Frequency control P(f)

Last modified: 05 Sep 2025

Note

The content on this page refers only to the blue'Log XC combined with the Hybrid EMS licence.

About frequency control

All countries have a standard grid frequency, for example, 50 Hz in Europe, and it is essential to maintain the frequency to prevent blackouts and damage. The grid code of each country requires power-generating systems to apply frequency control during inadmissible system frequency changes. Depending on the requirements, this can be carried out by the inverters or a power plant controller such as the blue’Log XC.

The blue’Log XC allows you to regulate the frequency-dependent active power output in three different modes:

Each of the control modes can be individually configured for the PV system as well as for the battery system.

Prerequisites

Hybrid EMS license
Only the user group Service can make changes. See Users
Meter/power analyzer is compatible with blue'Log. See Compatibility Check
Meter/power analyzer fulfills the measurement requirements for frequency control
Example: Your grid code requires the frequency to be measured with a certain accuracy
For configuration assistance, we recommend our project-specific controller tuning services. For more information, see Grid code compliance engineering services under Downloads - meteocontrol GmbH and contact Sales.

Required measured values

In order to perform frequency control properly, measurements of active power and frequency are required. You can configure the source of both values.

Active power source

You can choose between a physical meter or the sum of inverters as the data source for the active power measurement. The active power is only used to determine the setpoint. The control itself is in open loop.

Recommendation

We recommend using a physical meter. The sum of inverters is a value calculated by the blue’Log. It is based on the active power values reported by the connected inverters, which may not always be synchronized, fully available, and as accurate as a physical meter.

Frequency source

As the source of frequency measurement, you can configure a meter. This frequency source is
used for both PV and battery frequency control.

Additionally, you can select Simulated test frequency for testing purposes. In this case, the blue’Log XC uses the specified simulated test frequency instead of the measured frequency of the meter. Set the simulated test frequency via the Modbus Power Control interface.

Limited frequency sensitive mode - underfrequency (LFSM-U)

Underfrequency occurs when the frequency of the electrical grid falls below the nominal operating frequency. A deficit of generation power is opposed to a surplus of load. To correct this, the active power output of the system is increased according to a characteristic curve P(f) until the grid frequency returns to the permissible range.

The following parameters can be configured:

	Parameter	Description
1	Frequency bands	You can configure up to five frequency bands. For each frequency band, you can set a dedicated droop and frequency threshold.
2	Reset frequency f_reset	Frequency above which LFSM-U is deactivated.
3	Frequency threshold f_thd	Frequency below which LFSM-U is activated.
4	Minimum system frequency f_min	Frequency below which the active power output is not further increased.
5	Droop s_f	Ratio of the steady-state change of frequency Δf to the resulting steady-state change in active power output ΔP.
6	Reference active power P_ref	Power to which the calculated active power change ΔP refers: Momentary active power P_mom Installed active power P_inst

By default, the LFSM-U setpoint has priority over the grid operator setpoint. You can change this behavior by activating the function Prioritization grid operator. In this case, the setpoint of the grid operator is monitored during underfrequency. If this setpoint is smaller than the calculated LFSM-U setpoint, it is prioritized.

Limited frequency sensitive mode - overfrequency (LFSM-O)

Overfrequency occurs when the frequency of the electrical grid exceeds the nominal operating frequency. To correct this, the active power output of the system is limited according to a characteristic curve P(f) until the system frequency returns to the permissible range.

You can configure the following parameters:

	Parameter	Description
1	Frequency bands	You can configure up to five frequency bands. For each frequency band, you can set a dedicated droop and frequency threshold.
2	Reset frequency f_reset	Frequency below which LFSM-O is deactivated.
3	Frequency threshold f_thd	Frequency above which LFSM-O is activated.
4	Minimum system frequency f_max	Frequency above which the active power output is not further decreased.
5	Droop s_f	Ratio of the steady-state change of frequency Δf to the resulting steady-state change in active power output ΔP.
6	Reference active power P_ref	Power to which the calculated active power change ΔP refers: Momentary active power P_mom Installed active power P_inst

By default, the LFSM-O setpoint has priority over the grid operator setpoint. You can change this behavior by activating the function Prioritization grid operator. In this case, the setpoint of the grid operator is monitored during overfrequency. If this setpoint is smaller than the calculated LFSM-U setpoint, it is prioritized.

Frequency sensitive mode (FSM)

In this mode, the active power is adjusted according to a defined P(f) curve to provide the ancillary service frequency containment reserves (FCR, also known as primary control reserves). Thus, FSM is only relevant if your system participates in the electricity balancing market. Compared to the LFSM-O and LFSM-U modes, FSM mode is usually active during smaller frequency deviations. In case of overlap, LFSM-O and LFSM-U have priority over FSM.

	Parameter	Description
1	Active power range \|ΔP₁/P_inst\|	Indicates the maximum amount by which the active power can be increased or reduced by the FSM function. If the active power was increased or reduced by \|ΔP₁/P_inst\|, no further active power adjustment takes place even if the grid frequency continues to decrease or increase. Exception: When LFSM-O/LFSM-U are activated these functions may further decrease/increase active power output.
2	Deadband Δf_DB	Is centered around the nominal frequency f_n. As long as the system frequency f remains within the frequency range (f_n - Δf_DB) < f < (f_n+Δf_DB), there is no frequency-dependent active power adjustment according to the P(f) characteristic curve.
3	Base value P_base	The power change ΔP resulting from the P(f) characteristic curve is added to the base value to obtain the resulting active power setpoint. Momentary active power P_mom (default) Remote power control setpoint P_RPC^* ^*The remote power control setpoint P_RPC is the setpoint transmitted via the Remote Power Control (RPC) interface for the respective system (PV or battery). This option is only available if the Remote Power Control (RPC) license is installed.
4	Droop s_f	Ratio of the steady-state change of frequency Δf to the resulting steady-state change in active power output ΔP.
5	Reference active power P_ref	Power to which the calculated active power change ΔP refers: Momentary active power P_mom Installed active power P_inst

Note

By default, the P(f) characteristic curve is set up symmetrically.

To configure the parameters (|ΔP₁/P_inst|,s_f and Δf_DB ) separately for underfrequency and overfrequency, activate the toggle Asymmetric characteristic curve.

FSM commands via Modbus

The energy trader can activate FSM and set the active power range via Modbus. For this, consider the following:

The Modbus Power Control license is required.
Refer to the data sheet for the relevant registers: Downloads - meteocontrol GmbH.
You must first activate the parameters via the blue’Log user interface: Power control > Active power > Frequency control > Active power adjustment P(f) - FSM > Activate FSM via Modbus toggle.

Set a ramp rate limiter (after frequency restoration)

If the frequency event ends and the frequency is restored, a ramp rate limiter can be applied to the transition to normal operation.

This function applies only after LFSM-U and LFSM-O events and cannot be set if FSM is activated.

Example

The power was reduced to 500 kW during the frequency event, but the plant can deliver 800 kW after the frequency event ends, and no power limitations are present. In this scenario, if the ramp rate limiter is activated after frequency restoration, the power does not immediately return to maximum power (800 kW) but has a configurable rate of change.

Calculation of the active power change

During frequency control, the blue’Log XC monitors the frequency and calculates the
active power change according to the following formula:

ΔP/P_ref = (f_thd - f_measured) / (f_n * s_f)

ΔP calculated absolute active power change (W)
P_ref active power reference (W)
f_thd frequency threshold from that on frequency control is activated (Hz)
f_measured measured system frequency (Hz)
f_n nominal system frequency (Hz)
s_f droop (%)

Note

In FSM mode, the frequency threshold f_thdis calculated by the sum of the nominal frequency f_nand the frequency deadband Δf_DB: f_thd = f_n+ Δ𝑓_𝐷𝐵

The calculated active power change is added to the base active power 𝑃_{𝑏𝑎𝑠𝑒} to obtain the final
setpoint during frequency control:

Note

For LFSM-U and LFSM-O, the base value P_base corresponds to the momentary active power P_mom when the frequency threshold is exceeded. For FSM, the base value is alternatively configurable as the remote power control setpoint from the energy trader.

The frequency-dependent power output (operating point) permanently moves up and down along the configured P(f) characteristic curve depending on the measured grid frequency (see figure below):

P(f) characteristic curve for LFSM-O with *f_thd*= 50 Hz and *s_f* = 5%.