Short Definition
A feed-in limitation defines the maximum active power that an energy system may export to the public grid at the grid connection point. Zero export is the special case in which the feed-in limit is 0 kW. The generated energy must then be consumed on site, stored in a battery energy storage system (BESS), or curtailed if no capacity is available to absorb it.
Feed-in Limitation and Zero Export at a Glance
- The decisive factor is the energy flow at the grid connection point, not merely the current output of the PV system.
- An energy meter continuously measures grid import and grid export. Based on these measurements, an EMS adjusts the setpoints of controllable assets.
- Surplus PV power can initially be absorbed by loads or used to charge a BESS. At the same time, BESS discharge must be limited if it would cause the feed-in limit to be exceeded.
- Measurement intervals, communication delays, and response times can cause brief limit violations. For this reason, zero-export control often maintains a small amount of grid import as a safety buffer.
How Does Feed-in Limitation Work?
Unlike a static limitation of inverter output, dynamic feed-in limitation continuously considers current site consumption, additional generators, and the charging and discharging behaviour of a BESS. This allows generation output to be adjusted to the site’s current operating conditions while remaining within the permitted feed-in limit.
The control system is based on a bidirectional energy meter installed at the grid connection point. It continuously measures whether the site is currently importing power from or exporting power to the grid. The EMS compares this measurement with the permitted feed-in limit and calculates the required control action.
If grid export remains below the limit, the PV system can continue generating within its operating limits. As grid export approaches the limit, the EMS distributes setpoints among the flexible assets. Additional energy can be used to charge a BESS or absorbed by controllable loads. In doing so, the EMS considers factors including load power, maximum charging and discharging power, state of charge, and device availability. If the combined output of the PV system, BESS, and other generators would exceed the feed-in limit, BESS discharge must be limited or suspended. If insufficient flexibility remains available, the EMS reduces the active power output of the PV inverters.
For zero export, the limit is set to 0 kW. In practice, the control system is often configured to maintain a small amount of grid import. This safety buffer helps reduce short-term export peaks caused by sudden load changes or delayed inverter responses. The required size of the buffer depends on the dynamics of the site and the control system.
Where Are Feed-in Limits Used?
- Grid connections with limited export capacity: A PV or hybrid system may export power to the public grid only up to a specified limit.
- PV systems designed for self-consumption: If grid export is not intended or is contractually prohibited, the control system coordinates generation, consumption, and storage at the site.
- Sites with PV and BESS: The BESS temporarily absorbs surplus energy before curtailment of the PV system becomes necessary. BESS discharge is also incorporated into the feed-in control strategy.
- Hybrid sites with multiple generators: For grid-connected operation, a shared feed-in limit may apply at the grid connection point.
Benefits, Limitations, and Technical Requirements
Dynamic feed-in limitation enables a generating system to operate within the permitted grid connection conditions while maximising the amount of energy used on site. A BESS or flexible loads can reduce curtailment losses. However, the actual benefit depends on the load profile, generation profile, storage capacity, operating limits, and available flexibility.
Reliable control requires sufficiently fast and accurate measurement at the grid connection point, stable communication connections, and controllable interfaces for the inverters and other assets. The control cycle, setpoint transmission, and asset response times must also be coordinated. A project-specific fallback strategy is required for measurement errors, communication failures, or device outages. Depending on the applicable requirements, this strategy may include a substantial power reduction up to the complete curtailment of controllable generators, as well as the suspension of BESS discharge.
Even with technically implemented zero-export control, rapid load changes can cause brief control deviations. Whether such deviations are permitted and how compliance must be demonstrated depends on the project-specific grid connection conditions and the requirements of the grid operator. Requirements relating to protection concepts and certifications must also be assessed separately.
How Does EcoPhi Implement Feed-in Limitation and Zero Export?
EcoPhi can measure power at the grid connection point and use this information to derive dynamic setpoints for PV inverters and BESS. Flexible loads can also be incorporated, provided that suitable controllable interfaces are available and integrated into the respective project. The control system can account for available charging and discharging power, state of charge, load power, device availability, and a defined safety buffer. Measurements, setpoints, and control deviations can be monitored and analysed historically. Depending on the available data and project configuration, curtailment losses can be calculated.
The specific implementation depends on the available device interfaces, response times, control architecture, and project requirements. Depending on the system design, EcoPhi can perform local operational control or transmit setpoints and enable signals to other local controllers. However, the EMS is not automatically equivalent to an EZA controller—a power-generating plant controller certified for the respective grid connection.
Brief Summary
Feed-in limitation and zero-export control regulate the power exported to the public grid based on the combined balance of all generators, storage systems, and loads at the grid connection point. Measurement, setpoint distribution, safety buffers, and fallback behaviour must be coordinated for the specific project.
Frequently Asked Questions About Feed-in Limitation
What is the difference between feed-in limitation and zero export?
With feed-in limitation, grid export is permitted up to a specified limit. With zero export, this limit is 0 kW.
Why is a small amount of grid import often maintained during zero-export operation?
A project-specific grid import setpoint can serve as a safety buffer and reduce brief export peaks caused by sudden load changes and system response times.
Can a BESS completely prevent PV curtailment?
Only if sufficient charging power and available storage capacity are present. If the BESS is full, has reached its charging limit, or is unavailable, the EMS must use other sources of flexibility or reduce generation output.
Is an EMS used for zero export automatically an EZA controller?
No. An EMS can technically implement local control, but this does not mean that it automatically fulfils all project-specific requirements for an EZA controller. The required functions, documentation, compliance evidence, and certifications must be assessed for the respective grid connection.