Engr. Jet Andal has 6 years of experience in the design and installation of residential, commercial and utility-scale solar PV systems. Together, and with the use of solar energy, let us help make the world a better place. You can click here to read all of our other blogs. For aspiring solar PV engineers, you can also check out his Solar PV Engineering Ebook on Amazon on this link.
Types of Solar Inverters: String, Central, and Micro
Although there are three types of solar solar inverters available in the market today, they are basically the same in terms of their functions. The main difference between the three is their size and their application.
String Inverters – these are commonly used in residential and commercial applications and their sizes range from 1 to less than a hundred kilowatts. These can be on-grid, off-grid and hybrid inverters.
Central Inverters – these solar inverters are used for utility-scale or solar farm applications. Their sizes range from 100kW to a few megawatts. In solar farms, a power station is used with central inverters to be able to connect directly to the grid. Central inverters are used on utility-scale applications because they are cheaper in per kilowatt cost and are easier to install (less total number of units installed compared to if string inverters were used). Although, solar PV systems using central inverters have a disadvantage of having one point of entire system failure.
Microinverters – these are, as their name suggests, much smaller than string and central inverters. They can only usually accommodate 2 PV modules and are rated at 500W. These are used for residential applications but their use in commercial systems is slowly gaining more popularity. Microinverters have an advantage compared to string inverters of having module-level MPPT. We will discuss more about this later in this chapter: Inverter Functions: MPPT. This adds an advantage of increased flexibility to accommodate different roof sizes, increased efficiency and less effects of shading. But it is more difficult to install and maintain because of more units needed.
Inverter Functions: Basics
The inverter, on the most basic level, converts the DC electricity produced by the PV modules to AC electricity. It can be considered as the brain of the solar PV system as it controls many aspects of its operations, from PV array production, data collection, grid management, and system protection. Solar inverters have the advantage of being “automated” in its operations and this is because of how far solar inverter technology has come.
It is very important to really understand how the inverter functions and its properties as a solar PV engineer because they greatly affect system design. Its input parameters, voltage, current, number of MPPTs, DC rated power, etc., all dictate PV array design, while its output parameters are crucial to compatibility with the building’s electrical system.
The inverter also has functions other than converting DC power input to AC. These include MPPT, grid management, and protection functions, all of which are very crucial in maintaining smooth and proper operation for the whole solar PV system.
Inverter Properties: Input Parameters
The input part of SMA’s STP 15000TL inverter datasheet is shown below:
Max. Generator Power – the maximum DC input power to the inverter. It is important to note that there is a difference between the DC rated power of the array and the DC power that actually reaches the inverter. The sizing of the PV array with respect to this parameter is discussed more on the chapter on PV Array Sizing Ratio.
DC Rated Power – the maximum DC input power on one of the inverter’s inputs. The value on the datasheet is much less than the solar inverter's rated output of 25kW because this is the DC input power per input and this inverter has 2 MPP inputs. The MPPT function of the inverter is discussed in the next section, 5.4 Inverter Functions: MPPT.
Max. Input Voltage – this determines the maximum DC voltage that your PV array can have. 1,000V DC PV array systems are most commonly used so most inverter manufacturers have solar inverters that have a maximum input voltage of 1,000V. As 1,500V DC solar PV systems become more and more popular, manufacturers will surely adopt this and release more solar inverters with a maximum input voltage of 1,500V.
MPP voltage range – this will be discussed in the next section.
Min. Input Voltage – this is the minimum input voltage that must be met before the inverter starts to get DC power from the PV array for conversion to AC. Having a smaller minimum input voltage means that the inverter will start converting power from the PV array earlier in the morning and later in the afternoon, where the irradiance and voltage are lower.
Max. Input Current Input A/Input B – the maximum allowable current on each of the MPPT inputs of the inverter. This affects how many strings you can connect to each input.
Number of independent MPP inputs/strings per MPP input – this will also be discussed in the next section.
Inverter Functions: MPPT
MPPT stands for Maximum Power Point Tracking. Remember that on the PV module’s IV curve, there is a point where the PV module produces the maximum amount of power and is most efficient. This point is called the MPP or maximum power point. The inverter’s MPPT function forces the PV array to always operate on this point.
The inverter does this by varying the resistance of the input circuit. Remember in Ohm’s Law that the voltage, current, and resistance in a circuit are related by the equation:
V = I *R
Where: V = Voltage
I = Current
R = Resistance
When the resistance of the input circuit is changed, the voltage and current values can also be changed. The inverter tracks the output of the PV array for each change in input circuit resistance and does a quick trial and error-like algorithm to find the resistance value that will produce the highest PV array power. In this way, the inverter is able to choose which operating point the PV modules operate on.
You may ask the question: If we already know the MPP of the PV module from the IV curve, why don’t we just set the input circuit resistance that will produce the VMPP and IMPP? This is because the IV curve is only for a specific irradiance and temperature, specifically for STC where the irradiance is 1,000W/m2 and a cell temperature of 25OC. Actual conditions vary and thus, the MPP varies as well.
Central inverters usually only have 1 MPPT input for the whole PV array. This is because solar farms, in which they are used, have a uniform PV array in which all the PV modules have the same tilt and orientation. This means that at any point throughout the day, the PV modules receive the same amount of irradiance and thus, have the same MPP. There are still some problems with this, however. One problem is cloud cover. Clouds can cover only a part of the PV array which will make the MPP of the covered PV modules different. In these cases, the inverter will choose an operating point where the total power output of the PV array is at its maximum. This does not necessarily mean that each individual PV module will be operating at their MPP.
Some string inverters have 2 MPPT inputs. The purpose of having 2 MPPT inputs is for the inverter to accommodate PV modules installed on 2 different roof orientations because different roof orientations receive different amounts of irradiance. When this happens, these PV modules will also always have different MPPs. In the design of solar PV systems, strings on different roof orientations are always put on different MPPT inputs.
Lastly, microinverters have module-level MPPT. Microinverters usually has 2 inputs for 2 PV modules with an MPPT for each input. It means that each PV module will then operate independently from each other. When one PV module is shaded, it would not affect the other PV module’s production. This reduces the effects of shading and cloud cover to a minimum. It even completely eliminates mismatch losses as each MPPT can adjust to slight variations in the MPP of each module due to manufacturing imperfections.
We have 2 more inverter properties that we have not yet discussed in the previous section:
MPP voltage range – this is the voltage range of the string in which the inverter can track the MPP of the PV modules. The number of modules in a string of the PV array is always designed so that the string VMPP will always be in this range.
Number of independent MPP inputs/strings per MPP input – this represents the number of MPPTs that an inverter has. The strings per MPP input is the number of strings that you can input to each MPPT.
Inverter Properties: Output Parameters
The output part of SMA’s STP 15000TL inverter datasheet is shown below:
Rated Power – the rated maximum power output of the inverter.
Max. AC Apparent Power – the rated maximum apparent power of the inverter. The apparent power consists of two parts: real power and reactive power. The real power is the power absorbed by the loads themselves while reactive power can be seen as waste power because it is just absorbed by capacitors and inductors to maintain a constant voltage level. However, there are cases where apparent power is useful because this may be used to regulate grid voltage.
AC Nominal Voltage – specifies the characteristics of the inverter’s output socket. 3/N/PE means that its output has three line wires, a neutral and a protective earth or ground. The voltage values specify the line to neutral and line to line voltages, respectively.
AC Grid Frequency/Range – the AC grid frequency is the nominal frequency of the oscillations of alternating current in a utility grid. It varies according to the country-specific standards that the inverter is manufactured for. The next value is the acceptable range of grid frequency variations before the inverter calls its anti-islanding feature. This will be discussed more in the chapter on Inverter Functions: Grid Management.
Rated Power Frequency/Rated Grid Voltage – the rated frequency and line to neutral voltage of the inverter’s output.
Maximum Output Current/Rated Output Current – the maximum and rated output current that the inverter can produce during normal operation.
Power Factor at Rated Power/Adjustable Displacement Power Factor – the power factor cos is given as a numerical value between 0 and 1 and describes the quantitative ratio of the real power flowing to the load to the apparent power in the circuit. Having a power factor of 1 means that the inverter is only supplying real power to the load. The adjustable displacement power factor is the range in which the power factor of the inverter can be adjusted based on grid requirements. This will be discussed more in the chapter on Inverter Functions: Grid Management.
THD or Total Harmonic Distortion (THD) – defined as the ratio of the sum of the power of all the signal harmonics to that of the power of the fundamental frequency. What is important to note here is that the lower the THD is, the better is the output of the inverter as these signal harmonics are basically electrical noise and are wasted energy.
Feed-in Phases/Connection Phases – the number of phases on the inverter output. Most solar inverters designed for commercial solar PV systems have three phases to match the commercial building’s electrical system of having three phases. Solar inverters for residential systems, on the other hand, only have a single phase to match the residential electrical system.
Inverter Properties: Efficiency
The efficiency of the inverter is defined as the ratio of the output power to the input power expressed as a percentage. The more efficient the inverter is, the less input power is wasted by the inverter during its operation.
One source of losses in the inverter is the power required to keep the inverter “on”. This is a constant value which is why the inverter efficiency is lower when the power output is also lower. A typical inverter’s efficiency curve with respect to its output power is shown below:
There are three types of inverter efficiencies that you may see in inverter datasheets:
Peak efficiency – indicates the performance of the inverter at the optimal power output. It shows the maximum point for a particular inverter and can be used to get an idea of its quality.
European efficiency – the weighted number taking into account how often the inverter will operate at different power outputs. It is sometimes more useful than peak efficiency as it shows how the inverter performs at different output levels during a solar day.
California Energy Commission (CEC) efficiency – also a weighed efficiency that is similar to the European efficiency, but it uses different assumptions on weighing factors.
Inverter efficiency is commonly used to compare the quality of available solar inverters but usually, the differences in efficiencies of top inverter brands are very small that it would be better to base your selection on other aspects of the inverter like the number of MPPTs, additional features, etc.
Inverter Functions: Grid Management
One problem with solar energy is that our utility grid is not made for decentralized generation. The grid was designed to provide power from centralized power plants to end-users. Solar PV systems, however, sometimes feed into the grid the excess generated power that cannot be consumed by the user. This is called backfeeding. Even though it is not designed to operate this way, the utility grid can still handle backfeeding, but the amount of backfed power depends on the age and complexity of the local grid equipment.
Because of this, decentralized energy generation systems like solar energy were required to participate in grid management and provide grid services to an extent. As solar PV grows and the number of connected systems grows as well, it puts more responsibilities to the electrical grid to keep itself stable. Solar inverter manufacturers, however, are agreeing to help out and participate, which is why available on-grid inverters right now in the market all have built-in grid management functions.
Some of these functions are:
Active Power Curtailment – there may be times when specific portions of the grid are temporarily overloaded. When this happens, the inverter shifts the PV array’s operating point away from its MPP to reduce the output power. This is one way for the utility grid to balance out generated power to the load.
Reactive Power Control – reactive power can help regulate grid voltage. The utility can tell the solar PV system how much reactive power is needed to be fed into the grid.
Power Factor Control – the inverter can adjust its power output from having a power factor of 1 (active power only) to 0 (reactive power only) according to grid requirements.
Voltage Ride Through – during times when there is a dip in the grid voltage, power plants may have a chain reaction of disconnecting, leading to grid instability. This is called cascading. Voltage ride through is the inverter function that prevents it from disconnecting.
Frequency Ride Through – this is the same with voltage ride through, but with respect to the variations in grid frequency. Both functions aid in grid stability during these times.
Ramp-Rate Controls – the inverter can control the rate at which it transitions between different established power factor points. This ensures the plant output does not ramp up or down faster than a specified limit.
Inverter Functions: Safety and Protection
As we have said, the inverter is the brain of the solar PV system. Therefore, it is only logical that it should also be the one to check on fault conditions and do the appropriate actions. These safety and protection functions are industry standards, so all the top-quality solar inverters available on the market have these functions.
The main certifications that apply to the safety and protection systems of inverters are UL 1741 (Inverters, Converters, Controllers and Interconnection System Equipment for Use with Distributed Energy Resources), IEC 62109-1 (Safety of Power Converters for Use in Photovoltaic Power Systems – Part 1: General Requirements) and IEC 62109-2 (Safety of Power Converters for Use in Photovoltaic Power Systems – Part 2: Particular Requirements for Inverters).
The main inverter safety and protection functions are listed below:
Anti-islanding Protection – an island is defined as a scenario when there is a solar PV system connected to a feeder and a fault happens on that feeder which causes the substation breaker to open (a blackout or a power outage). The solar PV system will then unintentionally energize a portion of the grid through its interconnection point. This will be an electric shock hazard to personnel that will work on the electrical lines, with them knowing that these lines are not energized. Anti-islanding protection is required for UL1741 / IEEE 1547.
Ground-fault Protection – a ground fault happens when a current-carrying conductor makes inadvertent contact with an equipment grounding conductor or any piece of metal that is grounded. This can occur through damaged conductor insulation or with improper installation. This situation presents a serious electric shock hazard and therefore, the inverter is designed to be able to detect these faults. The inverter checks for ground fault before starting in its normal operation by checking the resistance to ground of both the positive and negative sides of the DC input terminals. If there is low resistance to ground, the inverter will not start operating. During its normal operation, the inverter continues to check for ground-fault conditions by measuring the current on both DC input terminals. Having a ground-fault would mean that some of the currents from either the positive or negative terminals are going through a grounded conductor or metal. Since the currents on both positive and negative terminals of any circuit should always be equal, if there is a difference in the currents flowing on both DC input terminals, this is an indication of a ground fault. The inverter then shuts down and stops producing power.
Surge Protection – surge protection devices or SPDs are used to protect electrical devices from lightning and overvoltage risks. The risks of lightning can be categorized into 2: direct, which is the direct lightning impact on the PV modules, and indirect, which are overvoltages on modules, on the converter/inverter and other connections.
Overheating Protection – if the temperature inside the inverter becomes too high, it will immediately stop producing power for a while.
Short-Circuit Protection – if the AC output circuit of the inverter experiences a short-circuit, the inverter will immediately stop producing power.
Besides all of this, off-grid inverters have additional protection functions that are related to the batteries of the system:
Automatic Voltage Stabilization Function – when the battery voltage fluctuates between the under-voltage and over-voltage values the inverter regulates the battery voltage.
Over-voltage Protection Function – when the battery voltage goes higher than the over-voltage point, the inverter will immediately stop producing power until the battery voltage goes back to normal.
Voltage Protection Function – when the battery voltage goes lower than the under-voltage point, in order to avoid over-discharge damage to the battery, the inverter will immediately stop producing power.
Data Monitoring
Some solar inverters that are available in the market today have built-in data monitoring capabilities while some require a separate monitoring device. Data monitoring is a function that tracks operating parameters like PV array power input, AC power output, etc.
This function is very important in operations and maintenance and in measuring system performance. Because of a monitoring system, the solar PV engineer can monitor system performance and condition remotely. In some cases, troubleshooting can also be done remotely, saving time and effort that would have been exerted in an actual site visit.
For large-scale solar PV systems using SMA solar inverters, they use a separate device called the Cluster Controller. This is not only used for monitoring and recording data but also for controlling the whole solar PV plant.
The SMA Power Plant Controller, and most monitoring devices, have input ports that can be used to also monitor weather data through sensors. This is useful, especially for larger solar PV systems as it can give you a direct basis of how well the solar PV system is performing. By knowing the exact weather conditions on-site at any given time, we could already get an idea of how much power the solar PV system should be producing.
The two parameters that directly affect solar PV production are irradiance and module temperature. Irradiance can be measured using a pyranometer or a solar-cell based irradiance sensor. PV module temperature can be directly measured using a temperature sensor mounted on the back of one module in the PV array. Other sensors that are also used are rain gauges, anemometers or wind speed sensors, humidity sensors, etc. But using these sensors is an “overkill” because as we said, the only parameters that directly affect solar PV production are irradiance and module temperature. For example, wind speed affects solar PV production but it does this indirectly. Having a higher wind speed means more ventilation for the PV modules. This reduces its temperature, which is the one that directly affects solar PV production.
Engr. Jet Andal has 6 years of experience in the design and installation of residential, commercial and utility-scale solar PV systems. Together, and with the use of solar energy, let us help make the world a better place. You can click here to read all of our other blogs. For aspiring solar PV engineers, you can also check out his Solar PV Engineering Ebook on Amazon on this link.