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11kv voltage collapse during peak load at
5

11kv voltage collapse during peak load at

11kv voltage collapse during peak load at

(OP)
Hi all,

we have 6 gas engines (0.4kv,1950kw each) connected to 11kv bus throw (0.4kv/11kv 3MVA transformers). this power station feeds residential loads in the countryside (air conditioners, water pumps, lighting, and workshops) throw 20km, 175mm2 OHL .

I noticed the following during the summer period when the station breaker is closed, the loads start to rise gradually within an hour until they reach about 380 amperes in the station breaker, then a sudden jump in the current occurs, as a result of this breaker trip, especially at peak times (at noon). And when we switch on this barker again it trip within 1 hour. But during the night or dawn period, this problem does not occur and the loads are about 320 amps in this braker.

If we observe this problem from the load side at the end of the line at the branching distributing lines, we notice that the voltage is initially about 10.6 kv when the distribution lines are disconnected, then when all the lines are connected and the load begins to rise , we notice that the voltage drops gradually from 10.6 to 8 kv within an hour and then a jump occurs in the current and a severe voltage breakdown then the main station circuit breaker trip.

The peak load is about 7MW

please see the attached SLD

RE: 11kv voltage collapse during peak load at

What do the relay event records show?

I’ll see your silver lining and raise you two black clouds. - Protection Operations

RE: 11kv voltage collapse during peak load at

It happens during noon time in summer, you said. This suggests that the large air conditioning load is being added in a short time which is causing voltage dip and consequent higher draw of current by the AC equipment.
One way you can handle this is by providing Automatic power factor correction capacitors at load end to minimise VAr drawal / voltage drop. Further, if switching on of ACs can be staggered, that will be great help by reducing the starting inrush.

RE: 11kv voltage collapse during peak load at

8 kV on an 11 kV line is 73%.
You may have large motors or a lot of air conditioners stalling.
As each unit stalls, the locked rotor current will drive the voltage lower resulting in a cascade effect.
Modern controls tend to restart A/Cs after a random delay. Some units may have to be restarted manually.
Have you considered installing a bank of voltage regulators to improve the downstream voltage?

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

RE: 11kv voltage collapse during peak load at

(OP)
the relay at Power station shows spike in current and voltage drop slightly.

RE: 11kv voltage collapse during peak load at

(OP)
bill

the voltage is (8 kV on an 11 kV line is 73%) if we install capistor bank do you think it will help even the voltage drop is very big

RE: 11kv voltage collapse during peak load at

I am amazed you only seem to be concerned about the breaker tripping and not the 27% voltage drop. The motors in customer equipment can be damaged by supplying such low voltage. In my region, we are required to have no more than a 5% drop, and customers sometimes complain about a 3-4% voltage drop.

Reactive power does not travel very far very well, so trying to ship ship several MVar a distance of 20 km on an 11 kV line results in terrible voltage drop. You may need multiple stages of capacitor bank so that the flows on the 20 km line stay somewhat close to unity power factor. Having a single very large capacitor bank could result in excessively high voltage under light load conditions.

RE: 11kv voltage collapse during peak load at

Over-voltage at light loading is always a concern with bulk (un-switched) capacitor banks.

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

RE: 11kv voltage collapse during peak load at

Based on your SLD, the installed capacity is 18MVA @ 11kV. That means your 11kV station breaker
should be able to handle comfortably upto 1000Amps @ 11kV. If you say that it trips at 380A, then in my
opinion there is some serious error in proetction relay settings.
Also your SLD shows 20km/ 175sq.mm (is it 185sqmm or 175 sqmm?) feeder. I am assuming it
is a cable feeder. The continous current carrying capacity of a 11kV/ 1Cx175sq.mm cable is 550A only.
It looks like the cable is not sized enough for the steady state voltage drop for 18MVA.
Therefore, could you please provide,
1)The make/ model/ AS LEFT settings of 50/51 protection relay associated with the 11kV station breaker?
2)The correct size/ type of the 11kV feeder?

RE: 11kv voltage collapse during peak load at

(OP)
Kiribanda

its is not cable its OHL 175mmsq. and the protection working fine its within set point i already carry out secondarily injection on the relays

RE: 11kv voltage collapse during peak load at

If it is 175sqmm then the conductor should be LYNX. It is having maximum ampacity of 400A.
How this can be used to trasmit 18MVA @ 11kV without a considerable voltage drop? Protection
settings has nothing to do about the steady state voltage drop.

RE: 11kv voltage collapse during peak load at

A couple of corrections:
The station has six times 1950 kW sets. At 0.8 PF that is a capacity of 14.625 MVA
The load is given as 7 MW at .85 PF = 8.24 MVA
The breaker holds in at 380 Amps. The breaker trips at some value above that.


newelecteng, what is the trip set at on the breaker?
What current are you seeing at the time of tripping?

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

RE: 11kv voltage collapse during peak load at

(OP)
hi bill

The maximum capacity of these ensigns is 8.5 MW due to derating. the maximum load during peak is around 6.9mw. the line is 20km, 175mm2 OHL, aluminum

This problem occurs when we reach maximum load at mid day or afternoon when the amps reach 370 to 380. but this proplem disappear when the amps below 350.

If we see this problem from Power station you will see big jump in amps it reach 480 amp and the barker trip on over current (the over current pickup is 450 amps).

but if you see this problem from load side at the end of the line you will notice that the voltage is initially about 10.6 kv when the distribution lines are disconnected, then when all the lines are connected and the load begins to rise , we notice that the voltage drops gradually from 10.6 to 8 kv within an hour and then a jump occurs in the current and a severe voltage breakdown then the main station circuit breaker trip.

RE: 11kv voltage collapse during peak load at

If you have no local loads or if you can make some arrangement to handle overvoltages on local loads, you can try compensating for line voltage drop.
Use a CT on each generator an outgoing line and drop the current across a resistor. Tap a voltage from the resistor and use it to bias your sense inputs to the AVRs.
As the load increases, the bias will increase the generator voltages to compensate for line voltage drop.
This may help if your problem is primarily caused by low voltage.
If it is just an overloading issue you may have to shed loads.

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

RE: 11kv voltage collapse during peak load at

(OP)
bill

the generators have max voltage output 0.42kv

is it possible for this line 175mm2,aluminum,11kv,20km to handle 6.8MW on this voltage level (what is the maximum amps)

RE: 11kv voltage collapse during peak load at

Do your transformers have taps?

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

RE: 11kv voltage collapse during peak load at

(OP)
bill

the transformers already on the max tab 12.1kv

is it possible for this line 175mm2,aluminum,11kv,20km to handle 6.8MW on this voltage level (what is the maximum amps)

RE: 11kv voltage collapse during peak load at

Do know the charging current of the line? Consider installing enough capacity to balance the charging current as a first step.
You can probably double that amount of capacity without serious voltage rise issues.
Do your load end distribution transformers have taps?

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

RE: 11kv voltage collapse during peak load at

(OP)
I dont know the harging current of the line

what you mean by "installing enough capacity to balance the charging current as a first step"

our distribution transformers is 11/0.4kv Dyn11 and its already on its min tap.

RE: 11kv voltage collapse during peak load at

Using a search key use of capacitors on distribution lines will bring up some information useful to your design question. Do you know what the power factor - or VAR load is when you trip?

RE: 11kv voltage collapse during peak load at

(OP)
hi FacEngrPE

the pf before trip reach 0.8

RE: 11kv voltage collapse during peak load at

Note - I am not a power distribution engineer, I sit next to one.
I just realized the PF was discussed further up in the thread.
The current situation you are describing contains these factors
Loads - current draw increases dramatically as voltage drops - typical of motor loads.
Power Line - Power lines do not transfer power efficiently at low power factors.
This slideshare goes into the subject in a bit more depth.

This slide share on voltage regulation is focused on tap changers, but it has a good discussion on where to place them which also applies to capacitors.

What Bill is proposing is that a good place to start is compensating for the reactive demand you know about by adding some capacitors out in your system. A useful place to start is just compensating for the power lines reactive demand (possibly including transformer reactive demand).

Fred





RE: 11kv voltage collapse during peak load at

You show the loads as 0.85 PF and the plant as 0.8 PF.
The difference represents the inductive reactance between the points that the two measurements were taken.
As for charging current, I misspoke.
What I meant to say was the no-load current. That is, the current needed to energize all of the transformers in the system.
One limit on the capacity of a transmission line is the ability of the On-LOad-Tap-Changers to compensate for line voltage drop.
It sounds as if your line is badly overloaded.
A long OHL will have significant inductive reactance. This will cause a voltage drop that may be mitigated by capacitors.
The magnetizing current of the transformers will add to the reactive current and the resulting line voltage drop.
A solution from long ago, when unswitched capacitor banks were used for power factor correction to avoid over voltage at light load was to connect capacitors to large loads, usually large motors or in some cases such as a lumber dry kiln, to groups of smaller motors so that the capacitors were switched by the same contactor which switched the motors.
This gave us switched capacitors without the cost or labour of adding capacitor switching.
This was seat of the pants or red neck engineering. Using a knowledge of the basics to solve a problem when neither the information nor the instruments to gather the information may have been available.
Subject always to actual conditions and judgement, we would often use 200% correction. That is, if a motor demanded 5 KVARs to correct the PF to unity we would connect 10 KVARs. This worked well for an industrial plant with a combination of a lot of smaller motors and a few large motors.
I am sure that some readers will be able to suggest examples where this solution would not be successful. That is where judgement came into play. We would look at the load profile of a plant and try to anticipate these situations and search for other solutions.
Back to your case.
As a first step in an economical improvement:
Determine the no-load or magnetizing current of each distribution transformer and from that calculate the VAR demand of the transformer.
Connect twice that amount of capacitors on the secondary, so that when a transformer is off-line the associated capacitors are also off line. This will go a long way towards avoiding over-voltages.
But this may not be enough to solve your problem.
It appears that your system is long overdue for a major upgrade.
I am not confident that we have identified the root cause of your problem.
The first rule of trouble shooting is to accurately determine the root cause of the problem. The second and third rules are: See the first rule.
We have two possible issues.
1. Is the problem motors stalling due to low voltage.
2. Is the problem too much load for the system capacity.
A solution to the first problem may be capacitor banks. A better but more expensive solution may be the installation of a bank of voltage regulators at the load end of the line.
A solution to the second issue may be load shedding or it may be measures to increase the output of the generating plant.

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

RE: 11kv voltage collapse during peak load at

FacEngrPE; I didn't see your post until after I had posted my reply. Thank you for the support.

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

RE: 11kv voltage collapse during peak load at

Bill your explanation is better than mine, we must have been typing at the same time.

Power system stability is a subject that has many contributors. Here we have a small system, which probably has limited resources and budget (a guess). As a result it is likely that a lower cost solution that provides a somewhat acceptable result is better than a more expensive technically perfect solution.

If this was my problem to solve I would want more information about where in the system the loads are. That might help answer questions about the best places to inject reactive power.

RE: 11kv voltage collapse during peak load at

Waross,

If you are seeing dips in voltage of 20%+, I don't think that voltage regulators are going to save you if you are that close to slipping. The voltage problem get worse as the voltage dips because the voltage dip causes more vars to be consumed, which causes it to dip further. You can get capacitors that are only in service to provide starting var support and not have to worry about normal running overvoltage.

One possible solution if the voltage is dipping when unit is started is get a VFD or soft start to limit the voltage sag during motor inrush.


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