Grid-connected Mode · Glossary

Grid-connected mode refers to the operation of a power generation plant, a storage system, an inverter or a microgrid in electrical connection with the public electricity grid. In German this is usually called parallel grid operation (Netzparallelbetrieb). The plant is not disconnected from the grid but connected to a higher-level grid. Its voltage, frequency and protection requirements largely determine local operation. In the technical literature on microgrids, grid-connected mode is the counterpart to island mode: a microgrid can connect to the grid or disconnect from it and then operate in one mode or the other.

In grid-connected mode, the public grid is the electrical reference. Inverters, generators and loads align with the grid frequency, nominally 50 hertz in the European interconnected grid, and with the specified voltage. A photovoltaic inverter, a battery storage system or a combined heat and power unit then feeds in synchronously with the grid or draws electricity from it. An illustrative image: the large grid acts like a very heavy flywheel to which small plants couple. They can supply energy locally, serve loads and feed in surpluses. Normally, however, they do not have to keep the voltage and frequency of a closed power system stable on their own.

The term therefore does not describe a specific technology but an operating state. Depending on its design and protection concept, the same plant can operate in grid-connected mode, in island mode or with switchover between the two. In parallel grid operation, a battery storage system can shave load peaks, increase self-consumption or, after appropriate prequalification, provide balancing energy. In a grid outage, the same storage system continues to supply power only if it is designed for island operation, has a suitable transfer switching device and the local circuits are safely disconnected from the public grid.

Distinction from island mode and emergency power operation

The most important neighbouring term is island mode. Here a local grid is separated from the public grid and must establish its own stability. This requires at least one unit that can operate in grid-forming mode, for example a synchronous generator or a grid-forming battery inverter. This unit must set voltage and frequency, balance load changes and supply enough fault current or suitable protection signals so that the protective devices trip. In grid-connected mode, the public grid generally takes over this reference function.

Emergency power operation is not automatically the same as island mode, but is often related to it. An uninterruptible power supply, a diesel generator set or a battery system can supply selected loads in a grid outage. The technical design determines whether this supply forms a true island grid, serves only individual circuits or shuts down in an orderly way after a short bridging period. Grid-connected mode only says that, in the state described, a connection to the public grid exists.

Self-consumption must also be distinguished. In grid-connected mode, a photovoltaic system can consume part of its electricity directly in the building and feed in the rest. This does not make the building independent of the grid. It still needs the grid when its own generation does not match the load, when the inverter requires a grid reference or when protection rules require disconnection in the event of grid faults. Even a plant with so-called zero feed-in is technically regarded as operated in parallel with the grid, because it works synchronously with the grid even if it delivers no energy to the public grid.

Technical function in parallel grid operation

In parallel grid operation, a plant must work in an electrically compatible way with the grid. This includes synchronisation, grid and plant protection (NA protection), defined behaviour in the event of voltage and frequency deviations, limited harmonics, the provision of reactive power and defined responses to grid disturbances. In small plants, it is mainly the inverter that implements these functions in accordance with the connection rules. In larger plants, plant certificates, higher-level decoupling protection and agreements with the grid operator are added.

Many grid-connected inverters operate in grid-following mode. They measure the voltage and frequency of the grid and feed in their current to match. If the grid reference is lost, they must not simply continue feeding in, otherwise an unintended island grid could arise. The protection against this is called anti-islanding or island grid detection and is central to grid safety and occupational safety: lines that are considered de-energised during a fault or maintenance must not be unintentionally kept live by distributed plants. The US Department of Energy describes the same principle: a photovoltaic system switches off on loss of the grid for safety reasons so that no electricity is fed back into the public grid.

Grid-forming inverters behave differently. They actively set voltage and frequency and are therefore particularly important for island grids, microgrids and some forms of system support. They too can run in grid-connected mode, but then coordinated with the higher-level grid. Whether a microgrid has grid-following or grid-forming inverters is often more important for its assessment than the mere question of whether storage is present. It should also be noted that converters usually supply only a limited fault current in a short circuit. Protection settings designed for the high short-circuit power of the public grid therefore do not readily work in island operation.

Why grid-connected mode matters for microgrids and storage

According to the widespread definition of the US Department of Energy, a microgrid is described as a group of interconnected loads and distributed energy resources that acts as a single controllable entity with respect to the grid. In practice, many microgrids run in grid-connected mode most of the time. They use local generation, storage and controllable loads while remaining connected to the distribution grid. In this way they can draw electricity, feed in surpluses, support voltage maintenance or limit load peaks at the grid connection point. Switching to island operation is usually limited to grid disturbances or planned special cases.

This changes the technical and economic assessment. A grid-connected microgrid does not have to cover every conceivable load on its own at all times. It can use the capacity of the public grid and deploy local flexibility where it pays off economically or operationally. This entails connection costs, grid charges, metering requirements, billing issues and obligations towards the grid operator. The grid connection point thus becomes the decisive interface between local optimisation and the public power system.

This is particularly evident with battery storage systems. A storage system in grid-connected mode can absorb electricity when the photovoltaic system generates more than the building consumes, and release it again later. It can reduce the maximum power drawn, increase self-consumption or respond to price signals. However, this does not automatically provide supply during a grid outage. For backup power, switchover, grid disconnection, protection, earthing, inrush currents and the supply of critical loads must be specifically planned. A hybrid inverter can also do this only if it explicitly supports backup power or island operation.

Connection rules in Germany (as of November 2026)

Grid-connected mode is not a purely technical term. As soon as a plant runs in parallel with the public grid, connection rules and operating requirements apply. In Germany, VDE-AR-N 4105 is authoritative for generation plants and storage systems on the low-voltage grid. Its current edition 2026-03 (TAR EZA NS) supplements the general low-voltage connection rule VDE-AR-N 4100. It applies to all generation plants and storage systems operated in parallel with the grid operator's grid, including those that do not feed in. It regulates, among other things, grid connection, NA protection, billing metering, operation and proofs of compliance. New features include Q(U) as the standard method for reactive power, extended system-supporting properties and simplified requirements for very small plants up to 800 VA.

For customer installations with a grid connection point in the medium-voltage grid, i.e. above 1 kV and below 60 kV, VDE-AR-N 4110 (TAR Mittelspannung) applies. Among other things, it implements requirements of the European network code Regulation (EU) 2016/631 for generation plants of types B and C. However, the allocation no longer strictly follows the voltage level. According to VDE-AR-N 4105:2026-03 and the draft E VDE-AR-N 4110:2024-11, the full medium-voltage requirements for generation plants and storage systems apply only above a cumulative installed capacity of 500 kW or an agreed feed-in capacity of 270 kW. Smaller plants, even at a higher voltage level, go through a simplified procedure. In addition there are supplementary technical connection conditions of the respective grid operator, metering concepts, registration and reporting obligations, and energy law requirements. For plants outside Germany, other national rules apply, which may be based on the same EU network codes.

These rules determine which business models are possible. A storage system can optimise self-consumption, reduce grid charges, participate in the electricity market or provide balancing energy. Not every combination is readily permissible or economical, because metering, balancing, grid charges, levies and taxes have different effects. A technical capability becomes a usable option only when connection conditions, billing and market roles fit.

Parallel grid operation is also important for grid operators. Many distributed plants change the power flows in the distribution grid. Photovoltaics can cause reverse feed-in at midday, heat pumps and charging points increase local loads, and storage systems can smooth or amplify load peaks, depending on how they are controlled. Grid-connected mode thus describes an operating reality in which distributed plants are part of the power system and directly influence its operation.

Common misconceptions

Grid connection does not mean dependence, and island operation does not mean self-sufficiency: a grid-connected system can use a great deal of its own generation and still remain reliant on the grid as a layer of stability and balancing. An island grid can be technically independent, but this requires sufficient generation, storage, control, protection technology and reserves. Independence is not a property of a single device but the result of a complete system design.
A storage system is not emergency power: the fact that a building has a battery storage system says little about whether it can keep running in a grid outage. Many home storage systems shut down without a grid reference or supply only a separate backup circuit. Other systems can form a three-phase island grid. The difference lies in the inverter, the transfer switching device, the protection concept and the sizing, not in the battery module alone.
The grid is more than a power source: in grid-connected mode, the public grid provides not only electricity when local supply falls short. It also provides the frequency reference, the voltage level, short-circuit power, the balancing of generation and consumption, and a structure for billing and responsibility. Anyone who wants to replace these functions in an island grid faces technical requirements and costs that are often missing from simple self-sufficiency calculations.

Grid-connected mode makes it visible that distributed energy plants act in two directions. They can reduce local costs, integrate renewable generation and provide flexibility. At the same time, they remain part of a grid whose stability, protection concepts and market rules depend on coordinated behaviour. The term therefore does not merely mean the state of being "connected", but the mode of operation at the interface between the local plant and the public power system.

Sources

VDE-AR-N 4105 Anwendungsregel:2026-03 – Anschluss und Betrieb von Erzeugungsanlagen am Niederspannungsnetz (TAR EZA NS) — VDE VERLAG / VDE FNN; 2026-03 (erschienen 2026-02-26). Produktbeschreibung: Anwendungsbereich und Änderungsvermerk
VDE-AR-N 4110:2018-11 – Technische Regeln für den Anschluss von Kundenanlagen an das Mittelspannungsnetz und deren Betrieb (TAR Mittelspannung) — VDE VERLAG / VDE FNN; 2018-11. Produktbeschreibung
E VDE-AR-N 4110 Anwendungsregel:2024-11 (Entwurf) – TAR Mittelspannung — VDE VERLAG / VDE FNN; 2024-11. Produktbeschreibung des Entwurfs
Microgrid Resource Center – What is a Microgrid? — Lawrence Berkeley National Laboratory; unbekannt. Startseite, Abschnitt „What is a Microgrid?“
Islanding a Microgrid — U.S. Department of Energy, Federal Energy Management Program; unbekannt. Erläuterung zur Animation

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Edited by eCMS on 12.11.2026 05:05