About three-phase unbalance standard

[YKC]

Hello everyone,

Recently, my manager asked me to collect the standard about three-phase unbalance include voltage and current.
I already read some standards, like 1)IEEE std.1159, 2)IEC 61000-2-2, 3)EN 51600, 4)ANSI C84.1, 5)NEMA MG-1.
However, most of them only mentioned about the limit of voltage unbalance (like "not reach over 3%.").

Following are my questions:
Q1: Does any standard mention about the current unbalance?
Q2: I would like to know how to decide the limit values in each area or country? E.g. IEEE std.1159 set the limit value = 5%, but ANSI C84.1 set the limit value = 3%. Could someone please tell me the reason why they choose 5% / 3% as their standard.

Thank you, and please have a nice day
YKC

 

[waross]

The current and the current balance depend on both the voltage and on the nature of the load.


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

 

[dpc]

Voltage imbalance can create damaging unbalanced current in standard (non-AFD) induction motors. Current imbalance is generally created by voltage imbalance and unbalanced loads. I have seen cases where loss of a phase (the ultimate unbalanced voltage) destroyed nearly every motor in an older sawmill. Conventional electromechanical overload relays do not fully protect motors from this condition (negative sequence current).

 

[YKC]

So, in your opinions, could I say that "Design the standard for voltage unbalance is enough, since the current unbalance is created by voltage unbalance."?

My project focuses on the renewable energy sources, and we both know that RES connect to grid by power electronic equipment. It might result in rich unbalance, so we need to design the standard for both voltage unbalance and current unbalance. Then, the RES owner will be required to follow the standard and might need to install some devices to mitigate the unbalance.

 

[bacon4life]

The current imbalance is specific to the equipment subject to the current imbalance. As an example of acceptable extreme current imbalance, it is quite common to serve a single phase load from a three phase source. If various loads are connected to each phase, the overall balance can be good enough at upstream location. Some common reasons for addressing current balancing include:
1)Excessive neutral current. In some locations it is common for the neutral conductor to be smaller than the phase conductors.
2)Excessive voltage drop. For balanced currents there is just voltage drop on the phase conductor, whereas unbalanced currents also cause voltage drop in the neutral.
3)Excessive voltage imbalance.
4)Neutral overcurrent relay sensitivity. For example, the phase pickup might be 800 amps and the neutral pickup might be 240 amps. Imbalances approaching 240 could trip the relay. On a 15 kV system, I try to keep the imbalance to less than 100 amps so that the circuit can tolerate a 100T fuse blowing without tripping the substation breaker.
5)Needing to add new load, but one phase exceeds the conductor ratings.

 

[stevenal]

Perhaps you could share with us the nature of the loads. If single phase loads are involved, your RES might be required to provide highly unbalanced current while maintaining good voltage balance.

 

[Padlock01]

Hi YKC
Perhaps the attached file may help you.

 

[YKC]

Thanks each of you for the response, I am really grateful.
Actually, I got the day-off from Feb.8 to Feb.14, and also I have schedule with my family, so I might not give a response in couple days.
After getting back, I will read the responses and make a discussion with you.

Please have a nice day !!!

P.S. This is my first time to ask the question on this forum, I am really surprise and thankful to get the responses from each of you.

 

[Gr8blu]

YKC: Voltage and current unbalance are related - but not linearly proportional. For example, 3% voltage unbalance may easily result in 10% current unbalance.

The problem, from an electrical equipment perspective, is almost always a thermal consideration. The amount of heat generated is going to be (roughly) proportional to the square of the current. In practical terms: 3% voltage => 10% current => 21% additional heat. For equipment that is allowed an 80 C rise-above-ambient, that could mean an additional 16-17 C in observed temperature ... which might be enough to damage the insulation.

So what usually happens is this: the equipment on a system has to be able to operate under non-ideal conditions ... which in this case means some amount of voltage and/or current unbalance. The standards applicable to the equipment manufacture and testing give these limits. The power supplier has another set of limits - usually a bit more strict - so that the load never sees those "out of range" conditions.

Hence NEMA MG 1, etc. limiting voltage variation to +/- 5% while the utility unbalance level is around +/- 3%.

Lastly, note that there are typically THREE limits to a power supply: voltage unbalance (or variation due to harmonic content), current unbalance (or variation due to harmonic content), and frequency variation.

Converting energy to motion for more than half a century

 

[jghrist]

We have used a phase current imbalance criteria in medium voltage distribution planning studies to improve circuit efficiency. I don't believe that there is a code requirement in the USA - maybe for particular utilities. The criteria we use is:

The current magnitude at the beginning of a circuit in any phase should not differ from the average of the three phases by more than 15 percent. If the imbalance exceeds 15 percent, loads should be shifted to achieve an imbalance of approximately 10 percent.

 

[stevenal]

jghrist,

15% might be good goal for normal operation, but I think we need to consider the abnormal as well. First of all we have little control over what customer loads do. Second of all, nature tends to take out big chunks of single phase load from time to time. I'm not sure how an inverter based resource would "correct" this current imbalance without a huge voltage imbalance. Tripping the resource off-line for such a condition might not be acceptable either.

 

[jghrist]

stevenal,

Our 15% criterion is applied in planning studies for peak load under normal operation. I wouldn't use it as an operations criterion.

 

[bacon4life]

jghrist-Does your criteria have any sort of minimum load? I have lots of locations with large imbalances on relatively small loads (e.g. a 600 amp rated circuit with 40/50/60 amps is a 20% imbalance). In many of those locations I see the balance shift by 10 amps or more throughout the day. I think my own practices roughly align with a limit of 15% of the circuit rated ampacity rather than 15% of measured amps.

 

[bacon4life]

This is an example of a circuit where the imbalances changes quite a bit through the day. At lunchtime the yellow phase is the highest by about 20 A, whereas at dinner time the yellow is the lowest phase by about 25 amps.

 

[jghrist]

Our criterion is for peak feeder loading and does not address the magnitude of the peak. We have found lightly loaded feeders that exceed 15% imbalance but had no voltage or capacity problems. We made exceptions to the 15% requirement and did not recommend rebalancing the loads. The 15% is not a code requirement, so engineering judgement can be used.

Sometimes rebalancing can be easily accomplished, but sometimes it is not worth it. CYMDIST has an analysis that will show how phase balance can be improved through several circuit modifications.

 

[YKC]

stevenal,

Load state is still investigated, so I could not give you an answer recently. However, our research focus on RES in grid in the future situation, we could not expect the penetration of RES in the future (based on gov. policy). But, if the penetration is too high, we need to design the standard in advance. In fact, my current work is just only collecting the international standard which mentioned about the unbalance of voltage and current.

 

[YKC]

jghrist,

In your personal experience, you give a rule for the current unbalance is "not exceed 15%", right? Could you please tell the voltage level in your system? In my opinion, the tolerance seem to be too large for distribution system, or your feeder is not operated under high demand?

Also, I want to find some solutions to deal with the unbalance issue, considering the RES in the grid, I would like to figure out the relation between the location of RES and the high unbalance and solve the issue by re-dispatching the RES.

 

[jghrist]

Most of the systems we have studied are 12.5 kV. The unbalance is not the only criterion. The system also has to meet voltage, capacity, and reliability criteria.

We haven't considered renewables on the feeders with relation to unbalance; they haven't been a large part of feeders studied. If you're concerned with small roof top solar on individual buildings, then they could influence unbalance if a large number are on a single phase. These may need to be balanced. A special case with RES in operation could be handled in the same manner as normal phase balancing.

If you're concerned with large installations, these will be balanced 3Ø installations. They would not cause unbalance, but could make existing unbalance a larger percentage of the reduced feeder load. The concern would be more with these sources causing overvoltage on the feeder.

RES are normally not dispatchable.

 

[YKC]

That's true, the grid must meet various criteria to ensure it operate safely and normally. Thank you for giving a good vision of small scale RES, I did not consider about the roof top PV system previously.

My wording needs to be corrected, it is not "re-dispatching RES", it more likes "installing RES at specific location to reduce the unbalance". In my thought, since RES might cause the unbalance, maybe we can use RES to solve the problem? if we could know the unbalance state of RES caused in advance. The gov. encourages installing RES. If RES have not installed or still planned, maybe we can arrange the location for those RES.

 

[bacon4life]

I would be hesitant to use RES to try to fix an imbalance problem. Often RES have much different output profile than the loads, so adding excessive RES on a particular phase may create imbalance problems during periods of high RES/low load.

When new customer want to add load, there is a process in place for determining if they will be a single phase or a three phase customer. If they are a single phase load, we evaluate which phase to add them to. In my area, any site with more than 167 kVA of load requires 3 phase service from a 12.5 kV system. For RES, I assume the same process would apply with no need to invent something specific for RES. Unless the RES is going a in an area with only single phase lines available, I would expect anything 100 kW or larger to be balanced as well as possible across all three phases. It should be acceptable to have +/-1 inverter string on a phase if the total number of inverters is not multiple of 3. For example if there are a total of 4 inverter strings, there may be 2 strings on one phase and 1 string on the other two phases.