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Thread: capacitor banks to lower electricity bill

  1. #1
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    capacitor banks to lower electricity bill

    on your own experience , does installing capacitor banks to correct power factor work to lower your monthly bill? , not in residences but in factories,I do maintenance to a fish house and a packing warehouse, at the fish house they have about 20 compressors for ice machines and coolers, and a few pumps, their pf are , .63 ,.42,.95 (in the three phases) at the main disco electrical bill around $8,000 a month, the packing warehouse is very similar.
    is it worthy to install them? what is experience on this?

  2. #2
    Join Date
    Dec 2007
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    Quote Originally Posted by wireperson View Post
    on your own experience , does installing capacitor banks to correct power factor work to lower your monthly bill? , not in residences but in factories,I do maintenance to a fish house and a packing warehouse, at the fish house they have about 20 compressors for ice machines and coolers, and a few pumps, their pf are , .63 ,.42,.95 (in the three phases) at the main disco electrical bill around $8,000 a month, the packing warehouse is very similar.
    is it worthy to install them? what is experience on this?
    If you are not charged a penalty for low power factor they will not save anything on energy bill. They could result in on site advantages such as smaller transformers feeders etc because of less total VA but would have to be installed at the load(s), but the savings would most likely be offset or even exceeded by capacitor costs.

  3. #3
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    For customers that are charged a penalty for poor Power Factor, capacitors pay back and sometimes within months. We have one customer that actually gets a credit for maintaining a PF above .9. Another we fell short of the .9 but did not feel the additional expense was worth it. I believe the best way PFC should be done is when the motors are installed but single large or switching of banks can be done later.

    We use "hardened" caps that are designed to withstand higher voltage levels and some problems caused by VFDs. In the case of the better correction above we found that the higher resultant voltage of a single added cap caused some problems with older existing capacitors. We had the POCO change the taps at the sub to lower the voltage. Measuring the current and applied voltage at the caps was text book. Voltage up = Current up. Voltage down = Current down.
    Tom
    TBLO

  4. #4
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    Quote Originally Posted by ptonsparky View Post
    We use "hardened" caps that are designed to withstand higher voltage levels and some problems caused by VFDs.
    This is wrong solution to the problem that may cause damage.

    The problem that you mean is harmonics. When you have harmonics (such as from VFDs), at certain point the the capacitor reactance (1/impedance) will be the same in negative sign as the transformer's. The result is that the capacitor and transformer branch impedance is infinity. The harmonic distortion, which acts as current source, goes on the infinity impedance to create infinity voltage (in theory, in real life it is no infinity). This cause the voltage to increase and the capacitors to blow up. If you use higher rated capacitors, that can withstand the higher voltage, the next component to go is the transformer itself. In the better case, it belongs to the POCO and you will only have long downtime. In the other case, you have to buy a new one.

    In such cases, or to my recommendation in any case of using capacitors, use detuned reactors to prevent this issue.
    Last edited by iwire; 04-25-10 at 05:39 AM. Reason: removed link
    Amir Broshi

    Certified engineer, PowerQualityDoctor

  5. #5
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    We looked long and hard at this before ordering the caps. I am definately no engineer but when pressed I can do some of the basic math and read all I can to find the pitfalls that salespersons tend to ignore when they want to sell you something. The customer is not cursed with low voltage even under the highest demands of summer. It is always high but not every customer has a sub that feeds only them 50' from the building. Add another cap and watch it climb higher. His VFD load is very small in comparison to the total motor load. They seemed to blow caps or their fuses when equipped, quite often before the vfds were installed. They have had no problems for the last 6 years.

    Not saying it is perfect and just slapping a couple hundred kvar on willy nilly is not recommended. Do your homework first.

    The OP was "on your own experience , does installing capacitor banks to correct power factor work to lower your monthly bill?" My answer is still Yes.
    Tom
    TBLO

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