Low voltage ride through

In electrical power engineering, fault ride through (FRT), sometimes under-voltage ride through (UVRT), or low voltage ride through (LVRT),<ref>IEC Glossary: UVRT</ref> is the capability of electric generators to stay connected in short periods of lower electric network voltage (cf. voltage sag). It is needed at distribution level (wind parks, PV systems, distributed cogeneration, etc.) to prevent a short circuit at HV or EHV level from causing a widespread loss of generation. Similar requirements for critical loads such as computer systems<ref>http://www.powerqualityworld.com/2011/04/cbema-curve-power-quality-standard.html CBEMA Curve – The Power Acceptability Curve for Computer Business Equipment, 2011-04-03</ref> and industrial processes are often handled through the use of an uninterruptible power supply (UPS) or capacitor bank to supply make-up power during these events.

General concept

Many generator designs use electric current flowing through windings to produce the magnetic field on which the motor or generator operates. This is in contrast to designs that use permanent magnets to generate this field instead. Such devices may have a minimum working voltage, below which the device does not work correctly, or does so at greatly reduced efficiency. Some will disconnect themselves from the circuit when these conditions apply. The effect is more pronounced in doubly-fed induction generators (DFIG),<ref name="IEEE Journals & Magazine 2019">Guo, Wenyong; Xiao, Liye; Dai, Shaotao; Xu, Xi; Li, Yuanhe; Wang, Yifei (2019-06-18). "Evaluation of the Performance of BTFCLs for Enhancing LVRT Capability of DFIG". IEEE Transactions on Power Electronics. 30 (7): 3623–3637. doi:10.1109/TPEL.2014.2340852.</ref> which have two sets of powered magnetic windings, than in squirrel-cage induction generators which have only one. Synchronous generators may slip and become unstable, if the voltage of the stator winding goes below a certain threshold.<ref name="IEEE Conference Publication 2019-1">Mahrouch, Assia; Ouassaid, Mohammed; Elyaalaoui, Kamal (2019-06-18). "LVRT Control for Wind Farm Based on Permanent Magnet Synchronous Generator Connected into the Grid". 2017 International Renewable and Sustainable Energy Conference (IRSEC). pp. 1–6. doi:10.1109/IRSEC.2017.8477281. ISBN 978-1-5386-2847-8.</ref>

Risk of chain reaction

In a grid containing many distributed generators subject to disconnection at under voltage, it is possible to cause a chain reaction that takes other generators offline as well. This can occur in the event of a voltage dip that causes one of the generators to disconnect from the grid. As voltage dips are often caused by too little generation for the load in a distribution grid, removing generation can cause the voltage to drop further. This may bring the voltage down enough to cause another generator to trip, lower the voltage even further, and may cause a cascading failure.

Ride through systems

Modern large-scale wind turbines, typically 1 MW and larger, are normally required to include systems that allow them to operate through such an event, and thereby “ride through” the voltage dip. Similar requirements are now becoming common on large solar power installations that likewise might cause instability in the event of a widespread disconnection of generating units. Depending on the application the device may, during and after the dip, be required to:<ref name="IEEE Conference Publication 2018">Liasi, Sahand Ghaseminejad; Afshar, Zakaria; Harandi, Mahdi Jafari; Kojori, Shokrollah Shokri (2018-12-18). "An Improved Control Strategy for DVR in order to Achieve both LVRT and HVRT in DFIG Wind Turbine". 2018 International Conference and Exposition on Electrical and Power Engineering (EPE). pp. 0724–0730. doi:10.1109/ICEPE.2018.8559605. ISBN 978-1-5386-5062-2.</ref>

• disconnect and stay disconnected until manually ordered to reconnect
• disconnect temporarily from the grid, but reconnect and continue operation after the dip
• stay operational and not disconnect from the grid<ref name="IEEE Conference Publication 2019">Harandi, Mahdi Jafari; Ghaseminejad Liasi, Sahand; Nikravesh, Esmail; Bina, Mohammad Tavakoli (2019-06-18). "An Improved Control Strategy for DFIG Low Voltage Ride-Through Using Optimal Demagnetizing method". 2019 10th International Power Electronics, Drive Systems and Technologies Conference (PEDSTC). pp. 464–469. doi:10.1109/PEDSTC.2019.8697267. ISBN 978-1-5386-9254-7.</ref>
• stay connected and support the grid with reactive power (defined as the reactive current of the positive sequence of the fundamental)<ref name="Akagi_2007">Akagi, H.; Edson Hirokazu Watanabe; Mauricio Aredes (2007). Instantaneous power theory and applications to power conditioning. IEEE Press Series of Power Engineering. John Wiley & Sons. p. 137. ISBN 978-0-470-10761-4.</ref>

Standards

A variety of standards exist and generally vary across jurisdictions. Examples of the such grid codes are the German BDEW grid code<ref>BDEW Medium Voltage Guideline Archived 2012-11-05 at the Wayback Machine retrieved on 9 November 2008</ref> and its supplements 2,<ref>BDEW MV Guideline 2nd Supplement retrieved in 07/2010</ref> 3,<ref>BDEW MV Guideline 3rd Supplement Archived 2013-01-27 at the Wayback Machine retrieved in 02/2011</ref> and 4<ref>BDEW MV Guideline 4th Supplement Archived 2013-08-16 at the Wayback Machine retrieved in 12/2015</ref> as well as the National Grid Code in UK.<ref>National Grid Code Archived 2010-02-14 at the Wayback Machine retrieved on 9 2008-11-9</ref>

Testing

For wind turbines, the FRT testing is described in the standard IEC 61400-21 (2nd edition August 2008). More detailed testing procedures are stated in the German guideline FGW TR3 (Rev. 22). Testing of devices with less than 16 Amp rated current is described in the EMC standard IEC 61000-4-11<ref>IEC 61000-4-11</ref> and for higher current devices in IEC 61000-4-34.<ref name="IEC Webstore 2005">"IEC 61000-4-34:2005 - electromagnetic compatibility, EMC, smart city". IEC Webstore. 2005-10-17. Retrieved 2019-07-04.</ref>

See also

• Voltage dip