Equipment to solve phase unbalance 5

[YKC]

Hi everyone,

I am collecting the information about the equipment to solve phase unbalance, but I am not sure about my search keyword is correct or not, since I could not find many information.
I use the words like: phase balancer; phase shifter; phase balance device.
Then, I would like to ask the experts on this forum, could anyone provide some keywords or the equipment names about this topic? Give the website would be best.

Thanks in advance and please have a nice day!

YKC

 

[waross]

I am not sure that either of us understands the question.
What is the nature of the phase unbalance and why do you think it must be solved?
While the issue may be clear in your mind, it is too broad a scope for an easy answer.
Is the unbalance a grid condition or a local condition?
Is the issue phase to phase unbalance, phase to neutral unbalance, voltage unbalance, current unbalance, phase angle unbalance or an interesting combination and interaction of the conditions.
From the information given, the best answer is:
"It depends."

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

 

[bacon4life]

Based on your previous request in thread

https://www.eng-tips.com/viewthread.cfm?qid=517145,

are you talking about utility level grid imbalance?

Normally imbalances are solved by reconnecting loads/generators to a new phase. That just involves an electrician using simple hand tools like screwdriver, a wrench, or a hot stick to make a permanent fix.

Your searches not finding anything is probably because you seem to be searching for an overly complex solution to a simple problem.

A "phase shifting transformer" is a totally different kind of device uses to control the flow of real power in a regional transmission grid. This if often called a phase shifter as a slang term.

 

[waross]

A four wire wye:delta transformer Abhors voltage and phase angle unbalances.
The transformer bank will use the delta winding to transfer power from higher voltage phases to approach a balance.
How close the approach depends on source impedance, line impedance and transformer impedance.
Unbalances are often the result of uneven phase loading.
The transferred power will reduce the unbalance seen by the source.
I have not done this as a mitigation method, but I have seen some accidental, real world examples.
I have also seen an extreme unbalance overload and burn up a transformer in wye:delta bank.
The actual delta voltage is unimportant. It just sits there with nothing connected to it.
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[YKC]

For bacon:

Yes, it is, glad to hear that you still remembered my previous question.
I am actually a beginner on this topic, so I might not describe the question clearly.
What my project manager ask me to do is to collect the information of the equipment or the techniques which can help to improve the phase unbalance.
He also mentioned the techniques about "switch phase" (sorry I'm not English native speaker, I'm not sure the translation is correct or not). The "switch phase" technique is to change two phases in ABC phase (or RTS in some country). He also gave me the example such as: the upstream branch current in phase A is 90A; B 95A C 105A, then the downstream branch will separate to two branches, let's call D1 and D2. D1 current in phase A is 65A; B 50A C 55A, where D2 current in phase A is 25A; B 45A C 50A, and we can see the unbalance in D1 is around 14.7%, where D2 is 37.5%. Follow up, we can find D1 has lower unbalance, so we decide to switch D2's phase. The current value in D1 from high to low is A -> C -> B, so current value in each phase of D2 must follow D1 "reverse" connection, which lead to change phase B and phase C.(higher value in D1 connect to lower value in D2, middle connect to middle, lower connect to higher) Final, the upstream current in phase A is still in 90A; B changes to 100A, C changes to 100A, and its unbalance reduces from 8% to 3.4%.

So, what i want to ask is "Is there any equipment or techniques can achieve such work?"

Thanks~~

 

[waross]

Yes.
An electrician with a clamp on ammeter and a screwdriver.

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

 

[YKC]

For waross,

Did you mean that directly adjust the phase by hand (Manual processing)? Is there any equipment could automatic detect the current signal and switch the phase?
But I think my thought might be too ideal.

 

[waross]

For phase currents 90 Amps, 95 Amps, 105 Amps, a much more important question may be;
"Is there enough coffee in the coffee room for tomorrows coffee break?"
A wye:delta transformer will solve that.
The larger,more important question is WHY?
I doubt that the unbalance is costing anything.
The loses inherent in a correction device will probably far outweigh any cost incurred by such a small unbalance.
And payback time on investment may be negative, ie: The investment will never be returned.
Forget the unbalance and check the coffee. It is more important.

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

 

[YKC]

Well, it 's just only an example, not the real situation, I do know that is a small unbalance by calculating.

You're right, coffee is more important than phase unbalance, I write down this comment with drinking my ice coffee.

 

[jghrist]

The "switch phase" technique is to change two phases in ABC phase (or RTS in some country).

Changing two phases will cause any 3Ø motors to run backwards (or try to until something trips or breaks).

 

[crshears]

In my world that was called swapping any two phases.

The other technique [ reconnect a loads' phase A to Phase B, phase B to phase C, and phase C to phase A ] we referred to as "rolling phases" - and the "electrician with a clamp on ammeter and a screwdriver" was combined with a careful charting of the current drawn on each phase of every device on site, and configuring the imbalances so as to minimize them.

Not rocket science, not equipment intensive, and definitely not all that expensive.

Don't overthink it.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

Voltage unbalance is much more serious than current unbalance.
Phase to phase voltage unbalance is common on long rural distribution lines even though the phase to neutral voltages are equal.
Most of the rural loads are single phase and are not affected.
Three phase loads are affected.
Construct a vector sketch with equal angles but unequal values.
The angles are still equal.
The vectors will not close the triangle.
Eg, 240 Volts, 240 Volts, 220 Volts.
Now connect a three phase motor.
Take the back EMF of the motor as 230 Volts.
What happens?
On two phases there is an effective voltage of 10 Volts driving current through the motor impedance. (240V - 230 V = 10 Volts.)
On one phase there is an effective voltage of 10 Volts driving current back into the grid. (220 V - 230 V = -10 Volts.)
So the work is done by only two phases.
Additionally, these two phases must provide any power delivered back into the grid by the third phase.
As result the current unbalance tends to be much greater than the voltage unbalance that causes the current unbalance.
The motor runs hot and may burn up due to overload at less than rated HP output.
Engineers and electricians familiar with rural conditions tend to over-size three phase motors to provide the capacity needed to survive unbalanced voltages.

These unbalanced currents tend to counter the line drops that cause them.
And the more three phase motors connected simultaneously, the less will be the effect on each motor as all motors share the correction duties.
In another post I will talk about wye:delta mitigation.

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

 

[stevenal]

It seems we've narrowed the unbalance question down to current unbalance. Current unbalance alone is not a problem requiring a solution. Is the current unbalance causing voltage unbalance or excessive losses? You may get better answers here if can express the real problem you and your manager are trying to solve.

 

[YKC]

For stevenal,

Why you think the current unbalance is not a problem? In my opinion, if there are three single-phase load connects on a branch and their loads are different, then the three phase current would be different, which causes the phase unbalance. Also the current transmission must have some line loss, and as I know the higher the unbalance, the higher the line loss is. Our project is to help the transmission operator improve the transmission quality (Reducing line loss by reducing phase unbalance), like tell them which region has higher unbalance, and you should do some improvement first.

 

[stevenal]

YKC,

Different single-phase loads doesn't cause current unbalance, they are current unbalance. If the current unbalance is sufficient to cause voltage unbalance of 3% or more, I would look at re-balancing. Losses are a problem only if the solution is less costly. Put some costs on the losses and solution. If the solution involves equipment as you suggest, don't forget to include maintenance and replacement costs.

 

[waross]

Voltage unbalance mitigation:
Consider 3 transformers in a wye delta bank.The primary neutral is connected to the source.
The secondaries are connected in delta.
Draw a vector sketch of the delta voltages.
This will be an equilateral triangle.
Now reduce the voltage on one primary phase.
The corresponding vector on the delta side will be shortened.
As a result, the delta will not close, there will be a gap representing the percentage voltage difference of the primary phase.
But how can that be? It is a bolted connection.
The voltage represented by the gap in the vector diagram causes a current to circulate in the delta.
The current is limited by three times the single phase impedance, or by the sum of the impedances of the three transformers.
The circulating current is equal in each transformer and so the back EMF developed by the circulating current is equal.
But, the higher voltage phase will inject power into the delta and the lower voltage phases will deliver power back into the grid.
That is a "Wyeelta for Dummies" explanation.
The extra current on the high phases will cause extra line loss and act to drop the supplied voltage.
The less current or reverse current on the lower voltage phases will reduce line voltage drop.
But wait! Different voltages don't transfer real power, they cause or transfer reactive power.
Moving on from the "Wyeelta for Dummies" explanation, look at the vector sketch of the delta.
With one vector shorter, this is no longer an equilateral triangle.
As a result the phase angle are no longer equal.
It is this phase shift that causes the real power to be transferred from one phase to another.
The wye:delta (and the back EMF of induction motors) both act to oppose or partially correct the condition that is causing them.

A real world example of an accidental mitigation by a wye;delta bank:
A distribution circuit extended three phases about 15 miles from the substation.
At that point there was a small radio transmitter fed from a wye:delta bank.
From there, two phases extended for an unknown number of miles picking up single phase loads.
A fiend who was an engineer in the distribution department of the utility asked me for an opinion on what happened next.
The circuit was reasonably well balanced on all of the substation meters.
The engineer noticed that the radio station was defunct and service had been discontinued for a couple of years.
He issued orders that the transformer bank be de-energized.
He told me that the metering in the sub station "went crazy".
What had been a reasonably balanced circuit was now a severely unbalanced circuit.
Accidental wye:delta mitigation.

Another case of accidental mitigation, this time by induction motors.
This was a fairly large saw mill on the end of about 50 miles of three phase line.
Two phases extended about 20 miles further.
There was a dairy with an open delta three phase supply at the end of that line.
Unbalance caused by open delta loading is further degraded by voltage drop on the neutral caused the neutral current.
The voltage unbalance at the mill was pretty bad.
When the first motor was started, the current was excessive in proportion to the load on the motor.
As more motors were started, the voltage balance got better and the excessive currents were reduced as the multiple motors shared the task of partially mitigating the voltage unbalance.



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

 

[YKC]

For stevenal,

OK, I can understand this situation IS current unbalance, and I would like to ask another question since you mentioned about the current unbalance cause voltage unbalance. Is there any mathematics' equation can describe the relationship between current unbalance and voltage unbalance?
Also on the other hand, the transmission operator we service mainly do the improvement by measuring the neutral current; if the value is over the setting threshold value, then improve it. I curious about how do we give a "reasonable" threshold value?

 

[Parchie]

YKC,
Please see it this way: if your load amps differ in every phase, naturally the voltage drop in each of the supply line will differ. Even if the source is balanced, the resulting terminal voltage at the point of common coupling at your location will not be equal. That is what @stevenal is talking about -> the load current imbalance is too great that the voltage drops along the lines caused a voltage imbalance that is also great.

 

[waross]

The basic formula is IR. Current times line Resistance = voltage drop.
The higher the current, the greater the voltage drop.
But, the greatest line loss is generally in the primary lines.
But let's consider that your panel is fed from a close in transformer with a 7.62 kV primary. (13.2 kV/7.62 kV system)
I can't find a voltage stated so I will use 220 Volts (220 V/380 V system)
7620 V / 220 V = 34.64
Your 100 Amps demands 100A/34.64 = 2.89 Amps on 7620 Volts. So the voltage drop in Volts will be quite low.
Now if we express that as a percentage, it is a percentage of 7620 Volts.
The small voltage drop is a very small percentage.
But we are not looking at the voltage dropcaused by the 100 Amp load.
We are concerned with the difference between the voltage drop with a 105 Amp load and the voltage drop caused by a 95 Amp load.
Our small percentage gets even smaller.

Disclaimers:
1. There are many exceptions. While in most cases the unequal voltage drops cause by those unequal currents is negligible, there are exceptions.This is presented as an illustration to explain the basics.
2. This is a simplified explanation to illustrate how transformer ratios act when considering line voltage drops. This example considered resistance only.
In the real world,the voltage drop across an impedance will also have a phase shift.
Things start to get too complicated for this discourse.
3. In the real world, distribution lines also have inductive reactance.
Inductive reactance in each phase conductor is affected by the position of the conductor relative to the other conductors and its relative proximity to the ground.
The inductive reactances of three untransposed lines are unequal.
This inequality will lead to unequal voltages under load.
This is mitigated with transpositions so that each conductor travels an equal distance in each position relative to the other conductors.
Another possibility is missed or poorly spaced transpositions resulting in incomplete balancing of the inductive reactances of the conductors.












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