Zero Export in Solar: How Export Control Works

Short answer

Zero export, or export control, makes a solar plant deliver to the facility only the energy it consumes and nothing to the utility grid. A meter at the point of connection reports the power flowing toward the grid; a controller, or the inverter itself, reduces production when that flow changes sign or approaches the limit. Like any control loop, it responds with a delay, so it is complemented by a protection that disconnects if the control falls short. It is the typical mode of a self-generator that has no permission, or no interest, to deliver surplus energy.

When it is used

• Self-generation without surplus export. The approval process is lighter than with export, but it does not disappear: the self-generator still needs the utility’s approval.
• Limited grid capacity. If the feeder cannot accept more injection, limiting export allows installing more peak power than the grid would receive.
• Surplus with little value. When what is paid for delivered energy is less than the cost of limiting it, some users prefer to use only what they consume.
• Solar with a diesel generator. There the limit is set not by the utility but by the engine, which does not tolerate reverse power. This is the case of solar with a diesel generator.

What utilities and codes typically ask

Requirements vary by country and by utility. The design and settings are adapted to the local code and to the utility requirements the client provides. In general terms:

• A self-generator that does not export is usually not subject to the same capacity limits or the same simplified connection study as one that does, but it still needs the utility’s approval.
• The application asks for the manual of the device that controls non-injection into the grid, or injection at a fixed power or energy level. If the inverter has that function, the inverter manual is provided. If the system does not deliver energy to the grid, the document is necessary.
• A self-generator that does not export is not usually required to change its metering conditions; one that does export must meet the generation boundary metering requirements. See bidirectional meter for self-generation.
• Protection requirements for the point of connection often include an over-power (forward power) function, ANSI 32, located at the point of connection. It applies to self-generators with and without export. The measurement must be taken at the point of connection or somewhere else at the same voltage level. The limitation must be implemented in the metering, control or protection systems. And the function must be able to limit the power delivered to the grid or disconnect the self-generator, as agreed with the utility. It is not required if the utility authorized delivery of the full installed capacity. Taking the measurement on the low side of a two-winding transformer is sometimes accepted, under conditions and with the utility’s validation.

Response time and flow tolerance are not fixed by a general rule. Those values are agreed with the utility and must be recorded in writing in the application.

How it is achieved

The principle is the same in all three: the meter gives the feedback, the controller calculates the setpoint and the inverter executes it. A practical detail is the meter location: it must see everything that leaves through the point of connection, including another generator if there is one, and take power with the correct sign. A reversed current transformer makes the control reduce production by mistake or fail to act.

Response times and why some export always occurs

The total time adds up several delays: the measurement interval, the communication between meter and controller, the inverter’s reaction and the ramp at which power drops. Manufacturers acknowledge this. SMA states in the documentation of its Sunny Home Manager that, due to the control cycles, a residual amount of energy may inevitably flow to the grid. In the manual of its Data Manager M it states that it is possible to limit injection to 0% within a few seconds. For a 0% limit, the active power gradient must also be adjusted. A negative margin can also be set on the setpoint to absorb load jumps. The specific times depend on the manufacturer, the model and the configuration, and must be taken from the document of the installed equipment.

Two consequences:

1. A brief export is normal when the load drops suddenly. The design must agree with the utility on whether it is tolerated and what value and duration it allows.
2. The protection must wait for the control. If function 32 trips before the controller has time to reduce, the plant disconnects with every load jump. Its setting is coordinated with the real loop time, measured in tests.

On loss of communication, the equipment must go to a safe state. SMA documents that, if communication with the manager fails, the inverter reduces to 0 W in the fallback function. Always verify that behavior on the equipment of your project.

Illustrative numerical example

Assumed values, not from a real project. A factory has a minimum daytime load of 35 kW and a plant with a 50 kW AC inverter. At one moment the irradiance would allow 48 kW, and the load is 35 kW.

• Power the inverter can deliver without exporting: 35 kW, minus a setpoint margin of 1 kW, that is 34 kW.
• Curtailed power: 48 − 34 = 14 kW, or 29% of what is available at that instant.
• If the load drops 20 kW suddenly and the loop takes 3 s to reduce, the exported energy is 20 kW × 3 s ÷ 3600 = 0.017 kWh.

Conclusion: zero export curtails production whenever the load is lower than the possible generation, and the export caused by the delay is small in energy.

Limitations

• Power is curtailed, not stored. Without batteries, unconsumed energy is lost. With storage the picture changes; see hybrid solar system with batteries.
• Size is limited by the daytime load. What is neither consumed nor stored is not used; see sizing a solar plant from demand.
• It depends on communication. A cable or a meter port that goes down leaves the control without feedback.
• It is not a protection. The protection is function 32 at the point of connection.

Relationship with anti-islanding

They are different functions. Zero export controls how much power leaves in normal operation; anti-islanding protection disconnects the plant when the utility grid opens. That the plant does not export does not release it from anti-islanding. Where utility requirements list anti-islanding methods for inverter-based generation, reverse power is typically not allowed as a standalone function; it is valid only within a hybrid logic. In other words, a 32 relay set for zero export does not work, by itself, as an anti-islanding method.

Common mistakes

• Placing the meter at a point that does not see all the current leaving, or with a reversed current transformer.
• Setting function 32 with a time shorter than the control loop and causing trips with every load jump.
• Not agreeing with the utility on the value and duration of export tolerated.
• Forgetting that a change in the installation, such as a new load or an additional generator, can leave the meter at a point that no longer serves its purpose; in that case a new measurement point is needed.

What to request or verify

1. The manual of the non-injection function of the inverter or controller, which utilities ask for in the application.
2. The point and type of measurement the utility will accept for function 32, and the agreed setting.
3. The communication scheme between meter, controller and inverters and its behavior on failure.
4. An on-site test that simulates load steps and loss of the meter, with the response time recorded. See on-site inverter testing and solar plant PPC testing.
5. The test of function 32 by secondary injection and its coordination with the control. See reverse power 32.

How we help

We define the limitation and protection scheme of your plant, prepare the information for the utility and test the whole system on site, in Colombia or elsewhere on request; the design is done remotely, worldwide. See our services for solar plant testing and commissioning and solar plant engineering. Write to us through contact with your single-line diagram and the inverter data.

Sources

• SMA, Sunny Home Manager 2.0 manual and Data Manager M manual (active injection limitation to 0%).

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