PV inverter — grid-support functions: reactive power control and frequency-based power reduction (LFSM-O)

The guide on anti-islanding covers how a PV inverter reacts when the grid drops out completely, and the guide on the RCMU covers the inverter's internal earth-fault detection. This article covers a third mechanism, unrelated to those two: the grid-support functions that are active as long as the inverter is simply connected to a functioning grid, which NEN-EN 50549 requires in order to prevent a large number of simultaneously feeding-in PV inverters from degrading local grid quality themselves.

Why an inverter has to do more than just deliver power

A low-voltage grid was originally designed for one-way traffic: power flows from the substation to the connection. With a high density of PV feed-in on a weak or long cable connection, the local voltage can actually rise during sunny hours instead of dropping, risking a breach of the permitted voltage band. In addition, a large, simultaneous power surplus on the grid can temporarily push grid frequency above the nominal 50 Hz. NEN-EN 50549 therefore requires that an inverter — beyond simply generating power — actively contributes to keeping both local voltage and grid frequency stable.

Reactive power control: Q(U) or cosφ(P)

To counteract local voltage rise, an inverter above a certain power threshold must be able to control its reactive power according to one of two characteristics specified by the grid operator:

Q(U) control: the inverter measures the local voltage and automatically absorbs more reactive power (inductive behaviour) as the voltage rises further above nominal, following a pre-set droop characteristic between voltage and reactive power.
Fixed cosφ(P) characteristic: the target power factor of the inverter varies with the delivered active power — close to cosφ = 1 at low power, dropping to a lower, pre-set power factor as power approaches the maximum.

Which of the two methods applies, and with which parameters, is set by the grid operator at the connection — this is not a free choice of the installer or the system owner.

Frequency-based power reduction: LFSM-O

LFSM-O (Limited Frequency Sensitive Mode – Overfrequency) is the function that requires an inverter to automatically reduce its active power as soon as grid frequency exceeds a set threshold — in the European grid code, the standard threshold is 50.2 Hz. Above that threshold the inverter reduces its power output according to a set droop as frequency rises further, and increases it again once frequency drops back below the threshold. This mechanism prevents a large share of distributed generation from pushing an already elevated grid frequency even higher, which without this function could lead to a large-scale, coordinated shutdown of generating capacity.

Note: LFSM-O is functionally different from anti-islanding. Anti-islanding shuts the inverter off entirely when the grid drops out or exceeds an absolute limit. LFSM-O ramps power down gradually, within the inverter's normal operating range, without the inverter disconnecting from the grid — it stays connected and active, just delivering less power.

Why factory defaults do not automatically comply

An inverter shipped with factory-default parameters for another country, or a generic NEN-EN 50549 profile, does not automatically comply with the specific settings the Dutch grid operator requires for a given connection. The frequency thresholds, the reactive power characteristic (Q(U) versus cosφ(P)), and the associated droop parameters must be explicitly parameterized at commissioning to match the profile appropriate to the voltage level and connection type (Type B under the European RfG classification, for most small-scale PV installations).

Practical relevance

When commissioning or inspecting a PV installation with an inverter above the threshold at which grid-support functions become mandatory, it is important to verify that the inverter has actually been parameterized according to the profile specified by the grid operator — not merely that the inverter is "NEN-EN 50549 compatible" per the nameplate, since compatibility says nothing about the actually active parameter settings.

Common mistakes

Assuming NEN-EN 50549 compatibility of the inverter is sufficient, without checking whether the grid-support functions have actually been activated and correctly parameterized at commissioning.
Leaving the inverter on factory defaults tuned for another country or a generic profile, instead of the specific profile the Dutch grid operator prescribes for the connection.
Confusing LFSM-O with anti-islanding — the former gradually ramps power down within the normal operating range, the latter shuts the inverter off entirely.
Not checking which reactive power characteristic (Q(U) or cosφ(P)) applies to the specific connection, and leaving the inverter on an arbitrary or default setting.