Can the photovoltaic inverter reduce power due to temperature?

What is temperature power reduction?

Inverters for photovoltaic systems are, in a simplified way, made up of a module responsible for converting DC levels and searching for MPP, a module that transforms DC energy to AC, a logic and control module and a sensor module.

In the DC level conversion and DC to AC energy conversion modules, the inverter is working with high current and voltage values, which cause heating of the electronic switching, filtering and energy conditioning components.

Figure 1 – Simplified block diagram of a photovoltaic inverter

The heat generated in these modules, when not dissipated effectively, can lead to overheating of the inverter components. To prevent this overheating from damaging its sensitive components, the inverter reduces its operating power.

This protection mechanism is called “temperature power reduction” or temperature derating. Temperature derating is a controlled way of reducing inverter power.

The power that the inverter receives from the PV array changes constantly and depends mainly on the level of solar irradiation and the temperature of the PV modules.

The inverter is capable of changing its operating point, that is, the voltage and current converted in the DC and AC module, to adapt to new generation levels. The greater the power received by the PV array, the more heat is generated in the inverter components.

Once the maximum temperature allowed in the components is reached, the inverter changes its operating point to reduce its operating power. The power is reduced in steps.

In extreme cases, the inverter may even shut down completely to protect itself. As soon as the temperature of sensitive components falls within a safe value, the inverter returns to operating at its optimum point.

Overheating and temperature derating occur for several reasons, including:

• The inverter is not able to dissipate heat due to unfavorable installation conditions;
• The inverter is operating exposed to the sun or in an environment with high temperatures that cause heat dissipation to be inadequate;
• The PV set and inverter are not well sized (comparing the power of the PV set with the power that the inverter can operate);
• The inverter installation location is at an unfavorable altitude (for example, altitudes close to the maximum operating altitude). As a result, temperature derating is more likely to occur since the air is less dense at higher altitudes and therefore less capable of cooling components;
• A high DC voltage (V_MPP) is present in the inverter.

The figure below illustrates the effect of ambient temperature on the power that the inverter can supply to the grid.

Figure 2 – Inverter output power normalized by ambient temperature

The DC voltage level has a considerable influence on the temperature rise of the inverter, therefore it can accelerate overheating and temperature derating. The graph below illustrates the output power reduction in relation to ambient temperature for 3 DC voltage levels (390 V, 600 V and 800 V). This reduction serves to prevent overheating of the inverter components.

Figure 3 – Inverter output power normalized by ambient temperature for PV array voltage of 390V, 600V, 800V

The following figure (Figure 4) shows as an example the different operating points of a PV system (Australia, Alice Springs; 140% oversizing) in relation to the ambient temperature and DC operating voltages of the system. The inverter can operate continuously at its maximum power point as long as it can remain on the left side of the characteristic curve (in blue).

In the figure, we can see that the inverter's highest DC operating level point decreases as the ambient temperature increases: from 800 VDC at 15°C to 720 VDC at 40°C. There are no inverter operating points allowed for temperatures above 31°C at 800 VDC.

Figure 4 – Characteristic curve and inverter operating points in relation to DC operating voltage and ambient temperature

The blue curve on the graph indicates the DC level operating limits in relation to the ambient temperature at which the inverter can operate.

System design and power reduction by temperature

Although unwanted, temperature derating does not need to be avoided at any cost. Systems must always be optimized for the highest total energy generation, even if in a short period of time, the inverter suffers power reduction due to temperature. As the energy generated is dependent on the power of the PV array and the efficiency of the inverter, the aim is to make these values as high as possible.

Figure 5 shows the amount of energy produced per year in a given city in Germany distributed across power ranges in which the inverter operated. The figure also shows the frequency at which the system operates in these power ranges.

Figure 5 – Distribution of power intervals and frequency with which these points occur

The graph shows, for example, that the system generated 5.5% of its total energy when the power supplied by the modules was 70% of the inverter's nominal power.

We then note that the share in total power generation when the power supplied by the PV array is 90% or more than the rated power of the inverter is small. The range in which most energy is generated is between 20% and 90% of the inverter's nominal power.

Let's assume that you want to mitigate the effects of temperature derating at peak generation times of a PV array by selecting an inverter with a nominal power greater than 100% of the array's power.

By doing this, we will be moving these generation intervals to regions where the inverter efficiency is lower. These losses caused by operating the inverter in regions of lower efficiency are greater than the gains from avoiding power reduction due to temperature at peak generation times.

Figure 6 – Efficiency, DC power and output of the inverter when the nominal power of the equipment is 90% to 100% of the power of the PV array

Figure 7 – Efficiency, DC power and output of the inverter when the nominal power of the equipment is greater than 100% of the power of the PV array

Derating rarely occurs when the PV system is well designed. Temperature derating is more common when the inverter is undersized in relation to the PV array.

Heat dissipation in inverters

The inverters are designed with cooling systems suited to their power. Inverters can be cooled in two different ways: passively, by dissipating heat to the atmosphere, or actively, by dissipating heat through cooling systems with fans.

In inverters with passive cooling, heat is conducted from the components to a metallic body with fins and dissipation occurs through natural air convection. The hot air is less dense and tends to move upwards, “pulling” the cold air towards the inverter heatsinks.

In inverters with active cooling, the cooling system takes effect as soon as a certain temperature is exceeded. When this temperature is reached, the fans come into operation, drawing cold air to the inverter's heat sinks, thus cooling its components.

Typically this design is adopted in inverters with higher power, since passive dissipation is not capable of cooling an inverter with the same efficiency as the active system.

To avoid power reduction due to temperature, the inverter must be installed appropriately to aid heat dissipation. Below are some installation recommendations:

• Install the inverter in less hot environments, e.g. a basement instead of an attic;
• Choose a location with sufficient air circulation. Install additional ventilation if necessary;
• Do not expose the inverter to direct solar radiation. If you are installing an inverter outdoors, place it under shade or install a roof over it;
• Maintain a minimum distance between adjacent inverters or other objects as specified in the installation manual. Increase the distance if the installation environment is likely to have high temperatures;
• When installing multiple inverters, arrange them so that the air intake from one device is not drawing hot air from another piece of equipment. Passively cooled equipment is oriented in such a way that the heat generated by the heatsinks flows upwards.

One of the possible arrangements that maximize the dissipation of a set of passively cooled inverters is shown as an example below.

Figure 8 – Optimal arrangement of inverters with passive cooling

The optimal arrangement for active cooling inverters depends on the position of the air inlet and the position of the hot air exhaust. Several examples can be seen below.

Figure 9 – Optimal arrangement of inverters with active cooling

Figure 10 – Optimal arrangement of inverters with active cooling for the SMA STP50-40 model

Other causes of power reduction due to temperature

Inverters are typically designed to be below the permitted operating temperature as long as the PV system is properly designed and operating under suitable environmental conditions. However, temperature derating can still occur due to the following reasons:

• The inverter could not dissipate enough heat to the atmosphere because the heat sink or fan grilles are clogged or the fan is broken. Clean the affected parts described in the inverter installation manual;
• The power of the selected inverter is very low compared to the power of the PV array. This configuration can make financial sense in some cases and is increasingly adopted in facilities. Even if the PV system is properly designed, the power of the PV array may exceed the rated power of the inverter in extreme weather events, such as high solar irradiation combined with low temperature of the modules;
• The inverter installation location does not have desirable climatic characteristics (see the technical data section in your inverter manual). In this case, the inverter must be relocated by a qualified installer to a more appropriate location. Be sure to maintain the recommended distances between multiple pieces of equipment. Increase this distance further in hot installation environments. Install inverters away from the hot air vents of other inverters. Provide additional cooling for the inverter if necessary. Ventilate the inverter assembly in such a way that the airflow can cool all equipment equally.