Low Voltage Ride Through

In electricity supply and generation, low voltage ride through (LVRT), or fault ride through (FRT), is a capability of electrical devices, especially wind generators, to be able to operate through periods of lower grid voltage. Similar requirements for critical loads such as computer systems and industrial processes are often handled through the use of an uninterruptible power supply (UPS) to supply make-up power during these events.

Many generator designs use electrical current flowing through windings to produce the magnetic field that the motor or generator operates on. 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 cut themselves out of the circuit when these conditions apply. This effect is more severe in doubly-fed Induction generators (DFIG), which have two sets of powered magnetic windings, than in squirrel-cage Induction generators which have only one.

In a grid containing many distributed generators subject to low-voltage disconnect, it is possible to create 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, removing generation can cause the voltage to drop further. This may bring the voltage low enough to cause another generator to trip out, lower it further, and causing a cascading failure.

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 low voltage. Similar requirements are now becoming common on large solar power installations that likewise might cause instability in the event of a disconnect. Depending on the application the device may, during and after the dip, be required to:

• disconnect temporarily from the grid, but reconnect and continue operation after the dip
• stay operational and not disconnect from the grid
• stay connected and support the grid with reactive power (defined as the reactive current of the positive sequence of the fundamental)

A variety of standards exist and generally vary across jurisdictions. Examples of the such grid codes are the German BDEW grid code and its supplements 2 and 3 as well as UK National Grid code.

Testing of the devices with less than 16 Amp rated current is described in the standard IEC 61000-4-11 and for higher current devices in IEC 61000-4-34. For wind turbines the LVRT 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).