A well-designed and well-installed grid-connected photovoltaic system should run trouble-free for many years, but there will still be problems. If the system does not work, it is generally due to one of two reasons: either part of the equipment is malfunctioning, or there may be a problem with the power grid. The cause of the problem must be found, and the problem must be fixed. Troubleshooting should be performed by appropriately qualified technicians, as this type of work involves risks such as high-altitude work, high voltage, etc. The following will discuss troubleshooting and problem solving in some common situations.
1. Identify the problem
By monitoring system performance, system owners will notice that the system is not working or its performance is too low. If the problem cannot be detected immediately by visual inspection, such as the shielding of the photovoltaic array, the owner should contact the installation company, who should go to the site for inspection.
The troubleshooting work should be carried out during the day, preferably with sufficient sunlight, so that the output of the photovoltaic module can be tested, and a complete investigation can be carried out under sufficient sunshine hours. The first thing to check is that the DC main circuit breaker/isolation device and the array AC main circuit breaker/isolation device are not disconnected.
If all circuit breakers/isolation devices are in the closed state, a visual inspection of the inverter should be carried out. Inverter failure is usually caused by electronic device failure, which may be accompanied by error messages or other displayed information. If the inverter fails, you need to decide whether to repair it on site or return it to the manufacturer. If the inverter is not turned on, but the problem is not immediately obvious, that is, there is no error message, it is usually necessary to determine whether the inverter has AC power, and then determine whether the inverter has DC power. If there is no AC power from the grid, all paths from the photovoltaic system to the grid connection point must be systematically checked to find faults. This involves measuring whether there is an AC voltage:
1) Measure at the meter.
2) Then measure the inverter side.
3) Next, measure the grid side of the main circuit breaker/isolation device of the photovoltaic array.
4) Then measure the grid connection point connected to the grid.
If the inverter and the AC circuit connected to the inverter are not faulty, the photovoltaic array should be checked.
2. Trouble shooting of photovoltaic array
This process varies according to the system configuration. The process here assumes that there is a PV combiner box between the PV array and the DC circuit breaker/isolator. If the fault occurs on the DC side of the inverter (that is, on the PV array side), the possible cause is either that the PV array power received by the inverter is insufficient and the inverter shuts down, or the system does not output the expected power due to the array. Performance is too low. All electrical service or maintenance work performed on the photovoltaic array must be performed by appropriately qualified personnel.
If the DC side of the inverter fails, the first task is to measure the open circuit voltage of the photovoltaic array at the input of the inverter and confirm whether the DC power reaches the inverter. If there is no DC power at the inverter, the technician should systematically and sequentially check the components and their connections (by measuring whether there is a DC voltage), starting from the inverter and ending with the photovoltaic array.
If there is no DC voltage at the inverter, the usual approach is to first check whether the DC main circuit breaker/isolation device is working, and whether the circuit breaker or fuse (if any) is closed and operational. If there is no DC voltage at the PV combiner box, the fault is inside the array. This generally indicates that one (or all) photovoltaic module string has failed. The possible reasons for the failure of the photovoltaic module string include:
• The cable is disconnected or the plug is faulty (if a plug is used).
·The connection inside the photovoltaic combiner box is loose.
·The photovoltaic module is faulty or broken.
The actual fault may only be found through physical inspection of cables and components and current and voltage measurements. Zero current indicates that a fault has occurred.
3. Troubleshooting for low system performance
When the system can work but the performance is too low, troubleshooting is also needed. If the system performance is too low, the following steps should be performed:
·Visual inspection of the system to check for other obvious faults such as obstruction or component damage.
·Use a clamp meter to measure the current of the array.
If there is no occlusion problem, but the current is lower than the expected value, the array test should be carried out by measuring means. If there are multiple PV module strings, each module string should be shut down systematically. If one of the photovoltaic module strings does not output power, the current will not change when the module string is turned off. Once the faulty component string is identified, check whether the circuit breaker or fuse is faulty. If there is no failure, you should perform the steps described in the previous section.
It is important to have a basic understanding of how photovoltaic modules work:
·The open circuit voltage and short circuit current can be measured only when the photovoltaic module is not loaded.
·Even when the solar irradiance is very low, the output voltage of the photovoltaic module is still close to the open circuit voltage, and the short-circuit current is proportional to the effective solar irradiance.
·The shaded photovoltaic module will not output current or reduce the current according to the shading situation.
·If one component in a long component string is blocked, due to the bypass diode, the working current of the whole component string may not change, but the open circuit voltage will decrease.
4. Troubleshooting of the inverter
There are DC voltage and AC voltage on the inverter terminals (and the photovoltaic array output is sufficient), but the inverter does not work, the inverter may have failed. Most manufacturers include troubleshooting chapters in the manual, and the manual should be consulted. In many cases, the inverter will indicate a problem with a fault indicator or error message; the manual should be read to identify and repair these errors. Some common problems are: the grid voltage is too high or too low: this indicates that the grid is faulty, so you should contact the distribution supplier.
·The grid frequency is out of range: This indicates that the grid is faulty, so the distribution supplier should be contacted.
· Array DC voltage is too low: need to troubleshoot the array.
·The DC voltage is too high: the array should be disconnected immediately, because it may damage the inverter.
·Line impedance is too high: Check the AC side wiring, it may be loose. If there are still problems, there may be a problem with the grid, so you should contact the distribution supplier.
·Excessive ground leakage current (only for transformerless inverters).
5. other common problems
In addition to technical issues, other common problems of photovoltaic arrays include customers’ unrealistic expectations of system performance. Unfortunately, there is no way to correct it, so it is important to communicate well with customers before installing the system so that they understand the capabilities of the system. When the system designer makes mistakes and the system loss calculation is not appropriate, the system output will be overestimated, and this type of problem will also occur. Such calculations should often be conservative, and the reasons for the degradation of system performance should be clearly explained to the customer.
Poor system design is also an important reason for photovoltaic system failures. Usually, this problem may occur when the photovoltaic array is not configured with the correct capacity according to the inverter used. Therefore, the output voltage or current of the photovoltaic column is often outside its input range, which forces the inverter to shut down or disconnect the grid. In these cases, errors in system design must be corrected, which may involve purchasing replacement components and installing team services. If the voltage is below the inverter voltage range, it may be due to high temperature that the voltage has dropped and is lower than expected. If the array heat dissipation is not good, the support structure must be changed to allow the convection cooling behind the photovoltaic module to correct this problem. This will lower the operating temperature of the component and reduce the amount of voltage drop under high temperature conditions. Another solution is to increase the number of photovoltaic modules connected in series in each module string. If you do this, you must carefully confirm that the system voltage will not exceed the maximum input voltage of the inverter in extremely cold weather. If the fault is caused by overvoltage, in order to reduce the overall system voltage, some components may need to be removed from the component string. Overvoltage is a serious fault that damages the inverter. Inverter manufacturers will specify the voltage operating range for their inverters, and some inverter data sheets will also display a “maximum DC voltage” parameter, which is higher than the maximum operating voltage. The inverter and photovoltaic array specifications must be confirmed in advance to avoid this problem.
Unstable power grids can also cause system failures. This type of problem generally only lasts for a short period of time, and it is difficult to identify it without the help of the grid company. Grid voltage and/or frequency fluctuations may mean that the grid is working outside the inverter’s AC voltage and/or frequency range, so the inverter will shut itself down. Most urban power grids are fairly stable, but people living in rural areas at the end of long-distance transmission and distribution lines may experience grid failures more frequently. In this case, contact the power company that manages the local grid.