Three Phase/Single Phase Voltage Fluctuations

I hope this is the right group for this post of mine - hoping someone will have some insight to this one.

Our house power supply is three phase (i.e. 230v/400v). Since I live in the southern hemisphere, it is our winter at the moment, so high load demand at peak times. The voltage on red phase dropped several times between 6pm and 7pm (also once or twice in the morning till around 10:30am), going as low as 200v, while the other two remain around 215v to 225v. With this problem, I contacted City Power, and eventually they stepped up the transformer tappings to 240v (420 roughly for 3 phase).

Things have been much better, but I am still not entirely happy. I have noticed that adding a load of 3KW (geyser) drops a phase to neutral voltage by 7v, while the next phase in the sequence picks up 2v or 3v at the same time. This to me sounds like a problem with the neutral return path somewhere. This is a bit worrying when I start getting readings of

247v phase to neutral! Electrical standards allow for 207v to 253v, so it is still within the limits. The transformer is 250m away from the overhead pole where our connection is made.

250 m that is a hoof..... [a very long distance] Where I live transformers are usually withing 250 feet, or ~77 meters.

I doubt that your going to get the utility to do much as long as they are withing tolerances.

7 volts at the voltages your dealing with is what ~ 3%.

I do not know where you are. Where I live the neutral is established at the service, i.e. your property. Have you checked your grounding lately?

Overhead cable size is a mix of copper=? and aluminium=185mm² and our cable feed is around 60m to the meter board using copper 16mm² cable. No stray voltages are present in the neutral or earth conductors.

I am not actually an electrician, but have picked up my knowledge over many years. What I need to ask is - does this sound plausible, or are these voltage drops indicative of a fault somewhere?

Thanks so much for the help

P.S. The transformer is (according to City Power) a 500kVA transformer.

I'm assuming that this is 3 phase 4 wire, i.e. a wye connected system (since you refer to a neutral farther on).

This could be due to imbalanced loads.

It could be loads of some of your neighbors (if any). Based on the secondary distances and transformer sizing, I'm guessing that you share the secondary system and transformer with a number of other services.

Did they send someone out with a clamp-on ammeter to check the transformer imbalance?

What's a geyser? A single phase load, I assume.

The voltage drop and rises you are seeing are characteristic of a single phase load on a three phase system. The load causes a drop on both the line and the neutral. The neutral's voltage drop is a voltage vector which will appear to be a rise with respect to the two unloaded phases.

I don't have my metric wire tables handy. At any rate, the '?' copper size makes any calculations only a guess anyway.

At first glance, it sounds OK, although comparing your voltage drop measurements due to the 3 kW load to a calculation should indicate whether these levels are out of line.

Hi SQLit and Paul Hovnaian P.E.

Thanks to both of you for the speedy answers. I hope you don't mind me answering both of your questions in one reply.

The distance of 250m I hate to say is not to the end of the transformer run - it still goes on a bit. We are not the only household feeding off this transformer - it feeds more than eleven houses - not sure how many down a side street. All were given 3 phase connections since they needed the higher voltage for boreholes long ago before there was a piped water supply (boreholes now for garden irrigation).

Where do I live - in good old Johannesburg, South Africa (normally sunny, but not this morning ;-)

A geyser I think is called a hot water cylinder in the States (a tank of water heated by an electric element - for the household hot water supply). It is a single phase load of 3KW, so around 13 amp.

We have a multiple earth neutral system in place - an earth connection is made at the service connection, and the neutral and earth of the house are connected together at this point. There are also several connections to the metal water pipes and an additional earth rod near the borehole. The neutral from outside to the service box is functioning, and has a parallel earth helping to carry current. The clamp of the neutral to the overhead cable outside has been replaced last month by City Power when I found out that there was no current flowing in the neutral, but all flowing back through the parallel earth (checked with clamp ammeter). All seems in order.

Load imbalance - it is definitely the neighbours causing this imbalance. I have been adjusting the balancing between phases - so far we only run pretty much at around 25 to 30 Amp per phase maximum (obviously we may go slightly over at times). The service breaker per phase is rated 60A (5KA). Unfortunately, City Power has not checked to see how unbalanced the load is from the transformer.

By the way, I have been looking for a formula to calculate the voltage drop that I should be expecting - do either of you have one?

Thanks again for your help - it is much appreciated. Regards,

Pierre

I would have never guessed the geyser meaning. Knew it drew power but would have never guess a water heater

This one is easy to work with. You may have to work the calculation in section due to the wire sizes and types.

From what you have said I fail to understand the concern. If your not the only load on the transformer then your trying to hit a moving target. Load balancing where I live is usually tried when the load is more than 20% difference. Even then it is hard to lower imbalanced phases much due to the types and times of loads. Your "geyser" may take up phases A and B the next single phase load might want to be on B and C. Problem lies is that they need to operate at the same time to maintain balance. A geyser will heat then shut off, then heat some more. Since your neighbors are working off the same transformer the solution changes second to second.

Is there a huge penalty assessed for unbalanced loads? Talk to you utility and see what exactly the meter does. I once assessed a hospital for imbalances. Their power bill was about a million dollars a month. I was lead around for 2 days and shown all sorts of "ideas". I finally got to the power bill and the imbalance was costing them less than $1000.00 a month. Not really worth the effort and money it was going to cost to control the loads.

I ran a quick calculation from your original post of the drop caused by a 3 Kw single phase load. The calculation for balanced 3 phase balanced loads are similar, but there is no current (and no drop) on the neutral.

185mm² Aluminum is close to our 350 kcmil Aluminum, which has R = 0.20 ohms/Km 16mm² Copper is close to our #6 Cu, which has R = 1.7 ohms/Km

For a 250 m run of 185mm² Al plus 60 m of 16mm² Cu, R = (0.25)*(0.20) + (0.06)*(1.7) = 0.152 ohms per conductor (there are two for single phase, the phase plus the neutral). A 3 Kw, 230 volt load draws 13 amps.

Vdrop = 2 * (0.152 ohms) * 13 A = 3.95 volts

A guestimate of the transformer bank resistance per phase is about 5 to

10 milliohms, so a 3 Kw (13 A) load adds negligible additional drop.

This is close to what you measured. It wasn't clear if your voltages were measured at the service entrance or at the water heater, which will add the drop of the branch circuit to the above figures.

It appears that your 7 volt drop figure may not be too far off from calculations.

The reading I took where there was a drop of 7v was at the DB that feeds the geyser - I have just checked at the service connection, and the readings went from 233v to 227v, so not much of a difference.

As to why I am so concerned about this, its not so much the drop in volts (as long as we remain within the limits that appliances can work within), but rather the fact that there is an increase in the adjacent phase voltage. Measurements (also taken now) indicate that there is an increase in the blue phase from 236v to 239v when the geyser is once again energised, the geyser being on white phase. (Phase colouring in South Africa is the same as Europe = Red, White (or yellow), Blue). Is this quite normal? I ask this because we had a situation a couple of years ago where the feeder neutral was faulty, and we ended up with roughly 280v on red and blue phase, with roughly 150v on white. I found out afterwards that we were only connecting to 4 other houses via the neutral at that time. Needless to say, all that was destroyed by the overvoltage was an old sprinkler controller, but it could have been far worse.

Thanks for all the advice and the links/calculations. I really appreciate you guys giving your time to help out!

Wouldn't a great number of customers on the line provide more diversity, hence a lesser probability of a load imbalance?

I once had a condo (in the US) in which the building was served by 3 phase delta-wye 120/208 connection. Each of the 12 dwelling units on eachfloor of this 3 story building connected to 2 of the possible 3 phase connections (single phase service). Three phase service was provided for the elevators, pumps, and blower fans in the garage.

As I recall there was seldom a problem with load imbalance. The 208 V (instead of 240 V) that was supplied to the dryers made the clothes dry slowly, however. But that's a different problem.

In the OP's example, wouldn't the electrical authorities intentionally spead the Geyser load equally among all three phases for every customer served by the section?

Beachcomber

| As to why I am so concerned about this, its not so much the drop in | volts (as long as we remain within the limits that appliances can work | within), but rather the fact that there is an increase in the adjacent | phase voltage. Measurements (also taken now) indicate that there is an | increase in the blue phase from 236v to 239v when the geyser is once | again energised, the geyser being on white phase. (Phase colouring in | South Africa is the same as Europe = Red, White (or yellow), Blue). | Is this quite normal? I ask this because we had a situation a couple | of years ago where the feeder neutral was faulty, and we ended up with | roughly 280v on red and blue phase, with roughly 150v on white. | I found out afterwards that we were only connecting to 4 other houses | via the neutral at that time. | Needless to say, all that was destroyed by the overvoltage was an old | sprinkler controller, but it could have been far worse.

In the US the problems of multiwire circuits are more common and well known because we have them even on single phase (2 hots 180 degrees to each side of a grounded neutral). The same problems also exist with three phase, but the calculations are a little more involved.

When you have an excessive load on the red phase, it "pulls" the neutral point towards the voltage point of the red phase (which pulling the red phase toward neutral). The degree of this pull is most extreme nearest the heavy load and decreases as you follow the power feed wires back to the source. The impedance of the wire and transformer(s) are part of why this happens. When current flows across an impedance, there is a voltage drop. This is simplest to figure when it is a pure resistance. The standard ohms law equation applies. Suppose for example the total resistance of the wiring is 0.1 ohms and you are drawing 13 amps through that wiring. The voltage drop will be 1.3 volts regardless of what the supply voltage is. Half of that will be on the hot phase and half will be on the neutral (assuming each has 0.05 ohms resistances). There will also be 16.9 watts of power heating up those wires, distributed across the length of them. That voltage on the neutral has a phase angle to it which is a vector in the direction of the phase having the extra load. That vector drawn on a polar plot shows a decreased distance to the end of the vector for the red phase, illustrating the voltage decrease seen in the red phase. That change in neutral vector position also shows an increase in voltage relative to the other 2 phases.

So, given the arbitrary parameters I picked, you'd have a vector change of 0.65 volts in the red phase back to the neutral, and a vector change of 0.65 volts in the neutral out to the red phase. That would raise the voltage between blue and neutral by 0.325687886 volts. Same for the white/yellow phase.

This gets more complicated when the loads are reactive and have current phase angles different than the voltage phase angles, such as you can see with motors. It gets even more complicated with non-linear loads that introduce harmonics (due to the 120 degree phase angle, not all the harmonics will cancel out even if all the phases are in balance). The vector diagrams in these cases would involve constantly changing vectors that spin and pulse.

Of course all the mathematical precision here is pointless with lots of voltage changes taking place with varying loads happening on the various phases. But the real point here is that a voltage RISE is very much to be expected on the opposing phase(s) as a result of the load on other phases. Keeping the phases in balance keeps the voltage drop vectors on the neutral close to zero (they add up as vectors). When neutral current is zero, voltage drop across the run of the neutral is zero.

There are things you can do about this. Running the water heater at

400 volts (if you can get a 400 volt element and the controls are rated for 400 volts) would help by spreading the load across 2 phases instead of 1, and avoiding the neutral entirely. In the US, we run our heavy appliances like a water heater mostly connected between the 2 hot phases (for a total of 240 volts). Although we have more issues with that due to our lower overall voltage and higher current (leading to more voltage drop that hits that lower voltage harder) the same idea could help in your situation.

The higher the voltage you can bring the power in with, the better the situation. You'd see noticeably less if you brought power in at 400/690 volts and transformed it down to 230/400 right there. But you'd also see noticeably less money in your bank account.

Some voltage rise is OK on the unloaded phases when a single phase load is applied.

A broken neutral will result in excessive voltage swings on the neutral. A really rough rule of thumb is that if the voltage rise on an unloaded phase is around half of the voltage drop on the loaded phase, you are OK. This indicates that the phase and neutral voltage drops due to the load are roughly equal, which is normal and due to a drop on the entire circuit. If the voltage rise on an unloaded phase is nearly equal to the drop on the loaded phase, it indicates that most of the drop is due to a high resistance on the neutral connection somewhere. Its a little trickier to visualize on a three phase system than on our simple single phase stuff.