unbalanced currents and voltages that occur during faults or normal operation. It reduces a complex math to simple, symmetrical math. (It is similar to thinking about watts and vars and RMS currents to simplify the vector math of sinusoidal circuits.)
One useful thing that drops out with this math analysis is that for a phase-ground fault, total fault current is = 3 x I0 at the fault = three times the Zero Sequence Current calculated at the fault. No positive or negative sequence current is present in the fault current.
In a normal balanced load, there are only positive sequence currents, no Negative or Zero Currents. A fault will produce negative or zero sequence currents, unless it is a balanced three-phase fault. So relay systems that measure negative sequence or zero sequence currents can distinguish between "good" load current (positive sequence) and "bad" negative or zero currents better than a breaker that just looks at amps.
A "Zero Sequence" CT is a doughnut CT encircling all current carrying conductors so it only reads the zero sequence current. (Actually 3 x the I0 current). And all the time you thought this was a ground fault CT. A ground fault relay is looking for zero sequence currents.
When a utility gives the system impedance at your location it will usually provide the positive sequence and zero sequence impedance, or the three-phase fault and ground fault current data. You can calculate one from the other. Negative sequence impedance is usually = to positive sequence so it is not normally given.
We use these values to calculate fault current levels in our systems.
The above explanation is not technically rigorous. Google ?Symmetrical Components training aids? to find some neat websites that show how this mathematical analysis works.
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