Written by Youssef Barreiros
A typical photovoltaic power generation system converts direct current (DC) into alternating current (AC). When the power generated by the photovoltaic system exceeds the demand of the local load, the additional electricity is directed to the power grid. The adoption of the zero grid solution is motivated by this need, as it aims to control the injection of excess energy and thus avoid the inversion of flow in the utility grid.
What does Zero Grid mean?
The concept of zero grid refers to a methodology for managing the export of energy to the power grid. In some cases, the injection of excess energy is reduced to zero, while in others it can reach a certain value. It is important to emphasize that the terms zero grid, zero export and grid zero refer to this same method of controlling power injection.
FIGURE 01 - Hybrid Solar Energy System with Exported Current
Generation and consumption in the photovoltaic system
To regulate the amount of energy that a photovoltaic system can send to the grid, it is essential to recognize that such a system is interconnected to the electrical infrastructure.
Therefore, it makes no sense to discuss the control of electricity export to the grid in the context of an off-grid photovoltaic system, which operates disconnected from it.
Therefore, any installation that incorporates zero grid control must be considered as connected to the electrical grid. This concept can be clearly seen in the image below.
FIGURE 02 - Energy balance of a consumer unit
During a few hours in a single day, energy is generated from the photovoltaic system, represented by the gray area under the dashed curve. The dark area refers to self-consumption, which corresponds to the energy generated by the photovoltaic system and consumed instantly by the loads of the consumer unit.
The curve outside the dotted line also indicates the energy consumed instantly by the loads, but this energy comes from the electrical grid, since consumption occurs at times when the photovoltaic system is not producing energy.
Therefore, the energy not consumed by the loads will be injected into the grid of the concessionaire. When the concessionaire does not support or does not allow the injection of this excess energy, the photovoltaic system must limit the export of current to the grid.
How does the zero-export function work?
To explain, there are Nansen Solar inverters, which have the anti-backflow function in all on-grid and hybrid models, both single-phase and three-phase. In addition to enabling this function, Nansen Solar inverters also allow control of the percentage of power that can be injected into the grid. It is important to note that this functionality is available internally and, with the appropriate external equipment, it is possible to take full advantage of it.
FIGURE 03 - Nansen Solar App configuration screen
In this way, to limit the excess power generated by the photovoltaic system, the inverter needs to identify whether there is current flowing into the electrical grid. This is made possible by means of current transformers (CT). The CT is connected to the AC wiring between the inverter and the electrical grid, before the utility meter.
FIGURE 04 - Connection diagram of zero export
However, the CT cannot measure the current; it only identifies the presence of current. Therefore, it is necessary to rely on the help of an energy meter, the smart meter, which reads the data from the CT and sends it to the inverter. With the function enabled, the inverter can regulate its injection. It is worth mentioning that some models of Nansen Solar inverters already have this meter internally, and it is only necessary to connect a CT directly to the photovoltaic inverter.
FIGURE 05 – Connection between CT and Smart Meter
When it detects current flowing into the grid, the inverter quickly adjusts its output until it becomes zero, ensuring zero-power access. When the smart meter identifies the electrical flow, it sends this information to the inverter, which immediately changes its operating mode, starting maximum power point tracking (MPPT). The operation is then transferred to a mode that controls the output power, equalizing it to the level of the local loads and thus implementing its anti-backflow function.
Adopting zero-export solutions is simpler than it seems, and choosing a solar inverter with this function enabled is essential to avoid installation problems. In addition, this helps to avoid the costs associated with installing energy storage systems.
Problems Caused by Flow Reversal
When a photovoltaic system generates more energy than the consumer unit needs, excess energy is created. If this energy is not stored in batteries, it will be injected into the distributor's electrical grid. For several reasons, this injection can be harmful to the grid and cause problems, such as:
Grid Overload: Equipment such as transformers and circuit breakers can be overloaded, leading to failures and interruptions in the power supply.
Voltage Imbalance: Power injection can cause voltage fluctuations, affecting the quality of the power supply.
Outages: Overload and imbalance can result in power outages, impacting consumers and equipment.
It is important to note that the grid zero system can be used in conjunction with an on-grid system. This integration makes it possible to maximize the potential for energy generation without sending this energy to the power grid. This configuration is especially advantageous in places where the generation of energy credits is not permitted and is widely adopted in projects with high energy consumption.
In this way, it becomes a solution to meet energy demand without the need for modifications to existing contracts with electricity providers. For this reason, sectors such as industry and agribusiness stand out as major users of grid zero systems, controlling the injection up to a limit or restricting it to zero.
On-grid System Approval with Zero Export
Photovoltaic systems that operate in zero grid mode are not exempt from approval by electricity concessionaires, as these systems need the reference provided by the grid to maintain synchronization and must meet the requirements of the anti-islanding function.
In addition, the regulation provides that the distributor may interrupt the energy supply if the generation installation occurs without authorization, especially if the continuous parallelism between the grid and the consumer's generator causes technical problems or risks to the safety of the grid and other consumers.
The approval requirement applies not only to on-grid photovoltaic systems, but to any generating unit belonging to the consumer that operates in parallel with the distribution grid. All these projects must be submitted for analysis and approval by the distributor.
The guidelines, requirements and instructions necessary for connecting the photovoltaic unit to the electrical system are available in the documentation prepared by each concessionaire.
For those who still have doubts or insist on the idea that a photovoltaic system operating in zero grid mode does not interact with the distributor's grid, a simple practical test can be carried out: when turning off the input point circuit breaker, if the photovoltaic inverter is deactivated, this indicates that it was working in conjunction with the grid and activated its anti-islanding function.
In case of uncertainty, it is always advisable to directly consult the distributor responsible for supplying electricity to the consumer unit and request a clear and objective response, based on its technical standards and regulations. Information disseminated by social networks or by photovoltaic system sellers without adequate proof should not be fully trusted.
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