Measuring multiple feeders but using single power factor correction???
Measuring multiple feeders but using single power factor correction???
(OP)
Hi all, I am new to the forum has see some quite interesting topics being discussed. I have one that I would like some suggestions on please.
We are looking at a remote standalone plant with multiple diesel generators (8 or 10 depending on load) feeding into three 3.3kV switchboards, all tied together effectively creating one generation bus.
The 3.3kV distribution board mirrors this setup, multiple feeders leaving to various locations on the site, supplied from three distribution switchboards, also tied together to form a single distribution bus. All buses are generally tied, unless out for maintenance etc.
There are three sets of cables between the generation and distribution boards The switchgear has PTs and VTs, different ratios on the PTs due to different brands/ages of switchgear.
I am looking for a solution to measure the power factor of all three sets of cables, sum the VArs required and control a single capacitor bank.
I have attached a quick sketch of the situation. Any suggestions would be appreciated.
Thank you very much,
Edwin
We are looking at a remote standalone plant with multiple diesel generators (8 or 10 depending on load) feeding into three 3.3kV switchboards, all tied together effectively creating one generation bus.
The 3.3kV distribution board mirrors this setup, multiple feeders leaving to various locations on the site, supplied from three distribution switchboards, also tied together to form a single distribution bus. All buses are generally tied, unless out for maintenance etc.
There are three sets of cables between the generation and distribution boards The switchgear has PTs and VTs, different ratios on the PTs due to different brands/ages of switchgear.
I am looking for a solution to measure the power factor of all three sets of cables, sum the VArs required and control a single capacitor bank.
I have attached a quick sketch of the situation. Any suggestions would be appreciated.
Thank you very much,
Edwin






RE: Measuring multiple feeders but using single power factor correction???
The generators will happily supply the required VARs. The is why the KVA rating is 25% higher than the kW rating of most generators.
Unless you have some very special conditions you may be about to spend a lot of money needlessly.
If you believe that you have a condition that warrants capacitors, tell us and we can take it from there.
As long as the plant KVA demand does not exceed the generating KVAs online, you don't need capacitors or correction.
If a bad power factor is causing feeder overloading the issue may not be properly addressed by block capacitors and the capacitors may even make the cable loading worse.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Measuring multiple feeders but using single power factor correction???
In addition, I can see some problems with your setup should it become necessary to split the buses if the VAR controller sees a low PF condition and switches caps in on the one bus.
RE: Measuring multiple feeders but using single power factor correction???
Yes in island mode.
PF varies all the time, at times running as low as 0.6.
All generators are 400V units, running through 400v to 3.3kV transformers. 50Hz system.
Most generators are 2,000kVA, 1,600kW.
Load on generators around 1,500kW, but at PF of 0.6, this is 2,500kVA.
Generators, 400V cables & transformers running hot, 3.3kV system holding up well.
Idea is to correct PF to between 0.8 and 0.9 to reduce the current in the generators and transformers.
If PF is not corrected, will need to install extra generators and transformers to reduce load on existing equipment.
Thanks very much,
Edwin
RE: Measuring multiple feeders but using single power factor correction???
Buses are tied 100% of the time, unless for maintenance, and then some plant is also turned off, so there will be sufficient kVA capacity in the generators without PF correction. PF correction can be turned off when bus ties are opened.
Edwin
RE: Measuring multiple feeders but using single power factor correction???
RE: Measuring multiple feeders but using single power factor correction???
I would check a couple of things before adding the capacitor bank.
I would look at the source of the bad power factor. If it was one or two large loads I would consider correcting the PF at the problem loads.
I always like to try to get an overall picture rather than just correcting the symptoms.
I would attempt to analyze the effect of the proposed correction on the load bus tie cables. If the source of the inductive VARs is on the left hand bus your tie cable may become overloaded.
A couple of further comments:
Often a drop in power factor is not an added VAR demand but a reduced real load demand.
How hot? What are the actual temperatures relative to the rated temperature rises? I have seen numerous instances where equipment was "Running hot" when an investigation showed that the equipment was designed to run hot and was well below maximum rated temperature.
Forgive me again for doubting you, but you asked for advice and I have to ask. grin
How about connecting the CT secondaries in parallel. If this is not possible (using existing CT outputs) add interposing CTs and put the secondaries of them in parallel.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Measuring multiple feeders but using single power factor correction???
In addition, the CT secondary outputs can be switched and shorted by tie breaker auxiliary contacts to remove their contribution from their summing junction. That would make the VAR controller "see" only the loads connected to the buss with the capacitor banks.
If that's too complicated, aux contacts can be set up to disable the vAR controller when the system is placed into an abnormal (isolated) state.
RE: Measuring multiple feeders but using single power factor correction???
First, thanks for your time thinking about this, there is no need to apologise, I'm learning stuff, and you don't learn if you don't get asked the tricky questions...
Also I am not on site, which is in the Pacific Islands, I am in New Zealand, so can't run off and check things easily...
Source of bad PF, good point, it's from all over. Only a few instruments are working on the outgoing feeders, but the ones that are, show ranging from 0.7 to 0.93 as a snapshot.
Most of the electronic instruments have failed after a major power failure from the grid recently (complete with exploding capacitors and fire, luckily outside in the switch yard), which is one reason why they are not (no longer) grid connected, other reason is that the grid can't supply the site's total requirement.
The tricky question is that we don't actually know how big the PF problem actually is, hence my reasoning to sum the 3 supply cables and find out before buying and installing any PF capacitors.
Overall load is fairly constant, although some feeders experience 100kW+ motors starting and stopping regularly, but this is relative small as a percentage of all load.
Transformers and cables too hot to touch. 400V current at times higher than name plate rating, although within kW rating of generator according to the generator instruments, PF that I have observed varied down to 0.7, although on-site staff have observed 0.6. There is no recording device anywhere, so all info I have are just snapshots.
CTs are different ratios (therefore not suitable for parallel connection???) due to different ages and brands of switchboard, so not practical to connect in series(?) I understand. Unless using a summing CT, which would need to be wound specially for the different CT ratios, and also adds lag I understand.
Changing thinking slightly, can I install three individual PF controllers, controlling say 4 steps each of a 12 step capacitor bank? At this stage we would install three recording PF controllers, find out what each supply needs as correction (may be the same for all three???), and then size a 12 step capacitor bank to suit???
I like the idea of isolating the PF controller when bus tie are opened to ensure we don't add to many VArs on the remaining bus.
Your thoughts are much appreciated,
Edwin
RE: Measuring multiple feeders but using single power factor correction???
I am still a little concerned with the total VARs on one bus and possible effects on the bus tie cable. It may make the heating of the bus tie worse. Without an overall picture (which I understand may not be available) it is difficult to predict the result.
Can you consider rearranging the bus tie feeder so that it is solidly connected to the center bus and connected to the free breaker on the right hand bus? That would free up a breaker on the center bus if the connected load indicates that the capacitors may perform better connected to the center bus.
I know, in practice a lot of suggestions prove impractical but we keep asking anyway.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Measuring multiple feeders but using single power factor correction???
Good thoughts, the bus tie is 2,300A rated (left hand side) (at 3.3kV), that's about 13MVA, I believe it will cope as the whole site uses about this.
The bus tie (right hand side) is made up from 4 x 500mm sq copper cable (each phase) at say 600A each (de-rated), we also have 2,400A or so. I am confident that there should not be any current issued in the busses and ties.
The centre bus does not have a cable box on the end unfortunately, and is hard up against a wall, can't make any changes there...
Building on the 3 controller idea, I am thinking like Plan A on attached sketch. However each controller can have 6 connections to the capacitors, can we do Plan B, where some capacitors can be turned on my more than 1 controller?
If PF improves to better than say 0.9, No1 controller will try and turn one cap off, however if this cap is controlled by No2 controller, No1 controller will turn off another cap, won't it.
Just running through some different scenarios...
Thanks very much guys,
Edwin
RE: Measuring multiple feeders but using single power factor correction???
raises a big red flag, I was suspecting right from the beginning as I saw the word "remote site" in the OP and hence did not jump in earlier but was following the thread. The paragraphs following that are even more disconcerting.
I am not sure the facts are on hand or even OP knows where the 'locals' are reading the data from. Need to know the "source" of the bad pf first before you go resolving the pf, if at all needed.
The operation with closed ties is also not a sign of a properly managed system. It is possible that gen controls are messed up and they are just exchanging vars between them and someone is reading the pf at the gens. But again, I would say get the facts first hand or from a reliable source before "assuming" a problem and trying to come up with a imaginary solution.
Rafiq Bulsara
http://www.srengineersct.com
RE: Measuring multiple feeders but using single power factor correction???
As to the shortage of information, I agree but I also salute the OP for getting as much background and "what if" scenarios as possible to prepare himself for whatever he finds on site when he gets there.
I think that your tip about possible VAR exchange between the generators is the type of heads up that edwinnz needs. It may not be happening, but it should be checked.
Keep the good suggestions coming.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Measuring multiple feeders but using single power factor correction???
I have obtained a bit more info, again it's a snapshot of what was happening at time of observation.
Eight generators running: kVA rating, instrument kW, instrument PF
2,000kVA, 1,325kW, 0.77
1,000kVA, 751kW, 0.89
2,000kVA, 1,325kW, 0.87
2,000kVA, 1,098kW, 0.90
2,000kVA, 1,260kW, 0.85
2,000kVA, 1,380kW, 0.89
2,000kVA, 1,225kW, 0.90
3,582kVA, 1,190kW, 0.60
The 3,582kVA generator is a 1995 unit. This is one of the original generators on site. There is one more 3,582kVA (also 1995), one at 2,750kVA (old?) and one at 1,775kVA (made in 1963).
These 4 generators are generating at 3.3kV, and connect straight to the generation bus via circuit breakers.
The newer units are containerised Caterpillar PRIME diesel generator sets, generating at 400V and stepping up through a transformer each. There are a few (3 or 4) more of the 2,000kVA units, but not running at the time.
The 3.3kV generators are very fuel hungry, and with the cost of fuel these days, these old generators are being de-commissioned. The same kW can be produced by the new generators with a 20% reduction in fuel. The sites uses 60,000 litres of diesel per 24 hours!!!
The power factor on the containerised generators appears to be OK (see above figures) as long as one of the old generators is running.
However when the old generators are all turned off, the PF drops to the reported 0.7 and as low as 0.6 at times.
So somehow one or more of the old generators are either:
(a) producing enough VArs to satisfy the site's poor PF (as shown by the 0.60 PF on this generator), or
(b) somehow stabilising the system to prevent the other generators from swapping VArs???
Any thoughts much appreciated,
Edwin
RE: Measuring multiple feeders but using single power factor correction???
The reason I'm asking is that here we use a lot of Multilin relays throughout our distribution system and they can measure VARS. See if your protection relays has the ability and then you can follow the VAR flow.
RE: Measuring multiple feeders but using single power factor correction???
Good thoughts, most of the relays are Alstom. The older ones have needle indicators, with only half actually indicating something trustworthy. And most of the COS-phi meters are not moving at all.
The newer GEC-Alstom relays have digital readout, but there are only a few of these.
At some point they installed electronic instruments, ND (Northern Design) PM305-6 panel meters on most circuit breakers. However most of these have failed as described in an earlier post. I have been unable to find a replacement meter to fit the hole in the steel panels (panel meters are 96mm x 48mm).
All in all not enough working instruments and therefore usable data to see what is happening unfortunately...
Edwin
RE: Measuring multiple feeders but using single power factor correction???
That unit is kicking out 3379 KVARs. When it drops off line the other 7 units must make up the shortfall and their PF will drop accordingly.
An old school suggestion may be to back off the throttle so that the engine is just loaded enough to prevent "wet stacking" or "slobbering". That will save a lot of fuel. Then push up the excitation as much as you dare to supply most of the KVARs that your loads demand.
Has the client looked into a combined cycle plant? The fuel saving may be enough to cover the payments.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Measuring multiple feeders but using single power factor correction???
Get a couple of good generator control technicians and a good engineer on site. Review source of low pf, generator var controls, and the overall system in general, including grounding and winding pitches. Quiet a bit of testing and adjustment will also be required.
Rafiq Bulsara
http://www.srengineersct.com
RE: Measuring multiple feeders but using single power factor correction???
Have you confirmed that all the machines are running at a lagging power factor? One leading machine could be causing a lot of trouble. Rafiq already hinted at this, but in different words.
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: Measuring multiple feeders but using single power factor correction???
That is not a problem if you "wind" the interposing CTs.
Example:
A 100:5 CT has a ratio of 20 to one. If you pass the secondary loop once through the window before connecting it you will have a ratio of either 19:1 or 21:1 depending on which direction you wind. Wind the primary conductor two or three times through the window and you divide the ratio by two or three.
A real life instance:
The DCS programmers wanted to display the percentage motor current for each motor in the plant and didn't want to be bothered with a lot of programming. They wanted a current signal where a 5 Amp signal would indicate 150% of full load current.
We used 100:5 CTs for every motor from about 1 HP to 75 HP.
We were supplied with CT winding charts for about 10 amps and up. and it fell to me to come up with the winding turns needed for all motors less than about 10 amps. There were quite a few.
My work was later verified by primary injection testing.
Just make sure that all the CTs are on the same phase.
You may want to reconsider this approach after the other issues are sorted out.
Bill
--------------------
"Why not the best?"
Jimmy Carter