Bad Power Factor? – A reason to oversize your inverter

In a previous blog, we discussed some good reasons to oversize your PV array. In this blog we will discuss how, by oversizing your inverter, you can correct a site’s poor power factor.

 

Electricity used in our homes and businesses is (almost always) alternating current. Put simply, voltage and current that are transmitted throughout the electric power grid in a sinusoidal waveform averaging 0. When these current and voltage waveforms are perfectly synchronised in time, they have a power factor of 1 or pure active power.

 

Example of pure active power (left) with current and voltage perfectly in-phase, and of pure reactive power (right) with current and voltage perfectly out-of-phase.

Example of pure active power (left) with current and voltage perfectly in-phase, and of pure reactive power (right) with current and voltage perfectly out-of-phase.

 

When we consume electricity (in pumps, fridges, lights, etc) current and voltage waveforms can  go out of alignment. This will lead to a power factor ≠ 1. As a site’s power factor moves further away from 1, they will typically incur increased grid quality supply charges from their electricity provider. This is where your SMA inverter can begin to help save you even more money. By utilising SMA inverter’s built in grid support functionality, you can correct a bad power factor by feeding reactive power as well as active power and hence reduce the grid quality charge component of your electricity bill. This can often be cheaper than using additional power factor correction equipment such a capacitor banks.  Often active power  is just as valuable to a site as reactive power for correcting power factor. This creates a financial driver to oversize your inverter.

 

How much should I oversize my inverter?

Since this is an abstract concept for a lot of system designers and installers, let’s work through an example.

First, we need to understand the relationship between ACTIVE, REACTIVE and APPARENT power. Apparent power consist of active and reactive power, two different types of power existing only in its pure nature. Because active and reactive power don’t have a relationship, it is impossible to convert one power type to the other. A graphical model of such relationship is a cartesian coordinate system with active power for the x-axis and reactive power in the y-axis. These 3 power types are related together according to Pythagoras’ theorem as shown in the following diagram.

 

Relationship between Apparent, Active and Reactive power.

Relationship between Apparent, Active and Reactive power.

 

Now let’s assume we have a site which is consuming 80kW of active power with a site power factor of 0.85 due to some inductive loads such as pumps and motors. This would result in the following relationship:

 

Power consumption without power factor correction or generation

Power consumption without power factor correction or generation

 

Now let’s assume the site needs to correct its power factor back to 0.90, and they also want to reduce their active power consumption by ~60%. If we begin with a 60kW solar system (60kW PV array, 60kW inverter), and this system generated power with a cos(φ) of 1.0, we would have the following power consumption. We can see that if we did nothing to the way the solar system operated, it could actually make the site’s power factor (and hence power quality charges) significantly worse from the utility’s point of view.

 

Power consumption with Generation at cos(φ) 1.0

Power consumption with Generation at cosphi 1.0

 

Now let’s operate the solar system with a cos (φ) of 0.82 to try and correct the site’s power factor. We would have the following power consumption and generation relationships:

 

Power consumption with generation at cosphi 0.82

Power consumption with generation at cosphi 0.82

 

This would then have the resultant power consumption for the site according to the following:

 

Resultant power consumption with inverters correcting power factor

Resultant power consumption with inverters correcting power factor

 

We could then consider to implement a cos(φ) function similar to the following could help to ensure that as the solar system increases its output power, it will change its cos(φ) to compensate for site power factor.

 

Dynamic cosphi function to assist correcting a poor site power factor

Dynamic cosphi function to assist correcting a poor site power factor

 

In this example, we require 60kVA of inverter capacity, but only 49kW of active power generation, meaning we can oversize our inverters by about 20% compared to the size of our PV array. SMA inverters can generate reactive power without using any active power. Within SMA, were refer to this capability as Q @ Night ( read more about Q @ Night here ).

 

Conclusion

By oversizing inverters, you have reserve reactive power capacity which can be utilised without sacrificing active power generation. Utilising the built-in grid support functionality in SMA inverters, such as a dynamic cos(φ) function, can help to improve a site’s power factor and in turn help to reduce grid quality supply charges a customer might incur from their electricity provider.

 

You can read more about this topic in detail by reading SMA’s PV Grid Integration technical compendium.

 

27 Comments
  1. Avatar
    benjamin Apraku says:

    Interesting stuff. Reminds me of my uni power systems.

    In future looking to reduce my electricity cost.

    Regards
    Ben

    Reply
  2. Avatar
    Farhad Khan says:

    Hi Scott
    thanks for these valuable information.how and where can i get more info.so to increase my knowledge to convince commercial/industrial and domestic customers of this advantage and savings and so to secure more sales for ourselves and SMA products.
    Regards
    Farhad Khan

    Reply
  3. Avatar
    Neranjan says:

    Very good illustrative presentation. However, i feel that over sizing inverter will not be required in some cases as the actual power generation of Solar PV arrays are often less than the name plate power due to factors like Temperature, irradiance, air flow rate etc.

    Reply
  4. Avatar
    Samer says:

    Hi Scott;
    Thanks for the interesting article.
    What if the inverter is not over sized?
    Am I going to lose the extra input DC power in order to fulfill the grid requirement?
    For STP60 Inverter, what is the procedure of setting the function of dynamic power factor

    Reply
  5. Avatar
    Hi Scott says:

    Hello Scott

    The information given here is valuable. Please, can you be of help as to providing more informations or materials as to where i can get more knowledge as to increase my technical knowledge so as to convince commercial/industrial customers of this advantage and savings and so to secure more sales for my organization thereby increasing more sales of SMA products.

    I’ve participated in an installation of a fuel-saving solutions for an oil lubricating plant here in my country using the STP 20000, but now I’m in a role that actively puts me in business development for commercial/industrial offerings. Informations given here will help a lot.

    Best Regards
    Adewale Bamgbade

    Reply
  6. Avatar
    P.Murugan says:

    Dear all,
    Is the inverter reqd to be oversized ?In the case of inductive loads are more in a factory where in solar plant has been commissioned and connected to grid under net metering policy . The power factor has down than the stipulated level by utility and hence we are paying penalty to the utility on account of lower power factor.Can you please give some idea to improve the PF without any additional capital expenditure?

    Reply
    • Carolyn Schlosser
      Carolyn Schlosser says:

      Hello Mr. Murugan,

      Without having any information about your system we are not able to help you. Please contact your installer, he knows yout system the best. If he can not help you feel free to sign in on our Online Service Center.

      Sunny regards,
      Carolyn

      Reply
  7. Avatar
    Comorin Solar says:

    In India there is harmonics, Power factor testing done by Electricity Company for grid connected systems.
    And if the inverter capacity is 8 kW and array is 6 kW in residential case, there are more chances the harmonic is more than 5% and power factor less than 0.6.

    Then Electricity company don’t give you approval..
    For residential systems it is always better to have correct capacity of Inverter and pv array.
    Even with SMA Inverter it happens.
    Oversizing of Inverter maybe successful in Industrial application not for residential

    Reply
  8. Avatar
    Jehad says:

    In this example, if we set the inverter to operate at 0.82 power factor, then we should oversize both the inverter and PV array, right?

    Reply
    • Carolyn Schlosser
      Carolyn Schlosser says:

      Hi Jehad,

      Thank you for your question. To provide addition reactive power only needs the inverter to be oversized. This is to allow all DC input energy to be used to generate kW, while still leaving a difference between the inverter’s kVA capacity and instantaneous kW output. This difference is able to be used to generate any required kVAr’s. The PV array does not need to be oversized, since oversizing would not increase reactive power capability.

      Over sizing your PV array is only helpful to increase the generation of real power. You can read about that topic in a previous blog here.

      Hope that answers your question.

      Sunny regards,
      Carolyn

      Reply
      • Avatar
        Jehad says:

        Thank you Carolyn for your helpful answer.

        Please, can I know in which mechanism does the inverter generates reactive power? In case there is no capacitor bank included inside the inverter!

      • Avatar
        Luq says:

        Hi Carolyn,

        I am currently working on a utility scale project and have a couple of questions regarding the inverter’s reactive power compensation. Is there someone I could email that can help answer my questions?

      • Avatar
        Maliki says:

        Jehad hightlighted a correct statement:
        S = P + jQ
        S comes from PV arrays through inverter. If you increase Q, you will sacrify P. Thus, you need to increase S to maintain P incase you want to increase Q. Increase the inverter power rating is not the only solution.

  9. Avatar
    Troy says:

    Hi Carolyn,
    This is interesting but your instructions to set up the inverters in this manner could be improved greatly. To date we’ve have many discussions with SMA-Service teams and are still unable to determine which steps to take to set the inverter, or the controller, to have the system respond and correct power factor dynamically.

    I have read these documents:
    WP QATNIGHT.AEN132110W.pdf
    Data Manager User Manual Version 2.1
    Operating Manual CORE1 50kW Version 1.4

    May you request that SMA HQ spends more time on providing sufficient instructions to your service offices and your clients? Thank you.

    Reply
  10. Avatar
    Prajval Suthar says:

    Thanks for the explanation.
    operating inverter not at unity power factor is good or bad in relation with cost?
    by this concept kwh generation will reduce. so it will increase amount of bill for grid tied system but obvious oversizing 20% the inverter required of smaller rating (means lower the cost).

    please correct me if i’m thinking in wrong way.

    Reply
  11. Avatar
    Murali Subramanian says:

    A very insightful article. In this example as the load is inductive, the inverter delivers part of the reactive power (49 kVAR) to the load with the rest (15 kVAR) being sourced from the grid. The cos phi of generation is at 0.82. Can you please clarify if the PF setting of the inverter would be +0.82 (capacitive) or -0.82(inductive)?

    Reply
  12. Avatar
    Ken L. says:

    Hey Carolyn,

    We have a residential PV site that we can see on our monitoring portal that the inverter turns on and off multiple times (3-6 times/15mins-60mins) throughout the day. Does bad power factor issues have anything to do with what this site is experiencing and causing the inverter to shut off? The site does have a pool/well pump and possibly motor equipment at the property. All DC and AC voltages/connections checked out fine every time we went out to check on the system. Can the pool/well pump or motors cause a spike in AC/DC voltage or AC/DC current to go beyond the inverters operating parameters for it to shut off and turn back on once those loads are off? Can you provide any insight as to what may be causing what this site is experiencing. Thanks!

    Reply
  13. Avatar
    Alex Zhang says:

    Dear SMA professionals, regarding an ongoing project, I’ve got a couple of queries about your product. To maximize the PV array
    Whether the inverter can set a fixed output PF, ie set a active power output cap?
    With the fixed PF, can user apply oversizing on PV array, say 1.4 DC-AC ratio?
    Can the inverter output or absorb Var without being affected by active power output fluctuation?
    Can the inverter autonomously adjust the PF as grid requirement, saying normally operates with PF @ 1 and lowers the PF in response to grid Var demand?

    Thanks!

    Reply
    • Anke Baars
      Anke Baars says:

      Dear Alex,
      Thanks for your questions.
      Which inverter are your questions about? Just asking, because answers may differ based on the inverter, so please find below answers given based on common properties.
      – Power output can be limited to a specified value. However, for further information (and additional devices and/or settings needed, if any), type of the inverter needs to be known.
      – Yes. The upper limit for oversizing is determined according to the total working hours in maximum power per year based on the PV array configuration. This can (and must) be checked with our free design tool Sunny Design. As long as the inverter is calculated to work below 2400 h/year at maximum power, oversizing is allowed.
      – In general, yes. However, for further information (and additional devices and/or settings needed, if any), type of the inverter needs to be known.
      – Yes. It is possible to adjust the PF based on the grid requirements with a signal coming from the grid operator. However, for further information (and additional devices and/or settings needed, if any), type of the inverter needs to be known.
      I hope this helps? You can also sign in our Online Service Center to get help.
      Kind regards,
      Anke

      Reply
  14. Avatar
    Sajal Aggarwal says:

    We have installed a 290 kWp system in India (at a school ) using 9 SMA , STP 25000 TL-30 inverters. We also have replaced the power factor bank and supply all reactive power using the solar inverters. The power factor is expected to rise to at least 99%.
    Thumbs up SMA , a deep dive in solar technology , now 7 years with SMA inverters . Thanks a lot
    video of the school here : https://www.youtube.com/watch?v=Jl3x9_46jFQ

    Reply
    • Carolyn Schlosser
      Carolyn Schlosser says:

      Thank you for your comment. That’s a really nice system! 🙂

      Sunny regards,
      Carolyn

      Reply

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