Overvoltage: main problems in photovoltaic installations?

Photovoltaic inverters increasingly have embedded technologies and monitoring and problem detection facilities, with different types of alerts for each possible anomaly identified. 

Therefore, their indications facilitate analysis by the technicians responsible for correcting the problem.

However, a greater amount of information about the system may not represent an advantage if the person responsible for the analysis does not know the concepts behind each of the errors. 

In equipment manuals, for example, there are lists with various error codes, as shown below.

In this article we will address the three most common errors that reach technical support, all of which indicate abnormalities in the installation and which require analysis and correction in the field. 

Most of the errors mentioned and presented in inverter alerts relate to a problem in the installation and not in the inverter, which only identified it.

Remembering that the codes vary for each manufacturer, however the concept behind the cause of the error is the same for the vast majority of alerts generated.

Table 1 – List of errors in an inverter installation and operation manual. Source: Sungrow

Among the most common errors in photovoltaic systems that reach technical support are AC overvoltage, islanding and low insulation resistance, each of which will be addressed exclusively in a series of three articles, starting with this one, where we will deal with the error. overvoltage, which is without a doubt the most common demand.

The overvoltage error, when the occurrence is on the utility side (alternating voltage), represents that the inverter is measuring, at its input, a voltage value per phase greater than that configured for protection in the inverter. 

All equipment of this type has a nominal operating voltage and a limit range of variation, defined by standards and resolutions of ANEEL (National Electric Energy Agency), which regulates the sector in Brazil, as well as ABNT (Brazilian Association of Technical Standards) . 

According to these, every photovoltaic inverter must necessarily go into protection and shut down if the values in the table below are reached:

In other words, in an electrical network with a nominal voltage of 220 V, the inverter will act to protect against overvoltage when reaching 242 V and undervoltage at 176 V. This should be uncommon, but it occurs extremely frequently in several installations in Brazil.

And what is the reason for this? There are some reasons that can cause an overvoltage error. A less common, and simple to resolve, is when the setup was configured incorrectly. For example, let's say that the network where the system was installed is a 254V rural single-phase network, common in Brazil. 

If the previously informed option is selected (Vname = 220 V), obviously the system will not operate, as the nominal voltage (254 V) is already higher than the configured protection value, making it necessary to correctly adjust the protection values.

However, the most common cases do not concern this, but rather the actual increase in AC voltage that reaches the inverter, which is caused by one of the following two options: poor utility power supply or internal problems in the AC installation of the photovoltaic system. 

However, before going into the details of each one, let's understand how overvoltage occurs. The voltage variation is related to the current that will be transported between the points of the installation or, in general, between the generator and the load. We can illustrate it as follows: The point where the generator is located, in our case represents the photovoltaic inverter. The load can be the customer's own internal consumption or, in cases where there is a generation surplus, the additional energy is transported to the concessionaire, serving neighboring consumers. 

In both, when transporting energy between two points, we have a voltage variation due to the impedance or resistance existing in the section, which is obtained by Ohm's 1st Law:

ΔV=Vgenerator–Vcharge=Zeq*I

Where Zeq is the equivalent impedance of the circuit and I is the current flowing through it. In other words, the greater my circulating current, and the greater the impedance (resistance) of the circuit, the greater the voltage variation.

This is why it is so common for overvoltage problems to occur, especially during peak hours of the photovoltaic system – this is when we have the highest value of electric current circulating through this section. 

And for the same reason, at times of lower generation or in the case of the inverter being turned off, the electrical voltage measured is “normal”. Of course, there is no current circulating, consequently there is no variation in the electrical voltage between the points.

And as it is not viable, much less interesting, to reduce the injected current (reduce power), we have to understand why the impedance rises to the point of tripping the equipment due to overvoltage.

As previously stated, this is due to the internal electrical installation to which the inverter is connected or to the power company's own electrical network.  N

In the first case, the precariousness of the electrical installation can be caused by a very long section of AC cabling without the appropriate cable section or some splice or connection in the panels that may be poorly tightened, generating a high resistance (hot spot) — all This can cause a considerable increase in voltage. 

Or, something very common to happen: you (or the engineer responsible for the work) correctly sized the cable section for the inverter, checked all the connections up to the customer's internal panel, and everything is ok.

However, what is the quality and reliability of the existing network, connecting your client's overall framework to the entry standard? That may be where the problem lies. 

In cases where the problem is in the concessionaire's network, it is quite common to occur mainly in rural networks and end-of-line networks, with lower quality and reliability in the supply of electrical energy. 

Problems generally occur in older electrical networks, with less maintenance. When they suffer a high injection of current from the inverters, they are unable to transmit this energy over a long distance, with satisfactory quality to maintain the electrical levels required by standards.

As stated at the beginning, the overvoltage error in the vast majority of cases will be a problem with the installation, not with the inverter. So, the first step is to try to identify the origin or cause of the overvoltage in the electrical network. 

Many installers go straight to expanding the inverter's operating voltage range, as the vast majority support higher voltages, reaching values such as 270 V, for example.  However, this has some implications:

• This high voltage range will not always make the inverter work at its optimum point, reducing system efficiency;
• If there is a problem in the installation, sooner or later it can get worse and, even with the overvoltage adjustment, the error continues to occur, causing loss of performance;
• The voltage, when adjusted, will be high as a whole in the installation. Therefore, there is a risk of burning the customer's electronic equipment, which may be more sensitive to this variation;
• Current standards require overvoltage protection to operate with 10% above the nominal voltage (as previously mentioned). Any damage to the electrical network resulting from these changes may be held responsible for the consumer unit where the inverter operates outside these limits.

Therefore, we must know the concepts that cause this elevation and act to correct the problem. Initially, it must be checked whether the electrical project really meets the minimum requirements, whether it is correctly dimensioned and, based on this, whether the installation matches what was designed. 

Once this is done, a possible way to get a first indication of the location of the problem is to measure the voltage at some points, simultaneously, to find out in which section the voltage variation is occurring. For example: In the figure above we can see the voltage measurement being carried out at 3 different points of the installation: at the photovoltaic inverter, at the customer's internal AC panel, where the photovoltaic system is interconnected, and finally at the utility's input standard.

The measurement must be carried out with the inverter running, of course, as this is when the overvoltage occurs. With this, we can have an initial diagnosis as follows:

• Voltage increase in the section between inverter and AC panel. Possible problem in PV installation: cables, connections, poor tightening, poor quality transformers (high impedance), etc.;
• Elevation in the AC frame section – input standard: Existing infrastructure at the customer with problems, whether of low quality, poorly dimensioned, with poor connection, etc.;
• “Joint” elevation at all points, that is, from the inverter to the standard, the voltage remains very close, even during the voltage elevation. Possible infrastructure problem at the dealership.

In the first two, a detailed inspection of the installation will be enough to find the problem and correct it. In the latter, it depends on a complaint being made to the energy company for reinforcement or repair of the network, as it is the only permanent solution in the case. 

Let us remember that when issuing the access opinion, the concessionaire guarantees us that we can make the PV connection with the indicated power and that the electrical grid will support such a system. 

A quality analyzer installed on site for a period of time, collecting information, will be of great value in this argument with them.

Conclusion

Overvoltage should not be the case, but it is a common problem in photovoltaic systems installed in Brazil. Poor quality electrical networks on the part of concessionaires, especially in more remote locations, make the generation of electrical energy by inverters unfeasible or detrimental. 

Add to this several installation errors, poor quality of materials and equipment used, and we have the most common error in Brazilian photovoltaic systems. 

To make the situation worse, in many cases the solution sought is to adjust the inverter protection parameters to their maximum limits, as simply as if we were adjusting the date and time of the equipment, without prior analysis of the problem and without understanding the risks involved in this process, without knowing the origin of the problem of rising AC voltage.  

These and other reasons support increasingly frequent complaints from end users (customers) dissatisfied with a product that should only bring security and savings on their energy bills.

These challenges highlight the importance of invest in solar energy training, where professionals can acquire specialized knowledge about identifying and solving problems related to the operation of photovoltaic systems. The search for suitable solutions and understanding the underlying principles are essential to ensure customer satisfaction and the efficiency of solar systems.