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Low voltage ride through in grid connected hybrid renewable energy systems

Researches so far shows that, by 2020, around 20% of the total energy production worldwide will be generated from renewable energy. But the major problem with the standalone system is that the sources are not continuous.

This intermittent nature of the sources can tamper the power system stability.

Low voltage ride through in grid connected hybrid renewable energy systems
Low voltage ride through in grid connected hybrid renewable energy systems

Hence we can combine two or more such renewable energy sources to form a hybrid renewable energy system. Such hybrid systems are more promising and effective in generating power, especially in remote areas, as compared to individual systems.

They could bring out the advantages of each renewable source being combined and also complement the demands of conventional power systems. There are so many renewable energy sources available, but Wind and solar power projects are widely getting implemented that they are of free access and environment – friendly.

But for the connection of new generation systems into our existing grids, the transmission system operators define minimum requirements that should be met, which is called Grid Code.

The major challenge with a grid connected hybrid system is that they must contribute with the power quality and power system stability. During a fault at the grid side, it’s the voltage at the point of common coupling (PCC) which drops suddenly. This will adversely affects the entire hybrid system as the drop in voltage abruptly increases the rotor speed of the wind energy conversion system (WECS) generator and also affects the normal operation of the PV system.

Thus, in order to protect the renewable systems, it was customary practice to disconnect the renewable systems upon faulty grid conditions.

But, nowadays, due to higher penetration of renewable systems into the grid, disconnection of such a large number of renewable systems instantly from the grid during fault can aggravate the power system stability issues.

Because removal of such large scale hybrid generation during voltage dip will further cause the voltage to go down, which in turn results in the disconnection of more generation units, leading to a cascading failure.

Result of a voltage drop test at a PV system
Figure 1 – Result of a voltage drop test at a PV system. In this diagram the voltage drops to about 20% of the nom­inal voltage for a time of approx. 550ms. The PV inverter recognizes the voltage drop and feeds a reactive current of approx. 100% of the nominal voltage into the system for the duration of the fault in order to support the grid. After fault clearance the active power output is increased to the value prior to the occurrence of the fault within 160ms. (credit: J. Dirksen; DEWI GmbH, Wilhelmshaven)

IEEE 1547a

Therefore, came a renewed grid code, IEEE 1547a standard in 2014, which is referred to as Amendment 1 to IEEE 1547 standard in 2003, demanding that these new large scale generation plants should possess Low Voltage Ride Through (LVRT) capability.

As per this Amendment 1, there will be coordination between the grid operators and those of the renewable systems, on how these renewable systems can regulate the voltage by changes to real and reactive power.

It aims at maintaining sustainable power delivery during the faulty conditions and instead of getting disconnected from the grid, the generation units should ride through the low voltage conditions and support the grid. This will provide a much robust grid, as the Distributed Energy Resource (DER) is clearly allowed to provide low voltage ride through.

By providing appropriate control to the DER systems, we could make the system ride through the voltage dip condition by injecting reactive current at the point of common coupling PCC, and thereby improving the voltage that has fallen due to fault. But this method involves inclusion of some complex control circuitry.

Hence, among the various solutions to accomplish LVRT capability, the better choice is to employ FACTS (Flexible AC Transmission Systems) devices.

Grid connected PV-Wind Hybrid system integrated with FACTS-ESS
Figure 1 – Grid connected PV-Wind Hybrid system integrated with FACTS-ESS

FACTS technologies provide advanced solutions as, Unified Power Flow Controller (UPFC) – like FACTS devices can provide independent and simultaneous control of both real and reactive power flow. Due to instantaneous reactive power injection, steady state is reached faster. During the time of large transients, the DC link storage of the FACTS device may not be sufficient as it is limited to a definite value.

So, a backup energy storage system (ESS) like Super Capacitor can be coupled to the device’s DC link to improve the dynamic performance of power systems. Integrating an ESS into a FACTS device can lead to improved controller flexibility by providing dynamic decentralized active power capabilities.

Thus, the enhanced performance of the combined FACTS/ESS will have greater appeal to transmission service providers, when it comes to the case of achieving LVRT capability in Grid connected hybrid systems.

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About Author //

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Sajna Soman

sajna soman - Graduate in Electrical and Electronics Engineering from Kerala University, India. Currently, pursuing Master's degree in Power Electronics & Power Systems. Interested in Power System Engineering as well as Illumination Technologies. Present on Google+ and LinkedIn.

4 Comments


  1. Kimberly King
    Jun 05, 2017

    Perhaps the readership would like to know about harmonization efforts to address LVRT since the 2014 rule? There have been great efforts to harmonize all efforts via SunSpec, not just IEEE.

    There was a discussion back in 2016 about these harmonization efforts that perhaps the readership might want to know about? See the SunSpec portion of this presentation: http://standards.ieee.org/events/2016_der_presentations/rule21.pdf

    BTW: In 2012, I composed the first draft document for installing a high penetration of residential PV on a pre-existing electrical distribution network when discussions on LVRT were in their infancy as a part of the UH-HNEI | DoE Grid, Photovoltaic & Battery Smart Grid Inverter Project; part of the US DoE Energy Efficiency & Renewable Energy SunShot Initiative.

    Also, I must respectfully challenge the image that was chosen for the beginning of this post. Seasoned professionals in wind energy know this UGE (?) green fashionistas who persistently undermine the science ‘kinetic architecture’ installation is a POORLY sited example, whereby it’s no wonder that the solar PV arrays are the workhorses for this hybrid renewable energy installation. Here’s some evidence from internationally known wind sage Paul Gipe:

    http://www.wind-works.org/cms/index.php?id=64&tx_ttnews%5Btt_news%5D=4293&cHash=3b8980cb12a91d879338a0846671e494

    [snip]
    UGE Dumps VAWT Business—What Happens Now to Eiffel Tower & Lincoln Field Turbines?

    Controversial developer of “urban” wind projects Urban Green Energy (UGE) is quitting the wind business. Using its in-house designed vertical-axis wind turbine, the New York City-based company installed dozens of the wind turbines on buildings in high-profile locations, including the much-hyped turbines installed inside the Eiffel Tower in Paris.

    Wind industry veterans criticized the installations in Philadelphia, Paris, and elsewhere as mere architectural bling at best, greenwashing at worse.

    Critics charged that the turbines were unproven, expensive, and would produce little electricity. They feared that if the helical turbines failed or otherwise did not perform well in high-visibility installations it would give the entire industry a black eye.

    And there was also this…

    http://www.peninsuladailynews.com/news/wind-turbines-generating-regret-100000-turbines-to-create-1-50-in-electricity-monthly/#civil-comments

    This all being said, don’t support this company who is ONLY fomenting mistrust in the mostly energy illiterate general public. I suggest locating images with a Skystream 3.7 and PV array as a better example.


  2. Kimberly King
    Jun 03, 2017

    Here’s a better example of a Skystream 3.7 and solar array, that perhaps Sajna Soman can use instead of the green fashionistas who undermine the science, Green Going Wrong UGE’s photo at the start of this post.

    https://media.treehugger.com/assets/images/2011/10/hybrid-wind-solar.png

    Anytime one sees a rooftop installation of a wind turbine in the built/urban environment, be wary, be VERY wary unless it’s a well-thought out, correctly sited, Windchallenge Windleaf. https://windchallenge.com/the-windleaf/


  3. Kimberly King
    Jun 03, 2017

    Edvard. I was so distracted by the photo of the green fashionistas who undermine science UGE (??) wind-solar installation you chose for this article that I could not get past it and had to issue this comment. I REALLY wish you could have selected a better photo of a hybrid RE system, instead of this one.

    Anyone who has installed small wind turbines in the built environment know is NOT a good practice to install wind turbines this close to a rooftop, be they VAWTs or HAWTs. The best wind is higher above a roof/structure. It annoys me to no end that UGE persists in poorly educating its clients and poorly siting its wind turbines, which in this case are only ‘kinetic architecture’, and the PV is the true workhorse of this particular installation.

    • Edvard
      Edvard
      Jun 04, 2017

      I got your message Kimberly, you’re right, thanks for the warning! I’ll replace the photo ASAP.

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