mitigating low voltage on 11kV feeder using 11kV/11kV autotap change or STATCOM 1

[emmjea001]

Hi, the 11kV feed to a factory with about 2 / 2.5 MVA demand is facing low voltage problems (9.6kV) from the utility side - lets forget about them fixing it, or reasons why, but look at how to fix it ourselves.

I have never used one, but assume you could put in a single 11kV/11kV auto tap changing TX upstream of the rest of our 11kV network?

or assume you could use a SVC or STATCOM etc?

The network drops relatively slowly, say sometimes it is at 10.8kV then it will take 5-10 mins to reduce down to 9.6kV.

How do we begin to assess which solution is preferred?

Any comments or experience or references much appreciated

Regards

 

[wayne440]

I readily admit that this is well outside my area of expertise. However, I suggest that a proper assessment of the situation must necessarily involve the utility. I would not assume that you can "fix it" from your side of the service. Perhaps some damage control with regard to your relationship with said utility might be worth while.

 

[itsmoked]

It is also out of my area, but in my area of expertise using various methods to continue to pull a constant load from a spongy source generally does not work. The source is sagging because it has problems supporting demand. By adding a mechanism to force your constant demand to be met you will necessarily force that rigid demand back onto the supply causing it to further sag in an endless spiral down to complete failure. Perhaps partnering with your power company you can spend money to help them meet your needs which will possibly give you some priority in keeping your supply in reasonable limits.

Think out of the box:
Help them finance a network upgrade?
Offer Co-Gen they can use for cheap fuel rates?
Shift your production to offload their peaks for a big price break?

Often you can make very fast relatively painless deals with your power supplier while they're having difficulties. Deals that pay your company dividends for years or decades and would be very hard or slow to get when they're in a "comfortable place". A power company with excess capacity doesn't want to talk to you about Co-Gen whereas one short of capacity will likely roll out a red carpet for it.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[MatthewDB]

Statcom does well when your factory is a large portion of the load on a long feeder. In that case a fairly reasonably sized statcom system can affect the voltage. In this case adding a voltage boosting transformer is prone to cause voltage collapse. (That happens when the incoming voltage drops more than the tap size when the transformer taps up.)

If there is significant other loads drawing the system down, statcom won't be able to raise voltage much. In this case a voltage regulating transformer works better.

 

[emmjea001]

Thanks for everyone's response.

I hear you loud and clear on working with the utility - and the idea that although a voltage regulating transformer may work, it simply passes the strain back to the utility - and it will be a parallel and slower moving process, but for now production must continue, and so there is a need to specify a voltage regulator.

@matthewDB I am interested to hear more about voltage collapse - is this when the source side cannot maintain the current demanded to produce the stabilized voltage and current draw on the load side, so the utility feeder effectively suffers a collapse - is this what you mean?

 

[dpc]

The step-type voltage regulators would be a much less expensive solution than the STATCOM and should solve the problem, provided that the utility voltage is not changing too rapidly and that your process can handle short-term voltage fluctuations.

The voltage regulators will take some time to adjust and will have an adjustable time delay to prevent them from constantly tapping. The newer regulators a quite fast, but you should assume delays of at least 15 seconds or so before the regulator will respond to a step change in utility voltage. Standard regulators have a range of plus/minus 10%. They are single-phase units, so you'll need three.

 

[itsmoked]

Consider adding your own generator? If your utility is that flaky it will only get flakier in time. Run your own generator as the voltage regulator.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[emmjea001]

Thanks @dpc - I believe you can also use 2 x single phase units in open delta to achieve reasonable regulation - I am not sure of the advantages of using 3 units vs 2 - I assume more capacity and hence greater range on limits?

@itsmoked - indeed - in fact this was the first thing we did, so we have a 4MVA syncronising diesel plant to back up whole site - problem is it costs a lot to run in terms of $/kWh vs grid costs - about 3 x the price, and it keeps syncronising in and taking over the load, which is correct, but it's expensive, so unless there is a failure we want to use the grid, even if it is flaky and invest in regulating it!

 

[bacon4life]

Since the voltage changes are relatively slow, seems like you could use mechanically switch capacitor banks instead of an SVC. As long as you address any harmonic resonance issues, seems like you could overcorrect to leading power factor.

I think @itsmoked was asking about running your generator in parallel to supply mostly reactive power rather than transferring all the load to the generator.

Voltage regulators in the USA typically are only +/- 10%, but it looks like you about +15%.

 

[dpc]

If you're worried about voltage regulation, using two regulators in open delta is a bad idea. Voltage regulation and phase balance will suffer. Do it right or don't bother.

 

[itsmoked]

Yes bacon, that was my thinking as that would not require the generator to support all the plant just the missing part. With the 4MVA generator you could study the result to see what the added energy needs are. Perhaps you only need a few 100KVA which a smaller prime generator could economically provide.

I'm not sure if this could be done without actually exporting power. If your plant is the actual load ultimately causing the sag you can probably do this without exporting power.

If ultimately you did export some power the power company might welcome that and pay for it since they must have other customers upset over the voltage sag.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[emmjea001]

@dpc - I am keen to find out more about the merits of the various options for connecting regulators - especially in terms of what you say about it's not a good idea using 2 for a 3 ph system? If you can point me to any reference would be much appreciated? Check out page 4 on this doc:

http://www.cooperindustries.com/con...urces/library/225_VoltageRegulators/77006.pdf

where they talk about using 2 regulators - as I said, I am keen to learn more about why this is not a good idea?

@itsmoked and @bacon4life (thanks for highlighting this) - ok so that is very interesting - we have 8 x 550kVA prime rated unit all sync'ing - so could definitely try this. I guess my question would be how do you practically take this forward?

[ul] 1. the network is sagging because of general high demand from all clients on this feeder, it is not due to great distances - just too many customers connected to a feeder not rated to deliver that much power.[/ul]

[ul]2. at no load from our side - grid will range around 10.5 - at 1MVA load grid down to 10 - at 1.5MVA grid down to 9.6 - at 2MVA down to 9.4 etc - so at first glance would it be fair to say that we need to start grid support at 1MVA, and actually co-generate about 1MVA if we have a 2MVA load?[/ul]

[ul]3. so to me it seem this is a capacity issue, of both kW and kvars, so I am not sure that reactive compensation would help that much - or do you think it would - The plant itself has PFC and runs at about .98 to .99 anyway?[/ul]

[ul]4. I am getting in a muddle now, as if the plant has vars being generated by the PFC to suit actual plant load, if we made generators send vars - that means they would be sent to other customers on the feeder - so would this help the sagging line - is this your point? [/ul]

Apologies in advance for the barrage of questions, but trying to get my head around this

 

[MatthewDB]

If you're worried about voltage regulation, using two regulators in open delta is a bad idea. Voltage regulation and phase balance will suffer. Do it right or don't bother.

Normally three independent regulators are connected in wye. Doing that is dependent on a 4-wire system being available. If the source is only 3-wire then 3 wye connected regulators are out because there isn't a stable neutral point.

Only open delta or a 3 phase regulator is viable. Three independent regulators in delta are a bad idea because they easily can setup excessive circulating currents.

 

[stevenal]

How's your phase balance? The open delta needs to step together, while three wye connected regulators
can be configured to operate independently and improve the phase balance.

 

[waross]

Several things come to mind.
Regarding three phase wye versus open delta:
Three phase wye connected regulators may be a good idea or a bad idea.
It depends.

Three line to line regulators in wye:
I see three single phase regulators most often on rural lines that may be unevenly loaded.
The regulators correct the phase to neutral voltages.
If voltages ahead of the regulators are unequal, as may be the case with uneven single phase loading, the regulators will introduce phase angle errors and uneven phase to phase voltages.
Motors don't like this. Motors don't care about phase to neutral voltages, they have no connection to the neutral.
The back EMF of the motors has equal voltages on each phase and equal phase angle errors. Connect this to an unbalanced supply and you get some understanding of why a small voltage unbalance results in large current draws. The motor is trying to correct the unbalance.
So before considering a three regulator solution, we must have some idea of the line to line voltage balance.
Hint: The utility may be able to help.
I worked in an area with a cooperative utility. The electronic meter recorded a wealth of data. Line to line voltages, line to neutral voltages, current per line, kW demand, KVAR demand. The records were taken at 15 minute intervals.
I supplied a floppy disk and a technician did a data dump from the revenue meter for the previous two months.
Wonderful data if it is available.
be wary of using three regulators in wye unless you are assured of balanced phase to phase voltages and phase angles.
The regulators should step together.

Two regulators connected line to line in open delta:
Not a bad idea.
The regulators should step together.

I regretfully disagree with two respected MVPs.
Using independent stepping on a wye bank to improve balance:
An unbalance implies that there may already be line to line unbalances and possible phase angle errors. Trying to balance the line to neutral voltages may make the line to line unbalance worse, and will introduce phase angle errors.

The issues with open delta stem not from the open delta as much as from the distribution system. An open delta on a solid supply has quite good performance.
The performance issues stem mostly, but not completely from voltage drop on the supply neutral.
When we think of open delta, we generally think of the classic rural application of two phase conductors and a neutral.

For regulation the regulators would be connected line to line rather than line to neutral. If the incoming phases are well balanced, the outgoing phases should be well balanced.
The downside of the open delta is that it is not compatible with a wye transformer primary. If your plant transformer has a delta primary, you should be good to go with an open delta regulator.
Again, regardless of type, wye or open delta, all regulators should step together.
Out of time just now. I'll post this now.
I will comment on possible uses of the generators in a later poast.
Thank you very much for the heads up and your confidence in me, Keith. Your semi private message means more to me than a lot of lps's.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

A couple of further notes on the open delta boost.
I have used this successfully many times to convert between 480 Volts and 600 Volts. That was a fixed adjustment rather than an adjustable adjustment but the open delta works well.
The open delta boost leaves you with uneven line to neutral voltages and so is unsuitable for any line to neutral loading.
Fault currents are not equal. Fault current on one phase is limited by the source impedance. Fault currents on the other two phases are further limited by the impedance of the regulators.

I should mention that whatever method you use it will be well to review your arc-flash calculations.

Generators:
Again, it depends. Any action other than supplying part of your own load will entail exporting either real power, reactive power or both.
Tip: If you switch your generator over to simple droop control you should be able to dial up the amount of load that you want the generator to take.
Do you know if the voltage drop is due to just line losses or is the voltage dropping at the head end of the line also?
Distribution lines are fairly reactive. Much of the voltage drop in the lines may be due to inductive reactance. This may be mitigated by exporting KVARs. From an electrical point of view, you don't need to burn any fuel in the diesel generator to export KVARs. Once up and running, the generator will export KVARs if the voltage is set high. The grid will motor the generator any spin the engine.
From the point of view of the diesel engine, you want to burn enough fuel and generate enough real power that the engine does not "Slobber" or lose unburned lube oil out the exhaust.
The generator will act as a synchronous condenser and mitigate the reactive voltage drop in the distribution lines. Very little fuel may be needed.
HOWEVER, any export of power, real or reactive, must have the consent and be on the terms of the utility.
If a large part of the voltage drop is resistive, you may need to export real power to offset the voltage drop. That will entail loading the generator and burning fuel. This is probably not economical.
Again, you should have the permission of the utility before running generators in parallel with the grid under any mode of operation.

Also, the generator may improve the voltage by producing KVARs with little fuel use, but any production of real power must be supported by fuel consumption.
Anecdote; A small country once supported the energy needs of a fairly large city with diesel power. This was back in the day when fuel was cheap. Then a transmission line was built from a new hydro facility and the diesels were shut down. As time went by, the city grew and the load increased. Eventually the line losses became greater than could be overcome by the transformer taps changers.
The line drop was mostly reactive and so the old generators were put back online. They produced very little real power and fuel costs were minimal. However the generators were run over-excited and produced enough KVARs to offset the reactive voltage drop in the line.
A couple of points to mention:
It was known that the voltage drop was mostly reactive. We don't know that in your case.
You will need the approval of the utility. In the example, the utility had no problem giving themselves permission to run the generators.

I tend to favour using three regulators connected in wye and arranged to step together.
The apparent advantage of considering the generators is that they are in place. However the utility may require additional protection and control.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Most interesting. Thanks Bill!

Keith Cress
kcress -

http://www.flaminsystems.com