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Glossary · Energy industry & energy transition

Fault Ride Through

fault ride through fault ride-through inverter wind energy photovoltaics grid connection grid stability protection technology

Fault Ride Through, FRT for short, is the ability of a generating plant, a storage system or a converter not to disconnect from the grid immediately during a short grid fault. The plant remains connected to the grid for a specified time and behaves according to prescribed rules. In German this is usually called Fehlerdurchfahrt, and in VDE rules also dynamische Netzstützung (dynamic grid support). The focus is on behaviour during a voltage dip. Such dips arise mainly from short circuits, switching operations or faults in equipment. The European network code for generators defines the term narrowly: devices are to remain connected and continue operating during temporarily low voltage at the grid connection point if the fault is one that is "to be managed in accordance with the design". This means a fault that the grid protection successfully clears according to the network operator's planning criteria.

Voltage-time curves and p.u. values

Technically, FRT is described by voltage-time curves. They specify a lower limit: how far may the voltage at the grid connection point drop before, during and after a fault without the plant being allowed to disconnect? The voltage is usually not stated in volts but as a relative value in p.u. (per unit). 1.0 p.u. corresponds to the reference or nominal voltage. A dip to 0.2 p.u. therefore means that only 20 percent of the nominal voltage is present at the point under consideration. Only when the actual voltage profile falls below the curve may or must the plant disconnect.

Regulation (EU) 2016/631 (Network Code Requirements for Generators, RfG) specifies parameter ranges for this. The transmission system operators set the concrete values nationally. For non-synchronous plants, so-called power park modules such as wind, PV or battery plants, the retained voltage during the fault for types B and C is in the range of 0.05 to 0.15 p.u. For synchronous plants of type D, Table 7.1 provides a retained voltage of 0 p.u. The fault clearing time, i.e. the time until the fault is cleared, is in each case 0.14 to 0.15 seconds. Where grid protection and secure operation require it, up to 0.25 seconds is possible. After that, the curve rises again in steps. Depending on the plant type, the recovery phase can last several seconds. The permissible durations thus range from milliseconds to a few seconds.

The curves in the RfG refer to symmetrical faults. Each transmission system operator itself determines the behaviour in the case of asymmetrical faults. In addition, network operators must publish the pre-fault and post-fault conditions that apply to the demonstration of compliance, for example the minimum short-circuit power at the connection point. The ability to ride through a fault depends strongly on how "stiff" the grid is, that is, on its short-circuit power.

LVRT, HVRT and reactive current

Riding through voltage dips is called Low Voltage Ride Through (LVRT), and riding through short overvoltages High Voltage Ride Through (HVRT). The RfG explicitly defines FRT only for phases of low voltage. Requirements for short-term overvoltages arise, depending on the region and set of rules, from national provisions or connection rules. For the power system it matters not only whether a plant stays connected. It is also important what current it delivers during the fault, whether it supports the voltage with reactive power, and how it ramps its active power back up afterwards. The RfG lets the transmission system operator specify whether active or reactive power has priority during the fault. If active power has priority, it must be available no later than 150 milliseconds after the onset of the fault.

In Germany, VDE-AR-N 4110 (Technical connection rule for medium voltage, current version 2023-09) implements these requirements for the medium-voltage grid. It distinguishes type 1 plants (directly coupled synchronous machines) from type 2 plants (mainly converter-coupled plants). There are two variants for type 2 plants. With full dynamic grid support, the plant feeds in additional reactive current, depending on how deep the voltage dips. A gain factor k, with k = 2 recommended, determines how strong the response is. Put simply: for a dip of 25 percent of the nominal voltage, k = 2 requires additional reactive current of about 50 percent of the rated current. With limited dynamic grid support, the plant remains connected but feeds in neither reactive nor active current during the fault. The network operator specifies which variant applies and at which location the k factor must be met. Separate application rules with differing details apply to high and extra-high voltage.

More than a delayed disconnection

FRT is not simply an obligation to stay connected to the grid. In a severe fault, the voltage is often so low that no normal active power can be transmitted. Power results from voltage, current and phase angle. If the voltage drops sharply and the current remains limited, the active power inevitably drops as well. Inverters limit their current because their semiconductors can only tolerate certain maximum currents. Their control must therefore decide how to split the available current between the active and reactive components. Connection rules may require that reactive current has priority and that active power rises again after the fault in an orderly manner rather than abruptly.

FRT also does not override protection technology. If a fault lasts too long or is too deep, i.e. the voltage profile lies below the curve, the plant must disconnect. It is a matter of coordination: the plant should not trip at every short disturbance, but it also must not blindly remain connected in dangerous conditions. The RfG therefore requires protection settings to be coordinated between the network operator and the plant operator. The curves of the VDE rules act directly on the settings of the decoupling protection. If an operator deactivates dynamic grid support, they must adjust these protection settings. FRT is thus an interplay of plant control, protection settings, connection conditions, protection concept and grid operation.

FRT must be clearly distinguished from the detection of an unintended island grid. If a plant were to unintentionally continue supplying a separated grid section, it must disconnect. There, voltage and frequency would not be reliably controlled, and work on the grid would become dangerous. FRT, by contrast, concerns faults in the interconnected grid that continues to exist, where the plant is to remain connected in a controlled manner. Black-start capability, frequency support and inertial response are likewise separate requirements, even though they are technically related to modern converter technology.

Significance for a renewable power system

In the past, large synchronous generators shaped the behaviour of the grid during faults. They deliver high fault currents during short circuits, and their behaviour has been taken into account in protection concepts for decades. Wind, PV and battery plants, by contrast, are usually connected via power electronics. How they behave during a fault is determined by control algorithms, measurements, parameter settings and the thermal capacity of the semiconductors. Without clear requirements, many plants could disconnect almost simultaneously during a voltage dip. A locally limited short circuit, which the grid protection actually clears in a targeted way, would then turn into a large loss of generation that can trigger further protection functions. FRT does not prevent the original fault but limits its consequences and is thus one building block of grid stability. It matters most when there is high simultaneous feed-in from wind and solar.

Under the RfG, only plants of type B and above must be FRT-capable. Type A plants, i.e. small plants, are exempt. However, national connection rules may contain their own grid support requirements for smaller plants. For larger plants, proofs are added: unit and plant certificates, simulation models, stability assessments and tests during commissioning. FRT is therefore also an institutional concept. It links physical behaviour with formal requirements from the network codes and clear responsibilities.

Legal status and outlook

As of November 2026, Regulation (EU) 2016/631 is authoritative in the EU, implemented through national rules such as the VDE application rules. On 8 July 2026, the European Commission published the draft of a revised version (RfG 2.0). The public consultation ran until 25 August 2026. Among other things, the draft is intended to explicitly include storage systems and other converter-based plants and to lay down requirements for grid-forming behaviour. For planning purposes, the following therefore applies: until adoption and national implementation, the existing rules remain binding. New requirements for grid-forming converters may, however, change the fault behaviour of future plants.

Typical misunderstandings and distinctions

  • FRT does not make disturbances consequence-free. The capability reduces the risk of cascading disconnections but replaces neither grid reinforcement nor coordinated protection nor sufficient reserves. In weak grids or with protection devices that are not designed for the fault current behaviour of converters, these tasks remain.

  • FRT does not mean feeding in active power without interruption. In deep dips, reactive current is often what is needed or, with limited grid support, no current at all. Active power is only rebuilt after the fault has been cleared.

  • Converters are not unsuitable in principle. Their behaviour can be shaped more strongly through control than that of synchronous machines. Problems can arise when many devices are set identically, when measurements become uncertain during a fault, or when controls influence one another. What is decisive is which grid functions they provide demonstrably and with what quality.

  • Distinction from voltage control: Voltage control concerns ongoing operation within the permissible voltage bands. FRT concerns short exceptional disturbances and the period immediately afterwards.

  • Distinction from frequency control: Frequency problems arise from an imbalance between generation and consumption in the entire synchronous area. Voltage dips are more local in character and depend on grid impedances and short-circuit power. A plant can support the frequency well and still have weak FRT properties, and vice versa.

  • Distinction from security of supply: FRT is only one technical aspect of security of supply. It shows, however, how strongly a reliable supply depends on events that last only fractions of a second.

FRT is thus not a comfort function but a minimum requirement for the behaviour of a plant in the interconnected grid. A plant is not system-compatible merely because it generates energy. In the event of faults it must also react in such a way that a limited grid fault remains limited.

Sources

Related glossary entries

Edited by eCMS on 11.11.2026 19:47

© 2026 Erik Schmidt. All rights reserved. Quotations with a link to the source are welcome.