Load Imbalance
Load Imbalance
(OP)
In a project I am working on, I have a 208Y/120V unit substation supplying a load that is essentially all 120V, 20A, receptacle circuits - many panels, each with 30 or so 20/1 breakers. The design loading is reasonably well balanced from phase to phase, the bulk of the load being 180VA receptacles.
Does anybody have any empirical data as to how much imbalance I might see in actual operation? The worst possible case would be, with the building fully loaded, somebody went through and turned off and the phase 'B' and phase 'C' breakers, giving 100% load on 'A' and nothing on the other two. But that wouldn't be very likely. On the other hand, there could be, by happenstance mostly, a noticeable unbalance. How unbalanced could one reasonably expect to see?
Thanks.
Does anybody have any empirical data as to how much imbalance I might see in actual operation? The worst possible case would be, with the building fully loaded, somebody went through and turned off and the phase 'B' and phase 'C' breakers, giving 100% load on 'A' and nothing on the other two. But that wouldn't be very likely. On the other hand, there could be, by happenstance mostly, a noticeable unbalance. How unbalanced could one reasonably expect to see?
Thanks.






RE: Load Imbalance
RE: Load Imbalance
RE: Load Imbalance
The description “load being 180VA receptacles” is fairly meaningless. Would you elaborate on what's plugged into the receptacles?
RE: Load Imbalance
The concern is not neutral loading, but rather negative and zero sequence currents in the three phase conductors on the primary of the 12,470V circuit feeding the grounded wye-grounded wye transformer of the unit substation.
RE: Load Imbalance
You may wish to consider using delta primary to stop propogation of triplen harmonics.
What specifically is the concern for negative and zero sequence currents with the transformer? Transformers can operate with any amount of unbalance.
RE: Load Imbalance
It is better to have the 12.47kV ungrounded (delta) to avoid the zero sequence current propagation and elevation of grounds during lightning on 12.47kV side.
RE: Load Imbalance
jbartos, 12.47kV line is underground, and a decent ground grid is being installed.
The transformers are power cast with fan cooling to 150% of nameplate base rating. I have found it very difficult to do fully selective coordination on delta - grounded wye transformers and make full use of the 150% of full load. With a delta - wye (grounded or not) the damage curve is at 58% of the current of a wye - wye (grounded or not at either side). The local utility is not much worried about the propagation of triplen harmonics as all of their service transformers are grounded wye - grounded wye anyhow.
RE: Load Imbalance
One description of the load may be ‘institutional’. With a grounded·wye grounded·wye transformer, secondary-winding [switchmode} triplen currents would be reflected into the primary neutral. I am not sure what the concern is as far as transformer load or thermal losses, with or without neutral current.
It could be that IEEE Std C57.96-1999 Guide for Loading Dry-Type Distribution and Power Transformers contains applicable information. [There is a table of contents for the 1989 edition at standards.ieee.org/reading/ieee/std_public/description/dtransformers/C57.96-1989_desc.html ]
RE: Load Imbalance
From your responses, I gather that you intend to use zero- and negative-sequence relaying to gain sensitivity for Ø-Ø and Ø-grd faults and need to know the expected phase imbalance to determine how sensitive you can make the settings. For this purpose, the third-harmonic question is probably irrelevant because most microprocessor relays respond only to the fundamental frequency. You might want to confirm this with the relay manufacturer that you are using.
You could use the experiences of ahleman and dpc (20-30% imbalance). You're situation may have more imbalance because there may be times when a teaching lab or two on one phase are not in use while other spaces are fully used. You could monitor the load on a similar facility to get a better estimate.
RE: Load Imbalance
I am looking to implement a fast bus trip scheme, sort of a poor man's bus differential, where outgoing breakers (which can include the mains and the ties depending on operational configuration, loads, and local generation) which see a fault block the fast trip of the next upstream breaker. The goal is to detect and clear any bus faults within 6-7 cycles while maintaining full selectivity.
Obviously any 3-phase fault will have to greater than maximum allowable load current as there is no way to distinguish a 3-phase fault from load. But any fault that does not involve all three phases can be distinguished from load through the zero- and negative-sequence currents.
If the maximum imbalance is in the range of 20-30%, and I've heard 30% elsewhere also, then I'll add some margin and set the fault detectors at 50-60% of the transformer full load rating.
RE: Load Imbalance
Any greek letter can be created by entering &[ig-nore]letter;[/ig-nore] (remove the "-" in ignore). Capitalize the written out letter to get a Greek capital letter. Omega for Ω and omega for ω for instance.
RE: Load Imbalance
The SEL-501-2 relays are protecting feeders to? 208/120 panels? Will there be intermediate distribution panels?
It is very difficult to coordinate the high side protection of a transformer. I don't like to do it but sometimes you have to use electronic fuses. There not cheap and you locked in to one supplier ( there may be more now ).
The 20- 30% imbalance is hard to design for. Some suggestions I have are.
1.) Spread the circuitry out, Put receptacles from at least 2 phases in each room or area. The cheap way to so it is to hang 10 receptacles on each circuit and locate then in one area. People get by with that for a while in spec built offices but i don't think it'll work in an engineering lab. Most of the office buildings designed like that get reworked sooner or later.
2.) Leave enough spares in each panel to relocate loads at a later date. Monitoring the operatiog conditions is the only real way to tell whats going on. The way things are used probably won't turn out to be what you or anyone else planned.
3.) To find out what is goin on for step 2 you have to monitor usage over time. Tha can mean temporary recording devices or better yet if the projcet has the money a smart metering system. Square D has power logic which is good but all the major equipment suppliers have systems that are just as good.
RE: Load Imbalance
But that's all in the theoritical world of design, where things are a balanced as possible. What I am interested in is how differently the real world behaves.
RE: Load Imbalance
Load imbalance will show up on negative sequence sensors, but zero sequence sensing should include neutral CT's to cancel the effect of normal neutral current.
SEL-501's are a little pricey. Have you looked at the zone selective interlocking capabilities in newer electronic trip units for low voltage breakers? I think ZSI can accomplish at least part of what you are wanting.
RE: Load Imbalance
RE: Load Imbalance
Even smaller projects, e.g. residential houses with 208V/120V 1Ø 3W, can have the load balanced, if 1Ø receptacles are balanced and larger loads, e.g. air conditioners, heaters are 208V instead of single phase 120V. Obviously, if there are all receptacles on one circuit, i.e. on one phase, in the house, the panel may become very unbalanced. However, this is the electrical contractor responsibility, if there are no electrical drawings.
Similar reasoning holds true for large projects. It may be too late to try to balanced loads, if the power distribution is improperly designed and installed by the electrical contractor. It may require an expensive re-work in some instances, (i.e. "pay now or pay later," which many of us very well know).
In old times, when there were fuses only, the rest of electrical power distribution was functional when one or two fuses were blown; approximately 33% unbalance for one fuse blown and 67% unbalance for two fuses blown.
RE: Load Imbalance
A lot of special characters can be created by holding the Alt key down and entering 3- or 4-digit numbers (128-255 and 0128-0255) with the numeric keypad. Alt 0216 gives Ø.
///The Numeric Lock light on the keyboard should be on\\\
Any greek letter can be created by entering &[ig-nore]letter;[/ig-nore] (remove the "-" in ignore). Capitalize the written out letter to get a Greek capital letter. Omega for O and omega for ? for instance.
///Use a dictionary for Greek letter spelling in the Greek alphabet.\\\
RE: Load Imbalance
What I've been looking for is the difference between that balanced, calculated, design loading and what might happen, worst case, when the users of the building have some portion of the potential load on and the rest off. So far it looks like an unbalance of 30% is the highest likely to be seen, so I'll set the fault detectors above 50% to leave some margin.
RE: Load Imbalance
RE: Load Imbalance
Will an unbalanced load try ot shift the secondary neutral point to somewhere other than the center of the secondary voltage triangle? IF I remember correctly the problem was current flowing in the link between the two grounds.
A 1000 kVA transormer is going to have a super fault current. I think industry experience will verify ( I don't have any statistics at hand to bear this out however.) that faults in the gear are man caused. That is someone leaves a tool in the gear or something like that. In this case I would opt for fully insulated bus. Some of that red plastic on the bus may save you a lot of blue smoke some time in the future.
RE: Load Imbalance
Reference:
Electrical Transmission and Distribution Reference Book by Central Station Engineers of the Westinghouse Electric Corporation, East Pittsburgh, Pennsylvania, 1964.
Table 15 Influence of transformer connections on third-harmonic voltages and currents on page 129