Project rejection: is power flow inversion really a problem?

(..) The (…) identified inversion of power flow (…) due to the new connection or the increase in power injected from its microgeneration. Among the options (…), the only viable and lowest overall cost (…) is the injection (…) restricted to the hours of 7:00 pm to 5:00 am (every day).

If you have requested a connection quote in recent months in the states of Minas Gerais, São Paulo, Mato Grosso and Rio Grande do Sul, it is very likely that you have received such a communication limiting the injection of energy into the electrical distribution network. In some cases, distributors limit the injection to the period between 7pm and 5am, precisely at times when there is no sunlight.

Distributors in these states are issuing connection quotes imposing limitations on energy injection, on the grounds that this measure is necessary to overcome problems that may be caused by power flow reversal.

In their provisions, energy distributors rely on Article 73 of REN 1000, which states: “If the new connection or increase in injected power from microgeneration or distributed minigeneration implies inversion of the power flow at the distributor's transformation station or in the feeder circuit breaker, the distributor must carry out studies to identify viable options that eliminate such inversion…”.

But what is power flow inversion? And how can an integrator or end consumer overcome this situation and enable the installation of a photovoltaic system under these conditions?

What is power flow inversion?

Before explaining the inversion of power flow, it is first necessary to understand what power flow is.

Geraldo Silveira, electrical engineer from the Canal Solar engineering team, explains that power flow or charge flow are the paths taken by active and reactive power in all the elements that make up the electrical system.

Power flow determines the behavior of electrical quantities (voltages, currents and powers) throughout the circuits of the electrical system.

To find out whether variations in these quantities will not degrade the stability of the network, a study of the power flow is carried out. The power flow study aims to predict the behavior of a system's electrical variables and ensure that all parameters are within the supportability limits of the equipment that makes up the network.

“Basically, the aim is to predict, for sinusoidal permanent regime operation situations, that is, in normal operating situations, what the values of the electrical variables are at different points of the system under analysis”, he explained.

He also highlights that for this type of operation it is necessary to use specific electrical grid simulation software, based on mathematical formulations, which can predict these values based on system modeling.

“This modeling takes into account the generations, loads, transmission and distribution lines, transformers and other equipment that make up the electrical system”, he points out.

In a classic SIN (National Interconnected System) model, the natural direction of power flow occurs from large generating centers to consumer centers. In other words, the consumer units (CUs) were consumers only.

To make it clearer, look at Figure 1 below. Note that the flow goes from generation to load.

It is important to highlight that this example is just for a didactic explanation, in practice there are different sources of generations and different loads. In other words, the system, mainly distribution, operates predominantly radially.

In cases of flow reversal, what happens is that the concessionaire identifies at some points in its system that the flow, instead of going in the direction shown in Figure 1, goes in the opposite direction, as shown in Figure 2.

This means that the set of distributed generations in that region, in addition to meeting all local load demand, generates a surplus that is being sent to the network operated by the concessionaire.

That's the reverse flow, that is, the flow is in the opposite direction to the conventional one due to the large concentration of generation close to the load.

“In a simulation considering all these factors, the software can indicate that flow reversal is occurring at specific points in the electrical system”, comments the engineer.

By itself, power flow reversal is not a problem for the system. “The problem is when the reverse power flow causes a violation of the charging limits of the electrical network equipment or when the busbar voltage modules exceed the operational limits. In other words, reverse flow is a problem when it causes the electrical network in which it flows to go from an operational state without violations to one with violations and instability”, he explains.

To illustrate, the reversal of power flow can cause a violation of the voltage limits of a distribution system; these limits are pre-established in module 8 of Prodist. Maintaining voltage levels within pre-established limits is essential for system stability.

Silveira highlights that these problems are previously identified in power flow simulations in specific software. “And that’s the importance of software like this, as they can identify problems even before they occur,” he says.

Although REN 1000 points out that flow reversal, if it happens, the distributor must carry out studies to identify viable options that eliminate such reversal, technically, the simple flow reversal should not be used as a disapproval criterion, which therefore does not always such a reversal will impact network operation.

In his assessment, the detailed study must be presented showing this reversal and pointing out the places in the system that will suffer their operational limits being exceeded.

“This point is important to demonstrate transparency with the customer and thus maintain a good relationship between distributors and users”, he highlights.

According to Dirceu Ferreira, an electrical engineer who worked at CPFL Energia for more than 33 years in the area of Electrical Distribution System Planning, Art. 73 of REN 1000/2023 “gave a blank check to the concessionaires to block Access Opinions and must be corrected.”

According to him, “flow reversal will not always cause degradation of network parameters. The rollback will only be harmful if it exceeds some operational limit of the system”, he added.

But what to do in this situation?

O Solar Channel heard from lawyers who are experts on the subject to better guide integrators who feel aggrieved. “What the entrepreneur needs to know is that there can be an inversion of the power flow, but he can only accept this claim from the distributor if the distribution concessionaire proves it, motivated and in detail”, he explains. Thiago Bao Ribeiro, attorney.

The majority of complaints point to standard responses issued by Concessionaires. They use §1 of Art 73 of REN 1,000/2021, included by 1,059/2023, as support for the statement, but without making available the studies that led to this conclusion.

For the lawyer, this is the first step to take after the denial. “Art. 78 of the same resolution 1,000 obliges the distributor to provide these studies within 10 days. From then on, it is up to the integrator to verify whether the company has evaluated each hypothesis described in art. 73 of resolution 1,000, one of which is the restriction of injections at certain times”.

According to the standard, the distributor has to analyze each of the hypotheses described and choose the one with the lowest overall connection cost. For new connections, the resolution provides for the possibility of indicating “viable options” so that inversions do not occur, pointing out five alternatives, including “reducing injectable power on pre-established days and times or dynamically”.

“The vast majority of connection quotes are being issued without demonstrating flow reversal and maximum connection/flow capacity without flow reversal, violating the resolution”, emphasizes Ribeiro.

Lawyer Lucas Pimentel reinforces that “the study must consider all alternatives, with the consumer being able to choose the one that is least expensive for them. As it stands, the Concessionaires simply throw out a hypothesis that makes the operation unfeasible, without demonstrating the analysis of the other hypotheses or demonstrating that there was a study that attests to the flow reversal”.

To begin resolving the problem, the first step is to request access to the Dealership to the studies that supported the instruction and review them. If necessary, there is the company's ombudsman and also the National Electric Energy Agency (Aneel). Finally, if the conflict is not resolved, the Judiciary may be called upon, “especially to force the distributor to present detailed studies and/or consider more economical and viable alternatives for consumers”, details Ribeiro.

The problem that started with Cemig D, according to a survey by ABSOLAR (Brazilian Association of Solar Photovoltaic Energy), has gained national scope, involving at least the following companies: CPFL Paulista, RGE, Celpe, Coelba, Enel (CE) , Energisa (MS), Enel (GO), Equatorial (PA) and Energisa (TO).

* With collaboration from Filipe Calmon, journalist at Canal Solar in Brasília (DF)