Hi all,

here's the situation; a 20MVA generator will be shutdown for 3 years at a papermill. This generator was providing all power factor correction needed by the plant. And about half of the load. (Total load is 12MW, 5.7MVar)

When the generator is down, pf at service entrance drops down to 0.87 and voltage on 13.8kV bus drops to about 13kV, and causes all kinds of problem.

Primary voltage is 22.9kV, and they use a 25kV/13.8kV transformer (16MVA), on the lowest tap (22.8kV/13.8kV). So no help here to raise the voltage.

NOTE: no pf penalities at all, they pay kw-h only!

I see three possible options:

-install a capacitor bank on 13.8kV bus: but these capacitors will be useless when the generator is put back on in three years.

-install a voltage regulator transformer(with automatic OLTC) upstream of the existing one

-change the existing transformer for a 22.9kV/13.8 kV with automatic OLTC

I would like to have your opinions about this project.

Thanks!



It is hard to comment without knowing the cost of the various options. I imagine that any installation will be superfluous in three years. An OLTC may provide more operating flexibility than a capacitor bank, in the future. A capacitor bank may be easier to sell if you decide to remove it when the generator is back online.
I would be checking the warehouse and talking to the head electrician about hiding places. It is possible that there may be an old, unused, synchronous motor available somewhere. A 4 MW motor would bring you up to 0.95 pf.
It's a long shot but worth checking. Also, of course, if you find an alternator that may be motored as a synchronous condensor that is also an option.

Bill
--------------------
"Why not the best?"
Jimmy Carter That is true davidbeach,

but since it is a 20MVA generator supplying only about 6 MW, there is no benefits to run it at 1.0 pf.

I think it will all come down to pricing...

Capacitor banks needs maintenance, protection study, harmonic study.. etc.

Voltage regulator tranformer might be more expensive but it is "plug and play" , plus, I think it would be maintained by the utility.

Thanks for the feedback.
JLuc; I'm very curious as to why a generator would "go away" for three years.?

Could you just lease one and drop it somewhere on site for the interim 3 years? No studies. It would be easy to sell later too, I'd think.

Keith Cress
kcress - itsmoked:

It is a steam powered generator, the steam comes from a gas boiler that will be replaced by three bark boilers. The steam generated by this gas boiler is also used in the process.

During the time of the project (3 years), the steam required by the process will be supplied by some other boiler.

When the three bark boilers are all set and hooked up to the generator, they will restart de generator.

So, they need to buy all power from utility during this replacement (3 years max).

The bottom line is one thing should be kept in mind, VR/OLTC can not provide you additional Var but only obsorb var from the system u connect.
Shunt capacitor bank does provide u additional Vars.

I would suggest if the system you connect is strong, then get a VRor/OLTC and bring Var from the system if no penalties. Of course you have to compare the cost.
Cheers Are you sure there will be no kVAR charges? Once the utility sees the plant will be requiring 5.7MVAR for 3 years they just might want to charge for supplying it. If the plant has a power contract stating no power factor penalty it just might be dependent on them correcting their power factor xx% of the time.

Have you determined that by eliminating most of that 5.7MVAR with a capacitor bank that the new voltage drop will be acceptable? You might correct the voltage drop from 6% you have now to say 3% so is that 3% suitable?

Changing the transformer could be very close to "plug and play".

Installing a new transformer or a capacitor bank are both large projects so to me the decision mostly comes down to cost. But don't forget delivery time too.

Have you considered getting a price from Cat or Onan/Cummins on rental gensets? They can get about 1.5 MW in a semi trailer. Check with them as to how many VARs you can produce without becoming unstable. It may only take one or two units to raise your voltage to an acceptable level.
Even if the cost is too much, your boss may be impressed with your diligence in evaluating all possible solutions.

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

Have you considered getting a price from Cat or Onan/Cummins on rental gensets? They can get about 1.5 MW in a semi trailer. Check with them as to how many VARs you can produce without becoming unstable. It may only take one or two units to raise your voltage to an acceptable level.

You will find that generating vars with diesel fuel may not compare favorably with using scrap wood chips!
Cap bank is the inexpensive answer since your volt drop is likely due to the power factor of the load and the voltage regulation of the transformer. -kVAr will likely provide the required voltage rise at minimal cost compared to the other options you suggest. 22.9 sounds like northern MN - you could rent a set of 400 amp 14.4kV vregs for three years if you're close and are interested in a rental.

Capacitor banks - give your appliances the rewards they deserve

In our daily life, you will use various electronic devices to save time. In order to work effectively, they need sufficient electricity. However, exceeding the requirements can cause insulation damage to the wires and machine short circuits. So, what can you do to balance voltage fluctuations? Place a capacitor bank so that they can obtain sufficient required power.What is a capacitor bank?

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A capacitor bank is a set of capacitors within the same range, connected in series or parallel to store more electrical energy. A capacitor can store a small amount of charge, and a device can embed a capacitor. Substations use them to provide consumers with ufficient energy without power lag or current phase shift. Now, this technology has spread to home use to support your device.

How does it work?

Manufacturers design capacitor banks to correct AC fault currents or provide DC power. A capacitor has two conductive plates separated by the dielectric of a capacitor (such as air or any other material). Electricity causes these two boards to charge in opposite directions, so that a static electric field can be maintained between them.

Over time, charges begin to move from one plate to another, dissipating energy and providing electricity to connected devices. These small capacitor units are connected in parallel or series, increasing their capacitance and allowing them to distribute energy to power plants and large factories.

Type of capacitor bank

1.External fuse type：in this arrangement, each capacitor unit has its own fuse unit connected externally. Once any problems occur, the fuse will be blown out and the faulty capacitor unit will be disconnected. The bank continues to work without any interruption, but the voltage provided is lower than before. Furthermore, even if all units are healthy, the failure of the fuse unit can affect the performance of the capacitor bank.

2.Internal fuse typ：the internal fuse type connects each component of the capacitor to the fuse, all enclosed in the same box. You can easily install and maintain it because it has low-level units. When any single component fails, the group of capacitors can continue to operate. The main drawback is that if more components stop working, you cannot change each of them; On the contrary, you must purchase a brand new group.

3.Reduce Fuze:in order to eliminate the need for fused capacitor banks, a "low fuse" connects all capacitor units in series to form a series. Then, it connects each series in parallel to form a bank. When one capacitor fails, other capacitors in series will mask its missing capacity. Therefore, this type of capacitor does not require a fuse to operate.

What is the purpose of capacitor banks?

1.Compensating reactive power

Adding an inductive load to the system will reduce the quality of electricity provided by the main power supply. To improve the lag of power factor, you can add capacitors. Therefore, they can reduce the actual power load and overcome the deficiency of reactive power.

2.Power factor correction

During the distribution process, capacitors improve the stage of power transfer. It appears as an additional load in the system and reduces the waste of unused electricity. Due to the power factor being the ratio of the power used to the total power provided, capacitors are added to make these two quantities equal, thereby achieving a pf of 1.

3.Energy storage

Capacitor banks store energy in the form of charges, and without them, the system may be wasted. The larger the capacitance of a capacitor bank, the greater the energy it can store. Electrical equipment that requires DC power also uses capacitors to maintain power while charging their batteries.

4.Noise bypass

When capacitors are placed on a circuit, they serve as a source of high resistance, rather than being used for high-frequency signals. Therefore, you can use parallel capacitor banks to block noise. Therefore, signals with higher carrier frequencies will not be transmitted to electrical appliances, nor will sudden fault currents occur.

Reduce the cost of fault current

Due to the low power factor of the system, the friction inside the wires will waste a large amount of energy in the form of heat and reaction forces. However, power suppliers are not concerned about this issue, as you receive less than the bills you pay. Installing a capacitor bank can reduce this issue and save additional energy to meet the requirements of your device.

Substation capacitor bank

Due to the fact that capacitor banks can improve power factor and provide voltage quality, they are installed in substations. They connected several capacitors in series to improve the voltage curve. Through its addition, the power factor angle is reduced because the current is now leading the voltage. As the angle decreases, you know that the power factor is improving.

By increasing the inductive load on the user side, the demand for reactive power increases. A load, such as a water pump, requires more reactive power to generate magnetic flux and move its coils. The increase in demand leads to a decrease in power factor, resulting in imbalanced power consumption. More energy loss becomes a burden on the factory. When you use capacitor banks in production, they become a source of reactive power for consumers, making the factory's work more efficient.

Can we only use capacitor banks on a larger scale?

Over time, the usage range of capacitor banks has also expanded to a smaller range. Mobile phones use low-power capacitor banks and supercapacitors to reduce their charging time. Supercapacitors can hold up to a hundred times more charge than ordinary capacitors, and we also use them as low-voltage rechargeable batteries in most cases.

In wireless space, tiny microelectromechanical systems, also known as MEMS, replace full-size capacitors. Research has shown that capacitors can also provide the large pulse current required by many pulse power devices and weapons. A high-energy bank equipped with modern semiconductor switches can create pulses of up to hundreds of kilojoules and high ampere electrical pulses. Radar, nuclear fusion research, and Marx generators are common and unique applications of this high-density pulse.

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How to protect capacitor banks?

During the installation process, in order to limit surge currents, reactors are connected in series on capacitor banks. These reactors can protect it:

1.Overcurrent (unbalanced) relay

Due to a blown fuse or internal short circuit in the capacitor bank, some capacitor units may malfunction. These units are no longer connected, causing a burden on other units within the chain. At first, this may not be a problem, but as the number of faulty units increases, the asymmetry becomes more pronounced. An overcurrent relay can detect this imbalance within the capacitor bank, so you can replace it in a timely manner without causing any damage to the entire circuit.

2.Overload relay

Capacitors are connected in series with reactors, so it is not easy to detect overvoltage if it occurs. Due to its ability to withstand only 110% of the rated voltage, after this increase, the capability curve begins to follow an inverse time characteristic, meaning that the current begins to decrease.

To detect this, an overload relay was introduced that measures the current from the battery pack and converts it into the corresponding voltage. It can prevent capacitors from releasing energy in the event of insufficient voltage supply, even if it is not necessary.

3.Short circuit protection

In addition to overcurrent and voltage issues, capacitors are also susceptible to short circuits and ground faults. For this situation, 2-phase or 3-phase short-circuit protection and ground overcurrent relays can be a good solution.

What should be considered when selecting a capacitor bank?

When selecting a capacitor bank in any situation, you must consider the following parameters:

1.Voltage level to maintain peak voltage and surge voltage. It has a normal peak voltage of up to 110% and a normal effective voltage of 120%.

2.KVaR level to ensure the system obtains sufficient reactive power. The calculation of capacitor banks requires initial and expected power factors.

3.Temperature level to maintain the temperature of the capacitor bank during operation. To calculate this rating, you must observe the temperature rise and other energy losses under sunlight.

4.Insulation level, as the medium used will play a significant role in determining the energy provided, just like in any other electrical equipment.

5.The rated current level, or the effective value of the normal current for charging capacitors.

6.Discharge time, see how long the capacitor will deplete its energy.

7.Single phase or three-phase, as both are useful for specific reasons.

Conclusion: 

Electricity is transmitted from the main power source to your home through wires. During this process, the frictional force inside the wire converts this energy into thermal energy and other forms of energy.

If you install a capacitor, the energy that heats the wires and strains your device will be directed towards it. Therefore, it reduces your bill and saves you the need to repair motors affected by power failures.

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