A typical photovoltaic power generation system converts direct current (DC) into alternating current (AC). When the power generated by the photovoltaic system exceeds the local load demand, the additional electricity is fed into the electrical grid. The adoption of a zero-grid solution is motivated by this need, as it aims to control the injection of excess energy and thus avoid reverse 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 electricity grid. In some cases, the injection of surplus 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 all refer to this same method of controlling power injection.
Generation and consumption in a photovoltaic system.
To regulate the amount of energy a photovoltaic system can send to the grid, it is essential to recognize that such a system is interconnected with the electrical infrastructure.
Therefore, it makes no sense to discuss controlling the export of electricity to the grid in the context of an off-grid photovoltaic system, which operates disconnected from the grid.
Therefore, any installation that incorporates zero-grid control should be considered as connected to the electrical grid. This concept can be clearly seen in the image below.
For a few hours on a single day, energy is generated from the photovoltaic system, represented by the gray area under the dashed curve. The dark orange area refers to self-consumption, which corresponds to the energy generated by the photovoltaic system and consumed instantaneously by the loads of the consumer unit.
The curve outside the dotted line also indicates the energy consumed instantaneously by the loads, but this energy comes from the electrical grid, since consumption occurs at times when the photovoltaic system is not producing energy.
Thus, the energy not consumed by the loads will be injected into the utility grid. When the utility does not support or does not allow the injection of this surplus energy, the photovoltaic system must limit the export of current to the grid.
How does the zero-export function work?
For example, Auxsol inverters feature an anti-backflow function in all on-grid and hybrid models, both single-phase and three-phase. In addition to this function, Auxsol inverters also allow control over the percentage of power that can be injected into the grid.
It is important to emphasize that this functionality is available internally and, with the appropriate external equipment, it is possible to take full advantage of it.
Therefore, to limit the excess power generated by the photovoltaic system, the inverter needs to identify if there is current flowing to the electrical grid. This is made possible by means of current transformers (CTs). The CT is connected to the AC cabling between the inverter and the electrical grid, before the utility meter.
However, the CT cannot measure the current; it only identifies the presence of current. Therefore, it is necessary to use an energy meter, the smart meter, which reads the data from the CT and sends it to the inverter. With this function enabled, the inverter can regulate its injection. It is worth noting that some Auxsol inverter models already have this meter internally, requiring only a CT to be connected directly to the photovoltaic inverter.
Upon detecting current flowing into the grid, the inverter rapidly 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, initiating maximum power point tracking (MPPT).
The operation is then transferred to a mode that controls the output power, matching it to the level of the local loads and thus fulfilling 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. Furthermore, it helps 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, it creates surplus energy. If this energy is not stored in batteries, it will be injected into the distributor's electrical grid. For various reasons, this injection can be harmful to the grid and cause problems such as:
- Network Overload: Equipment such as transformers and circuit breakers can be overloaded, leading to failures and interruptions in the power supply.
- Voltage Imbalance: Energy injection can cause voltage fluctuations, affecting the quality of the electrical supply.
- Interruptions: Overload and imbalance can result in power outages, impacting consumers and equipment.
It is important to emphasize that the Zero Grid 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 electrical grid. This configuration is especially advantageous in locations 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 utilities. For this reason, sectors such as industry and agribusiness stand out as major users of zero-grid systems, controlling injection up to a limit or restricting it to zero.
On-grid System Approval with Zero Export
Photovoltaic systems operating in zero-grid mode are not exempt from approval by electricity utilities, as these systems require the reference provided by the grid to maintain synchronization and must meet the requirements of the anti-islanding function.
Furthermore, the regulation stipulates that the distributor may interrupt the power supply if the generation installation occurs without authorization, especially if the continuous paralleling between the grid and the consumer's generator causes technical problems or risks to the safety of the grid and other consumers.
The homologation 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 network. All these projects must be submitted to the distributor for analysis and approval.
The guidelines, requirements, and instructions necessary for connecting a photovoltaic unit to the electrical system are available in the documentation prepared by each utility company.
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 performed: when switching off the circuit breaker at the point of entry, 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 consult directly with the distributor responsible for supplying electricity to the consumer unit and request a clear and objective answer, based on their technical standards and regulations. One should not fully trust information disseminated through social media or from sellers of photovoltaic systems without adequate verification.
The opinions and information expressed are the sole responsibility of the author and do not necessarily represent the official position of the author. Canal Solar.
