PV Inverter Performance:
The efficiency of any photovoltaic inverter on the solar market is affected not only by the losses produced by switching but also by losses due to passive elements, such as transformers, filters, capacitors, etc. … Solar inverters generally have a full load efficiency of between 90 and 94% (or even more today) for low voltage input systems (400 V).
In general the efficiency of an inverter is higher in inverters with low void losses, and the efficiency increases if the DC input voltage of the solar panels grows.
Fundamental invester requirements:
The flow of electric energy normally always goes in the direction of charges, when a power outage occurs, the inverter has to stop and stop injecting into the power grid.
The basic requirements for any solar inverter such as SMA 20kW and 25kW inverter are:
- The photovoltaic installation should be considered as part of a component of the electrical network.
- The inverter must meet the required specifications of the electrical network.
- The safety of operators must always be taken into account (there may be unknown active parts).
- Photovoltaic installations should not deliver power to a power line without protections.
- Disconnection of the inverter must be automatic as soon as a fault appears.
- There must be an easy disconnect point (border switch) that is accessible to employees of the power company at any time.
- The inverters must operate with a power factor equal to one.
- Electrical isolation between the photovoltaic system and the grid must be carried out.
- If the output signal of the inverter exceeds the predefined conditions for operation (over / under voltage, over / underfrequency) the inverter must be automatically disconnected from the grid.
- The new connection will be enabled after a certain time (3 minutes normally), time that the control system and network protection waits to try a new connection.
Inverter signal quality:
The two main technical issues to consider in an inverter, such as SMA STP25000TL and 20000TL, from the point of view of signal quality are: power factor and harmonic distortion.
The harmonic content in the output signal of an inverter for photovoltaic use is difficult to fix because there is not much information available. The effects of harmonic distortion on the electrical network itself and on the loads connected to it are not sufficiently studied and, finally, there are many difficulties in measuring the distortion obtained.
Within all specifications, the ones related with the current should be taken into account more than the voltage, since current harmonics are the most troublesome cause and because it is easier for the equipment manufacturer to demonstrate full compliance with the specification of current injection. This specification is usually given in terms of total harmonic distortion (THD). The magnitude representing any of the harmonics separately is called simple harmonic distortion (SHD) and is equally interesting for power companies.
The values normally used for both distortions are 5% and 3% respectively.
Another need of considerable importance in the connection with the electrical network is the obtaining of an adequate isolation. This can be obtained with a transformer located between the network and the inverter. Galvanic isolation is required for safety reasons.
Several regulations from EU countries indicate that a solar inverter can not be connected to the grid unless some form of isolation is provided. The self-generator and the electric company must use protections that can interrupt continuous fault currents. A fault in the inverter with continuous levels can saturate the distribution transformer of the electric company and cause service malfunction to other subscribers.
Both, the SMA Sunny Tripower 20000TL solar inverter and the SMA Sunny Tripower 25000TL meet these minimum requirements.
The SMA 20kW and 25kW solar inverter models includes control of the entire system. This includes detecting that the photovoltaic field has enough power to be able to connect to the grid, closing a contactor at that moment and starting to operate as soon as there is sun light. At night the inverter must be fully disconnected. The control logic of the photovoltaic inverter will include a protection system that detects abnormal operating situations such as:
- Lack of continuous ground.
- Abnormal conditions in the electrical network (voltage, frequency, etc.)
- Losses in one phase.
- Stop the solar inverter.
- The inverter must be protected against voltage transients using varistors on the DC and AC side.
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