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High System Voltage Curtailing Power Evacuation From Mini Hydro Plant

06/14/2017 3:02 PM

We have a 6.5 MW mini hydro plant which can only be evacuated through a 33 kV medium Voltage network. There is a challenge though given that the main load center is about 60 km away. At the time of synchronizing, the system Voltage is normally about 32 kV. When the MW output from the plant is gradually raised, the system Voltage starts to raise as well. At 3.5 MW output, the system Voltage is about 36 kV and the plant eventually trips on over Voltage.

What solution can solve this problem?

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#1

Re: High System Voltage Curtailing Power Evacuation From Mini Hydro Plant

06/14/2017 4:44 PM

Throw in a DSTATCOM to help regulate the voltage. That's what our customer did to solve a similar hydro+wind to grid voltage regulation problem.

DSTATCOM

There you go, useful free advice and you didn't have to pay a consultant to get it.

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#2
In reply to #1

Re: High System Voltage Curtailing Power Evacuation From Mini Hydro Plant

06/14/2017 4:58 PM

What he↑ said.

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#3

Re: High System Voltage Curtailing Power Evacuation From Mini Hydro Plant

06/15/2017 3:02 AM

Several things:

  • Increase the amount of cable between the generation and <...load center...>.
  • Reduce the voltage at the <...load center...>.
  • Not export so much as <...3.5MW...>.
  • Assess whether the <...36kV...> trip point is too low, and take carefully considered steps to raise it.
  • Add more load at the generation end of the cables.
  • Etc.
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#4

Re: High System Voltage Curtailing Power Evacuation From Mini Hydro Plant

06/15/2017 12:19 PM

It does take an electrical engineer to know Ohm's Law and to determine that you will have to deal with a certain voltage drop on the line in order to have the power flow you want and need.

This is why utilities use high voltage lines between generation centers and load centers. Doubling the voltage of the line will result in a factor of four reduction in line drop.

So, your solution is to increase the line voltage - substantially, say build a 66 kV line, or to reduce the impedance of the 33 kv transmission line. You figure out what you want to do.

Another item, . . . are you certain that the tripping is on over-voltage? Do you have any crystallographic evidence of this? How about system stability ? ? With the size of voltage drop you have in the line, . . . you are likely to approach system stability problems . . . and tripping on system instability . . .

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#5
In reply to #4

Re: High System Voltage Curtailing Power Evacuation From Mini Hydro Plant

06/15/2017 12:25 PM

sounds to me the system was set up with over-voltage threshold set too low to begin with for a long XMT run.

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#8
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Re: High System Voltage Curtailing Power Evacuation From Mini Hydro Plant

06/15/2017 2:00 PM

With the plant running at 32kV at no load, . . and 36 kV @ 3.5MW, . . . this means running the plant @ ~40kV to obtain its full output @ 6.5MW.

Running 32kV equipment @ 25% over-voltage is not recommended by anyone. Just think of the generator transformer, . . . stepping up the generator voltage (7?, 11?, 14.4kV?) to the 32kV line - will saturate, draw excessive excitation current, will not provide the required 40 kV, and WILL burn up in the process.

As the losses here are all reactive (inductive) losses, . . . they may be compensated for with capacitive compensation.

Yes, one can add a series capacitor to the existing line, and be able to operate the plant at 32kV, . . . and supply the load at 32kV at the end of the line. The capacitor will boost the line voltage by about 25% (8kV) which will be lost in the inductance of the transmission line.

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#9
In reply to #8

Re: High System Voltage Curtailing Power Evacuation From Mini Hydro Plant

06/15/2017 4:39 PM

<...Running 32kV equipment @ 25% over-voltage is not recommended by anyone...>

Er, <...Assess whether the <...36kV...> trip point is too low, and take carefully considered steps to raise it....> was already covered in #3⇑.

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#7
In reply to #4

Re: High System Voltage Curtailing Power Evacuation From Mini Hydro Plant

06/15/2017 1:46 PM

Yes tripping is on over Voltage for sure

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#6

Re: High System Voltage Curtailing Power Evacuation From Mini Hydro Plant

06/15/2017 1:17 PM

Your generator terminal voltage regulator (excitation) can be configured for remote sensing, by modeling the line resistance and reactance to the load center.

The adjustment should include terms like IR Compensation. As the kW output of the generator goes up, normally the voltage will go down (due to line resistance), so your generator sending end voltage will need to go up, so you end up with a satisfactory voltage at the load center.

If the engineer who set up the regulator over estimated the line losses under load, your voltage regulator will start over-charging the line, and your voltage will go up, rather quickly.

You might try putting the voltage regulator in MANUAL, monitor your reactive flow to the load center (kVAR) when your voltage is satisfactory. Then run the regulator with some small compensation, and keep increasing it until the kVAR is similar to the value you determined experimentally, when running under load.

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#10
In reply to #6

Re: High System Voltage Curtailing Power Evacuation From Mini Hydro Plant

06/16/2017 9:22 AM

Experimenting with the AVR and excitation level was my initial thought as well; however there are some problems with this. For starters, the line may not have the capacity to handle the load current, and since this is an overvoltage problem, reducing the excitation to lower the voltage may lead to steady-state and/or transient stability problems which can cause loss of synchronization (aka slipping poles).

Without knowing the conductor size, line configuration (radial or networked), and whether there any other loads/generation on this circuit, it is difficult at best to determine if there is a satisfactory solution. You might find that this paper provides some engineering information, if not then pass it along to the engineer in charge of the project.

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#11
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Re: High System Voltage Curtailing Power Evacuation From Mini Hydro Plant

06/16/2017 2:57 PM

It seems this whole scenario is unlikely, especially having to run your generator under-excited, as a prime candidate for stability problems. If the line doesn't have the capacity to handle the load current, then overvoltage would seem to be an unlikely problem (assuming no one hung a bunch of capacitors in the middle of the line).

To ignore other load configurations between the generator and load center would also seem unlikely, based on the original post, however...

That's why I'm taking a wild guess that the regulator is not set up properly, and someone thought incorrectly that highly conservative for the line compensation was the best starting point.

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#12

Re: High System Voltage Curtailing Power Evacuation From Mini Hydro Plant

06/16/2017 4:17 PM

It is time to take another look at this problem and take a different tack,

The problem starts with the engineering project itself. It must have been done by a bunch of incompetent engineers who know very little how power systems operate.

The designers should have known from the outset that it was not possible to evacuate 6.5 MW of power over a 33 kV line that was 60 km long. The impedance of the line is just to high for pushing so much power to the other end.

Just think that the 6.5 MW @ 33kV means a current of about 120 amperes, . . . and for this current the selected conductor would be in the order of 1/0. . . 4/0, providing a resistance of about 20 ohms for the 60 km. The line resistance alone would cause a loss of about 1 MW at full power - providing a 15% power loss. You can not operate a power system effectively or efficiently with losses of this magnitude.

The inductance of the line is even more serious providing an impedance of some 40 ohms. This is what is pushing the voltage UP when you try to supply the 6.5 MW. To overcome this inductance the generators have to supply some 1.8 MVAr.

The problem is that the generators do not like to operate at this power factor, but even more serious is the increase in terminal voltage trying to push the 6.5MW down the 60 km line. This pushes the voltage requirement 25% over the design voltage, and neither the generators nor the step-up transformers can handle it.

The proper solution to this situation is to build a higher voltage line and purchase higher voltage step-up transformers for each end.

A very inexpensive solution may be to compensate the 1.8 MVAr line loss with 1.8 MVAr of capacitor compensation connected in series with the line.

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#13
In reply to #12

Re: High System Voltage Curtailing Power Evacuation From Mini Hydro Plant

06/16/2017 5:43 PM

1.8MVAR on a 6MW generator is maybe high for line charging, but not out of this world as seen from the generator terminals. I would tend to think if anything, there was too much distributed capacitance on the lines already, good point to look at it from the line perspective, though adding more capacitors would make it worse.

If high capacitance was the case, then higher voltage would give even more reactive contribution. Seems unlikely, so I stick with bad VR setting.

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#14

Re: High System Voltage Curtailing Power Evacuation From Mini Hydro Plant

06/17/2017 4:07 PM

It may seem elementary but the whole system will not be at 36 kV....

  1. At 3.5 MW hydro, what are voltages at generator, at 33 kV line hydro end[transformer HV?] and at line end 60 km away?
  2. What are current per line and power factor at 33 kV at hydro end of line?
  3. What is power factor at load end of line?
  4. What are line parameters? or at least, line spacing, geometry, material and cross-section area?

Not sitting in Matsiko's position, it seems to me 3.5 MW might be too much generation - you are just raising the load voltage [and the whole system] in an attempt to get it to draw more power. Size of load and other generation [location?] were not given us.

Just taking some 132 kV line parameters for 113 mm2 copper conductors [current rating about 350 amp] I get for 60 km 6.9 ohm resistive & 25 ohm inductive per phase - about 26 ohm impedance. At 100 amps 33 kV [5.7 MW] one gets 2.6 kV drop on 26 ohm or about 36 kV in to get 33kV at remote load, with 3*1002*6.9 =~210 kW loss. So 33 kV line is possible & will have less inductance than 132 kV.

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#15
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Re: High System Voltage Curtailing Power Evacuation From Mini Hydro Plant

06/19/2017 9:22 AM

I gave you a good answer, since your calculations appear dead on, but also you looked at this from a perspective is driver versus driven. If the driven load is not available, then line voltage surges upward if load is attempted to be raised to a stated value.

There may be less load available when they are attempting to cram the lines with more electrons, thus overvoltage trip.

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#16
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Re: High System Voltage Curtailing Power Evacuation From Mini Hydro Plant

06/19/2017 12:46 PM

You should be able to control this with your generator excitation, though, even to the point of being under-excited (risking pullout in extreme cases...).

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#17
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Re: High System Voltage Curtailing Power Evacuation From Mini Hydro Plant

06/19/2017 3:33 PM

Thanks for the vote James, but I did "goof" typing the 1002 and the 26 ohms is per line & so its 2.6 kV is relevant to 19 kV line to ground voltage!

I was just hoping to provoke some usable data from the original poster.

Using the transformer regulation equation ε = εr cosφ + εx sinφ - that is assuming the line is just resistance and inductance and εr and εx are % volt drops across line resistance and inductance at 100 amps respectively and cosφ is load power factor at end of line - I get regulations of......

10.9% at 0.8 power factor. = (0.7kV/19)*0.8 + (2.5kV/19)*0.6

9.1% at 0.9 power factor.

3.7% at 1.0 power factor

---- so, as has been pointed out by other posters, power factor correction of the load end of line may be the only "fix" available.

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#18
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Re: High System Voltage Curtailing Power Evacuation From Mini Hydro Plant

06/19/2017 4:28 PM

It makes sense to me, but for all the data we really have on this, it could be the load center on far end of line is not drawing what they claim to be drawing. If producer is attempting to drive 3.8 MW into a single 3.6 MW load, I suspect chaos will ensue.

You cannot control a power plant based on telephone conversation with the purchaser.

You have to have dynamic control of the line and the load needs to be steady, within a small slice of time, only so much rate of change on load is allowed. IF this were one large machine with a small integer number of motors drawing current, with constant switching on/off, that could be a big part of the instability present that spikes the output voltage over the trip point.

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#19
In reply to #18

Re: High System Voltage Curtailing Power Evacuation From Mini Hydro Plant

06/19/2017 6:25 PM

I agree with all your points, James.

If you are still there, OP Richard Matsiko, give more information or further replies are pointless - because all the likely causes of trouble have already been given by posts and one cannot give any definite advice, like "you must upgrade your power line" without ratings of what you have.

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#20
In reply to #15

Re: High System Voltage Curtailing Power Evacuation From Mini Hydro Plant

06/20/2017 12:23 AM

As James has correctly stated, there is little load in the first 30-40 km from the hydro plant. The network operator has been requested to look at the option of mobilizing more load to connect to the hydro plant by changing the present network configuration. This is expected to be done over the next couple of weeks as it may involve construction of new line segments

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#21
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Re: High System Voltage Curtailing Power Evacuation From Mini Hydro Plant

06/20/2017 5:21 AM

Richard Matsiko,

Thanks for letting us know that the actual problem is too little load! I hope the comments did help. As you saw, too little information can cause a "storm" of speculation about problems you do not have!

My guess is that you are trying to get your hydro plant tested, signed-off and your due money received, but are frustrated by lack of load.

I trust you have written to the customer stating that you are ready to final test and complete but cannot procede due to lack of load and that delay may lead to claims for additional costs.

Deliver by hand with a witness and get a receipt for letter with its title/subject and date. If you do not, customer will later claim your costs seemed negligible because you would have told him if they were a problem or that he was not aware you were delayed by him. Time to read contract, consult a lawyer, if you have not already done so!

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#22
In reply to #20

Re: High System Voltage Curtailing Power Evacuation From Mini Hydro Plant

06/20/2017 9:57 AM

The kilowatt load from the generator is governed by the water passage, the voltage is the generator excitation, a separate knob. Take your voltage regulator out of automatic and your voltage will not go too high, and you can get your load test.

I still think your line drop compensation is set too high, when in auto, otherwise the excitation would not go to trip level at the local bus.

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#23
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Re: High System Voltage Curtailing Power Evacuation From Mini Hydro Plant

06/20/2017 10:03 AM

Sounds like the hydro plant needs to disconnect from the transmission line for their certification run, but rather install a dummy resistive load of 3MW up to 4 MW.

That load could be hard to come by, so instead, they could make a 3.5 MW induction coil heater, and heat a cylinder of steel of appropriate size. Use caution in applying the coolant to the coil, as this must be pure water to avoid heat transfer loss, and also to avoid direct conduction of the electricity.

A step-down transformer is also recommended. I don't know where he could rent any of this equipment, but that might be the best route to take.

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